MEMS

1

• MEMS
• Class 5
• Micromachining Technologies
• Mohammad Kilani

2
Photolithography Steps, Home Work 1
Surface Cleaning
Removal of particulates, organic films, adsorbed
metal ions
Adhesion Promoter
Sometimes used to achieve better adhesion of the
resist
Thickness varies with rotational speed of the
spinner and viscosity of the resist
Resist Application
Prebake (Soft Bake)
70 C 90 ºC, necessary to drive solvent out of
the resist
Exposure
Contact/proximity printing, projection printing
Developing
Negative resist solvent positive resist
alkaline developer
90 ºC 140 ºC, necessary to increase both
adherence and etch resistance
Post Bake
Etching/deposition/doping
Stripping solutions, plasma etching in oxygene
atmosphere
Resist Removal
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Surface Cleaning

• Why is it important?
• How is it done?
• Physics/chemistry?
• Parameters affecting it?

Report 10 min. PowerPoint presentation
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Micromachining Technologies

• Lithographic Technologies
• Bulk Micromachining
• Surface Micromachining
• LIGA
• Nonlithographic Technologies
• Ultraprecision Mechanical Machining
• Laser Machining
• Electrodischarge Machining
• Screen Printing
• Microcontact Printing
• Nanoimprint Lithography
• Hot Embossing
• Ultrasonic Machining

5
Bulk Micromachining

• Realize micromechanical structures within the
  bulk of a single-crystal silicon wafer by
  selectively removing (etching) wafer material.
• Significant amounts of silicon are removed from a
  substrate to form membranes and a variety of
  trenches, holes, or other structures.
• It emerged in the early 1960s and has been used
  since then in the fabrication of different
  microstructures.
• It is utilized in the manufacturing of the
  majority of commercial devices almost all
  pressure sensors and silicon valves and 90 of
  silicon accelerometers.
• The microstructures fabricated may cover the
  thickness range from submicron to full wafer
  thickness (200 to 500 µm) and the lateral size
  range from submicron to the lateral dimensions of
  a full wafer.
• Can be divided into wet etching and dry etching
  of silicon according to the phase of etchants.
  Wet etching relies on aqueous chemicals, while
  dry etching relies on vapor and plasma etchants.
• Wafer-bonding is often necessary for the
  assembled MEMS devices.

6
Bulk Micromachining Wet Etching

• Wet etching occurs by dipping substrate into an
  etching bath or spraying it with etchants which
  may be acid or alkaline.
• Can either be isotropic or anisotropic depending
  on the structure of the materials or the etchants
  used. If the material is amorphous or
  polycrystalline, wet etching is always isotropic
  etching. Single-crystal silicon can be
  anisotropically etched.
• During isotropic etching (etchants used are acid
  solution), resist is always undercut, meaning the
  deep etching is not practical for MEMS.
• Anisotropic wet etchants such as solutions of
  potassium hydroxide (KOH), ethylenediamine
  pyrocatechol (EDP), tetramethylammonium hydroxide
  (TMAH) and hydrazine-water are used. These
  etchants have different etch rates in different
  crystal orientations of the silicon.
• The etch process can be made selective by the use
  of dopants (heavily doped regions etch slowly),
  or may even be halted electrochemically (e.g.
  etching stops upon encountering a region of
  different polarity in a biased pn junction).
• By combining anisotropic etching with boron
  implantation (P etch-stop), and electrochemical
  etch-stop technique, varied silicon
  microstructures can be bulk machined

7
Bulk Micromachining Dry Etching

• Dry etching occurs through chemical or physical
  interaction between the ions in the gas and the
  atoms of the substrate.\
• Nonplasma, isotropic dry etching can be possible
  using xenon difluoride or a mixture of
  interhalogen gases and provides very high
  selectivity for aluminum, silicon dioxide,
  silicon nitride, photoresist, etc.
• The most common dry etching of bulk silicon are
  plasma etching and reactive ion etching (RIE)
  etching, where the external energy in the form of
  RF power drives chemical reactions in
  low-pressure reaction chambers. A wide variety of
  chlorofluorocarbon gases, sulfur hexafluoride,
  bromine compounds and oxygen are commonly used as
  reactants.
• The anisotropic dry etching processes are widely
  used in MEMS because of the geometry flexibility
  and less chemical contamination than in wet
  etching.
• Arbitrarily oriented features can be etched deep
  into silicon using anisotropic dry etching. Very
  deep silicon microstructures can be obtained by
  the deep RIE (DRIE) dry etching

