Wafer polishing method

Abstract

To provide a polishing method of wafer capable of accurately polishing an abrasive face of wafer.SOLUTION: A polishing method of wafer for polishing a circular recessed part of a wafer which has a circular recessed part and an annular protruded part of surrounding the circular recessed part on the rear surface includes a first polishing step 401 which positions a polishing pad having a diameter of radius or less of the circular recessed part onto a radius region attaining to an outer periphery from a center of the circular recessed part and rotates the wafer and the polishing pad into the same direction to each other to polish the circular recessed part, and a second polishing step 402 which positions a polishing pad having a diameter of a diameter or more of the polishing pad so that the whole surface of the polishing pad is brought into contact with a circular recessed part, rotates the wafer and the polishing pad in the direction opposite to each other and polishes the circular recessed part.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a method for polishing a wafer for polishing a wafer.

Background Art

[0002] In order to remove grinding marks after grinding and increase strength, polishing with a polishing pad or wet etching using a polishing liquid is performed (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In TAIKO (registered trademark) grinding, when polishing by pressing a polishing pad larger than the concave portion as in the method shown in Patent Document 1, pressure is applied to the annular convex portion, and it is difficult to uniformly press the polishing pad against the entire concave portion that becomes the grinding surface. Also, when using a polishing pad smaller than the concave portion, the contact time of the polishing pad with the outer peripheral region of the concave portion is shorter than that with the center, and the polishing amount is smaller than that with the center, so the thickness accuracy of the concave portion deteriorates.

[0005] Therefore, although polishing by wet etching is common, the etching liquid tends to stay around the annular convex portion, and the periphery of the annular convex portion tends to be thicker than the center, making it difficult to form a uniform thickness even by wet etching.

[0006] An object of the present invention is to provide a method for polishing a wafer that can accurately polish the grinding surface of the wafer.

Means for Solving the Problems

[0007] To solve the above-mentioned problems and achieve the objective, the present invention provides a wafer polishing method for polishing a recess in a wafer having a recess and an annular protrusion surrounding the recess on one surface, comprising: a first polishing step of polishing the recess by rotating a small polishing pad having a diameter less than or equal to the radius of the recess and the wafer in the same direction; and a second polishing step of positioning a large polishing pad having a diameter less than or equal to the diameter of the recess and greater than or equal to the diameter of the small polishing pad so that the entire surface of the large polishing pad is in contact with the recess, and polishing the recess by rotating the wafer and the large polishing pad in opposite directions. [Effects of the Invention]

[0008] This invention has the effect of enabling high-precision polishing of the wafer's grinding surface. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic perspective view showing the wafer to be polished in the wafer polishing method according to Embodiment 1. [Figure 2] Figure 2 is a schematic perspective view showing an example of the configuration of a polishing apparatus for carrying out the wafer polishing method according to Embodiment 1. [Figure 3] Figure 3 is a schematic side view showing a partial cross-section of the configuration of the small polishing unit of the polishing apparatus shown in Figure 2. [Figure 4] Figure 4 is a schematic side view showing a partial cross-section of the configuration of the large polishing unit of the polishing apparatus shown in Figure 2. [Figure 5] Figure 5 is a flowchart showing the flow of the wafer polishing method according to Embodiment 1. [Figure 6] Figure 6 is a schematic plan view showing the rotation direction of the polishing pad and holding table in the first polishing step of the wafer polishing method shown in Figure 5. [Figure 7] Figure 7 shows the amount of polishing on the bottom surface of the circular recess of the wafer in the first polishing step of the wafer polishing method shown in Figure 5. [Figure 8]Figure 8 is a schematic plan view showing the rotation direction of the polishing pad and holding table in the second polishing step of the wafer polishing method shown in Figure 5. [Figure 9] Figure 9 shows the amount of polishing on the bottom surface of the circular recess of the wafer in the second polishing step of the wafer polishing method shown in Figure 5. [Figure 10] Figure 10 is a schematic cross-sectional view showing the first polishing step of a wafer polishing method according to a modified example of Embodiment 1. [Figure 11] Figure 11 is a schematic cross-sectional view showing the second polishing step of a wafer polishing method according to a modified example of Embodiment 1. [Modes for carrying out the invention]

[0010] Embodiments for carrying out the present invention will be described in detail with reference to the drawings. The present invention is not limited to the contents described in the following embodiments. Furthermore, the components described below include those that can be easily imagined by those skilled in the art, and those that are substantially the same. In addition, the configurations described below can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the configurations can be made without departing from the spirit of the present invention.

