US3056740A

US3056740A - Vapourisation of metals

Info

Publication number: US3056740A
Application number: US689381A
Authority: US
Inventors: Holland Leslie Arthur; Laurenson Laurence
Original assignee: Edwards High Vacuum Ltd
Current assignee: Edwards High Vacuum Ltd
Priority date: 1956-10-12
Filing date: 1957-10-10
Publication date: 1962-10-02
Anticipated expiration: 1979-10-02
Also published as: FR1184364A; GB879256A; DE1242429B

Description

1962 L. A. HOLLAND ETAL 3,056,740

VAPOURISATION OF METALS Filed on. 10, 1957 4 Sheets-Sheet 1 COOLANT Fl G. I

COOLANT FlG.3

INVENTORS LESLIE ARTHUR HOLLAND,

LAURENCE LAURENSON ATTORNEYS Oct. 2, 1962 A. HOLLAND ETAL 3,056,740

VAPOURISATION OF METALS Filed Oct. 10, 1957 4 Sheets-Sheet 2 & lint/RENO; LJIJREIYSOIYI INVEN'TOR Oct 1962 A. HOLLAND ETAL 3,056,740

VAPOURISATION OF METALS Filed Oct. 10, 1957 4 $heecs-Sheet3 LESUE HOLLAND X/lflURE/WE ZAUKEIYSQIYI \NVENTOR 5 ATTORNEY Oct. 2, 1962 L. A. HOLLAND ETAL 3,956,740

VAPOURISATION OF METALS Filed OO'b. 10, 1957 4 Sheets-Sheet 4 LEjL/E f/OLLA/YP ,qu/fE/Y Jaw,

\NVENTOR 5,

ATTORNEY LLAuREME L United States Patent 3,tl56,740 VAPUURISATEON 0F METALS Leslie Arthur Holland, Northgatc, Crawley, and Laurence Laurenson, Langley Green, Crawiey, England, assignors to Edwards High Vacuum Limited, Crawley, England, a British company Filed Oct. 10, 1957, Ser. No. 689,381 Claims priority, application Great Britain Oct. 12, 1956 2 Claims. (Ci. N b-298) This invention relates to the vaporisation of metals and particularly to apparatus in which the heat for effecting vaporisation of a metal is produced by electron bombardment of the metal.

In vacuum vapour metallisation apparatus, it is known to feed a wire of the metal to be vapourised onto a heated refractory support. Suitable refractory materials are however expensive, and further, their useful life is short. In the specification of the present applicants Patent No. 754,102, it is proposed to provide an electron bombarded vapour source in which a wire is fed horizontally through a water-cooled nozzle into the path of an electron beam and the wire, being bombarded by electrons, reaches a sufficient temperature to evaporate. Normally a molten globule is formed at the end of the wire unless the metal evaporates rapidly by sublimation. This method of evaporation has the advantage that metals can be evaporated which would otherwise react and form alloys and compounds if heated on a support made, for example, from a metal or ceramic material. In carrying out the method referred to, the energy flowing into the molten globule must be sufficient, not only to maintain the energy lost by heat radiation and thermal evaporation of the metal, but also to maintain the high temperature difference between the point of evaporation and the water-cooled nozzle. However, this minor disadvantage is more than outweighed by the simplicity of the source and the avoidance of the use of costly refractory materials. For some metals, e.g. titanium, no satisfactory refractory support materials are as yet known.

In order to obtain reasonable rates of evaporation with most metals, it is necessary to raise their temperatures above those of their molten state, for example, aluminium melts at 650 C. and evaporates rapidly at 1300 C. and titanium melts at 1750 C. and evaporates rapidly at 2000 C. Thus, if evaporation is to take place with the apparatus described a temperature difference must be maintained between the molten zone of the globule and the solid wire, which is of the order of many hundreds of degrees in temperature. Further, a very high temperature diiference will be required between the evaporation point and the molten zone.

According to the present invention, a source of metal vapour comprises a downwardly fed metal wire and means providing electron bombardment of the end of the wire to produce a molten droplet which is subjected to the electron bombardment and emits metal vapour. By downwardly is meant vertically downwardly, or downward feed at any angle from 0 to such angles approaching 90 from the vertical as will result in the formation of a molten droplet as hereinafter explained.

In particular forms of metal vapour source according to the invention, the molten droplet is suspended from a cooled surface down or adjacent which the metal wire is fed, the extremity of the wire being subjected to electron bombardment to produce the molten droplet.

