JP7078005B2 - Silicon wafer flattening method - Google Patents

Description

The present invention relates to a method for flattening a silicon wafer.

As a semiconductor substrate, a single crystal silicon substrate is widely used. Further, an insulating film such as a SiO ₂ layer is provided on the surface of the single crystal silicon substrate, and a single crystal silicon layer (generally referred to as an "active layer") serving as a semiconductor device forming region is formed via the insulating film. SOI wafers having an SOI (Silicon on Insulator) structure are attracting attention. Wafers having these various structures are used properly according to the semiconductor device to be formed. Hereinafter, in the present specification, a single crystal silicon substrate and an SOI wafer are collectively referred to as a silicon wafer, and a single crystal silicon substrate is referred to as a “bulk” silicon wafer.

Here, in order to obtain a silicon wafer having a desired thickness and thickness uniformity, a wafer sliced from a single crystal silicon ingot is subjected to a thinning step by grinding and polishing and a step of performing a finish polishing process. A flattening process may be performed by a local dry etching method such as plasma etching. Further, in the flattening process using the local dry etching method, the silicon wafer for the active layer is bonded to the bulk silicon wafer via an insulating film, and the active layer is obtained from the silicon wafer for the active layer. Widely used.

In the local dry etching method, the relative position between the etching nozzle of the etching gas (active seed gas) provided facing the wafer surface and the local work area on the wafer surface is scanned on the wafer surface. , It is usual to inject an etching gas onto the work area. Then, when scanning the relative position between the spray nozzle and the work area, as illustrated in FIG. 1, the spray nozzle is folded back and scanned in one horizontal direction along the wafer surface of the silicon wafer 10. The folded scan is repeated with a predetermined scan pitch width P. The scanning of the spray nozzle on the wafer surface is usually performed by stage scanning of the stage on which the silicon wafer 10 is placed. Hereinafter, for convenience of explanation, the "horizontal one direction" which is the main scanning direction in the above-mentioned folded scan may be referred to as the Y direction, and the "scanning pitch width direction" may be referred to as the X direction. Further, when the spray nozzle is simply "scanned" on the wafer surface, it means scanning the "relative position" between the spray nozzle and the work area. The relative position may be scanned by stage scanning, the relative position may be scanned by scanning the spray nozzle, or both scans may be combined.

In order to adjust the etching amount for each local workpiece region of the silicon wafer 10, the in-plane thickness distribution of the silicon wafer 10 is measured in advance prior to the flattening process. Then, it is common to calculate the etching amount for each work area based on the measurement result and adjust the scanning speed in the Y direction with respect to the surface of the wafer according to the etching amount of each part.

Further, the wafer surface flattening process by such a local dry etching method is performed one by one for a plurality of (usually several tens) wafers in one batch.

By the way, in the flattening process by the local dry etching method, the natural oxide film is removed by cleaning the silicon wafer, and the exposed surface of bare silicon (hereinafter referred to as "bare silicon") is processed. It is known that the process is performed and the process is performed with the natural oxide film left. Of these, in the former case, it is known that impurities such as particles adhere to the exposed surface of bare silicon, which may affect the processing quality. On the other hand, in the latter case, for example, as described in Patent Document 1, uneven thickness distribution spots can be formed at the same interval as the scanning pitch width P in the X direction in the wafer thickness distribution after the flattening process. It has been known.

In Patent Document 1, in order to prevent the formation of uneven thickness distribution spots in the cycle corresponding to the scanning pitch width P, first, the first spray nozzle scanning for removing the natural oxide film on the surface of the silicon wafer is performed. Then, a second spray nozzle scan is performed on the exposed surface of the bare silicon to flatten the silicon wafer.

Japanese Unexamined Patent Publication No. 2004-55931

However, in the method of Patent Document 1, the spray nozzle scans twice on the wafer surface of the silicon wafer, and there is a concern in terms of productivity. Further, the scanning condition for performing the second scan after the removal of the natural oxide film in Patent Document 1 is based on the thickness distribution of the silicon wafer with the natural oxide film attached, and is the second scan after the removal of the natural oxide film. The film thickness distribution state of is not always the same as that before the removal of the natural oxide film. Originally, in order to adjust the etching rate, it is necessary to measure the film thickness distribution in the state after removing the natural oxide film and adjust the etching rate using the measurement result, but such measurement is not realistic. Therefore, it is desirable that the spray nozzle is scanned once on the wafer surface.

