KR102362488B1 - Atomic layer deposition apparatus - Google Patents

Abstract

The present invention relates to an atomic layer deposition apparatus capable of preventing the accumulation of foreign substances due to a reaction between gases remaining inside a shower head and the generation of particles due to this phenomenon, wherein an accommodating space is formed to accommodate a substrate. chamber; and a shower head installed in the chamber and capable of supplying a first gas and a second gas to the substrate, respectively, wherein the shower head includes,
A plurality of first gas injection holes spaced apart from each other are formed on the surface of the gas injection surface so that the first gas and the second gas can be evenly distributed on the surface of the substrate, respectively, and a plurality of second gas injection holes are formed therebetween. Gas injection hole portions may be formed.

Description

Atomic layer deposition apparatus

The present invention relates to an atomic layer deposition apparatus, and more particularly, to an atomic layer deposition apparatus capable of preventing the accumulation of foreign substances due to a reaction between gases remaining inside a shower head and generation of particles thereby. .

Due to the development of semiconductor integration technology, the process of depositing a high-purity, high-quality thin film has become an important part of the semiconductor manufacturing process. Representative methods of thin film formation include a chemical vapor deposition (CVD) method and a physical vapor deposition (PVD) method. A physical vapor deposition method such as a sputtering method cannot be used to form a film of a uniform thickness on an uneven surface because the step coverage of the formed thin film is poor.

Chemical vapor deposition is a method in which gaseous substances react on the surface of a heated substrate, and a compound produced by the reaction is deposited on the surface of the substrate. Compared to the physical vapor deposition method, the chemical vapor deposition method is widely applied because it has better step coverage, less damage to the substrate on which the thin film is deposited, low deposition cost of the thin film, and can mass-produce thin films.

However, as the degree of integration of semiconductor devices has recently improved to sub-micron units, a uniform thickness of sub-micron units can be obtained on a wafer substrate only by the conventional chemical vapor deposition method, or excellent step coverage is achieved. It is difficult to obtain a material film having a constant composition regardless of location when there is a step such as a sub-micron-sized contact hole, via, or trench on the wafer substrate. suffered from

Therefore, unlike the conventional chemical vapor deposition method in which all process gases are simultaneously injected, two or more process gases necessary to obtain a desired thin film are sequentially divided and supplied according to time so as not to meet in the gas phase, but these supply cycles are periodically repeated to form a thin film. A time-division atomic layer deposition (atomic layer deposition) method of forming a thin film is being applied as a new thin film formation method.

In addition, space-division atomic layer deposition in which two or more process gases are supplied in different spaces so that they do not meet in the vapor phase and the substrate is moved to different spaces is also applied.

In general, in the time-division atomic layer deposition method, two or more gases can be supplied to a substrate with a time difference using a single shower head. Gas can remain inside the shower head, and when the residual gas is deposited and accumulated inside the shower head for a long time, it not only contaminates the shower head, but also contaminates the shower head due to the nature of the shower head that contracts and expands repeatedly. As the thickness of these materials increases, they can be easily damaged and a large amount of particles can be generated, which adversely affects the substrate, adversely affects the subsequent process, or reduces the maintenance work time and cost of equipment due to the cleaning operation to remove these foreign substances. There were many problems, such as wasting.

In addition, the conventional atomic layer deposition apparatus, which generates plasma between the shower head and the substrate, has many problems, such as excessive energy being directly transferred to the substrate, lowering the safety of the equipment, and possibly damaging the substrate.

The present invention is intended to solve various problems including the above problems, and the remaining first gas and the remaining first gas and the It is possible to prevent the occurrence of particles due to an abnormal reaction between the second gas, thereby reducing the time and cost of the maintenance work of the equipment, thereby improving the productivity of the equipment and the yield of the substrate, and the inside of the shower head It is possible to minimize damage to the substrate by forming remote-type plasma in the An object of the present invention is to provide an atomic layer deposition apparatus. However, these problems are exemplary, and the scope of the present invention is not limited thereto.

