MEMS-based Resonant Sensor Arrays:
Selective Detection of Volatile and Toxic Chemicals
By
George C. Whitfield
S.B. Electrical Engineering and Computer Science
Massachusetts Institute of Technology, 2003
SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND
ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF
MASTER OF ENGINEERING IN MATERIALS SCIENCE AND ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SEPTEMBER 2004
OF TECH OLOGY
© 2004 George C. Whitfield, All rights reserved.
LIFEB
The author hereby grants MIT permission to reproduce
and to distribute publicly paper and electronic
copies of this thesis document in whole or in part.
Signatureof Author: .........
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Departmen6f
LIBRA .RIES
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Materials Science and Engineering
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Certifiedby: ......................................
6 2005
Sebtmber3,2004
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[Harry
\
L. Tuller
Professor of Cerahics and Elebtronic Materials
Department of Materials Science and Engineering
Thesis Supervisor
-
Acceptedby: ...........
....................................
Carl V. Thompson, II
Stavros Salapatas Professor of Materials Science and Engineering
Chair, Departmental Committee on Graduate Students
1
MEMS-based Resonant Sensor Arrays:
Selective Detection of Volatile and Toxic Chemicals
by
George C. Whitfield
Submitted to the Department of Materials Science and Engineering
September 3, 2004 in Partial Fulfillment of the Requirements for the Degree of
Master of Engineering in Materials Science and Engineering
ABSTRACT
With growing concerns about homeland security, public health, and environmental
cleanliness, there is a strong need today for robust chemical sensing systems that are
portable in addition to being highly sensitive. While there are many options available for
gaseous chemical detection and identification, not all are well-suited toward the creation
of a portable device.
Boston MicroSystems, Inc. (BMS) has developed a resonant chemical sensor that is
predicted to meet the performance needs of the current market in terms of gas sensitivity,
operational reliability, and overall device portability. Desirable device characteristics are
attained through integrating aluminum nitride and silicon carbide in processes that are
protected through a strong base of intellectual property. By developing a standardized
platform for gas detection based on this sensor technology, barriers to entering the
targeted markets may be overcome.
Thesis Supervisor: Harry L. Tuller
Title: Professor of Ceramics and Electronic Materials
Department of Materials Science and Engineering
2
Acknowledgments
The completion of this thesis was made possible by contributions received from many
individuals. Firstly, I would like to give special thanks to Professor Carl V. Thompson II,
who provided a great deal of input and guidance during the earlier portion of this work.
He provided an objective perspective on the technology that is the subject of this report,
and encouraged me to think critically and ask tough questions.
I would like to thank my thesis advisor, Professor Harry Tuller, for his support and for a
great deal of time and effort invested in guiding me to complete this thesis.
I would like to thank the individuals at Boston MicroSystems, Inc. who are developing
the technology that is the topic of this thesis. Dr. Dharanipal Doppalapudi provided very
useful discussions in relations to materials selection and processing. Dr. Richard Mlcak,
the president of the company, offered a great deal of technical advice and provided
insightful discussions regarding business development.
Finally, I would like to thank my family for their love and support, my wife for her
patience, and my son for being a daily inspiration.
3
Table of Contents
1 INTRODUCTION
.....................................................................................................................................
6
2 MARKETS FOR PORTABLE GAS SENSORS .................................................................................... 8
2.1 HOMELAND SECURITY........................................................................................................................
2.2 HOME ANDWORKPLACE SAFETY ......................................................................................................
2.3 QUALITY CONTROL .....................................................................................
8
9
10
2.4 SUMMARYOF CURRENT MARKET POTENTIAL...............................
........................
10
3 OVERVIEW OF COMMERCIAL GAS DETECTION SYSTEMS .................................................. 12
3.1 INTRODUCTION.......................................................
12
3.2 CHROMATOGRAPHY AND SPECTROMETRY .......................................................
3.3 OPTICAL ABSORPTION DETECTORS .......................................................
13
15
3.3.1 Infrared Spectroscopy ............................................................................................................... 15
3.3.2 Photo-ionization Detection ....................................................................................................... 16
3.5 CHEMORESISTIVESENSORS .......................................................
17
3.5.1 Semiconducting Metal Oxide Sensors .........................................................
17
3.5.2 ConductivePolymerSensors .........................................................
18
3.6 ACOUSTICSENSORS.......................................................
3.6.1 Bulk Acoustic Wave Devices.........................................................
3.6.2 Surface Acoustic Wave Devices ................................................................................................
3.7 TECHNOLOGY
COMPARISON
............................................................................................................
4 MEMS-BASED CHEMICAL SENSOR ARRAYS .......................................................
4.1 MOTIVATION FOR DEVELOPING MEMS .......................................................
4.2 MICROCANTILEVERBEAM RESONATORS........................................
20
20
21
23
25
25
...............
27
4.2.1 The MicrocantileverBeam as a Platformfor ChemicalDetection.........................................
27
4.2.2 Microcantilever Beam Theory .........................................................
31
4.2.3Issues in MaterialsSelection and Processing........................................ .................
4.3 THE BOSTONMICROSYSTEMS MICRORESONATOR ........................................
...............
4.3.1 Device Structure .........................................................
4.3.2 Materials Processing .................................................................................................................
4.3.3 Preliminary Device Characterization ........................................
.................
4.3.4 Comparison to Competing Technology .........................................................
5 INTELLECTUAL PROPERTY .......................................................
5.1 IP PROTECTING THE BOSTON MICROSYSTEMS MICRORESONATOR..............................................
32
37
37
39
41
42
45
45
5.1.1 Patents on Materials Processing .........................................................
45
5.1.2Patent on Device Structure.........................................................
5.1.3 ProcessInfrastructure.........................................................
46
47
5.2 PATENT ANALYSIS.......................................................
6 PENETRATING THE MARKET .......................................................
6.1 CHALLENGES..............................
.........................
48
51
51
6.2 A STANDARDIZED
SENSORPLATFORM.......................................................
52
6.3 BUSINESSSTRATEGY.......................................................
54
7 CONCLUSION .......................................................
55
APPENDIX.
56
SUMMARY OF EXAMINED PATENTS .......................................................
REFERENCES ........................................................................................................................................... 58
4
List of Figures
Figure
Figure
Figure
Figure
1.
2.
3.
4.
Chemical Analysis using a GC and MS ....................................................... 14
Diagram of a PID Sensor.......................................................
16
Diagram of a Quartz Crystal Microbalance...................................................... 20
Diagram of a SAW Sensor .......................................................
21
Figure 5.
(a) Number of U.S. MEMS Startups since 1980
(b) 2003 MEMS Industry Growth Forecast .......................................................
26
Figure 6. Common Readout Mechanisms in MCBs that are under
(a) static deflection or
(b) resonant excitation.......................................................
29
Figure 7. Admittance at resonance vs. fundamental frequency for a
piezoelectric doubly-clamped beam resonator and an
electrostatic parallel plate resonator.......................................................
30
Figure 8. Structure of a piezoelectric microcantilever beam ........................................... 33
Figure 9. Schematic of the BMS Microresonator ....................................................... 37
Figure 10. SEM of the BMS Microresonator.......................................................
38
Figure 11. I-V Characteristic of Si-HF interface under anodic bias ................................ 39
Figure 12. Temperature and Pressure Response of the BMS Microresonator................. 42
Figure 13. Preliminary Response Data ........................................
...............
42
Figure 14. Image taken from USPT#5,719,324 "Microcantilever Sensor" ..................... 50
Figure 15. The Detection Engine .......................................................
52
Figure 16. Comparison of supply chain of several business strategies............................ 53
List of Tables
Table 1.
Table 2.
Table 3.
Table 4.
Comparison of Performance Requirements, Across Several Markets............... 11
Qualitative Comparison of Gas Sensor Technologies ....................................... 23
Comparison of several mechanical support materials ...................................... 33
Several Companies that are Developing Portable Gas Sensors ......................... 43
5
1 Introduction
Chemical gas sensors are widely used today in automotive emissions control and by
Hazmat teams. However, many additional applications are highly likely in the coming
years. Terrorist attacks within the US and around the world have heightened the
awareness of the need for preventive measures to ensure public safety, and sensors can
provide early warnings of chemical weapons usage. When fires, chemical spills, or other
large scale disasters occur, portable chemical sensors are of critical importance in
emergency response, to probe for chemical species such as combustible and toxic gases
in the air or contamination to the environment. Chemical sensors are also commonly
employed to ensure the safety of individuals working in industrial sites where there is a
risk of toxic industrial chemicals (TICs) leaking into the air.
Another
important
application of chemical sensors is for the detection of potentially harmful volatile organic
compounds (VOCs) which degrade indoor air quality. VOCs may be emitted from
building materials, furniture, carpets, cleaning agents, and from the decomposition of
organic material, e.g., food spoilage.
To date, many different methods have been developed for the identification and
quantification of gaseous chemical compounds. Each individual method possesses
benefits and limitations, which determine the applications in which they are most
appropriate. Certain commercial chemical detection systems are able to very precisely
identify unknown compounds, however these are generally very large, bulky systems that
are used in research laboratories. There is a growing need for portable gas sensors that
meet a stricter set of requirements, combining high performance with small overall
device size and low power consumption.
A survey of the options that are commercially available in portable gas detection
shows that opportunities for improvement exist. One category of technology that is
particularly well-suited to meet the combined needs of sensitivity and portability is that
of microelectromechanical systems (MEMS), which combine decreased device sizes with
increased performance characteristics.
MEMS are also subject to technological
limitations, and with the large number of organizations that devote R&D resources
toward the development of MEMS, it is important for any new MEMS-based product to
6
have a clearly defined advantage over similar products that can be obtained through
conventional process technology.
Boston MicroSystems, Inc. is an MIT-startup company based in Woburn, MA that
is developing and commercializing MEMS technology that is fabricated through a unique
set of process capability. These processes give Boston MicroSystems the exclusive
capability of using single-crystal aluminum nitride and silicon carbide as structural
material in MEMS, which result in a number of desirable characteristics in the
development of sensors. To address the need for highly sensitive, selective, and portable
chemical detection systems, Boston MicroSystems has developed microscopic resonant
chemical sensor arrays in order to rapidly detect and identify volatile and toxic chemicals
that are present in the atmosphere, and preliminary device characterization has indicated
that they exhibit the high level of sensitivity.
A set of fundamental and broadly applicable intellectual property has been
secured in direct relation to this technology, which is critical as the technology is being
brought to the competitive arena. While challenges in commercialization are still present,
a business plan involving the establishment of consumer market share through
collaboration with suppliers is expected to significantly reduce the hurdles toward
product development and deployment.
7
2 Markets for Portable Gas Sensors
2.1 Homeland Security
Ever since the events of September 11, 2001, the U.S. government has had a heightened
awareness of the need for preventative measures that ensure public safety. As stated in
the Homeland Security Act of 2002, the main purpose of the U.S. Homeland Security
Dept. is to prevent terrorist attacks within the U.S., reduce the vulnerability of the U.S. to
terrorism, and minimize damage incurred by terrorist attacks that do occur within the U.S.
The Homeland Security budget totaled $38 billion in fiscal year 2003, and is expected to
continue to grow in years to come.