[0011] [Embodiment 1] A wafer polishing method according to Embodiment 1 of the present invention will be described with reference to the drawings. Figure 1 is a schematic perspective view showing a wafer to be polished in the wafer polishing method according to Embodiment 1. Figure 2 is a schematic perspective view showing an example of the configuration of a polishing apparatus for carrying out the wafer polishing method according to Embodiment 1.

[0012] (Waha) The wafer polishing method according to Embodiment 1 is a polishing method for polishing the wafer 1 shown in FIG. 1. The wafer 1 shown in FIG. 1, which is the object to be polished in the wafer polishing method according to Embodiment 1, is a wafer such as a disk-shaped semiconductor wafer or an optical device wafer having a substrate 2 made of silicon, sapphire, gallium, etc. As shown in FIG. 1, the wafer 1 includes a device region 4 and an outer peripheral surplus region 5 surrounding the device region 4 on the surface 3 of the substrate 2.

[0013] The device region 4 has a division planned line 6 set in a grid pattern on the surface 3 of the substrate 2 and a device 7 formed in each region partitioned by the division planned line 6.

[0014] The device 7 is, for example, an integrated circuit such as an IC (Integrated Circuit) or an LSI (Large Scale Integration), an image sensor such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), a MEMS (Micro Electro Mechanical Systems), or a semiconductor memory (semiconductor storage device). The outer peripheral surplus region 5 surrounds the device region 4 over the entire circumference and is a region where no device 7 is formed on the surface 3 of the substrate 2.

[0015] Also, in Embodiment 1, the back surface 8 (corresponding to one surface) of the device region 4 of the wafer 1 is ground by a well-known grinding device, and as shown in FIG. 1, a circular recess 10 (corresponding to a recess) is formed in the back surface 8 of the device region 4, and the outer peripheral surplus region 5 is formed into an annular convex portion 11 (corresponding to a convex portion) thicker than the device region 4, which is a so-called TAIKO (registered trademark) wafer.

[0016] The circular recess 10 is formed in a region that overlaps with the device region 4 of the back surface 8 in the thickness direction, and the annular protrusion 11 is formed in a region that overlaps with the outer peripheral surplus region 5 of the back surface 8 in the thickness direction. In this specification, a region that overlaps with the device region 4 of the back surface 8 in the thickness direction is referred to as the device region 4 of the back surface 8, and a region that overlaps with the outer peripheral surplus region 5 of the back surface 8 in the thickness direction is referred to as the outer peripheral surplus region 5 of the back surface 8.

[0017] Thus, the back surface 8 of the wafer 1 has a circular recess 10 formed at the center and an annular protrusion 11 surrounding the circular recess 10, and a step 12 is formed between the device region 4 and the outer peripheral surplus region 5. Also, the front surface 3 of the wafer 1 is formed in the same plane across the device region 4 and the outer peripheral surplus region 5. Note that the bottom surface 13 of the circular recess 10 is a grinding surface that has been subjected to grinding or the like by a grinding device.

[0018] The polishing method of the wafer according to Embodiment 1 is performed by the polishing device 100 shown in FIG. 2. In Embodiment 1, the polishing device 100 is a processing device that polishes the bottom surface 13 of the circular recess 10 of the wafer 1 shown in FIG. 1.

[0019] (Polishing Device) Next, the polishing device 100 will be described. FIG. 3 is a side view schematically showing a partial cross-section of the configuration of the small polishing unit of the polishing device shown in FIG. 2. FIG. 4 is a side view schematically showing a partial cross-section of the configuration of the large polishing unit of the polishing device shown in FIG. 2.