Here, the present inventors conducted an experiment in which the active layer of the SOI wafer was flattened by a dry etching method under predetermined conditions with one scanning of the spray nozzle. An example of the film thickness distribution before and after processing at that time is shown in the graph of FIG. In the experiment, the scanning pitch width was set to 4 mm. From FIG. 2, it can be confirmed that uneven film thickness distribution spots are formed at a cycle corresponding to the scanning pitch width of 4 mm. Further studies by the present inventors have revealed that such film thickness distribution spots may or may not occur remarkably in units of one batch of the substrate for the active layer. Furthermore, it was found that during the processing of one batch, although the film thickness distribution unevenness did not occur in the first half of the batch, the film thickness distribution unevenness may occur in the latter half of the batch.

Bulk silicon wafers and active layers of SOI wafers are required to have various characteristics depending on the application. Regardless of the specific wafer conditions, it is required to establish a flattening processing method capable of suppressing the occurrence of uneven thickness distribution unevenness at a cycle corresponding to the scanning pitch width.

Therefore, an object of the present invention is to provide a method for flattening a silicon wafer, which can suppress the formation of uneven thickness distribution spots in a cycle corresponding to the scanning pitch width.

The present inventors diligently studied in order to solve the above-mentioned problems, and first considered the reason why the period of the uneven thickness distribution spot corresponds to the scanning pitch width. With reference to FIG. 3, the nozzle diameter of the spray nozzle 100 is referred to as D, and the scanning pitch width is referred to as P. Under the condition of D> P, the processing process of a local work area on the wafer surface of the silicon wafer 10 will be considered. The silicon wafer 10 is provided with a natural oxide film 12 (usually, the thickness of the natural oxide film 12 is several Å to several tens Å) on the surface of the bare silicon 11. It is considered that the etching gas 110 ejected from the spray nozzle 100 comes into contact with the surface of the silicon wafer 10 while diffusing directly under the spray nozzle 100. Here, the natural oxide film 12 is less likely to be etched (the etching rate is smaller) than the bare silicon 11. Therefore, the etching amount in one scan in the Y direction shows a predetermined processing shape distribution, and in the X direction, the region 10A from which the natural oxide film is completely removed and a part of the natural oxide film are removed. The formed region 10B is mixed and formed. Then, in the folded scan (broken line portion in FIG. 3) after the spray nozzle 100 has moved with the scanning pitch width P, the region 10A of only the bare silicon 11 and the region 10B from which the natural oxide film 12 is partially removed and the natural oxidation The region 10C from which the film has not been removed is mixed and etched.

In FIG. 4A, the nozzle diameter D of the spray nozzle 100 is 8 mm, the scanning pitch width P is 4 mm, and the standardized etching amount per scanning in the Y direction in the pitch width direction (X direction) of 4 mm to 12 mm is examined. It is a graph. It is assumed that the etching gas 110 etches the silicon wafer 10 within the range of the nozzle diameter D of the spray nozzle 100, and that the etching amount exhibits a Gaussian distribution. Further, the film thickness of the natural oxide film was set to correspond to the standardized etching amount of 1.0, and the etching rate ratio between the natural oxide film and bare silicon was set to 1: 2. Further, for the sake of simplicity, the natural oxide film is completely removed from 0 to 8 mm in the X direction by the scanning immediately before, and the area 10A is only the bare silicon 11. The natural oxide film is formed in the X direction: 8 mm or later. It is assumed that it remains completely. As shown in FIG. 4A, etching is performed according to the Gaussian distribution in the range of X direction: 4 to 8 mm, but the etching amount is asymmetrical with the range of X direction: 4 to 8 mm in the range of X direction: 8 to 12 mm.

