TWI603026B - Apparatus for holding substrate - Google Patents

Description

Device for holding a substrate

The present invention relates to an apparatus for holding a substrate, and more particularly to an apparatus capable of stably holding a substrate on a holding table.

In general, the apparatus for holding a substrate is disposed in a processing chamber for manufacturing a display device such as a light crystal display (LCD), an organic light emitting diode (OLED), or the like. The universal substrate holding device includes a holding table on which the substrate is held, and a plurality of lift bars that are vertically passed through the holding table and raised. The lifter assists in supporting the substrate on the holding table. When the substrate is above the holding table, the lift bar is lifted to support the lower portion of the substrate and then lowered to allow the substrate to be held on the substrate holding table.

The size of a glass substrate used for a display device such as an LCD, an OLED, or the like is gradually increased. When the plurality of lift bars of the substrate holding device support the large substrate, the substrate may bend due to its own weight and slip when the substrate is held on the holding table. Thus, it is inconvenient to align the substrate again, and thus the processing time is extended.

A vacuum apparatus of a liquid crystal display device is disclosed in Korean Patent Publication No. 10-2004-0115542. Here, the vacuum device comprises: a base, which is held on top a base; a head disposed to correspond to a back surface of the substrate and having a predetermined width corresponding to a length of one side of the base; and a lift rod passing through the base to vertically move the head in the vacuum chamber The main body is composed.

The present invention provides an apparatus for stably holding a substrate on a holding table.

The present invention also provides an apparatus for holding a substrate that is capable of preventing the substrate from sliding when the substrate is held on the holding table.

According to an exemplary embodiment, an apparatus for holding a substrate on a top surface thereof includes: a holding table that is held on a top surface of the holding table; a plurality of suction channels that are vertically worn Passing through at least one portion of the holding table, and the plurality of suction channels are arranged in a width direction of the holding table to support the substrate by using vacuum suction; a plurality of ventilation passages vertically passing through the At least one portion of the holding station, the plurality of ventilation passages being disposed in the width direction of the holding table such that the plurality of ventilation passages are located between the plurality of suction passages to pass the plurality of ventilation passages a channel releases gas between the substrate and the holding table; and a plurality of substrate supports disposed in the width direction of the holding table, the plurality of substrate supports being raised or lowered to allow Loading the substrate on the holding table and unloading the substrate from the holding table.

A plurality of portions of the plurality of suction passages may extend in the X direction of the holding table and are spaced apart from each other in the Y direction, and the remaining plurality of suctions The passages may extend in the Y direction of the holding table and are spaced apart from each other in the X direction.

The suction passage extending in the X direction and the suction passage extending in the Y direction may cross each other.

Each of the plurality of ventilation passages may be defined between the plurality of suction passages, wherein portions of the plurality of ventilation passages extend in the X direction of the holding table and are mutually in the Y direction The plurality of ventilation passages are spaced apart in the Y direction of the holding table and are spaced apart from each other in the X direction.

The ventilation passages extending in the X direction and the ventilation passages extending in the Y direction may cross each other.

The height of the substrate support supporting the edge of the substrate in the plurality of substrate supports may be greater than the height of the support supporting the central region of the substrate.

The plurality of suction channels may include: a central suction channel defined in a region on a central portion of the substrate supported on the holding table; and a peripheral suction channel defined outside the central suction channel In the surrounding area.

The apparatus for holding a substrate may further include a suction control unit connected to the plurality of suction passages, wherein the suction control unit controls the suction passage such that the suction passage is drawn from the center The suction passage to the peripheral suction passage continuously generates vacuum suction at a time interval.

100‧‧‧Guide

110‧‧‧ Subject

120‧‧‧Support board

200‧‧‧ ventilation passage

300‧‧‧sucking channel

310‧‧‧Center suction channel

321, 322, 331, 332 ‧ ‧ suction channel

400a, 400b‧‧‧ base support

500a, 500b‧‧‧ transmission part

600‧‧‧sucking control unit

610‧‧‧vacuum pump

620‧‧‧ gas storage tank

630‧‧‧Vacuum valve

631‧‧‧vacuum pipeline

640‧‧‧ gas valve

641‧‧‧ gas pipeline

650‧‧‧Connected pipeline

660‧‧‧Central Pipeline

661‧‧‧First pipeline

662‧‧‧Second pipeline

663‧‧‧ Third pipeline

664‧‧‧fourth pipeline

670‧‧‧ center valve

671‧‧‧First valve

672‧‧‧Second valve

673‧‧‧third valve

674‧‧‧fourth valve

S‧‧‧ base

The exemplary embodiments may be understood in more detail by the following description in conjunction with the accompanying drawings in which: FIG. 1 and FIG. 2 are three-dimensional views of an apparatus for holding a substrate, according to an exemplary embodiment.

