Method and apparatus for coating thin foil with a boron coating

Abstract

An apparatus and a process is disclosed for applying a boron coating to a thin foil. Preferably, the process is a continuous, in-line process for applying a coating to a thin foil comprising wrapping the foil around a rotating and translating mandrel, cleaning the foil with glow discharge in an etching chamber as the mandrel with the foil moves through the chamber, sputtering the foil with boron carbide in a sputtering chamber as the mandrel moves through the sputtering chamber, and unwinding the foil off the mandrel after it has been coated. The apparatus for applying a coating to a thin foil comprises an elongated mandrel. Foil preferably passes from a reel to the mandrel by passing through a seal near the initial portion of an etching chamber. The mandrel has a translation drive system for moving the mandrel forward and a rotational drive system for rotating mandrel as it moves forward. The etching chamber utilizes glow discharge on a surface of the foil as the mandrel moves through said etching chamber. A sputtering chamber, downstream of the etching chamber, applies a thin layer comprising boron onto the surface of the foil as said mandrel moves through said sputtering chamber. Preferably, the coated foil passes from the mandrel to a second reel by passing through a seal near the terminal portion of the sputtering chamber.

Claims

1. An apparatus for applying a coating to a thin foil by a continuous, in-line process, the apparatus comprising: a first elongated, cylindrical mandrel, said first elongated, cylindrical mandrel being movable in a longitudinal direction; a foil input reel containing a roll of thin foil, said foil input reel supplying thin foil for winding onto said first elongated, cylindrical mandrel; a drive system for advancing said first elongated, cylindrical mandrel longitudinally via translation movement and for rotary translation of said first elongated, cylindrical mandrel, wherein the translation movement comprises longitudinally moving in a linear direction said first elongated, cylindrical mandrel sequentially through a first transition chamber, an etching chamber, a sputtering chamber, and a second transition chamber; the etching chamber for glow discharge cleaning and conditioning a surface of the thin foil as said first elongated, cylindrical mandrel moves longitudinally through said etching chamber; the sputtering chamber having a target comprising boron carbide for sputtering a thin layer comprising boron onto the surface of the thin foil as said first elongated, cylindrical mandrel moves longitudinally through said sputtering chamber; and a coated foil uptake reel for removing the thin foil from said first elongated, cylindrical mandrel after the thin layer comprising boron has been applied.

2. The apparatus of claim 1, wherein the thin foil comprises a copper foil.

3. The apparatus of claim 1, wherein the thin foil comprises an aluminum foil.

4. The apparatus of claim 1, wherein thin foil has a width of between about 0.25 and about three inches.

5. The apparatus of claim 1, further comprising a second elongated, cylindrical mandrel, said second elongated, cylindrical mandrel having a vacuum sealable coupling for disconnectably coupling said second elongated, cylindrical mandrel to said first elongated, cylindrical mandrel.

6. The apparatus of claim 5, wherein each mandrel has a coupling on one end and another coupling on the opposite end allowing the first elongated, cylindrical mandrel and the second elongated, cylindrical mandrel to be connected together from either end.

7. The apparatus of claim 5, wherein each mandrel has a male screw connection on one end and a female screw connection on the opposite end allowing the first elongated, cylindrical mandrel and the second elongated, cylindrical mandrel to be connected together from either end.

8. The apparatus of claim 1, wherein said first elongated, cylindrical mandrel comprises aluminum pipe.

9. The apparatus of claim 1, wherein said first elongated, cylindrical mandrel comprise nominal 4 inch OD pipe.

10. The apparatus of claim 1, wherein said first elongated, cylindrical mandrel comprises a wall thickness of about 0.25.

11. The apparatus of claim 1, wherein said first elongated, cylindrical mandrel is about 24 feet in length.

12. The apparatus of claim 1, wherein said drive system comprises a winch operably connected to said first elongated, cylindrical mandrel for translational movement.

13. The apparatus of claim 1, wherein said drive system comprises a high tension belt drive for rotational movement.

14. The apparatus of claim 1, wherein said drive system comprises an electric motor operably coupled to said first elongated, cylindrical mandrel for rotational movement.

15. The apparatus of claim 1, further comprising a control system for controlling rotational speed of said mandrel as a function of the speed of translation of said mandrel.

16. The apparatus of claim 1, wherein said foil input reel comprises drag torque control.

17. The apparatus of claim 1, wherein said sputtering chamber is configured to sputter a thin adhesion layer onto the thin foil prior to sputtering of the thin layer comprising boron.

