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Description

Polymethyl methacrylate (PMMA) was one of the first materials developed for e-beam lithography. It is the standard positive e-beam resist and remains one of the highest resolution resists available. PMMA is usually purchased in two high molecular weight forms (496k or 950k Series) in a casting solvent such as chlorobenzene or anisole. PMMA is spun onto the substrate and baked at 170C to 200C for 1 to 2 hours. Electron beam exposure breaks the polymer into fragments that are dissolved preferentially by a developer such as MIBK. MIBK alone is too strong a developer and removes some of the unexposed resist. Therefore, the developer is usually diluted by mixing in a weaker developer such as 2-Propanol (cmos) - IPA. A mixture of 1 part MIBK to 3 parts IPA produces very high contrast but low sensitivity. By making the developer stronger, say, 1:1 MIBK:IPA, the sensitivity is improved significantly with only a small loss of contrast.

The sensitivity of PMMA also scales roughly with electron acceleration voltage, with the critical dose at 50 kV being roughly twice that of exposures at 25 kV. Fortunately, electron guns are proportionally brighter at higher energies, providing twice the current in the same spot size at 50 kV. When using 50 kV electrons and 1:3 MIBK:IPA developer, the critical dose is around 350 uC/cm2. Most positive resists will show a bias of 20 to 150 nm (i.e. a hole in the resist will be larger than the electron beam size), depending on the resist type, thickness, and contrast and development conditions and beam voltage.

When exposed to more than 10 times the optimal positive dose, PMMA will crosslink, forming a negative resist. It is simple to see this effect after having exposed one spot for an extended time (for instance, when focusing on a mark). The center of the spot will be crosslinked, leaving resist on the substrate, while the surrounding area is exposed positively and is washed away. In its positive mode, PMMA has an intrinsic resolution of less than 10 nm. In negative mode, the resolution is at least 50 nm. By exposing PMMA (or any resist) on a thin membrane, the exposure due to secondary electrons can be greatly reduced and the process latitude thereby increased. PMMA has poor resistance to plasma etching, compared to novolac-based photoresists. Nevertheless, it has been used successfully as a mask for the etching of silicon nitride and silicon dioxide, with 1:1 etch selectivity. PMMA also makes a very effective mask for chemically assisted ion beam etching of GaAs and AlGaAs.

Processes


EXAMPLE PROCESS: PMMA POSITIVE EXPOSURE AND LIFTOFF (see E-beam Resist (PMMA) Development for approved process)

Start with 496K PMMA, 4% solids in chlorobenzene. Pour resist onto a Si wafer and spin at 2500 rpm for 40 to 60 seconds.

Bake in an oven or on a hotplate at 180 C for 1 h. Thickness after baking: 300 nm.

Expose in e-beam system at 50 kV, with doses between 300 and 500 uC/cm2. (Other accelerating voltages may be used. The dose scales roughly with the voltage.)

Develop for 1 min in 1:3 MIBK:IPA. Rinse in IPA. Blow dry with nitrogen.

Optional descum in a barrel etcher: 150W, 0.6 Torr O2.

Mount in evaporator and pump down to 210-6 Torr.

Evaporate 10 nm Cr, then 100 nm Au.

Remove from evaporator, soak sample in methelyne chloride for ~10 min. Agitate substrate and methylene chloride with an ultrasonic cleaner for ~1 min to complete the liftoff. Rinse in IPA. Blow dry.

References

See attachments for info on PMMA resists
  Name Size
- PMMA_Data_Sheet.pdf 1.14 MB
© 2011 Lurie Nanofabrication Facility, University of Michigan
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