JP2011129570A - Method of evaluating impurity in silicon epitaxial wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of carrying out a qualitative and quantitative analysis of a heavy metal impurity included in a silicon epitaxial wafer in a highly sensitive manner. <P>SOLUTION: The method of evaluating an impurity in a silicon epitaxial wafer has: a film forming process of vapor-phase growing a silicon single-crystal thin film on a silicon single-phase crystal substrate under a hydrogen atmosphere by supplying a source gas during the process; a cooling process of calculating a temperature at which a specified value or process average value for the concentration of a to-be-evaluated impurity present in the silicon single-phase crystal thin film matches the solid solution limit concentration of the to-be-evaluated impurity and cooling a silicon epitaxial wafer having the silicon single-crystal thin film formed thereon by the film forming process under temperatures at least ranging from 50°C below to 50°C above the calculated temperature at a 20°C/sec or greater cooling rate after silicon epitaxial wafer film-formation; and an evaluation process of chemically analyzing the surface layer of the silicon single-crystal thin film to evaluate the concentration of the to-be-evaluated impurity. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an impurity evaluation method for a silicon epitaxial wafer, and specifically relates to an evaluation method capable of evaluating a metal impurity concentration in a silicon epitaxial wafer with high sensitivity.

A silicon epitaxial wafer is manufactured as follows, for example.
That is, a silicon single crystal substrate is placed in a reaction vessel of a vapor phase growth apparatus, and the temperature in the reaction vessel is raised to 1100 ° C. to 1200 ° C. in a state where hydrogen gas is allowed to flow (temperature raising step).
When the temperature in the reaction vessel reaches 1100 ° C. or higher, the natural oxide film (SiO ₂ : Silicon Dioxide) formed on the substrate surface is removed.

In this state, a silicon source gas such as trichlorosilane (SiHCl ₃ : Trichlorosilane) or a dopant gas such as diborane (B ₂ H ₆ : Diborane) or phosphine (PH ₃ : Phosphine) is supplied into the reaction vessel together with hydrogen gas. In this way, a silicon single crystal thin film is vapor-phase grown on the main surface of the substrate (deposition process).

  After vapor-depositing the thin film in this manner, the supply of the source gas and the dopant gas is stopped, and the temperature in the reaction vessel is lowered while maintaining the hydrogen atmosphere (cooling step).

By the way, in the process of manufacturing a silicon epitaxial wafer as described above, if heavy metal impurities are mixed in the epitaxial layer (silicon single crystal thin film), the characteristics of a device manufactured using the substrate may become abnormal. .
In particular, if there is impurity contamination on the surface layer side of the epitaxial layer that becomes the device active layer in which the device is built, the adverse effect on the device is increased.

  Examples of conventional methods for evaluating heavy metal impurities in an epitaxial silicon wafer include AAS (Atomic Absorption Spectroscopy), ICP-MS (Inductively Coupled Plasma-Mass Spectroscopy), TRXF (Inductively Coupled Plasma Mass Analysis), TRXF -Ray Fluorescence: Total reflection fluorescent X-ray analysis), etc.

Also, in the cooling process at the time of manufacturing the silicon epitaxial wafer, an evaluation method for detecting Cu contamination with high sensitivity by precipitating Cu on the wafer surface by switching the atmosphere gas from a hydrogen atmosphere to a nitrogen atmosphere at 400 ° C. or lower. Is disclosed (see Patent Document 1).
In addition to this, the silicon wafer is etched for 30 minutes or more by using a treatment liquid having a higher ammonia concentration than hydrogen peroxide solution, and the number of LPDs formed on the surface is examined, whereby the silicon wafer is contaminated with Cu. Have been disclosed (see Patent Document 2).

Japanese Patent No. 3664101 Japanese Patent No. 3717691

However, even when trying to measure the amount of heavy metal impurities contained in a silicon epitaxial wafer by the conventional evaluation method, the amount of impurities present in the wafer is very small, so the sensitivity to analyze and evaluate may not be sufficient. .
Therefore, in the evaluation on the monitor wafer, even when the semiconductor device is manufactured using the silicon epitaxial wafer manufactured in the manufacturing process evaluated as good by the conventional analysis method, the device having poor device characteristics is manufactured. There was a problem that it might end.