18. The apparatus of claim 17, wherein said sputtering chamber is configured to sputter a thin adhesion layer comprising a metallic transition layer.

19. The apparatus of claim 17, wherein said sputtering chamber is configured to sputter a thin adhesion layer comprising chrome.

20. The apparatus of claim 1, further comprising the first transition chamber, prior to said etching chamber, through which said first elongated, cylindrical mandrel moves longitudinally, said first transition chamber for transitioning from atmospheric to vacuum pressure.

21. The apparatus of claim 20, wherein said first transition chamber comprises a secondary transition chamber for receiving and containing inert flowing gas maintained at a positive pressure relative to atmospheric pressure.

22. The apparatus of claim 21, wherein the inert gas is argon.

23. The apparatus of claim 20, wherein said first transition chamber comprises a plurality of energized seals.

24. The apparatus of claim 1, further comprising the second transition chamber, after said sputtering chamber, through which said first elongated, cylindrical mandrel moves longitudinally, said second transition chamber for transitioning from vacuum to atmospheric pressure.

25. The apparatus of claim 24, wherein said second transition chamber is of the same design as the first transition chamber.

26. The apparatus of claim 1, wherein said foil input reel is connected to said etching chamber allowing the thin foil to pass through an open flexible vacuum bellows into the etch chamber.

27. The apparatus of claim 1, wherein said foil input reel comprises pulleys for guiding the thin foil into and out of said etching chamber.

28. The apparatus of claim 1, wherein said coated foil uptake reel is connected to said sputtering chamber allowing the thin foil to pass through an open flexible vacuum bellows into the sputtering chamber.

29. The apparatus of claim 1, wherein said coated foil uptake reel comprises pulleys for guiding the thin foil out of said sputtering chamber.

30. The apparatus of claim 1, further comprising a flow restrictive chamber between said etching chamber and said sputtering chamber allowing said etching chamber and said sputtering chamber to operate at different pressures.

31. The apparatus of claim 1, wherein said sputtering chamber sputters a boron layer between about 0.1 and 3 microns thick.

32. The apparatus of claim 1, wherein said sputtering chamber further comprises a target comprising boron.

33. The apparatus of claim 1, wherein said thin layer comprising boron comprises 99% or greater pure boron carbide.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

(1) A better understanding of the invention can be obtained when the detailed description set forth below is reviewed in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a process flow diagram of an embodiment of the process for coating thin foil with a boron carbide coating;

(3) FIG. 2 is a depiction of an embodiment of the apparatus for coating thin foil with a boron carbide coating;

(4) FIG. 3 A-C depict an embodiment of a male mandrel coupling, a female mandrel coupling, and the male coupling of a first mandrel being connected to the female coupling of a second mandrel, respectively;

(5) FIG. 4 is a depiction of an embodiment of a rotational drive system such as may be used with the apparatus;

(6) FIG. 5 is a depiction of an embodiment of an input reel for use with the apparatus;

(7) FIG. 6 is a depiction of an embodiment of a transition chamber for use with the apparatus;

(8) FIG. 7 is a depiction of an embodiment of that portion of the apparatus wherein foil is fed into the chamber for wrapping around the mandrel;

(9) FIG. 8 is a depiction of an embodiment of an etching chamber for use with the apparatus;

(10) FIG. 9 is a depiction of an embodiment of a sputtering chamber for use with the apparatus;

(11) FIG. 10 A/B is a depiction of an embodiment of the inside, and the outside, respectively, of a partially constructed sputtering chamber for use in the apparatus; and

(12) FIG. 11 is a depiction of an embodiment of the coated foil uptake reel for use in the apparatus.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

(13) In a preferred embodiment of the present invention, a continuous reel to reel process is utilized for application of a boron coating onto the surface of a foil. The continuous process preferably comprises drawing the foil from a reel housed inside the vacuum system onto a rotating and translating mandrel, cleaning the foil with glow discharge in an etching chamber as the mandrel with the foil wrapped around it moves through the chamber, preferably applying a thin metallic adhesion coating of preferably Chrome by physical vapor deposition (PVD), then applying a boron coating, preferably boron carbide, by PVD as the mandrel moves through the PVD chamber, and finally unwinding the foil off the mandrel onto another reel also inside the vacuum system after it has been coated. A preferred embodiment of the process of the present invention can run continuously, twenty-four hours a day, for long periods of time limited only by exhaustion of PVD targets or input foil.

