JP4016954B2 - Method for manufacturing silicon carbide semiconductor device - Google Patents

Description

  The present invention relates to a method for manufacturing a silicon carbide semiconductor device.

Since silicon carbide (SiC) can form an insulating film made of high-quality silicon dioxide (SiO ₂ ) by thermal oxidation, a high output insulated gate field effect transistor (MOSFET) with high breakdown voltage and low loss can be manufactured. However, since a large number of interface states (traps) exist at the so-called MOS interface of SiO ₂ / SiC formed by the conventional thermal oxidation method, the channel conductance (channel mobility μ _ch ) becomes very low. There is a problem that the on-resistance increases and the loss at the on-time increases. In order to reduce the interface state existing at the MOS interface, high temperature heat treatment (nitriding treatment) using oxygen nitride (NO) or oxygen dinitride (N ₂ O) gas as in Non-Patent Document 1 and Non-Patent Document 2, for example. ) _Has been reported to improve the quality of the MOS interface and improve the channel mobility _μch .

G.Y.Chung et al., "Improved Inversion Channel Mobility for 4H-SiC MOSFETs Following High Temperature Anneals in Nitric Oxide", IEEE Electron Device Letters, USA. April 2001, vol. 22, No. 4, p. 176-178. L.A. Lipkin et al., "N2O Processing Improves the 4H-SiC: SiO2 Interface", Mater. Sci. Forum, Switzerland, 2002, vol. 389-393, p. 985-988.

Although the channel mobility μ _ch is improved by nitriding using NO or N ₂ O, further improvement is necessary to realize device characteristics expected from the physical properties inherent in SiC. Further, the reduction of the interface state due to the nitriding treatment reduces the threshold voltage V _th along with the improvement of the channel mobility μ _ch. However, considering the low voltage driving, although the reduction of the threshold voltage V _th is preferable, the threshold voltage V _th is too high. _{If th} is too small, there is a risk that normally-on characteristics that are undesirable as element characteristics may occur when applied to a semiconductor device having a complicated structure such as an accumulation channel field effect transistor (ACCUFET) or an epichannel MOSFET. It was. From these practical requirements, a technique for further improving the channel mobility _μch and simultaneously controlling the threshold voltage _Vth has been desired.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain a method for manufacturing a silicon carbide semiconductor device having a high channel mobility and a controlled threshold voltage.

  A method of manufacturing a silicon carbide semiconductor device, a nitriding treatment step of performing a nitriding treatment simultaneously with forming a silicon dioxide film on the silicon carbide layer by heat-treating a wafer having a silicon carbide layer formed in an oxygen dinitride atmosphere; A heat treatment step of heat-treating the wafer in an oxygen atmosphere containing water vapor.

According to another method of manufacturing a silicon carbide semiconductor device, a silicon dioxide film is formed by oxidizing a part of the surface side of the silicon carbide layer by thermally oxidizing a wafer having a silicon carbide layer formed on the surface in an oxygen atmosphere. A thermal oxidation step to be formed; a nitridation step in which the wafer on which the silicon dioxide film is formed is heat-treated in an oxygen nitride or dioxygen atmosphere; and the nitridated wafer is heat-treated in an oxygen atmosphere containing water vapor. And a heat treatment step.

According to the method for manufacturing the silicon carbide semiconductor device, without lowering the n-channel channel mobility mu _ch of the silicon carbide MOSFET, it is possible to easily control the threshold voltage V _th by heat treatment temperature and heat treatment time.

Embodiment 1 FIG.
FIG. 1 shows an element structure of a silicon carbide semiconductor device manufactured by the method for manufacturing a silicon carbide semiconductor device of the first embodiment of the present invention. As an example of the silicon carbide semiconductor device, a cross-sectional structure of an n-channel silicon carbide MOSFET is shown. Moreover, the manufacturing method of the silicon carbide semiconductor device of Embodiment 1 of this invention, specifically, the manufacturing method of n channel silicon carbide MOSFET, are shown in FIGS.

