iii
UNIVERSITY OF WATERLOO
Faculty of engineering
Nanotechnology Engineering
Design and Fabrication of Electrospray Sources for Electric
Propulsion
Prof. Mario Lanza
Dr. Enric Grsutan
Institute of Functional Nano and Soft Materials, Soochow University,
199 Renai Road, Suzhou Industrial Park,
Suzhou, Jiangsu Province, China
Prepared by
Chuqi (Steven) Wei
ID number: 20518399
Userid: c27wei
Previous academic term: 2B
Completion date: Sept. 15, 2016
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Suite 218, 50 Clegg Road,
Markham, Ontario, L1C 0C6
Sept.15, 2016
Dr. Shirley Tang, director
Nanotechnology Engineering
University of Waterloo
Waterloo, Ontario
N2L 3G1
Dear Madam,
The following report, titled “The design and fabrication of electrospray source for electric
propulsion”, was prepared as my 2B Work Term Report for Institute of Functional Nano and Soft
Materials (FUNSOM), at Soochow University. This report is submitted specifically for WKPRT
300 course, in fulfillment of the WatPD-Engineering program as required by my BASc
Nanotechnology Engineering degree. The purpose of this report is to present in details the design
and fabrication of a new type of electrospray source as well as the apparatus for performance testing
in the newly mounted laboratory. Some preliminary characterizations are also included in the report.
Soochow University is a public institution located in the city of Suzhou, Jiangsu Province, China.
The Institute of Functional Nano and Soft Materials (FUNSOM) is a leading institute for
nanotechnology research in China. I was employed by Professor Mario Lanza to work as a Research
Assistant on a new project with the focus of electrospray. This is a new area of research for
Professor Lanza whose main focus has been Resistive Random Access Memory devices and
Atomic Force Microscopy. The job not only requires me to do research work such as design and
fabrication, but I was also in charge of mounting a new laboratory.
I would like to thank Prof. Mario Lanza and Dr. Enric Grustan for providing me valuable advice
and resources, including critical trainings for numerous equipment, academic literatures,
fabrication experience and guidance for the design. I would also like to thank FUNSOM and
Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) of Chinese Academy of Science for
providing the facilities and equipment for me to do my job. They also proofread this report.
I also wish to thank all members in Prof. Lanza’s group, Bingru Wang, Yuanyuan Shi, Tingting
Han, Fei Hui, Lanlan Jiang, Na Xiao, Yanfeng Ji, Xiaoxue Song, Xu Jing and Chengbing Pan for
helping me with issues such as navigating through bureaucracy and paperwork, and most
importantly, the constant support to make me feel welcomed. I hereby confirm that I have received
no further help in writing this report, other than what is mentioned above. I also confirm that this
report has not been previously submitted for academic credit at this, or any other academic
institution.
Sincerely,
Chuqi (Steven) Wei 20518399
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Contributions
The team I worked with consisted only two people. During my cooperative work term, I was mainly
working exclusively with my supervisor, Dr. Enric Grustan who has a PhD degree in Mechanical
and Aerospace Engineering. Our work is under the oversight of Prof. Mario Lanza from FUNSOM
who grants me access to the cleanroom along with its fabrication and characterization equipment.
The overall goal of this project is to mount a new laboratory based on the model of Dr. Grustan’s
previous lab in Irvine, California, and use this new facility to explore more novel applications for
electrospray. Consequently, my work contains two major aspects, mounting the lab and working
on the electrospray source including design and fabrication. It’s noteworthy that while the lab was
being mounted, our research didn’t remain stand still, instead, processes were made on both fronts
at the same time.
For mounting the lab, my tasks include allocating funds, searching for suppliers for equipment,
devices and materials and acting as liaison between Dr. Grustan and Soochow University. Some of
the critical procured equipment, devices and materials consisted of a custom designed vacuum
chamber for prototype performance testing; mechanical and turbo-molecular pumps for
establishing vacuum environment; the electronic apparatus for performance testing which includes
a pulse generator, a pico-ammeter, an oscilloscope, two high voltage sources; an industrial camera
set to observe and document the working status of the prototype; ionic liquid 1-ethyl-3-
methylimidazolium bis(triflouromethylsulfonyl)amide (EMI-Im) to fuel the prototype; two types
of micro silica thread to act as feeding thread between the ionic liquid reservoir and the vacuum
chamber; hard and soft masks for photolithography; silicon wafers with different doping and
polishing requirements; and other laboratory supplies. These tasks are crucial for mounting the lab
successfully and achieving the overarching goal set by Prof. Lanza.
On the other hand, I was also in charge of design and fabrication of the electrospray source
prototype under the supervision of Dr. Grustan. There are two types of model, the single emitter
set for physical sputtering and the multi-emitter array for electric propulsion. The single emitter set
consists of two metal extractor layers, two plastic layers and a plastic case encompassing the whole
structure. There are holes extending from the outside to different layers for the purposes such as
fixation and acting as contact electrodes. I was in charge of designing this prototype using
Solidworks software and the final version was approved for prototype manufacture. The multi-
emitter array pattern was designed by Dr. Grustan and the fabrication and characterization part was
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carried out by me. Each mask for photolithography consists of seven arrays of emitters, while each
array consisting of sixty-four emitters. Using micro-electro-mechanical system (MEMS)
fabrication techniques, including photolithography, and deep reactive ion etching, emitter arrays
were fabricated and characterized. It’s worth mentioning that, since the multi-emitter array is
fabricated using MEMS techniques, many of the previous obstacles such as oversize and low power
to thrust efficiency have been overcome. Theoretically, the prototype should be able to generate a
thrust that is sufficient for micro-spacecraft.
The purpose of this report is to summarize the design process, document details and parameters for
singe emitter set, elaborate on the fabrication steps and characterization results for multi-emitter
array and present the apparatus for performance testing. The content of this report takes into account
of both aspects of my work and it’s a fair reflection of my experience in the institute.
Looking at the broader scheme of things, first and foremost, the successfully mounted lab is one of
its kind in China, so it puts our research group in a very advantageous position to further our
research in the field of electrospray and its potential applications. Secondly, electric propulsion has
immediate applications. Electric propulsion provides an alternative solution for thrust used for a
new generation of micro-spacecraft, given the competitiveness of 21st century space race, micro-
spacecraft with its high cost-performance ratio will be a major player in the field of civil and
military satellite competition.
