Microfluidic channels in glass and silicon chips are fabricated in the cleanroom. Their acoustic focusing properties are then tested and reported in this project report (MEMS 5801).
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photo lithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.
Etching
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes:
Wet etching where the material is dissolved when immersed in a chemical solution
Dry etching where the material is sputtered or dissolved using reactive ions or a vapour phase etchant
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photo lithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.
Etching
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes:
Wet etching where the material is dissolved when immersed in a chemical solution
Dry etching where the material is sputtered or dissolved using reactive ions or a vapour phase etchant
Semper Fi Systems: A technology consulting practice offering transformative innovations in the fields of infrared imaging science & artificial intelligence for data governance
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving, to form a patterned coating on a surface.
This is one of the fabrication technology used in IC fabrication. This technique is mostly used in flexible electronics. It contains various other subprocesses in it.
This paper explains the fabrication of thin film using modified Physical Vapor Deposition (PVD) Module. Physical Vapor Deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the material onto various surfaces. The surface morphology of various such as Titanium Dioxide and Aluminum thin film has been studied. The Titanium Dioxide and Aluminum thin film has been fabricated on Silicon (Si) substrate using modified Physical Vapor Deposition (PVD) module system. The process started with the establishment of process flow, process modules, and process parameters. Two modules were developed. The characteristics prior to the thin film fabrication namely surface morphology, metal thickness characterization and V-I characteristic were recorded. The samples were characterized by Optical Microscope, Atomic Force Microscope (AFM),X-ray diffraction (XRD) and I - V characterization. The result and data were analyzed and applied in the fabrication of thin film using various materials. The thin film fabrication process used Titanium Dioxide (TiO2) nanopowder and Aluminum (Al2O3) nanopowder for the coating process. The result for each processes are presented in this paper.
Semper Fi Systems: A technology consulting practice offering transformative innovations in the fields of infrared imaging science & artificial intelligence for data governance
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving, to form a patterned coating on a surface.
This is one of the fabrication technology used in IC fabrication. This technique is mostly used in flexible electronics. It contains various other subprocesses in it.
This paper explains the fabrication of thin film using modified Physical Vapor Deposition (PVD) Module. Physical Vapor Deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the material onto various surfaces. The surface morphology of various such as Titanium Dioxide and Aluminum thin film has been studied. The Titanium Dioxide and Aluminum thin film has been fabricated on Silicon (Si) substrate using modified Physical Vapor Deposition (PVD) module system. The process started with the establishment of process flow, process modules, and process parameters. Two modules were developed. The characteristics prior to the thin film fabrication namely surface morphology, metal thickness characterization and V-I characteristic were recorded. The samples were characterized by Optical Microscope, Atomic Force Microscope (AFM),X-ray diffraction (XRD) and I - V characterization. The result and data were analyzed and applied in the fabrication of thin film using various materials. The thin film fabrication process used Titanium Dioxide (TiO2) nanopowder and Aluminum (Al2O3) nanopowder for the coating process. The result for each processes are presented in this paper.
IJRET : International Journal of Research in Engineering and Technology is an international peer reviewed, online journal published by eSAT Publishing House for the enhancement of research in various disciplines of Engineering and Technology. The aim and scope of the journal is to provide an academic medium and an important reference for the advancement and dissemination of research results that support high-level learning, teaching and research in the fields of Engineering and Technology. We bring together Scientists, Academician, Field Engineers, Scholars and Students of related fields of Engineering and Technology.
Analysis Of Carbon Nanotubes And Quantum Dots In A Photovoltaic DeviceM. Faisal Halim
Analysis of Carbon Nanotubes and Quantum Dots in a Photovoltaic Device
A poster prepared by Francis and me; presented by Francis. I modified on of the photographs used, in this copy.
MONOLITHIC IC PROCESSES A monolithic integrated circuit (IC) is a set of circuitry on a single semiconductor plate or chip rather than built of separate elements as a discrete circuit is.
A micro-electromechanical system (MEMS) gyroscope is commonly used to monitor the angular rate of a moving body due to its benefits. The most promising advantages include its small size, low cost, and a high degree of integration. MEMS gyroscope has different fabrication processes and micromachining techniques. LIGA (Lithography-Galvanoformung-Abformung), bulk micromachining, surface micromachining, Silicon-on-glass (SOG) and Deep Reactive Ion Etching (DRIE) are the known fabrication techniques for MEMS gyroscope. This paper systematically reviewed the fabrication techniques used to fabricate the MEMS gyroscope. The current review paper also focuses on the performance of MEMS gyroscope which included several recent developments. For the conclusion of results, the variable typically used is the rate of turn (°/s) for MEMS angular rate sensors with respect to bandwidth frequency. Finally based on the review some analysis on fabrication technology, key principles, and performance parameters are discussed.
