Wafer testing process and grinding and polishing device

Abstract

A wafer inspection method for imaging a polished surface of a wafer (W) to check the polishing result of the wafer, the wafer inspection method comprising: a polishing step for polishing an upper surface of the wafer held on a chuck table (42) and ground by grinding elements (48, 49); an imaging step for imaging at least a predetermined area of ​​the polished surface of the wafer (W) along 360° around the wafer center to thereby obtain image data; and a determination step of Fourier transforming the image data to thereby extract a frequency distribution (Fd) corresponding to the saw marks (99), then inverse Fourier transforming the frequency distribution previously extracted to obtain an amplitude of each saw mark (99), and then determining defective polishing of the wafer in the case where the amplitude is greater than a predetermined limit.

Description

BACKGROUND OF THE INVENTIONTechnical field

[0001] The present invention relates to a wafer inspection method for inspecting a polished surface of a wafer for saw marks and further to a grinding and polishing apparatus for carrying out the wafer inspection method. Description of the state of the art

[0002] In a grinding and polishing apparatus, a wafer is polished after grinding to remove grinding damage remaining on the wafer and accordingly improve chip strength (see, for example, JP 2005-153090 A). The grinding and polishing apparatus described in JP 2005-153090 A includes a rotary table having a plurality of chuck tables and a series of grinding and polishing agents arranged around the rotary table. The rotary table is periodically rotated to sequentially move the wafer, each held on the chuck table, to the grinding agent and the polishing agent in that order. Thus, the grinding operation and the polishing operation can be performed continuously without removing the wafer from the respective chuck tables. The documents DE 10 2008 021 569 A1, CN 1 02 645 436 A, JP 2012 - 93 200 A and CN 1 01 383 281 A disclose further prior art. PRESENTATION OF THE INVENTION

[0003] In the grinding operation, the upper surface of the wafer, which is rotated, is ground by grinding elements of a rotating grinding wheel, so that arc-shaped saw marks are formed on the upper surface of the wafer by the grinding elements. In the grinding and polishing apparatus described in JP 2005-153090 A, the polishing operation is performed after the grinding operation to remove the saw marks. However, in the case where the saw marks formed on the wafer by grinding are deep, there is a possibility that the saw marks are not sufficiently removed from the wafer. As a result, the saw marks remain on the wafer despite the polishing operation, resulting in inferior polishing of the wafer.

[0004] It is therefore an object of the present invention to provide a wafer inspection method and a grinding and polishing apparatus which can remove the saw marks from the wafer to reduce the imperfection of polishing the wafer.

[0005] The present invention is defined by the wafer testing method according to the features of independent patent claim 1, the wafer testing method according to the features of independent patent claim 2 and the grinding and polishing device according to the features of independent patent claim 3. Dependent patent claim 4 relates to a preferred development.

[0006] According to one aspect of the present invention, there is provided a wafer inspection method for imaging a polished surface of a wafer to inspect the polishing result of the wafer, the wafer inspection method comprising: a polishing step of polishing an upper surface of the wafer held on a chuck table and ground by grinding elements; an imaging step of imaging at least a predetermined area of ​​the polished surface of the wafer around the wafer center over 360° to thereby obtain image data; and a determination step of Fourier transforming the image data to thereby extract a frequency distribution corresponding to saw marks, then inverse Fourier transforming the frequency distribution previously extracted to obtain an amplitude for each saw mark, and then determining that the polishing of the wafer is defective when the amplitude is larger than a predetermined limit.

[0007] With this configuration, the polished surface of the wafer is imaged to generate image data containing the amplitude (height difference) of each saw mark remaining on the polished surface of the wafer. The saw marks remaining on the polished surface of the wafer repeat periodically (are uniformly distributed in the circumferential direction of the wafer). The image data is Fourier transformed to extract the frequency distribution corresponding to the saw marks. Subsequently, the frequency distribution corresponding to the saw marks is inversely Fourier transformed to obtain the amplitude of each saw mark without noise. Afterward, the amplitude of each saw mark without noise is compared with the predetermined limit, so that the wafer's polishing deficiency can be accurately determined.If the amplitude of each saw mark exceeds the predetermined limit, it is determined that the wafer's polishing is inadequate. In this case, the wafer can be repolished to reduce the polishing imperfections.

[0008] According to another aspect of the present invention, there is provided a wafer inspection method for imaging a polished surface of a wafer to inspect the polishing result of the wafer, the wafer inspection method comprising: a polishing step of polishing an upper surface of the wafer held on a chuck table and ground by grinding elements; an imaging step of imaging at least a predetermined region of the polished surface of the wafer around the wafer center over 360° to thereby obtain image data; and a determination step of Fourier transforming the image data to generate a frequency distribution and then determining that the wafer has been defectively polished when the intensity at a part of the frequencies in the frequency distribution is greater than a predetermined threshold.