The cooled surface may consist of a nozzle, or the equivalent, through which the wire is fed into proximity with a ring shaped heated filament. It is a feature of the invention that the source may be used either in vacuum vapour metallisation apparatus or in a getter pump.

Forms of metal vapour sources constructed and arranged to operate in accordance with the invention and apparatus to utilise such sources will now be described, as examples, with reference to the accompanying drawings in which:

FIGS. 1 and 2 illustrate the invention diagrammatically,

FIG. 3 shows the addition of a detail improvement,

FIG. 4 shows diagrammatically a form of pump provided with a gettering vapour source embodying the invention,

FIG. 5 shows diagrammatically a general arrangement of a developed form of pump embodying the invention,

FIG. 6 is an elevation in section of a practical form of evaporation source and wire feed mechanism for use in the pump shown diagrammatically in FIGURE 5, and

FIG. 7 is a plan, partly in section and to a slightly larger scale than FIGURE 6, of a pump having the evaporation source shown in FIGURE 7.

Referring to FIGS. 1 and 2 of the drawings, it will be seen that a Wire 1 is fed downwardly and, for the reasons now to be explained, the problem already described is not encountered.

The wire 1 is driven by knurled wheels 2 through the hollow core 3 of a hollow block 4 cooled by water flowing through pipes 11. The end of the wire leaving the core 3 is heated by electron bombardment from a circular tungsten filament 5 which is energised by the low tension winding of a transformer T and constitutes a. cathode, the block 4 being connected to the positive terminal of a source of supply.

When the end of the wire 3 is heated by electron bombardment from the cathode 5 the less viscous and high temperature metal flows to the top of the globule, which is remote from the cooled feed. When the evaporation is commenced with the wire fed vertically downwards a globule forms on the end of the metal which remains suspended by surface tension forces, as shown in FIG. 1. The electron bombardment system is arranged so that the maximum bombardment occurs on the outer tip of the globule. Thus, this region of the gobule becomes the hottest zone from which evaporation occurs. The globule may run back up the wire, depending on thermal conduction of the metal wire and the length of wire bombarded, until the globule touches the cooled support where it freezes and forms a hemi-spherical droplet as shown, cross-hatched at 7 in FIG. 2. The wire is fed into the droplet to maintain the evaporation.

A simple apparatus for melting and evaporating metal by electron bombard-ment can be developed from the diagrammatic form in FIG. 2. The cathode consists of a circular turn of tungsten wire and the guide nozzle is made from copper and water cooled. Care is taken to avoid the use of brazed joints at the end of the watercooled nozzle. This is to obtain the highest thermal conductivity between the nozzle and the cooling water, which is necessary if the nozzle is not to melt when the molten evaporant has a temperature in excess of the melting point of the copper or brazing solder used. Where brazed joints are used, they must not form a junction or thermal barrier in the heat flow between the droplet and the cooling water.

A negative electrode in the form of a ring shield 8 surrounds the filament and prevents the electron stream from bombarding any metal fittings which may be at a positive potential and adjacent to the wire and feed tube. The shield may be enlarged and formed with a small aperture 9 as shown in FIG. 3 to prevent undue electron bombardment of the end of the wire feed nozzle. However, if the metal globule has been made to melt back and contact the cool support, as in FIG. 2 then the electron beam need not be greatly restricted because the end of the feed nozzle will be almost covered by evaporant which is molten on its outer surface. This is an advantage because it is often difficult to prevent the molten wire contacting the shield electrode and short circuiting the power supply if the central aperture is too small.

When the wire melts back and the droplet contacts the cooled nozzle it partially solidifies and sticks to the outer surface of the nozzle. Thus a much larger droplet can be supported on the nozzle than by the wire alone and this greater mass of droplet increases the surface area available for evaporation.

The vapour source described has been used by the applicants for evaporating titanium in a special form of high vacuum pump known as a getter pump. The evaporated metal atoms combine with gas and vapour molecules to form compounds on the walls of a receiver. The source is then used in a chamber as shown in FIG. 1 fitted with a large entry port for connection to the vessel to be exhausted. The chamber houses a vapour source as already described. The metal evaporates in a downward direction and condenses on the sides and base of the chamber. To obtain the maximum gettering efficiency the receiver is then arranged to receive more or less equal masses of evaporant per unit area of the receiver. Thus when the pump is in operation the removal of gas molecules by absorption at the receiver surface, is more or less constant over the whole of the receiver surface. When the evaporation rate is adiusted to provide a given pumping speed there is no region of the receiver where evaporated metal is condensed without maximum use.