In this work, we synthesized and characterized mesoporous thin fims
of SiO 2 and NiTiO3 structured by a surfactant called Brij58. These fims
were fabricated by the method of dip coating and the best conditions for
well-structured thin fims were investigated as a function of surfactant
concentration and diffrent types of substrates. These fims have been
characterized by X-ray reflctivity which was calculated using the matrix
formalism. We demonstrated that the silicon substrate had a great effct
on the structure and porosity of the fabricated fims for both SiO2 and
NiTiO3. Furthermore, we found that mesoporosity has been increased
as a function of the surfactant concentration in the solution. This experimental procedure allows also to produce NiTiO3 powders which have
been characterized by X-ray diffaction. The XRD coupled to the crystallographic software “Maud”shows that the samples are constituted by 98,
2% NiTiO 3 powders.
Fullerenes Synthesis Using Fabricated Arc Discharge System with Relatively La...iosrjce
Arc discharge technique is still one of the popular techniques for producing fullerenes. This work
describes an arc discharge installation with a chamber size of 140 mm diameter and 500 mm length that is able
to produce fullerenes. Fullerenes solid was extracted from carbon soot collected from the arc discharge system
at various experimental conditions of discharge current and chamber pressure using the solvent extraction
technique. Fullerenes yield of up to 6 to 8 % was extracted from the fabricated discharge system which was
higher when compared to the 3-4 % yield produced by an arc discharge system of 100 mm diameter and 200
mm length. The fabricated discharge system gave the best fullerenes percentage yield of 8 % at arc current of
150 A and 100 Torr of chamber pressure. Increase in inert atmosphere from 100 to 200 Torr on the amount of
carbon soot vapourization but inhibited the percentage fullerenes yield from 8 to 6 %. Scanning Electron
Microscopy (SEM) and Ultraviolet Visible (UV-Vis) Spectroscopy analysis carried out on all fullerenes solid
extracted revealed the presence of fullerenes. The fabricated arc discharge system suggests that large chamber
size could promote the formation the synthesis of fullerenes.
Extraction of Silica and other related products from Rice HuskIJERA Editor
The main objective of the present work is the synthesis of silica from rice husk and to study its characterization
studies. The alkaline method succeeded by acid precipitation is done for the extraction of silica. The
optimization of the concentration of sodium hydroxide and the amount of rice husk ash that goes with it for
attaining the maximum yield of silica is done. X-ray Diffraction and Fourier Transform Infrared Spectrometry
studies are executed for acquiring its characteristic. Activated carbon and silica gel are the intermediate products
formed during the synthesis process of silica.
Seminar of U.V. Spectroscopy by SAMIR PANDASAMIR PANDA
Spectroscopy is a branch of science dealing the study of interaction of electromagnetic radiation with matter.
Ultraviolet-visible spectroscopy refers to absorption spectroscopy or reflect spectroscopy in the UV-VIS spectral region.
Ultraviolet-visible spectroscopy is an analytical method that can measure the amount of light received by the analyte.
Nutraceutical market, scope and growth: Herbal drug technologyLokesh Patil
As consumer awareness of health and wellness rises, the nutraceutical market—which includes goods like functional meals, drinks, and dietary supplements that provide health advantages beyond basic nutrition—is growing significantly. As healthcare expenses rise, the population ages, and people want natural and preventative health solutions more and more, this industry is increasing quickly. Further driving market expansion are product formulation innovations and the use of cutting-edge technology for customized nutrition. With its worldwide reach, the nutraceutical industry is expected to keep growing and provide significant chances for research and investment in a number of categories, including vitamins, minerals, probiotics, and herbal supplements.
Professional air quality monitoring systems provide immediate, on-site data for analysis, compliance, and decision-making.
Monitor common gases, weather parameters, particulates.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
Cancer cell metabolism: special Reference to Lactate PathwayAADYARAJPANDEY1
Normal Cell Metabolism:
Cellular respiration describes the series of steps that cells use to break down sugar and other chemicals to get the energy we need to function.
Energy is stored in the bonds of glucose and when glucose is broken down, much of that energy is released.
Cell utilize energy in the form of ATP.
The first step of respiration is called glycolysis. In a series of steps, glycolysis breaks glucose into two smaller molecules - a chemical called pyruvate. A small amount of ATP is formed during this process.
Most healthy cells continue the breakdown in a second process, called the Kreb's cycle. The Kreb's cycle allows cells to “burn” the pyruvates made in glycolysis to get more ATP.
The last step in the breakdown of glucose is called oxidative phosphorylation (Ox-Phos).
