Process Control System Produces High Laser Yields

Stephanie A. Weiss

ENDICOTT, N.Y. -- A novel process-monitoring and -control system has produced excellent semiconductor laser wafer yields, according to the president of a company that has licensed the technology from the US government.

Semiconductor Laser International Corp. President Geoffrey Burnham said the desorption mass spectrometry technology it licensed from the US Air Force Wright Laboratories has enabled the firm to produce CW laser devices with wafer manufacturing yields of around 40 percent.

Burnham said that while wafer yields in silicon are about 80 percent, typical commercial-scale laser-wafer yields in GaAs lasers are much lower. Industry experts indicate that typical yields range from less than 10 percent to more than 90 percent, depending on the type of device being manufactured and the specifications the device must meet. "GaAs is hard to make into lasers," Burnham explained.

Controlling the deposition of the layers of gallium and arsenide is the key to high wafer yields. Commercial wafer-growers employ two techniques to manufacture wafers: metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). It is said to be easier to control growth in the latter to produce one structure repeatably. In practice, however, even slight temperature fluctuations alter the amount of deposited material that sticks to the surface of the wafer. Without knowing the exact temperature to within ±5 °C, absolute process control is not possible, and yields suffer.

Desorption mass spectrometry, Burnham said, "achieves the original hope for MBE. It allows you, not knowing what the temperature is, to interrogate the desorption rates off the surface and use feedback to automatically adjust the temperature of the substrate."

The company has used the technology to produce a line of commercial semiconductor laser bars that operate from 785 to 1020 nm, with powers from 20-W CW to 80-W quasi-CW (peak power). The firm is still compiling lifetime data, but Burnham said he is "pretty encouraged that the laser bars will have fairly long lives."