1. Lithography

3. Lithography Lithography
Lithography comes from two Greek words : “lithos " means stone, and "graphein" means write.
"writing a pattern in stone"
Lithography is the process of imprinting a geometric pattern from a mask onto a thin layer of material called a resist, which is sensitive to photons, electrons or ions.
Pattern transfer

4. Lithography
Optical Lithography System

5. Lithography Types of Photolithography Processes
Photoresists are photosensitive polymeric materials and are of two types: positive and negative.
Negative: Prints a pattern that is opposite of the pattern that is on the mask.
Positive: Prints a pattern that is the same as the pattern on the mask.
Negative Photoresists
Positive Photoresists

6. Lithography Negative Photoresists
Areas exposed to light become polymerized and resist the develop chemical.
Prints a pattern that is opposite of the pattern that is on the mask.
Resulting pattern after the resist is developed.

7. Lithography Positive Photoresists
Areas exposed to light become photosoluble.
Prints a pattern that is the same as the pattern on the mask.
Resulting pattern after the resist is developed.

8. Lithography Basic Steps of Photolithography
Spin coat a thin layer of photoresist on surface.
Soft bake :
- Partial evaporation of photo-resist solvents
- Improves adhesion, uniformity, etch resistance, linewidth
control
Soft Bake

9. Lithography
Precise alignment
of mask to wafer.

10. Lithography Exposure of photoresist. Negative resist is polymerized.
Development :
Removal of
unpolymerized
resist .
Hard bake

11. Lithography Etch : Oxide layer is removed through
opening in resist layer.
Photoresist strip : remove photoresist layer from
wafer.
Final inspection

12. Lithography Radiation Sources
The pursuit of submicron feature sizes has motivated extensive work in four major lithography technologies :
optical lithography
e-beam lithography
x-ray lithography
ion-beam lithography

13. Lithography Optical Lithography (Photolithography) UV - Mid-1950s
- Typically 300~450 nm
- Mercury spectrum g(436nm), h(405nm), i(365nm)
- 0.5 to 3 um film thickness(thin films)
- Resolution : 0.5 um

14. Lithography Deep UV - Mid-1980s - Typically 200-300 nm
- 0.5 to 3 um film thickness
- Resolution : 0.25 um
- Necessary to use masks made of quartz.
Two types of UV exposure sources
- Mercury arc lamp
- Excimer laser
13. 5 nm

15. Lithography Photomask Substrate
High transmission at exposure wavelength
Small thermal expansion coefficient
High degree of flatness
Low non-linear effect
Common material : quartz, fused-silica or borosilicate glasses
Opaque Material
No transmission at exposure wavelength
Good adhesion to the substrate
High degree of durability
Common material : Chrome, emulsion and ion oxide
Chrome-on quartz masks
- Formed by fused silica plates
- Plates are coated with a thin chrome layer(~800A).
Emulsion masks

16. Lithography Wafer Exposure System
Three optical lithographic techniques :
- Contact printing
- Proximity printing
- Projection printing
1:1 exposure systems

17. Lithography
Typical contact or proximity printing system

18. Lithography
Projection Printing

19. Calculating Resolution for a given , NA and k
Lithography
Performance of Lithographic Exposure Tool
Resolution
- The minimum feature size that can be transferred with high fidelity to
a resist film on the surface of the wafer.
Registration
Calculating Resolution for a given , NA and k
- A measure of how accurately pattern of successive masks can be aligned with respect to the previously define pattern on a wafer.
Throughput
- The number of wafers that can be exposed per hour for a given
mask level.