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An Introduction to Polysilicon Micromaching Robert W. Johnstone www.sfu.ca/~rjohnsto/ www.sfu.ca/immr/ Personal Information Robert W. Johnstone Graduate Student at Simon Fraser University School of Engineering Science Simon Fraser University 8888 University Drive, Burnaby, BC Canada V5A 1S6 Tel: (604) 291-4971 Fax (604) 291-4951 An Introduction to Polysilicon Micromachining 2 Outline Introduction Fabrication MEMS Technology Sensors Actuators Packaging Issues and Integration MUMPs Examples Design Issues Evaluations and Questions An Introduction to Polysilicon Micromachining 3 Introduction An Introduction to Polysilicon Micromachining 4 Introduction: Terminology Micromachining Microfabrication Microelectromechanical Systems (MEMS) Microsystems Technology (MST) An Introduction to Polysilicon Micromachining 5 Introduction Microfabrication Micromachining Bulk Micromachining LIGA Process Microelectronics Surface Micromachining Raised Structures An Introduction to Polysilicon Micromachining 6 Introduction: Features of MEMS Miniature mechanical systems Batch fabrication approach Utilizes microelectronic manufacturing base Common technology for sensors, actuators and systems An Introduction to Polysilicon Micromachining 7 Introduction: Why Miniaturize? System Integration Avoid assembly of discrete components Better reliability Lower costs Better Performance Better Response Smaller devices have less inertia, less thermal mass, less capacitance, etc. Increased Reliability Mass decreases faster than structural strength An Introduction to Polysilicon Micromachining 8 Introduction: Systems-on-a-Chip Traditional Hundreds of components Manual/semi automated assembly Plenty of solder joints Sensitive to shock and vibration Future Single chip No assembly Minimal solder joints Batch fabrication Insensitive to shock & vibration An Introduction to Polysilicon Micromachining 9 Introduction: Growth Prediction Technologies experiencing growth. Hard disk drive heads Inkjet print heads Heart pacemakers In vitro diagnostics Hearing aids Pressure sensors Chemical sensors Infrared imagers Accelerometers Gyroscopes Machine monitoring Micro fluidics Magnetoresistive sensors Microspectrometers Micro optical systems Military systems An Introduction to Polysilicon Micromachining 10 Introduction: Growth Prediction MEMS Device Revenues Source: SEMI MEMS use in existing systems Source: MST News An Introduction to Polysilicon Micromachining 11 Introduction: Applications Relevant Examples Telecommunication relies on routing optical signals Present systems use large and centralized networks A low cost optical switch can revolutionize telecommunications technology MEMS enables practical, low cost micro-mirrors An Introduction to Polysilicon Micromachining 12 Introduction: Applications Inertial Measurement MEMS enables low cost chips that can monitor motion and position Enables integration of inertial measurement in systems not possible with traditional technology Applications in air bags, skid control, machine tools, sports equipment etc An Introduction to Polysilicon Micromachining 13 Introduction: Applications Micro Fluidics Ink jet printing mTAS – Micro Total Analysis System (chemical analysis) Environmental monitoring: Detection of pollutants and pathogens Biomedical devices: heart/lung and kidney Dialysis machine, dosing systems etc DNA analysis systems for diagnostic, therapeutic And forensic studies An Introduction to Polysilicon Micromachining 14 Introduction: Applications Development Strategy Strategy #1 Build the best one possible to meet the most stringent requirements Strategy #2 Build them cheap and worry about performance later An Introduction to Polysilicon Micromachining 15 Introduction: Applications Industry’s Interest in MEMS New products in old fabs Seamless integration into existing fabrication plants Minimal additional investment Risk is low Logical next step An Introduction to Polysilicon Micromachining 16 Introduction: Major Challenge Technology Standards Application specific technologies Differently tuned technology for different devices/applications Presently low synergy or cooperation in formulating a common technology An Introduction to Polysilicon Micromachining 17 Fabrication An Introduction to Polysilicon Micromachining 18 Fabrication: Technology Basic fabrications processes based on IC technology An Introduction to Polysilicon Micromachining 19 Fabrication: Spectrum IC technology Bipolar CMOS BiCMOS MEMS related technology Bulk micromachining Surface micromachining LIGA, LIGA-like Micro EDM 3D stereo lithography Laser micromachining Focused ion beam milling An Introduction to Polysilicon Micromachining 20 Fabrication: Basic Processes Silicon Processing Lithography Oxidation Diffusion Thin film deposition CVD process Thermal evaporation Sputtering An Introduction to Polysilicon Micromachining 21 Fabrication: Lithography Lithography is the process of