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- Motivation
- History
- Methods and Theories
- Preparation and Priming
- Spin-Coating
- Photoresists
- Soft-baking
- Mask Alignment and Exposure
- Developing
- Hard-baking
- References
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- Key top-down manufacturing technology
- Applications:
- Microelectronics (ICs, transistors, etc.)
- Microelectromechanical Systems (MEMS)
- Sensors
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- Historically, lithography is a type of printing technology that is based
on the chemical repellence of oil and water.
- Photo-litho-graphy: latin: light-stone-writing
- In 1826, Joseph Nicephore Niepce, in Chalon, France, takes the first
photograph using bitumen of Judea on a pewter plate, developed using oil
of lavender and mineral spirits
- In 1935 Louis Minsk of Eastman Kodak developed the first negative
photoresist
- In 1940 Otto Suess developed the first positive photoresist.
- In 1954, Louis Plambeck, Jr., of Du Pont, develops the Dycryl polymeric
letterpress plate
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- In photolithography, the pattern is created photographically on a
substrate (silicon wafer)
- Photolithography is a binary pattern transfer: there is no gray-scale,
color, nor depth to the image
- This pattern can be used as a resist to substrate etchant, or a mold,
and other forms of design processes
- The steps involved are wafer cleaning, photoresist application, soft
baking, mask alignment, and exposure and development
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- Prepare the substrate (silicon wafer):
- Wash with appropriate solvent to remove any matter and other impurities
- Dry in Oven at 150°C for 10 min.
- Place on hotplate and cover with petri dish, let temp. stabilize at
115°C.
- Deposit Primer (optional)
- Chemical that coats the substrate and allows for better adhesion of the
resist
- TCE = trichloroethylene, MeOH = methanol
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- Deposit a layer of SiO2 (silicon dioxide) on the surface of
the wafer to serve as a barrier.
- Spin on the photoresist to the surface of the wafer
- Standard methods are to use high spin coaters
- Produces a thin uniform layer of photoresist on the wafer surface.
- Use red/amber light at this stage
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- Photoresist is an organic polymer which changes its chemical structure
when exposed to ultraviolet light.
- It contains a light-sensitive substance whose properties allow image
transfer onto a printed circuit board.
- There are two types of photoresist: positive and negative
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- Put on hotplate, or in oven
- Removes volatile solvents from the coating
- Makes photoresist non-sticky
- Hardens to amorphous solid
- Be careful not to overbake and destroy the sensitizer
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- Photomask is a square glass plate with a patterned emulsion of metal
film on one side
- After alignment, the photoresist is exposed to UV light
- Three primary exposure methods: contact, proximity, and projection
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- Highly-pure buffered alkaline solution
- Removes proper layer of photoresist upon contact or immersion
- Degree of exposure affects the resolution curves of the resist
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- Final step in the photolithographic process
- Not always necessary; depends on the resist
- Hardens the photoresist
- Improves adhesion of the photoresist to the wafer surface
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- Motivation
- http://www.mems-issys.com/html/singlestep.html#photolithography
- http://www.chipcenter.com/columns/bmcginty/col005.html
- http://www.adhesives.de/seiten/literature_highlights/gaynes.htm
- Background
- http://www.lib.udel.edu/ud/spec/exhibits/color/lithogr.htm
- http://www.ee.washington.edu/research/microtech/cam/PROCESSES/PDF%20FILES/Photolithography.pdf
- http://www.dbanks.demon.co.uk/ueng/plith.html
- Theories and Methods
- http://www.ece.gatech.edu/research/labs/vc/theory/PosNegRes.html
- http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html
- http://www.intl-light.com/photoresist.html
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