GB2159540A - Apparatus and methods for coating substrates - Google Patents

Abstract

Quartz crucibles (710) as illustrated in Fig. 8 for the melting of silicon and ceramic substrates are coated with protective materials (710') or metals at least in part evaporated from an electrode (701,702) with which an arc is struck at low voltage and current to deposit material from the electrode (701 or 702) on the substrate in a vacuum chamber. The electrode (701,702) may be heated and the substrate (710) may be sandblasted and preheated. Specifically a method of coating a quartz crucible for use in the melting of silicon comprises the steps of juxtaposing a pair of electrodes, composed of at least one component of a material adapted to coat said crucible, with an interior surface of said crucible; evacuating the space in which said electrodes are juxtaposed with said surface and maintaining a fixed pressure during deposition; and striking an electrical arc between said electrodes at one end of each of said electrodes at a voltage in the range 30-60 volts and with a current in the range 50-90 amperes by intermittently bringing said electrodes into contact with one another and separating them, thereby depositing material evaporated from said electrodes substantially over said interior surface of said crucible. <IMAGE>

Description

SPECIFICATION Apparatus and methods for coating substrates The invention relates to apparatus and methods for coating substrates. Such coatings can be effected with a material which is brought into the vapor phase by electrical means.

The deposition of material from a vapor phase onto a substrate is well known in the coating art and in the field of surface transfor mation of a substrate. Generally speaking, a body of the material to be transferred to the substrate is heated in the region of this substrate and transformed first into a molten state and then into a vapor state. The material thus undergoes two phase transformations, namely, the transformation from the solid phase to the liquid phase and then from the liquid phase to the vapor phase.

The coating is generally effected in a vacuum and usually a relatively high vacuum must be drawn to permit transfer of vapors from the source to the substrate.

Earlier systems may use induction heating to effect the aforementioned phase transformation.

A particular problem has been encountered with the respect to the quartz crucibles utilized in the production of silicon wafers for semiconductor purposes. Such crucibles in which the elemental silicon is melted, generally are composed of quartz and are received in a supporting carbon jacket within an induction coil which is utilized to melt the elemental silicon. A monocrystalline silicon seed can then be lowered into the silicon melt of the crucible and raised slowly to draw a silicon bar which -is controlledly cooled so that the monocrystalline product can then be cut into wafers. Such crucibles are, for the melting of the elemental silicon and during the drawing process brought to and held for many hours at an elevated temperature close to their softening point and at a temperature at which attack by the molten silicon can occur.This causes deterioration of the crucible and threatens the introduction of undesired impurities in the wafers which are produced.

Hence the semiconductor field has long sought a method of protecting such crucibles which will increase their life.

Another problem is the application of metals to ceramics. While many metals can be coated onto ceramics, the refractory metals such as tungsten and titanium have been applied heretofore in a manner which is less than satisfactory and practically in all cases adhesion problems are encountered by the methods used heretofore.

As will be described hereinafter there is provided a method for the vapor deposition of material on large-area and/or complex configuration substrates at relatively low-energy cost and with improved uniformity.

There is also provided a method for the high-speed coating of complex and/or largearea surfaces.

There is yet further provided an improved method of protecting quartz crucibles of the type utilized in the semiconductor field so as to increase the life thereof.

Also provided is an improved method of applying metal coatings to ceramics whereby the poor adhesion problems characterizing prior art systems are avoided.

The method of vapor desposition which will hereinafter be described is based upon the discovery that especially large-area surface deposits can be formed by juxtaposing an elongated electrode of the depositing material, laterally with the surface of the substrate to be coated over a substantial portion of the length of the electrode in a vacuum, and striking an arc between one end of this electrode and a counterelectrode such that the arc current should be between 50 and 90 amperes with a voltage applied across the electrode of 30 to 60 volts.

Surprisingly, once the arc is struck as the two electrodes are separated, the arc, a portion of the arc or a heating effect generatd by the arc appears to spiral around the long electrode and cause vaporization of the material of the electrode in a generally helical or spiral pattern progressively moving away from the counterelectrode.

It is indeed a remarkable surprise that the arc is not confined to the space between the two electrodes but rather has a component or an effect which spirals away from the counterelectrode toward a region of the length of the long electrode which is further removed from the counterelectrode in spite of the fact that the greatest conductivity would appear to lie in a line directly between the two electrodes where the major portion of the arc appears to be confined. This effect is manifest in the fact the long electrode, i.e. the deposition electrode, while originally of uniform cross section, develops a taper toward the counterelectrode and coating from the blank of the deposition electrode onto the substrate can be observed at considerable distance from the arc's striking face of the deposition electrode.

In fact, the effect appears to survive for a brief period following extinction of the original arc and hence it is preferred to periodically contact and separate the electrodes to generate the arc and then allow extinction thereof.

Means are provided at an end of the electrode of the material to be deposited, remote from the arc-striking electrode to control the temperature of the material-supplying electrode, generally to maintain it in the range of 800"F to 1000"F (427"C to 538to), the speed under the lower voltage, lower current and temperature conditions of the present invention, at which the material evaporates from the material-supplying electrode, can be increased by 1.5 to 2.0 times the speed of evaporation of the earlier systems. Practically all metals, alloys, carbides and silicides can be used in making the material-supplying electrode. In addition to metals and other alloys, carbides, borides, silicides and nitrides can be deposited on the substrate.