In the figure, 1 is an n-type (first conductivity type) substrate, 2 is a drift layer made of n-type (first conductivity type) silicon carbide, 3 is a p-type (second conductivity type) base region, and 4 is n The source region of the type (first conductivity type), 5 is a gate insulating film made of silicon dioxide (SiO ₂ ), 6 is a gate electrode, 7 is a source electrode, and 8 is a drain electrode.

  A method for manufacturing the silicon carbide semiconductor device of the first embodiment of the present invention will be outlined with reference to FIGS. First, a drift layer 2 made of n-type silicon carbide is formed on an n-type (first conductivity type) substrate 1 by an epitaxial crystal growth method (FIG. 2). For example, an n-type silicon carbide substrate is suitable as the n-type substrate 1.

  After the epitaxial crystal growth, a pair of p-type (second conductivity type) base regions 3 are formed by implanting impurities into portions of the drift layer 2 spaced apart by a predetermined distance using a resist as a mask. FIG. 3 shows a cross section of the element after removing the resist. Examples of the impurity that becomes p-type in the drift layer 2 include boron (B) and aluminum (Al).

  Further, an n-type (first conductivity type) source region 4 is formed by implanting impurities into each of the p-type base regions 3 using a resist as a mask. FIG. 4 shows a cross section of the element after removing the resist. Examples of the n-type impurity include phosphorus (P) and nitrogen (N).

  After the n-type and p-type impurity ions are implanted, the implanted ions are electrically activated by heat treating the wafer at a high temperature using a heat treatment apparatus.

Subsequently, in a thermal oxidation process, a gate insulating film 5 made of SiO _{2 is} formed on the entire surface of the wafer by a thermal oxidation method (FIG. 5). After thermal oxidation, nitriding is performed in an oxygen nitride (NO) or oxygen dinitride (N ₂ O) atmosphere. Further, in the heat treatment step, heat treatment is performed at a temperature higher than 800 ° C. and lower than 1100 ° C. in an oxygen atmosphere containing water vapor. Since this series of steps is a characteristic step in the present invention, it will be described in detail later.

  A gate electrode 6 is formed and patterned on the gate insulating film 5 (FIG. 6). The gate electrode 6 is patterned in such a shape that the pair of base region 3 and source region 4 are located at both ends, and the drift layer 2 exposed between the base regions 3 is located at the center.

  Further, the remaining part of the gate insulating film 5 on each source region 4 is removed by the lithography technique and the etching technique (FIG. 7), and after the removal, the source electrode 7 is formed at a portion where the source region 4 is exposed on the surface. Patterning is performed (FIG. 8). When the drain electrode 8 is formed on the back side of the substrate 1, the main part of the element structure as shown in FIG. 1 is completed.

Next, a characteristic thermal oxidation step and a nitridation treatment step and a heat treatment step in a NO or N ₂ O atmosphere after thermal oxidation in the method for manufacturing a silicon carbide semiconductor device of the present invention will be described in detail. 9 to 11 are diagrams showing temperature profiles in the reactor in each process from the thermal oxidation process for the formation of the gate insulating film 5 described above to the nitriding process after thermal oxidation and the heat treatment process.

Note that FIG. 9 shows that oxygen oxidation (O ₂ ) containing water vapor (H ₂ O), that is, oxygen (O ₂ ) containing water vapor (H ₂ O) after performing a thermal oxidation process in an O ₂ atmosphere and a subsequent nitriding process using one reactor, When the heat treatment process is performed in a wet O ₂ atmosphere, FIG. 10 shows the case where the heat oxidation process and the nitriding process at the same temperature are performed using three reactors and then the heat treatment process in a wet O ₂ atmosphere is performed. FIG. 11 shows temperature profiles in the case where a thermal oxidation and nitridation process using N ₂ O is simultaneously performed using one reactor and a heat treatment process in a wet O ₂ atmosphere. Hereinafter, this series of steps will be described.

First, the wafer after the formation of the base region 3 and the source region 4 is introduced into a reactor heated to about 700 ° C., and a thermal oxidation process can be performed in an argon (Ar) atmosphere or a nitrogen (N ₂ ) atmosphere. The temperature is raised until the temperature is reached.

