Control of nitrogen and/or oxygen in silicon via nitride oxide pressure during crystal growth

The disclosure relates to a method for producing single crystal silicon from a polycrystalline silicon melt wherein dopants such as oxygen and nitrogen are uniformly distributed in the crystal both along the crystal axis and radially therefrom. This is accomplished by identifying the correct species in the melt and above the melt and determining the thermochemical equilibrium between the two chemical species which lead to a change of the composition of the silicon single crystal during the entire growth process. This approach effectively circumvents the segregation coefficient during the growth process through the control of the concentration of the dopants in the melt.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the formation of substantially homogeneous single 
crystal silicon for use in the production of silicon slices for 
manufacture of semiconductor devices and the like. 
2. Description of the Prior Art and Background 
In the manufacture of single crystal silicon or semiconductor grade, single 
crystals are normally pulled from a melt of polycrystalline silicon 
utilizing a seed crystal and, under standard and well known conditions, 
pulling a single crystal of silicon from the melt. The crystal pulling 
normally takes place in an inert atmosphere, such as argon, and at 
elevated temperatures, usually in the vicinity of 1410.degree. C. It is 
known that in the processing of silicon in the above described manner, the 
single crystal, from which slices are later taken, become contaminated 
with heavy metals such as iron, copper and the like which are found in the 
furnaces and other processing materials and equipment being utilized. 
In order to getter these heavy metal impurities, it has been found that the 
addition of oxygen to the melt provides precipitated zones where the 
oxygen has been precipitated and which tends to getter the heavy metals. 
Furthermore, the addition of nitrogen to the melt appears to strengthen 
the crystal itself. It is therefore readily apparent that the controlled 
addition of dopants such as oxygen and nitrogen during single crystal 
growth of silicon is required in order to obtain a silicon slice that has 
improved physical strength and has internal defect and impurity gettering 
capabilities that are active during device manufacture. Furthermore, each 
slice provided from the formed single crystal of silicon must have exactly 
the same concentration of dopant and it must have excellent radial 
uniformity. This slice to slice control of dopant level can only be 
achieved during the crystal growth process. Current crystal growth 
techniques are not capable of introducing and controlling the required 
exact concentrations of nitrogen and oxygen throughout the crystal growth 
process. 
The concentration of dopant in the crystal during the single crystal growth 
process is a direct function of the dopant concentration in the melt and 
the segregation coefficient of that impurity. If the segregation 
coefficient is greater than 1 (oxygen is 1.25), then the concentration of 
oxygen will be high at the top of the crystal and low at the bottom. The 
reverse is true for nitrogen in silicon crystal growth. 
There is a thermodynamic equilibrium between the concentration of the 
dopant in the melt and the partial pressure of a chemically related gas 
species above the melt. If the partial pressure of this gas species can be 
precisely fixed during the growth process, then the melt concentration of 
that species will be fixed and the concentration of the associated dopant 
will thereby be fixed in the crystal. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a method for 
producing single crystal silicon from a polycrystalline silicon melt 
wherein dopants such as oxygen and nitrogen are uniformly distributed in 
the crystal both along the crystal axis and radially therefrom. This is 
accomplished by identifying the correct species in the melt and above the 
melt and determining the thermochemical equilibrium between the two 
chemical species which lead to a change of the composition of the silicon 
single crystal during the entire growth process. This approach effectively 
circumvents the segregation coefficient during the growth process through 
the control of the concentration of the dopant in the melt. 
