CA1037414A - Method for making thin film tungsten-throrium alloy - Google Patents

Abstract

METHOD FOR MAKING THIN FILM
TUNGSTEN-THORIUM ALLOY
ABSTRACT OF THE DISCLOSURE
A thin film tungsten-thorium alloy is fabricated by a method using an electroplating technique. A conductive substrate is situated in a heated plating bath and forms the cathode of the electrical system.
The plating bath comprises a first aqueous solution consisting of WO3, Na3PO4 and H2O. The thin film of tungsten-thorium alloy is formed by concurrently passing electrical current through the heated plating bath containing the substrate and adding a second aqueous solution consisting of Th(SO4)2 and H2O to the first solution at a predetermined rate. In some applications, the W-Th alloy subsequently remains as a layer on the object on which it is deposited. In other applications, it is sub-sequently completely or partially removed from the object. Also, in applications where it is desired to have the W-Th alloy in a single cry-stal state, it is subsequently annealed.

Description

BACKGROUND OF THE INVENTION
1. Field of the Invention This invention is related to a method for making tungsten-thorium alloys and in particular a method for making thin film tungsten-thorium alloys.
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` 2. Description of the Prior Art It is well known in the prior art to use tungsten-thorium alloy as wire filaments for incandescent lamps, displays and the like because it ~,,~, .
can be made in a single crystal state. A thermal compression extruding ; method is used in the prior art for making these devices. I~ requires 1 the providing of a powdery mixture oP the tungsten and thorium ingredi-.. . .
,) ents, concurrently subjecting the mixture to high temperature, circa 2000C., and high pressure conditions, and extruding the mixture under these conditions through a wire forming die. As such, the extruded wire is in a polycrystal state.

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,i..l ~L~374~L4 1 In order to make it a single crystal, the wire is subsequently annealed at an elevated ~, '~.. ',. '., .:

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~ l 1 temperature, e.g. 1800C., for approximately two hours. ~-i ~ 2 As is well known to those skilled in the art, a single ~ 3 crystal tungsten-thorium alloy filament has an extended :; .
life expectancy as compared to a polycrystal one.
The aforedescribed prior art extrusion ' 6 method is not conducive to making thin film W-Th alloys 7 such as thin film plates and sheets, or layers, and 8 particularly thin films of ten microns or less. Thus, 9 for example, it is not conducive to making a thin . , - .:
;~ 10 film W-Th alloy in the form of a protective layer or 11 coating used for anti-thermal, anti-corrosive, and/or ;;`
~;1 12 wear resistance purposes. Furthermore, this prior i 13 art method canno~ be employed for making W-Th alloy 1 14 components used in printed and/or integrated circuits.
It is known in the prior art to make ... ,, i:
il 16 planar, i.e. printed circuit type, pure W filaments by 17 chemical vapor deposition. Because W has a high boil-t 18 lng point, i.e. 5660C., chemical vapor deposition is 19 not particularly suitable for depositing pure W in ~^-these~type applications. Accordingly, a W compound, 21 such as WF6, is used in combination with a suitable 22 reducing agent, e.g. H, to provide the pure tungsten ~ I
~¦ 23 ~deposit at temperatures of 600C. approximately. In j 24 addition, the pure W filament when formed is in a poly-crystal state. As is well known to those amiliar with I 26 the art, pure W cannot be produced in single crystal ; 27 state form. Hence, the polycrystal pure W filament 28 suffers from the defect of short life expectancy.
, 29 It has been suggested to make printed :
circuit type filaments of a W-Th filament by the method 1 31 of chemical vapor deposition. However, because the -,` EN972045 -2-.. ~.