[0009] With this configuration, the polished surface of the wafer is imaged to generate image data including not only the amplitude (height difference) of the saw marks remaining on the polished surface of the wafer, but also the characteristics of grinding dust and defects generated during the wafer grinding process. The image data is Fourier transformed to obtain the height difference on the polished surface as a frequency distribution, which indicates the intensity (amplitude or amplitude sum) at each frequency. The intensity across the frequency distribution can be compared with the predetermined threshold, making it possible to detect not only poor polishing due to saw marks, but also poor polishing including a poor condition of the polished surface of the wafer due to grinding dust and defects at any position other than the saw marks.In the case where the amplitude at some of the frequencies is greater than the predetermined threshold and it is therefore determined that the polishing of the wafer is defective, the wafer may be polished again to thereby reduce the defective polishing.

[0010] According to another aspect of the invention, a grinding and polishing apparatus is provided, comprising: a chuck table having a holding surface for holding a lower surface of a wafer; rotating means for rotating the chuck table; grinding means having grinding elements arranged in a ring shape and configured to abut against an upper surface of the wafer held on the chuck table, thereby grinding the upper surface of the wafer; polishing means for polishing the upper surface of the wafer ground by the grinding means; and a wafer inspection mechanism for inspecting a polished surface of the wafer, which is the upper surface of the wafer polished by the polishing means; wherein the wafer inspection mechanism comprises: an imaging camera for imaging the polished surface of the wafer from the upper side thereof to obtain image data;a light source for horizontally applying light to a predetermined area of ​​the polished surface of the wafer to be imaged by the imaging camera; and a detection means for detecting a defective polish of the wafer based on the image data.

[0011] Preferably, the upper surface of the wafer is polished again by the polishing agent when the defective polishing is detected by the detecting means of the wafer inspection mechanism.

[0012] According to the present invention, the polished surface of the wafer can be accurately inspected for saw marks. In the case where the wafer's polishing is inadequate, the wafer is repolished to remove the saw marks from the polished surface and thus reduce the imperfect polish of the wafer.

[0013] The above and other objects, features and advantages of the present invention and the modes for carrying them out, as well as the invention itself, will be best understood by studying the following description and accompanying claims with reference to the accompanying drawings which show a preferred embodiment of the invention. SHORT DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a grinding and polishing apparatus according to a preferred embodiment of the present invention. Fig. 2A-2C are views showing image data on a polished surface of a wafer according to this preferred embodiment. Fig. 3A and Fig. 3B are diagrams for illustrating a detection step in a first wafer testing method according to this preferred embodiment. Fig. Figure 4 is a flowchart showing the first wafer testing method. Fig. 5A and Fig. 5B are diagrams for illustrating a detection step in a second wafer testing method according to this preferred embodiment. Fig. 6 is a flowchart showing the second wafer testing method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] A grinding and polishing apparatus according to a preferred embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. Figure 1 is a perspective view of the grinding and polishing device according to the preferred embodiment. The configuration of the grinding and polishing device shown in Fig. The grinding and polishing apparatus shown in Figure 1 is merely illustrative. The grinding and polishing apparatus according to the present invention may have any configuration capable of grinding and polishing a wafer.

[0015] Reference number 1, which in Fig. 1 generally characterizes the grinding and polishing apparatus according to this preferred embodiment. The grinding and polishing apparatus 1 is a fully automatic processing apparatus capable of fully automatically performing a series of operations including loading, rough grinding, fine grinding, polishing, cleaning, and unloading a wafer W. The wafer W is a substantially disk-shaped workpiece accommodated in a cassette C1 carried to the grinding and polishing apparatus 1. The wafer W may be any plate-like workpiece to be ground and polished. For example, the wafer W may be a semiconductor substrate made of silicon, gallium arsenide, etc., an inorganic substrate made of ceramics, glass, sapphire, etc., or a package substrate for a semiconductor product.

[0016] The grinding and polishing apparatus 1 includes a base 11. A pair of cassettes C1 and C2 are arranged at the front end of the base 11. A plurality of wafers W to be processed by the apparatus 1 are accommodated in the cassette C1, and a plurality of wafers W that have already been processed by the apparatus 1 are accommodated in the cassette C2. A cassette robot 16 is provided at the rear of the cassettes C1 and C2. The cassette robot 16 functions to take out any one of the wafers W from the cassette C1 before processing and further to place the wafer W into the cassette C2 after processing. A positioning mechanism 21 for positioning the wafer W before processing is provided on the right rear side of the cassette robot 16. A cleaning mechanism 26 for cleaning the wafer W after processing is provided on the left rear side of the cassette robot 16.A loading means 31 for loading the wafer W onto a chuck table 42 before processing is provided between the positioning mechanism 21 and the cleaning mechanism 26. An unloading means 36 for unloading the wafer W from the chuck table 42 after processing is provided adjacent to the loading mechanism 31 between the positioning mechanism 21 and the cleaning mechanism 26.