The oxygen and/or nitrogen are provided, for example, at least in part, 
from the quartz or silicon nitride liner inside the graphite crucible 
containing the melt. The dissolution of the quartz liner supplies oxygen 
to the crystal. Nitrogen can also be provided in this manner by utilizing 
a silicon nitride liner rather than the quartz liner. In each case, the 
formation of the crystal will take place in the presence of nitrogen gas 
introduced above the melt in the case of the quartz liner and with the 
introduction of oxygen above the melt in the case of the silicon nitride 
liner. This causes the formation of nitrous oxide as a result of the 
thermochemical equilibrium between the liquid phase and the gas phase. The 
nitrous oxide forms at the surface of the melt by controlling the 
concentration of nitrous oxide at the surface of the melt, either by 
adding nitrous oxide or by adding nitrogen for reaction with oxygen in the 
melt or by adding a mixture of nitrogen and oxygen or by adding oxygen for 
reaction with nitrogen in the melt. All of these procedures operate toward 
establishing the equilibrium gas phase over the melt. This controls the 
amount of nitrogen and oxygen dissolved in the melt at any time. 
The amounts of oxygen and nitrogen in the melt tend to be mutually 
exclusive. Where there is a high oxygen partial pressure there will be a 
low nitrogen partial pressure because nitrogen is removed from the melt to 
form the nitrous oxide. Where there is a high nitrogen partial pressure, 
there is a low oxygen partial pressure for the same reason. 
To control the nitrogen level in silicon, it is necessary to establish the 
thermochemical correlations between nitrogen content in the melt and a gas 
species containing nitrogen above the melt. In order to simultaneously 
control both nitrogen and oxygen, the gas species must be one of the 
oxides of nitrogen. Examination of the thermochemistry of all the gaseous 
oxides of nitrogen: N.sub.2 O.sub.5, N.sub.2 O.sub.4, N.sub.2 O.sub.3, NO 
and N.sub.2 O shows that only N.sub.2 O has a measurable pressure in the 
Si-C-O-N system at the melting point (1685.degree. K.) of silicon. 
Appendix 1 shows an example of the SOLGAS calculations which support this 
conclusion. 
Appendix 2 is an example of the calculation of the equilibrium between 
silicon and the dopants oxygen, carbon and nitrogen which shows the gas 
and liquid phase composition. It also shows that invariant solids can be 
formed during the process of establishing the equilibrium state. In this 
example, the silicon melt is saturated with nitrogen (120 ppma) and 
silicon nitride (Si.sub.3 N.sub.4) begins to be formed in the melt. A 
large amount of nitrous oxide in the system causes this nitrogen 
saturation in the melt resulting in the precipitation of Si.sub.3 N.sub.4 
and the depletion of oxygen in the melt. When the nitrous oxide pressures 
are lowered, the nitrogen level in the melt is lowered and no Si.sub.3 
N.sub.4 precipitates in the melt. These calculations can be used to 
establish a thermochemical model of this system.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the FIGURE, there is shown a schematic diagram of a 
standard crystal puller arrangement for use in conjunction with the 
present invention. The system includes an enclosure 1 within which is 
provided a crucible 3 formed of carbon with an interior liner 5 formed of 
quartz. The crucible 3 is positioned on a support 7 and a standard crystal 
puller mechanism 9 with a crystal 11 being pulled from the melt 13 is 
shown. Also shown are feeders 15 for feeding nitrogen into the system and 
17 for feeding oxygen into the system, there being a valve 19 in the case 
of the nitrogen and 21 in the case of the oxygen for controlling the flow 
of the respective gases into the chamber 1. Also shown in a vacuum pump 23 
for evacuating the chamber 1 to the desired vacuum level. A heater 
mechanism 25 is positioned about the crucible 3 to inductively heat the 
melt 13 therein to the desired temperature. 
To form a crystal in accordance with the present invention utilizing the 
above described system, polycrystalline silicon is entered into the 
crucible 3 in the melt 13 at room temperature. The system 1 is then sealed 
up and pumped down by means of the vacuum pump 23 to evacuate the system 
and remove air therefrom. The conditions desired in the melt are then 
established under vacuum conditions in order to maintain the system clean 
and then come up to the proper melt temperature. At the melt temperature, 
the necessary gases such as nitrogen from nitrogen feeder 15 and/or oxygen 
from oxygen feeder 17 are introduced into the system. (The system could 
start out with the gases introduced therein at room temperature with the 
temperature then being raised to the final operating temperature for the 
melt.) These amounts are determined by the model presented in Appendix 1. 