When the thermal oxidation temperature is reached, the reactor is switched from an Ar atmosphere or N ₂ atmosphere to an oxygen (O ₂ ) atmosphere containing water vapor (H ₂ O) or an atmosphere containing only O ₂ , and this state is maintained for a predetermined time. (Thermal oxidation process). By performing this thermal oxidation process, the silicon carbide layer (drift layer) on the wafer surface is oxidized to form the gate insulating film 5 made of SiO ₂ . That is, the portion of the thermal oxidation process in the temperature profile shown in FIGS. In the series of steps represented by the temperature profile shown in FIG. 11, the formation of the gate oxide film 5 described above is performed simultaneously with the nitridation process in the nitridation process described later.

In the thermal oxidation step, the first predetermined time may be performed in an atmosphere containing only O ₂ , and the remaining time may be performed in an O ₂ atmosphere containing water vapor (wet O ₂ atmosphere), or vice versa.

After the thermal oxidation step, the O ₂ atmosphere is switched to an Ar atmosphere or an N ₂ atmosphere, and the temperature is raised or lowered until reaching the temperature required for the nitriding treatment in the NO or N ₂ O atmosphere as the next step. This is because the heat treatment temperatures of the two are usually different. For example, in the series of steps shown in FIG. 10, since the thermal oxidation step and the nitriding step are performed in different reactors, the temperature is once lowered to a temperature of 700 ° C. or less at which the sample can be moved between the two steps. However, when the two temperatures coincide as shown in the temperature profile of FIG. 9, switching to the Ar atmosphere or the N ₂ atmosphere can be omitted.

When the inside of the reaction furnace reaches a predetermined temperature, the Ar atmosphere or the N ₂ atmosphere is switched to the NO or N ₂ O atmosphere, and the nitriding process is started. The nitriding treatment step in the NO or N ₂ O atmosphere preferably has a temperature range of 950 ° C. or higher and 1150 ° C. or lower, but similar effects can be obtained by using a special apparatus that can withstand high-temperature heat treatment even at 1150 ° C. or higher. It is done. The nitriding time is preferably about 3 hours.

As a result of the oxynitriding of the MOS interface by nitriding in an NO or N ₂ O atmosphere, the interface state is greatly reduced. Note that, in the nitriding treatment in the N ₂ O atmosphere, the formation of the gate insulating film 5 is continued by oxygen generated from the N ₂ O gas. Therefore, in this case, the thickness of the gate insulating film 5 is the sum of the amount formed in the thermal oxidation step and the amount formed in the nitriding treatment step in an N ₂ O atmosphere. Therefore, when N ₂ O is used as the atmosphere, the first thermal oxidation step can be omitted as shown in FIG.

NO or _{N 2} switch again Ar atmosphere or an _{N 2} atmosphere of NO or _{N 2} O atmosphere at the nitriding treatment ends under O atmosphere. The temperature is raised or lowered to the heat treatment temperature required for the heat treatment step, and when the temperature is stabilized, the wet O ₂ atmosphere is switched to start the heat treatment step. After performing a heat treatment for a predetermined time, the atmosphere is switched to an Ar or N ₂ atmosphere and the temperature is lowered to about 700 ° C. which is a wafer take-out temperature, and the wafer is taken out of the reactor. The heat treatment temperature in the heat treatment step will be described later.

  In the above-described series of steps, each step does not need to be performed in one reactor. When separate reactors are used, wafers are introduced and removed at 700 ° C., and the temperature is raised and lowered between the heat treatment temperature and 700 ° C. for each treatment.