[0020] As shown in FIG. 2, the polishing device 100 includes a device main body 101, a small polishing unit 102, a large polishing unit 103, a polishing feed unit 104, a turntable 105, a plurality (three in Embodiment 1) of holding tables 106 installed on the turntable 105, a cassette 107, an alignment unit 108, a loading unit 109, a cleaning unit 110, a loading / unloading unit 111, an unloading unit 114, and a control unit 200.

[0021] The turntable 105 is a disc-shaped table provided on the upper surface of the main body 101 of the apparatus, and is rotatable in the horizontal plane around an axis parallel to the Z-axis direction, and is driven to rotate at predetermined timings. Three holding tables 106 are arranged on this turntable 105 at equal intervals, for example, with a phase angle of 120 degrees. These three holding tables 106 have a holding table structure in which the holding surface is connected to a vacuum chuck with a suction source (not shown), and the surface 3 side of the wafer 1 is placed on the holding surface and is sucked in by the suction source to hold the wafer 1 on the holding surface.

[0022] During polishing, these holding tables 106 are rotated in the horizontal plane by a rotational drive mechanism around an axis parallel to the vertical direction, i.e., the Z-axis direction. The holding tables 106 are sequentially moved to the loading / unloading area 301, the small polishing area 302, the large polishing area 303, and back to the loading / unloading area 301 by the rotation of the turntable 105.

[0023] The loading / unloading area 301 is the area where wafers 1 are loaded into and unloaded from the holding table 106; the small polishing area 302 is the area where the small polishing unit 102 polishes the bottom surface 13 of the circular recess 10 of the wafer 1 held on the holding table 106; and the large polishing area 303 is the area where the large polishing unit 103 polishes the bottom surface 13 of the circular recess 10 of the wafer 1 held on the holding table 106.

[0024] The small polishing unit 102 is equipped with a disc-shaped polishing pad 120 (corresponding to the small polishing pad) that polishes the bottom surface 13 of the circular recess 10 on the back surface 8 of the wafer 1 held on the holding table 106, and is a polishing unit that polishes the bottom surface 13 of the circular recess 10 of the wafer 1 held on the holding surface of the holding table 106 in the small polishing area 302. The large polishing unit 103 is equipped with a disc-shaped polishing pad 130 (corresponding to the large polishing pad) that polishes the bottom surface 13 of the circular recess 10 of the wafer 1 held on the holding table 106, and is a polishing unit that polishes the bottom surface 13 of the circular recess 10 of the wafer 1 held on the holding surface of the holding table 106 in the large polishing area 303.

[0025] As shown in Figures 3 and 4, the polishing units 102 and 103 have polishing pads 120 and 130 attached to the lower ends of spindles 122 and 132 which are rotated around their axes by motors 121 and 131. The polishing units 102 and 103 polish the bottom surface 13 of the circular recess 10 of the wafer 1 by having the polishing pads 120 and 130 rotate around an axis parallel to the Z-axis direction, while the polishing feed unit 104 brings the polishing pads 120 and 130 closer to the holding table 106 at a predetermined feed speed.

[0026] Furthermore, the polishing units 102 and 103 are mounted on movable blocks 115, which are provided with a polishing feed unit 104 and are movable in the radial direction of the holding tables 106 in each region 302 and 303 by a moving mechanism (not shown). That is, the polishing pads 120 and 130 of the polishing units 102 and 103 are provided to be movable in the radial direction of the holding tables 106 in each region 302 and 303.

[0027] Furthermore, the polishing pad 120 of the small polishing unit 102, shown in Figure 3, has a diameter less than or equal to the radius of the circular recess 10. Also, the diameter of the polishing pad 120 is at least 5 mm smaller than the radius of the circular recess 10. In Embodiment 1, the diameter of the polishing pad 120 is the radius of the circular recess 10.

