JP3293688B2 - Manufacturing method of semiconductor substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an SOI semiconductor substrate obtained by a bonding method.

[0002]

2. Description of the Related Art The formation of a single crystal Si semiconductor layer on an insulator is performed by using a silicon on insulator (SO
I) Much research has been done because this substrate has numerous advantages that are widely known as technology and cannot be reached with a bulk silicon substrate for making ordinary silicon integrated circuits. That is, by using the SOI technology,
1. Easy dielectric separation and high integration. 2. Excellent radiation resistance. Stray capacitance is reduced, enabling higher speed,
4. 4. Well step can be omitted. 5. Latch-up can be prevented. Advantages such as the possibility of a fully depleted field-effect transistor by thinning can be obtained.

[0003] One of the SOI technologies that has attracted the most attention in recent years is a technology commonly called "bonded SOI". In this method, the mirror surfaces of two wafers having at least one of which has an insulating film formed by oxidation or the like are brought into close contact with each other, heat treatment is applied to strengthen the bond at the contact interface, and then the substrate is polished from one side. Alternatively, this is a technique in which a silicon single crystal thin film having an arbitrary thickness is left on an insulating film by etching. Since the thin film obtained by this bonded SOI is originally a single crystal substrate itself, not only the controllability of the crystal orientation but also extremely few crystal defects, and the most excellent crystal perfection among many SOI technologies. It is thought that it is.

[0004]

However, there are still problems to be solved in the prior art using this bonding method.

The first problem is the controllability of the film thickness in the step of uniformly thinning one side of two bonded silicon substrates. That is, usually, a silicon substrate having a thickness of several hundred μm must be uniformly polished or etched to a thickness of several μm or 1 μm or less, which is technically extremely difficult in terms of controllability and uniformity. Since the distribution of the film thickness causes a variation in the electrical characteristics of elements formed thereon, it is urgently necessary to solve this problem.

As a second problem, an unbonded portion (hereinafter referred to as a "void") generated at an interface between two substrates in close contact with each other.
There is control. The voids are small (several μm
Dust etc. is one of the causes, but it is also simply bubbles taken in at the time of bonding, water vapor generated by a chemical reaction at the interface during heat treatment of the bonded substrate, or It has been reported that hydrocarbon-based contamination that has been physically adsorbed on the substrate surface before bonding may sometimes occur. The size of voids generated by these causes ranges from a diameter of 1 μm or less to several cm. If a void is generated when two substrates are bonded together,
Most of these void regions are missing when thinning by polishing or etching, and holes are formed in the film. Naturally, an SOI device cannot be formed in a region where the thin film is missing. Even if a thin film remains in the void region, there is a very high possibility that the void region will be lost due to the element forming process.

[0007] Here, the bonding mechanism according to the conventional technique will be described.

FIG. 2 shows two substrates 210, 203 and 20.
It is a schematic diagram which shows the bonding state of the atom of 3 'of bonding interface, (A) shows the state of the interface by the prior art for every heat treatment temperature, (B) is this invention. FIG. 4 is a schematic diagram showing a state of an interface according to the manufacturing method of FIG.

As shown in FIG. 2A, an OH group is formed on the wafer surface before bonding. When the wafer is brought into contact at room temperature, the OH group causes a hydrogen bond and adheres as shown in FIG. As described above, the presence of minute dust adhering to the interface causes poor adhesion and causes voids. Adhesion at this time is weak 3
５5 kg / cm ² .

Next, heat treatment is performed to strengthen the bonding at the contact interface. At this time, first, a dehydration condensation reaction occurs, and the hydrogen bond is changed to a Si—O—Si bond as shown in (c), and at a higher temperature, Si atoms may be directly bonded as shown in (d). .

However, in the conventional bonding method,
As shown in FIGS. 2C and 2D, there is a problem that the possibility that a dangling bond is formed at the bonding interface by the termination of the Si atom occurs at a considerable probability.

The dangling bond increases the interface state between the single crystal Si of the SOI substrate and the insulator layer (I layer), and has a problem that voids are formed depending on the density.

[0013] Since a complete solution to these problems has not yet been found, a bonded SOI has the potential to provide the highest quality single crystal thin film in SOI technology, but has high quality and high reliability. Has not yet been produced.

