JPH0992588A - Single-crystal silicon substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain a single-crystal silicon substrate from separating out oxygen by a method wherein the composition ratio of isotopes Si<29> and Si<30> to Si<28> in a region of the single-crystal substrate where a device is to be formed is set deviating from that of natural Si by specific % or above, and each of Si<29> , Si<30> , and Si<28> is set lower than specific % in composition ratio. SOLUTION: The composition ratio of isotopes Si<29> and Si<30> to Si<28> in a region of a single-crystal substrate 1 where a device is to be formed is set deviating from that of natural Si by ±10% or above, and furthermore each of Si<28> , Si<29> , and Si<30> is set lower than 99% in composition ratio. Or, an Si epitaxial crystal layer 2 controlled in an isotope composition ratio as prescribed is formed on an Si substrate 3 which is not controlled in composition ratio. By this setup, oxygen separated out by a thermal treatment and thermal donors can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon single crystal substrate used as a substrate for semiconductor devices.

[0002]

2. Description of the Related Art As a silicon single crystal substrate for a semiconductor device, a so-called CZ substrate formed by a pulling method is used except for special applications. Since a crucible made of quartz (SiO ₂ ) is usually used for the pulling method, CZ
The substrate contains oxygen of about 10 ¹⁷ to 10 ¹⁸ / cm ³ . Initially, this oxygen exists in the interstitial state in a solid solution state, but is precipitated by the heat treatment. For this oxygen precipitation, a substance that becomes a precipitation nucleus is necessary.

For a silicon single crystal containing oxygen, 450 ° C.
Thermal donor (thermal donor)
Occurs and the resistivity decreases. It is considered that the precipitation of oxygen is closely related to the generation of this thermal donor. Thus, a silicon single crystal usually contains an isotope, and unless special processing such as concentration is added,
Its isotope ratio remains that of natural silicon.

[0004]

In the above-mentioned conventional silicon single crystal, there is a phenomenon that oxygen precipitates are generated in the crystal or a thermal donor is generated by the heat treatment. Therefore, the conventional silicon single crystal substrate is used. However, there is a problem in that the characteristics of the semiconductor device cannot be completely controlled, and there is a problem in that variations in the characteristics of the element increase. Therefore, it is an object of the present invention to provide a silicon single crystal substrate in which oxygen precipitation is suppressed.

[0005]

To achieve the above object, a silicon single crystal substrate according to the present invention is provided with at least a Si ²⁸ isotope Si ²⁹ , S in a region where a device is formed.
The composition ratio of i ³⁰ deviates from the composition ratio of natural Si by ± 10% or more, and the composition ratio of any of Si ²⁸ , Si ²⁹ , and Si ³⁰ is 99% or less. is there.

[0006]

Next, embodiments of the present invention will be described with reference to the drawings. In the silicon single crystal substrate of the present invention, at least in the device formation region, the abundance ratios of the isotopes Si ²⁹ and Si ³⁰ in the crystal deviate from the natural abundance ratio by ± 10% or more. The natural silicon isotope abundance ratio is Si ²⁸ : Si ²⁹ : Si ³⁰ = 92.23: 4.67:
Since it is 3.10, the abundance ratio of Si ²⁹ : 4.20% or less, or 5.14% or more, the abundance ratio of Si ³⁰ : 2.79% or less, or 3.41% or more. Another condition to be satisfied by the silicon single crystal substrate according to the present invention is Si ²⁸ , Si ²⁹ , Si ^30.
The existence ratio of any of the above is 99% or less.

In the silicon single crystal substrate according to the present invention, as shown in FIG. 1A, the entire substrate is the Si substrate 1 whose component ratio is adjusted to the above isotope ratio, or
As shown in (b), the Si substrate 3 whose component ratio is not adjusted
A crystal layer of the Si epitaxial layer 2 whose component ratio is adjusted to the above isotope ratio is provided on the top. Substrates or crystals with these component ratios adjusted are separated into their respective isotopes using a well-known isotope separation technique, and then the desired isotope ratio is prepared, or a specific isotope is isolated from natural silicon. It can be formed by drawing or by adding a specific isotope to natural silicon.

In a study on the oxygen precipitation process in a silicon single crystal, the present inventor found that oxygen is likely to precipitate in a silicon single crystal having a specific isotope ratio. Therefore, a silicon single crystal in which oxygen precipitation is suppressed can be obtained by avoiding this silicon having an isotope ratio that easily precipitates oxygen.

