JP2018157009A - Method for detecting carbon in silicon - Google Patents

Abstract

The present invention provides a method for detecting carbon in silicon with high sensitivity or stability. [Solution] Silicon is subjected to wet treatment using a mixed acid. The wet-processed silicone is allowed to stand for at least 144 hours or from 72 hours to 120 hours after wet processing. The density of the impurity level of the composite containing at least carbon and hydrogen is measured using the DLTS method for silicon that has been left for 144 hours or more or from 72 hours to 120 hours. The carbon concentration in silicon is evaluated based on the obtained level density. By performing wet treatment prior to DLTS measurement, it is possible to introduce hydrogen into the silicon and activate the levels of the complex containing at least carbon and hydrogen in the silicon. In comparison, carbon-related level density can be measured with high sensitivity. A stable level density can be obtained by leaving the wet treatment for 144 hours or more. By leaving it for 72 to 120 hours after the wet treatment, the level density can be measured with high sensitivity. [Selection diagram] Figure 1

Description

  The present invention relates to a method for detecting carbon in silicon.

As the density of devices such as semiconductor integrated circuits is increased and the integration is increased, stabilization of device operation has been desired. In particular, improvement of characteristic values such as leakage current and oxide film breakdown voltage is an important issue. However, when impurities are mixed into a silicon wafer which is an integrated circuit substrate, a stable operation of a device manufactured thereafter cannot be expected. For example, in a semiconductor integrated circuit manufacturing process, it is widely known that carbon in a wafer adversely affects device characteristics even at a concentration of 1 × 10 ¹⁵ atoms / cm ³ or less.

  Therefore, there is a need for an accurate concentration measurement technique to keep carbon in the wafer low. Usually, for this purpose, the carbon concentration is measured by Fourier Transform Infrared Spectroscopy (FT-IR method). This method is a method for indirectly determining the interstitial carbon concentration from the infrared absorption spectrum of a silicon wafer, and is widely used because it can be easily measured.

However, in reality, it is extremely difficult to accurately measure the concentration of 10 ¹⁴ atoms / cm ³ in the carbon concentration measurement by the FT-IR method. The same applies to SIMS (Secondary Ion Mass Spectroscopy) as another measurement technique.

In this regard, the DLTS (Deep Level Transient Spectroscopy) method is a method that has a possibility of measuring concentrations of 10 ¹³ atoms / cm ³ . The DLTS method is a method of operating a reverse bias voltage applied to a Schottky junction or a pn junction formed on a measurement target, and measuring a deep impurity level from the temperature dependence of a capacitance change of a depletion layer generated at the junction. It is a method of obtaining information about the position. The measurement result of this DLTS method is shown, for example, by a graph of DLTS signal intensity and measurement temperature. A peak formed on the graph indicates the presence of a certain deep impurity level. Further, the energy of roughly deep impurity levels is found from the temperature of the peak, and the height of the peak theoretically indicates the density of deep impurity levels.

  Here, for example, in the method shown in Non-Patent Document 1, the carbon-related levels E1, E2, and E3 present in the silicon crystal are deep impurity levels formed by HC and H—C—O complexes. In particular, it is said that the level of E3 is not affected by oxygen because it is a level caused by HC.

However, it cannot be said that the carbon concentration of 10 ¹³ atoms / cm ³ can be measured with high sensitivity as it is. Therefore, some ideas have been proposed. For example, Patent Document 1 shows that it is effective to add E1, E2, and E3, which are carbon-related impurity levels.

Minoru Yoneta, Yomiichi Kamiura, and Fumio Hashimoto, “Chemical etching-induced defects in physphorous-dopeped silicon”, J. Am. Appl. Phys. 70 (3), 1 August 1991, p. 1295-1308

JP, 2006-108159, A

  However, when carbon-related impurity levels in silicon are measured by the DLTS method, there is a problem that the DLTS signal intensity is not stable. In this regard, none of the conventional literature mentions the cause of the unstable level as an essential solution to stably measure carbon-related levels at low concentrations with high sensitivity. . For this reason, no real measures have been taken, and no effective method has been achieved with an essential solution.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a method capable of detecting carbon in silicon with high sensitivity or stability.