As an example of the method for manufacturing the silicon carbide semiconductor device of the present embodiment, after performing a thermal oxidation step in an O ₂ atmosphere and a nitriding step in an N ₂ O atmosphere, a heat treatment step in a wet O ₂ atmosphere at 950 ° C. FIG. 12 shows the gate characteristics of the respective silicon carbide MOSFETs when it is performed for 1 hour and when the heat treatment step in the wet O ₂ atmosphere as a comparative example is omitted. It can be seen that the threshold voltage V _th is about 1 V in the silicon carbide MOSFET according to the comparative example, whereas the threshold voltage V _th is increased to 9 V in the silicon carbide MOSFET subjected to the heat treatment step at 950 ° C. The channel mobility μ _ch obtained from the linear region of the gate characteristics was 24 cm ² / Vs in the silicon carbide MOSFET according to the comparative example, whereas it was 32 cm ² / Vs in the silicon carbide MOSFET according to the present embodiment. Higher channel mobility _μch was achieved. This is presumably because the H group or OH group in the wet O ₂ atmosphere was taken into the MOS interface or the gate oxide film and became a source of negative charge leading to an increase in the threshold voltage _Vth . In addition, these negative charges have no effect of lowering the channel mobility μ _ch improved by nitriding treatment, and traps at a level where the channel mobility μ _ch is lowered by heat treatment in a wet O ₂ atmosphere. It may be decreasing.

On the other hand, when the heat treatment step is performed at 950 ° C. in a dry O ₂ atmosphere instead of a wet O ₂ atmosphere, the channel mobility μ _ch is reduced to 13 cm ² / Vs and the threshold voltage V _th becomes 1.8V. Thus, when the heat treatment step was not performed, that is, the results were not so different from those of the comparative example described above.

Further, when the heat treatment temperature in the heat treatment step in the wet O ₂ atmosphere was 1100 ° C., the channel mobility μ _ch was 6.7 cm ² / Vs, which was significantly reduced to 1/3 or less. At the heat treatment temperature of 1100 ° C. or higher, the oxidation of the silicon carbide layer is promoted, so that the origin of the interface state is newly generated, and it is considered that these interface states cause the channel mobility μ _ch to decrease. . Further, even when the heat treatment temperature in the heat treatment step is set to 800 ° C. or lower, the channel mobility μ _ch is lowered. FIG. 13 shows the heat treatment dependence of the channel mobility μ _ch and the threshold voltage V _{th in} the heat treatment step in a wet O ₂ atmosphere. From the above experiments and considerations, the heat treatment temperature in the heat treatment step is preferably in the range of higher than 800 ° C. and lower than 1100 ° C., more preferably 900 ° C. or higher and 1000 ° C. or lower. The atmosphere during the heat treatment step is preferably wet O _2. By changing the heat treatment temperature, the threshold voltage _Vth can be controlled within a predetermined range while maintaining the channel mobility _μch at a certain value or higher. Note that the threshold voltage _Vth can be controlled not only by the heat treatment temperature but also by the heat treatment time.

As described above, according to the method for manufacturing the silicon carbide semiconductor device of the present embodiment, since the nitriding process is performed after the thermal oxidation process, the interface state existing at the MOS interface is reduced, so that the MOSFET characteristics are improved and further 800 ° C. Higher threshold value for normally-off characteristics while maintaining high channel mobility while maintaining the effect of nitriding treatment by a heat treatment step in a wet O ₂ atmosphere at a heat treatment temperature higher than 1100 ° C. Can be controlled to a desired value.

Embodiment 2. FIG.
As an example of the silicon carbide semiconductor device manufactured by the method for manufacturing the silicon carbide semiconductor device of the second embodiment of the present invention, a cross-sectional structure of an n-channel silicon carbide MOSFET is shown in FIGS. In the figure, 9 indicates an n-type silicon carbide layer, and 10 indicates a depletion portion provided in the n-type silicon carbide 9, respectively.

  In contrast to the silicon carbide semiconductor device of the first embodiment, that is, the element structure shown in FIG. 1, the element structure that is the object of the method for manufacturing the silicon carbide semiconductor device of the second embodiment is as shown in FIGS. The difference is that n-type silicon carbide layer 9 is provided directly under gate insulating film 5, and depletion portion 10 exists in n-type silicon carbide layer 9 in contact with drift layer 2.

  The n-type silicon carbide layer 9 is formed by ion implantation of n-type impurities or n-type impurity into the wafer after the formation of the base region 3 and the source region 4 shown in the first embodiment, that is, the wafer shown in FIG. The silicon carbide layer is formed by a new epitaxial crystal growth method. It should be noted that the depletion portion 10 can be easily formed by masking the portion corresponding to the depletion portion 10 during ion implantation so that impurities are not ion-implanted.