[0028] In Embodiment 1, as shown in Figure 3, the small polishing unit 102 is positioned so that the outer edge of the polishing pad 120 overlaps with the outer edge of the bottom surface 13 of the circular recess 10 and the center of the bottom surface 13 of the circular recess 10, and polishes the bottom surface 13 of the circular recess 10 of the wafer 1. If the diameter of the polishing pad 120 is smaller than the radius of the circular recess 10, the bottom surface 13 of the circular recess 10 of the wafer 1 is polished while being moved radially by the holding table 106 of the small polishing area 302 by a moving unit, spanning from a position where the outer edge of the polishing pad 120 overlaps with the outer edge of the bottom surface 13 of the circular recess 10 to a position where it overlaps with the center of the bottom surface 13 of the circular recess 10.

[0029] Furthermore, if the diameter of the polishing pad 120 exceeds the radius of the circular recess 10, the area near the center of the bottom surface 13 of the circular recess 10, which is constantly in contact with the polishing pad 120 in the first polishing step 401, will be excessively removed. This makes it difficult to compensate for the variation in the polishing distribution formed in the first polishing step 401 in the second polishing step 402, in which the polishing pad 130 is used to polish, thereby suppressing the variation in the thickness of the wafer 1 and flattening it. Therefore, it is preferable that the diameter of the polishing pad 120 be less than or equal to the radius of the circular recess 10. In addition, there is no particular lower limit for the diameter of the polishing pad 120, as it can be reduced by increasing the width over which the polishing pad 120 is moved. However, as the width over which it is moved increases, the distribution of the amount of polishing becomes more complex and difficult to compensate for with the polishing pad 130. Therefore, it is preferable that the diameter be at least 5 mm smaller than the radius of the circular recess 10.

[0030] Furthermore, the polishing pad 130 of the large polishing unit 103 has a diameter less than or equal to the diameter of the circular recess 10 and greater than or equal to the diameter of the polishing pad 120. In Embodiment 1, the diameter of the polishing pad 130 is the diameter of the circular recess 10. In Embodiment 1, as shown in Figure 4, the large polishing unit 103 is positioned so that the outer edge of the polishing pad 130 overlaps the outer edge of the bottom surface 13 of the circular recess 10 over its entire circumference, and polishes the bottom surface 13 of the circular recess 10 of the wafer 1. Also, if the diameter of the polishing pad 130 is smaller than the diameter of the circular recess 10, it is desirable that the diameter of the polishing pad 130 be at least 5 mm smaller than the diameter of the circular recess 10.

[0031] Furthermore, if the diameter of the polishing pad 130 exceeds the diameter of the circular recess 10, the polishing pad 130 will ride up onto the annular protrusion 11, making it difficult for the polishing pad 130 to contact the circular recess 10 and its outer edge, and thus making it impossible to polish the bottom surface 13 of the circular recess 10 with a uniform load. Therefore, it is preferable that the diameter of the polishing pad 130 be less than or equal to the diameter of the circular recess 10. Also, it is preferable that the diameter of the polishing pad 130 be greater than or equal to the diameter of the polishing pad 120 in order to flatten the bottom surface 13 of the circular recess 10 of the wafer 1 by offsetting the amount of polishing done by the polishing pad 120.

[0032] Furthermore, the closer the diameter of the polishing pad 120 is to the radius of the circular recess 10, and the closer the diameter of the polishing pad 130 is to the diameter of the circular recess 10, the more accurately the distribution of polishing amount can be canceled out, and the flatness of the bottom surface 13 of the circular recess 10 will increase. Taking into account the positioning accuracy of the polishing pads 120, 130, etc. of the polishing device 100 and the thickness variation of the bottom surface 13 of the circular recess 10 that is generally desired, it is preferable that the diameter of the polishing pad 120 is at least 5 mm smaller than the radius of the circular recess 10, and that the diameter of the polishing pad 130 is at least 5 mm smaller than the diameter of the circular recess 10.

