Lithography

A -Prepare wafer, spin on photoresist (photosensitive chemical), bake B -Align photomask (transparent plate with chromium patterns) , expose with light (UV), bake C- Develop – in this case removes non-exposed regions (negative resist) D- Do something: Etch, deposit, diffuse, then remove remaining resist. Repeat until device completed

Positive vs Negative Photoresists

Positive resist: exposed resist becomes soluble to developer. Unexposed resist is insoluble. Unexposed areas remain.

Negative resist: exposed resist hardens and becomes insoluble to developer. Unexposed resist is soluble. Exposed areas remain.

Property	Positive	Negative
Adhesion to Silicon	Fair	Excellent
Cost	More expensive	Less Expensive
Solubility in developer	Exposed region is insoluble	Exposed region is soluble
Min Feature	$0.5\mu m$	$<10nm$
Step Coverage	Better	Lower
Wet Chemical Resistance	Fair	Excellent

Spatial Resolution

$$\text{linewidth}\approx \sqrt{\lambda \ast \left(g+\frac{d}{2}\right)}$$

Where $\lambda$ is the wavelength, $g$ is the gap width, $d$ is the resist thickness
![[Pasted image 20260420172648.png]]

Linewidth defines how well we can define features

Resist Profiles

Binary & Phase Shift Masks

Phase shift mask uses interference effects to improve the contrast of the exposed and unexposed areas.

This can improve the resolution of the image on the wafer