Demonstration of a new mask structure using a bonded hard pellicle

As 193 nm lithography appears to be a long term solution for wafer patterning, we expect new resolution enhancement coming from advanced mask technologies. We have studied an assembly technique that could increase mask capability towards advanced wafer patterning. This paper presents a proof of concept for the use of bonded mask, obtained with two plates assembled together. The initial application targeted here is an alternative solution to pellicle. A special process has been worked out to obtain bonded test samples. Based on the knowledge of silicon wafer bonding techniques, we have developed a process that allows bonding of fused silica square plates. Constant progress allowed us to use specific materials used in mask manufacturing, such as chromium and fused silica, and also specific square shapes and rather large thickness. The final demonstrator is a Chromium on Glass mask (COG), on which a hard pellicle has been bonded without any additional material. The pellicle was 0.5 mm thick and 100 mm in diameter. This test sample has been qualified in a 248 nm AIMS tool. We made comparative measurements on different occurrence of the same chip, covered or not covered by the pellicle. We have shown evidence of induced spherical aberration for conventional illumination and this has been confirmed by simulation. Image fidelity was proven for positive and negative features. Through focus image capture showed that process windows were not impacted by the hard pellicle.