JP5347309B2 - Method for evaluating SOI wafer and method for manufacturing SOI wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for detecting the bonding defect of a bonding interface of an SOI wafer with high sensitivity. <P>SOLUTION: The method of evaluating the SOI wafer having an insulating layer and an active layer in this order on a support substrate comprises forming a pattern on the insulating layer after removing part of the active layer from the insulating layer by wet or dry etching; and evaluating the bonding interface between the active layer and insulating layer by observing whether the pattern peels and/or whether a bonding defect is caused. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for evaluating a bonding interface between an active layer and an insulating layer of an SOI (Silicon-on-Insulator) wafer, and more specifically, a supporting side substrate and an active side substrate in a bonded SOI wafer manufacturing process. The present invention relates to an SOI wafer bonding interface evaluation method capable of evaluating the adhesive strength of a local portion of a bonding interface after bonding.

  An SOI wafer is a wafer having a multilayer structure in which a thin silicon layer that is insulated and separated is formed on a silicon substrate, and a semiconductor device that operates at high speed and operates at low power consumption is formed by forming device elements on the silicon layer. Can do.

In an SOI wafer, an unjoined portion at the interface between an active layer (SOI layer) and an insulating layer (BOX layer) becomes a void called a void. This void is known to greatly affect the quality of SOI wafers because it causes device defects and the like (see, for example, Patent Document 1).
JP-A-08-250567

  As a technique for detecting voids at the bonding interface of SOI wafers, there is a void inspection apparatus using ultrasonic waves. This void inspection device can detect voids with a gap width greater than the detection limit of the bonding interface part, but there is no gap or gap with a gap width smaller than the detection limit, but it cannot detect a part with weak adhesive strength. There is.

  Peeling inspection is also employed as a method for detecting voids. The peeling inspection is a method in which an adhesive sticker is attached to the surface of the active layer after bonding and peeled off. According to this method, the active layer is locally peeled off when the seal is peeled off at the portion where the bonding strength of the bonding is weak. However, in the peeling inspection, it is very difficult to always keep the adhesive strength of the seal used for the inspection constant, so that the inspection results vary greatly. Therefore, there is a problem that the quality variation of the product becomes large in the peeling inspection.

  Conventionally, even in an SOI wafer that has passed inspection by a void inspection apparatus or peeling inspection, a defect may occur in a device process. This is thought to be because there are voids that cause device element failures after passing through the device process in places that could not be detected by these inspections, and parts with weak adhesive strength, which are manifested by various loads in the device process. It is done. Therefore, in order to reduce the device defect rate in the SOI wafer, on the side closer to the device process, it is more sensitive than the inspection by the void inspection apparatus or peeling inspection, for detecting the void at the bonding interface or the weak portion of the bonding strength. Means are sought.

  Accordingly, an object of the present invention is to provide means for detecting a bonding failure at a bonding interface of an SOI wafer with high sensitivity.

As a result of intensive studies in order to achieve the above object, the present inventors have found that a void portion that has not been detected by an ultrasonic void inspection apparatus or peeling inspection or a portion having low adhesive strength passes through a device process in an SOI wafer. As a result, we obtained new knowledge that various external loads can cause defects due to device element peeling and void swelling. Therefore, the present inventor has further studied based on the above knowledge, and found that a bonding failure that causes element peeling in the device can be evaluated with high sensitivity by subjecting the SOI wafer to a process that approximates the device process. .
The present invention has been completed based on the above findings.