The distribution of the accumulated normalized etching amount after scanning such local dry etching in the X direction with a scanning pitch width P: 4 mm is as shown in FIG. 4B. As shown in FIG. 4B, uneven distribution unevenness of the normalized etching amount in a cycle corresponding to the scanning pitch width P: 4 mm is formed. It is considered that such distribution unevenness of the etching amount is the cause of the thickness distribution unevenness after the flattening process.

Based on this consideration and the above-mentioned experimental facts with reference to FIG. 2, the formation of uneven thickness distribution spots in the period corresponding to the scanning pitch width is the film thickness of the natural oxide film of the silicon wafer and the local dryness. The present inventors have found that it largely depends on the periodic scanning conditions in the etching method. Therefore, the present inventors have conceived that the above problems can be solved by adjusting the periodic scanning conditions so as to suppress the formation of uneven thickness distribution spots according to the thickness of the natural oxide film of the silicon wafer. However, the present invention has been completed.
That is, the gist structure of the present invention is as follows.

(1) While spraying etching gas onto the wafer surface of a silicon wafer from a spray nozzle, the spray nozzle is folded back and scanned in one horizontal direction along the wafer surface, and the scanning is repeated at a predetermined scanning pitch width. A silicon wafer flattening method for flattening the entire surface of the wafer using a dry etching method.
The first step of obtaining the film thickness of the natural oxide film of the silicon wafer before the flattening process and
The second step of determining the periodic scanning conditions in the local dry etching method based on the obtained film thickness of the natural oxide film, and
A method for flattening a silicon wafer, which comprises a third step of flattening the silicon wafer based on the periodic scanning conditions.

(2) The periodic scanning condition in the second step is set to the scanning pitch width P in the direction perpendicular to the folded scanning direction of the spray nozzle with respect to the nozzle diameter D of the etching gas spray nozzle in the flattening apparatus. The method for flattening a silicon wafer according to (1) above, which is adjusted by using the ratio P / D.

(3) The method for flattening a silicon wafer according to (2) above, wherein the maximum value of the ratio P / D is 0.25, and the second step is performed within the range of the maximum value or less.

(4) The method for flattening a silicon wafer according to any one of (1) to (3) above, wherein the silicon wafer is an n-type silicon wafer having a specific resistance of 0.001 Ω · cm or less.

(5) The flatness of the silicon wafer according to any one of (1) to (4) above, wherein the silicon wafer in one batch contains a silicon wafer having a thickness of 0.7 μm or more of the natural oxide film. Chemical processing method.

(6) The wafer surface of the silicon wafer is the wafer surface of the silicon wafer for the active layer of an SOI wafer in which a silicon wafer for a support substrate and a silicon wafer for an active layer are bonded together via an insulating film. The method for flattening a silicon wafer according to any one of (1) to (5).

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a method for flattening a silicon wafer, which can suppress the formation of uneven thickness distribution spots in a cycle corresponding to the scanning pitch width.