While it is not fully understood why the rate of evaporation of the material to be deposited increases with the lower energy utilization, it is possible that the migration of the arc may spread the otherwise pooled moltion phase over a wider area of the material-supplying electrode to allow, in effect, evaporation of the molten metal in thin film form.

The principles set forth above can be utilized in the application of metal coating to synthetic resins, the synthetic resins in the form of cabinets or housings for electronic components. It has been found, most surprisingly, that since the substrate is unaffected by the large area coatings applied the method of coating described is highly advantageous when utilized to coat the interiors of synthetic resin cabinets or housings which may be utilized for electronic components, the coating forming an electromagnetic shield.

It has also been found, most surprisingly, that the described method is highly effective in applying pure silicon coatings, or other protective coatings, e.g. silicon carbide, silicon nitride or boron nitride, to the interior surfaces of the quartz crucibles hitherto utilized for the melting of silicon in the production of the monocrystalline bars to be drawn from the molton silicon in the semiconductor field. Furthermore, the method may be utilized in the application of metal coatings to ceramics, with improved adhesion, even when the applied metals are nickel, tungsten, titanium, tantalum and like refractory metals which have been difficult to apply heretofore to ceramic substrates.Practically any ceramic substrate may be utilized and in the case of the quartz crucibles and the ceramic substrates, it is preferred to subject the surface adapted to receive the coating to a sandblasting or other blast-roughening procedure. The term "sandblasting" is here utilized to describe the entrainment of abrasive particulates against the surface, the abrasive particulates being generally metal particles, silicon carbide, silicone nitride, diamond dust, iron oxide, silicon dioxide or any other material capable of surface roughening. The entraining gas can be air or any other available gas. In both cases, in addition, the substrate may be preheated within the vacuum chamber or prior to introduction into the vacuum chamber to a temperature less than the melting point of the metal. The preheating temperature should be at least several hundred degrees, however.

Ceramic coating is effected in accordance with the present invention, by juxtaposing two electrodes of different metals, preferably a highly conductive and a highly refractory metal with a substrate which is preferably a ceramic body, and striking an arc between these electrodes in an evacuating chamber containing the electrodes and the substrate.

According to the invention, the electrodes are first given a relative polarity, i.e. one is poled positively while the other is poled negatively to deposit the metal upon one of these electrodes selectively while at the same time, it appears, depositing a small amount of this latter metal on the second electrode.

When polarity is then reversed, the metal vaporizes preferentially from the second electrode, initially including the small portion of the metal from the first electrode which is deposited thereon so that at the interface between of the two layers a mixed composition of the metals is formed.

The disadvantages, which have hitherto been encountered when conductivity metals, especially copper but also gold and silver, are applied to a ceramic substrate with respect to adhesion and especially with respect to adhesion after or during soldering or other welding of conductivity elements thereto, can be obviated if, prior to the application of the high conductivity metal, the ceramic is coated with a refractory metal in a comparatively small thickness and this intermediate layer of coating is in turn coated with the conductive metal.

More particularly; it has been found that it is possible to deposit a coating of a thickness of, say, 5 to 10 microns a tungsten, molybdenum, titanium or zircomium as the refractory metal upon the substrate and thereafter to apply a coating of greater thickness, say, 0.001 to 0.02 inches (0.004 cm to 0.0041 cm) of copper or a copper alloy, gold, silver or some other nonrefractory metal, i.e. metal having a substantially lower boiling point than that of the refractory metal which is used.

It has been found that, when a two-electrode method is used, it is possible to constitute one electrode as the refractory metal and the other electrode as the nonrefractory metal and by regulating the polarity of the electrodes during the deposition, the particular metal which is deposited can be controlled.

It has also been found that it is possible to increase the adhesion, in terms of the force required to separate the coating from the substrate by 100 or more times, all other things being equal, when the thin refractory metal coating is applied between the copper coating and the ceramic substrate.

A ceramic substrate can be used in accordance with the method described and masking techniques can be employed to ensure the formation of the deposit in any desired pattern.

One of the two electrodes which are juxta posed with the substrate can be moved out of alignment with the other electrode and replaced by a substitute electrode and the process repeated with the latter to additionally deposit at least a layer of the metal of the third electrode upon the second layer.

Apparatus and methods for coating substrates will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a diagram in elevational view illustrating an apparatus for carrying out vapor deposition in accordance with an embodiment of the present invention; Figure 2 is a similar view of another apparatus wherein, however, the vapor deposited material is collected on a vertically reciprocal electrode; Figure 3 is a vertical section, also in diagrammatic form, illustrating an apparatus for depositing material upon a substrate disposed below the pool of metal; Figure 4 is a view similar to Fig. 3 illustrating another embodiment of the invention;; Figure 5 is an axial cross-sectional view of another apparatus for depositing material upon a substrate according to this invention Figure 6 is an axial cross-sectional view of a highly compact portable apparatus for carrying out the method of the invention; Figure 7 is a diagrammatic cross-sectional view of another apparatus for carrying out the present invention; Figure 8 is a a diagrammatic section illustrating the application of the invention to the coating of a quartz crucible for use in the production of semiconductor wafers; Figure 9 is a view of still another device diagrammatically illustrating the application of large area coatings to ceramic substrates according to the invention; Figure 10 is a diagram of an apparatus for carrying out the ceramic-coating method of the present invention;; Figure ii is a cross-sectional view drawn to a larger scale of a product of the present invention; Figure 12 is a view similar to Fig. 10 but illustrating another apparatus for carrying out the invention; Figure 13 is a diagram showing an effect obtained during the deposit of the metal for the first layer before the commencement of the second layer; and Figure 14 is a cross-sectional view through the product in the latter case.

