GB2145872A - Molten metal feed trough for electron beam evaporator crucible - Google Patents

Abstract

A trough 34 is provided in flow communication with the crucible 23 of an electron beam evaporator for the pre-melting of metal 31 supplied to the crucible and a cooled shield 36 is arranged so that molten masses ejected from the trough, e.g. as a result of outgassing of metal entering the melt in the trough, will strike the underside of the shield and not the target 13. <IMAGE>

Description

SPECIFICATION Molten metal feed trough for electron beam evaporator crucible This invention relates to high-vacuum coating systems utilizing electron beam technology, and, more specifically, to the construction of electron beam evaporation sources utilized therein.

In general, these systems comprise a vacuum chamber, wherein are located the devices for presenting the substrate to be coated and the electron beam evaporator. Adjacent thereto, but outside the vacuum chamber are the high vacuum pump system and the requisite valves and controls for the operation of the overall system. Typically, the electron beam evaporator comprises a crucible to contain molten material to be evaporated, the electron beam generator, and a deflection system whereby the electron beam from the electron beam generator is directed to a target area in the crucible. Conversion of the kinetic energy of the accelerated electrons heats up the source material in the crucible until the evaporation temperature is reached to drive off the source material as a vapor.The electron beam itself is generated by heating a cathode filament maintained at a negative high-voltage potential. The capability for deflection of the electron beam by the use of a magnetic field enables location of the cathode, where it is protected from being coated by the vaporized source material.

Such a vacuum coating system is one of those that may be utilized in the practice of the inventions disclosed and claimed in copending U.S. Patent Applications Serial No.

499,019-Lifshin, et al., filed May 27, 1983 and Serial No. 519,280-Grey, Jr. et al., filed August 1, 1983.

In the practice of each of these inventions, copper is deposited directly on a chemically clean aluminum foil carrier surface by vapor deposition while controlling the magnitude of the adhesion between the copper and the carrier surface. The copper coating applied to the aluminum foil carrier in the practice of the aforementioned inventions is most usually less than about 1 6 microns in thickness, and typically is less than about 5 microns in thickness. It is desired that this coating of copper be uniformly thick, continuous, smooth and pinhole-free at about 100 percent theoretical density. Also, under the conditions of copper deposition typically employed, the coating has a discernable columnar structure.The copper film/aluminum carrier combination is a precursor in the production of copper-clad assemblies having special use in the manufacture of high-resolution printed circuit boards.

In copper deposition production runs, it is necessary to evaporate copper from the crucible at high rates and, providing very high purity copper having a very low gas content is employed, the vapor deposition presents no particular problems.

It would be desirable to employ copper of lesser purity (with respect to gas content) in order to reduce the material cost of the copper/carrier product foil. However, when copper having impurities (e.g. significant gas content) is employed, outgassing occurring in the crucible frequently projects small molten masses of the copper out of the crucible. Impingement of such molten masses, typically globules about 50-70 microns in diameter, on the aluminum foil carrier production surface results in certain defects in the resulting laminate.These defects are of two types: if the molten mass of copper strikes the copper surface of already-coated carrier foil, as it cools it shrinks and tears the copper film; if the molten mass strikes uncoated foil carrier, it presents a sizeable lump, which cannot be accommodated on the high-resolution printed circuit boards for the preparation of which the copper/aluminum carrier laminate precursors of the Lifshin et al. and Grey, Jr., et al. inventions are employed.

The device of this invention restricts any outgassing of the melting copper and ejection of molten copper masses within the confines of an enclosure whereby such molten copper can be captured and prevented from impinging on the foil carrier.

The basic crucible constructions employed in electron beam evaporator are provided with a longitudinally-extending trough in flow communication with the crucible well and aligned with automated wire feed means. At least the distal end of the trough is covered with a liquid-cooled shield, or hood, which together with the trough provides a confining enclosure for the molten metal feed in the trough. Solid metal wire feed leaving the wire feed means at a controlled rate passes under the hood, enters the metal in the trough (kept molten by the heat conducted thereto from the crucible) and is melted. The hood is dimensioned in cross-section to be small enough to permit efficient melting and yet large enough to allow outgassing of the melting metal feed and escape of the gaseous products in a harmless manner.Any ejected molten masses of the feed metal impinge on the inside wall of the shield and are condensed. The shield is removed, cleaned and replaced when the need arises.

