阅读说明：本技术 静电卡盘 (Electrostatic chuck ) 是由 渡边玲雄 近藤畅之 海野丰 于 2018-10-16 设计创作，主要内容包括：本发明提供一种静电卡盘(10),其具有在圆板状的陶瓷板(12)埋设有静电电极(16)的构造,并利用约翰森-拉别克力吸附载置于陶瓷板(12)且直径比陶瓷板(12)的直径小的晶片(W)。该静电卡盘(10)在陶瓷板(12)的表面中的从陶瓷板(12)的外周缘至载置于陶瓷板(12)的晶片(W)的外周缘的内侧为止的环状区域具有电阻比陶瓷板(12)的电阻大的绝缘膜(14)。(The invention provides an electrostatic chuck (10) having a structure in which an electrostatic electrode (16) is embedded in a disk-shaped ceramic plate (12), and a wafer (W) which is placed on the ceramic plate (12) and has a smaller diameter than the ceramic plate (12) is adsorbed by Johnson-Rahbek force. The electrostatic chuck (10) has an insulating film (14) having a higher resistance than the ceramic plate (12) in an annular region of the surface of the ceramic plate (12) from the outer peripheral edge of the ceramic plate (12) to the inside of the outer peripheral edge of a wafer (W) placed on the ceramic plate (12).)

1. An electrostatic chuck having a structure in which an electrostatic electrode is buried in a disk-shaped ceramic plate, and a wafer having a smaller diameter than the ceramic plate and placed on the ceramic plate is adsorbed by Johnson-Rahbek force,

at least an annular region from a position corresponding to an outer peripheral edge of the electrostatic electrode to an inner side of an outer peripheral edge of the wafer placed on the ceramic plate in a surface of the ceramic plate has an insulating film having a higher resistance than the ceramic plate.

2. The electrostatic chuck of claim 1,

the annular region is a region from an outer peripheral edge of the ceramic plate to an inner side of an outer peripheral edge of the wafer mounted on the ceramic plate, in a surface of the ceramic plate.

3. The electrostatic chuck of claim 1 or 2,

the ceramic plate has a circular convex portion on a surface thereof for mounting the wafer,

the circular projection is provided so that when the wafer is placed on the circular projection, the outer peripheral edge of the wafer protrudes outward from the circular projection,

the insulating film is provided outside the circular convex portion on the surface of the ceramic plate.

4. The electrostatic chuck of claim 3,

a circumferential groove is formed along the periphery of the circular convex part,

the circumferential groove is provided such that the outer peripheral edge of the wafer placed on the circular projection is positioned in a space above the circumferential groove.

5. The electrostatic chuck of claim 4,

the ceramic plate has a circumferential slope on the outer side of the circumferential groove in the surface thereof.

6. The electrostatic chuck according to any one of claims 1 to 5,

the insulating film is also provided on the side surface of the ceramic plate.

Technical Field

The present invention relates to an electrostatic chuck.

Background

Conventionally, an electrostatic chuck for attracting and holding a wafer by using johnson-rahbek force has been known. In such an electrostatic chuck, an electrostatic electrode is usually embedded in the AlN ceramic plate, and when a voltage is applied to the electrostatic electrode, charges generated in the AlN ceramic plate move to the outermost surface of the AlN ceramic plate, and charges of opposite polarity are generated on the back surface of the wafer with respect to the charges on the outermost surface, and as a result, the wafer is strongly attracted to the AlN ceramic plate. Such an electrostatic chuck is shown in patent document 1, for example. In the electrostatic chuck of patent document 1, the AlN ceramic plate having an electrostatic electrode embedded therein has a circumferential slope on the outermost periphery of the surface. A circular convex portion for placing a wafer is provided on the inner side of the circumferential slope of the surface of the ceramic plate. The circular protrusion has a diameter smaller than the diameter of the wafer. Since the inner peripheral edge of the circumferential slope is formed slightly larger than the wafer, the wafer is guided by the circumferential slope when the wafer is placed on the circular convex portion, and thus the gap between the inner peripheral edge of the circumferential slope and the wafer falls within the range of the predetermined gap.

