Photolithography types and pre-steps

Photolithography is the process of using light to transfer a circuit pattern from a mask to a photoresist layer on the wafer, so the pattern can be etched or doped into the wafer.

Types

• Positive and negative photolithography, depending on the photoresist (exposed part removed or kept).
• Contact, proximity and projection printing, depending on how the mask is placed relative to the wafer.
• Optical (UV, DUV, EUV), electron-beam, X-ray and ion-beam lithography, depending on the radiation used.

Steps before photolithography

 Sand --> pure Si --> Crystal --> Ingot --> Wafers
 (SiO2)   (poly)      growth      shaping    slicing
                                               |
 Resist <-- HMDS <-- Oxidation <-- Clean <-- Polish
 coating    prime     / deposition

1. Raw material purification: quartz sand (SiO2) is reduced to metallurgical silicon and purified to electronic-grade polysilicon (99.9999999 %).
2. Crystal growth: the Czochralski method pulls a single-crystal ingot from molten silicon, doped p-type or n-type.
3. Ingot shaping: the ingot is ground to a standard diameter and a flat or notch marks the orientation.
4. Slicing: a diamond saw cuts the ingot into wafers (about 0.5-0.8 mm thick).
5. Lapping, etching and polishing: removes saw damage and gives a mirror-flat surface (chemical-mechanical polishing).
6. Cleaning: RCA cleaning removes particles, organics and metal ions.
7. Oxidation or film deposition: a thin SiO2 (thermal oxidation), poly-Si or metal layer is formed, which the first lithography will pattern.
8. Dehydration bake and priming: HMDS improves adhesion of the photoresist, which is then spin-coated.