12. Examples of a Microfluidic System
(from Sandia Laboratories)
World's Smallest Microsteam Engine
Water inside of the compression cylinder is heated by a flow of
electric current and vaporizes, pushing the piston out. Capillary
forces then retract the piston when the current is not flowing.
13. Photolithography
Photolithography process involves the use of an optical image
(on the photolitographic mask) and a photosensitive film (on the
substrate) to produce desired patterns on the substrate.
Photolithographic mask: A quartz plate with Chromium patterns
through which light can not pass.
Methods of mask fabrication : leaser writing, electron beam
writing.
The mask is then placed above the top-face of a silicon
substrate coated with thin film of photoresistive materials.
Then, the photoresist is exposed through the mask to an UV
light, X-rays or E-beam (depending on the type of the photoresist
and the process requirements).
14. Application of photoresists
● The process begins with securing the substrate wafer onto the top of a
vacuum chuck.
● A resist puddle is first applied to the center portion of the wafer from a
dispenser.
● The wafer is then subjected to high speed spinning at a rotational speed
from 1500 to 8000 rpm for 10 to 60 seconds. The speed is set depending on
the type of the resist, and on the desired thickness.
● The centrifugal forces applied to the resist puddle cause a uniform spread
of the fluid over the entire surface of the wafer.
● Typically the thickness is between 0.5 – 10 µm with ±5 nm variation.
● Pre-bake (e.g. 90°C, 60 min).
PR
Dispenser
● Exposure through the mask.
● Develop in appropriate solution.
● Post bake (e.g. 90°C, 30 min).
Wafer
Chuck
Spin Xrpm
15. The two main kinds of photoresists
Positive resist – destroy bonds, the exposed area is soluble
Negative resist – crosslinking, the exposed area is NOT soluble
16. Light sources
Photoresist materials used in micro fabrication are sensitive
to light with wavelength ranging from 300 to 500 nm.
Most popular light source for photolithography is the mercury
vapor lamps. This light source provides a wavelength
spectrum from 310 to 440 nm.
Deep UV (ultra violet) light has a wavelength of 150-300 nm.
In special applications for extremely high resolutions, x-ray is
used.
The wavelength of x-ray is in the range from 4 to 50 Angstrom.
(an Angstrom, = 0.1 nm or 10-4 µm).
21. Pure Silicon wafers
Pure silicon boules of 300 mm diameter and 30 ft
long,
can weigh up to 400 Kg.
22. Silicon Crystal Structure
● Single silicon crystals are basically of “facecubic-center” (FCC) structure.
● Total no. of atoms in a single silicon crystal = 18.
● The unsymmetrical distribution of atoms within the
crystal make pure silicon anisotropic in its
mechanical properties.
28. Isotropic Etching of Silicon
1. Isotropic Silicon etch
Isotropic etching of silicon is typically performed
using an etchant consisting of HNO3 and HF.
6HF + HNO3 + Si
H2SiF6 + HNO2 + H2O + H2
2. Isotropic Silicon etch
HNA
(160 ml acetic acid, 60 ml nitric acid, 20 ml
hydrofluoric acid)
This is an extremely aggressive acidic mixture
which will vigorously attack silicon. It is an
isotropic wet etchant which etches silicon at a rate
of approximately 1-3 microns per minute.
Substrate must be clean with hard nitride mask.
No photoresist.
29. Microsystem's manufacturing
technologies :
Bulk and Substrate micromashining
Bulk micromachining
Bulk micromachining is the oldest paradigm of silicon
based MEMS. The whole thickness of a silicon wafer is
used for building the micro-mechanical structures. Silicon
is machined using various etching processes. Anodic
bonding of glass plates or additional silicon wafers is
used for adding features in the third dimension and for
hermetic encapsulation. Bulk micromachining has been
essential in enabling high performance pressure sensors
and accelerometers that have changed the shape of the
sensor industry in the 80's and 90's.
