Battery-Powered deposition system and process for making reflective coatings

Abstract

A battery powered deposition system and process for applying aluminum, silver, and SiO films (and their derivatives such as aluminum oxide, aluminum nitride and silicon dioxide) especially for making broadband reflective coatings for mirrors. One or more filaments are wetted with a filament wetting material such as aluminum or a silver alloy. In a preferred embodiment filaments are heated quickly to a high temperature with an array of batteries and the filament wetting material is deposited as a reflective coating on the mirror.

Claims

1. A method of producing coatings by vapor deposition from at least one filament fabricated from a filament material selected from a group of filament materials consisting of tungsten, tantalum, molybdenum and columbium comprising the steps of: A) wetting the filament with a filament wetting material selected from a group filament wetting materials consisting of aluminum and a silver alloy; B) depositing at least a portion of the filament wetting material on a substrate positioned in a vacuum chamber or outer space by heating the filament to a temperature in excess of the boiling point or the sublimation point of the wetting material with electric energy supplied by at least one battery contained in a leak-proof container containing the battery and a circuit board programmed to control energy applied to the filament so as to control the rate of deposition of filament wetting material on the substrate.

2. The method as in claim 1 wherein a mechanical shutter is provided with respect to each of the at least one filament and adapted to precisely start and stop the deposition of the filament wetting material.

3. The method as in claim 1 wherein the at least one wherein the at least one filament and at least one battery are an array of filaments and batteries each contained in a leak proof vacuum compatible container, together defining an array of vapor deposit units.

4. The method as in claim 3 wherein the array vapor deposit units is a hexagonal array.

5. The method as in claim 4 wherein the hexagonal array of vapor deposit units comprises a plurality of vapor deposit units chosen to assure a desired coating uniformity.

6. The method as in claim 3 wherein the substrate is a mirror.

7. The method as in claim 6 wherein the mirror has a diameter of at least one meter.

8. The method as in claim 6 wherein the mirror has a diameter of at least eight meters.

9. The method as in claim 8 wherein the deposition takes place in outer space.

10. The method as in claim 8 wherein the deposition takes place in a vacuum chamber.

11. A system for producing thin film coatings on a substrate by vapor deposition comprising: A) At least one filament wetted with a filament wetting material selected from a group filament wetting materials consisting of aluminum and a silver alloy; B) a substrate positioned in a vacuum chamber or outer space; and C) at least one battery contained in a leak-proof vacuum compatible container containing the battery and a circuit board programmed to control energy applied to the filament by the battery so as to heat the filament to a temperature in excess of the boiling point or the sublimation point of the wetting material in order to produce a thin film not exceeding 5,000 angstroms on the substrate with a controlled deposition rate.

12. The system as in claim 11 and further comprising mechanical shutter for each of the filaments adapted to precisely start and stop the deposition of the filament wetting material.

13. The system as in claim 11 wherein the at least one filament and at least one battery are an array of filaments and batteries each contained in a leak proof vacuum compatible container, together defining an array of vapor deposit units.

14. The system as in claim 13 wherein the array of vapor deposit units is a hexagonal array.

15. The system as in claim 14 wherein the hexagonal array of filaments comprises a number of vapor deposit units is chosen to assure a desired coating uniformity.

16. The system as in claim 11 wherein the substrate is a mirror.

17. The system as in claim 14 wherein the mirror has a diameter of at least one meter.

18. The system as in claim 15 wherein the mirror has a diameter of at least eight meters.

19. The system as in claim 14 wherein the deposition takes place in outer space.

20. The system as in claim 14 wherein the deposition takes place in a vacuum chamber.

21. The system as in claim 14 wherein the mirror is a part of a telescope defining an optical path and the array of filaments are configured so as to be moved out of the optical path when the telescope is used for observation.

22. The system as in claim 15 wherein the coating is an aluminum coating adapted for reflecting far ultraviolet light in a frequency range of 30 nm to 2500 nm with an average reflectance greater than 90 percent over most of the band

23. The process as in claim 6 wherein the mirror is a part of a telescope defining an optical path and the array of filaments are configured so as to be moved out of the optical path when the telescope is used for observation.