Additionally, corporate spending on homeland
security, already estimated between $40B and $80B, is expected to follow the increase in
governmental spending. 1
The government recognizes that gas sensing technologies are essential in ensuring
the nation's security, and is becoming increasingly aware of the role that micro- and
nano-scale technology is playing in this area. This is evident by contracts that have been
awarded such as a $1B contract for Boeing and Siemens to install explosive detection
sensors in 429 US airports or a $500M contract for Lockheed Martin to install explosive
trace detection systems. Federal support for nano-scale R&D has been increasing in
recent years, with funding specifically allotted for the development of chemicalbiological-radioactive explosive detection.
The federal budget for sub-micrometer
technology totaled $604M in FY'02, with an increase to $710 proposed by the President
in FY'03. The National Science Foundation predicts that the total market for products
and services that utilize nanotechnology will reach one trillion US dollars by 2015.2
For fixed-location screening of individuals for explosives or harmful chemical
agents, many large-scale walk-in booths have already been deployed for use in
applications such as airport security3 , however there have also been recent initiatives for
the development of portable gas sensors for military use. The Department of Defense
recently pursued the development of a "joint chemical agent detector," a project which
would have required the development of over 257,000 chemical point detection systems4 ,
valued in the range of $500M. Similarly, fixed-location gas sensing technologies can be
8
very useful to monitor workplace safety in environments where there is a heightened risk
of the presence of harmful chemicals, such as in semiconductor microfabrication plants.
The global budget for new fabrication facilities has been in excess of $100B in recent
years. 5
2.2 Home and Workplace Safety
The quality of air in the home or at the workplace can potentially have a serious impact
on an individual's health.
Much indoor air pollution is caused by volatile organic
chemicals (VOCs) emitted from common household items such as paint, polishes, room
fresheners, floor adhesives, and new carpets. Benzene, used commonly in rubbers, dyes,
and detergents, is a particularly harmful VOC, and exposure to toxic levels has been
linked to cancer. Even in homes where the amount of benzene is below the toxic level,
its presence has been linked to an increased probability (three times more likely) of
occurrence of asthma in toddlers.6
While public awareness of the problems associated with VOCs is increasing, the
extent to which a market currently exists for consumer use in households is still low. In
order to capitalize on these needs, the public needs first to be educated of the dangers and
then offered a solution that is sufficiently low in price. One market that currently exists
with regard to indoor air quality control is in the monitoring of heat, ventilation and air
conditioning (HVAC) systems in large buildings. HVAC systems are typically used to
control air intake and flow, and with 800,000 systems deployed in large indoor facilities
nationwide, they represent a large market potential. The use of VOC sensors to monitor
air quality can make energy usage more efficient, by allowing the system to actively take
in more fresh-air from outdoors when indoor quality drops below acceptable levels, while
recycling indoor air when the quality is of sufficient purity. A VOC sensor on such a
system can also be used to monitor the air supply for any potential threat that may arise,
e.g. introduction of harmful chemicals into the air ducts by terrorists.
9
2.3 Quality Control
As an additional market of interest, VOC sensors can be used to detect the quality of food,
and other chemicals of interest, such as perfumes. In the case of monitoring food, it is of
particularly important for workers in the food industry to prevent dissemination of
spoiled materials, and the usage of chemical sensors or "electronic noses" in this
application is increasing.7 Microbial contamination of food can result in economic losses
to the manufacturer, through damage to crops and loss of nutritive value. Often, the
resultant mycotoxin production that accompanies fungal production in food is toxic to
humans on ingestion.8
Here, however, the systems are commonly larger and bulkier, operating at a fixed
location. The requirements in regard to portability are not as strict in this case, however,
the sensors still need to be highly sensitive and selective. As the requirements for this
area somewhat more relaxed than some of the other markets, a possible scenario in the
roadmap for this market seems to include the commoditization of products that meet
performance requirements by a larger number of competing companies.
2.4 Summary of Current Market Potential
In total, three key markets are identified where the detection of volatile organic chemicals
(VOCs), toxic industrial chemicals (TICs), chemical warfare agents (CWAs), and
explosives play a key role. The three markets are identified as Homeland Security and
the Military, Home and Workplace Safety, and Electronic Noses in Quality Control, and
are each distinguished by different levels of performance requirements.
Military
applications require the highest levels of importance, and the government is willing to
invest the amount of money necessary to obtain that performance. Consumer markets
such as home and workplace safety, by comparison, are inherently cost-driven, and while
performance requirements are not as stringent, the price of any product that is offered
will have a key role in how well it is received by the market. The additional market of
electronic noses in quality control is also included, as a host of products have recently
been developed to address needs in this area. This last market, however, does not have
10
the performance requirements that are as strict as the other areas, making the
commoditization of technology by many sources a possible resultant scenario as the
market continues to develop. The following table summarizes the key properties that
sensors need to satisfy, and to what degree, in three major application areas:
Market
Reuieent
Sensitivity
(high/moderate)
Selectivity
(high/moderate)
Speed
(fast/moderate/slow)
Size
(small/med/large)
Power Dissipation
(low,moderate, high)
Reliability
(high/moderate)
Allowable Cost
(high/moderate/low)
Homeland Security/
Military
Home/Workplace
Safety (TICs,
Electronic nose /
Quality Control
(CWAs, Explosives)
VOCs)
(VOCs)
High
High
Moderate
High
Moderate
Moderate-High
Fast
Moderate
Slow-Moderate
Small
Medium-Large
Medium-Large
Low
Low-Large
Moderate-High
High
Moderate
Moderate
High
Low-Moderate
Moderate
Table 1. Comparison of Performance Requirements, Across Several Markets
11
3 Overview of Commercial Gas Detection Systems
3.1 Introduction
There are many different types of physical/chemical interactions that a material can have
with a gas in order to enable detection.
In the case of gas chromatography,
a gas is
actually separated into its constituent components, for individual analysis. For some
sensors, such as conductive polymers or semiconductor metal oxide devices, the
adsorption or reaction of a gas with the sensor material causes a modulation in electrical
conductance. Resonant sensors respond directly to the increased mass induced by gas
adsorption by a change in resonance frequency. Other response mechanisms sometimes
utilized include changes in optical properties or in metal work function.
To address the need for chemical sensitivity combined with selectivity, sensors
have been developed that respond to a direct interaction between a gas and a chemically
sensitive material such as a polymer or a semiconducting metal oxide. In the case of
polymers, a specific material composition that shows an affinity for specific absorbing
certain gases is chosen. In general, this absorption results in an increase in material mass
and volume. In the case of semiconducting metal oxides, gases interact with the surface
of the material by means of an oxidation-reduction reactions which can be made more
selective by use of appropriate catalysts. These reactions add or deplete the surface of the
material of electronic charge carriers, thereby changing its conductivity.
In the case of sensors that preferentially respond, in part, to a range of different
gases, selectivity is improved when the responses of multiple devices are correlated. By
depositing a range of polymers of varying chemical sensitivity on an array of identical
structures, a higher level of chemical selectivity is achievable. Research indicates that
when an additional chemically sensitive material is added to a sensor array, even if its
individual response to a gas is not highly selective, the improvement in total sensitivity
and selectivity of the sensor array can enhanced. Each chemical species has a response
profile across each element in the sensor array, which can be analyzed by pattern
12
recognition techniques such as neural net algorithms in order to determine the
composition of the measurement environment.
The following sections provide an overview of the different commercial
technologies that are currently available for gas detection.
3.2 Chromatography and Spectrometry
One method of chemical detection that has been used for many years is gas
chromatography, which involves physically separating different molecules of a gas, for
individual analysis. Typical components of such a system include a gas chromatography
column, a carrier gas supply, sample injector, and an output detector. Chromatography
columns are made out of materials such as fused silca lined with a liquid stationary phase
that absorbs or desorbs different molecules of a vapor according to their respective
partition coefficients. The sample injector passes a test-vapor along with the carrier gas
into this capillary column, through which they migrate with differential speed. At the
output of the column, the presence of a gas being analyzed can be sensed e.g., by using a
thermal conductivity detector to monitor the gas. Individual gas species can be identified
by the speed at which they traversed the column's total length and then quantified by
analyzing the total area under a peak in measured thermal conductivity output vs. time.9
The gas chromatograph (GC) is typically used on a laboratory scale, as the
method is time consuming and the hardware involved tends to be fragile and bulky.
For
laboratory analysis, samples of a gas are collected, and stored in a special jar or in a
cryogenic container for transportation to the instrument, increasing total measurement
time. The GC column itself is very thermally sensitive and frequently also needs to be
replaced because of fouling. In spite of the challenges involved, attempts have been
made to miniaturize this detection process, and several laptop-sized GCs have been
developed, including Agilent's Micro GC and Photovac International Inc.'s PetroPD.
A mass spectrometer (MS) can be used to further improve the identification of
complex mixtures of molecules, after a GC has separated out individual groups of
molecules. In a mass spectrometer, molecules are ionized, accelerated with an electric
field, and passed through an applied magnetic field, a process that enables individual
13
molecules to be separated from one another based on the ratio of mass to electronic
charge. A number of detectors are used in such a system, to determine parameters such
as the time-of-flight of the ion as it travels, which decreases with smaller mass, and the
radius of curvature of the path of the ion, which decreases with increased ion charge.10
While mass spectrometers are very versatile tools for the identification and
quantification of chemicals, they are typically very large, generally having very high
power requirements.
Although they are usually used within a laboratory setting, a
prototype combined GC-MS has been developed by Lawrence Livermore National Labs
in a size as small as a briefcase.
Gas Chromatograph
Mass Spectrometer
GC column:
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
...
,
i l7e-senaratin
n
X
I
I
I
I
I
I
I
I
I
I
I
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I
I
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I
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I
I
I
Figure 1. Chemical Analysis using a GC and MS
Ion-mobility spectrometry (IMS) is another widely used technique that that
identifies a gas by sorting ions that are passed through an electrical field. One major
difference here, however, is that while MS typically requires vacuum in order to prevent
ions from colliding, IMS operates at atmospheric pressure. Ions are passed into a tube
that is charged with an electric field, where they experience a randomization in motion
due to ambient pressure and temperature, but an overall drift due to the applied field.
Smaller ions experience fewer collisions with neighboring ions, and consequently, have a
higher mobility in the tube. Ions that drift through the tube are sorted according to their
size, and although this method is not particularly useful for identifying the composition of
unknown compounds (as MSs can do), the entire process is much faster than GC-MS
14
technology, taking a few seconds to complete, as opposed to a few minutes up to an hour
for the GC-MS.
Because of their rapid response time and utility in detecting chemical signatures
of pre-determined, potentially harmful compounds, IMS has recently found wide use at
large, fixed-location portals are used for security screening.
In such as system, an
individual walks into the portal, and a blast of air dislodges chemicals from their clothing.
The chemicals are collected by a concentrator and passed to the IMS for analysis.
Initially developed at Sandia National Labs, the technology has been licensed to Smiths
Detection for locations such as airports, national landmarks, and government buildings.
3.3 Optical Absorption Detectors
Electromagnetic radiation can be utilized in many ways in the chemical detection process.
Some devices use light, by measuring a change in optical properties of a material that
interacts with gases. For example, the use of chemically sensitive luminescent11 and
phosphorescent materials has been proposed for use in conjunction with optical fibers
that probe the materials for optical response.
