1. SU-8 And Its Features
Anne Samuel

2. Contents Introduction - photoresist - positive vs negative photoresist
SU-8
- photosenstivity of SU-8
- process
- features
- application
References

3. Photoresists Photoresists consists of three components.
- Resin( binder that provides adhesion, chemical resistance etc- poly(cis-isoprene))
- Sensitizer(photoactive compound such as bis(aryl)azide,t-butyl benzoic acid)
- Solvent( which keeps the resist liquid)

4. Positive Vs Negative Photoresists
Positive Photoresists(PMMA)
- developer is not a solvent.
- resolution is 0.5(mm) and contrast is 2.2
- higher resistance to plasma etching
Negative Photoresists( rubber based resists)
-provides good adhesion and tougher films for wet etching.
- more sensitive,so faster reaction(exposure energy is 10 mJ/cm^2 vs 100 mJ/cm^2).
-contrast is ~ 1.5
- lower % of sensitizer, so these resists are cheaper.
- not sensitive to over-developing.

5. What is SU-8? EPON SU-8 - a negative epoxy based photoresist
- near UV sensitive
- consists of multifunctional highly branched polymeric epoxy resin
- dissolved in a organic solvent along with photoacid generator.
SU-8 was first formulated at IBM as early as 1982.

6. What is SU-8?
A single molecule contains 8 epoxy groups in a bisphenol- A novalac glycidyl ether.
8 in SU-8 means the 8 epoxy groups.
SU-8 polymer structure.
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7. Photosensitivity of SU-8
The photo acid generator with the epoxy resin in SU-8 is triarylsulfonium salt.
When SU-8 is exposed to light or photon, it is absorbed and photochemical reaction takes place.
The photochemical reaction produces an acid.
< http ee250a/lectures
/EE250A_Lecture_09_Thick-Film_Lithography_SU-8_files/frame.htm>

8. Photosensitivity (cont’d)
The acid formed in the photochemical reaction acts as a catalyst in the exposed regions during post baking exposure (PEB).
To promote a cross linking reaction.
During cross linking, a zipping process occurs between the epoxy groups creating a three dimensional network.
In order for the cross linking to takes place PEB must be above Tg
Cross-linking causes the film to shrink and become more dense.
< http ee250a/lectures
/EE250A_Lecture_09_Thick-Film_Lithography_SU-8_files/frame
.htm>

9. SU-8 Process A typical SU-8 process consists of - Spin-coating
- Soft Bake
- Exposure
- Post Exposure bake(PEB)-to cross link exposed regions
of the film
- Development
- Rinse & dry
- Hard Bake (or curing-optional)
- Imaged material
- Remove (optional)

10. Features of SU-8 Why should we use SU-8? Good adhesion
Near UV- Sensitive
Broad range of thickness can be obtainted from one spin with a conventional spin coater (750 nm to 500 mm)
High aspect ratios ( ~15 for lines and 10 for trenches).
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Thick-Film_Lithography_SU-8_files/frame.htm>

11. Applications of SU-8 Some of the applications include
- Electroplating molds
- Sensors
- Actuators
- Micro-to Milli scale structures
- Microfluidic channels

12. References
1. Judy, Jack. “Thick film Lithography & SU-8”
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2. IBM Research -Zurich Research Laboratary. “ Epon SU-8 Photoresist”.
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3. Micro.Chem.“Nano-SU-8: Negative Tone Photoresist Formulations” Feb.2002.
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4. Lorenz,H, Guerin,L et.al . “ SU-8 Technologies at EPFL”. June 2002.
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