Effect of exposure energy, focus range, and surface reflection on the photolithography process of contact hole patterning

The patterning the contact holes by photolithography process, in this case polysilicon contacts, is the process to produce electrical connection between metal layer and polysilicon gates or metal layer and source/drain. The photolithography process can control size and shape of the contact hole patterns on photoresist before permanently patterned by etching process and then metallization. The contact holes were specified to have the size of 0.8 micron after the photolithography process. In order to achieve this, the process parameters to be considered are exposure energy, focus length of the lens system, and the effect of light reflecting from the film topography underneath. Since in the previous processing step, the attempt to reduce the film topography was done by planarization using BPSG. By adjusting the exposure energy and focus, the optimal condition, to produce 0.8 micron contact holes, was found to be energy of 305 mJ/cm² and focus of -1.0 mum.