SCREEN PRINTING TECHNIQUE FOR DUV PATTERNING
OF PMMA SUBSTRATES
by
Aminreza Ahari Kaleibar
BSc, Iran University of Science and Technology, 2000
THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
In the
School of Engineering Science
© Aminreza Ahari Kaleibar 2011
SIMON FRASER UNIVERSITY
Summer 2011
All rights reserved. However, in accordance with the Copyright Act of Canada, this work
may be reproduced, without authorization, under the conditions for Fair Dealing.
Therefore, limited reproduction of this work for the purposes of private study, research,
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if cited appropriately
APPROVAL
Name:
Aminreza Ahari Kaleibar
Degree:
Master of Applied Science
Title of Thesis:
Screen Printing Technique for DUV Patterning of
PMMA Substrates
Examining Committee:
Chair:
Mr. Mike Sjoerdsma
Lecturer, School of Engineering Science
______________________________________
Dr. Ash Parameswaran, P.Eng
Senior Supervisor
Professor, School of Engineering Science
______________________________________
Dr. Behraad Bahreyni, P.Eng
Supervisor
Assistant Professor, School of Engineering Science
______________________________________
Dr. Woo Soo Kim
Internal Examiner
Assistant Professor, School of Engineering Science
Date Defended/Approved:
2 / A u g u s t / 2 0 1 1 ______________________
ii
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Last revision: Spring 09
ABSTRACT
Plastics and Poly-Methyl-Methacrylate (PMMA) is becoming the choice of
microfluidic components in medical diagnostic systems these days. These
materials offer excellent flexibility in manufacturing processes and also make the
technology economical compared to traditional glass and silicon based devices.
Deep-UV exposure based PMMA microfluidic devices fabrication has been
reported in the past. However it utilizes a fairly expensive metal deposition step
and standard photoresist based lithographic process steps. While this
demonstrates a direction toward economical processing of PMMA microfluidic
parts, a more cost effective process needs to be developed to address ultra
economical process for PMMA parts fabrication.
In this context, this thesis proposes the utilization of the ubiquitous silk screen
printing process to pattern PMMA surface for producing microfluidic components.
The ink utilized in the silk screen printing process is inherently opaque to DUV.
Further screen printed and dried ink is readily dissolved in isopropyl alcohol (IPA)
which is the standard developing and etching solution for PMMA. These features
allow us to configure a process sequence to ultra economically pattern PMMA
substrates and produce microfluidic components. This thesis outlines this novel
technique and also analyses the fabricated components for bio-compatibility by
studying the fabricated surface using XPS.
Keywords: Microfluidic device, Silk Screen Printing, ESCA, XPS.
iii
DEDICATION
To my parents
iv
ACKNOWLEDGEMENTS
At first and foremost, I wish to thank my senior supervisor, Professor Ash
Parameswaran, for his continuing support, guidance and encouragement and for
giving me independence to follow my ideas wherever they led. I wish also to
extend my thanks to Professor Behraad Bahreyni and Professor Woo Soo Kim,
my supervisor and internal examiner, in the committee, respectively as well as
Mr. Mike Sjoerdsma for serving as the chair of defense session. I would like to
thank Mr. Michael Wong of the 4D-Labs at SFU for his assistance and helpful
discussion in carrying out ESCA measurements. At last but not least, I wish to
thank all my lab-mates who helped and encouraged me during my graduate
studies at SFU; especially in my first days of moving to Canada made
memorable moments for me. Lastly, I must extend very special thanks to my
parents who invested their life in my education and my future.
v
TABLE OF CONTENTS
Approval .......................................................................................................................... ii
Abstract .......................................................................................................................... iii
Dedication ...................................................................................................................... iv
Acknowledgements ......................................................................................................... v
Table of Contents ........................................................................................................... vi
List of Figures................................................................................................................ viii
List of Tables ................................................................................................................... x
Glossary ......................................................................................................................... xi
1
Introduction .............................................................................................................. 1
1.1
Technological Challenges.................................................................................. 2
1.1.1
Materials for microfluidic devices................................................................ 2
1.1.2
Current Fabrication Techniques for Polymer Microfluidic Device ................ 3
1.1.2.1 Replica Molding ...................................................................................... 4
1.1.2.2 µContact Printing .................................................................................... 6
1.2
2
Silk Screen Printing Technique for DUV Patterning ........................................... 7
Ultra-economical PMMA Microfluidic Fabrication Process ...................................... 10
2.1
UV-based Fabrication Process ........................................................................ 10
2.2
SILK SCREEN PRINTING AS a MASK FOR 254nm UV ................................. 11
2.2.1
Silk Screen Mask Preparation .................................................................. 12
vi
2.2.2
Ink Image Registration ............................................................................. 14
2.2.3
The Exposure System .............................................................................. 16
3
Patterns of Printed-Ink............................................................................................ 18
4
Surface Properties of Patterned PMMA .................................................................. 23
4.1
5
ESCA as a Tool for Surface Analysis ............................................................... 24
Experimental .......................................................................................................... 27
5.1
Microfluidic channel fabrication ....................................................................... 27
5.1.1
5.2
Specimen Preparation for ESCA scan ............................................................. 32