This section reviews three of these
mechanisms, including infrared spectrometry, photo-ionization detection, and photoacoustic detection.
3.3.1 Infrared Spectroscopy
Infrared spectroscopy is a method for identification and quantification of a gaseous
compound, based on the absorption of infrared light at a wavelength corresponding to the
vibrational modes of the gas molecule. Optical absorption for an arbitrary gas can be
expressed in terms of Beer's law, a = sCL, where
= molar absorptivity (a function of
wavelength), C = molar concentration, and L = distance of light propagation into the
absorbing material. The light falls off exponentially as it passes through the medium
given by I = I0 exp-aL
where I
is the incident light intensity and I is the light
intensity after traveling a distance L through the gas. By passing a beam of infrared light
through a sample of gas and then measuring the resultant attenuation as a function of
wavelength, the gas's chemical composition can be identified and quantified.
15
The sensitivity of this detection mechanism is dependent on the amount of
absorption that can be detected. However, as shown in the above equation, absorption
decreases exponentially with decreasing path length. To achieve a reasonable path length
in equipment of practical size, mirrors are often used to provide multiple passes of the
light through the optical path of the gas to be analyzed. The reflectivity of the mirrors
will, however, limit the number of reflections possible and ultimately limit device
sensitivity. While it may be possible to miniaturize this type of detection scheme by
using micro-mirrors or an optical resonator, the total amount of absorption occurring in
the gas would also decrease, resulting in a decrease in sensitivity of the device to changes
in chemical concentration.
3.3.2 Photo-ionization Detection
Another method of chemical detection
U
that also relies on molecular absorption
lit
of electromagnetic radiation is photoionization
detection.
However,
this
anode
technique omits the detailed analysis of
the absorption spectrum that is necessary
*
'r'r
in IK
__
_r _
·
__
spectroscopy.
vJ
High
voltage
a
sn:
Instead, tnls
technique exploits the fact that the
Gasinlet
optical energy of molecular resonance of
many volatile organic compounds is at
Figure 2. Diagram of a PID Sensor
about the same energy level (8-10 eV).
When exposed to ultraviolet radiation of a slightly higher energy (e.g. 11.7 eV for a
xenon lamp), volatile organic chemicals will absorb the light and become ionized,
however molecules of higher ionization energy that are commonly found in air (such as
N 2 , 02, in the range of 12-15 eV) will remain unaffected. Consequently, if a high voltage
is applied across opposite sides of a gas-sampling chamber that is illuminated with light
of a given energy, an electrical current will be measured only when ionized molecules are
present.'2
16
This type of device has been shown to be capable of detecting volatile organic
chemicals in the parts-per-billion (ppb) range, and small, hand-held instruments are
commercially available.'3 The most significant drawback of the PID detection scheme is
the poor selectivity of chemical detection that results from the ionization of nearly all
VOCs within the sample chamber. The typical PID sensor will respond with high
sensitivity to the presence of many VOCs, however it will be incapable of distinguishing
one from the other.
3.5 Chemoresistive Sensors
3.5.1 Semiconducting Metal Oxide Sensors
Inorganic chemoresistive sensors are fabricated from semiconducting metal oxide (SMO)
materials such as ZnO, SnO2, or W0 3. Sensing is achieved through modulation of the
bulk or surface resistivity of the oxide as it interacts with gas in the form of an oxidationreduction reaction. Whether these devices interact primarily with oxidizing or reducing
gases, is based on whether the SMO is n-type or p-type. In the case of an n-type material,
the material operates best as a detector for reducing molecules such as combustible gases,
CO, or H2. The case of the hydrogen reaction is illustrated by the following equations: 1'4
02 + 2e-- 20-
H2 + 0- ' H20 + e
Consider an SMO that is left in air at sufficiently high temperature: oxygen will
adsorb on the material, diffuse inwards and fill oxygen vacancies within the material,
depleting it of electrons thereby increasing its resistance. If the material is exposed to
hydrogen, oxygen will diffuse outwards towards the surface and will react with the
hydrogen to form water vapor and electrons will be freed within the material, resulting in
a decrease in resistance that is correlated to gas concentration. At reduced temperatures,
bulk diffusion is largely frozen and the impact of the gases is largely due to adsorption
onto the surface of the SMOs upon which they form surface states. Adsorbed oxygen
acts like an electron trap leading to depletion of electrons near the surface.
17
Since adorption/desorption kinetics are extremely slow at room temperature,
measurements are typically performed at elevated temperatures (250-400 for thin films,
600-1000 for bulk devices). The use of a catalytic material such as copper, platinum, or
palladium can also increase gas sensitivity by lowering the activation barrier to specific
gases. The selection of operating temperature and catalyst can thus contribute toward
improved selectivity of sensors towards certain gases.
One benefit of using these sensors is the rapid response time associated with
increased chemical reaction rates at elevated temperatures.
The high temperature
required for operation, however, has negative implications on device operation as well by
increasing the power requirement for and affecting long-term device stability through the
acceleration of morphological changes in film structure. Device operation further suffers
from fouling through irreversible binding with sulfur compounds. Additionally, poor
response to volatile organic compounds is observed when ethanol is present, which
makes application of these devices toward many military and industrial markets (see
previous section on marketing) impractical15. In addition to these problems, typical
detection limits for these devices are in the range of ppm, which is large compared to
other options that are available.
Despite the associated operational problems, these devices are still very attractive
in many applications.
Simple processing techniques can be used to integrate these
devices directly with measurement circuitry that is also simple due to the DC resistance
readout. Due to elevated temperatures required for operation, these devices are often
useful in environments that are already hot, such as automobile exhaust systems.
Because of their low cost, thick film SMO sensors can be very desirable in applications
where sensitivity and stability requirements are not too demanding.
3.5.2 Conductive Polymer Sensors
Polymers are utilized as key components of gas sensors, due primarily to their selective
affinity for absorption of different classes of chemicals. One type of sensor involves a
composite material consisting of a matrix of conducting particles such as graphite
suspended in a polymer. Polymers that selectively absorb certain types of gases are used,
18
which upon absorption results in the swelling of the polymer.. The swelling results in a
relative increase in the distance between conductive particles and a corresponding
decrease in the conductivity of the material. Typical polymers used in this detection
mechanism are polyalklacrylate, poly-co-vinyl-acetate, or poly-vinyl butyral.
Another method of detection involves the use of polymers that are intrinsically
conducting, without need for a conductive second phase. Conductive polymers typically
used in gas sensing include polypyrrole, polythiophene, and polyaniline. These materials
exhibit conductivity along a linear backbone of organic molecular chains, which can be
doped p-type or n-type. Detection in this case is also related to the absorptive swelling of
the polymer, as increased volume will decrease electron density in the polymer chain.
The main benefit shared by both types of polymeric sensors is the broad range of
polymers that have been developed, which enable a high level of chemical selectivity
when these devices are used in an array-type of configuration. Detection limits for both
types of polymers have been reported to be on the 100 ppb level. Both types of sensors
operate by the modulation of DC conductivity, so the associated support electronics can
be relatively simple. Polymer matrices are further attractive given simplified processing
and low-cost.
These devices do share several drawbacks, however.
The absorption of the
polymers is typically a very temperature-dependent process, so the temperature of the
devices often must be actively controlled with some form of heater, increasing power
requirements.
Fluctuations in temperature and also humidity (which also induces
swelling) can lead to device inaccuracy if left uncompensated. Both types of polymeric
sensors suffer from significant aging effects, leading to sensor drift, response degradation,
and limitations on device lifetime (as low as 9 months for intrinsically conducting
polymers). In spite of the drawbacks, several products that are based on this technology
have been successfully marketed, including a portable gas sensor currently sold by
Smith's Group, formerly by Cyrano Sciences, Inc. 16
19
3.6 Acoustic Sensors
3.6.1 Bulk Acoustic Wave Devices
A bulk acoustic wave (BAW) sensor is made up of a bulk piezoelectric crystal upon
which typically a chemically-sensitive polymer film has been deposited. When an AC
voltage is applied across the crystal, it will oscillate at its associated resonant frequency.
Upon absorption of gas into the polymer, the effective mass of the resonator is increased,
resulting in a shift of resonant frequency, typically downwards. Quartz crystals are the
most common piezoelectric utilized leading to the common expression quartz crystal
microbalance (QCM).
- polymer
polymer-coated quartz disc
- Electrode
Figure 3. Diagram of a Quartz Crystal Microbalance
The sensitivity of the QCM sensor is given by:
Af /AC = (-2.3xlO-6 )f 2 /A
Eqn.1
Wherefis the resonant frequency, C the gas concentration, and A the area of the sensitive
polymer film. 17
An example of the sensitivity achievable by this technology is a 1.5 Hz
shift for 1 ppm of n-heptane.18 From this equation, it is evident that devices that are
scaled-down in size (f increases) will exhibit higher sensitivity. However, it has been
observed that signal to noise ratios tend to suffer from surface interference as the crystal
size decreases.
This tends to limit the minimum manufacturable size of a functional
BAW-device.
20
3.6.2 Surface Acoustic Wave Devices
Surface acoustic wave [SAW] devices operate by transmitting acoustic waves along the
surface of a substrate material, rather than through the bulk as in BAW devices. Acoustic
waves are generated and detected by sets of interdigitated transducers, which spatially
generate an alternating electric field within an underlying piezoelectric material. The
varying electric field is transduced into an acoustic wave by the piezoelectric material
and propagates along the surface of the piezoelectric, between two sets of transducers.
Piezoelectric materials typically used include ZnO, LiNbO3, PZT, or AN. The
chemically absorptive polymer film is deposited onto the piezoelectric layer.
IDTs
Figure 4. Diagram of a SAW Sensor
SAW devices have been extensively developed in the past two decades, and are
used largely in communications.
Since it takes a finite amount of time for the acoustic
wave to propagate between transducers, they can be used as electrical delay lines. Also,
since there is a resonant frequency associated with the acoustic wave that propagates
through the device surface, they find use as band-pass filters in electrical circuits. In the
chemical sensor application, a chemically absorptive film (typically a polymer) is
deposited on top of the piezoelectric that transmits the acoustic wave. Absorption of a
gas into the polymer film increases its mass and alters the resonant frequency and
attenuation of acoustic propagation.
SAW devices exhibit rapid response times,
determined by the time constant associated with absorption and desorption of the gas
molecules from the sensitive film coating.
21
An expression for the empirical sensitivity of this type of detection system is
given as follows:
Af/Acv = Afp*K/pp
Eqn. 2
Where Afpis the change in frequency caused by the change membrane characteristics, cv
is vapor concentration, Kp is the partition coefficient, and pp is the density of the polymer
membrane.
Another important performance metric is the electromechanical coupling
coefficient, which determines the amount of energy transferred between electrical and
acoustic waves at each IDT. This can be related to parallel resistance at resonance of the
SAW element, given by the following equation:
Rp = (8k 2 FoCsN 21)-
Eqn. 3
where N is the number of finger-pair electrodes in the IDT, Cs is the capacitance per
finger-pair, k is the piezoelectric coupling coefficient, and Fo is device resonant
frequency.
Detection limits of 700 ppb have been reported (in the case
tetrachloroethylene vapor), and limits as low as the ppb level may be achievable.
