Enhancing direct-write laser control techniques for bimetallic grayscale photomasks - art. no. 688312

Novel grayscale photomasks are being developed consisting of bimetallic thin-films of Bismuth on Indium (Bi/In) and Tin on Indium (Sn/In) with optical densities (OD) ranging from similar to 3.0 OD to 0.22 OD. To create precise three-dimensional (3D) microstructures such as microlenses, the mask's transparency must be finely controlled for accurate gray level steps. To improve the quality of our direct-write masks, the design of a feedback system is presented where the mask's transparency is measured and used to adjust the mask-patterning process while making the mask. The feedback would account for local variations in the bimetallic film and enhance the control over the mask's transparency such that >64 gray level photomasks become possible. A particular application of the feedback system is towards the production of beam-shaping masks. When placed in the unfocussed path for the photomask-patterning system, they can improve the consistency of the grayscale patterns by altering the laser to have a more uniform "top-hat" power distribution. The feedback system aids the production of beam-shaping masks since the processes of patterning, verifying, and using the mask are all performed using the same wavelength. In developing the feedback system, two methods were examined for verifying grayscale patterns. The first utilizes the mask-patterning system's focused beam along with two photodiode sensors; the second utilizes image analysis techniques on lower resolution microscope images. The completed feedback design would also account for drifts in the laser power used to pattern the bimetallic thin-film photomasks.