[0017] The cassette robot 16 includes a robot arm 17 configured by an articulated member and a hand portion 18 provided at the front end of the robot arm 7. The wafer W to be processed is moved from the cassette C1 to the positioning mechanism 21 by the cassette robot 16. Further, the wafer W that has already been processed is transferred from the cleaning mechanism 26 to the cassette C2 by the cassette robot 16. The positioning mechanism 21 includes an intermediate storage table 22 for temporarily storing the wafer W and a plurality of positioning pins 23 arranged around the intermediate storage table 22 so as to be movable toward and away from the center of the intermediate storage table 22.The wafer W placed on the intermediate storage table 22 is arranged in such a manner that the plurality of positioning pins 23 are moved to abut against the outer periphery of the wafer W, thereby causing the center of the wafer W to coincide with the center of the intermediate storage table 22.

[0018] The loading means 31 includes a loading arm 32 that swings horizontally above the base 11, and a loading pad 33 provided at the front end of the loading arm 32. The wafer W is fed by the loading means 31 in such a manner that the wafer W is lifted from the intermediate storage table 22 by the loading pad 33 and then moved to the chuck table 42 by swinging the loading arm 32. Similarly, the unloading means 36 includes an unloading arm 37 that swings horizontally above the base 11, and an unloading pad 38 provided at the front end of the unloading arm 37. The wafer W is carried away by the unloading means 36 in such a manner that the wafer W is lifted from the chuck table 42 by the unloading pad 38 and then moved to the cleaning mechanism 26 by swinging the unloading arm 37.

[0019] The cleaning mechanism 26 includes a spinner table 27 for holding the wafer W and various nozzles (not shown) for ejecting cleaning water and drying air toward the wafer W held on the spinner table 27. In the cleaning mechanism 26, the spinner table 27 holding the wafer W is lowered into the interior of the base 11, and cleaning water is ejected from the nozzle onto the wafer W to clean the wafer W as the spinner table 27 rotates. Then, drying air is blown against the wafer W to dry the wafer W. A turntable 41 is provided on the back of the loading means 31 and the unloading means 36. Four chuck tables 42 are provided on the turntable 41 so as to be equally spaced from each other in the circumferential direction of the turntable 41. Each chuck table 42 has an upper surface as a holding surface 43 for holding the lower surface of the wafer W.Each clamping table 42 is rotated about its center by a rotating means (not shown) provided in the base 11.

[0020] The turntable 41 is configured to be periodically rotated at 90° intervals, thereby positioning the wafer W at a loading / unloading position where the wafer W is loaded or unloaded, a rough grinding position where the wafer W faces a coarse abrasive 46, a fine grinding position where the wafer W faces a fine abrasive 51, and a polishing position where the wafer W faces a polishing agent 56. At the rough grinding position, the wafer W is roughly ground by the coarse abrasive 46 to reduce the thickness of the wafer W to a predetermined thickness. At the fine grinding position, the wafer W is finely ground by the fine abrasive 51 to reduce the thickness of the wafer W to a final thickness. At the polishing position, the wafer W is polished by the polishing agent 56. Three columns 12, 13, and 14 are provided around the turntable 41.

[0021] A moving means 61 for vertically moving the coarse grinding means 46 is provided on the column 12. The moving means 61 includes a pair of parallel guide rails 62 provided on the front surface of the column 12 so as to extend in the Z direction, and a motor-driven Z-table 63 slidably mounted on the guide rails 62. The coarse grinding means 46 is supported by a housing 64 on the front surface of the Z-table 63. A ball screw 65 is threadably engaged with a nut portion (not shown) formed on the rear surface of the Z-table 63. A drive motor 66 is connected to one end of the ball screw 65. When the ball screw 65 is driven in rotation by the drive motor 66, the grinding medium 64 is moved in the Z direction along the guide rails 62.

