SU-8 And Its Features Anne Samuel

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

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)

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.

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.

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. <http://www.zurich.ibm.com/st/mems/su8.html>

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

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

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)

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). <http://www.ee.ucla.edu/~jjudy/classes/ee250a/lectures/EE250A_Lecture_09_ Thick-Film_Lithography_SU-8_files/frame.htm>

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

References 1. Judy, Jack. “Thick film Lithography & SU-8”. 2003. <http://www.ee.ucla.edu/~jjudy/classes/ee250a/lectures/EE250A_Lecture_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>