24. The process as in claim 6 wherein the coating is a silver alloy coating adapted for reflecting UV to visible light in a frequency range of 310 nm to 20 microns with an average reflectance greater than 90 percent.

25. The process as in claim 1 wherein the silver alloy comprises trace impurities chosen to permit the silver alloy to wet the filament material.

26. The process as in claim 1 wherein the filament wetting material is a silver alloy and the process vacuum chamber is infiltrated with sufficient nitrogen or oxygen to permit deposition of one or more layers of aluminum nitride or aluminum oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] FIG. 1 is a sketch of a single battery-powered deposition (BPD) unit.

[0068] FIG. 2A shows a concept for a Cassegrain-Telescope integrated with a hexagonal battery-powered coating system in a coating position.

[0069] FIG. 2B shows a concept for a Cassegrain-Telescope integrated with a hexagonal battery-powered coating system in an observation position.

[0070] FIG. 3 is a sketch of a single BPD unit with a mechanical shutter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071] FIG. 1 is a sketch of a single battery-powered deposition (BPD) unit. The unit 1 includes a filament 2 and circuit board 4 and two batteries 6 both contained in leak-proof tube 8. FIGS. 2A and 2B show individual BPD units 1 arranged in a hexagonal pattern 10 In these two drawings, a Cassegrain-Telescope with a primary mirror 12 and a secondary mirror 14 integrated with the battery-powered coating system. In this integration concept, a disc-shaped array of evaporators swings in front of the mirror for coating, and then swings out of the way for observations as shown in FIGS. 2A and 2B. More complicated telescopes with additional small mirror bounces and gratings can be accommodated with single BPD units, which also swing in front of their optics. The optical design of the system must, of course, provide physical space for these devices and motions.

[0072] As describe in detail below with embodiments of the present invention mirrors can be coated in space with important advantages. One other benefit to coating in space is the possibility of removing dust contamination prior to coating. If dust contamination could be removed, a much less scattering optical surface could be created and maintained. Ultra-low scattering mirrors are ideal for extrasolar planet coronagraphy. Significant improvements in system performance may be realized, even if only the smaller mirrors near the image plane are cleaned just prior to coating in space.

[0073] The present invention provides substantial improvements in reflectance and coating uniformity compared to the state-of-the-art, particularly for very large mirrors several meters in diameter. However, the process may be applied to any size mirrors from mirrors as small as a few centimeters (such a 5-cm) to mirrors as large as or larger than eight meters. The present invention utilizes a new battery-powered deposition (BPD) device to apply the coating layer or layers. In preferred embodiments, the reflective layer may be bare aluminum coated in space, an aluminum reflector protected by metal-fluorides and manufactured in a vacuum chamber on earth for subsequent use in space, or a protected and enhanced silver reflector manufactured at a large observatory on a mountaintop on earth. The new processes use one or more evaporation filaments powered by lithium batteries contained in small pressurized vessels within a vacuum chamber, or in the vacuum of space. For large mirrors, many evaporation filaments are desirable to achieve high evaporation rates (for improved aluminum coatings) and for improved coating uniformity, which results in improved waverfront error and improved telescope system performance.

Three Preferred Embodiments

Battery-Powered Coating Process for Mirrors Coated in Space

[0074] Recent developments in battery technology allow small lithium batteries to rapidly discharge large amounts of energy. It is therefore conceivable to power an array of resistive evaporation filaments in a space environment, using a reasonable mass of batteries and other hardware. A battery-powered process, therefore offers the possibility of eliminating the need for a protective metal-fluoride over-coat altogether, which could yield reflectance values approaching 90% in the 90-190-nm region, and substantial reflectance in the EUV region from 30-90-nm. This improvement would be of great benefit to astrophysics. It is a primary objective of the invention described herein, to create a process that delivers exceptionally high deposition rates over very large coating areas by using batteries to power a large array of individual evaporation filaments.

[0075] Another possibility is the ability to change the coating from aluminum to silver once in orbit. For example, aluminum coatings have good FUV and EUV performance for investigating star formation, but are highly polarizing, which is not optimum for looking at reflected light from extra-solar planets. Silver coatings could be applied to mirrors in space using the battery-driven process to create a less polarizing coating for observing reflected energy from extra-solar planets. The ability to change the coating from aluminum to silver, once the telescope is in orbit using a battery-powered coating process, would allow both scientific objectives to be realized with a single large telescope.

