Rapid thermal annealing for semiconductor substrate by using incoherent light

Rapid thermal annealing for heat-treating a semiconductor substrate is provided without damaging the substrate surface After the semiconductor substrate is placed in an annealing apparatus having an incoherent light source, an inert gas containing a very small amount of an oxygen gas is introduced into the annealing apparatus, while applying an incoherent light to the substrate surface from the incoherent light source. In this case, the oxygen concentration in the inert gas is defined by 10 to 1000 ppm.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of making a semiconductor device, and 
more particularly to rapid thermal annealing of a semiconductor substrate 
by radiation of incoherent light. 
2. Description of the Related Art 
In a manufacturing process of a semiconductor device, rapid thermal 
annealing (hereinafter, referred to as RTA) of a semiconductor substrate 
has been used. The advantages of RTA lie in: first, since rapid heating 
and cooling are performed, it is possible to heat-treat a semiconductor 
substrate for a short time with an accuracy of second unit; and second, 
since semiconductor substrate is transferred to and from a heating furnace 
at a low temperature near the room temperature, an atmosphere of the 
heating furnace is accurately controlled during heating. In the 
heat-treatment of semiconductor substrates using an ordinary electric 
furnace, outside air enters into the furnace due to entrainment with 
transfer of the semiconductor substrate into the furnace. Accordingly, a 
problem of oxidizing the substrate may often occur, which should not 
occur. RTA is especially effective to avoid this problem, because RTA 
enables the heat-treatment in the inert gas atmosphere under the condition 
of the residual oxygen concentration being as low as almost zero. 
Conventionally, in the heat-treatment by RTA, a low concentration of 
residual oxygen has been considered as an advantage in controlling the 
atmosphere. 
The application of RTA includes the activation of ion implantation layers, 
the elimination of crystal defects in the diffused layers, and the 
formation of resistor layers with low resistance by heat-treating a 
semiconductor substrate, having a region containing a high concentration 
impurity, in an atmosphere of inert gas such as nitrogen. In such 
applications, it is possible to heat-treat the semiconductor substrate 
without oxidation. 
In the actual manufacturing process semiconductor devices, however, a 
problem arises in heating an exposed substrate. Particularly, when a 
semiconductor region having an impurity concentration of 
1.0.times.10.sup.19 cm.sup.-3 or more and formed in a semiconductor 
substrate is heat-treated by RTA in a nitrogen atmosphere, impurity atoms 
and atoms constituting the substrate will be removed from the surface of 
the exposed semiconductor region. This results in a nonuniform sheet 
resistance of the semiconductor region after the heat-treatment by RTA, 
which makes it difficult to form a low resistance element in the 
semiconductor substrate. The nonuniformity of the sheet resistance results 
inevitably from heat-treatment of an exposed substrate, but not from 
variation in the temperature of heat-treatment. Additionally, in a 
nitrogen atmosphere, heat-treatment at as high as 1100.degree. C. or more 
will create surface roughness in the substrate regardless of impurity 
concentration. Such nonuniformity of the sheet resistance and surface 
roughness (hereinafter, generically referred to as damage to the substrate 
surface) have an undesirable effect on the uniformity and reproducibility 
in the manufacturing process of semiconductor devices. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a method of 
heat-treating a semiconductor substrate whose major surface is partially 
exposed, using RTA. 
Another object of the present invention is to provide a method of making a 
semiconductor device having a uniform sheet resistance by heat-treating of 
a high-impurity concentration semiconductor region exposed to a 
semiconductor substrate surface, using RTA. 
Still another object of the present invention is to provide a method of 
making a semiconductor device for activating an ion implanted layer and 
removing crystal defects in a diffused layer and a semiconductor 
substrate, using RTA. 
A further object of the present invention is to provide a method of 
heat-treating a semiconductor substrate without damaging a surface of the 
semiconductor substrate, using RTA. 
According to an aspect of the present invention, there is provided a method 
of making a semiconductor device, which comprises placing a semiconductor 
substrate in an annealing apparatus having an incoherent light source, and 
introducing an inert gas containing a very small amount of an oxygen gas 
into the annealing apparatus, while applying an incoherent light to the 
major surface of the semiconductor substrate from the incoherent light 
source, thereby subjecting the semiconductor substrate to rapid thermal 
annealing. In this case, the oxygen concentration in the inert gas is 
defined by 10 to 1000 ppm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be explained in detail. 
