Note: Descriptions are shown in the official language in which they were submitted.
SPECIFICATION
Field of the Invention
The invention relates to a method of coating of a
substrate with a material which is brought into the vapor phase
by electrical jeans.
undo of the Invention
,
The deposition of material from a vapor phase onto a
substrate is well known in the coating art and in the field of
surface transformation 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 transform
matron from the solid phase to the liquid phase and then from
the liquid phase to the vapor phase.
The orating is generally effected in a vacuum and
usually a relatively high vacuum must be drawn to permit trays-
for of vapors from the source to the substrate.
, .
Earlier systems may use induction heating to effect
I; 20 the aforementioned phase transformation.
A particular problem has been encountered with respect
to the quartz~crucibles~utilized in the production of silicon
wafers for semiconductor purposes. Such crucibles in which the
elemental Solon lo melted, generally are composed of quartz
and are received in a supporting carbon jacket within an induct
lion toll which lo utillzed~to melt the elemental silicon. A
monocrystaIline silicon seed can then be lowered into the silicon
:, :
-1 - Jo .,
,. . : : ",, " .: . ,
I
molt of the crucible and raised slowly to draw a silicon bar
which is control1edly cooled so that the monocrystalline product
can then be cut into wafers. Such crucibles are, for the melt-
in 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 deterrer-
lion of the crucible and threatens the introduction of undesired
impurities in the waxers 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 core-
miss. While many metals can be coated onto ceramics, the refract
tory metals such as tungsten and titanium have been applied here-
tougher in a manner which is less than satisfactory and practically
in all cases adhesion problems are encountered by the methods
used heretofore.
: :
Jo
An object of this invention is to provide a method or
the vapor deposition ox material on large-area and/or complex
configuration substrates at relatively low-energy oust and with
improved uniformity.
It is also an object of the invention to provide a
method for the whopped coating of complex and/or large-area
surfaces.
Yet another object of the invention is to provide an
improved method ox protecting quartz crucibles of the type Utah-
I- lived in the semiconductor yield so asset increase the life
thereon.
, .
I
Sill]. another object of this invention it to provide
an improved method of applying metal coatings to ceramics whereby
the poor adhesion problems characterizing prior art systems are
avoided.
summary of the Invention
These objects and others which Wylie become apparent
hereinafter are attained in accordance with this invention, in
a method of vapor deposition which 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 a the electrode in a vacuum,
and striking an arc between one end of this electrode and a
counter electrode such that the arc current should be between 50
and 90 amperes with a voltage applied across the electrode of
30 to I volts.
Surprisingly, once the arc is struck as the two elect
troves are separated, the arc, a portion of the arc or a heating
effect generated 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 counter electrode.
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 counter
electrode toward a~;reglon of the length ox the one electrode
Jo which is further removed prom the counter electrode in spite, of
the foot 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, lye. the deposition electrode, while
; -3-
I
originally of uniform cross section, develops a taper toward
the counter electrode and coating from the blank of the deposit
lion electrode onto the substrate can be observed at consider-
able 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.
According to a feature of the invention, means is
provided at an end of the electrode of the material to be depot
sited, remote from the arc-striking electrode to control the
temperature of the material-supplying electrode, generally to
maintain it in the range of 800F to 1000F, 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 marine the material-supplying electrode. In addition
to metals and other alloys, carbides, brides, silicides and
nitrides can be deposited on the substrate.
While it is not fully understood why the rate of ova-
portion of the material to be deposited increases with the
lower energy utilization of the present invention, it is possible
that the migration of the arc may spread the otherwise pooled
moltion~phase over a wider area of the material-supplying elect
trove to allow, in effect, evaporation of the molten metal in
thin film forum
Lo
cording to another feature of the invention, the
principles set forth above are utilized in the application of
metal coating to synthetic resins, the synthetic resins in the
form of cabinets or housings for electronic components. It's been
found, most surprisingly, that since the substrate is unaffected
by the large area coatings which can be applies in accordance
with the invention, the invention 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
invention 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
I; production of the monocrystalline bars to be drawn from the
molten silicon in the semiconductor yield. Furthermore, the
o invention may reutilized in the application of metal coatings
to ceramics, with improved adhesion, even when the applied
metals are nickel, tungsten, titanium, tantalum and like refract
tory metals which have been difficult to apply heretofore to
ceramic substrates. Practically any ceramic substrate may be
utilized for the purpose ox the invention and in the case of
the quartz crucible sand the ceramic substrates, it is preferred
to subject the surfacs~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 particulatss against the surface, the abrasive part-
curates being generally metal particles, silicon carbide, sift-
3 con nitride, diamond dust, iron oxide, silicon dioxide or any
Jo
--5--
I
other materiel 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 from 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 prom 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 the two
layers a mixed composition of the metals is formed.