That is, the above object was achieved by the following means.
[1] A method for evaluating an SOI wafer having an insulating layer and an active layer in this order on a support substrate,
By removing a part of the active layer from the insulating layer, a pattern is formed on the insulating layer, and the appearance of the SOI wafer after the pattern formation is inspected. A method for evaluating an SOI wafer, characterized by evaluating a bonding interface between the insulating layer and the insulating layer.
[2] The method for evaluating an SOI wafer according to [1], wherein a part of the active layer is removed by etching.
[3] The method for evaluating an SOI wafer according to [2], wherein the etching is wet etching or dry etching with an alkaline solution.
[4] The method for evaluating an SOI wafer according to any one of [1] to [3], further comprising immersing the SOI wafer from which a part of the active layer has been removed in a hydrofluoric acid solution.
[5] The SOI wafer evaluation method according to any one of [1] to [4], wherein the pattern has a size of 10 μm square to 100 μm square.
[6] The SOI wafer evaluation method according to any one of [1] to [5], wherein an interval between the patterns on the insulating layer is in a range of 5 μm to 100 μm.
[7] preparing a lot of SOI wafers comprising a plurality of SOI wafers having an insulating layer and an active layer in this order on a support substrate;
Extracting at least one SOI wafer from the lot;
Evaluating the quality of the extracted SOI wafer;
A method for manufacturing an SOI wafer, comprising shipping another SOI wafer in the same lot as an SOI wafer determined to be non-defective by the evaluation as a product wafer,
The method according to claim 1, wherein the extracted SOI wafer is evaluated by the method according to any one of [1] to [6].

  ADVANTAGE OF THE INVENTION According to this invention, the bonding interface defect (part with weak adhesive strength and the part where a micro space | gap exists) which influences the quality of an SOI wafer can be detected with high sensitivity.

The present invention relates to a method for evaluating an SOI wafer having an insulating layer and an active layer in this order on a support substrate. According to the evaluation method of the present invention, a part of the active layer is removed from the insulating layer to form a pattern on the insulating layer. The bonding interface is evaluated.
In an SOI wafer, device elements are formed in an active layer through etching or the like in a device process. Passing various loads from the outside by passing through such a device process is considered to be a cause of failure due to peeling of device elements or swelling of voids. Therefore, in the present invention, a device element (island pattern) is simulated by patterning the active layer of the SOI wafer. As a result, the SOI wafer is placed in a state close to the device process. Therefore, the island-shaped pattern is peeled off at the portion where the device element is peeled off due to poor bonding in the device process. Thereby, it is possible to detect a bonding failure of the SOI wafer that causes a device failure. Such poor bonding includes a portion having a low adhesive strength that does not form a minute void or void. These bonding defects are difficult to detect by the conventional inspection method, although they cause device defects. On the other hand, according to the present invention, it is possible to detect even the above-mentioned bonding failure with high sensitivity.
Hereinafter, the evaluation method of the present invention will be described in more detail.

The evaluation object in the evaluation method of the present invention is an SOI wafer having an insulating layer and an active layer in this order on a support substrate. An SOI wafer is generally manufactured by the following method, but the evaluation method of the present invention can be applied to an SOI wafer manufactured by any method.
(1) Another silicon wafer is bonded to one silicon wafer on which an oxide film (insulating film) is formed, and one of the bonded silicon wafers is ground and polished to reduce the thickness of the SOI layer. Forming method (grinding polishing method)
(2) After forming an ion implantation layer by implanting hydrogen ions or the like into the surface layer of the silicon wafer (active layer wafer) on the SOI layer side, the ion implantation layer is bonded to the silicon wafer for the support substrate, and then subjected to the above heat treatment. A method of forming an SOI layer that is thinned by peeling with an ion implantation layer (smart cut method)

The material constituting the SOI wafer to be evaluated by the evaluation method of the present invention is not particularly limited. Usually, the support substrate is a silicon substrate, the active layer is a silicon layer, the insulating layer is an oxide film, and specifically Is a SiO ₂ film. The silicon substrate and the silicon layer may contain a dopant such as boron.

  The thicknesses of the support substrate and the insulating layer are not particularly limited. On the other hand, if the active layer is excessively thick, it takes a long time to remove a part of the layer and form a pattern. Therefore, the thickness is preferably 30 μm or less. The thickness of the active layer in the SOI wafer is generally 2 μm or more.

