521218A JOHDATUS
MIKROVALMISTUSTEKNIIKOIHIN
(INTRODUCTION TO MICROFABRICATION METHODS)
2.5 Fabrication of micromechanical structures (on silicon)
Materials, manufacturing & applications
Merja Teirikangas
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JMVT 2015_8
CONTENT
1.
2.
Introduction
Silicon based technologies
1. Silicon
2. Thin Films & Epitaxy
3. Basic processes
4. Component fabrication
5. MEMS
6. Interconnection & isolation
7. Wafer bonding & Package
3. Additive manufacturing
1. Traditional printed electronics
2. Printed electronics novel and new trends
3. Polymer based
4. Nanotechnology
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9. CONTENCT
1. Intro
2. Silicon based fabrication methods
• Materials used in MEMS
• Bulk micromachining:
• Dry/ Wet etching
• Wafer bonding
• Surface micromachining
• Basic
• Integration with electronics
3. Polymer based microfabrication
4. Other fabrication methods
• LIGA
• Deep reactive ion etching (DRIE)
• Hot embossing
5. Structures
• Cantilever, membrane, micromotor
6. Applications
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9.1 INTRO
• Micro-Electro-Mechanical Systems, MEMS, is a technology that in its most
general form can be defined as miniaturized mechanical and electromechanical elements that are made using the techniques of
microfabrication
• MEMS fabrication uses many of the same techniques that are used in the
integrated circuit domain such as oxidation, diffusion, ion implantation,
LPCVD, sputtering, etc., and combines these capabilities with highly
specialized micromachining processes
• The functional elements of MEMS are miniaturized structures, sensors,
actuators, and microelectronics, the most notable elements are the
microsensors and microactuators
- The dimensional spectrum of the microstructures that can be fabricated using
these techniques spans from 1 mm to 1 μm
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9.1 INTRO
• Traditional silicon micromachining technology
• Common microfabrication technology for IC
• Bulk micromachining (kappale mikrotyöstö)
• Etching, bonding, planarization
• Surface micromachining (pinta mikrotyöstö)
• Suspended structures, antistiction methods, 3D
microstructures
• Methods for merging micromechanics and IC
• High – Aspect ratio micromachining
• LIGA
• Deep reactive ion etching (DRIE)
• Polymer based microfabrication
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9.2.1 FABRICATION METHODS FOR SILICON BASED MEMS
MATERIALS
• Silicon carbide (SiC) has intrinsic properties that make it a material of great interest for microelectronic
and MEMS
• SiC-based thin films, such as SiCN, SiCO, SiCNO, SiCB, SiCBN and SiCP, have been extensively used
- in electronic and MEMS devices either as a semiconductor or as an insulator, depending on the film composition.
• Substrates
-Silicon
-Glass
-Quartz
• Thin Films
-Polysilicon
-Silicon Dioxide/ Silicon Nitride
-Metals
-Polymers
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9.2.2 INTRO - BULK MICROMACHINING
• Bulk micromachining (kappalemikrotyöstö) is the oldest of MEMS technology
• Technique involves the selective removal of the substrate material in order to realize miniaturized
mechanical components.
• Bulk micromachining can be accomplished using chemical or physical means, with chemical means being
far more widely used in the MEMS industry
• It is currently by far the most commercially successful one, helping to manufacture devices such as
pressure sensors and inkjet printheads
• The most important microfabrication techniques used in bulk micromachining are wet and dry etching
and substrate bonding
• Basic concept in bulk micromachining is selective removal of the substrate (silicon, glass, GaAs, etc.)
micromechanical components such as beams, plates, and membranes which can be used to fabricate a variety
of sensors and actuators
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9.2.2 INTRO - SURFACE MICROMACHINING
• Surface micromachining is another very popular technology used for the fabrication of MEMS devices.
• Number of variations of how surface micromachining is performed, depending on the materials and
etchant combinations that are used.
• A sequence of steps
- Deposition of thin-film material to act as a temporary mechanical layer (sacrificial layer) onto which the actual
device layers are built;
- Deposition and patterning of the thin-film device layer of material which is referred to as the structural layer;
- Removal of the temporary layer to release the mechanical structure layer from the constraint of the underlying
layer, thereby allowing the structural layer to move
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9.2.2 INTRO - SURFACE MICROMACHINING
• Surface micromachining provides for precise dimensional control in the vertical direction.
