Stationary mask for continuous vapor deposition

BACKGROUND OF THE INVENTION

The present invention relates generally to the deposition of $ metal stripes onto moving substrates and, more particularly, to masks used to define the edges of the stripe. While not limited thereto, the invention is particularly adapted to the deposition of an aluminum stripe onto a Kovar (trademark) substrate which is ultimately fabricated into an integrated circuit lead ¹frame. Kovar is an iron-cobalt-nickel alloy having thermal expansion properties similar to glass.

A variety of methods have been used in the past to coat a strip of one metal onto a substrate of another material. Perhaps the simplest and most widely employed method is to coat the entire surface of the substrate, mask the desired area, and etch the unmasked material away. The initial coating step could be accomplished in a variety of ways; vapor deposition, cladding and plating are examples. The coiled, coated strip is jq then wound on a second roll while a strip of etch-resistant tape is applied as a mask over the desired area. Etching follows. The major problems with this method are poor edge definition of the finished product, and inaccuracies in placement of the mask. Usually the desired stripe covers only a minor propor- 25 tion of the surface of the substrate, so many times more metal is etched away than is left on. This means that consumption of etching solution and metal are both high, though the latter is not generally a significant cost factor.

A more sophisticated method involves deposition of a stripe 30 of metal directly onto a moving substrate through a moving mask. Metal is vaporized in a high vacuum and moves, in all available directions, in a straight line. The substrate is unwound from a payout reel and, with guide rollers, moves across the deposition zone. Two loops of metal form the mask. They move directly beneath the moving substrate and are spaced apart the distance desired to be coated. After passing the deposition zone, the mask loops pass down below the source to a cleaning section, where metal deposited on the mask is removed, usually mechanically, as by scratch brushing. After cleaning, the mask returns for another pass through the deposition zone. Vaporization of the source metal is usually accomplished with a focused electron beam on a boat full of metal. Electron beam equipment is very expensive, however, and more recently vaporization has been accomplished by feeding a wire of the source onto a heated refractory bar. The latter is a mixture of carbides and nitrides and is resistively heated.

Many problems are associated with this kind of moving mask apparatus, most particularly alignment and cleaning. A stationary mask has not been considered practical because deposition of metal thereon, particularly at the edge, would permit no more than a few minutes operation before cleaning was necessary. This involves cooling the system and breaking the vacuum, a long and cumbersome procedure.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a workable, stationary mask for vapor deposition of a stripe of metal onto a moving substrate.

Various other objects and advantages of the invention will become clear from the following description of an embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional elevation of a mask system for simultaneously striping four strips of substrate metal from two sources; and

FIG. 2 is a detail view illustrating the preferred spacing between the mask, substrate and guide rollers.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a system designed to deposit a 0.155 inch stripe of aluminum on a moving, 1-inch strip of Kovar with a tolerance of ±0.005 inch. Five masks 10,12,14,16 and 18 are employed. The essence of the invention is the discovery that there is a critical angle between the line of sight of the metal from the source to the substrate and the side of the mask. This has been found to be about 10°. The precise allowable variations from 10° have not been determined, but it has been found that an angle as low as 5° is unsatisfactory. Thus, “about 10” is used herein as meaning well within these limits. Why this angle should be so critical is not known, but undoubtedly has to do with the mask surface “seen” by the source.

The edges of the masks defining the openings should, of course, be as sharp as possible. In any event, they should have a radius of under 0.005 inch.

Each mask is provided with a central channel 20 for water cooling. Cooling is imperative to prevent adherence of evaporated metal and to prevent distortion of the mask during long periods of operation. The material used to fabricate the mask is not critical but it should have good thermal conductivity; copper is the material of choice.

In operation, it is important that the substrate material pass very close to the top of the mask but not touch it. Contact between the mask and the substrate will cool the latter to below the proper deposition temperature. Too great a distance between the mask and substrate, on the other hand, will result in loss of edge definition of the stripe. In the system illustrated, it was determined that the distance between these two elements should be in the range of 0.010 to 0.020 inch.

How to maintain this rather close tolerance over the length of the deposition zone, and the precise alignment of the substrate material presented certain problems, the solution of which is shown in FIG. 2. In particular, the substrate strip is tensioned around at least two guide rollers 22 having grooves 24 therein. Grooves 24 are just wide enough to receive substrate 26 and hold some flat the prescribed distance above masks 28. 30. It has not been found necessary to heat rollers 22; they are heated by substrate 26 and assume the temperature of the system. Optionally, stationary edge guides of too] steel or the like may be employed between the guide rollers.

It is preferred to mount the masks in a manner which allows them to be moved when a change in the width or position of the deposited stripe is desired. Adjusting screws (not shown) can be provided for this purpose.

Various changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.