  In order to solve the above-mentioned problems, the method for detecting carbon in silicon according to the present invention includes a wet treatment with silicon by an acid, and after the wet treatment is left for 72 to 120 hours, it is contained in the silicon. The impurity level resulting from the composite containing at least carbon and hydrogen is measured by the DLTS method. According to the present invention, carbon in silicon can be detected with higher sensitivity than conventional methods.

  Further, the method for detecting carbon in silicon of the present invention includes at least carbon and hydrogen contained in silicon after performing wet treatment with acid on silicon and leaving it for 144 hours or more after performing the wet treatment. The impurity level resulting from the complex is measured by the DLTS method. According to the present invention, carbon in silicon can be detected more stably than in the conventional method.

  The composite is preferably a composite containing only carbon and hydrogen. Thereby, it can suppress that a DLTS measurement value varies by the influence of the density | concentration of elements other than carbon and hydrogen.

It is the figure which showed the change of the carbon-related level density obtained by DLTS method with respect to the leaving time after the wet process performed prior to DLTS measurement.

  Hereinafter, embodiments will be described. In the DLTS method, a metal electrode that forms a Schottky junction is usually formed on the surface of the measurement sample, a metal electrode having an ohmic junction is formed on the back surface, a reverse bias is applied between the two electrodes, and the resulting depletion layer When the temperature dependence of the capacitance change is acquired, the capacitance change forms a peak at a temperature corresponding to the energy level formed by the impurities. The impurity level density can be calculated from the change in capacitance at the peak position. Specifically, in an n-type silicon wafer, gold is often used for the Schottky electrode formed on the front surface, and gallium is often used for the ohmic electrode formed on the back surface. Any electrode is desirably formed on a clean silicon surface where no oxide film is present, and surface oxide film removal by HF is performed to obtain the clean surface.

One feature of the present embodiment is that, prior to the HF treatment, etching with an acid (wet treatment) is performed, and hydrogen is introduced deeply into the silicon wafer during the reaction. Specifically, it is known that it is effective in many cases that the acid to be used is a mixed acid containing HNO ₃ and HF.

  This wet process introduces hydrogen into the silicon. This is because the diffusion of hydrogen is extremely fast, so that it can be sufficiently introduced into silicon even in a wet process at room temperature.

  However, this indicates the instability of the introduced hydrogen, and the present inventors have conceived that investigating the stability is the key to the stability measurement of carbon-related levels.

  FIG. 1 shows the result of the confirmation experiment. Of the three carbon-related levels E1, E2, and E3, the E3 level that is not affected by oxygen is left for the time indicated on the horizontal axis of the graph after the wet treatment with mixed acid, and then the surface due to HF. Removal and Schottky electrode formation processing were performed, and DLTS measurement was performed. As shown in FIG. 1, the direction in which the level density gradually decreases immediately after the wet treatment with the mixed acid is shown. This large tendency is considered to indicate the instability of hydrogen introduced by wet processing. On the other hand, a peak of level density is observed at a standing time of 72 to 96 hours after the wet treatment. At present, the reason why this peak is formed is not clear, but it is considered that this reaction is a reaction in which hydrogen and carbon are combined into a complex to have a maximum efficiency overall. In addition, this peak is higher than the level density immediately after the wet processing (that is, the level density when the standing time is 0 hour). In addition, after the peak, the level density gradually decreases.

  Therefore, in order to obtain a stable and quantitatively accurate carbon concentration at a certain high level, 144 hours or more have passed since the wet treatment with a mixed acid in which the level density is stable beyond the peak. It can be seen that it is preferable to wait for it to be measured and perform DLTS measurement. In order to obtain the true carbon concentration in the wafer using this method, a calibration curve is created between the carbon concentration in silicon measured in advance by another method and the level density obtained by this method. (It will be described later in detail.)