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Summary
The purpose of this report is to present the outcomes of the 8-month cooperative work term in
Institute of Functional Nano and Soft Materials of Soochow University, China. The goals of the
project are to mount a new laboratory for electrospray experimentation and to design and fabricate
electrospray source for physical sputtering and electric propulsion.
The main points covered in this report are the introduction to the field of electrospray thus
explaining the motivation for this project, then the background knowledge about electrospray and
electric propulsion, the design process and fabrication steps of single emitter set and multi-emitter
array with a detailed analysis for fabrication characterization, and finally presentation of the
performance testing apparatus setup including different working modes with general conclusions
and recommendations.
A major conclusion in this report is that during the deep reactive ion etching (DRIE) during the
multi-emitter array fabrication, adding oxygen to the etching gas can significantly reduce the
formation of silica grass at the base of the emitter well and the emitter rod. Different amount of
oxygen addition have been tested including constant 45 sccm, constant 35 sccm, constant 25 sccm
and ramping addition from 0 to 30 sccm. Detailed results will be discussed in the report body.
Another major conclusion is that after intensive design process, the single emitter set can now meet
the electronic and mechanical mounting requirements for fabrication and performance testing.
The major recommendations in this report are to further complete and calibrate the performance
testing apparatus since by the time of the submission of this report, the mounting process is not yet
100% complete. After the completion of the mounting process, it is recommended to perform a test
run for single emitter set prototype since it has a lower degree of complexity and easier to make
modification based on the test results. For the fabrication of the multi-emitter array, it is
recommended that to try a different DRIE etching recipe with a slower rate but less chance of over-
etching the emitter rods. It would also be beneficial to investigate further into the relationship
between the mercury lamp power of photolithography machine and its effect on the etching process
since the prototype fabrication involves multiple photolithography and DRIE steps
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Table of Content Summary .......................................................................................................................................... v
List of Figures ................................................................................................................................ vii
List of Tables ................................................................................................................................ viii
1.0 Introduction .......................................................................................................................... 1
2.0 Background ................................................................................................................................ 1
2.1 Electrospray ionization for electric propulsion .................................................................... 2
3.0 Prototype design and fabrication.......................................................................................... 4
3.1 The Design and Fabrication of the Single Emitter Set ........................................................... 4
3.2 Multi-Emitter Array Fabrication and Characterization .......................................................... 7
3.2.1 Fabrication Process ......................................................................................................... 8
3.2.2 Photolithography ............................................................................................................. 9
3.2.3 Deep Reactive Ion Etching (DRIE) .............................................................................. 10
4.0 Performance Testing Apparatuses ..................................................................................... 14
4.1 Vacuum apparatus ................................................................................................................ 14
4.2 Electronic apparatus ............................................................................................................. 18
5.0 Conclusions .............................................................................................................................. 18
6.0 Recommendations .................................................................................................................... 20
Glossary ......................................................................................................................................... 21
Reference ....................................................................................................................................... 22
Appendix A: RCA-1 clean process ................................................................................................ 23
Appendix B: Parameters of all DRIE trials .................................................................................... 25
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List of Figures page
Figure 1: Demonstration of electrospray ionization. ....................................................................... 1
Figure 2: A structural breakdown of multi-emitter array prototype. ............................................... 3
Figure 3: The experimental setup for testing single emitter set. ...................................................... 5
Figure 4: The isometric view and front plane section view of the prototype................................... 6
Figure 5: Illustration of metal contacts in the inner plastic layer and inner metal layer. ................. 6
Figure 6: The base structure of the prototype and the feeding thread connector . ........................... 7
Figure 7: Breakdown for emitter and channel fabrication process. ................................................. 8
Figure 8: Images of developed emitter array wafer and channel wafer. ........................................ 10
Figure 9: SEM characterization of emitter well for trial No. 1. ..................................................... 11
Figure 10: SEM characterization of emitter array and single emitter well for trial No. 2. ............ 12
Figure 11: Ideal shape and geometry of emitter well and emitter rod. .......................................... 13
Figure 12: Comparison between different mode of O2 injection. ................................................. 13
Figure 13: Air circuit of the vacuum system .................................................................................. 15
Figure 14: Schematics of pumping mode. ..................................................................................... 15
Figure 15: The mounting of the prototype for performance testing at Dr. Grustan’s lab at UCI. . 16
Figure 16: Schematics of experimentation mode. .......................................................................... 17
Figure 17: Schematics of standby mode. ....................................................................................... 17
Figure 18: Electronic Setup of performance testing. ..................................................................... 18
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List of Tables page
Table 1: Comparison of photolithography parameters at FUNSOM and SINANO facilities ......... 9
Table 2: DRIE parameters for trial No. 1....................................................................................... 11
Table 3: DRIE parameters for trial No. 2....................................................................................... 12
Table B-1: Trial No.1: only etching gas and passivation gas ........................................................ 25
Table B-2: Trial No.2: adding O2 gas (10% of SF6 flow) in the etching phase ............................ 25
Table B-3: Trial No.3: reduce O2 flow to 25 sccm ........................................................................ 25
Table B-4: Trial No. 4: increase passivation time and O2 flow ..................................................... 25
Table B-5: Trial No. 5: reset passivation time and reduce O2 flow .............................................. 25
Table B-6: Trial No.6: reduce O2 flow .......................................................................................... 25
Table B-7: Trial No.7: Increase O2 flow in a ramping fashion ..................................................... 26
1
1.0 Introduction
Electrospray, also known as electrospray ionization (ESI), is a technique that ionizes or atomizes
particles or molecules from its original form to charged ions for various purposes. The most
commonly used method to achieve such effect is to apply high voltage to transform the source
liquid to a gaseous phase. Nano-sized neutral-charged droplets will form at the tip of the
electrospray emitter and due to the strong electric field generated by the high voltage sources as
shown in Fig. 1, the droplets will reach the ejection threshold and be ejected with accordance of
electric field [1][2][3]. The concept of ESI was first proposed by Lord Rayleigh in 1882 and after
a century of development, in the late 1980s, mass spectrometry with ESI was achieved by J. B.
Fenn, who was awarded the Nobel Prize for Chemistry in 2002 [4].
2.0 Background Electrospray ionization’s prime application is mass spectrometry, which can be used to analyze the
mass-to-charge ratio of ions within the sample. Signals from the analyzer generate a spectrum
which provides useful information regarding the composition of the sample, making mass
spectrometry an extremely useful characterization tool for biochemistry, gas analysis, space
technology and many other fields of research [5]. Although mass spectrometry remains the most
prominent application of ESI, recent explorations to expand the use of electrospray ionization have
yielded promising results in numerous fields.
