JP3451955B2 - Crystal defect evaluation method and crystal defect evaluation device - 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 and an evaluation apparatus for detecting and evaluating minute crystal defects existing in a semiconductor substrate or the like.

[0002]

2. Description of the Related Art In an LSI using a Si single crystal substrate, a crystal defect becomes a problem at various stages of the LSI manufacturing process and affects the characteristics and reliability of the device. In particular,
Defect control is one of the important key technologies in improving the reliability and the yield in the LSI process, which is progressing toward miniaturization, and active research is being conducted mainly by wafer manufacturers and LSI manufacturers. .

Although there are various types of crystal defects,
The oxygen precipitation defect is one of the defects to be controlled. this is,
This is because the oxygen precipitation defects inside the wafer effectively act as a gettering source of metal contamination, but the oxygen precipitation defects near the surface adversely affect the device characteristics. In addition, it is necessary to control even finer oxygen precipitation defects, which have not been a problem until now, by reducing the thickness of the gate oxide film accompanying the miniaturization of the device. In order to control minute oxygen precipitation defects, a technique for evaluating the defects is essential.

As the simplest evaluation method for such oxygen precipitation defects, selective etching with a chemical solution has been conventionally adopted. Further, as a method for evaluating smaller oxygen precipitates, a method using an infrared laser (infrared laser bright field interferometry; OPP: Optical Precipitate Profiler, infrared tomography method), a transmission electron microscope (TEM), etc. There is.

[0005]

The most convenient method for evaluating oxygen precipitation defects is selective etching with a chemical solution, but it does not become apparent as pits unless the defect size is about 1.0 μm. With the method using an infrared laser, it is easy to obtain a value of 0.
Although a defect having a size of about 03 μm can be detected, it takes about 2 hours to evaluate one wafer. Further, in the method using the infrared laser, the resolution in the depth direction is only about 2 μm in principle. Transmission electron microscope (TEM)
Although it is possible to detect extremely small defects of about several nm, there are problems that it takes a lot of time and effort to prepare a sample, and it is difficult to observe unless the defect density is 10 ¹⁰ cm ⁻³ or more.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to evaluate minute crystal defects easily and in a short time, and it is also possible to perform highly accurate crystal defect evaluation excellent in resolution in the depth direction. The object is to realize a method and an apparatus.

[0007]

In order to achieve the above-mentioned object, the crystal defect evaluation method according to the present invention uses anisotropic etching with a high selectivity for crystal defects contained in a substrate or in a predetermined layer. Characterized in that the substrate or a predetermined layer is etched, a conical etching residue formed with a crystal defect as an apex is exposed on the surface of the substrate or layer, and the crystal defect is evaluated based on the conical etching residue. To do.

The crystal defect evaluation apparatus according to the present invention is
A conical shape exposed on the surface of the substrate or layer formed by using the crystal defects as an apex by etching the substrate or the prescribed layer by anisotropic etching with a high selection ratio for crystal defects contained in the substrate or the prescribed layer The density or depth of the crystal defects in the substrate or in the predetermined layer is obtained by observing the etching residue and the number of the conical etching residue, or the size of the cone of the etching residue, or either data. Data processing means for obtaining both or one of the distributions in the depth direction.

As described above, in the present invention, defects are detected by performing anisotropic etching under the condition that the selection ratio of the crystal defects to be detected is large. The detection principle will be described below.

FIG. 1 shows the principle of detection of oxygen precipitation defects by high selectivity anisotropic etching. A sample in which defects (oxygen precipitation defects) are formed by heat treatment (for example, Si
When a (wafer) is etched (see FIGS. 1A to 1B) under a condition that the selection ratio with respect to SiO ₂ is large, a conical protrusion having an apex of an oxygen precipitate which is difficult to be etched as an apex is formed as an etching residue ( FIG. 1 (c)). If the number of the obtained conical etching residues is counted, the etched Si
The density of oxygen precipitation defects in the sample can be obtained. If there is a crystal defect (for example, oxygen precipitation defect), this conical etching residue is generated regardless of its density. Therefore, the crystal defect density is evaluated as in the case of performing the crystal defect evaluation using a conventional TEM. It can be evaluated regardless of. Anisotropic etching is dry etching (for example, reactive ion etching) using a gas containing a halogen-based (Br, Cl, F) gas when evaluating oxygen precipitation defects in a silicon substrate or a silicon film. By. If etching is performed under such conditions, FIG.
A conical protrusion resulting from oxygen precipitation defects such as

In the conical etching residue according to the present invention, if the etching conditions are the same, the base angle of each cone is
Since it is constant regardless of the size of the cone, as shown in FIG. 2, based on the size (diameter of the bottom surface) of the conical protrusion,
The position X _ox from the surface of the oxygen precipitation defect is calculated by the following equation (1).
Can be asked. Therefore, the distribution of the oxygen precipitation defect density in the depth direction can also be obtained. In the following equation, D is the etching depth from the surface, d is the bottom diameter of the conical protrusion, and θ is the base angle of the conical protrusion.

