Low temperature silicon epitaxy with germanium doping

A non-strained epitaxial layer is formed to have a small transition width and a low amount or no amount of oxygen incorporated therein. During the formation of non-strained epitaxial layer, a germanium source gas is introduced. Germanium reacts with water and/or oxygen to form GeO, which sublimates from the surface of the non-strained epitaxial layer, instead of oxygen being incorporated into the lattice. Thus, a low temperature epitaxial process can be used to obtain the small transition width without having oxygen incorporated into the non-strained epitaxial layer.

BACKGROUND OF THE INVENTION 
This invention relates, in general, to semiconductor materials, and more 
particularly, to a low temperature silicon epitaxy. 
It is desirable to use low temperature (less than 900.degree. C.) 
processing when growing silicon epitaxial layers on a semiconductor 
substrate to limit the autodoping effects and the diffusion length of 
dopants out of the substrate. The amount or thickness of the epitaxial 
layer formed before the desired dopant profile is obtained is called the 
transition width. 
The use of low temperature, however, increases the incorporation of oxygen 
(O.sub.2) into the silicon epitaxy. The oxygen can cause oxygen induced 
stacking faults or act as a contaminant which creates a barrier to 
electron mobility. Both of which can be detrimental to semiconductor 
devices formed in the silicon epitaxial layer. 
It would be desirable to remove or lower the oxygen incorporation while 
still using a low temperature epitaxial process. A lower oxygen 
incorporation would increase the quality of the epitaxial layer by 
lowering crystal defects and raising the silicon epitaxial layer's charge 
carrier lifetimes. 
A way of reducing the amount of oxygen incorporated into a silicon 
epitaxial layer is to use ultra-high vacuum CVD epitaxy, utilizing a base 
pressure of 1.times.10.sup.-9 torr and process pressure in the range of 
1.times.10.sup.-3 torr. This ultra-high vacuum CVD process removes oxygen 
and water (H.sub.2 O) vapor so that less oxygen is available to 
incorporate into the silicon epitaxy during growth. The use of ultra-high 
vacuum CVD, however, in a manufacturing setting is not very practical 
because of low throughput and high cost. 
Thus, it would be desirable to have a manufacturable process for reducing 
the amount of oxygen incorporation into a silicon epitaxial layer when 
forming epitaxial layers at low temperatures. 
SUMMARY OF THE INVENTION 
A non-strained, germanium doped, silicon epitaxial layer is chemically 
formed on a substrate in the presence of a germanium source gas resulting 
in a germanium percentage concentration in the non-strained, germanium 
doped, silicon epitaxial layer greater than 0% but less than an amount 
which gives a silicon germanium bandgap significantly different from the 
bandgap of silicon.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a cross-sectional view of an embodiment of the present 
invention. What is shown, is substrate 10 having a non-strained, germanium 
doped, silicon epitaxial layer 20 (hereinafter non-strained epitaxial 
layer) formed thereon. Non-strained epitaxial layer 20 of the present 
invention is formed to have a small transition width and a low amount or 
no amount of oxygen incorporated therein, as will be described below. 
Substrate 10 may be comprised of a suitable semiconductor material, 
preferably silicon. 
FIG. 2 illustrates a graph of the thickness of an epitaxial layer and a 
semiconductor substrate versus the dopant concentration therein. Curve 30 
illustrates a dopant (N or P type) concentration after the epitaxial layer 
is formed on the semiconductor substrate using a high temperature 
epitaxial process, while curve 35 illustrates a dopant concentration after 
an epitaxial layer is formed on a semiconductor substrate using a low 
temperature epitaxial process. Note that the transition width of the 
epitaxial layer formed at the high temperature is wider relative to the 
transition width of the epitaxial layer formed at a low temperature. It is 
desirable to have this transition width minimized because it is desirable 
to form thinner epitaxial layers in order to achieve better electrical 
performance of the semiconductor devices which are formed in the epitaxial 
layer. If the transition width is large, a thicker epitaxial layer must be 
formed in order to achieve an epitaxial layer thickness having the desired 
constant concentration. 
