US2444763A - Process and apparatus for depositing metals on a support by thermal evaporation in avacuum - Google Patents

Description

July 6, 1948. p ALEXANDER 2,444,76$

PROCESS AND APPARATUS FOR DEPOSIIING METALS ON A SUPPORT BY THERMAL EVAPORATION IN A VACUUM Filed 001 7. 1946 v v i 26 j 26 A 2 25 I! I! 26a ""5 V, I ill lll Inventor Patented July 6, 1948 UNIT-EDI STAT-Es PATENT oFFicE ING'ME'frA Js A SUPPORTfBY THERMAL EVAPORATIONIN A VACUUM Paul AlexandenBe'rkhamsted, Eng land Ali-mammo ram 7, 1946,"Ser ial' No. 7013158 ratus for depositing metals on asupport by thermal evaporation in a vacuum.

. The metal most commonly evaporated is aluminium and the heater required for its evaporationis one of the .rafractory metals, tungsten being: the most suitable.

Aluminium, however; at high temperature alloys with or dissolves tungsten, and the tungsten heater quickly becomes rotten and breaks if molten aluminium remains in contact with it.

. In U. S. A. patent specification No. 2,153,786 there is disclosed 'a process of and apparatus for depositing aluminium on a support ina vacuum, the heater being made of tungsten, and in which a means avoiding the rapid disintegratioh of the heater is avoided by feeding aluminium to the heater in the form of a wire so that an end only of the wire came intocontact with'the heater, and was evaporated before there was time for a pool of :molten aluminium to. form onthe bar. By this means the life of the tungsten heater can ,be very largely increased.

Nevertheless, the tungsten disintegrates in time, and the wastage of tungsten remains the most costly ite'm inthe process.

The principal object of the invention is to reduce the-wastage of tungsten .in heatersincorporated in apparatus of thekinddescribed.

Another object is to avoid dense aluminium vapour in the vicinity of'the heater.

Another object is to. so feed a fine wire of the metal to be evaporated so as to assure that it is brought into contact, with a tungsten heater of very small cross-section. e

I Another object isto lead currentto a tungsten heaterin rod orbar form by meanswhich avoid stressing-the heater whenit lengthens or contracts on heating and cooling.

To avoid accumulation of vapourin the vicinity of the heater the aluminium wire'must'be so fine, that it is .very flexible, and it has beenfound impracticable to feed such a wire, if straight, accurately to a given point.

Guiding means for the wire cannot be brought close to the heater bar, by reason of the high temperature radiation, .aridfalso' because any such' me'a'ris, close to the point of evaporation, becomes covered with a growing layer of deposited aluminium. A length of wire of 2.5 to 5 cms. must therefore be allowed to descend free of any guiding means, and this length is liable to be deflected from a straight path by five or more millimetres.

Apparatus for depositing metal on a support by tinuously 3 Claims. (01. 117-107) 2 thermal evaporation-of the metal in a vacuum. comprises a heater bar of-refractory metal, winding means disposedv with itsv axis vertically above theheater and perpendicular thereto. for condrawing a wire of the metal to be evaporated from a storage spool and a coaxial former'on which the wire is coiledintoa contlnuously rotating and vertically descending helix, said former terminating at a, point'di stant from the heater, whereby .a free length of helix is disposed in the vicinity of thehe'ater and the lower end only of the helix comes into contact' with'the heater bar.

Preferably the descending helix has a diam- 'eter which is larger than the width of the heater bar.

By the present invention, a fine aluminiumwire may be :fed to a'heater rod or bar of tungsten of as small a crosssection as is desired, with no 'risli of the wire failing to make contactwith it. In practice a round rod of tungsten is the most economical form of heater bar, and a rod of only two millimetres diameter may be used. The invention, thereforefenables a smallsamount of tungsten in'its most economical'form to be used in metal depositing apparatus of the kind described.

The invention comprises also a process ofrdepositing aluminium on a support .by thermal evaporation in a vacuum. wherein a tungsten heater is used to evaporate the metal and the metal in the form of a wire is continually ad'- vanced to the heater so that the end only of the wire touches theheater, characterised in that the metal is fed .to the heater in theform of a freely dependinghelix rotating about its'axis, the rate of descentof the wire in the helix being related to the temperature of the heater to effect instantaneous evaporation of the wire as it comes in contact with the heater, sents only a relatively small helix, the' radius of the helix being somewhat greater'than the extent of maximum deflection of the'free endof the helix laterally of the heater. One preferred form of the invention which has been" found successful in practice in the process of depositing aluminium on a support by thermal evaporation in a vacuum, will be described, by way of-exai'nple, with reference to the accompany' ing drawings, of which- Figure 1 is a view from below of the helix forming device, in which the helix itself is omitted, taken along the line AA of Figure 2.

Figure 2 is a side view of the device including the heater bar, a portion of the helix forming and'the heater p-recrosssection to the spindle 8 is turned 3 against the portion :and. serves to guide .as a former.

the heater bar and its deflection of even several millimetres is permissible.

