Technique for preparing defect-free high aspect ratio SU-8 resist structure using x-ray lithography

摘要

This paper describes a process of deep x-ray lithography (DXRL) using epoxy negative photoresist SU-8. Resist coating, soft bake, exposure dose, post exposure bake (PEB), and development of the resist are characterized. The negative resist SU-8 has been increasingly used in micro- and nanotechnologies due to its excellent coating and processing properties as well as its mechanical and chemical stability. A fabrication of polymer based high aspect ratio (HAR) micro-channel structure is investigated by x-ray lithography system. It is always important to optimize the processing parameters for any resist with a given radiation. SU-8 resist have been focused on understanding the process parametric effects in UV lithography applications. However, its use as a resist in deep x-ray lithography is not well-understood. This paper reports the results of our experiments in optimization of the process parameters in deep x-ray lithography for SU-8 resist. In order to fabricate high aspect ratio micro-structures, we used the New Subaru synchrotron radiation facility at the University of Hyogo. The x-ray lithography system known as beam line 2 (BL2) has a potential for large area patterning across an area up to full-A4-size with a highly uniform pattern thickness [1]. At New Subaru facility, the x-ray exposure energy can be set to either 1.0 GeV or 1.5 GeV in the synchrotron radiation storage ring. For the operating energy at 1.0 GeV, it provides a spectral range from 2 to 12 keV, with a peak at about 3.5 keV, while in the case of 1.5 GeV operating energy, it has a spectral range from 2 to 30 keV, with a peak at about 5 keV. A low-energy beam line (λ=1nm) of 1.0 GeV with carbon x-ray mask is used to create arrayed micro-channels on glass substrate. One of the major limitations in the use of SU-8 in lithographic processes is the occurrence of internal stress and hence the distortion of the microstructure [2]. Internal stress in SU-8 is significantly higher in samples where s--mall channels are removed from large areas of SU-8 than the case where the large areas are removed to leave tall and narrow towers. There are many examples of high aspect-ratio microstructures fabricated using SU-8 where the final microstructure is a “tower” rather than a “channel” [3, 4]. Very few work has been reported in the fabrication of high aspect ratio “channels,” most likely due to the problems of high internal stress, poor adhesion and slow development.