Disclosure of Invention

Problems to be solved by the invention

When a conductive film is deposited on the surface of the wafer using such an electrostatic chuck, the conductive film is also deposited on a portion (exposed surface) of the surface of the ceramic plate which is not covered with the wafer. When the wafer is connected to the AlN ceramic plate via the conductive film, the surfaces of the wafer and the ceramic plate have the same potential, and therefore, sufficient johansen-rahbek force is not developed, and the adsorption force is reduced.

The present invention has been made to solve the above problems, and a main object thereof is to continuously maintain the appearance of johnson-rahbeck force.

Means for solving the problems

The electrostatic chuck of the present invention has a structure in which an electrostatic electrode is buried in a disk-shaped ceramic plate, and a wafer having a smaller diameter than the ceramic plate and placed on the ceramic plate is adsorbed by johnson-lazy force,

at least an annular region from a position corresponding to an outer peripheral edge of the electrostatic electrode to an inner side of an outer peripheral edge of the wafer placed on the ceramic plate in a surface of the ceramic plate has an insulating film having a higher resistance than the ceramic plate.

When a conductive film is deposited on the surface of the wafer using the electrostatic chuck, the conductive film is also deposited on the surface of the ceramic plate in a region outside the wafer. In this case, even if the conductive film deposited on the surface of the wafer is electrically connected to the conductive film deposited on the surface of the ceramic plate, at least a portion directly above the electrostatic electrode in the conductive film deposited on the surface of the ceramic plate is electrically insulated from the ceramic plate by the insulating film. Therefore, the wafer and the ceramic plate do not have the same potential at the portion directly above the electrostatic electrode on the surface. After the wafer on which the conductive film is deposited is replaced with a new wafer, the outer peripheral edge of the wafer may be placed on the conductive film deposited on the insulating film of the ceramic plate. When a voltage is applied to the electrostatic electrode, the portion directly above the electrostatic electrode on the surfaces of the wafer and the ceramic plate becomes the same potential via the conductive film unless the insulating film is present. Thus, the electrostatic chuck can continue to develop johnson-rahbeck force. Further, even if the parts of the surfaces of the wafer and the ceramic plate outside the part directly above the electrostatic electrode have the same potential through the conductive film, the part directly above the electrostatic electrode on the surfaces of the wafer and the ceramic plate generates a potential difference, and therefore johanson-rahbeck force can be continuously developed.

In the electrostatic chuck according to the present invention, the annular region may be a region from an outer peripheral edge of the ceramic plate to an inner side of an outer peripheral edge of the wafer mounted on the ceramic plate, in a surface of the ceramic plate. In this way, the insulating film is provided on the surface of the ceramic plate at a position from the outer peripheral edge of the ceramic plate to the inner side of the outer peripheral edge of the wafer mounted on the ceramic plate, and therefore the wafer and the surface of the ceramic plate do not have the same potential. Therefore, the electrostatic chuck can more reliably continue to develop johnson-rahbeck force.

The electrostatic chuck according to the present invention may be configured such that the ceramic plate has a circular convex portion on a surface thereof on which the wafer is placed, the circular convex portion is provided such that, when the wafer is placed on the circular convex portion, an outer peripheral edge of the wafer protrudes outward from the circular convex portion, and the insulating film is provided on an outer side of the circular convex portion on the surface of the ceramic plate. In this case, the surface of the circular projection (wafer mounting surface) may be higher than the surface of the insulating film. Thus, the conductive film deposited on the surface of the wafer and the conductive film deposited on the surface of the insulating film of the ceramic plate are easily maintained in an electrically independent state due to the difference in height between the wafer mounting surface and the surface of the insulating film. Therefore, the electrostatic chuck can more reliably continue to develop johnson-rahbeck force.

The electrostatic chuck of the present invention having the circular convex portion may have a circumferential groove along an outer periphery of the circular convex portion, and the circumferential groove may be provided such that an outer peripheral edge of the wafer placed on the circular convex portion is positioned in a space above the circumferential groove. In this way, the conductive film deposited on the surface of the wafer and the conductive film deposited on the surface of the insulating film of the ceramic plate are more easily maintained in an electrically independent state due to the presence of the circumferential groove. Therefore, the electrostatic chuck can more reliably continue to develop johnson-rahbeck force.

The electrostatic chuck of the present invention having the circumferential groove may be configured to have a circumferential slope on the surface of the ceramic plate outside the circumferential groove. Thus, the wafer placed on the circular convex portion can be prevented from being largely deviated or from falling off the circular convex portion.