It takes place in specialized cell structures called mitochondria. This process produces a large amount of ATP. Importantly, cells need oxygen to complete oxidative phosphorylation.
If a cell completes only glycolysis, only 2 molecules of ATP are made per glucose. However, if the cell completes the entire respiration process (glycolysis - Kreb's - oxidative phosphorylation), about 36 molecules of ATP are created, giving it much more energy to use.
IN CANCER CELL:
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
Unlike healthy cells that "burn" the entire molecule of sugar to capture a large amount of energy as ATP, cancer cells are wasteful.
Cancer cells only partially break down sugar molecules. They overuse the first step of respiration, glycolysis. They frequently do not complete the second step, oxidative phosphorylation.
This results in only 2 molecules of ATP per each glucose molecule instead of the 36 or so ATPs healthy cells gain. As a result, cancer cells need to use a lot more sugar molecules to get enough energy to survive.
introduction to WARBERG PHENOMENA:
WARBURG EFFECT Usually, cancer cells are highly glycolytic (glucose addiction) and take up more glucose than do normal cells from outside.
Otto Heinrich Warburg (; 8 October 1883 – 1 August 1970) In 1931 was awarded the Nobel Prize in Physiology for his "discovery of the nature and mode of action of the respiratory enzyme.
WARNBURG EFFECT : cancer cells under aerobic (well-oxygenated) conditions to metabolize glucose to lactate (aerobic glycolysis) is known as the Warburg effect. Warburg made the observation that tumor slices consume glucose and secrete lactate at a higher rate than normal tissues.
A brief information about the SCOP protein database used in bioinformatics.
The Structural Classification of Proteins (SCOP) database is a comprehensive and authoritative resource for the structural and evolutionary relationships of proteins. It provides a detailed and curated classification of protein structures, grouping them into families, superfamilies, and folds based on their structural and sequence similarities.
Fabrication of microfluidic channels in glass and silicon
1. Fabrication of Microfluidic Channels In
Glass and Silicon
Brent Auyong, Yichen Sun
Abstract – Acoustic particle focusing microfluidic devices have recently received a growing interest as Point of Care (POC)
diagnostic tools.Effective fabrication techniques are fundamental to the functionality of these devices. In this report, we
discuss the fabrication techniques used to create acoustic particle focusing microfluidic devices in silicon and glass. We
also discuss experimental results that validate our devices’ effectiveness at focusing various particles. Despite difficulties
with our devices’ operation, the results illustrate that the micro channels were effective at focusing particles at the
expected focusing frequencies.
Index Terms – Anisotropic Wet Etching, Isotropic Wet Etching, Point of Care Diagnostics, Acoustic Particle Focusing
Introduction
The growing interest to develop robust Point
of Care diagnostic tools (POC), such as a device
to separate cells from particles using acoustic
waves in biological fluids, may have important
implications towards the improved detection of
protein biomarkers [1]. Other POCs use
dielectrophoresis to separate living cells from
non-living cells in biological fluids to improve on
particle separation capabilities [2]. Both
examples illustrate the importance of
understanding lithographic fabrication
techniques for both silicon and glass substrates.
Traditional microelectromechanical systems
(MEMS) were primarily fabricated from single-
crystal silicon wafers using processes such as
silicon, metal and oxide deposition to build
devices for a multitude of applications [3].
These processes allowed for a variety of
electrical components such as insulators,
conductors, resistors, transistors, etc, to be built
on the micro and nano scale level. In the field of
microfluidics, many of these fabrication
techniques have been carried over. These
techniques are categorized into three
approaches: surface micro-machining, buried-
channel and bulk micromachining [4]. The
surface micro-machining approach uses
structural layers coupled with patterned
sacrificial layers, to produce multi-layered micro
features on a substrate [4]. The buried-channel
approach uses anisotropic deep ion reactive
etching to create large cavities deep in a
substrate with small inlets connecting it to the
surface [4]. Bulk micromachining uses a variety
of etching techniques to remove areas of
material from a substrate [4]. The bulk micro
machining fabrication approach will be the
primary focus of this report.
Bulk micro machining in glass or silicon is
accomplished using two main fabrication
techniques, dry etching and wet etching. Dry
etching is a process that removes material from
a substrate through bombardment of ions in a
gas mixture. Typically, the gas mixture used in
the dry etching process depends on the
substrate and the material being removed. For
silicon substrates, dry etching creates isotropic
and anisotropic profiles. For glass substrates,
only anisotropic profiles can be achieved. In
contrast, wet etching uses solvents to remove
material from a substrate. Both silicon and glass
etch in an isotropic manner with the exception
that etch rates in silicon depend on the
crystallographic orientation. Solvent selection is
based on the material being etched away and
the desired etch rate. In our module design,
these different types of bulk micro machining
processes were implemented during the
fabrication of two microfluidic devices. We used
a combination of fabrication techniques to create
micro channel geometries in glass and silicon.
