Introduction to Microeletromechanical Systems (MEMS) Lecture 3 Topics

Transcription

1 Introduction to Microeletromechanical Systems (MEMS) Lecture 3 Topics MEMS Fabrication Techniques Review of the Si Crystal Lattice Review of Wet Etching Dry Etching Plasma Etching Reactive Ion Etching Additive Processes Sacrificial Processes MEMS Fabrication Techniques Dry Etching Vapor Phase Plasma RIE Additive Processes CVD Sputtering Electroplating Sacrificial Layers Lift-off Wet Release Issues

2 Review Of The Si Crystal Lattice Unit Cell: most basic structural element in a crystal, repeated regularly over all three dimensions IV group elements: Diamond Lattice (Figures: Campbell, 1996.) Review Of The Si Crystal Lattice Figures: etti@nmsu.edu (1996) Notation: (100) particular crystal plane {100} all equivalent planes: (100), (010), (001) in cubic lattice [100] direction normal to crystal plane Wafer characterization: in 100 wafer, 100 plane is parallel to the wafer surface Several useful Internet WEB sites for visualization of Si crystals ostc.physics.uiowa.edu/~wkchan/solidstate/crystal et.nmsu.edu/etclasses/vlsi/files/crystal.htm stm2.nrl.navy.mil/~lwhitman/projects.html#sisum Location of atoms in various planes of the diamond lattice.

3 Review Of Wet Etching Review Of Wet Etching Anisotropic Wet Etching: Convex corners are undercut Concave corners stop at [111] intersections Figures: G. Kovacs, 1996.

4 Overview Vapor Phase Etch: Use of reactive gases No drying necessary Dry Etching Plasma Etch: RF energy generates reactive ions and free radicals No high temperatures required (250 C down to room temperature) Reactive Ion Enhanced (RIE) Etch: Higher energy ions Higher anisotropy Vapor Phase Etch XeF 2 Isotropic Silicon Etch Simple setup Does not attack: - Silicon oxide - Silicon nitride - Metals - Photoresist Basic reaction: 2XeF 2 + Si 2Xe +SiF 4 Caveat: 2XeF 2 +2H 2 O Xe 2 +4HF+2O 2 exothermic! Hoffman et al., 1995 (UCLA)

5 XeF 2 Isotropic Silicon Etch Post processing for standard CMOS Suspended and 3D structures Fold-up structures with conducting Al hinges Tahhan et al., SPIE 1999 (UC Berkeley) Storment et al., JMEMS 1994 (Stanford) Plasma Etch RF energy drives etching reaction: accelerates stray electrons between pair of plates in low pressure gas Electrons generate reactive ions and free radicals (e.g., monoatomic fluorine) Substrate surface is bombarded with reactive ions (physical and chemical etching) Si or other materials are etched by forming volatile components

6 Plasma Etch RIE allows higher ion energies: higher etch rates, higher anisotropy Reactive Ion Etch Often, multiple etching and deposition reactions take place simultaneously and reach equilibrium Example: High concentration SF 6 etches Si Low concentration O 2 removes resputtered photoresist but also forms SiO 2 and polymeric films CHF 3 removes oxide and polymers Selection of etch parameters (concentration, pressure, RF power, bias, ) gives (limited) control over anisotropy, selectivity, etch rate, surface roughness

7 SCREAM (Single Crystal Reactive Etching And Metallization) Multiple anisotropic and isotropic dry etches Low temperature etching and deposition Reactive Ion Etch Zhang et al., 1993 (Cornell) Reactive Ion Etch Figure: G. Kovacs, RIE postprocessing of CMOS to release thin film structures (Fedder et al. 1996)

