Hi all,

I am doing photolithography using SU-8 2100 photoresist.

The target thickness is around 200 μm or more.

There is no problem with spin coat.

Coating even at 1000 rpm ensures uniform coating.

But during soft bake,

there's a lot of bubble generation inside film.

(I'm not sure it's bubble, but it looks like bubble)

As a result, films that are uniform after coating become extremely uneven after baking.

To solve this problem, I tried various methods such as dehydration or HMDS coating but it was not effective.

Is there anyone who can solve this problem?

(This problem also occurs on thin SU-8 films(~ 50 um))

Here's my experiment conditions

1. Substrate: (100)-oriented Si wafer

2. Substrate dicing: ~ 5 x 5 cm

3. Substrate cleaning: Acetone(sonication, 10 min) -> IPA(sonication, 10 min)-> Piranha(immersion, 20 min)

4. BOE treatment: ~ 60 sec

5. HMDS coat: Spin coat(6000 rpm, 40 sec)

6. Bake: 120 oC, 10 min

7. SU-8 2100 dispense on Si wafer (Pour directly from the vial)

8. Spreading using N2 gun(Gently)

9. Spin coat(1000 rpm, 60 sec or 2000 rpm, 60 sec)

10. Self-leveling(Leave on a flat surface for about 10 minutes)

11. Soft bake(hotplate)

1) Room temp. -> 55 oC

2) 55 oC, 5 min

3) 55 oC -> 65 oC

4) 65 oC, 5 min

5) 65 oC -> 75 oC

6) 75 oC, 5 min

7) 75 oC -> 85 oC

8) 85 oC, 5 min

9) 85 oC -> 95 oC

10) 95 oC, x min

(* Bubbles always occur near 65 oC)

(** It's not "wrinkle". I know what it looks like)