[0033] The polishing feed unit 104 is installed on the movable block 115 and moves the polishing units 102 and 103 in the Z-axis direction, thereby moving them further away from and closer to the holding table 106. In Embodiment 1, the movable block 115 on which the polishing feed unit 104 is installed is supported by a movable unit on an upright column 118 erected from one end of the device body 101 in the Y-axis direction parallel to the horizontal direction, so as to be movable in the radial direction of the holding table 106. The polishing feed unit 104 includes a well-known ball screw rotatably mounted around its axis, a well-known motor for rotating the ball screw around its axis, and a well-known guide rail for supporting the spindle housings of each polishing unit 102 and 103 so as to be movable in the Z-axis direction.

[0034] Cassette 107 is a storage container having multiple slots for accommodating multiple wafers 1. As shown in Figure 2, cassette 107 accommodates multiple wafers 1 before and after polishing. In Embodiment 1, a pair of cassettes 107 are provided, each mounted on a cassette mounting stand. The cassette mounting stand raises and lowers the cassette 107 in the Z-axis direction. The alignment unit 108 is a table on which wafers 1 removed from cassette 107 are temporarily placed and their center alignment is performed.

[0035] The loading unit 109 has a suction pad for picking up wafers 1. The loading unit 109 picks up and holds the wafers 1, which have been aligned by the alignment unit 108, before polishing, and loads them onto the holding table 106 located in the loading / unloading area 301. The unloading unit 114 has a suction pad for picking up wafers 1. The unloading unit 114 picks up and holds the wafers 1, which have been polished, on the holding table 106 located in the loading / unloading area 301, and unloads them from the holding table 106 and transports them to the washing unit 110. The washing unit 110 washes the wafers 1 after polishing to remove contaminants such as polishing debris.

[0036] The loading / unloading unit 111 removes the wafer 1 before polishing from the cassette 107 and transports the wafer 1 to the alignment unit 108, and also removes the wafer 1 after polishing from the washing unit 110 and transports it to the cassette 107. The loading / unloading unit 111 is, for example, a robot pick equipped with a U-shaped hand 119, which uses the U-shaped hand 119 to hold and transport the wafer 1 by suction.

[0037] The control unit 200 controls each of the aforementioned component units that make up the polishing apparatus 100. In other words, the control unit 200 causes the polishing apparatus 100 to perform a polishing operation on the wafer 1. The control unit 200 is a computer having a processing unit with a microprocessor such as a CPU (central processing unit), a storage device with memory such as ROM (read-only memory) or RAM (random access memory), and an input / output interface device.

[0038] The arithmetic processing unit of the control unit 200 performs calculations according to a computer program stored in the memory device and outputs control signals for controlling the polishing device 100 to the aforementioned components of the polishing device 100 via the input / output interface device. The control unit 200 is also connected to a display unit consisting of a liquid crystal display device that displays the status of the processing operation and images, an input unit used by the operator to register processing content information, and a notification unit that notifies the operator. The input unit consists of at least one of a touch panel provided on the display unit and a keyboard. The notification unit notifies the operator by emitting at least one of sound, light, and a message on the touch panel.

[0039] (Wafer polishing method) Next, a wafer polishing method according to Embodiment 1 will be described. Figure 5 is a flowchart showing the flow of the wafer polishing method according to Embodiment 1. The wafer polishing method according to Embodiment 1 is also a polishing operation of the polishing apparatus 100 having the configuration described above, in which the bottom surface 13 of the circular recess 10 of the wafer 1 is polished. As shown in Figure 5, the wafer polishing method according to Embodiment 1 comprises a first polishing step 401 and a second polishing step 402.

[0040] In the polishing apparatus 100 with the configuration described above, the operator places a cassette 107 containing the wafer 1 with the circular recess 10 facing upwards on the cassette mounting platform of the apparatus body 101, the polishing conditions are registered in the control unit 190, and when the control unit 190 receives an instruction from the operator to start the polishing operation, the polishing operation, i.e., the first polishing step 401, is started.

