Lecture 2: VLSI-based Fabrication for MEMS: Fundamentals Prasanna S. Gandhi Assistant Professor, Department of Mechanical Engineering, Indian Institute of Technology, Bombay, Recap: Last Class What is MEMS? Why study of MEMS important? Practical examples and products Contents of the course Fabrication Analysis Characterization
Today s Class Materials for MEMS VLSI-based fabrication processes for MEMS: an overview Lithography Material removal Material addition Combination of all these to make devices Materials for MEMS <111> <100> <001> <010> Silicon crystal orientation Silicon Stronger than steel Light as Aluminum Can be coated with varieties of materials Available in form of wafers 2, 4, 8, 12 dia Other materials
Materials for MEMS Other materials Polycrystalline silicon (polysilicon) Silicon dioxide (SiO 2 ) Silicon nitride (Si 3 N 4 ) Aluminum (thin film) Chromium, Gold (thin film) Several others now a days: various photoresists (polymers), tungston, copper, magnetic, etc. Doping of silicon Fabrication processes Material removal Material deposition Lithography: patterning Chemical etching Isotropic Anisotropic Plasma etching: RIE Oxidation Sputtering Chemical vapor deposition (CVD) Electroplating Surface micromachining LIGA
Lithography Positive Photoresist (PPR) U-V RAYS MASK Lithography Negative Photoresist (NPR) U-V RAYS MASK
Lithography E-beam Lithography Features are written by scanning electron beam No necessity of mask Can be used for preparation of mask Very fine size (sub-micron or <1 micron) features can be produced easily Not suitable for higher length features Typical resist PMMA Polymethylmethacrylate Chemical Etching Without agitation (5) With agitation (20) Isotropic etching Etchant: HNA mixture. HNA can dissolve 550µm thick silicon wafer in about 20 min. HNA mixture removes silicon equally in all directions. SiO2 etch: 10-30nm/min
<100> Chemical Etching <111> <001> <010> <100> surface wafer <110> surface wafer Anisotropic etching Etchant: KOH, EDP <111> direction has lower etching rates than <100> Can produce grooves, slanted/vertical walls Plasma Etching Vacuum chamber Electrode PLASMA Wafer Electrode V Gas is exposed to electric and magnetic fields Ionized gas hits the target surface to remove material Plasma + chemical Reactive ion etching (RIE) very efficient process
Oxidation Oxidation of Si: keep in air at high temp (1000-1200 o C) Well understood and controlled process 2 Parameters Tox + AT ox = B( t + τ ) Temperature A, B Constants Environment dt ox (2Tox + A) = B Time dt Oxide: important patterning material Problems: thermal stresses Sputtering Vacuum chamber Electrode Ar/He PLASMA Wafer Electrode Target V Target surface bombarded with a flux of inert ions (Ar, he) DC fields or RF for accelerating Deposition rates 1µm/min for Al Granular deposited film under stress Low temperatures
Chemical Vapor Deposition (CVD) Gases Wafer Temperature > 300 o C Chemical reaction in vacuum chamber High temperatures (>300 o C) Polysilicon, SiO2, Si3N4, tungston, titanium, copper etc. can be deposited Low pressure CVD (LPCVD) Plasma Enhanced CVD: low temperatures Pressure, temp, gas flow Surface Micromachining Wafer U-V RAYS Combination of lithography and etching Defined with respect to deposited films instead of Si substrate
Surface Micromachining At DARPA, USA Surface Micromachining Material Systems Structural Polysilicon Polyimide LPCVD Si 3 N 4 + Al Aluminum Sacrificial SiO 2 Aluminum Polysilicon Photoresist Release Etch Buffered HF PAN etch XeF 2 Oxygen plasma Isolation Si 3 N 4 + SiO 2 SiO 2 SiO 2 SiO 2
More Complex Fabrication Electroplating U V LIGHT OPTICAL MASK CHROMIUM LAYER COPPER LAYER ADHESION LAYER SUBSTRATE
LIGA Combined Lithography, electroplating and molding process for high aspect ratio (depth/width) structures MASK PMMA RESIST SUBSTRATE LIGA Micromotor fabricated using LIGA
Design Considerations Design for fabrication: example, long shaft manufacture not possible Fabrication: series of selective deposition and etching processes Array of devices can be fabricated Cost: no. of lithography steps should be minimum for cost effective design Other consideration Thermal expansion of devices Air damping Integration of electronics along with mechanical device Packaging and safety Sensing Methods Resistive sensing Based on strain: accelerometers Based on temperature: Capacitive sensing: accelerometers, pressure sensors Bimetalic strips Thermocouple effect Piezoelectric Using optics laser source and detectors Diffraction effects Interference effects Quadrature photo diodes
Conclusion VLSI-based fabrication processes for MEMS Lithography Material removal: Etching: Isotropic, anisotropic, RIE Material Deposition: CVD, LPCVD, PECVD Design: entirely new philosophy Fabrication Analysis Next class Optical Lithography Process details Various types Important parameters