- Due to the structural and sacrificial layer thicknesses that are defined by deposited film thicknesses which can be
accurately controlled.
• Also provides for precise dimensional control in the horizontal direction
- the structural layer tolerance is defined by the fidelity of the photolithography and etch processes used.
• Other benefits of surface micromachining are that a large variety of structure, sacrificial and etchant
combinations can be used;
- some are compatible with microelectronics devices to enable integrated MEMS devices.
• Surface micromachining frequently exploits the deposition characteristics of thin-films such as conformal
coverage using LPCVD.
• Surface micromachining uses single-sided wafer processing and is relatively simple.
 higher integration density and lower resultant per die cost compared to bulk micromachining.
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JMVT 2015_8
9.2.2 INTRO - SURFACE MICROMACHINING
• Disadvantages of surface micromachining is that the mechanical properties of most deposited thin-films
are usually unknown and must be measured.
- The reproducibility of the mechanical properties in these films can be difficult to achieve.
- The release of the structural layer can be difficult due to a stiction effect whereby the structural layer is pulled
down and stuck to the underlying substrate due to capillary forces during release
• The most commonly used surface micromachining process and material combination is a PSG sacrificial
layer, a doped polysilicon structural layer, and the use of Hydrofluoric acid as the etchant to remove the
PSG sacrificial layer and release the device.
- This type of surface micromachining process is used to fabricate the Analog Devices integrated MEMS
accelerometer device used for crash airbag deployment.
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9.2.2 BULK MICROMACHINING – ETCHING OF SILICON
• Relatively deep wet etching process on single-crystal silicon substrate
• Several structures that can be formed in single-crystal silicon substrate by: (a) (110) silicon and (b) (100)
silicon
• Etching modes
• Isotropic vs. anisotropic
• Reaction-limited
• Etch rate dependent on temperature
• Diffusion-limited
• Etch rate dependent on mixing
• Also dependent on layout and geometry, “loading”
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9.2.2 BULK MICROMACHINING –ETCHING
• In bulk micromachining it is desirable to make thin membranes of silicon or control the etch depths very
precisely.
• As with any chemical process, the uniformity of the etching can vary across the substrate, making this
difficult.
• To allow a higher level of precision in anisotropic etching the MEMS field has developed solutions to this
problem with etch stops.
• Etch stops are very useful to control the etching process and provide uniform etch depths across the
wafer, from wafer to wafer, and from wafer lot to wafer lot.
• There are two basic types of etch stop methods that are used in micromachining: dopant etch stops and
electrochemical etch stops.
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9.2.2 BULK MICROMACHINING – WET ETCHING
• Chemical wet etching is popular in MEMS because it can
provide a very high etch rate and selectivity
• Etch rates and selectivity can be modified by
-
altering the chemical composition of the etch solution
adjusting the etch solution temperature
modifying the dopant concentration of the substrate
modifying which crystallographic planes of the substrate are
exposed to the etchant solution
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9.2.2 BULK MICROMACHINING – DRY ETCHING
• Use of anisotropic wet etchants to remove silicon can be marked as the beginning of the micromachining
era
• Back-side etch was used to create movable structures such as beams, membranes, and plates
• The creation of thin structures (< 20 μ m) requires the use of various etch-stop techniques
• heavily boron-doped regions and electrochemical bias can be used to slow down the etch process drastically and
hence create controllable thickness microstructures
Fig. Wet anisotropic silicon back-side etch
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9.2.2 BULK MICROMACHINING – DRY ETCHING
p++ etch-stop
• requires epitaxial growth of a lightly doped region on top of a
p++ etch-stop layer
• can also be used to create isolated thin silicon structures
through the dissolution of the entire lightly doped region
- fabricate silicon recording and stimulating electrodes for
biomedical applications
• Combination of p++ etch-stop layers and silicon–glass anodic
bonding has also been developed
 Dissolved wafer process and it has been used to fabricate a
variety of microsensors and microactuators
Dissolved wafer process
sequence:
(a) KOH etch,
(b) deep B diffusion,
(c) shallow B diffusion
(d) silicon–glass anodic bond
(e) release in EDP
Wet micromachining etch-stop techniques:
(a) electrochemical with n-epi on psubstrate,
(b) p++ Etch stop with n-epi, and
(c) p++ etch stop without n-epi
Free-standing microstructure
fabrication using deep and
shallow boron diffusion and EDP
release
(a) silicon wafer, (b) deep and
shallow boron diffusion, and
(c) EDP etch
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9.2.3 SURFACE MICROMACHINING
• The surface micromachined structures are constructed by
- Structural layer
- Sacrificial layer
• After the sacrificial layer is removed, the structural layer will be released from the
substrate
• Space between components is limited by the thickness of the sacrificial layer.
