JPH1174493A - Inspecting method for defect of soi wafer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a defect estimating method wherein very fine defects on an SOI wafer can be inspected easily by a usual inspecting apparatus for the surface of a wafer. SOLUTION: As an inspecting method for defects S on an SOI wafer 3, the SOI wafer 3 is cleaned by an alkali-based cleaning liquid to make 0.1 μm the film thickness of the SOI layer of its surface. Thereafter, the SOI wafer 3 is dipped in a hydrofluoric acid solution to generate very fine etching pits caused by the defects S on the SOI layer. Then, these very fine etching pits are dipped into the foregoing hydrofluoric acid solution as they are to increase their etching pits sizes to measure thereafter their defect density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting defects near the SOI layer on the surface of an SOI wafer.
In particular, the present invention relates to a defect evaluation method for SOI wafers for evaluating minute defects.

[0002]

2. Description of the Related Art Defects are introduced into a semiconductor crystal formed by the CZ method during a crystal growth process, and these defects are called grown-in defects. Above all, the vacancies (crystal lattice points lacking Si atoms) introduced during the crystal growth process are gathered or grown during the cooling process to form small crystal cavities having a size of about 0.1 μm to 0.3 μm. The existence of a defect has been confirmed, and this minute defect is generally called COP (Crystal Originated Particle).

When a wafer is immersed in an alkaline solution or the like,
An etch pit is generated by the COP defect, and the COP defect can be evaluated by inspecting the etch pit using an evaluation device such as a laser beam. Conventionally, a method using a laser spectroscopy system for evaluating a crystal surface by irradiating a semiconductor wafer with laser light for minute defects such as COP and detecting scattering of laser light due to these crystal defects is known. . The aforementioned Green
LSTD (Laser Scattering Tomogra
phy Defect).

As another method for measuring defects, a scanning electron microscope (SE) is used.
M) and a transmission electron microscope (Transmis
A measurement method using a sionElectron Microscope (TEM) is known.

[0005] In recent years, as the integration of MOS memories becomes higher, the miniaturization of LSIs advances, and demand for thin-film SOI wafers is expected to increase in order to form DRAMs of 64/256 megabits in the future. In addition, 1
As the demand for gigabit DRAMs increases, the demand for thin-film SOI wafers having a thin-film SOI layer of 0.2 μm or less is expected to increase in order to improve the degree of integration of DRAMs.

Therefore, in order to evaluate a defect existing in a thin film SOI wafer, a highly accurate defect evaluation technique is required by a simple method.

[0007]

However, the COP present on a thin-film SOI wafer having a thin SOI layer as described above is difficult to inspect, unlike the COP present on a normal bulk wafer. That is, when inspection is performed by an apparatus using ordinary laser light, scattering noise occurs due to the thin SOI layer and the film interface of the buried oxide film interposed between the SOI layer and the support substrate, and the COP having a diameter of 0.2 μm or less Is very difficult. Also,
In addition, when an apparatus such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) is used, the inspection is difficult because the COP defect is very small.

On the other hand, as a method for evaluating defects of an SOI substrate having a thin SOI layer, a wafer is subjected to secco etching (an etching method using an etching solution containing hydrofluoric acid and potassium dichromate) to form an etch pit due to a minute defect. In addition, a method has been proposed in which the etch pits are enlarged to a measurable size, and then the surface SOI layer is removed to evaluate defects (J. Electr).
ochem. soc. , Vol. 140, no. 6, J
une 1993, 1713-1716).

However, this method mainly evaluates dislocation defects and is not suitable for evaluating COP. That is, C
When this method is applied to the evaluation of OP, since the seco etching has an effect of selectively etching silicon,
Dislocation defects are generated as etch pits together with the COP defects in the OI layer. Therefore, even if the etch pits formed by secondary enlargement are evaluated, whether they are due to the etch pit dislocations or COP defects It is not possible to distinguish whether it is due to the COP defect, and accurate density measurement of COP defects cannot be performed.

Also, metal particles such as chromium contained in the etching solution adhere to the surface of the SOI wafer,
There is a risk that the I wafer will be contaminated,
In this method, it is necessary to remove the surface SOI layer and the buried oxide film layer, and there is a problem that crystal evaluation becomes complicated. In addition, since the evaluation is performed by removing the SOI layer on the surface, there is a disadvantage that the external shape of the COP defect cannot be observed.