[0076] A first example of a preferred embodiment of the present invention is the coating of a future 6-meter primary mirror in the vacuum of space, such as NASA's proposed HABEX space telescope. The main advantage of coating a mirror in space is the bare aluminum coating will reflect energy down to 30-nm wavelengths, whereas an aluminum coating made on earth must be protected with a fluoride-based coating to prevent oxidation. Protected aluminum coatings are limited to a minimum wavelength of about 90-nm because the fluoride over-coats absorb almost all energy below 90-nm. Based on our mathematical model of a single battery-powered deposition device, a six-meter diameter primary mirror (such as a mirror proposed for NASA's HABEX), would require 81 simultaneously energized evaporation filaments per square meter (or 2,289 total for a 6-meter diameter mirror), to yield a coating uniformity of 6.4% peak-to-valley (PTV). This level of uniformity is within the anticipated wavefront error budget of 5-nm RMS (25-nm PTV) for the telescope's primary mirror. The current-state-of-the-art coating technology does not provide a means to rapidly apply an aluminum coating with sufficient speed and precision to meet NASA's current requirements for future telescopes, but Applicant's proposed battery-powered deposition process will meet these requirements. The predicted evaporation rate for this coating process is 133 A/sec, which has been shown in previous research, to be adequate to create high reflectance and low scatter aluminum coatings in the extreme ultraviolet portion of the electromagnetic spectrum (EUV). In a space-based coating operation, the other smaller mirrors in the telescope system must also be coated in space (to use bare, unprotected aluminum with the highest EUV reflectance), with similar processing conditions, and to meet similar reflectance and wavefront requirements.

Battery-Powered Coating Process Applied in Vacuum Chamber for Use in Space

[0077] A second example of a preferred embodiment of the present invention is the BPD coating of a large space telescope mirror (like the HABEX example in the first preferred embodiment) except the large primary mirror is coated in a vacuum chamber on earth, for use later in space. In this case, the battery powered filament evaporation process is again used to obtain the high evaporation rates needed for high reflectance in the FUV portion of the electromagnetic spectrum (between 90-nm and 190-nm), as well as, to achieve an approximately flat coating, which meets the anticipated wavefront requirements of a large, future space telescope. However, in this case, the aluminum-coated mirror is protected from oxidation using additional layers of fluoride materials such as aluminum fluoride, magnesium fluoride, or lithium fluoride and these protection schemes were developed by at NASA-JPL and NASA-Goddard Spaceflight Center (GSFC) (ref., (1) Kunjithapatham Balasubramanian, et. Al Aluminum mirror coatings for UVOIR telescope optics including the far UV. Proc. SPIE 9602, UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts VII, 960201 (Sep. 22, 2015); doi:10.1117/12.2188981. (2) M. A. Quijada, et. Al., Enhanced far-ultraviolet reflectance of MgF2 and LiF over-coated mirrors, Proc. SPIE Vol 9144 9144G-1 (2014).)

[0078] The protective fluoride layers in the NASA recipes need not be deposited with the filament evaporation process and may be deposited with a conventional resistive evaporation process, by electron beam evaporation, or by sputtering. The protective fluoride layers are typically applied very thin (2-nm to 20-nm thick) and have little impact on wavefront error for the telescope system.

Battery-Powered Silver Coating Process for Very Large Telescope Mirrors

[0079] A third example of a preferred embodiment of the present invention is the coating with the battery-powered deposition method of a large ground-based telescope primary mirror with a protected silver coating. An example is the future Giant Magellan Telescope (GMT) currently under design and construction in Chile. This telescope uses seven, 8-meter diameter mirror segments. Currently there is no existing coating technology to produce protected silver coatings for very large ground-based astronomy mirrors that have sufficient reflectance in the visible and UV portions of the magnetic spectrum, like those specified by GMT.

