Vacuum Evaporation Apparatus

Abstract

A crucible for holding a quantity of material to be evaporated in a vacuum evaporation system includes an elongated tubular upper portion and a heat shield to confine heat to the lower portion of the crucible so that the upper portion remains cooler than the lower portion. This prevents overflowing and serves to collimate the evaporant.

Description

BACKGROUND OF THE INVENTION

This invention relates to vacuum evaporation apparatus and, more particularly, to apparatus including a crucible for holding a relatively large quantity of a substance to be evaporated so that prolonged evaporation is made possible.

Vacuum evaporation is widely used in the electronics industry, for example, for the application of relatively thin coatings of materials such as aluminum to surfaces of electronic components. Evaporated aluminum leads and contacts for semiconductor devices are common. The manufacture of television picture tubes includes the application of a coating of aluminum to the surface of the envelope of the tube.

Other materials including semiconductors, insulators, and metals are also applied by the process of vacuum evaporation. For some of these materials, it has been common practice to place small bodies thereof directly on a coiled filament. This method, however, does not permit prolonged evaporation or relatively large quantities of material.

The evaporation of aluminum, in particular, over relatively long periods of time poses several problems. Molten aluminum tends to attack the usual crucible materials, such as alumina. Molten aluminum wets many crucible materials and capillary attraction causes the aluminum to creep up the sides of the crucible and overflow. Flash evaporation of small aluminum pellets dropped into a hot crucible requires an elaborate pellet feeding mechanism.

SUMMARY OF THE INVENTION

The present apparatus includes a crucible having an elongated tubular upper portion with an open end through which the material to be evaporated may pass and a closed lower end portion for holding a quantity of material. Means for heating the lower end portion of the crucible and a heat shield for confining the heat to the lower end of the crucible cooperate with the elongated upper end portion to solidify the material on its wall and thus prevent overflow of the material due to wetting.

IN THE DRAWINGS

The single FIGURE is a vertical cross-sectional view of the present apparatus.

THE PREFERRED EMBODIMENT

The present novel vacuum evaporation apparatus is illustrated generally at 10 in the drawing. As shown, the apparatus 10 includes a tubular housing 12, which is made of a refractory material, supported on a suitable thermal insulator 13. The housing 12 has a closed lower end 14 and a top surface 15 defining an open upper end for receiving the other elements of the apparatus 10.

A crucible 16, made of a material which is electrically and thermally insulating and chemically inert, such as pyrolytic boron nitride, is supported in the open upper end of the housing 12. The crucible 16 has an elongated tubular upper portion 18, a conical lower portion 20 defining a closed end, and a threaded midportion 21. A heat shield 22 having the shape of an annular disc with a concentric, internally threaded opening is assembled on the midportion 21 of the crucible 16. An annular stop 24 on the crucible 16 serves to define the position of the heat shield 22 thereon. The diameter of the heat shield 22 is such that the shield may rest on the surface 15 of the housing 12 to support the crucible 16 thereon and to define, with the housing 12, a chamber for confining heat to the vicinity of the lower end portion 20 of the crucible 16.

A resistance heating element 26, in conical coil form, is disposed tightly around the lower conical end portion 20 of the crucible 16. Typically, the heating element 26 is made of resilient tungsten or molybdenum wire. The size of the element 26 should be such that it is stretched when it is in place on the crucible 16 with its uppermost turn disposed in the lower thread of the midportion 21 of the crucible. The resiliency of the heating element 26 then holds it in tight, thermally efficient contact with the lower end portion 20 of the crucible 16.

Input and output conductors 28 and 30, respectively, extend from the heating element 26 to the outside of the housing 12 through suitable slots 32 and 34 in the walls thereof. A power source, represented by a battery 36, may be connected to the leads 28 and 30 to produce heat in the heating element 26 to melt the material in the crucible 16.

In the use of the present apparatus, a quantity of material such as aluminum is placed in the crucible 16 and the apparatus 10 is then placed in a vacuum system, not shown, with the open end of the crucible 16 directed upward toward the article to be coated. Low-pressure conditions are established and current is then passed through the heating element 26 to melt the material in the crucible 16 and form a bath of molten liquid, 38.

The present apparatus permits the use of crucible materials which are chemically inert to the material being evaporated even though they are wetted thereby. The result of wetting is that the material creeps up the walls of the crucible 16 by capillary action. This is indicated in the drawing by the quantity of material 40 which appears as a coating on the wall of the crucible 16.

The heat shield 22 confines heat from the coil 26 to the lower portion 20 of the crucible 16. Heat may flow by conduction through the walls of the midportion 21 of the crucible 16 to the upper portion 18 thereof, but this heat is minimized by the thinness and relatively low thermal conductivity of the crucible wall. Moreover, the relatively large surface area of the tubular portion 18 of the crucible 16 provides a large radiating surface for the dissipation of any heat which does reach the upper portion 18. Consequently, the upper portion 18 remains relatively cool.

Molten material 40 rising up the interior wall of the upper portion 18 cools and solidifies upon encountering the relatively cooler material thereof. A dam, 41, of solidified material is thereby established which prevents any further flow of material up the wall of the crucible. Consequently, the material cannot overflow from the crucible.

The elongation of the tubular upper portion 18 also serves to collimate the evaporation of the material 38. As is generally known in the vacuum evaporation art, the mean free path of atoms or molecules in the vacuum is relatively long so that material proceeds in straight lines from the molten zone. In the present apparatus, atoms leaving the crucible 16 will be largely confined to the conical region bounded by the arrows 42 and 44 in the drawing. Material is thus directed toward the article to be coated and does not condense on the walls of the evaporation chamber or elsewhere in the apparatus. Moreover, material is not reflected from the walls of the chamber.

Claims

I claim:

1. In a vacuum evaporation apparatus:

a crucible for holding a substance to be evaporated, said crucible having an elongated tubular upper portion with an open end and a lower portion with a closed end and being of a material which is wetted by said substance;

means adjacent to said lower portion of said crucible for heating said substance; and

heat shield means thermally coupled to said crucible for confining heat to said lower portion of said crucible and for maintaining said upper portion of said crucible at a substantially lower temperature than said lower portion of said crucible, whereby overflow of said substance from said crucible is prevented.

2. Apparatus as defined in claim 1 wherein said crucible is formed of boron nitride.

3. Apparatus as defined in claim 1 wherein said crucible has an externally threaded portion thereon and said heat shield means comprises a disc having a threaded opening therein adapted to be engaged with said threaded portion of said crucible.

4. Apparatus as defined in claim 1 wherein said heat shield means comprises a radially extending disc surrounding said crucible and wherein said tubular upper portion extends a substantial distance above said heat shield.