CVD reactor for uniform heating with radiant heating filaments

Apparatus for the growth of epitaxial layers is disclosed. The apparatus includes a wafer carrier mounted within a growth chamber, a reactant inlet for introducing a reactant into the chamber, and a heating element mounted within the chamber for heating wafers mounted on the wafer carrier. The heating element is mounted in a manner which permits unrestricted thermal expansion of the heating element therein.

FIELD OF THE INVENTION 
The present invention relates to apparatus for growing epitaxial layers on 
wafers. More particularly, the present invention relates to apparatus for 
growing such epitaxial layers in connection with chemical vapor deposition 
(CVD) processes for the processing of substrates such as semiconductor 
wafers. Still more particularly, the present invention relates to such 
apparatus for growing epitaxial layers which comprise oxide films and in 
which the apparatus operates at elevated temperatures. 
BACKGROUND OF THE INVENTION 
In general, in connection with the processing of semiconductor wafers by 
means of chemical vapor deposition (CVD) and like processes, various wafer 
substrates such as silicon wafers and the like, are generally heated 
within various temperature ranges which are designed to apply to specific 
thin film depositions or etching operations thereon. However, in 
connection with the processing of epitaxial oxide materials, such as 
superconductor thin films (e.g., YBa.sub.2 Cu.sub.3 O.sub.7), high 
dielectric thin films (e.g., Ba.sub.1-x Sr.sub.x TiO.sub.3) and 
ferroelectric thin films (e.g., SrBi.sub.2 (Ta.sub.1-x Nb.sub.x).sub.2 
O.sub.9), with high electro-optical coefficients have become of 
consideration interest in connection with the production of certain types 
of products, such as high-density capacitors integrated on dynamic random 
access memories (DRAM), microwave integrated circuits and the like. 
However, when CVD reactors need to be designed for the deposition of such 
oxide films, they generally are required to operate within considerably 
elevated temperature ranges, such as between about 600.degree. C. and 
800.degree. C., and even higher. This, in turn, has created certain 
difficulties in connection with the design of these reactors. That is, 
conventional CVD reactors employ materials of construction such as 
graphite, molybdenum, tungsten, tantalum, and the like. These materials, 
however, will generally begin to oxidize at temperatures above about 
300.degree. C. to 400.degree. C. of course, the oxidation of these 
materials itself can create problems within the reactor. For example, if 
these materials are used in connection with resistance heating or the 
like, oxidation can cause variations and/or interruptions in the current 
flow, and thus result in uneven or inconsistent heating of the 
semiconductor wafers within the reactor. The most significant need in 
these reactors, however, is to produce the uniform heating of each 
semiconductor, both within a single reactor use and more importantly as 
the reactor undergoes continued use from these processes. 
Attempts have been made to apply different coatings to these materials to, 
therefore, protect them in these high-temperature environments. In 
addition, small cracks can be generated in these materials during the 
frequent thermal temperature cycling which is required by the very nature 
of these processes. The existence of these factors has limited the number 
of specific possibilities for selection of materials of construction for 
these particular CVD reactors. One such material comprises various ceramic 
materials, but ceramics are quite expensive and difficult to manufacture, 
and, in fact, are not always available in the particular geometric shape 
or type which is necessary for a particular reactor. Furthermore, these 
ceramic materials are difficult to handle and relatively easy to damage or 
destroy. Thus, the use of ceramics as a material of construction for these 
CVD reactors has been limited to the use of ceramics for relatively small 
parts having simple shapes. 
One specific apparatus for treating semiconductor wafers is disclosed in 
Ohkase et al., U.S. Pat. No. 5,536,918. In this patent, there is shown a 
flat heat source 2 which can either be affixed to the heat retention 
material 4 or arranged below the processing surface 11 for semiconductor 
wafers. The specification of this patent discloses that the heating source 
2 can comprise a plurality of ring-shaped heating units 21a-e which can be 
configured from a superalloy such as Kanthal. 
Mahawili, U.S. Pat. No. 5,059,770, discloses heater assemblies with a 
plurality of heating zones, such as those shown in FIG. 3 thereof. The 
heating segments 22a-c are separated by gaps to provide radial spacing 
between the segments and electrode pairs 40a-c extending upwardly through 
the support base and heater base therefor. In this case, the heater 
element is said to be electrically conductive material, such as a metal 
deposited directly on the heater base 20, which is formed of a suitable 
dielectric material. The heater elements are said to be formed from 
pyrolytic graphite for the high temperatures utilized therein. 