Because of the large number of polymers available for use, many different types of gases
may be detected, and an array-type configuration, typically consisting of 4 sensors 9 , can
be utilized to increase chemical selectivity.
One particularly attractive aspect of SAW technology is its compatibility with
commercial photolithographic techniques that are commonly used with integrated circuit
technology. Traditionally, SAW devices have been fabricated on large single-crystal
piezoelectric substrates.
More recently developed SAW devices utilize a thin-film
piezoelectric deposited on a semiconductor substrate such as Si or GaAs, enabling
monolithic integration of sensors with support electronics. The use of a material bi-layer
results in dispersion of SAW propagation and electromechanical coupling, and
polycrystalline piezoelectric structures (due to sputtering or chemical vapor deposition
processes) further deteriorates device performance. Even so, the ability to monolithically
fabricate both sensor and support electronics onto silicon is highly attractive.
One potential limitation to operation is related to the general trend for SAW
chemical sensors to be fabricated on a macro- to messo-scale. Hank Wohltjen, inventor
of the SAW chemical sensor, quotes a typical SAW device active device area of 5 to
6mm that has a resonance frequency of about 500 MHz. Cernosek of Sandia National
22
Laboratories demonstrated a 7-element array of SAW devices in an active device
structural area of 1.25" x .7".20 While this size affords an advantage over several other
currently available commercial technologies, the absence of smaller functional a smaller
functional device raises the question whether such a technology is realizable. If it were,
additional benefits in portability and miniaturization would be afforded.
3.7 Technology Comparison
With an understanding of the benefits and limitations of the different methods that are
available for chemical detection, it is possible to select an appropriate detection system to
address the needs of any given application. Additionally, each of the markets that have
been discussed have generally different degrees of performance requirements and cost
allowances, which might immediately disqualify certain technologies from being suitable
to address the needs of a particular market. A table summarizing different commercial
technology in terms of performance and cost is presented below:
Type of Sensor.
Chromatography/Spectrometry
'Sensitivity* electiviy
(GC, MS, IMS)
Photo-Ionization Detection
Semiconducting Metal Oxides
Conductive Polymer Arrays
Ppb
Ppb
ppm
ppb - ppm
Acoustic Wave Devices
ppb-ppm
Excellent
Poor
Moderate
Excellent
Moderate
Reliability.
Portability.
Cost
Moderate-good
Good
Good
Poor
Poor
Moderate
Moderate
Moderate
High
Moderate
Moderate
Low
Good
Moderate
Moderate
*Here, sensitivity refers to the estimated minimum detectable concentration of gas when a device is
exposed to a single analyte, in the absence of interfering chemicals.
Table 2. Qualitative Comparison of Gas Sensor Technologies
It is clear that while the different methods of chromatography and spectrometry
offer high sensitivity and selectivity, they are clearly unsuited to many applications
because of the high costs and low portability of the available systems. Lab-scale systems
are clearly far too expensive for most consumer markets, and would more likely be
purchased by research facilities, forensic laboratories, or other high-budget fixedlocations. While some relatively portable gas chromatographs have been developed for
the detection of VOCs [reference: photovac], their utility in analyzing the chemical
content of an environment is limited without inclusion of a spectrometer (which
significantly adds to device weight, complexity, and cost). For portable gas sensing
applications, we it is necessary to look towards other technologies.
Conventional Optical IR spectroscopy also does not seem to be suitable for
portable applications, however the photo-ionization detection technology that was
discussed is
relatively portable. Unfortunately, the inability of PID to resolve the
different types of VOCs that could be present in the environment makes this an
impractical choice in situations where the content of the environment is unknown.
Similarly, the semiconductor metal oxide technology that was discussed, while wellsuited towards detection of reducing gases, experiences difficulty in distinguishing VOCs
when in the presence of ethanol. None of these technologies present a robust solution for
selective chemical detection when other interfering chemicals may be present.
In order to selectively and rapidly detect the widest range of chemicals possible
with a device that is portable and affordable, it seems that the most successful
technologies to-date have made use of polymer engineering. Because it is possible to
control a polymer's chemistry to create an affinity toward absorption of a select group of
chemicals, these materials are prime candidates for the analysis of multi-chemical
environments. While devices that are largely formed out of the polymeric material itself
suffer from nonlinear response output, measurement drift, and short device lifetimes,
increased success has been found by integrating the polymers with acoustic elements that
respond to changes in mass.
It is important to further consider whether this technology might be the most
optimal manifestation of the fundamental idea of mechanically detecting the absorption
response of polymeric material. This technology has demonstrated advantages over
many other currently available techniques with respect to reliability, portability,
sensitivity, and cost; however, can any further improvements be made? One shortcoming
is that the two types of acoustic sensors, SAW and BAW, must be constructed on a macro
to meso scale, in order to preserve a high quality of device operation. This ultimately
limits the number of elements that can be included within an array of a portable device,
which in turn limits the chemical selectivity that a device can exhibit. With the strong
demand for portable devices that can reliably detect specific chemicals in complex
environments, a device that utilizes polymer absorption, but on a smaller size scale, may
present an attractive solution.
24
4 MEMS-based Chemical Sensor Arrays
4.1 Motivation for Developing MEMS
Many different mechanical devices exhibit enhanced performance when constructed in
smaller dimensions, and over the past two decades, advancements in micromachining
techniques have led to the widespread commercialization of microelectromechanical
systems (MEMS) that capitalize on these desirable small-scale effects. MEMS include a
diverse group of devices and components that are used in a broad range of different
applications, all of which are constructed of miniscule components that take advantage of
some phenomenon that is not present or readily accessible on the macro-scale. MEMS
present a new paradigm for the design and fabrication of complex and integrated systems,
and the benefits gained by devices of this category are numerous:
·
High fundamental modes of resonance, which result in
o High gravimetric and inertial sensitivity of resonators
o Rapid actuator response times
o Resistance to mechanical shock
·
Low power consumption
·
Low fabrication cost per device, because of
o High manufacturing throughput in batch-processing
o
Small amount of material consumed per device
·
The ability to integrate complex structures into small packaging
·
Low thermal time constants for rapid component heating and cooling
The utility in developing products that are based on MEMS is illustrated through the
many examples of devices that have been successfully developed and brought to market.
It was estimated in 2001 that there were 1.6 MEMS devices per person in the U.S., and
that number was predicted to multiply to nearly 5 devices per person by 2004. Currently,
the worldwide market for MEMS compnses a multi-billion dollar industry that has been
steadily growing for the past two decades and is predicted to exhibit strong growth in
25
years soon to come.
Figure 5 displays results of studies conducted by the MEMS
Industry Group, detailing recent growth in the number of companies pursuing MEMS
devices and predicting future growth in markets for MEMS. As shown, the study
indicates that at least 83 MEMS-based startups have been established in the U.S. between
1980 and 2001, and 44 of those companies have been started between 1995 and 2001.
Source: In-Stat/MDR 07/03
M
14
"
12
Worldwide Revenue Forecast for MEMS
o
o
E
2002-2007 (US $ in Billions)
II
d An
:$ IU
-
ItA
-
8
z
6
4
$6
2
$4
i
I
A
----
-. I
$2
I
$0
I
.I
2002 2003 2004 2005 2006 2007
IiC.A: M,/b.
(a)
(b)
Figure 5. (a) Number of U.S. MEMS Startups since 198021
(b) 2003 MEMS Industry Growth Forecast 2 2
One example of a highly successful product is the MEMS accelerometer, which
was developed by many groups, including Sandia National Laboratories, Motorola, and
Analog Devices. These sensors are now pervasive in the automotive industry as key
components in airbag deployment systems and in the active control of the braking and
handling system. Another very similar class of inertial sensors is the MEMS gyroscope,
which because of their small mass and low weight are well suited for use precision
control of military guided weapons systems.23
MEMS have also come to find common use as components in optical systems.
High-portability, high-speed projection video displays have been developed by Motorola
26
and Texas Instruments using arrays of micro-mirrors that can be deflected to activate or
deactivate individual pixels of data. In the telecommunication industry, MEMS-based
optical switches have become widely adopted for precision control and modulation of
signal transmission. In both of these applications, the rapid electromechanical actuation
time that is observed in the device microstructure is essential in obtaining excellent
device characteristics.
One common microstructure in MEMS that is particularly well-suited for use in a
diverse range of applications is the microcantilever beam. This type of structure can be
used both as a sensor and an actuator, and with varying designs, can respond
preferentially to minute changes in force, mass, temperature, or pressure, to name a few
measurands.
One example of its successful commercial use is in atomic-force
microscopy (AFM), a method of imaging the surface of a material based on the forces of
molecular interaction between a sharpened probe on the tip of a cantilever and a material
topography across which it is scanned. Nanoindentation is another technique that makes
use of a cantilever beam with a sharp probe tip, however in this case the deformation
properties of a material are characterized by pushing the probe tip into a surface with a
known force and then measuring beam deformation.
4.2 Microcantilever Beam Resonators
4.2.1 The Microcantilever Beam as a Platform for Chemical Detection
Given the success that has been seen in the development and commercialization of so
many other MEMS-based devices, it seems likely that this design paradigm would have a
positive impact in the realm of chemical sensing as well. With knowledge of the benefits
that are commonly seen across varying types of MEMS devices, including increased
resonator sensitivity, rapid thermal and mechanical response times, low power dissipation,
and small device size, many of the previously discussed commercial technologies become
prime candidates for miniaturization. Currently, MEMS-scale redesign of many of those
technologies is the focus of active research and development, and there is a wealth of
information available in a wide range of varying design disciplines.
27
In the previously presented review of commercially available technology, it was
observed that of all of the gas sensor technologies that are currently commercially
available, one of the most promising technologies for the selective detection of VOCs
appeared to be the mass-sensitive acoustic sensors coated with chemically selective
polymer films. The diverse range of polymer coatings that can be chemically engineered
provide an viable route toward the required selectivity of chemical detection. The key
limitations that were observed in the commercial technology were related to device
portability and integration into array-based configuration. Since these design issues are
readily addressed through the use of MEMS-based processes, it is likely that a similar
approach that uses MEMS technology would also be successful.
One emerging platform for the selective detection of chemicals is the
microcantilever beam (MCB) structure, which in other applications has been shown to be
very sensitive to tiny forces, deflections, and changes in resonance characteristic. To
create a MCB-based chemical detector, it is possible to deposit a chemically-sensitive
material such as a polymer film onto the surface of the cantilever beam. In the case of a
polymer, an affinity for absorption of certain chemicals is utilized, and when the material
is exposed to one of these chemicals, the film will swell and increase in mass to
accommodate the additional molecules that have been absorbed.
There are mainly two ways that a MCB can respond to a change in the properties
of a deposited polymer film: through changes in its static and dynamic mechanical
characteristics. A change in mass and volume of the polymer will impose a stress on the
beam, which will cause it to deflect if its stiffness is sufficiently low. Additionally, the
additional mass and stress induced by the absorbed vapor molecules will affect the
dynamic characteristics of the beam through a measurable change in its fundamental
mechanical resonant frequency.
In order to detect the change in static or dynamic
mechanical properties, transduction to either a DC or an AC electrical signal is employed,
and examples of typical techniques for doing so are pictured in Figure 6.