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1 boiling point of Th is also relatively high, i.e. 4790C., this method is not suitable for these applications. Moreover, the system would require that the W and Th each be compounded with other elements and a suitable and compatible reducing agent be utilized. Thus, the system complexity is increased and thereby the system's reliability is decreased. Hence, the use of chemical vapor deposition for making thin film W-Th alloy bodies is not deemed practical. ;
SUMMARY OF THE INVENTION
It is an obiect of this invention to provide a method of making single crystal or polycrystal thin films of tungsten-thorium alloys.
Another object of this invention is to provide a method for making ~ thin film of W-Th alloys as incandescent filaments.
', Still another object of this inve~ntion ~s to prov~de a method o~
making thin film W-Th alloys as components for printed c~rcuits and/or . integrated circuits~
;. Still another object of this invention is to provide a method for , making thin film of W-Th alloy bodies and in particular bodies which i ... . .
are substantially planar such as plates or sheets and the like.
Another object of this invention is to provide a method for making thin film of W-Th alloy bodies as a coating or layer for an object and . . . .
in particular protecti~ve coatings used for anti-thermal, anti-corrosive and/or wear resistance purposes.
The method~for making thih films of tungsten-thorium alloy according to the invention ~
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1 includes providing a plating bath comprised of a first

2 aqueous solution of WO3, NaPO4 and H2O at a predeter-

3 mined temperature. The bath has immersed in it prede- -.;"

4 termined cathode and anode electrode means. The cathode ~
and anode electrode means are energized to provide a `
6 predetermined current density for a predetermined time .. ; . ~ .
period. During the energizing, another aqueous solution ~;i 8 of a predetermined concentration of Th(SO4)2 is added ' 1 g to the bath at a predetermined rate to produce plating --' 'i .
of the thLn film tungsten-thorium alloy on an appro-11 priate substrate which may be part of the cathode elec-,;
12 trode means.
13 The foregoing and other objects, ea-14 tures, and advankages oE khe invention will be appar-ent from the following more particular description of `
16 the preferred embodiments of the invention, as illus-17 trated in the accompanying drawing.
18 DESCRIPTION OF THE DRAWING `~
19 FIG. 1 is a perspective view of a member ~ .. , . ~ ., used to apply thereto a thin film W-Th alloy according 21 to the method of the present invention;
~; 22 ~ FIG. 2 is a perspective view of the W-Th ~ 23 alloy layer or coating applied to the member of FIG. l;
li ~ 24 FIG. 3 is a perspective view of a thin ¦ 25 film W-Th alloy body after the member of FIG. 1 has :, 26 been removed from the structure of FIG. 2;
27 FIG. 4 is an enlarged partial per~qpcc-' 28 tive view of another member useful in applying thereto ;~, 29 a W-Th alloy ac~ording to the method of the present ¦ 30 invention:

