Deep x-ray lithography with the SU-8 resist

Although the process of deep x-ray lithography with PMMA achieves good resolution, it requires significant exposure times because of the low sensitivity of PMMA to x-rays. Therefore resist materials, which can achieve high resolution, but which are inherently more sensitive than PMMA, are desirable. Here it is shown, that x-ray exposures of the SU-8 resist can achieve high resolution with substantially reduced exposure times. Irradiation at the synchrotron source of DCI at Lure (Paris) and MAXLAB (Lund, Sweden) demonstrated a reduced exposure time for a 600 mum thick SU-8 relative to PMMA. The dose needed to obtain standing structures was 30 J/cm(3) for DCI and 52 J/cm(3) for MAXLAB. A 600 mum thick PMMA resist requires a typical bottom dose of 4 kJ/cm(3), So SU-8 is considerably more sensistive to x-rays than PMMA. Preliminary critical dimension measurements (CD) of the 600 mum SU-8 resist structure have been obtained for the entire height of the structure, which was exposed at DCI. The CD measurements were made in a Scanning Electron Microscope (SEM) using 10 mum wide structures, which have a 20 mum pitch, this being used to calibrate the measurements. These measurements show that the gain in the critical dimension per structure edge is dependent on the bottom dose. Doses of 30 J/cm(3) achieved a CD gain per edge of +0.5 mum, while doses of 40 J/cm(3) yielded a CD gain per edge of 0.9 mum. However, the gain in the CD per edge is critically dependent on the solvent content in the resist. Doses of 40 J/cm(3) into a resist with a 2 % residual solvent content yielded CD gains per edge of 0.3 mum. In addition, the dose profile in the resist does not change the CD values significantly. It has been shown that the resolution of x-ray exposed SU-8 structures compare quite favourable with PMMA, but the exposure time for SU-8 is approximately 100 times less than that for PMMA. This significantly improves throughput for deep x-ray lithography processes.