MICROMACHINING – Detailed Overview for Mechanical Engineering
- 2. Micromachining • Photolithography • Etching • LIGA • Laser Ablation • Mechanical Micromachining
- 3. Micromachining Basics • Refers to techniques for fabrication of 3D structures on the micrometer scale • Applications include MEMS devices e.g. airbag sensor, medical devices, micro-dies and molds, etc. • Most methods use silicon as substrate material
- 4. Photolithography • Used in microelectronics fabrication • Used to pattern on silicon oxide/nitride/polysilicon films substrate • Basic steps - Photoresist development - Etching - Resist removal (a) (b) (c)
- 5. Photolithography Process Description • The wafers are chemically cleaned to remove particulate matter, organic, ionic, and metallic impurities • High-speed centrifugal whirling of silicon wafers known as "Spin Coating" produces a thin uniform layer of photoresist (a light sensitive polymer) on the wafers • Photoresist is exposed to a set of lights through a mask often made of quartz • Wavelength of light ranges from 300-500 nm (UV) and X-rays (wavelengths 4-50 Angstroms) • Two types of photoresist are used: – Positive: whatever shows, goes – Negative: whatever shows, stays
- 6. Etching Process Variations: 1. Wet etching 2. Dry etching Variations of wet etching
- 7. Wet Etching Process Description • The wet etching process involves: – Transport of reactants to the surface – Surface reaction – Transport of products from surfaces • The key ingredients are: – Oxidizer (e.g. H2O2, HNO3) – Acid or base to dissolve the oxidized surface (e.g. H2SO4, NH4OH) – Dilutent media to transport the products through (e.g. H2O)
- 8. Dry Etching Process Variations: 1. Plasma based 2. Non plasma based A typical parallel plate plasma etching
- 9. Bulk Micromachining • Process for producing 3D MEMS structures – older process • Uses anisotropic etching of single crystal silicon • Example: silicon cantilever beam for atomic force microscope
- 11. Surface Micromachining • Newer process for producing MEMS structures • Uses etching techniques to pattern micro- scale structures from polycrystalline (poly) silicon, or metal alloys • Examples: accelerometers, pressure sensors, micro gears and transmissions, micro mirrors etc.
- 12. Surface Micromachining (a) deposition of a phosphosilicate glass (PSG) spacer later; (b) etching of the spacer layer; (C) deposition of polysilicon; (d) etching of polysilicon; (e) selective wet etching of PSG, leaving the silicon substrate and deposited polysilicon unaffected
- 13. Comb Drives and Gears Spider Mites on Ring (slow) Spider Mite on Ring (faster)
- 14. Typical MEMS Parts Six gear chain
- 15. Typical MEMS Parts Silicon mirror assembly
- 18. LIGA - Basic Steps
- 19. LIGA Process Description • Deep X-ray lithography and mask technology – Deep X-ray (0.01 – 1nm wavelength) lithography can produce high aspect ratios (1 mm high and a lateral resolution of 0.2 µm) – X-rays break chemical bonds in the resist; exposed resist is dissolved using wet- etching process • Electroforming – The spaces generated by the removal of the irradiated plastic material are filled with metal (e.g. Ni) using electro-deposition process – Precision grinding with diamond slurry-based metal plate used to remove substrate layer/metal layer • Resist Removal – PMMA resist exposed to X-ray and removed by exposure to oxygen plasma or through wet-etching • Plastic Molding – Metal mold from LIGA used for injection molding of MEMS structures
- 20. LIGA Process Capability • High aspect ratio structures: 10-50 – Max. height 1-500 μm • Surface roughness < 50 nm • High accuracy < 1μm Any lateral shape High accuracy High aspect ratio
- 21. Laser Ablation
- 22. Laser Ablation Process Description • High-power laser pulses are used to evaporate matter from a target surface • A supersonic jet of particles (plume) is ejected normal to the target surface which condenses on substrate opposite to target • The ablation process takes place in a vacuum chamber - either in vacuum or in the presence of some background gas
- 23. Mechanical Micromachining • Lithography and/or etching methods not capable of making true 3D structures e.g. free form surfaces • Also, limited in range of materials • Mechanical machining is capable of making free form surfaces in wide range of materials • Can we scale conventional/non-traditional machining processes down to the micron level? Yes!
- 24. Mechanical Micromachining • Two approaches used to machine micron and sub- micron scale features – Design ultra precision (nanometer positioning resolution) machine tools and cutting tools • Ultra precision diamond turning machines – Design miniature but precise machine tools • Micro-lathe, micro-mill, micro-EDM, etc
- 25. Ultra Precision Machine Tools
- 26. Miniature Machine Tools Micro Lathe Micro Factory Source: MEL,AIST , Japan
- 27. Miniature Machine Tools Source: MEL, AIST , Japan Micro Turning Micro Milling
- 28. Micro Cutting Tools Cutting tools made by Focused Ion Beam (FIB) machining Source: http://www.sandia.gov Source:Adams et al, Prec. Eng., 24 (2000) 347-356
- 29. Stencil Machining φ= 50 μm, N = 50,000 rpm, feed = 100 mm/min, chip size = 100 nm
- 30. Mechanical Micromachining Process Description • Can produce extremely smooth, precise, high resolution true 3D structures • Expensive, non-parallel, but handles much larger substrates • Precision cutting on lathes produces miniature screws, etc with 12 μm accuracy • Relative tolerances are typically 1/10 to 1/1000 of feature • Absolute tolerances are typically similar to those for conventional precision machining (micrometer to sub-micrometer)
- 31. Summary • Micromachining methods – IC fabrication based processes – Mechanical machining based processes • Applications in MEMS, medical device fabrication, etc. • Still evolving field