5.2.1
5.3
Microwave Bonding .................................................................................. 30
Results and Discussion ............................................................................ 33
Summary ........................................................................................................ 38
6
Conclusion ............................................................................................................. 39
7
Future Work ........................................................................................................... 40
Appendix A .................................................................................................................... 41
References.................................................................................................................... 42
vii
LIST OF FIGURES
Figure 1.1 1) Deposited metal on top of a casted PMMA over substrate 2)PMMA
exposed to X-ray via a metal mask 3)Developed PMMA 4)Electroplated metal on
channels 5) PMMA easily removed ................................................................................. 4
Figure 1.2 1) Patterned metal over a substrate 2) Pouring PDMS over master 3) Curing
and releasing PDMS ....................................................................................................... 5
Figure 1.3 1) Depositing Au on the substrate 2) Forming SAMs by micro-contact printing
3) Wet etching the Au ...................................................................................................... 7
Figure 2.1 1) PMMA as a substrate 2) Depositing Au-Cr bi-layer 3) Pattering on the metal
layer 4) Deep-UV exposure 5)Developing Exposed PMMA 6)Bonding PMMA to produce
Microfluidic Device ........................................................................................................ 11
Figure 2.2 Cadence layout design of the test microfluidic patterns ................................ 12
Figure 2.3 Two sample mylar masks produced by Fineline Imaging Inc. ....................... 13
Figure 2.4 Silkscreen stretched on a frame and ready for imaging step......................... 14
Figure 2.5 Ink printed PMMA substrate.......................................................................... 15
Figure 2.6 The in-house built deep-UV irradiation box: front view (left) and rear view
(right). ........................................................................................................................... 17
Figure 3.1 A wetted surface with interfacial tensions labeled ......................................... 18
Figure 3.2 Microscopic Image of ink pattern for 58 micrometer wide Channel ............... 20
Figure 3.3 Profilometer Scan for Minimum Printed Channel using Screen Printing
Technique...................................................................................................................... 21
Figure 3.4 Microscopic Image of ink pattern of 150 micrometer wide Channel .............. 22
Figure 4.1 Sampling in depth of substrate ..................................................................... 26
viii
Figure 5.1 Microfluidic device process steps. (1) PMMA substrate (2) printing of the
negative image of the micro-channel on PMMA by the silk screen printing technique, (3)
254mn UV exposure. (4) development of the exposed substrate using IPA-water mixture
(5) microfluidic device.................................................................................................... 28
Figure 5.2 Profilometer plot for 7-hour exposure sample ............................................... 29
Figure 5.3 Etch Depth [micrometer] versus Exposure Time [hour] ................................. 30
Figure 5.4 The samples are clamped together using small binder clips ......................... 31
Figure 5.5 Example of PMMA Microfluidic Devices ........................................................ 32
Figure 5.6 C (1s) spectrum of the developed PMMA after 10 hours exposure in
comparison to an unprocessed PMMA sample .............................................................. 33
Figure 5.7 C (1s) spectrum of the over-developed PMMA after 5 hours exposure in
comparison to unprocessed PMMA sample ................................................................... 34
Figure 5.8 C (1s) spectra of the over-developed PMMA after 5 hours exposure at 0°
,18°,and 78° take-off angles .......................................................................................... 36
Figure 5.9 C (1s) spectra of the over-developed PMMA after 10 hours exposure at 0°
,18°,38°,and 78° take-off angles .................................................................................... 37
ix
LIST OF TABLES
Table 1.1 SAM formation on Au ....................................................................................... 7
Table 1.2 Comparison between lithographic and soft-lithographic techniques ................. 8
Table 5.1 Binding Energy of Carbon Composition ......................................................... 34
Table 5.2 Summary of Experiments ............................................................................... 38
x
GLOSSARY
ESCA (Electron Spectroscopy for Chemical Analysis)
XPS (X-ray Photoemission Spectroscopy)
LIGA (Lithography, Electroplating and Molding)
SAM (Self-Assembled Monolayer)
PMMA (Poly Methyl Methacrylate)
PDMS (PolyDiMethylSiloxane)
xi
1 Introduction
Plastic microfluidic components are becoming popular for medical
diagnostic systems, particularly for the bio-marker-based diagnostics. The
discovery of molecules associated with different diseases, biomarkers, not only
opens new horizon to medical practices to diagnose timely and accurately, is
beneficial for individual and public health sector and so called translational
medicine, but also creates new market opportunities for bio-analytics
measurements[1]. For individual patients, this leads to detection of diseases in
very early stage, and consequently results in a timely treatment. In the standpoint
of public health, this promotes quick and effective awareness of infectious
diseases outbreak. Agilent technologies and Affymetrix announced $2.4 billion
dollars (USD) in revenue for bio-analytics measurements, included diagnostic
tools, forensic testing and research, in 2010[2]. In the standpoint of bio-analytics
measurement, microfluidic analysis systems possess great potential as
mainstream tools over conventional methods in biology, biochemistry, and
medicine. Their sub-nano liter volumes and small feature size reduces required
sample size and enables integrating multiple analyses into a single device,
reducing analysis time, especially. This pattern of increasing device densities and
reducing the per-unit cost, often called lab-on-chip (LOC), analogous to those
achieved in microelectronics industry is still a challenge [3], [4], [5]. Particularly,
efficient design methods both in terms of process and material are required to
realize this promise.
