JP4370812B2 - SOI wafer inspection method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for inspecting defects existing in an SOI layer of an SOI wafer.
[0002]
[Prior art]
In recent years, an SOI wafer having an SOI (Silicon On Insulator) structure in which a silicon active layer is formed on an insulating silicon oxide film is used for a semiconductor device formed on the SOI layer at high speed, low power consumption, and high withstand voltage. In particular, it has attracted attention as a high-performance LSI wafer for electronic devices because of its excellent performance and environmental resistance. This is because an SOI wafer has a buried oxide (BOX) layer, which is an insulating layer, between the base wafer and the silicon active layer (SOI layer), so that an electronic device formed on the SOI layer has a charging property. This is because it has a great advantage that the soft error rate caused by minority carriers generated by, for example, α rays is also low because of high radiation resistance.
[0003]
In addition, with the recent high integration of semiconductor devices, there is a demand for the production of semiconductor wafers with high purity and low defects. For this purpose, it is important to accurately evaluate crystal defects and the like present in the semiconductor wafer. In order to reduce the defects in these semiconductor wafers, the substance of the defects is accurately grasped and appropriate for them. It is necessary to take appropriate measures. That is, it is necessary to improve the quality and productivity of semiconductor wafers by quickly inspecting defects and feeding back the results to the manufacturing process.
[0004]
As a method for detecting a crystal defect in a silicon single crystal wafer, the surface of the silicon single crystal wafer is subjected to a Secco solution (Secco solution: 0.15 mol of K ₂ Cr ₂ O ₇ aqueous solution and 49% hydrofluoric acid in a volume of 1: 2. Etch pits due to crystal defects are generated on the wafer surface by selective etching with an etching solution mixed at a ratio, and then defect detection is performed by observing the etch pits on the wafer surface with an optical microscope ( Patent Document 1).
[0005]
This selective etching solution is etched by a process of first oxidizing the silicon surface with an oxidizing agent K ₂ Cr ₂ O ₇ and then dissolving the silicon oxide with hydrofluoric acid. Usually, the defect region has a higher oxidation rate of silicon than the complete region without defects, so that the etching rate of the defect portion is increased, and as a result, the defect portion is selectively etched. This selective etching is performed until the etch pit becomes a size that can be observed with an optical microscope.
[0006]
However, when detecting a crystal defect in a thin SOI layer of an SOI wafer, unlike the above-described defect detection in a silicon single crystal wafer (bulk wafer), the etching cost for selective etching of the SOI layer is SOI. Since it is limited by the film thickness of the layer, there are cases where selective etching cannot be performed until the etch pit becomes a size that can be observed with an optical microscope. In addition, when an etching solution having a high etching rate such as Seco solution is used, it is difficult to appropriately control the etching amount and the remaining film thickness of the SOI layer, which may not be suitable for etching the thin film SOI layer. Therefore, a method of detecting defects in a thin-film SOI wafer by combining diluted Secco solution and etching with hydrofluoric acid is known (hereinafter referred to as Diluted Secco Etching method).
[0007]
In this diluted Secco etching method, first, crystal defects such as dislocations and OSF (Oxidation Induced Stacking Fault) existing in the SOI layer of the thin-film SOI wafer are selectively etched by the diluted Secco solution to generate etch pits. . Thereafter, when the SOI wafer is immersed in hydrofluoric acid, the hydrofluoric acid penetrates the buried oxide film from the portion where the etch pits of the SOI layer are generated, and the silicon oxide forming the buried oxide film is etched to generate etch pits. That is, the etch pit due to the defect in the SOI layer is transferred to the etch pit of the buried oxide film. Etch the etched oxide etch pits until they become observable, and expose the exposed etch pits from the surface of the SOI layer or by completely removing the SOI layer with a diluted secco solution. Defects can be detected by observing the surface of the oxide film with an optical microscope.
[0008]
Alternatively, the SOI wafer that has been etched with hydrofluoric acid as described above to generate etch pits in the buried oxide film is further etched with a dilute secoc solution to completely remove the SOI layer, and etch pits in the buried oxide film are formed. Then, an etch pit is generated on the surface of the base wafer, and the remaining buried oxide film is removed by etching with hydrofluoric acid, and the defect pit can be detected by observing the etch pit on the surface of the base wafer with an optical microscope. This method is called a four-stage dilution Secco etching method (Non-patent Document 1).