(14) As shown in FIG. 1, the process begins with the step 10 of providing a mandrel 12. The mandrel 12 is preferably prepared from a long section of aluminum or similar pipe having a diameter of between about 3 to 8 inches. In a preferred embodiment, the mandrel is approximately 24 feet in length. A coupling is attached to each end of the mandrel 12, preferably (See FIGS. 3A-C) a male screw connection 14 on one end and a female screw connection 16 on the opposite end. The couplings allow a second mandrel 12 to be connected to the first mandrel 12. Preferably, an O-ring is provided with connection 14 to provide a vacuum seal between the mandrels. As the first mandrel 12 passes out of the process, it can be disconnected, cleaned and reconnected to the opposite end of the second mandrel 12. This allows the process to proceed continuously.

(15) A drive train 24 rotates and translates the mandrel pair continuously through the process. This is preferably accomplished by driving a steel belt wrapped around the mandrel under tension and positioned at an appropriate non-orthogonal orientation providing both linear translation and rotation. In one embodiment of the invention, the drive train 22 comprises separate rotational and translational drive systems. Alternatively, the rotational drive system can be an electric motor in rotational engagement with the mandrel 12. The translation drive system 26 is preferably an automated winch system that can removably connect to the mandrel to advance the mandrel through the process.

(16) A transitioning chamber 28 (See FIG. 6) is utilized to transition the mandrel as it advances from atmosphere pressure to high level of vacuum that exists in the chamber 52. The transition is preferably achieved through energized seals which preferably utilize steel springs and Teflon or using Teflon coated soft O-ring seals. Multiple seals may be utilized to provide the sealing quality required for the transition chamber and the overall vacuum tightness of the system. In one embodiment, a second transition chamber is included sealed with similar seals and an inert gas, such as argon, is streamed through this chamber while maintaining positive pressure relative to atmosphere to prevent air and humidity contamination.

(17) The next step 30 of the process involves providing an input reel 32 containing a roll of foil 34. The foil 34 may be a thin metal, such as copper or aluminum, or a plastic material. Preferably, the foil 34 has a width of between about 0.25 and about three inches. The foil input reel 32 (See FIG. 5) is preferably housed in a large chamber connected through a large pass through port to the etching chamber. This input reel chamber preferably operates at the same pressure as the etching chamber. As mandrel 12 rotates, foil is pulled over a set of roller guides from the input reel 32 and winds on the mandrel 12 (See FIG. 7). The tension of the foil 34 entering the etching chamber 52 is preferably set by application of controlled torque to the reel. Preferably, torque is passed from a torque motor outside the vacuum into the vacuum through a ferrofluidic rotary seal.

(18) As the wrapped foil 34 advances, it enters the surface conditioning or etching step 50. The etching chamber 52 (or conditioning chamber) is used to clean the surface of the foil 34, preferably with glow discharge bombardment of Argon ions, before sputtering. (See FIG. 8). This cleaning step is important in achieving high quality coating adhesion quality of the sputtered materials occurring in the following chamber.

(19) In a preferred embodiment of the invention, a flow restrictive chamber (See FIG. 2) is utilized in between etching chamber 52 and sputtering chamber 62. Flow restrictive chamber 58 introduces a thin, lengthy annular gap allowing the etching chamber 52 and sputtering chamber 62 to operate at different pressures.

(20) Once the foil 34 has been conditioned properly in the etching step 50, the foil 34 enters the sputtering step 60. The second process chamber 62 is a magnetron sputter system (See FIG. 9). In this chamber 62, a thin adhesion layer, preferably chrome, is sputtered onto the foil. Following the application of the adhesion layer, a boron coating, preferably a boron carbide coating, is sputtered onto the adhesion layer. In a preferred embodiment, a Baratron system regulates the pressure in the sputter chamber by leaking high purity Argon gas into the system. At the same time, a high capacity turbo molecular vacuum pump pumps the chamber. By this manner, a very high purity Argon environment is achieved at the precise optimal pressure to support magnetron sputtering. In this process Argon is ionized and driven into the boron carbide target with sufficient energy to release B.sub.4C molecules from the surface of the cathode. Preferably, a cathode surface area of about 3.519.5 area is provided and a plasma power level of about 5000-7000 Watts supplying sufficient sputtering rate to achieve a product output rate of >3.7 m.sup.2 per 24 hour operation time. Levels of contaminant gas molecules such as O2, N2, and H2O are preferably maintained below about 1% of the Argon gas pressure in order to achieve low contaminations levels of less than 1% in the boron carbide coating when used in boron straw neutron detectors.