In Fig. 1 there is shown a system utilizing a simple arc method for obtaining mirror-iike protective coatings upon substrates or for evaporating various metals or metal alloys, including heat-resistant and refractory metals, to apply coatings thereof to the substrate.

As is apparent from Fig. 1, the basic apparatus can include a vacuum chamber, not shown, which can be similar to the vacuum chamber of Fig. 6 and in which a metal electrode 1 can be fed by an electrode feeder 7 toward an electrode body 2 to form the pool 3 of molten metal with which the arc 4 is struck.

The electrode body 2 is held in a fixure or holder 5 and the direct-current source applies the arc current across the electrode 1 and the body 2 via a conventional arc stabilizing circuit represented at 8.

It has been found to be advantageous to provide the relatively small cross-section electrode 1 with a thermal regulator 6 tending to prevent overheating of this electrode.

Since the cross-section of body 2 is substantially larger than that of the electrode 1, the pool 3 lies in a concave recess formed in situ in the body 2.

Example 1 The apparatus of Fig. 1, utilizing electrodes 1 and 2 of titanium, aluminium, tungsten, tantalum or copper, strikes an arc at a temperature of 5000 to 7000"F (2760on to 3871 C) to generate vapor of the metal of the pool 3 which traverses the distance of 10 to 1 5 cm to the substrate 10 and forms a coating of the metal thereon. The pool 3 can be formed by a mixture of metal contributed by the electrodes 1 and 2, thereby depositing an allow of the metals of the two electrodes upon the substrate. Preferably the electrode is composed of titanium while the molten metal predominantly consists of aluminium, tungsten, tantalum or copper.

The apparatus of Fig. 1, without substantial modification, can be utilized in a noncrucible method of generating protective coatings of carbides, for producing silicide coatings on the substrate or for forming carbide or silicide and even silicon carbide layers upon the substrate. To deposit silicon carbide-tungsten carbibe layers on the substrate, electrode 2 is composed of graphite and electrode 1 of tungsten silicide. The vacuum is initially drawn to 10-6 torr (133 x 10-6Pa) and maintained at 10-5 torr (133 x 10 5Pa) or lower. The direct current arc-generating voltage is 100 volts and the arc current 1 50 amperes. The deposit forms at a rate of about 0.2 grams per minute.

In this case, the apparatus of Fig. 1 is used, again in the usual vacuum chamber, although the electrode 1 can be composed of silicon or carbon while the electrode 2 is composed of a metal whose silicide or carbide is to be formed or, in the case of a deposit of silicon upon the substrate, can also consist of silicon.

For example, when a silicon carbide deposit upon the substrate 10 is desired, the electrode 1 may consist of silicon while the electrode 1 is a carbon block in which a pool 3 of silicon and solubilized carbon is received.

The vapors are transferred to the substrate and deposited in a silicon carbide layer ther eon. The substrate may be titanium and the deposit formed on the substrate may be a mixture of titanium silicide and titanium carbide.

Alternatively, when the electrode 1 is composed of silicon or carbon, and the electrode body 2 is composed of titanium, titanium carbide or silicide can be deposited on a substrate of a different composition.

When a slight oxidizing atmosphere is provided in the evacuated chamber, silicon dioxide deposits are formed on the substrate.

Obviously the apparatus of Fig. 1 is particularly effective in the production of semiconductors.

The theremoregulator 6 may be duplicated along the length of the electrode 1 and additional thermoregulators may be provided for the electrode body 2 to prevent overheating thereof.

When either the electrode 1 or the body 2 is composed of silicon and the other is composed of carbon, silicon carbon is generated by the reaction and deposits in a higher purity than that of the original silica and carbon.

When both of the electrodes are composed of silicon, high density silica and silicon deposits can be obtained as is particularly desirable for the coating of semiconductors.

The apparatus of Fig. 2 is generally similar to that of Fig. 1 but operates under somewhat different principles, the evaporation being effected at least in part from the wetted upper electrode 101.

In this Figure, elements which correspond to those of Fig. 1 utilize similar reference numerals differing in the hundreds position.

In Fig. 2, the electrode feeder 107 is coupled with a vertical reciprocator 11 2 which imparts a reciprocation to the electrode 101 in the direction of the arrow 1 14 so as to periodically plunge the tip of the electrode 101 into the pool 103 of the molten metal formed in the electrode body 102.

Upon rising from this pool to restrike the arc 104, the coating 11 3 of molten metal upon the electrode 101 is evaporated and the deposit is formed upon the substrate 110.

The electrode body 102 is shown in the holder 105 and the arc current supply is formed by the direct current source 109 and the stabilizer 108 in the manner described, the electrode 101 being provided with the thermoregulator 106.

This system has been found to be particularly effective, in a modification of the foregoing example, when the electrode 101 is composed of titanium and the pool 103 is formed of aluminium.