The present invention has been made in view of the above problems, and
Provided is an SOI wafer defect inspection method that can easily evaluate a COP defect existing in an OI wafer by using a normal wafer surface defect evaluation device and can also evaluate an appearance shape of the COP defect. The purpose is to do.

[0012]

According to the first aspect of the present invention, there is provided a method for inspecting a defect existing in an SOI wafer, wherein the SOI wafer is cleaned with an alkaline cleaning solution, and the SOI layer is formed to a thickness of 0.1 μm. With the following film thickness,
A defect inspection method for an SOI wafer having a configuration in which minute etch pits are generated in an SOI layer, and the SOI wafer is immersed in a hydrofluoric acid solution to remove a buried oxide film immediately below the minute etch pits and then measure a defect density. It is.

According to the present invention, since the SOI wafer is cleaned with an alkaline cleaning solution, there is no effect of etching dislocation defects in the SOI layer, and only COP defects can be generated as etch pits. By cleaning the surface of the SOI layer until the thickness becomes
The COP interposed in the OI layer is exposed on the surface of the SOI wafer, and the COP is exposed as minute etch pits. After that, when the SOI wafer is immersed in a hydrofluoric acid solution, the hydrofluoric acid solution is passed through the etch pits.
The buried oxide film immediately below the etch pit penetrates into the buried oxide film layer of the I wafer, and is partially removed by etching.
Secondary etch pits corresponding to each of the minute etch pits are formed in the oxide film layer, and the secondary etch pits can be easily measured by a normal wafer surface defect evaluation device.

As described above, even for a very small COP defect, the enlarged etch pit can be recognized using a normal surface defect device by intentionally expanding the secondary etch pit during the hydrofluoric acid immersion treatment. The COP defect density can be measured.

According to a second aspect of the present invention, there is provided the defect inspection method for an SOI wafer according to the first aspect, wherein the defect density is measured by a wafer surface inspection apparatus using a laser beam. .

The above-mentioned etch pits generated secondarily in the oxide film layer can be easily measured by a usual wafer surface defect evaluation apparatus. That is, the surface SOI layer is thinned to a thickness of 0.1 μm or less by the above-mentioned cleaning treatment, and the buried oxide film immediately below the minute etch pit is partially removed. Since the confusion intensity of the next etch pit becomes strong and is not influenced by the confusion light noise generated at the film interface between the SOI layer and the buried oxide film layer, the density of COP defects is measured by a defect evaluation device using laser light. Can be performed.

The invention described in claim 3 of the present application is an SOI
This is a method for inspecting a defect present in a wafer, wherein the SOI wafer is cleaned with an alkaline cleaning solution, and the SOI layer is removed from the wafer.
A thickness of 1 μm or less and minute etch pits are generated in the SOI layer. The SOI wafer is immersed in a hydrofluoric acid solution to remove a buried oxide film immediately below the trace etch pit. This is a defect inspection method for an SOI wafer configured to observe a defect shape by a microscope.

As described above, according to the present invention, COP defects can be evaluated without removing the surface SOI layer as in the prior art, so that a scanning electron microscope (SEM) and a transmission electron microscope (TEM) ) Etc., or
Using an atomic force mycroscope (AFM), the external shape of the COP defect can be observed, and the evaluation technique can be improved.

In the present invention, since the surface SOI layer is cleaned with an alkaline cleaning solution, it also has the function of removing organic substances and the like on the substrate surface.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

FIG. 1 is a flowchart showing the procedure of the method for inspecting defects of an SOI wafer according to the present invention. FIG. 1 (a)
FIG. 1 is a flowchart showing an evaluation method when an evaluation is performed on a normal bulk wafer, and FIG.
9 is a flowchart illustrating an evaluation method when an evaluation is performed on an SOI wafer.

First, a normal bulk wafer is arbitrarily selected, and SC1 cleaning (etching cleaning with an alkaline solution of NH ₄ OH, H ₂ O ₂ , H ₂ O solution) is performed a plurality of times, in this example, 5 times. Cleaning (80 ° C., 10 minutes) is performed to generate etch pits due to COP defects in the bulk wafer.

The etch pit density of minute etch pits having a particle size of 0.1 μm to 0.2 μm is measured using a defect inspection apparatus using a laser beam on the bulk wafer having generated the etch pits. Then, a bulk wafer having a different etch pit density is arbitrarily selected.

Next, a bulk wafer of the same lot as the selected bulk wafer is taken out and a thin film SOI wafer is formed.

The bulk wafer is formed by a conventional bonding method, for example, in the order shown in FIGS.
It is formed on an I substrate.