The helix making device is mounted on a plate I fixed to the wall 2 of the vacuum chamber, which latter may be constructed as described in the aforesaid United States patent, and com prises a floor 2c in which is a pipe 2?) through which the chamber is evacuated. The wire 3 is taken from a spool 4 through a guide tube 5, with lead in bell-mouth B, the tubebeing held in a support I fixed to the plate I.

- A spindle 8 is adapted to turn in a bush 9 which passes through the plate I and above the bush a chain wheel I 0 is fixed by a set screw II to the spindle. The chain wheel ill, with the slowly by a chain t2 from a driving. chain wheel, I3, which may be driven in any convenient way, such as the clockwork shown diagrammatically by the casing I 4, fixed to the plate I by the bracket Ma. A wire wind- ;ing portion I5 of the spindle 8 is knurled, and the tube 5 is located so that the wire 3 issues tangentially to the winding portion l5. A roller I6 heldin an arm II, pivoted to the plate I at I8 is urged towards the knurled portion I5 of the spindle by a spring I 9, and then presses the wire I5. A guide nib fixed a groove 2i in the spindle the wire towards a lower portion 22 of the spindle, which portion functions A support S on which the vapour is. to be deposited is carried in a frame F mounted on the floor 2a.

In operation, the wire is drawn by hand through the tube 5 between the knurled portion I5 and the roller I6, passed below the guide nib 20 and given one or two turns round the former portion 22. Then, when the spindle is turned counter-clockwise as seen in Figure l, the wire 3 is drawn from the spool by the winding portion I5 and continuously transferred to the former 22 where it is coiled into a continuously rotating and vertically descending helix 23 of about 8 mms. diameter which extends beyond the former to the plate I, enters thereby providing a free length of the helix .which reaches the heater only the free end of .the Wire touching the heater.

The helix-forming device is mounted 2!) cans. or more above the point of evaporation to avoid its becoming thickly coated with the evaporated metal. The helix 2 3 tends to descend to the point of evaporation in a straight line, but it may 'be guided by a rod 24, forming an extension of the spindle to within 2.5 to 5 cms. of the point of evaporation.

The heater bar consists of a thin rod 250i about 2 mms. diameter of tungsten and presents a relatively small cross section to the helix which has a diameter of 8 mm., and the invention commounted in the 'bar 30, the

-metal to be evaporated from a prises a novel means of supporting such heater.

To this end the heater is gripped between blocks 26 of copper supported on posts 25a passing through the floor 2a and insulated therefrom. The blocks 26 are carried on supports 21 of copper comprising strip portions 28 which are thin enough to yield and permit the expansion and contraction'of the tungsten rod 25. Each support 21 is electrically connected to a lead 29 by a bar so fitted to the respective post 2 6a, and is lead being clamped to its post by a nut 31.

' In operation, the rod 25 is heated to a temperaturein its central part of some 2000 0., the wire in the helix 23 is fed at a rate related to the temperature of the heater so that the end of the I helix melts and is evaporated as soon as it comes into contactwith the rod 25, and there is no risk of the helix deflecting enough to make it fail to contact the rod. j

I claim:

1. Apparatus for depositing a metal on a support by thermal evaporation of the metal in a vacuum, comprising a heater bar of refractory metal, winding means disposed with its axis vertically above the heater and perpendicular thereto for continuously drawing a wire of the storage spool and a coaxial former on which the wire is coiled into a continuously rotating and vertically descending helix, said former terminating at a point distant from the heater, whereby a free length of helix is disposed in the vicinity of the heater and the lower end only of the-helix comes into contact with the heater bar.

2. Apparatus for depositing a metal on a support by thermal evaporation of a metal in a vacuum, comprising a heater bar of refractory metal, winding means disposed with its axis vertically above the heater and perpendicular thereto for continuously drawing a wire of the metal to be evaporated from a storage spool and a coaxial former on which the wire is coiled into a continuously rotating and vertically descending helix of a diameter larger than the width of the heater bar, said. former terminating at a point distant from the heater, whereby a free length of helix is disposed in the vicinity of the heater and the lowerend only of the helix comes into contact with the heater bar. I

3. Process of depositing aluminium on a support by thermal evaporation in a vacuum wherein a tungsten heater is used. to evaporate the metal and the metal is in the form of a wire continually advanced to the heater so that the end only of the wire touches the heater to evaporate the metal, characterised in that the wire is fed to the heater in the form of a freely depending helix rotating about its axis, the rate of descent of the wire on thehelix being related to the temperature of the heater to eflect instantaneous evaporation of the wire as it comes in contact with the heater, and the'heater presents only a relatively small cross section to the helix, the radius of the helix being somewhat greater than the extent of maximum deflection of the free end of the'helix laterally of the heater.

. PAUL ALEXANDER.

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