Specific Process Knowledge/Lithography/4562
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AZ 4562 is a positive UV photoresist for thick coatings (above 5µm).
Priming
Using HMDS priming with AZ 4562 seems to cause more problems than it solves, and is generally not recommended. HMDS priming seems to make the adhesion worse, maybe due to accumulation of N2 at the resist-substrate interface during exposure. A combination of oxygen plasma treatment (7min @ 400ml/min O2 and 500W power) and HMDS has been successful for a 14µm coating, but has not been tested further (Taran Dec 2019).
If adhesion failure is an issue, a dehydration bake (e.g. 1min at 100°C) just before spin coating is recommended. Storing the coated substrates before exposure also leads to adhesion failure after as little as a couple of days.
Spin coating
The thickest achievable coating using a normal spin cycle is probably 10-15µm. However, reducing the spin-off time to a few seconds at 2000rpm has successfully been used to increase the coating thickness beyond 20µm on a Gamma coater. The substrate is rested for 1min before softbake in order to reduce the edge bead height.
Exposure
Before exposure, the resist must be rehydrated. A 1-2µm thick resist rehydrates in less than 1 minute, a 6µm layer requires 15-20min, and a 10µm layer requires an hour, maybe more.
After exposure, the nitrogen generated must be allowed to diffuse out, in order to avoid bubble formation (in particular before any heating). This process is similar to rehydration, but usually slower.
In order to avoid heating during exposure, leading to the formation of bubbles, it is recommended to use multiple exposure, limiting the exposure time for each cycle to 10-15s, and using a 10-15s pause between cycles.
Development
The recommended development speed for AZ 4562 is 2µm/min. A 6.2µm resist film thus requires 3min development, usually as three 60s puddles of TMAH (AZ 726 MIF). 10µm AZ 4562 has successfully been developed using 4x60s development