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MEMS devices: How do we make them?. A mechanism. Gear chain Hinge Gear within a gear. Sandia MEMS. Basic MEMS materials Silicon and its derivatives, mostly. Micro-electronics heritage Si is a good semiconductor, properties can be tuned Si oxide is very robust

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slide1

MEMS devices: How do we make them?

A mechanism

Gear chain Hinge Gear within a gear

Sandia MEMS

slide2

Basic MEMS materials

Silicon and its derivatives, mostly

  • Micro-electronics heritage
  • Si is a good semiconductor, properties can be tuned
  • Si oxide is very robust
  • Si nitride is a good electrical insulator
slide3

Materials in MEMS

Dominant: SEMICONDUCTORS (Silicon centric)

MEMS technology borrows heavily from the Si micro-electronics industry

The fabrication of MEMS devices relies on the processing of

Silicon and silicon compounds (silicon oxide, nitride …)

METALS: used in electrical contacts (Al,Cu),

magnetic elements (Ni, NiFe)

POLYMERS: used as sacrificial layers, for

patterning (photoresist/polyimide)

slide4

Making MEMS

  • Planar technology,
  • constructing components (MEMS & electronics) on initially flat wafers
  • > Wafer level processes
  • > Pattern transfer
  • Introduction to Micro-machining
  • - Wet and Dry etching
  • - Bulk and surface micro-machining
  • What kinds of materials are used in MEMS?
    • Semiconductors
    • Metals
    • Polymers
slide5

Photolithography

Light

Light

MASK

MASK

Deposit

Metal

Photoresist

Silicon substrate

Silicon substrate

Positive photoresist

Negative photoresist

slide6

Deposit and remove materials precisely to

  • create desired patterns

The photo-lithography process

Positive

Remove deposit and etch

J. Judy, Smart Materials & structures, 10, 1115, 2001

Negative

slide7

Surface micromachining

How a cantilever is made:

http://www.darpa.mil/mto/mems

slide8

One can make devices as complex as one wishes

using deposition and micromachining processes

http://mems.sandia.gov/

slide9

Any MEMS device is made from the processes

of deposition and removal of material

e.g. a state-of-the art MEMS electric motor

www.cronos.com

slide10

The History of MEMS

Y.C.Tai, Caltech

slide11

Bulk micromachining

  • Wet Chemical etching:

Masking layer

Bulk Si

Bulk Si

Isotropic Anisotropic

slide12

Bulk micromachining

  • Dry etching
  • Ions: Reactive ion etching (RIE), focused ion beams (FIB)
  • Laser drilling: using high powered lasers (CO2/YAG)
  • Electron-beam machining: sequential slow
slide13

Wet Etching: Isotropic

  • atomic layer by atomic layer removal possible
  • Isotropic etching:Hydrofluoric + nitric + acetic acids (HNA)

Bulk Si

Chemical reaction:

Si + 6 HNO3+6 HF H2SiF6 + HNO2 + H2O + H2

Principle:

HNO3 (Nitric acid) oxidizes Si  SiOx

HF (Hydrofluoric Acid) dissolves SiOx

Acetic acid/water is a diluent

slide14

Z

Y

X

Anisotropic etching, due to the Silicon crystal structure

- Diamond cubic crystal structure

Different planes of atoms in a Silicon crystal have different

densities of atoms

(111) (100) (110) (111)

This implies preferential/anisotropic etching is possible

slide15

fiber

Applications: Anisotropic Etching

Inkjet printers

Aligning fibers

slide16

Wet etching: Anisotropic Etching

(100)

(110)

(100)

(111)

Bulk Si

Bulk Si

Chemical recipes:

EDP (Ethylene diamine, pyrocatechol, water)

[NH2(CH2)2NH2, C6H4(OH)2]

- low SiO2 etch rate, - carcinogenic

KOH (Potassium hydroxide),

- high <110> / <111> and <100>/ <111> selectivity ( ~ 500)

- high SiO2 etching

TMAH (Tetra-methyl Ammonium Hydroxide: (CH3)4NOH)

- Low SiO2 and SixNy etch rate

- smaller <100> / <111> selectivity

slide17

Comparison of wet chemical etches

Reference: “Etch rates for Micromachining Processing”

- K. R. Williams, IEEE Journal of MEMS, vol. 5, page 256, 1996.

slide19

Micro-fluidic channels

based on (110) preferential etching

slide20

MEMS Process Sequence

Slide courtesy: Al Pisano

slide21

Surface micromachining

How a cantilever is made:

http://www.darpa.mil/mto/mems

Sacrificial material: Silicon oxide

Structural material: polycrystalline Si (poly-Si)

Isolating material (electrical/thermal):Silicon Nitride

slide22

MEMS Processing

Oxidation of Silicon  Silicon Oxide (Sacrificial material)

Dry Oxidation: flowing pure oxygen over Si @ 850 – 1100 oC

(thin oxides 1- 100 nm, high quality of oxide)

Uses the Deal-Grove Model: xoxide = (BDGt)1/2

Temperature (oC)BDG (mm2/ hour)

920 0.0049

1000 0.0117

1100 0.027

slide23

MEMS Processing

Oxidation of Silicon  Silicon Oxide

(Sacrificial material)

Wet Oxidation: uses steam

for thicker oxides (100nm – 1.5 mm, lower quality)