Atomic layer deposition apparatus according to the spirit of the present invention for solving the above problems, a chamber in which an accommodation space is formed to accommodate a substrate; and a shower head installed in the chamber and capable of supplying a first gas and a second gas to the substrate, respectively, wherein the shower head includes the first gas and the second gas on the surface of the substrate, respectively. A plurality of first gas injection hole portions spaced apart from each other may be formed on the surface of the gas injection surface to be evenly distributed, and a plurality of second gas injection hole portions may be formed therebetween.

In addition, according to the present invention, in the shower head, the first gas path to which the first gas is supplied and the second gas path to which the second gas is supplied are formed independently, respectively, so that the first gas and the second gas are formed independently. A first distribution space capable of distributing the first gas supplied through a first gas supply pipe is formed in order to prevent the occurrence of particles due to an abnormal reaction between gases, and a first distribution space communicating with the first distribution space is formed on a lower surface thereof. A first layer portion in which a plurality of the first gas injection hole portions are formed, and a plurality of bypass pipes having the second gas injection hole portions formed therebetween are formed to be distinguished from the first distribution space between the first gas injection hole portions ; and a second distribution space for distributing the second gas supplied through a second gas supply pipe is formed, and the lower side is opened so that the second distribution space communicates with the bypass pipes of the first layer part. It may include; a second layer portion formed of.

In addition, according to the present invention, the first layer portion, a plurality of the first gas injection hole portion is formed, the gas injection panel between which the bypass pipe receiving portions are formed therebetween; and a plurality of first grooves that are assembled with the gas injection panel and formed on a lower surface to form the first distribution space therein, and protrude from the first groove to be inserted into the bypass pipe accommodating portion of the gas injection panel. and a bypass panel in which the two bypass pipes are formed and the second gas injection hole portion of the bypass pipe is exposed on an upper surface of the bypass panel.

In addition, according to the present invention, the second layer portion is assembled with the bypass panel, and the second distribution space communicating with the second gas injection hole portion exposed on the upper surface of the bypass panel is formed therein. a spacer formed in a ring shape; and an upper cover formed in a shape to cover the upper surface of the spacer to seal the upper portion of the second distribution space and assembled with the upper panel of the chamber.

In addition, according to the present invention, the second layer portion is installed in the spacer so that plasma can be generated in the first distribution space or the second distribution space in correspondence with the grounded first layer portion, and the upper portion an electrode panel electrically connected to the terminal passing through the cover; and an insulating panel installed between the electrode panel and the upper cover, wherein the spacer of the second layer part may be made of an insulating material.

In addition, in the atomic layer deposition apparatus according to the present invention, a first gas flow path extending to a front end is formed in a central axis portion, a second gas flow passage extending to a middle portion is formed in an edge portion, and the front end portion is the bypass panel a multi-path gas supply pipe which can communicate with the first distribution space through the may include more.

Also, according to the present invention, the first gas flow path is connected to a source gas line connected to a source gas supply source for atomic layer deposition, and the second gas flow path is a reactive gas connected to a reactive gas supply source for atomic layer deposition. line can be connected.

In addition, according to the present invention, a purge gas line connected to a purge gas source for atomic layer deposition may be connected by selecting at least one of the first gas flow path, the second gas flow path, the chamber, and combinations thereof. .

According to some embodiments of the present invention made as described above, the first gas and the second gas remaining using a shower head configured independently so that the path of the first gas and the path of the second gas do not overlap each other. It is possible to prevent the occurrence of particles due to the abnormal reaction between It is possible to minimize damage to the substrate by forming a plasma of will be. Of course, the scope of the present invention is not limited by these effects.

1 is a cross-sectional view illustrating an atomic layer deposition apparatus according to some embodiments of the present invention.
FIG. 2 is an assembled perspective view of a shower head of the atomic layer deposition apparatus of FIG. 1 ;
3 is an exploded top perspective view illustrating a shower head of the atomic layer deposition apparatus of FIG. 1 ;
4 is an exploded bottom perspective view illustrating a shower head of the atomic layer deposition apparatus of FIG. 1 .
FIG. 5 is an exploded perspective view of partially cut parts showing a shower head of the atomic layer deposition apparatus of FIG. 1 .
FIG. 6 is an exploded bottom perspective view of a partially cut part showing a shower head of the atomic layer deposition apparatus of FIG. 1 .
7 is a diagram illustrating an evaluation result of an atomic layer deposition apparatus according to some embodiments of the present invention.

