1 ME 381R Fall 2003 Micro-Nano Scale Thermal-Fluid Science and Technology Lecture 18: Introduction to MEMS Dr. Li Shi Department of Mechanical Engineering The University of Texas at Austin Austin, TX

2 Outline MEMS: Applications Pattern transfer Method - Lithography Subtractive Method - Wet and Dry Etching Additive Method - Thin Film Deposition Example (Cantilever Beam) } Basic Fabrication Techniques

3 MEMS Applications

4 MEMS Applications

5 This image depicts a mirror system moved by a small rotating gear MEMS Applications

6 MEMS Fabrication Techniques Pattern Transfer Method : Photo Lithography E-beam Lithography Nano-imprinting Lithography Subtractive Method : Wet Etching Dry Etching Additive Method : Thin Film Deposition

7 Photolithography Energy Change cross linking of polymer chains of a photo-sensitive polymer called photoresist, and modify its dissociation rate in a developer similar to that for developing photos Mask Absorber (Dark Area) & window (Open area) Resist Transfer image from mask to wafer

8 Positive Photoresist - the polymer is weakened and more soluble in the developer Negative Photoresist - the polymer is hardened and less soluble Photolithography

9 Exposure Systems Contact – not limited by diffraction Proximity – 2 ~ 20  m gap, limited by diffraction Projection – limited by diffraction Photolithography

10 Direct writing on resist-coated substrate No Mask Minimum beam size (5nm) for optical & X-ray masks, and nano-devices Electron Beam Lithography

11 Energy Sources Energy Minimum Line Width Energy Sources

12 Comparison OpticalElectron Beam Advantage  Low ~High precision  Fast exposure speed  Relatively low cost  No diffraction  Easy to control  Available for small features Disadvantage  Light diffraction  Alignment problem  Debris between mask and wafer  Needs vacuum  High system cost  Slow Comparison

13 Nano-imprinting Lithography: Courtesy: Profs. C. G. Willson & S.V. Sreenivasan, UT Austin Step and Flash Imprinting Lithography (S-FIL TM )

14 Sub-40 nm contacts 30 nm dense lines 20 nm isolated lines S-FIL Patterned Nanowires Courtesy: Profs. C. G. Willson and S.V. Sreenivasan

15 Subtractive Method Wet and Dry Etching Wet Etching - Chemical Reaction - Liquid source Dry Etching - Chemical + Physical Reaction - Gas or Vapor phase source

16 Etching Mechanism

17 Dry Etching Pressure Ion Energy Physical Etching (Sputtering) Momentum transfer  bond breakage Particle Collisions Aniosotropic Physical- chemical Etching Ion bombardment to make the surface more reactive Anisotropic Chemical Etching Reactive etchant species Isotropic

18 Silicon Crystal Structure

19 Isotropic and Anisotropic in Wet and Dry Etching IsotropicAnisotropic Wet Dry For Silicon - HNA (HF, HNO 3, CH 3 COOH)

20 Comparison ParameterWet EtchingDry Etching Directionality Only with single crystal materials With most materials Cost LowHigh Selectivity Can be very goodPoor Typical Etch Rate Fast (1um/min)Slow (0.1 um/min) Control DifficultGood

21 Etching Process

22 Additive Method Thin Film Deposition Physical Vapor Deposition (PVD) Thermal Evaporation Sputtering Chemical Vapor Deposition (CVD) PECVD (Plasma Enhanced) LPCVD (Low Pressure) Electroplating

23 PVD: Thermal Evaporation Thermal Evaporation evaporacion_resistencia.html Based on the sublimation Fast Low surface damage Difficult to control Limited materials (metal) Low working pressure to increase mean free path Low surface damage Faster than sputtering Limited material

24 PVD: Sputtering Sputtering Based on Ion bombardment Unlimited material Possible surface damage Excellent adhesion Expensive

25 CVD: Plasma Enhanced PECVD research-pecvd.jpg Plasma helps reaction Low substrate temperature Good step coverage Chemical contamination

26 CVD: Low Pressure LPCVD < 10 Pa Excellent purity Low stress High temperature Low deposition rate images/cvd_reactor.jpg

27 Electroplating Various metal (Au, Ni, etc) Fast > 10  m Hydrogen bubble generation Difficult for sub-  m features Needs seed layer J.W. Judy, "Magnetic microactuators with polysilicon flexures," Masters Report, Department of EECS, University of California, Berkeley, August 29, 1994

28 Making a Cantilever Beam (Surface Micromachining)