JP2007141995A - Manufacturing method of semiconductor chip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a manufacturing method of a semiconductor chip capable of preventing a sheet or an adhesive layer from being modified, and of forming a sufficient amount of refined areas for dividing a semiconductor substrate aiming at the vicinity of an adhesion surface with the sheet in the semiconductor substrate. <P>SOLUTION: Because the rear surface 21b of the semiconductor substrate 21 includes the aluminum film 25 of the thickness of several μm over the entire surface formed thereon, even when a focal point of laser light L shifts to the side of a sheet 41 from a scheduled focused point Pa for forming a refined region Ks and when the focused point shifts to the focused point Pb in the sheet 41, the laser light is reflected by the aluminum film 25 before the laser light L is focused. It is therefore no fear that the adhesion layer 52 and the sheet 41 might be modified thermally because no laser light L actually converges to the focused point Pb. Moreover, it is possible to form a refined area K in the vicinity of the rear surface 21b that is a start point of the division. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a semiconductor chip by dividing a semiconductor substrate in its thickness direction.

Conventionally, in the manufacture of semiconductor chips, a semiconductor substrate that has been processed into a line to be divided and bonded to a sheet is expanded by stretching the sheet and applying stress in the surface direction of the semiconductor substrate. A method of manufacturing a semiconductor chip that is divided into semiconductor chips is used.
In recent years, as a processing method of a line to be divided, a study and research on a processing method using laser light (laser dicing) has been advanced. For example, Patent Document 1 discloses a processing technique for a semiconductor substrate using a laser. Has been. FIG. 6 is an explanatory diagram showing a dicing process using laser light. FIG. 6A is an explanatory diagram of a modified region forming process by laser light irradiation, and FIG. 6B is an explanatory diagram of a semiconductor substrate dividing process.
As shown in FIG. 6A, a semiconductor substrate W made of a semiconductor such as silicon and having a semiconductor element D formed on the substrate surface is prepared, and the back surface of the substrate surface is bonded to a stretchable resin sheet S. To do. An adhesive layer B coated with an ultraviolet curable adhesive or the like is formed on the entire surface of the sheet S to which the semiconductor substrate W is adhered, and the entire back surface of the semiconductor substrate W is adhered to the adhesive layer B.
The laser head H that irradiates the laser light L includes a condenser lens CV that condenses the laser light L, and condenses the laser light L at a predetermined focal length. In the modified region forming step, the planned dividing line that divides the semiconductor substrate W under the laser light irradiation conditions set so that the condensing point P of the laser light L is formed at a depth d from the surface of the semiconductor substrate W. The laser head H is moved along the DL (frontward in the figure), and the laser beam L is irradiated from the surface of the semiconductor substrate W. As a result, a modified region K by multiphoton absorption is formed in a path of depth d where the condensing point P of the laser beam L is scanned.
In the modified region K, the depth d of the condensing point P is adjusted along the planned dividing line DL, and the condensing point P is moved in the thickness direction of the semiconductor substrate W, thereby adjusting the thickness of the semiconductor substrate W. It can be formed at a plurality of locations at any depth within the range.
Here, multiphoton absorption means that a substance absorbs a plurality of the same or different photons. Due to the multiphoton absorption, a phenomenon called optical damage occurs at the condensing point P of the semiconductor substrate W and in the vicinity thereof, thereby inducing thermal strain, generating cracks in the portion, and a layer in which the cracks are gathered. That is, the modified region K is formed.
Subsequently, as shown in FIG. 6B, by applying a stress in the in-plane direction of the semiconductor substrate W (directions indicated by arrows F1 and F2 in the figure), the substrate thickness starts from the modified region K. Cracks are propagated in the vertical direction, and the semiconductor substrate W is divided along the division lines DL to obtain the semiconductor chip C.
Japanese Patent Laid-Open No. 2005-1001

However, in the conventional method, when the modified region K is introduced in the vicinity of the back surface, which is the adhesive surface with the sheet S, the laser light L is transmitted through the semiconductor substrate W, and the condensing point P is the adhesive layer B or the sheet. When they fit inside S, they may be altered by heat. The deteriorated portions of the adhesive layer B and the sheet S lose their stretchability and become brittle, so that they may be scattered as a powder when the semiconductor substrate W is divided and attached to the semiconductor element D.
Further, in order to avoid the above phenomenon, if the laser beam L is not irradiated in the vicinity of the back surface of the semiconductor substrate W, a sufficient amount of the modified region K may be formed aiming at the vicinity of the back surface that is the starting point of the division. Since this is not possible, a large force is required to divide the semiconductor substrate W, causing a problem that the semiconductor substrate W is left behind.

