JP2015138897A - Method for cleaning or inspection of vapor-phase growth apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for cleaning or inspection of a vapor-phase growth apparatus by which heavy metal contamination caused by the maintenance of a vapor-phase growth apparatus can be reduced.SOLUTION: A method for cleaning or inspection of a vapor-phase growth apparatus 1 comprises the step of reducing an oxygen density in a reaction chamber 3 to under an oxygen density in atmospheric air in cleaning or inspection which involves the opening of the reaction chamber 3 of the vapor-phase growth apparatus 1 put in an airtight condition, whereby the increase in heavy metal contamination caused by the maintenance of the vapor-phase growth apparatus 1 can be suppressed. Especially, the oxygen density in the reaction chamber 3 is made 100 ppm or below, whereby the heavy metal contamination can be prevented more effectively.

Description

  The present invention relates to a method for cleaning or checking a vapor phase growth apparatus.

  As a substrate processing apparatus used in the manufacturing process of a semiconductor substrate such as a silicon wafer, a CVD (Chemical Vapor Deposition) apparatus (vapor phase growth apparatus) is known. As an example of epitaxial processing of a silicon wafer, single crystal silicon is formed on the surface of the silicon wafer. A production method for vapor phase epitaxial growth of an epitaxial film made of is developed.

  As a manufacturing method thereof, a silicon wafer is horizontally arranged on a susceptor housed in an epitaxial growth chamber (reaction vessel) of a vapor phase growth apparatus, and then the silicon wafer is rotated while rotating the susceptor around a vertical rotation axis. Heating is performed at a high temperature (1000 to 1200 ° C.) with a heat source such as a halogen lamp, and a process gas is supplied. As a result, silicon produced by thermal decomposition and reduction of the reaction gas is deposited on the wafer surface, and an epitaxial film made of single crystal silicon grows on the wafer surface.

  In this way, contamination of reaction by-products, etc. due to process gas (silane-based gas or hydrochloric acid gas) accumulates in the reaction vessel while continuing operation with the vapor phase growth apparatus. From the viewpoint of management, maintenance work such as cleaning (cleaning) and inspection of the pipes connected to the reaction container and the reaction container is periodically performed by opening the reaction container to the atmosphere.

  However, generally, the level of metal impurities (heavy metal contamination) in an epitaxial wafer manufactured immediately after cleaning and inspection work that involves opening the reaction vessel to the atmosphere constitutes the reaction vessel that is a metal impurity adhering to the vapor phase growth apparatus during maintenance. Since the metal impurities contained in the material, the vapor phase growth apparatus, and the stainless steel components normally used in the piping system thereof are used as the contamination source, heavy metal contamination is increased as compared with the epitaxial wafer manufactured immediately before the cleaning and inspection work.

  On the other hand, in recent years, a silicon epitaxial wafer obtained by vapor-depositing a silicon film on a silicon wafer has been used as an image sensor substrate such as a CCD (Charge Coupled Device) or CIS (CMOS Image Sensor). In such an epitaxial wafer, when metal impurities are present in the wafer, defects called white scratches (white spots) are generated. Therefore, it is important to reduce the level of metal impurities (heavy metal contamination) in the wafer.

  Therefore, as a measure to reduce impurities (metal impurities) in the epitaxial wafer, it is conceivable to change the constituent material of the reaction vessel. However, it is necessary to make a large-scale renovation or to find a material that can replace the conventional material. This is difficult to implement.

Further, Patent Documents 1 to 3 disclose methods for suppressing impurities in an epitaxial wafer from another viewpoint. For example, Patent Document 1 proposes a method of depositing a thin film on a substrate in an atmosphere in which the oxygen concentration is controlled to 10 ⁻²¹ Pa or less in order to reduce the amount of oxygen taken into the thin film. Patent Documents 2 and 3 disclose a method for controlling the oxygen concentration in the load lock chamber to a predetermined value and preventing the wafer surface from being contaminated by an inappropriate natural oxide film formed on the wafer surface in the load lock chamber. Proposed.

JP 2009-2000158 A Japanese Unexamined Patent Publication No. 9-45597 JP 2000-58619 A

  However, the methods of Patent Documents 1 to 3 are methods for reducing moisture contamination, oxygen contamination, and natural oxide film contamination on a wafer during normal operation, and cleaning or inspection of a vapor phase growth apparatus (hereinafter referred to as maintenance). It is not a method for reducing heavy metal contamination caused by the above.

