ZEP is a Styrene Methyl Acrylate based positive ebeam resist and is used in ebeam mostly as a dry etch mask because it’s more dry etch resistant than PMMA. It has good resolution and is more sensitive than PMMA. Because it’s prime use is for dry etch it’s used in a standard single layer configuration because you want the sidewalls as vertical as possible.
Higher sensitivity means lower dose. Typical doses for ZEP are between 160 and 400µC/cm2.
Resolution should be below 30nm for isolated features.
For Si ZEP has very good dry etch selectivity with the Si-Opto process on the AMS. For large features you can expect ≈16:1 Si:ZEP. For SiO2 the selectivity is ≈4:1 with the SiO2 5:1 process. ZEP’s wet etch selectivity is similar to PMMA.
WARNING: Long aggressive dry etch processes cause ZEP to crack and burn. Once this happens it is impossible to remove ZEP even in oxygen plasma.
There are a few choices but we use n-amyl-acetate for 1 min for films less than around 300nm, for thicker films it’s best to develop for 2mins for make sure you have cleared the resist completely. We use 90:10 MiBK:IPA to rinse immediately after development. This MiBK:IPA solution is also a developer for ZEP so you should be careful when rinsing that you always use the same time for the rinse. Blow dry with Nitrogen.
Before processing choose 2 beakers from the ZEP shelf in the dry cabinet. Rinse one with a small amount of developer (Amyl-Acetate) and the 2nd with the ZEP rinse solution (90:10 MiBK:IPA).
|1||substrate dehydration||5min of O2 plasma at high power or alternatively 5mins at 180°C on the hotplate|
|3||ZEP coating||see curves below|
|4||ZEP baking||5min at 180°C on the hotplate|
|5||resists exposure||between 160 and 400µC/cm2|
|6||resists development||1-2min into developer Amyl-Acetate on the solvents wet bench|
|7||substrate rinsing and drying||1min 90:10 MiBK:IPA rinse solution and dry with nitrogen|
When you have finish empty beakers into the solvent waste in the solvent wet bench, rinse the beakers with IPA and put them into the ZEP recycle tray at the back of the solvents wet bench.