Dynamic Modelling of CVD for Real-Time Control of Microstructure
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M.A. Gevelber, M.T. Quifiones, M.L. Bufano Manufacturing Engineering, Boston University, 44 Curnmington St., Boston MA 02215
ABSTRACT A nonlinear dynamic model of the chemical vapor deposition (CVD) process has been developed and analyzed to obtain insight into the design of an appropriate control structure.
CVD's importance is related in part to the possibility of engineering the micro-structure of the coatings which determines coating properties. Our ultimate research objective is to develop a system to control the micro-structure of coatings produced by CVD based on in-situ measurements related to the growth process. The conventional practice is to regulate the inputs in a feedforward manner (i.e. maintaining temperature, pressure and gas flow rates set-points), and only limited work has been directed at developing control strategies to ensure that a desired micro-structure is achieved. Development of a measurement-based feedback system can enhance the ability to achieve coating microstructure, compensate for process variations (disturbances) and improve the capability of transferring process recipes to different systems. However, successful development of an appropriate control structure (i.e. selection and connection of measurements to specific inputs) requires an explicit understanding of the process dynamics. Extensive modelling and experimental investigations have considered the fluid/thermal/chemical nature of CVD, but most of this work has focused on the steady-state characteristics of the system [1, 2, 3, 4, 5]. To obtain insight into an appropriate control structure, we have developed a lumped, dynamic, nonlinear process model that relates the manipulable inputs to the local thermal/fluid behavior of the equipment and, in turn, to the physics of the deposition process [6, 7]. As a benchmark, we focus on TiN deposition since there has been significant experimental work reported and is an important coating for cutting tools. Specific features addressed in this paper are the transport delay and multicomponent aspect of the deposition process. An experimental CVD unit is being developed that is the basis for the physical parameters used in this analysis.
MODELLING The reactor is a 5 cm diameter tube one meter in length, L, enclosed by a resistance heater (figure 1). The mass flow rates are regulated by mass flow controllers (MFC). Pressure control is achieved by varying the volumetric flow rate of exhaust from the reactor by introducing a controlled flow of nitrogen gas upstream of the constant flow rate pump. The reactor is hot-wall, which is widely used in making protective coatings. As a result, there are small thermal gradients within the system, simplifying the flow behavior. Lumped control volume (CV) analysis is used to model the dynamics of both the equipment and process physics.
33 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
Equipment Model The equipment model represents the dominant dynamics linking the equipment's manipulable inputs to the variables that serve as in
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