  On the other hand, if it is a qualitative purpose to detect a trace amount of carbon based on quantitative accuracy, it is effective to use a peak that appears in a period of 72 to 120 hours after wet treatment with a mixed acid. Since this peak value has a concentration of more than twice the density of carbon-related levels that can be stably measured after 144 hours, if it is a qualitative analysis that confirms the presence of carbon with the appearance of this peak, It can be an effective measurement technique. However, using this peak concentration as a quantitative value is considered to be a technique to avoid from the instability of the peak height.

  Hereinafter, the details of the method for measuring the carbon concentration in silicon according to the present embodiment will be described. First, a method of performing DLTS measurement for the purpose of quantitative evaluation of the carbon concentration in silicon will be described.

(Quantitative evaluation of carbon concentration in silicon)
First, a plurality of first silicon crystals having different carbon concentrations are prepared. Specifically, for example, an n-type silicon crystal ingot pulled up by the FZ method is cut to a predetermined thickness, and the cut wafer is subjected to rough polishing, etching, polishing, etc., and the surface is mirror-finished (polished substrate). Wafer). In addition, this board | substrate can be made into the board | substrate produced for formation of electronic devices, such as a transistor and a diode, for example. Next, a silicon crystal is cut out from the substrate to produce a first silicon crystal. The carbon concentration of the first silicon crystal may be adjusted to a range (for example, 1 × 10 ¹⁵ to 1 × 10 ¹⁶ atoms / cm ³ ) measurable by FT-IR method or SIMS. The first silicon crystal may be formed by the CZ method (Czochralski method) or may be formed by the FZ method (floating zone method). Same as the law. Further, the carbon concentration of the first silicon crystal is measured in advance by a method other than the DLTS method, such as SIMS or FT-IR method, that is, known.

Next, prior to DLTS measurement, an etching process (wet process) using an acid is performed on the front surface, the back surface, or both surfaces of each first silicon crystal. Examples of the wet process include a mixed acid liquid process including HNO ₃ and HF. In the mixed acid solution treatment, the surface of the first silicon crystal is etched by setting the acid solution to a predetermined solution temperature, and then the first silicon crystal is rinsed with pure water. As the mixed acid solution treatment, for example, the surface of the first silicon crystal is etched by 50 μm or less at a liquid temperature of 30 ° C., and then the first silicon crystal is rinsed with pure water for 3 minutes. Incidentally, and concentration of each acid (HNO 3, _HF, etc.) constituting the mixed acid solution, the mixture ratio (volume ratio or mass ratio) may be set as appropriate. Moreover, what is necessary is just to set suitably the time and etching amount of a mixed acid liquid process.

  After performing this wet process, the first silicon crystal is left for a certain period of time. Specifically, it is left for 144 hours or more after the wet treatment with the mixed acid. By leaving it to stand for 144 hours or more, it is possible to obtain a level density in a form that suppresses the influence of the peak of the level density that appears during 72 to 120 hours after performing the wet processing by DLTS measurement to be performed later ( (See FIG. 1). That is, a stable level density can be obtained. In order to obtain a more stable level density, it is preferable to leave it for 192 hours or more after the wet treatment. As shown in FIG. 1, the degree of change of the level density with respect to the passage of time becomes smaller when left for 192 hours or more after the wet treatment. At this time, there is no particular upper limit value for the standing time after the wet treatment, but it may be appropriately determined in consideration of productivity and the like.

  Here, regarding the standing for 144 hours or more after the wet treatment, for example, when the wet treatment is performed at 12:00 pm on February 1, the treatment is left for 144 hours from the wet treatment at 12:00 pm on February 7. It becomes. Therefore, when the wet process is performed at 12:00 on February 1, the first silicon crystal is left at least until 12:00 on February 7.