  For Cu, a method has been proposed for improving the sensitivity by devising the atmosphere gas in the cooling process of the epitaxial wafer manufacturing process, but for other heavy metal impurities, effective methods for increasing the sensitivity are proposed. The processing method was not known.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide an impurity evaluation method capable of performing qualitative and quantitative analysis of heavy metal impurities contained in a silicon epitaxial wafer with high sensitivity.

  In order to solve the above-described problems, the present invention provides a method for evaluating impurities of a silicon epitaxial wafer, in which a silicon single crystal thin film is vapor-phase grown in a hydrogen atmosphere on a silicon single crystal substrate while supplying a source gas. And a silicon epitaxial wafer on which the silicon single crystal thin film is formed by the film forming step, a standard value or a process average value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film and a solid solution of the impurity to be evaluated A cooling step of calculating a temperature at which the critical concentrations coincide with each other, cooling in a temperature range of at least 50 ° C. above and below the calculated temperature at a cooling rate after film formation of the silicon epitaxial wafer of 20 ° C./sec or more; A chemical analysis of the surface layer of the crystal thin film, and an evaluation step of measuring the concentration of the impurity to be evaluated. To provide an impurity evaluation method of a silicon epitaxial wafer to be.

Most impurities in the silicon epitaxial wafer exist in a solid solution state in a high temperature region immediately after the epitaxial reaction for forming a silicon single crystal thin film. Precipitation begins when it reaches a temperature at which the solid solution limit is reached in the cooling step. Therefore, the temperature at which the standard value of the concentration of the impurity to be evaluated and the process average value (which can be calculated from the past production results of the silicon epitaxial wafer) matches the solid solution limit concentration of the impurity to be evaluated is calculated. When the cooling rate is controlled to 20 ° C./sec or more in a temperature range of at least 50 ° C. above and below the calculated temperature, impurities to be evaluated collect in the surface layer portion of the silicon single crystal thin film.
The concentration of the metal impurity in the silicon epitaxial wafer is conventionally determined by chemical analysis of the surface layer of the silicon single crystal thin film in which the impurities to be evaluated in the silicon single crystal thin film, particularly the metal impurities are collected, to measure the concentration of the impurity to be evaluated. Can be evaluated more sensitively and quantitatively.

Here, the cooling rate is preferably 30 ° C./sec or less.
Even if the cooling rate is increased further, the ability to detect the impurities to be evaluated cannot be further improved. On the other hand, a special cooling facility for further increasing the cooling rate is required, leading to an increase in cost. As described above, by setting the cooling rate to 30 ° C./sec or less, it is not necessary to provide an excessive cooling facility, and it is possible to improve both the detection capability of the impurities to be evaluated.

Moreover, it is preferable that the evaluation object impurity is Ni.
The content of Ni in a silicon single crystal thin film of a general silicon epitaxial wafer is assumed to be 1 × 10 ⁹ atoms / cm ³ to 1 × 10 ¹¹ atoms / cm ³ .
Therefore, referring to FIG. 2, the temperature range in which this concentration range becomes the solid solubility limit of Ni is 300 ° C. to 400 ° C. Therefore, when the impurity to be evaluated is Ni, by controlling the cooling rate in the range from at least 400 ° C. to 300 ° C. to 20 ° C./sec or more in the cooling process, Ni precipitation on the surface layer of the silicon single crystal thin film This makes it possible to control the Ni concentration, which has been difficult in the prior art.

As described above, in the cooling process after the film formation reaction of the silicon single crystal thin film, the temperature at which the standard value of the impurity to be evaluated and the process average value coincide with the solid solution limit concentration of the contaminating element, that is, the contaminating element becomes supersaturated. The silicon epitaxial wafer is rapidly cooled at 20 ° C./sec or more in the vicinity of the temperature range where it begins to become (about ± 50 ° C.).
As a result, the contaminating elements in the silicon single crystal thin film gather on the surface layer side. Therefore, by analyzing the region where the contaminating elements are concentrated by a method such as WSA (Wafer Surface Analysis) method or step etch analysis, It is possible to evaluate the contamination of the silicon epitaxial wafer in the process of forming the crystalline thin film with higher sensitivity than before.

