Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR DEPOSITING A SUBSTANCE
~ WITH TEMPERATURE CONTROL
FIELD OF THE INVENTION
This invention relates to temperature control during
material deposition and, more particularly, to controlling
temperature of a substance being deposited on a surface. The
invention is especially applicable to deposition of diamond by
plasma jet on a rotating surface.
BACKGROUND OF THE INVENTION
Techniques have long been known for depositing
substances, such as layers of semiconductor material, using a
plasma that is formed into a jet. For example, U.S. Patent
Numbers 4,471,003 and 4,487,162 disclose arc jet plasma
deposition equipments which utilize a plasma for deposition of
semiconductors and other materials. Ions and electrons are
obtained by injecting an appropriate compound, such as a
silicon compound, into an arc region, and a jet (or beam) is
formed by utilizing magnetic fields to accelerate and focus
the plasma. Recently, equipment of this type has been used to
deposit synthetic diamond. Superior physical and chemical
properties make diamond desirable for many mechanical,
thermal, optical and electronic applications, and the ability
to deposit synthetic diamond by plasma jet deposition holds
great promise, particularly if plasma jet techniques can be
improved for this and other purposes. A plasma of a
hydrocarbon and hydrogen can be obtained using electrical
arcing, and the resultant plasma focused and accelerated
toward a substrate, using focusing and accelerating magnets,
so that polycrystalline diamond film is deposited on the
substrate. Reference can be made, for example, to U.S. Patent
No. 5j204,144, assigned to the same assignee as the present
Application, for description of an example of a type of plasma
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jet deposition that can be utilized to deposit synthetic
diamond on a substrate.
In various commercial applications it is desirable to
have relatively large size diamond films. In plasma jet
deposition techniques there are various factors which limit
the practical size of the deposition area that is active on a
substrate at a particular moment. For example, when an arc is
employed to generate the heated gas mixture in an arc jet
plasma deposition system, the diameter of the beam can be
limited by a number of factors. Since the cross-section of
the plasma beam is generally limited in practical
applications, the area on which it is desired to deposit a
diamond film may be larger than the deposition beam. This
means that it may be desirable to move the beam and the target
substrate with respect to each other during the deposition
process. This has been achieved by spinning the substrate
during deposition, which helps to promote temperature
uniformity over the substrate, as well as to attain larger
area substrate coverage (see e.g. the referenced U.S. Patent
No. 5,204,144)
In plasma jet deposition of the type described, it is
typically necessary to cool the substrate (or mandrel) upon
which the diamond is being deposited, to prevent the hot
plasma from overheating the deposition surface, and to provide
an optimum deposition temperature for the particular product
characteristics desired. A coolant can be circulated in the
mandrel to provide cooling. In a rotating mandrel type of
deposition equipment, as described in U.S. Patent No.
5,204,144, cooling fluid can be circulated through a rotating
union. Figure 1 illustrates a prior art type of configuration
wherein a rotatable mandrel 110, coupled with a rotary union
115, is rotated, such as by a belt or gear drive (not shown),
and coolant fluid can be circulated through the mandrel, in
the direction indicated by the arrows, or in the opposite
direction. A disc-shaped spacer 150 and substrate 170 are
shown as being mounted on the rotating mandrel. Although the
described type of cooling and spinning of the substrate can be
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effective in obtaining a suitable temperature that is
generally uniform azimuthally, temperature at the deposition
surface of a disc-shaped substrate can vary substantially in
the radial direction, due to factors such as variation in
heating by the beam as a function of its cross section.
It is among the objects of the present invention to
provide improved temperature control, such as to achieve
temperature uniformity, in a plasma deposition apparatus, such
as the type used for deposition of synthetic diamond. It is
also among the objects of the present invention to provide
temperature control in a deposition system in which a
substance, such as synthetic diamond, is being deposited on a
rotating mandrel or substrate.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and
technique for use in systems where a substance is being
deposited on a mandrel or substrate, and where it is desirable
to control temperature at the deposition surface. In many
applications it is desirable to obtain spatial and temporal
temperature uniformity over the deposition surface, but the
invention is also applicable to situations where it is
desirable to obtain a particular temperature pattern at the
deposition surface. The deposition technique may require
heating or cooling of the deposition surface, with cooling
being necessary or desirable for most plasma deposition
applications, such as deposition of synthetic diamond film,
which is an important application of the invention. The
invention is also particularly advantageous in systems where
the mandrel (which may have a substrate mounted thereon) is
rotated during deposition. The above referenced U.S. Patent
No. 5,204,144 discloses a plasma jet deposition system wherein
the deposition surface is rotated at a relatively high rate,
and this facilitates obtaining temperature uniformity on the
surface, particularly where the depositing plasma jet has a
smaller cross-sectional area than the surface on which the
substance (e.g. diamond) is being deposited. As first noted
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above, however, temperature uniformity in the radial direction
could stand improvement, and a feature of the invention
achieves this objective.