[0012]

## EQU1 ## X _OX = D- (d / 2) tan θ (1) In addition, the oxygen precipitation defect density and its distribution in the depth direction are automatically obtained by combining a photography device and an image processing device. be able to.

In the present invention, the crystal defects contained in the substrate or the predetermined layer are anisotropically etched with a high selectivity to etch the substrate or the predetermined layer at different selection ratios. Predetermined observation means respectively capture the conical etching residues exposed on the surface of the substrate or layer, and the data processing means, based on the data of the conical etching residues obtained by etching with different selection ratios, the crystal Evaluating the size of defects can also be adopted.

Further, with respect to each of the conical etching residues obtained by the etching with the different selection ratios, as described above, the number of residues and / or the size of the cone are obtained, and the size of the crystal defect is obtained. Along with
Both or one of the density and / or the distribution in the depth direction of the defects in the substrate or in a predetermined layer may be obtained.

By carrying out such a treatment, it becomes possible to more accurately know the existence and state of crystal defects and the distribution state thereof.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

In the present invention, the substrate or the predetermined layer is etched by anisotropic etching having a high selection ratio for crystal defects contained in the substrate or the predetermined layer. By performing such etching, as shown in FIG. 1C, a conical etching residue having crystal defects as apexes is exposed on the surface of the substrate or layer. The obtained conical etching residue can be captured by an optical microscope, SEM, or the like, and the number thereof is measured by a data processing device, a particle counter, or the like. Also,
Based on the principle shown in FIG. 2, for example, the diameter d of the bottom surface of the cone is measured from an image obtained by an optical microscope or the like, the base angle of the cone is measured using SEM, and the substrate (layer) before etching is further measured. The etching depth D from the surface is measured, and the formula (1) is used.
The depth (position) X _OX from the surface of the crystal defect is obtained by performing the calculation of. By adopting such a method, the distribution of crystal defects in the depth direction can be obtained.

FIG. 3 conceptually shows a procedure and an apparatus for evaluating a crystal defect according to the present invention. In the following, a case will be described as an example in which an oxygen precipitation defect generated by heat treatment in a silicon substrate or a silicon layer used for various semiconductor devices is used as an evaluation target sample. The heat-treated sample is first carried to a cleaning device to remove the native oxide film formed on its surface, and the native oxide film is removed (S1). After the natural oxide film is removed, the sample is subjected to an anisotropic etching with a high selective ratio according to the present invention, for example, an RIE (reactive ion etchin) device.
g) Carried into the equipment. The natural oxide film may be etched not only by the cleaning method as described above, but also by the same etching device or another etching device before the high selectivity anisotropic etching.

An etching gas is separately supplied into the etching apparatus from a gas supply apparatus (S3). As an etching gas, for example, a general magnetron RIE apparatus (Appl
When etching is performed using Precision 5000ETCH manufactured by ied Materials, it is preferable to use a halogen-based mixed gas (for example, HBr / NF ₃ / He + O ₂ mixed gas). This halogen-based etching gas is
With respect to oxygen precipitation defects in silicon, the etching selectivity increases in the order of F, Cl and Br. Therefore,
The detection sensitivity, that is, in order to generate more conical etching residues by this anisotropic etching,
The r-based gas is most preferable, and Cl and F are in that order.

By carrying out such anisotropic etching,
A conical etching residue as shown in FIG. 2 is exposed on the surface of the etched silicon substrate. Where RIE
Since the etching is performed by, the reaction product may adhere to the conical side surface of the etching residue. During anisotropic etching, it is considered that the reaction products attached to the side surface of the cone serve as a protective film and contribute to the maintenance of the cone shape, but when measuring the bottom diameter of the cone, it adversely affects the measurement accuracy. there is a possibility. Therefore, in this embodiment,
After the anisotropic etching is performed (S2), the conical side surface protective film is removed by immersing the sample in, for example, dilute hydrofluoric acid prior to the observation of the conical etching residue (S4). However, this side surface protective film removing step is not always necessary and can be omitted.