In the electrostatic chuck according to the present invention, the insulating film may be further provided on a side surface of the ceramic plate. Thus, even if the conductive film is attached to the side surface of the ceramic plate, the wafer and the ceramic plate can be prevented from being at the same potential via the conductive film.

Drawings

Fig. 1 is a perspective view of an electrostatic chuck 10.

Fig. 2 is a sectional view a-a of fig. 1.

Fig. 3 is a cross-sectional view showing a state where a conductive film F1 is deposited on the wafer W.

Fig. 4 is a cross-sectional view showing a state where the wafer W is placed on the conductive film F2 on the insulating film 14.

Fig. 5 is a perspective view of the electrostatic chuck 20.

Fig. 6 is a sectional view B-B of fig. 5.

Fig. 7 is a cross-sectional view showing a state where a conductive film F1 is deposited on the wafer W.

Fig. 8 is a cross-sectional view showing a state where the wafer W is placed on the conductive film F2 on the insulating film 14.

Fig. 9 is a sectional view of a modification of the first embodiment.

Fig. 10 is a sectional view of a modification of the first embodiment.

Fig. 11 is a sectional view of a modification of the first embodiment.

Fig. 12 is a sectional view of a modification of the first embodiment.

Fig. 13 is a sectional view of a modification of the first embodiment.

Detailed Description

[ first embodiment ]

The electrostatic chuck 10 will be described below as a preferred embodiment of the present invention. Fig. 1 is a perspective view of the electrostatic chuck 10, and fig. 2 is a sectional view a-a of fig. 1.

The electrostatic chuck 10 is a component having an electrostatic electrode 16 built in a ceramic plate 12. The ceramic plate 12 is a circular plate made of AlN ceramic. A circular convex portion 12a having a small height concentric with the ceramic plate 12 is provided on the surface of the ceramic plate 12. The surface of the circular projection 12a serves as a wafer mounting surface 12 c. The annular region 12b is a region (region outside the circular convex portion 12 a) from the outer peripheral edge of the ceramic plate 12 to the inner side of the outer peripheral edge of the wafer W placed on the wafer placing surface 12c, of the front surface of the ceramic plate 12. The annular region 12b is lower than the wafer mounting surface 12c by one layer and is covered with an insulating film 14 having a resistance larger than that of AlN. The height of the insulating film 14 substantially matches the height of the wafer mounting surface 12 c. Examples of the material of the insulating film 14 include an alumina ceramic material and a high-resistance organic material (e.g., a resin). The wafer W has a diameter larger than the wafer mounting surface 12c and smaller than the diameter of the ceramic plate 12. The electrostatic electrode 16 is a circular flat plate having a diameter slightly smaller than that of the ceramic plate 12 and forming a concentric circle with the ceramic plate 12. The electrostatic electrode 16 is parallel to the wafer mounting surface 12 c. Examples of the material of the electrostatic electrode 16 include Mo, W, Mo-based alloys, W-based alloys, and the like. A dc voltage is applied to the electrostatic electrode 16 from the back surface of the ceramic plate 12 via a power supply rod not shown.

Next, a use example of the electrostatic chuck 10 configured as above will be described. First, the wafer W is placed on the wafer placement surface 12c of the electrostatic chuck 10 in a state where the electrostatic chuck 10 is placed in a vacuum chamber, not shown. At this time, the outer peripheral edge of the wafer W is placed on the insulating film 14 while extending outward from the wafer mounting surface 12 c. Then, the vacuum chamber is depressurized by a vacuum pump to a predetermined vacuum degree, and a dc voltage is applied to the electrostatic electrode 16 of the electrostatic chuck 10 to develop a johnson-rahbeck force (JR force) to thereby suction-fix the wafer W to the wafer mounting surface 12 c. Next, a reaction gas atmosphere of a predetermined pressure (for example, several tens to several hundreds Pa) is set in the vacuum chamber, and in this state, a high-frequency voltage is applied between an upper electrode (not shown) and the electrostatic electrode 16 of the electrostatic chuck 10, thereby generating plasma. In the present embodiment, the conductive film F1 is deposited on the surface of the wafer W by the generated plasma.

