Susceptor for vapor-phase growth apparatus

It was an objective of the present invention to provide a susceptor which can prevent a increasing phenomenon of the dopant concentration of the epitaxial layer at the peripheral portion of the wafer. By providing a through-hole 7 passing through to a rear side at the outer peripheral side of the wafer inside the wafer pocket 6, a down flow of a reacting source gas from the upper surface of the susceptor 5 is formed, so that the unwanted flow of the dopant species being exhausted at the rear surface onto the wafer surface can be avoided. As a result, a raise in the dopant concentration at the outer peripheral portion of the epitaxial layer 9 can be controlled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improvement of a susceptor which is 
employed to a vapor-phase growth apparatus to grow an epitaxial film onto 
the semiconductor wafer. More specifically, the present invention relates 
directly to a vapor-phase growth apparatus in which a through-hole portion 
extending to a rear side of the susceptor is provided at the most outer 
peripheral portion inside the wafer pocket in order to mount the wafer, 
and a raise in dopant concentration at the outer periphery of the grown 
epitaxial film can be controlled. 
2. Description of the Prior Art 
As to a vapor-phase growth apparatus in order to grow an epitaxial film 
onto the semiconductor wafer, there have been several conventional types 
of apparatus available; they may include (1) a vertical-type vapor-phase 
growth apparatus in which the susceptor being placed on a circular disc is 
heated from its bottom side, and (2) a single wafer type vapor-phase 
growth apparatus with which a good quality epitaxial film can be 
fabricated. 
For example, inside the rectangular chamber being fabricated of quartz in 
said single wafer type vapor-phase growth apparatus, semiconductor wafer 
is mounted on the disc-shaped susceptor which is a graphite being coated 
with SiC. The semiconductor wafer is heated using a heater which is 
provided outside of the chamber in order to react with various types of 
reacting source gases passing through the chamber, resulting in growing 
the epitaxial film on the semiconductor wafer. 
As seen in FIG. 3, said susceptor 5 is composed of a high purity graphite 
which is coated mainly by SiC. On the surface, a groove called as a wafer 
pocket 6 is formed in order to accomodate the silicon wafer. The size of 
said wafer pocket 6 is slightly larger than the dimension of the wafer 8 
and the depth is about 1 mm. After placing the wafer 8 inside the wafer 
pocket 6, the susceptor is held in the reacting source gaseous flow at a 
predetermined temperature to generate the silicon epitaxial film layer 9 
on the wafer surface. 
Moreover, several improvements have been proposed in order to minimize the 
surface contact between the inner surface of the pocket and the wafer rear 
surface. These proposed improvements may include (1) a structure to 
contact-hold the wafer to a plurality of convex portions by forming a 
mesh-shaped shallow fine groove so-called a roulette, (2) making a tapered 
surface in order to confine the contact of the wafer at its outer 
periphery, or (3) using much coarser surface roughness of coated SiC 
surface than the surface roughness of the wafer. 
As to a reacting source gas, a dopant source gas such as diborane (P type) 
or phosphine (N type) is added to a chloro-silane gas which is hydrogen 
diluted. Hence the silicon epitaxy as well as a bi-product of HCl are 
produced on the wafer surface through a heat CVD (chemical vapor 
deposition) reaction. As a result, although the silicon epitaxial growth 
on the wafer surface can proceed, the rear surface of said wafer is also 
exposed to diffusion reaction gas to create a Si--H--Cl atmosphere, which 
might furthermore lead to a precipitation/etching reaction in a 
microscopic scale. 
For instance when the epitaxial growth having lower concentration than the 
dopant concentration of the wafer is conducted such as an epitaxial growth 
of P type film (specific resistance is 1 .OMEGA.cm) against the wafer with 
the dopant concentration P ++ type (specific resistance is 5 m.OMEGA.cm), 
the dopant concentration in the epitaxial layer tends to increase at the 
outer peripheral portion of the wafer, as demonstrated in FIG. 4 which 
shows a change in dopant concentration from the center of the wafer and as 
a function of a distance from the center to the most outer periphery. 