  Therefore, the present invention prevents the sheet or the adhesive layer from being altered by the laser light irradiated to the division line being transmitted through the semiconductor substrate and condensed on the sheet or the adhesive layer, and An object of the present invention is to realize a semiconductor chip manufacturing method capable of forming a sufficient amount of the modified region K for dividing the semiconductor substrate, aiming at the vicinity of the adhesive surface with the sheet in the semiconductor substrate.

  In order to achieve the above object, according to the first aspect of the present invention, in the first aspect of the present invention, a semiconductor substrate is prepared in which the adhesive surface of the sheet is bonded to one substrate surface, and the semiconductor substrate is divided in the thickness direction. While the laser beam is moved relative to the semiconductor substrate along the planned dividing line, the laser beam is irradiated with the focusing point inside the semiconductor substrate, and the focusing point is caused by multiphoton absorption. A modified region forming step for forming a modified region, and the semiconductor substrate that has undergone the modified region forming step, by extending the sheet, the modified region as a starting point along the planned division line And a dividing step of obtaining a semiconductor chip by dividing in a thickness direction, a reflecting material that reflects the laser light is formed on at least the division line of the substrate surface. That, using the technical means of.

  According to a second aspect of the present invention, in the semiconductor chip manufacturing method according to the first aspect, a technical means is used in which the reflecting material is formed of a metal film.

  According to a third aspect of the present invention, in the method for manufacturing a semiconductor chip according to the second aspect, the technical means that the metal film is mainly formed of aluminum is used.

  According to a fourth aspect of the present invention, in the method for manufacturing a semiconductor chip according to any one of the first to third aspects, the laser beam irradiated into the semiconductor substrate in the modified region forming step. Is reflected by the reflecting material formed on the substrate surface and the modified region is formed by aligning the condensing point of the reflected laser light inside the semiconductor substrate.

  According to a fifth aspect of the present invention, in the semiconductor chip manufacturing method according to the fourth aspect, the modified region formed by the reflected laser light is formed in the vicinity of the one substrate surface. Use means.

According to the first aspect of the present invention, the laser beam for forming the modified region is reflected on at least the planned division line of the substrate surface of the semiconductor substrate to which the bonding surface of the sheet is bonded. Since the reflecting material is formed, it is possible to prevent the laser beam from passing through the semiconductor substrate and the focusing point on the sheet.
In addition, even if laser light is irradiated near the bonding surface of the semiconductor substrate, there is no possibility that the laser light will pass through the semiconductor substrate and be irradiated onto the sheet. An amount of modified region can be formed.
In other words, the laser beam applied to the planned dividing line is transmitted through the semiconductor substrate and focused on the sheet, thereby preventing the sheet from being altered and aiming near the substrate surface to which the adhesive surface of the sheet is bonded. Thus, a semiconductor chip manufacturing method capable of forming a sufficient amount of modified region for dividing the semiconductor substrate can be realized.

  According to the second aspect of the invention, since the reflecting material is formed of a metal film, the reflection efficiency of the laser beam is high, so that it is ensured that the laser beam passes through the semiconductor substrate and is condensed on the sheet. Can be prevented.

  In particular, when the metal film is mainly formed of aluminum as in the invention described in claim 3, the film formation is easy, the adhesion to the semiconductor substrate is high, and the reflection efficiency of the laser beam is further increased. Therefore, it is possible to reliably prevent the laser light from passing through the semiconductor substrate and being collected on the sheet.

  According to the invention described in claim 4, in the modified region forming step, the laser beam irradiated in the semiconductor substrate is reflected by the reflecting material formed on the substrate surface, and the condensing point of the reflected laser beam is set to the semiconductor. Since the modified region is formed by matching in the substrate, the modified region can be efficiently formed by effectively using the energy of the reflected laser beam.