The disadvantages, which have hitherto been encountered
when high conductlvlty~metals, especially copper but also gold
and silver, are applied to a ceramic substrate with respect to
: :
; adhesion and especlally~with respect to adhesion after or during
soldering or other welding ox conductivity elements thereto,
oan~be obviated if, prior to the application of the high conduct
3 ~iVlty metal, the ceramic is coated with a refractory metaling
-6
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 of
tungsten, molybdenum, titanium or zirconium as the refractory
metal upon the substrate and thereafter to apply a coating of
greater thickness, say 0.001 to 0.02 inch of copper of a copper
alloy, gold, silver or some other non refractory 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, according to the invention, it is possible to keenest-
lute one electrode as the refractory metal and the other elect
trove as the non refractory metal and by regulating the polarity
of the electrodes during the deposition, the particular metal
i
which is deposited can be controlled.
With the present invention, it has been found that it
is possible to increase the adhesion, in terms of the force no-
squired to separate the coating from the substrate by 100 or
more times, all other things being equal, when the thin refract
tory metal Catalina is applied between the copper coating and
the ceramic substrate.
A ceramic substrate can be used in accordance with
the present invention and masking techniques can be employed
to ensure the formation of the deposit in any desired pattern.
According to an important feature of the invention,
one of the two electrodes which are juxtaposed with the substrate
can be moved out of alignment with the other electrode and no-
; placed by a substitute electrode and the process repeated with
'
the latter to additionally deposit at least a layer of the meatloaf the third electrode upon the second layer.
Brief Description of the Drawing
__
The above and other objects, features and advantages
of the present invention will become more readily apparent from
the following description, reference being made to the accompany-
in drawing in which:
FIG. 1 is a diagram in elevation Al view illustrating
an apparatus for carrying out vapor deposition in accordance with
an embodiment of the present invention;
FIG. 2 is a similar view of another apparatus wherein,
however, the vapor deposited material is collected on a vertically
reciprocal electrode;
FIG. 3 is a vertical section, also in diagrammatic
form, illustrating an apparatus for depositing material upon a
substrate disposed below the pool of metal;
Jo FIG. 4 is a view similar to FIG. 3 illustrating
:
another embodiment of the invention;
FIG. 5 is an axial cross-sectional view of another
Jo 20 apparatus for depositing material upon a substrate according to I-
; this invention;
FIG. 6 Sian axial cross-sectional view of a highly
compact portable apparatus for carrying out the method of the
invention
FIG. issue a diagrammatic cross sectional view of
another apparatus for carrying out the present invention;
: FIG. 8 is a diagrammatic section illustrating the
application of the invention to the coating of a quartz crucible
or use in the production of semiconductor wafers;
Jo : :
I: :
US
FIG. 9 is a view of still another device diagram-
tidally illustrating the application of large area coatings to
ceramic substrates according to the invention;
FIG. 10 is a diagram of an apparatus for carrying
out the ceramic-coating method of the present invention;
FIG. 11 is a cross-sectional view drawn to a larger
scale of a product of the present invention;
FIG. 12 is a view similar to FIG. 10 but illustrating
another apparatus for carrying out the invention;
FIG. 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
FIX. 14 is a cross-sectional view through the product
in the latter case.
,:
Specific Description
In FIG. 1 there is shown a system utilizing a simple
!
arc method in accordance with the present invention for obtaining
mirror-like protective coaxings upon substrates or for evaporate
ing~various metals or metal alloys, including heat-resistant
; 20 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
own be fed by an electrode feeder 7 toward an electrode body 2
to form the pool 3 of molten metal with which the arc is
struck. ;
; The electrons body 2 is held in a fixture or holder
5 and the direct-current source applies the arc current across
the electrode 1 and the body 2 via a conventional art stabilizing
circuit represented at 8.