1
1.1 Technological Challenges
1.1.1 Materials for microfluidic devices
In terms of the materials utilized for microfluidic devices, silicon and glass
were first choice [6] [7], while polymers offer excellent mechanical and
biocompatible-surface properties for LOC applications. For polymers, both
PDMS(PolyDiMethylSiloxane) , a soft material with an excellent Young’s modulus
which is suitable specifically for fabricating valves [4] [5] [8] and pumps [9] as well
as fabrication of other fluidic device and sensors [10] [11], and PMMA (Poly
Methyl Methacrylate) [12] [13] [14] find vast applications in microfluidic
fabrication.
Soft materials, elastomeric polymers, hold key advantages over silicon for
fluidic device fabrication. First, microfluidic devices for bio-analytic applications
require delicate surface chemistry that is promising with less stringent condition
required to fabricate soft materials. Secondly, elastomers form tight bonding with
silicon and glass allowing fabricating hybrid devices with electronics integrated
with fluidic system in single devices. This can even lead to CMOS-compatible
fabrication processes in the future. Thirdly, the popular elastomer (PDMS) is 50
times cheaper than silicon [9].
PMMA has been widely utilized in biomedical implants, barriers,
membranes, microlithography, and fluidic devices. Indeed, the first polymeric
2
implanted biomedical device has been made from PMMA since 1950s [15].
Furthermore, it offers excellent mechanical stability, though the drawback of
PMMA is the lack of direct sealing properties and the limited resistance against
alcohol [16].
1.1.2 Current Fabrication Techniques for Polymer Microfluidic Device
Photolithography is ubiquitously utilized for fabricating polymer
microstructures, either for pattering or fabricating a mold to create the designed
pattern on the polymer [17]. Fabrication of polymeric microfluidic devices mostly
relies on master mold- assisted methods where a pre-fabricated mold serves as
a master for replication of devices.
A popular example of such a technique is called LIGA [18]. LIGA utilizes
deep X-ray photolithography, to produce master mold in combination with hot
embossing technique [12] [13] [14] to produce polymer devices. LIGA is capable
of generating high aspect ratios that made it quite popular in fluidic device
manufacturing. The steps required to make master mold for LIGA has been
shown in Figure 1.1. This process starts by casting a thick PMMA on top of the
substrate. Then PMMA is exposed to X-ray photons via a metal mask. The
exposed PMMA is developed in the next step to create the desired patterns on
PMMA. Using electroplating process the channels on the PMMA are filled by a
metal. In the end, PMMA can be removed easily and mold is ready to use.
3
1
2
3
PMMA
4
Au
Metal
5
Figure 1.1 1) Deposited metal on top of a casted PMMA over substrate 2)PMMA
exposed to X-ray via a metal mask 3)Developed PMMA 4)Electroplated metal
on channels 5) PMMA easily removed
1.1.2.1 Replica Molding
While, photolithography requires precise equipment and trained personnel,
another popular technique, so called replica molding[19] , also utilizes master
4
molds to produce fluidic devices [9][10][11]. In Figure 1.2, the steps for fabricating
by replica molding have been shown. In general, this process begins with
depositing and patterning either a metal layer or SU8 on silicon wafers. Then this
patterned structure used as a mold to pour PDMS over master. Finally, PDMS
cured and released of the mold to create the desired patterned device. The
curing of prepolymers is processed by either UV-cross linking or heating. PMMA
can also be molded in the same manner by replica molding but PDMS is more
popular. As well master mold can made of PDMS instead of a rigid material. The
fidelity of replica molding is depended on van der Waals interactions, wetting and
kinetic factors such as filing of the mold; therefore, feature sizes much smaller
than photolithography are achieved by this process.
1
2
Prepolymer
Substrate
3
Figure 1.2 1) Patterned metal over a substrate 2) Pouring PDMS
over master 3) Curing and releasing PDMS
5
SU8
1.1.2.2 µContact Printing
Another efficient method received much attention recently for patterning
polymers with large-area surfaces is micro-contact printing utilizing an
elastomeric stamp to form patterns of self-assembled monolayers (SAMs) on the
substrate. There are different configurations to realize this printing technique.
However, the fundamental principle is the same, and a very good example of
them is µcontact printing of alkanethiols on Au [17] [19]. In Figure 2.3 the steps
required for this µcontact printing has been shown. This process begins with
depositing a layer of Au onto the substrate. Then the PDMS stamp wetted with an
“ink”, a solution of hexadecanetheiol in ethanol, is brought into contact with the
gold surface. Upon contact to the surface, the ink chemically reacts and
generates patterns of SAMs on the gold surface. In the standpoint of generating
SAMs successfully, the contact time plays an important role and has to be
optimised. For SAMs on Au substrate, more details are presented in table 1.1.
Next, the unprinted areas are selectively wet etched. For patterning the polymeric
substrate the remainder of this process may be combined with other techniques
such as the UV-assisted method, described in detail in the next chapter.