Figure 1: Demonstration of electrospray ionization [1].
2
Physical sputtering is a widely used deposition, etching technique in semiconductor industry,
micro-electronics industry, 3D printing industry and so on. Some traditional methods of performing
physical sputtering such as reactive ion etching (RIE) and deep reactive ion etching (DRIE) are
effective against substrate that is susceptible of chemical attack, such as silicon. However, if the
substrate is chemically inert, those aforementioned techniques will not be as effective as ion
bombardment techniques such as ion beam milling or cluster ion beam. ESI will produce highly
energized nano-droplets in the process, and bombarding inert material’s surface with those droplets
will greatly increase the sputtering rate and yield [6]. Moreover, the size and energy the projectile
can be tuned by applying different magnitude of electric field and source fluid, so that the surface
properties of the target can be tuned mechanically. In addition to simply etching the surface,
bombarding the surface with nano-droplets can also alter the roughness of the target substrate or
achieve amorphization of a thin layer. ESI bombardment can also be a potential substitute of
focused ion beam (FIB) to create pillars, nanorods, and nanowires [7].
One of ESI’s more novel applications is electric propulsion for micro spacecraft which will be the
focus of this report. The idea of this application can be traced back to 1960s, Thanks to the
development of microfabrication techniques, using electrospray ionization becomes a feasible
solution to space propulsion problem.
2.1 Electrospray ionization for electric propulsion
As mentioned above, ESI has numerous applications, one of the more novel and promising paths
is electric propulsion, as known as colloid thruster, for micro spacecraft. Entering 21st century, the
need for smarter and more cost-effective spacecraft has become an urgent concern for those
countries that wish to share the enormous benefit from the exploration of space. So to develop a
cheaper, more controllable and more reliable source of propulsion is becoming a popular research
area. Not only for micro spacecraft, there are some missions that require the position of the
spacecraft reaches nanometer precision, such as the joint Laser Interferometer Space Antenna
(LISA) project by European Space Agency (ESA) and NASA. To achieve such effect, the required
thrust ranges from micro to milinewton with a resolution no greater than 0.1 micro newton [5].
Contestants for propulsion source include laser propulsion, electromagnetic drive, ESI and other
propulsion system.
The idea of using electrospray ionization to power spacecraft was proposed in the 1960s and its
feasibility was also discussed by NASA. The problem back then was the magnitude of the thrust
generated was insufficient to power spacecraft since only a handful of emitters can be fabricated
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on the limited surface area. In addition, the US government cut the budget for aerospace research
during that time period, so the research for ESI propulsion entered a halt. The development of
micro-electro-mechanical system (MEMS) techniques helps to bring ESI propulsion back to the
table for it is now possible to fabricate much more emitters on a limited surface than it could in the
1960s. In theory, if a single emitter can generate a thrust in the order of nano-newton, an array of
100 emitters could reach the order of micro-newton, and the 100-emitter array can now be
fabricated using MEMS techniques [8][9].
Some advantages of using ESI propulsion over other contestants are its extremely controllable flow
rate and its high power-thrust efficiency. For a satellite mission, in most of the cases, the power is
provided via solar panels, and how to utilize the limited power to generate the maximum thrust is
a mission-critical problem. In a colloid thruster, the amount of thrust is controlled by the magnitude
of the electric field across the extractors, and the magnitude of the electric field is controlled by the
voltage input which relies on the solar panel. A high power-thrust efficiency will not only enhance
the thrust generated, but can also extend the lifetime of the solar panels, making the mission more
cost-effective. Dr. Grustan’s lab at the University of California at Irvine (UCI) produced the
prototype with the maximum thrusting efficiency of 70%, which is the highest among all the other
colloid thruster prototypes previously fabricated [1].
To facilitate the reading of this report, a brief breakdown of the multi-emitter prototype structure
will be provided. There are three main parts of the prototype, an extractor, an emitter and a feeding
system, as shown in Fig. 2. The extractor(s) will be connected to a voltage source to generate an
electric field, thus “extracting” the ions from the emitter. The emitter array and channels underneath
it (integrate into the emitter die) will act as exits and conduits respectively. A silica feeding thread
will connect the reservoir filled with ionic fluid to the aforementioned structures to provide fuel to
the system.
Figure 2: A structural breakdown of multi-emitter array prototype [1].
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The work that is undertaken by Institute of Functional Nano and Soft Materials (FUNSOM) at
Soochow University is to reproduce some the results from the UCI experiments and modify the
design to improve the performance.
3.0 Prototype design and fabrication
There are two types of ESI source prototypes in which the team was involved of designing and
fabricating. The first one is the multi-emitter array and the other one is a single emitter set. The
design for the multi-emitter array was completed by Dr. Grustan when he was still at UCI, though
some minor modifications were required due to the difference in the fabrication equipment. Due to
time constraint, a fully functional multi-emitter array prototype was not yet finished, however, the
team was able to fabricate the emitter array and the channel to conduit the ionic fluid using
microfabrication techniques such as photolithography and DRIE, and was able to characterize the
quality of the fabrication via scanning electron microscopy. For the single emitter set, Dr. Grustan
made some major modifications based on his experience working with the previous prototype at
UCI. Building upon the rough sketch he provided, a new ESI source with single emitter was
designed using the software Solidworks, some new features such as an extra extractor and adjusted
dimensions, appear in the new design. This new single emitter prototype is currently being
fabricated by a 3D-printing company, which is also a development from the previous one, which
was fabricated via conventional means.
3.1 The Design and Fabrication of the Single Emitter Set
In order to understand how multi-emitter array works, it’s logical to understand the mechanism of
the single emitter set first. Similar to the aforementioned multi-emitter prototype breakdown, the
single emitter set also consists of three major parts, the extractor, the emitter and the feeding system.
However, there are also some features that are unique to the single emitter set, such as the
alternating metal/plastic layers encompassing the emitter rod, screws acting as metal contacts to
create a potential difference between different layers and a customized flange to fit the testing
chamber.