[0009]
On the other hand, a method of measuring defects in an SOI layer using a surface inspection apparatus using laser light is disclosed for detecting defects in the SOI layer (Patent Document 2). This method is a small crystal cavity having a size of about 0.1 μm to 0.3 μm formed by aggregation or growth of atomic vacancies (crystal lattice points lacking silicon atoms) introduced during the crystal growth process. This is suitable for the detection of defects, that is, defects generally called COP (Crystal Originated Particles), and is performed by the method described below.
[0010]
First, the thin-film SOI wafer is cleaned and etched with an alkaline cleaning solution to make the SOI layer about 0.1 μm thick, and minute etch pits due to COP defects are generated in the SOI layer, and then the SOI wafer is hooked. It is immersed in an acid solution, and the etch pits of the SOI layer are transferred to the buried oxide film, and then the defect is inspected by a surface inspection apparatus using laser light. By using the surface inspection apparatus in this way, high-precision defect inspection can be performed in a short time.
[0011]
[Patent Document 1]
Japanese Patent Publication No. 6-103714 [Patent Document 2]
Japanese Patent Laid-Open No. 11-74493 [Non-Patent Document 1]
UCS Semiconductor Fundamental Technology Study Group, SOI Science, p306-309, published by Realize Inc. [0012]
[Problems to be solved by the invention]
By the dilution etching method as described above, inspection by observing defects in the thin film SOI layer, for example, inspection of defect density and in-plane distribution can be performed. However, when defect density inspection is performed by observing etch pits with an optical microscope and counting them, the area that can be measured at once with an optical microscope is small, so it takes a lot of time to measure the entire wafer and at the same time The accuracy also deteriorates, and it may be impossible to measure the defect distribution in the wafer surface. In particular, when the defect density is low, there is a problem that the reproducibility of measurement is poor.
[0013]
In addition, the inspection method disclosed in Patent Document 2 uses a non-selective alkaline cleaning liquid in the first etching step, so that a defect is selectively etched using a selective etching liquid. Only atomic vacancy type defects such as COP are generated as minute etch pits. Therefore, etch pits cannot be generated from defects such as dislocation defects. That is, dislocation defects cannot be detected.
[0014]
On the other hand, even if an attempt is made to measure the surface of a thin-film SOI wafer on which etch pits are generated by a diluted Secco etching method using a surface inspection device such as a light scattering particle counter, the surface of the SOI layer or the buried oxide film is diluted Secco etching Since the surface is roughened, the laser light is scattered not only by the etch pit but also by the rough surface. This scattered light becomes noise with respect to the scattered light from the etch pit to be measured, making it difficult to measure with high accuracy. In some cases, the scattered light from the etch pit is buried in the noise, making it impossible to measure. There's a problem.
[0015]
In the case of the four-stage dilution Secco etching method, the buried oxide film is finally removed with a hydrofluoric acid solution to expose the surface of the base wafer with relatively little surface roughness, so that surface inspection using laser light is possible. However, in this method, the number of steps is increased, and etch pits are transferred from the thin film SOI layer to the buried oxide film and further to the base wafer. For this reason, for example, defects existing in the buried oxide film overlap. There is a fear.
For this reason, there has been a demand for a method capable of measuring various types of defects existing in a thin film SOI layer for manufacturing a semiconductor device, which is highly integrated, quickly, accurately and with good reproducibility.
[0016]
An object of the present invention is to provide an SOI wafer inspection method that solves the above-described problems and enables various types of defects existing in an SOI layer to be measured with high accuracy and good reproducibility in a short time. And
[0017]
[Means for Solving the Problems]
To achieve the above object, according to the present invention, there is provided a method for inspecting defects existing in an SOI layer of an SOI wafer having at least a base wafer, a buried oxide film, and an SOI layer, and selectively etching defects in the SOI layer. An etch pit is formed in the SOI layer by etching the SOI wafer with an etchant, and the buried oxide film is etched by immersing the SOI wafer in a hydrofluoric acid solution and etching the buried oxide film through the etch pit of the SOI layer. Etch pits are formed in the SOI wafer, and the SOI layer is etched away with an etchant whose silicon etching rate is higher than the etching rate of the buried oxide film. Of an SOI wafer characterized by measuring That provides査方method.
[0018]
In this way, etch pits due to defects are formed in the SOI layer of the SOI wafer with a selective etching solution, and the SOI wafer is immersed in hydrofluoric acid and transferred to the buried oxide film through the etch pits of the SOI layer. Then, the SOI wafer is etched away with an etchant whose silicon etching rate is faster than the etching rate of the buried oxide film to expose the buried oxide film with less surface roughness on the surface, and this is inspected. By doing so, various defects in the SOI layer can be inspected with high accuracy without being affected by surface roughness.
