Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-09-23
2004-10-12
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339010
Reexamination Certificate
active
06803576
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for determining the nitrogen concentration in single crystal silicon, and more particularly, to a method of using low temperature Fourier Transform infrared spectroscopy (LT-FTIR) in the far infrared spectral range (FIR) to determine the concentration of nitrogen in single crystal silicon produced in accordance with the Czochralski method.
BACKGROUND OF THE INVENTION
The properties of nitrogen in silicon have become a matter of interest in recent years for several reasons. For example, nitrogen is known to lock, or pin, crystal dislocations, it is know to impart strength and warp resistance to silicon with low oxygen concentrations, and it is known to form shallow thermal donors (STDs) with an ionization energy of 35-37 meV in as-grown and annealed silicon. In view of the important role nitrogen plays in silicon, several measurement techniques have been applied to study nitrogen in silicon, including infrared spectroscopy, luminescence, electron paramagnetic resonance, Hall effect, deep level transient spectroscopy, secondary ion mass spectroscopy (SIMS), and nuclear reaction analysis.
In addition to studying the effects of nitrogen in silicon, some of the foregoing methods have been used to determine the quantity of nitrogen in silicon. For example, SIMS has been used to determine the concentration of nitrogen in silicon. See R. S. Hockett and D. B. Sams, Electrochemical Society Proceeding, Volume 2000-17, p. 584. The detection limit for nitrogen in silicon using SIMS is about 1-2×10
14
atoms/cm
3
, and is limited primarily by background nitrogen (i.e., the residual nitrogen in the chamber atmosphere after evacuation).
Fourier Transform infrared spectroscopy (FTIR) has also been used to determine the concentration of nitrogen in silicon. The presence of nitrogen in silicon gives rise to several IR absorption peaks. The peaks of interest for these FTIR nitrogen measurement methods occur within the medium infrared region (MIR) between about 600 and about 1000 cm
−1
. Specifically, the two main nitrogen-related MIR absorption peaks are at about 963 and about 766 cm
−1
and are associated with the vibrational modes of the molecular nitrogen species (N—N). The most widely accepted of determining nitrogen concentration by FTIR uses the absorption peak at 963 cm
−1
for which a calibration coefficient of 1.83×10
17
cm
−2
has been determined. See Y. Itoh and T. Nozaki, Appl. Phys. Lett. 47, p. 488 (1985). The nitrogen concentration is determined by the equation:
[N](atoms/
cm
3
)=1.83×10
17
×&agr;(963
cm
−1
). (1)
The absorption peak at 766 cm
−1
has also been used and the calibration coefficient of is 4.45×1016 cm
−2
. See P Wagner et al., Apply Phys. A 46, p. 73 (1988); and Watanabe et al., Semiconductor Silicon, p. 126 (1981). The nitrogen concentration is determined by the equation:
[N](atoms/
cm
3
)=4.45×10
14
×&agr;(766
cm
−1
). (2)
The intensity of the nitrogen-related absorption bands, or peaks, in the MIR is always low for a silicon comprising about 10
15
-10
16
atoms/cm
3
(the maximum absorbance is 0.1), and much lower than oxygen and carbon peaks in the MIR. Thus, the accuracy of determining nitrogen according to these methods is less than desirable especially at low nitrogen concentrations. Additionally, these methods are even less accurate for determining the nitrogen concentration in CZ silicon than in FZ silicon. This is because the calibration coefficient for 963 cm
−1
absorption peak was determined using float zone-grown (FZ) single crystal silicon in which the vast majority of the nitrogen is in molecular form with the small remaining fraction being primarily atomic. In contrast, in CZ silicon, a significant fraction of the nitrogen is in the form of N—O complexes, and as a result, using the foregoing method underestimates the total nitrogen concentration in CZ silicon.
Thus, a need continues to exist for a method of quickly and accurately determining the concentration of nitrogen in CZ silicon, especially at low concentration levels such as below about 1×10
14
atoms/cm
3
.
SUMMARY OF THE INVENTION
Among the features of the invention, therefore, is the provision of a method for quantitatively measuring nitrogen in Czochralski silicon based on the detection of one or more N—O complexes by means of low temperature Fourier Transform infrared spectroscopy (LT-FTIR) in the far infrared spectral range (FIR); the provision of a method for quickly and accurately determining the concentration of nitrogen in single crystal silicon; and the provision of a method for detecting nitrogen at concentrations below about 1×10
14
atoms/cm
3
.
Briefly, therefore, the present invention is directed to a method for measuring a concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature T
a
for an annealing time t
a
to saturate the silicon sample with nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
The present invention is also directed to a method for measuring a concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature T
a
for an annealing time t
a
to saturate the silicon sample with nitrogen-oxygen complexes, wherein T
a
is selected as a function of a detection limit for the nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
Additionally, the present invention is directed to a method for detecting a measurement of concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature T
a
for an annealing time t
a
to saturate the silicon sample with nitrogen-oxygen complexes, wherein T
a
is selected as a function of annealing time available to saturate the silicon sample with nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
In another embodiment, the present invention is directed to a method for detecting a measurement of concentration of nitrogen in a silicon sample by Fourier Transform infrared spectroscopy, the method comprising: annealing the silicon sample at an annealing temperature T
a
for an annealing time t
a
to saturate the silicon sample with nitrogen-oxygen complexes, wherein t
a
is selected as a function of a detection limit for the nitrogen-oxygen complexes; performing Fourier Transform infrared spectroscopy to measure an absorbance value related to the concentration of nitrogen-oxygen complexes in the annealed silicon sample; and calibrating the absorbance value to a nitrogen concentration value.
Other objects will be in part apparent and in part pointed out hereinafter.
REFERENCES:
patent: 5066599 (1991-11-01), Kaneta et al.
Yatsurugi, Y., et al., Concentration, Solubility, and Equilibrium Distribution Coefficient of Nitrogen and Oxygen in Semiconductor Silicon, J. Electrochem. Soc. 120, 1973, p. 975-979.
Watanabe, M., et al., Oxygen-Free Silicon Single Crystal Grown from Silicon Nitride Crucible, Semiconductor Silicon, 1981, p. 126-137.
Itoh, Y., et al., Calibration curve for infrared spectrophotometry of nitrogen in silicon, Appl. Phys. Lett. 47, Sep. 1, 1985
Collareta Paolo
Falster Robert J.
Porrini Maria
Pretto Maria Giovanna
Scala Roberto
MEMC Electronic Materials, SPA
Moran Timothy
Senniger Powers
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