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Texas Christian University Department of Engineering Ed Kolesar
Introduction toMicroeletromechanical Systems
(MEMS)
Lecture 3 Topics• MEMS Fabrication Techniques• Review of the Si Crystal Lattice• Review of Wet Etching• Dry Etching• Plasma Etching• Reactive Ion Etching• Additive Processes• Sacrificial Processes
Texas Christian University Department of Engineering Ed Kolesar
MEMS Fabrication Techniques
• Dry EtchingVapor PhasePlasmaRIE
• Additive ProcessesCVDSputteringElectroplating
Sacrificial LayersLift-offWet Release Issues
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• Unit Cell: most basic structural element in a crystal, repeated regularly over all three dimensions
• IV group elements: Diamond Lattice
Review Of The Si Crystal Lattice
(Figures: Campbell, 1996.)
Texas Christian University Department of Engineering Ed Kolesar
Review Of The Si Crystal Lattice
Notation: (100) particular crystal plane{100} all equivalent planes: (100), (010), (001) in cubic lattice[100] direction normal to crystal plane
Wafer characterization: in 100 wafer, 100 plane is parallel to the wafer surface
Several useful Internet WEB sites for visualization of Si crystals• ostc.physics.uiowa.edu/~wkchan/SOLIDSTATE/CRYSTAL• et.nmsu.edu/ETCLASSES/vlsi/files/CRYSTAL.HTM• www.izzy.net/~jc/CrystalGallery/crystalgallery.html• stm2.nrl.navy.mil/~lwhitman/Projects.html#sisum
Location of atoms in various planes of the diamond lattice.
Figures: [email protected] (1996)
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Texas Christian University Department of Engineering Ed Kolesar
Review Of Wet Etching
Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Review Of Wet EtchingAnisotropic Wet Etching:• Convex corners are
undercut• Concave corners stop at
[111] intersections
Figures: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
Dry Etching
Overview• Vapor Phase Etch:
Use of reactive gasesNo drying necessary
• Plasma Etch:RF energy generates reactive ions and free radicalsNo high temperatures required (250°C down to room temperature)
• Reactive Ion Enhanced (RIE) Etch:Higher energy ionsHigher anisotropy
Texas Christian University Department of Engineering Ed Kolesar
Vapor Phase EtchXeF2 Isotropic Silicon Etch
Simple setupDoes not attack:
- Silicon oxide- Silicon nitride- Metals- Photoresist
• Basic reaction:2XeF2 + Si → 2Xe +SiF4
• Caveat:2XeF2+2H2O → Xe2+4HF+2O2exothermic!
Hoffman et al., 1995 (UCLA)
Figure: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
XeF2 Isotropic Silicon Etch• Post processing for
standard CMOS• Suspended and 3D structures• Fold-up structures with
conducting Al hinges
Storment et al., JMEMS 1994 (Stanford)Tahhan et al., SPIE 1999 (UC Berkeley)
Texas Christian University Department of Engineering Ed Kolesar
Plasma Etch• RF energy drives etching reaction: accelerates stray
electrons between pair of plates in low pressure gas
• Electrons generate reactive ions and free radicals (e.g., monoatomic fluorine)
• Substrate surface is bombarded with reactive ions (physical and chemical etching)
• Si or other materials are etched by forming volatile components
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Texas Christian University Department of Engineering Ed Kolesar
Plasma Etch
• RIE allows higher ion energies: higher etch rates, higher anisotropy
Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Reactive Ion Etch• Often, multiple etching and deposition reactions take
place simultaneously and reach equilibrium
• Example:High concentration SF6 etches SiLow concentration O2 removes resputtered photoresist but also forms SiO2 and polymeric filmsCHF3 removes oxide and polymers
• Selection of etch parameters (concentration, pressure, RF power, bias, …) gives (limited) control over anisotropy, selectivity, etch rate, surface roughness
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Texas Christian University Department of Engineering Ed Kolesar
Reactive Ion EtchSCREAM
(Single Crystal Reactive Etching And Metallization)
• Multiple anisotropic and isotropic dry etches
• Low temperature etching and deposition
Zhang et al., 1993 (Cornell) Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Reactive Ion Etch
RIE postprocessing of CMOS to release thin film structures(Fedder et al. 1996)
Figure: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
Deep RIE“Bosch Process” (Patent: Lärmer & Schilp, 1994)
• Idea: alternate between etching and thin film deposition that protects sidewalls but is removed in trenches
• Etching phase: SF6 / Ar• Polymerization phase: CHF3 (or C4F8/SF6) / Ar forms
Teflon-like polymer layer• Ion bombardment can prevent formation of polymer
on horizontal surfaces• Several DRIE systems are on the market (after only 5
years!): STS, Plasma Therm, Oxford Instruments, Trion
Texas Christian University Department of Engineering Ed Kolesar
Deep RIE Examples
Ayon et al., 1998 (MIT)