The mechanism for the single emitter set is quite straight forward. As shown in Fig. 3, the reservoir
contains ionic liquid and during experimentation, carefully adjust the pressurized system denoted
as P in the schematic, the pressure difference will draw the liquid to the emitter and the strong
electric field between the extractor and the emitter tip will atomize the ionic liquid thus forming
5
nano-scale droplets. First, apply voltage to the extractor creating a potential difference between the
first extractor and the emitter rod, this difference will generate an electric field. Then ionic fluid
which is fueled with silica thread via pressure difference will migrate toward the tip of the rod,
when a certain threshold is reached, those liquid particles will be ionized and ejected toward the
extractor,
Figure 3: The experimental setup for testing single emitter set [1].
However, to facilitate the formation of droplets instead of pure ions, a second extractor is added on
top of the first extractor acting as an accelerator. Because the single emitter set is mainly used as
an ESI source for physical sputtering, so it is necessary to use a lower voltage to form electrospray
consisting mostly of energized nano-droplets instead of ion beam, then to further accelerate it to
the target.
There are five layers of alternating metal/plastic layers encompassing the emitter rod. The metal is
aluminum and the plastic is a special material called polyether ether ketone (PEEK). The PEEK
material has a low leakage coefficient making it suitable for vacuum experiments. Three screws act
as contact electrodes which extend to various layers in the prototype body. The metal layers are
made of aluminum, which has great conductivity. The screws will connect the metal layer and the
voltage sources so when turned on, strong electric field can be generated in between layers,
specifically between the extractor layer and the innermost PEEK layer, and between the extractor
layer and acceleration layer (the outer metal layer). Fig. 4 (a) is the isometric view of the single
emitter set prototype in Solidworks. Metal part is painted orange and the PEEK part is painted
white. In the body of the outer PEEK shell, three holes can be observed, those are for the screws
extending to different layers inside the emitter body. On top of the emitter set is a metal “coin”
which has an opening in the middle, this is to block any excess spray and control the diameter of
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the output beam Fig. 4 (b) is the front plane section view of the emitter body, the layers have been
better illustrated and the tube in the center represents the tip of where the nano-droplets are created.
The inner metal layer is the extractor layer which generates electrospray droplets and the outer
metal layer which is the acceleration layer will energize (accelerate) those droplets so they carry
enough energy to bombard the surface of the target.
(a) Isometric view (b) Front plane section view
Figure 4: (a) The isometric view and (b) front plane section view of the prototype.
Fig 5 shows two of the three screw holes, the screw in 5 (a) will extend to the innermost layer of
the prototype which is a PEEK layer, and then connect to the ground; the screw in 5 (b) will extend
to the inner metal layer acting as a metal contact, and then connect to a high voltage source and
create a strong electric field between the extractor and tip, as shown in Fig. 4 (b). The height of this
prototype is 8.3 cm. This will ensure the emitter set can fit into the testing apparatus nicely.
The screw bodies have a dimension of metric 2mm. Since the screws are contacts, so it’s important
that they don't touch other layers, especially the other metal layers. So the holes are drilled with a
slightly larger diameter, 3mm for the PEEK layers and 4mm for the metal layers.
(a) Screw hole No. 1 (b) Screw hole No. 2
Figure 5: Illustration of metal contacts in the inner plastic layer and inner metal layer.
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The base of the emitter set will be fixed on top of a PEEK flange by two metric 3mm screws as
shown in Fig. 6 (a). The flange will seal the testing chamber opening. A connector which connects
the feeding thread to the flange and upper portion of the emitter is shown in Fig. 6 (b). An ultra-
thin silica thread will act as a feeding thread and when extending into the connector the feeding
thread will go through the center of the flange and into the main body to fuel the emitter tip. The
internal structure of the connector is designed such that it will enclose any space between the thread
and side walls and essentially become a seal for the flange with the thread as the only exit. This
structure will later be elaborated in section 4.1.
(a) The PEEK flange base (b) schematics of the feeding connector
Figure 6: (a) The base structure of the prototype and (b) the feeding thread connector.
As of the time of the completion of this report, the design has met all requirements set by Dr.
Grustan including dimensions and positions of metal contacts and the prototype has been 3D-
printed. The advantage of 3D-printing is that the entire structure can be manufactured at the same
time and it offers greater precision than the traditional methods such as a machine tool.
3.2 Multi-Emitter Array Fabrication and Characterization
As mentioned in the structure breakdown in section 2.1, there are three parts in the multi-emitter
array prototype, extractor(s), an emitter array and a feeding system. Due to time constraint and
current resource limitation, only sample emitter arrays and sample channels were fabricated,
complete fabrication process and system assembly are not yet finished. The fabrication was
conducted in the cleanrooms at FUNSOM and Suzhou Institute of Nano-Tech and Nano-Bionics
(SINANO) of Chinese Academy of Science. Techniques such as photolithography, and deep
reactive ion etching (DRIE) were used in the process, the detailed procedures using those
techniques will be further elaborated. To evaluate the quality of fabricated arrays and channels,
scanning electron microscopy (SEM) was used for characterization.
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3.2.1 Fabrication Process To complete a functional prototype, multiple steps of photolithography, DRIE, plasma-enhanced
chemical vapor deposition (PECVD), reactive ion etching (RIE) are needed. Fig. 7 is a step-by-step
breakdown of this process.
Figure 7: Breakdown for emitter and channel fabrication process [1].
The wafer used has a thickness of 450 µm. Step (1): the double-polished wafer is cleaned through
standard RCA-1 procedure (refer to Appendix A); step (2): pattern the channel via photolithography;
step (3): etch 20 µm anisotropically via DRIE, strip the photoresist afterward; step (4): oxidize the
wafer by heating the entire wafer in an oxidation oven for 2 hours, grow a 1 µm silicon dioxide
(SiO2) layer (denoted as thermal oxide in Fig. 7) at 1100 °C, this is to protect the channel from
external damage and smooth any etching defects; step (5): flip the wafer and use PECVD to grow
a 4 µm SiO2 layer (denoted as PECVD oxide in Fig. 7); step (6): pattern the emitter array (well
only) via photolithography; step (7): use RIE to etch the oxide layer and dip the wafer in
hydrofluoric acid to remove the unprotected oxide; step (8): pattern the emitter rod via
photolithography; step (9): use DRIE to etch 250 µm into the wafer and strip the photoresist; step
(10): the last DRIE carves the emitter rod to 300 µm deep, connecting the channels at the backside
with the emitter rod, also note the formation of the side well; step (11): clean the wafer and strip
the oxide via hydrofluoric acid.
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3.2.2 Photolithography
Photolithography is crucial in the fabrication process for it’s the technique for pattern transfer. The
quality of photolithography directly impacts the quality of latter steps and the performance of the
prototype. All the photomasks used in this process were designed by Dr. Grustan back at UCI.