Gilbert et al., U.S. Pat. No. 5,343,022, discusses dealing with the 
potential oxidation problem in these types of reactors by employing 
pyrolytic boron nitride heating units. In particular, this patent 
discusses protection of electrical contact areas from oxidation by 
employing a contact assembly which employs graphite posts and screws for 
the heating element 14 thereof. 
The search has, therefore, continued for improved reactors for growing 
epitaxial layers, particularly in connection with epitaxial oxide 
materials such as those discussed above. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, these and other objects have now 
been realized by the discovery of apparatus for the growth of epitaxial 
layers on a wafer comprising a chamber, a wafer carrier mounted within the 
chamber for mounting at least one of the wafers thereon, a reactant inlet 
for introducing a reactant for the epitaxial layer into the chamber, and 
heating means mounted within the chamber in juxtaposition with the wafer 
carrier for heating the wafer to a predetermined temperature for growing 
the epitaxial layer thereon, the heating means comprising at least one 
radiant heating element mounted with respect to the wafer carrier in a 
manner which permits unrestricted thermal expansion of the radiant heating 
elements. 
In accordance with one embodiment of the apparatus of the present 
invention, the apparatus includes at least one electrode bonded to the at 
least one radiant heating element at a first location therein, wherein 
substantially the entire radiant heating element apart from the first 
location is free of any rigid attachment to a immovable surface which 
could prevent the thermal expansion thereof. 
In accordance with a preferred embodiment of the apparatus of the present 
invention, the radiant heating element comprises a superalloy. Preferably, 
the superalloy is selected from the group consisting of nickel- and 
iron-based superalloy compositions. 
In accordance with another embodiment of the apparatus of the present 
invention, the wafer carrier comprises a rotatable disk mounted for 
rotation in a horizontal plane within a chamber. Preferably, the radiant 
heating element is mounted adjacent to the rotatable disk in a plane 
parallel to the horizontal plane thereof. In a preferred embodiment, the 
radiant heating element comprises a circular filament in the plane, most 
preferably an interrupted circular configuration comprising a first end 
and a second end defining an interruption therebetween. In another 
embodiment, the first and second ends of the interrupted circular 
configuration are permanently attached to electrodes for electrically 
connecting the radiant heating filament to a power source. Preferably, the 
electrodes extend outside of the chamber, and the apparatus includes a 
detachable electrical connector for detachably connecting the electrodes 
to the power source outside of the chamber. 
In accordance with another embodiment of the apparatus of the present 
invention, the heating means comprises a plurality of the radiant heating 
elements. Preferably, the wafer carrier comprises a rotatable disk mounted 
for rotation in a horizontal plane within the chamber, and the plurality 
of radiant heating elements are mounted adjacent to the rotatable disk at 
a plane parallel to the horizontal plane thereof. Preferably, the 
plurality of radiant heating elements comprise circular filaments in the 
plane, and preferably interrupted circular configurations comprising a 
first end and a second end defining the interruption therebetween. In a 
most preferred embodiment, the first and second ends of the interrupted 
circular configurations are permanently attached to electrodes for 
electrically connecting the plurality of radiant heating filaments to a 
power source, and the electrodes preferably extend outside of the chamber. 
Preferably, the apparatus includes detachable electrical connectors for 
detachably connecting the electrodes to the power source outside of the 
chamber itself. 
In accordance with another embodiment of the present invention, the radiant 
heating element comprises a metal having a coefficient of linear thermal 
expansion greater than about 10.times.10.sup.-6 (IOE-6) and preferably 
greater than about 15.times.10.sup.-6 (15E-6). 
In accordance with another embodiment of the apparatus of the present 
invention, the apparatus for the growth of epitaxial layers on a wafer 
comprises a chamber, a wafer carrier mounted within the chamber for 
mounting at least one of the wafers thereon, a reactant inlet for 
introducing a reactant for the epitaxial layer into the chamber, and 
heating means mounted within the chamber and in juxtaposition with the 
wafer carrier for heating the wafer to a predetermined temperature for 
growing the epitaxial layer thereon, the heating means comprising at least 
one radiant heating element mounted with respect to the wafer carrier, the 
radiant heating element including electrical connection means permanently 
affixed to the radiant heating element and extending outside of the 
chamber for connecting the radiant heating element to a power source, 
whereby the chamber is free of any removable electrical connections to the 
radiant heating elements. 
In accordance with one embodiment of the apparatus of the present 
invention, the electrical connection means comprises at least one 
electrode. 