Then, we used the fabricated channels to
conduct acoustic particle focusing and
separation testing. Based on the geometries of
each micro channel, an estimated focusing
frequency was calculated using Eq. 1.
𝑓𝑓 =
𝑛𝑛𝑛𝑛
2𝐿𝐿
(1)
2. These frequencies were then compared to
actual observed focusing frequencies for each of
the channels.
This report will discuss the following bulk
micro machining techniques specifically:
isotropic wet etching in glass, KOH wet etching
in silicon and reactive ion etching through an
oxide layer. The report will be organized into
four main sections: Introduction, Methods,
Results/Discussion and Conclusion.
Methods
Silicon Channel Fabrication
Oxidation of Silicon
A common starting point for fabricating
micro channels in silicon is to first grow an oxide
layer. Based on our desired channel result, it
was determined that an oxide layer thickness of
800 nm was needed. By using a simplified
Deal-Grove model of thermal oxidation, we
calculated the time necessary to grow an 800
nm oxide layer at 1100°C [7]. Eq. 2 is the
simplified Deal-Grove model used.
tOX=�B(t + τ) (2)
where tOX is the oxide thickness, B is the
parabolic rate constant determined by different
oxidation conditions, t is the required time of
oxidation in hours and τ is a time factor
determined by the initial oxide thickness. In our
case, constant B is 0.03 μm2/hr at 1100°C
according to Figure 1, and τ is 0.076 hr
according to Table 1. Therefore, it was
estimated that approximately 22 hours was
required to achieve an 800 nm thick oxide layer.
However, we determined experimentally that it
actually takes 80 hours to produce an 800 nm
oxide layer.
Figure 2 shows the Lindberg/Blue M 3-Zone
Tube Furnace, used to produce oxide layers on
silicon wafers. The furnace uses a multiple
chamber configuration to uniformly heat the
silicon wafers and produce a uniform layer of
oxide. We inserted the silicon wafers near the
center of the furnace, set a target temperature at
1100°C for each chamber and let the chamber
heat up with a ramp time of 2 hours. The
temperature was maintained at 1100°C for 80
hours; then we stopped heating to finish thermal
oxidation and let the chambers cool down to
ambient temperature at normal rate.
The thickness of the oxide layer was then
measured with a J.A. Woollam Co. α-SE
Ellipsometer to confirm our oxide growth
parameters.
Photolithography
Once the 800nm oxide layer was grown on a
silicon wafer, photolithography was used to
pattern the desired micro channel geometries.
Since the oxide layer is naturally hydrophilic,
steps need to be taken to prevent the developer
and etching chemicals from penetrating the
resist/substrate interface and causing extreme
etchant undercutting and complete delamination
of the photoresist film[8]. A layer of MCC Primer
80/20 (HMDS), was applied prior to spin-coating
photoresist, which helps to promote adhesion
between oxide layer and photoresist layer to be
coated later. A Brewer Science CEE Spin
Figure 1. Linear and Parabolic Oxidation Rate Constants
Figure 2. Lindberg/Blue M 3-Zone Tube Furnace
Table 1.Thermal Oxidation Parameters under Different Conditions
3. Coater was used to deposit a layer of positive
photoresist, Shipley S1827, onto the wafer
surface. The spin coater first ramped up by 100
rpm/s for 5 seconds to 500 rpm, then held at
2000 rpm for 30 seconds with ramp of 500
rpm/s, and finally decelerated with a ramp of 500
rpm/s for 4 seconds. Next, we soft-baked the
photoresist on a hotplate at 115°C for 40
seconds and exposed it using a Karl Suss MJB-
3 Mask Aligner. The wafer was exposed to UV
light at 7.71 mW/cm2 for 40 seconds to obtain a
manufacture-recommended exposure dose of
300 mJ/cm2. The wafer was then hard-baked on
a hot plate at 120°C for another 10 minutes to
make firm the photoresist layer and thus
preventing potential contamination of reactive
ion etching chamber later when clamped. A
profilometer was used to measure the
photoresist channel depth Pr and ensure the
correct photoresist layer was achieved.
Reactive Ion Etching (RIE)
Once the channels geometries were defined
by the photoresist, reactive ion etching (RIE)
was used to etch the channel geometries into
the oxide layer. Figure 3 shows the Oxford
Instrument Plasmalab System 100, which was
used to perform RIE. The machine uses a 2.54
GHz plasma source in the inductively coupled
plasma module (ICP) to generate high-density
plasma. The generated plasma ionizes reactive
gases and the resulting ions bombard the wafer
surface in the main reaction chamber. Through
chemical reaction and physical momentum
transfer, volatile products are produced and
pumped away.