28
(b) Resonant Excitation
(a) Stress Deflection
Photodiode
Photodiode
+
Laser
+Laser
VAC
Optical Readout
Optical Readout
+VDC
Piezoresistive Readout
VAC
Piezoelectric or
Capacitive Readout
Figure 6. Common Readout Mechanisms in MCBs that are under
(a) static deflection or (b) resonant excitation
The applications in which each of the above pictured methods find most
appropriate use are varied. For example, cantilever tip deflection is precisely detected
down to 10-'14 m by monitoring the position of a ray of light that is reflected from the tip
of the beam to a position sensitive photodetector.2 4 This technique is well suited for
AFM, nanoindentation, and other large-scale, high-precision systems, however the
additional hardware that is involved makes this option somewhat less attractive for use in
a portable device.
In order to determine whether it may be more desirable to optimize a MCB
chemical sensor for the detection of either a static or dynamic mechanical response, it is
important to first note that the design metrics that are necessary for high sensitivity in
each mode of operation are opposite to one another. In order to obtain a cantilever that
exhibits a large deflection with a given amount of material absorbed by the polymer film,
it is more desirable to increase cantilever length, decrease thickness and width, and use
material of a lower Young's modulus. All of these features would tend to decrease the
resonant frequency of the structure, which would give it a worse dynamic response to
chemical absorption than if the opposite were done (this will be shown in the proceeding
section on MCB theory). Since there is a tradeoff of sensitivity between static deflection
29
and resonant frequency shift, it is useful to consider which mode of operation might be
the most useful for optimization in the application being considered. One thing to note is
that cantilever beams that show a larger deflection response will also tend to be more
fragile, and more susceptible to yield issues in device processing, and somewhat more
susceptible to sudden acceleration. To address the need for robust portable chemical
sensors, and keeping with the MEMS-paradigm of miniaturization, it seems logical to
choose the more mechanically stable structure that will continue to improve in sensitivity
as its dimensions are reduced, the resonant MCB sensor.
Taking into account that the dynamic response of the MCB will be of primary
interest, it is relevant to note that the preferred choice of mechanical to electrical
transduction is by means of piezoelectric interaction. While alternatives could include
piezoresistance or parallel-plate capacitance, both are out-performed by piezoelectric
materials as device frequencies at high frequencies. While many successful resonant
devices that are manufactured make use of electrostatic actuation (e.g. accelerometers,
gyroscopes), theoretical analysis predict that the electromechanical coupling efficiency of
electrostatic actuation falls far below the typical electromechanical coupling efficiency of
piezoelectric actuation as frequencies are extended toward the RF regime.2 5
This is
illustrated in Figure 7, which plots a simulation of the admittance of two identicallydimensioned resonators, one operating by means of a piezoelectric and the other through
use of electrostatic interaction.
i
I
i
i
lxId
)P
Ply
i
%s,
I
'%
f50
100
150
f 1IkzI
._ ._. _.
200
...._A.v
250
Figure 7. Admittance at resonance vs. fundamental frequency for a piezoelectric doublyclamped beam resonator and an electrostatic parallel plate resonator.
30
The above plot was included in a journal publication by D.L. DeVoe, and is significant
because of an associated observation that DeVoe made with regard to the relative
admittances:
Ypiezo (piezqn.
Yplate1
4
plate
where Ypiezois admittance of the piezoelectric structure, Yplateis admittance of the parallel
plate capacitive structure, llpiezois the electromechanical coupling coefficient of the
piezoelectric structure, and
rlplate
is the electromechanical coupling coefficient of the
parallel plate capacitive structure.
This observation implies that for increasing
frequencies of operation, the piezoelectric material will enable a much higher
electromechanical coupling efficiency than capacitive actuation would allow in a device
of the same dimension.
4.2.2 Microcantilever Beam Theory
One method commonly used for analyzing the fundamental resonant frequencies of a
lightly damped microcantilever is to draw an analogy to a simple harmonic oscillator, the
resonant frequency of which is given by the following equation:
=A
Eqn. 5
where k is the spring constant and m is the mass of the resonant structure. In modeling a
rectangular cantilever beam, an effective suspended mass of m = .24 mb is used, where
mb is the actual mass of the beam. The spring constant is derived from the mechanical
parameters of the beamError! Bookmark not defined.
k=
Ewt 3
Eqn. 6
413
where E is Young's Modulus of the beam and 1, w, and t are beam length, width, and
thickness.
In analyzing the niechanical performance of a MCB esonator, two metrics are of
particular interest: the gravimetric sensitivity and minimum detectable mass of the
31
structure.
Gravimetric (mass-loading) sensitivity can be examined by computing
differential change in frequency with respect to mass:
&nb
2
Eqn. 7
o
mb
Combining equations thus far, considering the behavior of a lightly-damped
microcantilever beam, we can derive an expression for the gravimetric sensitivity of the
MCB in terms of several parameters intrinsic to the device:
fo_
gmb
1
5=/2
Ewt3
22 3m/ 3 2
241
8
qn.
E represents the overall stiffness of the MBC and is directly related to Young's modulus
integrated across each of the beam's constituent layers. mb is similarly related to the
density of each constituent material of the beam. This expression for sensitivity defines
the amount of change in frequency that will accompany a change in mass-loading of the
idealized cantilever structure, as a function of elastic parameters and device dimensions.
In general, the minimum detectable mass of the cantilever will be determined by
the level of noise present in the background of the entire system.
Regarding the
cantilever separately from the rest of the system, a major source of noise will be ambient
and fluctuating thermal energy, which cause physical fluctuations in cantilever tip
deflection. The amplitude of the average amount of oscillation induced in the MCB by
background effects will inherently limit minimum detectible shifts in resonant frequency.
4.2.3 Issues in Materials Selection and Processing
In order to obtain high frequency operation and resultant high device sensitivity,
piezoelectric actuation is selected as the preferred mechanism to excite the device into
motion. A direct way of integrating the effects of the piezoelectric material with the
mechanical properties of a cantilever beam is to include it as one of the constituent layers
of the microcantilever beam, such as depicted in Figure 8. The inclusion of multiple
materials on a single cantilever beam, however, significantly complicates the processing,
and in order to design a MCB resonator, materials for each of the layers must be selected
in light of their interaction with one another, in addition to their individual characteristics.
32
While it is be desirable for the manufactured devices to have high chemical sensitivity
and selectivity, the uniformity of performance characteristics across the range of
manufactured structures will also be of critical importance.
polymer film _
/
top electrode
piezoelectric
bottom
electrode
contact
bottom electrode,
mechanical suppor
Figure 8. Structure of a piezoelectric microcantilever beam.
MechanicalSupportMaterials
In considering how to manufacture the MCB structure, it is useful to first determine
which mechanical support materials have characteristics that would be most desirable in
this application. From Equation 4 of the previous discussion on MCB theory, it is seen
that the sensitivity to chemical absorption is proportional to the square-root of the beam's
Young modulus divided by the beam mass. This implies a figure of merit (FOM) of
Young's Modulus divided by density, also known as the specific modulus of the material,
which is listed for comparison between several materials in Table 3. The information
shown indicates that for a MCB of a fixed set of dimensions, the choice of SiC as a
structural material would give an improved resonator sensitivity over several other
options.
Material
E [GPa]
p [106g/m3 ]
FOM (E / p)
Thermal SiO2
69
2.2
31
Polysilicon
160
2.3
70
<100> Si
190
2.3
83
LPCVD Si3 N4
270
3.2
84
6H-SiC
473
3.2
148
Table 3. Comparison of several mechanical support materials.
3
The microstructure of the support material that is used has an important impact on its
mechanical properties. Comparing polysilicon to single crystal silicon in the above chart,
the single crystal silicon is observed to have a higher Young's modulus (due to the
continuity of crystalline bonds). Perhaps more important, however, is the fact that the
mechanical properties of the polycrystalline silicon will be difficult to reproduce
consistently. Randomization of the film structure lends some uncertainty toward the
mechanical characteristics that will result.
Additionally, as the polycrystalline material is deposited onto the silicon wafer, it
experiences varying levels of stress associated with film growth, which remain within the
film once growth is complete, in the form of residual stress. While it can be improved
with annealing, this residual stress is difficult to eliminate, and stress gradients across the
deposited wafer and across individual microstructures can further limit device uniformity
and manufacturability.
In his discussion on control of film stress in surface
micromachining, Madou proposes an expression that is analogous to Equation 1 for
resonance frequency of a micromachined structure of similar geometry, while taking into
account stress present in the beam:
3
1 2Ewt
24owt
f0,12Et
+2w
=
2i 13m
51m
Eqn.
9
where E, 1, w, t, and m are similarly defined as in Equation 5 and a is residual
stress in the beam. This equation shows that device resonant frequency will be a function
of stress within the material. Consequently, variations in the stress state can have a
strong effect on reproducibility in the manufacturing process. High stresses also tend to
induce defects within materials, in order to relieve the stress and allow the material to be
in a lower-energy state, which further adversely affects the general quality and
reproducibility of mechanical characteristics. In general, in order to obtain high-quality,
repeatable mechanical characteristics, it is desirable to keep any stresses introduced to the
microstructure at a minimum.
34
PiezoelectricMaterials
The issues regarding film uniformity and residual stress also affect the performance of
piezoelectric material to be used within the device structure.
However, for these
materials, the number of processing techniques that have been found to solve the
challenges present is limited.26 Piezoelectric materials that are used in MEMS include
ZnO, PZT, and A1N, which are typically deposited by sputtering or by sol-gel processing
and are polycrystalline in texture.
Because of its additional utility as an optoelectronic material, A1N is commonly
deposited in single-crystal form on SiC substrates by means of molecular beam epitaxy,
however this process is not practical on a Si substrate. While a <111> Si substrate is of
the proper orientation for single crystal AiN growth, mismatch of the lattice parameters
and thermal expansion coefficients between the material would result in a very high
residual stress that would be prohibitive of the formation of uniform, high-quality devices.
Since SiC is a refractory compound of very high chemical stability, bulk micromachining
of the substrate is not possible with conventional MEMS processing techniques. Even
though deposition processes for high-quality single crystal A1N have been developed,
conventional processing techniques do not allow the integration these materials with
piezoelectric MEMS.
Bulk vs. Surface Micromachining
In general, MEMS processing techniques are grouped into two categories:
bulk
micromachining and surface micromachining. Bulk micromachining is characterized by
the removal of material from a substrate (subtractive processes) in order to pattern a
mechanical structure, while surface micromachining generally makes use of additive
processes followed by subsequent patterning of material into mechanical elements on the
surface of a substrate. While both of these techniques are versatile and have enabled the
fabrication of many kinds of MEMS structures, they each are subject to limitations in the
application of fabricating MCB devices.
Bulk micromachining can be used to form single-crystal MCB structures by
under-etching the Si substrate around a patterned epi-layer of material. KOH is the most
commonly used etchant for bulk-micromachining of Si, and would be appropriate in this
35
case. In order to control the etching process, an etch-stop must be employed that defines
the boundaries of regions which are to be etched. One technique that could possibly be
used in this circumstance is an electrochemical etch stop, which, through light doping of
Si, affords more precise thickness control at lower residual stress in materials, in
comparison to other options. Even in this circumstance, however, there remains a level
of uncertainty in dimensional control of the resultant structure.