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Bulk Micromachining Example Vibration Sensor
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Vibration Sensor Masks Layout
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Vibration Sensor Masks Layout
Diffusion
Insulation
Si Over
Si Under
Metal
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Bulk Micromachining Example Vibration Sensor
Cleaning in Acetone Cleaning in H2O2/H2SO4
Deposition of PECVD SiO2
Diffusion Mask
Patterning of SiO2 Photolithography through
Diffusion Mask
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Bulk Micromachining Example Vibration Sensor
Etching of SiO2 Stripping of Resist Deposition of
Si3N4 Deposition of SiO2
Patterning of SiO2 and Si3N4 Photolithography
through Si Under mask
Si Under mask
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Bulk Micromachining Example Vibration Sensor
Etching of Si bulk in KOH
Patterning of SiO2 and Si3N4
SiO2 / Si3N4 mask
001 Silicon
111 Silicon
Silicon membrane
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Bulk Micromachining Example Vibration Sensor
SiO2 deposition, etching of Si3N4
Diffusion doping of phosphorous, etching of SiO2
mask.
Deposition and patterning of SiO2 insulation
layer.
Insulation mask
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Bulk Micromachining Example Vibration Sensor
Deposition and structuring of conduction layer
Metal mask
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Bulk Micromachining Example Vibration Sensor
After diffusion doping of piezoresistor
After sputtter desposition of Au (metalization)
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Bulk Micromachining Example Vibration Sensor
Deposition and structuring of SiO2 using Silicon
Over mask
Through etching of silicon to release tongue
Silicon Over Mask
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Bulk Micromachining Example Vibration Sensor
Silicon Oxide mask
Before etching of silicon
Free Silicon Paddle
After through etching of silicon to release
paddle
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Surface Micromachining

• Surface micromachining does not shape the bulk
  silicon but builds structures on the surface of
  the silicon by depositing thin films of
  sacrificial layers and structural layers and
  by removing eventually the sacrificial layers to
  release the mechanical structures.
• The dimensions of surface micromachined
  structures can be several orders of magnitude
  smaller than bulk micromachined structures.
• The prime advantages of surface-micromachined
  structures is their self assembly and easy
  integration with IC components.
• As miniaturization in immensely increased by
  surface micromachining, the small mass structure
  involved may be insufficient for a number of
  mechanical sensing and actuation applications.

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Surface Micromachining

• Silicon microstructures fabricated by surface
  micromachining are usually planar structures (or
  are two dimensional). Other techniques involving
  the use of thin-film structural materials
  released by the removal of an underlying
  sacrificial layer have helped to extend
  conventional surface micromachining into the
  third dimension.
• By connecting polysilicon plates to the substrate
  and to each other with hinges, 3D micromechanical
  structures can with rotational freedom can be
  realized.

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Surface Micromachining

• Surface micromachining requires a compatible set
  of structural materials, sacrificial materials
  and chemical etchants.
• The structural materials must possess
  satisfactory mechanical properties e.g. high
  yield and fracture stresses, minimal creep and
  fatigue and good wear resistance.
• The sacrificial materials must have good
  mechanical properties to avoid device failure
  during fabrication. These properties include good
  adhesion and low residual stresses in order to
  eliminate device failure by delamination and/or
  cracking.
• The etchants to remove the sacrificial materials
  must have excellent etch selectivity and they
  must be able to etch off the sacrificial
  materials without affecting the structural ones.
  In addition the etchants must have proper
  viscosity and surface tension characteristics.