It is a schematic diagram explaining the scanning direction and the scanning pitch width direction in a general local dry etching method. It is a graph which shows an example of the active layer film thickness distribution of the SOI wafer before and after the flattening processing using the local dry etching method by the experiment of the present inventors. It is a schematic diagram explaining the processing process of a local work area based on the consideration of the present inventors. It is a schematic diagram explaining the etching amount per scan of the work area by the consideration of the present inventors. It is a schematic diagram explaining the cumulative etching amount of the work area by the consideration of the present inventors. It is a flowchart explaining the flattening processing method of the silicon wafer by one Embodiment of this invention. It is a schematic diagram explaining the flattening processing method of the silicon wafer by one Embodiment of this invention. It is a graph which shows an example of the etching amount distribution in the X direction at the time of flattening a silicon wafer by the experiment of the present inventors. It is a graph which shows the relationship between the film thickness of the natural oxide film of the silicon wafer, and the size of the processing distribution unevenness by the experiment of the present inventors. It is a graph which shows the etching amount distribution in the X direction at the time of flattening a silicon wafer in Example 1. FIG. It is a graph which shows the etching amount distribution of another batch in Example 1. FIG. It is a graph which shows the film thickness distribution of the sir sound wafer before and after the flattening process using the local dry etching method in Example 1. FIG. It is a graph which shows the etching amount distribution in the X direction at the time of flattening a silicon wafer in Example 2. FIG. It is a graph which shows the etching amount distribution of another batch in Example 2. FIG. It is a graph which shows the etching amount distribution in the X direction at the time of flattening a silicon wafer in the comparative example 1. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 5A is a flowchart illustrating a method for flattening a silicon wafer according to an embodiment of the present invention, and FIG. 5B is a schematic diagram illustrating a preferred embodiment of the method for flattening a silicon wafer. The thicknesses of the bare silicon 11 and the natural oxide film 12 in FIG. 5B are exaggerated, unlike the actual thickness ratios. Further, although the natural oxide film 12 actually covers the entire surface of the bare silicon 11 in the silicon wafer 10, the natural oxide film 12 is shown on only one side of the silicon wafer 10 for the sake of simplification of the drawings. For the reference numerals, the above-mentioned FIGS. 1 and 2 are also referred to.

In the method for flattening a silicon wafer according to an embodiment of the present invention, as shown in the flowchart of FIG. 5A and the schematic diagram of FIG. 5B, the active seed gas is sprayed onto the wafer surface of the silicon wafer 10 from a spray nozzle. The entire surface of the wafer surface is flattened by using a local dry etching method in which the spray nozzle is folded back and scanned in one horizontal direction (Y direction) along the wafer surface and the scanning is repeated with a predetermined scanning pitch width P. The flattening method according to the present embodiment is a periodic method in the local dry etching method based on the first step S10 for obtaining the film thickness of the natural oxide film 12 of the silicon wafer 10 and the obtained film thickness of the natural oxide film 12. It includes at least a second step S20 for determining scanning conditions and a third step S30 for flattening the silicon wafer 10 based on periodic scanning conditions.

Prior to the description of the first step S10 to the third step S30, the scanning mode of the spray nozzle 100 for spraying the etching gas (active seed gas) of the flattening process in the present embodiment will be described again.

In the flattening process using the local dry etching method in the present embodiment, the relative position between the spray nozzle 100 provided facing the wafer surface of the silicon wafer 10 and the local workpiece region on the wafer surface is set on the wafer surface. The etching gas 110 is ejected to the work area while sequentially scanning above.

The local dry etching method is used in, for example, a dry flattening apparatus such as DCP (Dry Chemical Planarization) manufactured by Speedfam Co., Ltd. and PACE (Plasma Assisted Chemical Etching) manufactured by the same company.

Here, referring to FIG. 1, when scanning the relative position between the spray nozzle 100 and the work area, the spray nozzle is horizontal to the wafer surface of the silicon wafer 10 in one direction (Y direction). Is folded back, and this folded scanning is repeated with a predetermined scanning pitch width P. In general, in both the folded scan from the + Y direction to the −Y direction and the folded scan from the −Y direction to the + Y direction, the scan pitch width direction is the predetermined scan pitch width P each time. The spray nozzle 100 is scanned in the (X direction) direction. Further, although the scanning of the spray nozzle 100 on the wafer surface is generally performed by stage scanning of the stage on which the silicon wafer 10 is placed, the silicon wafer 10 is fixedly installed and the spray nozzle 100 is scanned. Alternatively, both may be investigated at the same time to adjust the relative position.

In order to adjust the etching amount for each processed region of the silicon wafer 10, it is common to adjust the etching amount as follows. That is, the in-plane thickness distribution of the silicon wafer 10 is measured in advance prior to processing, the target etching amount for each area to be processed is calculated based on the measurement result, and the target etching amount for each area is calculated. Adjust the scanning speed in the Y direction with respect to the wafer surface. In this way, the etching allowance to be actually processed is adjusted. Hereinafter, the details of each step will be described in sequence.