Photolithography contains several steps which can be briefly summarized as the following: 1. Clean
the wafers and photomasks, normally with the RCA-1 procedure, but sometimes for convenience,
acetone and ethanol rinse is sufficient; 2. Apply bias (trimethylsilyl) amine, which is also known
as HMDS to the surface of the wafer. This is to enhance the adhesiveness between the wafer and
photoresist. In the cleanroom at FUNSOM, HMDS was applied via a spinner while at SINANO, a
programed treatment system is used to spray HMDS to the wafer; 3. Apply photoresist and evenly
spread it on the wafer; 4. Bake the coated wafer; 5. Load the photomask and wafer onto
photolithography machine, expose the wafer to UV light; 6. Unload the wafer and immerse it in
developer solution for development; 7. Rinse the developed wafer with deionized water and dry it
with nitrogen gun; 8. Hard-bake the wafer at 110°C for 8mins. Table 1 details the parameters of
some of the aforementioned steps and compares the difference between the FUNSOM and
SINANO facilities.
Table 1: Comparison of photolithography parameters at FUNSOM and SINANO facilities
Name of procedure
Parameters (FUNSOM)
Parameters (SINANO)
HMDS coating 1000 RPM (10s) /3500 RPM
(20s)
10mins standard procedure in
HMDS pre-treating machine
Photoresist (AZ4620) coating
500 RPM (10s) /1500 RPM
(30s)
500 RPM (10s) /1500 RPM
(30s)
Baking
90°C for 30mins 100°C for 6mins
Exposure
Soft contact for 25s (mercury
lamp power 20mW/cm2)
Soft contact for 27s (mercury
lamp power 9mW/cm2)
Developing
AZ400K Developer: water=1: 4
Time: 3mins 33s; the time varies
due to the quality of previous
steps
RZX-3038 Developer:
water=1:8
Time: 3mins; the time varies due
to the quality of previous steps
10
The parameters used at FUNSOM are customized based on Dr. Grustan’s experience working on
microfabrication at UCI while the parameters at SINANO are standardized. The effect of those
differences is an issue worth investigating given the importance of photolithography in this
fabrication process. Fig. 8 shows pictures of developed wafer pieces, 8(a) is the emitter array
whose photolithography process was conducted at FUNSOM cleanroom; 8(b) is the channel
whose photolithography process was conducted at SINANO cleanroom.
(a)developed emitter array wafer piece (b) developed channel wafer piece
Figure 8: (a) Images of developed emitter array wafer and (b)channel wafer.
Generally speaking, the cleanroom at FUNSOM is at a more inferior quality compared to the
facility at SINANO which has a much stricter system of regulations. For photolithography,
especially for the fabrication of the channel part, the class of cleanroom is a critical factor because
even the slightest blockage in the channel could cause the failure of the entire prototype.
3.2.3 Deep Reactive Ion Etching (DRIE)
After the photolithography process is completed, the wafer is ready to be etched. And since in the
prototype, the emitter well is 300 µm deep in average with high aspect ratio, deep reactive ion
etching is the most suitable method. This process was carried out in SINANO facility.
DRIE is a cyclic process, alternating between an etching and passivation phase. It’s well-
established that SF6 works as the etching gas and CF4 as the passivation gas. The etching
environment is under Argon protection and etching gas will be ionized and form plasma from
where radical fluorine particles bombard the surface of the target guided by a strong electric field.
The areas covered by photoresist are protected from fluorine attacks and after a set amount of time,
normally in seconds, a passivation phase will follow. The purpose of passivation is to protect side
edges from etchant’s chemical attacks so that the etching can maintain largely isotropic along the
vertical axis. However, too much of passivation gas will form a deposition layer at the bottom of
11
the emitter well thus preventing further etching. Multiple trials were run to find the optimal recipe
for the process, table 2 is the conditions for trial No. 1 and Fig. 9 corresponds to its scanning
electron microscope (SEM) characterization. The sample holder was tilted 45° to get a better look
at the depth profile, and it can be clearly observed that at the bottom of the emitter well and the
bottom of the emitter rod, large amount of under-etched sharp silicon known as the silicon “grass”
are present. Their presence is not desirable thus the recipe needs to be modified.
Table 2: DRIE parameters for trial No. 1
Etching time 8s
SF6 flow 450 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 30mins
* Sccm corresponds to gas flow unit which is standard cubic centimeters per minute.
Figure 9: SEM characterization of emitter well for trial No. 1.
After the first trial, Dr. Grustan suggested adding oxygen gas during the etching phase to increase
the etching efficiency. The theoretical basis for this is that when SF6 molecule is ionized, fluorine
radicals have the tendency to recombine with sulfur ions, so by steadily adding O2 gas, sulfur ions
can bind with O atoms thus preventing this recombination. With more fluorine radicals available,
the etching efficiency is improved, and a second effect of adding O2 gas is that it can consume any
passivation deposition at the bottom of the well so the etchant can bombard the target’s surface
without any hindrance thus also improving the etching efficiency. Trial No. 2 is modified based on
Dr. Grustan’s suggestion of adding O2 gas during the etching phase, parameters are shown in Table
3. Fig. 10 is the corresponding SEM characterization image, Fig. 10 (a) is the overview of the
emitter array, and smooth surface at the bottom of the wells is observed, which is a significant
improvement compared to the first trial, Fig. 10 (b) gives a closer look at an individual well, and
12
further confirmed that no “grass” is formed at either the bottom of the side wall or the bottom of
emitter rod.
Table 3: DRIE parameters for trial No. 2
Etching time 8s
SF6 flow 450 sccm
O2 flow 45 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 50mins
(a) SEM image of the emitter array (b) SEM image of a single emitter well
Figure 10: (a) SEM characterization of emitter array and (b) single emitter well for trial No. 2.
Trial No. 2 revealed promising results which proved that adding oxygen gas during the etching
phase can significantly increase the etching efficiency, thus eliminating the formation of silicon
“grass”. However, it was also clear that the diameter at the bottom of the rod is narrower than that
of the top of the rod. The diameter is designed to be 100 µm and by estimation, the diameter at the
bottom is almost halved, which has exceeded the tolerable margin of error. Ideally, there should
not have been any reduction in diameter of the rod, a margin of 10-20 um reduction is considered
within the margin of error. This “over-etching” effect could cause the rod to break and increase the
impedance of fluid flow, which potentially can compromise the integrity and performance of the
prototype. Fig. 11 shows the ideal shape of the emitter rod, which was fabricated at UCI, 11 (a) is
the overview of the array while 11 (b) shows an individual emitter rod which largely maintains a
uniform diameter along the vertical axis. To achieve such effect, further modification of the recipe
is required.