8 Deep RIE Bosch Process (Patent: Lärmer & Schilp, 1994) Idea: alternate between etching and thin film deposition that protects sidewalls but is removed in trenches Etching phase: SF 6 / Ar Polymerization phase: CHF 3 (or C 4 F 8 /SF 6 ) / Ar forms Teflon-like polymer layer Ion bombardment can prevent formation of polymer on horizontal surfaces Several DRIE systems are on the market (after only 5 years!): STS, Plasma Therm, Oxford Instruments, Trion Deep RIE Examples 20µm STS 1999 Klaassen et al., 1995 (Stanford) Ayon et al., 1998 (MIT)

9 Additive Processes Formation of films on surface of substrate ( surface micromachining ) Structural layers Sacrificial layers (spacers to be removed later) Wide Variety Of Techniques: Oxidation of Si CVD, PECVD Evaporation Sputtering Epitaxial growth Molding Chemical Vapor Deposition CVD uses thermal energy to drive reactions that deposit thin films on substrate surface PECVD (Plasma Enhanced CVD) substitutes thermal energy (partially) by RF energy: greater control over stresses and other film properties Note analogy to Plasma Etching, RIE etching Commonly deposited thin films with PECVD: SiO 2, Si 3 N 4, SiC, poly-si

10 Epitaxial Growth SCS grows selectively on exposed Si surfaces 2H 2 + SiCl 4 Si + 4HCl (hydrogen reduction) SiH 4 Si + 2H 2 (pyrolysis) Electroplating Plating processes use the reduction of metal ions in solution to form solid metal Many metals and alloys (Au, Ag, Cu, Hg, Ni, Pt, Permalloy [NiFe], ) Electroplating uses electrical current to drive the reduction Electroless plating uses reducing agents to drive metal deposition Pulsing the electroplating current allows to replenish reactants (stress control, control over morphology, etc., possible) Under diffusion-limited conditions, amorphous metal layers can be plated (very high surface areas, e.g., platinum black )

11 Electroplating Fastest growing crystal planes disappear Figure: G. Kovacs, 1996, after Bockris, Reddy, Note analogy to anisotropic etching Evaporation And Sputtering Evaporation of metals by Heating (thermal evaporation) Bombardment with electron beam (e-beam evaporation) Sputtering: bombardment of target with inert ions (Ar + ) Metals Si Compounds Dielectrics Better stress control

12 Sputtering vs. Evaporation Geometry of evaporation and sputtering chambers (as well as electromagnetic fields) determine directionality of deposition: Good or bad step coverage (can be advantage or disadvantage) Shadowing Directionality of evaporation can be exploited to form features smaller than the lithographic resolution

13 Sub-Resolution Feature Sizes How can we build structures that are smaller than the resolution of our lithography equipment? Sacrificial Layers Frequent goal in MEMS: released, movable structures Concept: use spacer layers as temporary support between structural materials Commonly used sacrificial layers: SiO 2 (etched with HF) Photoresist (etched with acetone, O 2 plasma) Others Example: SiO 2 in multi-layer polysilicon structures

14 Sealed Cavity Formation More complex example for sacrificial layers: Form cavity with SiO 2 layer Removal of sacrificial layer Reactive sealing Sacrificial Layer In Electroplating Note: requires sufficient step coverage, otherwise

15 Lift-Off Process Removal of deposited thin film (usually metal) without etching: resist Substrate resist Lift-Off Substrate Positive resist: Negative resist: Substrate Substrate Wet Release Issues Attractive Forces Between Surfaces: Electrostatic forces Surface tension Hydrophilic surfaces: hydrogen bonds (attraction between a hydrogen atom of one molecule and a pair of unshared electrons of another molecule) Hydrophobic surfaces: van der Waals forces (attractive and repulsive electrostatic dipole-dipole interactions between molecules)

16 Critical Point Drying Adhesion forces during wet release can be a major problem. Possible solutions: Geometric surface modification (dimples) Chemical surface modification Sublimation methods Critical point drying: CO 2 : 25 C at 1200psi (liquid) 35 C at 1200psi (supercritical) gas is then removed Figure: G. Kovacs, 1996, after Mulhern et al., 1993.