The present invention will be further described, by way of exampie oniy, with reference to the accompanying drawings, in which: Figure 1 is a schematic view partially in section of a vacuum chamber showing the cruciblewith-trough construction of this invention disposed in operative relationship with the rest of an electron beam evaporator arranged for the vapor deposition of metal on a moving substrate; Figure 2 is an enlarged view in section of the crucible-with-trough shown in Fig. 1; Figure 3 is a sectional view taken on line 3-3 of Fig. 2 and Figure 4 is a sectional view showing a modified construction according to this invention.

Similar elements in the several views are designated by the same numeral.

In the arrangement of Fig. 1, vacuum chamber 11 of the vapor deposition coating system 1 2 contains an arrangement of operating components enabling, for example, the practice of the invention in S.N. 499,019. Aluminum foil carrier sheet 13 moves from payout roll 14 over and in contact with the surface of coating drum 1 6 to be received by takeup roll 17, after the application of an ultrathin film of copper thereto. Liquid coolant (not shown) is circulated within coating drum 1 6 in order to control the temperature of the drum and of foil 1 3 in contact therewith during the deposition process.Other components needed for an operative system, such as vacuum pumps, controls, power sources, means for circulating coolant through the electron beam evaporator, etc., form no part of this invention and are not described.

Electron beam evaporator 20 comprises in its essential parts, the electron source (tungsten cathode filament 21), permanent magnet 22 and graphite crucible 23. Cathode 21 is heated to emit electrons and the beam of electrons so produced is programmably deflected by magnet 22 (and auxiliary permanent and electro-magnets not shown) so that the electron beam is directed in the general path identified by dotted line 24 to strike a predefined target area of the molten metal 26 disposed in crucible 23, comprising a graphite liner in a water-cooled copper support block. The molten metal, in this case copper, in the target area on which the electron beam impinges, is evaporated and replenished by adjacent liquid copper.

The resultant vapor makes contact with foil 1 3 and is deposited thereon in the desired thickness and with the desired strength of bond. Replenishment of the copper to pool 26 is accomplished by the controlied feeding of coppper wire thereto, the wire 31 advancing from spool 32 via motorized wire feed drive mechanism 33, which directs metal feed wire 31 into contact with already-molten copper in the extension 26a of pool 26 contained in trough 34, which is in flow communication with crucible 23.

As is shown in Figs. 1 and 2, solid metal feed in wire configuration passes under liquidcooled shield 36 at a downward angle so that its point of impact with molten pool extension 26a is well under the region covered by shield 36. Upon contacting the molten metal, wire 31 melts and whatever outgassing occurs upon melting (i.e., evolution of dissolved gaseous impurities, such as oxygen, nitrogen, and carbon dioxide and/or decomposition of certain oxide or nitride contaminants) will be under shield 36. Thus, any ejection of molten masses of metal caused by the outgassing will impinge on the underside of shield 36 cooled by the circulation of coolant through coils 36a affixed to shield 36.By the time molten copper moves from the location at which the melting occurs to the target area in crucible 23, where the evaporation occurs, the outgassing effect from impurity content will have been substantially completely eliminated. In order not to alter the magnetic fields established for the desired operation of evaporator 20, shield 36 is made of material, which is not magnetizable, such as austenitic stainless steel.

Tests of one construction have shown that copper could be successfully evaporated at rates as high as 25 grams/minute without encountering the problem of molten copper ejection from the evaporator. In these tests both the crucible and trough were 3/8" (9mm) deep and made of graphite the crucible had an inner diameter of 1-1 /2" (38mm) and the trough (made integral with the crucible) was about 3-1/2" (89mm) long and 3/4" (18mm) wide. The shield was made as a U-shaped hood and was cooled as shown in Fig. 2. Copper condensed on the inner surface of the shield was easily removed, since cooling of the surface prevented formation of an intermetallic.