Etching is preferably used as a method for forming a pattern by partially removing the active layer of the SOI wafer. Etching is preferably performed by forming an oxide film as a mask on the active layer, particularly when alkaline etching is performed. Alkali etching has a large selection ratio between silicon and a silicon oxide film, so that a sufficient protective effect can be obtained by forming a thin oxide film.
Below, the process of the pattern formation by etching and bonding interface evaluation is demonstrated. FIG. 1 shows an example of a flow of pattern formation by etching and an evaluation process. In FIG. 1, (a) shows the case of wet etching, and (b) shows the case of dry etching. However, the present invention is not limited to the embodiment shown below, and it is of course possible to change or modify each process as appropriate.

(A) In case of wet etching A protective oxide film against wet etching is formed on the active layer side surface of the bonded SOI wafer. The protective oxide film can be formed by dry oxidation or wet oxidation. The oxidation is preferably performed in an oxygen-only atmosphere or a mixed atmosphere of oxygen and nitrogen. The thickness of the protective oxide film is preferably determined by the etching time and the silicon / silicon oxide film etching rate ratio.

  Next, after applying a photoresist solution on the protective oxide film, an unnecessary portion is removed by exposure to produce an etching resist on the protective oxide film. The above process can be performed by known photolithography.

  Thereafter, the portion of the surface of the protective oxide film that is not covered with the resist is removed. The protective oxide film can be removed by immersing the wafer in a hydrofluoric acid aqueous solution (HF solution) or a buffered HF solution usually used in the device process.

  Next, the wafer from which the mask oxide film has been removed as described above is wet etched. As an etchant, an alkali solution generally used in a device process is preferable. Among them, tetramethylammonium hydroxide (TMAH) and potassium hydroxide (KOH), which are anisotropic etching and etching is inhibited by a BOX layer (oxide film layer). ) Is preferred. The concentration of the etching solution is preferably in the range of 3% by mass to 40% by mass, and the temperature is preferably in the range of 60 ° C. to 90 ° C. In general, the etching rate of the TMAH aqueous solution is silicon / silicon oxide film = 1000/1, and the etching rate of the KOH aqueous solution is around 100/1. When an aqueous solution of TMAH or KOH is used as an etchant and the etching time is about 5 to 30 minutes, the thickness of the protective oxide film is preferably 200 to 5000 mm.

  In the case of wet etching, attention should be paid to the relationship between the size of a pattern formed by partially removing the active layer (hereinafter also referred to as “simulated island”) and the thickness of the active layer. When the scrub line portion is etched from the active layer surface to the BOX layer, the surface of the simulated island is covered with a protective oxide film, but is etched from the lateral direction. For this reason, if the etching time is long, the simulated island may disappear due to etching. In order to shorten the etching time, the thickness of the active layer can be previously reduced to, for example, about 1 μm to 5 μm by polishing or etching. This makes it possible to produce a small-sized simulated island.

  Through the above steps, a part of the active layer in the SOI wafer is removed from the insulating layer, whereby a pattern of the active layer is formed on the insulating layer. The pattern can be produced only in a part of the surface, or only in a part where bonding failure is likely to occur. However, it is preferable that the pattern is formed on the entire surface of the SOI wafer in order to detect the in-plane bonding failure evaluation without omission. The shape of the pattern to be left as a simulated island may be any shape such as a square or a circle. For example, the square pattern preferably has a size of 10 μm square to 100 μm square. The circular pattern preferably has a diameter of 10 to 100 μm. The interval between the patterns is preferably in the range of 5 μm to 100 μm. If the pattern of the said size is formed at the said space | interval, the bonding defect in a wafer can be detected accurately.