In the case of surface micromachining, materials are deposited on top of a
substrate and then patterned into three-dimensional strurctures on its surface.
Polycrystalline materials such as polysilicon are most commonly used, in which case the
previously mentioned difficulties in maintaining reproducibility and managing stress
states in the films apply. One emerging technology that enables the processing of singlecrystal devices with surface-micromachining techniques is the use of a silicon-oninsulator (SOI) substrate. The SOI substrate is made up of a thick single-crystal Si layer,
a thin oxide layer, and a thin single-crystal Si layer, stacked consecutively. The thin Si
layer is of a controlled thickness that can be specified as thin as .2 mm, and would serve
to form the mechanical support material of the MCB. By patterning the Si layer and then
under-etching the SiO2 as a sacrificial material with standard surface-micromachining
processes, cantilever-type MEMS structures have been realized. A potential drawback in
this method, however, is the fact that the process of fusion bonding process that is used to
form the SOI wafer often leaves significant levels of stress within the material.
Limitations in Conventional MEMS Technology
In order to attain a finished product that is of high quality, each of the individual steps in
device processing must be precisely controlled.
Limitations that are present in
conventional MEMS micromachining techniques can make the fabrication of a reliable
MCB-based chemical sensor a challenging task. These limitations are perceived as:
·
Insufficient reliability due to variation in morphology across polycrystalline films
·
Difficulty controlling residual stresses in mechanically-released structures
·
Inability to integrate single-crystal piezoelectric materials
·
Insufficient dimensional control in conventional etch-stops
36
4.3 The Boston MicroSystems Microresonator
4.3.1 Device Structure
In order to address the difficulties that are present in the development of piezoelectric
MEMS-based devices, Boston MicroSystems, Inc. has developed a set of processes that
facilitate the integration of high-quality, precisely dimensioned, single-crystal materials
in a MCB structure. The cantilever is formed by selective underetching of appropriately
doped epitaxial layers of silicon carbide deposited onto a single crystal SiC substrate.
This is accomplished by utilizing a proprietary photoelectrochemical etching procedure.
An epitaxial piezoelectric aluminum nitride layer is then deposited onto the SiC beam. A
metal such as Ti or Al lies above the ALN layer, and serves as the upper contact.
The
alternating electrical field is applied across the AN, between the metal and the
underlying SiC. The topmost layer of the structure is comprised of the chemically
sensitive polymer film. [Figure 9]
signal
chemically
sensitive
epitaxial AIN
piezoelectri
@ \ /
Figure 9. Schematic of the BMS Microresonator
_.7 /
Figure 10. SEM of the BMS Microresonator
The choice of AiN and SiC as functional and structural materials of the cantilever
uniquely distinguishes this technology from other alternatives and provides excellent
resonant characteristics. As discussed previously, both of these materials are very well
suited for use in a resonant device, and since both materials have a single-crystal
structure, problems commonly seen in uniformity and reliability of polycrystalline films
are avoided. The benefits of selecting the AIN/ SiC bi-layer as the cantilever structural
material are summarized as follows:
·
The SiC and A1N layers are both single crystal structures, which exhibit high
stability, uniformity, and reproducibility of materials characteristics.
* AIN and SiC have a very low lattice and thermal mismatch, which facilitate the
epitaxial growth of virtually stress-free microstructures.
* High specific moduli of SiC and AiN contribute toward a high-frequency, highsensitivity resonant structure.
* The high piezoelectric coupling coefficient and low DC conductivity of A1N
make it well-suited for use in a resonant mass sensor.
* Both SiC and AIN are compatible with harsh-environments, meaning that they
will resist interaction with many chemicals. This allows the interaction between
the polymer and the environment to be isolated for detection.
·
High thermal conductivity of SiC and AIN facilitate the correlation of resonator
measurements with a reference device of equal temperature
O8
* The process capability that has been developed exclusively at Boston
MicroSystems provides a competitive advantage.
4.3.2 Materials Processing
PhotoelectrochemicalEtching of Bulk Silicon Carbide
One process that is critical in the fabrication of the BMS Microresonator is the
micromachining of single crystal SiC by a photoelectrochemical etch process. Having
been initially investigated in the Ph.D. thesis of Dr. Richard Mlcak (President, Boston
MicroSystems)2 7, the process was subsequently developed and patented both at MIT28
and at the company29 . While SiC is a very stable compound and generally very resistant
to harsh chemicals, this process enables the isotropic etch of the bulk material, facilitating
the release of freely-moving structures over etched-out cavities in the substrate.
i
I
I
U
Bias(vot vrs
SCE)
Figure 11. I-V Characteristic of Si-HF interface under anodic bias30
The etch-stops utilized in this process have also been patented, and enable precise
dimensional control of virtually stress-flee mechanical structures. The stop is formed by
means of a p-n junction in the SiC material that limits electrical current flow to the
39
semiconductor-electrolyte interface by means of a potential barrier. Standard ionimplantation techniques allow the depth of the p-n junction (and consequently the
thickness of the etched structure) to be controlled to sub-micrometer dimensions. Since
only very low defect concentrations (ppm) are required for the doping, the stress of
mechanical structures released by this method is also very low.
MolecularBeam Epitaxy ofAluminum Nitride
Another processing technique that is essential is the deposition of epitaxial A1N on top of
single-crystal SiC device structures. The processes typically used to deposit piezoelectric
materials for MEMS are sputtering and chemical vapor deposition, which generally
produce polycrystalline films. With this conventional process technology, it is generally
difficult to obtain high quality piezoelectric films in the fabrication of MEMS, as
structures are often formed with high residual stresses and poor uniformity. The use of
single-crystal materials enables large improvements in issues of uniformity, stability, and
reliability. Molecular beam epitaxy is used to deposit single-crystal A1N on the SiC, and
is currently performed outside of BMS under contract.
Ink-jetdepositionoffunctionalpolymers
An additional critical component of the BMS Microresonator is the chemically sensitive
film that coats the device surface. While polymer design and deposition have been
thoroughly investigated on other chemical sensing platforms such as SAW and BAW
devices, the processes that are commonly used there such as wafer spin-coating or dipcoating are not compatible with 3D-type structure that are seen in MEMS. Since the
MCB structure includes a large etched-out cavity underneath the mechanical element, it
is not practical to deposit a polymer across the entire wafer and then pattern it, as
polymer material may amass within the etch cavities.
In order to selectively deposit polymers on the surface of the cantilever beams, an
ink-jet deposition process is used, where several microscopic drops of a dilute solution
that contains a polymeric material is deposited on the surface of the cantilever beam.
Other groups that are developing MCB-based devices have demonstrated continuous
coverage of the MCB structure31 . While deposition has been performed thus far outside
40
of BMS under contract, the tools necessary to perform the process are commercially
available, enabling in-house customization of the process in the future. With much
progress having been demonstrated in the design of the materials32 and of the deposition
process, it is thought that chemically sensitive materials will continue to be readily
available for integration with the MEMS devices.
4.3.3 Preliminary Device Characterization
These devices have been fabricated, and characterization of their chemical sensitivity is
currently under way.
The measurement data shown in Figure 4 shows the strong
temperature and pressure-sensitivity of the devices, which is accounted for by comparing
the response between a polymer-coated and an uncoated sensing element. Figure 5
shows preliminary chemical response data taken on a sensor in the presence of DMMP, a
chemical stimulant for sarin nerve gas. This initial data indicates that it is possible to
resolve 150 ppb of gas over a frequency shift of 100 Hz (easily detectible in electronics),
however the method of characterization is still being developed, and further testing is
under way to verify predictions of much higher levels of gas sensitivity.
Since these resonant microstructures are also highly sensitive to fluctuations in
temperature and pressure, it is also very important to reference the response of polymercoated devices to uncoated devices as measurements are made. Studies that have been
conducted indicate that frequency response to changes in temperature and pressure are
very well defined. This facilitates the compensation of measurement error that may be
seen from temperature or pressure drift, by simultaneously measuring an un-coated
reference cantilever.
41
_
___
'---
4.22
.__.·_ · _ .
·_ __·
____l__·i_____C
_C___····
· _·___
4.205
4.215
N
X
2
IN 4.2
4.21
0
4.205
@ 4.195
01
o
06d 4.2
C
(t0 4.195
U.
,
0
C)
4.19
4.19
uL
10
20
30
40
50
60
70
80
4.185
0
Temperature
(C)
10
20
30
40
Pressure(psi)
500 Hz / psi
400 Hz / deg.
Figure 12. Temperature and Pressure Response of the BMS Microresonator
f
200
0
N
-200
a Temperaturecorrecteddata
*Rawdata
i
l
.
Z -400
oo
C -600
_____________
___________________________________________
Cr
,
-800
-1000
-1200
0
100
200
300
400
500
600
ppB DMMP in argon
700
800
Figure 13. Preliminary Response Data
Exposed to Dimethyl Methyl Phosphate (DMMP) Pressure: 771.1 +/- 0.1 torr,
Temperature: 19.7 - 19.9 deg. C
4.3.4 Comparison to Competing Technology
Several companies listed in Table 1 currently offer products targeted toward the same
market areas that the BMS Microresonator addresses. As seen in the chart, most of the
devices with which the microresonator directly competes utilize surface acoustic wave
'It,
"tL.
[SAW] technology. Typically, these devices are several hundred microns along a single
planar device dimension, several times larger than a typical microresonator that is
dimensioned at ten's of microns on a side.
The microresonators are theoretically
predicted to be able to detect a much lower minimum concentration of gas, however the
preliminary measurements given earlier at least confirm that ppb sensitivity comparable
to SAW is achievable. The smaller device footprint used to achieve this sensitivity
already puts the microresonator at an advantage over SAW, in terms of portability and
selectivity.
Company
BAE Systems
Gas Sensor
Market
hem/bio agents,
Gas Sensor
Technology
SAW
Min
Detectable
Gas Conc.
Ppb
SAW
ppb-ppm
tICs*
Microsensor
Emergency
Systems, Inc. response, defense,
tIC
Mine Safety
Appliances
Workplace safety, SAW, PID
Emerg. Resp.,
Co.
defense
RAE Systems Defense, TICs,
Ppb
PID
ppb-ppm
Conductive
Ppb
Emerg. resp.
Smiths Group Defense, TICs,
Photovac
Process Control
Polymer
VOC Detection
Swelling
PID, GC
ppb for GC
Table 4. Several Companies that are Developing Portable Gas Sensors
TIC: Toxic Industrial Chemical, VOC: Volatile Organic Chemical, SAW: Surface
Acoustic Wave, PID: Photo-ionization Detector, GC: Gas Chromatograph.
The BMS Microresonator holds potential to replace a number of different sensor
technologies that are currently in the market today, offering increased device sensitivity
and uniformity (due to single-crystal materials integration) with decreased footprint and
power dissipation. Whether or not this device will be able to overtake the competition
depends also on the cost at which these improved performance characteristics are offered.
Certain application areas may intrinsically require the cost of each device to be below a
43
certain point, and may even sacrifice an increase in performance for a decrease in cost.
This is a key point that deserves a detailed analysis, especially since the cost of SiC is
currently considerably higher than other materials alternatives such as Si.
While
breakthroughs in engineering of single crystal SiC substrates may lower its price in the
future, for purposes of the present analysis, it will be important to employ an evaluation
of the trade-offs that are given and taken between cost and performance within individual
applications and evaluate any consequences or possible alternatives to obtain more
optimal product-to-market matching.