transferring a pattern from a mask to a photoresist using a photographic tool (mask aligner), and to the silicon substrate using etching techniques. PATTERN TRANSFER An Introduction to Polysilicon Micromachining 22 Fabrication: Lithography Coat the wafer with an adherent and etchresistant photoresist Selectively remove the resist to leave the desired pattern by exposure and development steps Etch to transfer the mask pattern to the underlying material Remove (strip) the photoresist and clean the wafer An Introduction to Polysilicon Micromachining 23 Fabrication: Lithography Mask Pattern UV Light Transparent region Opaque region Photomask Photoresist Oxide Silicon substrate An Introduction to Polysilicon Micromachining 24 Fabrication: Lithography Positive Photoresist Silicon substrate Expose and develop Silicon substrate Strip resist Silicon substrate Etch oxide An Introduction to Polysilicon Micromachining 25 Fabrication: Lithography Negative Photoresist Silicon substrate Expose and develop Silicon substrate Strip resist Silicon substrate Etch oxide An Introduction to Polysilicon Micromachining 26 Fabrication: Lithography Subtractive vs. Additive Pattern Transfer Film Mask Mask After lithography Film Etch After mask removal An Introduction to Polysilicon Micromachining 27 Fabrication: Lithography Spin Coating of Photoresist Dispense resist Spin PR Spinner Spin complete An Introduction to Polysilicon Micromachining 28 Fabrication: Lithography Types of Lithographic Tools Mask and wafer in direct contact Very high resolution 1X magnification Proximity Printers Mask and wafer separated by a few micron gap Moderate resolution Contact Printers Projection Printers Accomplished via mirror and lenses Step and Repeat Projection Printers High resolution 5X and 10X reduction possible Relaxes reticle tolerances and defect requirements An Introduction to Polysilicon Micromachining 29 Fabrication: Lithography Contact Printing Wafer and mask out-of-contact during alignment Wafer and mask in-contact during exposure An Introduction to Polysilicon Micromachining 30 Fabrication: Lithography Projection Printing (using Wafer Stepper) An Introduction to Polysilicon Micromachining 31 Fabrication: Oxidation Thermal oxidation is a high temperature process used to grow a continuous layer of high-quality silicon dioxide on silicon substrate Dry oxidation: oxidizing species is oxygen Wet oxidation: oxidizing species is water vapour An Introduction to Polysilicon Micromachining 32 Fabrication: Oxidation After oxidation Oxidation process An Introduction to Polysilicon Micromachining 33 Fabrication: Oxidation Ref: Fundamentals of Silicon Integrated Device Technology An Introduction to Polysilicon Micromachining 34 Fabrication: Oxidation Ref: Fundamentals of Silicon Integrated Device Technology An Introduction to Polysilicon Micromachining 35 Fabrication: Oxidation Oxidation Through a Window in the Oxide Oxidation complete Oxidation process Oxide removed An Introduction to Polysilicon Micromachining 36 Fabrication: Oxidation Local Oxidation Silicon nitride deposition Oxidation complete Oxidation Silicon nitride removed An Introduction to Polysilicon Micromachining 37 Fabrication: Oxidation Oxide Layer Color Chart Film Thickness (Microns) 0.05 Color and Comments Tan Film Thickness (Microns) 0.60 Color and Comments Carnation pink 0.07 Brown 0.58 Light orange or yellow to pink borderline 0.10 Dark violet to red violet 0.57 0.12 Royal blue 0.56 Yellow to "yellowish" (At times it appears to be light creamy gray or metallic) Green yellow 0.15 Light blue to metallic blue 0.54 Yellow green 0.1 Metallic to very light yellow green 0.52 Green (broad) 0.20 Light gold or yellow - slightly metallic 0.50 Blue green 0.22 Gold with slight yellow orange 0.49 Blue 0.25 Orange to melon 0.48 Blue violet 0.27 Red violet 0.47 Violet 0.30 Blue to violet blue 0.46 Red violet 0.31 Blue 0.44 Violet red 0.32 Blue to blue green 0.42 Carnation pink 0.34 Light green 0.41 Light orange 0.35 Green to yellow green 0.39 Yellow 0.36 Yellow green 0.37 Green yellow Silicon Processing for the VLSI Era: Volume 1- Process Technology An Introduction to Polysilicon Micromachining 38 Fabrication: Diffusion Diffusion is a process by which atoms of impurities (eg., B, P, As, Sb) move into solid silicon as a result of the presence of a concentration gradient and high temperatures. An Introduction to Polysilicon Micromachining 39 Fabrication: Diffusion Diffusion Through an Oxide Window An Introduction to Polysilicon Micromachining 40 Fabrication: Diffusion Diffusion Profiles Diffusion from unlimited source Diffusion from limited source Diffusion from concentration step An Introduction to Polysilicon Micromachining 41 Fabrication: Diffusion Resistivity of Diffused Layers in Silicon Irvines’s Curves An Introduction to Polysilicon Micromachining 42 Fabrication: Diffusion Oxidation/Diffusion Furnace Separate furnaces for oxidation and diffusion processes An Introduction to Polysilicon