In accordance with one form of the invention, temperature
control at the surface of a substrate, such as a substrate
rotating on a cooled mandre~, is controlled by using a special
type of spacer. In an embodiment of this form of the
invention, there is provided an apparatus for depositing a
substance, which comprises: a mandrel rotatable on an axis; a
spacer mounted on the mandrel; a substrate mounted on the
spacer; means for directing, toward the substrate, a plasma
containing constituents of the substance being deposited; the
spacer having a thermal conductance in its thickness direction
that varies with radial dimension. In a preferred embodiment
of this form of the invention, the thermal conductance of the
spacer in the thickness direction varies by at least 5 percent
from minimum to maximum.
An advantage of the summarized feature of the invention
is that the radial temperature profile on the deposition
surface can be better controlled. For example, in a situation
where the center of the substrate tends to be hotter than the
outside thereof, and where temperature uniformity is desired,
a spacer having greater thermal conductance toward the center
can provide effectively greater heat exchange near the center
(by providing a higher thermal conductance path for heat
exchange with the cooled mandrel near the center of the
substrate), or vice versa for the opposite situation. In one
preferred embodiment of this form of the invention, the spacer
comprises a disc having concentric grooves therein. For a
metal spacer, the metal will generally have much higher
thermal conductivity than the open space of the grooves, so by
providing, for example, more groove volume near the center of
the spacer, the thermal conductance thereof will be higher
toward the periphery of the spacer. In this example, if the
mandrel would otherwise tend to provide relatively uniform
heat exchange over its~surface, the result will be more
cooling toward the center, where the thermal conductance of
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the spacer (in the thickness direction) is higher. The
thermal conductance of the spacer, in the thickness direction,
can vary linearly or non-linearly with radial dimension. In
accordance with a further feature of the invention, the radial
heat flux characteristic at the deposition surface is
determined, and a spacer is provided that has a thermal
conductivity in its thickness direction that varies in
accordance with (i.e., inversely with, for most situations)
the determined radial heat flux characteristic.
While spinning of a mandrel (or a mandrel with a
substrate thereon) is advantageous in obtaining temperature
uniformity on its deposition surface, there can be practical
difficulties in reliably providing heat exchange (typically, a
cooling fluid) to and from a rotating mandrel to remove heat
therefrom. A further form of the invention addresses this
problem. In accordance with an embodiment of this form of the
invention, there is provided a method for depositing diamond
film that includes the following steps: providing a
deposition chamber; providing, in the chamber, a rotating
mandrel; directing a plasma containing-a hydrocarbon gas and
hydrogen gas toward the mandrel; and controlling temperature
on the surface of the mandrel by passing coolant fluid
radially through the mandrel at several angular reference
positions in the mandrel. In a preferred embodiment of this
form of the invention, the step of passing coolant fluid
radially through the mandrel comprises providing radial bores
through the mandrel at several angular reference positions.
In this embodiment, the fluid is hydrogen, and the fluid
enters the environment of the chamber after passing radially
through the mandrel to the periphery of the mandrel. Since
most of the environment of the chamber is hydrogen, permitting
hydrogen gas to enter the chamber does not contaminate the
deposition environment. An advantage of this feature is that
it becomes unnecessary to return the heat exchange fluid
through a rotating seal, as the hydrogen in the chamber is
ultimately handled by the vacuum system that controls the
chamber pressure. In a further embodiment of this form of the
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invention, the fluid is passed through the mandrel at an angle
with respect to the surface of the mandrel. Preferably, the
angle is at least 2 degrees with respect to the plane of the
mandrel surface. In this manner, heat exchange can be varied
radially. For example, depending on the direction of the
taper with respect to the mandrel surface, the heat exchange
can be concentrated either more toward the center of rotation
of the mandrel or more toward the periphery of the mandrel.
