EUVL dosimetry at NIST

As part of its role in providing radiometric standards in support of industry, the National Institute of Standards and Technology has been active in advancing extreme ultraviolet dosimetry on various fronts. Recently, we undertook a major effort in accurately measuring the sensitivity of three extreme ultraviolet photoresists. It has been common practice to use photoresists as a transfer “standards” to determine the intensity and uniformity of the radiation transmitted by extreme ultraviolet steppers. In response to preliminary results from Lawrence Berkeley National Laboratory that showed that two “standard” photoresists were almost twice as sensitive as had been previously believed, NIST carried out similar measurements and confirmed the Berkeley results. However, we have found that the assumed sensitivities are more a question of system calibration than of absolute resist dose sensitivity. We will describe the facility we used to make these measurements. Photoresists make less than perfect radiometers. They are very non-linear, sensitive to atmosphere, and difficult to calibrate. All of these characteristics led to the disparate results in assumed sensitivity values. We have developed an alternate wafer-plane dosimeter based on image plates. The dosimeter is linear over several orders of magnitude, comparatively insensitive to atmosphere, and can be re-calibrated as necessary. Moreover it can pass through a stepper as any other wafer. We will describe this dosimeter in detail.