  According to the fifth aspect of the present invention, since the modified region formed by the reflected laser beam is formed in the vicinity of one substrate surface, a sufficient amount to be a starting point of division in the vicinity of the substrate surface. A modified region can be formed. The modified region formed in the vicinity of the substrate surface becomes a starting point for crack propagation in the planned dividing line when the semiconductor substrate is divided by expanding the sheet. Therefore, by forming a sufficient amount of the modified region, a smaller force can be obtained. The semiconductor substrate can be divided. Therefore, the semiconductor substrate can be surely divided, and the remainder can be eliminated.

<First Embodiment>
A first embodiment of a semiconductor chip manufacturing method according to the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram showing a configuration of a semiconductor substrate. 1A is an explanatory plan view of a semiconductor substrate, and FIG. 1B is a cross-sectional view taken along the line 1A-1A in FIG. 1A. FIG. 2 is an explanatory diagram showing a method for irradiating a semiconductor substrate with laser light. FIG. 3 is a schematic view showing a state in which laser light is reflected by an aluminum film formed on a semiconductor substrate.
In each figure, a part is enlarged and exaggerated for explanation.

(Structure of semiconductor substrate)
As shown in FIG. 1A, a thin disk-shaped semiconductor substrate 21 made of silicon is prepared. The semiconductor substrate 21 has a stretchable resin sheet 41 in which the back surface 21b (FIG. 1B) of the substrate surface 21a has an adhesive layer 52 (FIG. 1B) formed on the entire surface thereof by an adhesive or the like. It is glued to. The outer periphery of the sheet 41 is held by an annular frame 42 so that the sheet 41 is stretched.
An orientation flat OF showing a crystal orientation is formed on a part of the outer periphery of the semiconductor substrate 21. On the substrate surface 21a of the semiconductor substrate 21, semiconductor elements 24 formed through a diffusion process or the like are arranged and arranged like a grid.
On the substrate surface 21a between the semiconductor elements 24, planned division lines DL1 to DL14, which are lines scheduled to divide the semiconductor substrate 21 in the thickness direction, are set in the thickness direction of the semiconductor substrate 21 toward the back surface 21b. ing. The division lines DL1 to DL7 are provided in a direction substantially perpendicular to the orientation flat OF and are set to be parallel to each other. The division lines DL8 to DL14 are provided in a direction substantially parallel to the orientation flat OF and are set to be parallel to each other. That is, the planned division lines DL1 to DL7 and the planned division lines DL8 to DL14 intersect each other vertically.

Each semiconductor element 24 is surrounded on its four sides by a planned division line DL. The semiconductor substrate 21 is divided in the thickness direction along the division lines DL, and a plurality of semiconductor chips 22 having the semiconductor elements 24 are obtained.
An aluminum film 25 having a thickness of several μm is formed on the entire back surface 21b of the semiconductor substrate 21 by sputtering (FIG. 1B). The aluminum film 25 is easy to form, and has high adhesion to the semiconductor substrate 21 and high laser beam reflection efficiency. Further, since the aluminum film 25 is formed by the sputtering method which is a dry process, the semiconductor element 24 is not affected by the film forming process.

  In the following description, a portion that is not divided from the semiconductor substrate 21 and that originally becomes a semiconductor chip after being divided is also referred to as a semiconductor chip. These semiconductor chips 22 are divided into a thickness direction along the division line DL by a dicing process, and then packaged ICs and LSIs through various processes such as a mounting process, a bonding process, and an encapsulation process. Complete.

As shown in FIG. 1B, six semiconductor chips 22 a to 22 f are formed on the 1A-1A line of the semiconductor substrate 21. The semiconductor substrate 21 has the back surface 21b bonded to the adhesive layer 52 via the aluminum film 25, and the aluminum film 25, the adhesive layer 52, and the sheet 41 are arranged in this order from the semiconductor substrate 21 side.
In order to divide these semiconductor chips 22a to 22f, seven division lines DL1 to DL7 and division lines DL11 and DL12 (FIG. 1A) (not shown in FIG. 1B) are set. Yes. In the planned division lines DL1 to DL7, DL11, and DL12, a modified region K (FIG. 2) serving as a starting point of division is formed in the thickness direction of the semiconductor substrate 21 by a method described later.

