MEMS devices: How do we make them?

Sandia MEMS

Gear chain Hinge Gear within a gear

A mechanism

Basic MEMS materials Silicon and its derivatives, mostly

• Micro-electronics heritageSi is a good semiconductor, properties can be tunedSi oxide is very robustSi nitride is a good electrical insulator

Substrate Cost Metallization Machinability

Silicon High Good Very good

Plastic Low Poor Fair

Ceramic Medium Fair Poor

Glass Low Good Poor

Surface micromachining

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One can make devices as complex as one wishes using deposition and micromachining processes

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Any MEMS device is made from the processesof 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:

Bulk Si Bulk Si

Masking layer

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

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

Chemical reaction:

Principle:HNO3 (Nitric acid) oxidizes Si SiOx

HF (Hydrofluoric Acid) dissolves SiOx

Acetic acid/water is a diluent

Anisotropic etching, due to the Silicon crystal structure

Different planes of atoms in a Silicon crystal have different densities of atoms

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

XY

Z- Diamond cubic crystal structure

This implies preferential/anisotropic etching is possible

Applications: Anisotropic Etching

fiber

Aligning fibers Inkjet printers

Wet etching: Anisotropic Etching(100) (110)

(100) (111)

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

Bulk Si Bulk Si

Comparison of wet chemical etches

Etchant Typical etching conditions

Anisotropic <100>/<111> etching ratio

Etch rate of masking layers

EDP 50-115 oC

20-80 m/hr

10-35 SiO2(2 Å/min)

SiN(1 Å/min)

KOH 50-90 oC

10-100 m/hr100-400 SiO2(2 Å/min)

SiN(1 Å/min)

TMAH 60-90 oC

10-60 m/hr10-20 SiO2(2 Å/min)

SiN(1 Å/min)

Reference: “Etch rates for Micromachining Processing” - K. R. Williams, IEEE Journal of MEMS, vol. 5, page 256, 1996.

Sensors based on (100) preferential etching

Honeywell sensor

Micro-fluidic channels based on (110) preferential etching

MEMS Process Sequence

Slide courtesy: Al Pisano

Surface micromachininght

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Sacrificial material: Silicon oxide

Structural material: polycrystalline Si (poly-Si)

Isolating material (electrical/thermal): Silicon Nitride

How a cantilever is made:

MEMS ProcessingOxidation 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 m2/ hour 920 0.0049 1000 0.0117 1100 0.027

Wet Oxidation: uses steamfor 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

Oxidation of Silicon Silicon Oxide (Sacrificial material)

MEMS Processing

Silicon oxide deposition

For deposition at lower temperatures, useLow Pressure Chemical Vapor Deposition (LPCVD)

SiH4 + O2 SiO2 + 2 H2 : 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)

SiH4 + O2

425-450 oC0.2-0.4 Torr

LTO: Low Temperature Oxidation process

Case study: Poly-silicon growth

- by Low Pressure Chemical Vapor Deposition- T: 580-650 oC, P: 0.1-0.4 Torr

Effect of temperature

Amorphous Crystalline: 570 oCEqui-axed grains: 600 oCColumnar grains: 625 oC

(110) crystal orientation: 600 – 650 oC (100) crystal orientation: 650 – 700 oC

SiH4

Amorphous film570 oC

Crystalline film620 oC

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Poly-silicon growthTemperature has to be very accurately controlledas grains grow with temperature, increasing surface roughness, causing loss of pattern resolution and stresses in MEMS

Mechanisms of grain growth:

1. 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 (500 MPa)

- 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

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 filmsPECVD: Stress-free Silicon Nitride films

(for electrical and thermal isolation of devices)

1016 cm, Ebreakdown: 107 kV/cm

SiH2Cl2 + NH3

x SiH2Cl2 + y NH3 SixNy + HCl + 3 H2

700 - 900 oC0.2-0.5 Torr

Depositing materialsPVD (Physical vapor deposition)

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

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Depositing materialsPVD (Physical vapor deposition)

• Evaporation (electron-beam/thermal)

Commercial electron-beam evaporator (ITL, UCSD)

Electroplating

e.g. can be used to form porous Silicon, used for sensors due to the large surface to volume ratio

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Issues: •Micro-void formation• Roughness on top surfaces• Uneven deposition speeds

Used extensively for LIGA processing

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

Si wafer

Dropper

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 micromachiningCarving 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

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HF can etch Silicon oxide but does not affect Silicon

Release step

Release of MEMS structures A difficult step, due to surface tension forces:

Surface Tension forces are greater than gravitational forces ( L) ( L)3

Release of MEMS structures To overcome this problem:

(1) Use of alcohols/ethers, which sublimate, at release step

(2) Surface texturing

(3) Supercritical CO2 drying: avoids the liquid phase

35oC, 1100 psi

Si substrate

Cantilever

A comparison of conventional vs. supercritical drying

Reactive Ion Etching (RIE) DRY plasma based etching

Deep RIE (DRIE): • Excellent selectivity to mask material (30:1)• Moderate etch rate (1-10 m/minute)• High aspect ratio (10:1), large etch depths possible

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Deep Reactive Ion Etching (DRIE)

Bosch Process Alternate etching (SF6) +Passivation (C4F8)

• Bowing: bottom is wider• Lag: uneven formation

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

Gas phase Silicon etching

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

• Room temperature process• No surface tension forces• No charging effects• Isotropic

Etching rate: 1-10 m/minute

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-For thick films (> 100 m)

- HEXSIL/PDMS, compatible with Bio-MEMS

- 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 armsElectromagnetic motor

Capability Bulk Surface LIGA

Max. structural thickness Wafer thickness < 50 m 500 m

Planar geometry Rectangular Unrestricted Unrestricted

Min. planar feature size 2 depth < 1 m < 3 m

Side-wall features 54.7o slope Limited by dry etch 0.2 m

Surface & edge

definitions

Excellent Adequate Very good

Material properties Very well controlled

Adequate Well controlled

Integration with electronics

Demonstrated Demonstrated Difficult

Capital Investment Low Moderate High

Published knowledge Very high High Moderate

Technology Comparison

Bulk vs. Surface micromachining vs. LIGA