Figure 3 is a cross-sectional view of the apparatus for holding a substrate taken along line A-A' of Figure 1.

Figure 4 is a cross-sectional view of the apparatus for holding a substrate taken along line B-B' of Figure 1.

FIGS. 5 to 8(b) are views illustrating a process of holding a substrate on a substrate holding device.

Specific embodiments will be described in detail below with reference to the accompanying drawings. However, the invention may be embodied in different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and the scope of the invention will be fully conveyed to those skilled in the art. Throughout the text, the same reference numerals will be used to refer to the same elements.

1 and 2 are three-dimensional views of an apparatus for holding a substrate, according to an exemplary embodiment. Here, FIG. 1 is a view illustrating a state in which a plurality of substrate supports are raised to protrude upward from a holding table, and FIG. 2 is a view illustrating that the plurality of substrate supports are lowered such that the plurality of substrate supports are placed on a holding table A view of the internal state. Figure 3 is a cross section of the apparatus for holding a substrate taken along line A-A' of Figure 1 Figure 4, and Figure 4 is a cross-sectional view of the apparatus for holding a substrate taken along line B-B' of Figure 1.

The apparatus for holding a substrate according to an exemplary embodiment may be an apparatus for holding a glass substrate, specifically for holding a large substrate. The substrate holding device is disposed in a processing chamber (not shown) for processing the substrate. For example, a processing chamber in which a substrate holding device according to an exemplary embodiment is disposed may be a chamber in which laser induced thermal imaging (LITI) is performed.

Referring to FIGS. 1 through 3, a substrate holding device according to an exemplary embodiment includes a holding table 100 having an upper portion on which a substrate is held, and a plurality of suction channels 300 vertically passing through at least one of the holding stations 100 Partially to support the substrate by using vacuum suction; a plurality of ventilation passages 200 vertically passing through at least a portion of the holding table 100 to serve as a passage through which gas between the release substrate and the holding table 100 passes; a suction control unit 600 connected to each of the plurality of suction channels 300 to generate vacuum suction; a plurality of substrate supports 400a and 400b disposed in a width direction of the holding table 100, the plurality of substrate supports The pieces 400a and 400b are raised to support loading/unloading of the substrate; and a plurality of transmission units 500a and 500b for lifting the plurality of substrate supports 400a and 400b.

The holding table 100 may be a unit for placing a substrate on an upper portion thereof to support the substrate. The holding stage 100 includes a body 110 and a support plate 120 disposed on the body 110 to support the substrate. Here, the size of the support plate 120 is smaller than the size of the body 110, and the support plate 120 is disposed on the body 110. The area of the main body 110 is larger than The area of the substrate. Although the support plate 120 according to an exemplary embodiment has a holding plate shape, the present invention is not limited thereto. For example, the support plate 120 can be fabricated in a variety of shapes that correspond to the shape of the substrate.

The suction passage 300 extends in a linear shape along both width directions of the holding stage 100, thereby forming a lattice pattern shape on the holding stage 100. Specifically, portions of the plurality of suction passages 300 extend in the X direction of the holding table 100 and are spaced apart from each other in the Y direction, as illustrated in FIGS. 1 and 2. The remaining plurality of suction passages 300 extend in the Y direction of the holding table 100 and are spaced apart from each other in the X direction. That is, at least a plurality of portions of the plurality of suction channels 300 extending in the X direction and the plurality of suction channels 300 extending in the Y direction cross each other. The plurality of suction channels 300 can be configured to be filled in the support plate 120.

When the substrate is held on the holding table 100, the substrate is held such that the substrate continuously contacts the support plate 120 from the central region of the substrate toward the edge region of the substrate. To this end, the suction channel 300 is controlled using a suction control unit 600, which will be described later, such that the suction channel 310 defined from the central region of the substrate holding region of the plurality of suction channels 300 is oriented toward a plurality of suctions. Suction channels 321, 322, 331, and 332 defined in the edge regions of channel 300 continuously create vacuum suction.