  The subject of this invention is providing the cleaning or inspection method of the vapor phase growth apparatus which can reduce heavy metal contamination resulting from the maintenance of a vapor phase growth apparatus.

Means for Solving the Problems and Effects of the Invention

The first cleaning or inspection method of the vapor phase growth apparatus of the present invention is:
It is characterized in that the oxygen concentration in the reaction vessel is reduced from that in the air atmosphere at the time of cleaning or inspection accompanied by opening of the reaction vessel of the gas phase growth apparatus which is made airtight.

In addition, the second cleaning or inspection method of the vapor phase growth apparatus of the present invention,
In a method for cleaning or inspecting a vapor phase growth apparatus, comprising a preparation step of removing the lid-like member of the reaction vessel and opening the reaction vessel in preparation for cleaning or inspecting the inside of the reaction vessel of the vapor phase growth apparatus for producing an epitaxial wafer ,
Covers the lid-like member before being removed and is positioned so that the reaction vessel is further closed from the top of the lid-like member so that the lid-like member becomes a double lid, and the atmosphere inside and outside can be isolated, and the atmosphere inside An attachment process for attaching an adjustable purge box to the reaction vessel;
Adjusting the oxygen concentration inside the purge box attached to the reaction vessel to be lower than the oxygen concentration in the outside air atmosphere, and
A preparatory step is performed after the adjustment step.

  The present inventor focused on the point of opening the reaction vessel to the atmosphere at the time of maintenance for contamination of the vapor phase growth apparatus caused by maintenance, and in the intensive study, when oxygen enters the reaction vessel, heavy metal contamination in the epitaxial wafer As a result of investigating the reaction vessel opened to the atmosphere during maintenance based on this knowledge, the oxygen concentration in the reaction vessel opened to the atmosphere rises to about 200000 ppm, which is the same as the oxygen concentration in the atmosphere. It was confirmed.

  As a result, the increase in heavy metal contamination of the vapor phase growth apparatus due to maintenance is caused not only by using the materials constituting the reaction vessel as a contamination source, but also by the reaction of oxygen mixed in the reaction vessel by opening the reaction vessel to the atmosphere. The conclusion was reached that heavy metal contamination would increase.

  Therefore, heavy metal contamination caused by maintenance of the vapor phase growth apparatus is reduced by reducing the oxygen concentration in the reaction vessel from the air atmosphere during cleaning or inspection involving opening the inside of the reaction vessel of the gas phase growth apparatus that has been made airtight. Can be suppressed. At this time, heavy metal contamination can be more effectively prevented by setting the oxygen concentration in the reaction vessel to 100 ppm or less. In the embodiment of the present invention, a purge box is attached to the reaction vessel in advance for preparation for cleaning or inspection, the oxygen concentration in the purge box is adjusted to be lower than the oxygen concentration in the outside (atmospheric atmosphere), and maintenance of the vapor phase growth apparatus is performed. The increase in heavy metal contamination caused by can be suppressed. In addition, the present invention is particularly effective for suppressing an increase in metal impurities in an epitaxial wafer manufactured immediately after maintenance work.

  The purge box has a glove box structure, which facilitates cleaning or inspection work inside the reaction vessel.

The schematic side surface sectional view which shows an example of the vapor phase growth apparatus which attached the purge box used for the cleaning or inspection method of this invention. The schematic side surface sectional view which shows an example of the vapor phase growth apparatus used in the comparative example. The flowchart which shows the outline | summary of the evaluation method of the maintenance contamination degree of the reaction container in a comparative example. The flowchart which shows the outline | summary of the evaluation method of the maintenance contamination degree of the reaction container in an Example. The graph which showed Mo concentration ratio of the wafer produced before and after the maintenance in an Example and a comparative example. The graph which showed the relationship between Mo concentration ratio of the wafer produced before and after the maintenance, and the oxygen concentration in the reaction container at the time of maintenance.