As described above, a technique capable of providing a high-quality SOI substrate sufficient for manufacturing a high-performance electronic device with high productivity has not yet been achieved.

[Object of the Invention] An object of the present invention is to manufacture a high-performance SOI substrate by a bonding method.
While providing substrates with good film thickness distribution with high productivity,
A method for manufacturing an SOI substrate in which the bonding between single crystal Si and an insulator layer (I layer) is made more reliable and of high quality, thereby reducing the interface state and suppressing the generation rate of minute voids. Is to provide.

[0016]

According to the present invention, as a means for solving the above-mentioned problems, two substrates, at least one of which is a Si substrate having an oxide film, are bonded via the oxide film. In the method for manufacturing a semiconductor substrate having a step, an impurity having a stronger bond with Si than hydrogen is introduced into the surface of the oxide film before the bonding step. . Ma
The present invention also provides a non-porous single crystal silicon layer on a porous silicon layer.
A first substrate having a silicon oxide layer via a recon layer
Preparing a silicon oxide layer and a second silicon
Bonded to a substrate to form a multilayer structure,
By removing the porous silicon layer from the structure
Through the silicon oxide layer on the second substrate.
Fabrication of semiconductor substrate to form non-porous single-crystal silicon layer
In the manufacturing method, before the bonding, the silicon oxide is used.
Impurities whose bonding to silicon is stronger than hydrogen
By introducing and then performing a heat treatment after the bonding.
The bonding force between the silicon oxide layer and the second substrate.
It is characterized by strengthening.

Further , a non-porous single bond is formed on the porous silicon layer.
The first substrate having an oxide layer via a crystalline silicon layer
The oxide layer is formed between the first substrate and the second substrate.
Laminate so that it is located inside to form a multilayer structure
Bonding step, wherein the non-porous layer is formed on the oxide layer.
Method for manufacturing a semiconductor substrate having a porous single-crystal silicon layer
Before the bonding step, the oxide layer surface is impure.
A method of manufacturing a semiconductor substrate characterized by introducing a substance.
There is also.

Also, a non-porous single bond is formed on the porous silicon layer.
Providing a first substrate having a crystalline silicon layer,
First substrate and second substrate having silicon oxide layer on surface
Bonded so that the silicon oxide layer is located inside
Including a bonding step of forming a multilayer structure,
The non-porous single-crystal silicon layer on the silicon oxide layer
In the method for manufacturing a semiconductor substrate having
Before the process, impurities are introduced into the surface of the silicon oxide layer.
The present invention also provides a method for manufacturing a semiconductor substrate.

Also, a non-porous single bond is formed on the porous silicon layer.
Providing a first substrate having a crystalline silicon layer,
Bonding the first substrate and the second substrate via an oxide layer,
A bonding step of forming a multilayer structure,
Having the non-porous single-crystal silicon layer on a passivation layer
In the method for manufacturing a substrate, before the bonding step,
A semiconductor characterized by introducing impurities to the surface of the oxide layer
It is also a method of manufacturing a body substrate.

[0020]

According to the present invention, two physical effects of the porous silicon play an important role in solving the problem of uneven distribution of the film thickness.

One is the etching characteristics of porous silicon. Normally, silicon is hardly etched with hydrofluoric acid, but by making it porous, etching with hydrofluoric acid becomes possible. In addition, when a mixed etching solution of hydrofluoric acid, aqueous hydrogen peroxide and alcohol is used, an etching rate ratio of about 10 times or more can be obtained for non-porous and porous materials. Therefore, even a thin layer having a thickness of about 1 μm can be selectively etched uniformly and with good controllability.

Another effect is an epitaxial growth characteristic. Porous silicon has a single crystal structure as a crystal structure, and has a thickness of several tens to several hundreds
The holes having a large diameter exist at a high density. The epitaxial layer grown on this surface has the property that the same crystallinity as the epitaxial layer on the non-porous single crystal substrate can be obtained. Therefore, since a single crystal thin film equivalent to an epitaxial layer on a single crystal silicon substrate having high reliability is used as an active layer, an SOI substrate having better crystallinity than a conventional SOI substrate can be provided.

Of the above problems, in order to solve the problem of voids to the element, the adhesion at the bonding interface is increased and the interface state between the single crystal Si and the insulator layer (I layer) is reduced. Therefore, the introduction of impurities into the oxide film surface of the present invention plays an important role.