[0019]
This measurement when the etch pits, Ru can be performed by using a particle counter or an optical microscope of the light scattering etch pits of the buried oxide film.
In this way, by measuring the etch pits with a light scattering type particle counter, it is possible to detect the defect density and the in-plane distribution with good reproducibility in a short time. Further, by using an optical microscope, it is possible to directly observe the shape of etch pits and to count the density.
[0020]
Further, Jirutoru solution as a selective etchant, dash solution, seco liquid, light liquid, SATO solution, Cymel solution or hydrofluoric acid, nitric acid, Ru can be used at least one kind of mixed acid solution containing acetic acid.
In this way, by using an optimal selective etching solution according to the purpose such as the crystal orientation and the type of defect, it is possible to detect a defect with higher accuracy.
[0021]
Moreover, the selective etching solution further iodine or not preferable to contain an iodide.
Thus, by including, for example, potassium iodide in the selective etching solution, it is possible to prevent the occurrence of stains (stains) adhering to the wafer surface and at the same time to control the etching rate.
[0022]
Further, the SOI layer as an etchant to etch away, tetramethylammonium hydroxide, sodium hydroxide, potassium hydroxide, hydrazine, Ru can be used at least one of ethylenediamine.
Thus, by using an alkaline etching solution, the SOI layer can be removed without causing surface roughness of the buried oxide film.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, although embodiment of this invention is described, this invention is not limited to this.
As described above, a light scattering type surface inspection apparatus using a laser beam in a wafer inspection method in which defects in a thin film SOI layer are transferred as an etch pit to a buried oxide film or a base wafer to reveal the defects. By using this, the defect density and in-plane distribution can be inspected with high measurement accuracy and reproducibility in a short time.
[0024]
However, in the conventional diluted Secco etching method, the diluted Secco solution is used for etching removal of the SOI layer after transferring the etch pits to the buried oxide film, and the surface of the exposed buried oxide film was roughened. When the surface was irradiated with laser light, light scattering noise was generated due to surface roughness, and the surface inspection apparatus could not be applied. In the four-stage dilution Secco etching method, other problems such as an increase in the number of processes and multiple transfer of etch pits occurred.
[0025]
As a result of intensive studies to eliminate the above-described measurement difficulties due to surface roughness, the present inventors have found that after etching pit transfer to a buried oxide film using hydrofluoric acid, the etching rate of silicon becomes the etching rate of silicon oxide. The present invention was completed by finding that the above-mentioned problems can be solved by performing etching after removing the roughened SOI layer using a faster alkaline etching solution and exposing the buried oxide film.
[0026]
Hereinafter, an example of the process of the surface inspection method according to the present invention will be described with reference to FIG.
First, an SOI wafer including at least an SOI layer 1, a buried oxide film 2, and a base wafer 3 as shown in FIG. The SOI wafer may be produced by a bonding method in which a silicon oxide film is formed on at least one of two silicon single crystal wafers and bonded via the silicon oxide film, or oxygen ions are applied to the silicon single crystal wafer from the surface. May be produced by another method such as a SIMOX (Separation by IM planted OXide) method in which a silicon oxide film is formed by high-temperature heat treatment after the ion implantation is formed.
[0027]
Thinning for forming the SOI layer 1 of the SOI wafer can be performed by grinding and polishing, or may be performed by an ion implantation separation method. In the ion implantation separation method, for example, a silicon oxide film is formed on at least one of two silicon wafers, and at least one of hydrogen ions or rare gas ions is implanted from the surface of the bond wafer, and the inside of the bond wafer, for example, near the surface. After the microbubble layer (encapsulation layer) is formed on the substrate, the bond wafer is brought into close contact with the base wafer through the silicon oxide film on the ion implantation surface side, and then heat treatment (peeling heat treatment) is applied to cleave the microbubble layer ( This is a technology that peels the bond wafer into a thin film as a peeling surface, and further heat treatment (bonding heat treatment) to firmly bond the two silicon wafers to form an SOI wafer. A high thin film SOI wafer can be obtained relatively easily.
