Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
AUTONOMOUS SENSORS ON FLEXIBLE FOILS POWERED
BY RFID AND ENERGY SCAVENGING FOR
ENVIRONMENTAL AND GOODS MONITORING
1
D. Briand, J. Courbat, N.F. de Rooij
Institute of Microtechnology, University of Neuchâtel, Neuchâtel, Switzerland
Abstract: In this communication, we report on the development of a technology platform for the realization
of the next generation of autonomous smart sensing systems. The technology platform is based on flexible
foils envisioning low-cost and environmentally friendly printing processes for the technological realization
of the devices. Printing being an additive manufacturing process at low energy compared to the traditional
silicon semiconductor industry. The objective is to develop smart sensor systems realized on flexible
substrates that have sensing, communication and powering capabilities. Besides RF and battery powering,
energy scavenging is considered for prolonging or replacing batteries for the realisation of autonomous long
life sensor nodes. These smart flexible labels are intended to be used as intelligent RFID tags and
autonomous sensing systems for the monitoring of the environment. Foreseen applications are in the field of
wearable systems, smart buildings, and along the transportation chain of perishable products.
Key words: Printed sensors, Sensors on plastic foil, Autonomous sensors, Smart sensing systems, RFID, Logistics
very sensitive to the environmental conditions in
which they are transported. This includes temperature,
humidity, light, pressure, shock, and the presence of
gases or vapors resulting from goods’ deteriorating.
According to the March-April, 2007 issue of the
RFID Journal:
•
90% of perishable goods are transported in
containers world-wide;
•
Just in the area of perishable food one faces an
average of 56% losses in the US and 50% in the UK;
•
20% of the vegetables distributed in China are
lost during or because transportation.
RFID tags which allow identification and
traceability of objects have become ubiquitous in the
logistics and retail industries. Originally used in retail
as a security measure, RFID is now widely used for
smart stock control (the retailer knows what is in the
warehouse, what is in the shop and when an item is
purchased). In logistics, RFID is used to identify the
content of shipments and to provide a unique identifier
to allow items to be tracked between transit points.
Recently the logistics industry has been investigating
the possibility of passive and active RFID tags to
provide information about the environmental
conditions of goods in transit.
Certain high value goods such as pharmaceutical
products and various categories of foodstuff are very
sensitive to the environmental conditions in which they
are transported. This includes temperature, humidity,
light, and the presence of gases or vapors resulting
from goods’ deteriorating (mixed volatile organic
compounds). It can be seen that sensors embedded in
1. INTRODUCTION
Monitoring of environmental parameters in a
distributed manner can contribute to improve the
energy consumption and efficiency in some applicative
areas. For instance, in the field of transport of goods,
the identification of their deterioration could allow to
stop their transport along the way and then to transport
only valuable goods for the retailers and customers. In
the field of building climate control, a larger
distribution of environmental sensors would allow a
more efficient control of heating, ventilation and air
conditioning (HVAC) systems, which consume an
enormous amount of energy, therefore leading to
substantial savings of energy. For both of applications
mentioned here, either in monitoring the environment
in containers/pellets/boxes or in buildings, there is
definitely a need for low-cost and ultra-low power
environmental sensors that could be implemented in
large scale wireless sensor network. The two
applications have however relatively significant
different requirements, with one (logistic of goods)
requiring sensors for operation for few weeks, months
and the other one (smart buildings) requiring long
lifetime, several years, without or very limited
maintenance. The cost of production and the power
consumption are two very important parameters that
should be minimized to allow a large distribution of
these sensors.
If we focus now on the logistic application, certain
high value goods such as pharmaceutical products,
various categories of foodstuff, high-tech products are
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Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
RFID have the potential to improve control of
transportation, reducing wastage and maintaining
product quality. They will allow the individual parts of
the logistics chain to take responsibility for
environmental conditions and enable liabilities for
failures in the system to be identified. These benefits
can only be achieved by the realization of lowcost/low-power multi-sensor platforms on flexible
substrates by using processes compatible with large
scale fabrication on foils (roll to roll, printed
electronics). The application specific target prices for
RFID tags is in the range of 5 to 10€ for the
pharmaceutical products and between 1 and 3€ for the
food applications.