6
1
2
Substrate
“Ink”
3
Figure 1.3 1) Depositing Au on the substrate 2) Forming SAMs by
micro-contact printing 3) Wet etching the Au
Au
Substrate
Ink
SAM
Optimum
Contact Time
Au
( ) ( ) 10-20 sec
Table 1.1 SAM formation on Au
1.2 Silk Screen Printing Technique for DUV Patterning
Soft-lithographic, replica molding and contact printing, and lithographic
techniques can be compared in table 1.2.
7
Soft-Lithography
Lithography
µ-contact
printing
Replica
molding
PDMS
stamp
PDMS
mold
Pattern Definition
Mask
Fabricated Structures
2-D structures
Both 2-D and 3-D
structures
Costs
Needs capital investment
In long-term leverages its
initial mold and stamp
expenses
Minimum feature size
In µ-scale range
Both µ and nano-scale
range
Table 1.2 Comparison between lithographic and soft-lithographic techniques
Though these techniques are promising, most of the microfluidic
diagnostic tools fabricated by these methods can be located in modern hospitals
and clinics only in developed countries. While in developing countries, only a
small fraction of people have access to such cutting-edge facilities and majority
of the diagnostic tools designed and deployed to developing countries are failed
[20]. Hence, additional major improvements both in formulating low-cost
fabrication process and detection mechanism are required. This thesis aims at
leveraging rapidness, cost-effectiveness and accuracy in fabrication process
development, and inspires its idea from a DUV-assisted technique introduced first
in [21]. This process eliminated the need for the master mold and utilizes silk
screen printing technique and UV-opaqueness of ink to further reduce the
manufacturing costs.
8
Arguably, silkscreen printing process is the most widely used image
transfer technique employed from textile industry to printed circuit board making
[22] and thick-film technology [18]. Silkscreen printing technology is fairly well
advanced and interestingly enough there are plenty of silkscreen imaging outlets
in most of the cities, even in developing countries. This demand is mainly due to
the custom T-Shirt and business sign-board manufacturing industry. Most of the
silkscreen imaging outlets accepts designs using the commonly used image
formats such as jpeg, tiff, pdf as well as wmf. Further, the highest resolution silk
screen can produce an image registration as fine as 20. The second most
interesting as well as important parameter that encouraged us to explore this line
of process for PMMA microfluidics is the ability of the printing ink to completely
mask-out the 254nm radiation. Even the thinnest, uniform layer of dried printing
ink is completely opaque to 254nm radiation and above all this dried ink can be
readily dissolved using IPA-water mixture. These two combinations offer a unique
advantage for the PMMA microfluidics manufacture. The next chapter describes
the silk screen printing technique for DUV pattering of PMMA in detail.
9
2 Ultra-economical PMMA Microfluidic Fabrication Process
2.1
UV-based Fabrication Process
A novel technique for the fabrication of PMMA microfluidic devices has
been first reported in [21]. The process is illustrated in figure 2.1. This technique
begins with depositing a bi-layer of Cr-Au on PMMA. Then, the micro-channel
design is patterned on Cr-Au by the process of photolithography and etching. The
patterned Cr-Au serves as a shadow mask for exposing PMMA. In the next step
of this process the long chains of the exposed PMMA are broken using deep-UV
(λ=254nm). The 254 nm radiation is the most economical deep-UV radiation
source available commercially in the market due to its wide spread utilization in
water disinfection and DNA cross-linking [23]. Exposed PMMA can be easily
dissolved using IPA-water mixture [24]; therefore, the designed micro-channel is
developed by a solution of 7:3 IPA: water mixture. While this technique is
relatively inexpensive, still the process requires metal deposition, metal etching
which require processing equipment that are not necessarily cheap.
10
1
2
3
DUV
4
PMMA
5
Au
6
Figure 2.1 1) PMMA as a substrate 2) Depositing Au-Cr bi-layer 3) Pattering on
the metal layer 4) Deep-UV exposure 5)Developing Exposed PMMA 6)Bonding
PMMA to produce Microfluidic Device
2.2
SILK SCREEN PRINTING AS a MASK FOR 254nm UV
This thesis describes an adaptation of PMMA microfluidic device
fabrication utilizing a more economical processing technique, silk screen printing
technique, and ink in lieu of gold [25] by the virtue of excellent UV-absorption of
ink, measured by our Cary 300 Bio spectrometer. Although another printing
technology, inkjet printing, is also available for ink printing; it results in additional
costs in microfluidic device fabrication. However, inkjet printing technology has
been considered in [26] for cost-effective IC design fabrication purposes.
11
2.2.1 Silk Screen Mask Preparation
To produce a microfluidic chip the design of the channel can be created
using any available CAD program such as Cadence, L-Edit or Autocad. For our
experiments, a set of test microfluidic channels has been designed using
Cadence. This is typically a single layer design and the layout pattern is shown in
Figure 2.2. This design was sent to Fineline Imaging [27] to obtain a mylar highcontrast image as shown in Figure 2.3. Typically this image will be a negative
image (dark field) of the channel design.
Figure 2.2 Cadence layout design of the test microfluidic patterns
12
Figure 2.3 Two sample mylar masks produced by Fineline Imaging Inc.
The mylar mask was delivered to a local silkscreen manufacturing outlet
called “Ink-Plus”[28]. Using a photo exposure process, Ink-Plus produces a
silkscreen. The silk screen is then stretched on a metal frame that is attached to
a raised hinge. This arrangement is shown in Figure 2.4. The hinge arrangement
allows us to place a cleaned PMMA sheet below the screen and the image can
be transferred by a pushing the printing ink using a squeegee.