It is the flowchart which showed an example of the outline of the evaluation method of the silicon epitaxial wafer of this invention. It is the figure which showed the temperature dependence of the solid solubility of Ni in silicon. It is the figure which showed the relationship between the cooling rate of 350 degreeC vicinity in the cooling process after the film-forming reaction of a silicon single crystal thin film, and the density | concentration of Ni collected in the silicon single crystal thin film surface layer part.

Hereinafter, the present invention will be described more specifically.
In some cases, the conventional evaluation method has insufficient sensitivity to detect the amount of heavy metal impurities contained in the silicon epitaxial wafer, and the evaluation cannot be performed with high accuracy.
Therefore, even when a semiconductor device is manufactured using a silicon epitaxial wafer evaluated as good by a conventional analysis method, there is a problem that a device having low device characteristics may be manufactured.

Therefore, the present inventor conducted intensive studies and experiments in order to solve such problems.
As a result, attention was paid to cooling conditions after growth of the epitaxial layer (silicon single crystal thin film) as a condition affecting the impurity concentration of the surface layer of the silicon epitaxial wafer. In particular, we focused on the cooling rate in the temperature range where the heavy metal impurities contained are supersaturated, and have conceived of changing this cooling rate.

  As a result of further intensive studies and experiments, the temperature at which the standard value or the process average value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film coincides with the solid solution limit concentration is calculated, and the calculated temperature The impurity to be evaluated can be collected in the surface layer portion of the silicon single crystal thin film by setting the cooling rate of the silicon epitaxial wafer after film formation to 20 ° C./sec or more at least in the temperature range of 50 ° C. or above. As a result of chemical analysis, it has been found that the concentration of impurities can be evaluated with higher sensitivity than in the past, and the present invention has been completed.

Hereinafter, the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto.
First, a method for evaluating a silicon epitaxial wafer according to the present invention will be described with reference to FIG. FIG. 1 is a flowchart showing an example of an outline of the silicon epitaxial wafer evaluation method of the present invention.

First, as shown in FIG. 1, a silicon single crystal substrate is placed on a susceptor provided in a reaction vessel of a vapor phase growth apparatus using a transfer device (FIG. 1 (a), preparation).
Next, with the hydrogen gas flowing in the reaction vessel, the temperature in the reaction vessel is raised to a film formation temperature for vapor phase growth of the silicon single crystal thin film (FIG. 1B, temperature rise). The film forming temperature is set to 1000 ° C. or higher at which the natural oxide film on the substrate surface can be removed with hydrogen.

  Next, while keeping the inside of the reaction vessel at the film forming temperature, the source gas and the dopant gas are supplied at a predetermined flow rate together with the hydrogen gas, and the silicon single crystal is obtained until the silicon single crystal thin film reaches a predetermined film thickness in a hydrogen atmosphere. A silicon single crystal thin film is grown on the substrate (FIG. 1C, film forming step).

Thereafter, the supply of the source gas and the dopant gas is stopped, and the silicon epitaxial wafer is cooled by lowering the temperature in the reaction vessel while flowing hydrogen as the carrier gas (FIG. 1 (d), cooling step).
In this cooling step, a temperature at which the standard value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film or the process average value matches the solid solution limit concentration of the impurity to be evaluated is calculated, and at least 50 ° C. above and below the calculated temperature. In the temperature range, the cooling rate after film formation of the silicon epitaxial wafer is set to 20 ° C./sec or more for cooling.
Further, the hydrogen atmosphere can be switched to the nitrogen atmosphere between about 800 ° C. and 400 ° C.