That is, when no impurity is introduced, there is a considerable probability that the termination of Si atoms at the interface forms dangling bonds. In this case, depending on the density, the thin film is separated from the substrate in the absence of the supporting base, and if a void is generated, the film is peeled. Further, when the termination of the Si atom forms a dangling bond, the interface state increases and the device characteristics are changed. For this, there is a method of fixing the potential of a substrate serving as a supporting base to stabilize the element characteristics, but this is not possible in the case of a complementary MOS.

Therefore, in the present invention, by introducing an impurity into the oxide film surface, the impurity terminates the dangling bond at the interface, thereby suppressing the generation of minute voids, enhancing the adhesion, and improving the interface state. Order reduction can be achieved.

FIG. 2B is a schematic view showing the bonding state of the bonding interface between the two substrates 210 and 203 showing the method of the present invention. As shown in FIGS. 3C and 3D, according to the present invention, by introducing fluorine or the like having a larger bonding force than Si as hydrogen into the silicon oxide surface as the impurity 207 in advance, Si Terminating with fluorine or the like, it is possible to prevent generation of dangling bonds.

(Embodiment) FIG. 1 shows an embodiment of the present invention.
This will be described with reference to FIG.

Description of FIG. 1A: A single-crystal silicon substrate 100 is anodized to form porous silicon 101. At this time, the region to be made porous may be only one surface layer of the substrate or the entire substrate. When only one surface layer is made porous, the region may have a thickness of 10 to 100 μm.

A method for forming porous silicon will be described with reference to FIG.

First, a P-type single crystal silicon substrate 300 is prepared as a substrate. Although it is not impossible even with an N-type, in that case, the substrate is limited to a low-resistance substrate. FIG.
Set in the device as shown in (a). That is, one side of the substrate is in contact with the hydrofluoric acid-based solution 304, the solution side is provided with a negative electrode 306, and the opposite side is provided with a positive metal electrode 3
It touches 05.

As shown in FIG. 3 (b), the positive electrode side 305 'may also take a potential via the solution 304'.

In any case, porosity occurs from the negative electrode side in contact with the hydrofluoric acid-based solution. Hydrofluoric acid solution 304
In general, concentrated hydrofluoric acid (49% HF) is used. Diluting with pure water (H ₂ O) is not preferable because etching occurs at a certain concentration, depending on the value of the current flowing.

During the anodization, bubbles are generated from the surface of the substrate 300, and alcohol may be added as a surfactant for the purpose of efficiently removing the bubbles. As the alcohol, methanol, ethanol, propanol, isopropanol and the like are used. Alternatively, anodizing may be performed while stirring the solution using a stirrer instead of the surfactant.

For the negative electrode 306, a material that is not eroded by a hydrofluoric acid solution, for example, gold (Au), platinum (Pt), or the like is used. The material of the positive electrode 305 may be a commonly used metal material, but when the anodization is performed on all the substrates 300, the hydrofluoric acid solution 304 is used.
Reaches the positive electrode 305, the surface of the positive electrode 305 may be coated with a metal film resistant to hydrofluoric acid solution.

The current value for anodizing is up to several hundred mA / c.
m ² and the minimum value need not be zero. This value is determined within a range where good quality epitaxial growth can be performed on the surface of the porous silicon. Usually, when the current value is large, the rate of anodization increases, and at the same time, the density of the porous silicon layer decreases. That is, the volume occupied by the holes increases. This changes the conditions for epitaxial growth.

Description of FIG. 1B: The porous silicon substrate or porous layer 10 formed as described above
1, a non-porous single-crystal silicon layer 102 is epitaxially grown. Epitaxial growth is common thermal CV
D, low pressure CVD, plasma CVD, molecular beam epitaxy, sputtering or the like. The grown film thickness may be the same as the design value of the SOI layer.

Description of FIG. 1C: The surface of the grown epitaxial layer 102 is oxidized to form a SiO ₂ layer 103.
To form This oxide film 103 becomes an insulator layer (I layer) having an SOI structure. Oxidizing the epitaxial layer 102 also means reducing the interface state density of the epitaxial layer 102, which is the active layer, with the underlying insulator interface when forming a device on the completed SOI substrate.
At this time, the thickness of the epi oxide film is preferably 0.5 to 1.0 μm in order to make use of the characteristics of the SOI device.

