JP2012066943A - Substrate for forming nitride semiconductor, and nitride semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon wafer-based substrate which is suitable for forming a nitride semiconductor and is low in cost; and to provide a substrate for forming a nitride semiconductor which reduces the occurrence of a curvature and crack, which are caused by differences in lattice constant and thermal expansion coefficient between materials, even if a nitride semiconductor epitaxial layer whose film is large in thickness is grown, the substrate being excellent in mechanical strength and thermal strength.SOLUTION: In a silicon wafer, boron and germanium are doped at a specific concentration. For a ratio in concentration of the boron to the germanium, the concentration of the germanium is controlled preferably to be 5-8 times as much as that of the boron.

Description

  The present invention relates to a substrate for forming a nitride semiconductor, and more specifically, even when a nitride semiconductor film is formed on the substrate, the warp and stress generated in the film can be reduced, and a nitride semiconductor that provides a good nitride semiconductor is provided. The present invention relates to a forming substrate.

As compound semiconductors, for example, nitride semiconductors typified by GaN and AlN have higher mobility, light emission characteristics, higher band gaps, higher breakdown electric fields, and the like than silicon semiconductors. High power devices) and optical devices (eg laser diodes, light emitting diodes) are attracting attention.
However, since bulk crystal growth of nitride semiconductors has problems in terms of cost and technology, research has been accelerated on nitride compound semiconductors obtained by heteroepitaxially growing a nitride semiconductor single crystal on a silicon substrate.

Usually, an electronic device using a nitride semiconductor is manufactured using a substrate made of, for example, silicon carbide (SiC), sapphire, ZnO, or silicon. In particular, a substrate made of silicon is very effective as a substrate for an electronic device because a large-diameter substrate can be obtained at a low price.
However, since there is a very large difference in lattice constant and thermal expansion coefficient between silicon and nitride semiconductors such as GaN, when a nitride semiconductor layer is directly epitaxially grown on a silicon substrate, a large tensile strain is inherent in the nitride semiconductor layer. As a result, the entire epitaxial substrate on which the nitride semiconductor layer is epitaxially grown may cause a concave warp or deteriorate crystallinity. Further, if the inherent strain is large, cracks are generated in the nitride semiconductor layer.

  As described above, the nitride semiconductor epitaxial growth on the silicon substrate is heteroepitaxial growth with different lattice constants and thermal expansion coefficients, so that the nitride semiconductor epitaxial layer has a large warp, large cracks, few crystal defects, and a large thickness after film formation. It has been a big problem in using a silicon substrate.

Patent Document 1 discloses a silicon wafer produced by slicing a silicon single crystal ingot to which boron and germanium are added as a silicon wafer for forming a silicon epitaxial layer.
However, the silicon wafer disclosed in this document only provides a substrate for forming a silicon epitaxial layer, and when a nitride semiconductor layer such as GaN is formed, warping still occurs greatly. Since it is not suitable as a substrate for forming a nitride semiconductor, a substrate for forming a nitride semiconductor that can be used for forming a nitride semiconductor layer is strongly desired.

JP 2004-175658 A

  An object of the present invention is to provide a nitride semiconductor forming substrate.

  Specifically, it is an object to provide an inexpensive formation substrate based on a silicon wafer, which is suitable for forming a nitride semiconductor such as GaN. In addition, even when a nitride semiconductor epitaxial layer with a large number of lattice defects and a large thickness is grown, the generation of a warp and cracks is reduced, and a substrate for forming a nitride semiconductor having excellent mechanical strength and thermal strength is provided. It is to be.

  As a result of intensive studies, the present inventors have determined that boron and germanium are doped at a specific concentration, and preferably a silicon wafer in which the germanium concentration is controlled to be 5 to 8 times the boron concentration in the concentration ratio of boron and germanium. Was prepared, and a GaN layer was formed as a nitride semiconductor layer. As a result, it was found that the amount of warpage can be significantly reduced and a nitride semiconductor having good characteristics can be provided, and the present invention has been completed.

  That is, the present invention is as follows.