[0022] Similarly, a moving means 71 for vertically moving the fine grinding means 51 is provided on the column 13. The moving means 71 includes a pair of parallel guide rails 72 provided on the front surface of the column 13 so as to extend in the Z direction, and a motor-driven Z-table 73 slidably mounted on the guide rails 72. The fine grinding means 51 is supported by a housing 74 on the front surface of the Z-table 73. A ball screw 75 is threadably engaged with a nut portion (not shown) formed on the rear surface of the Z-table 73. A drive motor 76 is connected to one end of the ball screw 75. When the ball screw 75 is driven rotationally by the drive motor 76, the fine abrasive 51 is moved in the Z direction along the guide rails 72.

[0023] The coarse abrasive 46 has a cylindrical spindle and an attachment 47 attached to the lower end of this spindle. A grinding wheel 49 is attached to the lower surface of the attachment 47 of the coarse abrasive 46. The grinding wheel 49 includes a plurality of coarse grinding abrasive elements 48 arranged in a ring shape. For example, each grinding element 48 is a diamond grinding element formed by bonding diamond abrasive grains with a binder such as a metal binder and a resin binder. Similarly, the fine abrasive 51 has a cylindrical spindle and an attachment 52 attached to the lower end of this spindle. A grinding wheel 54 is attached to the lower surface of the attachment 52 of the fine abrasive 51. The grinding wheel 54 includes a plurality of fine grinding abrasive elements 53 arranged in a ring shape.Each grinding element 53 is formed by bonding abrasive grains having a grain size smaller than that of the abrasive grains constituting the grinding element 48. In the rough grinding operation, the thickness of the wafer W is reduced to a predetermined thickness. In the fine grinding operation, the thickness of the wafer W is further reduced to a final thickness.

[0024] A moving means 81 for moving the polishing agent 56 to a predetermined polishing position with respect to the wafer W is provided on the column 14. The moving means 81 includes a pair of parallel guide rails 82 provided on the front surface of the column 14 so as to extend in the Y direction, and a motor-driven Y-table 83 slidably mounted on the guide rails 82. The moving means 81 further includes a pair of parallel guide rails 84 provided on the front surface of the Y-table 83 so as to extend in the Z direction, and a motor-driven Z-table 85 slidably mounted on the guide rails 84. The polishing agent 56 is supported by a housing 86 on the front surface of the Z-table 85.

[0025] A Y-ball screw (not shown) is threadably engaged with a nut portion (not shown) formed on the rear surface of the Y-table 83. A drive motor 87 is connected to one end of the Y-ball screw. When the Y-ball screw is rotationally driven by the drive motor 87, the polishing agent 56 is moved in the Y direction along the guide rails 82. Further, a Z-ball screw (not shown) is threadably engaged with a nut portion (not shown) formed on the rear surface of the Z-table 85. A drive motor 88 is connected to one end of the Z-ball screw. When the Z-ball screw is rotationally driven by the drive motor 88, the polishing agent 56 is moved in the Z direction along the guide rails 84. The polishing means 56 comprises a cylindrical spindle and an attachment 57 attached to the lower end of this spindle.A polishing pad 58 is attached to the lower surface of the attachment 57 of the polishing agent 56. The polishing pad 58 is formed of a foamed material, fibrous material, etc. During the polishing operation, the upper surface of the wafer W is lightly polished to thereby remove grinding damage left on the wafer W during the rough grinding operation and the fine grinding operation.

[0026] The surface of the wafer W, which has been processed by rough grinding and fine grinding, has arc-shaped saw marks 99 (see Fig. 2C) as grinding damage. Although the upper surface of the wafer W is polished after performing fine grinding, there is a case where the saw marks 99 have a large height difference and cannot be completely removed. To deal with this problem, a wafer inspection mechanism 91 for inspecting the polished surface of the wafer W near the loading / unloading position is provided to detect imperfect polishing of the wafer. If the polishing of the wafer W is determined to be imperfect, the wafer W is polished again to completely remove the saw marks 99 from the polished surface of the wafer W.

[0027] The wafer inspection mechanism 91 includes a light source (not shown) for horizontally applying light to a predetermined area of ​​the polished surface of the wafer W and an imaging camera 92 for imaging the polished surface of the wafer W held on the chuck table 42 from the upper side of the wafer W. Accordingly, the light from the light source is applied horizontally to the polished surface of the wafer W so that the saw marks 99 (see Fig. 2C) are imaged three-dimensionally by the imaging camera 92, thereby forming a clear detection image of the saw marks 99 left on the wafer W. The wafer inspection mechanism 91 further includes a determination means 93 for determining the defective polishing of the wafer W from the image data obtained from the detected image. The determination processing by the determination means 93 will be described in detail below.

