Intelligent Process Control Of Silicon Nitride Chemical Vapor Deposition

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ABSTRACT The chemical vapor deposition of silicon nitride can be used to protect advanced materials and composites from high temperature, corrosive, and oxidative environments. Desired coating characteristics, such as uniformity and morphology, cannot be measured in-situ by traditional sensors due to the adverse conditions within the high-temperature reactor. A control strategy has been developed which utilizes a process model and an advanced laser-based sensor to measure the deposition rate of the silicon nitride coating in real-time. The control system is based on a three level hierarchical architecture which functionally separates the process control into PID, supervisory and advanced sensor-based control. Optimal setpoint schedules for the supervisory level are derived from a quasi-fuzzy logic inverse mapping of the process model. An advanced sensor utilizing laser ultrasonics provides real-time coating thickness estimates. Model bias is characterized for each reactor and is correlated on-line with the sensor's deposit thickness estimate. Deviations from model predictions may result in parametric changes to the process model. New setpoint schedules are then created as input to the supervisory control level by regenerating the inverse map of the updated process model. INTRODUCTION The use of chemical vapor deposition (CVD) for the synthesis of advanced refractory coatings has become increasingly important for high-temperature structural applications. Silicon nitride (Si3N 4) can be used to protect advanced materials and composites from high temperature, corrosive, and oxidative environments. Since the performance of this advanced coating is dependent on the microstructure, 1 a primary objective of the CVD control system is to ensure that the desired microstructural characteristic is achieved. Also, since the application may include high speed rotating equipment such as turbine engine components, it is necessary to provide a uniform coating distribution on the substrate surface. A final objective is to optimize the efficiency of the process as measured by material costs and process cycle time. To achieve these objectives a hierarchical control architecture and a laser ultrasound sensor were developed and demonstrated* on a laboratory scale reactor at United Technologies Research Center (UTRC). The advanced laser ultrasound sensor estimates deposit thickness from acoustic waves in the coating generated by a yttrium aluminum garnet (YAG) laser and detected with laser interferometry. The sensor thickness estimate is based on a sliding-window least squares fit, based on an average of 100 samples taken every 5 minutes. The control architecture consists of 3 levels - process parameter control (PPC), supervisory control (SupCon) and process management (PM) *

Supported by ARPA-DSO through WRDC contract no. F33615-89-C-5628. Maj. Joseph Hager was the WRDC monitor, and Mr. William Barker was the ARPA sponsor. 57 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society

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