At this time, the conductive film F2 is also deposited on the surface of the ceramic plate 12 in the region outside the wafer W. Fig. 3 is a cross-sectional view showing the situation at this time. In fig. 3, even if the conductive film F1 deposited on the surface of the wafer W is electrically connected to the conductive film F2 deposited on the surface of the ceramic plate 12, the conductive film F2 is electrically insulated from the ceramic plate 12 by the insulating film 14. Therefore, the surfaces of the wafer W and the ceramic plate 12 do not have the same potential, and the electrostatic chuck 10 can continue to exhibit a sufficient JR force.

When the conductive film F1 is deposited on the front surface of the wafer W using the electrostatic chuck 10, if the conductive film F1 having a predetermined thickness is deposited on the front surface of the wafer W, the wafer W is removed from the wafer mounting surface 12c, a new wafer W is mounted on the wafer mounting surface 12c, and the conductive film F1 having a predetermined thickness is deposited on the front surface of the wafer W. When this process is repeated and the position of the wafer W is slightly shifted, the outer peripheral edge of the wafer W may be placed on the conductive film F2 on the insulating film 14. Fig. 4 is an explanatory diagram showing a situation at this time. At this time, if the portion of the insulating film 14 is AlN ceramic, the surfaces of the wafer W and the ceramic plate 12 are at the same potential via the conductive film F2, and therefore, a sufficient JR force is not obtained even when a voltage is applied to the electrostatic electrode 16. However, in the present embodiment, the insulating film 14 is present, so that the surfaces of the wafer W and the ceramic plate 12 are not at the same potential via the conductive film F2. Therefore, the electrostatic chuck 10 can continue to develop a sufficient JR force when a voltage is applied to the electrostatic electrode 16.

According to the electrostatic chuck 10 described above, even if the process of depositing the conductive film F1 on the wafer W using the electrostatic chuck 10 is repeated, a sufficient JR force can be continuously developed. Further, since the ceramic plate 12 is made of AlN ceramic, it has a volume resistivity (e.g., 10) suitable for developing JR force⁸～10¹²About Ω · cm).

[ second embodiment ]

The electrostatic chuck 20 will be described below as a preferred embodiment of the present invention. Fig. 5 is a perspective view of the electrostatic chuck 20, and fig. 6 is a sectional view B-B of fig. 5.

The electrostatic chuck 20 is a component having an electrostatic electrode 26 built in a ceramic plate 22. The ceramic plate 22 is a circular plate made of AlN ceramic. A circular convex portion 22a having a lower height and concentric with the ceramic plate 22 is provided on the surface of the ceramic plate 22, a circumferential groove 22d is provided along the outer periphery of the circular convex portion 22a, and a circumferential slope 22e is provided outside the circumferential groove 22 d. The surface of the circular projection 22a serves as a wafer mounting surface 22 c. The bottom surface of the circumferential groove 22d is lower than the wafer mounting surface 22 c. The circumferential slope 22e is a bowl-shaped surface inclined obliquely downward from the outside toward the inside. An annular flat surface 22f is provided outside the circumferential slope 22e and at a position higher than the wafer mounting surface 22 c. The region from the outer peripheral edge of the ceramic plate 22 to the inner side of the outer peripheral edge of the wafer W placed on the wafer placing surface 22c (i.e., the annular flat surface 22f, the circumferential slope 22e, and the bottom surface of the circumferential groove 22 d) is covered with an insulating film 24 having a resistance larger than that of AlN. Examples of the material of the insulating film 24 include an alumina ceramic material and a high-resistance organic material (e.g., a resin). The height of the insulating film 24 covering the bottom surface of the circumferential groove 22d is lower than the wafer mounting surface 22 c. The diameter of the wafer W is larger than the diameter of the wafer mounting surface 22c and smaller than the diameter of the inner peripheral edge of the circumferential slope 22 e. Therefore, when the wafer W is placed on the wafer mounting surface 22c so as to be concentric with the wafer mounting surface 22c, the outer peripheral edge of the wafer W is positioned in the space above the circumferential groove 22 d. The electrostatic electrode 26 is the same as the electrostatic electrode 16 of the first embodiment, and therefore, the description thereof is omitted here.