After the conical side surface protective film is removed, in this embodiment, the conical etching residue is captured and observed, for example, the size of the bottom portion of the conical etching residue is evaluated (S5). To capture and observe this residue, use an optical microscope,
SEM or the like is used. By observing the sample from above (etching direction) with an optical microscope, the number of conical etching residues in the exposed surface can be counted, and the bottom diameter d of the cone can be determined. Further, since the SEM can be used to observe each cone-shaped etching residue in more detail, by observing the cone from the side, in addition to accurate measurement of the bottom diameter d of each cone, the cone bottom angle θ Can also be detected. If the anisotropic etching conditions are the same, the base angle θ of the cone is almost constant regardless of the size of the cone. Therefore, the base angle θ of the cone corresponding to the anisotropic etching conditions performed is If known, the calculation process of the base angle θ using the SEM is not always necessary.

Further, in order to more accurately obtain the distance from the surface of the sample before anisotropic etching to the surface after etching, that is, the etching depth D as shown in FIG. At least a part of the surface of the sample is masked and left in the case of the reactive etching, and the remaining area,
The step (etching depth D) with the sample surface after anisotropic etching is measured by a stylus-type step measuring device (for example, a stylus-type surface shape measuring device manufactured by Tencor Co., Ltd., or a scanning tunnel which requires some time and effort. Measure with a microscope etc. (S
6). However, this step difference measurement can be omitted when the etching depth can be obtained from the etching speed and etching time of anisotropic etching.

The values obtained by measuring the number of conical residues, the bottom diameter d and the bottom angle θ, the etching depth D, etc. from the images and data obtained from the optical microscope, SEM, step measuring device, etc. as described above. The position X _ox of the crystal defect from the sample surface can be obtained by calculating the above-mentioned formula (1) based on the above, and the distribution of the crystal defect in the sample depth direction can be obtained.

In the present embodiment, in order to automate the evaluation of crystal defects, the image data obtained by the observing device or the step measuring device is sent to a data processing device such as a CPU, and the image data is processed by this processing device. (For example, binarization, contour enhancement, contour extraction, etc.), the bottom diameter d of the cone, the bottom angle θ, or the etching depth D is automatically obtained (S7).
If a particle counter is used for the same sample, the number of conical etching residues in the exposed surface can be automatically counted.

The result obtained by the above data processing is output to an output device such as a printer or a monitor (S8), and the evaluator can know the distribution of crystal defects from the output result, and the crystal Defect evaluation can be performed.

Here, FIG. 4 shows HBr / NF ₃ / He +.
A micrograph of oxygen precipitation defects detected when high selective ratio anisotropic etching was performed using a silicon substrate as a sample using an O ₂ mixed gas (FIG. 4A is an optical microscope image, FIG.
(B) is an SEM image.

As a comparative example, FIG. 5 shows Se of the same sample.
The micrograph after etching with the cco solution (FIG. 5A is a surface optical microscope photograph, and FIG. 5B is a sectional optical microscope photograph). As can be understood from FIG. 4, a conical etching residue is formed on the Si surface after the high selective ratio anisotropic etching with the oxygen precipitation defects as apexes (FIG. 4 (b)). It is observed as a round dot (black dot in the figure) (Fig. 4 (a)). The conical etching residues are randomly distributed in the surface and the heights thereof are different, but none higher than the etching depth D. Further, it can be seen from FIG. 4B that the sectional shapes of the conical projections are almost the same regardless of the size (in the example of FIG. 4, the base angle is about 80 °). This supports the fact that the distribution of oxygen precipitation defect densities in the depth direction can be easily obtained by measuring the diameter d of the conical etching residue by observation from the surface and determining the size distribution. On the other hand, as shown in FIG. 5, in the etching with the Secco solution, pits due to oxygen precipitation defects were observed near the center of the sample substrate in the sample cross section (white dots are pits corresponding to oxygen precipitation defects), but on the sample surface. , No oxygen precipitation defect was observed. From this, it is understood that the high selective ratio anisotropic etching as in the present invention can detect minute oxygen precipitation defects on the sample surface or the like which cannot be detected by the selective etching with the chemical liquid such as Secco liquid.

Further, according to the highly selective anisotropic etching of the present invention, it is possible to evaluate oxygen precipitation defects of at least about ten and several nm, and to detect when defects are detected using an infrared laser. It is possible to detect defects of the same size as or smaller than the possible size of defects (30 nm). Moreover, even with respect to the resolution in the depth direction of the sample, which can be obtained only by the infrared laser of about 2 μm, the method of the present invention can obtain the resolution in the depth direction of about 0.1 μm.

