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UNCLASSIFIED, PUBLIC RELEASE
Ion Beam Sputter Deposition of high LIDT Optical Coatings on Large Substrates
Abstract: Recent applications of high energy lasers have created a demand for large area complex coatings with low absorption and high LIDT on large optics in production. We describe system design considerations and results for production of such coatings.
1. Introduction:
Recent advancements in laser driven fusion research has spurred a significant amount of activity in commercial research and development aimed at fusion energy production. Some embodiments of these efforts require large, high laser damage threshold coated optics. Telescopes used as beam directors for high energy lasers require at least one large diameter primary mirror. The mirrors are often off axis aspheric substrates. These mirrors often require multi spectral, thick, uniform coatings with very low absorption and high laser damage threshold. These two types of efforts have created a need for ion beam sputtered (IBS) coatings on large substrates in the one meter scale.
2. Evolution of Ion Beam Sputtering:
IBS coatings were originally developed in the 1970s for applications in inertial navigation [1]. The process demonstrated exceptionally low scatter and relatively low absorption. In the following five decades a number of companies worked on refining this process for a variety of applications all requiring low optical losses. Absorption and laser induced damage threshold of the coatings continued to improve [2]. Other companies around the world developed IBS deposition systems. Advent of wavelength division multiplexing and the telecom boom of the late 1990s pushed the industry to develop highly reliable deposition equipment capable of producing coatings with thickness of up to 30 µm on substrates of up to 30 cm in diameter with reasonably good uniformity. During the following 20 years a large number of companies further developed the process for a variety of applications including the production of coatings for high energy and high power density laser applications as well as complex coatings for ultra fast lasers. The LIGO and VIRGO collaboration pushed the development of very uniform low loss coatings on substrates up to 35 cm diameter [3].
3. Large Area IBS coatings at FiveNine Optics:
Recent advancements in high energy and high power laser systems has created demand for larger and more complex IBS coated optics and mirrors. While the demand for such large optics is limited, they are critical to continued development of these laser systems. At least two system manufacturers offer machines for coating of substrates up to 600 mm in diameter.
Our first project in large IBS coatings required coating of substrates with up to 750 mm in diameter. Our goal was the deposition of low absorption, low loss, and high laser damage threshold coatings over 750 mm diameter aperture with uniformity of better than +/-1%. The deposition rates needed to be fast enough to allow for the reliable production of coatings of up to 30 µm in thickness.
We made significant modifications to an existing commercial IBS system for this application. The modifications included but were not limited to the location and orientation of the ion source, target and the substrate. We evaluated the possibility of using a 220 mm diameter high current (power) ion source but abandoned that option in favor of using highly reliable and proven 160 mm ion sources and power supplies capable of producing a beam with up to 1.5 amps of current at 1,200 volts.
We started by characterizing the deposition plume from our sputtering source and targets and used vacuum deposition software models to optimize the geometry of the system for obtaining the best possible uniformity without the use of masks. After implementing the required geometry, we added masks to refine the uniformity.
Fig. 1. Uniformity over a 700 mm diameter substrate
A deposition rate of ~0.5 A/sec was achieved for SiO2 and Ta2O5 with the aggregate uniformity of figure 1. This rate was just under 1/4th of the deposition rate we use in our commercial production systems for substrate diameters in the range of 300 mm. The reduction in the rate of deposition of a factor of 4 when increasing the substrate diameter from 300 mm to 700 mm was lower than we had expected.
5. References:
[1] US Patent # 4,142,958. Method for fabricating multi-layer optical films, issued March 06,1979, Litton Systems,
David T. Wei, Anthony W. Lauderback.
[2] G. Rempe, R. J. Thompson, H. J. Kimble, and R. Lalezari, Measurement of ultralow losses in an optical
interferometer," Optics Letters 17, 363{365 (1992).
[3] Steven Penn, LIGO DCC: LIGO-G080303-00-z
UNCLASSIFIED, PUBLIC RELEASE
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