1Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05• 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) Fabrication2Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Axial Stress and StrainStress σ: force per unit area acting on a material[unit: Newtons/m2 (pascal)]σ = F/A , A = areaσ > 0 tensileσ < 0 compressiveStrain ε: displacement per unit length (dimensionless)ε = ∆L/ Lo* Figure assumes there is no change in lateral dimensions3Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05E = σ / ε [ in N/m2 (Pascal) ]Poisson’s Ratio νν = 0.5 ⇒ volume conservedE in GPa ( 1E9 N/m2)Si 190SiO2 73Diamond 1035Young’s Modulus of a material4Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Stress-Strain CharacteristicFor low stress:• material responds in elastic fashion• (Hooke’s Law) stress/strain = constantσy= yield stressUltimate stress - material will break;For Si (brittle) ultimate stress ~ yield stress5Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Mechanical Properties of Microelectronic Materials6Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Material Choices(a) Stiffness(b) Strength7Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Stressing along the x-direction, all layers take the same strainEx= fAEA+ fBEBfAand 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 Layers8Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Poly-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)9Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Poly-Si Structure• Effect of substrate:single-crystal substrate(clean surface)⇒ epitaxial layeramorphous substrate⇒ polycrystalline film• Average grain size depends on deposition & annealing conditions10Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Stress in LPCVD Poly-Si Films• Stress varies significantly with process conditions– strong correlation between microstructure and stressStrain vs. tanneal:Tdep~620oC11Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Origins of Thin-film Stress• Extrinsic– Applied stress– Thermal expansion– Plastic deformation• Intrinsic–Growthmorphology– Lattice misfit– Phase transformationσtot= σth+ σint+ σext12Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05substratesubstrateEffect 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 substrateCantilever13Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Thin-films Stress Gradient Effects on MEMS Structures Top of beam more tensileTop of beam more compressive14Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Use of Stressed Composite layer to reduce bending15Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Thermal Strain16Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Biaxial Stress in Thin Film on Thick SubstrateNo stress occurs in direction normal to substrate (σz=0)Assume isotropic film (εx=εy=ε so that σx=σy=σ)* See derivation in EE143 handout(Tu et al, Electronic Thin Film Science)17Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Radius of Curvature of warpaget s= substrate thicknesst f= film thicknessE = Young’s modulus of substraten= Poisson’s ratio of substrate“Stoney Equation” r = Es × ts2 ( 1- ν)s × 6 ×σf ×tf See handout for derivationSubstrate Warpage18Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Typical Thin Film stress: 108to 5x1010dynes/cm2(107dynes/cm2= 1 MPa)• Compressive (ε <0)– film tends to expand upon release--> buckling, blistering, delamination• Tensile (ε >0)– film tends to contract upon release--> cracking if forces > fracture limit19Professor N Cheung , U.C. BerkeleyLecture 25EE143 F0520Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05The oxide stress is compressivesince 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 m21Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Deflection of Microstructures - Thin Plate approximationCantilever Beam with length L, width w, and thickness tF in Newtonρ 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 only22Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Deflection of Circular thin membraner = radius, t=thickness, P= uniform pressure (in N/m2)For small deflections, maximum deflection in centerA more accuraterelationshipFor reference only23Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05kHz24Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05Stiction Poly-Si beam released without stiction after sacrificial layer etchingPoly-Si beam with two stiction pointsafter sacrificial layer etching25Professor N Cheung , U.C. BerkeleyLecture 25EE143 F05As 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
View Full Document
Unlocking...