阅读说明：本技术 外延硅晶片的杂质吸附能力的评价方法及外延硅晶片 (Method for evaluating impurity adsorption capacity of epitaxial silicon wafer and epitaxial silicon wafer ) 是由 重松理史 奥山亮辅 栗田一成 于 2018-01-12 设计创作，主要内容包括：本发明提供一种外延硅晶片的杂质吸附能力的评价方法,其能够以高精度对形成于外延层正下方且固溶有碳的改性层中的杂质的吸附举动进行评价。本公开的方法的特征在于,通过三维原子探针法分析形成于外延层正下方且固溶有碳的改性层,并根据通过该分析获得的所述改性层中的碳的三维分布图,对所述改性层中的杂质的吸附能力进行评价。(the invention provides a method for evaluating impurity adsorption capacity of an epitaxial silicon wafer, which can evaluate the adsorption behavior of impurities in a modified layer which is formed under an epitaxial layer and has carbon dissolved therein with high precision. The method of the present disclosure is characterized by analyzing a modified layer formed directly below an epitaxial layer and having carbon dissolved therein by a three-dimensional atom probe method, and evaluating the adsorption capacity of impurities in the modified layer based on a three-dimensional distribution map of carbon in the modified layer obtained by the analysis.)

1. A method for evaluating impurity adsorption capacity of an epitaxial silicon wafer, the epitaxial silicon wafer being produced by the steps of:

A step 1 of implanting ions containing carbon from the surface of a silicon wafer to form a modified layer in which carbon is dissolved in the surface layer of the silicon wafer; and

A 2 nd step of forming an epitaxial layer on the modified layer of the silicon wafer,

The method for evaluating the impurity adsorption capability of an epitaxial silicon wafer is characterized in that,

Performing a heat treatment on the silicon wafer after the 1 st step,

Then, the modified layer of the silicon wafer was analyzed by a three-dimensional atom probe method,

and evaluating the adsorption capacity of impurities in the modified layer based on the three-dimensional distribution map of carbon in the modified layer obtained by the analysis.

2. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to claim 1, wherein,

Determining a carbon aggregate in a range of a depth of 200nm from a surface of the modified layer in the three-dimensional distribution map of carbon,

Obtaining the concentration distribution of the carbon aggregate and the carbon and impurities around the carbon aggregate,

From this concentration distribution, the adsorption capacity of impurities in the modified layer was evaluated.

3. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to claim 1, wherein,

determining a carbon aggregate in a range of a depth of 200nm from a surface of the modified layer in the three-dimensional distribution map of carbon,

the adsorption capacity of impurities in the modified layer was evaluated based on the density of the carbon aggregates.

4. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to claim 1, wherein,

in the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

Further, a 2 nd equal concentration surface with the impurity concentration of a predetermined value is prepared in the three-dimensional distribution diagram,

and evaluating the adsorption capacity of the impurities in the modified layer according to the position relation of the 1 st equal concentration surface and the 2 nd equal concentration surface.

5. the method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to claim 1, wherein,

in the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

Using the 1 st iso-concentration surface as a reference surface, a near histogram of carbon and impurities is prepared,

From the proximity histogram, the adsorption capacity of the impurities in the modified layer was evaluated.

6. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to claim 1, wherein,

In the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

The adsorption capacity of impurities in the modified layer was evaluated based on the density of spheroids formed on the 1 st iso-concentration surface.

7. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to any one of claims 1 to 6, wherein,

the impurities are heavy metal elements.

8. the method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to any one of claims 1 to 7, wherein,

The heat treatment is performed in a non-oxidizing atmosphere at 900 ℃ to 1300 ℃.

9. the method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to any one of claims 1 to 8, wherein,

the heat treatment is a heat treatment at the time of epitaxial growth in the step 2.

10. The method of evaluating impurity adsorption capacity of an epitaxial silicon wafer according to any one of claims 1 to 9, wherein,

The ions containing carbon are cluster ions containing carbon and hydrogen.

11. an epitaxial silicon wafer, comprising:

a silicon wafer;

A modified layer formed on a surface layer portion of the silicon wafer and having carbon dissolved in the silicon wafer; and

an epitaxial layer formed on the modified layer,

In a three-dimensional distribution diagram of carbon obtained by analyzing the modified layer by a three-dimensional atom probe method, carbon aggregates are present in a range of a depth of 200nm from the surface of the modified layer, and the carbon aggregates have an average diameter of 5nm or more and a density of 1 × 1015 pieces/cm 3 or more.

Technical Field

The present invention relates to a method for evaluating impurity adsorption capacity of an epitaxial silicon wafer, and an epitaxial silicon wafer.

Background

An epitaxial silicon wafer obtained by forming an epitaxial layer of a single crystal silicon on a silicon wafer has been developed from various viewpoints to be used as a raw material wafer in the production of various semiconductor devices such as a solid-state imaging device. For example, when the epitaxial layer is contaminated with a heavy metal, the dark current of the solid-state imaging element increases, and a defect called a white damage defect occurs.

Therefore, in order to suppress such heavy metal contamination, a technique of forming an adsorption site for capturing heavy metals in advance just below an epitaxial layer of an epitaxial silicon wafer has been developed. As one of the methods, a technique of forming adsorption sites in a silicon wafer by ion implantation is known.

for example, patent document 1 describes the following production method: after carbon ions are implanted from one surface of the silicon wafer to form a carbon ion implanted region, a silicon epitaxial layer is formed on the surface to form an epitaxial silicon wafer.