  Next, the density of carbon-related impurity levels is measured by DLTS method for each first silicon crystal left for 144 hours or more after wet processing. Specifically, the density of impurity levels of a complex containing at least carbon and hydrogen is measured. Among the composites containing at least carbon and hydrogen, in particular, a composite containing no elements other than carbon and hydrogen (oxygen, etc.) (that is, a composite containing only carbon and hydrogen (HC complex)). It is preferable to measure the unit density. This is to prevent the DLTS signal from varying due to the influence of the concentration of other elements such as oxygen. When the conductivity type of the first silicon crystal is n-type, among the carbon-related levels E1, E2, and E3 shown in Non-Patent Document 1, the level E3 caused by the HC complex is measured. Is preferred. The three impurity levels E1, E2, and E3 are carbon-related impurity levels formed in a range of about 0.11 to 0.15 eV detected by measuring an n-type silicon crystal by the DLTS method. The energy of the level E1 is 0.11 eV, the energy of the level E2 is 0.13 eV, and the energy of the level E3 is 0.15 eV.

  In the DLTS measurement, after the surface oxide film removal process by HF is performed on the surface and the back surface of the first silicon crystal, Au is deposited on the surface to form a Schottky electrode, and the back surface is coated with, for example, Ga. Thus, an ohmic electrode is produced. And a reverse bias (for example, -5V) is applied between two electrodes, and it sweeps in a predetermined temperature range (for example, the range of 30-300K), and measures a carbon-related level density.

  Next, based on the carbon-related level density obtained from each first silicon crystal and the carbon concentration of each first silicon crystal, the correlation between the carbon-related level density and the carbon concentration in silicon is obtained. A calibration curve is derived. Note that when the carbon-related level density and the carbon concentration in silicon are in a substantially proportional relationship, the carbon-related level density obtained from the first silicon crystal is the carbon concentration of the first silicon crystal. The level formation rate, which is the rate at which carbon in silicon forms impurity levels, may be derived by dividing by. This level formation rate is also an index indicating the correlation between the carbon-related level density and the carbon concentration in silicon.

  Next, a second silicon crystal having an unknown carbon concentration is prepared. The manufacturing method of the second silicon crystal is the same as that of the first silicon crystal.

  The second silicon crystal is subjected to the same wet process as that performed on the first silicon crystal, and left for 144 hours or longer after the wet process. At this time, the leaving time from the wet treatment to the second silicon crystal is the same as the leaving time from the wet treatment to the first silicon crystal. That is, for example, when the standing time for the first silicon crystal is 192 hours, the standing time for the second silicon crystal is also 192 hours.

  Next, the density of carbon-related impurity levels is measured by the DLTS method for the second silicon crystal that has been left for 144 hours or longer. At this time, when the previously derived calibration curve or level formation rate is obtained based on the level E3 caused by the HC complex, the density of the level E3 also with respect to the second silicon crystal. Measure. That is, the same impurity level as the impurity level constituting the calibration curve or level formation rate is measured from the second silicon crystal. The DLTS measurement procedure for the second silicon crystal is the same as the DLTS measurement procedure for the first silicon crystal.

  Next, the carbon-related level density of the second silicon crystal and the calibration curve or level formation rate (correlation between the carbon concentration in silicon and the carbon-related level density) obtained from the first silicon crystal Based on the above, the carbon concentration in the second silicon crystal is obtained. As described above, the carbon concentration in the silicon crystal can be obtained by the DLTS method.

(Qualitative evaluation of carbon concentration in silicon)
Next, a method for performing DLTS measurement for the purpose of qualitative evaluation of the carbon concentration in silicon will be described.

  First, a silicon crystal to be evaluated for carbon concentration is prepared. The silicon crystal may be manufactured by the CZ method or may be manufactured by the FZ method.

  Next, wet processing similar to that performed on the first silicon crystal is performed on the prepared silicon crystal.

  After carrying out this wet process, the silicon crystal is left for a certain period of time. Specifically, it is left for 72 to 120 hours after the wet treatment. As described with reference to FIG. 1, since the peak of the level density appears when the standing time after the wet treatment is 72 to 120 hours, the silicon crystal is allowed to stand for 72 to 120 hours after the wet treatment. The level density in the vicinity of this peak can be measured. That is, the level density can be measured with high sensitivity.