Hereinafter, several preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Examples of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, and the scope of the present invention is as follows It is not limited to an Example. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the spirit of the present invention to those skilled in the art. In addition, in the drawings, the thickness or size of each layer is exaggerated for convenience and clarity of description.

Throughout the specification, when referring to one component, such as a film, region, or substrate, being located "on", "connected to," "stacked" or "coupled to" another component, the one component It may be construed that an element may be directly in contact with, “on,” “connected to,” “stacked with,” or “coupled to,” another element, or that there may be other elements interposed therebetween. On the other hand, when it is stated that one element is located "directly on," "directly connected to," or "directly coupled to" another element, it is interpreted that there are no other elements interposed therebetween. do. Like numbers refer to like elements. As used herein, the term “and/or” includes any one and all combinations of one or more of those listed items.

Although the terms first, second, etc. are used herein to describe various members, parts, regions, layers, and/or parts, these members, parts, regions, layers and/or parts are limited by these terms so that they It is self-evident that These terms are used only to distinguish one member, component, region, layer or portion from another region, layer or portion. Accordingly, a first member, component, region, layer or portion discussed below may refer to a second member, component, region, layer or portion without departing from the teachings of the present invention.

Also, relative terms such as "above" or "above" and "below" or "below" may be used herein to describe the relationship of certain elements to other elements as illustrated in the drawings. It may be understood that relative terms are intended to include other orientations of the element in addition to the orientation depicted in the drawings. For example, if an element is turned over in the figures, elements depicted as being on the face above the other elements will have orientation on the face below the other elements. Thus, the term “top” by way of example may include both “bottom” and “top” directions depending on the particular orientation of the drawing. If the device is oriented in a different orientation (rotated 90 degrees relative to the other orientation), the relative descriptions used herein may be interpreted accordingly.

The terminology used herein is used to describe specific embodiments, not to limit the present invention. As used herein, the singular form may include the plural form unless the context clearly dictates otherwise. Also, as used herein, “comprise” and/or “comprising” refers to specifying the presence of the recited shapes, numbers, steps, actions, members, elements, and/or groups thereof. and does not exclude the presence or addition of one or more other shapes, numbers, movements, members, elements and/or groups.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings schematically illustrating ideal embodiments of the present invention. In the drawings, variations of the illustrated shape can be envisaged, for example depending on manufacturing technology and/or tolerances. Accordingly, embodiments of the spirit of the present invention should not be construed as limited to the specific shape of the region shown in the present specification, but should include, for example, changes in shape caused by manufacturing.

1 is a cross-sectional view illustrating an atomic layer deposition apparatus 100 according to some embodiments of the present invention.

First, as shown in FIG. 1 , the atomic layer deposition apparatus 100 according to some embodiments of the present invention may largely include a chamber 10 and a shower head 20 .

For example, as shown in FIG. 1 , the chamber 10 is a type of closed box-shaped structure in which an accommodating space A is formed to accommodate various substrates 1 such as semiconductor wafers or display substrates, the substrate The susceptor to be seated may be installed therein, and may be formed of various members such as the upper panel 11 and the side wall and the bottom.

However, the chamber 10 is not necessarily limited to the drawings, and various types of chambers having sufficient strength or durability to support a susceptor or a shower head supporting a substrate therein may be applied.

Also, for example, the shower head 20 may be installed on the upper panel 11 of the chamber 10 , and may include a first gas G1 and a second gas G2 on the substrate 1 . It may be a kind of gas distributor that can supply each.

Accordingly, the shower head 20 may evenly distribute the supplied first gas G1 and the second gas G2 on the surface of the substrate 1 with a time difference.

For example, as shown in FIG. 1 , the shower head 20 is configured such that the first gas G1 and the second gas G2 are evenly distributed on the surface of the substrate 1 , respectively. A plurality of first gas injection hole portions H1 spaced apart from each other may be formed on the surface of the gas injection surface 20a, and a plurality of second gas injection hole portions H2 may be formed therebetween.

