1. Photolithography
PEOPLE Program

2. How is Lithography Related to Computers?
Computer chips are made using photolithography
Instead of drawing with a sharp tip, it uses light to transfer a tiny pattern from a photomask to a light-sensitive chemical
Packaging
Central Processing Unit (CPU)

3. How is Lithography Related to Computers?
ENIAC (Electronic Numerical Integrator and Computer)
First general-purpose electronic computer
Apple II
An early personal computer
Apple MacBook Air
Modern laptop computer weighing only 3 pounds!
65 years… and many advances in photolithography!

4. Photolithography
Role of the Resist: Light exposure changes solubility and allows mask formation
Applications: Making tiny, detailed stuff! For example, microelectronics, microfluidics, MEMS, bioanalytics, sensing, and many more
Main Benefit: High throughput!
Geissler, et al. Adv. Mater. 2004, 16,

5. Exposure of Photosensitive Material to Light
What do you think “photosensitive” means?

6. Exposure of Photosensitive Material to Light
What do you think “photosensitive” means?
Photosensitive materials react when exposed to light.
Examples:
Suntan
Faded Paint

7. Transferring the Pattern: Make a Mask!
Substrate that will “receive” the pattern
Mask that controls the passage of light
Silicon wafer
Transparency with desired pattern

8. ? Transferring the Pattern to the Silicon Wafer Mask Silicon wafer
1. Coat silicon wafer with photoresist
Photoresist
Silicon Wafer
Mask
2. Tightly place mask on top
3. Expose to UV light
Silicon wafer
4. Develop and Rinse ?
Depends on the type of photoresist

9. What it mean if something can dissolve
What it mean if something can dissolve? What does it mean if something can’t dissolve?

10. What could be some examples of things that can dissolve in H2O
What could be some examples of things that can dissolve in H2O? What could be some examples of things that cannot dissolve in H2O?

11. Our definition
Dissolve: A solid loses its structure and becomes dispersed into a liquid.*
Examples:
Table salt (NaCl) dissolves in water.
Sand (SiO2) does not dissolve in water.
Gasoline does not dissolve in water.
A breath-strip dissolves in your saliva.
*Dissolving can take other forms, as well. For example a gas can dissolve into another gas, or a gas can dissolve into a liquid.

12. Positive vs. Negative Photoresist
Positive Photoresist: Exposed areas become able to dissolve in the developer (so the exposed portion is dissolved by the developer)
Negative Photoresist: Exposed areas become not able to be dissolved in the developer (so the unexposed portion is dissolved by the developer)
We will use S-1813, a common positive photoresist

13. Positive vs. Negative Photoresist
3. Expose to UV light
POSITIVE
Photoresist
NEGATIVE
Photoresist
4. Develop and rinse
4. Develop and rinse
5. Etch patterns into wafer
5. Etch patterns into wafer
6. Remove residual photoresist
6. Remove residual photoresist

14. Positive vs. Negative Photoresist
3. Expose to UV light
POSITIVE
Photoresist
NEGATIVE
Photoresist
4. Develop and rinse
4. Develop and rinse
5. Etch patterns into wafer
5. Etch patterns into wafer
Positive vs Negative Painting
6. Remove residual photoresist
6. Remove residual photoresist

15. Make sure you get it! Fill in the blanks:
In a _____ photoresist, _____ light reacts with the photoresist to make it unable to be dissolved, so it is rinsed away by the _____.
3. Expose to UV light
POSITIVE
Photoresist
NEGATIVE
Photoresist
4. Develop and rinse
4. Develop and rinse
5. Etch patterns into wafer
5. Etch patterns into wafer
6. Remove residual photoresist
6. Remove residual photoresist

16. Review! What is cross-linking?
Do you think that a substance that has a lot of cross-links would be more easily or less easily dissolved? Why do you think so?

17. Today’s Activity: Photolithography
Bucky masks have been created for you
Glass slides have been primed and coated S1813 photoresist
You will be performing photolithography using these masks
Your Mask:
PEOPLE Program 2016
Your Name

18. Today’s Activity: Photolithography
7) Inspect
4) Post-exposure
bake
5) Develop
6) Hard bake
UV Light
Mask 
3) Alignment
and Exposure
Resist
1) Spin coat
2) Soft bake