(Laser irradiation)
As shown in FIG. 2, the semiconductor chip manufacturing apparatus 1 is provided with a laser head 31 that irradiates a laser beam L. The laser head 31 includes a condensing lens 32 that condenses the laser light L, and can condense the laser light L at a predetermined focal length. Here, the condensing point P of the laser beam L is set so as to be formed at a depth d from the substrate surface 21 a of the semiconductor substrate 21.

In order to form the modified region K in the semiconductor substrate 21, first, one of the division lines DL shown in FIG. 1A is scanned with the laser light for detecting the semiconductor substrate, and the irradiation range of the laser light L Set. Here, a case where the modified region K is formed in the planned division line DL4 will be described.
Subsequently, as shown in FIG. 2, the laser head 31 is scanned along the planned division line DL4 (in the direction F4 in the figure), and the laser beam L is irradiated from the substrate surface 21a, thereby condensing the laser beam L. A modified region K by multiphoton absorption is appropriately formed in a path of depth d scanned by P.
Here, by adjusting the depth d of the condensing point P of the laser light L, the modified region K having an arbitrary number of layers can be formed at an arbitrary depth within the thickness range of the semiconductor substrate 21. it can. For example, when the thickness is relatively thick, the condensing point P is moved in the thickness direction, and the modified region K is formed continuously in the thickness direction of the planned division line DL or at a plurality of locations. The semiconductor substrate 21 can be reliably divided.

As shown in FIG. 3, the case where the modified region Ks is formed in the vicinity of the back surface 21b of the semiconductor substrate 21 on the planned division line DL4 will be described as an example. This is a case where the condensing point of the laser beam L is shifted to the sheet 41 side from the condensing point Pa which is planned to form the modified region Ks, and moves to the condensing point Pb inside the sheet 41. However, since the aluminum film 25 is formed on the back surface 21b of the semiconductor substrate 21, the laser light L is reflected by the aluminum film 25 before being condensed. Therefore, since the laser beam L is not actually condensed at the condensing point Pb, there is no possibility that the adhesive layer 52 and the sheet 41 are deteriorated due to thermal influence.
That is, even if the laser beam L is irradiated in the vicinity of the back surface 21b of the semiconductor substrate 21, there is no possibility that the laser beam L passes through the semiconductor substrate 21 and is condensed on the sheet 41. The modified region K can be formed aiming at the vicinity.
For the other planned dividing lines DL, the modified region K is formed in the vicinity of the back surface 21b, similarly to the planned dividing line DL4.

(Division of the semiconductor substrate 21)
Subsequently, by expanding the sheet 41 in the surface direction, stress is applied to the semiconductor substrate 21, cracks are developed starting from the modified region K, and the thickness of the semiconductor substrate 21 is increased along the division line DL. Split in direction.
As a method of expanding the sheet 41, for example, a semiconductor substrate is mounted from the back side of the sheet 41 using a pressing device (not shown) having a flat surface substantially the same size as the back surface 21b of the semiconductor substrate 21 with the frame 42 fixed. A known method of expanding the sheet 41 in the surface direction and applying a stress in the in-plane direction of the semiconductor substrate 21 can be used by pressing the substrate 21 so as to push it up.

  Here, when the modified region K is formed in the vicinity of the back surface 21b of the semiconductor substrate 21, it effectively acts as a starting point of crack generation when the semiconductor substrate 21 is divided by expanding the sheet 41. The semiconductor substrate 21 can be reliably divided.

  In the present embodiment, as long as the material reflects the laser beam L, a material other than the aluminum film 25 can be used as the film formed on the back surface 21b of the semiconductor substrate 21. For example, another metal film such as a titanium film may be formed. Further, the film forming method is not limited to the sputtering method, and may be a plating method or application of a paint reflecting laser light. In the case where the metal film is formed by a plating method, it is possible to form a metal film having high adhesion to the semiconductor substrate in a short time and at a low cost.

