Oxygen quenching effect in ultra-deep X-ray lithography with SU-8 resist

An ultra-thick (1.5 mm) SU-8 resist was prepared and patterned using synchrotron x-rays. The lithographic sensitivity of the SU-8 resist is over 100 times higher than that of the traditional PMMA resist. However, preliminary results showed that the standard resist process produced poor lithographic quality. The fluorescence induced by high-energy x-ray irradiation was believed to blur the transfer pattern. The lithographic quality improved significantly if the resists were relaxed in an oxygen atmosphere before being soft baked. The FTIR spectrum displayed new absorption peaks, corresponding to the C=O bond, in the oxygen-relaxed resists. The development rates of the low-dosed resists increased if they were held in the oxygen atmosphere, implying that trapped oxygen restrains the cross linking of the SU-8 resist. Based on these observations, this study proposes an oxygen quenching mechanism for interpreting these phenomena. When the SU-8 is oxygen relaxed, the oxygen molecules will gradually diffuse into the resist. Oxygen is a strong free-radical scavenger, and thus kills the free radicals generated by fluorescence, providing a clear pattern defined by x-ray irradiation. After the SU-8 resist was properly oxygen treated, the maximum dimension error was only about 2 mum for a 1.5 mm-thick SU-8 resist. This lithographic quality is good enough to meet the micromachining requirements for mm-wave RF (radio frequency) cavity applications.