Semiconductor substrate evaluation method

Abstract

The present invention relates to a method for evaluating a semiconductor substrate, which evaluates the electrical characteristics of the semiconductor substrate, and includes the following steps: forming a pn junction on the surface of the semiconductor substrate; mounting the semiconductor substrate on a wafer chuck, the wafer chuck being provided with a device for irradiating the surface of the semiconductor substrate with light and a device for measuring the amount of the irradiated light; irradiating the surface of the semiconductor substrate with light for a predetermined time; and measuring at least the amount of carriers generated after the light irradiation when the pn junction is closed after the light irradiation. Thus, a method for evaluating a semiconductor substrate is provided, which, when evaluating the characteristics corresponding to the afterimage characteristics of a wafer used for a product requiring a high yield, such as a CCD or CMOS image sensor, does not use a process equipment to produce a component, and can also perform the same evaluation as when forming an actual solid-state imaging element in the wafer state.

Description

Technical Field

[0001] The present invention relates to a method for evaluating a semiconductor substrate. Background Art

[0002] With the miniaturization and high performance of semiconductor devices such as memory, CCD (Charge-Coupled Device) and other solid-state imaging elements, in order to improve their product yield, the silicon wafers used as materials are also required to be of higher quality, and various silicon wafers corresponding to them are being developed. In particular, the crystallinity of the surface layer of the wafer is particularly important, which is presumed to have a direct impact on product characteristics. As a guideline for improving it, the following are developed: 1) annealed wafers that are subjected to high-temperature heat treatment in an atmosphere containing an inert gas or hydrogen, 2) defect-free polished wafers that reduce native (Grown-in) defects by improving pull-up conditions, and 3) epitaxial wafers that have undergone epitaxial growth.

[0003] As an existing method for evaluating the electrical characteristics of the surface quality of silicon wafers, the oxide film withstand voltage (GOI) evaluation is used. This method forms a gate oxide film on the surface of the silicon wafer by thermal oxidation, and applies electrical stress to the insulator, that is, the silicon oxide film, by forming an electrode on it, and evaluates the surface quality of the silicon wafer by its insulation degree. That is, the surface quality of the silicon wafer is evaluated by the following characteristics: if there are defects or metal impurities on the surface of the original silicon wafer, the defects or metal impurities enter the silicon oxide film through thermal oxidation, or form an oxide film corresponding to the surface shape to become an uneven insulator, etc., so that if there are defects or impurities, the insulation is reduced.

[0004] This is a reliability evaluation of the gate oxide film of MOSFET (metal-oxide-semiconductor field-effect transistor) in actual devices, and various wafers have been developed to improve it. However, even if there are no problems in the GOI evaluation, there will certainly be cases where the device yield is reduced. In recent years, especially with the high integration of devices, such a phenomenon has increased. First, consider the case of a solid-state imaging element, for example, where the dark current is reduced and the sensitivity is improved. The reduction in leakage current caused by the wafer leads to a reduction in dark current, which ultimately helps to improve the device characteristics.

[0005] In the case of metal contamination, trace metals have gradually become an issue as components have become more high-performance in recent years. In the results of chemical analysis, various metals have been detected through efforts to increase sensitivity, but the current situation is that it is very difficult to understand which metal among the metal elements detected by chemical analysis has the greatest impact on actual component and joint leakage. In addition, since metal impurity analysis is a method of analyzing the etching solution by, for example, etching the wafer surface, the method of analyzing the information on the wafer surface as a representative generally cannot obtain information about the in-plane distribution. On the other hand, in the evaluation of leakage current, by forming multiple pn junctions on the surface of the silicon substrate and finding the respective reverse leakage currents, the leakage distribution within the substrate surface can be obtained.