FIG. 6 shows the relationship between the minimum defect size detectable by the present invention and the anisotropic etching selection ratio of the present invention. As is clear from the figure, the size of the detectable defect changes depending on the etching selection ratio. Specifically, when the etching selection ratio is high (s2), a smaller crystal defect size (d2) can be detected than the detectable crystal defect size (d1) when the selection ratio is low (s1). it can. For example, when detecting an oxygen precipitation defect in a silicon crystal material, if a Br-based gas is used as an etching gas as described above, another halogen-based (Cl
The selection ratio is higher than that in the case of using a system gas or F system gas, and it becomes possible to detect a finer etching residue. Further, anisotropic etching is performed under two conditions with different selection ratios, the density distribution of the conical etching residue obtained in each case is measured, and if the difference between the two is taken, size d1 and size d
It is also possible to obtain the distribution of crystal defects having a size between 2 and 1. Then, by repeating this, it becomes possible to obtain the defect of each size in all the selection ratios capable of obtaining the distribution in the depth direction without etching.

In the present invention, if the base angle θ of the conical etching residue exposed on the etching surface becomes too large,
More conical etching residue is obtained in the plane, but the area of the bottom of the cone is very small. Therefore, it becomes difficult to observe the residue using an optical microscope and detect the diameter d of the bottom surface thereof. On the contrary, if the base angle θ of the etching residue becomes too small, the area of the bottom surface of the cone becomes large, so that the detection of the conical etching residue and the measurement of the diameter d of the bottom surface become easy, but the flat surface in the etching direction. A plurality of conical etching residues overlap each other, and the number of detected residues includes many errors with respect to the actual crystal defect density. Therefore, in the present embodiment, the etching conditions are set so that the base angle θ of the conical etching residue is easy to detect and measure by an optical microscope and is appropriate for the crystal defect density assumed in the sample. It is preferable to set.

When the number of conical etching residues is measured by the above particle counter, it is preferable to calibrate the particle counter in advance. Specifically, this calibration is performed by, for example, creating a conical etching residue of a certain size and measuring its size (for example, the diameter d of the bottom surface) in advance, and measuring another conical etching residue of a different size. It is realized by creating it, measuring its size, and storing them. If such a calibration is performed, when the crystal defects are actually evaluated, the size of the residue counted by the particle counter can be determined (whether the measured residue is a conical residue or not). You can know (including judgment). Furthermore, by using such a particle counter, it becomes easy to know the number of conical etching residues by size. For example, if the number of residues can be known for each base angle diameter d, the sample depth of crystal defects can be determined. The distribution in direction can be more easily evaluated.

In the present embodiment, in order to detect the crystal defect distribution in the thickness direction of the sample more easily, the cross section of the sample is exposed and the above-described high selective ratio anisotropic etching is applied to this cross section. Good. As a result, a conical etching residue as shown in FIG. 4B is generated on the exposed surface in the cross-sectional direction of the sample. If the residue is observed using an optical microscope and the number is measured, the above formula ( Without calculating 1),
It is possible to easily evaluate the crystal defect distribution in the sample thickness direction.

In the present embodiment, the sample targeted for crystal defect detection is not limited to the silicon substrate, but may be another material substrate, or a silicon layer formed on the substrate. Further, not only oxygen (SiO ₂ ) in the Si material, but also the etching gas and the etching conditions are made appropriate according to the material, so that nitrogen (SiN) and carbon (SiC) in the Si material can be removed. It is also possible to adopt a configuration in which detection is performed by high selective ratio anisotropic etching. In this case, Si
As the etching material for N and SiC, the above-mentioned Si is used.
It is possible to use a fluorine-based gas similarly to O _2, and the same effect as described above can be obtained by etching SiN and SiC with, for example, a fluorine-based gas material. In addition, Si in the SiO ₂ material and S in the SiN material
It is also possible to evaluate i in the Si material.

Although the crystal defect evaluation according to the present invention is a destructive test, for example, if a pattern is formed on a scribe line and other portions are masked to carry out the anisotropic etching of the present invention. It is possible to evaluate with a real wafer. Therefore, the crystal defects of the actual wafer can be examined at any time in the semiconductor manufacturing process.

[Embodiment] Next, the results of investigating the applicability of the high-selectivity anisotropic etching of the present invention to wafers subjected to various semiconductor manufacturing processes will be described.

In FIG. 7, CMOS processes are performed on Si wafers having different substrate oxygen concentrations, and at the time of drive-in heat treatment (diffusion of dopants for forming wells and formation of a defect-free layer on the surface). Of the wafer after the heat treatment of (1) is used as a sample, and high selective ratio anisotropic etching and
The micrograph of the Si surface and the cross section when etching with the Secco solution is shown.