That is, first, as shown in FIG. 2A, the bulk wafer 1 is a first semiconductor wafer for forming an SOI layer, and the other wafer 2 is a second semiconductor wafer on a support side. . Where s is the bulk wafer 1
It is a COP defect present in it.

Next, as shown in FIG. 2B, an oxide film 1a to be a derivative layer is formed on the surface of the first semiconductor wafer A1. After that, a cleaning treatment is performed on the bonding surfaces of both the first semiconductor wafer 1 and the second semiconductor wafer 2,
As shown in FIG. 2 (3), the semiconductor wafers 1 and 2 are adhered at room temperature and heat-treated at a temperature of 800 ° C. or higher to increase the adhesive strength. Then, as shown in FIG.
An unbonded portion generated in a peripheral portion of the bonded wafers 1 and 2 is ground. After that, as shown in FIG. 2 (5), the surface of the first semiconductor wafer 1 is ground and thinned, and the SOI wafer 3 is manufactured.

FIGS. 3 (1) to 3 (3) are cross-sectional views showing the state of the SOI wafer 3 which has been subjected to processing for COP defect evaluation.

First, the SOI wafer 3 was subjected to SC1 cleaning (etching cleaning with an alkaline solution of NH ₄ OH, H ₂ O ₂ , and H ₂ O solution) five times (80 ° C., 10 minutes),
The thickness of the surface SOI layer of the I-wafer 3 was set to 0.1 μm or less.

At this time, as shown in FIG.
The COP defects s, s existing in the I layer are exposed on the surface of the SOI layer, and etch pits p, p are generated.

Thereafter, as shown in FIG.
When the OI wafer 3 is immersed in a hydrofluoric acid solution having a concentration of 25% for 4 minutes, the hydrofluoric acid solution penetrates to the buried oxide film 1a through the etch pits p, and the buried oxide film 1a around the etch pits p and p is partially etched. The removed circular pits p ′ and p ′ having a size of about 4 μm are formed in the oxide film 1.
a.

When the treated SOI wafer 3 is inspected by a surface defect evaluation device using a normal laser beam, the circular pits p 'and p' are found to have an interface between the SOI layer and the buried oxide film layer 1a. As a result, a high level of scattering intensity not affected by the confusion noise of light is obtained, so that the circular pits p ′ and p ′ can be easily recognized and measured.

As described above, conventionally, it has been difficult to inspect COP defects having a diameter of 0.2 μm or less in the SOI layer by the ordinary wafer surface inspection method, whereas in the present invention, the SOI wafer 3 is cleaned with an alkaline cleaning solution. The SOI layer is reduced to 0.1 μm or less by immersion in a hydrofluoric acid solution, thereby forming circular pits p ′ and p ′ in the oxide film layer 1 a in which the buried oxide film 1 a immediately below the COP is removed by etching. By doing so, it becomes possible to perform defect evaluation using a normal wafer surface defect evaluation apparatus.

Further, since the SOI layer is not deleted in the present invention, the appearance shape of the COP defect is determined using a microscope such as a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an atomic force microscope (AFM). Can be observed.

Next, the results of evaluation of an SOI wafer using the method of the present embodiment will be described.

FIG. 4 is a diagram showing inspection results of inspection of the bulk wafers A, B, and C and the thin film SOI wafers A ', B', and C 'using a laser spectroscopy system.

The bulk wafer A is a wafer formed from an ingot obtained by the CZ method, and A ′ is a thin film SOI wafer formed from the bulk wafer of the same lot as the bulk wafer A. B is an annealed bulk wafer, and B 'is an annealed thin-film SOI wafer formed from the same lot of bulk wafers as B. Also, C
Is a bulk wafer that has been subjected to a hydrogen annealing treatment,
C ′ is obtained by subjecting a thin film SOI wafer formed from a bulk wafer of the same lot as C to a hydrogen annealing treatment.

As shown in FIG. 4, each of the bulk wafers A,
The etch pit densities of B and C and the thin film SOI wafers A ', B' and C 'which have been subjected to the same processing as those of the bulk wafers show substantially the same value. , And C, it was confirmed that COP defects could be evaluated in the same manner as for bulk wafers.

As described above, according to the method of this embodiment, COP defects existing in a thin-film SOI wafer can be reliably evaluated using a wafer surface defect apparatus using ordinary laser light. It was confirmed that the COP defect can be evaluated even when each processing such as the processing is performed.