In Fig. 3 the vapor is deposited upon a substrate 210 disposed below a crucible 217 in the form of an upwardly open ring containing the molten metal 203, the crucible being mounted in a holder or frame 205.

Here the upper electrode 201 is in the form of a spherical segment which functions as a reflector so that, when an arc 204 is struck between the electrode 201 and the melt in the crucible 217, the vapors pass upwardly as represented by the arrows 219 and are reflected downwardly to focus upon the substrate 210 as represented by the arrows 218.

The direct current source 209 is here connected across the electrode 201 and the crucible 21 7 via the arc stabilizer 208 and the upper electrode 201, mounted on the rod 216, is vertically positioned by the feeder 207 and horizontally positioned by an auxillary mechanism 215 which adjusts the position of the electrode 201 over the evaporating metal.

In this embodiment, the electrode 201 can be composed of titanium, molybdenum or tungsten while the molten metal can be composed of aluminium or copper and the crucible 217 of graphite.

In Fig. 4 there is shown another embodiment of the invention in which the vapors flow downwardly to deposit upon the substrate 310.

In this case, the upwardly open crucible 317 containing the molten metal 303 can be supplied with additional molten metal from a ladle or other sources represented at 322 or with solid metal which is melted in the crucible 317. The latter can be heated by auxillary means such as an inductive heater 323 and is supported in a holder 305.

The bottom of the crucible 317 is formed with apertures 321 at which droplets of the molten metal appear, these droplets being vaporized by the arc 304 struck between the electrode 301 and the bottom of the crucible 317.

The temperature in the region of the arc can be controlled by an auxillary inductive means 324 and the electrode 301 can be cooled as represented by the cooling element 306.

Electrode 301 is fed toward the crucible 317 by the electrode holder 307 and the arc is maintained by an arc stabilizer 308 connected to the direct current source 309.

In this embodiment, the molten metal may be copper.

In place of the auxillary device 324, a substrate to be coated may be provided at this location, e.g. in the form of a titanium ring, which can collect the vapor in the form of a coating.

The embodiment of Fig. 5 evaporates the molten metal as it is formed in a closed space, the vapors being discharged through apertures 425 on the substrate 410.

In this case, the pool of liquid is formed by melting the electrode 402 supported by the holder 405 by feeding the counter electrode 401 via the electrode feeder 407 through a central bore 426 in the electrode 402, the electrode 401 passing through an insulating sleeve 427 forming a guide. A temperature regulator 406 is provided coaxially around the two electrodes adjacent the arc 404 to prevent overheating in the region ahead of the apertures 425. The deposit is formed on the substrate 410.

The current is supplied between the electrodes through the arc stabilizer 408 and the direct current source 409 in the manner described previously.

Fig. 6 shows a portable voltaic are device for depositing reflective, anticorrosive, protective and semiconductor type metal, silicide and carbide coatings using the principles described previously.

This apparatus comprises a vacuum chamber 500 which is formed at its upper end with a handle 530 enabling the portable unit to be readily transported.

Within this chamber, there is provided a hollow sphere 517, the lower part of which forms a crucible for the molten metal 503, coated internally with a high-temperature heat resistant (refractory) material such as aluminium oxide.

The upper portion of this sphere is coated at 531 with a reflective layer concentrating the heat reflected from the bath back onto the latter.

An arc 504 is struck between the electrode 501 and the bath 503, the electrode being fed by the unit 507 toward the bath as the electrode material is consumed.

Additional metal, e.g. in solid form, is fed to the bath as a rod 532 which also is connected to the feeder 533' so that as the bath is consumed, additional metal is supplied thereto.

The electrode 501 and the bath 503 are connected to opposite terminals of an arc stabilizer and a direct current source in the manner previously described.

A tubular electrode 502 surrounds the rod 532.

The lower part of the chamber 500 is provided with an airpump as represented at 533, the latter evacuating the chamber containing the hollow sphere 51 7 and, via a vacuum hose 534, via a valve 535, an adaptor 536 of outwardly divergent configuration which can be connected to a lateral aperture 525 of the hollow sphere 517.

The chamber 500 can be formed with a heating coil 537 to prevent undesired condensation of vapor thereon.

Between the aperture 525 and the adaptor 536 there is provided a vacuum lock 538 and a mounting arrangement 539 for holding a variety of adaptors of different shapes and sizes.

The adapter 536 is also formed with a vacuum gasket 540 whereby the adaptor can bear against the substrate 510 to be coated.

The portable unit shown in Fig. 6 is carried to the location of the substrate 510 to be coated and the appropriate adaptor 536 is mounted on-the fitting 539 and the gasket 540 pressed against the surface 510 to be coated. The arc current is supplied and the system is evacuated by the air pump 533, thereby melting the metal and forming the bath 503 within the hollow sphere. The gate 538 is then opened and the vapors permitted to pass onto the substrate 510 at least in part by pressure differential as controlled by the valve 535 maintained between the interior of the sphere 517 and the adaptor 536.

Practically any product at any site can be coated and the use of a variety of adaptors of different shapes and sizes enables coating of even intricate bodies without moving them from the area in which they are to be used.

The device can be collapsible so as to be used to provide coatings inside ducts and the like.

The apparatus shown in the drawing, without the adaptor 536, can be used as a propellant for individuals or equipment in space.

