高感度SU-8光阻之超深X光光刻技術研究

Abstract

本研究主要探討以SU-8作為高感度X光深刻光阻的可行性，並建立發展一套高效率、高精度的X光深刻技術。該技術除了可應用於各式微機械或微流體系統外，未來也將實際應用於高精度(<2μm)與高深度(~1.5mm)要求的毫米波源精密共振腔體之製作。 SU-8厚膜光阻藉由化學放大(chemical amplify)的光化學反應，而有相當高的曝光敏感度。研究結果顯示，SU-8對於X光的曝光敏感度為傳統PMMA光阻的160倍，而且其光刻側璧表面品質(Ra~12nm)與PMMA一樣優異。樹脂系SU-8的附著性、化學穩定性及抗應力腐蝕性均遠勝於傳統的PMMA光阻，可有效提升厚膜製程能力及良率。此外，由於SU-8光阻呈現高感度及高感光對比的特性，因此X光光罩可使用較厚的鼓膜與較薄的吸收體，其結果將可有效降低傳統X光光罩製程的困難度及複雜性，大幅提升X光微加工技術的工業應用潛力。 研究結果亦發現，以X光深刻SU-8光阻時其頂部劑量必須小於200 J/cm3，方能獲致優異的光刻品質，因此曝光時必須施加適當的濾片以調變頂部劑量。此外，當高能量的X光穿透光阻時往往會引發螢光的產生進而影響光刻的精度。此時可藉由增加光阻內的氧含量，應用氧淬火(oxygen quench)效應抑制螢光所引發的光阻聚合反應，進而提昇X光深刻的精準度。 實驗中同時基於前述各項技術基礎，實際深刻1.5mm厚的SU-8光阻，並量測該製程技術的光刻品質。量測結果顯示，X光深刻SU-8光阻技術有極佳的圖形轉移精準度(精度~1μm、準度＜3μm)，其光刻微結構側壁亦相當筆直，光刻側壁的傾斜角度大約只有0.06°。該技術的微加工品質可完全滿足毫米波源共振腔體對於結構深度及精度的嚴苛要求。

This study investigates the feasibility of using SU-8 as a highly sensitive X-ray resist. An ultra-deep X-ray lithography (UDXL) technique will be developed to give a high- efficiency and high-precision micromachining process. This technique will be used to fabricate a deep (~1.5mm) and precise (＜2μm) resonant cavity of an mm-wave power supply in the future. Via chemical amplification mechanism, the SU-8 resist revealed very high sensitivity under X-ray irradiating. The result showed that its sensitivity is about 160 times higher than that of the traditional PMMA resist; meanwhile, the lithographic surface quality (Ra~12nm) is as good as that of PMMA. The epoxy-based SU-8 resist also has higher adhesion strength, chemical stability and stress-corrosion resistance than that of PMMA. In addition, due to the high contrast and sensitivity behaviors of the SU-8 resist, the corresponding mask absorber can be thinner and the mask membrane can be thicker. As a result, the fabrication process of the X-ray mask will be much simpler and easier than ever before. The results showed that the maximum dosage of the SU-8 resist must be lower than 200 J/cm3 to achieve an excellent lithographic quality. This can be done applying a proper filter to control the dose distribution in the resist. The results also indicated that high-energy X-ray will induce fluorescence, which will activate unwanted cross-linking reaction in the resist and then give very poor pattern definition. This problem is successively resolved in this study by increasing the oxygen concentration in the resist to “quench” the cross linking reaction under the masked area. Based on the established capability in this study, a 1.5mm thick SU-8 resist was exposed by synchrotron-radiating X-ray. The results showed that the DXL SU-8 technique could provide excellent pattern-transfer quality with high precision (~1μm) and high accuracy (＜3μm). The sidewall is quite perpendicular to the substrate with an inclined angle of only about 0.06°. The micromachining quality of the DXL SU-8 technique can fulfill the severe demands both in depth and precision of the mm-wave microcavity.