FIG. 5 is a diagram showing one of the circular pits existing in the thin film SOI wafers A ′, B ′, and C ′ observed with an optical microscope. FIG. FIG. 2 shows a diagram observed by an intermittent microscope (AFM).

As shown in FIG. 5, when the etch pits existing in the SOI wafers A ', B', and C 'are observed by an optical microscope, a minute etch pit p exists at the center, and a lower layer exists under the etch pit p. It can be confirmed that the periphery of a certain oxide film 1a is etched to form an enlarged circular etch pit p '.

As shown in FIG. 6, when the above-mentioned circular etch pit is observed with an atomic force microscope (AFM),
The appearance shape of the hollow COP defect can be clearly confirmed, and a useful result can be obtained in performing the defect evaluation.

In this example, the SOI wafer was immersed in a 25% hydrofluoric acid solution for 4 minutes to form circular etch pits. By changing the immersion time or the concentration of the immersion liquid, the etch pits were changed. Can be changed, and wafer surface defects can be evaluated according to various evaluation devices.

[0044]

As described above, according to the method of the present invention, since the SOI wafer is cleaned with an alkaline cleaning solution, there is no effect of etching dislocation defects in the SOI layer, and only COP defects are etched. By cleaning the surface of the SOI layer to a thickness of 0.1 μm or less, CO2 existing in the SOI layer can be generated.
P defects are exposed on the surface of the SOI wafer, and COPs appear as minute etch pits. Thereafter, when the SOI wafer is immersed in a hydrofluoric acid solution, the hydrofluoric acid solution penetrates into the buried oxide film layer of the SOI wafer through the etch pits, and the buried oxide film immediately below the etch pits is partially removed by etching. Secondary etch pits corresponding to each pit are formed in the oxide film layer.

The etch pits secondarily generated in the oxide film layer can be easily measured by a usual wafer surface defect evaluation apparatus. That is, since the surface SOI layer is thinned to a thickness of 0.1 μm or less by the cleaning process and the buried oxide film immediately below the minute etch pit is partially removed, the scattering at the etch pit is secondary. Since the intensity is high and is not affected by the influence of scattered light noise generated at the film interface between the SOI layer and the buried oxide film layer, the density of COP defects can be measured by a defect evaluation apparatus using laser light.

According to the method of the present invention, COP defects of 0.2 μm or less existing in SOI wafers, which have been difficult to evaluate in the past, can be easily inspected using a normal wafer surface defect evaluation device. In addition, since defect inspection can be performed without removing the SOI layer as in the related art, it is also possible to observe the external shape of the defect, thereby improving the defect evaluation technology.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a defect evaluation method for an SOI wafer according to the present invention.

FIG. 2 is a cross-sectional view illustrating a method for manufacturing an SOI wafer according to the present invention.

FIG. 3 is a cross-sectional view showing a state of an SOI wafer according to the present invention.

FIG. 4 shows test results, each bulk wafer and each SOI.
It is a figure showing the result of having measured the etch pit density of a wafer.

FIG. 5 is a view showing test results and observing circular etch pits present on an SOI wafer with an optical microscope.

FIG. 6 is a view showing a test result and observing an external shape of an etch pit existing in an SOI wafer by AFM.

[Explanation of symbols]

 Reference Signs List 1 First semiconductor wafer 1a Oxide film 2 Second semiconductor wafer 3 SOI wafer A Bulk wafer A 'SOI wafer B Bulk wafer B' SOI wafer C Bulk wafer C 'SOI wafer s Defect p Etch pit p' Circular pit

Claims (3)

[Claims] 1. A method for inspecting a defect present on an SOI wafer, comprising: cleaning an SOI wafer with an alkaline cleaning solution;
The layer is formed to a thickness of 0.1 μm or less, and minute etch pits are generated in the SOI layer. The SOI wafer is immersed in a hydrofluoric acid solution to remove the buried oxide film immediately below the minute etch pits. A defect inspection method for an SOI wafer, comprising measuring a density. 2. The defect inspection method for an SOI wafer according to claim 1, wherein the defect density is measured by a wafer surface inspection device using a laser beam. 3. A method for inspecting defects present in an SOI wafer, comprising: cleaning an SOI wafer with an alkaline cleaning solution;
The layer has a thickness of 0.1 μm or less, and minute etch pits are generated in the SOI layer. The SOI wafer is immersed in a hydrofluoric acid solution to remove a buried oxide film immediately below the trace etch pit. A defect inspection method for an SOI wafer, wherein a defect shape is observed by a microscope or an atomic microscope.