Investigation of Advanced Process Control Methods for Exposure Controlled Projection Lithography

The DMD based Exposure Controlled Projection Lithography (ECPL) process has promising applications in fabrication of microfluidics and micro optics components. Unlike a conventional layer-stacking projection stereolithography process, ECPL cures a 3D feature by projecting radiation through a stationary, transparent substrate by varying exposure patterns and durations implemented by a sequence of DMD bitmaps. Due to the unavailability of an in situ metrology for cured part dimensions, unmeasurable time-varying disturbances such as oxygen inhibition and light source fluctuations, and the complex chemical & physics interactions in photopolymerization, a common practice in stereolithography process planning is to use experimental characterization and statistics models in an open-loop mode, which yields poor accuracy. This paper reviewed existing process control methods for ECPL and defined a need for advanced control methods. As a first proposal for advanced control methods to mask projection stereolithography, the paper surveyed relevant processes and put forward a hierarchical framework of advanced control methods for ECPL, including evolutionary cycle-to-cycle (EC2C) and adaptive neural network (ANN) backstepping control methods. The goal is to identify some advanced control methods, which are capable of tracking the process dynamics by online updating the model parameters with real-time measurement feedback. Such closed-loop control methods are promising to be able to improve the process precision and robustness.