31. Microsystem's manufacturing technologies :
Bulk and Substrate micromashining
Surface micromachining
Surface micromachining uses layers deposited on the surface of a
substrate as the structural materials, rather than using the substrate
itself. Surface micromachining was created in the late 1980s to render
micromachining of silicon more compatible with planar integrated
circuit technology, with the goal of combining MEMS and integrated
circuits on the same silicon wafer. The original surface
micromachining concept was based on thin polycrystalline silicon
layers patterned as movable mechanical structures and released by
sacrificial etching of the underlying oxide layer. Interdigital comb
electrodes were used to produce in-plane forces and to detect in-plane
movement capacitively. This MEMS paradigm has enabled the
manufacturing of low cost accelerometers for e.g. automotive air-bag
systems and other applications where low performance and/or high granges are sufficient. Analog Devices have pioneered the
industrialization of surface micromachining and have realized the cointegration of MEMS and integrated circuits.
33. Surface Micromachining
Most commonly used materials for surface micromachining:
• substrate:
silicon
• sacrificial material:
SiO2
• structural material:
polysilicon, Si3N4
Alternative materials :
Substrates
Sacrificial
Structural
Glass
Plastic
metals
Polymer
Metals
silicon nitride
Thin film silicon (a-Si:H, c-Si)
silicon nitrides
Silicon carbide
Metals
polymers
bilayer composites
34. Surface micromachining on glass
Sacrificial Layer D
eposition and Patterning
Structural Layer D
eposition and Patterning
Sacrificial Layer R oval
em
d=1 m; h=500 nm; b=10 m
Lmax(bridge) ~ 60 m ; Lmax(cantilever) ~ 30 m
36. Etching with BHF of SiO2 or glass
Buffered oxide etch (BOE), also known as buffered HF or BHF, is a wet
etchant used in microfabrication. Its primary use is in etching thin films
of silicon dioxide (SiO2) or silicon nitride (Si3N4).
It is comprised of a mixture of a buffering agent, such as ammonium
fluoride (NH4F), and hydrofluoric acid (HF). Concentrated HF (typically
49% water) etches silicon dioxide too quickly for good process control.
Buffered oxide etch is commonly used for more controllable etching.
Some oxides produce insoluble products in HF solutions. Thus, HCl is
often added to BHF solutions in order to dissolve these insoluble
products and produce a higher quality etch.
A common buffered oxide etch solution comprises a 6:1 volume ratio of
40% NH4F in water to 49% HF in water. This solution will etch thermally
grown oxide at approximately 2 nanometres per second at 25 degrees
Celsius.
37. Example : etchnig of microfluidic channels
in soda-lime-silica glass
Technologie optimized in our group :
xº‚ x#Û#¸{"O¸¥GB�Yø####opendo
1) Substrat of soda-lime-silica glass of
Corning.
2) Cleaning in « piranha mixture » :
perhydrol : H2SO4
3) Deposition (by sputtering) of Cr mask
and of a photoresist
4) Photolithography.
5) Selective etching of the Cr
layer(commercial chromium etchant).
6) Etching of glass substrate in BHF + HCl
7) Etchnig of remaining Cr layer.
38. Example : etchnig of microfluidic channels
in soda-lime-silica glass
Etching in BHF
Etching in BHF + HCl
Channel width : 70 µm
39. Dry Etching
Dry etching involves the removal of substrate materials
by gaseous etchants without wet chemical or rinsing.
Dry etching is one of the core processes in microsystem's
technology.
Compared to wet etching RIE enables new possibilities such as
etching of vertical structures independent of the crystal structure
in the material.
The dry etching also leaves you with the possibility to etch delicate
structures without exposing them to a liquid that might ruin the
structures by breakage or sticking particles.
Several dry etching techniques :
1) The etching of silicon with XeF2 vapor (no plasma)
2) Plasma Etching (PE)
3) Reactive Ion Etching (RIE)
4) Deep Reactive Ion Etching (DRIE)
5) Inductive Coupled Plasma (ICP)
6) Focused Ion Beam (FIB) or Ion Milling
40. Potential distribution
in the Plasma Reactor
The etching mechanisms can be influenced by ion bombardment. Ion bombardment
is caused by positive ions which are accelerated by the negative DC voltage
towards the LOWER electrode and the wafer placed on this electrode.