[0006] Solid-state imaging elements such as CCD and CMOS (Complementary MOS) image sensors have different methods for extracting the charge generated by the electron-hole pairs generated by the incident light, but the principle of converting light into charge (photoelectric conversion) is the same, which is to form a pn junction and have a depletion layer as a structure. Here, the phenomenon that although no light is incident, electron-hole pairs are generated in the depletion layer due to defects or impurities and charge is generated is called white defect or dark current. In this way, the dark current of the solid-state imaging element can be evaluated for the reverse leakage current characteristics of the wafer with a pn junction, which can be used as one of the improvement indicators in the estimation of the cause or material development.

[0007] However, as the influence of material properties on solid-state imaging elements, in addition to dark current, there is also an afterimage characteristic. It is known that the afterimage characteristic is closely related to the material, especially the substrate (Non-Patent Document 1 and Non-Patent Document 2). For example, in Non-Patent Document 1 and Non-Patent Document 2, the light elements in the silicon substrate cause an influence, and the defects causing the influence are considered to be a complex of boron and oxygen. In this way, since the influence of the substrate on the afterimage characteristic becomes clear, in order to evaluate the substrate characteristics, in addition to the evaluation of the reverse leakage current characteristic that conforms to the dark current evaluation, a substrate evaluation method corresponding to the afterimage characteristic is also required.

[0008] As one of the methods, there is a method described in Patent Document 1, but in order to perform this method, it is necessary to form the light receiving part of the solid-state imaging element, that is, the photodiode. Therefore, in addition to the diffusion of dopants, it is also necessary to form an element separation structure. In order to form this element separation structure, process equipment such as photolithography and subsequent etching is required, and further, time and large-scale equipment are required for evaluation. Thus, in order to evaluate the afterimage characteristics, it was necessary to produce actual elements in the past, and there was a problem that it was difficult to evaluate in the wafer state.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2019-9212

[0012] Non-patent literature

[0013] Non-patent document 1: Draft lectures of the 77th Autumn Symposium of the Society of Applied Physics, 14p-P6-10, Kaneda Tsubasa and Otani Akira, “Elucidation of the mechanism of afterimage development in CMOS image sensors 1”

[0014] Non-patent document 2: Draft lectures of the 77th Autumn Symposium of the Society of Applied Physics, 14p-P6-11, Akira Otani and Tsubasa Kaneda, “Elucidation of the mechanism of afterimage development in CMOS image sensors 2”

[0015] Non-patent document 3: S. Rein "Lifetime Spectroscopy" p398, Springer, 2005 Summary of the invention

[0016] 1. Technical issues to be resolved

[0017] The present invention is made in view of the above-mentioned problems, and its purpose is to evaluate the characteristics corresponding to the afterimage characteristics of wafers used for products requiring high yields such as CCD and CMOS image sensors, without using process equipment to produce components, and to be able to perform the same evaluation as when an actual solid-state imaging element is formed in the wafer state. In addition, by simply performing wafer-level measurement in this way, it is helpful to improve the quality of semiconductor substrates.

[0018] (II) Technical solution

[0019] In order to achieve the above object, the present invention relates to a method for evaluating a semiconductor substrate, which evaluates the electrical characteristics of a semiconductor substrate and is characterized by comprising the following steps:

[0020] forming a pn junction on a surface of the semiconductor substrate;

[0021] The semiconductor substrate is mounted on a wafer chuck, wherein the wafer chuck is provided with a device for irradiating light to the surface of the semiconductor substrate and a device for measuring the amount of light irradiated;

[0022] irradiating the surface of the semiconductor substrate with light for a predetermined time; and

[0023] At least the amount of carriers generated after light irradiation of the pn junction after the light irradiation is turned off is measured.

[0024] According to such a method, it is possible to easily and accurately evaluate the afterimage characteristic failure caused by the semiconductor substrate, which is a concern in CCD, CMOS image sensors, etc., at the substrate level, and to provide a high-quality semiconductor substrate.

[0025] At this time, in the step of performing the light irradiation, it is preferable to measure the amount of carriers generated during the light irradiation of the pn junction.

[0026] By measuring the amount of carriers generated during light irradiation, it is possible to more reliably avoid the influence of the difference in the amount of carriers generated initially on the afterimage characteristics.