Typical sizing for the components of this invention is as follows: Trough width. . about 1 /2" (12.5mm) to about 1-1 /4" (32mm) Trough depth... about 1 /4" (6mm) to about 1" (25mm) Trough length . . . about 2" (51 mm) to about 5" (126mm) overall Crucible inner radius... about 3/8" (9mm) to about 2" (51 mm) Crucible inner depth... about 1 /4" (6mm) to 1" (25mm) Crucible inner wall thickness. . about 1 /8" (6mm) to about 3/4" (19mm).

Although the dimensions given above are for a unitary crucible-with-trough structure having (a) a uniform depth, (b) the crucible in the general shape of a right circuiar cylinder and (c) the trough of rectangu-cross-section, other shapes, cross-sectional areas and volumes can be employed so as to provide comparable volumetric relationships between crucible volume and trough volume. In this way the heat capacity of the molten copper in the crucible will maintain copper in the trough in the liquid state.

Fig. 4 shows a flat cooled shield 40 requiring separate support means (not shown) and, as well, the use of slag barrier 41 to prevent the entry of any slag developing in container 34 into crucible 23. Means 42 for circulating coolant in copper support block 43 are shown in part.

Shield 40 is dimensioned (i.e., length and width) so as to block any line-of-sight molten copper ejection from bath 26a to impinge upon aluminum carrier 1 3. The use of a flat plate shield is preferred in apparatus in which more than one evaporator is used and the units are mounted side-by-side covered by a single shield.

The crucible liner may be of graphite or other material inert to molten copper, such as magnesia or alumina. Shield material other than austenitic stainless steel may be used as long as it is non-magnetic and is not deleteriously affected by exposure to the operating conditions during vapor deposition.

The slag barrier should be mounted extending across and flush with the top of the trough or slightly recessed into the trough. Typical dimensions should range from about 1 /4" X 1 /4" (6mm X 6mm) in cross-section to about 1" X 1" (25mm X 25mm) in cross-section.

It may, therefore, be seen that the device of this invention by isolating the melting and outgassing of the metal feed with structure to interrupt any line-of-sight movement of molten copper ejected during outgassing from the copper bath to the substrate to be coated, greatly improves the conduct of the coating operation as well as the quality of the product. Thus, by this expedient it becomes feasible to use metal feed stock of significantly lower purity in respect both to dissolved gas content and to the content of other impurities, which emit gaseous products upon decomposing. Savings can, therefore, be realised both in material costs and in the utilization of capital equipment.

Claims (9)

1. An arrangement for the metered feed of a metal to the crucible of an electron beam evaporator located in a vacuum chamber wherein during operation the metal wire is dispensed at a controllable rate of feed from a source via wire feed drive means and is introduced in the solid state into molten metal collected in said crucible, a longitudinally-extending trough-iike container connected to and in flow communication with said crucible at one end of said container, said container being closed at the opposite end thereof and having volumetric dimensions relative to the volumetric dimensions of said crucible such that the heat capacity of molten copper in said crucible will maintain copper in said container in the liquid state, non-magnetic shielding means in juxtaposition with said container and spaced therefrom a distance at least sufficient to permit metal wire directed thereto by said wire feed drive means to pass under said shielding means and enter said container and means for cooling said shielding means, whereby molten copper masses caused to be ejected from said container by outgassing of metal wire entering the molten copper in said container and melting will impinge upon the underside of said shielding means.

2. An arrangement as claimed in claim 1 wherein the crucible and container are made of graphite.

3. An arrangement as claimed in claims 1 or 2 further including barrier means located over the container near the crucible for preventing slag floating upon molten copper in said container from entering said crucible.

4. An arrangement as claimed in any one of claims 1 to 3 wherein the shielding means is made of austenitic stainless steel.

5. An arrangement as claimed in any one of the preceding claims wherein the shielding means is a flat plate.

6. An arrangement as claimed in any one of claims 1 to 4 wherein the shielding means is Ushaped in cross-section and straddles the container and is spaced therefrom.

7. An arrangement as claimed in any one of the preceding claims wherein the crucible and container are integrally formed.

8. An arrangement as claimed in claim 7 wherein the crucible is circular in plan, has an inner radius ranging from 3/8" (9mm) to 2" (51mm) and is integrally formed with a trough having a width of from 1/2" (12.5mm) to about 1-1/4" (31 mm) and a length from 2" (51mm) to 5" (125mm).

9. An arrangement as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.