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(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.

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 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>

Applications of SU-8• Some of the applications include

- Electroplating molds

- Sensors

- Actuators

- Micro-to Milli scale structures

- Microfluidic channels

References1. 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>