EEL5225: Principles of MEMS Transducers (Fall 2003)1
EEL5225: Principles of MEMS Transducers (Fall 2003)
Fabrication Technology, Part II
Agenda:Surface micromachiningWafer bondingLIGA
Reading: Senturia, pp. 57-78.
Lecture 8 by H.K. Xie 9/12/2003
EEL5225: Principles of MEMS Transducers (Fall 2003)2
Surface Micromachining
OverviewSacrificial OxideSealed CavityVariationsMaterials
SurfaceMicro-
Machining
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Surface Micromachining
Basic process sequenceBegin with substrate
A. Deposit sacrificial layer- Also called spacer layer
B. Pattern holes in sacrificial layer- Anchor
C. Deposit structural layerD. Modify mechanical properties
via heat treatmentE. Remove sacrificial layer, or
“release”
Ref. Madou, Fundamentals of Microfabrication, p. 232.
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Surface MicromachiningSacrificial Materials
Sacrificial oxidePECVD SiO2
PSGBPSGMuch higher etch rate for doped oxide (PSG or BPSG)
Other materialsPhotoresist PolysiliconAluminum
Ref. Madou, Fundamentals of Microfabrication, p. 235.
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Surface MicromachiningStructural/Sacrificial Materials
Structural layer/ sacrificial layer combinations
Ref. Madou, Fundamentals of Microfabrication, p. 236.
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Surface MicromachiningStiction Problem
Potential problems with etch of sacrificial layer
“Stick-tion” = stiction– Pull-down via capillary forces due
to surface tension of liquid etchant
Solutions employedStand-off bumps (or dimples)Less polar solvents (methanol, ethanol)Phase change (avoiding liquid phase) via sublimation (freeze-drying) and critical point dryingHF vaporMonolayerPolymer spacer
Ref. Madou, Fundamentals of Microfabrication, p. 236. Adapted from Core et al., Solid State Technology, 1993.
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Surface MicromachiningSealed Cavity
Sealed cavity processes
Surface micromachining of cavitySealing
– Thin film sealing layer
– Reactive growth sealing
ApplicationsPressure sensorsVacuum packaging
Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139.
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Surface MicromachiningExamples
Variations on surface micromachining processPorous siliconHinged polysiliconMolded polysilicon structuresSilicon-on-insulatorPhotoresist and polymer structural elements
Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139.
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Surface MicromachiningPorous Silicon
Porous siliconFormed using low current density in HF solution
Very high aspect ratio poresPorous Si is highly reactive, oxidizes and etches at very high ratePorosity varies with current density20 angstrom to 10 um pore sizeApplications:
– Dielectric isolation– Capillaries for electrochemical
reference electrode– High surface area gas sensor– Humidity sensor– Light emission?
(photoluminescence demonstrated)
Si + 2F- + 2h+ → SiF2SiF2 + 2HF → SiF4 + H2
Micro optical filter (Lammel et al), composed of multiple porous silicon layers with different refractive indices obtained by varying current density
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Surface MicromachiningHinged Polysilicon
Hinged polysilicon
Issues include friction and assemblyPlastically deformable polyimide hingeHinged aluminum fabricated using XeF2etch
Hinged polysilicon and aluminum structures by Pister’s research group. Ref. Madou, Fundamentals of Microfabrication, p. 244-245.
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Surface MicromachiningMolded Polysilicon Structures (HEXSIL)
Molded polysilicon structures by Keller. Ref. Madou, Fundamentals of Microfabrication, p. 246.
Microtweezerwww.memspi.com
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Surface MicromachiningSOI Substrates
Silicon-on-insulator (SOI) substrateSeparation by Implanted Oxygen (SIMOX)Wafer bonding
Bonded wafer production
SIMOX wafer production
From Dunn, Solid State Technology, 1993 on How SOI wafers are made. Ref. Madou, Fundamentals of Microfabrication, p. 248.
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Surface MicromachiningSOI Substrates
Silicon-on-insulator (SOI) substrateAdvantages for micromachining
– Fewer process steps required for release– Mechanical uniformity of single crystalline silicon– Inherent dielectric isolation of electronics– Reduced parasitic capacitances
Disadvantages– Electronic silicon damage near buried oxide (BOX)/c-Si
interfaceIncreased electronic trapping/detrapping related 1/f noiseIncreased P/N junction leakage currents
– Higher cost of SOI wafers
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Surface MicromachiningSOI Substrates – Pressure Sensor
SOI surface micromachined absolute pressure sensor
Diem et al, “SOI as a substrate for surface micromachining of single-crystalline silicon sensors and actuators,” 7th Intl. Conf. on Solid-State Sensors and Actuators, 1993. Ref. Madou, Fundamentals of Microfabrication, p.250.
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Surface Micromachining – Materials
Commonly used materialsCVD poly-crystalline siliconCVD silicon nitrideCVD silicon dioxide
Key propertiesDeposition
– Temperature– Stress– Thickness
Etch– Etch rate– Selectivity
Stress– Compressive or tensile– Thermal expansion coefficient
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Surface Micromachining – Materials
compressive
CVD poly-crystalline SiDependence of morphology and mechanical properties on deposition temperature
Schematic of compressive polysilicon formed at 620-650C from Krulevitch. Ref. Madou, Fundamentals of Microfabrication, p. 256-257.
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Surface Micromachining – Materials
CVD poly-crystalline siliconEffect of heat treatment fine-grain polysilicon strainModification from initial compressive strain to tensile strain
Anneal curves from Guckel et al., Ref. Madou, Fundamentals of Microfabrication, p. 258.
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Surface Micromachining – Materials
CVD silicon nitrideExtremely good diffusion barrier for water and Na+ ionsInsulator for nonvolatile memoriesLow oxidation rateOxidation barrierImplantation maskEtch mask
Stoichiometry depends on deposition conditionsHence deposition dependence of morphology, electrical, and mechanical properties
Stoichiometric Si3N4
Variable SixNy:Other
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Surface Micromachining – Materials
CVD silicon nitridePECVD nitride
– Very high hydrogen content (SiN:H)
– As-deposited: high compressive stress
Leads to wafer warping and film cracking or delamination
LPCVD nitride– Tensile stress– Increasing silicon
content allows reduction of residual stress (low-stress silicon-rich nitride)
Effect of gas ratio on nitride residual stress from Sakimoto et al. Ref. Madou, Fundamentals of Microfabrication, p. 261.
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Surface Micromachining – Materials
Properties of CVD silicon nitride
Highly dependent on deposition temperature, rate, and composition
Ref. Madou, Fundamentals of Microfabrication, p. 261.
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Surface Micromachining – Materials
CVD silicon dioxideInterlayer dielectric between metal layers on integrated circuits (process determined by maximum temperature for metal lines, less than 350C for aluminum lines)Implantation maskDiffusion barrier for moisturePlanarization by reflow (not chemical-mechanical planarization)High etch rate for sacrificial oxide
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Surface Micromachining – Materials
Properties of CVD silicon dioxideHighly dependent on deposition temperature, rate, and compositionThermal SiO2 and higher temp CVD SiO2 are highly compressive
TEOS:tetra-ethoxy-silane
Ref. Kovacs, MicromachinedTransducers Sourcebook, p81.