[0028] A control means 96 for centrally controlling all components of the grinding and polishing device 1 is provided near the base 11. Specifically, the control means 96 functions to perform various controls for a rough grinding step by the coarse grinding means 46, a fine grinding step by the fine grinding means 51, a polishing step by the polishing means 56, and a determination step (first determination step or second determination step) for defective polishing by the determination means 93. The control means 96 is configured by a processor for executing various processing, a memory, etc. The memory is configured by one or more storage media, such as a ROM (Read Only Memory) and a RAM (Random Access Memory), depending on the uses.

[0029] In this grinding and polishing apparatus 1, the wafer W to be processed is transferred from the cassette C1 to the positioning mechanism 21 and then centered by the positioning mechanism 21. The wafer W is then loaded onto the chuck table 42 set to the loading / unloading position. The turntable 41 is then rotated stepwise to sequentially move the wafer W to the rough grinding position, the fine grinding position, and the polishing position in this order. The wafer W is roughly ground at the rough grinding position, then fine ground at the fine grinding position, and then polished at the polishing position. The polished surface of the wafer W is then inspected at the loading / unloading surface by the wafer inspection mechanism 91. After that, the wafer W is cleaned by the cleaning mechanism 26. After that, the wafer W is transferred from the cleaning mechanism 26 to the cassette C2.

[0030] A wafer inspection method by the wafer inspection mechanism 91 will now be described. The wafer inspection mechanism 91 can perform a first wafer inspection method for detecting the defective polishing from the height difference of the polished surface due to saw marks 99, and further perform a second wafer inspection method for detecting the defective polishing from the height difference of the polished surface due to defects or grinding dust in addition to the saw marks 99.

[0031] First, a determination step (first determination step) in the first wafer testing method is performed by the determination means 93 with reference to Fig. 2A-3B. Fig. 2A-2C show image data on the polished surface of the wafer W according to this preferred embodiment, wherein Fig. 3A and Fig. 3B are diagrams for illustrating the detection step in the first wafer testing method according to this preferred embodiment.

[0032] The image data stored in Fig. 2A are obtained from the detected image of the polished surface of the wafer W and show the height difference of the polished surface of the wafer W. In Fig. 2A, the vertical axis indicates the radius R of the wafer W, and the horizontal axis indicates the angle θ in a 360° rotation of the wafer W, with the state of the entire polished surface of the wafer W being shown by Rθ coordinates. The waveform W1, which is indicated by a solid line in Fig. 2A represents a region of the polished surface of the wafer W where the height difference is large. This waveform W1 is mainly formed by the saw marks 99 (see Fig. 2C). Accordingly, the waveform W1, which is shown in Fig. 2A, as a cyclic repetition of the saw marks 99 left on the polished surface of the wafer W.

[0033] While the height difference of the polished surface of the wafer W is represented by the waveform W1 for simplified representation, the height difference is actually represented by a difference in color brightness. More specifically, the image data contained in Fig. 2A, a light area and a dark area, wherein the boundary between the light area and the dark area is marked by the waveform W1. Consequently, the image data shown in Fig. 2A, information regarding the height difference in the area where the waveform W1 is not shown. In addition, the height difference changes depending on the position of the waveform W1. The image data is detected by the detection means 93 (see Fig. 1) depending on the brightness of each pixel, which forms the detected image of the polished surface of the wafer W.

[0034] The image data stored in Fig. 2B represent the height difference in a region obtained by cutting the wafer W in the circumferential direction at a radial distance a from the center of the wafer W (see Fig. 2C). In other words, the image data stored in Fig. 2B, by extracting the data at the radial distance a from the image data shown in Fig. 2A are shown. In Fig. 2B, the vertical axis indicates the amplitude (height difference of the polished surface of the wafer W) in the cross section taken along the line aa in Fig. 2A, where the horizontal axis represents the angle θ along a 360° rotation of the wafer W. As in Fig. As shown in Figure 2B, a waveform W2 repeats cyclically due to the saw marks 99 remaining on the polished surface of the wafer W and any noise such as grinding dust and defects.

[0035] The saw marks 99 are formed cyclically on the polished surface of the wafer W, with the cycle of the saw marks 99 being determined depending on the number of saw marks 99 in a 360° rotation of the wafer W. Accordingly, the cycle of the saw marks 99 decreases with an increase in the number of saw marks 99, so that the number of cycles of the saw marks 99 increases, resulting in a higher frequency. On the other hand, noise, such as grinding dust and defects, does not repeat cyclically on the polished surface of the wafer W. Accordingly, the waveform W2 includes both the saw marks 99 as a specific cyclic component, more precisely, a specific frequency component, and noise as another frequency component.