44
5 Intellectual Property
5.1 IP Protecting the Boston MicroSystems Microresonator
Boston MicroSystems holds key intellectual property in several aspects of the design and
fabrication of the MCB device that effectively prohibit any other groups from creating a
duplicate of the product. In addition, the patents that have been obtained are very general
and create opportunities for revenue through licensing to other companies in the future.
The following sections detail the individual pieces of intellectual property that together
create the exclusive capability of BMS to manufacture highly sensitive, highly uniform
MCB chemical sensors:
5.1.1 Patents on Materials Processing
The two patents on that have been obtained in regard to materials processing techniques
are directly relevant to the fabrication of the MCB devices are, however they cover a
much broader range of process capability
USPT# 6,511,915 "Electrochemical Etching Process" covers the process by which
bulk single-crystal SiC can be etched for the release of suspended, movable MEMS
components. This patent is essential because bulk SiC is by nature very difficult to etch
in wet chemistries, because of the materials resistance to chemical attack. The addition
of UV radiation enables the weakening of surface bonds through creation of electron-hole
pairs in the material, and resultantly allows the etching of bulk SiC in hydrofluoric acid.
The patent itself is applicable to other materials systems, as the process itself was
originally developed on silicon.
USPT# 5,464,509 "P-n junction etch-stop technique for electrochemical etching of
semiconductors" is a very general patent that enables precise dimensional control on the
thickness of structures created with the above mentioned process.
The patent itself,
however, is general enough to not only apply to this particular etching process. The first
claim of the patent is written as follows:
45
1. An electrochemical etching process comprising:
forming an n-type region on a p-type indirectband gap semiconductorsubstrate,a p-n
junction separatingthe n-typeregionfrom thep-type substrate,at leastportionsof the ntype region being provided with means for inhibiting injection of holes from the p-type
substratethroughsaidportionsof the n-type region;and
exposingthe n-type region and p-type substrate to an electrochemicaletchantwith an
electrical bias supplied between the p-type substrate and etchant such that the p-type
substrateis selectivelyetchedwith substantiallyno etchingof saidportions of the n-type
region,the electricalbiasforward biasingthep-n function.
Since this is the first claim, it is the most general of all that proceed, and as written it
is not specific of the type of semiconductor being etched, nor is it specific to the type of
etchant chemical being used, nor is it specific to the fact that light is also used in the
processing of SiC substrates. Any party wishing to utilize this sort of p-n junction etch
stop would have to license from the patent assignee.
In this case, the two inventors
named on the patent are the co-founders of Boston MicroSystems, and the assignee of the
patent is MIT.
5.1.2 Patent on Device Structure
USPT# 6,627,965 "Micromechanical device with an epitaxial layer", through more
general specification, patents the use of epitaxial AN with single-crystal SiC, a critical
design point that affects the quality of the resonant structure. The broad applicability of
this patent can be seen through examination of the first claim:
1. A micromechanical device comprising:
a single crystal micromachinedmicromechanicalstructure, at least a portion of the
micromechanicalstructurecapableofperforminga mechanicalmotion;and
an epitaxial layer covering at least a part of said portion of the micromechanical
structure that is capable of performing a mechanical motion, the micromechanical
structureand the epitaxial layer beingformed of different materials,the epitaxiallayer
46
beingformed of a materialthat provides at least one of protective,strength,frequency,
damping, piezoelectric, pyroelectric, electro-optic, magno-resistive, variable reflectivity,
chemically sensitive and biologically sensitive properties.
As written, this first claim states that any single crystal micromechanical structure
that has an epitaxial film on its surface is protected by the patent. One interesting detail
about this patent is that the epitaxial layer does not have to be single-crystal. The
American Heritage Dictionary of the English Language defines epitaxy as33
The growth of the crystals of one mineral on the crystal face of another mineral, such
that the crystalline substrates of both minerals have the same structural orientation
The first claim of this patent is very general, to such a point that it almost seems that
there must be devices in existence that make use of this type of technology. A patent of
this nature provides the possibility for future income for the company in terms of
licensing of technology.
5.1.3 Process Infrastructure
The BMS Microresonator is fabricated by means of unique process technology that has
been developed over a number of years. In order to perform the process, a significant
amount of hardware and control software had to be built. Additionally, a significant
amount of R&D was invested in order to fine-tune process variables. The trade secrets in
process infrastructure that have been amassed give BMS an strong advantage over any
other party that would want to begin to try to develop technology that utilizes the same
materials system.
Perhaps most importantly of all, the original inventors of the key features of this
technology are the ones that are currently pursuing its development. A recent study
indicates that 70% of all university patents can't be used without involvement of the
original inventor.3 4 With legally-protected processes and structures, complex and finetuned fabrication infrastructure, and a core base of expertise with which to further
develop the technology, Boston MicroSystems is in a very secure position to bring a new
product to the market.
47
5.2 Patent Analysis
A patent search was conducted to analyze the intellectual property space that currently
exists around technology related to the BMS Microresonator and gauge whether there
might be any challenges or roadblocks in the continued development of a product for the
markets being considered. Patents dating back to 1976 were searched online at the US
Patent Office's website (www.uspto.org), via three main techniques:
1. Patents were searched by general query for patents containing key words related to
the Microresonator technology (e.g. "cantilever and reson$ and 'chemical sensor')
2. Patents were searched by US classification numbers, which are assigned to each
patent by the Patent Office in order to group and classify patents by similar
applications or characteristics
3. Patents were searched according to names of researchers and companies that are
known to be actively pursuing similar technology or market areas
Additionally, citations of several key older patents as well as references given in more
relevant newer patents were investigated. The following is a summary of some of the
results that were obtained during this patent analysis.
Many of patents have been filed around technology that has some similar traits to
the BMS Microresonator. US Patent #4,596,697 on a "Chemical Sensor Matrix" was the
earliest reference that was found describing gas measurement in a similar (but not
identical) array-based measurement configuration.
Surface acoustic wave (SAW)
technology appears to be the most widely used for resonant chemical sensor arrays, and
the patent space for SAW technology is very crowded with over 44,529 patents awarded,
although this includes applications largely outside of gas sensing.
This is a key
technology that the BMS Microresonator is well poised to replace, as sensor technology
progresses. More relevant to the nature of the microresonater are the 2446 patents on
devices that relate to a "resonator or cantilever", although this still includes many
unrelated applications.
Conducting a search with the more specific query terms
"cantilever and ((gas sensor) or (chemical sensor))" reveals 73 patents that have been
48
awarded, but in actuality only a few of these patents are directly relevant to the
microresonator.
Following along with the second method in the above list, another search
technique that is of use is to query the database by the classification that has been
assigned to the patent by the government official that worked with the patent during its
approval. Every patent that is granted by the government is assigned one or more pairs of
class and subclass numbers. If a relevant class and subclass of the invention can be
determined, then an examination of all elements within that subclass can provide a more
thorough patent search.
An example is US Patent Class 73, subclass 24.01, "Gas Analysis by Vibration,"
under which 158 patents have been filed. Of these patents, only 36 seemed relevant by
title, many of the other patents having to do with technology such as SAW and QCM
devices, and some of the patents pertaining to methods of probing gases by means of an
acoustic signal sent into the gas. A summary of the examination conducted on these
patents is included in the Appendix.
In examining the patent space, an effort was made to locate sensor technology that
had very similar characteristics to the BMS microresonator, in case the patent claims
posed any restriction on the types of design or application that are allowed with the
device. It appeared that a good deal of the technology that currently is being researched,
while very similar to the microresonator, does not pose much of a risk within the markets
being considered for product development. Additionally, no technology was found that
directly blocks the core technology or proposed measurement system.
One example of such a closely related technology is a "Microcantilever Sensor"
(USPT #5,719,324) that is being developed at Oak Ridge National Laboratories.
Although these types of cantilevers are coated with polymer films and driven by a
piezoelectric, several key differences include the fact that all cantilever devices contact a
single mechanically oscillating structure and also the fact that response measurements are
taken from the structures optically. This device is shown in an image taken from the
patent document, displayed in Figure 14 of this report.
49
Fig. 4
-42
I4
54
Fgla.
5
7
Ca
I~q
l
Figure
14.
Image
USPT#5,719,324
taken
from
"Microcantilever
Sensor"
Figure 14. Image taken from USPT#5,719,324 "Microcantilever Sensor"
(element 52 is a common piezoelectric, elements 56 and 58 are
LED-photodiode pairs that measure each structure's oscillation)
Throughout examination of this patent and many others, no claims were found
that block the core sensor technology of the BMS Microresonator, however it is also
necessary to consider other IP that also facilitates the operation of the device but might
not directly specify a sensor. Additional patents of interest include USPT # 6,321,588
"Chemical Sensor Array", which focuses on the use of multiple sensors in a single
system and protects the use of a certain type of scheme of device multiplexing. USPT #
6,041,642 specifies a "Method and apparatus for sensing the natural frequency of a
cantilevered body," a task that is of key importance to the operation of the microresonator,
however within the claims of this patent the sensing of device frequency is specific to
analysis of an optical (LED-photodiode pair) response of the mechanical structure. As
products
are developed and commercialized
it is important
to keep in mind that
companies will often patent a much larger amount of IP than their own technology
encompasses.,
50
6 Penetrating the Market
6.1 Challenges
The issues of product sale price aside, there are additional challenges related to the
amount of resources that must necessarily be expended in order to break into the different
markets to which the BMS Microresonator technology is most well suited. Across
different areas such as homeland security, workplace safety, or process control, there are
a large number of niche markets that have been formed around various applications in
chemical detection. Each of the companies that are providing solutions to the needs in
these various applications have developed complete devices that not only include the core
sensing technology, but also elements such as supporting electronics, packaging, and
software for analysis and interpretation of data.
The task that a company faces in developing a complete product that can be
delivered directly to the end user, when starting only from a single core sensor
technology, can be quite daunting. A large capital investment is required to develop
these additional components of a measurement instrument, and then even when this is
finished, further capital is still required for sales and marketing. For a small business that
has developed a strong core technology but has a limited amount of capital to work with,
it is advantageous to try to put out a product that can address a need without requiring
excessive amounts of money to develop additional technology that does not give the
company an exclusive advantage over competition. Fortunately, with all of the IP
protection that has been amassed, BMS is in an excellent position to leverage directly off
of their core technology to offer a more fundamental product that holds potential to tap a
broad range of markets.
51
6.2 A Standardized Sensor Platform
A product idea that is currently under consideration at BMS would utilize the
Microresonator technology to simultaneously address needs across many specialized
markets, through proposal of a standardized sensor platform called a Detection Engine.
This product would include an array of gas sensing elements connected to electronics that
drive resonator oscillation, compensate for variations in temperature and pressure, apply
signal processing to filter out other noise, and output a frequency "profile" corresponding
to the individual responses of each of the devices within the sensor array. This product
would be marketed as a discreet component for use in a larger system. It would take as
input a gas to be analyzed and then output an electrical response profile, which could be
analyzed by any number of different pattern recognition or numeric quantification
At the same time, it would also be possible to tailor the platform
algorithmic techniques.
to specific desired application areas, by proper choice of the polymer coatings within the
sensor array. A simple schematic of the proposed system is shown below.