From the distribution profile, damage to the substrate surface described in 
the prior art has been considered to stem basically from the same 
phenomenon. Specifically, the direct cause is that a dopant impurity in 
the substrate, such as arsenic (As) atoms, or silicon (Si) atoms, 
themselves are removed from the surface. It has been considered that the 
primary cause of such removal of atoms is the fact that a barrier to the 
removed of atoms from the surface is lost because a so-called natural 
oxide film, which normally exists on the surface of the semiconductor 
substrate located in the atmosphere, is lost due to heating in an inert 
gas atmosphere. It has been known that such a phenomenon takes place when 
the oxygen concentration in the inert gas atmosphere is approximately 1 
ppm or less at a heat-treatment temperature of approximately 1000.degree. 
C., typical conditions for use of RTA. The phenomenon which occurs will be 
explained in detail. In a sufficiently low concentration of oxygen, a 
normal thermal oxidation takes place as follows: 
EQU Si+O.sub.2 .fwdarw.SiO.sub.2 
Instead of this reaction, however, the following reaction is predominant in 
determining an equilibrium state: 
EQU SiO.sub.2 .fwdarw.SiO+1/2O.sub.2 
As a result of this, SiO whose vapor pressure is relatively high vaporizes 
easily, and is removed from the substrate surface. 
It can be apparent that RTA provides more accurate control of atmosphere 
than the heat-treatment by normal electric furnace, but on the other hand, 
it permits the above-described phenomenon to take place. 
To avoid this problem, in the present invention, when a semiconductor 
substrate whose major surface is partially exposed is heat-treated by TRA 
in an atmosphere of inert gas such as nitrogen, a very small amount of 
oxygen is purposely added to the inert gas atmosphere. A suitable oxygen 
concentration required for the present invention must meet the following 
two conditions: 
First condition--the oxygen concentration must be high enough to allow the 
oxide film (including natural oxide film) on the substrate surface to 
remain stably. 
Second condition--the oxygen concentration must be low enough to prevent 
the oxide film from growing on the substrate surface to a thickness more 
than necessary. 
Of these two conditions, an oxygen concentration that meets the first 
condition must be approximately 1 ppm or more as mentioned above, and when 
taking into account different variations caused in the process, it is 
desirable that the concentration should be approximately 10 ppm or more. 
On the other hand, for an oxygen concentration that fulfills the second 
condition, this condition must be defined more specifically. 
That is, the new second condition is that the thickness of an oxide film 
grown on the substrate must be approximately 50 .ANG. or less in spite of 
the substrate condition including impurity concentration when a 
heat-treatment is carried out at 900.degree.-1200.degree. C. for about 60 
seconds or less that is a typical condition of RTA. Experiment has shown 
that an oxygen concentration fulfilling the new second condition is 
approximately 1000 ppm or less. 
It is not desirable to expect the residual oxygen from the outside air to 
achieve an oxygen concentration that meets the above two conditions, 
because such residual oxygen is not controllable and the inert gas 
atmosphere is contaminated. For obtaining a controlled oxygen 
concentration that meets the two conditions, it is preferable to employ a 
mass flow controller (hereinafter, referred to as an MFC). Since a normal 
flow rate of process gas used in RTA is approximately 10 slm, the 
condition of 1000 ppm or less can be satisfied provided that the flow rate 
of oxygen gas is 10 sccm or less. The flow rate accurately controlled by 
MFC is 1 sccm or more, so that it is sufficiently possible to achieve the 
oxygen concentration that meets the two conditions by using an MFC. 
Therefore, only minor change to the conventional RTA apparatus makes it 
possible to heat-treating the exposed substrate surface in an inert gas 
atmosphere without the unwanted damages of the substrate surface. 
FIGS. 1A and 1B show a semiconductor substrate to be heat-treated, whose 
major surface is partially exposed. 
In FIG. 1A, a P-type semiconductor substrate 1 has an N.sup.+ -type ion 
implanted layer 3 (or diffused layer) with an impurity (As) concentration 
of 1.0.times.10.sup.19 cm.sup.-3 or more, the N.sup.+ -type ion implanted 
layer 3 being exposed through an opening formed in a silicon oxide film 2. 
In FIG. 1B, part of the surface of the P-type semiconductor substrate 1 is 
similarly exposed through an opening formed in the silicon oxide film 2. 
