Study of a Chemically Amplified Resist for X-Ray Lithography by Fourier Transform Infrared Spectroscopy

• Published in: Applied spectroscopy 2004-11, Vol.58 (11), p.1288-1294
• Main Authors: Tan, T. L., Wong, D., Lee, P., Rawat, R. S., Patran, A.
• Format: Article
• Language: English
• ISSN: 0003-7028; 1943-3530
• DOI: 10.1366/0003702042475402

Future applications of microelectromechanical systems (MEMS) require lithographic performance of very high aspect ratio. Chemically amplified resists (CARs) such as the negative tone commercial SU-8 provide critical advantages in sensitivity, resolution, and process efficiency in deep ultraviolet, electron-beam, and X-ray lithographies (XRLs), which result in a very high aspect ratio. In this investigation, an SU-8 resist was characterized and optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of resist thickness and X-ray exposure dose. The exposure dose of soft X-ray (SXR) irradiation at the average weighted wavelength of 1.20 nm from a plasma focus device ranges from 100 to 1600 mJ/cm2 on the resist surface. Resist thickness varies from 3.5 to 15 μm. The cross-linking process of the resist during post-exposure bake (PEB) was accurately monitored using Fourier transform infrared (FT-IR) spectroscopy. The infrared absorption peaks at 862, 914, 972, and 1128 cm−1 in the spectrum of the SU-8 resist were found to be useful indicators for the completion of cross-linking in the resist. Results of the experiments showed that the cross-linking of SU-8 was optimized at the exposure dose of 800 mJ/cm2 for resist thicknesses of 3.5, 9.5, and 15 μm. PEB temperature was set at 95 °C and time at 3 min. The resist thickness was measured using interference patterns in the FT-IR spectra of the resist. Test structures with an aspect ratio 3:1 on 10 μm thick SU-8 resist film were obtained using scanning electron microscopy (SEM).