Photolithography on Three Dimensional Substrates

Abstract

Photolithography is the primary technique for patterning planar substrates. However, some higher-density packaging solutions require fine features to be patterned onto grossly non-planar substrates, for example, in mechanical, optical and fluidic microsystems and in novel packaging schemes. Standard photolithography cannot be used in these cases because the inevitable gap between the (planar) mask and (non-planar) substrate causes diffractive line broadening and loss of resolution. We address this issue by realising the mask as a Computer Generated Hologram (CGH), which can then be illuminated to generate an image in space corresponding to the required non-planar profile. The CGHs are derived from analytical expressions and encode both amplitude and phase information. We illustrate the performance with a 100μm line exposed onto a substrate in the form of a plane/slope/plane, in which the change in depth is 40mm. Enhancements to the line shape are discussed that make the technique more robust to manufacturing process variations. The fact that features in the range 10-100μm can be imaged at large distance whilst coping with significant changes of depth indicates that the technique shows great potential in the microelectronics packaging industry.