Methods of heat-treating semiconductor wafers

With a view to optimizing the donor killing process performed in the semiconductor wafer fabricating process, a heat-treating operation is performed in a thermal furnace above at least 900 .degree. C. for a predetermined time so that growth of the initial oxygen precipitates, induced into the crystal lattices during single-crystal growth, is suppressed. Thus, the oxygen precipitates are easily suppressed, irrespective of the concentration of the initial oxygen, so that the yield of the semiconductor device is improved

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor wafer and a method of 
heat-treating the same and, more particularly, to a heat-treating method 
in a donor killing process which is performed to remove a role of donor of 
interstitial oxygen in the crystal during single-crystal growth in a 
semiconductor wafer fabricating process and a semiconductor wafer shaped 
by the heat-treating method. 
2. Discussion of Related Art 
Very large scale integration circuit (VLSI) is that high-density devices 
are integrated on a single-crystal semiconductor wafer. A general 
semiconductor wafer fabricating process will be described with reference 
to FIG. 1. 
First, a single crystal is grown from raw materials. The single-crystal 
growing process is that after the raw materials like quartzite, etc. are 
shaped into a polycrystalline silicon through complex and multilevel 
purifying process, it is grown into a single-crystal ingot by the 
Czochralski (CZ) technique or the Float Zone (FZ) technique. 
Following the growth of the single-crystal ingot, a wafer, being an 
adequate material in semiconductor device fabrication, is shaped through 
performing a series of complex shaping and polishing process. This process 
is called "watering". 
That is, since the surface of the single-crystal ingot is ruggedly formed, 
a trimming process is necessary for trimming the ingot surface to have an 
adequate shape and size. The ingot trimmed is oriented in the desired 
direction along its length after one or more flats have been examined by 
X-ray (Orientation Flattening). Then, an ingot etching operation is 
performed to remove contaminants from the ingot surface and a slicing 
process is performed to make a silicon slice out of the ingot. The slicing 
process is carefully performed in order to correctly keep the crystal 
direction of the slice. Thereafter, the edge of each silicon slice, sliced 
into that having a predetermined thickness, is rounded to give a continual 
convenience in treating the wafer and not to accumulate a layer on the 
edge of the wafer in the subsequent device fabricating (Edge Rounding). 
Thereafter, the slice is lapped by the use of mixtures of oxide aluminum 
and glycerin to prevent it from bending so that its flatness increases. 
The previously described shaping operations leave the surface and edges of 
the wafer damaged and contaminated. The damaged and contaminated regions 
can be removed by chemical etching (Slice Etching). 
Following that operation, a heat-treating process called "donor killing" is 
performed. The donor killing process is generally performed in a thermal 
furnace at 600 to 650.degree. C. for 30 minutes. While, in a Rapid thermal 
Annealing (RTA) device, the process is performed around 700.degree. C. for 
30 seconds. Since the interstitial oxygen existed in the single-crystal 
lattices of the silicon is positively or negatively charged to thereby 
have a role of donor in the crystal lattices, the donor killing operation 
is to remove the interstitial oxygen which deteriorates the electrical 
control by use of an implantation process with respect to the wafer. The 
phenomenon caused by the interstitial oxygen is eradicated by performing a 
heat-treating operation above 500.degree. C. 
Then, the surface of the heat-treated slice is polished chemically or 
mechanically and cleaned. An inspection in the defects and orientation of 
the cleaned wafer is performed and an packing is performed for the passed 
wafer. 
Meanwhile, when a single-crystal silicon is grown by the conventional 
Czochralski technique, plentiful of oxygen is generated. It typically 
exists in the range of 5.times.10.sup.17 to 1.times.10.sup.18 
atoms/cm.sup.3 (or 10 to 20 ppma). The initial oxygen (Oi) induced in the 
crystal growing process is atomically dissolved and occupies the 
interstitial sites of the lattice. It becomes the most important 
precipitation material in the silicon wafer shaped by the Czochralski 
technique due to its characteristics that the degree of diffusion is very 
high and the solubility rapidly falls at low temperature. The oxygen 
precipitates grown from the initial oxygen is extremely undesirable in the 
electrical characteristic of the semiconductor device and is closely 
connected with the Electrical Die Sorting (EDS) process or the yield of 
the package. Thus, it should be removed or suppressed. 