Hereinafter, a suction channel defined in a central region of the plurality of suction channels 300 in the base plate 120 is referred to as a "central suction channel 310", and a suction channel defined in a peripheral region of the central suction channel 310 It is referred to as "peripheral suction passages 321, 322, 331, and 332". Referring to Figure 3, the peripheral suction channels 321, 322, 331 The sum 332 is defined in both directions of the central suction channel 310. Each of the peripheral suction channels 321 and 322 defined in one direction of the central suction channel 310 may be divided into a plurality of channels. Also, each of the peripheral suction channels 331 and 332 defined in other directions of the central suction channel 310 can be divided into a plurality of channels. For example, two peripheral suction channels 321 and 322 can be defined in one direction of the central suction channel 310, and two peripheral suction channels 331 and 332 can be defined in the other direction of the central suction channel 310. Hereinafter, the two peripheral suction passages 321 and 322 defined in one direction of the center suction passage 310 are referred to as "first and second peripheral suction passages 321 and 322". Here, the suction passage defined adjacent to the central suction passage 310 is referred to as a first peripheral suction passage 321 and the other suction passage is referred to as a second peripheral suction passage 322. Also, the two peripheral suction passages 331 and 332 defined in the other direction of the center suction passage 310 are referred to as "third and fourth peripheral suction passages 331 and 332". Here, the suction passage defined relative to the central suction passage 310 is referred to as a third peripheral suction passage 331 and the other suction passage is referred to as a fourth peripheral suction passage 332.

Although suction control unit 600 is used to continuously supply vacuum suction from the center suction passage 310 to the peripheral suction passages 321, 322, 331, and 332 as described below. More specifically, the vacuum suction is continuously provided in the order of the center suction passage 310, the first and third peripheral suction passages 321 and 331 and the second and fourth peripheral suction passages 322 and 332.

In the foregoing description, the central suction passage 310 and the peripheral suction passages 321, 322 are described above with reference to Fig. 3 illustrating a section cut along the line A-A' of Fig. 1. Configuration and arrangement of 331 and 332. Although not separately illustrated, the central suction channel 310, the first and the first defined in one direction of the central suction channel 310, may be illustrated in a cross section along a line perpendicular to line AA' of FIG. Two peripheral suction channels 321 and 322 and third and fourth peripheral suction channels 331 and 332 defined in the other direction of the central suction channel 310.

The ventilation passage 200 is defined between the plurality of suction passages 300 to extend in a linear shape along both width directions of the holding table 100, thereby forming a lattice pattern shape on the holding table 100. More specifically, as illustrated in FIGS. 1 and 2, each of the plurality of ventilation passages 200 is defined between the suction passages 300. Here, the plurality of portions of the plurality of ventilation passages 200 extend in the X direction of the holding table 100, and are arranged in the Y direction and spaced apart from each other. The remaining plurality of ventilation passages 200 extend in the Y direction of the holding table and are arranged in the X direction and spaced apart from each other. That is, at least portions of the plurality of ventilation passages 200 extending in the X and Y directions cross each other. A plurality of ventilation passages 200 may be filled in at least the support plate 120 of the holding table 100 and may be extended such that both ends of the ventilation passages are located outside the support plate 120 on the holding table 100.

The suction control unit 600 is coupled to each of the plurality of suction passages 300 provided in a lattice shape to allow the suction passage 300 to be directed from the suction passage 300 defined in the central region of the holding table 100 toward the holding table The suction channel 300 defined in the edge of 100 continuously produces vacuum suction. Here, vacuum suction is continuously generated from the suction passages 300 defined in the center of the support plate 120 in all directions.

Hereinafter, as an example, the description will be made to connect to extend in the Y direction and And a suction control unit 600 of a plurality of suction channels (see FIG. 3) arranged in the X direction and spaced apart from each other.

