How to Determine Carbon and Oxygen Content in Silicon Single Crystal?

How to Determine Carbon and Oxygen Content in Silicon Single Crystal?

In the production process of single crystal silicon, impurities such as carbon and oxygen are inevitably introduced due to factors such as raw materials and methods, which directly affect the performance of single crystal silicon. For example, the annealed silicon wafer supplied by us, shown as Fig.1, more details please refer to https://www.powerwaywafer.com/what-are-annealed-silicon-wafer%ef%bc%9f.html.

annealed silicon wafer

Fig. 1 Annealed Silicon Wafer with Oxygen and Carbon Content

The oxygen content in silicon is the key data that must be mastered for acceptance, process monitoring and research and development of today’s silicon material and device manufacturing. Carbon is the second most important impurity in silicon after oxygen and has a decisive influence on the properties of single crystal silicon, especially the behavior of oxygen during heat treatment. Therefore, accurate determination and control of the oxygen content in silicon single crystal is an indispensable link in the manufacture of silicon materials and device processing. Here we recommend a standard method for measuring the substituted carbon and interstitial oxygen content in silicon single crystals using low temperature (LT) Fourier transform infrared spectroscopy.

1. Applicable Scope of Low Temperature Fourier Transform Infrared Spectroscopy

This method is suitable for the determination of substitutional carbon and interstitial oxygen impurities in N-type silicon single crystals with room temperature resistivity greater than 0.1 Ω*cm and P-type silicon single crystals with room temperature resistivity greater than 0.5Ω*cm.

The effective range of this standard for the determination of carbon and oxygen content is from 5X1014atoms.cm-3 (0.01ppma) to the maximum solid solubility of substituted carbon and interstitial oxygen in silicon. The actual measurement standard will be higher than that specified by the standard.

2. Measuring Principle of LT Fourier Transform Infrared Spectroscopy

Cool the silicon single crystal sample to a temperature below 15 K. Directly transmit the sample with an infrared beam, collect the absorption spectrum, and use the reference method to determine the absorption coefficient of the infrared absorption peak at the wavenumber of 607.5cm-1 by the substitutional carbon atom in silicon. Determine the carbon content and the absorption coefficient of the infrared absorption peak of the interstitial oxygen atoms in silicon at the wave number 1136.3 cm-1 to determine the oxygen content.

3. Interfering Factors for Measuring Carbon & Oxygen in Silicon

Factors below listed will make influences on measuring carbon/silicon oxygen in silicon:

1) Interference factors exist in the absorption bands of carbon and oxygen, and there are vibrational absorption bands of silicon lattice, which will affect the determination of carbon and oxygen. A zone fused silicon single wafer with carbon and oxygen content less than 5X 1014 atoms*cm-3 (0.01 ppma) should be used as a reference sample, and the thickness of the reference sample and the test sample should be as consistent as possible to eliminate the influence of the silicon lattice absorption vibration band;

2) Multilevel internal reflections can produce secondary interference and baseline deviation. Secondary interference and baseline drift can be eliminated by changing sample thickness, surface handling, or resolution;

3) The test sample and the reference sample should be kept at the same temperature as possible to avoid the effect of temperature-dependent lattice absorption on the test results;

4) The infrared spectral absorption peak positions and calibration factors of substituted carbon and interstitial oxygen vary with temperature, and the corresponding absorption peak positions and calibration factors are also different at different temperatures, see Table A;

A.1 Carbon Absorption Peak Position and Calibration Factor

Temperature / K Carbon Infrared Absorption Peak Position / cm-1 Calibration Factor(Fc) / cm-2
300 607.2 0.82×1017
78 607.5 0.40×1017
10 607.5 0.37×1017

 

A.2 Oxygen Absorption Peak Positions and Calibration Factors

Temperature / K Carbon Infrared Absorption Peak Position / cm-1 Calibration Factor(Fc) / cm-2
300 1106 3.14×1017
78 1127 1.32×1017
10 1136 0.20×1017

 

5) At low temperature, the absorption of free carriers can be suppressed to a certain extent. However, for heavily doped silicon single crystals, the concentration of free carriers is very high, and it is also difficult to measure the infrared absorption spectrum of silicon due to the influence of severe carrier absorption.

4. Instruments

1) Low-temperature Fourier transform infrared spectrometer: with optical components and detectors for wave numbers 250 cm-1 ~ 1300 cm-1, the resolution of the spectrometer should reach 1 cm-1 or better at a temperature of 15 K;

2) Sample holder: It is made of metal material with high thermal conductivity, with small holes and can block any infrared light passing through the sample;

3) Micrometer or other equipment, which are suitable for measuring the thickness of the sample with an accuracy of 0.001mm.

5. Samples of Silicon

1) Cut the silicon single crystal into silicon single crystal samples, grind both sides, and polish both sides to a mirror surface by mechanical or chemical methods;

2) The thickness change of the test area on both surfaces of the processed sample shall not be greater than 0.05mm, and the surface shall be free of oxide layer;

3) The thickness of the prepared sample is between 2.0 mm and 4.0 mm. The diameter is suitable for the size of the sample holder;

4) For polycrystalline silicon samples, silicon single crystals should be prepared in advance with reference to other methods.

6. Carbon and Oxygen Content Measurement Steps

1) Wipe the surface of the sample with absolute ethanol;

2) Measure the thickness of the sample according to the provisions of GB/T 6618, accurate to 0.001 mm, and record the thickness of the sample;

3) Load the sample into the sample holder. Then fix the sample holder in the sample chamber;

4) Set the instrument parameters, and cool the sample to a temperature lower than 15K through the cryostat device configured by the instrument;

5) Run the analysis program, scan the empty aperture, and collect the background spectrum; scan the reference sample, and collect the reference spectrum; scan the sample to be tested, and collect the spectrum of the sample to be tested. The content of substitutional carbon and interstitial oxygen was calculated by the reference method. The absorption peak positions of 10.6 substituted carbon and interstitial oxygen are shown in Table 1. The infrared spectra are shown in Figures 2 and 3:

Table 1 Substituted Carbon and Interstitial Oxygen Absorption Peak Positions

Element Peak Position / cm-1
Carbon 607.5
Oxygen 1136.3

 

Fig.2 Low Temperature Infrared Spectrum of Substituted Carbon

Fig.2 Low Temperature Infrared Spectrum of Substituted Carbon

 

Fig.3 Low Temperature Infrared Spectrum of Interstitial Oxygen

Fig.3 Low Temperature Infrared Spectrum of Interstitial Oxygen

For more information, please contact us email at victorchan@powerwaywafer.com and powerwaymaterial@gmail.com.

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