The appearance of the SOI wafer after the above wet etching is inspected, and the interface state is observed by observing the pattern (simulated island) adhesion failure (floating, partial peeling) and the presence or absence of pattern peeling. The presence or absence of voids due to locally weak parts or poor adhesion can be evaluated. For example, it can be determined that there is a bonding failure at the interface as described above in a portion where the pattern is lifted or partially peeled off or peeled off. In the present invention, based on the above determination criteria, an SOI wafer in which the number of simulated islands causing adhesion failure and peeling is equal to or greater than a certain value can be determined as a defective product including many bonding failures. According to the study by the present inventor, it has been clarified that the above evaluation method can detect an SOI wafer which causes a defect in the device process even though it is determined as a non-defective product in the conventional void inspection. Thereby, an SOI wafer with higher reliability can be provided.
The appearance inspection can be performed visually or with an optical microscope, and is preferably performed under a condenser lamp or a fluorescent lamp.

  It is also preferable to immerse in a hydrofluoric acid solution (HF solution) in order to emphasize the weakly bonded portion of the simulated island produced in the active layer portion. The HF solution used here is preferably a hydrofluoric acid aqueous solution having a concentration of 1% by mass to 50% by mass. The immersion time in the HF liquid is preferably in the range of 1 minute to 50 minutes per immersion. There is no restriction | limiting in particular about the frequency | count of immersion, What is necessary is just to set suitably. The strength of the adhesive strength can be relatively evaluated by the relationship between the HF immersion time and the timing at which the simulated island is peeled off. Regarding the size, when the size of the simulated island is sufficiently smaller than the size of the poorly bonded portion (void or low adhesion portion), it can be evaluated by counting the number of the simulated islands peeled off.

(B) In the case of dry etching In dry etching, a mask oxide film is unnecessary and a resist film is used as a mask.
Since the isotropic etching is performed from the portion where the resist film is removed and the etching is inhibited by the oxide film, a simulated island can be formed only in the active layer.
Processes other than etching are the same as in the case of the wet etching.

  Through the above steps, it is possible to detect with high sensitivity even a bonding failure that cannot be detected by a conventional inspection method.

[SOI wafer manufacturing method]
The method for producing an SOI wafer according to the present invention comprises a step of preparing a lot of SOI wafers comprising a plurality of SOI wafers having an insulating layer and an active layer in this order on a support substrate, and extracting at least one SOI wafer from the lot. A process for evaluating the quality of the extracted SOI wafer, and manufacturing another SOI wafer in the same lot as the SOI wafer determined to be non-defective by the evaluation as a product wafer. In the method, the extracted SOI wafer is evaluated by the SOI wafer evaluation method of the present invention.

  As described above, according to the SOI wafer evaluation method of the present invention, the bonding failure at the interface between the active layer and the insulating layer of the SOI wafer, in particular, microvoids and low adhesion portions that could not be detected by the conventional inspection method. , Can be detected with high sensitivity. According to the method for manufacturing an SOI wafer of the present invention, a product wafer can be shipped after evaluating and confirming the presence or absence of a bonding defect that cannot be detected by a conventional inspection method but causes a problem in an actual device process. Therefore, a high-quality SOI wafer can be provided with high reliability. In addition, the reference | standard which determines with a non-defective product, and the number of sample wafers extracted from 1 lot can be set in consideration of the physical property calculated | required by a wafer according to the use etc. of a wafer.

  Hereinafter, the present invention will be further described based on examples. However, this invention is not limited to the aspect shown in the Example.

