text: Photo-reflectance is a highly sensitive measurement technique and provides unmatched capability for characterizing the material and electronic properties of thin films. Photo-reflectance has been particularly important in basic research on semiconductors due to its ability to precisely determine semiconductor bandstructures (even at room temperature). As an optical technique, photo-reflectance would appear suited to industrial applications because it is non-contact, and because it has good spatial resolution. However, the need for spectroscopic information limits measurement speed, and consequently the adoption of spectroscopic photo-reflectance in industrial applications such as process control of microelectronics manufacturing.
  Nevertheless, where spectroscopic information is not required, photo-reflectance techniques have been implemented in semiconductor manufacturing process control.  For example, in the late 1980s, Therma-Wave, Inc. introduced the "Therma-Probe" photo-modulated reflectance system to the market for semiconductor process control equipment. The original Therma-Probe focused an intensity modulated pump laser beam onto a spot on a silicon sample, modulating the sample reflectance. The reflectance changes were detected by a coincident laser probe beam of 633 nanometer wavelength. At this wavelength no electro-reflectance signal is present, since it is far removed from any interband transitions in silicon. Rather, the mechanisms responsible for the Therma-Probe signal are thermo-modulation and the Drude free carrier effect. The Therma-Probe was used primarily for monitoring of the ion implantation process in silicon semiconductor manufacturing. Measurement systems such as the Therma-Probe are particularly desirable in process control of microelectronics manufacturing because they provide the ability to quickly verify the correct execution of process steps, without contacting the wafer or removing the wafer from the clean room. Generally a number of measurements will be made on certain areas of the wafer and compared with expected values. As long as the measured values are within a certain range, the wafers are passed for continued processing. (This is known as statistical process control.) Other photo-modulated reflectance systems marketed for process control of implant processes are the "TWIN" metrology system marketed by [http://www.pvatepla.com/en/home PVA TePla AG], and the "PMR-3000" marketed by [https://www.semilab.hu/ Semilab Co. Ltd] (originally Boxer-Cross, Inc.).
  However, by the mid 2000s, new manufacturing processes were requiring new process control capabilities, for example the need for control of new "diffusion-less" annealing processes and advanced strained silicon processes.  To address these new process control requirements, in 2007, [https://xcalipr.com Xitronix Corporation] introduced a photo-reflectance system to the semiconductor process control market.  Like the Therma-Probe, the Xitronix metrology system utilized a fixed wavelength probe beam generated by a laser.  However, the probe beam of the Xitronix system had a wavelength of approximately 375 nanometers, near the first major interband transition in silicon.  At this wavelength the electro-modulation signal is dominant, which enabled the Xitronix system to precisely measure active doping concentration in diffusion-less annealing processes. This probe beam wavelength also provided excellent sensitivity to strain in strained silicon processes. More recently, the use of laser photo-reflectance technology for precision measurement of carrier diffusion lengths, recombination lifetimes, and mobilities has been demonstrated.