Next, a use example of the electrostatic chuck 20 configured as above will be described. First, the wafer W is placed on the wafer placement surface 22c of the electrostatic chuck 20 in a state where the electrostatic chuck 20 is placed in a vacuum chamber, not shown. At this time, the wafer W is guided by the circumferential slope 22e and is placed so that the gap between the inner peripheral edge of the circumferential slope 22e and the wafer W falls within the range of a predetermined gap. Therefore, the wafer W is placed at a substantially determined position without being greatly displaced from a position concentric with the wafer placement surface 22 c. The outer peripheral edge of the wafer W is located in a space above the circumferential groove 22 d. Then, the vacuum chamber is depressurized by a vacuum pump to a predetermined vacuum degree, and a dc voltage is applied to the electrostatic electrode 26 of the electrostatic chuck 20 to develop a JR force, thereby attracting and fixing the wafer W to the wafer mounting surface 22 c. Next, a reaction gas atmosphere of a predetermined pressure (for example, several tens to several hundreds Pa) is set in the vacuum chamber, and in this state, a high-frequency voltage is applied between an upper electrode (not shown) and the electrostatic electrode 26 of the electrostatic chuck 20, thereby generating plasma. In the present embodiment, the conductive film F1 is deposited on the surface of the wafer W by the generated plasma.

At this time, the conductive film F2 is also deposited on the surface of the ceramic plate 22 in the region outside the wafer W. Fig. 5 is a cross-sectional view showing the situation at this time. In fig. 5, since the outer peripheral edge of the wafer W is located in the space above the circumferential groove 22d, the conductive film F1 deposited on the surface of the wafer W and the conductive film F2 deposited on the surface of the ceramic plate 12 are hardly electrically connected. Even if connected, the conductive film F2 is electrically insulated from the ceramic plate 22 by the insulating film 24. Therefore, the surfaces of the wafer W and the ceramic plate 22 do not have the same potential, and the electrostatic chuck 20 can continue to exhibit a sufficient JR force.

When the conductive film F1 is deposited on the front surface of the wafer W using the electrostatic chuck 20, if the conductive film F1 having a predetermined thickness is deposited on the front surface of the wafer W, the wafer W is removed from the wafer mounting surface 22c, a new wafer W is mounted on the wafer mounting surface 22c, and the conductive film F1 having a predetermined thickness is deposited on the front surface of the wafer W. When this process is repeated, the outer peripheral edge of the wafer W may contact the upper surface of the conductive film F2 on the insulating film 24 when the position of the wafer W is slightly displaced. Fig. 6 is an explanatory diagram showing a situation at this time. At this time, if the portion of the insulating film 24 is AlN ceramic, the surfaces of the wafer W and the ceramic plate 22 are at the same potential as the ceramic plate 22 via the conductive film F2, and therefore, a sufficient JR force is not obtained even when a voltage is applied to the electrostatic electrode 26. However, in the present embodiment, the surfaces of the wafer W and the ceramic plate 22 are not at the same potential via the conductive film F2 because of the presence of the insulating film 24. Therefore, the electrostatic chuck 20 can continue to develop a sufficient JR force when a voltage is applied to the electrostatic electrode 26.

According to the electrostatic chuck 20 described above, even if the process of depositing the conductive film F1 on the wafer W using the electrostatic chuck 20 is repeated, a sufficient JR force can be continuously developed. Further, since the ceramic plate 12 is made of AlN ceramic, it has a volume resistivity (e.g., 10) suitable for developing JR force⁸～10¹²About Ω · cm).

Further, the conductive film F1 deposited on the surface of the wafer W and the conductive film F2 deposited on the insulating film 24 of the ceramic plate 22 are more likely to be electrically independent from each other due to the presence of the circumferential groove 22 d. Therefore, the electrostatic chuck 20 can continue to develop the JR force more reliably.

Further, since the wafer W is guided by the circumferential slope 22e and placed on the wafer placement surface 22c, the wafer W placed on the wafer placement surface 22c can be prevented from being largely deviated or from falling off from the wafer placement surface 22 c.

The present invention is not limited to the above-described embodiments, and can be implemented in various forms as long as the technical scope of the present invention is achieved.

For example, in the first embodiment described above, the surface of the circular projection 12a (the wafer mounting surface 12c) is set to the same height as the surface of the insulating film 14, but the wafer mounting surface 12c may be set higher than the surface of the insulating film 14 as shown in fig. 9. In this way, the conductive film deposited on the surface of the wafer W and the conductive film deposited on the surface of the insulating film 14 are easily maintained in an electrically independent state by the difference in level between the wafer mounting surface 12c and the surface of the insulating film 14. Therefore, the JR force can be continued to be developed more reliably.