• Advantage of surface micromachining is its capability of fabricating movable micro
machined structures with geometries less restricted through the conventional IC
process
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9.2.3 SURFACE MICROMACHINING– PROCESSES
• A variation of the surface micromachining process is to use
- a metal structural layer,
- a polymer layer as the sacrificial layer,
- an O2 plasma as the etchant.
• The advantage of this process is that the low deposition temperature of the sacrificial and structural layer
- not to degrade any microelectronics in the underlying silicon substrate
- integrating MEMS with electronics
• Also, since the sacrificial layer is removed without immersion in a liquid, problems associated with stiction during
release are avoided.
• Uses structural layers and sacrificial layers (example car crash sensor)
Aluminum
Nitride
(insulator)
n+ silicon (conductor)
Electrical
contact
to wafer
C
A
d
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9.2.4 INTEGRATION – BULK MICROMACHINING
• It is also possible to merge wet bulk micromachining and microelectronics fabrication processes to build
micromechanical components on the same substrate as the integrated circuits (CMOS, bipolar, or biopolar
complementary metal oxide semiconductor
• It allows the integration of interface and signal-processing circuitry with MEMS structures on a single chip
• Post-processing of CMOS integrated circuits by front-side etching in TMAH solutions (silicon-rich TMAH does not
attack aluminum and therefore can be used) to undercut microstructures in an already processed CMOS chip.
• used to fabricate a variety of microsensors (e.g., humidity, gas, chemical, and pressure)
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9.2.4 INTEGRATION - SURFACE MICROMACHINING
• Integration of surface-micromachined structures with on-chip
• The two most common techniques are MEMS-first and MEMS-last techniques.
- In the MEMS-last technique, the integrated circuit is first fabricated and surface-micromachined structures are
subsequently built on top of the silicon wafer.
- The MEMS-first technique fabricate the microstructures at the very be-ginning of the process. [if the
microstructures are processed first, they have to be buried in a sealed trench to eliminate the interference of
microstructures with subsequent CMOS processes.]
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9.2.5 SURFACE VS BULK MICROMACHINING
BULK MICROMACHINING
SURFACE MICROMACHINING
• Portion of substrate removed to create
structure.
- Microturbine
- Microchannels for on-chip cooling system
- AFM tip
• Structural and sacrificial layers created on wafer
surface.
• Sacrificial layers removed to free structures.
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9.2.5 SURFACE VS BULK MICROMACHINING
• Basic description of micromachining processes steps
(a) Bulk
micromachining
(b) surface
micromachining
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OTHER TECHNIQUES – DRIE
• Deep reactive ion etching (DRIE) is a relatively new fabrication technology that has been adopted by the
MEMS community.
• This technology enables very high aspect ratio etches to be performed into silicon substrates.
• The sidewalls of the etched holes are nearly vertical and the depth of the etch can be hundreds or even
thousands of microns into the silicon substrate.
• The sidewalls are not perfected or optically smooth and if the sidewall is magnified under SEM inspection, a
characteristic washboard or scalloping pattern is seen in the sidewalls.
• The etch rates on most commercial DRIE systems varies from 1 to 4 microns per minute
• The aspect ratio of the etch can be as high as 30 to 1, but in practice tends to be 15 to 1.