Most impurities in the silicon wafer exist in a solid solution state in the high temperature region immediately after the epitaxial reaction for forming the silicon single crystal thin film, and precipitation begins when the temperature reaches the solid solution limit in the cooling process. Begins.
Therefore, a temperature at which the standard value or process average value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film coincides with the solid solution limit concentration of the impurity to be evaluated is calculated. In the temperature range, when the cooling rate in the cooling step after the silicon single crystal thin film forming step is controlled to 20 ° C./sec or more, impurities to be evaluated in the silicon epitaxial wafer gather in the surface layer region of the silicon single crystal thin film.
Then, by utilizing this, by chemically analyzing this region where the evaluation target impurities are collected in the evaluation process described later, the impurity concentration in the silicon epitaxial wafer can be measured with high sensitivity.

Here, the evaluation target impurity may be Ni.
The content of Ni in a silicon single crystal thin film of a general silicon epitaxial wafer is assumed to be a level of 1 × 10 ⁹ to 1 × 10 ¹¹ atoms / cm ³ .
As shown in FIG. 2, when the contamination amount of Ni is about 5 × 10 ¹⁰ atoms / cm ³ within the above range, the temperature at which the contamination amount and the solid solubility coincide with each other is about 350 ° C.
Accordingly, when the impurity to be evaluated is Ni, the cooling rate is controlled to 20 ° C./sec or higher when the temperature of the silicon epitaxial wafer being cooled passes through a temperature range of at least 400 ° C. to 300 ° C.
FIG. 2 is a diagram showing the temperature dependence of the solid solubility of Ni in silicon.

Ni that adversely affects the device characteristics is selected as an impurity to be evaluated, and the temperature of the silicon epitaxial wafer is controlled to a temperature of at least 400 ° C. to 300 ° C. so that the cooling rate is 20 ° C./sec or more. Collect on the surface layer of the crystal thin film.
And Ni density | concentration can be evaluated in the evaluation process mentioned later.

Moreover, a cooling rate can be 30 degrees C / sec or less.
The higher the cooling rate of the silicon epitaxial wafer after film formation in the range of at least 50 ° C. above and below the temperature at which the standard concentration of the impurity to be evaluated existing in the silicon single crystal thin film and the solid solution limit concentration match, Although special cooling equipment is required and costs are high, the amount of impurities to be evaluated becomes saturated, and even if the cooling rate is increased further, no significant change is observed.
However, if the cooling rate is 30 ° C./sec or less, an excessive cooling facility is not necessary, and the amount of impurities to be evaluated can be kept sufficiently large, which is convenient.

  When the extraction temperature is reached in the nitrogen atmosphere, the silicon epitaxial wafer is taken out from the vapor phase growth apparatus (FIG. 1 (e), taking out).

  Subsequently, the concentration of the impurity to be evaluated collected in the surface layer region of the silicon single crystal thin film of the silicon epitaxial wafer is measured by chemical analysis, for example, step etch method, WSA method, AAS, ICP-MS, TRXF (FIG. 1 ( f) Surface layer impurity evaluation step).

Here, FIG. 3 is a diagram showing the relationship between the cooling rate around 350 ° C. and the concentration of Ni collected in the surface layer portion of the silicon single crystal thin film in the cooling step after the film formation reaction of the silicon single crystal thin film.
As shown in FIG. 3, the higher the cooling rate, the more Ni is collected near the surface layer of the silicon single crystal thin film. That is, after quenching a temperature range in which the standard value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film and the solid solution limit concentration match, the surface layer is subjected to the step etch method, WSA method, AAS, ICP-MS, If analyzed by TRXF or the like, impurities can be detected with high sensitivity.

  As described above, the cooling rate is controlled to 20 ° C./sec or more in a temperature range at least 50 ° C. above and below the temperature range in which the standard value or process average value of the concentration of the impurity to be evaluated matches the solid solution limit concentration of the impurity to be evaluated Then, the concentration of the impurity to be evaluated collected in the surface layer portion of the silicon single crystal thin film by cooling is chemically analyzed, whereby the concentration of the impurity to be evaluated in the silicon epitaxial wafer can be evaluated with higher accuracy than before.

  Then, by determining whether the evaluation target impurity concentration of the silicon epitaxial wafer to be monitored is lower than the standard value (determining that it is a non-defective product), the manufacturing process of the silicon epitaxial wafer can be managed.