Micromachining 43 Fabrication: Thin Film Deposition Chemical Vapor Deposition (CVD) Processes Physical Vapor Deposition (PVD) Processes Thermal evaporation Sputtering An Introduction to Polysilicon Micromachining 44 Fabrication: Thin Film Deposition CVD is the formation of a solid film on a substrate by the reaction of vapour phase chemicals which are decomposed or reacted on or near the substrate. An Introduction to Polysilicon Micromachining 45 Fabrication: Thin Film Deposition Reaction Energy Thermal Photons Electrons Reaction Types Heterogeneous reaction Chemical reaction takes place Very close to the surface Good quality films Homogeneous reaction Processes APCVD – Atmospheric pressure CVD LPCVD – Low pressure CVD PECVD – Plasma enhanced CVD Chemical reaction takes place In the gas phase Poor quality films An Introduction to Polysilicon Micromachining 46 Fabrication: Thin Film Deposition Deposition Conditions Mass Transport Limited Reaction Rate Limited Temperature not critical Regulation of reactant Temperature sensitive Reactant flux not critical species on wafer surface is important An Introduction to Polysilicon Micromachining 47 Fabrication: Thin Film Deposition Crystallographic Forms Deposition condition and reaction chemistry determine the crystalline nature of the film An Introduction to Polysilicon Micromachining 48 Fabrication: Thin Film Deposition APCVD Atmospheric pressure chemical vapor deposition Large volume of carrier gases needed Poor step coverage Low throughput Primarily used for LTO Process gases An Introduction to Polysilicon Micromachining 49 Fabrication: Thin Film Deposition LPCVD Low-pressure chemical vapor deposition Reaction rate limited operation Operates at 0.1 to 1Torr pressure Good quality films Conformal coverage Typically used for HTO, Poly-silicon, some metal films and nitride An Introduction to Polysilicon Micromachining 50 Fabrication: Thin Film Deposition PECVD Plasma enhanced chemical vapor deposition Low temperature operation Good conformal step coverage Primarily used for passivation and inter-level dielectrics An Introduction to Polysilicon Micromachining 51 Fabrication: Thin Film Deposition CVD Chemistry Film Reactant Gases (Carrier) Temp °C Deposition Rate nm/min APCVD Epitaxial Si Cold Wall (CW) SiCl4H2 (H2) SiHCl3 / H2 (H2) SiH2Cl2 (H2) SiH4 (H2) 1125 – 1200 1100 – 1150 1050 – 1100 1000 - 1075 500 – 1500 500 – 1500 500 – 1000 100 - 300 Poly Silicon (CW) SiH4 (H2) 850 - 1000 100 Si3N4 (CW) SiH4 / NH3 (H2) 900 - 1100 20 SiO2 SiH4 / O2 (N2) 200 - 500 100 An Introduction to Polysilicon Micromachining 52 Fabrication: Thin Film Deposition Film Epitaxial Silicon Poly Si Reactant Gases Temp (Carrier) °C LPCVD SiH2Cl2 (H2) 1000 - 1075 (30 – 80 Torr) Deposition Rate nm/min 100 100% SiH4 (0.2 Torr) 23% SiH4 (N2) (1.0 Torr) 620 10 640 19 Si3N4 SiH2Cl2 / NH3 (0.3 Torr) 800 4 SiO2 SiH2Cl2 / N2O (0.4 Torr) 900 8 SiO2 SiH4 / O2 SiH4 / PH3 / O2 (0.7 Torr) 450 450 10 12 300 10 Si3N4 PECVD SiH4 / NH3 (N2) An Introduction to Polysilicon Micromachining 53 Fabrication: Thin Film Deposition PECVD Systems Parallel plate PECVD (Low throughput) High throughput PECVD An Introduction to Polysilicon Micromachining 54 Fabrication: Thin Film Deposition Physical Vapor Deposition (PVD) Physical vapor deposition is a process in which the material to be deposited is converted from a solid phase into vapor phase, then moved through a region of low pressure, with the vapor condensing on the substrate, to form a solid thin film. Evaporation: Source material is converted into liquid phase and next into vapour phase usually by thermal process Sputtering: Physical dislodging of atoms from a target Primarily used for interconnect metal deposition An Introduction to Polysilicon Micromachining 55 Fabrication: Thin Film Deposition Thermal Evaporation Evaporator Evaporation Sources An Introduction to Polysilicon Micromachining 56 Fabrication: Thin Film Deposition Electron Beam Evaporation Provides very clean and high purity metal films An Introduction to Polysilicon Micromachining 57 Fabrication: Thin Film Deposition Sputtering Systems DC Sputtering DC voltage between target and substrate, used for conductive targets (metal films) RF Sputtering RF voltage between target and substrate, used for insulators (dielectrics) Magnetron Sputtering Magnetic field confines electrons near the target, increasing the number of electrons causing ionization collisions and, thereby, deposition rates Reactive Sputtering Sputtering a target material in presence of a reactive gas, thereby, depositing a compound An Introduction to Polysilicon Micromachining 58 Fabrication: Etching Thin Films Typically photoresist is used as a masking layer Wet Etch Liquid phase wet chemical etch Under cut problems Not useful for fine dimension control Dry Etch Use of a gas plasma to abrasively etch the thin film Excellent dimension control Reactive Ion Etch Use of a reactive gas species that reacts with the thin film and