Limitations of using a normal-incidence, in-situ, optical growth rate monitor for controlling nitride semiconductor deposition by metalorganic chemical vapor deposition

The feasibility of using a normal-incidence, in-situ, four color optical reflectance monitor for real-time feedback process control was investigated. The monitor was constructed using four visible wavelength lasers primarily for monitoring the Al_xGa_1-xN film deposition rate by metalorganic chemical vapor deposition (MOVCD). The feasibility of extracting other film parameters from the reflectance oscillations during growth was also investigated. The frequency of the reflectance oscillations was extracted in real time using the IEEE-STD-1057 sine wave fit algorithm. The method of extrapolating the index of refraction as a function of aluminum composition and film temperature is explained and shown to give accurate growth rate and thickness measurements at growth temperatures ranging from 1000°C to 1300°C. Post growth thickness was verified by Filmetrics (spectral interference fitting) and our method averaged thickness measurements 2.6% higher than the filmetrics measurements with a standard deviation of 1.4%. Our system was found to be unable to accurately extract, in-situ, other desired film properties from the oscillation frequencies due to resolution problems. Other considerations in using this popular MOCVD monitoring technique for real time process control are also discussed.