February, 2004

EEEL researchers, Chris Cromer, Darryl Keenan, and Marla Dowell, have completed the first exhaustive intercomparison of NIST excimer laser calorimeters. This work includes measurements taken over the course of a two-year period in which the performance of the NIST 157, 193, and 248 nm excimer laser calorimeters was monitored at the design wavelengths as well as at the other excimer laser wavelengths. The results show good agreement among our transfer standards and excellent stability over time.

From these data we determined that the responsivity of the NIST UV laser calorimeters all agree within their stated uncertainties. In all but one case, the calorimeters' responsivities agree to better than 0.3 %. The comparison between the QDUV (193 nm) and QUV (248 nm) calorimeters at 248 nm uncovered a 1 % difference between the calorimeters' responsivities. This difference is due to partial transmission of the 248 nm radiation through the absorbing glass of the QDUV calorimeter which reduces the calorimeter's absorptance and alters its response. It is important to note that the QVUV (157 nm) calorimeter design is based on a multiple-reflection absorbing cavity, a fundamentally different design than that of the QDUV and QUV designs, both of which use volume absorbers.

These studies, part of an on-going metrology program for excimer laser measurements, represent the first quantitative intercomparison of UV laser primary standards. The measurements are performed using three beamsplitter-based calibration systems at 157, 193, and 248 nm. At 157 and 193 nm, a spatially uniform beam from an excimer laser is generated using a special beam homogenizer. The beam propagation properties, including uniformity or homogeneity, are fully characterized with a state-of-the-art beam profile measurement system based on a pyroelectric camera array. This uniform beam is then used to calibrate laser dose (i.e., energy density) meters like those used in high-resolution semiconductor photolithography systems and other excimer laser applications.

Since the inception of excimer services in 1995 the Division has performed over 200 calibrations. Our excimer laser calibration services supports calibrations with a total deposited energy in the range of 0.1 to 15 J and total power from 100 µW to 10 W, with other values available upon request. We also provide calibrations of dose at 193 nm, and can provide dose measurements at 157 nm or 248 nm as a special test. In addition, detector linearity special tests can be performed for all three wavelengths.

Contact:

Chris Cromer, (303) 497-5620