[Effect of the first embodiment]
(1) Since the aluminum film 25 that reflects the laser beam L is formed on the back surface 21 b of the semiconductor substrate 21, the laser beam L is prevented from passing through the semiconductor substrate 21 and the converging point P is aligned with the sheet 41. it can.
Further, even if the laser beam L is irradiated near the back surface 21b of the semiconductor substrate 21, there is no possibility that the laser beam L will be transmitted through the semiconductor substrate 21 and irradiated onto the sheet 41. A sufficient amount of the modified region K serving as a starting point can be formed.
In other words, the laser beam L applied to the division line DL is transmitted through the semiconductor substrate 21 and condensed inside the sheet 41 to prevent the sheet 41 from being altered, and in the vicinity of the back surface 21b of the semiconductor substrate 21. In addition, it is possible to realize a method of manufacturing the semiconductor chip 22 that can form the modified region K sufficient for the division.

Second Embodiment
A second embodiment of the semiconductor substrate bonding method according to the present invention will be described with reference to the drawings. FIG. 4 is an explanatory view showing a method of forming the modified region K by condensing the laser beam L reflected by the aluminum film 25.
Hereinafter, only the process of forming the modified region K by irradiation with the laser beam L will be described, and the same reference numerals are used for the same configurations as those in the first embodiment, and the description thereof will be omitted.

In the present embodiment, the modified region K is formed by condensing the laser light L reflected by the aluminum film 25.
The position of the condensing point P is determined by the distance M between the emission surface of the laser beam L of the laser head 31 shown in FIG. 2 and the substrate surface 21a. If the aluminum film 25 is not formed, this distance M is short. The condensing point P moves to the sheet 41 side.
As shown in FIG. 4, when the laser beams L1 to L5 are irradiated, the distance M is set to be shorter in this order.

  The laser beam L1 is condensed at a condensing point P1 set closest to the substrate surface 21a among the condensing points P1 to P5, and a modified region K1 is formed around the condensing point P1. Similarly, the modified region K2 is formed around the condensing point P2 by the laser light L2, and the modified region K3 is formed around the condensing point P3 by the laser light L3. That is, the modified regions K1 to K3 are formed by condensing the laser beams L1 to L3 incident on the semiconductor substrate 21 directly on the condensing points P1 to P3.

  When there is no aluminum film 25, the laser beam L4 is condensed at the condensing point Pm inside the adhesive layer 52, but is reflected by the aluminum film 25 before being condensed, and the condensing point P3 and the back surface 21b. Is condensed at a condensing point P4 set between and a reformed region K4 is formed around it. Similarly, when there is no aluminum film 25, the laser beam L5 is condensed at the condensing point Pn inside the sheet 41, but is reflected by the aluminum film 25 before being condensed, and the condensing point P2. The light is condensed at a condensing point P5 set between the condensing point P3 and a reformed region K5 is formed around it.

Here, in the case of introducing a plurality of modified regions K in the thickness direction of the semiconductor substrate 21, if the modified region K is formed from the side closer to the substrate surface 21a, the modified laser beam L is formed first. Since the light is scattered when passing through the region K and the condensing point P becomes difficult to match, the modified region K having a sufficient size may not be formed. Therefore, the modified region K is preferably formed in order from the far side from the substrate surface 21a.
Therefore, it is desirable to form the modified region K in the order of K4 → K3 → K5 → K2 → K1, and the laser head 31 and the substrate surface so that the laser light L is irradiated in the order of L4 → L3 → L5 → L2 → L1. The distance M (FIG. 2) with 21a is controlled.

[Effects of Second Embodiment]
(1) In the modified region forming step, the laser beam L irradiated in the semiconductor substrate 21 is reflected by the aluminum film 25 formed on the back surface 21b, and the condensing point P of the reflected laser beam L is set inside the semiconductor substrate 21. In order to form the modified region K, the modified region K can be efficiently formed by effectively using the energy of the reflected laser light L.

(2) By condensing the laser beam L reflected by the aluminum film 25 and forming the modified regions K4 and K5 in the vicinity of the back surface 21b, a sufficient amount of modification is made to be the starting point of the division in the vicinity of the back surface 21b. A quality region K can be formed. Since the modified region K formed in the vicinity of the back surface 21b serves as a starting point for crack growth of the planned dividing line DL when the semiconductor substrate 21 is divided by the expansion of the sheet 41, a sufficient amount of the modified region K is formed. Thus, the semiconductor substrate 21 can be divided with a smaller force. Therefore, the semiconductor substrate 21 can be surely divided, and the remainder can be eliminated.