13
(a) the overview of the emitter array (b) an individual emitter rod
Figure 11: Ideal shape and geometry of (a) emitter well and (b) emitter rod [1].
To prevent this “over-etching” effect, reduction of the oxygen flow was applied to limit the number
of fluorine radicals so it can etch “less”, but the problem persisted. Increasing the passivation time
was also experimented aiming to give the edges more protection, but the deposit layer ended up too
thick to etch, Fig 12 shows the SEM characterization from different attempts to solve the “over-
etching” issue.
(a) O2 flow decreased (b) passivation time increased (c) ramping O2 increase
Figure 12: Comparison between different mode of O2 injection:
Fig. 12 (a) shows the result after decreasing the steady O2 flow to 27 sccm, though there’s
improvement compared to 45 sccm, but the shrink in diameter is still too significant to tolerate; Fig.
12 (b) shows the result after increasing the passivation from 3s to 5s and as it clearly shows, the
amount of silicon “grass” present increased and the etching depth is not enough. This is due to the
excess passivation gas forming a deposition layer at the bottom of the well during passivation cycles,
so the etchant couldn’t penetrate and attack the surface of the wafer; Fig. 12 (c) shows the result
after adding the O2 in a ramping fashion instead of a steady flow at a constant rate. However, due
to a functional problem of the machine, a consistent ramping wasn’t applicable, so a step-increase
was adopted instead. The result was not good, not only broken rods were present, but it also had
14
deformed rod and presence of silicon “grass”. Parameters of all DRIE trials are listed in Appendix
B.
It’s worth mentioning that the cost of using SINANO’s DRIE machine is 1000 RMB/hr which
approximately $200/hr, and every trial required at least two hours due to the preparation process.
So with a limited budget, after several unsuccessful trials, it was decided that it was more
economical and convenient to adjust the geometry of the photomask instead of modifying the recipe.
The newly designed pattern has a decreased diameter of the emitter rod and emitter well, making
it more durable during the DRIE process.
Photolithography and DRIE were the main focus of this co-op term, other steps outlined in Fig. 7
were not yet completed due to time constraint and resource limitation. It is expected that a working
prototype can be fabricated before August. The total cost of fabrication including photolithography,
DRIE and the use of SINANO cleanroom is estimated to be 15000$ for the completion of the
prototype.
4.0 Performance Testing Apparatuses
After a prototype is fabricated, performance testing is required to analysis its properties and
determine what modifications are required. The procurement of performance testing apparatuses
was a major task, the total cost of equipment exceeded 40,000$. Equipment can be grouped into
three categories, electronic apparatus, data acquisition (DAQ) system and the vacuum apparatus
which is the most expensive part costing about 25000$. The vacuum setup consists of a main
vacuum chamber, two mechanical pumps, two turbo-molecular pumps and valves. The electronic
setup consists of two high voltage sources, a pulse generator, an oscilloscope, a pico-ammeter and
a 3D positioning system which includes three sets of tracks and step-motors. The data acquisition
system consists of a central DAQ card, an optical camera and a computer to process collected data.
This report focuses on the vacuum setup and electronic apparatus.
4.1 Vacuum apparatus
The performance testing will be conducted in a vacuum environment whose pressure is maintained
below 9*10-5 Pa. The general air circuit of the vacuum system is shown in Fig. 13. In the figure, T
represents turbo-molecular pumps, M1 and M2 represent mechanical pumps, P represents a
pressurized system which in our case, the atmosphere. The reservoir and chamber are connected
15
with a silica thread (painted orange in the figure), and the orange box at the bottom right corner is
a pressure gauge.
Figure 13: Air circuit of the vacuum system
The vacuum system has three different modes, pumping, experimentation and standby.
For the pumping mode, first, the mechanical pump No. 1 (denoted as M1 in Fig. 13) will start pre-
pumping, the purpose of this step is to accelerate the pumping rate by quickly pumping large
amounts of air out of the chamber as illustrated in Fig 14 (a). At the step, all valves are open so that
both the reservoir and the chamber can be pumped at the same time; when the chamber pressure
drops to 10-1 Pa, the turbo-molecular pumps are turned on to further pump air molecules such as
N2 and O2 out of the system as illustrated in Fig 14 (b). The reason for turning on turbo-molecular
pump only when a certain pressure threshold is reached is that if a large amount of air molecules
enter the pumps, it will damage the turbans in the pumps and therefore damage the pumps.
Normally, the pumping process will last for two hours for the chamber pressure to drop to 9*10-5
Pa. In Fig. 14, 16 and 17, orange arrows indicate the air flow directions.
(a) Mechanical pre-pumping (b) turbo-molecular pumping
Figure 14: Schematics of pumping mode.
16
The experimentation will start once the pressure of the chamber drops to 9*10-5 Pa. The pressurized
system (denoted P in Fig. 13) will be connected to the reservoir to pressure the ionic fluid through
silica thread into the prototype which will be fixed in the chamber. The thread is connected to the
PEEK flange which acts as a seal of the chamber, Fig. 15 shows the mounting the prototype inside
the chamber for performance testing. To maintain the vacuum environment, the turbo-molecular
pumps will stay on, so it’s crucial to close the valve connecting the chamber and the reservoir so
that ionic fluid won’t eject into the chamber due to pressure difference and protect the pumps from
a sudden pressure change due to the exposure of pressurized system as illustrated in Fig. 16 (a).
The ionic liquid used in the testing is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
imide (EMI-Im). This particular ionic fluid has been tested multiple times by different research
groups and is believed to be the ideal source for electric propulsion testing [10]. After the
experiment is concluded, it’s necessary to level the pressure between the reservoir and the chamber
so the system can proceed to standby mode. To achieve that, mechanical pump No. 2 (denoted as
M2 in Fig. 13 will be turned on to lower the pressure in the reservoir so that it reaches approximately
the same level as the main chamber as illustrated in Fig. 16 (b), and the system is now ready to
proceed to standby.
Figure 15: The mounting of the prototype for performance testing at Dr. Grustan’s lab at UCI
[1].