A first invention is a semiconductor substrate for forming a nitride semiconductor,
Boron and germanium are doped in silicon,
When the concentrations of boron and germanium are [B] atoms / cm ³ and [Ge] atoms / cm ³ , respectively.
The following conditions,
2.5 × 10 ¹⁸ ≦ [Ge] ≦ 2.5 × 10 ²⁰ ,
4.0 × 10 ¹⁶ ≦ [B] ≦ 4.0 × 10 ¹⁹ ,
The nitride semiconductor forming substrate is characterized in that:

  The second invention is the nitride semiconductor forming substrate according to the first invention, further satisfying 5 [B] ≦ [Ge] ≦ 8 [B].

The third invention is 1.5 × 10 ¹⁹ ≦ [Ge] ≦ 2.2 × 10 ²⁰ ,
3.0 × 10 ¹⁸ ≦ [B] ≦ 2.7 × 10 ¹⁹ ,
The nitride semiconductor forming substrate according to the second invention, characterized in that:

The fourth invention is:
Heat treatment at 800 ° C. for 4 hours in a nitrogen atmosphere,
Thereafter, after heat treatment at 1000 ° C. for 16 hours in an oxygen atmosphere,
The oxygen precipitate density inside the wafer is 1.0 × 10 ⁷ pieces / cm ³ or more and less than 2.0 × 10 ¹⁰ pieces / cm ^3. It is a nitride semiconductor formation substrate described in one.

The fifth invention is:
Any one of the first to fourth inventions, wherein the oxygen concentration (ASTM F-121 1979) in the wafer is 9 × 10 ¹⁷ atoms / cm ³ or more and 14 × 10 ¹⁷ atoms / cm ³ or less. It is a nitride semiconductor formation substrate described in one.

  A sixth invention is a nitride semiconductor comprising a nitride semiconductor thin film formed on the semiconductor forming substrate according to any one of the first to fifth inventions.

The substrate for forming a nitride semiconductor of the present invention can provide a substrate for a nitride semiconductor with reduced warpage and cracks even when a thick nitride semiconductor layer is formed.
In addition, the nitride semiconductor of the present invention has a thick nitride semiconductor layer formed and is extremely useful as an electronic device made of a nitride semiconductor.

  Hereinafter, the present invention will be described in more detail.

The nitride semiconductor forming substrate of the present invention is characterized in that germanium and boron are contained in a silicon wafer.
The silicon wafer is doped with germanium and boron, so that even when a nitride semiconductor layer such as GaN is formed on the upper layer, it is excellent in thermal shock resistance and becomes a tough silicon wafer free from warping or cracking. .

(substrate)
The substrate for forming a nitride semiconductor of the present invention is a silicon substrate doped with boron and germanium.
The silicon substrate doped with boron and germanium of the present invention reduces warpage and cracks due to lattice stress due to the lattice constant difference of the nitride semiconductor layer formed thereon and thermal stress strain caused by the difference in thermal expansion coefficient, This contributes to improving the yield of electronic devices made of nitride semiconductors manufactured from the obtained substrate.

(boron)
As the concentration of boron doped in the nitride semiconductor formation substrate of the present invention, when the boron concentration is [B] atoms / cm ³ ,
4.0 × 10 ¹⁶ ≦ [B] ≦ 4.0 × 10 ¹⁹ , more preferably 3.0 × 10 ¹⁸ ≦ [B] ≦ 2.7 × 10 ¹⁹ .
By doping boron at the above concentration in silicon, dislocations in the silicon crystal can be suppressed.
The silicon substrate, which is a substrate for a nitride semiconductor according to the present invention, further increases the mechanical strength of the silicon single crystal by coexisting germanium, thereby obtaining an extremely tough silicon single crystal.

(germanium)
Here, as a suitable germanium concentration, if the germanium concentration is [Ge] atoms / cm ³ ,
2.5 × 10 ¹⁸ ≦ [Ge] ≦ 2.5 × 10 ²⁰ , more preferably 1.5 × 10 ¹⁹ ≦ [Ge] ≦ 2.2 × 10 ²⁰ .