  FIG. 1 shows an example of a single-wafer type vapor phase growth apparatus 1 used in the present invention and a purge area in which the inside is a work area for cleaning or checking the vapor phase growth apparatus 1 and the oxygen concentration in the work area can be adjusted. Box 2 is shown. The vapor phase growth apparatus 1 produces a silicon epitaxial wafer used for an image sensor substrate such as a CCD or CIS by vapor-phase growing a silicon single crystal film (epitaxial layer) on the main surface of the silicon single crystal substrate. It is a device to do.

  The vapor phase growth apparatus 1 includes a reaction vessel 3 that is a vapor phase growth furnace composed of a transparent quartz member, a metal member such as stainless steel, or the like. The reaction vessel 3 (chamber, reaction furnace) is located in a base ring 4 made of stainless steel (SUS) and connected to the lower side of the base ring 4, and is formed in an inverted dome shape so as to constitute the bottom of the reaction vessel 3. And a ceiling member 6 that is positioned above the base ring 4 and that forms the ceiling of the reaction vessel 3 so as to sandwich the base ring 4 together with the rowardome 5, and each of the lowomeme 5 and the ceiling member 6 is made of a transparent quartz member. Composed.

  The reaction vessel 3 is configured to be airtight (isolated from the atmosphere) so that impurities are not mixed into the reaction vessel 3 from the outside, and the reaction vessel 3 is isolated from the atmospheric air so as to cover the base ring 4. A quartz member (not shown), a susceptor 7 (for example, made of graphite) that horizontally supports a silicon single crystal substrate (hereinafter simply referred to as wafer W) that is a base substrate of a silicon epitaxial wafer, and a support that supports the susceptor 7 A rotation drive unit 9 that rotates the susceptor 7 around the central axis O via the support unit 8 and the support unit 8; and an oxygen concentration measurement unit (not shown).

  At one end of the reaction vessel 3, a vapor phase growth gas containing a source gas (for example, trichlorosilane) in the reaction vessel 3, a carrier gas (for example, hydrogen) for diluting the source gas, and a dopant gas for imparting conductivity to the epitaxial layer. G is introduced into the upper region of the susceptor 7, and a gas introduction pipe 10 that supplies the main surface of the wafer W on the susceptor 7 is connected. A gas discharge pipe 11 for discharging the gas in the reaction vessel 3 is connected to the opposite side of the gas introduction pipe 10.

  Although not shown, around the reaction vessel 3 (for example, above and below the reaction vessel 3), a lamp such as a halogen lamp for heating the wafer W to an epitaxial growth temperature (for example, 900 to 1200 ° C.) at the time of vapor phase growth is provided. .

  On the other hand, as shown in FIG. 1, the purge box 2 attached to the vapor phase growth apparatus 1 covers the ceiling member 6 and further closes the reaction vessel 3 from above the ceiling member 6 so that the ceiling member 6 becomes a double lid. Projecting outside the reaction vessel 3, a space (inside) is defined by the outer surface of the ceiling member 6 and the inner surface of the purge box 2, and the inside serves as a work area for cleaning or inspecting the reaction vessel 3. In order to facilitate the work in the reaction vessel 3, the purge box 2 is configured in a glove box shape.

  Further, the purge box 2 has a purge gas introduction pipe 2a that replaces the atmosphere in the purge box 2 by introducing an inert gas P such as nitrogen, and an air introduction valve 2b 'that can adjust the amount of air introduced. An atmosphere introduction pipe 2b capable of introducing the atmosphere A into the purge box 2, an oxygen concentration meter 2c for detecting the oxygen concentration in the purge box 2, and the purge box 2 (vapor phase growth) The apparatus 1) is provided with a purge exhaust pipe (not shown) for discharging outside, can isolate the atmosphere inside the purge box 2 from the outside, is airtight so that impurities are not mixed into the inside from the outside, The atmosphere can also be adjusted. Specifically, it is confirmed by the oxygen concentration meter 2c, and the introduction amount of the inert gas P such as nitrogen gas introduced by the purge gas introduction tube 2a and the introduction amount of the atmospheric gas introduced by the atmosphere introduction tube 2b are adjusted. Thus, the oxygen concentration of the atmosphere in the purge box 2 can be controlled within a range of 1 ppm to 200000 ppm, for example.