In accordance with another embodiment of this apparatus of the present 
invention, the radiant heating filament and the electrical connection 
means comprise a superalloy. Preferably, the superalloy is selected from 
the group consisting of nickel- and iron-based superalloy compositions. 
In accordance with another embodiment of the apparatus of the present 
invention, the wafer carrier comprises a rotatable disk mounted for 
rotation in a horizontal plane within a chamber. Preferably, the radiant 
heating element is mounted adjacent to the rotatable disk in a plane 
parallel to the horizontal plane thereof. In a preferred embodiment, the 
radiant heating element comprises a circular filament in that plane, and 
preferably the radiant heating filament comprises an interrupted circular 
configuration comprising a first end and a second end defining the 
interruption therebetween. 
In accordance with another embodiment of the apparatus of the present 
invention, the heating means comprises a plurality of the radiant heating 
elements.

DETAILED DESCRIPTION 
A solution to the problems created by high-temperature oxidating conditions 
in certain CVD reactors relates to the use of superalloys as the material 
of construction for the heating elements, such as heating filaments used 
therein. However, the use of these superalloy materials themselves creates 
additional problems. For example, for typical superalloy materials, which 
can be used in this invention, the ratio between the working and melting 
temperatures thus defining the mechanical properties of these superalloys, 
including strength, creep, and the like, is much higher than is the case 
for materials which have conventionally been used as reactor materials, 
such as graphite, molybdenum and tungsten. Thus, that ratio for 
superalloys is usually about 60%, while for compounds such as molybdenum 
and graphite, for example, that ratio ranges from 20% to about 30%. 
In addition, the coefficient of linear thermal expansion for these 
superalloys is far higher, approximately by a factor of about three, for 
the superalloys than it is for these conventional reactor materials, thus 
resulting in far higher thermal stress for the superalloy materials. 
In addition, the high-temperature properties of these superalloys, when 
used in the presence of oxidative materials at these elevated 
temperatures, will result in the generation of thin oxide layers on the 
surface thereof. These oxide layers have a much lower electrical 
resistance than the superalloys themselves and can thus result in 
alteration of the resistance between the filament and the electrode, for 
example. In the case of extremely low filament resistance, most if not all 
of the power will thus be generated at the contact between the filament 
and the electrode instead of in the filament itself. 
The specific properties of superalloys useful in accordance with the 
present invention as compared to materials conventionally used in CVD 
reactors is set forth in TABLE 1 below. 
TABLE 1 
__________________________________________________________________________ 
Superalloys 
Conventional Materials 
Kenthal 
Haynes 
Inconell 
Properties 
Unit 
Graphite 
Mo W Alloy AF 
Alloy 230 
Alloy 601 
__________________________________________________________________________ 
CTE* 1/C 
(5-8)E-6 
4.9E-6 
4.3E-6 
15E-6 18.6E-6 
16.4E-6 
melting 
C 3700 2610 
3410 
1500 1400 1354 
point (subl.) 
Tprocess/ 
% 23 32 25 57 60 63 
Tmelting 
(Tprocess = 
850.degree. C.) 
Nominal Cr - 22.0% 
Cr - 16.0% 
Cr - 23.0% 
composition Al - 5.0% 
Al - 5.0% 
Al - 1.4% 
Fe - 73.0% 
Fe - 3.0% 
Fe - 15.0% 
Ni - 0% 
Ni - 75.0% 
Ni - 60.0% 
__________________________________________________________________________ 
By mounting the radiant heating filaments of the present invention adjacent 
to the wafer carrier in a manner which permits unrestricted thermal 
expansion of the heating element, any potential bending or warping of the 
filament due to increased thermal expansion can be avoided. Thus, the high 
thermal stress and low mechanical strength of these filaments does not 
create the problem which would have been anticipated under these 
conditions. Furthermore, utilizing a radiant heating filament of this type 
which is directly connected to electrical connection means such as 
electrodes which are permanently affixed thereto within the reactor, and 
which extend outside the reaction chamber for ultimate connection to a 
power source, such as by the use of a removable electrical connector 
outside the reaction chamber itself, the potential nonstable resistance of 
the heating elements because of surface oxidation resulting in changes in 
the contact resistance can also be avoided. 
