Surface Micromachining II Dr. Thara Srinivasan Lecture 4 Picture credit: Sandia National Lab Lecture 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 Passivation 2
Lateral Resonator Process Flow electrostatic comb drive bumper shuttle spring suspension Shuttle with attached combs are spring-suspended 2 µm above ground plane poly 3 Lecture 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 Passivation 4
MultiUser 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 cm 2 die area 5 MUMPS Micromotor 6
MUMPS Process Flow I 7 MUMPS Process Flow II 8
MUMPS Process Layers Layer properties Thickness Stress 9 MUMPS Masks Mask conventions Light field: draw features that will stay through fabrication Dark field: draw holes to be cut out 10
MUMPS Minimum Features Minimum feature size Determined by MUMPS photolithography precision Violations results in missing (unanchored), under/oversized, or fused features Use minimum feature only when absolutely necessary nominal min feature min space poly0, 1, 2, hole0, poly1_poly2_via 3 µm 2 2 anchor1, 2 3 3 2 dimple 3 2 3 metal 3 3 3 hole1, hole2 4 3 3 holem 5 4 4 11 A. Enclosure B. Spacing MUMPS Design Rules C. Cut-in D. Cut-out 12
Design Rule Summary 13 Design Rule Example 14
Design Rule Example 15 Stringers and Planarization Sidewall stringers Planarization 16
Lecture 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 Passivation 17 Sandia SUMMiT Process 1 mechanical layer 2 mechanical layers 3 mechanical layers 18
Sandia 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 chemicalmechanical polishing (CMP) 4-poly process stack 19 SUMMIT Devices Comb drive microengine actuates hinged mirror through gear transmission 20
Digital 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 brands 21 Digital Micromirror Display 16 µm 22
DMD Fabrication Maluf 23 Lecture 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 Passivation 24
Thin 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 structures 25 Residual 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-direction A bad day at MCNC! (1996) 26
Stress 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 deposited σ = E T 2 6Rt 27 MEMS 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 measurable EI σ cr 2 L 28
MEMS Test Structure: Stress Vernier pointers Expansion or contraction of beams causes deflection of pointer, read on vernier Single structure indicates compressive or tensile stress 29 Stress Gradient Measurement Beam cantilevers Strain gradient Γ causes beams to deflect up or down Assuming linear Γ [L -1 ], z = ΓL 2 / 2 L.S. Fan Ph.D. Spiral cantilevers Krulevitch Ph.D. + compressive tensile 30
Young s Modulus Definition: slope of stress-strain curve in elastic region [N/m² = Pa] σ = Eε ε = L / L Silicon (ave.) Silicon nitride Silicon dioxide Polysilicon 160 GPa 323 GPa 73 GPa 140-190 GPa On-chip measurement Resonating structures f 0 1 2π 4E tw y ML 3 3 31 MEMS Test Structure: Fracture Fracture testing by beam bending Test structure shuttle pushed by probe tip so test beams hit and push against bumpers Fracture limit is 1-3 GPa (2.8 GPa) Fracture surface examined using SEM test beams folded flexure structure P.T.Jones PhD vernier shuttle 32
MEMS Test Structure: Fatigue Fatigue testing Microdevice with notched flexure resonated until stiffness change measured C. Muhlstein et al. 33 Variations in Microstructure Dimensions Sources of variation: Lithography-to-etch variation Non-vertical sidewalls; trapezoidal cross sections Tolerance ± 5% (±0.1 µm for t = W = 2 µm) Resulting Resonant Frequency Variation f (W/L) 3/2, σ negligible f = 15% for W = 2 µm f (W/L) 1/2, σ dominant f = 5% Compensation Laser trimming Isotropic etch Electrical tuning 34
Lecture Outline Today s Lecture Lateral Resonator Process Flow MEMS Test Structures Foundries and Design Rules TI s Digital Micromirror Display Process Flow Microstructure Release and Surface Passivation 35 Microstructure Release and Stiction Stiction ~ the unintended sticking of MEMS surfaces Release stiction ~ While drying after release etch, capillary forces of droplets pull surfaces into contact leading to permanent sticking In-use stiction ~ During device use, surfaces may come into contact and adhere due to Capillary condensation Electrostatic forces Hydrogen bonding van der Waals forces CJ Kim et al. 36
Avoiding Stiction Reducing droplet area with mechanical approaches ~ standoff bumps, meniscus-shaping features and tethers Avoiding liquid-vapor meniscus formation completely Supercritical CO 2, sublimated solvents Vapor-phase sacrificial layer etch Surface modification to change meniscus shape from concave to convex Teflon-like films Hydrophobic self-assembled monolayers (SAMs) P solid liquid STP Sublimation Supercritical drying Critical point Evaporation vapor T 37 Dry Release Dry sacrificial layer etches Etch sacrificial oxide with HF vapor Etch sacrificial polymer layer using O 2 plasma Spin-on polymer spacer, etch with plasma CJ Kim et al. Kobayashi et al. 38
CO 2 Supercritical Drying Release with supercritical CO 2 Supercritical phase avoids liquidvapor meniscus Procedure HF etching of oxide Thorough water rinses Methanol rinses and soaks, then put wafer into chamber Liquid CO 2 displaces methanol CO 2 goes from liquid to supercritical to gas P solid liquid Supercritical drying Critical point Mulhern et al. STP vapor T 39 Hydrophilic, Hydrophobic Hydrophilic, θ water < 90 Hydrophobic, θ water > 90 1 2 θ 3 contact angle Hydrophilic case P 2 d Lotus surface, Univ. Mainz Hydrophobic case P 1 P 1 P 2 40
Self-Assembled Monolayers SAMs as nonstick coatings Conformal, ultrathin Low surface energy Covalently bound wear resistant Thermally stable θ 1 Substrate OTS 2 CH3(CH2)17SiCl3 3 contact angle Substrate θwater ODT SAM 112 ± 0.7 SiO2 <10 41 Adhesion Test Structures Cantilever beam array Electrostatically actuated Beam length that remains stuck after voltage turned off determines adhesion energy between surfaces Clamped-clamped beams anchor actuation pad beam landing pad 2 µm ground-plane polysilicon Si substrate voltage on 42 21
Friction in MEMS Friction between MEMS surfaces Consumes significant portion of motive force Dominant failure mode is intermittent sticking followed by seizure Results in wear at contacting surfaces beam y post beam x Friction test structures Srinivasan, Howe, Maboudian et al.... equilibrium position... displaced and clamped 43 Stiction, Friction Reduction Stiction results With OTS self-assembled monolayer or Teflon coating Can release extremely compliant beams (up to 2 mm long, 2 µm thick, 10 µm wide for SAM) Coefficient of friction results from MEMS test structures friction-testing microstructures and rotating gears plain polysi (oxide-coated) µ s = 4.9 ± 1.2, µ k 0.26-0.5 OTS self-assembled monolayer µ s = 0.09 ± 0.01, µ k = 0.07 ± 0.01 Teflon-coated polysilicon µ k 0.035-0.12 Sandia friction tester: 350 longer until device seizure Texas Instruments DMD: mean time to failure 100,000 h 44