An Introduction to Polysilicon Micromaching Robert W. Johnstone www. sfu. ca/~rjohnsto/ www. sfu. ca/immr/ An Introduction to Polysilicon Micromachining
Personal Information § § § § Robert W. Johnstone Graduate Student at Simon Fraser University School of Engineering Science Simon Fraser University 8888 University Drive, Burnaby, BC Canada V 5 A 1 S 6 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 u Better reliability u Lower costs u Better Performance u § Better Response u § Smaller devices have less inertia, less thermal mass, less capacitance, etc. Increased Reliability u 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 m. TAS – 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 u § IC technology u u u § u Bipolar CMOS Bi. CMOS u u u MEMS related technology u u Bulk micromachining Surface micromachining LIGA, LIGA-like Micro EDM 3 D 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 u u u CVD process Thermal evaporation Sputtering An Introduction to Polysilicon Micromachining 21
Fabrication: 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 Film Subtractive vs. Additive Pattern Transfer 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 Contact Printers Proximity Printers §Mask and wafer in direct contact §Very high resolution § 1 X magnification §Mask and wafer separated by a few micron gap §Moderate resolution Projection Printers §Accomplished via mirror and lenses Step and Repeat Projection Printers §High resolution § 5 X and 10 X 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
Dry Oxidation Rate Fabrication: Oxidation Ref: Fundamentals of Silicon Integrated Device Technology An Introduction to Polysilicon Micromachining 34
Wet Oxidation Rate Fabrication: Oxidation Ref: Fundamentals of Silicon Integrated Device Technology An Introduction to Polysilicon Micromachining 35
Fabrication: Oxidation Through a Window in the 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 Through an Oxide Window An Introduction to Polysilicon Micromachining 40
Fabrication: Diffusion Profiles Diffusion from unlimited 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 u Sputtering u 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 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 1 Torr 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) Si. Cl 4 H 2 (H 2) Si. HCl 3 / H 2 (H 2) Si. H 2 Cl 2 (H 2) Si. H 4 (H 2) 1125 – 1200 1100 – 1150 1050 – 1100 1000 - 1075 500 – 1500 500 – 1000 100 - 300 Poly Silicon (CW) Si. H 4 (H 2) 850 - 1000 100 Si 3 N 4 (CW) Si. H 4 / NH 3 (H 2) 900 - 1100 20 Si. O 2 Si. H 4 / O 2 (N 2) 200 - 500 100 An Introduction to Polysilicon Micromachining 52
Fabrication: Thin Film Deposition Film Epitaxial Silicon Reactant Gases Temp (Carrier) °C LPCVD Si. H 2 Cl 2 (H 2) 1000 - 1075 (30 – 80 Torr) Deposition Rate nm/min 100 Poly Si 100% Si. H 4 (0. 2 Torr) 23% Si. H 4 (N 2) (1. 0 Torr) 620 640 10 19 Si 3 N 4 Si. H 2 Cl 2 / NH 3 (0. 3 Torr) 800 4 Si. O 2 Si. H 2 Cl 2 / N 2 O (0. 4 Torr) 900 8 Si. O 2 Si. H 4 / PH 3 / O 2 (0. 7 Torr) 450 10 12 300 10 Si 3 N 4 PECVD Si. H 4 / NH 3 (N 2) 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 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 u. Under cut problems u. 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 u Non planarized surface micromachining produces stringers and non-flat surfaces u Yield and reliability problems u 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 u Surface micromachining u LIGA u… u 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 Oxidize 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 Surface Micromachining §Deposit thin films on substrate §Pattern thin films lithographically §Selectively etch away a portion of §Selectively etch away one (or the substrate to form a free standing 3 D microstructure bound by a cavity more) of the intermediate thin films to form a free standing 3 D 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) Si. O 2, Si. N Boron > 1020 cm-3 reduce etch rate by 20 ~85 ~400 Ethylene Diamine Pyrocatechol (EDP) Si. O 2, Si. N, Au Boron > 5 x 1019 cm-3 reduces etch rate by 50 ~70 ~35 Tetramethyl Ammonium Hydroxide (TMAH) Si. O 2, Si. N 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) BOSCH Patent STS, Alcatel, Trion, Oxford Instruments … Unconstrained geometry Uses high density plasma to alternatively 90 o side walls etch silicon and deposit a etch-resistant High aspect ratio 1: 30 polymer on side walls Easily masked (PR, Si. O 2) Polymer deposition Process recipe depends on geometry Silicon etch using SF 6 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 u u u Silicon dioxide Doped silicon oxides Photoresist Polyimides Carbon and few metals § Structural Materials u u u Polysilicon Aluminium Silicon nitride Silicon carbide Nickel An Introduction to Polysilicon Micromachining 74
MEMS Technology: Surface § § Prototypical surface micromachining process Three structural layers u u u 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 Substrat e An Introduction to Polysilicon Micromachining 76
MEMS Technology: Surface Metal Silicon Dioxide Polysilicon Silicon Nitride Substrat e An Introduction to Polysilicon Micromachining 77
MEMS Technology: Surface Silicon Dioxide Polysilicon Silicon Nitride Substrat e An Introduction to Polysilicon Micromachining 78
MEMS Technology: Surface Polysilicon Silicon Nitride Substrat e 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 An Introduction to Polysilicon Micromachining 80
MEMS Technology: Surface Micro bridge Rotating part Structural Layer (Poly) 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 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 SUMMi. T 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 40 mm 20 mm Simple sensors & actuator 4 -Level Gears gear train CRONOS and various university technologies Sandia’s SUMMi. T 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 § p. H Detector u § 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 (Bi. CMOS) 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 u u Foreign chemical species enter interstitial or bonding sites at or near the surface Changes the interface properties Absorption u u 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 u Thermal u Magnetic u Piezoelectric u 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 e. A(V 2/d 2) Consider plate 2 inserted between plate 1 and 3 Force of attraction (along x direction) Constant with x-directional translation Fc = e (t/d) V 2 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 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 § § § 2 cm Muscle-cell analysis Plant pathogen detector Dr. Paul Li, 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 CO 2 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 CO 2 u 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 § Auto. CAD 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 § 1 st Oxide § Polysilicon-1 § 2 nd Oxide § Polysilicon-2 § Metal Design Layers § POLY 0 § ANCHOR 1 § POLY 1 § ANCHOR 2 § P 1 P 2 VIA § POLY 2 § 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 Poly 1 -Poly 2 -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 Poly 1 -Poly 2 -Via Poly-2 Silicon Substrate Metal An Introduction to Polysilicon Micromachining 138
MUMPs Examples Sample Design: 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 Poly-1 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 P 1 -P 2 Via Poly-2 Metal An Introduction to Polysilicon Micromachining 141
MUMPs Examples Sample Design: Gear Train Anchor-1 Dimple Poly-1 P 1 -P 2 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 P 1 P 2 V Poly-1 Poly-2 Anchor-2 An Introduction to Polysilicon Micromachining Metal 146
MUMPs Examples Simulation § § § At device level, simulation requires building 3 D model. Layout 3 D model www. sfu. ca/immr/ u u u § § ANSYS Cif-input to 3 D 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: Doublethickness § § Contact surfaces should be avoided in surface micro-machined devices. Where contact surface are needed, double-thickness parts will often be needed u u 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 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
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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, Mc. Graw-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), pp 2 O 3 -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. La. Vigne, M. S. Rodgers and P. J. Mc. Whorter, Monolithic Geared. Mechanisms 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: //www 2. ncsu. edu/eos/project/erl_html /erl_damemi. html http: //www. laas. fr/mc 2_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
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