The suction control unit 600 includes a vacuum pump 610 that generates vacuum suction, a gas storage tank 620 that stores gas for releasing the vacuum when the substrate is unloaded, a vacuum line 631 that has one end connected to the vacuum pump 610, and one gas end 641, one end of which Connected to the gas storage tank 620; a central line 660 connected to the central suction passage 310; first and second lines 661 and 662 connected to the first and second peripheral suction passages 321 and 322, respectively; third and fourth Lines 663 and 664 are connected to the third and fourth peripheral suction passages 331 and 332, respectively; a center line 660; and a connection line 650 connecting the first to fourth lines 661 to 664 to the vacuum line 631 and the gas line 641 Pipeline between. Also, the suction control unit 600 includes a vacuum valve 630 disposed in the vacuum line 631 to control communication between the connection line 650 and the vacuum pump 610, and a gas valve 640 disposed in the gas line 641 to control the connection line 650 and the gas storage a communication between the slots 620; a center valve 670 disposed in the center line 660 to control communication between the center suction passage 310 and the connecting line 650; and first to fourth valves 671 to 674, respectively disposed at the first to The fourth lines 661 to 664 control the communication between the first to fourth peripheral suction passages 321, 322, 331, and 332 and the connection line 650. Here, the gas stored in the gas storage tank 620 may be supplied into the suction passages 310, 321, 322, 331, and 332 to help unload the substrate. Here, a plurality of inert gases (for example, nitrogen, argon, etc.) or air may be used as the gas.

When the substrate is held on the holding table 100, the vacuum pump 610 operates to open the vacuum valve 630, and then successively opens the center valve 670, the first and third valves 671, and 673 and second and fourth valves 672 and 674. Thus, suction is generated in the order of the central suction channel 310, the first and third peripheral suction channels 321 and 331 and the second and fourth peripheral suction channels 322 and 332 to allow the substrate to pass from the central region of the substrate to the edge of the substrate. The region is continuously contacted or continuously attached tightly to the top surface of the holding table 100. Also, since the substrate is continuously and tightly attached to the top surface of the holding table 100 from the center region to the edge region, the gas remaining between the substrate and the holding table 100 (i.e., air) is released through the venting passage 200. Here, since the substrate is continuously and tightly attached to the holding table 100 from the central region to the edge region, air flows from the ventilation passage 200 defined in the center of the support plate 120 to the ventilation passage 200 defined in the edge of the support plate 120, And then released to the outside of the support plate 120. Thus, since the air is naturally released when the substrate is held on the holding table 100, the substrate can be closely attached and fixed to the holding table 100 without being lifted.

The substrate supports 400a and 400b hold the substrate on the holding table or separate the held substrate from the holding table 100. Each of the substrate supports 400a and 400b has a rod shape to pass vertically through the holding table 100 and provide a plurality of substrate supports. For example, as illustrated in FIG. 1 , the substrate supports 400 a and 400 b include a plurality of first substrate supports 400 b disposed outside the edges of the support plates 120 of the holding table 100 in the X direction, and are disposed on the support plate 120 A plurality of second substrate supports 400a disposed inside and in the X direction. Here, the first substrate supports 400b are respectively disposed outside the two edges of the support plate 120 in the X direction to face each other. That is, the first substrate supports 400b are disposed in two rows to face each other. Like the first substrate support 400b, the second substrate support 400a is also disposed in two rows, and Thus the second substrate support 400a is disposed between the first substrate supports 400b disposed in two rows. The top ends of each of the first substrate supports 400b are located at a higher position than the top end of each of the second substrate supports 400a. Thus, when viewing a plurality of substrate supports 400a and 400b from the Y direction with reference to Figure 1, two second substrate supports 400a (each having a relatively lower top end) are disposed on the two first substrate supports 400b Between (each having a relatively high top end). Referring to FIG. 4, the top end of the first substrate support 400b supporting the edge of the substrate is located at a height higher than the height of the top end of the second substrate support 400a at the center of the support substrate. The height difference between the first substrate support 400b and the second substrate support 400a is defined in consideration of the height difference due to the sag caused by the weight of the large substrate itself.

The transmission portions 500a and 500b are used to lift each of the plurality of substrate supports 400a and 400b and provide a plurality of transmission portions. The number of transmission portions 500a and 500b may correspond to the number of the plurality of substrate supports 400a and 400b to lift each of the substrate supports. The invention is not limited to the stated number of transmission parts. For example, the number of transmission portions may be less than the number of the plurality of substrate supports 400a and 400b, thereby simultaneously lifting a plurality of substrate supports disposed on the same line. Of course, a transmission portion can be provided to lift the plurality of substrate supports 400a and 400b at a time.