[0041] (First polishing step) Figure 6 is a schematic plan view showing the rotation direction of the polishing pad and holding table in the first polishing step of the wafer polishing method shown in Figure 5. Figure 7 is a diagram showing the amount of polishing of the bottom surface of the circular recess of the wafer in the first polishing step of the wafer polishing method shown in Figure 5. The first polishing step 401 is a step in which a polishing pad 120 having a diameter less than or equal to the radius of the circular recess 10 is positioned in the radial region from the center to the outer circumference of the circular recess 10, and the wafer 1 and the polishing pad 120 are rotated in the same direction relative to each other to polish the bottom surface 13 of the circular recess 10.

[0042] In the first polishing step 401, the polishing apparatus 100 rotates the spindles 122 and 132 of each polishing unit 102 and 103 around their axes at a rotational speed determined by the processing conditions, for example, in the direction of arrows 123 and 133 (shown in Figures 3 and 4, and in Embodiment 1, the same direction), causing the loading / unloading unit 111 to take out one wafer 1 from any of the cassettes 107 and load it to the alignment unit 108. The polishing apparatus 100 causes the alignment unit 108 to perform center alignment of the wafer 1, and loads the aligned wafer 1 onto the holding table 106 located in the loading / unloading area 301 to the loading / unloading unit 109.

[0043] In the first polishing step 401, the polishing apparatus 100 suction-holds the wafer 1 to the holding table 106 in the loading / unloading area 301, and rotates the turntable 105 to move the holding table 106, which is holding the wafer 1 in the loading / unloading area 301, to the small polishing area 302.

[0044] In the first polishing step 401, the polishing apparatus 100 rotates the holding table 106 of the small polishing area 302 around its axis in the direction of the arrow 116 in Figure 6, which is in the same direction as the polishing pad 120 of the small polishing unit 102. The outer edge of the polishing pad 120 is positioned in the radial region from the center of the circular recess 10 to the outer circumference, where it coincides with the outer edge of the bottom surface 13 of the circular recess 10 and the center of the bottom surface 13 of the circular recess 10. The polishing feed unit 104 lowers the polishing pad 120 of the small polishing unit 102, and the bottom surface 13 of the circular recess 10 of the wafer 1 is polished with the polishing pad 120, as shown in Figures 3 and 6.

[0045] In the first polishing step 401, as shown in Figure 6, the outer edge of the polishing pad 120 is positioned so that it overlaps with the outer edge of the bottom surface 13 of the circular recess 10 and the center of the bottom surface 13 of the circular recess 10. As a result, the contact time between the bottom surface 13 and the polishing pad 120 increases as it moves towards the center of the circular recess 10. Consequently, in the first polishing step 401, as shown in Figure 7, the amount of polishing is greatest at the center of the bottom surface 13 of the circular recess 10, and the amount of polishing gradually decreases as it moves towards the outer edge of the bottom surface 13 of the circular recess 10.

[0046] The amount of polishing refers to the thickness of the circular recess 10 that is thinned by the polishing process. The horizontal axis in Figure 7 shows each position on a straight line passing through the center of the circular recess 10, with both ends representing the outer edges of the bottom surface 13 of the circular recess 10 and the center representing the center of the bottom surface 13 of the circular recess 10. The vertical axis in Figure 7 shows the amount of polishing.

[0047] In the first polishing step 401, the polishing apparatus 100 polishes the bottom surface 13 of the circular recess 10 with the polishing pad 120 for a predetermined time. Then, the polishing feed unit 104 raises the polishing pad 120 and rotates the turntable 105 to move the holding table 106 holding the wafer 1 after the first polishing step 401 to the large polishing area 303, and proceeds to the second polishing step 402. In the first polishing step 401, if the diameter of the polishing pad 120 is smaller than the radius of the circular recess 10, the polishing pad 120 is moved radially across the holding table 106 in the small polishing area 302 by the moving unit, from a position where the outer edge of the polishing pad 120 overlaps with the outer edge of the bottom surface 13 of the circular recess 10 to a position where it overlaps with the center of the bottom surface 13 of the circular recess 10, while polishing the bottom surface 13 of the circular recess 10 of the wafer 1.