I
>
I
Gas
I
I
I
Resonator
Array
+
--
Profile of
data
> Frequency
>
Measurement
Electronics
Figure 15. The Detection Engine
This embodiment of the core technology takes the marketing focus away from the end
user and shifts it one step higher in the supply chain, to companies that develop full gas
sensing
instrumentation
and then sell to the end user (compare Fig. 8.a to 8.b).
Companies that have already developed instrumentation may be interested in upgrading
their core sensor technology, in which case the Detection Engine provides an attractive
solution.
From the perspective of BMS, there are several advantages to taking this
approach in product development:
·
Lower capital costs are required in developing a simpler product
*
Benefits of BMS' exclusive technology are captured with minimal additional
R&D necessary
* A standard, discrete component can be applied to different measurement
instrumentation, reaching a larger number of markets
·
The component is customizable simply through materials selection in a final
processing step
~'?Vc~
, .)·l"~i
Sell Instruments
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;-AfN~~~~~~~~~~~i,
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k.'ick
e
,
.-
:..
'1:'
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(a) Complete instruments are sold directly to End User
[very high capital costs for R&D, sales, and marketing]
Sell
Sell
Detection
,.
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*p
Engine
.'.-
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(b) Detection Engine is sold to Instrument Providers
[reduced capital costs, broader market penetration]
Strategic
Pnrtnorehin
a-
Sell
.-.
.."--;:;-:?
:;':':4
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(c) Strategic Partnerships formed with Instrument Providers
[Generation of capital; mutually beneficial]
Figure 16. Comparison of supply chain of several business strategies
53
6.3 Business Strategy
Although the Detection Engine makes more efficient use of core technology in bridging
the gap to the market for gas sensors, development of this product will still require
significant (albeit reduced) capital costs for development of sensor electronics, packaging,
and specification of the technology to each individual application. Additionally, in order
to offer the product at low price to potential buyers, a high production volume is required
to drive down manufacturing costs.
By negotiating with companies that might be interested in using the Detection
Engine in order to form strategic partnerships or joint business ventures, a more mutually
beneficial agreement may be found. In this case, instead of simply selling the Detection
Engine to different companies at a fixed price, negotiations would be initiated with
companies in order to find an agreement that more directly addresses the immediate
needs of all parties involved. As an example, a company might agree to provide BMS
with an amount of capital to invest in development of the Detection Engine, in exchange
for exclusive rights to use the technology for a limited period of time, once the product
has been completed. Exclusive IP, expertise, process infrastructure, and proven results
all contribute very strongly to the value proposition that BMS would present in such a
negotiation. During these negotiations, care would need to be taken in order to make best
use of the company's bargaining position and ensure future integrity of existing IP,
however the choice of going this route could be very advantageous by simultaneously
providing much-needed capital and ensuring a place for the technology within the market.
Once a market position has been secured and this product has been established as
a sustaining technology that continues to generate revenue influx for the company, BMS
would have more flexibility to explore new markets for their technology or open up paths
directly to the end user through continued product development, if so desired.
54
7 Conclusion
Especially in light of the requirements exerted by the markets of homeland security and
home and workplace safety, there is a need for highly sensitive, highly selective, portable
chemical sensors. Many different kinds of chemical detection systems are commercially
available for use in a broad range of applications. However, while some technologies
offer portability and others provide sensitivity and/or selectivity, few, if any, technologies
are capable of meeting all of these requirements.
MEMS provide an excellent platform for the integration of arrays of highly
sensitive, miniaturized sensors. The Boston MicroSystems microresonator is a MEMSbased gas sensor that has been developed using unique processes that facilitate markedly
improved quality of performance over alternative choices of materials and device
structures.
Because of strong protection via a broad-reaching base of intellectual
property, Boston MicroSystems is uniquely positioned to take advantage of this
technology in the commercial arena. Although challenges are present in the introduction
of this product to existing markets, a viable route for the establishment of an initial
position in the market can be found, through embodiment of the core technology in a
standardized sensor platform.
55
Appendix.
Summary of Examined Patents
The following table includes the portion of the patents that were examined in the patent
analysis that came from Patent class 73 subclass 24.01, gas analysis by vibration.
Comments are included where appropriate.
Patent #
Title
Comments
6,523,392
Microcantilever sensor
6,418,782
Gas concentration sensor
Subjacent polymer
coating
Uses ultrasonic
waves in the gas
6,357,278
6,321,588
Polymer coatings for chemical sensors
Chemical sensor array
6,308,572
Gas concentration sensor
6,305,212
6,171,867
6,167,748
Method and apparatus for real time gas analysis
Piezoelectric gas sensor
Capacitively readout multi-element sensor array with
common-mode cancellation
Method and apparatus for sensing the natural frequency
of a cantilevered body
6,041,642
6,014,889
5,958,787
5,955,659
5,729,207
5,719,324
5,644,070
5,445,008
Gas analyzer
Polymer sensor
Electrostatically-actuated structures for fluid property
measurements and related methods
Method and apparatus for phase for and amplitude
detection
Corrosive gas detecting sensor
Microcantilever sensor
Ozone concentration sensor
Microbar sensor
5,351,522
Gas sensor
5,343,760
Gas concentration and flow rate sensor
5,325,703
Method for identifying the concentration of fuels or
gases
Method and apparatus for monitoring the content of
binary gas mixtures
Method of sensing contamination in the atmosphere
Chemical sensors
Vibrating type transducer
Syste for determining gas concentration
Molecular as detector and analyzer
Electronic nerve agent detector
5,763,283
5,060,506
5,042,288
5,028,394
4,872,335
4,616,501
4,503,703
4,549,427
A patent on the
multiplexing of
multiple sensors
Capacitive
Patent on opticalfeedback phaselock
Capacitive
Claim explicitly
includes
photodetector
uartz
Similar device
Claims are not
arranged well
(there's one claim),
but sounds exactly
56
-
like BMS
microresonators
4,424,703
4,424,702
4,418,566
4,385,516
4,380,167
4,280,183
4,155,246
4,119,950
4,003,242
Device for monitoring the concentration of an air-vapor
mixture
Device for monitoring the concentration of an air-vapor
mixture
Gas analyzing techniques
System for the detection of the presence of a
predetermined chemical vapor distributed in the
atmosphere
Apparatus and method for detecting a fraction of a gas
Gas analyzer
Rapid gas analyzing system
Gas detection
Device for determining the mixing ratio of binary gases
Detects absorption
of radiation
57
References
1"http://www.altassets.com/casefor/sectors/2003/nz3016.php", Internet, 5/17/04
2 "Sensors will Benefit from Nanotechnology and Microfabrication", P. Adrian, Sensor Business Digest,
May, 2002.
3 "Chemistry 101: Ion Mobility Spectrometry", Website of the American Chemical Society,
"http://www.chemistry.org/portal/a/c/s/l/feature-tea.html?id=c373e9fe32edd98c8f6al7245d830100", the
internet, 8/9/2004.
4 "http://www.arofe.army.mil/Conferences/CWD2001/36-CWD2001.htm", Internet, 5/17/04
s "http://www.towardfreedom.com/1999/nov99/toxicchips.htm", Internet, 5/17/04
6 "Clean Houses May Trigger Asthma", BBC News Webiste,
"http://news.bbc.co.uk/l/hi/health/3597404.stm", Internet, 8/25/04.
7 'The Electronic Nose," Ray Marsili, Food Product Design Magazine, Weeks Publishing, June 1995 QA,
"http://www.foodproductdesign.com/archive/1995/0695QA.html", Internet 3/1/04
8"http://www.flair-flow.com/industry-docs/sme-syn9.pdf", Internet 9/3/04
9 Madou, Marc J. Fundamentals of Microfabrication,
10"Chemistry 101: Organic Mass Spectrometry", Website of the American Chemical Society,
"http://www.chemistry.org/portal/a/c/s/l/featuretea.html?DOC=teachers%5Ctea-mass-spec.html", The
Internet, 4/8/2004.
n H.T. Nagle, et al. "The how and why of electronic noses", IEEE Spectrum 35 (1998) 22-31.
1
2 M. Liess, M. Leonhardt. "New operation principle for ultra-stable photo-ionization detectors".
Measurement Science and Technology 14 (2003) 427-432.
13 http://www.photovac.com, internet, 8/25/04
14S.M. Sze, Semiconductor Sensors. 1994 John Wiley & Sons, Inc., USA.
15 K. Arshak et. al. "A review of gas sensors employed in electronic nose applications." Sensor Review 24
(2004) 181-198.
16 http://www.cyranoscience.com, Internet, 3/04
K. Arshak et. al. Sensor Review 24 (2004) 181-198.
Arshak et. al. Sensor Review 24 (2004) 181-198.
196/12/04 Interview with Hank Wohltjen, President, Microsensor Systems, Inc.
17
18 K.
20 Cernosek,
et.al., "Detection of volatile organics using a surface acoustic wave array system", SPIE
Proceedings 3857 (1999) 146-157.
21 MEMS Industry Group Annual Report,
"http://www.memsindustrygroup.org/pdf/2001 MIG_Annual_Report.pdf", Internet, 9/1/04.
22
MEMS Industry Group Website, http://www.memsindustrygroup.org/industrystatistics.asp, Internet,
9/1/04.
23 "Redstone arsenal rides into the future on BAE Systems inertial sensor for precision weapons"
"http://www.na.baesystems.com/releasesDetail.cfm?a=82", Internet Press Release, 9/3/2004.
24"Cantilever transducers as a platform for chemical and biological sensors", N.V. Lavrik et al, Review of
Scientific Instruments v75 n7 (2004), 2229-2253.
25 "Piezoelectric thin film micromechanical beam resonators", D.L. DeVoe, Sensors and Actuators A 88
(2001) 263-272.
27 J. Baborowski, "Microfabrication of Piezoelectric MEMS." Journal of Electroceramics 12 (2004), 33-51.
28 "Electrochemical and Photoelectrochemical Micromachining of Silicon in HF Electrolytes", R. Mlcak,
Ph. D Thesis, The Massachusetts Institute of Technology, 1994.
29 "P-N junction etch-stop techniques for electrochemical etching of semiconductors", US Patent #
5,464,509.
30 "Electrochemical Etching Process", US Patent # 6,511,915.
31 "Photo-assisted silicon micromachining: opportunities for chemical sensing", H. L. Tuller, R. Mlcak,
Sensors and Actuators B 35-36 (1996) 255-261
32 "Rapid functionalization of cantilever array sensors by inkjet printing", A. Bietsch, Nanotechnology 15
(2004) 873-880.
33 "Rational materials design of sorbent coatings for explosives: Applications with chemical sensors",
Houser, E.J. (Naval Research Laboratory, Material Science Division); Mlsna, T.E.; Nguyen, V.K.; Chung,
R.; Mowery, R.L.; McGill, R.A., Talanta, v 54, n 3, May 10, 2001, p 469-485
58
33 The American
Heritage® Dictionary of the English Language, Fourth Edition. 2000, Houghton Mifflin
Company
3
4 M. Darby, L. Zucker. "Grilichesian Breakthroughs: Inventions of Methods of Inventing and Firm Entry
in Nanotechnology", National Bureau of Economic Research, July 2003
59