Control of lithography in semiconductor manufacturing

Lithography is important in semiconductor manufacturing because it affects both the performance and yield of the devices in each wafer. The formation of a layer in lithography equipment involves a series of steps, each of which requires process control, namely, surface preparation, spin coat, soft bake, alignment and exposure, post-exposure bake, resist development, after-development inspection, resist removal, and final inspection. Lithography process control must address both chemical and mechanical aspects. In addition, the correct alignment of each layer must be obtained with respect to the previous layer and the overall alignment of the device. Advanced process control is used in various steps in lithography. For example, alignment and exposure may require multivariable model predictive control of a 10 times 10 system. Factory control of lithography equipment in a multiprocess/multiproduct environment requires special modifications for data handling and control since each tool has slightly different operating characteristics