FIG. 2 is a graph measuring the correlation between the initial oxygen 
concentration and the oxygen precipitate density after performing the 
Dynamic Random Access Memory (DRAM) process on the particular wafers 
passing through the conventional donor killing process. 
The wafers #1 to #6 are silicon wafers fabricated through the 
single-crystal growing process by the Czochralski technique, and are 
products of particular wafer manufacturers. Each wafer previously passed 
the gate oxide module formation during the DRAM process. Concerning each 
wafer, the density of the oxygen precipitates existed in the active region 
of DRAM is measured by means of the Laser Scattering Tomography (LST) 
device. Distribution of the oxygen precipitates measured lies in the range 
of 200 to 400 .mu.m. 
From FIG. 2, it can be known that the density of the oxygen precipitates in 
each wafer increases as does the initial oxygen concentration. That is, 
the initial interstitial oxygen induced into the crystal during 
single-crystal growth acts as the nucleation-element of the oxygen 
precipitates. 
FIG. 3 is a graph measuring the correlation between the initial oxygen 
concentration and the substrate leakage current after performing the 
semiconductor DRAM process concerning each wafer processed from the given 
ingots. 
These measuring operations are performed with respect to the ingots #1 to 
#6 in the substrate voltage 20 V. From FIG. 3, it might be expected that 
the substrate leakage current usually increases with respect to the ingot 
having higher initial oxygen concentration. The reason is that the oxygen 
precipitates increase in accordance with the increase of the initial 
oxygen concentration so that the leakage element of the semiconductor 
substrate increases. 
FIG. 4 is a graph measuring the correlation between the initial oxygen 
concentration and the Bin 1 yield with respect to the wafers passed the 
semiconductor 16 M DRAM. 
The Bin 1 yield indicates that passed the Bin 1 test (so-called, prime 
good) which is done as a step of the Electrical Die Sorting (EDS) process 
performing an electrical characteristic test of each chip with respect to 
the wafer embodying particular devices. 
FIG. 4 shows that the Bin 1 yield decreases in the relation of the 
complementary error function, indicated by a solid line, as the initial 
oxygen concentration increases. That is, rapid decrease in the Bin 1 yield 
occurs when the initial oxygen concentration is above 12.50 ppma (parts 
per million atoms). 
FIG. 5 is a graph representing difference in oxygen concentration 
(.DELTA.Oi) before and after the heat-treatment versus the initial oxygen 
concentration for obtaining the good yield in the semiconductor DRAM 
fabricated by the use of the usual semiconductor wafer. The oxygen 
concentration difference is measured with the Fourier Transform Infrared 
(FTIR) spectrometer. As shown in the drawing, the wafer is heat-treated in 
a thermal furnace at 700.degree. C. for 20 hours, and in succession, at 
1000.degree. C. for 10 hours. 
As might be expected from the correlation between FIGS. 4 and 5, the Bin 1 
yield rapidly decreases around the initial oxygen concentration 12.50 
ppma, being an alteration point. And correspondingly, when the initial 
oxygen concentration is above 12.50 ppma, the oxygen concentration 
difference (.DELTA.Oi) rapidly increases. On the contrary, when the 
initial oxygen concentration is below 12.50 ppma, the Bin 1 yield is kept 
very high above 35 % and the oxygen concentration difference is kept very 
stably below 2 ppma. 
Therefore, it is required in the semiconductor wafer fabrication that the 
initial oxygen concentration in the semiconductor wafer is kept below 
12.50 ppma to obtain the good yield of the device. 
However, to keep the initial oxygen concentration in the semiconductor 
wafer below an permissible value, for example, 12.50 ppma, not only a 
careful interest should be made from the beginning of single-crystal 
growth, but also a high cost of single-crystal growing device is required. 
Thus, the cost of the semiconductor wafer increases. 
Also, even in a high precision of single-crystal growing device, it is very 
difficult to precisely control the concentration of the initial oxygen 
induced during crystal growth. 
Furthermore, although the initial oxygen concentration in the semiconductor 
wafer might be precisely controlled, distribution of the initial oxygen 
concentration with respect to each wafer is variously dispersed so that it 
is difficult to precisely control the characteristics of the wafer. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to a method of heat-treating 
a semiconductor wafer which substantially obviates one or more of the 
problems due to limitations and disadvantages of the related art. 