13
Figure 2.4 Silkscreen stretched on a frame and ready for imaging step
2.2.2 Ink Image Registration
The silkscreen behaves like a sieve allowing the ink through the screen
where it is transparent and not allowing the ink to pass through the opaque
14
regions. The silk screen is placed at a certain distance, typically 1mm, from the
substrate and an inked-squeegee is swept over the silk screen. By this simple
process step, ink penetrates into very fine mesh of silk screen and the image is
transferred from the screen to the substrate in the form of the inked-pattern.
However, it should be emphasized that the roughly one-millimetre distance
between silk screen and substrate is essential to transfer patterns precisely;
otherwise, the ink will spread over the substrate and no good quality prints will be
produced. A PMMA sample after the printing process is shown in Figure 2.5.
Figure 2.5 Ink printed PMMA substrate
In general, two different types of ink are available for silk screen printing
process: ceramic inks, consists of both ceramic component and metallic
component, and metallo-organic inks, containing a metal and an organic
component dissolved in a large quantity of solvent. They differ in terms of binding
mechanism to the substrate in which the former’s is more mechanical than
chemical and the thickness of printed-ink after being printed and dried changes
slightly. The binding mechanism of metallo-organic ink mostly is chemical and the
15
thickness of wet printed-ink decreases substantially after being dried. Moreover,
for the latter, achieving a uniform bubble-free printed-ink layer requires stringent
consideration on the amount of ink applied on the substrate [18].
For the
experiments of this thesis, the ceramic inks are chosen, supplied by a local shop:
InkPlus, to exploit its binding mechanism for generating precise features.
2.2.3 The Exposure System
The PMMA exposure for this work was performed using a radiation source
equipped with low pressure mercury vapour ultraviolet lamps. Usually, these
bulbs are also called germicidal lamps because they are normally used for killing
pathogenic organisms on exposed surfaces and for producing ozone for water
disinfection. The strongest peak in the emission spectrum of these lamps is
located at 254 nm while their radiation is non-collimated.
The exposure system was built in-house and it served initially to
characterize the patterning of commercial grade PMMA using deep-UV
"
illumination [29]. Structurally, the system, which is made of 14 thick of aluminum
sheets, is cube-shaped, with a side of 21”. The twelve 25 Watt germicidal lamps,
representing the irradiation source are mounted on the ceiling of the box, as
shown in Figure 2.6.
16
Figure 2.6 The in-house built deep-UV irradiation box: front view (left)
and rear view (right).
The irradiation box is equipped with a safety switch, which shuts off the
power to the lamps when the door is open. A 8”x6”x3.5” thin aluminum box,
located on top of the exposure system, shelters the electronic control circuitry.
The lamps are fed by ballasts, mounted at the back of the box, and being driven
by an AC relay, which could be controlled either automatically or manually. The
exposure time can be pre-set using a commercial appliance power timer, which
can be overridden if a complete manual operation is desired.
17
3 Patterns of Printed-Ink
Printing on the surface is primarily determined by the wetting characteristic
of the surface, though other characteristic such as colloidal dynamics, phase
change, and chemical reaction may also be a factor [25]. In this thesis, however,
the binding mechanism of the ink is mechanical promising that the wetting is the
dominant factor in the printing. When the ink is applied to the surface of the
PMMA via a silk screen to print a line, the droplets of ink slowly fall through very
fine mesh of silk screen and shape itself on the substrate in a manner to
minimize its total interfacial energy. Given by Young-Dupre equation 3.1, it forms
a contact angle θeq, shown in Figure 3.1., with the surface of substrate, PMMA, at
its equilibrium state.
= − Equation 3.1
Where is ink’s surface tension, and and ! are PMMA-Ink and
PMMA-air interfacial tensions, respectively, and these parameters must be
empirically calculated for each substrate.
"#$
!
Figure3.2
3.1AAwetted
wettedsurface
surfacewith
withits
interfacial
tensions
labeled
Figure
interfacial
tensions
labeled
18
Given these parameters, from equation 3.1., a unique value is expected for
θeq that doesn’t conform to experiments and is a phenomenon known as contact
angle hysteresis. In reality θeq varies between two certain values, so called
receding contact angle("%#& ) and advancing contact angle ("'() ) , while the
width of printed-line remains constant. Recalling that silk screen printing is
accomplished by ink droplets fallen on PMMA via a silkscreen, equation 3.1 turns
out a geometrical understanding of the printed-lines in terms of the volume of ink
applied on the silkscreen.
From the standpoint of silk screen printing, since the negative image of the
designed channels is transferred to the substrate, the most important factor is the
stability of the printed-ink line on PMMA. They tend to decompose into droplets,
and uniform and continuous lines of ink are only achieved by stable ink patterns.