A silicon single crystal is mainly produced by gradually cooling from a liquid phase to a solid phase. At that time, Si ²⁹ and Si ³⁰ having a slightly higher freezing point first agglomerate to form a fine lump in the crystal. create. This mass of elements having slightly different mass numbers can serve as a nucleus for oxygen precipitation. As described above, the natural silicon isotope abundance ratio is Si ²⁸ : Si ²⁹ : Si ³⁰ = 92.23:
Although it is 4.67: 3.10, abundance is likely to occur especially in the case of this abundance ratio. However, for example, Si ²⁹ and S
Isotope abundance ratio in which i ³⁰ is half or less of natural one: Si ²⁸ :
In the case of Si ²⁹ : Si ³⁰ = 96.5: 2.0: 1.5, silicon is less likely to be solidified, and Si ²⁹ and Si ³⁰ tend to be evenly dispersed in the Si ²⁸ crystal. Further, even if a lump is formed, its size and density can be kept low. When the size of the lump is small, the possibility of becoming a nucleus of oxygen precipitation is low. On the contrary, in the silicon in which the abundance ratio of Si ²⁹ and Si ³⁰ is increased to, for example, Si ²⁸ : Si ²⁹ = 50: 50, isotopes are not mixed to form a lump.

In the present invention, by changing the isotope ratio of silicon from the natural ratio in this way, the size and density of isotope clusters that can become precipitation nuclei are controlled, whereby oxygen precipitates and thermal donors can be formed. The occurrence is suppressed. Then, if you try to greatly increase the purity of a particular isotope,
The cost increases exponentially according to the purity. However, according to the experiments of the present inventor, even if the purity was increased to a high degree of purity, no special effect was obtained from the viewpoint of oxygen precipitates. Therefore, in the present invention, the purity of each isotope is 99
It is limited to% or less. When manufacturing a semiconductor device using such a silicon single crystal substrate of the present invention, it is possible to suppress the occurrence of defects, improve the manufacturing yield of semiconductor devices, and obtain devices with extremely uniform characteristics. it can.

[0011]

Next, an embodiment of the present invention will be described. [First Example] The ratio of Si ²⁹ and Si ³⁰ to Si ²⁸ was increased to twice the natural ratio. That is, Si ²⁸ : Si ²⁹ :
A silicon substrate having an isotope ratio of Si ³⁰ = 84.46: 9.34: 6.20 was obtained, and this was heat-treated in an electric furnace at 1000 ° C. for 16 hours in a nitrogen atmosphere. Figure 2 shows Si after heat treatment.
It is a top view which shows the oxygen precipitation condition of the board | substrate 1. FIG. As shown in the figure, the amount of oxygen precipitates 4 is extremely small.

Comparative Example 1 For comparison, a similar heat treatment was performed on a silicon substrate having a natural isotope abundance ratio.
FIG. 3 shows a Si substrate 3 whose composition ratio is not adjusted after heat treatment.
FIG. 3 is a plan view showing the oxygen precipitation state of FIG. As shown in the figure, many oxygen precipitates 4 were observed in the conventional Si substrate.

[Second Embodiment] The ratio of Si ²⁹ and Si ³⁰ to Si ²⁸ was reduced to one-third of the natural value. That is,
Si ²⁸ : Si ²⁹ : Si ³⁰ = 97.41: 1.56: 1.
A silicon substrate with an isotope ratio of 03 was obtained, and 450 was obtained in an electric furnace.
Heat treatment was performed in a nitrogen atmosphere at 30 ° C. for 30 minutes. The generation density of thermal donors after the heat treatment was below the measurement sensitivity of the measuring device.

Comparative Example 2 For comparison, when a similar heat treatment was performed on a silicon substrate having a natural isotope abundance ratio, the generation density of thermal donors was 7 × 10 ¹⁵ cm ^-3 .

[0015]

As described above, the silicon single crystal substrate according to the present invention has an isotope ratio that makes it difficult for silicon isotopes to form clusters when the crystal is pulled, or the size and density of the clusters become low. Therefore, oxygen precipitation and thermal donor generated by heat treatment can be suppressed to be low. Therefore, according to the present invention, it is possible to improve the manufacturing yield and suppress variations in the characteristics of semiconductor devices.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a silicon single crystal substrate for explaining an embodiment of the present invention.

FIG. 2 is a plan view showing the state of oxygen precipitation in an example of the present invention.

FIG. 3 is a plan view showing the state of oxygen precipitation in a conventional example.

[Explanation of symbols]

 1 Si substrate with adjusted component ratio 2 Si epitaxial layer with adjusted component ratio 3 Si substrate with unadjusted component ratio 4 Oxygen precipitate

Claims (2)

[Claims] 1. The composition ratio of isotopes Si ²⁹ and Si ³⁰ of Si ²⁸ at least in a region where a device is formed is natural Si.
Deviates from the composition ratio in ± 10% or more, and S
A silicon single crystal substrate having a composition ratio of any of i ²⁸ , Si ²⁹ , and Si ³⁰ of 99% or less. 2. A silicon single crystal substrate, wherein an epitaxial layer made of a material having a composition ratio limited in claim 1 is formed on a single crystal silicon substrate whose composition ratio is not adjusted.