Although each of the transmission portions 500a and 500b according to the embodiment is a cylinder, the present invention is not limited thereto. For example, the transmission portion can include various units that can cause the substrate supports 400a and 400b to reciprocate vertically, for example, a unit that includes a motor.

FIGS. 5 to 8(b) are views illustrating a process of holding a substrate on a substrate holding device.

Hereinafter, a process of holding a substrate on a substrate holding device will be described with reference to FIGS. 1 to 8(b), and a repetitive description will be simply described or omitted.

A method of holding a substrate using a substrate holding device according to an exemplary embodiment includes: a process of positioning a substrate S at a corresponding position above the holding table 100; lifting a plurality of substrate supports 400a and 400b to allow the substrate support 400a and The process of supporting the substrate S on 400b; the process of allowing the plurality of substrate supports 400a and 400b to be lowered; and the suction channel defined in the central region of the support plate 120 to the suction channel defined in the edge of the support plate 120 321 , 322 , 331 , and 332 continuously generate vacuum suction to closely attach the substrate S to the support plate 120 .

Hereinafter, a method of holding a substrate will be described in detail.

First, the substrate S (for example, a glass substrate) is positioned at a corresponding position above the support plate 120 provided on the holding table 100 using an arm. The transmission portions 500a and 500b operate to lift the plurality of substrate supports 400a and 400b such that the plurality of substrate supports 400a and 400b protrude upward from the holding table 100 to allow the top ends of the plurality of substrate supports 400a and 400b to contact the substrate S The bottom surface is as illustrated in FIG. 6. Here, since the top end of the first substrate support 400b is located at a higher height than the height of the top end of the second substrate support 400a, the edge region of the substrate S is supported at a position higher than the position of the central region of the substrate. At the office. Thus, the central area of the substrate hangs down.

Then, the transmission portions 500a and 500b operate to allow a plurality of substrate supports The pieces 400a and 400b are lowered, thereby holding the bottom surface of the substrate S on the support plate 120. Here, due to the difference in height between the first and second substrate supports 400b and 400a, the central region of the bottom surface of the substrate S first contacts the support plate 120, and then the bottom surface of the substrate S continuously contacts the support plate 120 from the central region toward the edge, This is illustrated in (a) of FIG. 7 and (b) of FIG. 7. Thus, the entire top surface of the support plate 120 is in contact with the bottom surface of the substrate S as illustrated in (a) of FIG. 8 and (b) of FIG. The base supports 400a and 400b are lowered by the transmission portions 500a and 500b such that the bottom surface of the substrate S is fitted to the top surface of the holding table 100.

As described above, when the bottom surface of the substrate S continuously contacts the support plate 120 from the central region to the edge region, the suction control unit 600 allows the suction passage 300 to continuously generate vacuum suction, thereby closely attaching the substrate S to the support plate 120. .

Hereinafter, a process of closely attaching a substrate using a suction channel will be described in detail with reference to FIG.

When the central area of the bottom surface of the substrate S contacts the top surface of the support plate 120, the center valve 670 opens to allow the central suction passage 310 to generate vacuum suction. Here, due to the vacuum suction in the central suction passage 310, the bottom surface of the substrate S is closely attached to the top surface of the support plate.

Next, the bottom surface of the substrate S continuously contacts the support plate 120 from the central region to the edge region of the bottom surface of the substrate S. Here, vacuum suction is provided to the peripheral suction passages 321, 322, 331, and 332 defined in the region corresponding to the valve region. For example, referring to FIG. 3, the first and third valves 671 and 673 are opened to provide vacuum suction into the first and third peripheral suction passages 321 and 331 relatively close to the center suction passage 310, and then to be predetermined The time intervals open the second and fourth valves 672 and 674 to provide vacuum suction into the second and fourth peripheral suction channels 322 and 332. Thus, the substrate S The bottom surface is continuously and tightly attached from the central portion of the substrate S to the edge region to the top surface of the support plate.