Description of FIG. 1D: Impurity 107 is introduced into the surface of oxide layer 103. The impurities 107 are fluorine, a fluorine compound, carbon and a carbon compound,
Anything that has a stronger bond with Si than hydrogen can be used. The method of introduction is not particularly limited, and an ion implantation method, solid phase diffusion, spin coating, a CVD method, or the like is used. The introduction amount of the impurity 107 may be arbitrarily determined, but the greater the introduction amount, the higher the probability of terminating dangling bonds at the interface. This leads to a reduction in interface states and suppression of generation of minute voids. Therefore, the impurity 10 should be within a range that can be easily prepared under the normal process conditions of the equipment used.
7 may be determined. For example, when the ion implantation method is used to introduce the impurity 107, the dose amount is 1 × 1
It is about 0 ¹⁵ to 1 × 10 ¹⁶ [atom / cm ² ].

When the impurity 107 is introduced by ion implantation or the like, the epitaxial growth layer 102 may become amorphous. This becomes a single crystal again after the heat treatment, but there is a step of performing a heat treatment after the subsequent bonding, so that it is not necessary to perform a special heat treatment.

Further, when the impurity 107 is introduced by the CVD method, there may be considerable unevenness on the surface.
When the substrates are bonded, the flatness of the substrate surface is important. Therefore, when unevenness occurs, the substrate surface is polished and flattened, or after the impurity 107 is introduced, the surface is flattened by etching. Good to do.

The substrate obtained by the above steps is hereinafter referred to as "first substrate".

Description of (FIG. 1E): a first substrate;
The separately prepared second substrates 110 are bonded to each other with a mirror surface, and subsequently the bonded substrates are subjected to a heat treatment. The heat treatment is performed at a temperature at which a bonding force that does not cause peeling of the bonding interface or the like is obtained in the next polishing or etching step. Specifically, about 100 ° C. or higher is preferable. However, if the polishing is performed at a relatively low temperature, polishing or etching is completed, and after obtaining the final SOI form, 1000
It is preferable to perform a heat treatment at about ° C. This is to prevent film peeling and the like due to thermal stress during the device process.

The second substrate 110 is completely arbitrary, and may be selected from a silicon substrate, a quartz substrate, another ceramic substrate, or the like.

Description of FIG. 1 (f): Next, the porous portion 101 and the like are selectively removed from the first substrate side except the epitaxial growth layer 102. If the part to be removed at this time is entirely porous, the whole of the porous part 101 is selectively etched by immersing the whole of the bonded substrates in a hydrofluoric acid-based solution. In the case where the portion to be etched includes a region where the single crystal silicon substrate 100 remains, it is preferable to remove only the region of the silicon substrate 100 by polishing. Polishing is completed when the porous portion 101 is exposed, and selective etching can be performed later in a hydrofluoric acid-based solution. In any case, the non-porous single crystal epitaxially grown portion 102 hardly reacts with hydrofluoric acid and remains as a thin film.

If the second substrate 110 is mainly composed of SiO ₂ , it easily reacts with a hydrofluoric acid-based solution. Therefore, a silicon nitride film is previously formed on the surface opposite to the bonding surface by CVD or the like. And other substances that are difficult to react with hydrofluoric acid should be deposited. Alternatively, if the porous portion 101 is thinned to some extent before the substrate is immersed in the etching solution, the time required for the porous selective etching can be shortened, and the second substrate does not need to react much. Of course the second
There is no problem if the substrate does not react with hydrofluoric acid such as silicon.

As the hydrofluoric acid solution used for the selective etching, a mixture of hydrofluoric acid, hydrogen peroxide (H ₂ O ₂ ) and alcohols is used. Selective etching of porous silicon is possible with hydrofluoric acid and nitric acid, or a mixture of acetic acid and acetic acid. However, in this case, the remaining epitaxial silicon film 102 is also etched to some extent. There is.

Through the above steps, an SOI substrate having the deposit 107, the silicon oxide film 103, and the epitaxial silicon layer 102 sequentially on the second substrate 110 is obtained.

[0048]

The present invention will be described in detail with reference to specific examples, but the present invention is not limited to these examples.