31 FIG. 5 is an enlarged partial front view il.
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037~:14 1 of the member of FIG. 4 at a different stage of its formation; -FIG. 6 is an enlarged partial cross-sectional view of the member of FIG. 5 taken along the line 6-6 thereof; -.
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~ FIG. 7 is an enlarged partial cross-sectional view of a thin f;lm ! "
of W-Th alloy formed on the member of FIGS. 5 - 6 according to the method ~i of the present invention; and FIG. 8 is an enlarged front view, shown partially in schematic, of a W-Th alloy filament display of FIG. 7. -,.. ~ .~
; In the figures, like elements are designated with similar reference `
numerals. ;
.i'' DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method for making a thin film tungsten-thor~um alloy according to the present invention broadly contemplates electrodeposit~ng, i.e. ,~
'; electroplating, a W-Th alloy by pass~ng an electric current through a ,, heated bath consisting essentially of an aqueous solution of W03 and Na3P04 and H20 is in the ratio of the order of about 35:100:200 parts ~;
by weight, respectively. A 10% concentration of Th(S04)2 ls used as a second aqueous~solution. The bath is heated to a temperature of about i 80C.
The W-Th a~lloy is deposited on a substrate which is immersed in the bath. The substrate~is conductive and may be temporary or partially ~ temporary dependlng on the particular ,,,~, ,' :' ':'.. '' ` : .' ' :"
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1 application. For example, in the application where the W-Th alloy is to be used as a protective type coating such as a thermal, anti-corrosive and/or anti-abrasive type for an object, the object would ,~ ~
be immersed in the bath. As such, the object acts as the substrate ~-- and is of the permanent type. In an application, for example, where ;`
an W-Th alloy body is required such as W-Th sheet it can be deposited on a planar substrate which substrate is subsequently removed by, for example, a suitable etchant or other means. Since the substrate is '1 conductive, it also acts as the cathode electrode of the electroplating system.
;, Accordingly, referring to FIG. 1, member 1 is a planar conduc-tive body of, for example, Cu. Assume, for example, it is desired to coat surface la of member I w~th a thin film of W-Th alloy. An "ql ., approprlate anti-plating masking process is used to mask the opposite .1 surface and edges of member 1. In addit~on, a suitable lead-~n con-ductor is attached to the member I ZZn non-obstructing relationship with surface la so that member 1 will also serve as the cathode electrode. Thé anode electrode, not shown, and cathode electrode are immersed in the~aforedescribed heated bath and connected to a suitable i 20 electrlcal supply that passes the plating current through the bath at the same time,~ i.e.~concurrently, the Th(S04)2 aqueous solution is belng added. As a~result, the W-Th alloy coating 2, cf. FIG. 2, is -deposited on the surface la of member 1.
In certain~applications the member 1 is retained after the elctro-, ~
~ depositing such as, for example, the aforementioned applications where ` i ~
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;, 2 However, if it is desired in certain ap-;, 3 plications to provide a body, such as flat sheet of the ~' 4 W-Th alloy, the member 1 is removed by any suitable means such as etching with a compatible etchant that ;
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-1 6 removes the member l without adverse efects to the l 7 W-Th alloy. For the yiven copper substrate example, the :~ ! 8 etchant may, for example, be FeC13. As a result, the ~ g W-Th alloy member 2, cf. FIG. 3, becomes an independent ''.. ~: 1 . .
~;1 10 body which is substantiaIly rigid and of high strength I ll even at thicknesses of 10 microns or less.
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,~ 12 ¦ In aertain applications the W-Th alloy 13 is annealed after the electrodepositing process. On~
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~1 14 such application is for use o~ the W-Th alloy as an incandescent filament where annealing changes the W-Th `¦l 16 alloy to a single crystal state. The annealing may be i ¦ 17 done with or without the member 1 being attached to the 18 W-Th alloy film 2. Preferably, the annealing is done ~¦
19 below the melting point of the member 1 when it is at~
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tached to the film 2. Thus, for the given Cu example :-; 21 which has a melting point of 1083C., the annealing 22 temperature~may~be 900C., for example. If the member 23 ~ 1 is not attached, then the film 2 may be annealed at 24 higher temperatures, e.g. 1800C.
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1 suitable ceramic for this purpose is A1203. Imbedded in the ceramic member are an array of conductive posts 5 of a suitable material.
Preferably, posts 5 are high temperature materials such as Au.
. Alternatively, posts 5 may be of W. The posts 5 may be formed, for , . .
example, by drilling holes in the ceramic member 4 at spatial ,,:i .
locations corresponding to the desired post array. Next, the insides ~; and edges of the holes so formed are electroplated with gold using well known sensitizing and electroplating techni!ques.
Substrate 3 also includes a conductive layer 6 which is different from the materlal of the posts 5. Preferably the layer 6 is copper.
It is formed on the face of the ceramic member 4 by printed circuit deposition techniques. The exposed surface of the layer 6 is co-planar with the end surfaces of the posts 5 which protrude above the front surface 4a, as viewed ln FIG. 4. This can be done, for example, by ~; controlling the thickness of the layer 6 during its deposition and/or by subsequent lapping.
Referring now to FIGS. 5 and 6, in the preferred embodiment, the copper layer 6 is next processed using well known printed circuit techniques to form a composite conductive pattern 6' made up of individual conductive segments formed between mutually-exclusive pairs of posts 5. One such technlque which is preferred is to form the pat-tern using photorçslst methods. The composite pattern corresponds to the pattern desired for the W-Th alloy filament to be formed on the substrate 3 and more~particularly on the members S, 6' thereof. The ~`
pattern 6' comprises a rectangular "'. ~ .
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lO~i ` . -1 array of individual segments each of which is s~uare 2 wave-shaped in form and onLy one segment of which is 3 shown in FIGS. 5 and 6 for sake of clarity. By way `
4 of example, a typical array includes 35 such segments - ;
arranged in a 5x7 matrix as will be apparent herein- ~;
6 after. Preferably, the ends of post 5 protruding out- `
7 wardly from the rear surface 4b of member 4 are coated 8 with a suitable masking material to prevent depositing 9 of the W-Th alloy thereat in the subsequent electro-plating, i.e. electrodepositing, process. ~, 11 In accordance with the principles of the ;!~ ' . j ~ ~ . ' ,i .
~ 12 present invention, an aqueous solution o WO3, Na3PO~, ;;
'1 13 and l12O is provided as part of the plating bath. The ~l 14 bath is heated to an elevated temperature o~ approxi-;l 15 mately 80C. Immersed in the bath is the substrate 3 16 of FIG. 5. The copper segments 6' and gold posts 5 1 17 act as the cathode of the electroplating system. A
i 18 platinum anode is also immersed into the solution and ;; :
`', ~ 19 a plating current density of approximately 160 milli- .
amperes per square inch is conducted between the elec- ;~
21 trodes and through the bath. An appropriate electrical 22 ~ supply is~provided for this purpose. A plating time ;~!'' , '', 23 ~of between~;l5 to 30 minutes is utilized. During the 24 plating process~an aqueous solution having a concentra-tion of about 10% of Th(SO4)2 is added to and is part ~;l 26 of the plating bath at the rate oE approximately 20 27 drops per minute. As shown in phantom outline, the 28 tungsten-thorium alloy film 7 is deposited at the com-29 pletion of the plating process on the exposed copper pattern 6' and exposed gold end surfaces of -th~ posts 1 31 5 of the substrate 3 and thus, has the desired display :~
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After the layer 7 has been deposited and the composite structure of elements 4, 5, 6' and 7 is removed from the bath, an etching process takes place with an etchant that attacks only the conductive segments .. : .
6' but does not affect the ceramic member 4, W-Th alloy film 7, or ~ ~
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posts 5. For the preferred choice of materials, to wit: Cu, A1203 and Au, an etchant such as the aforementioned FeC13 is preferred. As a result, the conductive segments 6' are removed and the W-Th filament pattern 7 is affixedly supported to the posts 5 as shown in FIG. 7.
Subsequently, the film 7 is annealed to place the film 7 in a ; single crystal state.
In one way of annealing, a temperature is utilized which is below the temperatureassociated with the material of the members 4, 5 and 7 having the lowest melting polnt. For the preferred materials of , A1203, Au and W-Th, a suitable annealing temperature is 900C., for ; example.
, A preferred way of annealing the W-Th filament pattern 7 is to pass ;
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~` an electrical current through it while it is located in a vacuum or inert atmosphere. The relative high resistance W-Th generates heats ~ .
and incandesces. The W-Th is thus at a very`high temperature which is !
controlled by the amount of current being passed. Preferably, the `~
~ temperature is about 1800C. Ev,en though the material of posts 5 may ,~; have a melting point below this temperature, it will not be adversely i~ affected if it is judiciously selected to have a low resistance, i.e.
be a good conductor such ~::, '''''' ., ~, , .
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1 as gold, so that the heat generated therein ky the .
2 current passing through the posts 5 is at a temperature 3 well below their melting point~ This operation may be . ` .
; 4 performed prior to mounting the W-Th filament pattern 7 in its associated display housing, or alternatively .:~
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: 6 may be performed after it is mounted in the housing -. 7 and the housing is evacuated~
,,: .. , .:, 8 After annealing, a suitable printed : :
. 9 circuit conductive pattern may be provided on the rear .~
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' 10 surface 4b of the ceramic member 4 to provide lead-in ... .
11 electrode connections to the individual filament seg-.: 12 ments located on the front face 4a, c~. FIG. 8. When . .
13 incorporated in a suitabIe evacuated display housing, 14 the ~ilament segments via their respective electrode j. .
''''' .i , l 15 connections are selectively energized to provide the .`
j 16 desired chaxacter to be read out. In FIG. 8, the ,,,j ~
i. 17 re~erence numbers S' indicate the W-Th alloy plated end ` .
18 surfaces of the posts 5 for sake of clarity.
19 An example of the plating bath suitable 20~ for use in:the present method, is as follows: -21 ~ WO3~ ~ 35 grams -~ :
22 ~ 3po4 100 grams .., ~
; 2~3 ~ H2O~ 200 grams 24 In one such thin film W-Th alloy built :; :
:! 25 in accordanae with the principles of the present inven- . .
~ 26 tion, the ~ilm was approximately 4 microns thick.
;.~ 27 It should be understood that alterna- .