[0028]
A defect in the SOI layer 1 of this SOI wafer is treated with a selective etching solution for selectively etching (FIG. 1B). As the selective etching solution, in addition to the secco solution described above, a Zirtor solution (Sirtl solution: an etching solution comprising CrO ₃ , water and hydrofluoric acid), a dash solution (Dash solution: an etching solution comprising nitric acid, water, acetic acid and hydrofluoric acid). ), Light solution (Wright solution: etching solution made of CrO ₃ , nitric acid, water, acetic acid, hydrofluoric acid, Cu (NO ₃ ) ₂ ), Sato solution (Sato solution: etching solution made of nitric acid, water, acetic acid, hydrofluoric acid) ), Simmel liquid (Schimmel liquid: an etching liquid composed of CrO ₃ , water, hydrofluoric acid), or other ratio of mixed acid liquid (mixed liquid of hydrofluoric acid, nitric acid, acetic acid). As the selective etching solution, for example, an optimum one can be selected according to the crystal plane orientation of the SOI layer 1 to be etched and the type of defect to be detected. Further, the component ratio of the mixed acid solution may be changed in order to control the etching rate and the like.
[0029]
Further, an appropriate amount of iodine or iodide, such as potassium iodide, may be added to the selective etching solution. Thus, by adding iodine or iodide to the selective etching solution, it is possible to prevent the occurrence of stains attached to the surface of the SOI layer and at the same time to control the etching rate.
[0030]
By this etching process, the defective portion is etched faster than other portions having no defect, so that etch pits 4 are generated in the defective portion as shown in FIG. As the defects to be selected, for example, OSF, Dislocation (dislocation), BMD (Bulk Micro Defect), Shallow Pits, etc. are selectively etched with the above selective etching solution.
[0031]
This selective etching is preferably performed until the remaining film thickness of the SOI layer becomes 10 to 50 nm. By etching to such a thickness, the etch pits 4 due to defects in the SOI layer can be made sufficiently large, and the etch pits 4 are deep enough to allow the hydrofluoric acid to penetrate into the buried oxide film 2. As a result, the buried oxide film 2 is reached.
[0032]
Next, the SOI wafer is immersed in hydrofluoric acid. Then, as shown in FIG. 1C, hydrofluoric acid permeates the buried oxide film 2 through the etch pits 4 in the SOI layer, and the buried oxide film 2 is etched around the portion where the etch pits 4 in the SOI layer are generated. . That is, the defects existing in the SOI layer 1 are transferred to the etch pits 5 of the buried oxide film 2. While the SOI wafer is immersed, the permeated hydrofluoric acid continues to etch the buried oxide film 2 under the SOI layer, and this etching is large enough to be detected by the etch pits 5 of the buried oxide film 2. It is done until it expands. For example, the immersion time in hydrofluoric acid is about 5 to 20 minutes when the HF concentration is 50 wt% and about 2 to 5 hours when the HF concentration is 25 wt%.
[0033]
In this way, the SOI layer 1 of the SOI wafer in which the etch pits 5 are generated in the buried oxide film 2 is roughened by the selective etching solution, so that measurement using light scattering cannot be performed as it is. Therefore, the SOI layer 1 is removed. At this time, the roughened SOI layer 1 is removed sufficiently and quickly by using an etchant whose silicon etching rate is faster than the etching rate of the buried oxide film, and is buried in the surface. The oxide film 2 is exposed and the surface of the buried oxide film 2 is appropriately etched to obtain a sufficient surface state for subsequent optical measurement. As such an etchant, an alkaline solution such as tetramethylammonium hydroxide, sodium hydroxide, potassium hydroxide, hydrazine, or ethylenediamine, or a mixture of alcohol and this can be used by heating.
Thus, a sample from which the SOI layer 1 is removed by etching as shown in FIG.
[0034]
Next, in order to inspect the obtained sample with a surface inspection apparatus, fine foreign matters and dust such as organic matter and dirt on the surface are removed by washing. Cleaning is performed with a mixed solution of ammonia water and hydrogen peroxide solution (referred to as Standard Clean 1: SC1).
[0035]
Thus, since the surface of the cleaned sample as shown in FIG. 1 (e) is free from surface roughness and surface foreign matter, a highly accurate measurement can be performed in a short time using a surface inspection apparatus using a laser beam, for example, a light scattering particle counter. It can be performed with good reproducibility. It is also possible to observe the shape of etch pits with an optical microscope and to measure the defect density.
[0036]
【Example】
EXAMPLES Hereinafter, although an Example and comparative example of this invention are given and this invention is demonstrated concretely, this invention is not limited to these.