FIG. 2 is an assembled perspective view of the shower head 20 of the atomic layer deposition apparatus 100 of FIG. 1 , and FIG. 3 is an exploded top view of the parts showing the shower head 20 of the atomic layer deposition apparatus 100 of FIG. 1 . It is a perspective view, FIG. 4 is an exploded bottom perspective view of parts showing the shower head 20 of the atomic layer deposition apparatus 100 of FIG. 1 , and FIG. 5 is the shower head 20 of the atomic layer deposition apparatus 100 of FIG. 1 . It is an exploded top perspective view of partially cut parts, and FIG. 6 is an exploded bottom perspective view of partially cut parts showing the shower head 20 of the atomic layer deposition apparatus 100 of FIG. 1 .

More specifically, as shown in FIGS. 1 to 6 , in the shower head 20 of the atomic layer deposition apparatus 100 according to some embodiments of the present invention, the first gas G1 is The first gas path to be supplied and the second gas path to which the second gas G2 is supplied are formed independently, respectively, so that particles are generated due to an abnormal reaction between the first gas G1 and the second gas G2. In order to prevent development, the first layer portion 21 and the second layer portion 22 may be included as a whole.

For example, as shown in FIGS. 1 to 6 , the first layer part 21 includes a first distribution space ( B1) is formed, and a plurality of first gas injection hole portions H1 communicating with the first distribution space B1 are formed on a lower surface of the first gas injection hole portion H1, and the first gas injection hole portion H1 is disposed between the first gas injection hole portions H1. A plurality of bypass pipes BP in which the second gas injection hole H2 is formed may be formed to be distinguished from the distribution space B1.

More specifically, for example, as shown in FIGS. 1 to 6 , in the first layer part 21 , a plurality of the first gas injection hole parts H1 are formed, and the bypass pipe is formed therebetween. It is assembled with the gas distributing panel 21-1 and the gas distributing panel 21-1 in which the accommodating portions BPa are formed, and a first groove portion K on a lower surface such that the first distribution space B1 is formed therein. ) is formed, and a plurality of bypass pipes BP are formed so as to protrude from the first groove part K and be inserted into the bypass pipe accommodating part BPa of the gas injection panel 21-1, A bypass panel 21 - 2 through which the second gas injection hole H2 of the bypass pipe BP is exposed may be included on the upper surface.

On the other hand, for example, as shown in FIGS. 1 to 6 , the second layer part 22 is installed above the first layer part 21 , and the second layer part 22 is supplied through the second gas supply pipe P2 . A second distribution space B2 capable of distributing the second gas G2 is formed, and the second distribution space B2 is lowered so that the second distribution space B2 communicates with the bypass pipes BP of the first layer part 21 . It can be formed in this open form.

More specifically, for example, as shown in FIGS. 1 to 6 , the second layer part 22 is assembled with the bypass panel 21-2, and The spacer 22-1 and the second distribution space B2 are formed in a ring shape as a whole so that the second distribution space B2 communicating with the second gas injection hole H2 exposed on the upper surface is formed therein. It may include an upper cover 22-2 that is formed to cover the upper surface of the spacer 22-1 to seal the upper side of the chamber 10 and is assembled with the upper panel 23 of the chamber 10.

In addition, for example, as shown in FIGS. 1 to 6 , the second layer part 22 corresponds to the grounded first layer part 21 so that capacitively coupled plasma can be generated in a remote manner. The terminal T is installed inside the spacer 22-1 to generate plasma in the first distribution space B1 or the second distribution space B2 and passes through the upper cover 22-2. ) may further include an electrode panel (E) electrically connected to the electrode panel (E) and an insulating panel (I) installed between the electrode panel (E) and the upper cover (22-2).

Here, the spacer 22-1 of the second layer part 22 is made of an insulating material, and thus, the electrode panel E is formed by the insulating panel I and the spacer 22-1. It is possible to maintain an insulating state insulated from the chamber 10 , and through this, the first distribution space B1 between the electrode panel E and the first layer part 21 or the second layer part 22 or the Plasma may be generated in the second distribution space B2 by a capacitive coupling method.

Therefore, it is possible to minimize damage to the substrate 1 by forming a remote type plasma inside the shower head 20 , increase the safety of equipment, and generate a capacitively coupled plasma very quickly Because it can generate plasma in a high degree of productivity, it is possible to greatly increase the productivity.