I US
It has been found to be advantageous to provide the
relatively small cross-section electrode 1 with a thermal regular
ion 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.
Jo
The apparatus of FIG. 1, utilizing electrodes Lund
2 of titanium, aluminum, tungsten, tantalum or copper, strikes
an arc at a temperature of 5000 to 7000F to generate vapor of
the metal of the pool 3 which traverses the distance of 10 to
15 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 alloy of the metals
; of the two electrodes upon the substrate. Preferably the elect
strode lo composed of titanium while the molten metal predominantly
consists of aluminum, tungsten, tantalum or copper.
The apparatus of FIG. 1, without substantial midlife-
cation, can be utilized in a non crucible method of generating
protective coatings of carbides, for producing solaced coatings
on the substrate or for forming carbide or solaced and even
silicon carbide layers upon the substrate. To deposit silicon
carbide-tungsten carbide layers on the substrate, electrode 2
is composed ox graphite and electrode 1 of tungsten solaced,
The vacuum is initially drawn to 10 6 torn and maintained at
10 5 torn or lower Roy direct current arc-generating voltage
; is 100 volts and the arc current 150 amperes. The deposit forms
at a rate of about 0~2 grams per minute.
:
--10--
s
In this case, the apparatus of FIX. 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 solaced or carbide is to be
formed or, in the case of a deposit of silicon upon the sub-
striate, 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 2 is a carbon block in which a pool
3 of silicon and solubilized carbon is received.
The vapors are transferred to the substrate and
Jo deposited in a silicon carbide layer thereon. The substrate
may be titanium and the deposit formed on the substrate may be
a mixture of titanium solaced 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 solaced 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 th~ermoregulator 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 come
, :
posed of silicon and the other is composed ox carbon, silicon
!
carbide is generated by the reaction and deposits in a higher
purity than that of the original silica and carbon.
Jo :
'
, - , ,
OX
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 112 which imparts a reciprocation to the
electrode 101 in the direction of the arrow 114 so as to periodic
gaily 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 I04,
the coating 113 of molten metal upon the electrode lulls evapora-
ted and the deposit is formed upon the substrate 110.
The electrode body 102 is shown in the holder I05
and the arc current supply is formed by the direct current source
I,
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 effect
live, in a modification of the foregoing example, when the
electrode 101 is composed of titanium and the pool 103 is
formed of aluminum.
In FIG. 3 there is shown an embodiment of the invent
lion in which the vapor is deposited upon a substrate 210 disk
posed below a crucible 217 in the form of an upwardly open
3 ring containing the molten metal 203, the crucible being mounted
-I in a holder or frame 205.
-12-
I
Here the upper electrode 201 is in the form of a spheric
eel 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 sub-
striate 210 as represented by the arrows 218.
The direct current source 209 is here connected across
the electrode 201 and the crucible 217 via the arc stabilizer
208 and the upper electrode 201, mounted on the rod 216, is Yen-
tidally positioned by the feeder 207 and horizontally positioned
by an auxiliary mechanism 215 which adjusts the position of the
electrode 201 over the evaporating metal.
In this embodiment, the electrode Sol can be composed
of titanium, molybdenum or tungsten while the molten metal can
be composed of aluminum or copper and the crucible 217 of gray
White.
In FIG. 4 there is shown another embodiment of the invent
lion in which the vapors flow downwardly to deposit upon the
substrate 310.
Z In this oases 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
Jo heated by auxiliary means such as an inductive heater 323 and is
supported in a holder 305.
Jo : 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.
-13-
The temperature in the region of the arc can be con-
trolled by an auxiliary 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 stab-
liver 308 connected to the direct current source 309.
In this embodiment, the molten metal may be copper.
In place of the auxiliary 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 coat-
in.
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
I, 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 lo provided coccal 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
I; the arc stabilizer 408 and the direct current source 409 in the
manner described previously.
FIG. 6 shows a portable voltaic arc device for~depositlng
'I; reflective anticorrosive, protective and semiconductor type
metal, silicide~and carbide coatings using the principles dyes
cried 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.
-14-
. .