The vertical etch rate is increased, while the horizontal etch rate remains constant.
For this reason, it is possible to obtain an anisotropic etching with plasmas, even
for noncrystalline structures, We go from « PE etchning conditions » to «RIE »
etching conditions. However, the most common mechanism to obtain an
anisotropic vertical etch process is through the use of a passivation layer at the
vertical surfaces (DRIE process).
41. Example of a capacitively coupled plasma
(CCP) reactor
Capacitively coupled RF plasmas are still the most common plasmas used in
dry etching. A typical reactor chamber is shown above. The power is applied
to the lower or the upper electrode (or in some special cases to the reactor
walls ). In general the frequency of the applied power is 13.56 MHz.
43. (PE) Plasma Etching in practice
standard method for low damage isotropic etch
available as large batch system
substrate electrode on ground potential, cooled
top electrode RF driven (13.56 MHz)
shower head gas inlet (in the top electrode)
parameter: gas flows, pressure, RF power
typical process pressure: 200 - 1.000 mtorr
low ion energies ( 3 - 20 eV)
Typical Applications :
- isotropic low damage
- photoresist stripping
- isotropic SiN removal
- plasma cleaning
44. Reactive Ion Etching (RIE)
In reactive ion etching (RIE), the substrate is placed inside a reactor in
which several gases are introduced.
A plasma is struck in the gas mixture using an RF power source,
breaking the gas molecules into ions. The ions are accelerated towards,
and react with, the surface of the material being etched, forming another
gaseous material. This is known as the chemical part of reactive ion
etching.
There is also a physical part which is similar in nature to the sputtering
deposition process. If the ions have high enough energy, they can knock
atoms out of the material to be etched without a chemical reaction.
It is a very complex task to develop dry etch processes that balance
chemical and physical etching, since there are many parameters to
adjust.
By changing the balance it is possible to influence the anisotropy of the
etching, since the chemical part is isotropic and the physical part highly
anisotropic the combination can form sidewalls that have shapes from
rounded to vertical. Another versionn of RIE can be deep and its name
will be Deep RIE or DRIE Deep reactive ion etching (DRIE)
45. Deep Reactive Ion Etching (DRIE)
DRIE is a highly anisotropic etch process used to create deep holes and
trenches in wafers, with aspect ratios of 30:1 or more.
It was developed for microelectromechanical systems (MEMS), which require
these features, but is also used to excavate for creating through wafer via's in
advanced 3D wafer level packaging technology.
The primary technology is based on the so-called "Bosch process", which
can fabricate truly vertical walls.
The Bosch process, also known as pulsed or time-multiplexed etching,
alternates repeatedly 3 modes to achieve nearly vertical structures.
What also distinguishes DRIE from RIE is etch depth: Practical etch depths
for RIE is limited to around 10 µm at a rate up to 1 µm/min, while DRIE can
etch features much greater, up to 1000 µm or more with rates up to 20 µm/min
or more in some applications
Mostly we are doing RIE of dielectric films and DRIE of silicon.
47. Deep Reactive Ion Etching (DRIE)
EXAMPLE
DRIE is a high aspect ratio, deep
trench silicon etching process
(Bosch process). The principle of the
deep trench silicon etching process
is an alternating fluorine based
etching and passivation of the
structures. Masking layers can be
made of photo resist or silicon oxide.
Main Benefits of DRIE
- etch rate of up to 10 µm/min
- aspect ratio up to 40:1
- selectivity to positive resist > 75:1
- selectivity to silicon oxide >150:1
- etch depth capability 10 to 675 µm
(through wafer etching)
- sidewall profile 90°±1°
- eature size 1 to >500 µm
48. ICP Deep Plasma Etching of
Polymethylmethacrylate (PMMA) to a depth of 120
microns
50. Focused Ion Beam (FIB)
Focused ion beam (FIB) systems have been produced commercially
for approximately twenty years, primarily for large semiconductor
manufacturers.
