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EEL5225: Principles of MEMS Transducers (Fall 2003) Fabrication Technology, Part II Agenda: Ê Process Examples – TI Micromirror fabrication process – SCREAM – CMOS-MEMS processes Ê Ê 1 Wafer Bonding LIGA Reading: Senturia, pp. 79-98. EEL5225: Principles of MEMS Transducers (Fall 2003) Lecture 9 by H.K. Xie 9/15/2003 Process Examples Surface micromachining: TI Digital Micromirror Devices (DMDs) Ref. Maluf, Introduction to MEMS Engineering, p.145. 2 EEL5225: Principles of MEMS Transducers (Fall 2003) Invented by L.J. Hornbeck Process Examples Bulk-Micromachined Pressure Sensor Ê Ê Ê Ê Ê Thermal oxide Boron implantation Boron drive-in LPCVD Si3N4 Backside KOH etch – Electrochemical etch stop Ê Metallization Detailed process steps refer to Senturia p.93 Ref. Senturia, Microsystem Design, p.97 3 EEL5225: Principles of MEMS Transducers (Fall 2003) Process Examples Single Crystal Reactive Etching and Metallization (SCREAM) Process Ê Ê Ê Ê Ê First demonstrated by MacDonald’s group at Cornell University Single crystal silicon (SCS) microstructures Large force Large displacement Post-CMOS process for electronics integration Ref. Maluf, Introduction to MEMS Engineering, p.82. 4 EEL5225: Principles of MEMS Transducers (Fall 2003) CMOS-MEMS Why CMOS-MEMS? Ê Ê Ê Ê Ê Ê MEMS structures can be made Ê Ê Ê 5 “Smart” on-chip CMOS circuitry Multi-vendor accessibility Scalability Compact size More functions Low cost Before CMOS processes (“pre-CMOS”) In-between CMOS processes (“intermediate-CMOS”) After CMOS processes (“post-CMOS”) EEL5225: Principles of MEMS Transducers (Fall 2003) Pre-CMOS MEMS Process www.sandia.gov Pre-etched trench to house MEMS structures CMP to planarize the wafer for regular CMOS processing Wet etch to release MEMS structures Need a dedicated production line 6 EEL5225: Principles of MEMS Transducers (Fall 2003) Intermediate-CMOS MEMS Form transistors on bare wafers first Then deposit and anneal MEMS structural materials No CMP needed Only one interconnect metal layer Wet etch to release MEMS structures Need a dedicated production line NPN NMOS Sensor Area Sensor Poly Met Passivations BPSG Thox Nwell 7 Emitter Base NSD EEL5225: Principles of MEMS Transducers (Fall 2003) Courtesy of Mr. John Geen of Analog Devices, Inc. Thin-Film Post CMOS-MEMS (a) After standard CMOS processes CMOS region microstructural region metallization layers dielectric layers silicon substrate gate polysilicon (b) Pattern microstructure • Metal as etching mask • Anisotropic etch • CHF3 + O2 metal-3 metal-2 metal-1 (c) Release microstructure • Metal as etching mask • Undercut Si substrate • Isotropic etch • SF6 or XeF2 8 Spring beams movable microstructure anchored stator Sensing comb fingers Proof mass G. Fedder et al., Sensors & Actuators A, v.57, no.2, 1996 EEL5225: Principles of MEMS Transducers (Fall 2003) H. Xie et al., Thin-film z-axis accelerometer DRIE CMOS-MEMS Process (a) Backside etch STS: 12-sec etching, 130-sccm SF6, 13-sccm O2, 23 mT, 600 W coil power, 12 W platen power; 8-sec passivation 85-sccm C4F8, 12 mT, 600 W coil power, 0 platen power. CMOS-region Single-crystal Si (SCS) membrane metal-3 metal-2 metal-1 oxide poly-Si (b) Oxide etch PlasmaTherm-790: 22.5-sccm CHF3, 16-sccm O2, 100 W, 125 mT for 125 minutes and then 100 mT for 10 minutes. CMOS layer (c) Deep Si etch STS: same as Step (a). H. Xie et al, Journal of Microelectromechanical Systems, April 2002 9 EEL5225: Principles of MEMS Transducers (Fall 2003) Flat structure Thin-film structure (d) Si undercut SCS layer (20~100µm) STS: 130-sccm SF6, 13-sccm O2, 23 mT, 600 W coil power, and 0 platen power. bimorph actuator mirror Spring beams Sensing comb fingers Proof mass Xie et al, 1-D Scanning Micromirror Xie et al, DRIE z-axis accelerometer 10 EEL5225: Principles of MEMS Transducers (Fall 2003) Wafer Bonding Wafer bonding Ê Addresses need to obtain greater vertical dimensions and vacuum packaging, and to seal channels Methods Ê Ê Ê Ê Ê Epoxy bonding (low temperature 100°C) Metal eutectic bonding (low-moderate temperature 100-400°C) Glass frit bonding (low-moderate temperature 450°C) Anodic bonding (moderate temperature 450-500°C) Silicon fusion bonding (high temperature 10001100°C) Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139. 11 EEL5225: Principles of MEMS Transducers (Fall 2003) Wafer Bonding Anodic bonding Ê Ê Ê Moderate temperature 450-500°C Sodium-rich glass plate (7740 Pyrex) 500-1000V at 500°C to diffuse ions and to form electrostatic bond Bond chamber Ref. Kovacs, Micromachined Transducers Sourcebook, p. 120. 12 EEL5225: Principles of MEMS Transducers (Fall 2003) EVG501 Wafer Bonding System LIGA LIGA LIGA: German acronym for X-ray Lithographie, electrodeposition (Galvanoformung), and molding (Abformung) Issues Ê High energy radiation source – Alternatives Assembly Enables fabrication of microsize metal parts Ê Ref. Maluf, Introduction to MEMS Engineering, p.76. 13 EEL5225: Principles of MEMS Transducers (Fall 2003) Thick UV sensitive resist (‘poor man’s LIGA’) Project Issues 14 Teams Schedule EEL5225: Principles of MEMS Transducers (Fall 2003)