Klaassen et al., 1995 (Stanford)
STS 1999
20µm
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Texas Christian University Department of Engineering Ed Kolesar
Additive Processes• Formation of films on surface of substrate
(“surface micromachining”)• Structural layers• Sacrificial layers (spacers to be removed later)
Wide Variety Of Techniques:• Oxidation of Si• CVD, PECVD• Evaporation• Sputtering• Epitaxial growth• Molding
Texas Christian University Department of Engineering Ed Kolesar
Chemical Vapor Deposition
• CVD uses thermal energy to drive reactions that deposit thin films on substrate surface
• PECVD (Plasma Enhanced CVD) substitutes thermal energy (partially) by RF energy: greater control over stresses and other film properties
• Note analogy to Plasma Etching, RIE etching• Commonly deposited thin films with PECVD:
SiO2, Si3N4, SiC, poly-Si
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Texas Christian University Department of Engineering Ed Kolesar
Epitaxial Growth
• SCS grows selectively on exposed Si surfaces• 2H2 + SiCl4 → Si + 4HCl (hydrogen reduction)• SiH4 → Si + 2H2 (pyrolysis)
Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Electroplating• Plating processes use the reduction of metal ions in solution to
form solid metal• Many metals and alloys (Au, Ag, Cu, Hg, Ni, Pt, Permalloy
[NiFe], …)
• Electroplating uses electrical current to drive the reduction• Electroless plating uses reducing agents to drive metal
deposition
• Pulsing the electroplating current allows to replenish reactants(stress control, control over morphology, etc., possible)
• Under diffusion-limited conditions, amorphous metal layers can be plated (very high surface areas, e.g., “platinum black”)
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Texas Christian University Department of Engineering Ed Kolesar
ElectroplatingFastest growing crystal planes disappear
Note analogy to anisotropic etching
Figure: G. Kovacs, 1996,after Bockris, Reddy, 1970.
Texas Christian University Department of Engineering Ed Kolesar
Evaporation And SputteringEvaporation of metals by
Heating (thermal evaporation)Bombardment with electron beam (e-beam evaporation)
Sputtering: bombardment of target with inert ions (Ar+)MetalsSiCompoundsDielectrics
Better stress control
Figure: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
Sputtering vs. Evaporation
Geometry of evaporation and sputtering chambers (as well as electromagnetic fields) determine directionality of deposition:
Good or bad step coverage (can be advantage or disadvantage) Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Shadowing
• Directionality of evaporation can be exploited to form features smaller than the lithographic resolution
Figure: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
Sub-Resolution Feature Sizes
How can we build structures that are smaller than the resolution of our lithography equipment?
Texas Christian University Department of Engineering Ed Kolesar
Sacrificial Layers• Frequent goal in MEMS: released, movable structures• Concept: use spacer layers as temporary support
between structural materials• Commonly used sacrificial layers:
SiO2 (etched with HF)Photoresist (etched with acetone, O2 plasma)Others
• Example: SiO2 in multi-layer polysilicon structures
Figure: G. Kovacs, 1996.
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Sealed Cavity FormationMore complex
example for sacrificial layers:
• Form cavity with SiO2 layer
• Removal of sacrificial layer
• Reactive sealing
Figure: G. Kovacs, 1996.
Texas Christian University Department of Engineering Ed Kolesar
Sacrificial Layer In Electroplating
• Note: requires sufficient step coverage, otherwise…
Figure: G. Kovacs, 1996.
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Removal of deposited thin film (usually metal) without etching:
Positive resist: Negative resist:
Lift-Off Process
Substrate
resist resist
Substrate
Substrate Substrate
Lift-Off
Texas Christian University Department of Engineering Ed Kolesar
Wet Release IssuesAttractive Forces Between
Surfaces:• Electrostatic forces• Surface tension• Hydrophilic surfaces:
hydrogen bonds (attraction between a hydrogen atom of one molecule and a pair of unshared electrons of another molecule)
• Hydrophobic surfaces: van der Waals forces (attractive and repulsive electrostatic dipole-dipole interactions between molecules)
Figure: G. Kovacs, 1996.
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Texas Christian University Department of Engineering Ed Kolesar
Critical Point Drying• Adhesion forces during wet release can be a major
problem. Possible solutions:• Geometric surface modification
(dimples)• Chemical surface modification• Sublimation methods
• Critical point drying:CO2: 25°C at 1200psi (liquid)
35°C at 1200psi (supercritical)gas is then removed Figure: G. Kovacs, 1996, after
Mulhern et al., 1993.
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