Photolithography on Grossly Non-Planar Substrates

Within the electronics industry, photolithography is the primary technique by which patterns are transferred from mask to substrate. The substrates are either semiconductor wafers or printed circuit boards, both of which are nominally flat. The growth of micro-electro-mechanical and micro-electro-opto-mechanical systems (MEMS, MOEMS) and the search for higher-density electronics packaging solutions is leading to the requirement to pattern fine features onto grossly non-planar substrates. Standard photolithographic techniques cannot be used with these surfaces because the unavoidably large gap between mask and substrate allows diffractive line broadening, with a consequent loss of resolution. In this paper, we outline a method for realising photolithography on grossly non-planar substrates by using computer-generated holographic (CGH) masks. The technique that we describe enables photolithography to be realised in three dimensions, leading to the potential for a wide range of novel microelectronics packaging schemes to be realised