In the first step, the film thickness of the natural oxide film 12 of the silicon wafer 10 is obtained. It is known that the natural oxide film growth rate depends on the conductive type and the specific resistance of the silicon wafer 10, and in particular, it may be called an n-type silicon wafer (n ++ wafer) having a specific resistance of 0.001 Ω · cm or less. (There is), the growth rate of the natural oxide film is high (for example, "Matsutaka Hirose" Si Natural Oxide Film Control ", Applied Physics, Vol. 61, No. 11, pp. 1124-1131, 1992"). Therefore, the film thickness of the natural oxide film increases as the time required for the flattening process increases after the wafer cleaning step immediately before the flattening process is completed. Assuming that the silicon wafer 10 is actually flattened in one batch, in such a silicon wafer having a high growth rate of a natural oxide film, the silicon wafer to be flattened first and the silicon wafer to be flattened at the end of the batch are flattened. The film thickness of these natural oxide films is different from that of silicon wafers to be processed. According to the above-mentioned consideration by the present inventors, the larger the film thickness of the natural oxide film 12 of the silicon wafer 10, the more uneven thickness distribution unevenness occurs in the cycle corresponding to the scanning pitch width after the flattening process. It is estimated that it will be easier. Therefore, in the second and subsequent steps, the flattening conditions are set using the film thickness of the natural oxide film obtained in the first step.

In order to obtain the film thickness of the natural oxide film 12, it is possible to measure the film thickness of the natural oxide film 12 by a spectroscopic ellipsometer or the like. Further, the film thickness of the natural oxide film 12 is determined from the cleaning conditions in the process immediately before the flattening process, for example, the wafer cleaning process, and the film thickness, the time until the flattening process is performed, and the silicon wafer 10 are obtained. It is also possible to obtain the thickness 12 of the natural oxide film of the silicon wafer 10 based on the natural oxide film growth rate of the above. again,

By the way, prior to the detailed explanation of the second step, the experimental results which are the basis for reaching the present invention will be described. The present inventors conducted the following experiments in order to verify the hypothesis of the above-mentioned considerations with reference to FIGS. 3 and 4A and 4B.

As a flattening apparatus using a dry etching method, a plasma etching apparatus (manufactured by Speedfam Co., Ltd., model number: DCP-200X) was used to flatten the surface of the silicon wafer after HF (hydrofluoric acid) cleaning. The nozzle diameter of the spray nozzle 100 is fixed at 8 mm for three types of silicon wafers having different thicknesses of natural oxide films, and the scanning pitch width P of the stage on which the silicon wafer 10 is placed is 4 mm, 2 mm, and 1 mm. The flattening process was performed by adjusting each of the above. The film thickness of the natural oxide film was adjusted by adjusting the washing time. FIG. 6 shows the etching amount distribution in the X-axis (scanning pitch width direction) cross section when the silicon wafer 10 having a natural oxide film thickness of 7 Å is flattened. The etching amount distribution is obtained from the difference in film thickness distribution of the silicon wafer before and after processing, and noise caused by the wafer thickness distribution is removed.

With reference to FIG. 6, the average value of the difference between the maximum etching amount and the minimum etching amount in the etching amount distribution with the scanning pitch width P as the cycle is defined as “the height difference of the etching processing spot”. FIG. 7 shows the correspondence between the film thickness of the natural oxide film, the scanning pitch width P, and the height difference of the etching processing spots.

From the graph of FIG. 7, the following experimental facts were found. First, when the film thickness of the natural oxide film is 3 Å, no significant difference is observed in the height difference of the etching-processed spots regardless of whether the scanning pitch width is 4 mm, 2 mm, or 1 mm. Further, when the film thickness of the natural oxide film is 7 Å, the height difference of the etching-processed spots becomes large when the scanning pitch width is 4 mm, but the height difference of the etching-processed spots is observed when the scanning pitch width is 2 mm or less. do not have. Further, when the film thickness of the natural oxide film is 15 Å, the larger the scanning pitch width, the larger the height difference of the etching processing spots.

