We report on the development of a prototype polarization tag based system for detecting chemical vapors. The system primarily consists of two components, a chemically sensitive tag that experiences a change in its optical polarization properties when exposed to a specific chemical of interest, and an optical imaging polarimeter that is used to measure the polarization properties of the tags. Although the system concept could be extended to other chemicals, for the initial system prototype presented here the tags were developed to be sensitive to hydrogen fluoride (HF) vapors. HF is used in many industrial processes but is highly toxic and thus monitoring for its presence and concentration is often of interest for personnel and environmental safety. The tags are periodic multilayer structures that are produced using standard photolithographic processes. The polarimetric imager has been designed to measure the degree of linear polarization reflected from the tags in the short wave infrared. By monitoring the change in the reflected polarization signature from the tags, the polarimeter can be used to determine if the tag was exposed to HF gas. In this paper, a review of the system development effort and preliminary test results are presented and discussed, as well as our plan for future work.
Gas correlation imagers are important instruments for remotely detecting effluent emissions. However, making a
functional design for field testing is non-trivial given the range of environmental conditions the system may be operated under and the required matched imaging performance for both channels. We present a dual channel 7 degree full field of view f/2.5 athermal optical design athermalized from 0 to 50 degrees C that operates in the wavelength range of 2.0 to 2.5 microns suitable for methane imaging. We present the optical design, tolerance budget, and alignment plan used for the system. Predicted and as-built performance data including interferometric and ensquared energy measurements for both imaging channels are also shown.
As photolithographic processes utilize ever shorter wavelengths to produce more densely packed circuitry on silicon chips, the choice of materials suitable for use in the DUV spectral region becomes severely limited. We report here on preliminary life test results for calcium fluoride irradiated at 157 nm by F2 laser beams. The sample housing and beam delivery tubes were purged continuously with high purity nitrogen to keep the background oxygen level as low as possible and to sweep away any potential organic gases liberated from the sample mounting hardware and overall experimental apparatus. Data were collected to evaluate induced changes in transmission, wavefront distortion and birefringence over the course of billions of shots at a nominal fluence of 0.1 mJ/cm2.
Fused silica samples from six suppliers were irradiated with a range of fluences (0.004 mJ/cm2 to 0.2 mJ/cm2) using an ArF 193-nm excimer laser. The test was performed in an effort to determine fluence level dependency of induced wavefront distortion and birefringence. Each sample was irradiated with four beams of different fluence levels for 22 billion pulses over a period of 133 days. Wavefront distortion in the irradiated areas was observed for all samples. The sign and magnitude of the distortion were dependent upon the fluence level and the particular sample under irradiation. Birefringence measurements were also made. The birefringence characteristic varied among the samples, possibly as a function of fluence level and material. FTIR spectrum measurements were made and were correlated with wavefront distortion measurements. A description of the test and measurements is presented along with data covering a pulse count of 22 billion pulses.
Exposure tools for 193nm lithography are expected to use Argon-Fluoride lasers at repetition rates of at least 2kHz. We are showing that, by revisiting several key technologies, the performance and reliability of ArF lasers at 2 kHz are trending towards a level comparable to KrF lasers.
The present day notion of the extensibility of KrF laser technology to ArF is revisited. We show that a robust solution to ArF requirements can be met by significantly altering the laser's core technology-discharge chamber, pulsed power and optics. With these changes, a practical ArF tool can be developed. Some of the laser specifications are: Bandwidth: 0.6 pm (FWHM) 1.75 pm (95% Included Energy); Average Power: 5 W; Repetition Rate: 1000 Hz; Energy Stability (3(sigma) ): 20% (burst mode) 8% (continuous); Pulse Width: 25 ns.