An object of the present invention is to provide a semiconductor wafer 
heat-treating method for removing the potentially harmful effect that the 
interstitial oxygen induced in the crystal lattice during single-crystal 
growth is settled into oxygen precipitates and influences the 
semiconductor device succeedingly shaped on the wafer. 
Another object of the present invention is to provide a semiconductor wafer 
heat-treating method for removing the evil effect that the initial oxygen 
influences the semiconductor device, irrespective of the concentration of 
the interstitial oxygen induced into the crystal lattices during 
single-crystal growth. 
Still another object of the present invention is to provide a semiconductor 
wafer heat-treating method for removing the evil effect that the initial 
oxygen influences the semiconductor device, in a simple way without 
precisely controlling the interstitial oxygen induced into the crystal 
lattices during single-crystal growth. 
Yet another object of the present invention is to provide a semiconductor 
wafer embodying good semiconductor devices, irrespective of the initial 
oxygen concentration of the wafer. 
Yet another object of the present invention is to provide a semiconductor 
wafer in which the oxygen precipitates are suppressed, irrespective of the 
initial oxygen concentration of the wafer. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the present invention, as embodied and broadly described, the inventive 
semiconductor wafer heat-treating method for removing defects induced into 
the crystal when the semiconductor crystal, grown into a single crystal, 
is fabricated into the unit wafers, performs the heat-treating operation 
with respect to the semiconductor wafer above 900.degree. C. for a 
predetermined time. 
Growth of the precipitates is advantageously suppressed by performing the 
heat-treating operation in the thermal furnace at 900.degree. C. for at 
least 20 minutes or more, and otherwise, at 1000.degree. C. for at least 
10 minutes or more. 
The inventive semiconductor wafer fabricated out of the semiconductor 
crystal, grown into a single crystal, keeps the oxygen concentration 
difference before and after the heat-treatment below 2 ppma, irrespective 
of the initial oxygen concentration in the crystal. 
With a view to controlling the yield, the oxygen concentration difference 
is kept below 2 ppma in condition that the initial oxygen concentration is 
in the range of 10 to 20 ppma, and more advantageously, 11 to 15 ppma. 
As described above, the method of the present invention entirely differs 
from the conventional one which reduces the initial oxygen concentration 
during single-crystal growth in the semiconductor wafer shaping process 
and is used for suppressing generation of the oxygen precipitates causing 
deterioration in the yield of the semiconductor device. The inventive 
method is to suppress the nucleation and nucleus-growth of the initial 
oxygen, irrespective of the concentration of the initial precipitates 
induced during single-crystal growth. That is, the method is to control 
the generation and growth of the oxygen precipitates through the 
heat-treating process in which temperature and time are controlled. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
The present invention concerns development of the heat-treating method, not 
for suppressing the concentration of the initial oxygen to being induced 
into the crystal during single-crystal growth, but for preventing the 
initial oxygen pre-existed in the interstitial sites-of the crystal from 
growing into the oxygen precipitates in the semiconductor device 
fabricating process. Experiments concerning optimization of the 
heat-treating temperature and time condition are, therefore, performed 
with respect to the semiconductor wafer. 
Generally, as is in the previously described semiconductor wafering 
process, the single-crystal semiconductor is sliced into wafer units. 
Then, chemical etching is performed with respect to the sliced wafers to 
remove the contaminants from them. Thereafter, each wafer suffers 
heat-treatment called a donor-killing process to suppress the role of 
donor of the initial oxygen. The present invention is to realize an 
optimal condition of the donor killing process. 
First, to perform the donor killing process, each wafer sampled suffers a 
heat-treating operation in the thermal furnace for a predetermined time, 
differing in temperature. 
The heat-treating operations are respectively performed at 650.degree. C. 
800.degree. C. 900.degree. C. 1000.degree. C. and 1100.degree. C. for 30 
minutes with an identical criterion. The thermal furnace is that generally 
used in the semiconductor fabricating industry and basically installing 
with a heating unit, a temperature control unit, a gas feeding unit, a 
wafer loading unit, etc. For example, a crosswise or lengthwise furnace 
can be used. 
In the meantime, the wafer according to a preferred embodiment of the 
present invention is a silicon semiconductor and is grown into a 
single-crystal semiconductor by the Czochralski technique. 
FIG. 6 is a graph measuring the difference in oxygen concentration 
.DELTA.Oi before and after the heat-treatment versus the initial oxygen 
concentration with the FTIR spectrometer after performing the DRAM 
simulation test with respect to each wafer heat-treated. The oxygen 
concentration difference is is proportional to the density of the oxygen 
precipitates grown in the heat-treating process. 