Also provided in Appendix 2 is a set of mathematical correlations which is 
taken from the SOLGAS computer program. The SOLGAS program is well known 
in the art to find the equilibrium conditions in the systems containing 
gases, solids, and liquids together. The thermochemical constants are 
recorded in tables as a function of temperature so that a determination 
can be made as to what the equilibrium condition would be when the atoms 
are placed together in a confined volume as an isolated system. The SOLGAS 
calculations herein are restricted specifically to the materials utilized 
in the present system. 
Once the system of the FIGURE is set up as noted above, the conditions are 
made to conform to the following mathematical correlations in the addition 
of oxygen and/or nitrogen to the system at the temperature and pressure 
therein and the experimental data is examined: The nitrous oxide gas, 
oxygen and nitrogen in the melt must be established in order to control 
the concentrations of oxygen and/or nitrogen in the single crystal 
silicon. When the equilibrium: 
EQU O(Si)+2N(Si)=N.sub.2 O(gas) (1) 
was examined at 1685.degree. K. the following equations were derived from 
the SOLGAS calculations. 
The constant of reaction (1): K at silicon melting point is equal to: 
EQU K=p(N.sub.2 O)/O(Si)*(N(Si)).sup.2 =2.0325E-05 (2) 
The equilibrium partial pressure of nitrous oxide in the ambient atmosphere 
above the melt can be expressed as the following: 
EQU p(N.sub.2 O)=2.0325E-05*O(Si)*(N(Si)).sup.2 (3) 
Also the nitrogen concentration in the melt is obtained: 
EQU N(Si)=221.8118*(p(N.sub.2 O)/O(Si)).sup.1/2 (4) 
Or the oxygen concentration in the melt is equal to: 
EQU O(Si)=49200.429*p(N.sub.2 O)/(N(Si)).sup.2. (5) 
Knowing the nitrogen and oxygen segregation coefficient between molten and 
solid silicon (0.0007 and 1.25 respectively) the following correlations 
between nitrous oxide pressure and nitrogen concentration in silicon 
crystal can be calculated: 
EQU N(Si)=0.15552*(p(N.sub.2 O)/O(Si)).sup.1/2 (6) 
EQU O(Si)=0.02419*p(N.sub.2 O)/(N(Si)).sup.2. (7) 
Pressure of N.sub.2 O is expressed in atm, oxygen and nitrogen 
concentration is in ppma and reaction constant K is in atm/(ppma).sup.3. 
After the polycrystalline silicon has melted to form the melt 13, a seed 
crystal is placed in contact with the melt 13 and a crystal 11 is pulled 
by means of a crystal puller 9. During this crystal growth process, there 
is an equilibrium segregation coefficient of the nitrogen as to whether it 
will go into the crystal or whether it will stay in the melt. If the 
pressure in the melt is increased, then the concentration of gas in the 
melt is increased and the required oxygen partial pressure above the 
surface of the melt is increased. Therefore, by virtue of increasing 
concentration, material is being supplied from an equilibrium condition 
because some of the material is being frozen out. Gas is being applied to 
the atmosphere which then can be pumped away as vacuum so that the 
pressure in the chamber 1 is being controlled. Also, material is added, if 
necessary, due to its being depleted from the melt into the crystal. So 
oxygen is being segregated into the crystal, the crystal is being pulled 
and more oxygen is being pumped into the system to replace the oxygen 
segregated into the crystal. This is all apparent from the model in 
Appendix 3. The gases are introduced to maintain a layer of nitrous oxide 
above the melt. The nitrous oxide is measured on-line by means of a mass 
spectrometer on the system wherein gas can be removed from the surface of 
the melt and fed back into a lower pressure quadripole electron multiplier 
mass spectrometer (not shown). From these results, the amount of nitrogen 
and/or oxygen being introduced to the system is controlled on-line by a 
mass flow controller (not shown). Also, the pressure is controlled on-line 
by means of the vacuum pump. 
