When electron beam lithography must be performed on insulating substrates (e.g. Quartz, SiC, soda lime, etc.) negative charge buildup can occur on the substrate surface causing beam deflection, and thus pattern distortion.
Anti-charging methods (pdf) (ppt)
A thin film of metal (e.g. 200 Ang of Au, or Al) can be applied to the top of the resist, but it must be done with thermal evaporation. E-beam evaporation cannot be used as it will expose the resist. Au can be removed with Transene Au etchant, or Al with Al etchant. The benefit of using metal, is that it can provide for a reflective layer for height measurement in the EBL system as the substrates that are insulating tend to be transparent (e.g. SiO2, SiC, etc.)
ESpacer is a commercially avaialable conductive polymer which can be simply applied by spin coating, and simply removed by rinsing with water. It's disadvantage is high cost.
ESpacer 300Z: for use with ZEP520A, PMMA, maN-2403, and HSQ*
ESpacer 300HX02: for use with HSQ
ESpacer 300AX01: for use with chemically amplified resists
How to apply ESpacer:
1. get a disposable pipette. use compressed N2 gun on the tip of the pipette to clean it off.
2. ESpacer bottles are stored in refrigerator. Pipette out a small quantity (1mL) as it coats very easily and is very expensive ($3000 for 100mL). Make sure you have cleaned the tip of the pipette with a compressed N2 gun (step 1) to prevent contaminating the bottle with particles.
3. put bottle back in refrigerator.
4. Espacer will come to room temperature after a few minutes in pipette.
5. obtain sample that is resist coated. the resist on the sample must already be baked.
6. spin coat ESpacer at 2000 RPM, 2000 RPM/s, for 30 sec. no baking is necessary. this will create a uniform film of ESpacer on sample which is ~200 Angsroms in thickness. Note that those spin conditions have worked well for silicon and glass substrates at sizes 1x1 cm and larger. It has been found that for 3 x 5 mm SiC samples, higher spin speed and longer time is needed. 4000 RPM, 2000 RPM/sec, 60 sec.
7. expose sample with EBL.
8. remove ESpacer with DI water by immersion for 1 min, then rinse off with low DI flow from faucet.
9. N2 blow dry sample.
10. perform resist develop procedure as appropriate for resist being used.
*ESpacer 300Z does not coat HSQ very well. It will coat in a non-uniform manner. However, we have gotten ESpacer 300Z to coat HSQ by using a double step spin coat process of 1) 500 RPM, 500 RPM/sec, 10 sec and 2) 2000 RPM, 1000 RPM/sec, 30sec. Also, it helps to "paint" the ESPacer on with the pipette by holding the pipette close to the substrate and maintaining a bubble of ESPacer between the pipette and substrate.