Moreover, this determines the minimum width of channels printable on the PMMA
using silk screen printing technique. Several works has been done to derive
boundary conditions of a stable patterns of the liquid on a surface and to develop
a stability model. Mostly acceptable is the work done by Davis [30], and
demonstrates that wet printed-lines with contact angles less than 90° are stable
as long as the contact lines are fixed; otherwise they are unstable and will
decompose into droplets. For the silk screen printing, the pattern of the minimum
printed-channel are taken by a microscope and presented in Figure 3.2. Using
Alpha-Step Profilometer, the width of this printed-channel is also scanned and
shown in Figure 3.3. This shows a width of printed-channel equal to 58µm which
is the minimum channel feature size printed on PMMA using silk screen printing
19
technique. Observing its microscopic image shows non-uniformities in the
printing on the boundary of the channel, which tends to form a better shape in a
wider channel width as per Figure 3.4. This latter has width of 150µm.
Figure 3.2 Microscopic Image of ink pattern for 58 micrometer wide Channel
20
Figure 3.3 Profilometer Scan for Minimum Printed Channel using Screen Printing
Technique
21
Figure 3.4 Microscopic Image of ink pattern of 150 micrometer wide Channel
22
4 Surface Properties of Patterned PMMA
Most of chemical and mechanical properties such as wettability and
biocompatibility of polymers depend chiefly on their molecular surface structure
such as coverage and orientation of surface functional groups [15]. While UV
patterning of PMMA offers the most cost-effective, fairly high precision and at the
same time an ultra-economical fabrication process reported to date, it demands
characterization of the developed PMMA surface properties. In the standpoint of
PDMS, its fabrication methods are performed at low temperatures under less
stringent condition; therefore, its surface properties are not likely altered by
fabrication process.
For DUV-patterning of PMMA, while the molecules of the exposed area
are broken down by UV irradiation, it is too complicated to study the bond
breaking-mechanism, and they most likely recompose again to constitute new
chemical elements. However, these molecules associated with broken chains are
removed by the designated solvent. Studying the chemical elements, at the
surface of the exposed PMMA after been removed and washed by the solvent, is
required in a sense that is essential for determining the fidelity of the designated
solvent whether or not any residue of the broken PMMA left inside the channels.
Most of the published literature to date on PMMA microfluidic fabrication
processes seldom addresses the surface property analysis of PMMA microfluidic
structure. This work took the study of semi-quantitatively analyzing the surface of
DUV-patterned PMMA channels.
23
In the next section, a surface analysis tool, Electron Spectroscopy for
Chemical Analysis (ESCA) aka X-ray Photoemission Spectroscopy (XPS), are
introduced. The objective of ESCA scan is to compare the data collected over
DUV-patterned surfaces to the unprocessed PMMA in order to investigate
whether its chemical composition has been altered by DUV patterning process or
not. ESCA offers several advantages over other surface analysis techniques that
made it popular for biomaterial surface analysis. Primarily, this technique is nondestructive compared to Secondary Ion Mass Spectroscopy (SIMS) and Auger
spectroscopy. Secondly, ESCA data can be easily interpreted. Further, ESCA
sampling depth of polymer surface is relevant to biomaterial surface analysis
(~10-100 Å), due to the mean free path of the emitted photoelectrons and the
angle of the sample with respect to the analyser, so called take-off angle.
Therefore, the data collected by ESCA complements other surface analysis
techniques such as contact angle method to have full picture of surface
properties [31], [33].
4.1 ESCA as a Tool for Surface Analysis
ESCA also referred to as XPS is a method to study geometric, electronic
and chemical properties of a sample. ESCA operates based on photoelectron
effect that the specimen is illuminated by soft-Xray, and the emanating
photoelectrons are energy analysed [31]. If the energy of photons ( ℎ+ ), soft-Xray
beams, are larger than the binding energy of the electron (,- ), the excess energy
24
is converted to the kinetic energy of the emitted photoelectron (,. ). In light of
measured intensity and energy distribution of photoemission the binding energies
of the electron can be calculated, given by the equation 4.1 in which / is the
work function of the spectrometer.
Equation 4.1
01 = 23 − 04 − 5
Since the binding energy is a characteristic of the elements in a certain
chemical environment, ESCA allows determining the atomic composition present
on the surface of the specimen except to hydrogen. Moreover, the chemical state
of a certain element, electronic structure and band structure can be determined.
Additionally the different adsorption sites of a molecule adsorbed on the surface
can be distinguished by the chemical shifts in the ESCA spectrum. An example of
such direct conclusion on local coordination has been studied in [32] for whether
6 molecules adsorbed perpendicularly on a 67 (100) surface.
While survey ESCA scan spanning from 0 to 1200eV in .5eV step size
reveals basic information of the surface chemistry of the material, high-resolution
scan can be performed to collect data on a particular element of interest. An
example of such high-resolution ESCA scan is C (1s) spanning from 275 eV to
290eV that differentiates all types of carbon composition such as carbonyl,
alkene and aliphatic on the surface. ESCA may be collected at different so called
take-off angles (θ), shown in figure 5.1. Given the equation 4.2, it results in the
detection of the chemical composition located near to surface of material.
25
Equation 4.2
9 = : ;<= @
X-ray
"
d
Figure 4.1 Sampling in depth of substrate
In which > is the depth of the illuminated material, ? is the mean free path
of the emitted photoelectron, and " is the take-off angle. ESCA can scan depth of
material up to 10nm, depended on the mean free path of the emitted
photoelectrons that is relevant to biomaterial surface analysis.