Further features and advantages of the invention will
become more readily apparent from the following detailed
description when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a simplified cross-sectional diagram which
illustrates a prior art rotating mandrel assembly with heat
exchange.
Figure 2 is a schematic diagram, partially in block form,
of an apparatus in accordance with the invention, and which
can be used to practice the method of the invention.
Figure 3 is a plan view of a spacer in accordance with an
embodiment hereof.
Figure 4 is a cross-sectional view of the Figure 3
spacer, as taken through a section defined by arrows 4-4 of
Figure 3.
Figure 5 is a plan view of another spacer, in accordance
with an embodiment of the invention.
Figure 6 is a cross-sectional view of the Figure 5
spacer, as taken through a section defined by the arrows 6-6
of Figure 5.
Figure 7 is a plan view of another spacer, in accordance
with an embodiment of the invention.
Figure 8 is a cross-sectional view of the Figure 7
spacer, as taken through a section defined by the arrows 8-8
of Figure 7.
Figure 9 is a cross-sectional view of a spacer in
accordance with a further embodiment of the invention.
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Figure 10 is a cross-sectional view of a spacer in
accordance with a further embodiment of the invention.
Figure 11 is a cross-sectional view of a spacer in
accordance with a further embodiment of the invention.
Figure 12 illustrates an example of a graph of incoming
heat flux as a function of radial dimension on the substrate
disc surface.
Figure 13-15 are cross-sectional views of spacers having
conductances, in the thickness direction, that vary non-
linearly with radial dimension, and are inversely matched to
the heat flux characteristic of Figure 12.
Figure 16 is an operational flow diagram of a method in
accordance with an embodiment of the invention.
Figure 17 is a side sectional view of a temperature
controlled rotatable mandrel in accordance with a further
embodiment of the invention.
Figure 18 is a cross-sectional view of the mandrel of
Figure 17, as taken through a section defined by arrows 17-17
of Figure 17.
Figure 19 is a side cross-sectional view of a temperature
controlled rotatable mandrel in accordance with another
embodiment of the invention.
Figure 20 is a slide cross-sectional view of a
temperature controlled rotatable mandrel in accordance with
another embodiment of the invention.
DETAILED DESCRIPTION
Referring to Figure 2, there is shown a chemical vapor
deposition ("CVD") apparatus of a type which can be utilized
in practicing embodiments of the invention. A deposition
chamber 100 is the lower section of a plasma jet CVD
deposition system 200, evacuated by one or more vacuum pumping
systems (not shown).
The system 200 is contained within a vacuum housing 211
and includes an arc-forming section 215 which comprises a ~
cylindrical holder 294, a rod-like cathode 292, and an
injector 295 mounted adjacent to the cathode so as to permit
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injected fluid to pass over the cathode. A cylindrical anode
is provided at 291. In the illustrated system, where
synthetic diamond is to be deposited, the input fluid may be,
for example, a mixture of hydrogen and methane. The methane
could alternatively be fed in downstream. The anode 291 and
cathode 292 are energized by a source of electrical power (not
shown), for example a DC potential. Cylindrical magnets,
designated by reference numeral 217, are utilized to help
control the plasma generation. A nozzle, represented at 115,
can be used to control beam size, within limitations.
Optional cooling coils 234, in which a coolant can be
circulated, can be located within the magnets.
In an example of operation, a mixture of hydrogen and
methane is fed into the injector 295, and a plasma is obtained
in front of the arc forming section and accelerated and
focused toward the deposition region at which a substrate is
located. As is known in the art, synthetic polycrystalline
diamond can be formed from the described plasma, as the carbon
in the methane is selectively deposited as diamond, and the
graphite which forms is dissipated by combination with the
hydrogen facilitating gas. For further description of plasma
jet deposition systems, reference can be made to U.S. Patent
No.s 4,471,003, 4,487,162, and 5,204,144. It will be
understood that other suitable types of deposition equipment,
including, for example, other types of CVD plasma deposition
equipment, or physical vapor deposition equipment, can be used
in conjunction with the features of the invention to be
described.