<Other embodiments>
(1) FIG. 5 is a cross-sectional explanatory diagram of a configuration in which the aluminum film 25 is formed on at least the planned division line DL of the back surface 21b. As shown in FIG. 5, the aluminum film 25 is formed in a strip shape having a width of, for example, about 1/10 of one side of the semiconductor chip 22 on the planned dividing lines DL3 to DL5. When the entire back surface 21b of the semiconductor substrate 21 is viewed, the aluminum film 25 is formed in a lattice shape on the planned dividing lines DL1 to DL14. Even when this configuration is used, since the aluminum film 25 can reflect the laser light irradiated to the division line DL, the effects of the first and second embodiments can be achieved. Further, since most of the region other than the region where the aluminum film 25 is formed on the back surface 21b is directly bonded to the adhesive layer 52, the semiconductor substrate 21 can be firmly bonded to the sheet 41, so that the division is possible. In the process, the semiconductor substrate 21 can be reliably stressed and divided reliably.
Further, a metal foil such as an aluminum foil may be arranged in a lattice shape at a position corresponding to the division line DL on the adhesive layer 52.

(2) Although the semiconductor substrate made of only silicon is used as the semiconductor substrate 21, the application of the present invention is not limited to this. For example, an oxide film made of silicon oxide is used as the substrate surface of the semiconductor substrate 21. The present invention can also be applied to those formed in 21a and SOI (Silicon On Insulator) wafers.

[Correspondence between each claim and embodiment]
The back surface 21 b corresponds to one substrate surface according to claim 1, and the aluminum film 25 corresponds to a reflective material.

1A is an explanatory plan view of a semiconductor substrate, and FIG. 1B is a cross-sectional view taken along the line 1A-1A in FIG. 1A. It is explanatory drawing which shows the method of irradiating a semiconductor substrate with a laser beam. It is a schematic diagram of a state in which laser light is reflected by an aluminum film formed on a semiconductor substrate. It is explanatory drawing which shows the method of condensing the laser beam reflected by the aluminum film, and forming a modification area | region. It is sectional explanatory drawing of the structure which forms an aluminum film on the division line at least of a back surface. It is explanatory drawing which shows the dicing process using a laser beam. FIG. 6A is an explanatory diagram of a modified region forming process by laser light irradiation, and FIG. 6B is an explanatory diagram of a semiconductor substrate dividing process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor chip manufacturing apparatus 21 Semiconductor substrate 21b Back surface (one substrate surface)
22, 22a-22f Semiconductor chip 24 Semiconductor element 25 Aluminum film (reflective material)
31 Laser head 41 Sheet 52 Adhesive layer DL, DL1 to DL14 Scheduled division lines K, K1 to K5 Modified region L Laser light M Distance between laser light emission surface of laser head and substrate surface P, P1 to P5, Pa, Pb, Pm, Pn Condensing point

Claims (5)

Prepare a semiconductor substrate with the adhesive surface of the sheet bonded to one substrate surface,
While moving the laser beam relative to the semiconductor substrate along the division line for dividing the semiconductor substrate in the thickness direction, the laser beam is irradiated with the focusing point inside the semiconductor substrate. A modified region forming step of forming a modified region by multiphoton absorption at the condensing point;
A dividing step of obtaining the semiconductor chip by dividing the semiconductor substrate that has undergone the modified region forming step by dividing the sheet in the thickness direction from the modified region as a starting point by extending the sheet. In a method for manufacturing a semiconductor chip comprising:
A method of manufacturing a semiconductor chip, wherein a reflective material that reflects the laser beam is formed on at least the division line of the substrate surface.   The method of manufacturing a semiconductor chip according to claim 1, wherein the reflective material is formed of a metal film.   3. The method of manufacturing a semiconductor chip according to claim 2, wherein the metal film is mainly formed of aluminum.   In the modified region forming step, the laser light irradiated in the semiconductor substrate is reflected by the reflecting material formed on the substrate surface, and a focused point of the reflected laser light is adjusted inside the semiconductor substrate. The method for manufacturing a semiconductor chip according to claim 1, wherein the modified region is formed. 5. The method of manufacturing a semiconductor chip according to claim 4, wherein the modified region formed by the reflected laser light is formed in the vicinity of the one substrate surface.

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