produce a gaseous by product Fluorine, Chlorine Excellent dimension and sidewall control based chemistry An Introduction to Polysilicon Micromachining 59 Fabrication: Planarization Chemical mechanical polishing (CMP) Planarization process used in IC technology Non planarized surface micromachining produces stringers and non-flat surfaces Yield and reliability problems Not suitable for micro-optics An Introduction to Polysilicon Micromachining 60 Fabrication: Planarization Non CMP Stringers Non uniform staple Non planar link Non planar hinge CMP Sandia National labs Uniform and flat staple Planar link An Introduction to Polysilicon Micromachining Planar hinge 61 MEMS Technology An Introduction to Polysilicon Micromachining 62 MEMS Technology Major MEMS technologies Bulk micromachining Surface micromachining LIGA … An Introduction to Polysilicon Micromachining 63 MEMS Technology Historically: Silicon Micromachining 3-D Sculpting of silicon and silicon compounds Offshoot of IC fabrication technology Uses lithography & mass production Modern: Non silicon MEMS Electroforming Molding An Introduction to Polysilicon Micromachining 64 MEMS Technology: Roots IC Technology Micromachining Silicon wafer Silicon wafer Oxidize Oxidize Lithography Lithography Diffuse impurity Etch the substrate An Introduction to Polysilicon Micromachining 65 MEMS Technology: Roots Basic Etching Processes Isotropic Etching Anisotropic Etching Etch cavity bound by the crystal planes An Introduction to Polysilicon Micromachining 66 MEMS Technology: Roots Micromachining Classification Bulk Micromachining Deposit thin films on substrate Pattern thin films lithographically Selectively etch away a portion of Surface Micromachining Deposit thin films on substrate Pattern thin films lithographically Selectively etch away one (or the substrate to form a free standing 3D microstructure bound by a cavity more) of the intermediate thin films to form a free standing 3D structure standing on top of the substrate surface An Introduction to Polysilicon Micromachining 67 MEMS Technology: Bulk Relies mostly on anisotropic etching (wet as well as dry etch) An Introduction to Polysilicon Micromachining 68 MEMS Technology: Bulk Silicon Anisotropic Etchants Etchant Mask Etch Stop Etch Rate mm/hr Etch Ratio (100):(111) Potassium Hydroxide (KOH) SiO2, SiN Boron > 1020 cm-3 reduce etch rate by 20 ~85 ~400 Ethylene Diamine Pyrocatechol (EDP) SiO2, SiN, Au Boron > 5x1019 cm-3 reduces etch rate by 50 ~70 ~35 Tetramethyl Ammonium Hydroxide (TMAH) SiO2, SiN Boron > 1020 cm-3 reduce etch rate by 40 ~60 ~10 An Introduction to Polysilicon Micromachining 69 MEMS Technology MEMS Specific Etching Etch Stop Techniques Heavily boron doped silicon can act as an etch stop For more precise thickness control use electrochemical techniques Technology developed for silicon pressure sensors and single crystal silicon resonators An Introduction to Polysilicon Micromachining 70 MEMS Technology Deep Reactive Ion Etching (DRIE) STS, Alcatel, Trion, Oxford Instruments … Unconstrained geometry Uses high density plasma to alternatively 90o side walls etch silicon and deposit a etch-resistant High aspect ratio 1:30 polymer on side walls Easily masked (PR, SiO2) BOSCH Patent Polymer Polymer deposition Process recipe depends on geometry Silicon etch using SF6 chemistry An Introduction to Polysilicon Micromachining 71 MEMS Technology: LIGA X-Rays High aspect ratio X-Ray mask Thick Photoresist (PMMA) Electroplate metal Metallic microstructures Suitable for magnetic actuation and sensing Enables micro assembly Dissolve resist Liga-like technique uses thick photoresist and UV lithography An Introduction to Polysilicon Micromachining 72 MEMS Technology: Surface Sacrificial and Structural Layers Structural Layer: Must have good mechanical and electrical properties Sacrificial Layer: Must be stable during deposition and processing should etch quickly during release step Both layers should be IC process compatible and should have excellent etch selectivity An Introduction to Polysilicon Micromachining 73 MEMS Technology: Surface Sacrificial and Structural Layers Sacrificial Materials Silicon dioxide Doped silicon oxides Photoresist Polyimides Carbon and few metals Structural Materials Polysilicon Aluminium Silicon nitride Silicon carbide Nickel An Introduction to Polysilicon Micromachining 74 MEMS Technology: Surface Prototypical surface micromachining process Three structural layers Polycrystalline silicon (polysilicon) First layer is not moveable Often called zero layer An Introduction to Polysilicon Micromachining 75 MEMS Technology: Surface Metal Silicon Dioxide Polysilicon Silicon Nitride Substrate An Introduction to Polysilicon Micromachining 76 MEMS Technology: Surface Metal Silicon Dioxide Polysilicon Silicon Nitride Substrate An Introduction to Polysilicon Micromachining 77 MEMS Technology: Surface Silicon Dioxide Polysilicon Silicon Nitride Substrate An Introduction to Polysilicon