[0048] (Second polishing step) Figure 8 is a schematic plan view showing the rotation direction of the polishing pad and holding table in the second polishing step of the wafer polishing method shown in Figure 5. Figure 9 is a diagram showing the amount of polishing of the bottom surface of the circular recess of the wafer in the second polishing step of the wafer polishing method shown in Figure 5. The second polishing step 402 is a step in which a polishing pad 130, which has a diameter less than or equal to the diameter of the circular recess 10 and greater than or equal to the diameter of the polishing pad 120, is positioned so that the entire surface of the polishing pad 130 is in contact with the circular recess 10, and the wafer 1 and the polishing pad 130 are rotated in opposite directions to polish the bottom surface 13 of the circular recess 10.

[0049] In the second polishing step 402, the polishing apparatus 100 rotates the holding table 106 of the large polishing area 303 around its axis in the direction of arrow 117, which is opposite to the direction of rotation of the polishing pad 130 of the large polishing unit 103 (arrow 133). The outer edge of the polishing pad 130 is positioned to overlap with the outer edge of the bottom surface 13 of the circular recess 10. The polishing feed unit 104 then lowers the polishing pad 130 of the large polishing unit 103, bringing the entire surface of the polishing pad 130 into contact with the bottom surface 13 of the circular recess 10, as shown in Figures 4 and 8, and polishing the bottom surface 13 of the circular recess 10 of the wafer 1 with the polishing pad 130.

[0050] In the second polishing step 402, as shown in Figure 8, the polishing pad 130 is positioned so that its entire surface is in contact with the circular recess 10, with the outer edge of the pad overlapping the outer edge of the bottom surface 13 of the circular recess 10. The polishing pad 130 and the holding table 106, i.e., the wafer 1, are rotated around their axes in opposite directions to polish the bottom surface 13 of the circular recess 10. As a result, the relative speed between the polishing pad 130 and the bottom surface 13 of the circular recess 10 increases as you move towards the outer edge of the circular recess 10. Consequently, in the second polishing step 402, as shown in Figure 9, the amount of polishing is least at the center of the bottom surface 13 of the circular recess 10, and gradually increases as you move towards the outer edge of the bottom surface 13 of the circular recess 10.

[0051] In Figure 9, the horizontal axis represents each position on a straight line passing through the center of the circular recess 10, with both ends representing the outer edges of the bottom surface 13 of the circular recess 10 and the center representing the center of the bottom surface 13 of the circular recess 10. The vertical axis in Figure 9 represents the amount of polishing.

[0052] In the second polishing step 402, the polishing apparatus 100 polishes the bottom surface 13 of the circular recess 10 with the polishing pad 130 for a predetermined time. Then, the polishing feed unit 104 raises the polishing pad 130 and rotates the turntable 105 to move the holding table 106, which holds the wafer 1 after the second polishing step 402, to the loading / unloading area 301.

[0053] In the second polishing step 402, the polishing apparatus 100 transports the wafer 1 after the second polishing step 402 from the holding table 106 in the loading / unloading area 301 to the cleaning unit 110, where it is cleaned and then placed in the cassette 107. During the polishing operation, each time the turntable 105 rotates, the polishing apparatus 100 transports the wafer 1 from the holding table 106 in the loading / unloading area 301, which is holding the wafer 1 after the second polishing step 402, to the cleaning unit 110, and loads the wafer 1 before polishing into the holding table 106 in the loading / unloading area 301, which is not holding any wafers. The polishing apparatus 100 completes the polishing operation after performing the first polishing step 401 and the second polishing step 402 on all the wafers 1 in the cassette 107.

[0054] Furthermore, after the first polishing step 401 and the second polishing step 402 have been performed, a metal film is formed on the bottom surface 13 of the circular recess 10 on the back surface 8 by plating or the like. After the metal film is formed on the bottom surface 13 of the circular recess 10, the wafer 1 is divided into individual devices 7 along the division line 6 and cut along the outer edge of the circular recess 10, separating the device region 4 from the outer peripheral excess region 5, that is, separating the circular recess 10 from the annular protrusion 11.