[0027] In addition, at this time, preferably, the amount of carriers generated during the light irradiation and the amount of carriers generated after the light irradiation are measured using a carrier measurement probe provided separately from the device for performing the light irradiation and the device for measuring the amount of light irradiated.

[0028] Such a method can make measurement easier.

[0029] In this case, the carrier measurement probe is preferably a non-contact Kelvin probe.

[0030] As the carrier measurement probe, a non-contact Kelvin probe can be appropriately used.

[0031] Furthermore, in this case, the carrier measurement probe is preferably a mercury probe.

[0032] As the carrier measurement probe, a mercury probe can also be appropriately used.

[0033] Furthermore, preferably, after the step of forming the pn junction and before the step of mounting the semiconductor substrate, heat treatment is performed on the semiconductor substrate in advance.

[0034] By pre-heating the semiconductor substrate, defects in the semiconductor substrate are formed and grown, and the residual image characteristic defect can be more clearly shown. In addition, the behavior during the heat treatment of the device process can be reproduced.

[0035] Preferably, a semiconductor substrate for a solid-state imaging element is used as the semiconductor substrate, and the afterimage characteristic of the solid-state imaging element is evaluated based on the ratio between the amount of carriers generated during the light irradiation and the amount of carriers generated after the light irradiation.

[0036] The carrier generation during light irradiation varies depending on the type of semiconductor substrate, but if this method is used, the amount of carriers generated during light irradiation can be standardized by taking the ratio of the amount of carriers generated after light irradiation, thereby enabling evaluation regardless of the type of semiconductor substrate.

[0037] (III) Beneficial effects

[0038] According to the semiconductor substrate evaluation method of the present invention, afterimage characteristic failure caused by the semiconductor substrate, which is a concern of CCD, CMOS image sensors, etc., can be easily and accurately evaluated at the substrate level, and a high-quality semiconductor substrate can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic diagram showing an example of an embodiment of the present invention.

[0040] Figure 2 This is a diagram showing an example of a junction structure formed by the semiconductor substrate evaluation method of the present invention.

[0041] Figure 3 This is a diagram showing an example of the measurement procedure of the semiconductor wafer evaluation method of the present invention.

[0042] Figure 4 This is a graph plotting the relationship between the ratio of the amount of carriers generated during light irradiation and the amount of carriers generated after light irradiation in Examples 1 and 2 and the substrate oxygen concentration. DETAILED DESCRIPTION

[0043] As described above, there is a need for a semiconductor substrate evaluation method capable of measuring, at substrate level, characteristics corresponding to the afterimage characteristics of a semiconductor substrate used in products requiring high yields, such as CCDs and CMOS image sensors.

[0044] The inventors of this case have conducted intensive research to achieve the above-mentioned purpose, and found that the above-mentioned problem can be solved if the following semiconductor substrate evaluation method is used, thereby completing the present invention: a semiconductor substrate evaluation method, which evaluates the electrical characteristics of a semiconductor substrate, is characterized in that it includes the following steps: forming a pn junction on the surface of the semiconductor substrate; mounting the semiconductor substrate on a wafer chuck, the wafer chuck being provided with a device for irradiating the surface of the semiconductor substrate with light and a device for measuring the amount of the irradiated light; irradiating the surface of the semiconductor substrate with light for a predetermined time; and at least measuring the amount of carriers generated after the light irradiation of the pn junction after the light irradiation is turned off.

[0045] Hereinafter, the present invention will be described with reference to the drawings, but the present invention is not limited thereto.

[0046] First, a pn junction structure is made in a semiconductor substrate (a step of forming a pn junction on the surface of a semiconductor substrate). The pn junction structure is not particularly limited, and any pn junction structure is acceptable, but a structure that can reduce leakage current (surface component) caused by the pn junction structure as much as possible is preferred.