[0036] The image data is then Fourier transformed to obtain separate frequency components for analysis, as shown in Fig. 3A. Consequently, the waveform W2 is separated into a frequency distribution Fd corresponding to the saw marks 99 and another frequency distribution corresponding to noise such as grinding dust and defects. In Fig. In FIG. 3A, the vertical axis represents the sum of amplitudes at each frequency (this sum is referred to as the amplitude sum from now on). The height difference of the polished surface of the wafer W is large at the position where the saw marks 99 exist, so the amplitude sum is large at the frequency distribution Fd corresponding to the saw marks 99. Subsequently, the data obtained by Fourier transform is filtered to extract the frequency distribution Fd corresponding to the saw marks 99. In this case, a frequency band where the saw marks 99 appear can be predicted depending on the grinding conditions in the grinding operation, so the data in this frequency band is filtered to extract the frequency distribution Fd corresponding to the saw marks 99.

[0037] The amplitude sum of the frequency distribution Fd corresponding to the saw marks 99 is identical to the sum of the amplitudes (height differences) of the individual saw marks 99, so the amplitude sum increases with an increase in the number of saw marks 99. Consequently, even if the amplitude (height difference) of each saw mark 99 is small, the amplitude sum increases with an increase in frequency. This means that it is difficult to accurately detect polishing defects from only the amplitude sum. To address this problem, the first wafer inspection method includes the step of performing an inverse Fourier transform of the frequency distribution Fd corresponding to the saw marks 99, thereby inspecting the amplitude of each saw mark 99.

[0038] As in Fig. As shown in Figure 3B, the frequency distribution Fd corresponding to the saw marks 99 is inversely Fourier transformed to generate a waveform W3 without any noise. At this time, the amplitude sum is separated into the amplitudes of the individual saw marks 99. The amplitude of the waveform W3 indicates the height difference of each saw mark 99. The defective polishing is determined depending on whether the height difference of each saw mark 99 is greater than a predetermined limit D1 over the circumference of the wafer W or not. In the case where the height difference of each saw mark 99 is greater than the predetermined limit D1, as shown in Fig. 3B, it is determined that each saw mark 99 has not been completely removed in the polishing process, and thus the polishing is defective. In the case where the height difference of each saw mark 99 is less than or equal to the predetermined limit D1, it is determined that each saw mark 99 has been sufficiently removed in the polishing process, and therefore the polishing is not defective.

[0039] While the image data stored in Fig. 2B are subjected to a frequency analysis to determine the defective polish in this preferred embodiment, the image data shown in Fig. 2A are subjected to frequency analysis to determine the lack of polishing. Furthermore, although the vertical axis of the image data after Fourier transformation indicates the amplitude sum as the sum of amplitudes at each frequency, the vertical axis of the image data after Fourier transformation may indicate the amplitude at each frequency instead of the amplitude sum.

[0040] The flow of the first wafer testing method by the wafer testing mechanism 91 will now be described with reference to Fig. 4 described. Fig. 4 is a flowchart showing the first wafer testing method according to this preferred embodiment. In Fig. 4, the wafer has already been subjected to the polishing step, in which the upper surface of the wafer held on the chuck table was ground by the grinding elements and the ground surface of the wafer was polished.

[0041] As in Fig. 4, an imaging step for imaging the polished surface of the wafer W is performed (step S01). In the imaging step, the imaging camera 92 (see Fig. 1) swung horizontally across the wafer W in the radial direction thereof, at the same time continuously rotating the wafer W so that the entire polished surface of the wafer W is imaged spirally. Alternatively, a line sensor having a length identical to the radius of the wafer W may be used to perform the imaging step. In this case too, the wafer W is rotated when the line sensor is operated, thereby imaging the entire polished surface of the wafer W. Furthermore, although the entire polished surface of the wafer W is imaged in this preferred embodiment, a predetermined region of the polished surface of the wafer W around the center of the wafer W along 360° may be imaged.

[0042] In the imaging step, image data is generated from the detected image of the polished surface of the wafer W. The image data represents a height difference on the polished surface of the wafer W in the Rθ coordinate system, where the vertical axis represents the radius R of the wafer W and the horizontal axes represent the angle θ along a 360° rotation of the wafer W (see Fig. 2A). In this case, light is applied horizontally to the polished surface of the wafer W during imaging of the polished surface, so that the height difference on the polished surface of the wafer W appears as a difference in brightness in the detected image. The height difference on the polished surface of the wafer W is determined from the brightness of each pixel constituting the detected image, thereby generating image data. The image data includes not only the saw marks 99, but also noise due to grinding dust and defects.

