We are having the opposite problem from many: Where our SU-8 is exposed it develops nicely (i.e., SU-8 remains), but where it is *not* exposed does not fully develop , leaving either a nearly full-thickness SU-8 layer or sometimes more of a thin haze. Our process is:
• Substrate: GaAs 100 wafer
• Clean in acetone/methanol, N2 dry
• Other surface prep: 30% HCl/H20 dip 5 s and or O2 plasma ash. (These don’t seem to make much difference whether used or not.)
• Spin coat 5-10s at 500 rpm, acceleration 100 rpm/s, 30s at 3000 rpm, acceleration 300 rpm/s.
• Soft bake 1 minute at 95°
• Expose 1000 mJ/cm2 using MicroTech maskless lithography system @ 375 nm (single-wavelength laser). This exposure is much higher than the data sheet recommends, but for lower exposures the SU-8 does not properly crosslink. It is conceivable that our 375 nm laser is not properly calibrated in terms of intensity. But maybe this is a clue of something else we're doing wrong.
• Post-exposure bake 2 min @ 95°
• Develop in SU-8 developer for 60 s (longer doesn’t help)
After development the wafer looks like that shown in the attached file. There are 4 SU-8 structures (one colored blue for reference) that appear to have developed nicely. But outside of these structures, where there was no exposure, there remains a "haze" of SU-8 that won't develop. Even stranger, whether this haze remains or not depends on the treatment of the underlying substrate. In certain areas we have etched down into the substrate by 100 nm. There is no haze in these regions. The haze remains where we have not etched.
What could cause this behavior? I would have thought that it would always be easy to develop away SU-8 that has *not* been exposed. Or does development of unexposed areas depend crucially on various bake times, etc? Also, it seems very odd that whether the SU-8 develops or not depends on the nature of the underlying substrate. Since development presumably proceeds from the top surface of the SU-8 down, how would the developer "know" what is at the very bottom of the SU-8?
Any hints would be greatly appreciated.
There are several potential reasons why an SU8 photoresist (such as SU8 2000.5) may not properly develop in unexposed areas. Some possible causes include:
To troubleshoot this issue, it may be necessary to carefully review the photolithography process and ensure that all steps are being carried out correctly. It may also be helpful to check the age and storage conditions of the SU8 photoresist, as well as the compatibility of the substrate with the photoresist.
hello, I hope you must figure out what is the reason for SU-8 haze on the substrate. Though while i doing development, I find that instead of only dipping substrate in the developer if we shake it very slowly the SU-8 removed more betterly.
Though few of my collegues also facing same issue with their small 30um size square shapes. But photoresist purity, handling are major reason for SU-8 haze.
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