The above phenomenon might be due to the fact that the dopant species of 
the wafer 8 might be exhausted in Si--H--Cl atmosphere at the rear surface 
of wafer 8, and the exhausted dopant species might migrate to the front 
surface through the gaseous diffusion flow 11, resulting in increasing the 
dopant gaseous concentration locally. As a result, a particular region of 
the epitaxial layer where the dopant concentration is out of the range 
defined by the specification, leading to a poor production efficiency of 
the device. 
SUMMARY OF THE INVENTION 
Objective of the Invention 
All of the forgoing have resulted in a requirement for improvement of the 
apparatus of the present invention in which it is an objective of the 
present invention to provide a susceptor which can prevent the increasing 
phenomenon of the dopant concentration in the epitaxial layer at its 
peripheral portion, as it would be obvious when the epitaxial growth 
proceeds at lower concentration than the dopant concentration of the 
wafer. It is, accordingly, another objective to provide a susceptor for 
the vapor-phase growth apparatus which can avoid the unwanted flow of the 
dopant species being exhausted at the rear side to the wafer surface. 
Disclosure of the Invention 
The present inventors found that, in a suscpetor for the vapor-phase growth 
apparatus, the aforementioned localized nonuniform distribution of the 
dopant concentration can be minimized by forming a vapor flow in order to 
prevent the unwanted flow of the dopant species being exhausted at the 
rear side to the wafer surface. After investigating various designs for 
the susceptor to achieve said objectives, the following design was 
evaluated to perform the best efficiency. By providing a through-hole 
passing through to the rear side at the outer peripheral portion of the 
wafer inside the wafer pocket, the down-flow from the upper surface of the 
susceptor is generated, so that the unwanted flow of the dopant species 
being exhausted toward the wafer surface can be prevented. As a result, 
the raise in the dopant concentration can be controlled at the outer 
peripheral portion of the epitaxial layer. 
Namely, according to the present invention, a susceptor can be provided 
which is characterized by providing a through-hole passing through to the 
rear side at the most outer peripheral portion inside the wafer pocket 
which is a concave portion for mounting the wafer. 
According to the present invention, the localized raise of the dopant 
concentration at the most outer peripheral portion of the grown epitaxial 
layer can be prevented by providing a through-hole passing through to the 
rear side of the susceptor at the outer periphery inside the wafer pocket 
which is used for mounting the wafer. Specifically, the raise in the 
dopant concentration in the epitaxial layer can be avoided when the 
epitaxial growth with a lower concentration than the dopant concentration 
of the wafer is progressing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A vapor-phase growth apparatus seen in FIG. 1a has a rectangular chamber 1 
made of quartz, in which the semiconductor wafer 8 is mounted on the 
circular disc-shaped susceptor 5 which is a graphite substrate being 
coated with SiC. The semiconductor wafer 8 is heated by a heater (not 
shown) which is placed outside of the chamber 1 to react with the reacting 
source gas flowing horizontally inside the chamber 1 in order to generate 
the epitaxial film 9 on the semiconductor wafer surface 8. 
The susceptor 5 of the present invention has an arc-shaped groove-type 
through-hole portion 7 at the most outer peripheral portion inside the 
wafer pocket 6 for mounting the semiconductor wafer 8. The inner plain 
portion of the wafer pocket is a coated SiC layer. 
In FIG. 1a, although the reacting source gas 10 is introduced horizontally 
from the gas inlet opening 2 of the chamber 1 to the gas exhaust opening 
3, the source gas is supplied and heated particularly to the wafer surface 
8 and the bi-product gas is generated during the epitaxial reaction, so 
that the gas can be subjected to the volumetric expansion. By providing a 
through-hole portion 7 at the outer peripheral portion inside the wafer 
pocket of the susceptor 5, a localized gas flow is generated from the 
wafer surface 8, and the boron species being exhausted from the rear 
surface of the wafer 8 can be exhausted without the unwanted flow-back to 
the front surface of the wafer 8. 
