Epitaxial front seal for a wafer

A thin high resistivity epitaxial layer is provided over the entire surface of a semiconductor wafer in order to minimize autodoping while growing a desired epitaxial layer over the entire semiconductor wafer. The thin low resistivity epitaxial layer acts as a seal and is of the same conductivity type as the semiconductor wafer or substrate. The thin epitaxial layer effectively becomes a part of the substrate or semiconductor wafer. The seal layer or thin epitaxial layer is needed when growing an epitaxial layer over a very low resistivity silicon semiconductor wafer.

BACKGROUND OF THE INVENTION 
This invention relates, in general, to the manufacture of semiconductor 
wafers, and more particularly, to reducing autodoping during the growing 
of an epitaxial layer on a semiconductor wafer. 
Autodoping has been a persistent problem to the deposition of epitaxial 
layers on a semiconductor substrate. This is particularly true for a low 
resistivity semiconductor substrate. Autodoping is a result of two 
distinct occurrences; one is that caused by outdiffusion from the low 
resistivity substrate into the epitaxial layer, and the other is that 
caused by vaporization of the impurities in the substrate which enter into 
a gaseous phase of the epitaxial layer being deposited. The autodoping 
affect produces an epitaxial layer having non-uniform impurity 
concentrations. Therefore, it is difficult if not impossible to accurately 
control the impurity concentration within the epitaxial layer because of 
the autodoping affects. Autodoping can cause a semiconductor wafer to be a 
low yielding wafer. 
Since antimony outdiffuses less than arsenic, boron, or phosphorous, many 
semiconductor wafer manufacturers use antimony as the impurity dopant in 
the semiconductor wafer. However, some semiconductor devices require low 
resistivity substrates and therefore the semiconductor manufacturer is 
left with no choice but to use phosphorous or arsenic. Since autodoping 
can also result from vaporization of impurities from the backside of the 
wafer, it is generally customary to seal the backside of the wafer with a 
backseal such as oxide or nitride. There is not believed to be an 
effective front seal for the front or top of the wafer. 
Autodoping from a highly doped diffused region in a substrate during growth 
of an epitaxial layer has been reduced by the growing of a thin epitaxial 
layer over the entire surface of the substrate and then removing the 
epitaxial layer except for the portion over the highly doped diffused 
region. Typically this thin epitaxial portion then becomes part of the 
subsequently grown epitaxial layer, the substrate is of a different 
conductivity type than the small portion of thin epitaxial layer, and such 
autodoping minimization is believed to have only been used for integrated 
circuits. Examples of this technique can be found in U.S. Pat. No. 
3,660,180 which issued to Wajda, and in U.S. Pat. No. 3,716,422 which 
issued to Ing et al. Minimizing autodoping is also discussed in IBM 
Technical Disclosure Bulletin, Vol. 14., No. 11, April 1972, page 3218; 
IBM Technical Disclosure Bulletin, Vol. 15, No. 11, April 1973, page 3385; 
and in IBM Technical Disclosure Bulletin, Vol. 20, No. 3, August 1977, 
pages 1083-1084. However, it would be desirable to provide a front seal 
for an entire wafer and not just for a localized highly doped region. 
Accordingly, it is an object of the present invention to minimize 
autodoping from a low resistivity semiconductor wafer. 
Another object of the present invention is to minimize autodoping from a 
low resistivity wafer during the growing of an epitaxial layer over the 
wafer by using a thin epitaxial layer of the same conductivity as the 
wafer and which serves as a seal. 
Yet a further object of the present invention is to provide an epitaxial 
front seal for a low resistivity N-type substrate so that aluminum can be 
used as a back metal. 
SUMMARY OF THE INVENTION 
The above and other objects and advantages of the present invention are 
provided by a thin epitaxial layer deposited on a very low resistivity 
semiconductor substrate. The thin epitaxial layer is also low resistivity 
and is of the same conductivity type as the substrate. The low resistivity 
thin epitaxial layer becomes a part of the substrate. The desired 
resistivity of the thin epitaxial layer which serves as the front seal 
should have a resistivity of 0.1 to 1.5 ohms cm for an N-type conductivity 
substrate or 0.5 to 4.5 ohms cm for a P-type conductivity substrate. The 
desired epitaxial layer is then grown over the thin epitaxial layer which 
serves as a seal. The seal eliminates or minimizes the autodoping affects 
occurring in the disired epitaxial layers.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a structure which minimizes the outdiffusion or 
autodoping of heavily doped substrates. Semiconductor substrate or wafer 
10 is a heavily doped substrate having a maximum resistivity of 
approximately 0.0055 ohm cm for an N-type conductivity substrate or a 
maximum 0.02 ohm cm resistivity for a P-type conductivity substrate. 