Upon generation of the arc, one need only open the gate 538 to discharge a stream through the aperture 525 and effect propulsion in the opposite direction. The vacuum in space provides a natural vacuum for the device and no air pump 533 is then required.

Practically any waste found in space applications can be utilized in the vessel 517 to generate such propulsion.

Fig. 7 combines features previously described and concepts developed above.

In this system, which can be used to deposit a coating 610' on the inner surface 61 0a of a tube 610, forming a substrate. of complex shape, a material-supplying electrode 602 of corresponding shape is mounted centrally of the tube on a support 602a and is provided with an induction heating coil 606a of a temperature controller 606 which can have a thermocouple 606b or a like temperature sensor responsive to the temperature of the material-supplying electrode 602 for maintaining the temperature of the latter constant in the range of 800 to 1000"F (427"C to 538"C) by conventional feed-back control circuitry.

As in the previous embodiment, the substrate and the source of the material to be deposited on the substrate are enclosed in a vacuum chamber 600 which can be evacuated to 1 0 6 torr so that vapor deposition can be effected at a pressure of 10 - 5 torr (133x 10-5Pa).

The end of the material-supplying electrode 602 is provided with an arc-striking electrode 601 which can be reciprocated toward and away from the electrode 602 by an electrically controlled reciprocating drive 607. The latter can be operated in response to a zero current detector 607a so that when the arc current decays completely, the electrode 601 is displaced to the left into contact with the end 602a of the electrode 602 and is then withdrawn to reestablish an arc. The arc current is provided by a pulsating direct current source 609 across which an arc stabilizer 608 the parameters of the arc current and arc voltage are adjusted within the range of 50 to 90 amperes and 30 to 60 volts by these circuit elements.

In practice, utilizing the system illustrated once the arc is struck, the arc itself, an evaporation effect or some other electromagnetic phenomenon appears to progress as represented by the arrow A generally helical and spiral where arc-striking location and vapor deposition takes place over the entire length of the material-supply electrode 602 which is subjected to this phenomenon, i.e.

over the length at which the phenomenon is effective until the arc decays.

The material loss from the electrode 602 gradually transforms it into a tapered shape as represented by the dot-dash lines as 602b in Fig. 7.

The fact that the taper results in a recession of the electrode from the substrate does not create any problem of significance because the greatest deposit is at the region of greatest recession and consequently, the ultimate coating as it progresses along the substrate is highly uniform.

The system of the invention is especially useful in coating temperature-sensitive materials with very small thicknesses of coating material since the coating is especially rapid and it is possible to carry out the deposition without significantly heating up the substrate.

Example 2 A copper electrode 602 of the shape shown is provided in a substrate tube with an initial spacing of electrode 602 from the substrate of about 10 cm. The electrode is maintained at a temperature of 900"F (428"C) and an arc is struck in the manner previously described at one end. The arc current is about 70 amperes and the voltage applied after the electrode 601 is withdrawn to form the arc is about 40 volts. The speed of evaporation from electrode 602 under these conditions exceeds the speed of evaporation in Example 1.

In Fig. 8 there is shown an arrangement for applying a silicon coating 710' to a quartz crucible 710 of the type utilized for melting of silicon and from the melt of which a monocrystalline bar of silicon can be drawn for subsequent slicing into silicon wafers and use in the semiconductor industry. According to the invention, the interior surfaces of the crucible are sand blasted and the crucible is preheated, e.g. to a temperature of 200 to 600"C before the crucible is placed in the vacuum chamber. A pair of silicon electrodes 701 and 702 are juxtaposed with one another within the crucible and, utilizing the electrode reciprocating means of the type shown at 607, the electrodes are brought together and then separated as represented by the arrows 707a and 707b so they touch and then are drawn apart to stroke the arc.The power supply which can also be of the type described is represented at 709. The arc current can again be 50 to 90 amperes and the arc voltage some 30 to 60 volts. A substantially uniform highly adherent silicon coating can be obtained, especially when the silicon is initially heated, e.g. by means similar to that utilized in the embodiment of Fig. 7.

When a nitrogen atmosphere is released in the region of the arc, the deposit is of the silicon nitride Si3N4. When one of the electrodes is composed of carbon, a silicon carbide deposit is formed. The same system can be used for coating any substrate with pure Si or one of the other coatings mentioned. After the initial arc is produced the electrodes can be cooled.

In Fig. 9 there is shown a system for the large area coating of a ceramic substrate 801 which prior to introduction into the vacuum chamber can be initially preheated, after its coating-receiving surface has been sandblasted, by the movement of a burner 820 along the underside of the substrate. The electrode 801 can be composed of a refractory metal or nickel and via the actuator 807 is urged into contact with and withdrawn from contact from the counterelectrode 802 which may also be composed of the same metal.

The power supply has been represented at 809. The electrodes are here mounted on a track 821 and are moved along the substrate so that the arc is repeatedly struck and the arc travels along the electrode 801 in the vacuum chamber receiving the entire assembly, the entire surface of the substrate is coated utilizing the principles described in Fig. 7.

Example 3 Utilizing an apparatus operating with the principles shown in Fig. 9, an aluminium oxide plate is coated to a thickness of 1 to 2 mils with tungsten utilizing tungsten electrodes. The arc current is 50 amperes and the arc voltage 40 volts for a maximum electrode spacing of approximately 4 mm. The electrode diameter was about 1 cm. The tungsten coat ing was highly adherent to the alumina plate.