Technological breakthroughs in multi-sensor RFID
will have an extended impact due to the fact that they
will also open up new opportunities in the application
areas such as distributed wireless sensors for
environment monitoring, smart buildings, wearable
systems, and smart textiles.
ethylene, VOCs… [1-3]. This environmental sensing
platform contains the necessary read-out circuitry and
a microcontroller for data acquisition and processing,
allowing the wireless data transfer. Our System in Foil
(SiF) concept for the integration of long life sensor
nodes powered by scavenging energy in a flexible
credit card planar module will also be presented.
3. MULTI-SENSOR PLATFORM
ENVIRONMENTAL MONITORING
FOR
The technology platform to integrate multi-sensors
is based on large scale processing on flexible plastic
foils, such as roll to roll and printing technologies.
Using a hybrid combination of organic and inorganic
materials different types of transducers and sensing
layers can be integrated on the flexible plastic foils.
We will focus here on three types of chemical gas
sensors that are integrated on thin polyimide/PET
sheets (Fig. 1): metal-oxide gas sensors, capacitive
gas sensors and colorimetric gas sensors, which can
be combined with a temperature sensor for the
monitoring of the ambient environment. For these
three different sensing principles, we are targeting
their large scale processing on flexible substrates at
low-cost and operation at ultra-low power. The latter
is of high importance for the integration of these
devices in wireless sensor systems.
2. TECHNOLOGY
During the last years, a growing interest has
developed to replace silicon-based sensors by
polymeric substrates, which brings numerous
advantages. Their simplified fabrication processes and
their flexibility offer new opportunities and
applications. A significant decrease in manufacturing
costs and energy may be achieved by using large scale
fabrication techniques such roll to roll and printing
processes. With reduced price and power
consumption, it is now possible to integrated sensors
where it was impossible to imagine them a decade
ago. This technology platform could allow the
realization of flexible environmental sensors (gas,
humidity, temperature, flow, wind…) for applications
in autonomous wireless sensors networks or RFID
tags and make them compatible with large scale
production on foil such as roll to roll or printing
processes. Indeed the device fabrication is simplified
and sensors could be directly printed on the smart
sensing label, being more cost effective, avoiding thus
the need of additional assembly steps which is
required for the integration of Si-based sensors on
flexible substrates.
We will report in this communication of the recent
advances we have achieved in the development of a
multi-sensor platform on flexible plastic substrate for
environmental monitoring and its interfacing with
RFID communication. We have integrated different
sensing principles on flexible plastic foils such as
capacitive, optical and resistive read-outs for the
detection of several types of environmental
parameters such as temperature, humidity, ammonia,
Fig. 1: Chemical gas sensors integrated on flexible
plastic foil.
3.1 Metal-oxide gas sensors
Metal-oxide gas sensors operate usually at
temperatures between 200 and 400°C and allow the
detection of several different reducing and oxidizing
gases. For the food transport monitoring, they can be
of interest for the detection of ethylene and ammonia
for instance.
They consist in a resistive gas-sensitive metaloxide layer with a heater to warm it up and two
electrodes for the signal read-out. Because of the
relatively high temperature of operation, polyimide
was chosen as the base platform for the integration of
MOX sensors on flexible plastic foils. The micro174
Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
simplified and more cost effective by making it
compatible with mass production such as roll to roll
and printed processes, avoiding thus the need of an
additional assembly step which is required for Si-based
sensors on flexible substrates.
