MEMS_02 guiren photolithography_01

MEMS_02 guiren photolithography_01 - Photolithography...

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Photolithography
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Photolithography overview IC: fabricate thin film almost exclusively using photolithography This essentially 2-D process has limited tolerance for non-IC miniaturization system having extreme topographies
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Steps Design and fabricate a mask Select a substrate Coat the substrate with photoresist Align the coated substrate with UV light for exposure to cause photochemical reaction Developing Etching
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Masks Mask: stencil used to generate a desired pattern on photoresist-coated wafers Photomask: An optically flat glass (transparent to near UV light) or quartz plate (transparent to deep UV light) An absorber patterned metal layer (e.g. a chromium layer opaque to UV light) Mask polarity: light field image or dark (inset 1.2) Result in 1:1 image of entire mask onto the silicon wafer Can be repeatedly used Light field Dark field
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Masks Shadow printing Hard contact: direct physical contact Degrade faster Not suitable for VLSI But used in R&D for Mask Prototyping Soft (proximity) contact: 10-20 micronm above the wafer Projection printing Using a lens system to image onto the wafer The lens can reduce mask image pattern by 1:5 or 1:10 Mask life depends operator handling
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Masks Fabrication of mask: Generated by e-beam lithography (higher resolution than photolithography) CAD software Platform There are other cheaper faster methods: Laser Transparency Using Several others
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Spinning resist and Soft baking Before spinning: grow a thin layer of oxide on a wafer surface by heating it to 900-1150 ˚ C in Steam Humidified O 2 stream (Fig. 1.1A) Dry O 2 is OK, but wet O 2 and steam are fast The oxide can serve as a mask for subsequent processes Wet etch Boron implant 1 st : deposited by spinning a photoresist layer on the oxide surface (Fig. 1.1B) The wafer is lying on a wafer platen (vacuum chuck of a resist spinner (Inset 1.3)
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Spinning resist and Soft baking Photoresist is dispensed from a viscous polymer solution onto the wafer Spinning speed: 1500-8000 rpm depends on viscosity, required film thickness Thickness is primary important: determined by equ. (1.1) Th sc = K C β η γ / ω α Quality of coating determines the density of defect transferred to the device Uniformity: +/- 5 nm for a 1.5um film, i.e. = 1 0.3% to ensure reproducible line widths and development (i.e. etching) times
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Th sc depends on the Chemical resistance required for image transfer Fineness of line and space to be resolved Too thick : result in edge covering, hillucks and ridges Too thin : leave uncovered area Typical Th sc for IC: 0.5 – 2 μ m after prebaking Typical Th sc for miniaturization : much thicker, and can be fabricated by other methods (e.g. LIGA) Small structure requires thinner Th sc Soft baking (or prebaking): to
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This note was uploaded on 10/03/2011 for the course BMEN 589 taught by Professor Wang during the Spring '11 term at South Carolina.

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MEMS_02 guiren photolithography_01 - Photolithography...

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