  A silicon epitaxial wafer is manufactured by the vapor phase growth apparatus 1 (excluding the purge box 2) having the above configuration. During operation, the vapor growth gas G is introduced from the gas introduction pipe 10, passes through the susceptor 7, is discharged from the gas discharge pipe 11, and transports the wafer W into the reaction vessel 3. The environment is completely isolated from the atmosphere. Further, even when the reaction container 3 is opened when the wafer W is transferred, the wafer W is not transferred directly from the atmospheric environment but is transferred in an environment where the gas is replaced with nitrogen.

  When the vapor phase growth is repeated by the vapor phase growth apparatus 1, silicon by-products and the like constituting the silicon epitaxial wafer gradually accumulate in the reaction vessel 3 on the base ring 4 and the rowardome 5 inside the reaction vessel 3. The by-products and the like adversely affect the quality of the epitaxial wafer. Therefore, the ceiling member 6 is periodically removed from the reaction vessel 3 and the susceptor 7 and the support 8 are exchanged with the inside of the reaction vessel 3 open to the atmosphere, and the base ring 4, the rowardome 5 and the gas in the reaction vessel 3 are exchanged. The introduction pipe 10 and the gas discharge pipe 11 are cleaned to remove accumulated by-products. In addition, as a maintenance check of the reaction vessel 3, the inside of the reaction vessel 3 is checked with the inside of the reaction vessel 3 open to the atmosphere.

  When the ceiling member 6 is removed during such cleaning and inspection and the reaction vessel 3 is opened to the atmosphere, oxygen in the atmosphere is mixed into the reaction vessel 3 and promotes corrosion of the members constituting the reaction vessel 3. Increase the metal contamination in the reaction vessel 3. Therefore, the metal in the reaction vessel 3 is reduced by removing the ceiling member 6 for cleaning and inspection and reducing the oxygen concentration mixed in the reaction vessel 3 when the reaction vessel 3 is opened, compared to the oxygen concentration in the atmosphere. Contamination can be reduced.

  Then, next, the detail of the method of cleaning or inspecting the vapor phase growth apparatus 1 using the vapor phase growth apparatus 1 and the purge box 2 of FIG. 1 is demonstrated. First, prior to cleaning or inspection of the vapor phase growth apparatus 1, the purge box 2 is vapor phase grown so as to cover the ceiling member 6 and further close the reaction vessel 3 from above the ceiling member 6 so that the ceiling member 6 becomes a double lid. It attaches to the apparatus 1 (attachment process). The attached purge box 2 is attached to the vapor phase growth apparatus 1 in a state where the inside of the purge box 2 is isolated from the atmosphere and hermetically sealed so that impurities are not mixed into the purge box 2 from the outside.

  Next, the oxygen concentration inside the attached purge box 2 is reduced below the oxygen concentration in the air atmosphere outside the purge box 2 (adjustment step). Specifically, the air introduction valve 2b ′ is closed, oxygen or the like in the purge box 2 is purged with an inert gas P of nitrogen, and the oxygen concentration in the purge box 2 is reduced to 100 ppm or less by the oxygen concentration meter 2c. , Purging.

  When the oxygen concentration in the purge box 2 is adjusted to 100 ppm or less, the inside of the reaction vessel 3 is adjusted to a working atmosphere suitable for the work for cleaning or checking, and the purge box 2 is prepared for cleaning or checking in the reaction vessel 3. The reaction chamber 3 is opened by removing the ceiling member 6 (preparation step). When the ceiling member 6 is removed, the purge box 2 and the reaction vessel 3 communicate with each other to form a new vessel-shaped space. Since the purge box 2 has a glove box structure, the ceiling member 6 can be easily removed in the working atmosphere.

  After the reaction container 3 is opened, the replacement susceptor and support part prepared in the purge box 2 in advance, and the susceptor 7 and support part 8 in the reaction container 3 are exchanged, and the base ring 4 and the rowardome in the reaction container 3 are exchanged. 5 and the gas introduction pipe 10 and the gas discharge pipe 11 are cleaned of by-products and the like. After the cleaning, the ceiling member 6 is attached again, and then the purge box 2 is removed to remove the purge box 1. Cleaning and inspection are complete.

