EE143 F05 Lecture 25 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 F05 Lecture 25 Axial Stress and Strain Stress force per unit area acting on a material unit Newtons m2 pascal F A A area 0 tensile 0 compressive Strain displacement per unit length dimensionless L Lo Figure assumes there is no change in lateral dimensions Professor N Cheung U C Berkeley 2 EE143 F05 Lecture 25 Young s Modulus of a material E Si SiO2 Diamond 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 F05 Lecture 25 Stress Strain Characteristic For low stress material responds in elastic fashion Hooke s Law stress strain constant y yield stress Ultimate stress material will break For Si brittle ultimate stress yield stress Professor N Cheung U C Berkeley 4 EE143 F05 Lecture 25 Mechanical Properties of Microelectronic Materials Professor N Cheung U C Berkeley 5 EE143 F05 Lecture 25 Material Choices a Stiffness Professor N Cheung U C Berkeley b Strength 6 EE143 F05 Lecture 25 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 7 EE143 F05 Lecture 25 Poly Si as a Structural Material in MEMS Stronger than steel Not quite but close poly 1 6E11Pa steel 1 6E11 to2E11Pa Does not readily fatigue Directly compatible with modern IC fabrication processes batch fabrication in foundry high volume production at low unit cost Mechanical properties depend on film microstructure microstructure determined by fabrication process deposition and annealing conditions Professor N Cheung U C Berkeley 8 EE143 F05 Lecture 25 Poly Si 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 9 EE143 F05 Stress in LPCVD Poly Si Films Lecture 25 Stress varies significantly with process conditions Strain vs tanneal strong correlation between microstructure and stress Tdep 620oC Professor N Cheung U C Berkeley 10 EE143 F05 Lecture 25 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 tot th int ext 11 EE143 F05 Lecture 25 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 12 EE143 F05 Lecture 25 Thin films Stress Gradient Effects on MEMS Structures Top of beam more tensile Top of beam more compressive Professor N Cheung U C Berkeley 13 EE143 F05 Lecture 25 Use of Stressed Composite layer to reduce bending Professor N Cheung U C Berkeley 14 EE143 F05 Lecture 25 Thermal Strain Professor N Cheung U C Berkeley 15 EE143 F05 Lecture 25 Biaxial Stress in Thin Film on Thick Substrate No stress occurs in direction normal to substrate z 0 Assume isotropic film x y so that x y See derivation in EE143 handout Tu et al Electronic Thin Film Science Professor N Cheung U C Berkeley 16 EE143 F05 Lecture 25 Substrate Warpage Radius of Curvature of warpage r Es ts2 1 s 6 f 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 17 EE143 F05 Lecture 25 Typical Thin Film stress 108 to 5x1010 dynes cm2 107 dynes cm2 1 MPa Compressive 0 film tends to expand upon release buckling blistering delamination Tensile 0 film tends to contract upon release cracking if forces fracture limit Professor N Cheung U C Berkeley 18 EE143 F05 Professor N Cheung U C Berkeley Lecture 25 19 EE143 F05 Lecture 25 The oxide stress is compressive since r changes from 300m to 200m Si wafer more curved The wafer is less curved than with oxide alone Therefore the nitride film has a tensile stress However the total stress of the dual films is still compressive since r 240 m and is still smaller than the original curvature of 300 m Professor N Cheung U C Berkeley 20 EE143 F05 Lecture 25 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 21 EE143 F05 Lecture 25 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 22 EE143 F05 Lecture 25 kHz Professor N Cheung U C Berkeley 23 EE143 F05 Lecture 25 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 24 EE143 F05 Lecture 25 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 Mechanisms in Micromechanical Devices Tribology Letters 1997 Professor N Cheung U C Berkeley 25 EE143 F05 Lecture 25 Wafer Bonding Anodic bonding E field enhanced Adhesive bonding molten metal epoxy Direct wafer bonding can produce
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