Overcoming Difficulties in Photoreflectance Measurements on Product HBTs

Photoreflectance (PR) is an optical measurement technique used to study material properties and device structures. It is uniquely able to non-destructively measure electric fields within devices. Some device structures such as HBTs have a rough highly doped InGaAs surface layer. In some cases the surface roughness is sufficient to produce light scattering that impedes optical measurements such as PR. We have developed a new measurement configuration separating the pump laser, which causes the light scattering, from the probe laser. Rapid, non-destructive measurements of the emitter and collector depletion regions are possible with this configuration on product wafers with rough surface layers INTRODUCTION The production of heterojunction bipolar transistors (HBT) is a high volume process involving multiple production tools operating 24 hours a day 7 days a week. This production achieves tight specifications and high levels of quality assurance. Quality assurance relies on destructive device fabrication and measurement of individual wafers to indicate the overall quality of production. It is important to reduce the number of product wafers consumed for this testing yet maintain the high levels of quality. This has driven the industry to develop non-destructive measurements of product wafers. Measurements of sheet resistance, particles, and surface haze are commonly performed. We have previously demonstrated the utility of using a fully automated X-ray diffraction tool to monitor the base doping of GaAs HBTs [1]. This is complemented by a new configuration for a photoreflectance (PR) measurement that is able to measure product HBT wafers, which previously produced too much light scattering to be measured. PR has been used for decades to study material and device properties [2]. The measurement is routinely used to measure band-gap, ordering of InGaP alloys, and electric field. Its ability to measure electric fields distinguishes it from other optical measurement techniques such as ellipsometry or photoluminescence. This ability allows key information about the collector and emitter depletion regions to be analyzed. Production HBTs utilize a rough InGaAs contact layer to reduce emitter contact resistance. This rough layer strongly scatters light, which interferes with the PR measurement. EXPERIMENTAL We have found a new measurement configuration that allows a rough sample to be measured and nullifies the problem of the light scattering. The typical PR configuration is drawn in Figure 1. The measurement involves two light sources. Here a red diode laser is used as the pump laser, which is electronically chopped. The probe beam is generated from a halogen lamp with the wavelength selected by a monochromator. The electrons and holes generated by the chopped pump beam modify the electric