Study of X-ray lithographic conditions for SU-8 by Fourier transform infrared spectroscopy

There is growing interest in the use of chemically-amplified resists (CARs) such as SU-8 in the field of microelectromechanical systems (MEMS) research. This is due to its outstanding lithographic performance and its ability for use in the fabrication of stable structures with very high aspect ratio. However, it is important to control the processing conditions for optimum results in the desired application. In this investigation, the thickness (10–25 μm) of SU-8 resist film, due to different spin coating speeds on silicon wafers, was measured using Fourier transform infrared (FT-IR) spectroscopy. The effect of thermal-initiated cross-linking at various temperatures (95–160 °C) for 15 min baking time on the 25 μm SU-8 resist was studied by monitoring the 914 cm−1 absorption peak in the FT-IR spectrum. Results of the experiments showed that the onset of thermal-initiated cross-linking begins at about 120 °C. Furthermore, 25 μm SU-8 resist was optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of post-exposure bake (PEB) temperatures. The exposure dose of soft X-ray (SXR) irradiation with energies about 1 keV from a dense plasma focus (DPF) device was fixed at 2500 mJ/cm2 on the resist surface. Results showed that the optimum processing conditions consisted of an intermediate PEB at 65 °C for 5 min, with the PEB temperature ramped up to 95 °C over 1.5 min and then followed by a final PEB at 95 °C for 5 min. The scanning electron microscopy (SEM) images showed SU-8 test structures successfully imprinted, without affecting the resolution, and with aspect ratios of up to 20:1 on 25 μm SU-8 resist.