Example 1 Description of FIG. 1A: A 4-inch P-type (100) single-crystal silicon substrate having a thickness of 200 microns (0.1
.About.0.2 .OMEGA.cm), which was set in a device as shown in FIG. 3A and anodized to obtain porous silicon 101. At this time, the solution 304 used was a 49% HF solution, and the current density was 100 mA / cm ² . At this time, the rate of making porous is 8.4 μm / min. The P-type (100) silicon substrate having a thickness of 200 μm was entirely made porous in 24 minutes.

Description of (FIG. 1B): The P-type (10
0) A single-crystal silicon layer 102 was epitaxially grown on the porous silicon substrate 101 by a CVD method to a thickness of 1.0 μm. The deposition conditions are as follows.

Gas used: SiH ₄ / H ₂ gas flow rate: 0.62 / 140 (1 / min) Temperature: 750 ° C. Pressure: 80 Torr Growth rate: 0.12 μm / min. Description of FIG. 1C: Epitaxial growth layer 102
Is oxidized in a steam atmosphere at 1000 ° C.
A μm silicon oxide film 103 was obtained.

Description of FIG. 1D: Monovalent F ions 107 are formed on the oxide film 103 at 5 × 10 ¹⁵ [atom / c].
m ² ] and 30 [keV]. The substrate thus formed was used as a first substrate.

Description of (FIG. 1E): A 4-inch silicon substrate 110 was prepared as a second substrate, and was cleaned in a CH1: H ₂ O ₂ : H ₂ O solution together with the first substrate. After sufficient washing with water, the mirror surfaces of the first and second substrates were bonded together. Further, this substrate was subjected to a heat treatment at 1100 ° C. for 2 hours in a nitrogen atmosphere to strengthen the bonding force at the interface of the bonded substrates.

Description of FIG. 1 (f): The substrate adhered after the heat treatment is immersed in a selective etching solution,
Only 01 was selectively etched. At this time, the composition of the etching solution and the etching rate for the porous silicon were as follows: HF: H ₂ O ₂ : C ₂ H ₅ OH = 5: 25: 6 1.6 μm / min. Met. Thus, a 200 μm porous section is about 125
Everything was etched in minutes. Incidentally, the etching rate of the single crystal silicon layer 102 at this time is 0.0006 μm.
m / hour and remained almost without being etched.

Through the above steps, the silicon oxide film 103 and the epitaxial growth layer 10 are formed on the silicon substrate 110.
2 were sequentially obtained.

In order to compare the effects of voids, an SOI substrate (hereinafter referred to as a comparative SOI substrate) prepared by omitting only the step of implanting F ions 107 on the silicon oxide film 103 was observed with an optical microscope. 0.1 μm or more
Fine voids of about 1 μm were present at a density of about 5 / cm ² , and about half of these voids had film breakage. On the other hand, no void was observed with the same optical microscope in the SOI substrate implanted with the F ions 107, which was formed in the above process of this example.

The adhesion was measured for the above two types of SOI substrates using a tensile tester. The comparison SOI substrate was 600 to 700 [kg / cm ² ]. SOI with ion implantation
The substrate had a weight of 700 to 800 [kg / cm ² ].

Subsequently, the two types of SOI substrates have 100
Heat treatment is performed in an oxygen atmosphere at 0 ° C. to form a gate oxide film of 1000 [Å]. After that, the EB evaporation machine
(Aluminum) was formed, and this was patterned using a photolithography technique to form an electrode.

The two types of SO prepared by the above steps
When the CV characteristics were measured for the I substrate, the fixed charge density was 10 ¹² [cm ⁻² ] for the comparative SOI substrate, and 10 ¹⁰ [c] for the SOI substrate implanted with the F ions 107.
m ^-2 ] and two digits. This is due to the gate oxide and Si
Obviously because there is no difference between the interfaces, Si and the insulating film layer (I layer)
And the difference between the interfaces.

Example 2 In this example, a substrate having an oxide film on the surface was used as the second substrate, and impurities characteristic of the present invention were introduced into the surface of the oxide film of the second substrate.

A silicon single crystal surface of a first substrate having an epitaxial growth layer on a porous silicon substrate,
The substrate was similarly bonded to the surface of the oxide film into which impurities were introduced, and the porous portion was removed by etching.