(Concentration relationship between germanium and boron)
Here, the boron concentration indicated by [B] and the germanium concentration indicated by [Ge] are represented by the following relational expression (1).
5 [B] ≦ [Ge] ≦ 8 [B] (1)
Here, it is extremely important for the silicon substrate for forming the nitride semiconductor layer that the germanium concentration is 5 times or more the boron concentration.
That is, when the concentration of germanium is less than 5 times, the formation of a Ge-oxygen-B complex formed in the silicon substrate is difficult to occur, the thermal strength and the dislocation fixing effect are lowered, and the nitride semiconductor layer is not formed. The substrate is likely to warp due to the lattice strain due to the lattice constant difference and the thermal stress strain caused by the difference in thermal expansion coefficient.
In addition, when the germanium concentration exceeds 8 times the boron concentration, germanium is very expensive, so that the manufacturing cost is increased and the lattice strain difference in the obtained silicon substrate is increased, the thermal strength and Since the dislocation fixing effect is lowered, the warpage of the substrate may be increased.
Therefore, the appropriate ratio of the boron concentration to the germanium concentration is 5 to 8 and more preferably 6 to 8 for germanium with respect to 1 for boron concentration.

(Oxygen concentration)
The silicon wafer doped with boron and germanium is greatly affected by the oxygen concentration. The oxygen concentration in the wafer of the present invention is preferably in the range of 9 × 10 ^{17 to} 14 × 10 ¹⁷ atoms / cm ³ (ASTM F-121 1979).
When the oxygen concentration is less than 9 × 10 ¹⁷ atoms / cm ³ , the mechanical strength is lowered and the formation of the Ge-oxygen-B complex is insufficient, and a sufficient warpage suppressing effect cannot be obtained. There is.
On the other hand, if it exceeds 14 × 10 ¹⁷ atoms / cm ³ , the density of oxygen precipitates in the wafer may increase, and the mechanical strength of the resulting semiconductor substrate may be inferior and warpage may be increased.

(Manufacturing method of substrate for forming nitride semiconductor)
The nitride semiconductor substrate of the present invention can be manufactured by the following method.
A silicon melt is doped with a predetermined amount of boron and germanium, and a silicon single crystal is grown using a normal single crystal growth method such as a CZ method.
A silicon wafer is sliced from the obtained silicon single crystal, and each sliced wafer is subjected to various processes such as chamfering, lapping, acid etching, and mirror polishing by a known method to produce a silicon wafer.

(Nitride semiconductor manufacturing method)
In order to grow and form a nitride semiconductor thin film on a silicon wafer doped with boron and germanium of the present invention, a known technique can be used, and there is no particular limitation. Known techniques include atmospheric pressure metalorganic vapor phase epitaxy (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), sublimation, liquid phase epitaxy, and the like.
The silicon wafer doped with boron and germanium according to the present invention can grow and form various nitride semiconductor thin films, but as a buffer layer, a nitride compound semiconductor layer such as AlN or AlGaN can be used. Then, a nitride compound semiconductor layer such as a GaN layer is grown on the buffer layer formed of these nitride compound semiconductor layers using a known technique. It is preferable to make it.
The silicon wafer doped with boron and germanium of the present invention reduces the lattice strain caused by the lattice constant difference and the stress strain caused by the difference in thermal expansion coefficient even when the GaN layer is formed on the silicon wafer, and nitride. When an electronic device made of a semiconductor is manufactured, even if various thermal histories are repeated, warpage is unlikely to occur, cracks and lattice strain are reduced, and the yield of nitride semiconductors is improved.

(Evaluation methods)
(Measurement of boron, germanium, oxygen concentration)
As a method for measuring the boron concentration, the germanium concentration, and the oxygen concentration, the following methods are used.
The concentration of boron and germanium and the oxygen concentration can be measured using a secondary ion mass spectrometer (SIMS).
(Measurement method of oxygen precipitate density)
(1) As a method for evaluating oxygen precipitates, a silicon wafer doped with boron and germanium of the present invention was prepared, and then heat treatment was performed at 800 ° C. for 4 hours in a nitrogen atmosphere, and then in an oxygen atmosphere. Heat treatment is performed at 1000 ° C. for 16 hours.
(2) The thermal oxide film formed by the evaluation heat treatment is removed with an etching solution of HF: H ₂ O = 1: 1.
(3) The wafer is cleaved and the wafer is selectively etched using a Zirtor etchant (composition: HF: CrO ₃ (5M) = 1: 1).
(4) Using an optical microscope with a magnification of 500 to 1000, the oxygen precipitates on the wafer cleavage plane are observed, and the number of oxygen precipitates per unit volume is measured.
In this evaluation method, when the number of oxygen precipitates exceeds 2.0 × 10 ¹⁰ pieces / cm ³ , the formation density of the oxygen precipitates in the silicon wafer becomes excessive, the mechanical strength decreases, and the warpage of the wafer increases. There is a case.
In this evaluation method, the lower limit of the oxygen precipitate density (pieces / cm ³ ) is preferably 1.0 × 10 ⁷ or more. If the oxygen precipitate density is less than this, formation of a Ge-oxygen-B complex that is inevitably formed in the silicon single crystal substrate and is considered to improve the mechanical strength of the silicon single crystal substrate is difficult to occur. Therefore, there is a case where a sufficient warp suppressing effect cannot be obtained.
(Measurement method of warpage)
A value obtained by measuring the height difference between the central portion and the outer peripheral portion of the wafer on the same surface in the thickness direction of the silicon wafer by the laser length measuring device is defined as a warp amount.

