Anne Samuel

• Introduction
- photoresist
- positive vs negative photoresist
• SU-8
- photosenstivity of SU-8
- process
- features
- application
• References

• 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)

• Positive Photoresists(PMMA)
- developer is not a solvent.
- resolution is 0.5( m m) 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.

• 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.

• 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.
<http://www.zurich.ibm.com/st/mems/su8.ht
ml>

• 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
<http://www.ee.ucla.edu/~jjudy/classes/ http ee250a/lectures
/EE250A_Lecture_09_Thick-Film_Lithography_SU-
8_files/frame.htm> acid.

•
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://www.ee.ucla.edu/~jjudy/classes/ http ee250a/lectures
/EE250A_Lecture_09_Thick-Film_Lithography_SU-8_files/frame
.htm>

• 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)

• 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 m m)
• High aspect ratios ( ~15 for lines and 10 for trenches).
<http://www.ee.ucla.edu/~jjudy/classes/ee250a/lectures/EE250A_Lecture_09_
Thick-Film_Lithography_SU-8_files/frame.htm>

• Some of the applications include
- Electroplating molds
- Sensors
- Actuators
- Micro-to Milli scale structures
- Microfluidic channels

1.
Judy, Jack. “Thick film Lithography & SU-8”. 2003.
<http://www.ee.ucla.edu/~jjudy/classes/ee250a/lectures/EE250A_Lect ure_09_Thick-Film_Lithography_SU-8_files/frame.htm
2. IBM Research -Zurich Research Laboratary. “ Epon SU-8
Photoresist”.
<http://www.zurich.ibm.com/st/mems/su8.html>
3. Micro.Chem.“Nano-SU-8: Negative Tone Photoresist Formulations”
Feb.2002.
<http://www.ee.byu.edu/cleanroom/SU8_2-25.pdf>
4. Lorenz,H, Guerin,L et.al . “ SU-8 Technologies at EPFL”. June 2002.
<http://dmtwww.epfl.ch/ims/micsys/projects/prtop.html>