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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. Silicon oxide deposition. SiH 4 + O 2.

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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


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


    Reactive Ion Etching (RIE)

    DRY plasma based etching

    http://www.memsguide.com

    • Deep RIE (DRIE):

    • Excellent selectivity to mask material (30:1)

    • Moderate etch rate (1-10 mm/minute)

    • High aspect ratio (10:1), large etch depths possible


    Deep Reactive Ion Etching (DRIE)

    A side effect of a glow discharge  polymeric species created

    Plasma processes:

    Deposition of polymeric material from plasma vs. removal of material

    Usual etching processes result in a V-shaped profile

    Bosch ProcessAlternate etching (SF6) +Passivation (C4F8)

    • Bowing: bottom is wider

    • Lag: uneven formation


    Gas phase Silicon etching

    • Room temperature process

    • No surface tension forces

    • No charging effects

    • Isotropic

    XeF2 BrF3

    Developed at IBM (1962) Developed at Bell labs (1984)

    2 XeF2 + Si  2 Xe + SiF4 4 BrF3 + 3 Si  2 Br2 + 3 SiF4

    Cost: $150 to etch 1 g of Si $16 for 1 g of Si

    Etching rate: 1-10 mm/minute


    Micro-molding

    • For thick films (> 100 mm)

    • HEXSIL/PDMS, compatible with Bio-MEMS

    C. Keller et al, Solid state sensor & actuator workshop, 1994

    - loss of feature definition after repeated replication

    - Thermal and mechanical stability


    LIGA

    (LIthographie, Galvanoformung, Abformung)

    • For high aspect ratio structures

    • Thick resists (> 1 mm)

    • high –energy x-ray lithography ( > 1 GeV)

    • Millimeter/sub-mm sized objects which require precision

    Mass spectrometer with hyperbolic arms

    Electromagnetic motor


    Technology Comparison

    Bulk vs. Surface micromachining vs. LIGA


    Materials in MEMS

    Mechanical MEMS (for micro-motors etc.)

    Si, quartz (SiO2), Si3N4, Ti, Ni, permalloy (NiFe),

    polycrystalline Si …

    RF-MEMS (for wireless communications):

    Compound semiconductors: GaAs, InP, GaN

    Si, SiO2 …

    Bio-MEMS (micro-electrode arrays, DNA probes)

    enzymes, antigen/antibody pairs, DNA,

    polyimides, hydrogels, plastics, porous Si, C, AgCl…


    METALS

    used for wiring (Al, Cu), etch masks (Cr),

    structural elements (Al, W)

    - excellent electrical conductors

    - prone to fatigue

    SMA : Shape memory alloys(NiTi: Nitinol)

    Reversible temperature induced transformation from a

    stiff austenite phase (Y.S.: 550 MPa) to a

    ductile martensite (Y. S.: 100 MPa) phase.

    - used for thermal actuation

    - Can exert stresses of up to 100 MN/m2

    - Maximum operating temperature ~ 70 oC

    - very slow actuation mechanism

    Polymers: poly-norbornene


    Photoresist (PR)

    Cr/Au

    Glass substrate

    Si oxidation

    Si

    SiO2

    Pattern PR

    Si etch

    (KOH)

    Etch Cr/Au

    Pattern &

    deposit NiFe

    Etch Glass

    RIE to release

    cantilever

    Remove Cr/Au

    Magnetic materials

    • prevalent: Ni, NiFe (permalloy), Co alloys

    • - Not as widely used as electrostatic actuation

    • - Needs thick films (10-20 mm); using electro-deposition

    A magnetically actuated cantilever


    New applications demand new materials

    Silicon Carbide (SiC): structural & isolating layer

    - mechanically robust, E(500 GPa)  higher resonance frequency

    - high temperature material (>200 oC)

    - difficult to shape (chemically inert)

    - used in micro-gas turbines

    Diamond: very hard, for electrical isolation

    - E: 1035 GPa

    - excellent thermal conductor, easy heat dissipation

    - difficult to machine, needs oxygen-plasmas

    - used in Atomic Force Microscope cantilevers

    GaAs/InP: opto-electronics

    - good combination of electrical and mechanical properties

    - high piezo-electric coefficients

    - sophisticated manufacture for GaAs and InP substrates

    (Molecular Beam Epitaxy)


    • Polymers: structurally compliant

    • - 50 times lower E compared to Si/Silicon nitride

    • - Can withstand large strains (100%)

    • Polyimide: used in force sensor, shear stress

    • sensor skin

    • Piezoelectrics: have a mechanical response to an

    • electric field: ZnO, (Pb,Zr)TiO3

    • - Large mechanical transduction, force sensors



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