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Appendix II.
Steps in Optical Lithography
(Film-and-chemistry photography)
Blank Patch on a Silicon Wafer(12¨- Ø)
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Silicon Wafer is held on the chuk of a spinner by vacuum suction
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Next it is coated with photoresist and spun at a high r.p.m. to ensure uniform layer of photo resist across the wafer
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The correct mask is placed in contact with the photo resist layer
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The photo resist is exposed to Ultra-violet light for 1 minute through the mask
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The exposed region gets polymerized and hence hardened whereas the unexposed portion is soft and can be dissolved away
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Next wafer with the exposed Photo-Resist coating is dipped In a developer
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The developer dissolves away the soft portion leaving a window pattern on the hardened portion of the photo-resist layer
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Next the wafer along with the the window pattern printed on the photo resist is dipped in HF acid which is an etchant of SiO2. HF acid is a very strong solvent. It is placed in Teflon beaker and Teflon twigger is used for handling the wafer. HF acid must never come in touch with our skin. If it comes it will reach the bone and dissolve it. So the personnels will have to use Teflon gloves and suitable aprons for safety as well as for cleanliness
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The window pattern of the hardened resist is stripped off and the wafer is ready for diffusion or ion implantation or for metallization
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Hundreds of identical microchip pattern is inscribed on the wafer by optical photo-lithography, plasma etching and a combination of ion-implantation or diffusion
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Sliced into microchips using a diamond tipped scriber
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Individual microchips also referred to as dies are mounted on headers
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Gold wires are connected from the bonding pads on the die to the external leads on the header by thermo-compression bonding or by Ultra-sonic bonding
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Hermetically sealed
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