[0043] The image data is then Fourier transformed (step S02). As a result, the image data is divided into a frequency distribution Fd corresponding to the saw tracks 99 and a frequency distribution corresponding to the noise (see Fig. 3A). In this case, the image data stored in Fig. 2A are subjected to a Fourier transformation, or the image data shown in Fig. 2B (which are taken along the line aa from the image data shown in Fig. 2A, are subjected to a Fourier transform. Subsequently, the data obtained by the Fourier transform are filtered to extract the frequency distribution Fd corresponding to the saw marks 99, wherein the frequency distribution Fd extracted as above is subjected to an inverse Fourier transform (step S03). As a result, the noise included in the image data is removed, and the amplitude indicating the height difference of each saw mark 99 is obtained (see Fig. 3B).

[0044] Subsequently, it is determined whether the amplitude of each saw mark 99 is greater than the predetermined limit D1 (for example, 0.1 µm) or not (step S04). If the amplitude of each saw mark 99 is less than or equal to the predetermined limit D1 (No in step S04), it is determined that the polishing is not defective, and the inspection of the wafer W is terminated. If the amplitude of each saw mark 99 is greater than the predetermined limit D1 (Yes in step S04), it is determined that the polishing is defective. In this case, it is determined whether the polishing step should be performed again on the wafer W or not (step S05). In this case, a tolerance of 1 µm is previously set for the thickness of the wafer W being polished. Accordingly, depending on whether repolishing is performed, it is determined whether the thickness of the wafer W being polished falls within this tolerance or not.

[0045] If the polishing step is not performed again on the wafer W (No in step S05), it is determined that the polishing is defective, and the inspection of the wafer W is terminated. If the polishing step is to be performed again on the wafer W (Yes in step S05), the wafer W is polished again so that the amplitude of each saw mark 99 becomes less than or equal to the predetermined limit D1 (step S06). In this way, the frequency distribution Fd corresponding to the saw marks 99 is extracted from the image data, and the defective polishing of the wafer W is determined depending on the height difference of each saw mark 99 in the first wafer inspection step.

[0046] In the first wafer inspection method according to this preferred embodiment described above, the image data is Fourier transformed to extract the frequency distribution Fd corresponding to the saw marks 99. The frequency distribution Fd corresponding to the saw marks 99 is then inversely Fourier transformed to thereby obtain the amplitude of each saw mark 99 without noise. The amplitude of each saw mark 99 without noise is then compared with the predetermined limit D1. Thus, the defective polishing of the wafer W can be accurately detected. In the case where the amplitude of each saw mark 99 is larger than the predetermined limit D1 and thus the polishing of the wafer W is determined to be defective, the wafer W is polished again to thereby reduce the defective polishing.

[0047] A detection step (second detection step) in the second wafer testing method by the detection means 93 will now be described with reference to Fig. 5A and Fig. 5B. Fig. 5A and Fig. 5B are diagrams illustrating the determination step in the second wafer inspection method according to this preferred embodiment. The determination step in the second wafer inspection method differs from the determination step in the first wafer inspection method in that the polishing defect is determined from the amplitude sum of the image data obtained by Fourier transform. In the second wafer inspection step, image data similar to that used in the first wafer inspection method is used.

[0048] As in Fig. 5A and Fig. 5B, the image data (see Fig. 2A or Fig. 2B) is Fourier transformed to obtain separate frequency components for analysis. Thus, the amplitude sum can be obtained as the sum of amplitudes at each frequency component. However, in the case where a large amount of grinding dust or defects is present on the entire polished surface of the wafer W, there is a possibility that a large amplitude sum may be present not only at the frequency corresponding to the saw marks 99 (see Fig. 2C), but also at any other frequencies. In such a case, it is assumed that even if the height difference of the saw marks 99 remaining on the polished surface of the wafer W is not large, the condition of the polished surface of the wafer W is abnormal due to grinding dust or defects.

[0049] In the second wafer inspection method, the image data is Fourier transformed to generate a frequency distribution, and the polishing defect is determined based on whether the amplitude sum (intensity) of at least a portion of the frequency distribution is greater than a predetermined amplitude sum (threshold) D2. The predetermined amplitude sum D2 is set as a determination criterion for the amplitude sum of the saw marks 99 and also as a determination criterion for the amplitude sum of the grinding dust and defects. Accordingly, not only the height difference on the polished surface of the wafer W due to the saw marks 99 but also the height difference on the polished surface of the wafer W due to the grinding dust and defects is inspected.Accordingly, regardless of the position of the frequency distribution obtained by the Fourier transform of the image data, the defective polishing is detected in the case that the amplitude sum at some of the frequencies is larger than the predetermined amplitude sum D2.