Identifying the correct species in the melt and above the melt and 
determining the thermochemical equilibrium between the two chemical 
species leads to a way of changing the composition of the silicon single 
crystal during the entire crystal growth process. This approach 
effectively circumvents the segregation coefficient during the growth 
process through the control of the concentration of the dopant in the 
melt. 
In summary, depending on the crystal growth conditions (e.g crucible 
material), the nitrogen concentration (for constant oxygen) or the oxygen 
concentration (for constant nitrogen) can be controlled by changing the 
nitrous oxide partial pressure above the melt. 
When a quartz crucible dissolves with a constant rate it fixes the oxygen 
at a constant level and the equilibrium in the system adjusts to this 
oxygen value. The nitrogen concentration in the melt, the N.sub.2 O 
partial pressure and the other species pressures or concentrations are 
established in the process. By changing the nitrous oxide pressure it is 
possible to change the mass fractions of all species and this will force 
the system to new steady state equilibrium following equation (3). 
The same conditions prevail when a silicon nitride crucible dissolves at a 
constant rate. This fixes the nitrogen at a constant level and the change 
in the N.sub.2 O pressure causes the oxygen concentration in the melt to 
change according to equation (3). 
When there is not an outside source of oxygen or nitrogen in the melt there 
is one unique equilibrium state and one unique oxygen and nitrogen 
concentration in the system. Changing the nitrous oxide partial pressure 
changes the concentrations of oxygen and nitrogen in the melt following 
equation (3). 
Though the invention has been described with respect to a specific 
preferred embodiment thereof, many variations and modifications will 
immediately become apparent to those skilled in the art. It is therefore 
the intention that the appended claims be interpreted as broadly as 
possible in view of the prior art to include all such variations and 
modifications. 
APPENDIX 1 
SI/H/HE/C/O/N EQUILIBRIUM SYSTEM 
______________________________________ 
T = 1685.00.degree. K. 
P = 2.600E-02 ATM 
GAS PHASE: P/ATM 
______________________________________ 
N205 0.29893E-28 
N2O4 0.78851E-22 
N2O3 0.37603E-15 
NO 0.55052E-15 
N2O 0.46557E-02 
N2 0.38203E-05 
He 0.17041E-01 
CO 0.42995E-02 
CO2 0.29511E-08 
O2 0.15184E-20 
______________________________________ 
APPENDIX 2 
SOLGAS EQUILIBRIUM CALCULATIONS 
TABLE 
__________________________________________________________________________ 
SI/H/He/C/O/N EQUILIBRIUM SYSTEM 
T = 1685.00.degree. K. 
P = 2.600D-02 ATM 
IN/MOLE 
OUT/MOLE ACTIVITY 
__________________________________________________________________________ 
GAS PHASE: P/ATM 
He 0.11920E+02 
0.11920E+02 
0.12331E-01 
0.12331E-01 
CO2 0.00000E+00 
0.63559E-13 
0.65752E-16 
0.65752E-16 
CO 0.00000E+00 
0.10939E-04 
0.11316E-07 
0.11316E-07 
SiO(g) 0.00000E+00 
0.97627E-02 
0.10100E-04 
0.10100E-04 
O2 0.60000E+01 
0.10518E-21 
0.10881E-24 
0.10881E-24 
Si(g) 0.00000E+00 
0.47273E-03 
0.48904E-06 
0.48904E-06 
H2 0.50000E-01 
0.50000E-01 
0.51725E-04 
0.51725E-04 
H2O 0.00000E+00 
0.10255E-08 
0.10609E-11 
0.10609E-11 
HNO2 0.00000E+00 
0.40284E-29 
0.41674E-32 
0.41674E-32 
NH3 0.00000E+00 
0.47671E-09 
0.49316E-12 
0.49316E-12 
N2O 0.00000E+00 
0.11990E+02 
0.12404E-01 
0.12404E-01 
N2 0.26750E+02 
0.11622E+01 
0.12024E-02 
0.12024E-02 
LIQUID PHASE: MOLE FRACTION 
O(Si) 0.00000E+00 
0.11858E-04 
0.42366E-07 
0.42366E-07 
C(Si) 0.00000E+00 
0.88906E-03 
0.31764E-05 
0.31764E-05 
N(Si) 0.00000E+00 
0.33618E-01 
0.12011E-03 
0.12011E-03 
SI 0.30025E+03 
0.27987E+03 
0.99988E+00 
0.99988E+00 
INVIARIANT SOLIDS: 
Si3N4 0.00000D+00 
0.67904D+01 
Si2N2O 0.00100D+00 
0.00000D+00 
C 0.90000D-03 
0.00000D+00 
SiC 0.00000D+00 
0.00000D+00 
SiO2 0.00000D+00 
0.00000D+00 
__________________________________________________________________________ 
APPENDIX 3 
O(Si)--N(Si)--N.sub.2 O SYSTEM ANALYSIS 
The following table shows the equilibrium nitrogen concentration in the 
melt and crystal versus the equilibrium p(N.sub.2 O) in the gas phase 
above the melt when oxygen content is held at a constant value. 