Further, patent document 2 describes a method for producing an epitaxial silicon wafer, the method comprising: a first step of irradiating the surface of a silicon wafer with cluster ions containing carbon to form a modified layer containing carbon dissolved therein on the surface layer portion of the silicon wafer; and a step 2 of forming an epitaxial layer on the modified layer of the silicon wafer.

Further, patent document 3 describes, as an improvement technique of the technique described in patent document 2, that the adsorption capability of heavy metals can be improved by irradiating cluster ions so that a part of the modified layer in the thickness direction becomes an amorphous layer.

disclosure of Invention

Technical problem to be solved by the invention

In the techniques for forming an epitaxial layer after implanting carbon ions into a silicon wafer as in patent documents 1 to 3, it is considered that the carbon ion implanted region functions as an adsorption site. However, the detailed adsorption behavior in the carbon ion implantation region is not sufficiently clear at present.

patent document 1 describes that ion-implanted carbon accelerates oxygen precipitation to form high-density crystal defects, which serve as adsorption sites, and that Si and carbon generate stress due to a difference in covalent bond radius, and that the stress itself also serves as an adsorption site. However, no analysis was performed on the carbon ion-implanted region, and the mechanism of adsorption was limited to the range assumed.

in patent document 2, the depth direction of an epitaxial silicon wafer is analyzed by Secondary Ion Mass Spectrometry (SIMS), and it is clarified that a peak of a carbon concentration distribution exists in a modified layer. Further, SIMS measurement was performed after the forced contamination test with nickel and copper was performed, and it was found that there was a peak in the concentration distribution of nickel and copper at the position of the peak in the concentration distribution of carbon. However, this result also shows only the case where nickel and copper are adsorbed in the carbon ion implanted region (modified layer), and does not show the detailed adsorption behavior of the heavy metal in the modified layer.

In patent document 3, the cross section of the modified layer is observed with a Transmission Electron Microscope (TEM), and it is confirmed that a part of the modified layer in the thickness direction is an amorphous layer. Further, it was found by TEM observation that after the epitaxial layer was formed, the amorphous layer was recovered by the heat treatment at that time and the crystallinity was lost, and instead, black-dot defects were generated in the modified layer. However, this document only shows that the adsorption capacity is high in this case, and detailed adsorption behavior of the heavy metal in the modified layer is not clarified.

As described above, it is important in the design guidelines of ion irradiation and heat treatment conditions in the future to clarify the adsorption behavior of impurities in the modified layer, which has not been clarified by the conventional methods such as SIMS and TEM applied to the analysis of the ion-implanted region directly below the epitaxial layer, and to evaluate the impurity adsorption capability of the epitaxial silicon wafer.

In view of the above-described problems, it is an object of the present invention to provide a method for evaluating the impurity adsorption capability of an epitaxial silicon wafer, which can evaluate the adsorption behavior of impurities in a modified layer formed directly below an epitaxial layer and having carbon dissolved therein with high accuracy, and an epitaxial silicon wafer having excellent adsorption capability evaluated by the evaluation method.

Means for solving the technical problem

In order to solve the above-described problems, the present inventors considered to apply the three-dimensional atom probe (3DAP) method to analysis of a modified layer formed directly below an epitaxial layer. The 3DAP method has higher resolution than SIMS, and can perform three-dimensional element mapping/analysis as well as cross-section or surface observation as in TEM. Therefore, it is considered that a three-dimensional distribution map of carbon or a three-dimensional distribution map of impurities as a subject is obtained by analyzing the modified layer by the 3DAP method.

as a result, the following findings were obtained. First, immediately after the modified layer is formed by carbon ion implantation, carbon atoms are uniformly distributed in the modified layer even if the implanted ions are cluster ions. Further, it was found that by performing heat treatment after ion implantation, carbon atoms are aggregated in the modified layer, particularly in the region of about 200nm in the surface layer, to form an aggregate. Then, it was confirmed that the adsorbed impurities were segregated to the carbon aggregate and the periphery thereof in the modified layer. As described above, the present inventors have found that the adsorption behavior of impurities in a modified layer can be evaluated with high accuracy by applying the 3DAP method to the analysis of the modified layer, and have completed the present invention.

the gist of the present invention completed based on the above findings is as follows.

(1) a method for evaluating impurity adsorption capacity of an epitaxial silicon wafer, the epitaxial silicon wafer being produced by the steps of:

a step 1 of implanting ions containing carbon from the surface of a silicon wafer to form a modified layer in which carbon is dissolved in the surface layer of the silicon wafer; and

A 2 nd step of forming an epitaxial layer on the modified layer of the silicon wafer,

The method for evaluating the impurity adsorption capability of an epitaxial silicon wafer is characterized in that,

Performing a heat treatment on the silicon wafer after the 1 st step,

then, the modified layer of the silicon wafer was analyzed by a three-dimensional atom probe method,

and evaluating the adsorption capacity of impurities in the modified layer based on the three-dimensional distribution map of carbon in the modified layer obtained by the analysis.

(2) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to the above (1), wherein,

Determining a carbon aggregate in a range of a depth of 200nm from a surface of the modified layer in the three-dimensional distribution map of carbon,

Obtaining the concentration distribution of the carbon aggregate and the carbon and impurities around the carbon aggregate,

From this concentration distribution, the adsorption capacity of impurities in the modified layer was evaluated.