Two further advantages are gained by using this form of apparatus- Firstly, there is no obstacle in front of the evaporating globule to prevent the complete expansion of the vapour beam and which would reduce the gettering action. For example one form of pump is known in which a wire is fed on to an electron bombarded refractory support. Obviously the metal wire must be directed by a nozzle on to the anode support and the evaporated metal tends to condense on the end of the feed tube. This may block the feed tube and also reduce the amount of metal available for gettering purposes.

Secondly, when a gettering pump has been in operation for some time, the thick metal deposit peels from the walls of the receiver. With the source in the base of the chamber evaporating upwards the metal coating may fall on the electrodes and short circuit the supply. With an arrangement embodying the invention, this cannot occur.

If desired, the pumping port may be placed opposite the dead evaporation zone at one side of the vapour source as in FIG. 1 or above the vapour source as at 13 in FIG. 4. The latter arrangement is not so convenient for mounting the source and furthermore the path of the gas molecules into the pump may be restricted by the feed mechanism and auxiliary items. However, this is a matter of practical detail, which can be catered for in the design of the apparatus where it is of advantage to locate the pumping port as shown in FIG. 4.

The developed form of apparatus embodying the invention illustrated diagrammatically in FIG. 5 and of which essential parts are shown in more practical form in FIGS. 6 and 7 overcomes a difiiculty which is apt to arise with the use of a guide tube, namely a tendency for the wire slightly to melt back by thermal conduction into the guide tube with the resultant need periodically to service the tube in order to maintain uniformity of the bore. Uniformity of the bore is desirable in order that the feed wire may be smoothly fed without flowing into irregularities in the tube surface which would cause jamming of the feed mechanism.

Referring to FIGS. 5, 6 and 7, the water cooled block 4 of the preceding figures is replaced by a solid copper block 14 formed at its upper end with a cooling water chamber 15 having water inlet and

outlet pipes

16 and 17 respectively. Heat may also be removed from the block 14 by air cooling at its outer end, using a blower.

The block 14 is supported from the pump flange 18 by a cover 19, a sealing gasket 20 being provided. The gasket 20 may also serve as an insulator for isolating the anode block 14 from earth and may conveniently be made of polytetrafluorethylene which is an insulator and a vacuum sealing medium. The wire feed mechanism is mounted in the block 14 so as to prevent the mechanism from becoming overheated during the operation of the pump.

The wire 1 to be evaporated is fed from a stock reel 21 via a guide nozzle 22 and feed tube 23 through

knurled driving wheels

2a and 2b which drive the wire on to an inclined plane 24 formed on the block 14. At the ends of each of the

knurled wheels

2a, 2b are

gears

20, 2d which mesh with each other so that drive imparted to one wheel will cause the other also to be driven. A toothed wheel 25 fixed on the shaft of the wheel 2a is intermittently driven by a ratchet 27 carried at the end of a lever 28 pivoted on the shaft of the wheel 2a. The opposite end of the lever 28 has attached to it a push rod 29 which passes through a bellows vacuum seal 30 which permits the rod 29 to be vertically reciprocated by the outer casing of a ball race 31 driven by a cam 32 on a rotatable shaft 33. A high tension insulator 34 is interposed in the rod 29 to isolate electrically the rod driving means from the anode block 14. The bellows 30, driving cam 32 and motor, not shown, can thus be at earth potential. Reciprocation of the rod 29 results in actuation of the ratchet 27 via the lever 28 to drive the

wheels

25, 2a and 2b with consequential feeding of a small length of wire over the edge of the inclined plane 24. The protruding tip of the wire is bombarded by electrons emitted by a filament 35 and a molten droplet 36 is produced.

In the arrangement described, it will be seen that, as melting of the wire does not occur at the exit of the feed tube 23, that exit cannot be blocked or restricted so that no constraint on the wire will be imposed by the tube. Evaporation may not be confined to the single droplet 36 and more than one droplet may be formed beneath the edge of the inclined plane 24, particularly if the underside edge is inclined so that the droplets move under gravity.