:~ 28 tively a co-extensive W-Th alloy thin film can be de-i 29 posited directly on the exposed sur~ace 6a of layer 6 `1 30 and the exposed co-planar end surfaces of posts 5 of ~`l 31 substrate 3 shown in FIG. 4 in accordance with the :
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l principles of the inventive method. After the W-Th 2 film is deposited, the pattern 7 is formed using printed 3 circuit techniques. Preferably, a conventional photo-4 resist is applied to the exposed surface of the deposited W-Th film. Photoresist is exposed through a mask con- -6 taining the desired conductive display pattern and `
7 developed. Thereafter, subsequent etching processes are ; 8 employed to remove the unwanted portions of the W-Th 9 alloy film and to remove the temporary copper layer 6.
A mixture o~ K Fe(CN) and NaOH is a suitable etchant ll for removing the W-Th alloy and FeC13 is a suitable 12 etchant ~or removing the copper layer 6. Thereaeter, ~' 13 the remaining W-Th ~ilament pattern i~ annealed to 14 place the W-Th alloy in a single crystal state. As a result, a W-Th alloy ~ilament display identical to the 16 one shown in FIGS. 7 - 8 is provided.
i~, 17 Thus, it has been demonstrated from the 18 foregoing description that the W-Th alloy in some ap-~, 19 plications remains on the object on which it is deposited, ;
~''''.'! 1 20 cf. FIG. 2. In other applications, it is completely re-21 moved from the ob~ect, cf. FIG. 3. Still in other ap-22 plications, it is only partially removed, cf. FIGS. 7 - 8.
23 However, while the invention has been ~ 24 particularly shown and described with reference to pre-;~ 25 ferred embodiments thereof, it will be understood by ;
26 those skilled in the art that various changes in form 27 and details may be made therein without departing from ~l 28 the spirit and scope of the invention.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of electrodepositing thin film of W-Th alloy on a conductive substrate, said method comprising the steps of:
passing an electric current through a heated plating bath containing the substrate, said substrate forming the cathode of the electrical system used to pass said electrical current, said bath comprising a first aqueous solution consisting of WO3, Na3PO4 and H2O, adding, concurrently to passing said current through said bath, a second aqueous solution to said bath at a predetermined rate, said second solution consisting of Th(SO4)2 and H2O.