[0047] As an example of such a pn structure, Figure 2An example of a junction structure formed by the semiconductor substrate evaluation method of the present invention is shown. Such a junction structure can be produced, for example, by diffusing boron or the like on a semiconductor substrate 1 doped with phosphorus or the like to form a high-concentration diffusion layer 2 having a conductivity type opposite to that of the semiconductor substrate 1, and by bringing the semiconductor substrate 1 having conductivity types opposite to each other into contact with the high-concentration diffusion layer 2 to form a pn junction. Here, near the pn junction, electrons and holes combine to form a depletion layer 3 where no carriers exist. In addition, as described above, Figure 2 This is merely an example for explaining a pn junction that can be formed by the present invention, and the conductivity type of the semiconductor substrate or the pn junction structure is not particularly limited.

[0048] In order to explain the evaluation method of the semiconductor substrate of the present invention, Figure 1 An example of an embodiment of the present invention is shown in FIG. The following steps are performed: a semiconductor substrate 1 having a pn junction structure manufactured as described above is mounted on Figure 1 The wafer chuck 8 shown is provided with a device for performing light irradiation (illumination unit) 6 and a device for measuring the light amount (illuminance meter) 7. Then, the following process is performed: after the surface of the semiconductor substrate 1 is irradiated with light 4 of a predetermined illuminance for a predetermined time (light irradiation step), the amount of carriers generated after the light irradiation is turned off 4 is measured.

[0049] Here, it is considered that the light irradiation 4 preferably uses an actual device with lighting such as LED, such as white light. However, if, for example, a device dedicated to infrared light is envisioned, a light source (illumination unit) with a wavelength suitable for it can also be selected. In addition, it is preferred that the amount of light (illuminance) does not differ in each measurement, so it is preferable to use a component having both a mechanism for measuring illumination and a mechanism for adjusting illumination.

[0050] In addition, although it is the amount of light during measurement, for the afterimage characteristic, in an actual device, after strong light is incident, the shutter is temporarily closed to obtain an image, and then the shutter is opened to obtain the next image, the carriers generated by the previous light are not fully eliminated and their influence remains, so it is considered that a stronger amount of light is required. In actual testing, there are also cases where the illumination is changed to find the optimal illumination in advance, but generally speaking, it is sufficient to set the illumination to the maximum level with a commercially available lighting unit. As a specific illumination, it is preferably about 500 lux.

[0051] In addition, the light irradiation time is preferably 1 to 10 seconds, and more preferably 3 to 7 seconds. If the light irradiation time is more than 1 second, the time from the start of the light irradiation to the stabilization of the light amount of the illumination can be obtained, and the illumination can be more reliably constant. In addition, if it is less than 10 seconds, the measurement time can be shortened.

[0052] The amount of generated carriers at the pn junction thus formed was measured. Figure 3A conceptual diagram showing the specific timing of light irradiation and measurement. Figure 3 This is a diagram showing an example of the measurement procedure of the semiconductor wafer evaluation method of the present invention.

[0053] The inventors have found that the amount of carriers generated by the light irradiation 4 is affected by the type of semiconductor substrate 1 and the light elements contained in the semiconductor substrate 1, especially carbon. Therefore, in order to avoid the influence of the difference in the amount of carriers initially generated by the light irradiation 4 on the afterimage characteristics, as Figure 3 As shown, it is preferable to measure the amount of carriers generated at the time (the amount of carriers generated during light irradiation) while performing light irradiation. In this way, the semiconductor substrate can be evaluated in consideration of the difference in the amount of carriers generated initially.

[0054] In addition, it is preferred that the amount of carriers generated during light irradiation is measured, the light irradiation is turned off, and the amount of carriers generated is measured again (the amount of carriers generated after light irradiation), and the ratio of the amount of carriers generated during light irradiation to the amount of carriers generated when the light irradiation is turned off is obtained. The carrier generation during light irradiation varies depending on the type of semiconductor substrate, but by obtaining the ratio of the amount of carriers generated during light irradiation to the amount of carriers generated after light irradiation and standardizing them, it is possible to evaluate regardless of the type of semiconductor substrate.