On the other hand, for example, as shown in FIGS. 1 to 6 , in the atomic layer deposition apparatus 100 according to some embodiments of the present invention, a first gas flow path F1 extending to a front end is formed in a central axis portion, and , a second gas flow path F2 extending to a middle portion is formed on the edge portion, and the front end portion penetrates the bypass panel 21-2 to communicate with the first distribution space B1, and the The intermediate portion may further include a multi-path gas supply pipe 30 in which the front end is formed to protrude from the middle portion so that the intermediate portion passes through the upper cover 22-2 to communicate with the second distribution space B2. .

More specifically, for example, as shown in FIG. 1 , the first gas flow path F1 is connected to a source gas supply source S1 for atomic layer deposition, and a source gas having a first valve V1 installed therein. It is connected to the line L1, and the second gas flow path F2 is connected to the reactive gas supply source S2 for atomic layer deposition, and may be connected to the reactive gas line L2 in which the second valve V2 is installed. have.

In addition, for example, as shown in FIG. 1 , at least one of the first gas flow path F1 , the second gas flow path F2 , the chamber 10 and combinations thereof is selected for atomic layer deposition may be connected to a purge gas supply source S3 for

Accordingly, as shown in FIG. 1 , when explaining the operation process of the atomic layer deposition apparatus 100 according to some embodiments of the present invention, the first gas G1 is a source gas, and the first gas supply source From ( S1 ), the source gas line L1 , the first gas supply pipe P1 , the first gas flow path F1 of the multi-path gas supply pipe 30 , and the first distribution space of the first layer part 21 . (B1), through the first gas distribution hole (H1) may finally be evenly sprayed onto the substrate (1).

Then, the purge gas may be injected from the purge gas supply source S3 to the path of the first gas described above. In this case, the purge gas may simultaneously pass through a path of the second gas G2 to be described later and be injected into the substrate 1 at the same time.

Next, the second gas G2 is a reactive gas, and is supplied from the second gas supply source S2 to the reactive gas line L2 , the second gas supply pipe P2 , and the multi-path gas supply pipe 30 . The second gas flow path F2, the second distribution space B2 of the second layer portion 22, the bypass pipe BP, and the second gas distribution hole H2 are finally transferred to the substrate 1 through It can be sprayed evenly.

At this time, the path of the second gas G2 cannot react with the first gas G1 because it does not overlap the path of the first gas G1 and follows a path formed independently. As a result, it is possible to prevent the occurrence of foreign substances.

In addition, the purge gas may simultaneously pass through the path of the above-described first gas G1 and be injected into the substrate 1 at the same time.

However, the injection path of the purge gas is not necessarily limited to the drawings. For example, the source gas line L1 and the reaction gas line L2 may be formed separately as well as integrally formed. .

Subsequently, an atomic layer may be formed on the surface of the substrate 1 while the source gas, the purge gas, the reactive gas, and the purge gas are repeatedly supplied in this order, and these gases are exhausted from the chamber 10 . It may be discharged to the outside along the line L4.

7 is a diagram illustrating an evaluation result of the atomic layer deposition apparatus 100 according to some embodiments of the present invention.

7, when the number of particles generated in the shower head generated after the main drop deposition in the chamber is measured, the number of particles in the conventional shower head increases exponentially according to the accumulated deposition thickness in the chamber, whereas the number of particles increases exponentially, In the case of the shower head in which the precursor material (source gas) and the reaction gas of the present invention are separated, even if the deposition layer is accumulated in the chamber, it is not foreign substances due to the reaction of the source gas and the reaction gas. It was confirmed that the number of particles hardly increased.