Therefore, it is confirmed that the shorter the scanning pitch width P, the more the height difference of the etching processing spots can be suppressed. However, as the scanning pitch width P is shortened, the number of stage scans in the scanning pitch width direction (X direction) increases, so that the processing time per wafer increases, and the processing time per wafer increases. When it comes to processing time, the effect affects productivity. Therefore, when performing the flattening process, the scanning pitch width P should be determined in consideration of productivity and processing quality. Based on this idea, in the case of this experimental example, if the film thickness of the natural oxide film is 3 Å, the scanning pitch width may be any of 4 mm, 2 mm, and 1 mm, but it can be judged that 4 mm is preferable. Further, if the film thickness of the natural oxide film is 7 Å, the scanning pitch width may be either 2 mm or 1 mm, but in consideration of productivity, it is preferably 2 mm. It is understood that when the film thickness of the natural oxide film is 15 Å, the scanning pitch width may be used properly according to the allowable height difference of the etching processing spots.

Therefore, in the second step S20 according to the present embodiment, the periodic scanning conditions in the local dry etching method are determined based on the film thickness of the natural oxide film 12 obtained in the first step S10. For example, as shown in FIG. 7, the thickness of the natural oxide film and the periodic scanning conditions are set for each loading and flattening device, with the etching allowance in the flattening process and the gas type of the etching gas 110 as fixed conditions. Periodic scanning conditions may be appropriately determined according to the desired productivity and processing quality, such as by obtaining a calibration line indicating the correspondence with the height difference of the etching processing spots in advance.

The periodic scanning conditions in the second step are set in a direction perpendicular to the folded scanning direction (Y direction) of the spray nozzle with respect to the nozzle diameter D of the etching gas spray nozzle 100 in the flattening apparatus using the local dry etching method. It is preferable to adjust by using the ratio P / D of the pitch width P in the scanning pitch width direction, that is, the X direction. Only the scanning pitch width P may be adjusted, only the nozzle diameter D may be adjusted, or both may be adjusted. Since the adjustment of the nozzle diameter D involves replacement of equipment parts, it is easier and more efficient to adjust only the scanning pitch width P. In particular, it is preferable that the maximum value of the ratio P / D is 0.25 and the second step is performed within the range of the maximum value or less. When the thickness of the natural oxide film 12 of the silicon wafer 10 is 7 Å or more as in the above experimental example, for example, the nozzle diameter D is set to 8 mm and the scanning pitch width P is set to be within the range of 1 to 2 mm. It is also preferable to determine the periodic scanning conditions.

Then, in the third step S30, the silicon wafer 10 is flattened based on the periodic scanning conditions determined in the second step S20. In the flattening process by the third step, the silicon wafer 10 is flattened based on the scanning speed in the Y direction of the etching gas spray nozzle 100 adjusted based on the in-plane thickness distribution of the silicon wafer 10 measured in advance prior to the process. It is preferable to make a silicon wafer.

By the above first step S10 to third step S30, it is possible to suppress the formation of uneven thickness distribution unevenness in the cycle corresponding to the scanning pitch width P in the silicon wafer after the flattening process.

It is also preferable to sequentially apply the flattening processing method according to the present invention to a plurality of silicon wafers 10 in one batch. That is, the first sheet, the second sheet, ... Nth sheet (N is a natural number, which is a numerical value less than the number of sheets included in one batch), ... One sheet to the final silicon wafer. It is also preferable to apply the flattening processing method according to the present invention one by one (see FIG. 5B). As described above, the growth rate of the natural oxide film is particularly high in an n-type silicon wafer having a specific resistance of 0.001 Ω · cm or less. Therefore, in such a case, the film thickness of the natural oxide film of the silicon wafer in one batch may be different between the beginning and the end of the flattening process. However, if the method of the present invention is applied, the flattening process is performed in consideration of the film thickness of the natural oxide film, so that it is possible to suppress the height difference of the etching process spots in one batch.