It has been found advantageous to mount both the

wheels

2a, 2b on ball races as shown to prevent the high friction which may develop in dry bearings to be operated under high vacuum. The wheel 2b is mounted on a shaft 37 which can be adjusted to vary the compression on the feed wire. For this purpose, the ends of the shaft 37 are supported, below the centre line of the shaft by arms 38 linked by a rod 39 which can be raised or lowered by a nut 40 (FIG. 6) on a screw-threaded rod 41 which passes through a support 59. A block 51 at the end of the rod 41 is carried by the rod 39 and on rotation of the nut 40 in a clockwise or a counterclockwise direction, the rod 39 is raised or depressed respectively. The arms 38 linked by the rod 39, are fixed by bolts '52 (FIG- URE 7) to a disc 53 (FIGURES 6 and 7) forming a pivot bearing and the shaft 37 is rotatable in bearings 54 mounted in the disc 53. Rotation of the nut 40 produces slight rocking of the shaft 37 to vary the grip on the feed wire between the

wheels

2a, 2b, without afiecting the driving connection between the

gears

2c, 2d.

In operation of the mechanism there is sometimes a tendency for backlash in the mechanism to permit the feed wire to spring upwards after feeding with the result that only a small quantity of the original wire fed enters the droplet. To prevent this, a gravity compensation pawl 42 is mounted above the ratchet wheel 25 and this pawl permits the ratchet wheel to move forwards but engages in the teeth if the compression in the feed wire places a reaction strain on the mechanism,

While successful operation can be achieved with the use of a tungsten filament electron source, it has been found that the filament is subject to erosion due to positive ion bombardment and reaction with active gases, the effect being intensified with increased gas pressures. Erosion is contributed to also by ionised metal atoms striking the filament. Accordingly, as an alternative or in addition to a filament source of electrons, an arc discharge may be used with advantage for evaporation at high pressures. An electrode 35a shown in broken lines in FIG. 5 and which may conveniently be a pointed tungsten rod may therefore be used and will function as the negative electrode in an arc for-med between it and the positive block 14.

In one method of using the arc and because initial striking of the arc may be difficult to achieve, the evaporation is commenced by heating the evaporant using the hot filament. As the resultant droplet is brought to temperature by electron bombardment from the wire cathode the tungsten arc rod begins thermally to emit electrons as its temperature rises under bombardment of positive ions of gas molecules and metal atoms. An arc discharge then occurs between the negative tungsten rod and the positive metal droplet and it i possible then to disconnect the filament cathode until the pressure of the gas drops to below that necessary for an arc discharge to be sustained.

Apart from titanium, the properties of which make it ideal for evaporation by this method, many metals, for example, aluminum, copper and nickel-chrome alloy can be evaporated from the vapour source described. A low pressure to provide a clean wettable surface on the end of block 14 is required for these metal because, unlike titanium, the surface oxide skin which forms at high pressures prevents the droplets from adhering to the block.

It is a known practice when operating a getter pump, in which the metal is evaporated by electron bombardment heating, to make the condensing wall surface of the pump a cathode electrode to which positive gas ions may be attracted and trapped on the wall by subsequently condensing metal atoms. This process is found particularly advantageous when pumping inert gases, e.g. argon, etc., which do not chemically combine with the gettering or condensing metal. However, it is not possible to operate an electron bombarded source at a high gas pressure, approximately above 0.1 micron of mercury, if both the metal collecting walls of the pump or other condensing surfaces disposed in the path of the gettering metal vapour and on which vapour atoms condense, and the electron emitting filament are at the same potential. This is because the electron beam ionises sufficient of the residual atmosphere for a cold cathode glow discharge to pass from the condenser walls and the electron current flowing from the hot cathode to the anode is usually so reduced and dispersed that the evaporant cannot be volatilised. Such a system also becomes unstable because minute metal particles on the condenser walls become incandescent under positive ion bombardment and promote the formation of arc discharges.

In applying a further feature of the invention the vapour source described is operated in a getter pump in which the metal is evaporated by electron bombardment at a high pressure, that is to say above about 0.1 millimetre of mercury, with the heated cathode insulated from the condenser wall which is connected to the positive side of the supply. Positive ions of metal or gas atoms cannot then flow to the wall and a cold cathode glow discharge cannot occur. Such a stable arrangement of an electron bombarded source has been described by L. Holland in Vacuum Deposition of Thin Films (Chapman & Hall Ltd., 1957, p. 137), and in the specification of Patent No. 754,102. If desired the condenser wall potential may be permitted to float intermediate between that of the cathode and anode electrode. Thus if both electrodes are insulated from the earthed metal condenser only very Weak currents can flow to the wall.