A specific preferred embodiment of the present invention is shown in the 
Figures and will now be discussed in more detail. Referring, for example, 
to FIG. 1, a typical CVD reactor 1 is schematically shown therein. The 
reactor 1 is generally cylindrical in shape, and includes a top flange 3 
for entry into the reactor. Associated with the top flange 3 is a manifold 
arrangement of some type 5 for handling various gases to be utilized as 
reactants for producing epitaxial layers within the reactor 1. The gases 
thus enter the manifold through reactant inlet 7 for distribution to 
manifold 5 and application onto wafers within the reactor 1. 
In addition, the inlets shown in FIG. 1, or other inlets into the manifold 
5, can be used for applying carrier gases and other inert gases for 
application along with these reactant gases within the reactor chamber 1. 
Within the reactor chamber 1 itself, there is mounted a rotating susceptor 
9 which is mounted on a central spindle 11 rotated by a motor 13. The 
spindle 11 is sealed using a high vacuum rotary feedthrough 32, which is 
commercially available and is usually of the ferrofluidic type. The 
rotating susceptor is located essentially in a horizontal plane within the 
reactor 1 so that wafers can be mounted on the top of the rotating 
susceptor 9 in or parallel to that plane. Thus, the gases exiting from 
manifold 5 in a downward direction as seen in FIG. 1 can impinge directly 
onto the surface of the wafers mounted on the susceptor 9 to grow 
epitaxial layers as required thereon. The upper surface 12 of the 
susceptor 9 can include means for carrying one or more wafers thereon. 
This can include indentations, or separate wafer carriers mounted on the 
top of the susceptor 9 for effecting such rotation, and for maintaining 
the wafers in a predetermined position thereon during same. The spindle 11 
is sealably contained within the reactor so that desirable low pressure 
conditions can be maintained within the reactor. 
In order to heat these wafers to effect such epitaxial growth, a pair of 
radiant heating elements 15a and 15b are shown in FIG. 1. In this case, 
the radiant heating elements 15a and 15b are disposed below the rotatable 
susceptor 9 and generally in the same plane as is the upper surface 12 of 
the susceptor 9. Each of these heating elements 15a and 15b is shown in an 
upper view in FIG. 2 hereof. Details of these heating elements will be 
discussed in more detail below. 
In general, heat shields 7 are mounted below the radiant heating elements 
themselves in order to contain the heat generated therein at the 
locations, namely for heating the wafers on the susceptor 9. 
The specific arrangement for the radiant heating elements 15a and 15b shown 
in FIG. 2 includes a two-zone heating system. This is intended to provide 
the necessary uniform wafer temperature, which usually must be controlled 
within about .+-.1.degree. C. in order to ensure the deposition of 
high-quality epitaxial films therein. In the arrangement shown in FIG. 2, 
there is included a two-zone heating system with an outer filament 15a 
connected to an outer zone power supply 24a and an inner film in 15b 
connected to an inner zone power supply 24b. Independent control of these 
inner and outer filaments permits one to compensate for the significant 
heat flow which occurs from the susceptor 9 to the colder reactor walls 1, 
and therefore to provide for the necessary uniform wafer temperature 
therein. 
As can also be seen in FIG. 1, the heating filaments 15a and 15b are 
supported on the heat shield 7 by means of a number of support elements 
30. These support elements are produced from a high-temperature insulated 
ceramic such as alumina (Al.sub.2 O.sub.3) or zirconia. They are thus 
installed between the heating elements 15a and 15b and the heat shield 7, 
which are generally far thicker than the filaments of the heating elements 
15a and 15b and have much lower temperatures than these filaments. 
The heating filaments themselves, such as those shown in FIG. 2, are 
directly and permanently connected to electrodes 20 which then extend 
sealingly outside of the reactor 1. Thus, the connections between the 
filament and the external end of the electrodes 20 extending outside of 
the reactor chamber 1 are continuous and permanent, and can be a single, 
continuous element, or can be prepared by welding together or some other 
means of creating a permanent bond between any individual electrode 
portions in order to form single continuous electrode-filament 
arrangements. The elimination of any detachable electrical connections 
within the reaction chamber in this manner, in turn, thus eliminates the 
problems created by potential oxidation of the detachable electrode 
sections. 
The electrodes 20 themselves are sealed with respect to the reactor using 
high-vacuum seals 34 as seen in FIG. 1. These seals 34 can be produced 
from commercially available components, as particularly shown in FIG. 4 
hereof. In this case, the high vacuum seal 34 for the electrode 20 
comprises a high vacuum flange 36 which is part of the reactor 1 itself. A 
hollow electrical insulator element 39 consists of two high vacuum flanges 
38 at either end thereof, and identical to flange 36 for attachment 
thereto. The hollow electrical insulator element 39 includes a ceramic 
tube 41 which is soldered to flanges 38. A compressed O-ring made of 
vacuum-compatible elastomer 42 provides a high vacuum seal and is welded 
to flange 40 which is attached to lower flange 38. 