EXAMPLES Hereinafter, an Example is given and this invention is demonstrated in detail.
In addition, this invention is not limited at all by this Example.

Examples 1-6
Boron and germanium are doped into the silicon melt, and a 6-inch <111> silicon single crystal is grown by the CZ method. From this silicon single crystal, oxygen precipitates inside the silicon wafer detected by Zirtor etching are 1 0.0 × 10 ⁷ pieces / cm ³ or more and less than 2.0 × 10 ¹⁰ pieces / cm ³ , and the oxygen concentration (ASTM F-121 1979) in the wafer is 9 × 10 ¹⁷ atoms / cm ³ or more and 14 A mirror silicon wafer having a thickness of ≦ 10 ¹⁷ atoms / cm ³ or less was produced with a thickness of 625 μm by a known processing method.

Various changes were made while controlling the doping amounts of boron and germanium, and wafers were prepared in the same manner except that the boron concentration and the germanium concentration shown in Table 1 were changed to Examples 1 to 6, respectively.
An AlN layer is formed as a buffer layer on the mirror surface of each silicon wafer, and then a GaN layer is grown as a nitride semiconductor layer at a high temperature by metal organic vapor phase epitaxy. The nitride semiconductor layer was formed.
After the growth, the temperature was lowered to room temperature, and the amount of warpage generated on the main surface was measured with a laser length meter.
The results are shown in Table 1.

Comparative Examples 1-6
Comparative wafers in which the concentrations of boron and germanium were changed to those shown in Table 1 were prepared and used as Comparative Examples 1 to 6, respectively. After forming a GaN layer for each wafer in the same manner as in the example, the warpage was evaluated. The results are shown in Table 1.

According to Table 1, the boron concentration is less than 4.0 × 10 ¹⁶ atoms / cm ³ , the germanium concentration is less than 2.5 × 10 ¹⁸ atoms / cm ³ , and the boron concentration [B] and the germanium concentration [Ge] are It can be seen that in any of 5 [B]> [Ge], the warpage occurring on the main surface of the wafer increases.

The boron concentration is more than 4.0 × 10 ¹⁹ atoms / cm ³ , the germanium concentration is more than 2.5 × 10 ²⁰ atoms / cm ³ , and the boron concentration [B] and the germanium concentration [Ge] are 8 [B ] <[Ge] It can be seen that the warpage generated on the main surface of the wafer is similarly increased.

The boron concentration is 4.0 × 10 ^{16 to} 4.0 × 10 ¹⁹ atoms / cm ³ , the germanium concentration is 2.5 × 10 ^{18 to} 2.5 × 10 ²⁰ atoms / cm ³ , and the boron concentration [B ] And the germanium concentration [Ge] satisfy 5 [B] ≦ [Ge] ≦ 8 [B], warpage on the wafer main surface is effectively suppressed, and excellent mechanical strength, and thus warpage suppression. It was confirmed to have an effect.

Experimental example 1
For boron with a concentration of 3.0 × 10 ^{18 to} 2.7 × 10 ¹⁹ atoms / cm ³ and germanium with a concentration of 1.5 × 10 ^{19 to} 2.2 × 10 ²⁰ atoms / cm ³ , the boron concentration [B] A germanium concentration [Ge] was doped so as to satisfy 5 [B] ≦ [Ge] ≦ 8 [B], and a 6-inch <111> silicon single crystal was grown by the CZ method.