Fig. 10 shows an apparatus, in a highly diagrammatic form. for carrying out the method of the invention. Thus apparatus com prises a chamber 1010 which can be evacu ated by a suction pump 1011 to the desired degree of vacuum, generally 10-5 to 10 torr (133 X 10-to 133 x 10-6Pa). Within this chamber, by means not shown, a ceramic substrate 1012 can be disposed and can be shielded by a mask diagrammatically illustrated at 101 3 so that coating can only occur in regions defined by the windows 1014 in the mask.

Within the vacuum chamber the portion of the substrate to be coated is juxtaposed with a pair of electrodes, i.e. a copper electrode 1015 and a tungsten electrode 1016, the electrodes being provided with means such as the electromagnetic motors (solenoids) 101 7 and 1018 for briefly bringing them into contact to strike the arc and then drawing them apart. The pulser for periodically energizing the devices 1017 and 1018 have been shown at 1019.

The power supply comprises the alternating current source 1020 which is connected to a rectifier 1021 and the latter is provided with a reversing switch 1022 which can reverse the polarity of the electrodes 1015 and 1016 under the control of a timer 1023.

In operation with the copper electrode 101 5 poled positively and the tungsten 16 poled negatively an arc can be struck by passing an electric current of 30 to 100 amperes at a voltage of 40 to 100 volts through and across the gap after the electrodes briefly touch to preferentially vaporize tungsten and thus deposit tungsten through the window 1014 of the mask 1013 on the substrate. The duration of coating is controlled by the timer 1023 which, after the coating of the order of microns in thickness has been applied, reverses the polarity so that the copper electrode 101 5 is now poled negatively and the tungsten electrode 101 6 is poled positively whereupon copper is vaporized from the electrode 101 5 and deposited upon the substrate.

As can be seen from Fig. 11, the resulting article has a substrate 1030, e.g. of aluminium oxide, bearing a copper coating 1032 which is separated by the refractory metal coating 1031 (tungsten) of smaller thickness.

Example 4 Utilizing the principles described, a current of about 70 amperes, a voltage of 80 watts and a vacuum of about 10-5 torr (133 X 10-5Pa), an aluminium oxide plate is coated with tungsten to a thickness of about 8 microns and with copper to a thickness of about 0.002 inches (0.008 cm). The adhesion is measured and for the coating is found to be 500 to 700 Ibs per square inch (87500 to 122,500 N/m) (force required to remove the coating). When under identical conditions a copper coating of the same thickness is applied to the same substrate, the adhesion is only 6 to 8 Ibs per square inch (1050 to 1400 N/m). The direct copper-to-ceramic bond is found to be sensitive to both mechanical and thermal effects when a solder connection is made to it and with the copper/tungsten contact, formed according to the invention, no similar sensitivity was found.

Practically identical results could be obtained by substituting molybdenum, titanium and zirconium for the tungsten and with combinations of these refractory metals with one another and with tungsten as intermediate layers. Similarly, high degrees of adhesion were obtained with nickel, gold, silver and alloys thereof with one another and with copper.

Fig. 1 2 shows a modification of the apparatus of Fig. 10 in which the chamber 1110, evacuated by the pump 1111, includes a ceramic substrate 111 2 which is to be coated with a plurality of metals. In this case, in addition to the common electrode 111 6 and its actuator 111 8 driven by the pulser/timer 1119, a pair of counter-electrodes 111 5 and 1115a, respectively of copper and gold, are provided each with a respective actuator 1117, 1117a. The counter-electrode assembly is provided on a track 111 24 having a drive enabling shifting of the two electrodes to the left as illustrated by the showing of the electrode 111 5 in its shifted position in dotdash lines.Naturally, in the latter position, the electrode 1115a is aligned with the common electrode 1116. A reversing switch 11 22 as described in connection with Fig. 10 is here also provided and the apparatus is energized from the alternating current lines 11 20 through the rectifier 1121.

In this mode of operation, once the chamber is evacuated, the actuators 111 7 and 111 8 are operated to move the electrodes 111 5 and 111 6 together and apart to strike an arc, the electrode 111 6 of tungsten being poled positively while the electrode 111 5 of copper is poled negatively.

This process is continued in the manner described until the initial coating of tungsten has been brought up to the desired thickness.

As can be seen from Fig. 13, this procedure not only results in erosion of the tungsten electrode 1016 but it also gives rise to a small deposit 1025 of tungsten on the copper electrode 1015.

When the polarity is now reversed, i.e. the copper electrode 1015 is poled positively and the tungsten electrode is poled negatively, the arc is struck and evaporation is effected from the copper electrode, the tungsten deposit 1025, which has been exaggerated in thickness in Fig. 13, vaporizes together with copper and a mixed tungsten/copper deposit is produced as in interface.

In Fig. 14, for example, the substrate 111 2 is shown to have been coated at 1131 with the tungsten layer. The mixed or tungsten layer 11 26 is then applied thereto before, with continuation of the generation of vapor by arc-striking between the electrodes 111 5 and 1116, the copper coating 11 32 is applied.

When the copper coating has reached the desired thickness, the electrode assembly 1115, 111 7 is shifted to the left and replaced by the assembly 111 spa, 111 7a and the arc is struck between the electrodes 1115a, 111 6 to deposit gold in a layer 11 33 upon the copper coating.