(Example)
An SOI wafer having an SOI layer thickness of 110 nm is made to have HF (50 wt%): HNO ₃ (61 wt%): CH ₃ COOH (99 wt%): H ₂ O: KI (0.1 mol solution) 1: 15: 6: 6: Etching with a selective etching solution (23 ° C.) mixed at a ratio of 0.0067 to set the remaining film thickness of the SOI layer to 50 nm.
[0037]
Next, it is immersed in a 50 wt% HF solution for 1 minute to form etch pits in the buried oxide film, and then the SOI layer is etched away with tetramethylammonium hydroxide (TMAH) solution. Furthermore, it wash | cleaned with the washing | cleaning liquid containing SC1, and produced the sample. The sample thus prepared was measured using a light scattering type particle counter (SP-1 manufactured by KLA-Tencor). The result is shown in FIG. 2 (however, the wafer periphery of 3 mm was excluded as a measurement target). As shown by the black dots in FIG. 2, pits due to defects were detected by the particle counter, and defect distribution (defect map) in the wafer surface was obtained. According to the defect map of FIG. 2, it is observed that there is a region where defects are slightly dense in the peripheral portion on the lower side of the drawing.
[0038]
(Comparative example)
Dipping in a 50 wt% HF solution was performed under the same conditions as in the examples, and then measurement was performed with a particle counter (SP-1 manufactured by KLA-Tencor) without removing the SOI layer by etching. However, the scattering noise was strong due to the rough surface of the SOI layer, and the measurement range of the measuring instrument was exceeded, and etch pits could not be detected.
[0039]
That is, as shown in the results of the examples and comparative examples, according to the present invention, by removing the SOI layer with a non-selective etching solution, it becomes possible to detect etch pits with a light scattering type particle counter, and more quickly and Defect inspection with high accuracy and reproducibility is possible. Therefore, if this inspection method is used, it will contribute to the improvement of the quality and productivity of the SOI wafer.
[0040]
The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.
[0041]
【The invention's effect】
As described above, etch pits caused by defects in the SOI layer of the SOI wafer are formed with a selective etching solution, and the SOI wafer is immersed in hydrofluoric acid and etched into the buried oxide film through the etch pits of the SOI layer. After the SOI wafer is transferred, the SOI layer is etched away with an etchant whose silicon etching rate is higher than the etching rate of the buried oxide film to eliminate surface roughness, and then the buried oxide film exposed on the surface is removed. By measuring etch pits, it is possible to measure etch pits with a surface inspection device such as a light scattering particle counter, and reproduce the in-plane distribution and density of defects with high accuracy in a short time. Detection can be performed with good quality.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a measurement flow of etch pits caused by defects in an SOI layer of an SOI wafer according to the present invention.
FIG. 2 is a diagram showing a defect map obtained by measuring an SOI wafer sample manufactured according to an embodiment of the present invention with a particle counter.
[Explanation of symbols]
1. . . SOI layer,
2. . . Buried oxide film,
3. . . Base wafer,
4). . . Etch pits generated in the SOI layer,
5. . . Etch pits generated in the buried oxide film.

Claims (4)

A method for inspecting defects existing in an SOI layer of an SOI wafer having at least a base wafer, a buried oxide film, and an SOI layer, wherein the SOI wafer is etched with a selective etchant that selectively etches defects in the SOI layer. Etch pits are formed in the SOI layer by immersing the SOI wafer in a hydrofluoric acid solution and etching the buried oxide film through the etch pits of the SOI layer to form etch pits in the buried oxide film. The SOI layer is formed using at least one of tetramethylammonium hydroxide, sodium hydroxide, potassium hydroxide, hydrazine, and ethylenediamine, which is an etchant whose silicon etching rate is higher than that of the buried oxide film. Etched away The inspection method of an SOI wafer, comprising the measurement of etch pits of the buried oxide film exposed on the surface by a particle counter of light scattering. 2. The method for inspecting an SOI wafer according to claim 1, wherein the selective etching of the SOI layer is performed until the remaining film thickness of the SOI layer reaches 10 to 50 nm.   3. The selective etching solution according to claim 1 or 2, wherein at least one of a Zirtor solution, a dash solution, a Seco solution, a light solution, a Sato solution, a Simer solution or a mixed acid solution containing hydrofluoric acid, nitric acid, and acetic acid is used. The method for inspecting an SOI wafer according to the description.   4. The SOI wafer inspection method according to claim 1, wherein the selective etching solution further contains iodine or iodide.

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