Hotplates were coated with a metal-oxide film using
a drop coating procedures to form complete gas
sensors. Gas measurements performed with a 50 um
wide MOX sensor on PI foil operated at 10 mW
(~300°C) are presented in Fig. 3. We are now working
on evaluating the performances of smaller devices and
to reduce the power consumption in the µWs range by
establishing a temperature pulse mode of operation for
these devices.
hotplates were made out of a 50 µm thick polyimide
(PI) sheet. Five square-shape heater sizes were
designed: 100 µm, 50 µm, 25 µm, 15 µm and 10 µm
in side length, in order to evaluate the minimum size
achievable to minimize as much as possible the power
consumption. Moreover, two designs were evaluated
referred to as bulk (Fig. 2a) and closed membrane
(Fig. 2b). The bulk hotplates consisted in a heater
lying on the PI sheet. The electrodes deposited on top
of the device were electrically insulated from the
heater by a very thin PI layer. Closed membranes
relate devices where the polyimide sheet was locally
made thinner to thermally insulate the heated area to
reduce the losses.
1.00E+06
CH4 700 ppm
CH4 1250 ppm
10, 15, 25, 50, 100 µm
CH4 2500 ppm
Resistance [ohm]
(a)
10, 15, 25, 50, 100 µm
NO2 0.5 ppm
CO 50 ppm
1.00E+05
CO 50 ppm
CO 30 ppm
(b)
CO 50 ppm
1.00E+04
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
Cr electrodes
3.0
Spin-coated PI
2.0
Ti/Pt heater
1.0
UPILEX-50S
NO2 0.5 ppm +
CO 20 ppm
CO 10 ppm
Time [hour]
Fig. 2: Schematic cross-section view of microhotplates made of polyimide (a) bulk, (b) closed
membrane.
Fig. 3:Gas measurements on a MOX gas sensor on
PI, membrane design, 50 µm wide active area,
operating at about 300°C / 10 mW, synthetic air + 50%
Rh., gas flow of 200 ml/min.
Standard clean room equipment for the processing
of Si wafers was used for the whole fabrication, which
began with the deposition and patterning of the 150 nm
thick Ti/Pt heater by a lift-off technique on a 50 µm
thick polyimide sheet (Upilex 50S, UBE
Technologies). The resistance of the heater varied from
120 down to 25 Ω depending on the hotplate size. A
thin photosensitive polyimide layer used as insulator
was then spin-coated and contacts were opened to
allow the electrical contact to the heater. Thermal
curing of the insulating layer was performed at 375°C,
allowing operation up to 450°C [1]. Deposition and
patterning by lift-off of 150 nm thick Ti/Pt electrodes
completed the process to produce bulk hotplate. Closed
membranes were obtained by dry etching the polyimide
sheet from the backside and opened membrane with an
additional etching step from the top side.
A power consumption of 6 mW at 300°C was
reached with a 15 µm wide hotplate with a membrane,
making the PI-based devices as low power as those
fabricated on silicon. Finally, a very interesting
power consumption of 10 mW was achieved with a
15 µm wide heater patterned on a bulk polyimide sheet,
which is not conceivable with silicon due to its very
high thermal conductivity. By avoiding bulk
micromachining of PI, the device fabrication is
3.2 Capacitive gas and humidity sensors
Using the same technology, we have investigated the
integration of capacitive based gas sensors on the
polyimide foil to minimize the power consumption of
these devices to allow their integration on passive tags
(no battery on board, remote powering by RF). The
sensing array is based on differential sensor platforms
(Fig. 4), each containing a sensing capacitor, a
reference capacitor and a resistive thermometer [2].
They have been realized on polyimide foils by
depositing and patterning a Pt film and sensing
polymer layers using standard photolithography and
drop coating processes. Their gas sensing
characteristics, the total amount and the type of
analytes to be detected can be tuned by an appropriate
selection of the polymeric sensing layers.
Fig. 4. Differential capacitive sensor platform.
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Proceedings of PowerMEMS 2008+ microEMS 2008, Sendai, Japan, November 9-12, (2008)
The experimental output of a differential platform
consists of a “sensing layer + substrate response”, a
“substrate response” and a temperature value. From
the first two a “sensing layer only” signal can be
obtained. The detection limit and the resolution of the
“sensor & readout” assembly is high in spite of the
poor sensing layer sensitivity and substrate response to
humidity. Another interesting characteristic of the
proposed technology is that a temperature sensor can
be directly integrated on the platform. In reference [2],
detection of several VOCs in environments with
different levels of relative humidity and temperature
fluctuations was demonstrated.