As explained previously, the disadvantage of using a low temperature 
epitaxial process is that it causes an increased incorporation of oxygen 
into the epitaxial layer. Oxygen which is incorporated into the epitaxial 
layer lattice causes oxygen induced stacking faults and the oxygen creates 
a barrier to electron mobility. As will be described below, the present 
invention allows for the use of a low temperature epitaxial process, 
without resulting in increased incorporation of oxygen into the epitaxial 
layer. 
FIG. 3 illustrates a simplified view of the process utilized in the present 
invention. A reaction chamber 40 which forms non-strained epitaxial layer 
20 on substrate 10 is equipped with silicon source gases 45, dopant gases 
50, and a germanium source gas 55. At the present time, germanium source 
gas 55 can be comprised of germane (GeH.sub.4), however, other 
compositions may be available in the future. 
In a preferred embodiment, silicon source gases 45 are comprised of 
SiCl.sub.3 H, SiCl.sub.2 H.sub.2, SiH.sub.4, or Si.sub.2 H.sub.6. These 
gases are typical silicon source gases used in the art. Other silicon 
source gases may be used, except that silicon source gases 45 being able 
to react at a reaction temperature of above 1100.degree. C. will not be as 
effective in the present invention, due to the decreased reactiveness of 
germanium source gas 55. For example, SiCl.sub.4 requires the use of a 
temperature approximately 1150.degree. C. or higher. Most preferably, 
silicon source gases 45 requiring a temperature of less than or equal to 
900.degree. C. are used. This includes silicon source gases 45 such as 
SiCl.sub.2 H.sub.2, SiH.sub.4, or Si.sub.2 H.sub.6. A temperature of less 
than 900.degree. C. will be effective in reducing the transition width 
between substrate 10 and non-strained epitaxial layer 20 and will also 
increase the reactiveness of germanium source gas 55. 
Dopant gas 35 is used to achieve desired doping types and levels in 
non-strained epitaxial layer 20. Dopant gas 35 may be chosen, for example, 
from the list comprising arsine, phosphine, diborane, or other gases. 
Epitaxial layer 20 may be doped N- or P-type. 
The temperature and pressure used to form the non-strained epitaxial layer 
will affect the growth rate. In this particular example, a base pressure 
of 1 millitorr is used. It is preferable that the present invention be 
utilized in a chemical reaction rather than a physical reaction (as 
molecular beam epitaxy) because, physical reactions normally operate at 
ultra-high pressures at which oxygen is removed. 
Before the non-strained epitaxial layer is formed on substrate 10, a 
pre-clean of substrate 10 is performed to remove any silicon dioxide 
(oxide) thereon. In a preferred embodiment, the pre-clean is comprised of 
a wet chemical clean to remove any silicon dioxide present. Then, 
semiconductor substrate 10 is placed in reaction chamber 40. A high 
temperature pre-bake may be utilized to remove any remaining silicon 
dioxide on substrate 10. The pre-clean and/or the high temperature 
pre-bake may be utilized. 
In a preferred embodiment, germanium source gas 55 is introduced into 
reaction chamber 40 before the formation of non-strained epitaxial layer 
20 begins. Germanium source gas 55 acts as a pre-clean to remove any 
oxygen present in the chamber 40. Germanium reacts with H.sub.2 O and 
O.sub.2 to form a germanium monoxide (GeO) which can be removed through an 
exhaust (not shown). 
During the formation of non-strained epitaxial layer 20, germanium source 
gas 55 is introduced into reaction chamber 40. During the formation of 
non-strained epitaxial layer 20, the reaction between germanium and water 
or oxygen forms GeO, which sublimates from the surface of the non-strained 
epitaxial layer, instead of oxygen being incorporated into the lattice. 
Thus, the use of germanium source gas 55 allows one to go to a lower 
temperature to form non-strained epitaxial layer 20 to obtain the shorter 
transition width, while not incorporating or minimizing the incorporation 
of oxygen into non-strained epitaxial layer 20. A lower temperature also 
decreases the growth rate of an epitaxial layer. In the past, a higher 
silicon source gas flow rate was used to increase the growth rate, but 
this caused even more incorporation of oxygen into the epitaxial layer. In 
the present invention, a higher silicon source gas 45 flow rate can be 
used without incorporation or with minimal incorporation of oxygen, which 
results in a better quality non-strained epitaxial layer 20 and higher 
throughput. 
