Patterning Over Topography

In modern production processes, it is imperative that a patterning technique facilitate patterning over existing topography, since the substrates commonly posses at least marginal and sometimes extreme topography. Projection lithography has the ability to expose patterns among topography, as shown in Figure 1.  SFIL requires a planar substrate on which to imprint, and so an imprint planarization step is added to the SFIL process flow, as shown in Figure 2.
Figure 1. Schematic of conventional resist processing.
Figure 2. SFIL process flow with the addition of an imprint planarization layer.
In a collaborative effort with Agilent Technologies, PMMA (497,000 MW) was spun at 6000 RPM to provide a thin organic layer over a pre-patterned substrate, typically a grating structure. Then, a planarization layer of an acrylate photopolymer was cured over the hard-baked PMMA using a non-patterned optical flat for the imprint template. Finally, the etch barrier was patterned over the planarized organic layer. High aspect ratio resist features such as those shown in Figure 3 were generated using the same etch transfer process as that used for flat substrates.
a) b) c)
Figure 3. Examples of polymer features on extreme topography. The underlying line'space array is a 700 nm tall Fresnel lens in Si.