High aspect ratio patterning with a proximity ultraviolet source

A considerably less expensive option to synchrotron exposures of thick film photoresists is to use a proximity UV exposure tool. The use of UV radiation, however, is potentially limited by aerial image degradation as the image propagates through the thick photoresist layer. In addition to diffraction, run-out from uncollimated light or absorption in the resist easily dominate the aerial image problems for thick films. Alternatively, thick film lithography may be limited by the ability of present photoresists to print the aerial image. With some formulations, calculated line-and-space theoretical limits can be printed for 150-mum thick films. Typical aspect ratios for films greater than 200-mum thick exceed 20:1 with good process linearity and sidewall profiles and extension to 700-mum thick films is shown. We show that present commercial formulations of photoresists and not aerial image are likely the limiting factor in the practical resolution of final features from proximity UV printing. In particular, the redeposition of partially dissolved resist during drying after development leads to feature degradation. Released electrodeposited metal parts are also produced and demonstrated. (C) 2002 Elsevier Science B.V. All rights reserved.