1. Lecture 4
Photolithography

2. Introduction The aim is to produce the features on the
mask with the highest possible resolution
using photoresist onto the wafer surface.

3. Photolithography Process Steps
Dehydration Bake
HMDS Priming and Cool Down Wafer
Photoresist Coating
Softbake
Align and Expose
Post Exposure Bake and Cool Down Wafer
Photoresist Development
Hardbake

4. Step 1: Dehydration bake
To remove moisture from the wafer surface
Moisture on wafer surface reduces resist adhesion
Wafer on an oxide surface will allow wet etchants to penetrate easily between the resist and the wafer surface
Parameters : Temperature

5. To promote adhesion of the photoresist to the wafer surface
Hexamethyldisilazane
Parameters:
Temperature
Time
Step 2: HMDS PRIME

6. To cool down a wafer to the ambient temperature after HMDS baking process and before resist coating process
Parameters :
Cool down rate
Temperature uniformity
Substrate temperature
Step 3: Cool plate

7. Step 4: PHOTORESIST COATING
To coat a resist onto the Si wafer for patterning with a required resist thickness and uniformity
Parameters :
Resist viscosity
Resist dispense method
Resist dispense velocity
Resist dispense volume
Resist dispense time
Nozzle position
Spin speed
Spread time
Acceleration / deceleration
Final spin speed
Final spin time
Chuck diameter
Chuck vacuum
Controlled exhaust
Ambient atmosphere (RH & Temperature)
Cup ambient (RH & Temperature)
Wafer centering
Machine and chuck leveling
Resist temperature control

8. Step 5: SOFTBAKE
To remove a solvent from photoresist and activates photoactive compound
To remove stress in the photoresist
To improve photoresist adhesion
Parameters
Type of heat transfer (Hot plate or convection)
Temperature
Temperature uniformity
Time
Exhaust
Cool down rate

9. Step 6: Align and exposure
Si wafer with a layer of photoresist on its top will be aligned to the mask and exposed to the UV light.

10. Step 7: POST EXPOSURE BAKE
To reduce the effect of standing waves which occur during exposure
To increase the resistance of the resist prior to the etching process
Parameters :
Type of plate (hot plate or convection oven)
Temperature
Temperature uniformity
Time
Exhaust
Cool down rate

11. Step 8: Cool plate
To cool down a wafer to the ambient temperature before development process
Parameters :
Cool down rate
Temperature uniformity
Substrate temperature

12. Step 9: Development process
To remove area of resists that have been exposed/unexposed to form a pattern
The development rate for positive resist much less than negative resist
Parameters :
Development technique (Immersion/Spray/Puddle)
Developer type (Metal Ion/metal ion free)
Developer concentration
Developer temperature
Development time
Carbon dioxide absorption
Agitation
Time between exposure and develop
Time between develop and rinse
Resist thickness
Post exposure bake temperature
Exposure energy

13. Step 10: Hardbake To increase the resistance of the resist
To remove any residual solvent
Parameters :
Type of plate (conduction or convection)
Temperature
Temperature uniformity
Time
Exhaust
Cool down rate

14. Photolithography equipment: Exposure tools

15. Photolithography equipment: Printing methods

18. MASK A glass plate with a thin film of chromium
Contain complete pattern for the whole wafer
Same feature size on the wafer
The pattern is transferred by using mask aligner

21. RETICLE A glass plate with a thin film of chromium
Contains multiple image fields (two or three) and each field contains numerous die patterned
Images on the reticles are protected by a pellicle

22. Reticle
Chrome Pattern
Quartz Substrate
Pellicle

23. Mask vs. Reticles Mask Reticles
Chrome glass has an image that covers the entire wafer
Chrome glass image covers only a part of the wafer
A mask normally transfers the image to the wafer surface in 1:1 ratio
A reticle has a larger image and feature size than image it projects on the wafer surface (usually with 4:1. 5:1 or 10:1 reduction ratios)
Exposure systems such as projection printers, proximity printers, and contact printers
Exposure systems need to expose several times to cover the whole wafer. The step and repeat processes need an exposure system called steppers.

24. Photoresist Components
Polymer
Phenol-formaldehyde
React when exposed to energy (Photosolublization)
Dark
Solvent
Vehide for polymers and sensitizers
Ethoxyethylacetate & methoxyethylacetate
-Determine the thickness of resist
Sensitizers
Control / modify chemical reaction

25. Photoresist Performance Factors
Resolution
Smallest opening can be resolved in photoresistlayer
Positive resist > negative resist
Adhesion
Ability to adhere to the variety of surface
Negative resist > positive resist
Exposure Speed
The speed with which resist react to exposure
Pinhole count
Pinhole increase –layer thickness decrease
Negative resist has fewer pinholes than negative photoresist

26. Negative PR vs. Positive PR

27. Storage and Handling of Photoresist
Photoresist should be contained in the dark bottle
Photoresist should be operated in the yellow or gold lighting area
Photoresist must be stored under constant temperature condition
Photoresist bottles should be tightly closed
Photoresist should be filtered before use