17
(a) experimentation initiation (b) experimentation termination
Figure 16: Schematics of experimentation mode.
The standby mode preserves the vacuum after the experiments are concluded. When the pressure
between the reservoir and the main chamber reaches equilibrium, turn off the M2 pump and close
the valve between the M2 pump and the reservoir. To further lower the pressure by opening the
valves as the pre-pumping step as illustrated in Fig 17 (a). When the pressure stabilizes at 5*10-4
or lower, close the valve between the chamber and the turbo-molecular pumps and the valve
between the M1 pump and turbo-molecular pumps to seal the system. When the system is sealed
as illustrated in Fig. 17 (b), there’s a minimum air exchange with the atmosphere since a complete
isolation is virtually impossible. The preservation of vacuum reduces the time required for pumping
when next time the experiment is conducted. The pressurized system has been tested and the time
needed to reach the desired pressure met the requirement which is two hours if the chamber had
been previously exposed to the atmosphere or half an hour if the chamber had been sealed under
standby mode.
(a) standby mode---lower system pressure (b) standby mode---seal the system
Figure 17: Schematics of standby mode.
18
4.2 Electronic apparatus
The electronic setup which is illustrated in Fig. 18, is responsible for data measurements. The
high voltage supplies will be connected to the prototype to generate the electric field needed for
electrospray. The operation voltage ranges from negative 1500V to positive 1500V. When the
electrospray beam is formed, it will shoot toward the main chamber and be collected in the
Faraday cup which is connected with a pico-ammeter. The grid placed in front of the cup is to
suppress secondary electrons [5]. Current and current intensity are measured as a function of
experimentation time, those data will provide valuable information about the stability, emitting
efficiency, emitting intensity and so on. The thrust generated is closely related to the intensity and
magnitude of the current from ionic beam, and a vacuum chamber can simulate the working
environment of the thruster which offers researchers a more accurate assessment of the prototype.
The interior setup will be placed on an XYZ positioner which can move the Faraday cup in all
three directions. However, the assembly and calibration of the electronic setup were not
completed by the time the co-op term ended.
Figure 18: Electronic Setup of performance testing [5].
5.0 Conclusions
From the analysis of the report body, it was concluded that during the deep reactive ion etching
(DRIE), adding oxygen gas as part of etching gas can help eliminate the formation of silica grass
at the base of the emitter well and emitter rods. The ideal amount of oxygen added was thoroughly
investigated. The main etching gas was SF6, adding oxygen can prevent the recombination between
sulfur ions and fluorine ions which is mainly responsible for the etching. The addition of O2 gas
enhanced the efficiency of etching by allowing more fluorine ions to attack the surface of the silicon
wafer, thus increasing the etching rate and eliminating silicon grass.
19
The increased etching rate and efficiency is preferable, but it also comes with over-etching which
is an undesirable effect that weakens the emitter structure. Over-etching thins the base of the emitter
rods and in some cases even cause the rod to break. The integrity of the emitter rods is crucial to
the performance of the prototype and after several adjustments to the DRIE etching recipe, the
problem remains. Eventually, a modified design for multi-emitter array has been adapted. The new
array consists of the same number of emitters with higher durability to withstand the bombardment
from etchants.
The design of the single emitter set must meet the mounting requirements for performance testing.
Those include mechanical requirements such as the diameter, height of the emitter set and the
position of the fixation screws, which are important for placing the prototype onto the apparatus
and electrical requirements such as the positions of contact electrodes for the extractors to work
properly which are important for data collection. The final version of the design met all
requirements and is approved by Dr. Grustan for prototype fabrication.
Considering the economical aspect of the project, the mounting of the new laboratory, the testing
trails leading to the fabrication of prototype and the prototype fabrication itself would cost more
than 55000$. And if the first prototypes (single emitter and multi-emitter array) don’t deliver good
results, modifications are required either to the design or to the fabrication parameters. Both types
of modification cost a considerable amount of resources which include the cost of using SINANO
equipment, 3D-print new emitter set and so on. However, if the project can move on to the next
stage after promising results are delivered, then its potential application in the aerospace industry
can draw more attention and investment. Electric propulsion is a candidate for next generation
satellite propulsion system, so the potential economic gain is enormous.
Finally, the main components of the performance testing apparatus which consists of a vacuum
chamber, two turbo-molecular pumps, two mechanical pumps, an industrial camera set and
electronic measuring devices, is mounted and ready to perform some preliminary tests. However,
the mounting process is not yet completed, several key pieces are still in the process of procurement,
including an XYZ positioning system, a data acquisition system and a computer to process
experimental data. The chance of acquiring the aforementioned pieces within a short period of time
is promising and in the meantime, some preliminary results can be obtained so that Dr. Grustan’s
team can learn more about the prototypes and the performance testing apparatus.
20
6.0 Recommendations
Based on the analysis and conclusions drawn in this report, it is recommended that more research
should be done on the relationship between the flow of etching gas injected, etching/passivation
cycle time and their effects at a different aspect ratio. The study would be beneficial to improve the
quality of the DRIE process. While the over-etching problem was solved by redesigning the
geometry of the emitter rod, it comes at the cost of compromising some performance indices. It is
evident that injecting O2 during the etching cycle can help eliminate silica grass, but cause over-
etching at the base of the emitter rods, so to avoid that, it’s recommended that a new recipe should
be developed with a gentler rate of etching but producing a more evenly etched surface.
Another factor that significantly affects the quality of multi-emitter array fabrication is the quality
of photolithography. The cleanroom facilities in Institute of Functional Nano and Soft Materials
(FUNSOM) and Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) use different
parameters for photolithography, specifically the power of the mercury lamp. Due to time constraint,
the relationship between the power of the lamp and its effect on the quality of the photolithography
was not thoroughly studied, so it is recommended that more research should be done on this topic
to ensure the quality of photolithography which can significantly affect the quality of the DRIE
process thus affecting the final quality of the prototype.
Lastly, it is recommended that the missing pieces aforementioned in the conclusion section should
be purchased and assembled as soon as possible so that prototypes can be tested and modified
accordingly. Based on the degree of complexity of the two electrospray sources, it’s recommended
that the single emitter set should be tested first since it’s more easily manufactured and can help
the team to understand the capacity of the testing apparatus in the meantime.
21
Glossary
This list contains some of the acronyms appeared throughout the text with a brief description for
each term.
