WEB EXCLUSIVE: Optical in situ monitoring in LED device production

Dec. 19, 2008
The most significant issue in LED MOCVD growth today is precise control of the wafer temperature throughout the whole growth process.
As wafer temperature is the key parameter for controlling the final indium content of active layers for LED emission (InGaN multi-quantum wells), even a small deviation results in a shift in emission wavelength of the final LED. Process optimization methods that provide wafer temperature control are described.
Fig. 1 As both the demand for high brightness LEDS and number of applications increases, higher throughput and yield are mandatory to satisfy current and future markets. Reactor temperature control and wafer bowing figure prominently in achieving yield.

Temperature control
Currently, reactor temperature is mostly controlled by a thermocouple or a back-side pyrometer. Therefore, the true wafer temperature on the wafer top side might change during the run and go unnoticed by the temperature control loop.

Figure 1 gives a typical example of these effects. It shows reflectance and temperature measurements during a typical GaN growth on sapphire. At around 10000s, the reactor pressure is reduced, leading to a drop in wafer temperature (red curve). This remains unnoticed by the back-side light pipe providing the control temperature for the recipe (black curve).

The drop in temperature is accompanied by a change in curvature (blue curve, lower graph). While the process temperature is constant during the run, the measured wafer temperature is changing significantly due to changed process conditions (in this case, reduced pressure).

Furthermore, run-to-run wafer temperature variations can occur due to different pre-coating conditions of quartz parts in the reactor, or different satellite rotation speeds, which would remain unnoticed without true temperature measurement, and could be fatal for the final LED devices.

Read the complete article at Solid State Technology magazine's web site.