1.5 Lithography and Patterning

• In IC technology, lithography is the process of forming patterns on the surface of a semiconductor wafer
• Steps to form a pattern on a silicon wafer:
1. Silicon with a layer of silicon oxide is coated with a layer of photoresist, a light-sensitive organic film similar to the emulsion on a photographic film
2. The resist is then exposed to ultraviolet light that passes through the clear portions of a "mask", a glass plate with precise patterns of opaque material that block the light
3. In the case of positive resist, the areas of opaque material on the surface of the mask are located where SiO2 is to remain on the surface
4. After exposure to light in selected areas, the photoresist long-chain molecules have been broken (scission process) and can be removed in an organic solvent
5. The remaining photoresist after hardening is resistant to hydrofluoric acid (HF), which etches the SiO2 but does not etch the surface of silicon
6. Final Step is to remove (strip-off) the photoresist
• The geometric pattern of a mask is now transferred to the SiO2 layer 
• Other variations on lithography process: Negative resist; Electron-beam resist; X-ray lithography; Ion-beam resist
• A liftoff process is often used to define metal lines:

1. The substrate is covered with photoresist, which is then exposed in patterns so that openings are made where the metal is to remain
2. Remove unexposed resist
3. A metal film is then deposited over the surface 
4. Remove exposed resist and metal on resist
5. Metal pattern remains on Si

• Chemical etching - liquid or gas is used to remove any material not protected by a hardened resist (negative resist)
• Dry etching with ions:
• Plasma etching uses fluorine or chlorine ions to attack SiO2
• Reactive ion beam etching (RIBE) uses reactive ions
• A complete IC generally requires between 10 - 20 lithography processing steps