The amount of germanium source gas 55 (flowrate and % of Ge) used will 
depend on how much oxygen is present in reaction chamber 40. An amount of 
germanium may not react with the oxygen; that amount will be incorporated 
into non-strained epitaxial layer 20. However, the maximum amount of 
germanium source gas 55 which is used in the present invention is an 
amount which, when incorporated into non-strained epitaxial layer 20, does 
not substantially change the electrical characteristics of a silicon 
epitaxial layer. The bandgap of non-strained epitaxial layer 20 (silicon 
germanium bandgap) remains substantially unchanged relative to that of 
silicon by introduction or incorporation of germanium into non-strained 
epitaxial layer 20. In other words, the electrical characteristics of 
non-strained epitaxial layer 20 are essentially the same as a silicon 
epitaxial layer. 
In a preferred embodiment, the amount of germanium incorporated into 
non-strained epitaxial layer 20 is in the range of greater than 0% but 
less than or equal to 1%. This amount has been found not to alter the 
bandgap of silicon in a significant way. It is also important to note that 
non-strained epitaxial layer 20 is formed without any strain and remains 
non-strained. Below a certain Ge fraction in SiGe, it does not matter what 
the thickness of the SiGe layer is; it is formed and remains non-strained. 
This germanium fraction is believed to be approximately 3%. Non-strained 
epitaxial layer 20 of the present invention should not be confused with a 
strained epitaxial layer in which the bandgap of silicon is altered. With 
a germanium fraction above 3%, the SiGe layer as formed will be 
"strained." Strained epitaxial layers are typically used to form 
heterojunction devices. The thickness of the SiGe will have to be above a 
certain critical thickness so that the SiGe lattice relaxes and becomes 
"unstrained." It is also important to note that an unstrained SiGe layer 
is not the same as non-strained epitaxial layer 20 of the present 
invention. 
FIG. 4 illustrates a graph of the depth of non-strained epitaxial layer 20 
and substrate 10 formed in the present invention versus the concentration 
of various dopants therein. Curve 70 illustrates the doping of substrate 
10 and into non-strained epitaxial layer 20 formed using the low 
temperature process of the present invention. Dashed line 72 illustrates 
the portion of the dopant concentration that would be formed utilizing a 
high temperature process used in the prior art. Note that if a small 
transition width is not critical, the present invention can still be used 
to reduce oxygen incorporation. Curve 75 illustrates the concentration of 
germanium present in non-strained epitaxial layer 20. The resolution of 
detection of germanium only goes down to a certain level thus, substrate 
10 is shown to have germanium present, although none may be detectable. 
Curve 77 shows the concentration of oxygen present in substrate 10 and 
non-strained epitaxial layer 20. Note that the concentration of oxygen in 
the non-strained epitaxial layer 20 is approximately 5.times.10.sup.18 
atoms/cm.sup.3 by using germanium during the formation of non-strained 
epitaxial layer 20. This oxygen concentration using a low temperature 
epitaxial process of the prior art, without germanium, would probably be 
in the range of 1.times.10.sup.19 atoms/cm.sup.3 or greater. 
FIG. 5 illustrates a second embodiment of the present invention. What is 
shown, is a substrate 10 having an insulative layer 15 formed thereon, 
having selected portions removed. FIG. 5 illustrates that a selective 
epitaxial process can be used in the present invention to form a selective 
epitaxy 25. An advantage obtained when using a selective epitaxial process 
with the present invention is that a lower temperature increases the 
selectivity and reduces the amount of faceting of selective non-strained 
epitaxial layer 25 adjacent to insulative layer 15. Adding germanium 
source gas 55 during the selective epitaxial process also increases the 
selectivity because the lower the oxygen level present, the more selective 
the process is. Higher selectivity means that selective non-strained 
epitaxial layers 35 will not be formed on insulative layer 15. 
As can be seen, the present invention enables one to obtain a small 
transition width by using a low temperature epitaxial process without 
having a detrimental amount of oxygen incorporated into the epitaxial 
layer. When the present invention is used with a selective epitaxial 
process, all of the advantages disclosed above are obtained, as well as 
increased selectivity.