26
5 Experimental
5.1 Microfluidic channel fabrication
Figure 5.1 shows the process steps involved in producing the microfluidic
device. The silkscreen for the desired pattern is prepared on a frame. A clean
Plexiglass plate cut to appropriate size is used as the substrate. Using the
squeegee process the ink is applied on the substrate. The sample is then dried in
a clean environment for 10 minutes. Now this sample is transferred to the UV
exposure chamber.
The sample is then exposed for 7 hours and developed using 7:3 IPAwater mixture at 28°C. This development process instantly removes all the ink
from the substrate and starts to dissolve the exposed portion of the PMMA
substrate. Typical the development time is 30 minutes. The developed sample is
then cleaned using deionized water and dried.
27
1
2
DUV
3
PMMA
4
Ink
5
Figure 5.1 Microfluidic device process steps. (1) PMMA substrate (2) printing of
the negative image of the micro-channel on PMMA by the silk screen printing
technique, (3) 254mn UV exposure. (4) development of the exposed substrate
using IPA-water mixture (5) microfluidic device
In order to characterize the exposure time to the channel depth relation we
exposed different samples for 5, 7, 10, 12, and 15 hours followed by the
development step. The samples were then scanned through an Alpha-Step
profilometer to measure the etch depth. A profilometer scan plot for the 7 hour
exposure is shown in Figure5.2. The exposure time versus etch depth
relationship is graphed in Figure5.3.
28
Figure 5.2 Profilometer plot for 7-hour exposure sample
29
Figure 5.3 Etch Depth [micrometer] versus Exposure Time [hour]
5.1.1 Microwave Bonding
In the final step, the channels need sealing by bonding the PMMA sample
with groves on it to a blank piece of PMMA. Microwave-induced, thermal-assisted
solvent bonding method [34], are used. In this bonding method low cost
household devices are utilized instead of specialized costly equipment,
30
consistent with the goal of creating low cost microfluidic units. Figure 5.4 shows
the setting for the bonding.
Figure 5.4 The samples are clamped together using small binder clips
Two halves of PMMA microfluidic device is clamped together using small
size binder clips. A solvent that does not considerably affect the PMMA at room
temperature such as ethanol is applied to the edges of the interface. The
capillary action drives the solvent between the two surfaces. Any solvent that
leaks into the channels can be sucked out of the channels without effecting
channels preventing the clogging of the channels during the microwave heating.
Afterwards the sample is placed in the commercial microwave oven for about 1
minute. Metal of the clips absorbs the microwaves and heats up causing the
solvent to also heat up. At higher temperature the solvent dissolves the PMMA
and bonds the interface. Figure 5.5 shows an example of the bonded microfluidic
channels.
31
Figure 5.5 Example of PMMA Microfluidic Devices
5.2 Specimen Preparation for ESCA scan
A PMMA piece from OPTIX Plaskolite (Molecular Weight =55700 Da) cut
to 2 B 2Cis used as the substrate. A simple channel structure was patterned
using the technique as described in previous chapters. Three sets of samples
were prepared using 5, 10 and 15 hours of UV exposure followed by a 30 minute
development and deionized water cleaning and drying. The samples were then
scanned using an Alpha-Step profilometer to measure the channel depth.
ESCA measurements were carried out with Kratos Analytical Axis ULTRA
spectrometer using a monochromatic Al Kα X-ray excitation source with a DLD
detector. The beam size, resolution and dwell time were set 700 × 200 , 0.1
eV/step and 1000 ms/step, respectively. In order to minimize the positive charge
build up on the surface of specimen, due to electrons removed from surface, the
charge neutralizer is set to operate at filament current, filament bias and charge
balance equal to 1.91[A], 1[V] ,and 3.2[V] respectively. The double-sided sticky
tapes are utilized to attach aluminum foils onto surface to further reduce charging
effects. Furthermore, for each sample the signal-to-noise ratio are optimized.
With these settings, minimal charging effects were observed during our ESCA
scans.
32
5.2.1 Results and Discussion
The specimen exposed to 10 hour Deep-UV (λ=254nm) was developed
using 7:3 IPA-water mixture at 28°C. Scanning by Alpha-Step profilometer
presents 65µm etching from the surface of PMMA. Surface scanning using ESCA
for this area was performed in the C (1s) spectrum from 276eV to 290 eV. This
scan will reveal the chemical composition of surface by determining bonding
energy of carbon with carbon and oxygen. The C (1s) spectrum is shown in
Figure 5.6., and its peak is commensurate with bonding energy of carbon with
carbon.
Figure 5.6 C (1s) spectrum of the developed PMMA after 10 hours exposure in
comparison to an unprocessed PMMA sample
33
Theoretically the number of detected electrons in bonding with carbon for
PMMA is three times larger than oxygen that the former located at 285eV and the
latter at 286.7 and 289eV [35], referring to table 5.1. As seen in Figure 5.6., a
discrepancy is also observed in correspondent bonding energy. This could
perhaps be due to the charging effect that shifted C (1s) spectrum to right.
However, the proportion of detected electrons in our experiment conforms to
theory.