Micromachining 78 MEMS Technology: Surface Polysilicon Silicon Nitride Substrate An Introduction to Polysilicon Micromachining 79 MEMS Technology: Surface A combination of sacrificial and structural layers Micro bridge Rotating part Structural Layer (Poly) Sacrificial Layer Isotropic Etching Isotropic Etching An Introduction to Polysilicon Micromachining 80 MEMS Technology: Surface Micro bridge Rotating part Structural Layer (Poly) Isotropic Etching Isotropic Etching Sacrificial Layer An Introduction to Polysilicon Micromachining 81 MEMS Technology: Surface Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 82 MEMS Technology: Surface Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 83 MEMS Technology: Surface Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 84 MEMS Technology: Surface Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 85 MEMS Technology: Surface Silicon Dioxide Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 86 MEMS Technology: Surface Photolithography Silicon Dioxide Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 87 MEMS Technology: Surface Photolithography Poly-silicon Photolithography Silicon Dioxide Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 88 MEMS Technology: Surface Photolithography Metal Photolithography Poly-silicon Photolithography Silicon Dioxide Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 89 MEMS Technology: Surface Release Photolithography Metal Photolithography Poly-silicon Photolithography Silicon Dioxide Photolithography Poly-silicon Silicon Dioxide Silicon Nitride Wafer (Silicon) An Introduction to Polysilicon Micromachining 90 MEMS Technology: Surface An Introduction to Polysilicon Micromachining 91 MEMS Technology: Surface An Introduction to Polysilicon Micromachining 92 MEMS Technology: Surface State-of-the-Art Surface Micromachining Number of structural layers determine the complexity/advancement 2-Level 3-Level 4-Level Actuator Actuator Gear Hub Drive link Gear 5-Level Actuator Hub Drive link Gear Hub Actuator Movable plate Simple sensors & actuator Gears gear train CRONOS and various university technology Pin-joints, gear train Multilevel gears and advanced MEMS Sandia’s SUMMiT technology An Introduction to Polysilicon Micromachining 93 MEMS Technology: Surface State-of-the-Art Surface Micromachining 2-Level 3-Level 5-Level Pin-joints, gear train Multilevel gears and advanced MEMS 40mm 20mm Simple sensors & actuator 4-Level Gears gear train CRONOS and various university technologies Sandia’s SUMMiT technology An Introduction to Polysilicon Micromachining 94 Sensors An Introduction to Polysilicon Micromachining 95 Sensors: Transduction Principles Physical Sensors Accelerometer Gyroscope Pressure sensor Mass flow sensor Temperature sensor Proximity sensor Magnetic sensor Chemical Sensors Gas detector pH Detector Micro-fluidics Bio-analysis An Introduction to Polysilicon Micromachining 96 Sensors: Acceleration Transduction: Piezoresistive Capacitive Resonance based Analog Devices ADXL-50 integrated Accelerometer with on board electronics (BiCMOS) Based on comb structure and capacitive pick-up Translation direction 180° out-of-phase signals fed to this pair of stationary electrodes An Introduction to Polysilicon Micromachining 97 Sensors: Pressure Piezoresistive Bulk micromachining Electrochemical etching Anodic bonding to a PYREX base An Introduction to Polysilicon Micromachining 98 Sensors: Pressure Cross-Sectional and Side Views of a commercial Bulk Micromachined Pressure Sensor An Introduction to Polysilicon Micromachining 99 Sensors: Gas Adsorption Foreign chemical species enter interstitial or bonding sites at or near the surface Changes the interface properties Absorption Foreign chemical species enter interstitial or bonding sites within the bulk material Changes the materials bulk properties An Introduction to Polysilicon Micromachining 100 Sensors: Gas Concentration Gas Sensor Principles Metal Oxide Gas sensor FET Gas Sensor Adsorbed gas molecule alters the conductivity Adsorbed gas molecule alters the threshold voltage An Introduction to Polysilicon Micromachining 101 Sensors: Sandia Developments An Introduction to Polysilicon Micromachining 102 Sensors: 3-axis Acceleration Sensor Sandia National Labs An Introduction to Polysilicon Micromachining 103 Sensors: Surface Micromachined Pressure Sensor Sandia National Labs An Introduction to Polysilicon Micromachining 104 Sensors: Combustible Gas Sensor Sandia National Labs An Introduction to Polysilicon Micromachining 105 Actuators An Introduction to Polysilicon Micromachining 106 Actuators Microactuators are the special contribution of MEMS technology Actuation Mechanisms Electrostatic Thermal Magnetic Piezoelectric Can be easily implemented using most of the surface micromachining technology Silicon is neither piezoelectric nor magnetostrictive, therefore, additional thin films have to be added to the microstructures An Introduction to Polysilicon Micromachining 