A scatterometer for measuring the bidirectional reflectance and transmittance of semiconductor wafers with rough surfaces

A three-axis automated scatterometer (TAAS) with a high accuracy goniometric table, a fiber-coupled power stabilized diode laser, and a large dynamic range detector has been constructed and characterized to measure the bidirectional reflectance of rough surfaces. This scatterometer is capable of both the in-plane and out-of-plane measurements, and furthermore, it can also measure the bidirectional transmittance for semitransparent wafers. The goniometric table is comprised of three computer-controlled rotary stages of high angular resolution. The optical sources are fiber-coupled diode lasers at wavelengths λ=635, 785, and 1550 nm, respectively. These diode lasers exhibit excellent power stability and wavelength stability. Silicon and germanium photodiode detectors are used with a lock-in amplifier. The estimated combined uncertainty of the measurement is 0.5% at θr=45° and 2% at θr=80° without considering the effects of stray light and misalignment. Several rough silicon wafers have been measured and the results have been compared with those obtained from a reference instrument at the National Institute of Standards and Technology. The average relative difference is approximately 5% at λ=635 and 785 nm, and 24% at λ=1550 nm, much larger than the estimated uncertainty. The discrepancy may be attributed to the effects of stray light and in addition, the possible misalignment at λ=1550 nm. It is expected that the accuracy of the TAAS can be improved by reducing stray light and bettering the alignment.