(3) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to the item (1), wherein,

Determining a carbon aggregate in a range of a depth of 200nm from a surface of the modified layer in the three-dimensional distribution map of carbon,

The adsorption capacity of impurities in the modified layer was evaluated based on the density of the carbon aggregates.

(4) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to the item (1), wherein,

in the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

Further, a 2 nd equal concentration surface with the impurity concentration of a predetermined value is prepared in the three-dimensional distribution diagram,

And evaluating the adsorption capacity of the impurities in the modified layer according to the position relation of the 1 st equal concentration surface and the 2 nd equal concentration surface.

(5) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to the above (1), wherein,

In the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

Using the 1 st iso-concentration surface as a reference surface, a proximity histogram (program) of carbon and impurities is prepared,

From the proximity histogram, the adsorption capacity of the impurities in the modified layer was evaluated.

(6) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to the above (1), wherein,

In the three-dimensional distribution diagram of carbon, a 1 st equal concentration surface with a carbon concentration of a predetermined value is prepared in a depth range of 200nm from the surface of the modified layer,

The adsorption capacity of impurities in the modified layer was evaluated based on the density of spheroids formed on the 1 st iso-concentration surface.

(7) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to any one of the above (1) to (6), wherein,

the impurities are heavy metal elements.

(8) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to any one of the above (1) to (7), wherein,

the heat treatment is performed in a non-oxidizing atmosphere at 900 ℃ to 1300 ℃.

(9) The method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to any one of the above (1) to (8), wherein,

The heat treatment is a heat treatment at the time of epitaxial growth in the step 2.

(10) the method for evaluating impurity adsorption capability of an epitaxial silicon wafer according to any one of the above (1) to (9), wherein,

The ions containing carbon are cluster ions containing carbon and hydrogen.

(11) An epitaxial silicon wafer, comprising:

A silicon wafer;

A modified layer formed on a surface layer portion of the silicon wafer and having carbon dissolved in the silicon wafer; and

An epitaxial layer formed on the modified layer,

In a three-dimensional distribution diagram of carbon obtained by analyzing the modified layer by a three-dimensional atom probe method, carbon aggregates are present in a range of a depth of 200nm from the surface of the modified layer, and the carbon aggregates have an average diameter of 5nm or more and a density of 1 × 1015 pieces/cm 3 or more.

Effects of the invention

according to the method for evaluating the impurity adsorption capability of an epitaxial silicon wafer of the present invention, the adsorption behavior of impurities in a modified layer formed directly below an epitaxial layer and having carbon dissolved therein can be evaluated with high accuracy. The epitaxial silicon wafer of the present invention is excellent in the adsorption capability evaluated by the evaluation method.

Drawings

FIG. 1 is a schematic diagram illustrating the measurement principle of the three-dimensional atom probe (3DAP) method.

Fig. 2 is a view relating to experimental example 1, (a) is a three-dimensional distribution diagram of carbon obtained by the 3DAP method in the surface portion of the silicon wafer immediately after C3H5 cluster ions were implanted, and (B) is a three-dimensional distribution diagram of carbon obtained by the 3DAP method in the surface portion of the silicon wafer after heat treatment at 1100 ℃ for 30 minutes after C3H5 cluster ions were implanted.

fig. 3 is a view focusing on the carbon aggregate surrounded by a circle in the three-dimensional distribution chart of fig. 2(B), and showing the concentration distribution of carbon and oxygen along the radial direction from the center of the carbon aggregate.

fig. 4 is a schematic diagram for explaining the definition of the carbon aggregate in the present embodiment.

Fig. 5 is a graph showing the relationship between the implanted carbon dose and the carbon aggregate density in experimental example 2.

Fig. 6 shows C, O and Cu concentration distributions in the depth direction from the epitaxial layer surface obtained by SIMS measurement in experimental example 3.

fig. 7 is a three-dimensional distribution diagram obtained by the 3DAP method for experimental example 3, where (a) is an atomic distribution diagram of C, (B) is an atomic distribution diagram of Cu, and (C) is an atomic distribution diagram of C and Cu.

Fig. 8 is a graph showing an iso-concentration plane of a C concentration of 3.0% and an iso-concentration plane of a Cu concentration of 0.5% in a three-dimensional distribution chart, with respect to experimental example 3.

fig. 9 is a histogram of C and Cu in the experimental example 3, with the C concentration of 3.0% as a reference plane.

Fig. 10 is a graph showing an iso-concentration plane of a C concentration of 3.0%, an iso-concentration plane of a Cu concentration of 0.5%, and an iso-concentration plane of an O concentration of 1.0% in a three-dimensional distribution chart, with respect to experimental example 3.

Fig. 11 is an approximate histogram of C, Cu and O with the C concentration of 3.0% as the reference surface for experimental example 3.

Fig. 12 is a conceptual diagram of a proximity histogram.

Detailed Description

(method of evaluating impurity adsorption capability of epitaxial silicon wafer)

One embodiment of the present invention is a method for evaluating impurity adsorption capacity of an epitaxial silicon wafer, which is manufactured through the following steps: a step 1 of implanting ions containing carbon from the surface of a silicon wafer to form a modified layer in which carbon is dissolved in the surface layer of the silicon wafer; and a step 2 of forming an epitaxial layer on the modified layer of the silicon wafer.

examples of the silicon wafer include a bulk single crystal silicon wafer having no epitaxial layer on the surface. Further, carbon and/or nitrogen may be added to the silicon wafer in order to obtain higher adsorption capacity. Furthermore, a so-called n + type or p + type or n-type or p-type substrate may be formed by adding an arbitrary dopant to a silicon wafer at a predetermined concentration.