I
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 nests-
lent (refractory) material such as aluminum 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 an 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
air pump as represented at 533, the latter evacuating the chamber
- 20 containing the hollow sphere 517 and, via a vacuum hose 534,
via a valve 535, an adapter 536 of outwardly divergent configure-
lion which can be connected to a lateral aperture 525 of the
hollow sphere 517.
` The adapter 536 can be formed with a heating coil 537
to prevent undesired condensation of vapor thereon.
. .
Between the aperture 525 and the adapter 536 there is
provided a vacuum lock 538 and a mounting arrangement 539 for
holding a variety of adapters of different shapes and sizes.
The adapter 536 is also formed with a vacuum gasket 5~0
3 whereby the adapter can bear against the substrate 510 to be
coated.
-15-
;
s
The portable unit shown in FIG. 6 is carried to the
location of the substrate 510 to be coated and the appropriate
adapter 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 per-
milted to pass onto the substrate 510 at least in part by pressure
differential as controlled by the valve 535 maintained between
the interior ox the sphere 517 and the adapter 536.
Practically any product at any site can be coated and
the use of a variety of adapters 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 adapter
536, can be used as a propellant for individuals or equipment
in space.
20~ Upon generation of the arc, one need only open the
gate 53~ to discharge a stream through the aperture 525 and
effect propulsion in the opposite direction. The vacuum in spate
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.
In FIG. 'Thor is shown an embodiment of the invention
,
which combines features previously described and concepts
developed above.
In this system, which can be used to deposit a coating
3 610' on the inter surface aye of a tube 610, forming a sub-
striate, of complex shape, a material-supplying electrode 602 of
corresponding shape is mounted centrally of the tube on a support
-16-
I S
aye and is provided with an induction heating coil aye 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 tempera-
lure of the latter constant in the range of 800 to 1000F 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 en-
closed in a vacuum chamber 600 which can be evacuated to 10 6
torn so that vapor deposition can be effected at a pressure of
10-5 torn.
The end of the material-supplying electrode 602 is
provided with an arc-striking electrode 601 which can be respire-
acted 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 aye so that when the arc
current decays completely, the electrode 601 is displaced to the
Jo left into contact with the end aye of the electrode 602 and is
then withdrawn to reestablish an arc. The arc current is pro-
voided 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 ~aterial-supply electrode 602 which is sub-
jetted to this phenomenon, i.e. over the length at which the
phenomenon is effective until the arc decays.
I'
-17-
:
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 thick-
messes of coating material since the coating is especially
rapid and it is possible to carry out the deposition without
significantly heating up the substrate.
A copper electrode 602 of the shape shown is prove-
Dodd Inca substrate tube with an initial spacing of electrode
602 from the substrate of about 10 cm. The electrode is main-
twined at a temperature of 900F 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 ox-
coeds the speed of evaporation in Example 1.
; In FIG. theorization an arrangement for applying
a silicon coating 710' to a quartz crucible 710 of the type
utilized for the 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 in-
dusty. According to the invention, the interior surfaces of
3 the crucible are sandblasted and the crucible is preheated,
:;; :: :
-18-
- '
Lo
e.g. to a temperature of 200 to 600C 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 aye 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 Sweeney. 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 So or one of the other coatings mentioned.
After the initial arc is produced the electrodes can be cooled.
In FIG. 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 elect
trove 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 counter electrode 802 which may also be
composed of the same metal.
:
The power supply has been represented at 809. The
3 electrodes are here mounted on a track 821 and are moved along
the substrate so that as the arc is repeatedly struck and the
I
-19-
,
~3~9~
arc travels along the electrode 801 in the vacuum chamber receive
in the entire assembly, the entire surface of the substrate is
coated utilizing the principles described in FIG. 7.
E pie
Utilizing an apparatus operating with the principles
shown in FIG. 9, an aluminum oxide plate is coated to a thick-
news of` l to 2 miss 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 l cm. The tungsten coating was highly ad-
hornet to the alumina plate.
FIG. 10 shows an apparatus, in a highly diagrammatic
form, for carrying out the method of the invention. This Papa-
fetus comprises a chamber 1010 which can be evacuated by a
suction pump 1011 to the desired degree of` vacuum, generally
lo 5 to 10 6 torn. Within this chamber, by means not shown, a
ceramic substrate 1012 can be disposed and can be shielded by
a mask diagr~ammatica]ly~illustrated at 1013 so that coating can
only occur in regions defined by the windows 1014 in the mask. 'I
Within the vacuum chamber the portion of the substrate
to be coated is juxtaposed with a pair ox electrodes, i.e. a
copper electrode 1015 and a tungsten electrode 1016, the elect
triodes being provided with means such as the electromagnetic
motors (solenoids) 1017 and 1018 for briefly bringing them into
contact to strike the arc and then drawing them apart. The
purser or periodically energizing the devices 1017 and 1018
have been shown at loll.
