30 June 1987 Transition Radiation As An X-Ray Source For Lithography
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Abstract
We have demonstrated that high-intensity soft x rays (900 eV to 3 keV) from transition radiation can be generated by passing moderate-energy (17 to 109 MeV) electron beams through targets consisting of thin (1 pm) multiple foils of beryllium and aluminum. We measured the absolute photon flux from five foil targets using a 44-pA, 109-MeV-elec-tron beam, and estimated the photon flux by exposing 4" silicon wafers coated with PGMA photoresist. All previous measurements of photon production used photon-counting methods. We destructively tested foils at high-average cur-rents for the first time. We exposed photoresist using a mask to produce a soft-x-ray lithograph. This research constitutes the first use of transition radiation as a source of soft x rays for lithography. These results indicate that moderate-energy linacs with transition radiators offer an alternative method of high power production of soft x rays for lithography. The radiation produced is at least three orders of magnitude brighter on a per-electron basis than synchrotron radiation. Storage rings have higher average beam currents than linacs; however, with a 10-2 difference in average currents, our experiments show that transition radiation will have the same average flux. Small, inexpensive accelerators such as one-klystron linacs can produce the required beam energies needed and would be less expensive than larger synchrotron sources. The Lawrence Livermore National Laboratory's (LLNL) electron-positron linac was used in a series of short experi-ments (two 8-hr. shifts) to show feasibility. The foils were located in the accelerator cave in a direct line with the linac primary beamline. Immediately downstream from the foils, the electron beam was deflected into a beam-monitoring system. The lithography station and photon detectors were located 6.4 m downstream of the foils. The silicon wafers were partially shielded from other ionizing radiation produced in the accelerator cave. The soft-x-ray radiation was emitted in a cone of apex angle ± 5 mrad, with a diameter at the wafer of approximately 10 cm. The average power at the silicon wafer was 0.28 mw using the 44-pA beam. The x-ray flux produced a smooth exposure of the resist, result-ing in the "doughnut" pattern that is characteristic of the transition-radiation cone. No attempt was made to determine the optimum exposure time during these runs, and the wafers were left in the photon beam for time periods between 1 to 2 hrs.
© (1987) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Melvin A. Piestrup, Michael J. Moran, Barry L. Berman, Piero Pianetta, Daniel Seligson, "Transition Radiation As An X-Ray Source For Lithography", Proc. SPIE 0773, Electron-Beam, X-Ray, and Ion-Beam Lithographies VI, (30 June 1987); doi: 10.1117/12.940351; https://doi.org/10.1117/12.940351
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