Further, as the silicon wafer, an epitaxial silicon wafer in which a silicon epitaxial layer is formed on the surface of a bulk single crystal silicon wafer may be used. The silicon epitaxial layer can be formed by a CVD method under normal conditions. The thickness of the epitaxial layer is preferably in the range of 0.1 to 10 μm, more preferably in the range of 0.2 to 5 μm.

Examples of the ion containing carbon include a monomer ion (monoanion) of carbon. The acceleration voltage of the monomer ions is usually set to 150 to 2000keV/atom, and it is preferable to set the acceleration voltage within this range. The dose of the monomer ions is also not particularly limited, and may be, for example, 1X 1013 to 1X 1016atoms/cm 2.

As the ion containing carbon, a cluster ion containing carbon can be suitably exemplified. In the present specification, the "cluster ion" refers to an ion ionized by imparting a positive or negative charge to a cluster in which a plurality of atoms or molecules are aggregated and agglomerated. The cluster is a block-shaped group in which a plurality of (usually about 2 to 2000) atoms or molecules are bonded to each other.

when a silicon wafer is irradiated with cluster ions composed of carbon and hydrogen, for example, the energy of the cluster ions is instantaneously brought to a high temperature of about 1350 to 1400 ℃ when the silicon wafer is irradiated with the cluster ions, and the silicon is melted. Thereafter, the silicon is rapidly cooled, and carbon and hydrogen are dissolved in the vicinity of the surface of the silicon wafer. That is, the "modified layer" in the present specification means a layer in which the constituent elements of the irradiated ions are dissolved in the interstitial positions or substitution positions of the crystal in the surface layer portion of the silicon wafer. The concentration distribution of carbon in the depth direction of a silicon wafer by SIMS depends on the acceleration voltage of cluster ions and the cluster size, but is narrower than the case of monomer ions, and the thickness of a region (i.e., a modified layer) where irradiated carbon is locally present is approximately 500nm or less (e.g., approximately 50 to 400 nm). The modified layer serves as an adsorption site. This is also described in patent documents 2 and 3.

The conditions for cluster irradiation include the constituent elements of the cluster ions, the dose of the cluster ions, the cluster size, the acceleration voltage and the beam current value of the cluster ions, and the like.

in the present embodiment, the constituent element of the cluster ion is carbon, preferably carbon and hydrogen. For example, when cyclohexane (C6H12) is used as a material gas, cluster ions composed of carbon and hydrogen can be generated. Furthermore, as the carbon source compound, it is particularly preferable to use a cluster CnHm (3. ltoreq. n.ltoreq.16, 3. ltoreq. m.ltoreq.10) formed from pyrene (C16H10), dibenzyl (C14H14), or the like. This is because the small-sized cluster ion beam is easily controlled.

The cluster size can be set appropriately to 2 to 100, preferably 60 or less, and more preferably 50 or less. In the present specification, "cluster size" indicates the number of atoms or molecules constituting 1 cluster. In the experimental examples described later, C3H5 having a cluster size of 8 was used. The adjustment of the cluster size can be performed by adjusting the gas pressure of the gas ejected from the nozzle, the pressure of the vacuum vessel, the voltage applied to the filament during ionization, and the like. The cluster size can be obtained by obtaining a cluster number distribution by mass analysis or time-of-flight mass analysis based on a quadrupole high-frequency electric field, and averaging the cluster number.

The dose of cluster ions can be adjusted by controlling the ion irradiation time. The amount of carbon is not particularly limited, and can be set to a range of approximately 1X 1014atoms/cm2 or more and 1X 1016atoms/cm2 or less.

The acceleration voltage of the cluster ions can be preferably 40keV/atom or less, when CnHm (3. ltoreq. n.ltoreq.16, 3. ltoreq. m.ltoreq.10) is used as the cluster ions, in terms of 1 carbon atom, more than OkeV/atom and 50keV/atom or less.

the beam current value can be set to approximately 0.3mA or more and 3.0mA or less.

The silicon epitaxial layer formed on the modified layer can be formed under normal conditions. First, a silicon wafer is put into an epitaxial growth apparatus and subjected to hydrogen baking treatment. In the usual conditions for the hydrogen baking treatment, the inside of the epitaxial growth apparatus is set to a hydrogen atmosphere, the silicon wafer is charged into the furnace at a furnace temperature of 600 ℃ to 900 ℃ inclusive, the temperature is raised to a temperature range of 1100 ℃ to 1200 ℃ at a temperature raising rate of 1 ℃/sec to 15 ℃/sec, and the temperature is maintained at the temperature for 30 seconds to 1 minute. The hydrogen baking treatment is a treatment of removing a natural oxide film formed on the wafer surface before the epitaxial layer is grown. Next, a source gas such as dichlorosilane or trichlorosilane is introduced into the chamber using hydrogen as a carrier gas, and a silicon single crystal can be epitaxially grown on the silicon wafer by a CVD method at a temperature in a range of approximately 1000 to 1200 ℃. The thickness of the epitaxial layer is not particularly limited, and can be about 1 to 15 μm.