To heat the exposed surface of the N.sup.+ -type ion implanted layer 3 of 
FIG. 1A by radiation of incoherent light, after the P-type semiconductor 
substrate 1 is placed in an RTA apparatus, an inert gas (nitrogen gas) is 
introduced into the RTA apparatus. The oxygen concentration in the inert 
gas atmosphere is controlled to approximately 100 ppm to accomplish a 
heat-treatment at 1000.degree. C. for 60 seconds. FIG. 2 shows the 
distribution of the sheet resistance of the N.sup.+ -type ion implanted 
layer 3 after the heat-treatment. 
On the other hand, the P-type semiconductor substrate 1 of FIG. 1A is 
placed in the RTA apparatus and then heat-treated at 1000.degree. C. for 
60 seconds in an inert gas (nitrogen gas) atmosphere free from oxygen. 
FIG. 3 illustrates the distribution of the sheet resistance of the N.sup.+ 
-type ion implanted layer 3 after such a heat-treatment. 
As seen from FIGS. 2 and 3, variation in the sheet resistance of the 
N.sup.+ -type ion implanted layer 3 is significantly improved by adding a 
very small amount of oxygen to the inert gas, which shows the prominent 
effect of controlling the oxygen concentration. 
To heat the exposed surface of the P-type semiconductor substrate of FIG. 
1B by incoherent radiation, after the P-type semiconductor substrate 1 is 
placed in the RTA apparatus, an inert gas (nitrogen gas) containing 
approximately 100 ppm of oxygen is introduced into the RTA apparatus, 
thereby performing a heat-treatment at 1000.degree. C. for 60 seconds. 
On the other hand, when the same substrate 1 is heat-treated in an inert 
gas (nitrogen gas) atmosphere free from oxygen under the same condition as 
describe above, undesirable surface roughness occurs at the exposed 
surface. 
As described above, the nonuniformity of the sheet resistance results from 
the heat-treatment of the exposed substrate surface, but not from 
variation in the temperature of heat-treatment. In the nitrogen 
atmosphere, the heat-treatment at the temperature higher than 1100.degree. 
C. may create noticeable roughness of the substrate surface. In either 
case, such nonuniformity of the sheet resistance and surface roughness 
have an undesirable effect on the uniformity and reproducibility in the 
manufacturing process of semiconductor devices. The disadvantage, however, 
can be overcome by adding a very small amount of oxygen to the inert gas. 
FIG. 4 is a comprehensive illustration of the existence of damage to the 
substrate surface, using the oxygen concentration as a parameter. Here, 
100 ppm and 1000 ppm of oxygen concentration respectively indicate that 
the oxygen concentration flow rate is controlled to 1 sccm and 10 sccm by 
the MFC at the inert gas (e.g., nitrogen gas or argon gas) flow rate of 10 
slm. Zero oxygen concentration means that there is no control of oxygen 
concentration (this may be considered as similar to the prior art). That 
is, it is a case where an exact oxygen concentration is unknown, but may 
be considered at least 1 ppm or less. 
To prevent the damage of the substrate surface effectively by purposely 
adding a very small amount of oxygen to the inert gas atmosphere, the most 
effective oxygen concentration to be added is in the range more than 10 
ppm and less than 1000 ppm. The thickness of the oxide film formed on the 
substrate under the above heat-treatment condition is in the range of 
measurement errors and almost the same as that before the heat-treatment. 
As a result, the requirement is sufficiently satisfied in that extra oxide 
film is prevented from growing. 
As described above, according to the method of making the semiconductor 
device of the present invention, the following advantages will be 
provided. 
When a semiconductor substrate having an exposed surface is heat-treated by 
RTA, a very small amount of oxygen gas is purposely added to an inert gas 
atmosphere. This makes it possible to carry out RTA to the extent that an 
oxide film is stably remained on the substrate surface and also to the 
extent that an excessively thick oxide film is prevented from growing on 
the substrate surface. As a consequence, it is possible to perform RTA 
without causing damage to the substrate surface, which provides a good 
uniformity and reproducibility in the manufacturing process of 
semiconductor devices. 
It is further understood by those skilled in the art that the foregoing 
description is the preferred embodiment and that various changes and 
modifications may be made in the invention without departing from the 
spirit and scope thereof.