[0080] In this example of a preferred embodiment, hundreds of silver-coated tungsten filaments and hundreds of aluminum-coated tungsten filaments would be energized in groups, with battery power, sometimes in the presence of ionized oxygen or nitrogen species supplied by multiple End-Hall ion sources, to create a multi-layer protected silver coating comprised of protective alternating aluminum nitride and aluminum oxide layers placed on top of a silver reflector. Protective silver coating recipes using aluminum oxide and aluminum nitride as protective layers, are described in previous patents (Adams, Harvey N., Protective coating for surfaces of silver and mirror fabrication. U.S. Pat. No. 3,687,713. 29 Aug. 1972; Wolfe, Jesse D., Durable low-emissivity solar control thin film coating, U.S. Pat. No. 5,377,045, 27 Dec. 1994). For earth-based telescopes astronomers are not concerned with very short UV wavelengths since these wavelengths are almost completely absorbed in the earth's atmosphere. Also In a previous patent (W. H. Colbert, 1946, Method or Process of Evaporating Metals, U.S. Pat. No. 2,413,604) small amounts of alloying metals such as platinum, cobalt or nickel (1% to 10%) were added to a silver melt to make a tungsten evaporation filaments receptive to the molten silver, allowing wetting of the tungsten filament.

[0081] The new protective silver design also utilizes optical interference effects to increase the reflectance in the ultraviolet portion of the spectrum between 320-nm and 400-nm. The coating design may be constructed using only battery-powered filament evaporation processes as follows, with layer thickness adjusted to meet durability and reflectance requirements:

Layer 0. Substrate

[0082] Layer 1. Aluminum oxide (adhesion layer)
Layer 2. Silver (reflector)
Layer 3. Aluminum nitride (protection/UV-blue enhancement)
Layer 4. Aluminum oxide (protection/UV-blue enhancement)
Layer 5. Aluminum nitride (protection/UV-blue enhancement)
Layer 6. Aluminum oxide ((protection/UV-blue enhancement)

[0083] Additional preferred embodiments of the present invention are described below:

Battery-Powered Filaments for Radiant Heating

[0084] Large mirror assemblies often utilize bonded attachments, which have upper temperature limits of less than 100 C. Since it is desirable to coat mirror assemblies rather than bare mirror substrates, a relatively low-temperature coating process is usually desirable. One solution to this dilemma is to quickly heat only the surface of the mirror substrate, rather than the entire mirror assembly. Using many battery-powered heating filaments the mirror substrate may be heated if necessary for subsequent thin film coating deposition.

Battery-powered Filaments for Reduced Outgassing

[0085] Using batteries to power the deposition process allows the power supply (the battery) to be near the evaporation filament, eliminating the need for large copper cables to carry the low-voltage, high current over large distances (from the transformer outside the chamber, to the filaments at various positions inside the chamber). Reduced electrical wiring means less outgassing when the system is energized, hence better quality aluminum. For example, the tungsten filament used in our demonstration experiment required about 4-volts and 250-amps to power it. To coat a large mirror 6-meters in diameter, thousands of simultaneously fired filaments may be required releasing over a megawatt of power for a few seconds! This power requirement is burdensome to achieve in space, or on earth using conventional AC line power, however, this electrical discharge is quite easy with modern lithium batteries (which have greatly improved over the last decade).

Variations

[0086] Persons skilled in mirror coating art will recognize that there are many variations and additions possible to the systems and processes described in detail above. For example; a sublimation box could be powered by batteries with a similar construction to a battery-powered filament device, to create SiO or ZnS films. These dielectric films may be used for protective layer materials or optical materials. The vapor from a sublimation box may be emitted in any direction, like filament evaporation. Oxygen or oxygen ions may be added to SiO films to create SiO2 films. A second example; aluminum coatings applied to large ground-based telescope mirrors often suffer from slightly lower reflectance near the atmospheric UV wavelengths of 320-400-nm and reflectance may be improved by adding more filament evaporation sources to the system. A third example; other elements, compounds, or alloys may wet with various filaments comprised of tungsten, tantalum, platinum, or columbium, or their alloys, and be used in a battery-powered filament evaporation process.

[0087] Therefore, the scope of the present invention should be determined by the appended claims and their equivalence.