Control of photoresist film thickness: Iterative feedback tuning approach

The microlithography process is the most critical step in the fabrication of nanostructures for integrated circuit manufacturing. The most important variable in the microlithography process is the linewidth or critical dimension (CD), which perhaps is the single variable with the most direct impact on the device speed and performance. In this paper we control one of the key parameters that impact the final CD: the photoresist film thickness. Due to thin film interference, CD varies with photoresist thickness. In the past, fixed controllers were used. However, as different recipes, including different photoresists, may be used for different batches, a fixed controller has to be retuned each time. We proposed the use of self-tuning control – iterative feedback tuning – in this paper and have obtained improvement in both thickness uniformity and convergence time.