ME 141B: The MEMS Class Introduction to MEMS and MEMS Design Sumita Pennathur UCSBOptical MEMS Case Study: MEMS-Based Projection Displays Sumita Pennathur UCSBOptical MEMS • MEMS = good for light  Structural dimension on same order as wavelength of IR or visible light  Can control reflection/diffraction with small movements  Microfabricating smooth surfaces = easy  Actuators for control of light  not a lot of work 6/2/09 3/45Outline • Reflection vs. diffraction  Texas Instruments DMD reflective display  Silicon Light Machines diffractive display • DMD-based display: the basics  What it is  How it’s made  How it works • DMD-based display: the details  Reliability: why might this fail, and why doesn’t it usually fail?  Packaging  Test Procedures 6/1/09 4/45The Texas Instruments DMD 6/1/09 5/45Projecting with the DMD 6/1/09 6/45The Silicon Light Machines Approach  Instead of using mirror, array of small electrostatically actuated diffraction gratings  When unactuated  array reflects incident light back to source  When actuated  array diffracts light @specific angle collected by optics  Max diffraction -> of by quarter wavelength 6/2/09 7/45Pixel Operation • Incoming light is directed onto pixel by centrally located mirror • No actuation  screen is dark • 2D array  linear array 6/1/09 8/45Linear Array – Projection • Linear array  can still get 2D projection • Has horizontal scan mirror that moves • Grayscale  adjusting the amount of time, but also can be manual  amplitude of grating display within a pixel 6/2/09 9/45Both use suspended microstructures • DMD  Supported by elastically linear torsional spring  As one electrode is actuated, electrostatic actuation tips mirror toward active electrode  Pull-in exceeded and mirror tips until it contacts landing pad • GLV  Original device also used vertical pull-in until it contacted electrodes  Ok, but introduces problems with charging  Silicon Light Machines uses analog gray scale  amplitude of grating displacement within a pixel  No pull-in needed 6/2/09 10/45Outline • Reflection vs. diffraction  Texas Instruments DMD reflective display  Silicon Light Machines diffractive display • DMD-based display: the basics  What it is  How it’s made  How it works • DMD-based display: the details  Reliability: why might this fail, and why doesn’t it usually fail?  Packaging  Test Procedures 6/1/09 11/45Timeline of the DMD at TI • 1977: Initial Explorations (DARPA contract) • 1987: Demonstration of the DMD • 1992: Is this commercially viable? • 1994: Public demonstration of prototype • 1996: First units shipped • More than ten million units shipped • Initial focus limited to projectors to establish base market • Jump to TVs, theater projection • Now branching out into other market: lithography, medical imaging, scientific imagine 6/1/09 12/45The pixels • One mechanical mirror per optical pixel • 16 um aluminum mirrors, 17um on center • Address electronics under each pixel 6/1/09 13/45DMD Image 6/1/09 14/45SEMs of DMD 6/1/09 15/45SEMs of DMD 6/1/09 16/45Damaged mirrors 6/1/09 17/45Paper Clip Abrasion • Abrasion by a paper clip 6/1/09 18/45Mirrors with 5V bias 6/1/09 19/45  Off 5V bias   5V bias Near electrodeColored SEMs of DMD devices 6/1/09 20/45 Address system Bias reset Pixel substructureColored SEM 6/1/09 21/45Pixel Operation • Pixels rotate 10 degrees in either direction • Mirrors pull in • Motion is limited by mechanical stops • On: +10 degrees • Off: -10 degrees 6/1/09 22/45System Operation • Grayscale obtained by alternating each mirror between on and off positions in time  Multiple switch events per frame update • Color obtained by rotating color wheel  Mirror switching events are synchronized with wheel • Color alternative: use three chips • Other system elements: light source, drive electronics, switching electronics, switching algorithm, projection optics 6/1/09 23/45The Product • MEMS are fun, but products sell • The core of the product is the “digital display engine”, or DDE 6/1/09 24/45Fabrication considerations • MEMS parts must be fabricated over SRAM memory cells • MEMS processing must not damage circuits, inclding aluminum interconnects • Polysilicon? High Temperature Oxides? • Alternate approach: aluminum as a structural material, with photoresist as a sacrificial layer • Dry release by plasma strip is a benefit 6/1/09 25/45Fabrication Process 6/1/09 26/45Pull-in Analysis • 2 methods of analysis  Energy-based method of calculating capacitance as a function of angle • Demonstrates that resulting torque is nonlinear and increasing as a function of angle • There will be an angle where equilibrium between torque and linear restoring force will become unstable  Hornbeck • Calculate torque directly from parallel plate approximation of a tilted capacitor 6/2/09 27/45Torsional Pull-in Model 6/1/09 28/45Capacitance Modeling • Calculate capacitance vs. tilt angle • Fit to cubic polynomial • Perform conventional pull-in analysis 6/1/09 29/45Outline • Reflection vs. diffraction  Texas Instruments DMD reflective display  Silicon Light Machines diffractive display • DMD-based display: the basics  What it is  How it’s made  How it works • DMD-based display: the details  Reliability: why might this fail, and why doesn’t it usually fail?  Packaging  Test Procedures 6/1/09 30/45Brainstorm: why might this fail? • Breakage due to handling/shock • Stiction (from surface contamination, moisture, or van der Waals forces) • Light exposure • Thermal cycling • Particle effects (electrical short, stuck mirrors, etc.) • Metal fatigue in hinges • Hinge memory (permanent deformation) • Other mechanisms can impact yield right out of the fab: CMOS defects, particles 6/1/09 31/45Brainstorm: why might this fail? • Breakage due to handling/shock • Stiction (from surface contamination,