107 Actuators: Electrostatic In surface micromachining, often popularly known as comb drives. Plate-1 V (volts) d Principle y Plate-2 x Plate-3 Consider parallel plate 1 & 2 Force of attraction (along y direction) Fp = ½ eA(V2/d2) Consider plate 2 inserted between plate 1 and 3 Force of attraction (along x direction) Fc = e (t/d) V2 Constant with x-directional translation An Introduction to Polysilicon Micromachining 108 Actuators: Electrostatic Comb Drives CRONOS comb drive Sandia cascaded comb drive (High force) An Introduction to Polysilicon Micromachining Close-up view of the shuttle 109 Actuators: Electrostatic Squeeze Film: Texas Instruments DMD An Introduction to Polysilicon Micromachining 110 Actuators: Thermal Uses thermal expansion for actuation Small thermal expansion is mechanical amplified Very effective and high force output per unit area An Introduction to Polysilicon Micromachining 111 Actuators: Thermal Cold arm Direction of actuation Current output terminal Ground plane Current output pad Hot arm An Introduction to Polysilicon Micromachining 112 Actuators: Thermal Acknowledging ZYVEX (www.zyvex.com) An Introduction to Polysilicon Micromachining 113 Actuators: Motors Electrostatic Micromotor Wobble Micromotor (also electrostatic) Stator Rotor Stator Hub Hub Rotor An Introduction to Polysilicon Micromachining 114 Actuators: Motors Sandia’s wedge stepping motor LIGA – electro-magnetic microactuator An Introduction to Polysilicon Micromachining 115 Actuators: Motors Acknowledging Sandia National Labs (www.sandia.gov) An Introduction to Polysilicon Micromachining 116 Actuators: Motors Acknowledging Rotary Stepper Motor (www.zyvex.com) An Introduction to Polysilicon Micromachining 117 Actuators: Motors Linear Stepper Motor An Introduction to Polysilicon Micromachining 118 Actuators: Motors Vibromotor An Introduction to Polysilicon Micromachining 119 Actuators: Steam Engine Sandia National Labs Vapour Pvapour Heater Element Piston Liquid Cylinder Structure immersed in working fluid (DI water) Vapor bubble formed at right end Vapor condenses at the piston end Expansion of vapor bubble moves the piston An Introduction to Polysilicon Micromachining 120 Actuators: Steam Engine Sandia National Labs Single piston Multi piston An Introduction to Polysilicon Micromachining 121 Actuators: Fluidics Glass micromachined DNA purification system DRIE etched fluidic handling system An Introduction to Polysilicon Micromachining 122 Actuators: Fluidics Muscle-cell analysis Plant pathogen detector Dr. Paul Li, 2 cm department of chemistry, Simon Fraser university An Introduction to Polysilicon Micromachining 123 Actuators: Fluidics Phase Transformation Fluid Pump An Introduction to Polysilicon Micromachining 124 Actuators: Photonics Fresnel Zone Plate and Laser Diode (UCLA) UC Berkeley micromirror Sandia micromirror Clip-on and virtual retinal displays An Introduction to Polysilicon Micromachining 125 Actuators: Movies Acknowledging Sandia National Labs (www.sandia.gov) An Introduction to Polysilicon Micromachining 126 Packaging An Introduction to Polysilicon Micromachining 127 Packaging: Process Wafer dicing (diamond saw) Release and dry Die attach Wire bonding Multi-chip modules and flip-chip bonding Hermetic sealing (for physical sensor) Potting to protect from shock Orientation of sensors (inertial sensors) An Introduction to Polysilicon Micromachining 128 Packaging: Release Normal drying after deionized (DI) water rinse creates a meniscus between the substrate and microstructure This process collapses the freestanding microstructure In general, want to avoid surface coming into contact to avoid adhesion An Introduction to Polysilicon Micromachining 129 Packaging: Release Supercritical CO2 Drying Solutions Make structures stiffer in Zdirection (high aspect ratio) Add dimples to reduce surface contact area Treat surfaces to make them hydrophobic Avoid creating the meniscus by drying in supercritical CO2 Freezing and sublimating the solvent An Introduction to Polysilicon Micromachining 130 Packaging: Electronics CMOS Compatible Micromachining Technique Use an existing industrial CMOS technology as a base Introduce special layout design techniques Perform micromachining step as a post-process Advantages No need for a in-house fab Can integrate microstructure and electronics on the same chip Disadvantages Limited assortment of microstructures A CMOS Micromachined integrated IR emitter pixel An Introduction to Polysilicon Micromachining 131 Packaging: Why Integrate? Cost: Batch fabrication Performance: Reduced parasitics Manufacturability: Integrated contacts have higher yield than wire bonds and flip-chip bonds Reliability: Integrated systems are more reliable than hybrids Size: Offers the ultimate level of miniaturization An Introduction to Polysilicon Micromachining 132 MUMPs Examples An Introduction to Polysilicon Micromachining 133 MUMPs Examples Design Layout Generation Layout Tool L-Edit (Tanner Tools) Cadence AutoCAD Masks for fabrication Output File Format CIF GDS II DXF An Introduction to Polysilicon Micromachining 134 MUMPs Examples The Multi-User Micromachining Process (MUMPs) is a polysilicon surface-micromachining process Provided to public by Cronos Uses three structural layers Uses two sacrificial layers An Introduction to Polysilicon Micromachining 135 MUMPs Examples Process Layers Nitride Polysilicon-0 1st Oxide Polysilicon-1 2nd Oxide Polysilicon-2 Metal Design Layers POLY0 ANCHOR1 POLY1 ANCHOR2 P1P2VIA POLY2 METAL An Introduction to Polysilicon Micromachining 136 MUMPs Examples Design layers’ functions Poly-0 Defines the geometry of Polysilicon-0 Anchor-1 Attaches Poly-1 to Poly-0 or attaches Poly-1 to Nitride Poly-1 Defines the geometry of Poly-1 Anchor-2 Attaches Poly-2 to Poly-0 or attaches Poly-2 to Nitride Poly1-Poly2-Via Attaches Poly-2 to Poly-1 Poly-2 Defines geometry of Poly-2 Metal Defines geometry of metal. Preferably on top of Poly-2 An Introduction to Polysilicon Micromachining 137 MUMPs Examples Design layers’ functions Poly-0 First-Oxide Anchor-1 Poly-1 Second-Oxide Silicon Nitride Anchor-2 Poly1-Poly2-Via Poly-2 Silicon Substrate Metal An Introduction to Polysilicon Micromachining 138 MUMPs Examples Sample Design: Hinge Hinge Poly-1 Anchor-2 Poly-2 Metal An Introduction to Polysilicon Micromachining 139 MUMPs Examples Dimples Rotor without dimples Rotor with dimples When flat structures are released the surface contact will glue parts together and prevent movement Very small indentations (bumps) created on Poly1 and Poly-2 so that when structures are released they rest on the bumps An Introduction to Polysilicon Micromachining 140 MUMPs Examples Sample Design: Thermal Actuator Poly-0 Anchor-1 Dimple Poly-1 P1-P2 Via Poly-2 Metal An Introduction to Polysilicon Micromachining 141 MUMPs Examples Sample Design: Gear Train Anchor-1 Dimple Poly-1 P1-P2 Via An Introduction to Polysilicon Micromachining Poly-2 142 MUMPs Examples Sample Design: Gear Train An Introduction to Polysilicon Micromachining 143 MUMPs Examples Sample Design: Why double thickness structures Pawl can slip underneath gear teeth An Introduction to Polysilicon Micromachining 144 MUMPs Examples Sample Design: Why double thickness structures Teeth between gear and motor can slip An Introduction to Polysilicon Micromachining 145 MUMPs Examples Sample Design: Tower Poly-0 Dimple Anchor-1 P1P2V Poly-1 Poly-2 Anchor-2 An Introduction to Polysilicon Micromachining Metal 146 MUMPs Examples Simulation At device level, simulation requires building 3D model. Layout 3D model www.sfu.ca/immr/ ANSYS Cif-input to 3D output. Ansys VRML MEMS Pro Intellisuite An Introduction to Polysilicon Micromachining 147 MUMPs Examples Mirrors An Introduction to Polysilicon Micromachining 148 MUMPs Examples Bistable Mechanism An Introduction to Polysilicon Micromachining 149 MUMPs Examples Bistable Mechanism An Introduction to Polysilicon Micromachining 150 Design Issues An Introduction to Polysilicon Micromachining 151 Design Issues: Topography In the MUMPs process, all growth is conformal Processing steps depend heavily on the preceding steps Managing topology is an important An Introduction to Polysilicon Micromachining 152 Design Issues: Topography Thin films conform closely to the topology of the previously deposited and patterned layers Topology can trap a structure that was intended to move freely Unless the preceding layers are designed to ensure the upper structural layers are flat where needed, the induced topology can have detrimental effects on device operation An Introduction to Polysilicon Micromachining 153 Design Issues: Topography Topography can create structural weaknesses Topography provides stress concentration points and beam thinning may occur due to variation in film thickness across steps Problems are compounded since lithography is more difficult along height changes An Introduction to Polysilicon Micromachining 154 Design Issues: Topography Both actuators are composed of a wide arm and narrow arm Differential heating due to an applied current causes differential thermal expansion This was supposed to cause the arm to curve upwards However, the wide arm has conformed and is no longer flat The wide arm’s bending stiffness is thus significantly higher, reducing any motion An Introduction to Polysilicon Micromachining 155 Design Issues: Residual Stresses Uniform stress is the average stress through the thickness of the film For singly supported structures, one should expect a dimensional change as the structure relaxes to a non-stressed state Doubly supported structures can be more reliable, in that their length will remain fixed Over a critical compressive stress they will buckle An Introduction to Polysilicon Micromachining 156 Design Issues: Residual Stresses A non-uniform stress is a residual stress with a gradient Non-uniform stresses are both more difficult to handle theoretically and more