The present embodiment is characterized in that after the ion implantation (after the 1 st step), the silicon wafer is subjected to a heat treatment, and then the modified layer of the silicon wafer is analyzed by the 3DAP method, and the adsorption capacity of impurities in the modified layer is evaluated from the three-dimensional distribution map of carbon in the modified layer obtained by the analysis. The following is a description of the principle of measurement by the 3DAP method and experimental examples leading to the completion of the present invention.

(3DAP method)

the measurement procedure and measurement principle of the 3DAP method will be described with reference to fig. 1. First, a measurement object was processed by a focused ion beam to prepare a sharp needle-like sample having a diameter of at most about 100 nm. The sample was mounted on a 3DAP apparatus, evacuated and cooled, and a high voltage of about 1.5 to 5kV was applied to the sample. In this state, when the sample is irradiated with a pulse laser beam at an energy of about 20pJ/pulse, a high electric field is generated at the very tip of the sample, and a phenomenon of electric field evaporation (in which neutral atoms on the sample surface are positively ionized and detached from the surface) occurs. The ions in which the electric field evaporation has occurred are detected by a two-dimensional position detector, whereby the two-dimensional coordinates (x, y) of each atom are determined. Further, the TOF (Time of Flight) analyzer measures the Time of Flight from the irradiation of the pulsed laser light to the arrival of the ions at the detector, and as a result, the ion type can be identified. By repeating the irradiation with the laser light, the same measurement can be performed in the depth direction of the sample, and the depth coordinate (z) of each atom can be specified. The data thus obtained is processed to construct a three-dimensional map of atoms. Instead of the pulsed laser, the electric field may be evaporated by applying a pulsed voltage.

(Experimental example 1)

An n-type silicon wafer (diameter: 300mm, thickness: 725 μm, dopant: phosphorus, dopant concentration: 5.0X 1014atoms/cm3) obtained from a CZ single crystal silicon ingot was prepared. Next, a C3H5 cluster was generated by a cluster ION generator (nitrogen ION equivalent co. ltd., manufactured and clariis (registered trademark)), and the surface of the silicon wafer was irradiated with carbon at a dose of 1.0 × 1015atoms/cm2, thereby forming a modified layer. The acceleration voltage for each Cluster was set to 80keV/Cluster, the beam current value was set to 800 μ A, Tilt was set to 0 degrees, and Twist was set to 0 degrees.

the silicon wafer thus obtained was processed by a focused ion beam to produce a sharp needle-like sample having a trunk portion with a diameter of about 70nm and a tip with a diameter of about 20 nm. The tip of the needle-like sample was the surface of the modified layer of the silicon wafer, and the longitudinal direction of the needle-like sample was aligned with the depth direction from the surface of the modified layer of the silicon wafer. This sample was mounted on a 3DAP apparatus (AMETEK, inc., manufactured, LEAP4000XSi), and a three-dimensional distribution chart of carbon was obtained by performing 3DAP method measurement. Fig. 2(a) shows the three-dimensional distribution map.

Next, the silicon wafer obtained by the same method as described above was subjected to heat treatment at 1100℃ × 30 minutes under a nitrogen atmosphere. Thereafter, the modified layer was analyzed by the 3DAP method in the same manner as described above, and a three-dimensional distribution chart of carbon was obtained. Fig. 2(B) shows the three-dimensional distribution map.

As is clear from fig. 2(a), immediately after the modified layer is formed by carbon ion implantation, carbon atoms are uniformly distributed in the modified layer although the implanted ions are cluster ions. However, as is clear from fig. 2(B), by performing heat treatment after ion implantation, carbon atoms are aggregated in the modified layer, particularly in the region of about 200nm in the surface layer, to form an aggregate.

Next, in the carbon distribution diagram of fig. 2(B), focusing on a specific carbon aggregate, the concentration distribution of carbon and oxygen along the radial direction from the center of the carbon aggregate was obtained. In fig. 2(B), the carbon aggregate of interest is shown in a circle. Fig. 3 shows the concentration distribution of carbon and oxygen in the radial direction from the center of the carbon aggregate. In this way, in the present experimental example, it was confirmed that oxygen in the silicon wafer was segregated to the carbon aggregates in the modified layer and the periphery thereof. This is a finding that cannot be obtained by the conventional methods such as SIMS and TEM.

In the carbon distribution diagram of fig. 2(B), the carbon aggregates were identified in the range of a depth of 200nm from the surface of the modified layer, and the diameter of each carbon aggregate was determined. The definition of the carbon aggregate used in the present experimental example will be explained with reference to fig. 4. In this experimental example, the carbon aggregates were defined by determining 2 parameters dmax and Nmin. Focusing on a certain carbon atom a in the three-dimensional distribution map, carbon atom B, C, D … … present within a range of distance dmax from carbon atom a (i.e., within a spherical range) is considered as an aggregate candidate. Next, carbon atoms existing within a range dmax from the carbon atom B, C, D … … were taken as aggregation candidates. This operation is continued until there are no aggregation candidates. When Nmin or more carbon atoms are aggregation candidates, the carbon atoms of these aggregation candidates and atoms (silicon, oxygen, etc.) other than carbon present in dmax range from these carbon atoms are collectively defined as a carbon aggregate. In this experimental example, dmax is 1nm, and Nmin is 30.

However, the present invention is not limited to dmax being 1nm and Nmin being 30, dmax may be appropriately selected from a range of 0.5 to 5nm, and Nmin may be appropriately selected from a range of 10 to 100.

as shown in fig. 4, the carbon aggregates further include atoms other than carbon such as silicon included in the range dmax from the carbon atom candidate for each aggregate. The equivalent volume spherical equivalent diameter is obtained from the volume of the carbon aggregate defined as above, and this is taken as the diameter of the carbon aggregate.