A controller 11 27 may be provided for the electrode shifting device 1124, the pulser/ timer 1119 and the reversing switch 11 22 and may be controlled by a preprogrammed microprocessor to effect the polarity reversal and the switching of electrodes when layer thicknesses of the desired magnitude have been achieved.

Example 5 The method of Example 4 is practised except that the copper electrode is shifted away and replaced by a gold electrode. Utilizing the same vacuum and striking a similar arc, a flash coating in the order of the 5 microns range of gold was deposited upon the copper coating under the condition recited for copper deposition.

The adhesion was not diminished and the resulting gold layer was found to make an ideal contact for micro-electronic purposes.

Investigations of the interface between copper and tungsten showed mixed transition zone 11 26 which was traced to the vaporization of a minor deposit of tungsten from the copper electrode.

Claims (9)

1. A method of coating a quartz crucible for use in the melting of silicon which comprises the steps of: juxtaposing a pair of electrodes, composed of at least one component of a material adapted to coat said crucible, with an interior surface of said crucible; evacuating the space in which said electrodes are juxtaposed with said surface to a pressure of at most 10 - torr (133 X 10-5Pa) and maintaining the pressure in said space substantially no higher than 1 0 - S torr (133 x 10-5Pa) during deposition; and striking an electrical arc between said electrodes at one end of each of said electrodes at a voltage in the range of from 30 to 60 volts and with a current in the range of from 50 to 90 amperes by intermittently bringing said electrodes into contact with one another and separating them, thereby depositing material evaporated from said electrodes substantially uniformly over said interior surface of said crucible.

2. The method defined in Claim 1 wherein at least one of said electrodes is composed of silicon, further comprising the step of controlling the temperature of said one of said electrodes to maintain said temperature in the range of from 800"F to 1000"F (427"C to 538"C).

3. The method defined in Claim 1 or Claim 2 comprising the step of roughening said surface by blasting particles thereagainst.

4. The method defined in any preceding Claim comprising the step of preheating said crucible before striking said arc.

5. A method of coating a ceramic with a metal, comprising the steps of roughening a surface of a ceramic substrate by subjecting it to blasting with particles; preheating said substrate to a temperature of at least 200to but less than the melting point of a metal to be applied thereto; juxtaposing said surface of said substrate with an electrode composed of said metal; evacuating the space in which said electrode is juxtaposed with said substrate to at most 10-5 torr (133 X 10-5Pa) and maintaining the pressure in said space substantially no higher than 10-5 torr (133 X 1 0-5Pa); and striking an arc with said electrode by intermittently attacking same with another electrode while applying a voltage in the range of from 30 to 60 volts across said electrodes and passing a current in the range of from 50 to 90 amperes through said electrodes to evaporate the electrode composed of said metal and deposit said metal on said surface.

6. The method defined in Claim 5 wherein said metal is composed of nickel, copper, tungsten, titanium or tantalum.

7. Apparatus for effecting a bond by the method of Claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

7. The method defined in Claim 5 or Claim 6 wherein said ceramic is a body of alumina.

8. A method of bonding a material to a ceramic which comprises the steps of applying to a ceramic substance a thin layer of refractory metal; and thereafter bonding said material to said layer.

9. The method defined in Claim 8 wherein said material is a high-conductivity metal.

10. The method defined in Claim 9 wherein said high-conductivity metal is copper, gold, silver or an alloy thereof.

11. The method defined in any of Claims 8 to 10 wherein said refractory metal is tungsten, molybdenum, titanium, zirconium or an alloy or combination thereof.

1 2. The method defined in any one of Claims 8 to 11 wherein said refractory metal is applied to said substrate to a thickness of the order of microns and said metal is applied to a thickness of 0.001 to 0.02 inch (0.0004 to 0.0081 cm).

1 3. The method defined in Claim 1 2 wherein said refractory metal is applied in a layer of 5 to 10 microns thick.

1 4. The method defined in Claim 1 3 wherein said high-conductivity metal is copper and said refractory metal is tungsten.

1 5. The method defined in any one of Claims 8 to 1 4 wherein the metal is applied by striking an arc between a pair of electrodes to vaporize the metal from one of said electrodes in an evacuating chamber.

1 6. The method defined in Claim 1 5 wherein an electrode of said refractory metal is juxtaposed with an electrode of said highconductivity metal, said electrodes are brought into contact and drawn apart to strike said arc, and said electrodes are initially ener gized with positive and negative polarities in one sense to initially deposit said refractory metal on said substrate and the polarity is thereafter reversed to deposit said high-conductivity metal upon said substrate.

1 7. A ceramic body comprising a ceramic substrate having a thin refractory metal bonded thereto in a first layer and a second relatively thick layer of a high-conductivity metal bonded to said first layer.

1

8. A method of making multilayer metal coatings on a substrate which comprises the steps of juxtaposing a first electrode of a first metal with a second electrode of a second metal with one another and disposing a substrate in vapor-reversing relationship with said electrodes in a chamber; evacuating said chamber; striking an arc between said electrodes in said evacuated chamber while applying one electrical polarity to said first electrode and another electrical polarity to said second electrode to selectively vaporize metal from said first electrode and deposit same upon said substrate; thereafter reversing the polarities of said electrodes and striking an arc between them to selectively deposit metal from said second electrode on the metal from said first electrode previously deposited upon said substrate; and thereafter juxtaposing with one of said electrodes a substitute electrode and striking an arc between said substitute electrode and said one of said electrodes in said chamber to vaporize metal from said substitute electrode and deposit same on said layer of the metal of said second electrode on said substrate.