The prepared wafer was attached to a 4’’
wafer to mount in the machine and a small piece
of silicon was added to surface. The surface
below will not be etched which will allow the etch
rate and selectivity after a 15 minute RIE etch to
be measured. The wafer was then placed in the
small loading chamber, and after pumping to
2.5×10-2 torr by a roughing pump, the locked
gate connecting the loading chamber to the full
chamber was lifted and the wafer was inserted
into the full chamber.
The loading chamber has a small volume
that is easy to pull rough vacuum, and the full
chamber has its volume 30 times larger than
that of the loading chamber, and is always
maintained at high vacuum. So after the wafer
was inserted and the connecting gate locked up
again, the full chamber was pumped to 7.5×10-9
torr to pump away ambience and other
unwanted molecules, preventing undesired ions
during RIE. We next started oxide etch recipe
and 40 sccm (standard cubic centimeters per
minute) CF4 and 5 sccm oxygen were introduced
into the reaction chamber to etch SiO2 layer. The
plasma ionizes the CF4 to create reactive
fluoride ions which react with SiO2 to form SiF4
(gas) and other gaseous products. After 15
minutes etch, the reaction chamber was pumped
down to 7.5×10-9 torr again to pump away used
gases, preventing potential pollution to the
cleanroom.
The wafer was then taken out for a
measurement. A schematic diagram of the
measurement is illustrated in Figure 4. Etched
photoresist height ΔPr and height between
etched photoresist and etched oxide ΔPr-Ox
were measured.
The etch rates of photoresist and oxide layer
was then determined by Eq. 3.
RPR=ΔPr/t; ROX= (ΔPr-Ox+ΔPr - Pr)/t (3)
where t is the etching time, and we could
estimate the remaining RIE etch time. According
to our estimate, we repeated previous RIE
procedures for another 17-minute etch.
Figure 4. Schematic Diagram of Etch Rates
Measurement
Figure 3. Oxford Instruments Plasmalab System
100 and Schematic Diagram of RIE
4. Stripping of Photoresist
We used acetone to strip away any
remaining photoresist, cleaned the wafer with
deionized water and dried with nitrogen spray.
We next took the wafer for another profilometry
measurement to determine the oxide channel
depth and checked if the oxide layer was totally
removed.
Wet Etching of Silicon
We next aimed to etch channels into silicon
wafer masked by patterned oxide layer by wet
etching. Potassium hydroxide (KOH) can etch in
both [1 0 0] and [1 1 0] directions but almost no
etching effect on [1 1 1] direction. KOH has a
much higher etching rate of silicon compared to
the etching rate of silicon dioxide, so we can use
silicon dioxide as an etching mask in KOH
etching.
In our experiment, the silicon wafer has its
surface an [1 0 0] plane, and we carefully
aligned the channels so that they would be
widened and deepened but not extended. Figure
5 shows our silicon wafer after fabrication.
The patterned silicon wafer was mounted in
a PEEK holder, and its back surface was
protected while its top surface exposed to the
KOH solution. Figure 6 and 7 shows the wet
etching setup and its schematic diagram
respectively.
The hotplate was set at 125°C, and the Teflon
magnetic stirrer was kept stirring at 600 rpm to
increase diffusion of produced hydrogen
bubbles. The solution temperatures at the
beginning and in the end were 60.8°C and 63°C
respectively, and the temperature was
monitored by a thermometer throughout the
etching. To etch a 60-80 um deep channel, an
etching time of 7 hours was expected. During
etching, the oxide mask layer was more
resistant to KOH and protected the backside of
the wafer from etching. Upon completion of the 7
hour etching, the wafer was removed from the
solution and holder, then rinsed with deionized
water, and took for a measurement under
microscope for its etched thickness.
Stripping of Oxide Layer
The next step was to remove the remaining
oxide layer after the KOH etch. A buffered oxide
etchant (BOE) was chosen to accomplish this
task due to its controllable etching rates. A
buffered oxide etchant is a mixture of a buffering
agent and an acid. In this case, ammonium
fluoride (NH4F) and hydrofluoric acid (HF)
created the BOE used.
The wafer was placed into a Teflon tray and
immersed in BOE etchant for 1 minute. The
wafer was rinsed with DI water to stop etching
process and nitrogen-dried. The wafer was then
measured with ellipsometry for remaining oxide
layer thickness, and the 1 minute step BOE
etching was repeated 4 times until the oxide
layer was totally removed. Figure 8 shows the
equipment used in BOE etching.