Outside of the reaction chamber 1, the electrodes 20 can be attached by 
detachable members 22 to the power source 24 by means of wiring 26. 
Turning to the material of the heating filaments 15a and 15b themselves, 
as indicated above the filament comprises a superalloy. In particular, the 
specific nature of the superalloys such as Kanthal, Haynes and Inconell 
are set forth in Table 1 hereabove. The use of ceramic materials for the 
heating filaments 15a and 15b is not presently practical for such an 
application primarily due to the difficulties of obtaining and machining 
flat plates from such ceramic materials, such as silicon carbide and the 
like. There are also problems in providing reliable electrical contacts 
with such materials aside from the extremely high cost and low strength 
resulting in maintenance problems and the like. 
As can be seen in FIGS. 1 and 2, the filaments 15a and 15b are mounted 
below the susceptor 9 in a manner which permits their thermal expansion 
without restriction. Referring to FIG. 5, for example, safilament 15 has a 
circular configuration, and the ends of the circle are separated by a 
small distance A. 
These ends are connected to electrodes 20, and thus the remaining portion 
of the circle is free to expand upon increasing temperatures. Thus, as 
shown by the interrupted line in FIG. 5, the filament 15a will expand 
considerably to the configuration shown as 15c, for example, under 
increased temperatures of about 1000.degree. C. Any support for the bottom 
surface of the filament which would be required would be accomplished by 
utilizing ceramic and/or non-conductive members merely for support 
purposes, but again without permanent attachment and/or in any way 
preventing the thermal expansion shown in FIG. 5. The thermal expansion of 
the filament 15a (as well as that of filament 15b) is not restricted by 
the presence of the support elements 30 since they will restrict movement 
of the filaments only in the vertical direction. Furthermore, the 
connection between electrode 20 and cables 26, as particularly shown in 
FIG. 3, which are located outside of the reactor 1, are provided by 
commercially available electrical connections 22 (e.g., of the clamp type) 
rated by maximum current provided by power supplies 24a and 24b. Thus, in 
the embodiment shown in FIG. 5, the only restriction upon thermal 
expansion is at the point where the ends of the filaments 15 are connected 
to electrodes 20. In this manner, expansion occurs by rotation about these 
ends connected to electrodes 20. The angle of rotation thus effected by 
such thermal expansion is shown by angle .phi. shown in FIG. 5. This angle 
is proportional to the distance A between the two ends of the filament and 
between the electrodes 20, and can further be estimated from the following 
equation: 
EQU .phi.=ASIN (A/R)-ASIN (A/R(1-.alpha.t)) 
in which A is the distance between the electrodes (generally in the 
embodiment shown in FIG. 5 of about 0.5"), R is the average filament 
radius (in the embodiment shown in FIG. 5 about 3.5"), .alpha. is the 
coefficient of thermal expansion, and T is the filament temperature. Thus, 
in the case discussed above in which the superalloy Kanthal is employed, 
for example, having a coefficient of thermal expansion of 15e-6 1/C at a 
filament temperature of 1000.degree. C., the angle .phi. will be about 
0.070.degree.. In order to fully compensate for this minor rotational 
movement, the electrodes 20 can be sealed within the reactor 1 at a 
distance separated from the filaments themselves. 
Torque M required to twist electrode 20 on this angle (the same torque will 
be applied to the filament) can be calculated as: 
EQU M-GJ.phi./L, where (2) 
G=10 E7 psi - torque module of elasticity, 
J=1.57 R*4 polar moment of inertia, J=3.83 E-4 for electrode with 0.25" 
diameter, 
L=20.0" - length of the electrode 20, see FIG. 1, 
.phi.=0.07 degree - 1.22E-3 rad - torque angle calculated above. 
Moment calculated by eq. 2 above equal only - 0.2 lb. In. So small a torque 
moment (achieved by using long electrode 20) will not damage filament 15a. 
Although the invention herein has been described with reference to 
particular embodiments, it is to be understood that these embodiments are 
merely illustrative of the principles and applications of the present 
invention. It is therefore to be understood that numerous modifications 
may be made to the illustrative embodiments and that other arrangements 
may be devised without departing from the spirit and scope of the present 
invention as defined by the appended claims.