HIGH-PRECISION REFLECTOMETER FOR SUBMILLIMETER WAVELENGTHS

A high-precision reflectometer has been designed and implemented to me asure directly the specular reflectance (R) of materials in the submil limeter (SM) region of the spectrum (300 GHz < upsilon < 3000 GHz). Pr evious laser-based measurement systems were limited to an uncertainty in R of +/-1.0% because of a number of issues such as lack of an absol ute reflection standard, difficulties in the interchange of sample and standard in the laser beam, and instabilities in the laser system. We realized a SM reflection standard by ellipsometrically characterizing the complex index of refraction of high-purity, single-crystal silico n to a precision such that its SM reflectivity could be calculated to better than +/-0.038. To deal with alignment issues, a precision sampl e holder was designed and built to accommodate both sample and silicon reflection standard on an air-bearing rotary stage. The entire measur ement system operated under computer control and included ratioing of the reflected signal to a reference laser signal, measured simultaneou sly, to help to eliminate short-term laser instabilities. Many such me asurements taken rapidly in succession helped to eliminate the effects of both source and detector drift. A liquid-helium-cooled bolometer w as modified with a large area detecting element to help to compensate for the slight residual misalignment between sample and reflection sta ndard as they were positioned into and out of the laser beam. These mo difications enabled the final measurement precision for R to be reduce d to less than 0.1%. The major contribution to this uncertainty was th e difficulty in precisely exchanging the positions of sample and stand ard into and out of the laser beam and was not due to laser or detecto r noise or instabilities. In other words, further averaging would not help to reduce this uncertainty. This order-of-magnitude improvement m akes possible, for the first time to our knowledge, high-precision ref lectance measurements of common metals such as copper, gold, aluminum, and chromium whose predicted reflectivities exceed 99% in the SM regi on. Furthermore, precise measurement of the high-frequency losses in h igh-temperature superconducting materials is now also possible. Measur ements reported here of metals at a laser wavelength of lambda = 513.0 1 mu m (upsilon approximate to 584 GHz) indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling 0.1-0.3% below predicted values.