As described above, since the bottom surface of the substrate S is continuously closely attached to the top surface of the holding stage 100, the gas remaining between the substrate S and the support plate 120 (i.e., air) is released through the ventilation passage 200. Here, since the substrate S is continuously attached to the holding stage 100 from the central area to the edge area, air flows from the ventilation passage 200 defined in the center of the support plate 120 to the ventilation passage 200 defined in the edge of the support plate 120, and It is then released to the outside of the support plate 120. Thus, since the air is naturally released when the substrate S is held on the holding table, the substrate can be closely attached and fixed to the support plate without being lifted.

In the foregoing description, the substrate may be continuously and tightly attached to the support plate 120 from the central region to the edge region in one direction of the support plate 120, and continuously attached to the edge region continuously in the other direction of the support plate 120. Above, as illustrated in Figure 3. However, although not separately described, vacuum suction may be continuously generated from the suction passage defined in the central region of the support plate 120 to the edge of the support plate 120 in all directions to closely attach the substrate to the support plate 120.

As described above, the substrate holding device continuously and closely attaches the bottom of the substrate S to the holding stage 100 from the central region of the bottom surface of the substrate S toward the edge region. Thus, the substrate holding device can prevent the large substrate S from slipping due to its own weight when the large substrate S is held on the holding table 100. Therefore, the substrate holding device can minimize misalignment of the substrate S, shorten processing time, and stabilize the facility.

According to an embodiment of the invention, the substrate holding device is from the bottom surface of the substrate S The central region faces the edge region continuously and tightly attaches the bottom of the substrate S to the holding table 100. Thus, the substrate holding device can prevent the large substrate S from slipping due to its own weight when the large substrate S is held on the holding table 100. Therefore, the substrate holding device can prevent the substrate from being misaligned and shorten the processing time because it is not necessary to realign the substrate.

Although the substrate holding device of the present invention has been disclosed in the above embodiments, it is not intended to limit the present invention, and those skilled in the art can make some changes without departing from the spirit and scope of the present invention. The scope of protection of the present invention is defined by the scope of the appended claims.

100‧‧‧Guide

110‧‧‧ Subject

120‧‧‧Support board

200‧‧‧ ventilation passage

300‧‧‧sucking channel

Claims (7)

An apparatus for holding a substrate, the apparatus comprising: a holding table comprising a support plate for holding the substrate on a top surface of the holding table; a plurality of suction channels, vertically penetrating Passing at least one portion of the holding table, and the plurality of suction channels are arranged in a width direction of the holding table to support the substrate by using vacuum suction; a plurality of ventilation channels, the plurality of ventilation channels Each of the plurality is vertically defined in the holding table and extends in a width direction of the holding table, wherein the ventilation passage has an extension to be external to the support plate on the holding table Both ends such that gas between the substrate and the holding stage is released to the outside in the width direction of the support plate; and a plurality of substrate supports disposed in the width direction of the holding table, A plurality of substrate supports are raised or lowered to allow loading and unloading the substrate from the holding table, wherein the plurality of ventilation channels are disposed at the holding table In the width direction, the plurality of ventilation channels are located between the plurality of suction channels, wherein a plurality of the substrate supports are lifted to support an edge of the substrate in a plurality of the substrate supports The height of the substrate support is higher than the height of the support supporting the central region of the substrate. The apparatus of claim 1, wherein the plurality of portions of the plurality of suction passages extend in the X direction of the holding table, and in the Y direction This is spaced apart, and the remaining plurality of suction channels extend in the Y direction of the holding table and are spaced apart from each other in the X direction. The apparatus of claim 2, wherein the suction passage extending in the X direction and the suction passage extending in the Y direction cross each other. The apparatus of claim 2, wherein each of the plurality of ventilation channels is defined between the plurality of suction channels, wherein portions of the plurality of ventilation channels are The X-direction of the holding table extends and is spaced apart from each other in the Y direction, and the remaining plurality of ventilation channels extend in the Y direction of the holding table, and in the X direction They are spaced apart from each other. The apparatus of claim 4, wherein the ventilation passage extending in the X direction and the ventilation passage extending in the Y direction cross each other. The apparatus of claim 1, wherein the plurality of suction channels comprise: a central suction channel defined in a region, a central region of the substrate being supported above the region at the retention And a peripheral suction channel defined in a periphery of the exterior of the central suction channel. The apparatus of claim 6, further comprising a suction control unit coupled to the plurality of suction channels, wherein the suction control unit controls the suction channel such that from the center The suction channel to the peripheral suction channel continuously produces the vacuum suction at a time interval.