  When n-type silicon carbide layer 9 is formed by ion implantation, high-temperature heat treatment is performed. This is because the implanted ions are electrically activated by high-temperature heat treatment. Note that the pre-process and post-process other than the above-described characteristic steps are the same as those in the first embodiment, and are omitted.

In the element structure shown in FIG. 14, a case where nitriding treatment is performed in a N ₂ O atmosphere at the time of gate oxidation formation, and a case where heat treatment is performed at a heat treatment temperature of 950 ° C. in a wet O ₂ atmosphere is performed. The gate characteristics are shown in FIG. When the heat treatment was performed, the threshold voltage V _th increased to 3.8 V, compared to the threshold voltage V _th of 2.4 V when the heat treatment was not performed. The channel mobility μ _ch was about 10 cm ² / Vs in any case, and no decrease in channel mobility μ _ch due to the heat treatment after nitriding was observed. These results, also in the silicon carbide MOSFET forming the n-type region in the channel part, has been found to be able to control the threshold voltage V _th while maintaining a high channel mobility mu _ch by heat treatment in wet O ₂ atmosphere.

  In the above description, the silicon carbide MOSFET is taken as an example of the silicon carbide semiconductor device. However, other silicon carbide semiconductor devices having an element structure in which an insulating film is formed on a silicon carbide layer are also shown in the present embodiment. Needless to say, the same effect can be obtained by applying the above manufacturing method.

  In the above description, the gate insulating film 5 is made of silicon dioxide, but other insulating films such as a silicon nitride film or a silicon oxynitride film have the same effect.

2 is a cross sectional view of a silicon carbide semiconductor device (MOSFET) manufactured by the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. FIG. 5 is a diagram showing a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. It is a figure showing the temperature profile in the reaction furnace in a series of processes including a thermal oxidation process, a nitriding process, and a heat treatment process which are a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. It is a figure showing the temperature profile in the reaction furnace in a series of processes including a thermal oxidation process, a nitriding process, and a heat treatment process which are a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. It is a figure showing the temperature profile in the reaction furnace in a series of processes including nitriding treatment and heat treatment which are a part of the method for manufacturing the silicon carbide semiconductor device of the first embodiment. It is a figure which shows the element characteristic of the silicon carbide MOSFET produced by the manufacturing method of the silicon carbide semiconductor device of Embodiment 1, and its comparative example. It is a figure which shows the heat processing temperature dependence in the heat processing process of the threshold voltage in the silicon carbide MOSFET produced by the manufacturing method of the silicon carbide semiconductor device of Embodiment 1. FIG. FIG. 6 is a cross sectional view of a semiconductor device manufactured by a method for manufacturing a silicon carbide semiconductor device of a second embodiment. FIG. 6 is a cross sectional view of a semiconductor device manufactured by a method for manufacturing a silicon carbide semiconductor device of a second embodiment. It is a figure which shows the gate voltage dependence of the drain current in the semiconductor device manufactured by the manufacturing method of the silicon carbide semiconductor device of Embodiment 2. FIG.

Explanation of symbols

1 n-type (first conductivity type) substrate, 2 n-type (first conductivity type) silicon carbide drift layer, 3 p-type (second conductivity type) base region, 4 n-type (first conductivity type) ) Source region, 5 gate insulating film made of silicon dioxide (SiO ₂ ), 6 gate electrode, 7 source electrode, 8 drain electrode, 9 n-type silicon carbide layer, 10 depletion part.