With respect to the SOI substrate obtained in this embodiment,
When the adhesion and the CV characteristics were measured, the same effects as those of the SOI substrate obtained in Example 1 were obtained.

It should be noted that the method for preventing dangling bonds by introducing impurities into the oxide film on the bonding surface according to the present invention can be applied as long as it is a method of manufacturing a semiconductor substrate having a bonding step. The effect can be obtained.

[0064]

As described above, according to the present invention,
A single-crystal epitaxial growth layer and an oxide film are formed on porous silicon, and the first substrate is bonded to an arbitrary second substrate, heat treatment is performed, and porous silicon is selectively removed. In a bonded SOI substrate, a step of introducing an impurity into an oxide film at a bonding interface is provided before a bonding step, so that Si atoms are terminated with the impurity, and a dangling bond generated at the bonding interface is terminated by the impurity. As a result, the interface state can be reduced.

Further, by strengthening the bonding at the bonding interface, minute voids could be eliminated.

Thus, all substrates in which voids have occurred at the bonding interface have been treated as defective products. However, according to the present invention, the occurrence of voids can be tolerated to some extent. Improved.

Further, by reducing the interface state, the effect of increasing the speed peculiar to the SOI substrate and omitting the well process can be made more ideal.

Further, an SOI substrate having a uniform film thickness could be obtained by the porous selective etching.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a process of the present invention.

FIG. 2 is a schematic diagram of a bonding process and a mechanism at a bonding interface of the present invention and a conventional example.

FIG. 3 is a schematic view of an apparatus used for making a silicon substrate porous.

[Explanation of symbols]

 100, 300 single crystal silicon substrate 101 porous silicon substrate 102 epitaxial growth layer 103, 203 oxide film of epitaxial layer 107, 207 impurity 110, 210 single crystal silicon substrate 304, 304 'etching solution 305, 305' positive electrode 306, 306 'negative electrode

Continuation of the front page (56) References JP-A-5-47726 (JP, A) JP-A-5-55230 (JP, A) JP-A-4-346418 (JP, A) JP-A-4-137625 (JP) JP-A-5-198549 (JP, A) JP-A-1-137652 (JP, A) JP-A-2-9127 (JP, A) JP-A-2-18961 (JP, A) 3-91227 (JP, A) Takayuki Takahagi, "Inactivation of Si Surface", Applied Physics, November 5, 1990, Vol. 11, pp. 1441-1450 R.C. STENGL, et. al. , "A Model for the Silicon Wafer Bonding Process", Jpn. J. A ppl. Phys. , October 31, 1989, VOL. 28, NO. 10, pp. 1735− 1741 (58) Surveyed fields (Int.Cl. ⁷ , DB name) H01L 27/12 H01L 21/02 H01L 21/26-21/268 H01L 21/322-21/326 H01L 21/304

Claims (5)

(57) [Claims] A non-porous single-crystal silicon layer is formed on a porous silicon layer.
A first substrate having a silicon oxide layer via a recon layer
Preparing the silicon oxide layer and a second substrate made of silicon;
To form a multilayer structure, and removing the porous silicon layer from the multilayer structure.
With this, the silicon oxide layer is interposed on the second substrate.
Semiconductor substrate forming the non-porous single crystal silicon layer by
In the method of manufacturing a plate, before the bonding , a silicon oxide layer is formed on the surface of the silicon oxide layer.
By introducing an impurity having a bond with hydrogen that is stronger than that of hydrogen and performing heat treatment after the bonding, the bonding strength between the silicon oxide layer and the second substrate is increased.
A method for manufacturing a semiconductor substrate, comprising: Wherein said impurity is a method for manufacturing a semiconductor substrate according to claim 1, wherein the fluorine or fluorine compound. Wherein the impurity to a method for manufacturing a semiconductor substrate according to claim 1, wherein the carbon or carbon compound. Wherein the introduction of said impurities, ion implantation method, solid phase diffusion method, a method for manufacturing a semiconductor substrate according to claim 1, which is selected from among a spin coating method and CVD method. 5. The method according to claim 1, wherein the dose of the introduced impurity is 1 × 1.
^{2. The} method of manufacturing a semiconductor substrate according to claim 1 , wherein the concentration is from 0 ¹⁵ atom / cm ² to 1 × 10 ¹⁶ atom / cm ² .