The silicon wafer to which the present invention is applied is not particularly limited. As the bulk silicon wafer, a single crystal silicon wafer made of a silicon single crystal can be used. As the single crystal silicon wafer, a single crystal silicon ingot grown by the Czochralski method (CZ method) or the floating zone melting method (FZ method) can be sliced with a wire saw or the like. Further, carbon and / or nitrogen may be added to the single crystal silicon wafer. Further, any impurities may be added to obtain n-type or p-type. In addition, both an epitaxial silicon wafer in which a silicon epitaxial layer is formed on the surface of a bulk silicon wafer and an SOI wafer in which a silicon wafer for a support substrate and a silicon wafer for an active layer are bonded are subject to the application of the present invention. Is.

It is preferable to apply the present invention to an n-type silicon wafer having a specific resistance of 0.001 Ω · cm or less. As mentioned above, since the growth rate of the natural oxide film of the so-called n ++ silicon wafer is particularly fast, the film thickness of the natural oxide film increases during the flattening process of one batch, and the height difference of the etching process spots increases in the first half of the batch. This is because even if the processing effect is small, the processing effect can be large in the latter half of the batch.

Further, it is preferable to apply the flattening processing method according to the present invention to the silicon wafer 10 in which the film thickness of the natural oxide film 12 is 0.7 μm or more. This is because, according to the above-mentioned experimental results, when the film thickness of the natural oxide film is 0.7 μm or more, the height difference of the etching processing spots due to the periodic scanning conditions is significantly exhibited.

Further, it is also preferable that the flattening processing method according to the present invention is applied to an SOI wafer. That is, it is preferable that the wafer surface of the silicon wafer is the wafer surface of the silicon wafer for the active layer, which is an SOI wafer formed by laminating a silicon wafer for a support substrate and a silicon wafer for an active layer via an insulating film. This is because when the active layer of the SOI wafer is thinned by grinding and polishing, the processing quality of the flattening process by local dry etching is particularly important. In this case, the size of the film thickness of the natural oxide film of the support substrate wafer is not limited at all, and the conductive type and the specific resistance of the support substrate wafer are also arbitrary. On the other hand, it is preferable to apply the present invention when the wafer for the active layer is an n-type silicon wafer having a specific resistance of 0.001 Ω · cm or less. Further, it is preferable to apply the present invention when the film thickness of the natural oxide film of the active layer wafer is 0.7 μm or more.

Although a silicon oxide film is generally used as the insulating film between the silicon wafer for the support substrate and the wafer for the active layer, it is not limited as long as it is an electrical insulator, and diamond-like carbon (DLC; Diamond Like Carbon) etc. can also be used.

Further, when the present invention is applied to a plurality of silicon wafers 10 in one batch, the film thickness of the natural oxide film 12 of the first silicon wafer 10 is taken into consideration as described above in consideration of the growth rate of the natural oxide film. Is the minimum. Further, the film thickness of the natural oxide film 12 increases as the time until the processing process elapses. Therefore, when the number of processed sheets in one batch is large, the film thickness may affect the height difference of the etching processing spots from the Nth sheet (N is a natural number). Therefore, the periodic scanning condition may be determined so as to change the periodic scanning condition from the Nth sheet. Generally, about 25 silicon wafers are subjected to flattening in one batch.

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

<Example 1>
Two batches (25 wafers per batch) of n-type silicon wafers (n ++ wafers) with a specific resistance of 0.001 Ω · cm were prepared. The cleaning conditions by HF cleaning were adjusted so that the film thickness of the natural oxide film immediately after cleaning was set to 3 Å. The film thickness of the natural oxide film was measured by a spectroscopic ellipsometer. The film thickness of the final natural oxide film of each batch is described in "Matsutaka Hirose" Si Natural Oxide Film Control ", Applied Physics, Vol. 61, No. 11, pp. Based on "1124-1131, 1992", it is about 8 Å.

Two batches of this silicon wafer were flattened using a plasma etching apparatus (manufactured by Speedfam, model number: DCP-200X). Then, referring to the result of the graph of FIG. 7 described above, the nozzle diameter D of the etching gas spray nozzle was set to 8 mm, and the scanning pitch width P was set to 2 mm (ratio P / D was 0.25). The etching amount distributions in the X-axis (scanning pitch width direction) cross section of the final silicon wafer of each batch are shown in FIGS. 8A and 8B, respectively. The height difference between the etching-processed spots of the first batch and the second batch was 1 nm or less, respectively, and the thickness distribution of the silicon wafer after processing was also good for each batch. As a typical example, FIG. 9 shows the thickness distribution of the wafer before and after the flattening process of the final sheet of the first batch.