Claims (12)

A method for manufacturing a silicon carbide semiconductor device, comprising:
A nitriding treatment step of performing a nitriding treatment simultaneously with forming a silicon dioxide film on the silicon carbide layer by heat-treating the wafer on which the silicon carbide layer is formed in an oxygen dinitride atmosphere;
A heat treatment step of heat-treating the wafer in an oxygen atmosphere containing water vapor;
A method for manufacturing a silicon carbide semiconductor device comprising: A method for manufacturing a silicon carbide semiconductor device, comprising:
A thermal oxidation step of forming a silicon dioxide film by oxidizing a part of the surface side of the silicon carbide layer by thermally oxidizing a wafer having a silicon carbide layer formed on the surface in an oxygen atmosphere ;
A nitriding treatment step of heat-treating the wafer on which the silicon dioxide film has been formed in an oxygen nitride or dioxygen atmosphere;
A heat treatment step of heat treating the wafer after nitriding in an oxygen atmosphere containing water vapor;
A method for manufacturing a silicon carbide semiconductor device comprising: 3. The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein the conductivity type of the silicon carbide layer is n-type. A method for manufacturing a silicon carbide semiconductor device, comprising:
Epitaxially growing a drift layer made of silicon carbide of the first conductivity type on a substrate of the first conductivity type;
Forming a pair of second conductivity type base regions spaced apart by a predetermined distance in the drift layer by ion implantation of impurities;
Forming each source region of the first conductivity type in each of the base regions by ion implantation of impurities;
A gate insulating film made of a silicon dioxide film is formed on the silicon carbide layer by heat-treating the substrate, the drift layer, a wafer made of the base region and the source region formed in the drift layer in an oxygen dinitride atmosphere. A nitriding process for performing nitriding at the same time as forming the film;
A heat treatment step of heat treating the wafer after nitriding in an oxygen atmosphere containing water vapor;
Forming the gate electrode in a region where the drift layer is located in the center on the gate insulating film and the base region and the source region are located at both ends;
A method for manufacturing a silicon carbide semiconductor device comprising: A method for manufacturing a silicon carbide semiconductor device, comprising:
Epitaxially growing a drift layer made of silicon carbide of the first conductivity type on a substrate of the first conductivity type;
Forming a pair of second conductivity type base regions spaced apart by a predetermined distance in the drift layer by ion implantation of impurities;
Forming each source region of the first conductivity type in each of the base regions by ion implantation of impurities;
By oxidizing a part of the surface side of the drift layer made of silicon carbide by thermal oxidation in an oxygen atmosphere, the substrate, the drift layer, the base region formed in the drift layer, and the source region A thermal oxidation step of forming a gate insulating film made of silicon dioxide on the wafer;
A nitriding treatment step of heat-treating the wafer after forming the gate insulating film in an oxygen nitride or dinitrogen atmosphere;
A heat treatment step of heat treating the wafer after nitriding in an oxygen atmosphere containing water vapor;
Forming the gate electrode in a region where the drift layer is located in the center on the gate insulating film and the base region and the source region are located at both ends;
A method for manufacturing a silicon carbide semiconductor device comprising: The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein a heat treatment temperature in the heat treatment step is a temperature higher than 800 ° C. and lower than 1100 ° C. 6. 6. The method of manufacturing a silicon carbide semiconductor device according to claim 2, wherein water vapor is contained in an oxygen atmosphere in the thermal oxidation step. 8. The silicon carbide semiconductor according to claim 1, wherein a nitrogen or argon atmosphere is used when a processing temperature is changed between the thermal oxidation step or the heat treatment step and the nitriding step. 9. Device manufacturing method. Forming a first conductivity type impurity region by ion implantation of impurities in each source region, each base region, and each drift layer located between each base region facing the gate electrode through the gate insulating film; A method for manufacturing a silicon carbide semiconductor device according to claim 4, comprising: Forming a first conductivity type silicon carbide layer by epitaxial crystal growth on each source region, each base region, and each drift layer located between each base region facing the gate electrode through the gate insulating film; A method for manufacturing a silicon carbide semiconductor device according to claim 4, comprising: The method of manufacturing a silicon carbide semiconductor device according to any one of claims 4, 5, 9, and 10, wherein the threshold voltage is controlled by one or both of a heat treatment temperature and a heat treatment time in the heat treatment step. 12. The silicon carbide semiconductor device according to claim 1, wherein the silicon carbide semiconductor device is an insulated gate field effect transistor or a storage channel field effect transistor including a gate insulating film made of the silicon dioxide film. A method for manufacturing a silicon carbide semiconductor device.

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