Therefore, the first gas G1 and the second gas remaining using the shower head 20 independently configured so that the path of the first gas G1 and the path of the second gas G2 do not overlap each other. It is possible to prevent the occurrence of particles due to an abnormal reaction between the gases (G2), thereby reducing the time and cost of maintenance of the equipment, thereby improving the productivity of the equipment and the yield of the substrate, and the shower head ( 20) can minimize damage to the substrate by forming a remote-type plasma inside, and increase the safety of equipment. can be increased

Although the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

1: Substrate
G1: first gas
G2: second gas
10: chamber
A: accommodating space
20: shower head
20a: gas injection surface
H1: first gas injection hole portion
H2: second gas injection hole portion
P1: first gas supply pipe
P2: second gas supply pipe
B1: first distribution space
B2: second distribution space
21: first layer
21-1: gas injection panel
21-2: bypass panel
BP: Bypass pipe
BPa: Bypass pipe receiving part
22: second layer part
22-1: spacer
22-2: top cover
E: electrode panel
T: terminal
I: Insulation panel
30: multi-path gas supply pipe
F1: first gas flow path
F2: second gas flow path
S1: source gas source
S2: reactive gas source
S3: purge gas source
L1: source gas line
L2: reactive gas line
L3: purge gas line
L4: exhaust gas line
V1: first valve
V2: second valve
V3: 3rd valve
100: atomic layer deposition device

Claims (8)

a chamber in which an accommodating space is formed to accommodate the substrate; and
a shower head installed in the chamber and capable of supplying a first gas and a second gas to the substrate, respectively;
The shower head is
A plurality of first gas injection holes spaced apart from each other are formed on the surface of the gas injection surface so that the first gas and the second gas can be evenly distributed on the surface of the substrate, respectively, and a plurality of second gas injection holes are formed therebetween. Gas injection holes are formed,
The shower head is
The first gas path to which the first gas is supplied and the second gas path to which the second gas is supplied are each independently formed to prevent particle generation due to an abnormal reaction between the first gas and the second gas. a first distribution space capable of distributing the first gas supplied through the first gas supply pipe is formed, and a plurality of first gas injection holes communicating with the first distribution space are formed on a lower surface thereof , a first layer portion in which a plurality of bypass pipes in which the second gas injection hole portion is formed are formed to be distinguished from the first distribution space between the first gas injection hole portions; and
A second distribution space capable of distributing the second gas supplied through the second gas supply pipe is formed, and the second distribution space is opened downward so that the second distribution space communicates with the bypass pipes of the first layer part. A second layer portion is formed; including,
The second layer portion,
a spacer formed in a ring shape as a whole so that the second distribution space communicating with the second gas injection hole is formed therein;
an upper cover formed in a shape to cover an upper surface of the spacer to seal an upper portion of the second distribution space and assembled with an upper panel of the chamber;
an electrode panel that is installed in the spacer so as to correspond to the grounded first layer to generate a capacitively coupled plasma in a remote manner in the second distribution space, and is electrically connected to a terminal passing through the upper cover; and
an insulating panel installed between the electrode panel and the upper cover;
further comprising,
Atomic Layer Deposition Apparatus. delete The method of claim 1,
The first layer portion,
a gas injection panel in which a plurality of the first gas injection hole portions are formed and the bypass tube receiving portions are formed therebetween; and
It is assembled with the gas injection panel, and a first groove portion is formed on a lower surface so that the first distribution space is formed therein, and a plurality of the plurality of grooves are protruded from the first groove portion and inserted into the bypass tube receiving portion of the gas injection panel. a bypass panel on which the bypass tube is formed and the second gas injection hole portion of the bypass tube is exposed on an upper surface;
Including, atomic layer deposition apparatus. delete 4. The method of claim 3,
The spacer of the second layer portion is made of an insulating material, atomic layer deposition apparatus. 4. The method of claim 3,
A first gas flow path extending to a front end is formed in the central shaft portion, a second gas flow passage extending to a middle portion is formed in the edge portion, and the front end portion passes through the bypass panel to communicate with the first distribution space. a multi-path gas supply pipe in which the front end portion is formed to protrude from the intermediate portion so that the intermediate portion penetrates the upper cover and communicates with the second distribution space;
Further comprising a, atomic layer deposition apparatus. 7. The method of claim 6,
The first gas flow path is connected to a source gas line connected to a source gas supply source for atomic layer deposition, and the second gas flow path is connected to a reactive gas line connected to a reactive gas supply source for atomic layer deposition. deposition apparatus. 8. The method of claim 7,
A purge gas line connected to a purge gas supply source for atomic layer deposition is connected by selecting at least one of the first gas flow path, the second gas flow path, the chamber, and combinations thereof.

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