A Real-Time Chemical Vapor Deposition Optical Monitoring System for Ge1-XSnx Growth Optimization
Authors: Grant Joshua M , Golden Alexander , Yang Enbo , Yu Shui-Qing 
Published: December 2024

GeSn has attracted tremendous attention due to its tunable bandgap in recent decades. While the material has been produced through various methods, the Chemical Vapor Deposition (CVD) process has become one of the most essential for fabricating GeSn material since it was first demonstrated. However, the process is generally studied as a black box through input versus output. For example, one could study the change in temperature versus growth rate where changes in film thickness due to temperature change can be plotted only after the growth is finished. Process improvements from the study of these black box studies have yielded understandings in the growth kinematics and the development of growth recipes defining specific growth windows for different films that in many cases are very well understood. The black box treatment of CVD growth studies lacks near real-time feedback for the growth of the film leading to unsuccessful growths that in some cases slow research and in others lead to reduced yields of useful samples for device fabrication.
Real-time monitoring of the process both in-situ and ex-situ is essential, as these could enhance understanding of the CVD process and growth dynamics. In this research, we have been developing a sophisticated optical imaging system to monitor the wafer surface morphology with desired magnification and specific areas of a sample during growth. By coupling an optical microscope system and a high-resolution portable imaging system via ports into the CVD reactor, the growth of a sample can be monitored in real time at both the micro and macro scale. As schematically shown in Figure 1, a microscope and an LED light source are mounted to the two 2¾” flanges with an angle of incidence symmetrical along the chamber's vertical center line, and the portable camera is nearly vertically mounted at one of the reactor's optical windows below the horizontal axis. The microscope and the portable camera are designated for distinct functions, a characteristic that is determined by their respective locations and magnification capabilities. By meticulously calculating and applying geometric and optical design principles, such as the imaging area, solid angles, and reflection, we can capture real-time images of the sample surface during its growth.
Specifically, using this optical monitoring setup, we could observe the surface morphological change during a GeSn growth on Ge-Vs sample on a Si (100) substrate. This observation led to the early identification of defect formation and post-growth progression of defect formation during the cooling down process. Figure 2 is an example picture taken during the growth for an as-grown GeSn sample showing surface defects indicated by grey spots and edge and surface roughening by the orangish center region noted in the image. This capability would allow for the optimization of growth parameters throughout the entire process that result in larger areas of useful material or provide feedback that allows CVD operators to promptly respond and decide on the subsequent growth early to prevent lost time due to failed growths.
Future work is to be accomplished by improvements in the optical system to not only allow for better image quality by using a different camera allowing better focusing, and higher frame rates as well as adjusting light source angle and/or incident camera angle.

Acknowledgments

The work is supported by the Air Force Office of Scientific Research (AFOSR) (Grant No.: FA9550-19-1-0341, FA9550-20-1-0168).

Figure 1