FIB systems operate in a similar fashion to a scanning electron
microscope (SEM) except, that FIB systems use a finely focused
beam of ions (usually gallium) that can be operated at low beam
currents for imaging or high beam currents for site specific
sputtering or deposition.
Why ions ?
In summary, ions are positive, large, heavy and slow, whereas
electrons are negative, small, light and fast. The most important
consequence of the properties listed above is that ion beams will
remove atoms from the substrate and because the beam position,
and size are so well controlled it can be applied to remove material
locally in a highly controlled manner, down to the nanometer scale.
57. Evaporation
Vacuum thermal evaporation (VTA):
Uses resistive heating, laser
heating, or magnetic induction to
elevate the source temperature.
Electron beam evaporation (EBE):
The electron beamis focused on the
target material, which locally melts.
59. Chemical Vapor Deposition
● Chemical vapor deposition (CVD) is the most important process in
Microfabrication.
● It is used extensively for producing thin films by depositing many different
kind of foreign materials over the surface of silicon substrates, or over other
thin films that have already been deposited to the silicon substrate.
● Materials for CVD may include:
(a) Metals: Al, Ag, Au, W, Cu, Pt and Sn.
(b) Other materials: Al2O3, polysilicon, SiO2, Si3N4, piezoelectric
ZnO...
● There are three (3) available CVD processes in microfabrication:
(a) APCVD: (Atmospheric-pressure CVD)
(b) LPCVD (Low-pressure CVD),
(c) PECVD (Plasma-enhanced CVD).
● CVD usually takes place at elevated temperatures
61. Plasma Enhanced CVD (PECVD)
● Both APCVD and LPCVD operate at elevated temperatures, which
often
damage the silicon substrates. High substrate surface temperature
is required to provide sufficient
energy for diffusion and chemical reactions.
● The operating temperatures may be
avoided if alternative form of energy
supply can be found.
● CVD using plasmas generated from high
energy RF (radio-frequency) sources
is one of such alternative methods.
● This popular deposition method is called
“Plasma Enhanced CVD” or PECVD.
● A typical PECVD reactor is shown here :
73. Polymer structuring methods
Casting
Casting is a manufacturing process by which a liquid material is poured
into a mold, which contains a hollow cavity of the desired shape, and then
allowed to solidify. The solidified part is also known as a casting, which is
ejected or broken out of the mold to complete the process. A typical
casting material used in microfluidics is PDMS.
Hot embossing
Hot embossing is a technique to produce microstructures in thermoplastic
polymers. During the process the substrate is heated slightly above the
glass transition point. Then a structuring tool is pressed on the polymer.
This tool is a stamp that has the microstructure that will be transferred on
the substrate. After the polymer has cooled down, the tool is removed and
the structure stays in the substrate.
Injection moulding
Injection moulding is a manufacturing process for producing parts from
both thermoplastic and thermosetting plastic materials. Material is fed into
a heated barrel, mixed, and forced into a mold cavity where it cools and
hardens to the configuration of the mold cavity.
75. Polymethylmethacrylat – PMMA
- Often use as an alternative to glass
- Easily scratched
- Not malleable
- It can come in the form of a powder mixed with liquid methyl
methacrylate (MMA), which is an irritant and possible carcinogen
Fabrication of high aspect ratio structures requires the use of a photoresist
able to form a mold with vertical sidewalls. Thus the photoresist should
have a high selectivity between the exposed and the unexposed area in the
developer. It should be relatively free from stress when applied in thick
layers necessary to make high aspect ratio structures. PMMA is the
photoresist of choice in the LIGA process, mainly for its ability to hold
vertical sidewalls for tall structures.
Part of microsystems, in particular in microfluidics and micro-optics..
In polymer-based microfabrication techniques, microinjection molding is
most popular and generally used for micromolding in the industry. However,
compared to the microinjection molding, hot embossing provides several
advantages such as a relatively low-cost for embossing tools, simple
operation and higher accuracy in the replication of small features.