  As described above, in this embodiment, the working atmosphere suitable for cleaning or checking the inside of the reaction vessel 3 by adjusting the oxygen concentration in the purge box 2 (the purge box 2 and the reaction vessel 3 are integrated and integrated). By cleaning or inspecting the reaction vessel 3 in a container-like space), the oxygen concentration mixed in the reaction vessel 3 can be suppressed even when the reaction vessel 3 is opened. As a result, as shown in the following examples, metal contamination taken into the epitaxial wafer can be greatly reduced.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, these do not limit this invention. First, the vapor phase growth apparatus 1 of FIG. 1 and the vapor phase growth apparatus 100 similar to the vapor phase growth apparatus 1 (see FIG. 2) having the same level of metal contamination before cleaning or inspection (maintenance) are prepared. In the phase growth apparatus 1, the cleaning or inspection method of the present invention was implemented as an example, and in the vapor phase growth apparatus 100, cleaning or inspection was performed by a conventional method as a comparative example. In FIG. 2, the same components as those in the vapor phase growth apparatus 1 are denoted by the same reference numerals and description thereof is omitted.

  In addition, as a method for evaluating metal contamination of the reaction vessel 3 of the vapor phase growth apparatus 1 or 100, an epitaxial wafer is manufactured as a sample before and after maintenance, and the manufactured wafer is used as an ICP (Inductively Coupled Plasma) -MS (Mass Spectrometry). ) (Inductively coupled plasma mass spectrometry) was used to compare the metal quantitative values, and the value was taken as the degree of contamination due to maintenance. Specifically, a division value (concentration increase ratio) obtained by dividing the Mo (molybdenum) concentration after maintenance by the Mo concentration before maintenance is defined as the maintenance contamination degree. Thereby, the correlation regarding the maintenance contamination degree of the reaction container 3 in the maintenance environment which differs simply and highly sensitively can be obtained.

(Comparative example)
As shown in FIG. 3, a silicon epitaxial wafer having a diameter of 200 mm was prepared as a sample before maintenance, and a Mo metal quantitative value was obtained by ICP-MS from the wafer before maintenance (S1). Next, after removing the ceiling member 6 and opening the reaction vessel 3 to the atmosphere as in the prior art (S2), the susceptor 7 and the support 8 in the reaction vessel 3 are replaced, and the base ring 4 in the reaction vessel 3 and Cleaning of by-products and the like accumulated in Rowardome 5 was performed (S3). At this time, the oxygen concentration in the reaction vessel 3 was measured by an oxygen concentration measurement unit, and was 100000 ppm or more.

  After the cleaning operation, the ceiling member 6 is attached again, a silicon epitaxial wafer is produced as a sample after maintenance, a Mo metal quantitative value is obtained from the wafer after maintenance by ICP-MS, and the maintenance contamination degree of the reaction vessel 3 is determined. As a result of the evaluation, the Mo concentration ratio before and after maintenance was 6.3, and metal contamination increased (see FIG. 5). This is probably because the base ring 4, the rotation drive unit 9, the gas introduction pipe 10, the gas discharge pipe 11 and the like are generally made of a stainless steel member, and Mo contained in the stainless steel reacts with oxygen in the atmosphere. It is done.

(Example)
Next, examples will be described. As shown in FIG. 4, a silicon epitaxial wafer having a diameter of 200 mm was prepared as a sample before performing maintenance in the same manner as in the comparative example, and a Mo metal quantitative value was obtained from the wafer before maintenance by ICP-MS (S101). Unlike the comparative example, the purge box 2 becomes a work area where maintenance is performed before the ceiling member 6 is removed, and the work area is isolated from the air atmosphere and can be adjusted to a work atmosphere (oxygen concentration) suitable for the maintenance work. Was attached to the vapor phase growth apparatus 1 (S102).

  Next, in order to replace the atmosphere in the purge box 2, the air introduction valve 2 b ′ is completely closed (S 103), nitrogen gas is introduced from the purge gas introduction pipe 2 a, purge is performed, and the oxygen concentration meter 2 c is used to purge the purge box. Purge was performed until the oxygen concentration in 2 became 100 ppm or less.