1

9. The method defined in Claim 1 8 wherein said substrate is a ceramic.

20. The method defined in Claim 1 7 or Claim 1 8 wherein said first electrode is composed of a refractory metal.

21. The method defined in Claim 20 wherein said refractory metal is selected from the group which consists of tungsten, molybdenum, titanium, zirconium and alloys and combinations thereof.

22. The method defined in any one of Claims 1 8 to 21 wherein at least one of said second and substitute electrodes is composed of a metal selected from the group which consists of nickel, copper, gold, silver and alloys thereof.

23. The method defined in any one of Claims 1 8 to 22 wherein the layer of the metal of said first electrode is applied in a thickness of the order of microns and the layer formed by one of said second and substitute electrodes has a thickness of 0.01 to 0.02 inches (0.0004 cm to 0.0081 cm).

24. A method of coating a substrate which comprises the steps of juxtaposing a pair of electrodes of different metals with a substrate in a chamber; evacuating said chamber; applying one electrical polarity to one of said electrodes and the opposite electrical polarity to the other of said electrodes and striking an arc between said electrodes by approximately them into contact and drawing them apart to vaporize material from said one of said electrodes and deposit the vaporized material on said substrate in a first layer while simultaneously transferring a portion of said material onto the other of said electrodes; and thereupon reversing the electrical polarity of said electrodes and striking an arc between them to vaporize material from said other electrode including said portion and deposit a mixed layer of materials from both of said electrodes on said first layer as a transition layer and thereafter deposit material from said second electrode onto said transition layer.

25. The method defined in Claim 24 wherein said substrate is a ceramic.

26. The method defined in Claim 24 or Claim 25 wherein the material of said one of said electrodes is a refractory material.

27. The method defined in Claim 26 wherein said refractory metal is tungsten, molybdenum, zirconium or an alloy or combination thereof.

28. The method defined in any one of Claims 24 to 27 wherein said other electrode is composed of nickel, copper, gold, silver or an alloy thereof.

29. The method defined in any one Claims 24 to 28 wherein said one of said electrodes is composed of tungsten.

30. The method defined in any one of Claims 24 to 29 wherein said other electrode is thereafter shifted out of alignment with said one of said electrodes and is replaced by a substitute electrode with which an arc is struck in said chamber to deposit a further material on said material of said other electrode.

31. An apparatus for multilayer coating of a substrate as defined in one of the preceding claims which comprises a chamber; means for evacuating said chamber; a substrate adapted to be coated being positioned in said chamber; a pair of first electrodes juxtaposed with one another in said chamber; means for applying an electrical potential to said electrodes with one of said electrodes having one electrical polarity and the other of said electrodes having the opposite electrical polarity; means for approximately said electrodes and drawing them apart to strike an arc between said electrodes selectively vaporizing material of said one of said electrodes for deposit upon said substrate in a first layer; means for reversing the polarities on said electrodes whereby the electrodes strike an arc to deposit material selectively from said other of said electrodes on said first layer; and means in said chamber for automatically replacing one of the first mentioned electrodes with a substitute electrode and striking an arc between the remaining one of the first mentioned electrodes and said substitute electrode to deposit material of said substitute electrode selectively on said substrate.

32. A body made by the method of any one of Claims 9-30.

33. A method of coating substantially as herein before described with reference to the accompanying drawings.

34. Apparatus for effecting a bond substantially as hereinbefore described with reference to the accompanying drawings.

35. Apparatus for carrying out the method of any one of Claims 1 to 30 and 33.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 5-35 above have been deleted.

New or textually amended claims have been filed as follows:

1. A method of coating a quartz crucible for use in the melting of silicon which comprises the steps of: juxtaposing a pair of electrodes, composed of at least one component of a material adapted to coat said crucible, with an interior surface of said crucible; evacuating the space in which said electrodes are juxtaposed with said surface to a pressure of at most 10-6 torr (133 x 10-6Pa)and maintaining the pressure in said space substantially no higher than 10-5 torr (133 X 10-5Pa) during deposition; and striking an electrical arc between said electrodes at one end of each of said -electrodes at a voltage in the range of from 50 to 90 amperes by intermittently bringing said electrodes into contact with one another and separating them, thereby depositing material evaporated from said electrodes substantially uniformly over said interior surface of said crucible.

2. The method defined in Claim 1 wherein at least one of said electrodes is composed of silicon, further comprising the step of controlling the temperature of said one of said electrodes to maintain said temperature in the range of from 800"F to 1000"F (427"C to 538"C).

3. The method defined in Claim 1 or Claim 2 comprising the step of roughening said surface by blasting particles thereagainst.

4. The method defined in any preceding Claim comprising the step of preheating said crucible before striking said arc.

5. A method according to any preceding Claim wherein the material deposited from said electrodes is selected from the group consisting of silicon carbide or silicon nitride.

6. A method of coating according to Claim 1 substantially as hereinbefore described with reference to the accompanying drawings.