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9.5 STRUCTURES
• Cantilevers
• Membranes
• Micro motors
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STRUCTURES – CANTILEVERS & MEMBRANE
(BULK MICROMACHINING )
• Surface micromachining can be used to construct cantilevers, overhangs, and similar structures on a
silicon substrate
- The cantilevered beams are parallel to but separated by a gap from the silicon surface
- Gap size and beam thickness are in the micron range
Cantilever
1)
2)
3)
4)
5)
SiO2 layer formed on Si
substrate - thickness will
set the gap size for the
cantilever
portions of the SiO2 layer
are etched using
lithography
poly-Si layer is applied
portions of the poly-Si
layer are etched using
lithography
SiO2 layer beneath the
cantilevers is selectively
etched
Thin membrane
1)
2)
3)
4)
5)
Si substrate is doped
with boron
thick layer of Si is
applied on doped layer
by epitaxial deposition
both sides are thermally
oxidized to form a SiO2
resist on the surfaces,
resist is patterned by
lithography,
anisotropic etching
removes the Si except in
the boron doped layer
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STRUCTURES – MICRO MOTOR
1. Starting wafer
- Start with blank silicon wafer. Wafer orientation is not critical. The thickness of the wafer is
typically 0.3-0.5 mm.
2. Deposition of sacrificial layer
- Deposit silicon oxide film as the sacrificial layer.
- conformal coating
- thickness 1-3 micrometers
3. Deposition of structural layer
- Deposit polycrystalline silicon film as the structural layer.
- conformal coating
4. Pattern the top polysilicon layer
- Pattern the silicon layer with the first mask to form the shape of the rotor and the hole for
the anchor.
5. Deposit a second sacrificial layer
- Conformal deposition of P-doped oxide again
6. Pattern and Etch the sacrificial layers
- Pattern the wafer with the photoresist layer and the first mask.
- Using HF solutions to etch through the two oxide layers.
- Lateral etching will occur and the dimension control is critical
7. Deposit polysilicon structural layer
- Conformal deposition of polysilicon again.
8. Pattern Polysilicon
- Pattern the top layer polysilicon to form the confinement structure and anchor.
9. Sacrificial layer removal and freeing of structures
- Remove the oxide using 49% HF solutions, which etches oxide fast (1 micron/ minute) and
the polysilicon slowly.
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APPLICATIONS
• Biotechnology
- Polymerase Chain Reaction (PCR) microsystems for DNA amplification and
identification, enzyme linked immunosorbent assay (ELISA), capillary
electrophoresis, electroporation, micromachined Scanning Tunneling
Microscopes (STMs), biochips for detection of hazardous chemical and
biological agents, and microsystems for high-throughput drug screening
and selection.
• Medicine
- Largest market for MEMS pressure sensors in the medical sector is the
disposable sensor used to monitor blood pressure in IV lines of patients in
intensive care
- Pressure sensors (blood pressure and respiration)
• Communications
- RF-MEMS technology
- Resonators as mechanical filters
• Inertial Sensing
- Accelerometers and gyroscopes
Figure 5: Polysilicon micromotor fabricated
using a surface micromachining process.
Polysilicon
resonator
structure
fabricated using a
surface
micromachining
process.
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APPLICATIONS - PIEZORESISTIVE AND CAPACITIVE SENSORS
• Bulk micromachined pressure sensors
• Piezoresistivity – change in electrical resistance due to
mechanical stress
• In response to pressure load on thin Si film, piezoresistive
elements change resistance
• Membrane deflection < 1 µm
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APPLICATION - LATERAL RESONATORS
• The single crystalline (100) SiC film was produced by APCVD
whereas the polycrystalline (111) SiC film by LPCVD
• Both films were deposited on SiO2 /Si substrates
• 3C-SiC lateral resonators exhibit a resonant frequency
similar to polysilicon devices and temperature coefficient
of 22 ppm/ºC comparable to quartz oscillators (from 14 to
100 ppm/ºC)
Lateral resonator
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APPLICATIONS - RF MEMS SWITCH
• This model uses a beam that is made of two materials:
• the SiC film to give mechanical stability and the Au to provide the conducting path to the ground.
• A process using four masks was employed to fabricate it
• high-resistivity p-type (100) Si wafer with 1.0 µm thermal SiO2 was used as substrate
• 800 nm of Au was deposited and patterned using lift-off to define the coplanar waveguide (CPW), to form the
switch
• 1.5 µm of polyimide sacrificial layer was patterned to define the anchors
• The anchors and the beam are made by depositing a 0.3 µm and 0.9 µm layers of SiC and Au, subsequent etching
of the polyimide sacrificial layer
• The switch exhibited an isolation of -40 db at 10 GHz and pull down voltage of 3 V.
Switch
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