Epitaxial Process Monitoring and Development:
Use of a Residual Gas Analyzer for Epitaxial Process Monitoring and Development
B. Ramakrishnan, Dennis Liu, J. Town and S. Krishnakumar
Mitsubishi Silicon America
Salem, Oregon, 97303.
Residual Gas Analyzers (RGA) have proven to be a useful tool and hold promise to help optimize epitaxial processes against costly yield losses and extensive tool downtime. They also provide a detailed understanding of the semiconductor material and device processes. In the epitaxial process, RGA can be used as a monitoring tool to ensure that groups of process equipment which run the identical process recipes have nearly identical characteristics. In this study, an RGA is used with an Atmospheric Pressure Chemical Vapor Deposition (APCVD) reactor to study the gas phase reaction during various steps of the epitaxial process. Several examples of cases where the RGA provide valuable information about the epitaxial process is presented.
The cost of producing semiconductor devices has a direct dependence on maintaining a low contamination atmosphere. It also has impact on high machine uptime, throughput and a more complete utilization of the process equipment. In particular, to produce high quality epitaxial silicon layers, monitoring the trace levels of oxygen, moisture, particles and other contaminants is critical to prevent defects in the final film. In order to reduce the troubleshooting time and obtain a quick feedback, it has become critical to monitor insitu reactor processes. An RGA serves the purpose of sampling directly from the process chamber so that there is a continuous monitoring of the process and the process tool.
The use of RGAs in the semiconductor industry has been limited to leak checking and fingerprinting the processes. RGA can be used as both a process monitor and a diagnostic tool in a manufacturing environment and can help identify the problems, trigger alarms if the process conditions are not met or out of control, and provide an insight into the process chemistry itself. Recent studies have shown the importance of RGAs as general diagnostic tools in the manufacturing environment[2,3]. One study identified the source of foreign material on wafer surfaces which was released from high temp processes. Another report discussed how an intermittent nitrogen leak into an argon line affected an aluminum sputter system. An RGA was used in various stages of tool ramp-up for a silicon epitaxy CVD reactor. By monitoring both inlet gases and epitaxial reactor ambient conditions, process engineers have been able to reduce contamination sources, improve process control and increase epitaxial reactor yield by 2.5%. In this study, various steps of the epitaxial process as monitored by the RGA is presented. In addition, RGA was used to optimize the epitaxial high-etch process.
The epitaxial operation at Mitsubishi Silicon America consists of Epitaxial tools from different vendors such as Applied Materials (AMT), ASM and Concept Systems. In this study, an RGA from Spectra Instruments Inc. was used with an AMT 7700 APCVD reactor to monitor the gases coming out of the reactor. Figure 1 illustrates a schematic of the RGA system linked with a AMT 7700 reactor. The sensor head of the RGA has a mass range of 300 atomic mass units (AMU), which features a closed ion source design with dual tungsten filament. One rotary pump is used to create a pressure differential in the capillary line for sampling. In the other sensor chamber, a turbomolecular pump in conjunction with a rotary pump is used to create a background of the chamber. The best vacuum in the chamber can reach 10-9 Torr. The gas analyzer itself, using a quadruple mass spectrometer system, requires an internal pressure of less than 10-4 Torr.
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