Experimentally this condition can be approached through controlled 
dissolution of the quartz crucible containg the melt. In this case a value 
of 29 ppma oxygen was chosen for the melt. 
TABLE 
______________________________________ 
NITROGEN IN MOLTEN SILICON AND 
SILICON CRYSTAL AS A FUNCTION OF NITROUS 
OXIDE TIAL PRESSURE ABOVE THE 
MELT IN THE AMBIENT ATMOSPHERE 
AT SILICON MELTING POINT ( 1685.degree. K. ) 
FOR CONSTANT OXYGEN ( 29 ppma in the melt ) 
nitrogen in the melt 
nitrogen in the crystal 
N.sub.2 O partial pressure 
ppma ppma atoms/cc atm 
______________________________________ 
120 (soluble. limit) 
0.084 4.5E+15 8.475 
100 0.070 3.5E+15 5.886 
50 0.035 1.9E+15 1.471 
20 0.014 7.5E+14 0.235 
10 0.007 3.8E+14 0.059 
______________________________________ 
This table shows that there is an ambient gas pressure problem that arises 
under these conditions. The pressure of N.sub.2 O exceeds ambient pressure 
during the silicon crystal growth, i.e., 1 atm. This means that under 
equilibrium conditions it is impossible to achieve a nitrogen saturation 
(120 ppma) at 29 ppma of oxygen in the melt and still maintain a pressure 
of about 1 atm. 
The following table shows the equilibrium nitrogen and oxygen 
concentrations in the silicon melt for nitrous oxide partial pressures of 
1 and 0.026 atm. The nitrogen and oxygen concentrations are inversely 
proportional (see equation (2) ) and at constant nitrous oxide pressure 
are limited between two values. At 1 atm the highest nitrogen 
concentration is equal to the solubility limit (120 ppma). This 
corresponds to an oxygen concentration of 3.4 ppma and this concentration 
cannot be lowered until the nitrous oxide pressure is lowered. As the 
concentration of oxygen increases in the melt the nitrogen concentration 
of the melt decreases. When the oxygen content achieves its limit (41.7 
ppma) the nitrogen concentration in the melt cannot be lower than 34.3 
ppma. The only way to lower the nitrogen concentration is to lower the 
nitrous oxide pressure. 
TABLE 
______________________________________ 
NITROGEN AND OXYGEN IN MOLTEN SILICON 
IN EQUILIBRIUM WITH NITROUS OXIDE 
IN THE AMBIENT ATMOSPHERE 
AT SILICON MELTING POINT ( 1685.degree. K. ) 
nitrogen in the melt 
oxygen in the melt 
N.sub.2 O partial pressure 
ppma ppma atm 
______________________________________ 
120 (solub. limit) 
3.42 1.0 
120 0.09 0.026 
100 4.92 1.0 
100 0.13 0.026 
50 19.68 1.0 
50 0.51 0.026 
34.3 41.7 (solub. limit) 
1.0 
0.9 41.7 0.026 
______________________________________