Step and flash imprint lithography for sub-100nm patterning

Step and Flash Imprint Lithography (SFIL) is an alternative to photolithography that efficiently generates high aspect-ratio, sub-micron patterns in resist materials. Other imprint lithography techniques based on physical deformation of a polymer to generate surface relief structures have produced features in PMMA as small as 10nn [1], but it is very difficult to imprint large depressed features or to imprint a thick films of resist with high aspect-ratio features by these techniques. SFIL overcomes these difficulties by exploiting the selectivity and anisotropy of reactive ion etch(RTE). First, a thick organic "transfer" layer (0.3 mu m to 1.1 mu m) is spin coated to planarize the wafer surface. A low viscosity, liquid organosilicon photopolymer precursor is then applied to the substrate and a quartz template applied at 2psi. Once the master is in contact with the organosilicon solution, a crosslinking photopolymerization is initiated via backside illumination with broadband UV light. When the layer is cured the template is removed. This process relies on being able to imprint the photopolymer while leaving the minimal residual material in the depressed areas. Any excess material is etched away using a CHF3/He/O-2 RIE. The exposed transfer layer is then etched with O-2 RIE. The silicon incorporated in the photopolymer allows amplification of the low aspect ratio relief structure in the silylated resist into a high aspect ratio feature in the transfer layer. The aspect ratio is limited only by the mechanical stability of the transfer layer material and the O-2 RTE selectivity and anisotropy. This method has produced 60nm features with 6:1 aspect ratios. This lithography process was also used to fabricate alternating arrays of 100nm Ti lines on a 200nm pitch that function as efficient micropolarizers. Several types of optical devices including gratings, polarizers, and sub-wavelength structures can be easily patterned by SFIL.