Binding Energy
−
−D
D− =D
285.0 eV
286.7 eV
289.0 eV
Table 5.1 Binding Energy of Carbon Composition [35]
Figure 5.7 C (1s) spectrum of the over-developed PMMA after 5 hours exposure in
comparison to unprocessed PMMA sample
34
The C (1s) spectrum for 5 hour exposure time specimen demonstrates an
interesting observation. For this sample, instead of typical development time, the
specimen was developed for 1 hour. While no change was observed in the
measured etching depth in comparison to the typical developing time, the
spectra, as shown in Figure 5.7., exhibits another peak at bonding energy of
carbon and oxygen which is higher than the binding with carbon. The same
spectrum was observed on polymer surfaces which were studied in the water
contact experiment [33]. This phenomenon accounts for carboxylic groups
appearing at polymer surface. The ESCA scans were performed at different
take-off angles on the PMMA surface of the etched grooves. The spectra of 18°
and 78° take-off angles are shown in Figure 5.8.
35
Figure 5.8 C (1s) spectra of the over-developed PMMA after 5 hours exposure at 0°
,18°,and 78° take-off angles
The spectrum in 78° is commensurate with PMMA spectrum, and the
sampling depth of this angle is approximately 100 Å. However, the spectrum of
18° shows more oxygen content. In order to reconfirm the scan observations,
ESCA scans on the surface of 10-hour exposure PMMA was performed at
different angles and this result is shown in Figure 5.9.
Observing the same energy distribution for scans at 18°, 38°, and 78°
take-off angles shown in Figure 5.9., and comparing the scans presented in
Figures 5.7 and 5.8, it can be concluded that the oxygen content on the surface
36
is simply due the carboxylic groups’ composition on the surface. It reveals the
sensitivity of the surface chemistry of the patterned PMMA to the developing
time, and turns out to further characterizing of our previously offered developer in
[36].
Figure 5.9 C (1s) spectra of the over-developed PMMA after 10 hours exposure at 0°
,18°,38°,and 78° take-off angles
37
5.3 Summary
The experimental results can be summarized in table 5.2.
Maximum developed depth@15 hours
exposure
70µm
Typical Development Time
30 minutes, beyond this no depth
development was observed. However,
prolonged development time results in
oxygen bonding to the surface
Minimum Width Printed Reproducibly
50 µm
Maximum Exposure [Hours]
20 hours
Table 5.2 Summary of Experiments
38
6 Conclusion
PMMA offers excellent mechanical and bio-compatible surface properties
that are suitable for microfluidic components for medical and bio-analysis
applications.
An
ultra-economical
method
of
manufacturing
microfluidic
components on PMMA substrates has been presented using Silk Screen Printing
process. This novel technique allows producing microfluidic components without
the need for any metal sputtering process for masking. All the processing
chemicals are non-toxic and relatively environment friendly. In conjunction with
the microwave assisted bonding process fully functional microfluidic components
can be produced easily and reproducibly. An example of such prototyped device
is fabrication of an electrophoretic pinch injection device published in [37].
Microfluidic channels with dimensions as small as 50 micrometers has been
successfully fabricated.
Furthermore, the surface properties of PMMA that has undergone the
DUV-exposure based microfluidic component manufacturing process has been
semi-quantitatively studied using ESCA. The scan data were collected over C
(1s) spectrum which carbon has chemically bonded to oxygen and carbon. The
experimental data shows that the UV exposure based PMMA processing may
produce carboxyl groups at the surface provided that the sample is overdeveloped in the solution of 7:3 IPA: water. Therefore, the development time in
UV-exposure based PMMA microfluidic components is an important parameter.
This observation will be useful for the applications of UV exposure based PMMA
microfluidic components to bio-chemical analysis and systems.
39
7 Future Work
Future work of this thesis may direct to study whether the change of
surface composition during channel development will prejudice the performance
of a practical microfluidic analysis system. However, silk screen printing process
has already been applied to fabricate an electrophoretic pinch injection device in
our lab [37].
The entire process of printing and UV exposure can be optimized. For silk
screen printing, the space between screen mesh and substrate can be vacuumed
to generate finer patterns resulting in smaller feature size. In standpoint of UV
exposure, a vacuumed chamber results in more directional and energy-efficient
exposure system that generates planner channels after development.
Another direction in a foreseeable future, in a broader and generalized
perspective, is to formulate CMOS-compatible microfluidic fabrication processes
enabling integrating ICs with microfluidic device in a single chip [38]. This can be
performed by hybrid PMMA and PDMS devices.
40
Appendix A
Bath Preparation for Developing Exposed PMMA (the recommended
substrate size for this bath is3.5" B 3.5" )
1) Mix 280H of Isopropyl Alcohol (IPA) with 120 Hdeionised water
2) Set the temperature of heater to 28°C
3) Set the mixer to the speed of 300 rpm
4) Wait for 2hour to have the solvent reach to 28°C ( it is a typical time for
equilibrium)
Developing PMMA Samples after UV-Exposure
1) Wash the surface of samples using Isopropyl Alcohol (IPA) to remove ink
2) Put the sample into the bath such that the exposed surface is facing
bottom of container
3) Every 10 minutes it is suggested to bring the sample out of bath ,and wash
it by IPA and deionised water ,then dry and return it back to bath
4) After 30 to 40 minutes it is expected channels are pattered onto the PMMA
41
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