A class of DC-sputter deposited bimetallic thermal resists (BiIn, SnIn, BiSn) have been investigated with exposure sensitivities (7mj/cm 2 ) near those of current organic photoresists. A prototype resist consists of stacked bismuth on indium layers (30-45nm thick) with thickness ratios matching the eutectic alloy points in BiIn binary phase diagram. Laser exposed areas have significantly different optical characteristics, with lower reflectivity and higher transparency, hence showing the imaged pattern before development, thus confirming correct exposures. Predicted exposure levels are almost wavelength invariant: <5% change between 248 and 157nm. Profilometery results and SEM images showed no signs of ablation in the exposed areas. The BiIn resist was wet developed using a HNO 3 :CH 3 COOH:H 2 O (1:3:6) acid solution which preferentially removed the unexposed areas leaving written patterns of alloyed lines seen both under profilometery and SEM, and stripped in a dilute HCl:H 2 O 2 :H 2 O bath. Shelf tests shows that the BiIn film is stable under 50ûC/90% humidity conditions. XRD and TEM show that the film is polycrystalline. Both SnIn and BiSn exhibit similar resist property as BiIn that the exposed area converts to greater transparencies that unexposed areas, indicating these three bimetallic films can be a group of potential inorganic thermal resist.
Bimetallic thermal resist Bi/In has shown many applications in the areas of microfabrication, photomasks and data storage. Optical modeling shows that this class of thermal resists is wavelength invariant, and Bi/In can perform even better at 13.4 nm than at 248 nm due to the increase of absorption and the reduction of reflection. Images were successfully made on Bi/In films with both proximity (0.5 mm) and projection exposures with Nd-YAG laser running at 2 nd harmonic wavelength. A new kind of developing solution (HCl:H 2 O 2 :H 2 O=1:1:48) used at room temperature was found to be more effective in descumming than nitric acetic acid solution. Both have the etching selectivity of unexposed area to exposed area >60:1. Developed Bi/In resist shows good conductivity, which can be used as both a metal plating masking and seeding layer. 2 to 10 µm wide Cu and Ni lines and squares were successfully plated on the developed Bi/In patterns on glass slides and silicon wafers. Shelf test shows that the properties of Bi/In film do not change after being kept in a humid temperature-lifted environment for 10 days. Large optical transmission changes (OD>3.5 before laser exposure and <0.3 after exposure) indicate Bi/In can be used for direct-write photomasks and data storage media. Heat-treatment enhances the OD exposed/unexposed OD change.
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