In the carbon distribution diagram of FIG. 2(B), the average diameter of the carbon aggregates in the depth range of 200nm from the surface of the modified layer was 6.3nm, and the density was 6.0X 1016 pieces/cm 3. Since the adsorption behavior in the modified layer is known to be segregation of impurities into the interior and the periphery of the carbon aggregate, it is considered that the average diameter and density of the carbon aggregate can be evaluated based on the carbon distribution diagram, and thus the adsorption capacity that can be obtained can be evaluated. It is considered that the higher the density of the carbon aggregate is, the higher the adsorption capacity can be obtained.

(Experimental example 2)

The modified layer was analyzed by the 3DAP method to obtain a three-dimensional distribution chart of carbon, except that the dose of carbon was changed in various ways within the range of 1.0X 1014 to 1.0X 1016atoms/cm2, and the cluster ion irradiation and the heat treatment were performed under the same conditions and by the same method as in Experimental example 1.

the density of the carbon aggregates in the depth range of 200nm from the surface of the modified layer was obtained from each three-dimensional distribution map. The relationship between the implanted carbon dose and the carbon aggregate density is shown in fig. 5. As can be seen from fig. 5, the correlation that the larger the implanted carbon dose, the larger the carbon aggregate density becomes is obtained. As described in patent document 3, it is known that a higher adsorption capacity can be obtained as the implanted carbon dose is increased. From this, it is understood that the density of the carbon aggregate is obtained from the three-dimensional distribution diagram of carbon as in the present embodiment, and the adsorption capacity of the impurity can be evaluated based on the density.

(Experimental example 3)

next, the following experimental examples are shown: after the modified layer was formed on the silicon wafer, an epitaxial layer was actually formed, and the adsorption ability was evaluated for impurities such as copper as a heavy metal as well as oxygen.

An n-type silicon wafer (diameter: 300mm, thickness: 725 μm, dopant: phosphorus, dopant concentration: 5.0X 1014atoms/cm3) obtained from a CZ single crystal silicon ingot was prepared. Next, a C3H5 cluster was generated by a cluster ION generator (n.i. ION equivalent C0.ltd., manufactured and clariis (registered trademark)), and the surface of the silicon wafer was irradiated with carbon at a dose of 5.0 × 1015atoms/cm2, thereby forming a modified layer. The acceleration voltage for each Cluster was set to 80keV/Cluster, the beam current value was set to 800 μ A, Tilt was set to 0 degrees, and Twist was set to 0 degrees.

Then, the silicon wafer was carried into a single wafer type epitaxial growth apparatus (manufactured by Applied Materials, Inc.), hydrogen baking treatment was performed at a temperature of 1120 ℃ for 30 seconds in the apparatus, and then a silicon epitaxial layer (thickness: 0.5 μm, dopant: phosphorus, dopant concentration: 1.0X 1015atoms/cm3) was epitaxially grown on the modified layer of the silicon wafer by CVD method at 1150 ℃ using hydrogen as carrier gas and trichlorosilane as source gas, thereby obtaining an epitaxial silicon wafer.

Thereafter, in order to evaluate the adsorption capability of Cu, Cu ions were implanted into the epitaxial layer of the epitaxial silicon wafer at a dose of 1X 1015atoms/cm 2. Thereafter, the epitaxial silicon wafer was subjected to a heat treatment at 1000 ℃ for 1 hour in order to diffuse the implanted Cu.

the thus obtained epitaxial silicon wafer samples were used for measurement and evaluation of SIMS, TEM, and 3 DAP. The following describes methods and results of the respective measurements and evaluations.

＜SIMS＞

Fig. 6 shows C, O and Cu concentration distributions in the depth direction from the epitaxial layer surface obtained by SIMS measurement. As is clear from fig. 6, a sharp peak of C was observed just below the epitaxial layer, thereby defining a modified layer. Further, peaks of O and Cu were also observed at the same positions as those of C. Thus, it was confirmed that O and intentionally implanted Cu in the silicon wafer were adsorbed (trapped) in the modified layer of the solid solution C. This result is a result matching the disclosure of patent document 2.

＜TEM＞

As a result of TEM observation of the cross section of the modified layer of the epitaxial silicon wafer, black-dot defects that are considered to be caused by implantation of C3H5 cluster ions were observed. This is a result of matching with the disclosure of patent document 3.

＜3DAP＞

In order to evaluate the adsorption behavior in more detail than SIMS and TEM, the 3DAP method was performed. Specifically, the epitaxial silicon wafer thus produced was processed with a focused ion beam to remove the epitaxial layer, and further processed with a focused ion beam to produce a sharp needle-like sample having a trunk portion with a diameter of about 70nm and a tip with a diameter of about 20 nm. The tip of the needle-like sample was the modified layer surface of the silicon wafer (i.e., the interface between the silicon wafer and the epitaxial layer), and the longitudinal direction of the needle-like sample was aligned with the depth direction from the modified layer surface of the silicon wafer (the surface of the silicon wafer). The sample was mounted on a 3DAP apparatus (AMETEK, manufactured by NC., LEAP4000XSi) and measured by the 3DAP method, thereby obtaining a three-dimensional distribution chart of carbon, copper, and oxygen.

