mems fabrication - oakland university...asme biomems technology seminar (pd 437) may 19, 2003 mems...

ASME BioMEMS Technology Seminar (PD 437) May 19, 2003

MEMS Fabrication

Beyond Integrated Circuits…

MEMS Basic Concepts Uses integrated circuit fabrication techniques to make mechanical as well as electrical components on a single chip.Small size ⇒ 1µm – 1mmTypically a batch fabrication process, i.e. make many devices simultaneously.

2 MEMS pressure sensors in the eye of a needle

Photo courtesy of Integrated Sensing Systems (ISSYS) Inc.


MEMS Differs from ICsEtching the silicon substrate away– Bulk micromachining

Etching selected layers away entirely– Surface micromachining

Additional processing methods used– LIGA, spin-casting, molding…

Additional materials used– Gold, Titanium, SiC, polymers, epoxies…

Computer Aided Design (CAD)Idea ⇒ CAD ⇒ Simulate ⇒ Masks (for fabrication)

Comb Drive Resonator


MEMS Layout CAD ToolsMechanical considerations: cross section, mechanical simulation models, etc. MEMS component librariesSimulate fluid flow, stress gradients, etc.Predict how processing parameters affect mechanical performance

Can check cross-section of structures

Bulk Micromachining


= FCC cell

= offset FCC cell

Silicon has a covalently bonded diamond cubic (zinc blende) structure.Unit cell can be imagined as 2 FCC cells with 1 cell offset by [a/4, a/4, a/4], where a = length of unit cell = 5.431Å.

(top view)a/4

Silicon Crystal Structure

a/4

Silicon unit cell

a

Miller IndicesUsed to label crystal planes and directions.Miller indices of a plane:

– Determine the intercepts (x,y,z) of the plane along each of the three crystallographic directions.

– Take the reciprocals of the intercepts (1/x, 1/y, 1/z)– If fractions result, multiply each by the denominator of the

smallest fraction, resulting in (h, k, l).

Plane and Direction symbols:– Plane: (hkl)– Family of planes: {hkl}– Direction: [hkl]– Family of directions: <hkl>


Example: Miller DirectionsImagine you are looking down the Z axis of the crystal…

X

Y

[100] [310]

[010]

[120]

[210][110]

[110]

Example: Miller Planes

(100)

a=5.431Å

(111)

(110)

NOTE: Miller directions are perpendicular to Miller planes! Example: [100] vs. (100)

(111)(111)

(111)

(111)(111)

(111)

Z

X

Y


Isotropic Silicon Wet EtchingIsotropic etchants etch equally in all crystallographic directions– Examples: HNO3 (nitric) and CH3COOH (acetic), HNA– Etching done at or around room temperature (< 50 °C)– Etching is very fast (e.g. up to 100 µm/min)– Undercuts mask

Masks materials include SiO2, Au/Cr, or Si3N4

Anisotropic Silicon Wet EtchingAnisotropic etch rates depend on crystal orientation– Examples: KOH, EDP, TMAH– Etching done at higher temperatures (> 50 °C e.g. 85 to 115 °C)– Etching is slow ⇒ 1 µm/min (for <100> direction)– Does not undercut the mask

Etch rates differ: (100) > (110) >> (111)Masking materials include SiO2 , Si3N4

(100) Silicon

[100]

[111]

54.7°


Anisotropic Silicon Wet Etching

V-grooveInvertedpyramid

w

~0.7w

Through-wafer vias

Trenches

Anisotropic Silicon Wet Etching

NOTE: In (100) silicon, etchants will seek out the {111} planes, regardless of mask alignment or shape


• Silicon micro-blades etched via chemical etching• May be used for delicate microsurgery• Microneedles fabricated in a similar fashion

MEMS Microblades

Fabricated at Standard MEMS, Inc.

Etch Stop Technology:Wet Silicon Etching

Requires ion-implanted regions in substrate.Typical doping level greater than 1020 cm-3 boron in silicon.Etch rate in TMAH, KOH, decreases up to 40:1 doped:undoped (electrochemical etch stop).Good technique for forming membranes, needles.

silicon

boron Resulting doped region


Etch Stop Technology - Wet

Drawing and photo courtesy of K. D. Wise, University of Michigan.

Microelectrode array for central nervous system.

Isotropic Silicon Dry Etch: XeF2XeF2 crystals sublimate at vacuum, vapor etches silicon.Masking materials include SiO2, photoresist, metals (Aluminum, etc.)Gentle – no meniscus forces

(100) Silicon

etch mask


Anisotropic Silicon Dry Etch:Reactive Ion Etching (RIE)

Typical gases used: O2, SF6, CF4For silicon etching: CF4 + O2

Drawing courtesy of G. O’Brien, U. Michigan

Deep Reactive Ion Etch (DRIE)High aspect ratio structures via RIE!



DRIE Passivation/Etch Cycle

Each cycle = 4 – 12 seconds


RIE vs. DRIE

• Diffusion of reacting species causes lateral etching, nonvertical sidewalls as etch progresses beyond a few µm.

• Best aspect ratio ≈ 5:1• Etch rate of silicon ≈ 0.3µm/min• Fairly selective to oxide, PR

RIE DRIE

• Passivation prevents lateral etching. Nanometer scalloping.

• Aspect ratio > 40:1• Etch rate of silicon > 3µm/min• Extremely selective to oxide, PR


DRIE High Aspect Ratios

15µm deep, 300nm wide!

Photos courtesy of G. O’Brien, U. Michigan

20µm deep, 1µm wide walls.Scalloping on nm scale, decreases with etch depth

Produced using Deep Reactive Ion Etching of Silicon (DRIE)

Example – Leaf Spring Tether

Photo courtesy of Lucas NovaSensor

Capacitive plates


High Aspect Ratio - LIGALIGA = lithography, galvanoformung/electroplating, abformung/plastic molding – (Ehrfeld et al., Karlsruhe Center, Germany)

Lines/spaces: 5µmAspect ratios of 10:1 to 20:1Requires X-Ray (synchrotron) source!!

PMMA

Si substrate

oxideTi

Cu

1. Deposit oxide, Cu, Ti, PMMA 2. x-ray lithography 3. Electroplate Ni

Ni Ni

High Aspect Ratio - LIGA4. Mill back to planarize 5. etch PMMA 6. Release Ni (sacrificial Ti)

Photos courtesy of U. Wisconsin


High Aspect Ratio with SU-8

Photosensitive epoxy, processing similar to PRStandard equipment (no X-ray source required)Thickness up to 700µm with single spin applicationSmooth sidewalls, aspect ratios up to 20:1

800µm thick Su-8 100µm Ni plated via Su-8 mold

Photos courtesy of Micro Resist Technology, Berlin, Germany.

mems fabrication - oakland university...asme biomems technology seminar (pd 437) may 19, 2003 mems...

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