The imprint resist characteristics can be grouped into fluid (rheological and surface) properties prior to UV curing and solid phase properties after UV curing. Fluid properties include viscosity, wettability, evaporation characteristics and dispensability. Solid phase properties include cohesive yield strength, elongation to failure, adhesion to substrate, surface contact angle, and ease of release from imprint mask. The MonoMat formulas delicately balance the fluid and solid phase considerations. This balancing act is the key to MonoMat’s success.
Imprint resists must have dispensability with appropriate flow characteristics, requiring certain specific viscosity and evaporation characteristics. Process uniformity and shelf life add additional fluid requirements. When all features are fully filled with resist, UV cross-linking step follows. Efficient cross-linking materials require short exposure times without causing excessive heating to the template and substrate. All of these demands must to be carefully balanced.
After curing, the mask or template is separated from the substrate. It is essential to assure the cured resist adheres to the substrate, not to the mask or template. To reduce the surface energy of the template, a high surface concentration of fluorine is required, but this then restricts the wettability and filling speed, requiring a yet another balancing act. Typically, very thin substrate adhesion promoting layers are used to facilitate clean separation. The cured resist must also be mechanically strong and tough to resist cohesive failure and enable high aspect ratios. Adding certain polar components improves these characteristics but too much increases the surface tension and can reduce the fill speed. Finally, the material must have the necessary etch selectivity while meeting the most stringent CMOS purity requirements of <10ppb, not only as formulated, but after passage through the inkjet head assembly.