Temperature (oC)BDG (mm2/ hour)

920 0.203

1000 0.287

1100 0.510

Higher thicknesses of oxide: CVD or high pressure steam oxidation

slide24

Silicon oxide deposition

SiH4 + O2

LTO: Low Temperature Oxidation process

  • For deposition at lower temperatures, use
  • Low Pressure Chemical Vapor Deposition (LPCVD)
  • SiH4 + O2 SiO2 + 2H2 : 450 oC
  • Other advantages:
  • Can dope Silicon oxide to create PSG (phospho-silicate glass)
  • SiH4 + 7/2 O2 + 2 PH3 SiO2:P + 5 H2O : 700 oC
  • PSG: higher etch rate, flows easier (better topography)

425-450 oC

0.2-0.4 Torr

slide25

Case study: Poly-silicon growth

SiH4

Amorphous film

570 oC

Crystalline film

620 oC

  • by Low Pressure Chemical Vapor Deposition
  • T: 580-650 oC, P: 0.1-0.4 Torr
      • Effect of temperature
  • Amorphous  Crystalline: 570 oC
  • Equi-axed grains: 600 oC
  • Columnar grains: 625 oC
  • (110) crystal orientation: 600 – 650 oC
  • (100) crystal orientation: 650 – 700 oC

Kamins,T. 1998 Poly-Si for ICs and diplays, 1998

slide26

Poly-silicon growth

  • Temperature has to be very accurately controlled
  • as grains grow with temperature, increasing surface
  • roughness, causing loss of pattern resolution and stresses in
  • MEMS
  • Mechanisms of grain growth:
  • Strain induced growth
  • - Minimize strain energy due to mechanical deformation, doping …
  • - Grain growth  time
  • 2. Grain boundary growth
  • - To reduce surface energy (and grain boundary area)
  • - Grain growth  (time)1/2
  • 3. Impurity drag
  • - Can accelerate/prevent grain boundary movement
  • - Grain growth  (time)1/3
slide27

Grains control properties

  • Mechanical properties
  • Stress state:Residual compressive stress (500MPa)
  • - Amorphous/columnar grained structures: Compressive stress
  • - Equiaxed grained structures: Tensile stress
  • Thick films have less stress than thinner films
  • ANNEALING CAN REDUCE STRESSES BY A
  • FACTOR OF 10-100
  • Thermal and electrical properties
  • Grain boundaries are a barrier for electrons
  • e.g. thermal conductivity could be 5-10 times lower (0.2 W/cm-K)
  • Optical properties
  • Rough surfaces!
slide28

Silicon Nitride

SiH2Cl2 + NH3

(for electrical and thermal isolation of devices)

r: 1016W cm, Ebreakdown: 107 kV/cm

  • Is also used for encapsulation and packaging
  • Used as an etch mask, resistant to chemical attack
  • High mechanical strength (260-330 GPa) for SixNy, provides structural integrity (membranes in pressure sensors)
  • Deposited by LPCVD or Plasma –enhanced CVD (PECVD)

LPCVD: Less defective Silicon Nitride films

PECVD: Stress-free Silicon Nitride films

x SiH2Cl2 + y NH3 SixNy + HCl + 3 H2

700 - 900 oC

0.2-0.5 Torr

slide29

Depositing materialsPVD (Physical vapor deposition)

  • Sputtering: DC (conducting films: Silicon nitride) RF (Insulating films: Silicon oxide)

http://web.kth.se/fakulteter/TFY/cmp/research/sputtering/sputtering.html

slide30

Depositing materialsPVD (Physical vapor deposition)

  • Evaporation (electron-beam/thermal)

Commercial electron-beam evaporator (ITL, UCSD)

slide31

Electroplating

Courtesy: Jack Judy

  • Issues:
  • Micro-void formation
  • Roughness on top surfaces
  • Uneven deposition speeds
  • Used extensively for LIGA processing

e.g. can be used to form porous Silicon, used for

sensors due to the large surface to volume ratio

slide32

Depositing materials –contd.-

  • Spin-on (sol-gel)
  • e.g. Spin-on-Glass (SOG) used as a sacrificial molding material, processing can be done at low temperatures

Dropper

Si wafer

slide33

Surface micromachining

- Technique and issues

- Dry etching (DRIE)

Other MEMS fabrication techniques

- Micro-molding

- LIGA

Other materials in MEMS

- SiC, diamond, piezo-electrics,

magnetic materials, shape memory alloys …

MEMS foundry processes

- How to make a micro-motor

slide34

Surface micromachining

Carving of layers put down sequentially on the substrate by using selective etching of sacrificial thin films to form free-standing/completely released thin-film microstructures

http://www.darpa.mil/mto/mems

HF can etch Silicon oxide but does not affect Silicon

Release step

crucial

slide35

Release of MEMS structures

  • A difficult step, due to surface tension forces:

Surface Tension forces are greater than gravitational forces

( L) ( L)3

slide36

Cantilever

Si substrate

Release of MEMS structures

  • To overcome this problem:
  • Use of alcohols/ethers, which sublimate, at release step
  • Surface texturing
  • Supercritical CO2 drying: avoids the liquid phase

35oC,

1100 psi

slide37

A comparison of conventional

vs. supercritical drying

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