  Here, with respect to leaving the wet process for 72 to 120 hours, for example, when the wet process is performed at 12:00 on February 1, it is left until 12:00 on February 4 to 12:00 on February 6. It means to do.

  Next, the density of carbon-related impurity levels is measured by DLTS method on silicon crystals left for 72 to 120 hours after wet processing. This level density measurement is the same as the level density measurement for the first silicon crystal described above.

  Then, the carbon concentration in the silicon crystal is evaluated based on the obtained level density. For example, the fact that the carbon-related level density could be measured by the DLTS method means that a small amount of carbon is present in the silicon crystal, so the presence of carbon in the silicon crystal is determined based on the level density. Confirm and evaluate.

  Thus, in this embodiment, since the wet process with an acid is performed with respect to a silicon crystal prior to DLTS measurement, hydrogen can be introduced into the silicon crystal. With this introduction, a composite level containing carbon and hydrogen among the impurity levels in the silicon crystal can be activated, and this level can be measured with high sensitivity by DLTS measurement.

  In addition, since the DLTS measurement is performed after being left for 144 hours or more after the wet treatment or after being left for 72 to 120 hours after the wet treatment, the carbon-related level density can be measured stably or with high sensitivity. That is, when the DLTS measurement is performed after being left for 144 hours or more after the wet processing, the measurement can be performed when the change in the carbon-related level density with respect to the passage of time is small. Can be obtained. That is, the carbon concentration in silicon can be accurately measured.

On the other hand, when DLTS measurement is performed after standing for 72 to 120 hours after wet processing, the measurement can be performed in the vicinity of the peak of the level density, so that carbon in silicon can be detected with high sensitivity. Even if the amount of carbon is very small (for example, the carbon concentration is 10 ¹³ atoms / cm ³ ), the presence of the carbon can be confirmed.

  Hereinafter, although an example and a comparative example of the present invention are given and explained concretely, the present invention is not limited to these.

(Comparative Example 1)
A silicon crystal rod having a diameter of 6 inches, an orientation <100>, an n-type, a resistivity of 10 Ωcm and containing about 0.03 ppma of carbon was pulled up by the FZ method. This silicon crystal rod was processed into a silicon wafer. This silicon wafer was divided into two groups, and one group did nothing, and the other group etched (wet process) the surface layer with a mixed acid of HNO ₃ and HF. This wet treatment was performed by setting the temperature of the mixed acid to 30 ° C., etching the surface of the silicon crystal by 3 μm, and rinsing the silicon crystal with pure water for 3 minutes. Thereafter, the two groups of silicon wafers were immediately subjected to removal of the oxide film by HF, and a Schottky electrode made of Au and an ohmic electrode made of Ga were formed on the front and back surfaces, respectively, and DLTS measurement was performed. In the DLTS measurement, −5 V (reverse bias) was applied between the electrodes, and the density of the level E3 of the HC complex was measured by sweeping in a temperature range of 30 to 300K.

As a result, the density of the carbon-related level E3 of about 8 × 10 ¹¹ atoms / cm ³ was obtained in the sample subjected to the mixed acid treatment, whereas 2 × 10 ¹⁰ atoms was obtained in the sample not subjected to the mixed acid treatment. A density of carbon-related level E3 of about / cm ³ was obtained, and it was found that the mixed acid treatment had an effect of activating the E3 level. In addition, the level density (about 8 × 10 ¹¹ atoms / cm ³ ) obtained from the sample subjected to the mixed acid treatment is shown as a point when the standing time in FIG. 1 is 0 hour.