[0055] As described above, in the wafer polishing method according to Embodiment 1, the amount of polishing is gradually increased towards the center of the bottom surface 13 of the circular recess 10 in the first polishing step 401, and the amount of polishing is gradually increased towards the outer edge of the bottom surface 13 of the circular recess 10 in the second polishing step 402. Therefore, the thickness of the circular recess 10 of the wafer 1 after the second polishing step 402 can be maintained uniformly.

[0056] As a result, the wafer polishing method according to Embodiment 1 has the effect of being able to accurately polish the bottom surface 13 of the circular recess 10, which is the ground surface of the so-called TAIKO wafer 1.

[0057] It should be noted that the present invention is not limited to the embodiments described above. That is, it can be implemented with various modifications without departing from the core of the present invention. For example, the present invention may be implemented with a polishing device 100 equipped with only one polishing unit 126, as shown in Figures 10 and 11. Note that in Figures 10 and 11, the same reference numerals are used for the same parts as in Embodiment 1, and their descriptions are omitted.

[0058] The polishing apparatus 100 shown in Figures 10 and 11 has a polishing pad 120, which corresponds to a small polishing pad, attached to the tip of the spindle 122 of the polishing unit 126, and an outer peripheral polishing pad 125 that moves up and down in the Z-axis direction by a cylinder unit 124 attached to the spindle 122. When the outer peripheral polishing pad 125 is raised by the cylinder unit 124, as shown in Figure 10, its lower surface is positioned above the lower surface of the polishing pad 120. When the outer peripheral polishing pad 125 is lowered by the cylinder unit 124, as shown in Figure 11, its lower surfaces are positioned on the same plane as the polishing pad 120, and together with the polishing pad 120, it forms a polishing pad 130, which corresponds to a large polishing pad.

[0059] In the first polishing step 401, the polishing device 100 raises the outer peripheral polishing pad 125 using the cylinder unit 124 and polishes the bottom surface 13 of the circular recess 10 using only the polishing pad 120, as shown in Figure 10, similar to Embodiment 1. In the second polishing step 402, the polishing device 100 lowers the outer peripheral polishing pad 125 using the cylinder unit 124 and polishes the bottom surface 13 of the circular recess 10 using the polishing pads 120, 125, i.e., the polishing pad 130 which corresponds to the polishing pad, as shown in Figure 11, similar to Embodiment 1.

[0060] Figure 10 is a schematic cross-sectional view showing the first polishing step of the wafer polishing method according to a modified example of Embodiment 1. Figure 11 is a schematic cross-sectional view showing the second polishing step of the wafer polishing method according to a modified example of Embodiment 1.

[0061] Furthermore, the present invention may be implemented using a polishing apparatus having only the small polishing unit 102 and a polishing apparatus having only the large polishing unit 103 among the polishing units 102 and 103. While the present invention is also effective for ordinary wafers that do not have annular protrusions 11 and circular recesses 10, ordinary wafers are designed to process only the radial region of a holding table formed in a conical shape. Therefore, the polishing distribution can be adjusted by changing the inclination of the holding surface, which constitutes the processing area. On the other hand, TAIKO wafers are often designed to be held on a flat holding table 106. Therefore, it is difficult to adjust minute polishing distributions by changing the inclination of the holding surface, making the present invention particularly effective. [Explanation of Symbols]

[0062] 1 wafer 8. Reverse side (one side) 10. Circular recess (recess) 11 Annular protrusion 120 Polishing Pad (Small Polishing Pad) 130 Polishing Pad (Large Polishing Pad) 401 First polishing step 402 Second polishing step

Claims

[Claim 1] A method for polishing a wafer having a recess and an annular protrusion surrounding the recess on one surface, wherein the wafer is polished. A first polishing step involves polishing the recess by rotating a small polishing pad having a diameter less than or equal to the radius of the recess and the wafer in the same direction relative to each other. A second polishing step involves positioning a large polishing pad, whose diameter is less than or equal to the diameter of the recess and greater than or equal to the diameter of the small polishing pad, so that the entire surface of the large polishing pad is in contact with the recess, and then rotating the wafer and the large polishing pad in opposite directions to polish the recess. A method for polishing wafers, characterized by comprising the following:

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