Photolithography Outline n n Motivation History n Methods

Photolithography

Outline n n Motivation History ¡ n Methods and Theories ¡ ¡ ¡ ¡ n Photolithography Preparation and Priming Spin-Coating Photoresists Soft-baking Mask Alignment and Exposure Developing Hard-baking References

Motivation n Key top-down manufacturing technology n Applications: ¡ Microelectronics (ICs, transistors, etc. ) ¡ Microelectromechanical Systems (MEMS) ¡ Sensors

History n Historically, lithography is a type of printing technology that is based on the chemical repellence of oil and water. n Photo-litho-graphy: latin: light-stone-writing n 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 n In 1935 Louis Minsk of Eastman Kodak developed the first negative photoresist n In 1940 Otto Suess developed the first positive photoresist. n In 1954, Louis Plambeck, Jr. , of Du Pont, develops the Dycryl polymeric letterpress plate

Photolithography n In photolithography, the pattern is created photographically on a substrate (silicon wafer) n Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image n This pattern can be used as a resist to substrate etchant, or a mold, and other forms of design processes n The steps involved are wafer cleaning, photoresist application, soft baking, mask alignment, and exposure and development

Preparation and Priming n Prepare the substrate (silicon wafer): ¡ Wash with appropriate solvent to remove any matter and other impurities n n TCE, Acetone, Me. OH ¡ 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, Me. OH = methanol

Spin-Coating the Resist n Deposit a layer of Si. O 2 (silicon dioxide) on the surface of the wafer to serve as a barrier. n Spin on the photoresist to the surface of the wafer ¡ ¡ n Standard methods are to use high spin coaters n RPM: n Time: Produces a thin uniform layer of photoresist on the wafer surface. Use red/amber light at this stage

Photoresist n Photoresist is an organic polymer which changes its chemical structure when exposed to ultraviolet light. n It contains a light-sensitive substance whose properties allow image transfer onto a printed circuit board. n There are two types of photoresist: positive and negative

Diagram: n Exposure to UV light makes it more soluble in the developer n Exposed resist is washed away by developer so that the unexposed substrate remains n Results in an exact copy of the original design n Exposure to UV light causes the resist to polymerize, and thus be more difficult to dissolve n Developer removes the unexposed resist n This is like a photographic negative of the pattern

Soft-Baking n Put on hotplate, or in oven ¡ n n Temperature: , Time: Removes volatile solvents from the coating Makes photoresist non-sticky Hardens to amorphous solid Be careful not to overbake and destroy the sensitizer

Mask Alignment and Exposure n n n 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

Exposure Methods

Photoresist Developer n n n Highly-pure buffered alkaline solution Removes proper layer of photoresist upon contact or immersion Degree of exposure affects the resolution curves of the resist

Hard Baking n Final step in the photolithographic process ¡ n n Not always necessary; depends on the resist Hardens the photoresist Improves adhesion of the photoresist to the wafer surface

References n Motivation ¡ ¡ ¡ n Background ¡ ¡ ¡ n http: //www. memsissys. com/html/singlestep. html#photolithography http: //www. chipcenter. com/columns/bmcginty/col 005. html http: //www. adhesives. de/seiten/literature_highlights/gaynes. htm http: //www. lib. udel. edu/ud/spec/exhibits/color/lithogr. htm http: //www. ee. washington. edu/research/microtech/cam/PROCES SES/PDF%20 FILES/Photolithography. pdf http: //www. dbanks. demon. co. uk/ueng/plith. html Theories and Methods ¡ ¡ ¡ http: //www. ece. gatech. edu/research/labs/vc/theory/Pos. Neg. Res. html http: //www. ece. gatech. edu/research/labs/vc/theory/photolith. html http: //www. intl-light. com/photoresist. html