A mandrel 110 is rotatable on a shaft 111, and has a
spacer 120 and a substrate 170 mounted thereon (by means not
shown, bolting or clamping being typical). The mandrel 110
can be cooled by any suitable means, for example by using a
heat exchange fluid (e.g. water) that is circulated through
the mandrel, as in the prior art arrangement first illustrated
in Figure 1. The mandrel can be tilted with respect to the ~
direction of the plasma jet, as disclosed in U.S. Patent No.
5,342,660. The heat exchange fluid can be fed and returned
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through a rotary union, as shown in Fig. 1, and a suitable
motor (not shown) can be used for rotating the shaft, as is
also disclosed in U.S. Patent No. 5,342,660. The rotational
drive will conventionally be above the rotary union.
Figures 3 and 4 illustrate, respectively, a plan view and
a cross-sectional view of a spacer disc 120 which may be
formed, for example, of metal (for example molybdenum or
titanium), graphite, or ceramic, and has a plurality of
concentric annular grooves 121 formed therein, such as by
machining. The grooves can be empty, or can be filled with a
material having a different thermal conductivity than the rest
of the spacer disc. In the configuration of Figures 3 and 4,
the grooves are uniformly spaced so that variation in thermal
conductance in the thickness direction, as a function of
radius, will be local, and will result in an overall lower
thermal conductance of the spacer (as compared to a solid
spacer), since the open grooves have a lower thermal
conductivity than the solid material of the disc. This
generally symmetrical configuration will tend to result in
macroscopically uniform thermal conductance, in the thickness
direction, as a function of radial dimension. [The "ripple"
in conductance over relatively short radial distance will tend
to be smoothed by the remaining spacer thickness and substrate
thickness.]
Figures 5 and 6 illustrate a configuration that results
in greater thermal conductance (and, accordingly, more heat
transfer) near the center than near the periphery of the
spacer. This is achieved by providing more groove volume
(greater cross-sectional area and/or greater groove depth)
near the outside of the spacer, such as by providing more
grooves or wider grooves or deeper grooves near the periphery.
[The illustration of Figure 5 has more grooves near the
periphery.] Thus, for example, in a plasma deposition system
of the type illustrated in Figure 2, there may typically tend
to be a substantial radial temperature gradient caused by the
plasma beam, with the center of the substrate usually having a
higher temperature than the periphery of the substrate. If
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the mandrel is cooled relatively uniformly, and if one uses a
prior art configuration such as that of Figure l (with a solid
spacer, or, for example, with no spacer) there may be a
substantial radial temperature gradient, which could result in
non-uniform diamond deposition and quality. Using a spacer of
the type shown in Figures 5 and 6 between a cooled mandrel and
a substrate will tend to provide more efficient cooling toward
the center of the substrate, as the grooves reduce thermal
conduction from substrate to cooled mandrel near the
periphery. Thus, a more uniform radial temperature
characteristic can be achieved by using this form of the
invention. The groove pattern can be designed to obtain the
desired thermal conductivity characteristic. A supply of
spacers, having different thermal conductivity characteristics
in their thickness direction, as a function of radial
dimension, can be provided for use in different situations and
to "match" the inverse of the expected radial temperature
gradient on the substrate deposition surface.
Figures 7 and 8 illustrate the opposite case; that is, a
spacer configuration which has relatively less thermal
conductivity (in the thickness direction) toward the center of
the spacer disc and relatively greater thermal conductivity
toward the periphery thereof.
In Figures 3-8, the grooves are illustrated as being in
one surface of the spacer, but it will be understood that
other configurations can be provided, for example as shown in
Figure 9 where both surfaces of the spacer have grooves, the
grooves on the bottom being labelled 122. As shown in Figure
10, the grooves on opposing sides of the spacer need not
necessarily align in radial position. Also, it will be
understood that other means can be provided for obtaining a
radial thermal conductivity variation in the spacer. For
example, the grooves may be filled with a material, such as a
ceramic, metal, graphite or gas, having a different thermal
conductivity than the original spacer material. [It will be
understood that, as a heat transfer mechanism in the spacer
embodiments hereof, conduction will generally dominate over
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convection or radiation.] Alternatively, a tapered
configuration, such as that shown in Figure 11, could be
~ employed. As an example, the material 1122 can have a higher
thermal conductivity than the material 1121, which will result
in a higher thermal conductance toward the center of the
spacer 120.