When the substrate oxygen concentration is low (oxygen concentration: 1.
1 × 10 ¹⁸ cm ^-3 ), as shown in the lower left column of FIG.
In the etching with the cco solution, no oxygen precipitation defect could be detected in the vicinity of the surface, whether observed from the surface or cross section. On the other hand, when the high selective ratio anisotropic etching according to the present invention is performed, a conical etching residue caused by oxygen precipitation defects can be confirmed on the surface of the sample exposed by the etching. On the other hand, when the substrate oxygen concentration is high (oxygen concentration: 1.3 × 10 ¹⁸ cm ⁻³ , 1.6 × 10 ¹⁸ c
m ^-3 ), even in the etching with the Secco solution, in the cross-section observation shown in the lower part of FIG. 7, etch pits which are considered to be caused by oxygen precipitation defects are observed near the Si surface, and corresponding to this, the high selectivity of the present invention is selected. A large number of conical protrusions are observed by the anisotropic etching.

FIG. 8 shows the density of conical protrusions (etching residues) obtained by high selectivity anisotropic etching for each sample used in FIG. 7 and the like, and Secco solution etching at the wafer center for the same sample. Detected by BMD (bulk
micro defects) Plots of density relationships. As shown in FIG. 8, the high selectivity anisotropic etching of the present invention was applied to the defect-free region (Denuded Zone layer: D) near the sample surface.
(Z layer), the detected conical etching residue density was found to be BMD near the wafer center of the same sample.
It was found that it was almost proportional to the density. That is, it is understood that even with respect to minute crystal defects near the surface which cannot be detected by Secco liquid etching as described above, these can be detected by the high selective ratio anisotropic etching of the present invention.

[0039]

As described above, in the crystal defect evaluation of the present invention, by using high selective ratio anisotropic etching, it is possible to detect minute oxygen precipitation defects that cannot be detected by the conventional selective etching with a chemical solution. It is effective in evaluating oxygen precipitation defects when various processes are carried out.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a crystal defect evaluation principle of the present invention.

FIG. 2 is an explanatory diagram showing the principle of determining the position of oxygen precipitation defects from the surface based on the conical etching residue obtained in the present invention.

FIG. 3 is a diagram for explaining a crystal defect evaluation method and device according to the present invention.

FIG. 4 is a view showing a photomicrograph of a silicon substrate sample after high-selectivity anisotropic etching according to the present invention.

FIG. 5 is a view showing a micrograph of a silicon substrate sample obtained by a conventional crystal defect evaluation method after Secco solution etching.

FIG. 6 is a diagram showing the relationship between the selection ratio and the minimum detectable defect size in high-selectivity anisotropic etching according to the present invention.

FIG. 7 is a diagram showing a micrograph showing the relationship between the oxygen concentration in a silicon substrate sample and detected oxygen precipitation defects.

FIG. 8 is a conical etching residue density detected by high selective ratio anisotropic etching and a BM detected by Secco liquid etching at the center of a wafer sample according to an embodiment of the present invention.
It is a figure which shows the relationship with D density.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoyuki Yoshida, Nagakute-cho, Aichi-gun, Aichi Prefecture, Nagachoji, No. 41 Yokomichi, Yokosuka Central Research Institute Co., Ltd. (72) Koichi Mitsushima, Nagachite-cho, Aichi-gun, Aichi Prefecture 1 41, Yorokomichi Toyota Central Research Institute Co., Ltd. (56) Reference JP-A-8-17803 (JP, A) JP-A-9-162252 (JP, A) JP-A-5-273296 (JP, A) JP-A-7-118100 (JP, A) JP-A-55-68648 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/3065 H01L 21/66

Claims (2)

(57) [Claims] 1. The substrate or the predetermined layer is etched by anisotropic etching having a high selection ratio with respect to the crystal defect contained in the substrate or the predetermined layer, and the crystal defect is formed at the apex.
A method for evaluating a crystal defect, which comprises exposing a conical etching residue to be exposed to the surface of the substrate or layer, and evaluating the crystal defect based on the conical etching residue. 2. The substrate or the predetermined layer is etched by anisotropic etching having a high selectivity with respect to the crystal defects contained in the substrate or the predetermined layer, and the crystal defect is formed at the apex.
The observation means for capturing the conical etching residue exposed on the surface of the substrate or the layer, and the number of the conical etching residue, or the size of the cone of the etching residue, or both, and data are obtained to obtain the substrate. A crystal defect evaluation apparatus comprising: a data processing unit for determining both or one of a density and / or a distribution of crystal defects in a predetermined layer in a depth direction.