22
Surface Micromachining Compatible Sets

• Polysilicon-Silicon dioxide-HF
• LPCVD polysilicon as the structural material and
  LPCVD oxide as the sacrificial material. The
  oxide is readily dissolved in HF solution without
  the polysilicon being affected. Together with
  this material system, silicon nitride is often
  used for electrical insulation. This is the
  common IC compatible materials used in surface
  micromachining e,g. MUMPS and SUMMiT.
• Polyimide-aluminum- Acid-based etchants
• Polyimide is the structural material and
  aluminum is the sacrificial material. Acid-based
  etchants are used to dissolve the aluminum
  sacrificial layer.
• Silicon nitride-polysilicon- KOH and EDP
• Silicon nitride is used as the structural
  material, whereas polysilicon is the sacrificial
  material. For this material system, silicon
  anisotropic etchants such as KOH and EDP are used
  to dissolve polysilicon.
• Tungsten-silicon dioxide- HF
• CVD deposited tungsten is used as the structural
  material with oxide as the sacrificial material.
  HF solution is used to remove the sacrificial
  oxide.

23
Surface Micromachining
Single crystal wafer
LPCVD sacrificial oxide
Spin on Photoresist
Expose to UV
Develop PR
Etch unprotected oxide
PECVD conformal poly
Spin on PR
Strip off PR
Expose to UV
Develop PR
Etch unprotected Poly
Strip off PR
Etch Sacrificial oxide
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Multilevel Surface Micromachining
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LIGA

• LIGA is a German acronym for Lithographie,
  Galvanoformung, Abformung (lithography,
  galvanoforming, moulding). It was developed in
  Germany in the early 1980s using X-ray
  lithography for mask exposure to form the
  metallic parts and moulding to produce microparts
  with plastic, metal, ceramics, or their
  combinations
• Used to produce complex microstructures that are
  thick and three-dimensional achieve
  high-aspect-ratio (height-to-width) and 3D
  devices.
• Microstructures height can be up to hundreds of
  microns to millimeter scale, while the lateral
  resolution is kept at the submicron scale.

26
LIGA

• Electroplated MEMS structures can take the shape
  of the underlying substrate and a photoresist
  mold.
• First, a conducting seed layer (e.g., of gold or
  nickel) is deposited on the substrate.
• 5- to 100-µm thick resist is then deposited and
  patterned using optical or x-ray lithography.
• X-ray lithography is used to define very high
  aspect ratio features (gt100) in very thick (up to
  1,000 µm) poly(methylmethacrylate) (PMMA), the
  material on which Plexiglas is based.
• The desired metal is then plated. Finally, the
  resist and possibly the seed layer outside the
  plated areas are stripped off.

27
LIGA

• Various materials can be incorporated into the
  LIGA process, allowing electric, magnetic,
  piezoelectric, optic and insulating properties in
  sensors and actuators with a high-aspect ratio,
  which are not possible to make with the
  silicon-based processes.
• By combining the sacrificial layer technique and
  LIGA process, advanced MEMS with moveable
  microstructures can be built.
• Disadvantage is high production cost due to the
  fact that it is not easy to access X-ray sources
  limits the application of LIGA.
• Another disadvantage is that structures are not
  truly three-dimensional, because the third
  dimension is always in a straight feature.

