<title>Use Of The National Bureau Of Standards (NBS) Antireflective (AR)-Chromium Optical Linewidth Standard For Measurements On Other Types Of Chromium Photomasks</title>

Abstract

Both antireflective (AR)-chromium and bright-chromium photomasks are currently used in the production of integrated circuits. Differences in the optical transmittance and reflectance of these photomasks can significantly change the line-image threshold required for accurate edge detection in optical microscope linewidth measurements. The suitability of using a calibration curve based on an AR-chromium optical linewidth-measurement standard (SRM 474) from the National Bureau of Standards (NBS) to correct linewidth measurements on other types of photomasks is discussed. Linewidths on each of three chromium photomasks of different chromium thicknesses were measured on four different types of optical microscope linewidth measurement systems. These measurements were corrected using an SRM 474 and compared with measurements made on the NBS optical linewidth calibration system. For the two bright-chromium specimens with low transmittance, the residual differences between the corrected values and the NBS values as measured on the NBS calibration system are generally less than ±0.05 μm for three of the measurement systems. For the see-through AR-chromium photomask with a higher transmittance, the calibration curve does not correct all systematic errors greater than ±0.05 μm. These results support theoretical studies showing that the degree of correction for systematic linewidth errors varies with the transmittance of the chromium photomask being measured and with the measurement system.