78. Cycloolefincopolymer – COC
Cyclic olefin copolymer (COC) draws attention as a primary
substrate material for the microfluidic lab on a chip (LOC)
applications based on its various advantages with regards to
physical and chemical properties.
For the replication of small structures (especially less than 10
μm), hot embossing is preferable and provides the simple
fabrication process compared to the injection molding
technique.
However, as far as productivity is concerned, an injection
molding process is the most effective replication technique for
the polymer based LOC which has microstructures with feature
size greater than 10 μm.
For the injection molding of the COC LOC, the fabrication
process of nickel mold inserts is introduced based on UVphotolithography and subsequent electroplating process.
80. Polydimethylsiloxane – PDMS
- Silicon-based, organic polymer
- Non-toxic
- Non-flammable
- Gas permeable
- Most organic solvents can diffuse and cause
it to swell
PDMS is commonly used as a stamp resin in the procedure of
soft lithography, The process of soft lithography consists of
creating an elastic stamp, which enables the transfer of
patterns of only a few nanometers in size onto glass, silicon or
polymer surfaces. With this type of technique, it is possible to
produce devices that can be used in the areas of microsystems
for optic telecommunications or biomedical research. The
resolution depends on the mask used and can reach 6 nm.
PDMS is commonly used as a material for fast prototyping of
microfluidic devices.
85. Stereolithography
Stereolithography is a common rapid manufacturing and rapid prototyping technology
for producing parts with high accuracy and good surface finish. Stereolithography is an
additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin"
and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a
part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser
light cures, or, solidifies the pattern traced on the resin and adheres it to the layer
below. After a pattern has been traced, the elevator platform descends by a single layer
thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006"). Then, a resin-filled blade
sweeps across the part cross section, re-coating it with fresh material. On this new
liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A
complete 3-D part is formed by this process. After building, parts are cleaned of excess
resin by immersion in a chemical bath and then cured in a UV oven.
86. Structuring methods
Laser photo ablation
Laser photo ablation is a direct micromachining method. It is based on the removal of
polymer material by using intense UV or infrared irradiation provided by a laser. The
radiation wavelength used affects the removal mechanism. If infrared lasers are used,
the irradiated material is heated and decomposes, leaving a void in the polymer material.
If UV radiation is used, the irradiated polymer decomposes, presumably of a mixture of
two mechanisms: thermal and direct bond breaking. Thermal bond breaking is induced
by heat, as with infrared irradiation. In direct bond breaking, polymer molecules directly
absorb ultraviolet photons, often absorbing enough energy so that the chemical bonds
within the polymer chains are broken.
Micro-milling
Micro milling is a mechanical way to structure a substrate. It works like milling, but on a
microscopic scale. The material is removed by a fast rotating cutter. By moving this tool,
the structure is created on the substrate.
Micro-sandblasting
Sandblasting is a generic term for the process of smoothing, shaping and cleaning a
hard surface by forcing solid particles across that surface at high speeds. Micro
sandblasting uses air to create structures on a micrometric scale.
87. Laser Micromachining of Microfluidic Devices
in Glass Substrates
SEM image of a laser machined microfluidic pattern in
glass. Left Bar denotes 1mm, Right bar denotes 0.1mm
89. Nanopatterning
Nanopatterning can be defined as nanolitography techiques that do not
use mask or direct radiation writing to expose a resist layer, but imprint
a nanoscale patern by means of mechanical tools. When the physical
action is based on preasure assisted by heat or light, the techniques
are called NIL (Nano-Imprint-Litography). Examples :
90. Soft Litography,
Microcontact Printing (µCP)
Contrary to NIL, the µCP stamp is "inked" with a product which form
a self-assembled monolayers.Self-assembly is ubiquitous in nature.
Self-assembly is the spontaneous organization of molecules into
stable, well defined structires by noncovalent forces.
Another difference between NIL and µCP is that in the case of µCP
the material is deposited from the stemp on the substrate.
Furthermore, this material can be chosen over a wide range of
materials, which can not be "imprinted" by NIL.