From FIG. 6, it can be known that the oxygen concentration difference 
.DELTA.Oi before and after the heat-treatment increases as does the 
initial oxygen concentration, in all the wafers measured after the 
experiment. But, the characteristic of the oxygen concentration difference 
before and after the heat-treatment is different when the heat-treating 
operation is performed at 650.degree. C. or 800.degree. C. and when it is 
performed at 900.degree. C. 1000.degree. C. and 1100.degree. C. 
respectively. That is, as stated previously with reference to FIGS. 4 and 
5, the oxygen concentration difference before and after the heat-treatment 
for keeping the Bin 1 yield in a good state is below 2 ppma, and as 
illustrated in FIG. 6, the heat-treating temperature condition, in which 
the oxygen concentration difference before and after the heat-treatment is 
below 2 ppma, is above 900.degree. C. 
FIG. 7 is a graph conceptually representing the temperature dependence of 
the nucleation number versus the nucleus size computed by the classical 
nucleation theory with respect to above experimental data. In the drawing, 
the abscissa represents the nucleus size of the oxygen precipitates, the 
ordinate the nucleation number of the oxygen precipitates, and N* of the 
abscissa the critical nucleus size. When the nucleus size of the oxygen 
precipitates is reduced below the critical nucleus size, the nucleus 
growth does not occur even in the nucleating operation. 
As illustrated in FIG. 7, when the donor killing process is performed below 
800.degree. C., the nuclei having the size above the critical one of 
nucleus are present so that the oxygen precipitates is nucleated. On the 
contrary, when the process is performed above 900.degree. C., nucleation 
is suppressed even in the existence of the nuclei in accordance with the 
initial oxygen. Therefore, when the heat-treating operation is performed 
above at least 900.degree. C., the generation and growth of the oxygen 
precipitates are suppressed so that the oxygen concentration difference 
before and after the heat-treatment becomes lower below 2 ppma. 
Thereafter, to optimize the time condition of the donor killing process, 
the oxygen concentration difference before and after the heat-treatment is 
measured with respect to each wafer after fixing the temperature condition 
of the thermal furnace at 900.degree. C. and 1000.degree. C. and changing 
the time condition. 
FIG. 8 is a graph measuring the oxygen concentration difference before and 
after the heat-treatment versus the initial oxygen concentration after 
performing the DRAM simulation test with respect to each wafer, 
heat-treated at 900.degree. C. while changing the annealing time. FIG. 9 
is a graph measuring the oxygen concentration difference before and after 
the heat-treatment versus the initial oxygen concentration after 
performing the DRAM simulation test with respect to each wafer, 
heat-treated at 1000.degree. C. while changing the annealing time. 
As shown in FIG. 8, it might be expected that when the donor killing 
process is performed at 900.degree. C. for at least 20 minutes or more, 
the oxygen concentration difference before and after the heat-treatment is 
kept below 2 ppma. In particular, when the initial oxygen concentration is 
in the range of 11.00 to 15.00 ppma, the oxygen concentration difference 
before and after the heat-treatment is kept below 2 ppma, irrespective of 
the initial oxygen concentration. 
In the meantime, as shown in FIG. 9, when the donor killing process is 
performed at 1000.degree. C. for at least 10 minutes or more, the oxygen 
concentration difference before and after the heat-treatment is kept below 
2 ppma. In particular, when the initial oxygen concentration is in the 
range of 11.00 to 15.00, the oxygen concentration difference before and 
after the heat-treatment is satisfactorily kept below 2 ppma, irrespective 
of the initial oxygen concentration. 
Therefore, in the semiconductor device fabricated using the semiconductor 
wafer according to the present invention, generation of the oxygen 
precipitates is suppressed and the oxygen concentration difference before 
and after the heat-treatment is kept below 2 ppma so that the yield of the 
semiconductor device is greatly improved. 
Also, generation of the oxygen precipitates is suppressed below a 
predetermined range, irrespective of the initial oxygen concentration so 
that the yield of the semiconductor device is improved. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the semiconductor wafer heat-treating method 
of the present invention without departing from the spirit or scope of the 
invention. Thus, it is intended that the present invention cover the 
modifications and variations of this invention provided they come within 
the scope of the appended claims and their equivalents.