EEL5225: Principles of MEMS Transducers (Fall 2003) Fabrication Technology, Part II Agenda: Process Examples TI Micromirror fabrication process SCREAM CMOS-MEMS processes Wafer Bonding LIGA Reading: Senturia, pp. 79-98. 1 Lecture 9 by H.K. Xie 9/15/2003
Process Examples Surface micromachining: TI Digital Micromirror Devices (DMDs) Invented by L.J. Hornbeck Ref. Maluf, Introduction to MEMS Engineering, p.145. 2
Process Examples Bulk-Micromachined Pressure Sensor Thermal oxide Boron implantation Boron drive-in LPCVD Si 3 N 4 Backside KOH etch Electrochemical etch stop Metallization Detailed process steps refer to Senturia p.93 Ref. Senturia, Microsystem Design, p.97 3
Process Examples Single Crystal Reactive Etching and Metallization (SCREAM) Process 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
CMOS-MEMS Why CMOS-MEMS? Smart on-chip CMOS circuitry Multi-vendor accessibility Scalability Compact size More functions Low cost MEMS structures can be made Before CMOS processes ( pre-cmos ) In-between CMOS processes ( intermediate-cmos ) After CMOS processes ( post-cmos ) 5
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
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 Passivations BPSG Met Thox Nwell Emitter Base NSD Courtesy of Mr. John Geen of Analog Devices, Inc. 7
Thin-Film Post CMOS-MEMS (a) After standard CMOS processes CMOS region microstructural region metallization layers (b) Pattern microstructure Metal as etching mask Anisotropic etch CHF3 + O2 (c) Release microstructure Metal as etching mask Undercut Si substrate Isotropic etch movable microstructure SF6 or XeF2 G. Fedder et al., Sensors & Actuators A, v.57, no.2, 1996 dielectric layers silicon substrate gate polysilicon metal-3 metal-2 metal-1 anchored stator Spring beams Sensing comb fingers Proof mass H. Xie et al., Thin-film z-axis accelerometer 8
DRIE CMOS-MEMS Process (a) Backside etch STS: 12-sec etching, 130-sccm SF 6, 13-sccm O 2, 23 mt, 600 W coil power, 12 W platen power; 8-sec passivation 85-sccm C 4 F 8, 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 CHF 3, 16-sccm O 2, 100 W, 125 mt for 125 minutes and then 100 mt for 10 minutes. (c) Deep Si etch CMOS layer STS: same as Step (a). 9 H. Xie et al, Journal of Microelectromechanical Systems, April 2002
Flat structure Thin-film structure (d) Si undercut STS: 130-sccm SF 6, 13-sccm O 2, 23 mt, 600 W coil power, and 0 platen power. SCS layer (20~100µm) bimorph actuator Spring beams mirror Sensing comb fingers Proof mass Xie et al, 1-D Scanning Micromirror Xie et al, DRIE z-axis accelerometer 10
Wafer Bonding Wafer bonding Addresses need to obtain greater vertical dimensions and vacuum packaging, and to seal channels Methods 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 1000-1100 C) Ref. Kovacs, Micromachined Transducers Sourcebook, p. 139. 11
Wafer Bonding Anodic bonding 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. EVG501 Wafer Bonding System 12
LIGA LIGA LIGA: German acronym for X-ray Lithographie, electrodeposition (Galvanoformung), and molding (Abformung) Ref. Maluf, Introduction to MEMS Engineering, p.76. Issues High energy radiation source Alternatives Assembly Thick UV sensitive resist ( poor man s LIGA ) Enables fabrication of microsize metal parts 13
Project Issues Teams Schedule 14