Effect of surface treatments/coatings and soft bake profile on surface uniformity and adhesion of SU-8 on a glass substrate

We present the effect of surface treatments/coatings and soft bake temperatures aimed at improving adhesion and surface uniformity of SU-8 on glass substrates. While the adhesion strength of SU-8 to metal layers on glass and silicon has been previously investigated, our research examines the influence of additional surface treatments (RCA, Acetone/IPA rinse) and coatings (fresh/one-day-aged Ti, fresh/one-day-aged Cr, SU-8 2005®) on adhesion strength as well as surface uniformity for 100 μm thick SU-8 films. Additionally, we vary the soft bake times and temperatures while keeping all other process parameters constant, to correlate adhesion strength with surface uniformity of SU-8 films for each surface modification. We have found that for all surface treatments/coatings, a soft bake temperature of 65°C for 90 minutes yielded a more uniform SU-8 film (σ = 5.18 μm) as compared to the manufacturer-recommended soft bake temperature of 95°C (σ = 12.66 μm) for 30 minutes. Consequently, a more uniform SU-8 film provided excellent adhesion strength (> 2 MPa, as determined by stress testing using an Instron® microtester) for both metallic seed layers while the adhesion strength of films baked at 95°C was determined to be < 0.5 MPa. This study, for the first time, has been able to quantitatively determine the adhesion strength of SU-8 films on different seed layers deposited on glass substrates, for varying soft bake temperatures.