Fig. 7(a) shows an atomic distribution diagram of C, fig. 7(B) shows an atomic distribution diagram of Cu, and fig. 7(C) shows atomic distribution diagrams of C and Cu. As is clear from fig. 7(a), in the modified layer subjected to the heat treatment during the formation of the epitaxial layer, the carbon aggregates were confirmed particularly in the region of about 50 to 200nm in the surface layer. As is clear from fig. 7(B) and (C), it was confirmed that Cu as a heavy metal impurity was present in a large amount at the position of the carbon aggregate.

In order to analyze the adsorption state in more detail, the present inventors prepared an iso-concentration surface shown in fig. 8 from the three-dimensional atomic distribution diagram shown in fig. 7 (C). FIG. 8 is a view showing an isoconcentration plane of 3.0% C concentration and an isoconcentration plane of 0.5% Cu concentration in a cylindrical region of 20nm height extracted from the tip of a needle-like sample to a depth of 70 to 90nm from the tip side of the sample. In the three-dimensional distribution map, a cube of volume 1nm3 called a voxel has concentration information of each element. Further, by connecting the outermost voxels among the voxels having a C concentration of 3.0%, an iso-concentration plane having a C concentration of 3.0% can be created. Similarly, an iso-concentration plane having a Cu concentration of 0.5% can be created by connecting voxels located on the outermost side among voxels having a Cu concentration of 0.5%. As is clear from fig. 8, the iso-concentration plane having a Cu concentration of 0.5% exists so as to be included in the iso-concentration plane having a C concentration of 3.0%. From this, it was found that Cu was segregated inside the spheroid (region having a C concentration of 3.0% or more) constituted by the surface having an equal concentration of 3.0% C concentration.

In order to analyze the more detailed adsorption state, the present inventors produced approximate histograms of C and Cu with the C concentration 3.0% as a reference plane shown in fig. 9 from the iso-concentration plane shown in fig. 8. In the proximity histogram, when the density distribution of the density is calculated from each point of the reference plane in the normal direction with reference to the iso-density plane as the reference plane (distance zero), the proximity histogram will be described with reference to fig. 12. From the proximity histogram, the concentration distribution in which the influence of the interface roughness is eliminated can be analyzed. In fig. 9, the iso-concentration plane having a C concentration of 3.0% is defined as a reference plane, the positive direction of the abscissa indicates the inner (i.e., inner side with respect to the iso-concentration plane) direction of the sphere constituted by the iso-concentration plane having a C concentration of 3.0%, and the negative direction of the abscissa indicates the outer (i.e., outer side with respect to the iso-concentration plane) direction of the sphere. For example, the horizontal axis shows the average value and the maximum/minimum value (error bar) of the C concentration and the Cu concentration at a distance of 1nm in the normal direction from each point on the reference surface at a position of 1 nm. As is clear from fig. 9, Cu segregates inside a spheroid (region having a C concentration of 3.0% or more) formed by an iso-concentration surface having a C concentration of 3.0%.

The present inventors further superimposed a three-dimensional distribution chart of oxygen on the 3-dimensional atomic distribution chart of fig. 7(C), and produced an iso-concentration surface shown in fig. 10 based on the superimposed three-dimensional distribution chart. FIG. 10 is a view showing an isoconcentration plane of 3.0% C concentration, an isoconcentration plane of 0.5% Cu concentration and an isoconcentration plane of 1.0% O concentration in a cylindrical region of 20nm height extracted from the tip of a sample in a depth range of 70 to 90nm from the tip end side of the sample. As is clear from fig. 10, not only the iso-concentration plane having a Cu concentration of 0.5%, but also the iso-concentration plane having an O concentration of 1.0% is included in the iso-concentration plane having a C concentration of 3.0%.

Fig. 11 is an approximate histogram of C, Cu and O with the C concentration of 3.0% as the reference plane, which is created from the iso-concentration plane shown in fig. 10. As can be seen from fig. 11, Cu and O are segregated inside the spheroid (region having a C concentration of 3.0% or more) constituted by the equal concentration plane having a C concentration of 3.0%.

(method of evaluating adsorption Capacity)

as shown in experimental examples 1 to 3, the modified layer of the silicon wafer was analyzed by the three-dimensional atom probe method, and the adsorption ability of impurities in the modified layer was evaluated from the three-dimensional distribution diagram of carbon in the modified layer obtained by the analysis.

As a specific method, as shown in fig. 3, a carbon aggregate in a depth range of 200nm from the surface of the modified layer is specified in a three-dimensional distribution diagram of carbon, the concentration distribution of carbon and impurities around the carbon aggregate is obtained, and the adsorption ability of impurities in the modified layer can be evaluated based on the concentration distribution.

As can be understood from the results shown in fig. 5, the adsorption ability of impurities in the modified layer can also be evaluated based on the density of the carbon aggregates. It can be evaluated that the carbon aggregate has a higher adsorption capacity as the density thereof is higher. According to this method, with respect to impurities such as heavy metals, there is no need to intentionally contaminate them to obtain an atomic profile of the impurities.

