mems fabrication - oakland university...asme biomems technology seminar (pd 437) may 19, 2003 mems...
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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.
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
= 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>
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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°
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
• 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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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!
Drawing courtesy of G. O’Brien, U. Michigan
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
DRIE Passivation/Etch Cycle
Each cycle = 4 – 12 seconds
Drawing courtesy of G. O’Brien, U. Michigan
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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
ASME BioMEMS Technology Seminar (PD 437) May 19, 2003
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.