difficult to measure. An Introduction to Polysilicon Micromachining 157 Design Issues: Ground Planes Ground planes are necessary to electrically shield devices from the wafer Conducting bodies at different potentials will experience an attractive force; this includes surface micro-machined devices and the substrate If the attractive force is strong enough, the device will be pulled down to the substrate surface Even if the device does not adhere, significant friction will be present An Introduction to Polysilicon Micromachining 158 Design Issues: Double-thickness Contact surfaces should be avoided in surface micro-machined devices . Where contact surface are needed, double-thickness parts will often be needed Because they are so thin, surface micromachined devices will have significant vertical flexibility There may be bowing. An Introduction to Polysilicon Micromachining 159 Design Issues: Tethers Free moving structures will flap around during release This can damage the device itself as well as nearby devices The device should be tethered to the wafer surface These tethers are broken after release An Introduction to Polysilicon Micromachining 160 Design Issues: Dimples Dimples are small bumps on the underside of the first structural layer A short wet etch is used to isotropically etch small cavities the first sacrificial layer The first structural layer will then have bumps, as it will conformally fill the holes Dimple Dimple An Introduction to Polysilicon Micromachining Dimple 161 Design Issues: Raised Structures Hinges allow parts to rotate Properly design parts can rotate off the wafer surface An Introduction to Polysilicon Micromachining 162 Design Issues: Raised Structures An Introduction to Polysilicon Micromachining 163 Design Issues: Raised Structures An Introduction to Polysilicon Micromachining 164 Design Issues: Raised Structures An Introduction to Polysilicon Micromachining 165 Design Issues: Raised Structures An Introduction to Polysilicon Micromachining 166 Please fill out the course evaluation Thank-you An Introduction to Polysilicon Micromachining 167 Selected References S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era, Volume 1 – Process Technology, California, Lattice Press 1986. S. Ghandi, VLSI Fabrication Principles, Silicon and Gallium Arsenide, Second Edition, John Wiley & Sons, Inc., New York, 1994. S. M. Sze, VLSI Technology, McGraw-Hill, 1983. Madou M., Fundamentals of Microfabrication, CRC Press, New York, 1997. M. Parameswaran, M. Paranjape, Layout Design Rules for Microstructure Fabrication Using Commercialy Available CMOS Technology, Sensors and Materials, 5, 2, 1993, pp. 113-123. How Semiconductors are Made, Harris Semiconductor Ernest Garcia, Jeff Sniegowski, Surface Micromachined Microengine, Sensors and Actuators A 48 (1995), pp2O3-214. Kurt E. Peterson , Silicon as a Mechanical Material, Proc. of IEEE, vol 70 no 5, May 1982. Joseph Shigley , Mechanical Engineering Design, 1989, ISBN 0-07-056899-5 S.M. Sze , Semiconductor Sensors, 1994, John Wiley & Sons, ISBN 0-471-54609-7 J. J. Sniegowski and E. J. Garcia, Surface Micromachined Gear Trains Driven by an On-Chip Electrostatic Microengine, IEEE Electron Device Letters, Vol. 17, No. 7, 366, July 1996. J. J. Sniegowski, S. M. Miller, G. F. LaVigne, M.S. Rodgers and P.J. McWhorter, Monolithic GearedMechanisms Driven by a Polysilicon Surface-micromachined On-Chip Electrostatic Microengine, Solid-State Sensor and Actuator Workshop, Hilton Head Is., South Carolina, June 2-6, 19969 pp. 178182. J. J. Sniegowski, Moving the World with Surface Micromachining, , Solid State Technology, Feb. 1996, pp. 83-90. An Introduction to Polysilicon Micromachining 168 WWW MEMS References http://www.mems.sandia.gov http://www.onixmicrosystems.com/ http://www.siliconsense.com/ http://www.intellisense.com/index.html http://www.siliconlight.com/ http://www.css.sfu.ca/sites/immr/ http://www.memsrus.com/ http://www.zyvex.com/Research/MEMS/M EMS.html http://mems.engr.wisc.edu/liga.html http://www.biomems.net/ http://bsac.eecs.berkeley.edu/ http://www.cmc.ca/beams.html http://www.ece.cmu.edu/~mems/ http://www.mech.kuleuven.ac.be/ http://mishkin.jpl.nasa.gov/CSMT_PAGE http://muresh.et.tudelft.nl/dimes/index.html http://wwwbsac.eecs.berkeley.edu/~ptjones/databas e.html http://dolphin.eng.uc.edu/index.html http://mems.eeap.cwru.edu http://www.ida.org/MEMS/ http://www-mtl.mit.edu/home.html http://synergy.icsl.ucla.edu/index.html http://www.rgraceassoc.com http://cdr.stanford.edu/ http://www.shef.ac.uk/uni/projects/mesu/ http://www.mems.ecs.soton.ac.uk/title.htm http://www.trimmer.net/ http://www-mtl.mit.edu/semisubway.html http://www2.ncsu.edu/eos/project/erl_html /erl_damemi.html http://www.laas.fr/mc2_Europractice/ http://www.tanner.com/ http://www.omminc.com/ http://www.microsensors.com/ An Introduction to Polysilicon Micromachining 169 An Introduction to Polysilicon Micromachining 170 An Introduction to Polysilicon Micromachining 171