(21) As shown in FIGS. 10A and 10B, the sputtering chamber 62 preferably has a cylindrical shaped interior 64 with one or more rectangular openings 66 in the horizontal side portion thereof.

(22) One advantage of the laying out the process as this series of long cylindrical chambers (See FIG. 2) is that the process can be easily disassembled for cleaning and maintenance. Also, the volume of the vacuum chamber that must be pumped and outgassed is considerably reduced, preferably to less than 10 ft.sup.3 which is classed as a small chamber by industrial standards. Cleaning and maintenance of the system can be accomplished in less than 6 hours of down time per week of continuous operation.

(23) As will be recognized by those of skill in the art, the speed of translation of the mandrel through the sputtering chamber 62, together with the power level on the chrome and boron carbide cathodes determines the coating thicknesses of these two films on the foil 34. The uniformity of the critical coating thickness of the B.sub.4C is preferably controlled to better than 0.1% by precise regulation of the cathode power level using an Advanced Energy regulating DC supply and the mandrel drive speed is preferably controlled to much better than 0.1% with a precision DC motor whose speed is monitored and controlled, preferably by a live time LabView program running on a powerful PC with backup power system. For any given foil width, the rotational speed of the mandrel is set as a precise ratio to the translation speed in order that the foil covers the mandrel with minimal gap and does not overlap. The LabView system can perform this task by controlling the rotational motor to turn at the precise fraction of the speed of the translation motor. In both cases the actual speed of rotation as detected by a motor transducer is controlled.

(24) Once the foil 34 has been coated, the next step 80 is to unwind the coated foil from the mandrel 12. A coated foil uptake reel 72 is provided for this function (step 70). As shown in FIG. 11, the take up reel 72 is of similar design and construction as the input reel 32. Preferably, the foil 34 is removed from the mandrel 12 prior to exiting the sputtering chamber 62. An exit reel chamber 68 connected to the sputter chamber vacuum in this case, of similar construction to the entry reel chamber 28, is preferably utilized receive coated foil. The reel 72, in this case, draws foil from the mandrel 12 as a result of torque applied with a torque motor. Preferably, torque is passed from torque motor into the vacuum through a rotary ferrofluidic seal. The bare mandrel 12 then passes out of the vacuum through similar set of transition seals as used on the input.

(25) After the foil has been removed from mandrel 12, the first mandrel 12 continues to advance through the system until coupling 16 is exposed, at this step 90, the first mandrel 12 can be disconnected from the system. Preferably, mandrel 12 is then cleaned and reconnected to coupling 16 of the second mandrel 12 which is advancing through the process. In this manner, the process can continue indefinitely without interruption.

(26) As will now be recognized by one of skill in the art, the process and apparatus of the present invention have significant economic advantages over the prior art systems. The embodiment of the invention, as was reduced to practice, produced significantly reduced cost of manufacturing boron-coated straw detectors by eliminating significant set up and down time costs through the continuous operation of the process. This embodiment produced enough coated foil to produce as much as 439 meters of straw per day, thereby reducing the cost of the coating process from about $58 per meter of straw to roughly $10 per meter of straw. As demonstrated in the table below, this equates to significant cost savings for detector products that utilize boron-coated straws.

(27) TABLE-US-00001 Cost Benefit Coated Time to area coat w/2 Detector Number of required cathodes In-house Contract name Detector description straws (m.sup.2) (hrs) Coating coating BRD Backpack, equivelant to 8 31 1.14 4.15 $1,244 $4,275 2 3He-tubes NDM-95 Portal, equivelant to 1 95 2.64 9.60 $2,880 $9,900 3He-tube NDM-171 Portal, equivelant to 2 171 4.76 17.3 $5,193 $17,850 3He-tubes VMDS Ultra-light sensitivity 300 8.6 31.3 $9,382 $32,250 vehicle mounted system HLNCC-II Multiplicitity/ 650 5.25 19.1 $5,727 $19,688 AWCC Coincidence counters 1450 11.7 42.5 $12764 $43,875 Imager Imaging panel for 1470 34.6 1.26 $37,745 $129,750 neutron science (1 sq. meter, 5 layers)

(28) Although the invention has been described in reference to its preferred embodiments, those of skill in the art may appreciate from this description various changes and modifications which can be made thereto which do not depart from the spirit and scope of the invention as described and claimed herein.