EE143 F2010 Lecture 24 Micro Electro Mechanical Systems MEMS Fabrication Fabrication Considerations Stress Strain Thin film Stress Stiction Special Process Modules for MEMS Bonding Cavity Sealing Deep RIE Spatial forming Molding Layer Transfer Principle of Sensing and Actuation Beam and Thin Plate Deflections Micromachining Process Flows MEMS IC Integration BioMEMS PhotoMEMS Professor N Cheung U C Berkeley 1 EE143 F2010 Lecture 24 Axial Stress and Strain Stress s force per unit area acting on a material unit Newtons m2 pascal s F A A area s 0 tensile s 0 compressive Strain e displacement per unit length dimensionless e L Lo Figure assumes there is no change in lateral dimensions Professor N Cheung U C Berkeley 2 EE143 F2010 Lecture 24 Young s Modulus of a material E s Si SiO2 Diamond e in N m2 Pascal E in GPa 1E9 N m2 190 73 1035 Poisson s Ratio 0 5 volume conserved Professor N Cheung U C Berkeley 3 EE143 F2010 Lecture 24 Stress Strain Characteristic For low stress material responds in elastic fashion Hooke s Law stress strain constant sy yield stress Ultimate stress material will break For Si brittle ultimate stress yield stress Professor N Cheung U C Berkeley 4 EE143 F2010 Lecture 24 Mechanical Properties of Microelectronic Materials Professor N Cheung U C Berkeley 5 EE143 F2010 Lecture 24 Material Choices a Stiffness Professor N Cheung U C Berkeley b Strength 6 EE143 F2010 Lecture 24 Poly Si For MEMS Structure Effect of substrate single crystal substrate clean surface epitaxial layer amorphous substrate polycrystalline film Average grain size depends on deposition annealing conditions Professor N Cheung U C Berkeley 7 EE143 F2010 Lecture 24 Stress in LPCVD Poly Si Films Stress varies significantly with process conditions Strain vs tanneal strong correlation between microstructure and stress Tdep 620oC Professor N Cheung U C Berkeley 8 EE143 F2010 Lecture 24 Use of SOI for MEMS Process oxide mask layer Si device layer 20 m thick 1 Begin with a bonded SOI wafer Grow and etch a thin thermal oxide layer to act as a mask for the silicon etch buried oxide layer Si handle wafer silicon 2 Etch the silicon device layer to expose the buried oxide layer Thermal oxide 3 Etch the buried oxide layer in buffered HF to release free standing structures Professor N Cheung U C Berkeley 9 EE143 F2010 Lecture 24 Origins of Thin film Stress Extrinsic Applied stress Thermal expansion Plastic deformation Intrinsic Growth morphology Lattice misfit Phase transformation Professor N Cheung U C Berkeley stot sth sint sext 10 EE143 F2010 Lecture 24 Effect of Thin film Stress Gradient on Cantilever Deflection z Cantilever substrate 1 No stress gradient along z direction substrate 2 Higher tensile stress near top surface of cantilever before release from substarte Professor N Cheung U C Berkeley substrate 3 Higher compressive stress near top surface of cantilever before release from substrate 11 EE143 F2010 Lecture 24 Thin films Stress Gradient Effects on MEMS Structures Top of beam more tensile Top of beam more compressive Professor N Cheung U C Berkeley 12 EE143 F2010 Lecture 24 Effective Young s Modulus of Composite Layers B A fA and fB are fractional volumes Stressing along the x direction all layers take the same strain Ex fA EA fB EB Material with larger E takes the larger stress Stressing along the y direction all layers take the same stress Ey 1 fA EA fB EB Professor N Cheung U C Berkeley Material with smaller E takes the larger strain 13 EE143 F2010 Lecture 24 PECVD silicon nitride using the SiH NH N chemistry Substrate RF bias is used to induce ion bombardment Because of the light mass H ions can be assumed as the dominant ion bombardment flux Mechanical Stress in nitride in 1E8 Pa 6 3 Compressive 0 Tensile 3 6 Professor N Cheung U C Berkeley 1000eV H bombardment energy eV 14 EE143 F2010 Lecture 24 Use of Stressed Composite layer to reduce bending Professor N Cheung U C Berkeley 15 EE143 F2010 Lecture 24 Thermal Strain Professor N Cheung U C Berkeley 16 EE143 F2010 Lecture 24 Biaxial Stress in Thin Film on Thick Substrate No stress occurs in direction normal to substrate sz 0 Assume isotropic film ex ey e so that sx sy s See derivation in EE143 handout Tu et al Electronic Thin Film Science Professor N Cheung U C Berkeley 17 EE143 F2010 Lecture 24 Substrate Warpage Radius of Curvature of warpage r Es ts2 1 s 6 sf tf Stoney Equation t s substrate thickness t f film thickness E Young s modulus of substrate n Poisson s ratio of substrate See handout for derivation Professor N Cheung U C Berkeley 18 EE143 F2010 Lecture 24 Typical Thin Film stress 108 to 5x1010 dynes cm2 107 dynes cm2 1 MPa Compressive e 0 film tends to expand upon release buckling blistering delamination Tensile e 0 film tends to contract upon release cracking if forces fracture limit Professor N Cheung U C Berkeley 19 EE143 F2010 Professor N Cheung U C Berkeley Lecture 24 20 EE143 F2010 Lecture 24 Calculate Film Stress from change of curvature The oxide stress is compressive since r changes from 300m to 200m Si wafer more curved Professor N Cheung U C Berkeley 21 EE143 F2010 Lecture 24 Deflection of Microstructures Thin Plate approximation Cantilever Beam with length L width w and thickness t Assumes L w and t small deflection approximation where L length of beam in meter t thickness of beam in meter I bending moment of inertia wt3 12 in meter4 Professor N Cheung U C Berkeley F in Newton in N meter For reference only 22 EE143 F2010 Lecture 24 Deflection of Circular thin membrane r radius t thickness P uniform pressure in N m2 For small deflections maximum deflection in center A more accurate relationship For reference only Professor N Cheung U C Berkeley 23 EE143 F2010 Lecture 24 kHz Professor N Cheung U C Berkeley 24 EE143 F2010 Lecture 24 Stiction Poly Si beam released without stiction after sacrificial layer etching Poly Si beam with two stiction points after sacrificial layer etching Professor N Cheung U C Berkeley 25 EE143 F2010 Lecture 24 As the etching liquid is removed during a dehydration cycle a liquid bridge is formed between the suspended member and the substrate An attractive capillary force which may be sufficiently strong to collapse it Even after drying the inter solid adhesion will not release the structure Solutions Dry etching e g XeF2 Super critical drying e g rinse solution gradually replaced by liquid CO2 under high pressure Hydrophobic Coatings Use textured surfaces See C H Mastrangelo Adhesion Related Failure
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