Substrate 10 has a backside seal 13 which can be oxide, nitride, or the 
like. The use of aluminum as a back metal improves the silicon adherence 
to a smooth silicon and is useful in some semiconductor devices such as 
NPN or PNP transistor structures, for example. A front seal 11 covers the 
top side of substrate 10 has a thin epitaxial layer of the same 
conductivity type as substrate 10. The desired resistivity range for seal 
layer 11 is 0.1 to 1.5 ohm cm an N-type conductivity substrate 10 or 0.5 
to 4.5 ohm cm for a P-type conductivity substrate 10. The exact 
resistivity chosen within this range will depend largely on the particular 
resistivity of the following epi layer. If the resistivity of seal layer 
11 is too high it will act like a part of the active device structure, 
i.e., it will contribute to the extended basewidth of a resultant bipolar 
device thereby lowering high level current gain, H.sub.fe, and decreasing 
switching speed. If the resistivity of seal layer 11 is too low the seal 
layer itself will begin to act as a strong source of unwanted doping as 
would a low resistivity substrate that is unprotected by seal layer 11. 
The thickness of seal layer 11 is somewhat critical. If seal layer 11 is 
too thick its cost will be unnecessarily high due to the longer time spent 
in the reactor chamber. If seal layer 11 is too thin the dopant atoms from 
the substrate will diffuse through it and hence it will no longer be 
effective. A practical range of thickness for seal layer 11 seems to be 
approximately 10 micrometers to 15 micrometers. 
Seal layer 11 involves no patterning nor removal (partial or otherwise) and 
becomes a passive part of the final device structure, i.e., with the 
correctly chosen resistivity seal layer 11 becomes a part of substrate 10 
and not part of an active device structure which would be made in 
subsequent epitaxial layers. 
Additional epitaxial layers can be grown over seal layer 11 such as 
illustrated by dotted line 12. Epitaxial layer 12 can be any desired 
thickness, resistivity or conductivity type and can consist of one or a 
series of multiple epitaxial layer structures. Seal layer 11 will 
eliminate or substantially decrease autodoping from substrate 10 and 
therefore the uniformity of the dopant concentration of epitaxial layer 12 
can be accurately controlled. 
FIG. 2 is a cross-sectional view of a semiconductor wafer having a 
plurality of epitaxial layers. Substrate 10 is illustrated as being an N+ 
low resistivity substrate FIG. 1. Epitaxial layer 14 covers epitaxial seal 
layer 11 and has a higher resistivity than seal layer 11. Epitaxial layer 
15 covers epitaxial layer 14 and has a higher resistivity than does 
epitaxial layer 14. In the structure illustrated in FIG. 2, substrate 10 
is a silicon wafer being heavily doped with arsenic or the like to provide 
a maximum resistivity of 0.0055 ohm cm. A first epitaxial layer serves as 
seal layer 11 and has approximately 10 micrometers thickness, and has a 
resistivity of approximately 1 ohm cm. Second epitaxial layer 14 has a 
thickness in the range of 22 to 28 micrometers and a resistivity of 3.5 to 
5.5 ohm cm. Third, epitaxial layer 15 has a thickness in the range of 55 
to 66 micrometers and a resistivity in the range of 45 to 80 ohm cm. 
Epitaxial layers 11, 14, and 15 can be doped with phosphorous, arsenic, 
antimony or the like to obtain the desired resistivities. 
By now it should be appreciated that there has been provided an epitaxial 
front seal useful in the manufacturing of semiconductor wafers wherein the 
seal is of the same conductivity type as the substrate. In addition, the 
seal becomes a part of the substrate as opposed to becoming a region in 
the resulting device.