Figures 12-15 illustrate further embodiments hereof
wherein the thermal conduction of the spacer in its thickness
direction, as a function of radial dimension, can be selected
to obtain a generally uniform temperature at the deposition
surface by providing a thermal conduction in the thickness
direction, as a function of radial dimension, that is
substantially the inverse of the radial heat flux pattern at
the deposition surface. For example, Figure 12 illustrates a
typical gaussian bell-shaped distribution of incoming heat
flux (such as in units of watts per cm2) as a function of
radial dimension on the substrate disc surface at which
deposition is taking place. Figures 13, 14, and 15 illustrate
embodiments of spacers (120) which have thermal conduction in
their thickness direction, as a function radial dimension,
that is an inverse of the heat flux at the deposition surface.
Accordingly, these spacers can be used to promote a generally
uniform temperature at the deposition surface of the substrate
during plasma deposition. In the embodiment of Figure 13, the
thermal conduction characteristic is obtained by using equally
spaced grooves (124), with groove depths increasing (non-
linearly, in this case) as a function of radial dimension. In
the embodiment of Figure 14, the depths of grooves (124) are
the same, but groove widths are increased (again, non-linearly
for this case) as a function of radial dimension. In the
embodiment of Figure 15, the spacer 120 has an upper region
1501 formed of a relatively higher thermal conductivity
material, and a lower region 1502 formed of a relatively lower
thermal conductivity material, so that, again, the thermal
conduction in the thickness direction will decrease as a
function of radial dimension in a non-linear fashion. The
interface curve between the materials can be selected to
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12
obtain the desired conductance to be the inverse of the heat
flux characteristic. As one example, the material 1501 may be
molybdenum, and the material 1502 may be graphite of a type
having lower thermal conductivity than molybdenum.
Figure 16 illustrates an operational flow diagram of the
steps for practicing an embodiment of a method hereof. The
block 1610 represents the determining of the incoming heat
flux characteristic at the deposition surface as a function of
radial dimension. The incoming heat flux pattern can be
determined, for example, by measuring temperature using known
pyrometer or spectroscopy techniques, or from direct
measurement of temperature at the substrate surface. The
block 1620 represents providing of a spacer having a thermal
conduction in its thickness direction, as a function of radial
dimension, that is an inverse of the determined heat flux
characteristic. Then, with the spacer provided between the
cooled mandrel and the substrate, the block 1630 represents
deposition of the substance to be deposited, synthetic diamond
being a substance of particular interest herein.
In accordance with a further embodiment hereof, a fluid
coolant is utilized in a mandrel in an advantageous way.
Figures 17 and 18 illustrate a configuration, which can be
used, for example, in the plasma jet deposition system of
Figure 2, wherein coolant fluid is passed radially through a
mandrel 1710 via radial bores 1715, leaves the mandrel through
peripheral openings, and enters the environment of the
deposition chamber. By using a fluid that is normally
contained in the environment, in this case hydrogen gas, the
need to return coolant fluid through the fluid through the
mandrel is eliminated. This is particularly advantageous for
a rotating mandrel, in that one need only provide for
traversal of the gas one way through the mandrel, its rotating
shaft 1711, and the rotary union 1718. The hydrogen gas, as
the major constituent of the chamber environment, is
eventually evacuated by the vacuum system (Figure 2).
The configuration illustrated in Figures 17 and 18 will
tend to provide more cooling toward the center of the mandrel
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since the spacing between coolant-carrying bores increases as
a function as radial position. For applications where greater
cooling is needed toward the mandrel center to obtain
temperature uniformity (e.g. for deposition on the mandrel
surface or on a substrate mounted on the mandrel or on a
spacer), this configuration may be suitable. The radial
cooling characteristic can be tailored for a particular
application by selecting the taper angle of the bores 1715
with respect to the deposition surface. For example, Figure
19 illustrates a configuration wherein the coolant-carrying
bores taper toward the deposition surface as they extend
radially, this configuration producing more cooling toward the
periphery of the deposition surface than the one shown in
Figures 17 and 18. The opposite type of taper is shown in
Figure 20, which will tend to provide relatively less cooling
toward the periphery of the deposition surface.
The invention has been described with reference to
particular preferred embodiments, but variations within the
spirit and scope of the invention will occur to those skilled
in the art. For example, it will be understood that other
materials can be used, and that the principles hereof are
applicable to other deposition techniques.
