The Lithography Process
The Lithography Process
Photolithography is how circuit patterns are transferred onto wafers. It's the most critical and repeated step in chip manufacturing — performed 50–100+ times per chip. The basic sequence:
- Coat: Spin a thin layer of photoresist (light-sensitive polymer) onto the wafer
- Soft bake: Drive off solvents from the resist
- Align & Expose: Project the circuit pattern through a mask (reticle) onto the resist using UV light
- Post-exposure bake (PEB): Complete chemical reactions triggered by exposure
- Develop: Dissolve away the exposed (positive tone) or unexposed (negative tone) resist
- Hard bake: Harden the remaining resist pattern
- Etch or implant: Use the resist pattern as a mask for the actual process step
- Strip: Remove the resist after it's served its purpose
Analogy: Stencil Printing
Lithography is like using a stencil to spray-paint a pattern — but with light instead of paint, and at the nanometer scale. The mask is the stencil, the light is the "spray," and the photoresist is the surface that records the pattern.
Photoresist Chemistry
Photoresist Chemistry
Photoresists are engineered polymers that change solubility when exposed to light:
- Positive resist: Exposed areas become MORE soluble and are dissolved away during development. Used for most advanced patterning.
- Negative resist: Exposed areas become LESS soluble (crosslink). Unexposed areas are dissolved away. Used for some specialized applications.
Modern chemically amplified resists (CAR) use a photoacid generator (PAG). One photon generates an acid molecule that catalyzes many chemical reactions during PEB — amplifying the effect of each photon. This is essential for EUV lithography where photon counts are limited.
Key Concept: Resolution
The minimum feature size (resolution) is governed by the Rayleigh equation: R = k₁ × λ / NA, where λ is wavelength, NA is numerical aperture, and k₁ is a process factor. Smaller wavelength and larger NA give better resolution.
Knowledge Check
Knowledge Check
1 / 2In positive photoresist, what happens to exposed areas?