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Part III. Functional Polymers for Semiconductor Applications Outline Functional Polymer/1st Semester, 2006 _________________________________________ Part III. Functional Polymers for Semiconductor Applications Outline of Part Photoresist for Semiconductor Applications Introduction of photolithography Photoresist Materials for Exposure at 193 nm Wavelength Chemically Amplified Resists for F2 Excimer laser Lithography Prof. Jin-Heong Yim Motivations Creation of integrated circuits, which are a major component in computer technology An extension of photolithography processes are used to create standard semiconductor chips Play a key role in the production of technically demanding components of advanced microsensors Used to make adhesives in electronics Prof. Jin-Heong Yim History 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 Prof. Jin-Heong Yim Microlithography A process that involves transferring an integrated circuit pattern into a polymer film and subsequently replicating that pattern in an underlying thin conductor or dielectric film Prof. Jin-Heong Yim How Small Can We Print ? SEM picture of typical lithographic pattern Comparison of the dimensions of lithographic images and familiar objects Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994 Prof. Jin-Heong Yim Roadmap of Semiconductor Technology 10 - C.Rubber Novolak KrF ArF F2, - Bisazide - DNQ CAR CAR EUV, X-Ray CAR 16 K 1977 Resolution 5.0 64 K 1980 λ 3.0 256 K R = k 1.0 1983 64 M NA 2.0 1 M 1995 1986 0.35 256 M 1.2 4 M 1998 1989 0.25 1 G 0.8 16 M 2001 1992 0.18 4G 0.5 2004 16 G 0.13 2007 0.09 0.1 F , Contact g-line i-line 2 KrF ArF EUV, Printer Stepper Stepper X-Ray 19801985 1990 1995 2000 2005 2010 Prof. Jin-Heong Yim Light intensity(output) of Hg-lamp as a function of wavelength I-line G-line DUV Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994 Prof. Jin-Heong Yim Photolithography 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 Prof. Jin-Heong Yim Photoresist 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 Prof. Jin-Heong Yim PR Process Photoresist Coating Substrate hu Mask Exposure Negative Positive Develop Etch Strip Prof. Jin-Heong Yim Chemical Amplification Solution : 1. More sensitive photoresists (Very high efficiency) Photoacid Generator(PAG) hv Strong Acid H + H H3 C CH CH 2 CH CH2 + CO2 + + C CH2 n n H3C CH3 O O C CH3 OH C CH3 O "Catalytic Cycle" H3C + t-BOC H + C CH2 H3C 2. Brighter light source (e.g. KrF laser for 248nm and ArF laser for 193nm) Prof. Jin-Heong Yim How Novolac/DNQ Resists Work O OH COOH N2 I I I I I I UV CH2 I I I I IICoat R R Si CH3 n (Base insoluble) (Base soluble) DNQ(Diazonaphthoquinone) Carboxylic acid Novolac Resin (N) Inhibitor (I) Photoproduct (P) UV novolac + photoproducts I I P P P I I Expose pure I I P P P II Develop. novolac Rate Si I I I I I I Development novolac I I + DNQ Si Reaction Coordinate Prof. Jin-Heong Yim Two Types of Photoresist Positive Photoresist Negative Photoresist Exposure to UV light Exposure to UV light makes it more soluble causes the resist to in the developer polymerize, and thus be more difficult to Exposed resist is dissolve washed away by developer so that the Developer removes the unexposed substrate unexposed resist remains This is like a Results in an exact photographic negative copy of the original of the pattern design Prof. Jin-Heong Yim Preparation and Priming Prepare the substrate (silicon wafer): Wash with appropriate solvent to remove any dirt and other impurities Acetone, MeOH, TCE 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 Prof. Jin-Heong Yim Spin Coating Spin-coat the photoresist onto the surface of the wafer RPM: 1000-7000 Time: ~30 sec Produces a thin uniform layer of photoresist on the wafer surface. Use red/amber safe light at this stage Prof. Jin-Heong Yim Soft Baking Put on hotplate, or in oven Temperature: 65°C-115°C, Time: 1-5 min Removes volatile solvents from the coating Makes photoresist imageable Hardens to amorphous solid Be careful not to overbake and destroy the sensitizer Prof. Jin-Heong Yim Mask Alignment and Exposure 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 Prof. Jin-Heong Yim Exposure Methods Prof. Jin-Heong Yim Resolution of Lithographic Process * Use of shorter wavelength for exposure to get smaller feature size “Lens Equation” Light R= k λ 1 NA Lens θ R: Resolution (feature size) Coated λ:wavelength of light source Wafer k1:Process factor NA: Numerical aperture (NA= sin θ) Prof. Jin-Heong Yim Photoresist Developer Highly pure buffered alkaline solution Removes proper layer of photoresist upon contact or immersion Degree of exposure affects the resolution curves of the resist Prof. Jin-Heong Yim Hard Baking 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 Prof. Jin-Heong Yim.
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