1EE C245Surface Micromachining IIDr. Thara SrinivasanLecture 4Picture credit: Sandia National Lab2U. Srinivasan ©EE C245Lecture Outline• Reading• From reader: Bustillo, J. et al., “Surface Micromachining of Microelectromechanical Systems,” pp. 1552-56, 1559-63.• Problem set #1 due; problem set #2 on website• Today’s Lecture• Lateral Resonator Process Flow (from Lecture 3)• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes • MEMS Test Structures• Microstructure Release and Surface Passivation23U. Srinivasan ©EE C245Lateral Resonator Process Flowbumperelectrostatic comb driveshuttlespring suspensionShuttle with attached combs are spring-suspended 2 µm above ground plane poly4U. Srinivasan ©EE C245Lecture Outline• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes• MEMS Test Structures• Microstructure Release and Surface Passivation35U. Srinivasan ©EE C245MultiUser MEMS Process• Microelectronics Center of North Carolina, MultiUser MEMS Process (MUMPS), now owned by MEMSCAP, France.• Three-level polySi surface micromachining prototyping and foundry service• 8 photomasks• $4,900 for 1 cm2die area 6U. Srinivasan ©EE C245MUMPS Micromotor47U. Srinivasan ©EE C245MUMPS Process Flow I8U. Srinivasan ©EE C245MUMPS Process Flow II59U. Srinivasan ©EE C245MUMPS Process Layers• Layer properties• Thickness• Stress10U. Srinivasan ©EE C245MUMPS Masks• Mask conventions• Light field: draw features that will stay through fabrication• Dark field: draw holes to be cut out611U. Srinivasan ©EE C245• Minimum feature size• Determined by MUMPS’ photolithography precision• Violations results in missing (unanchored), under/oversized, or fused features• Use minimum feature only when absolutely necessarynominal min feature min spacepoly0, 1, 2, hole0, poly1_poly2_via 3 µm 2 2anchor1, 2332dimple 323metal 333hole1, hole2 4 3 3holem544MUMPS Minimum Features12U. Srinivasan ©EE C245MUMPS Design RulesC. Cut-in D. Cut-outA. Enclosure B. Spacing713U. Srinivasan ©EE C245Design Rule Summary14U. Srinivasan ©EE C245Design Rule Example815U. Srinivasan ©EE C245Design Rule Example16U. Srinivasan ©EE C245Stringers and Planarization• Sidewall stringers• Planarization917U. Srinivasan ©EE C245Lecture Outline• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules • Sandia and Texas Instruments Processes • MEMS Test Structures• Microstructure Release and Surface Passivation18U. Srinivasan ©EE C245Sandia SUMMiT Process2 mechanical layers3 mechanical layers1 mechanical layer1019U. Srinivasan ©EE C245Sandia SUMMIT Process• Sandia Ultraplanar Multilevel MEMS Technology (SUMMiT) is a 5-layer polysilicon process• 14 masks, up to 240 process steps; most complex poly surface micromachining process• 1 ground plane/electrical interconnect layer• 4 mechanical layers• Residual film stress < 5 MPa • Device topography is planarized using chemical-mechanical polishing (CMP)4-poly process stack20U. Srinivasan ©EE C245SUMMIT DevicesComb drive microengine actuates hinged mirror through gear transmission1121U. Srinivasan ©EE C245Digital Micromirror Display• Texas Instruments DMD• 2-D array of optical switching pixels on silicon substrate.• Pixel is a reflective micromirror supported on a central post• Post is mounted on lower metal platform, yoke, suspended by torsional hinges from posts anchored to substrate.• 2 electrodes under yoke are used to tilt mirror ±10°• Component in >17 projector brands22U. Srinivasan ©EE C245Digital Micromirror Display16 µm1223U. Srinivasan ©EE C245DMD FabricationMaluf24U. Srinivasan ©EE C245Lecture Outline• Today’s Lecture• Lateral Resonator Process Flow• MUMPS Foundry and Design Rules• Sandia and Texas Instruments Processes• MEMS Test Structures• Microstructure Release and Surface Passivation1325U. Srinivasan ©EE C245Thin Films Mechanical Properties• Mechanical properties which are critical • Adhesion• Residual stress, σ • Stress gradient, Γ• Pinhole density• Density • Mechanical strength• Young’s modulus, Ε • Fracture strength• Fatigue• Need for on-wafer measurement• Local measurement of film properties• Difficult to handle and align small structures26U. Srinivasan ©EE C245Residual Stress• Origins of residual stress, σ• Growth processes• Non-equilibrium deposition– Grain morphology change• Gas entrapment• Doping• Thermal stresses• Deposition, Coefficient of thermal expansion mismatch• Annealing • Stress gradient• Variation of residual stress in the direction of film growth• Can warp released structures in z-directionA bad day at MCNC! (1996)1427U. Srinivasan ©EE C245Stress Measurement• Wafer curvature method (Tencor Flexus)• Compressive stress makes wafer convex, tensile stress makes wafer concave.• Optically measure deflection of wafer before and after film is depositedσ = E’ T26Rt28U. Srinivasan ©EE C245MEMS Test Structure: Stress• Clamped-clamped beams (bridges)• Compressive stress causes buckling• Arrays with increasing length are used to determine critical buckling load • Only compressive stress is measurable2LEIcr≈σ1529U. Srinivasan ©EE C245MEMS Test Structure: Stress• Vernier pointers• Expansion or contraction of beams causes deflection of pointer, read on vernier• Single structure indicates compressive or tensile stress30U. Srinivasan ©EE C245Stress Gradient Measurement• Beam cantilevers• Strain gradient Γ causes beams to deflect up or down• Assuming linear Γ [L-1], z = ΓL2 / 2• Spiral cantileverscompressivetensile+–Krulevitch Ph.D.L.S. Fan Ph.D.1631U. Srinivasan ©EE C245Young’s Modulus• Definition: slope of stress-strain curve in elastic region [N/m² = Pa]• σ = Eεε= ∆L / L• On-chip measurement• Resonating structures140-190 GPaPolysilicon73 GPaSilicon dioxide323 GPaSilicon nitride160 GPaSilicon (ave.)330421MLtWEfyπ≈32U. Srinivasan ©EE C245• Fracture testing by beam bending• Test structure shuttle pushed by probe tip so test beams hit andpush against bumpers• Fracture limit is 1-3 GPa (2.8 GPa)• Fracture surface examined using SEMMEMS Test Structure: FractureP.T.Jones PhDfolded flexure structureshuttleverniertest beams1733U. Srinivasan ©EE C245• Fatigue testing • Microdevice with notched flexure resonated until
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