ESI: Electrospray ionization. It’s a technique which ionizes molecules via strong electric field. It’s
a technique mainly used for mass spectrometry. In the context of this report, the main focus is to
use ESI as a source for electric propulsion.
MEMS: Micro-Electronic-Mechanical-System. Electronic and mechanical devices which have a
dimension equal or below micro-scale. The techniques to fabricate these devices are different from
conventional methods and more sophisticated.
FUNSOM: Institute for Nano and Soft Materials. It’s an advanced research institute at Soochow
University, China. It is located in the Suzhou Industrial Park and it’s the place where I did most of
my work during the coop terms.
SINANO: Suzhou Institute of Nano-Tech and Nano-Bionics. It’s an advanced research and
fabrication facility belongs to Chinese Academy of Science. I conducted most of the fabrication
work for the prototype using their facility and equipment. Its facility contains a complete set of
micro-fabrication equipment and state-of-art cleanroom.
PEEK: Polyether ether ketone. It’s a special plastic which has a very low leakage coefficient,
making it an ideal choice for vacuum experiments. The plastic layers and the base of the single
emitter prototype and the flange to seal the chamber are made of this material.
DRIE: Deep reactive ion etching. It’s a micro-fabrication technique to perform the high aspect ratio
etching of silicon.
SEM: Scanning electron microscopy. It’s a widely used method to characterize micro-scale samples.
The imaging principle is to collect backscattering electrons and secondary electrons from the
sample after bombarding it with an electron gun.
Sccm: Standard cubic centimeter. It’s a gas flow unit which measures the different gas flow into
the DRIE chamber.
PECVD: Plasma enhanced chemical vapor deposition. It’s a technique to deposit thin films which
involves exciting the reaction gas to its plasma state. This technique was used to deposit a thin layer
of SiO2 on top of the silicon wafer.
RCA: Radio Corporation of America.
UCI: University of California at Irvine, it is the university which Dr. Grustan worked as the director
of the cleanroom and received his PhD. Many of the experimental parameters were based on his
work there.
22
Reference [1] E. Grustan Gutiérrez, "Multiplexing of electrospray sources for space propulsion and
physical sputtering", Ph.D, University of California at Irvine, 2015.
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Ng, M. W. M. Suen, and H. L. Tai, “Electrospray ionisation mass spectrometry: principles and
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[3] J. J. Pitt, “Principles and applications of liquid chromatography-mass spectrometry in
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[5] R. Krpoun and H. R. Shea, “Integrated out-of-plane nanoelectrospray thruster arrays for
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[6] R. Borrajo-Pelaez, E. Grustan-Gutierrez, and M. Gamero-Castaño, “Sputtering of Si, SiC,
InAs, InP, Ge, GaAs, GaSb, and GaN by electrosprayed nanodroplets,” J. Appl. Phys., vol. 114, no.
18, 2013.
[7] P. Sigmund, “Mechanisms and theory of physical sputtering by particle impact,” Nucl. Inst.
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[8] M. S. Alexander, K. L. Smith, M. D. Paine, and J. P. W. Stark, “Voltage-Modulated Flow
Rate for Precise Thrust Control in Colloid Electrospray Propulsion,” J. Propuls. Power, vol. 23,
no. 5, pp. 1042–1048, 2007.
[9] L. Konermann, E. Ahadi, A. D. Rodriguez, and S. Vahidi, “Unraveling the mechanism of
electrospray ionization,” Anal. Chem., vol. 85, no. 1, pp. 2–9, 2013.
[10] M. Gamero-Castaño, “Characterization of the electrosprays of 1-ethyl-3-
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1–11, 2008.
[11] W. Kern, Ed, Handbook of Semiconductor Cleaning Technology, Noyes publishing; Park
Ridge, NJ, 1993, Ch 1.
23
Appendix A: RCA-1 clean process11
Overview:
RCA-1 process was developed by Werner Kern in late 1960s. It’s an effective cleaning process to
remove organic residue and thin films from silicon wafer. Time required for the process is
approximately 30 mins.
Material required:
Ammonium Hydroxide (27%)
Hydrogen Peroxide (30%)
Bath container
Hot plate
Preparation:
The recipe requires the ratio of DI water, Hydrogen Peroxide (H2O2) and Ammonium Hydroxide
(NH4OH) to be 5:1:1. Turn on the hot plate for pre-heating. Pour sufficient amount of
aforementioned reactants to beakers based on the size of the bath container, for example 325ml of
H2O, 65ml of H2O2 and 65ml of NH4OH.
Procedure:
Add 65ml NH4OH to DI water, and heat the solution to about 70°C, remove the solution from the
hot plate and add 65ml H2O2 to the solution. The solution will start to bubble and violent
exothermic reaction will start which indicates the solution is ready for use. Fully immerse the
wafer into the solution and soak it for 15 mins, then retrieve the wafer and rinse it with DI water.
Repeat the process for all wafers needed for the fabrication.
11 W. Kern, Ed, Handbook of Semiconductor Cleaning Technology, Noyes publishing; Park Ridge, NJ,
1993, Ch 1.
25
Appendix B: Parameters of all DRIE trials
Table B-1: Trial No.1: only etching gas and passivation gas
Etching time 8s
SF6 flow 450 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 30mins
Table B-2: Trial No.2: adding O2 gas (10% of SF6 flow) in the etching phase
Etching time 8s
SF6 flow 450 sccm
O2 flow 45 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 50mins
Table B-3: Trial No.3: reduce O2 flow to 25 sccm
Etching time 8s
SF6 flow 450 sccm
O2 flow 25 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 50mins
Table B-4: Trial No. 4: increase passivation time and O2 flow
Etching time 8s
SF6 flow 450 sccm
O2 flow 45 sccm
Passivation time 5s
CF4 flow 190 sccm
Total time 50mins
Table B-5: Trial No. 5: reset passivation time and reduce O2 flow
Etching time 8s
SF6 flow 450 sccm
O2 flow 35 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 50mins
Table B-6: Trial No.6: reduce O2 flow
Etching time 8s
SF6 flow 450 sccm
O2 flow 27 sccm
Passivation time 3s
CF4 flow 190 sccm
Total time 50mins
26
Table B-7: Trial No.7: Increase O2 flow in a ramping fashion
Etching time 8s
SF6 flow 450 sccm
O2 flow 0 sccm for 10mins
9 sccm for 15mins
18 sccm for 15mins
27 sccm for 10mins
30 sccm for 20mins
Passivation time 3s
CF4 flow 190 sccm
Total time 70mins