At that time, the oxygen concentration at the time of manufacturing the silicon single crystal is set to 9 × 10 ¹⁷ atoms / cm ^{3 to} 14 × 10 ¹⁷ atoms / cm ³ , and the oxygen concentration is higher than 14 × 10 ¹⁷ atoms / cm ^3. Prepared each one.

From these silicon single crystals, a mirror-surface silicon wafer in which the density of oxygen precipitates inside the silicon wafer detected by Zirtor etching is 1.0 × 10 ⁷ pieces / cm ³ to 1.0 × 10 ¹¹ pieces / cm ³ is obtained. Obtained.

  A silicon wafer having a thickness of 625 μm was prepared by a known processing method, and a nitride semiconductor layer was formed on the main surface on the mirror surface side of each silicon wafer in the same manner as in Examples 1 to 6, and then warpage was evaluated.

FIG. 1 shows the relationship between the oxygen precipitate density inside a silicon wafer and the amount of warpage on the wafer main surface after the GaN layer is formed.
Wafers produced with a silicon single crystal having an oxygen concentration of 9 × 10 ¹⁷ atoms / cm ^{3 to} 14 × 10 ¹⁷ atoms / cm ³ (plotted with a circle in FIG. 1) have a small amount of warpage. Those produced at an oxygen concentration of more than 14 × 10 ¹⁷ atoms / cm ³ (plotted with x in FIG. 1) showed a significant increase in the amount of warpage.

Each silicon wafer was heat-treated at 800 ° C. for 4 hours in a nitrogen atmosphere, and then heat-treated at 1000 ° C. for 16 hours in an oxygen atmosphere, and then subjected to oxygen precipitation by the evaluation method shown in paragraph 0027 above. There is a clear relationship between the oxygen precipitate density measured for the object density and the amount of warpage. In order to suppress the amount of warpage on the main surface of the wafer after forming the GaN layer, the oxygen precipitate density is set to 2.0 × 10 ^10. It has been found desirable to stay below < ³ / cm ³ .

The figure which showed the relationship between a board | substrate oxygen precipitate density and the curvature amount after GaN layer formation.

  The nitride semiconductor forming substrate of the present invention can reduce warpage and suppress cracks when forming a nitride semiconductor layer.

  Further, according to the present invention, the nitride semiconductor forming substrate can form a thick nitride semiconductor layer, and is extremely useful as an electronic device made of a nitride-based semiconductor.

Claims (6)

A semiconductor substrate for forming a nitride semiconductor,
Boron and germanium are doped in silicon,
When the concentrations of boron and germanium are [B] atoms / cm ³ and [Ge] atoms / cm ³ , respectively.
The following conditions,
2.5 × 10 ¹⁸ ≦ [Ge] ≦ 2.5 × 10 ²⁰ ,
4.0 × 10 ¹⁶ ≦ [B] ≦ 4.0 × 10 ¹⁹ ,
A nitride semiconductor forming substrate characterized in that:   The nitride semiconductor forming substrate according to claim 1, further satisfying 5 [B] ≦ [Ge] ≦ 8 [B]. 1.5 × 10 ¹⁹ ≦ [Ge] ≦ 2.2 × 10 ²⁰ ,
3.0 × 10 ¹⁸ ≦ [B] ≦ 2.7 × 10 ¹⁹ ,
The substrate for forming a nitride semiconductor according to claim 2, wherein: Heat treatment at 800 ° C. for 4 hours in a nitrogen atmosphere,
Thereafter, after heat treatment at 1000 ° C. for 16 hours in an oxygen atmosphere,
The oxygen precipitate density inside the wafer is 1.0 × 10 ⁷ pieces / cm ³ or more and less than 2.0 × 10 ¹⁰ pieces / cm ^3. The nitride semiconductor forming substrate according to 1. 5. The oxygen concentration (ASTM F-121 1979) in a wafer is 9 × 10 ¹⁷ atoms / cm ³ or more and 14 × 10 ¹⁷ atoms / cm ³ or less. The substrate for forming a nitride semiconductor according to one.   A nitride semiconductor, comprising a nitride semiconductor thin film formed on the semiconductor forming substrate according to claim 1.

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