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The invention relates to a susceptor of the type comprising a hollow
polyhedron adapted to support a substrate on an outer planar surface of a
wall thereof.
In the production of certain semiconductor devices, an epitaxial layer of
silicon on a substrate, such as a silicon wafer, is used as a starting
material. The epitaxial layer of silicon is deposited upon the silicon
wafer in a chemical vapor-deposition (CVD) process wherein the wafer is
supported on a graphite susceptor and heated to a high temperature. A
volatile compound of silicon is introduced and thermally decomposed, or
reacted with other gases or vapors, at the surface of the wafer to yield
silicon which deposits on the wafer surface.
Various types of susceptors have been utilized for supporting substrates
during the chemical vapor-deposition process. In one type of apparatus,
the substrates are placed on the upper side of a flat plate-shaped
susceptor (slab) surrounded by an rf coil by which the susceptor is
heated, as disclosed in U.S. Pat. 3,892,940, issued to Bloem et al. on
July 1, 1975. In this type of susceptor, when the rf coil is energized,
induced eddy currents will flow on the upper and lower sides of the
susceptor, which currents are directed opposite to each other at the
center of the slab. Bloem et. al. discloses the use of recesses in the
lower side of such a susceptor in order to thereby decrease the heat
generated by the rf field in the thinner regions adjacent the recesses.
For effective slab heating, the thickness of the slab is usually a minimum
of two .delta., where .delta. is the skin depth or depth of penetration
equal to the depth below the surface where the current strength has a
value 1/e times the current strength at the surface.
In another type of apparatus, the substrates are mounted on the sides of a
susceptor having the shape of a hollow polyhedron, for example, a hollow
truncated pyramid. Such a susceptor is typically made by starting with an
integral pyramid-shaped piece of conventional graphite and then, starting
at the base thereof, hollowing-out the inside into the shape of a cone.
The susceptor is heated by circular rf induction coils which surround the
graphite pyramid and induce a continuous circular current therein which
flows in one direction only. Due to the fact that the outer surfaces upon
which the substrates are mounted are planar and the inner surface is
curved, the wall of such a susceptor has corner portions which are thicker
than the central portions thereof. This variation in the thickness of the
wall adjacent to the mounted substrates causes the substrate to heat
unevenly, which results in the deposition of a non-uniform epitaxial layer
upon the substrate.
In order to achieve uniform heating of the mounted substrates, truncated
pyramid-shaped susceptors have been hollowed-out so that the wall thereof
has outer and inner surfaces which are substantially similar and planar,
as illustrated in U.S. Pat. No. 3,980,854, issued to Berkman et al. on
Sept. 14, 1976 and assigned to RCA Corporation. However, due to the
converging pyramid-shaped wall, it is extremely difficult and expensive to
hollow-out such a structure, since the width of the wall continually
changes as one "machines out" the graphite. Also, the corners of such a
structure, due to their closer proximity to the surrounding rf coils heat
at a faster rate than the central portions between the corners. This
requires the continual use of cooling blowers in order to achieve
acceptable epitaxial layers of uniform thickness.
In the drawings:
FIG. 1 is a plan view illustrating one embodiment of the present novel
susceptor.
FIG. 2 is a partial, cross-sectional view taken along lines 2--2 of FIG. 1.
FIG. 3 is a plan view illustrating a second embodiment of the present novel
susceptor.
FIG. 4 is a partial, cross-sectional view taken along line 4--4 of FIG. 3.
FIGS. 1 and 2 show a novel susceptor 10 having a wall 12 adapted to support
semiconductor substrates 14 on a plurality of outer planar surfaces 15,
16, 17, 18, 19, 20 and 21 thereof, which form a hollow truncated pyramid.
Although the structure of the susceptor 10 is shown as a heptagonal
pyramid, it may take the shape of any hollowed-out polyhedron, adapted to
suit the requirements of a particular manufacturing process.
The susceptor 10 preferably comprises an integral piece of conventional
graphite which has been hollowed-out by a machine tool. The susceptor 10
shown in FIGS. 1 and 2 typically is made by starting with a pyramid-shaped
piece of graphite and then, starting at the base 22 thereof, machining-out
the inside wherein the inner surface 24 of the wall 12 has the shape of a
cone. Due to the fact that the outer surfaces 15-21 upon which the
substrates 14 are mounted are planar and the inner surface 24 is curved,
the wall 12 of such a susceptor 10 has, where the planar surfaces 15-21
intercept, corner portions 26, 27, 28, 29, 30, 31 and 32 which are thicker
than the central portions of the wall 12 disposed between the corner
portions 26-32.
A plurality of ledges 34 extend outwardly from the outer surfaces 15-21 of
the wall 12, as shown in FIGS. 1 and 2. The ledges 34 support the
semiconductor substrates 14 against the outer surfaces 15-21 of the wall
12, as shown in FIG. 2. The upper and lower surfaces of the ledges 34
preferably extend substantially perpendicular from the planar surfaces
15-21 to an extent of about 0.6-0.7 millimeters. The thickness of the
ledges 34 is about 1.2-1.5 millimeters.
