Process measurement and control for exposure controlled projection lithography

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Zhao, Xiayun
Rosen, David W.
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The Exposure Controlled Projection Lithography (ECPL) is an additive manufacturing process, in which liquid photopolymer monomers cross-link into solid polymer with controlled ultraviolet irradiation. Unlike other types of stereolithography processes, the ECPL system cures a 3D part by projecting from beneath a stationary and transparent substrate ultraviolet radiation, which is modulated by a sequence of DMD bitmaps varying the exposure intensities, patterns and durations. It has become promising in fabrication of micro optics and fluidics components. Due to the complex chemical & physics interactions in photopolymerization process, unavailable in-situ metrology and the unmeasurable time-varying disturbances such as oxygen inhibition and light source fluctuations, a common practice in stereolithography process planning is to use experimental characterization and statistics models in an open-loop control mode, which cannot effectively control the nonlinear black/grey-box process. Hence, the ECPL system still suffers loss of accuracy, which limited it from becoming a more capable micro manufacturing method for wider applications. A potential solution for controlling the not-fully-known ECPL process is the methodology of closed-loop control, which requires measurement feedback to link input and output variables. An in-situ interferometric curing monitoring (ICM) system has been developed to monitor the ECPL process, but it is not accurate or ready for real-time measurement yet and only able to provide interferograms for posterior analysis of cured heights combined with offline microscope measurements. In all, to improve the ECPL process accuracy and precision involves with extensive research in process modeling, measurement and control. A new research is needed to realize an automated, accurate and precise ECPL system. To attain this goal, two research questions will be investigated by this research. • How to develop a real-time metrology based on the existing in-situ interferometric curing monitoring system to measure the cured part dimensions, specifically the cured height profile across the curing area? • As a baseline control, without a constitutive process model of first principle differential equations, what is an applicable ECPL process control approach, which could utilize the real-time measurement system to improve the process accuracy? The intellectual merit of this research lies in developing real-time measurement and control methods for the ECPL process, with answers for the research questions. The scientific and engineering outcomes from this research will help achieve better manufacturing accuracy and precision thereby facilitating applications of the ECPL system in micro fabrication, and will offer an exemplification of the applicability and benefits of real-time feedback control methodologies for unknown nonlinear processes in other additive manufacturing methods.
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