A getter pump operated as described provides stability at high pressures when removing chemically active gases and additional ion pumping at low pressures as, for example when inert gases are admitted to the system being exhausted. To facilitate such operation, there is shown in FIG. 5 a single high tension source 43 provided with a change over switch 44 by means of which either

output terminal

45 or 46 of the source may be connected to the condensing wall or the contact arm may be left on the neutral terminal 47. For convenience, the condenser wall is always at earth potential if this forms part of the outer casing of the vacuum vessel. Obviously a circuit may be used in which two high tension power supplies are used, one for pumping at high pressures and the other for i011 pumping at low pressures.

In a getter pump it is necessary to remove absorbed gases from the condenser walls before commencing evaporation and getter pumping. Such absorbed gases would result from exposure of the pump to atmosphere if for any reason it has to be opened up, and these absorbed gases constitute a very serious gas load on the pump until they are gettered. During this period they drastically reduce the pumping speed of the pump and as these absorbed gases are released from th walls of the pump as they become heated during operation of the pump there can be a very undesirable rise in the system pressure which could be harmful to the system operation. Such pressure rise will persist until the absorbed gases are reduced by gettering when, of course, the pump begins to getter the gases coming from the system itself, i.e. it begins to function as a pump in its own right. This absorption of atmospheric gases is very much more serious in the case of the getter pump than in normal vacuum apparatus because the walls of the pump, covered as they are by porous layers of gettering material, are ideal for the taking up of enormous quantities of gas. The removal of the absorbed gases is usually achieved by heating the vessel to a high temperature under vacuum. It is known that a glow discharge will remov absorbed gases from the surface of an electrode under electron or positive ion bombardment and also absorbed gases if the temperature of the body rises under bombardment. Electron bombardment heating of the condenser wall is preferred because if, as is usual, the metal condenser is grounded than all of the earthed metal fittings in the apparatus at the same potential would form a cold cathode discharge if exposed to positive ion bombardment. The connection of the condenser wall to a positive source of supply while the source of bombarding electrons is energised, results in bombardment heating of the condenser wall. A preferred method of operating the pump described therefore involves use of the output of a source or sources of high tension supply to obtain the following sequence of events, high tension cleaning by removal of absorbed gases by electron bombardment of the vessel and condenser walls while these are at a positive potential, intermediate pumping and then final pumping which may be referred to as chemical and ion pumping.

Applying the above outline of an operational cycle to a specific case, the vacuum vessel under exhaustion and the chamber of the getter pump are exhausted to about 0.1 millimetre of mercury using a rotary oil pump. The cathode filament is then energised and the getter pump condenser wall is made positive with respect to the filament by appropriate operation of the switch 44. The wall is then degassed by electron bombardment at 3,000 volts with a current consumption of 2 amperes which raises the temperature of th wall to about 400 C. in about ten minutes. Initially the gas pressure rises and then falls as the pump Walls are freed from occluded gases. The electron bombardment of the pump wall is then stopped and the getter pump is isolated from the rotary pump. Next the Water flow to the cooling pipes 48 surrounding the pump condensing wall is commenced.

Positive potential is now applied to the copper block 14 with the result that the feed Wire itself is made positive with respect to the hot filament cathode 35. If an arc electrode is used, the electrons gradually become emitted from the arc electrode as the gases become ionised and the filament 35 may be disconnected until the pressure has fallen to about 1 micron of mercury. The condenser wall of the pump is either insulated from both the anode block 14 and the filament, or is connected to positive potential so that a glow discharge cannot pass from its surface. The wire feed mechanism is then brought into operation and the wire feeds into the molten droplet suspended at the tip of the block 14. Conveniently, No. 20 S.W.G. titanium wire may be used and in this case the feed rate may be adjusted from 0 to 1 gramme per minute. A typical evaporation rate of 100 to 200 milligrammes per minute may be obtained with a source input of 4 to 6 kilowatts at a potential of 2,000 volts. During the operation at this stage, gas atoms and molecules are removed by chemical combining with metal atoms in transit and chemical combination and absorption at the condenser walls. As already stated, a cold cathode discharge cannot occur and the heating bombardment is stable. This intermediate pumping really embraces two stages, firstly with the arc and secondly with the filament as a source of electrons.