The lower portions of the wall 12, near the bottom sections of the planar
surfaces 15-21, are cut away to form a plurality of relatively small
triangular surfaces 36 adjacent to the base 22 of the susceptor 10. Hence,
a horizontal cross-section of the susceptor 10 near the base 22 is a
polygon of 14 sides, and a horizontal cross-section near the top portion
of the susceptor 10 is a polygon of 7 sides. This structure allows the
susceptor 10 to be relatively large for a given sized vertical furnace,
whereby to support and process a maximum number of substrates 14.
The susceptor 10 is adapted for use in a typical vertical reactor furnace
heated by electrical induction energy (about 10 to 400 KHz) so that a
material can be deposited onto the substrates 14 from reacting chemical
components in a vapor-deposition process, well known in the art. The
susceptor 10 is usually heated by circular rf induction coils (not shown)
which surround the graphite susceptor 10 and induce a current therein.
While the dimensions of the susceptor 10 described herein are not
critical, the values given are for illustrative purposes. The wall 12 of
the susceptor 10 at the corner portions 26-32 thereof is about 15-20
millimeters in thickness, and about 8-12 millimeters in thickness at the
central portions at the wall 12. The height of the susceptor 10 is about
300-350 millimeters, and the ledges protrude just enough so as to maintain
the substrates 14 in place without substantially interfering with the
sensitive gas flow dynamics of reacting gases within the furnace. It has
been found that best results are obtained where there is a minimum of
interference with the gas flow dynamics of the reacting chemical
components within the furnace. This is accomplished when each of the
planar surfaces 15-21 makes about a 3.degree. angle with the vertical.
With such a structure the susceptor 10 can support a maximum number of
substrates 14 in an efficient chemical vapor-deposition process.
The susceptor 10 further comprises a plurality of recessed cavities 38
adjacent to the inner surface 24 of the susceptor 10. The structure of
each cavity 38 is such that the floor 40 thereof is parallel to the outer
planar surface adjacent thereto. In the embodiment shown in FIG. 2, each
cavity 38 comprises a circular recess machined into the wall 12 through
the inner surface 24 thereof. The diameter of each parallel surface 40 is
approximately equal to the diameter of the substrates 14 supported by the
susceptor 10, and the thickness of the wall 12 at the parallel surfaces 40
is about 8-12 millimeters.
Also shown in FIG. 2 are pyrolytic graphite heat shields 42 which are
inserted into the cavities 38. The heat shields 42 have a lower heat
conductivity than that of the wall 12 along a direction transverse to the
wall 12. Such heat shields 42 enhance the desirable heating effect gained
by utilizing the recessed cavities 38. For more detailed information on
use of pyrolytic heat shields, see U.S. Pat. No. 3,980,854 issued to
Berkman et al. on Sept. 14, 1976 and assigned to RCA Corporation
Referring to FIGS. 3 and 4 of the drawings, there is shown a second
embodiment of the present novel susceptor 10. In this susceptor 10 the
recessed cavities 38 comprise oblong-shaped slots which are machined into
the wall 12 through the inner surface 24. The slots have a constant width
which is approximately equal to the diameter of the substrates 14
supported by the susceptor. The thickness of the wall 12 at the parallel
surfaces 40 is about 8-12 millimeters. As shown in FIGS. 3 and 4, such
slots are machined straight up through the top of the susceptor 10 where
the parallel surfaces 40 intersect and slightly overlap each other.
Although not shown in this embodiment, pyrolitic graphite may also be
inserted in the oblong-shaped slots.
The structure of the novel susceptor 10 allows for the deposition of more
uniform epitaxial layers upon the semiconductor substrates 14. Since the
wall 12 of the susceptor 10 adjacent to the planar surfaces 40 is of a
constant thickness which extends over most of the diameter of the
supported substrates 14, the heating of the substrates 14, by means of an
rf induction coil, is much more uniform and thereby allows for more
uniform deposition of material across the surfaces of the substrates 14.
The use of recessed cavities 38 also allows the corner portions 26-32,
where the planar surfaces 15-21 intersect, to remain thicker than the
central portions of the wall 12 adjacent to the supported substrates 14.
Consequently, such corner portions 26-32 remain cooler than if the entire
wall 12 was of constant thickness, and the use of cooling air as described
above may be minimized with subsequent savings in power consumption. Such
a heating effect is in direct contrast with that obtained from recesses
disposed in a flat plateshaped susceptor wherein the thinner portions
become cooler than the thicker portions, due to the opposing eddy
currents. The present invention is particularly significant when the
"hollow-cylinder" type of susceptor is operated at relatively low
frequencies of the order of 10 KHz, due to the fact that, when the average
wall thickness is less than one .delta., variations in wall thickness at
low frequencies have a much more pronounced effect on the uniformity of
heating across the surfaces of the supported substrates. In addition, such
a novel susceptor 10 is easier and cheaper to manufacture than one where
the walls are of constant thickness, because the thickness of the recessed
cavities 38 remain constant, making the machining-out process much easier.
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