Professor N Cheung , U.C. BerkeleyLecture 24EE143 F20101• 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, PhotoMEMSMicro-Electro-Mechanical Systems (MEMS) FabricationProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20102Axial Stress and StrainStress s: force per unit area acting on a material[unit: Newtons/m2 (pascal)]s = F/A , A = areas > 0 tensiles < 0 compressiveStrain e: displacement per unit length (dimensionless)e = L/ Lo* Figure assumes there is no change in lateral dimensionsProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20103E = s / e [ in N/m2 (Pascal) ]Poisson’s Ratio = 0.5 volume conservedE in GPa ( 1E9 N/m2)Si190SiO273Diamond1035Young’s Modulus of a materialProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20104Stress-Strain CharacteristicFor low stress:• material responds in elastic fashion• (Hooke’s Law) stress/strain = constantsy= yield stressUltimate stress - material will break;For Si (brittle) ultimate stress ~ yield stressProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20105Mechanical Properties of Microelectronic MaterialsProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20106Material Choices(a) Stiffness(b) StrengthProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20107Poly-Si For MEMS Structure• Effect of substrate:single-crystal substrate(clean surface) epitaxial layeramorphous substrate polycrystalline film• Average grain size depends on deposition & annealing conditionsProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F20108Stress in LPCVD Poly-Si Films• Stress varies significantly with process conditions– strong correlation between microstructure and stressStrain vs. tanneal:Tdep~620oCProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201091) Begin with a bonded SOI wafer. Grow and etch a thin thermal oxide layer to act as a mask for the silicon etch.2) Etch the silicon device layer to exposethe buried oxide layer.3) Etch the buried oxide layer in buffered HF to release free-standing structures.Si device layer, 20 µm thickburied oxide layerSi handle waferoxide mask layersiliconThermal oxideUse of SOI for MEMS ProcessProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201010Origins of Thin-film Stress• Extrinsic– Applied stress– Thermal expansion– Plastic deformation• Intrinsic– Growthmorphology– Lattice misfit– Phase transformationstot= sth+ sint+ sextProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201011substratesubstrateEffect of Thin-film Stress Gradient on Cantilever Deflectionsubstratez(1) No stress gradient along z-direction (2) Higher tensile stress near top surface of cantileverbefore release from substarte(3) Higher compressive stress near top surface of cantileverbefore release from substrateCantileverProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201012Thin-films Stress Gradient Effects on MEMS Structures Top of beam more tensileTop of beam more compressiveProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201013Stressing along the x-direction, all layers take the same strainEx= fA EA+ fB EBfAand fBare fractional volumes* Material with larger E takes the larger stressStressing along the y-direction, all layers take the same stressEy=1/ [ fA / EA+ fB / EB ]* Material with smaller E takes the larger strainABEffective Young’s Modulus of Composite LayersProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201014PECVD 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 H+ bombardment energy (eV)MechanicalStress in nitride (in 1E8 Pa)01000eVCompressive Tensile-3-6+3+6Professor N Cheung , U.C. BerkeleyLecture 24EE143 F201015Use of Stressed Composite layer to reduce bendingProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201016Thermal StrainProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201017Biaxial Stress in Thin Film on Thick SubstrateNo 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. BerkeleyLecture 24EE143 F201018t s= substrate thicknesst f= film thicknessE = Young’s modulus of substraten= Poisson’s ratio of substrateRadius of Curvature of warpage“Stoney Equation” r = Es ts2 ( 1- )s 6 sf tf See handout for derivationSubstrate WarpageProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201019Typical Thin Film stress: 108to 5x1010dynes/cm2(107dynes/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 limitProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201020Professor N Cheung , U.C. BerkeleyLecture 24EE143 F201021The oxide stress is compressivesince r changes from 300m to 200m (Si wafer more curved)Calculate Film Stress from change of curvatureProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201022Deflection of Microstructures - Thin Plate approximationCantilever Beam with length L, width w, and thickness tF in Newton in N/meter* Assumes L >> w and t, small deflection approximationwhere L = length of beam (in meter)t =thickness of beam (in meter)I = bending moment of inertia = wt3/12 (in meter4)For reference onlyProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201023Deflection of Circular thin membraner = radius, t=thickness, P= uniform pressure (in N/m2)For small deflections, maximum deflection in centerA more accuraterelationshipFor reference onlyProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201024kHzProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201025Stiction Poly-Si beam released without stiction after sacrificial layer etchingPoly-Si beam with two stiction pointsafter sacrificial layer etchingProfessor N Cheung , U.C. BerkeleyLecture 24EE143 F201026As 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
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