Furthermore, by blowing the source gas directly toward the front surface of 
the wafer 8, the gas flow 12 passing from the front surface to the rear 
surface of the wafer 8 through the through-hole can be promoted, so that 
the growth efficiency can be enhanced. Moreover, by providing an exhaust 
opening 4 at the rear side of the susceptor 5 in the chamber 1, the gas 
flow passing from the front surface to the rear surface of the wafer 8 
through the through-hole portion 7 can also be promoted, resulting in that 
the epitaxial growth efficiency can be improved. 
In the above, although the single wafer type vapor-phase growth apparatus 
has been described, the type of the apparatus to which the present 
invention is applicable includes any prior art types including a 
vertical-type vapor-phase growth apparatus or a barrel-type vapor-phase 
growth apparatus. With any one of these types of growth apparatus, 
according to the present invention, the reacting source gas can flow in 
parallel to the wafer surface being placed in the susceptor. 
The through-hole portion of the susceptor in this invention can be various 
types including an arc-shaped groove-type through-hole portion as 
mentioned previously, an ovalshaped through-hole portion, or a plurality 
of small size of holes. Moreover, with the single wafer type of 
vapor-phase growth apparatus supporting wafers at the central portion of 
the susceptor, if through-holes as many as possible can be provided, the 
exhausting efficiency of the boron species from the rear surface of the 
wafer could be enhanced in such a way that said many through-holes are 
designed and fabricated with relatively large connecting area left, so 
that the area can be strong enough to withstand the weight of the outer 
peripheral portion area by the wafer pocket. Similarly, with any other 
susceptor types than the single wafer type, it is recommended to provide 
through-holes as many as possible if there is an enough connecting portion 
left to withstand the structural strength under considering the wafer 
weight. Furthermore, it is preferable to define the diameter (or width) of 
the through-hole along the wafer direction to be, at most, equal to the 
wafer's outer periphery under taking the heating effect into account. 
EMBODIMENTS 
Using the horizontal single wafer type vapor-phase growth apparatus with a 
lamp-heating method as seen in FIG. 1, an epitaxial film with the film 
thickness of about 10 pm was formed at a reaction temperature of 
1,150.degree. C. onto the P++ type (100) plane silicon semiconductor base 
plate (with 200 mm diameter) having the specific resistance of 5 
m.OMEGA.cm using SiHCl.sub.3 diluted with hydrogen as the silicon 
supplying source gas. Two tests were conducted; one was with susceptor 
having the through-hole of the present invention and the other was with 
the conventional type of susceptor without any through-holes as seen in 
FIG. 3. 
As seen in FIGS. 2a and 2b, the suscpetor according to the present 
invention has an arcshaped groove-type through-hole portion 7 at the most 
outer peripheral portion of the wafer pocket 6. In FIG. 2a, four locations 
are installed with the through-hole portions 7 leaving the connecting area 
of 75 mm on its peripheral portion. On the other hand, in FIG. 2b, four 
locations are provided with through-hole portions 7 leaving 5 mm 
connecting area on its peripheral portion. The length fractions of said 
each through-hole portion per the total peripheral length was 
approximately 50% and 90%, respectively. 
The raise in the dopant concentration of the outer peripheral portion of 
the grown epitaxial film was listed in Table 1 and presented in FIG. 4. By 
comparing with the conventional type as seen in FIG. 3, it was found that 
the longer the through-hole portion in both FIG. 2a and FIG. 2b, the 
lesser the dopant concentration increase. Table I. Comparison of dopant 
concentration at center and 3 mm from the edge for conventional type 
susceptor and the susceptor of the present invention. 
TABLE I 
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3 mm from 
center location 
the edge increment 
(.times. 10.sup.16 atoms/ 
(.times. l0.sup.16 atoms/ 
(.times. 10.sup.15 atoms/ 
susceptor cm.sup.3) 
cm.sup.3) 
cm.sup.3) 
______________________________________ 
conventional type 
1.00 1.20 2.0 
(FIG. 3) 
Example 1 1.00 
1.0 
(FIG. 2a) 
Example 2 1.00 
0.5 
(FIG. 2b) 
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While the invention has been explained with reference to the structure 
disclosed herein, it is not confined to the details as set forth, and this 
application is intended to cover modifications and changes as may come 
within the scope of the following claims.