[0055] In addition, the measurement time of the amount of carriers generated after light irradiation is after light irradiation is turned off, but since it also depends on the performance of the measurement device, it is preferably verified in advance. For example, the measurement time can be set to 1 second for integration.

[0056] In addition, in the present invention, the time from turning off the light irradiation to starting to measure the amount of carriers generated after the light irradiation is not particularly limited, and the measurement can be started at the same time as the light irradiation is turned off, or it can be started after a specified interval after the light irradiation is turned off. In the measurement of the amount of carriers generated after the light irradiation, the phenomenon that the carriers generated by the light irradiation are captured and released again is captured, and the re-released carriers are measured. Here, the timing of the carrier re-release depends on the type of carrier trap and the measurement environment, so it is preferably verified in advance. For example, the measurement can be started at the same time as the light irradiation is turned off, and the measurement can be performed with a measurement time of 1 second.

[0057] In addition, Figure 3 In the embodiment, the measurement is stopped before measuring the amount of carriers generated after the light irradiation is turned off. This is to more reliably avoid noise when the light irradiation is turned off. However, it is not necessary and depends on the performance of the measuring instrument and the condition of the measurement object.

[0058] In addition, the measurement of the amount of generated carriers during and after light irradiation is preferably performed as follows: Figure 1As shown, the measurement is performed by a carrier measurement probe 5 provided separately from the device for irradiating light and the device for measuring the amount of irradiated light. By using a non-contact Kelvin probe or a mercury probe as the carrier measurement probe 5, measurement can be performed more simply without performing element isolation.

[0059] After that, the afterimage characteristics can be evaluated based on the ratio of the current values ​​of the carrier measurement probe when the light irradiation is turned on and off. The phenomenon that the current value is high after the light irradiation is turned off indicates that the corresponding carriers are trapped, and it can be inferred that the afterimage characteristics are poor.

[0060] In the example of an actual solid-state imaging element, charges are generated by electron-hole pairs generated by incident light when the shutter is open, and these charges are introduced to form an image. However, it is important to quickly expel the electron-hole pairs after the shutter is closed. If this expulsion is slow, it becomes an afterimage and affects the next frame.

[0061] In addition, if a pn junction is formed only on a semiconductor substrate, there may be a case where a clear difference cannot be obtained. In this case, it is preferred to pre-heat treat the semiconductor substrate after the step of forming the pn junction. For example, it is effective to add a heat treatment to form defects on the semiconductor substrate. By such a heat treatment, the defects of the semiconductor substrate are formed and grown, and the poor afterimage characteristics can be made clearer. In addition, the afterimage characteristics are considered to be a complex of boron and oxygen in the substrate as described in non-patent literature 1 and non-patent literature 2. Even in the case where the defect reproduces the behavior in the heat treatment of the device process, it is effective to measure after the heat treatment. It is known that defects composed of clusters of light elements such as boron and oxygen are formed at a relatively low temperature, and become unstable if it becomes a high temperature, so the additional heat treatment temperature is preferably a relatively low temperature of about 100 to 500°C as described in non-patent literature 3.

[0062] According to such a method, afterimage characteristic failure caused by a semiconductor substrate, which is a concern in CCD and CMOS image sensors, can be easily and accurately evaluated at the substrate level, thereby providing a high-quality semiconductor substrate.

[0063] Example

[0064] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[0065] [Example 1]

[0066] Phosphorus-doped CZ silicon wafers with a resistivity of 10 Ω·cm and a diameter of 200 mm were prepared, and three samples were prepared with substrate oxygen concentrations (Oi) of 3.38, 3.58, and 3.71 ppma (JEITA). Boron was doped on the silicon wafers at a dose of 6.0×10 13atoms / cm 2 After ion implantation, recovery annealing is performed at 1000°C in a nitrogen atmosphere to form a pn structure.

[0067] Then, in Figure 3 In the measurement sequence shown, after measuring the amount of carriers (current value) generated by the non-contact Kelvin probe while performing light irradiation for 1 second, the light irradiation is turned off, and the current value is similarly obtained within 1 second. The afterimage characteristics are evaluated based on the ratio of the current values ​​before and after light irradiation (also called the current ratio). Figure 4 The results are shown in .

