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 1
Axial Stress and Strain Stress s: force per unit area acting on a material [unit: Newtons/m 2 (pascal)] s = F/A, A = area s > 0 tensile s < 0 compressive Strain e: displacement per unit length (dimensionless) e = L/ L o * Figure assumes there is no change in lateral dimensions 2
Young s Modulus of a material E in GPa ( 1E9 N/m2) Si 190 SiO2 73 Diamond 1035 Poisson s Ratio E = s / e [ in N/m 2 (Pascal) ] = 0.5 volume conserved 3
Stress-Strain Characteristic For low stress: material responds in elastic fashion (Hooke s Law) stress/strain = constant s y = yield stress Ultimate stress - material will break; For Si (brittle) ultimate stress ~ yield stress 4
Mechanical Properties of Microelectronic Materials 5
Material Choices (a) Stiffness (b) Strength 6
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 7
Strain vs. t anneal : EE143 F2010 Stress in LPCVD Poly-Si Films Stress varies significantly with process conditions strong correlation between microstructure and stress T dep ~620 o C 8
Use of SOI for MEMS Process 1) Begin with a bonded SOI wafer. Grow and etch a thin thermal oxide layer to act as a mask for the silicon etch. oxide mask layer Si device layer, 20 µm thick buried oxide layer Si handle wafer 2) Etch the silicon device layer to expose the buried oxide layer. silicon Thermal oxide 3) Etch the buried oxide layer in buffered HF to release free-standing structures. 9
Origins of Thin-film Stress Extrinsic Applied stress Thermal expansion Plastic deformation Intrinsic Growth morphology Lattice misfit Phase transformation s tot = s th + s int + s ext 10
Effect of Thin-film Stress Gradient on Cantilever Deflection Cantilever z substrate (1) No stress gradient along z-direction substrate substrate (2) Higher tensile stress near top surface of cantilever before release from substarte (3) Higher compressive stress near top surface of cantilever before release from substrate 11
Thin-films Stress Gradient Effects on MEMS Structures Top of beam more tensile op of beam more compressive 12
Effective Young s Modulus of Composite Layers B A f A and f B are fractional volumes Stressing along the x-direction, all layers take the same strain E x = f A E A + f B E B * Material with larger E takes the larger stress Stressing along the y-direction, all layers take the same stress E y =1/ [ f A / E A + f B / E B ] * Material with smaller E takes the larger strain 13
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 +3 +6 Tensile 1000eV H+ bombardment energy (ev) 14
Use of Stressed Composite layer to reduce bending 15
Thermal Strain 16
Biaxial Stress in Thin Film on Thick Substrate No stress occurs in direction normal to substrate (s z =0) Assume isotropic film (e x =e y =e so that s x =s y =s) * See derivation in EE143 handout (Tu et al, Electronic Thin Film Science) 17
Substrate Warpage Radius of Curvature of warpage r = E s t s 2 ( 1- ) s 6 s f t f Stoney Equation t s t f = substrate thickness = film thickness E = Young s modulus of substrate n = Poisson s ratio of substrate See handout for derivation 18
Typical Thin Film stress: 10 8 to 5x10 10 dynes/cm 2 (10 7 dynes/cm 2 = 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 19
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Calculate Film Stress from change of curvature The oxide stress is compressive since r changes from 300m to 200m (Si wafer more curved) 21
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 = wt 3 /12 (in meter 4 ) For reference only F in Newton in N/meter 22
Deflection of Circular thin membrane r = radius, t=thickness, P= uniform pressure (in N/m 2 ) For small deflections, maximum deflection in center A more accurate relationship For reference only 23
khz 24
Stiction Poly-Si beam released without stiction after sacrificial layer etching Poly-Si beam with two stiction points after sacrificial layer etching 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. XeF 2 ) Super-critical drying (e.g. rinse solution gradually replaced by liquid CO 2 under high pressure) Hydrophobic Coatings Use textured surfaces See C. H. Mastrangelo, Adhesion-Related Failure Mechanisms in Micromechanical Devices, Tribology Letters, 1997 26
Super-critical drying i) release by immersion in aqueous HF; ii) substrate and structure hydrophilic passivation by immersion in a sulfuric peroxide or hydrogen peroxide solution resulting in hydrophilic silicon surfaces; iii) thorough deionized water rinses followed by a methanol soak to displace the water; iv) methanol-soaked samples placed in the supercritical drying chamber for drying. 27
XeF 2 Etching of Si * Dry, isotropic, vapor-phase etch XeF 2 vapor pressure (~3.8 Torr at 25 C) 2 XeF 2 + Si 2 Xe (g) + SiF 4 (g) Advantages : Highly selective to silicon with respect to Al, photoresist, and SiO 2. Isotropic, large structures can be undercut. Fast ( ~10mm per hour) Gas phase etching, no stiction between freed structure and substrate Disadvantages: No known etch stops for Si substrate 28
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Wafer Bonding Anodic bonding (E-field enhanced) Adhesive bonding (molten metal, epoxy) Direct wafer bonding *can produce unique MEMS structures 30
Anodic Bonding Anodic Bonding: Low to moderate Temp, Rapid Process -1kV glass silicon T=300 o C * works mainly with alkaline-containing glass 31
Example of Anodic Bonding Pressure Transducer using membrane deflection glass glass glass glass 32
Liquid Phase Bonding After RTP for 750 o C/10secs Al sealing ring width=125mm Water is blocked outside Al does not wet glass well. Add Cr adhesion layer between Al and glass Vibrating resonator Nitride Al width=125 mm Air inside Water outside Top view Bump Cross-section Chiao and Lin, UCB 33
Direct Bonding Heat Examples: Si-Si bonding and Si-SiO2 bonding 34
Plasma Assisted Direct Bonding Gases Used: O 2, He, N 2, Ar Pressure 200mTorr Power 50W-300W Time 15-30 seconds Plasma Treatment + O 2 - Room Temp Bonding Chemical Cleaning: Piranha + RCA Bond Strengthening Annealing 35
Requirements of Direct Bonding Surfaces Surface micro-roughness ~ nm No macroscopic wafer warpage Minimal particle density and size: 1mm particle will give 1000mm void Contamination free surface 36
Permanent Bond Formation Si O O O O Si Si O Si O Si Si O Si Si Si Hydrogen Bonding Formation of Covalent bond 37
3-D circuit with a metal interconnect (at top), followed by a memory substrate, a bond interface, then logic metal, logic transistors and a logic substrate. Metal interconnects Memory substrate Bond memory substrate and logic substrate. Thin both substrates with grinding and CMP. Etch vias and metallization to connect the two die. Logic substrate http://www.ziptronix.com 38
Deep Reactive Ion Etching Uses high density plasma to alternatively etch silicon and deposit an etch-resistant polymer on side walls Polymer Polymer deposition Silicon etch using SF 6 chemistry 39
Molding Example:LIGA Process (Lithographie, Galvanoformung, Abformung) Lithography, electroplating, and molding processes to produce microstructures. Metal plating Plastic molding Very thick resist Final microstructure 40
1) Deep silicon mold etch. 2) Sacrificial layer deposition. 3) Structural layer deposition. 4) Chemical-mechanical polish (optional). and Deposition and patterning of a second polysilicon layer forms cross linkages between the high-aspect-ratio molded polysilicon structures. 5) Release and extract molded part. Source: Keller and Ferrari 41
Sealing of Cavities Deposition Thermal Oxidation 42