2. The method of claim 1, wherein the WO3, Na3PO4 and H2O of said first solution are in the ratio of the order of about 35:100:200, respectively, by weight, and said second solution has a 10% concentration of Th(SO4)2.

3. The method of claim 2, wherein said bath is at a temperature of about 80°C.

4. The method of claim 3, wherein said electric current has a current density of about 160 milliamperes per square inch.

5. The method of claim 1, further comprising the subsequent step of annealing said thin film of W-Th alloy to place said alloy in a single crystal state.

6. The method according to claim 2, wherein said predetermined rate is about 20 drops per minute.

7. The method according to claim 1, wherein said current is passed through said bath for a predetermined time period of not more than 30 minutes or less than 15 minutes.

8. The method according to claim 1, comprising in combination there-with the subsequent step of separating the W-Th film from said substrate.

9. The method according to claim 1, comprising in combination there-with the subsequent step of etching the substrate to separate the W-Th film therefrom.

10. The method of making a single crystal thin film of W-Th alloy, said method comprising the steps of:
providing a first aqueous solution consisting of WO3, Na3PO4 and H2O in the ratio of the order of about 35:100:200, respectively, by weight, heating said bath to maintain it at a temperature of about 80°C., providing a second aqueous solution consisting of Th(SO4)2 and H2O, said second solution having a 10% concentration of Th(SO4)2, adding said second solution to said first solution at the rate of about 20 drops per minute to provide an electroplating bath, immersing predetermined anode and cathode electroplating means in said bath, said cathode electrode means comprising a substrate on which the W-Th film is to be formed, passing an electric current having a current density of about 160 milliamperes per square inch between the two said electrode means and through said bath to electrodeposit said thin film of W-Th alloy on the substrate, said steps of passing and adding being performed concurrently, removing the substrate having thin film deposited therein, and subsequently annealing said thin film of W-Th alloy at a temp-erature of about 1800°C. to place said W-Th alloy in a single crystal state.

11. The method according to claim 10, further comprising the step of:
removing at least Cart of said substrate prior to the step of annealing.