[Example 1]
FIG. 2 is an explanatory diagram of a process for preparing a simulated island by wet etching using TMAH. The sample used is an SOI wafer having an active layer thickness of 15 μm, a BOX layer thickness of 1.5 μm, a supporting substrate thickness of 600 μm, and a diameter of 150 mm.
The wafer was subjected to oxidation treatment by placing it in an oxygen atmosphere (5 L / min) at 950 ° C. for 45 minutes to form a protective oxide film having a thickness of 300 mm on the active layer (1 in FIG. 2).
Next, a photoresist was applied on the protective oxide film (2 in FIG. 2), and the protective oxide film was patterned by exposing the coated surface through a photomask (3 in FIG. 2). Next, in order to remove the mask oxide film from the patterned wafer, the wafer was immersed in a buffered HF (HF: ammonium fluoride = 1: 15) solution for 70 seconds. Next, in order to remove the resist film, cleaning with sulfuric acid / hydrogen peroxide (sulfuric acid: peroxide water = 5: 1) was performed for 2 minutes, followed by pure water rinsing for 10 minutes.
After that, by immersing in a TMAH aqueous solution having a concentration of 5% by mass and a liquid temperature of 85 ° C. for 17 minutes and performing wet etching, a part of the active layer was removed in a pattern to produce a simulated island on the BOX layer (6 in FIG. 2). .

[Example 2]
When manufacturing an SOI wafer having an active layer thickness of 15 μm, a BOX layer thickness of 1.5 μm, a supporting substrate thickness of 600 μm, and a diameter of 150 mm, a foreign substance is intentionally attached to the BOX layer before bonding the BOX layer and the active layer. About the performed wafer, the wet etching was performed by the method similar to Example 1. FIG. The wafer after wet etching was immersed in a hydrofluoric acid aqueous solution (concentration: 25% by mass) for 5 minutes in order to emphasize the weak bonding strength. The result of having observed the wafer surface after the said process with the optical microscope is shown in FIG. As shown in FIG. 3, peeling of the simulated island was confirmed.
A device failure was observed when a device was fabricated using an SOI wafer that was intentionally adhered and bonded together in the same manner as described above.

[Example 3]
FIG. 4 shows a photograph of a simulated island produced by dry etching. Since dry etching is isotropic, the cross section of the simulated island is like Mt. Fuji. Development was performed after exposure to form a 100 μm square pattern. Thereafter, a simulated island was formed in the active layer portion by chemical dry etching.

[Example 4]
As with the SOI wafer used in Example 2, a simulated island was formed by the same method as in Example 3 for a SOI wafer that was intentionally adhered and adhered to a foreign object. Peeling was confirmed.

  According to the present invention, a high-quality bonded SOI wafer can be provided.

An example of the flow of pattern formation by etching and an evaluation process is shown. It is explanatory drawing of the simulation island preparation process (Example 1) by the wet etching using TMAH. 3 is an optical micrograph of a wafer surface processed in Example 2. FIG. 4 is an optical micrograph of the wafer surface processed in Example 3. FIG.

Claims (7)

A method for evaluating an SOI wafer having an insulating layer and an active layer in this order on a support substrate,
By removing a part of the active layer from the insulating layer, a pattern is formed on the insulating layer, and the appearance of the SOI wafer after the pattern formation is inspected. A method for evaluating an SOI wafer, characterized by evaluating a bonding interface between the insulating layer and the insulating layer. The method for evaluating an SOI wafer according to claim 1, wherein a part of the active layer is removed by etching. The method for evaluating an SOI wafer according to claim 2, wherein the etching is wet etching or dry etching using an alkaline solution. The method for evaluating an SOI wafer according to claim 1, further comprising immersing the SOI wafer from which a part of the active layer has been removed in a hydrofluoric acid solution. The SOI pattern evaluation method according to claim 1, wherein the pattern has a size of 10 μm square to 100 μm square. The method for evaluating an SOI wafer according to claim 1, wherein an interval between the patterns on the insulating layer is in a range of 5 μm to 100 μm. Preparing a lot of SOI wafers comprising a plurality of SOI wafers having an insulating layer and an active layer in this order on a support substrate;
Extracting at least one SOI wafer from the lot;
Evaluating the quality of the extracted SOI wafer;
A method for manufacturing an SOI wafer, comprising shipping another SOI wafer in the same lot as an SOI wafer determined to be non-defective by the evaluation as a product wafer,
The said method of evaluating the extracted SOI wafer by the method of any one of Claims 1-6 characterized by the above-mentioned.