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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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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

  • Si nitride is a good electrical insulator


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)


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


Photolithography

Light

Light

MASK

MASK

Deposit

Metal

Photoresist

Silicon substrate

Silicon substrate

Positive photoresist

Negative photoresist


  • 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


Surface micromachining

How a cantilever is made:

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


One can make devices as complex as one wishes

using deposition and micromachining processes

http://mems.sandia.gov/


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


The History of MEMS

Y.C.Tai, Caltech


Bulk micromachining

  • Wet Chemical etching:

Masking layer

Bulk Si

Bulk Si

Isotropic Anisotropic


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


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


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


fiber

Applications: Anisotropic Etching

Inkjet printers

Aligning fibers


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


Comparison of wet chemical etches

Reference: “Etch rates for Micromachining Processing”

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



Micro-fluidic channels

based on (110) preferential etching


MEMS Process Sequence

Slide courtesy: Al Pisano


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


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


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


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


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


    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


    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!


    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


    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


    Depositing materialsPVD (Physical vapor deposition)

    • Evaporation (electron-beam/thermal)

    Commercial electron-beam evaporator (ITL, UCSD)


    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


    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


    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


    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


    Release of MEMS structures

    • A difficult step, due to surface tension forces:

    Surface Tension forces are greater than gravitational forces

    ( L) ( L)3


    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


    A comparison of conventional

    vs. supercritical drying


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