  When the oxygen concentration in the purge box 2 becomes 100 ppm or less, the ceiling member 6 is removed (S104), and the reaction vessel 3 is opened. After the reaction vessel 3 is opened, the susceptor 7 and the support portion 8 in the reaction vessel 3 are exchanged, and cleaning of by-products and the like deposited on the base ring 4 and the rowardome 5 in the reaction vessel 3 is performed (S105). At this time, when the oxygen concentration in the reaction vessel 3 during the cleaning operation was constantly monitored by the oxygen concentration measuring unit, the oxygen concentration in the reaction vessel 3 did not exceed 100 ppm.

  After the cleaning operation, the ceiling member 6 is attached again, a wafer is prepared as a sample after maintenance, the Mo metal quantitative value is obtained by ICP-MS, and the maintenance contamination degree of the reaction vessel 3 is evaluated. The Mo concentration ratio was 0.96, indicating no increase in metal contamination (see FIG. 5). From this, it was shown that by increasing the oxygen concentration during the opening operation of the reaction vessel 3 in an environment of 100 ppm or less, the increase in Mo contamination can be suppressed.

  In the above embodiment, the air introduction valve 2b ′ was closed, and maintenance was performed with an oxygen concentration of 100 ppm or less. Separately, the air introduction valve 2b ′ is opened, the oxygen concentration in the purge box 2 (working atmosphere for performing maintenance) is controlled in the range of 1 ppm to 200,000 ppm, and the maintenance work is performed and reacted in the same manner as in the example. The maintenance contamination degree of the container 3 was evaluated. As shown in FIG. 6, the lower the oxygen concentration in the reaction vessel 3 during maintenance, the smaller the Mo concentration ratio. In particular, when the oxygen concentration is 100 ppm or less, the Mo concentration ratio is about 1, and the increase in maintenance contamination is suppressed. Rukoto has been shown.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits the same effects. Are included in the technical scope.

  For example, the vapor phase growth apparatus is not limited to a sheet format, and the present invention can be applied to maintenance of various vapor phase growth apparatuses such as a vertical type (pancake type) and a barrel type (cylinder type). Further, the atmosphere opening operation of the reaction vessel 3 includes a gas introduction pipe 10 and a gas discharge pipe 11. Further, in this embodiment, the purge box 2 is used to isolate the inside (working atmosphere) and the outside (atmospheric atmosphere) of the purge box 2 and reduce the oxygen concentration inside the purge box 2. If the effect is obtained, the purge box 2 may not be used.

DESCRIPTION OF SYMBOLS 1 Vapor growth apparatus 2 Purge box 2a Purge gas introduction pipe 2b Atmospheric introduction pipe 2c Oxygen concentration meter 3 Reaction container 4 Base ring 5 Rowardome 6 Ceiling member (lid member) 7 Susceptor 8 Support part 9 Rotation drive part 10 Gas introduction pipe 11 Gas exhaust pipe O Central axis G Vapor growth gas A Atmosphere P Inert gas

Claims (5)

  A method for cleaning or inspecting a vapor phase growth apparatus, characterized in that the oxygen concentration in the reaction vessel is reduced from the atmospheric atmosphere at the time of cleaning or inspection involving opening of the reaction vessel of an airtight vapor phase growth apparatus.   The method for cleaning or checking a vapor phase growth apparatus according to claim 1, wherein the oxygen concentration is 100 ppm or less. Cleaning or inspection of a vapor phase growth apparatus including a preparation step of removing the lid member of the reaction container and opening the reaction container in preparation for cleaning or checking the inside of the reaction container of the vapor phase growth apparatus for manufacturing an epitaxial wafer In the method
Covers the lid-like member before being removed and is positioned so as to further close the reaction vessel from the top of the lid-like member so that the lid-like member becomes a double lid, and the atmosphere inside and outside can be isolated, and An attachment step of attaching a purge box capable of adjusting the inner atmosphere to the reaction vessel;
Adjusting the oxygen concentration inside the purge box attached to the reaction vessel to be lower than the oxygen concentration in the outside air atmosphere, and
A method of cleaning or checking a vapor phase growth apparatus, wherein the preparation step is performed after the adjustment step.   The method for cleaning or checking a vapor phase growth apparatus according to claim 3, wherein the adjusting step sets the oxygen concentration inside the purge box to 100 ppm or less.   The method for cleaning or checking a vapor phase growth apparatus according to claim 3 or 4, wherein the purge box has a glove box structure.

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