When the pressure in the system is reduced to lower than 10 to 4 millimetres of mercury, the pump wall is switched to a negative potential so that positive gaseous ions formed by the electron stream from th cathode are transported to the condenser wall where they may be absorbed by deposition or embedded by the condensing metal in the manner known as ion pumping.

The method of pump operation described is applicable to a getter pump in which the metal to be evaporated is fed in the manner described or is fed on to an electron bombarded support or is heated directly by bombardment as described in the specification of Patent No. 754,102. If the pump system has been degassed by heating by known methods other than electron bombardment then the initial stage of degassing described may be omitted. Again, the method of connecting the condenser walls of the pump either for collecting or repelling positive ions may be used in pumps in which the number of positive ions produced per electron flowing through the gas is increased by the use of more complex electrode systems or electrodes combined with a magnetic field.

While some detailed reference has been made to the application of the invention to getter pumps, reference has also already been made to the use of the invention in connection wtih vacuum vapour metallisation. In such use of the invention, it will readily be appreciated that objects which are to be vacuum coated in accordance with well known techniques, may be placed in the path of the vapour beam, the objects being either statically mounted or mounted on rotating jigs.

It may be found desirable to effect various modifications in design of the apparatus described and, for example, in order to reduce or eliminate the possibility of melting back of the downwardly fed wire at the point where it leaves the cooled block or nozzle, the nozzle exit may be flared with the result that, while a solid zone of metal may be produced on the flared portion of the nozzle, a liquid zone will be maintained immediately below the exit. Alternatively, the nozzle may be mounted slightly out of the vertical so that the bead formed by melting back flows to one side and allows the wire to be fed forwardly without obstruction from partial solidification of the bottom of the bead.

We claim:

1. Apparatus providing a metal droplet as a source of metal vapor and comprising: cooling surface means for supporting a metal droplet, means for feeding metal wire downwardly through and past said cooling surface means, and electron bombardment means disposed below said cooling surface means and arranged directly to bombard a zone starting at the lower surface of said cooling surface means for melting back the end of the wire into contact with said cooling surface means to cool and partially solidify a droplet thereby formed for providing a droplet of larger size and thus larger evaporation surface area than a droplet supported solely by said wire.

2. Apparatus providing a source of metal vapour and comprising driving means for downwardly feeding metal wire and means providing electron bombardment of the end of the Wire to produce a molten droplet which is subjected to the electron bombardment and emits metal vapour, said apparatus including a cooled surface down which the metal wire is fed and from which the molten droplet becomes suspended, said cooled surface including a solid block of metal of high heat conductivity and having an inclined plane on to which said driving means feeds said wire and in which said means providing electron bombardment are disposed adjacent the lower extremity of said inclined plane.

References Cited in the file of this patent UNITED STATES PATENTS 2,157,498 Reinecke et a1 May 9, 1939 2,206,020 Berghaus et al. July 2, 1940 2,509,053 Calbick May 23, 1950 2,527,747 Lewis et al. Oct. 31, 1950 2,717,962 Wouters Sept. 13, 1955 2,791,371 Foster et al May 7, 1957 2,808,980 Alpert Oct. 8, 1957 2,960,457 Kuhlman Nov. 15, 1960 FOREIGN PATENTS 513,257 Great Britain Oct. 9, 1939 754,102 Great Britain Aug. 1, 1956

Claims

1. APPARATUS PROVIDING A METAL DROPLET AS A SOURCE OF METAL VAPOR AND COMPRISING: COOLING SURFACE MEANS FOR SUPPORTING A METAL DROPLET, MEANS FOR FEEDING METAL WIRE DOWNWARDLY THROUGH AND PAST SAID COOLING SURFACE MEANS, AND ELECTRON BOMBARDMENT MEANS DISPOSED BELOW SAID COOLING SURFACE MEANS AND ARRANGED DIRECTLY TO BOMBARD A ZONE STARTLING AT THE LOWER SURFACE OF SAID COOLING SURFACE MEANS FOR MELTING BACK THE END OF THE WIRE INTO CONTACT WITH SAID COOLING SURFACE MEANS TO COOL AND PARTIALLY SOLIDIFY A DROPLET THEREBY FORMED FOR PROVIDING A DROPLET OF LARGER SIZE AND THUS LARGER EVAPORATION SURFACE AREA THAN A DROPLET SUPPORTED SOLELY BY SAID WIRE.