It is conceivable to modify a NIL machine and make it working for
µCP.
Soft Lithography was pioneered by George Whitesides at MIT and David
Beebe at University of Wisconsin.
94. µCP – Fabrication of microfluidic
Channels in glass substrates
µCP is performed using patterned PDMS stamp inked in alkanethiols (e.g. Hexadecanethiol - HDT), that forms self-assembled
monolayers (SAMs) on glass substrates possessing on the top
layers of Cr and Ag (our own experience).
95. µCP – Fabrication of microfluidic
Channels in glass substrates
Top view of the glass etched channel cross-junction and
reservoir (optical microscope).
(our
own
98. Micro-contact
Printing (µCP)
View of the Microcontact Printer. The microscope slide is placed in the
substrate holder, which has three degrees of freedom (YZX) to position and
align the pattern of the stamp on the substrate. The red PDMS stamp,
placed in a stamp holder with two degrees of freedom (YZX), is made
parallel to the substrate. The stamp is inked by means of the miniaturized
pulverization head and is guided toward the substrate by a linear slide
actuated with a pneumatic jack. The load on the stamp during the contact
with the substrate is controlled via air pressure from the piston. The stamp
is released from the substrate by reversing the direction of airflow in the
piston (our own experience).
101. Bonding
Adhesive bonding
Adhesive Bonding is a modern joining process in which a liquid or semi
liquid substance is applied to adjoining work pieces to provide a long lasting
bond. This process is highly useful in bonding dis-similar materials that can
not be welded. Materials that have the ability to be bonded together are
virtually unlimited. Adhesives used in bonding can exist in many forms and
be made from various natural and/or artificial compounds. A hindrance to this
process is that adhesive bonds are not instantaneous such as welding or
nailing. Adhesive bonds take more time to process, in order to allow the
adhesives to cure.
Anodic bonding
Anodic bonding is the bonding of two substrates , usually glass and silicon,
by an electrical potential. The substrates are placed between two electrodes
and at temperatures around 400° C a high potential (around 1 kV) is applied to
the substrates. This forces sodium ions in the glass to move away from the
bonding surface. Therefore the surface is highly reactive and bonds easily to
the other substrate
102. Bonding
Direct bonding
The substrates are first exposed to a chemical treatment, for example with a
mixture of hydrogen peroxide and sulphuric acid. This makes them
hydrophilic what is essential for the bonding. By a high contact force, two
substrates (e.g two silicon wafers) are compressed. The materials get so
close, that molecular adhesive forces begin to act. The material then is
annealed at high temperature so that the direct bonding is strong enough to
keep the substrates together.
Plasma bonding
Plasma activation has to be done to improve adhesion properties of surfaces
prior to coating. Weakly ionised oxygen plasma is used. Plasma removes
surface layers with the lowest molecular weight, at the same time it oxidises
the uppermost atomic layer of the polymer. Oxygen radicals help break up
bonds and promote the three dimensional bonding of molecules. Oxidation of
the polymer is responsible for the increase in polar groups which is related to
the adhesion properties of the polymer surface. After having exposed the
substrates to the oxygen plasma they are pressed together and then heated
to a temperature slightly below the glass transition temperature.
103. Bonding
Roll laminating
In roll laminating, two polymer foils are bonded. Usually one of them is
structured. Roll laminating is a continuous process and therefore suitable
for mass production. The rolls of foil are unwound during the process and
pressed together. High temperature and pressure are usually necessary but
variable. Optionally, chemical agents may be added.
Thermal bonding
Thermal bonding exploits the fact that thermoplastic polymers become soft
at elevated temperatures close to the glass transition temperature.
Therefore the substrates to be bonded are heated and then pressed
together. As the polymer is soft, bindings between the two layers establish.
Care must be taken to choose the right temperature to bond the substrate
without damaging the microstructure.
104. One can say :
Micro-fabrication is DIFFICULT: Many of
them are still vibrant fields of
research…
Micro-fabrication is EXPENSIVE at
developing stage...
Micro-fabrication is expected to be EASY
and CHEAP at high volume production...