[0068] As a result, the higher the substrate oxygen concentration, the larger the current ratio becomes, and it is known that the afterimage characteristics deteriorate. The complex of boron and oxygen is considered to be the cause of the afterimage of the solid-state imaging element. In this result, a phosphorus-doped substrate was used, and the concentration of boron was controlled by ion implantation. Therefore, the difference in oxygen concentration of the semiconductor substrate determines the concentration of boron-oxygen complex. It is believed that as the oxygen concentration increases, the concentration of boron-oxygen complex also increases, indicating that the afterimage characteristics are affected.

[0069] [Table 1]

[0070] Oi(ppma) Current ratio before and after light irradiation (au) 3.38 1.32 3.58 1.41 3.71 1.43

[0071] [Example 2]

[0072] Next, three samples with the same CZ silicon wafer and the same oxygen concentration as in Example 1 were subjected to the same process as in Example 1 to produce a pn structure. After annealing for 70 hours at 450°C in a nitrogen atmosphere, the same measurements as in Example 1 were performed with reference to Non-Patent Document 1.

[0073] In Table 2 and Figure 4 The relationship between the substrate oxygen concentration and the current ratio after annealing at 450°C is shown in FIG. 1 . It is known that the higher the substrate oxygen concentration, the larger the current ratio becomes, indicating that the afterimage characteristic deteriorates. In addition, the difference between the samples becomes larger when annealing at 450°C is performed than when the annealing is not performed. The boron-oxygen complex is considered to be the cause of the afterimage of the solid-state imaging element, indicating that the defect grows due to the annealing at 450°C. This indicates that the poor afterimage characteristic can be made clearer through this heat treatment.

[0074] [Table 2]

[0075] Oi(ppma) Current ratio before and after light irradiation (au) 3.38 2.70 3.58 3.25 3.71 3.63

[0076] Thus, by applying the method of the present invention, it is possible to evaluate the afterimage characteristics at the substrate level by a simple and rapid method without using photolithography equipment or etching equipment compared to the past. It can be seen that the method of the present invention is an effective method as an evaluation method for semiconductor substrates used for solid-state imaging elements.

[0077] The present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are illustrative, and any scheme having substantially the same structure and achieving the same function and effect as the technical idea described in the claims of the present invention is included in the technical scope of the present invention.

Claims

1. A method for evaluating a semiconductor substrate, wherein the method evaluates the electrical characteristics of the semiconductor substrate, wherein: The process includes: forming a pn junction on a surface of the semiconductor substrate; The semiconductor substrate is mounted on a wafer chuck, wherein the wafer chuck is provided with a device for irradiating light to the surface of the semiconductor substrate and a device for measuring the amount of light irradiated; irradiating the surface of the semiconductor substrate with light for a predetermined time; as well as at least measuring the amount of carriers generated after light irradiation of the pn junction after turning off the light irradiation, In the step of performing the light irradiation, the amount of carriers generated during the light irradiation of the pn junction is measured. The amount of carriers generated during the light irradiation and the amount of carriers generated after the light irradiation are measured using a carrier measurement probe provided separately from the device for performing the light irradiation and the device for measuring the amount of light of the irradiated light, A semiconductor substrate for a solid-state imaging element is used as the semiconductor substrate, and the afterimage characteristic of the solid-state imaging element is evaluated based on the ratio between the amount of carriers generated during the light irradiation and the amount of carriers generated after the light irradiation.

2. The semiconductor substrate evaluation method according to claim 1, wherein: The carrier measurement probe is a non-contact Kelvin probe.

3. The semiconductor substrate evaluation method according to claim 1, wherein: The carrier measurement probe is a mercury probe.

4. The method for evaluating a semiconductor substrate according to any one of claims 1 to 3, characterized in that: After the step of forming the pn junction and before the step of mounting the semiconductor substrate, the semiconductor substrate is subjected to a heat treatment in advance.

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