<Example 2>
Two batches of the same silicon wafers as in Example 1 were prepared, and the cleaning conditions by HF cleaning were the same as in Example 1. Then, two batches of silicon wafer flattening treatments were performed in the same manner as in Example 1 except that the scanning pitch width P in Example 1 was changed from 2 mm to 1 mm (ratio P / D was 0.125). .. The etching amount distributions in the X-axis (scanning pitch width direction) cross section of the final silicon wafer of each batch are shown in FIGS. 10A and 10B, respectively. The height difference between the etching-processed spots of the first batch and the second batch was 1 nm or less, respectively, and the thickness distribution of the silicon wafer after processing was also good for each batch. However, the total processing time per batch was increased by 30% as compared with Example 1.

<Comparative Example 1>
Two batches of the same silicon wafers as in Example 1 were prepared, and the cleaning conditions by HF cleaning were the same as in Example 1. Then, one batch of silicon wafer flattening treatment was performed in the same manner as in Example 1 except that the scanning pitch width P in Example 1 was changed from 2 mm to 4 mm (ratio P / D was 0.50). .. FIG. 11 shows the etching amount distribution in the X-axis (scanning pitch width direction) cross section of the final silicon wafer. The height difference of the etched spots is 17 nm. In the thickness distribution of the silicon wafer after processing, uneven thickness distribution spots with a cycle corresponding to a scanning pitch width of 4 mm were confirmed.

From the above results, it was confirmed that by applying the present invention, it is possible to suppress the formation of uneven thickness distribution spots in the cycle corresponding to the scanning pitch width.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a method for flattening a silicon wafer, which can suppress the formation of uneven thickness distribution spots in a cycle corresponding to the scanning pitch width.

10 Silicon wafer 11 Bare silicon 12 Natural oxide film 100 Spray nozzle 110 Etching gas D Nozzle diameter P Scanning pitch width

Claims (6)

A local dry etching method in which an etching gas is sprayed onto the wafer surface of a silicon wafer from a spray nozzle, the spray nozzle is folded back and scanned in one horizontal direction along the wafer surface, and the scanning is repeated at a predetermined scanning pitch width. Is a silicon wafer flattening method for flattening the entire surface of the wafer surface using the above method.
The first step of obtaining the film thickness of the natural oxide film of the silicon wafer before the flattening process, and
The second step of determining the periodic scanning conditions in the local dry etching method based on the obtained film thickness of the natural oxide film, and
A method for flattening a silicon wafer, which comprises a third step of flattening the silicon wafer based on the periodic scanning conditions. The periodic scanning condition in the second step is set in a direction perpendicular to the folded scanning direction of the spray nozzle with respect to the nozzle diameter D of the etching gas spray nozzle in the flattening apparatus for flattening the silicon wafer . The method for flattening a silicon wafer according to claim 1, wherein the ratio P / D of the scanning pitch width P is used for adjustment. The method for flattening a silicon wafer according to claim 2, wherein the maximum value of the ratio P / D is 0.25, and the second step is performed within the range of the maximum value or less. The method for flattening a silicon wafer according to any one of claims 1 to 3, wherein the silicon wafer is an n-type silicon wafer having a specific resistance of 0.001 Ω · cm or less. Claims 1 to 1, wherein a silicon wafer having a thickness of 0.7 nm or more of the natural oxide film is included when the flattening process is performed one by one on a plurality of the silicon wafers in one batch. The method for flattening a silicon wafer according to any one of 4. The wafer surface of the silicon wafer is the wafer surface of the silicon wafer for the active layer of an SOI wafer in which a silicon wafer for a support substrate and a silicon wafer for an active layer are bonded together via an insulating film. 5. The method for flattening a silicon wafer according to any one of 5.

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