ME 141B: The MEMS Class Introduction to MEMS and MEMS Design Sumita Pennathur UCSBOutline • Class odds and ends • Lithography I • Photolithography • Ebeam lithography • Nanoimprint • Dip-pen • SoftPhotolithography • Photo-litho-graphy… from Latin: light-stone-writing • Photolithography is an optical means for transferring patterns onto a substrate. It is essentially the same process that is used in lithographic printing • Patterns are first transferred to an imagable photoresist layer • Photoresist is a liquid film that can be spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer from subsequent processing • Photolithography is a binary pattern transfer: there is no gray-scale, color, not depth to the image 10/12/10 3/45Photolithography Processing Steps 10/12/10 4/45Photolithography Processing Steps • Clean wafer: to remove particles on the surface as well as any traces of organic, ionic, and metal impurities • Dehydration bake: to drive off the absorbed water on the surface to promote the adhesion of PR • Coating:  Coat wafer with adhesion promoting form (e.g. HMDS)  Coat with photoresist • Soft bake: to drive off excess solvent and to promote adhesion • Exposure • Post-exposure bake: (optional): to supress standing wave-effect • Develop • Clean, Dry • Hard Bake: to harden the PR and improve adhesion to the substrate 10/12/10 5/45Photoresist Spin Coating • Wafer is held on a spinner chuck by vacuum and resis it coated to uniform thickness by spin coating • Typically 3000-6000 rpm for 15-30 seconds • Resist thickness is set by:  Primarily resist viscosity  Secondarily spinner rotational speed • Resist thickness is given by t = kp^2/w^(1/2), where  k = spinner constant, typically 80-100  p = resist solids content in percent  w = spinner rotational speed in rpm/1000 • Most resist thickesses are 1-2 um for commercial processes 10/12/10 6/45Photoresist Spin Coating 10/12/10 7/45Spinning Artifacts • Striations  ~30 nm variations in resist thickness due to nonuniform drying of solvent during spin coating  ~80 – 100 um periodicity, radially out from center of wafer • Edge Bead  Residual ridge in resist at edge of wafer  Can be up to 20-30 times the nominal thickness of the resist  Radius on wafer edge greatly reduces the edge bead height  Non-circular wafers greatly increase the edge bead height  Edge bead removers are solvents that are spun on after resist coating and which partially dissolve away the edge bead • Streaks  Radial patterns caused by hard particulates whose diameter are greater than the resist thickness 10/12/10 8/45Automated Resist 10/12/10 9/45Spray Development 10/12/10 10/45More about resist • Resist thickness (0.6-1 um) is a function of  Spin speed  Resist viscosity  Spin time • Positive resists are mostly used  Negative resists swell and are more toxic • G-line and i-line resists have three components  Inactive resin (that resists etching and ion implantation/milling)  PAC- Photoactive Compound  PAG – Photo-acid generator (acts as a chemical amplified or catalyst) • PAC is a diazonaphthoquinones (inhibits dissolution unless exposed to light) 10/12/10 11/45Positive Thick Photoresist • Spin cast • Thicknesses of order 10 microns • Sloped profiles  Slope somewhat controllable through process conditions • Some planarizing capability • Typical Applications:  Prolonged or low selectivity dry etch  Deep reactive ion etch  Masking any etch over topography • Not a standard front end material, but not inherently incompatible with it 10/12/10 12/45Very Thick Photoresists • SU-8 epoxy  Spin cast  Negative resist, optical exposure  Can planarize extreme topographies  Can be structural, not easily dissolved • Poyimide  Spin cast  Can planarize topographies  Humidity sensitive 10/12/10 13/45Photomasks • Master patterns which are transferred to wafers • Types:  Transparency mask (cheapest)  Photographic emulsion on soda lime glass  Fe2O3 on soda lime glass  Cr on soda lime glass • Dimensions:  4” x 4” x 0.060” for 3-inch wafers  5” x 5” x 0.060” for 4-inch wafers • Polarity:  “light-field” = mostly clean, drawn feature = opaque  “dark-field” = mostly opaque, drawn feature = clear 10/12/10 14/45Methods of optical lithography I 10/12/10 15/45 • Contact  Mask touches wafer  Inexpensive  Contact degrades mask  No die size limit  Resolution: down to 1 micron nervously; down to several microns comfortably • Proximity  Mask of order 10 microns from wafer  Inexpensive  Less mask damage  Diffraction means lower resolution  No die size limit  Resolution: down to several microns nervously, somewhat larger comfortablyExposure Systems 10/12/10 16/45Alignment and Exposure Hardware • For simple contact, proximity, and projection systems, the mask is the same size and scale as the printed wafer patter. Ie. the reproduction ratio is 1:1 • Projection systems give the ability to change the reproduction ratio. Going to 10:1 reduction allows larger size patterns on the mask, which is more robust to mask defect. • Mask size can get unwieldy for large wafers. • Most wafers contain an array of the same pattern, so only one cell of the array is needed on the mask. The system is called Direct Step on Wafer – “Steppers”  Advantage: only ne cell of wafer needed • The one cell must be PERFECT, since it is used for all die 10/12/10 17/45Projection Lithography • Projection lithography, especially when combined with an optical imaging system that reduces the image size, is used for high-resolution patterning (submicron to very submicron) • Larger mask features, no contact with mask • Wafer steppers expose one die at a time, assuring good focus and registration • Something to consider: if your device needs a fine features, a stepper may be required. But steppers have limits on dies size of about 1 cm. 10/12/10 18/45Stepper