As shown in fig. 8 and 10, a 1 st equal concentration plane having a predetermined carbon concentration in a range of a depth of 200nm from the surface of the modified layer in a three-dimensional distribution chart of carbon, a 2 nd equal concentration plane having a predetermined impurity concentration in the three-dimensional distribution chart, and the adsorption capacity of impurities in the modified layer can be evaluated based on the positional relationship between the 1 st equal concentration plane and the 2 nd equal concentration plane. In experimental example 3, the C concentration was 3.0%, the Cu concentration was 0.5%, and the O concentration was 1.0% as the impurity concentration, but the present invention is not limited to this, and for example, the C concentration can be appropriately selected from the range of 0.1 to 50%, the Cu concentration can be appropriately selected from the range of 0.1 to 5.0%, and the O concentration can be appropriately selected from the range of 0.1 to 5.0%.

As shown in fig. 9 and 11, it is also possible to create a 1 st equal concentration plane having a predetermined carbon concentration in a range of a depth of 200nm from the surface of the modified layer in the three-dimensional distribution diagram of carbon, create an approximate histogram of carbon and impurities using the 1 st equal concentration plane as a reference plane, and evaluate the adsorption capacity of impurities in the modified layer based on the approximate histogram.

From the results of experimental example 3, it was found that impurities were segregated into the interior of the spheroid constituted by the equal concentration planes of carbon. Therefore, the adsorption capacity of impurities in the modified layer can be evaluated based on the density of spheroids formed by the equal concentration surface of carbon. According to this method, with respect to impurities such as heavy metals, there is no need to intentionally contaminate them to obtain an atomic profile of the impurities.

(preferred embodiment of the present invention)

in the above experimental example 1, oxygen in the silicon wafer was noted as an impurity. By segregating oxygen into the carbon aggregates in the modified layer, diffusion of oxygen into the epitaxial layer can be suppressed, and degradation of device characteristics due to oxygen can be suppressed.

According to the studies of the present inventors, from the viewpoint of forming the carbon aggregate, it is preferable to use a non-oxidizing atmosphere at an atmospheric temperature of 900 ℃ to 1300 ℃. The non-oxidizing atmosphere is preferably a hydrogen atmosphere, a nitrogen atmosphere, or an argon atmosphere. When the atmospheric temperature is lower than 900 ℃, it is difficult to form carbon aggregates in the silicon wafer, while when it exceeds 1300 ℃, the thermal load on the apparatus is large and there is a possibility that slip or the like occurs in the silicon wafer. From these viewpoints, the ambient temperature is more preferably set to 1000 ℃ or higher and 1250 ℃ or lower.

The heat treatment time is preferably 10 seconds to 2 hours. If the time is less than 10 seconds, it is difficult to form the carbon aggregate, and even if the time for performing the heat treatment exceeds 2 hours, the carbon aggregate does not further increase, and from the viewpoint of reducing the heat treatment cost, the heat treatment is preferably limited to 2 hours or less. The heat treatment may be performed by a rapid temperature rise and fall heat treatment apparatus or a batch heat treatment apparatus (vertical heat treatment apparatus or horizontal heat treatment apparatus), or may be performed in an epitaxial apparatus.

The conditions for this heat treatment also satisfy the heat treatment at the time of epitaxial growth. That is, in the experimental examples 1 and 2, the silicon wafer after ion implantation was subjected to another heat treatment without forming an epitaxial layer. However, as shown in experimental example 3, a carbon aggregate was also formed in an epitaxial silicon wafer in which an epitaxial layer was formed on a silicon wafer after ion implantation. In addition, according to the processing technique based on the focused ion beam, it is also possible to process a part of the modified layer into a needle-like sample after removing the epitaxial layer.

In the above experimental example 1, oxygen in the silicon wafer was noted as an impurity. However, in the present invention, the impurities are not limited to oxygen, and similar results can be obtained with heavy metals as shown in experimental example 3. That is, after the epitaxial silicon wafer is subjected to the forced contamination by the heavy metal, the modified layer is analyzed by the 3DAP method, the carbon aggregate in the depth range of 200nm from the surface of the modified layer is identified in the three-dimensional distribution map of carbon, and the concentration distribution of the carbon and the heavy metal around the carbon aggregate is obtained, whereby the adsorption behavior of the heavy metal in the modified layer can be evaluated.

(epitaxial silicon wafer)

The epitaxial silicon wafer of the present embodiment is characterized by comprising: a silicon wafer; a modified layer formed on a surface layer portion of the silicon wafer and having carbon dissolved in the silicon wafer; and an epitaxial layer formed on the modified layer, wherein carbon aggregates are present in a depth of 200nm from the surface of the modified layer in a three-dimensional distribution diagram of carbon obtained by analyzing the modified layer by a three-dimensional atom probe method, and the carbon aggregates have an average diameter of 5nm or more and a density of 1 × 1015/cm 3 or more.

In this way, by setting the average diameter and density of the carbon aggregates in the reforming layer to predetermined values or more, high impurity adsorption capacity can be obtained.

As for conditions for producing the epitaxial silicon wafer according to the present embodiment, irradiation conditions such as dose, cluster type, acceleration voltage, and beam current are appropriately selected based on the cluster ion irradiation conditions adopted in the above experimental examples, and the correlation between the size and density of the carbon aggregates obtained from the selected combination of the cluster ion irradiation conditions and the heat treatment conditions can be obtained in advance, thereby producing the epitaxial silicon wafer according to the present embodiment.

Industrial applicability

By evaluating the adsorption behavior of impurities in the modified layer formed directly below the epitaxial layer and having carbon dissolved therein with high accuracy by the evaluation method of the present invention, it is possible to appropriately evaluate an epitaxial silicon wafer used as a raw material wafer in the production of various semiconductor devices such as a solid-state image pickup device.