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Multilevel Surface Micromachining Gear Example
Poly 0
Dimple 1 Cut
Sac Ox 1 Cut
Pin Joint Cut
Sac Ox 2 Cut
Poly 2
Poly 2 Cut (All layers)
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Multilevel Surface Micromachining Gear Example
Section through Gear
SI Substrate UNIFORM DEPOSITION
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Multilevel Surface Micromachining Gear Example
Grow Thermal Oxide CONFORMAL DEPOSITION
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Multilevel Surface Micromachining Gear Example
Nitride Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Nitride Etch DRY ETCH
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Multilevel Surface Micromachining Gear Example
Thermal Oxide Etch DRY ETCH
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Multilevel Surface Micromachining Gear Example
Poly0 Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Section through Gear
Poly 0
Poly0 Etch Dry Etch
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Multilevel Surface Micromachining Gear Example
Section through Gear
Sacrificial Oxide 1 Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Section through Gear
Dimple 1 Cut
Dimple 1 etch DRY ETCH (Timed)
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Multilevel Surface Micromachining Gear Example
Section through Gear
Sac Ox 1 Cut
Sac Ox 1 etch DRY ETCH
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Multilevel Surface Micromachining Gear Example
Section through Gear
Poly 1 Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Section through Gear
Pinjoint Cut
Pinjoint cut etch DRY ETCH
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Multilevel Surface Micromachining Gear Example
Section through Gear
Pinjoint Cut
Pinjoint undercut etch part 1 DRY ETCH
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Multilevel Surface Micromachining Gear Example
Section through Gear
Pinjoint Cut
Pinjoint undercut etch part 2 WET ETCH
(ISOTROPIC)
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Multilevel Surface Micromachining Gear Example
Section through Gear
Sac Ox2 Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Section through Gear
SacOx2 Mask
Sac Ox2 Etch Dry Etch
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Multilevel Surface Micromachining Gear Example
Section through Gear
Poly 2 Deposition LPCVD
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Multilevel Surface Micromachining Gear Example
Section through Gear
SacOx2 Mask
Poly 2 Etch DRY Etch
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Multilevel Surface Micromachining Gear Example
Section through Gear
Release RELEASE ETCH
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Homework 2

• Consider Photochemical Fabrication Processes
  available in Jordan, and which can be used to
  demonstrate MEMS fabrication processes.
  Examples
• BW Photography
• Zincograph
• PCB fabrication
• Glass Etching
• Describe the following
• The process, photos etc.
• The physics and chemistry
• Equipment cost
• Possibility of implementing in a MEMS lab

49
LIGA

• LIGA is a German acronym for Lithographie,
  Galvanoformung, Abformung (lithography,
  galvanoforming, moulding). It was developed in
  Germany in the early 1980s using X-ray
  lithography for mask exposure to form the
  metallic parts and moulding to produce microparts
  with plastic, metal, ceramics, or their
  combinations.
• Used to produce complex microstructures that are
  thick and three-dimensional achieve
  high-aspect-ratio (height-to-width) and 3D
  devices.
• Microstructures height can be up to hundreds of
  microns to millimeter scale, while the lateral
  resolution is kept at the submicron scale.

50
Electroplating

• Electroplating (galvanoforming) is the process of
  using electrical current to coat an electrically
  conductive object with a relatively thin layer of
  metal.
• The process used in electroplating is called
  electrodeposition. The part to be plated is the
  cathode of the circuit. In one technique, the
  anode is made of the metal to be plated on the
  part.
• Both components are immersed in a solution called
  an "Electrolyte" containing one or more dissolved
  metal salts as well as other ions that permit the
  flow of electricity. A rectifier supplies a
  direct current to the cathode causing the metal
  ions in the electrolyte solution to lose their
  charge and plate out on the cathode. As the
  electrical current flows through the circuit, the
  anode slowly dissolves and replenishes the ions
  in the bath.

51
LIGA

• Electroplated MEMS structures can take the shape
  of the underlying substrate and a photoresist
  mold.
• First, a conducting seed layer (e.g., of gold or
  nickel) is deposited on the substrate.
• 5- to 100-µm thick resist is then deposited and
  patterned using optical or x-ray lithography.
• X-ray lithography is used to define very high
  aspect ratio features (gt100) in very thick (up to
  1,000 µm) polymethylmethacrylate (PMMA), the
  material on which Plexiglas is based

52
LIGA

• The desired metal is then plated. Finally, the
  resist and possibly the seed layer outside the
  plated areas are stripped off.

Planarize
Electroform
Release
Remove PMMA
53
LIGA

• Various materials can be incorporated into the
  LIGA process, allowing electric, magnetic,
  piezoelectric, optic and insulating properties in
  sensors and actuators with a high-aspect ratio,
  which are not possible to make with the
  silicon-based processes.
• By combining the sacrificial layer technique and
  LIGA process, advanced MEMS with moveable
  microstructures can be built.
• Disadvantage is high production cost due to the
  fact that it is not easy to access X-ray sources
  limits the application of LIGA.
• Another disadvantage is that structures need to
  be manually assembled, and they are not truly
  three-dimensional, because the third dimension is
  always in a straight feature.