Real-Time Composition and Thickness Control Techniques in A Metalorganic Chemical Vapor Deposition Process
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ABSTRACT Metalorganic chemical vapor deposition (MOCVD) is a process used to manufacture electronic and optoelectronic devices that has traditionally lacked real-time growth monitoring and control. We have developed control strategies that incorporate monitors as real-time control sensors to improve MOCVD growth. An analog control system with an ultrasonic concentration monitor was used to reject bubbler concentration disturbances which exist under normal operation, during the growth of a four-period GaInAs/InP superlattice. Using X-ray diffraction, it was determined that the normally occurring concentration variations led to a wider GaInAs peak in the uncompensated growths as compared to the compensated growths, indicating that closed loop control improved GaInAs composition regulation. In further analysis of the X-ray diffraction curves, superlattice peaks were used as a measure of high crystalline quality. The compensated curve clearly displayed eight orders of satellite peaks, whereas the uncompensated curve shows little evidence of satellite peaks.
INTRODUCTION Stricter semiconductor device tolerance requirements [1, 2, 3] are driving the need for better monitoring and control in semiconductor fabrication processes. Metalorganic chemical vapor deposition (MOCVD) is a widely used technique for epitaxial growth of semiconductor layers, but is limited by a high degree of process variance. Recently, the MOCVD process has been improved with real-time closed loop control [4, 5], using a concentration monitor. In previous work [5], a control system was designed to improve both epitaxial thickness and composition precision. The controller is designed to maintain a constant delivery rate of one component of the semiconductor alloy being grown in the reactor. The primary goal of this paper is to outline general procedures for the design and implementation of a control system for epitaxial growth. A secondary goal consists of describing the control system implementation and a growth demonstration. To achieve these goals, the paper contains a brief overview of the system and experiment, followed by a detailed discussion of the control issues.
SYSTEM OVERVIEW Metalorganic chemical vapor deposition (MOCVD) is a vapor-phase growth technique used to grow a wide range of semiconductors and other materials. Fig. 1 is a schematic overview of the MOCVD delivery system for a single source. The atoms that are needed to create the thin films found in single-crystal compound semiconductors are supplied from precursors that can be solid, liquid, or gas. In the case of solid and liquid precursors, the vapor that arises from evaporation is transported to the reaction chamber by passing hydrogen gas (or another pure carrier gas) through the source vessel that stores the liquid or 133 Mat. Res. Soc. Symp. Proc. Vol. 406 01996 Materials Research Society
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