  In this way, manufacturing process of silicon epitaxial wafer based on the evaluation result in the previous evaluation process, manufacturing high-quality silicon epitaxial wafers with good device characteristics whose evaluation target impurity concentration is below standard value Can do.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
(Example 1-3, Comparative Example 1)
The plane orientation (100), P ⁺ type (0) in which it was confirmed in advance that the same batch of silicon single crystal substrates had a Ni concentration of 1 × 10 ¹⁰ atoms / cm ³ or less (lower detection limit) using total dissolution chemical analysis. .015 Ωcm) of silicon single crystal substrates were prepared, and a P ^- type (10 Ωcm) silicon single crystal thin film of 5 μm was vapor-phase grown on the main surface at a film forming temperature of 1130 ° C.

  When the silicon epitaxial wafer after film formation is cooled, the cooling rate between 400 ° C. and 300 ° C. is 15 ° C./sec (Comparative Example 1), 20 ° C./sec (Example 1), 30 ° C. A silicon epitaxial wafer was manufactured by changing to / sec (Example 2) and 35 ° C./sec (Example 3).

  A total of 4 silicon epitaxial wafers were extracted by a step etching method (see Japanese Patent Application Laid-Open No. 2005-265718, Japanese Patent No. 3755586, etc.) and a surface layer of a silicon single crystal thin film of 1.5 μm was extracted. The concentration of heavy metals containing was measured. The results are shown in Table 1.

As a result, as shown in Table 1, under the cooling conditions of Comparative Example 1, the Ni concentration in the surface layer of the silicon single crystal thin film is below the detection lower limit (1 × 10 ¹⁰ atoms / cm ³ ) of the ICP-MS device, It was not possible to confirm even the presence or absence of impurities, and it was not possible to perform highly sensitive impurity concentration evaluation.
On the other hand, in the cooling conditions of Example 1-3, the Ni concentration was 8.0 × 10 ¹⁰ atoms / cm ³ (Example 1), 4.0 × 10 ¹¹ atoms / cm ³ (Example 2), respectively. It was detected as 4.2 × 10 ¹¹ atoms / cm ³ (Example 3), and it was possible to evaluate the Ni concentration in the surface layer portion of the silicon single crystal thin film with higher sensitivity than in the case of Comparative Example 1. It was.
Further, when Example 1 and Example 2 were compared, it was found that in Example 2, the Ni concentration was higher and the evaluation ability was higher as the cooling rate was faster. However, in the case of the condition of Example 3 where the cooling rate was further increased than that of Example 2, the detected Ni concentration was slightly higher than that of Example 2, but there was no significant difference. That is, even if the cooling rate is increased from that in Example 2, further improvement in Ni detection capability cannot be expected. In addition, a special cooling facility for further increasing the cooling rate is required, which leads to an increase in cost. Therefore, it has been found that the upper limit of the cooling rate is preferably about 30 ° C./sec.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.
For example, the vapor phase growth apparatus for vapor phase growth of a thin film in the present invention is not limited, and can be applied to various vapor phase growth apparatuses such as a vertical type (pancake type), a barrel type (cylinder type), and a single wafer type. .

Claims (3)

An impurity evaluation method for a silicon epitaxial wafer,
A film forming step of vapor-phase-growing a silicon single crystal thin film in a hydrogen atmosphere on a silicon single crystal substrate while supplying a source gas;
A silicon epitaxial wafer in which the silicon single crystal thin film is formed by the film forming step, a standard value or a process average value of the concentration of the impurity to be evaluated existing in the silicon single crystal thin film, and a solid solution limit concentration of the impurity to be evaluated And a cooling step of cooling the silicon epitaxial wafer at a cooling rate of 20 ° C./sec or more in a temperature range at least 50 ° C. above and below the calculated temperature,
A method for evaluating an impurity of a silicon epitaxial wafer, comprising: chemically analyzing a surface layer of the silicon single crystal thin film to measure a concentration of the impurity to be evaluated.   The impurity evaluation method for a silicon epitaxial wafer according to claim 1, wherein the cooling rate is set to 30 ° C / sec or less.   The impurity evaluation method for a silicon epitaxial wafer according to claim 1 or 2, wherein the impurity to be evaluated is Ni.

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