Figure 5. Fabricated Silicon Wafer and Channels
Figure 6. Experimental Setup
of KOH Wet Etching
Figure 7. Schematic Diagram of KOH Wet Etching
Figure 8. BOE Etching Setup
5. It is necessary to notice that during glass
etching, special personal protective equipment
(PPE) was needed for safety purposes.
Neoprene gloves, an apron and a face shield
were required to be worn during the BOE
etching process. And during operation, we use a
Teflon tray to contain all equipment and avoid
agitating the hydrofluoric acid solution. After
etching, we rinsed the wafer twice to dilute
remaining etchant and collected liquid waste in
special containers.
Silicon Channel Bonding
To prepare our silicon channel for acoustic
testing, holes are drilled at the back of the silicon
wafer as inlets and outlets, and a thin oxide
layer with a thickness of 200 nm was grown to
make the channels hydrophillic. The front
surface of silicon micro channels were then
plasma treated and bonded to PDMS.
Glass Channel Fabrication
Photolithography
A chrome coated glass blank was used as
the starting point for micro channel fabrication.
A 0.5 um thick layer of positive photoresist AZ-
1500 was spun on the chrome layer and
softbaked. We exposed the substrate under UV
light of an intensity 7.83mW/cm2 for 16 seconds,
and the exposed photoresist received a dose
around 120mJ/cm2.
We then used developer MF-319 to develop
for exactly 55 seconds, and as AZ-1500 is a
positive photoresist, regions exposed to UV light
become preferentially soluble in the developer. It
should be noted that overdevelopment of the
unexposed photoresist is a possibility. Lastly,
we put the glass substrate into chrome etchant
1020 for approximately 1 minute
Wet Etching of Glass
After chrome etching, we used deionized
water to clean the substrate and prepared to
etch into glass. We mixed 69% (w%w) nitric
acid, 49% (w%w) hydrofluoric acid and
deionized water at a ratio of 1:2:6 as the glass
etchant; and to be specific, we put 20 ml 69%
nitric acid and 40 ml 49% hydrofluoric acid into
120ml deionized water. We first etched the chip
for 10 minutes, then rinsed with deionized water
and blew into channels with nitrogen for a
couple of times to blow away by-product
molecules accumulated at the bottom of the
channel. The channel was measured with
ellipsometry to determine etch rate, and to
produce an 80 um deep channel, we thereby
etched for another 6 minutes based on the etch
rate. After the second etching, we cleaned the
channel again with deionized water and nitrogen
spray.
Glass Channel Bonding
For sealing the glass channels, we first
drilled holes as inlets and outlets from the
backside into the glass chip with glass channels,
and then spin coated a blank glass mask with
SU-8 2025. The SU-8 is will act as an adhesive
for that will bind the glass blank to the glass
channels. The SU-8 was applied using a spin
coater with the following speed parameters:
Ramp of 100 rpm/s for 5 seconds from 0-500
rpm, then held at 6000 rpm for 60 seconds with
ramp of 300 rpm/s, and finally a ramp of down of
300 rpm/s for 10 seconds. This produced a thin
SU-8 film around 10 to 15 micrometer thick on
the glass blank. Before bonding the glass blank
to the glass channels, solvent evaporation and
exposure occurred. Figure 9 shows our glass
chip after fabrication.
Physical Vapor Deposition
Physical vapor deposition (PVD) is a
process that uses heating and sputtering to
produce a vapor of material, which is then
deposited on a surface as a thin film. In our lab,
we used PVD to deposit a thin aluminum layer
onto the glass blank. Figure 10 shows the PVD
chamber used in our lab.
Figure 9. Fabricated Glass Chip and Channels
Figure 10. Physical Vapor Deposition Chamber
6. Ellipsometry
We used J.A. Woollam Co. α-SE
Ellipsometer, which is shown in Figure 11, for
measuring oxide layer thicknesses. A laser
generator generates a laser beam onto the
oxide surface, and after reflection and refraction,
the signals are received by a detector. The film
thickness is determined by comparing the
polarization states of the incident and reflected
signals.
In our experiment, we used standard
measurement with a 73ºimpact angle. After
calibration by measuring a 60 nm silicon dioxide
substrate, we could then measure the thickness
of our oxide layers.
Micro Channel Characterization
Acoustic particle focusing and separation
testing was conducted on the glass and silicon
devices fabricated in the previous steps listed.
The device was first placed on a light source
and under a microscope for better observation.
The light source was oriented on the bottom of
the device for silicon and on top for glass. A
standing acoustic wave was generated with a
function generator with a 10 mV amplitude.
Polystyrene beads (d = 10um) were added to
the glass channels and glass beads (d = 6um)
were added to the silicon channels so focusing
could easily be observed. Since the focusing
frequency of a micro channel depends on its
geometry, we calculated a focusing frequency
for each channel under test. Then each channel
was tested and compared with its expected
focusing frequency.