Example 1
A silicon crystal rod having a diameter of 6 inches, an orientation <100>, an n-type, a resistivity of 10 Ωcm and containing about 0.03 ppma of carbon was pulled up by the FZ method. This silicon crystal rod was processed into a silicon wafer. The silicon wafer was subjected to surface etching with a mixed acid of HNO ₃ and HF. This wet treatment was performed by setting the temperature of the mixed acid to 30 ° C., etching the surface of the silicon crystal by 3 μm, and rinsing the silicon crystal with pure water for 3 minutes. The silicon wafer was left for 72 hours after the wet treatment. Thereafter, the oxide film was removed by HF, and a Schottky electrode made of Au and an ohmic electrode made of Ga were formed on the front and back surfaces, respectively, and then DLTS measurement was performed. In the DLTS measurement, −5 V (reverse bias) was applied between the electrodes, and the density of the level E3 of the HC complex was measured by sweeping in a temperature range of 30 to 300K.

As a result, a density of carbon-related level E3 of about 1.8 × 10 ¹² atoms / cm ³ is obtained, and the density of E3 level is more than twice that of the sample subjected to electrode formation and DLTS measurement immediately after the mixed acid treatment. The qualitative judgment of the presence of carbon could be made reliably. This level density (about 1.8 × 10 ¹² atoms / cm ³ ) is shown as a point when the standing time in FIG. 1 is 72 hours.

(Example 2)
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 192 hours.

As a result, a density of carbon-related level E3 of about 1.3 × 10 ¹¹ atoms / cm ³ was obtained, and the E3 level activity of about two times that of the sample not subjected to the mixed acid treatment as shown in Comparative Example 1 was obtained. A stable E3 level density could be obtained while having a conversion efficiency, and a quantitative determination of the presence of carbon could be made reliably. This level density (about 1.3 × 10 ¹¹ atoms / cm ³ ) is shown as a point at the standing time of 192 hours in FIG.

(Example 3)
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 96 hours.

As a result, a density of carbon-related level E3 of about 1.2 × 10 ¹² atoms / cm ³ is obtained, and an E3 level density higher than that of the sample in which electrode formation and DLTS measurement are performed immediately after the mixed acid treatment can be obtained. It was possible to make a qualitative judgment on the existence of carbon. This level density (about 1.2 × 10 ¹² atoms / cm ³ ) is shown as a point when the standing time in FIG. 1 is 96 hours.

Example 4
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 336 hours.

As a result, a density of carbon-related level E3 of about 1.0 × 10 ¹¹ atoms / cm ³ was obtained, and the E3 level activation efficiency was higher than that of the sample not subjected to the mixed acid treatment as shown in Comparative Example 1. While having it, a stable E3 level density could be obtained, and quantitative determination of the presence of carbon could be made reliably. This level density (about 1.0 × 10 ¹¹ atoms / cm ³ ) is shown as a point at the standing time of 336 hours in FIG.

Further, the difference in level density obtained in Examples 2 and 4 is about 0.3 × 10 ¹¹ atoms / cm ³ , and after the standing time after the wet treatment is 192 hours, it is before 120 hours. Compared with the passage of time, the change in level density is small. Therefore, it can be said that the carbon-related level density is stabilized by setting the standing time after the wet treatment to 144 hours or longer (particularly 192 hours or longer).

  The present invention is not limited to the above embodiment. The above embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

  For example, in the above embodiment, an example in which the density of the level E3 of the HC complex is measured by DLTS is shown. However, impurity levels other than the level E3 including at least H and C (HCCO complex) Or the like), and the concentration of carbon in silicon may be evaluated based on the measured value. As in Patent Document 1, the H-C complex and the H-C-O complex The density of the levels E1, E2, and E3 may be measured by DLTS, and the carbon concentration in silicon may be evaluated based on the total density obtained.

Claims (3)

  After performing wet treatment with acid on silicon and leaving it for 72 hours to 120 hours after performing the wet treatment, the impurity level caused by the composite containing at least carbon and hydrogen contained in the silicon is determined by the DLTS method. A method for detecting carbon in silicon, characterized by measuring by:   After performing wet treatment with silicon on acid and allowing it to stand for 144 hours or more after performing the wet treatment, the impurity level due to the composite containing at least carbon and hydrogen contained in the silicon is measured by the DLTS method. A method for detecting carbon in silicon.   The method for detecting carbon in silicon according to claim 1, wherein the complex is a complex containing only carbon and hydrogen.

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