In the first embodiment described above, the insulating film 14 may be provided also on the side surface of the ceramic plate 12 as shown in fig. 10. Thus, even if the conductive film is attached to the side surface of the ceramic plate 12, the wafer W and the ceramic plate 12 can be prevented from having the same potential. This also applies to the second embodiment.

In the first embodiment described above, the ceramic plate 12 incorporates the electrostatic electrode 16, but a heating electrode or an RF electrode may be incorporated in addition to this. A voltage is applied to the heating electrode from the back surface of the ceramic plate 12 via a power supply rod not shown. The temperature of the wafer W can be adjusted by using such a heating electrode. A high-frequency voltage is applied to the RF electrode from the back surface of the ceramic plate 12 via a power supply rod not shown. This also applies to the second embodiment.

A disc-shaped cooling plate may be joined to the back surface of the ceramic plate 12 of the first embodiment. The cooling plate may also be made of aluminum or an aluminum alloy, and may also have a refrigerant passage inside. This also applies to the second embodiment.

A cylindrical ceramic shaft may be joined to the back surface of the ceramic plate 12 of the first embodiment. The ceramic shaft is preferably made of the same material as the ceramic plate 12. This also applies to the second embodiment.

In the first embodiment described above, the unipolar electrostatic electrode 16 is shown as an example, but a bipolar electrostatic electrode may be used. This also applies to the second embodiment.

In the first embodiment described above, the annular region 12b covered with the insulating film 14 is a region from the outer peripheral edge of the ceramic plate 12 to the inner side of the outer peripheral edge of the wafer W placed on the wafer placing surface 12c of the surface of the ceramic plate 12, but may be a region shown in fig. 11. That is, the annular region 12b in fig. 11 is a region from a position P corresponding to the outer peripheral edge of the electrostatic electrode 16 on the surface of the ceramic plate 12 to the inner side of the outer peripheral edge of the wafer W placed on the wafer placing surface 12 c. The annular region 12b is covered with the insulating film 14, but a region 12d outside the annular region 12b is not covered with the insulating film 14. At this time, as shown in fig. 12, the conductive film F2 is also deposited on the surface of the ceramic plate 12 in the region outside the wafer W, and even if the conductive film F2 is electrically connected to the conductive film F1 deposited on the surface of the wafer W, at least the portion of the conductive film F2 immediately above the electrostatic electrode 16 is electrically insulated from the ceramic plate 12 by the insulating film 14. Therefore, the wafer W and the portion of the surface of the ceramic plate 12 immediately above the electrostatic electrode 16 do not have the same potential. After the wafer W is replaced, the outer peripheral edge of the wafer W may be placed on the conductive film F2 as shown in fig. 13. Since the insulating film 14 is present, even when a voltage is applied to the electrostatic electrode 16 in this state, the wafer W and the portion of the surface of the ceramic plate 12 immediately above the electrostatic electrode 16 do not have the same potential. Thus, the electrostatic chuck can continue to develop johnson-rahbeck force. Even if the regions 12d of the surfaces of the wafer W and the ceramic plate 12, which are located outside the portion directly above the electrostatic electrode 16, have the same potential via the conductive film F2, a potential difference is generated between the wafer W and the portion of the surface of the ceramic plate 12, which is directly above the electrostatic electrode 16, and therefore johanson-rahbeck force can be continuously developed. This also applies to the second embodiment.

This application is based on the priority of U.S. provisional application No. 62/572697, filed on 2017, 10, 16 and incorporated by reference in its entirety into this specification.

Industrial applicability of the invention

The present invention can be used in the industry of forming a conductive film on a wafer.

Description of the symbols

10-electrostatic chuck, 12-ceramic plate, 12 a-circular convex portion, 12 b-annular region, 12 c-wafer mounting surface, 14-insulating film, 16-electrostatic electrode, 20-electrostatic chuck, 22-ceramic plate, 22 a-circular convex portion, 22 c-wafer mounting surface, 22 d-circumferential groove, 22 e-circumferential slope, 22F-annular flat surface, 24-insulating film, 26-electrostatic electrode, F1, F2-conductive film, W-wafer.