Results and Discussion
Silicon Micro Channels
Oxidation of Silicon
By experience, the etch rates of silicon and
silicon dioxide in KOH solution is 20 um to 90
nm. In our case, as we desire a final silicon
channel depth of around 80 um, a minimum
silicon dioxide layer depth of 360 nm is required
to provide enough protection for silicon during its
wet etching. And we actually fabricated an 800
nm thick oxide layer to provide sufficient
protection during the wet etch.
The oxide layer had a measured thickness
of 862 nm after 80 hours. This result was
somewhat consistent with what was expected
experimentally. Since the theoretical calculation
was so far off from the experimental results, the
parameters used to estimate the time needed
has much to be desired. The layer growth could
be improved with better controls of the
temperature and other parameters. Although
the thermostat of the furnace was set to 1100°C,
the actual temperature of the chambers were
between 1000°C and 1050°C. Improving this
one factor would lead to more accurate
experimental results.
Photolithography
According to experience, reactive ion
etching etches photoresist four times as quick as
it etches silicon dioxide, so we aimed to coat a 4
um thick photoresist layer to provide enough
protection for silicon dioxide mask during dry
etching. After photolithography, the photoresist
film was measured to be 3.899 um thick, which
was close to our 4 um goal, and this layer of
photoresist would provide sufficient protection
over the oxide layer.
Reactive Ion Etching
After 15 minutes dry etch, the measured
depth change of the photoresist was 1.921 um.
The measured depth change from photoresist to
the channel base was 2.997 um. According to
equation 3, the measured etch rate of the
photoresist by RIE was 0.128 um/min and the
etch rate of the oxide was 0.03 um/min. The
selectivity of RIE was observed as a 4:1 ratio
between the photoresist and the oxide. By
extrapolating this calculation for our known
photoresist depth, it was determined that it
would take another 17 minutes to remove the
entire oxide layer.
Figure 11. J.A. Woollam Co. α-SE Ellipsometer
7. Stripping of Photoresist
The final channel depth was measured twice
by profilometry twice, and the results were 883
nm and 881 nm respectively. These values are
both greater than the 862 nm oxide layer
thickness, demonstrating that the oxide layer
was totally removed. And for the approximate 20
nm etch into the silicon wafer, it is negligible
after wet etching as the final depth is at the
order of 60-80 um.
Wet Etching of Silicon
Wet etching in silicon produced an
anisotropic profile as expected. The wet etching
in silicon resulted in final channel depths of 55.8
um, 55.1 um and 56.2 um. There was no
expected channel depth for this experiment but
the observed etch rate was around 8 um/h and it
took approximately 7 hours to obtain the final
channel depth. The expected etch rate was
approximately 20 um/h according to literature for
a stirred 45% KOH solution at 60°C [5]. The
observed etch rate was very slow compared to
literature values which could be attributed to
inconsistent heating, imprecise KOH solution or
improper sample preparation. As expected, the
observed microchannels had slanted sidewalls
at two ends and straight sidewalls along the
channel due to the anisotropic nature of the
etchant and the orientation of the channels. The
resulting micro channel had a high surface
roughness due to the hydrogen bubbles that
form on the surface during the etching process.
The hydrogen bubbles that form act as a mask,
which leads to a non-uniform material removal.
The stirring speed was not varied so no
definitive conclusions can be drawn on whether
increasing the speed would produce a smoother
surface. However, in literature, it can be
concluded that stirring and heating does improve
etch rates for all KOH molarities [6].
Stripping of Oxide Layer
We measured by ellipsometry that the
original oxide layer thickness was 348.8 nm, and
after 4 iterations of 1 minute BOE etch, the
remaining oxide layer thicknesses were 256.7
nm, 166.6 nm, 55.9 nm and 0 nm. The average
etching rate of oxide layer was 97.6 nm/min, and
throughout etching, the oxide layer had its color
shifted from geen-blue to blue to cloudy grey
and to shiny silver.
Silicon Channel Bonding
After oxide deposition and plasma
treatment, part of the silicon wafer broke when
pressed against PDMS, which is shown in
Figure 5. This is likely due to residual stresses in
the wafer plane and therefore had some
detrimental effects on final testing.
Glass Micro Channels
Wet Etching of Glass
After 10 minute’s etch, the channel was
measured with profilometry to be 52.2 um deep,
and the etch rate was around 5.2 um/min. And
after another 6 minute etch, the photolithography
and isotropic wet etching in glass resulted in a
final channel depth of 80.6 um for the wide
channel, 78.7 um for the medium channel and
60.2 um for the narrow channel.