The presented work highlights the potential of
capacitive gas sensor arrays realized on plastic
substrate to meet the practical application demands, in
terms of sensing performances and also of market
requirements (power, cost, weight, size). It also
demonstrated the possibility to use multivariate
analysis in the case of capacitive gas sensors. This
opens up the way to a new generation of ultra-low
power and low cost sensors due to the simplified
sensor architecture and processing. We are at the
moment working on replacing the patterning process
of the metallic film by an ink-jet printing process.
3.4 Multi-sensor platform
Finally, we used this polyimide technology
platform to integrate arrays of MOX and capacitive
gas sensors with a temperature sensor. Multi
environmental parameter recording is possible at very
low power consumption. Devices are now being
integrated to RFID tags with a wireless interface for
the remote powering of the tags and the transfer of
data following the concept illustrated below in Fig. 6.
passive transponder
µC
flexible
substrate
sensors
Fig. 7. Smart systems based on a system in foil (SiF)
integration concept: lamination of different layers
REFERENCES
[1] Briand D et al., 2008 Integration of MOX gas sensors
on polyimide hotplates, Sensors and Actuators B, 130,
430-435.
[2] Courbat J et al., Evaluation of pH indicator-based
colorimetric
films
for
ammonia
detection
using optical waveguides, International Meeting on
Chemical Sensors (IMCS 2008), Columbus, OH, USA,
13-16th July 2008.
[3] Oprea A et al. Integrated temperature, humidity and
gas sensors on flexible substrates for low-power
applications,
Proceedings
of
IEEE
Sensors
Conference, Atlanta, USA, October 28-31 2007, 158161.
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Transmission [%]
65
60
55
200 ppm Ethylene in 50%RH air
10500 ppm H in 50%RH air
2
40
2
30 ppm NO in 50%RH air
2
35
200 ppm CO in 50%RH air
100 ppm NH in 50%RH air
3
30
300
350
400
450
500
550
Wavelength [nm]
600
650
external
interface
A multi-sensor platform on flexible polyimide foil
has been developed for the environmental monitoring
of different gases, humidity and temperature. The
characteristics of these platforms are of high interest
for the realization of ultra-low power devices that
could be processed at low-cost using printing
processes. These platforms are at the moment
evaluated in food logistic scenarios with their
integration on passive and active RFID Tags. The
vision is to develop autonomous sensor nodes based
on a SiF concept with the integration of power sources
(battery, scavenging: solar, thermal, mechanical,
combined with supercapacitors), sensors, electronics
and communication. A simplified illustration of the
concept is given in Fig. 7.
75
50%RH air
30% O in 50%RH air
battery
4. AUTONOMOUS SENSOR NODES
80
45
comunication unit (RF-module) /
power source
Fig. 6. Sensors platform with a RFID read-out.
3.3 Colorimetric gas sensors
The use of colorimetric sensors has shown to be
very selective towards a specific gas, CO, CO2, NH3,
among others. The sensing principle of a colorimetric
sensor is based on the color change of a pH indicator
embedded in a polymeric matrix. An optical system
measures the color change of the dye, which depends
on the gas to be detected and on its concentration.
Sub-micrometric colorimetric films are spin
coated to form an optical waveguide and silicon LED
and photodiodes. Selective optical detection of low
concentrations of ammonia was achieved with a film
of PMMA incorporating a BPB (Bromophenol blue)
color dye using silicon LED (590 nm) and
photodetector (Fig. 5) [3]. The next steps are to
integrate these films by ink-jet printing on polymeric
waveguides (on PET) with printed organic LEDS and
photodetectors.
50
read-out unit
700
Fig. 5. Selective ammonia response of the spin-coated
colorimetric polymeric film.
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