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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 no-
verse the polarity of the electrodes 1015 and 1016 under the
control of a timer 1023.
In operation with the copper electrode 1015 poled
positively and the tungsten 16 poled negatively an arc can be
struck by passing an electric current of 30 to log 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 1015 is now poled negatively and the tungsten
electrode 1016 is poled positively whereupon copper is vaporized
from the electrode 1015 and deposited upon the substrate.
As can be seen from FIG. 11, the resulting article has
a substrate 1030, ego us aluminum oxide, bearing a copper coat-
in 1032 which is separated by the refractory metal coating 1031
(tungsten) of smaller thickness.
Example
Utilizing the principles described, a current of about
,
70 amperes, a voltage of 80 volts and a vacuum of about 10 5
torn, an aluminum ogle plate is coated with tungsten to a thick- ;
news of about 8 microns and with copper to a thickness of about
0.002 inches. The adhesion is measured and for the coating is
found to be 500 to 700 lobs per square inch (force required to
remove the coating). When under identical conditions a copper
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coating of the same thickness is applied to the same substrate,
the adhesion is only 6 to 8 lobs per square inch. The direct
copper-to-ceramic bond is found to be sensitive to both mechanic
eel 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. 12 shows a modification of the apparatus of FIG.
10 in which the chamber 1110, evacuated by the pump 1111, in-
eludes a ceramic substrate 1112 which is to be coated with a
plurality of metals. In this case, in addition to the common
electrode 1116 and its actuator 1118 driven by the pulserltimer
1119, a pair of counter-electrodes 1115 and aye, respectively
of copper and gold, are provided each with a respective actuator
1117, aye. The counter-electrode assembly is provided on a
track 11124 having a drive enabling shifting of the two electrodes
to the left as illustrated by the showing of the electrode 1115
in its shifted position in dot-dash lines. Naturally in the
latter position, the electrode aye is aligned with the common
electrode 1116. A reversing switch 1122 as described in connect
lion with FIG. 10 19 here also provided and the apparatus is
energized from the alternating current lines 1120 through the
rectifier 1121.
In this mode of operation, once the chamber is evoke
3 ted, the actuators 1117 and 1118 are operated to move the elect
troves 1115 and 1116 together and apart to strike an arc, the
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electrode 1116 of tungsten being poled positively while the
electrode 1115 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 7 i.e. the copper
electrode 1015 is poled positively and the tungsten electrode
is poled negatively, the arc is struck and evaporation is effect
ted from the copper electrode, the tungsten deposit 1025, which
has been exaggerated in thickness in FIG. 13, vaporizes together
with copper and a mixed tungstentcopper deposit is produced as
an interface.
In FIG. 14, for example, the substrate 1112 is shown to
have been coated at 1131 with the tungsten layer. The mixed
or tungsten layer 1126 is then applied thereto before, with con-
Tunisian of the generation of vapor by arc-striking between
the electrodes 1115 and 1116, the copper coating 1132 is applied.
When the topper coating has reached the desired thick- ;
news, the electrode assembly 1115, 1117 is shifted to the left
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and replaced Beth assembly aye, aye and the arc is struck
between the electrodes aye, 1116 to deposit gold in a layer
; 1133 upon the topper coating.
A controller 1127 may be provided for the electrode
shifting device 1124, the pulser/timer 1119 and the reversing
switch 1122 and may be controlled by a preprogrammed MicroPro
3 censor to effect the polarity reversal and the switching of electrodes when layer thicknesses of the desired magnitude have
been achieved.
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eye
The method of Example 4 is practiced except that the
copper electrode is shifted away and replaced by a gold elect
trove. 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-elec-
ironic purposes. Investigations of the interface between copper and tungsten showed mixed transition zone 1126 which was
traced to the vaporization of a minor deposit of tungsten from
the copper electrode.
.
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