The expected final channel depth was 80
um. The difference in channel depths may be
partly resulted from different etchants flow inside
the channels, in which case, the wider the
channel is, the more etchant flow is likely to be.
As the measured etch rate was
approximately 5 um/min and it took 16 minutes
to obtain the final channel depth. It can be
concluded that the glass etchant has a linear
etch rate through the photoresist. It can also be
concluded that extrapolating the etch rate to
obtain a desired channel depth is a highly
effective technique. The predictable nature of
the glass etchant makes fabricating
microchannels in glass a highly repeatable task.
Accuracy of the final channel depth could be
improved by taking more measurements as the
desired channel depth is approached. Due to
time constraints, we did not perform multiple
measurements. During microscopic
observation, the glass microchannel had sloped
sidewalls and a higher degree of surface
roughness. This observation was expected due
to isotropic nature of the glass etchant. It was
also observed that the glass microchannels had
larger defects but less of them when compared
with silicon microchannels. Figure 12 provides a
side-by-side comparison of the microchannels in
silicon and glass.
Micro Channel Characterization
Particle Focusing/Separation
Table 2 depicts the actual and calculated
focusing frequencies for each of the micro
channels in silicon. We were unsuccessful in
obtaining actual focusing frequencies due to
complications with our test set up and the
8. broken silicon channels. Since we were
successful in measuring glass channel focusing
frequencies, we decided to replace the silicon
channels with two more glass channels.
Table 3 depicts the actual and calculated
focusing frequencies for each of the micro
channels in glass. The data clearly shows that
the predicted focusing frequency was very close
to the actual observed focusing frequency for 4
out of the 5 channels. As expected, we
observed the larger channels to have lower
focusing frequencies.
Figure 13 shows the two observed channels
during acoustic particle focusing testing. As you
can see in Figure 13, the polystyrene beads
move towards the center of the channel when
the frequency of the sound wave was near the
focusing frequency for the channel. This
movement was consistent with our expectations
of how polystyrene beads should behave in
acoustic standing waves. This phenomenon is
called positive acoustic contrast and the beads
are moving to the nodes of the standing wave
where the lowest oscillation is occurring. We did
not observe any movement by the glass beads
in the silicon channels; however, we expected
the glass beads to exhibit the opposite behavior.
Glass beads exhibit negative acoustic contrast
in acoustic waves, which means we should have
observed the glass beads moving to zones of
highest oscillation.
We also were not able to draw any
conclusions on whether smaller or larger
particles focus faster since only the polystyrene
beads in glass channels were able to focus.
However, we expected the polystyrene or larger
particles to focus faster because they are
moving towards the center of the channel.
Since the fluid is flowing through the channel in
the laminar flow regime, a parabolic flow is
expected which means the flow near the center
will be faster than flow near the side walls.
Since the polystyrene particles exhibit positive
acoustic contrast, they will focus at the nodes of
the acoustic standing wave and thus, focus
faster in the center of the channel where flow is
fastest.
Channel
Widths
(um)
Predicted
Focusing
Frequency
(MHz)
Actual
Focusing
Frequency
(MHz)
Silicon 1 550 1.36 -
Silicon 2 663 1.13 -
Silicon 3 764 0.98 -
Channel
Widths
(um)
Predicted
Focusing
Frequency
(MHz)
Actual
Focusing
Frequency
(MHz)
Glass 1 352 2.13 2.39
Glass 2 472 1.59 1.65
Glass 3 727 1.03 -
Glass 4* 627 1.20 1.10
Glass 5* 260 2.88 3.03
Figure 12 Comparison of geometries for silicon
channel (left) and glass channel (right)
Table 3. Glass Channel Measurements
*Glass channels 4 and 5 were not made
during our lab section.
Table 2. Silicon Channel Measurements
Figure 13. Acoustic particle focusing
in glass channels (top) and silicon
channels (bottom)
9. Conclusion
The oxide growth and physical vapor
deposition results were both consistent with our
predictions. The success of the oxide growth
allowed the following fabrication steps to occur
seamlessly. The glass and silicon etched
channels were fabricated as we expectedwith no
major flaws or errors, which shows that the
techniques described are very effective. Despite
successfully fabricating these two devices,
acoustic particle focusing testing did not
impress. The silicon device did not provide any
observed particle focusing at all. Furthermore,
the glass device yielded results for four out of
the five different micro channel geometries.
Although testing struggled to produce results in
every case, the results that were obtained were
very consistent with theory. The causes for poor
testing results were most likely from improper
cleaning and human error. The devices were
tested once prior to laboratory testing in order to
troubleshoot the remaining test set up
components. Future attempts to replicate these
testing procedures should take precautions to
maintain these devices properly.
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s Photoresist Adhesion and HMDS Processing