SU-8 3000 Film Stress Reduction Attempts

Summary:  Annealing to above glass transition T (200 C) doesn’t reduce film stress.  Adjusting post-exposure bake protocol seems to reduce it.

A short post to document my efforts at reducing tensile stress in SU-8-3000 coatings by annealing in vacuum to its glass transition temperature (200 °C according to MicroChem).  This is in relation to the Nanoporous nitride lift-off work that Josh Miller and I are doing.  We are finding that the NPN membrane is wrinkled after lift-off, presumably caused by a mismatch in the film stress of the SU-8 scaffold and the NPN membrane.  Chris has suggested possibly reducing the stress through annealing to above the glass transition T.

I coated a new wafer with 10 µm of SU-8-3010 and patterned it with our hex scaffold (10 µm wide struts at a 100 µm, flat-to-flat spacing).  I used our “standard” procedures which are: 95 °C drying bake for 1  in prior to coating, spin-coat at 500 RPM for 10 s (5 s ramp), followed by 3000 RPM for 45 s (5 s ramp); soft bake for 2.5 min at 95 C; expose to 200 mJ/cm2 UV on KS MA-150 contact aligner; PE bake at 65 °C for 1 min followed by 95°C for 2.5 min; develop for 3 min in SU-8 developer, IPA rinse, then DIW rinse and air dry; Hard bake at 150 °C for 2.5 min.  The stress measurements were done by measuring the wafer’s profile from the rear face using the Tencor P2.  The timeline and stress measurement results follow:

• Date           Notes                                                                       Stress (MPa, Tensile)
• 4/8/15:  Wafer coated/patterned with SU-8-3010            2.2
• 5/1/15:   Recheck stress                                                           2.0
•                 Anneal in vacuum for 1 hr at 200 °C                    1.9
• 5/15/15: Anneal in vacuum for 3 hr at 230 °C                    4.3

So, it appears that the anneal treatment is increasing the stress level, which is perhaps not to surprising for a cross-linked epoxy.   Next, I tried using longer softbake and PE bake time, particularly at lower temperatures.  The new protocol is as follows:

• precoat dry bake: 150 C for 1 min
• same spin coat recipe
• softbake at 65 °C for 5 min followed by 95 °C for 30 min.
• same UV exposure, but allow wafer to sit at room T for 10 min. prior to PE bakes
• PE bake: 5 min at 65 °C, heat at nominally 2 °C/min to 95 °C, and hold for additional 20 min.
• Same develop process

The stress for this coating is 1.2 MPa, which is significantly lower that what I obtain with the standard process.