[0050] For example, in the case of Fig. 5A, where the amplitude sum at a frequency F1 corresponding to the saw marks 99 is greater than the predetermined amplitude sum D2, it is determined that the height difference on the polished surface of the wafer W due to the saw marks 99 is large, so that the defective polishing is determined. In the case of Fig. 5B, where the amplitude sum at each of the frequencies F2 and F3, which are different from the frequency F1 corresponding to the saw marks 99, is greater than the predetermined amplitude sum D2, it is determined that the height difference on the polished surface of the wafer W is large due to a large amount of grinding dust or any other dust (F2) present on the polished surface of the wafer W, or due to large-sized grinding dust or deep defects (F3) present on the polished surface of the wafer W, so that the polishing is determined to be defective. In the case where the amplitude sum at each frequency in the frequency distribution is less than or identical to the predetermined amplitude sum D2, it is determined that the polishing is not defective.

[0051] The image data stored in Fig. 2A can be subjected to a frequency analysis to determine the lack of polish or the image data shown in Fig. 2B can be subjected to frequency analysis to determine the lack of polishing. Furthermore, while the vertical axis of the image data after Fourier transformation represents the amplitude sum as the sum of amplitudes of each frequency, as shown in Fig. 5A and Fig. As shown in Figure 5B, the vertical axis of the image data after Fourier transformation represents the amplitude at each frequency rather than the sum of the amplitudes.

[0052] The flow of the second wafer testing method by the wafer testing mechanism 91 will now be described with reference to Fig. 6 described. Fig. 6 is a flowchart showing the second wafer testing method according to this preferred embodiment. In Fig. 6, the wafer has already undergone the polishing step, in which the upper surface of the wafer held on the chuck table is ground by the grinding elements, and the ground surface of the wafer is polished. Furthermore, the description of similar parts compared to the first wafer inspection method is omitted in the following description.

[0053] As in Fig. 6, an imaging step is performed to image the polished surface of the wafer W as in the first wafer inspection method (step S11). In the imaging step, the imaging camera 92 (see Fig. 1) is used to image at least a predetermined region of the polished surface of the wafer W around the center of the wafer W along 360°. Furthermore, in the imaging step, image data is generated from the detected image of the polished surface of the wafer W.

[0054] The image data is then Fourier transformed (step S12). As a result, the frequency distribution of the image data is obtained, with the amplitude sum being the sum of amplitudes at each frequency component. In this case, the image data contained in Fig. 2A, or the image data shown in Fig. 2B (which are taken along the line aa from the image data shown in Fig. 2A) are subjected to a Fourier transformation.

[0055] Then, it is determined whether the amplitude sum at each frequency in the frequency distribution is greater than the predetermined amplitude sum D2 or not (step S13). If the amplitude sum at each frequency is less than or equal to the predetermined amplitude sum D2 (No in step S13), it is determined that the polishing is not defective, and the inspection of the wafer W is terminated. If the amplitude sum at some of the frequencies is greater than the predetermined amplitude sum D2 (Yes in step S13), it is determined that the polishing is defective. In this case, it is determined whether the polishing step of the wafer W should be performed again or not (step S14). In this case, a tolerance of 1 µm is set in advance for the thickness of the polished wafer W. Therefore, the re-polishing is determined depending on whether the thickness of the wafer W that has been polished falls within the tolerance or not.

[0056] If the polishing step is not performed again on the wafer W (No in step S14), it is determined that the polishing is defective, and the inspection of the wafer W is terminated. If the polishing step is to be performed again on the wafer W (Yes in step S14), the polishing step is performed again on the wafer W such that the amplitude sum of the polished surface of the wafer W becomes less than or identical to the predetermined amplitude sum D2 (step S15). In this way, the image data is subjected to frequency analysis, and the defective polishing of the wafer W is determined depending on the height difference of the polished surface of the wafer W due to grinding dust and defects in addition to the saw marks 99 in the second wafer inspection process.

[0057] In the second wafer inspection method according to this preferred embodiment described above, the polished surface of the wafer W is imaged to generate image data including not only the amplitude (height difference) of the saw marks 99 left on the polished surface of the wafer W, but also characteristics of the grinding dust and defects generated during grinding of the wafer W. The image data is Fourier transformed to obtain the height difference on the polished surface as the frequency distribution indicating the intensity (amplitude or amplitude sum) at each frequency.The amplitude sum over the frequency distribution can be compared with the predetermined amplitude sum D2, so that it is possible to detect not only the defective polishing due to saw marks 99, but also the defective polishing including a poor condition of the polished surface of the wafer W due to grinding dust and defects at any position other than the saw marks 99. In the case where the amplitude sum at a part of the frequencies is larger than the predetermined amplitude sum D2 and it is thus determined that the polishing of the wafer W is defective, the wafer W is polished again to thereby reduce the defective polishing.

[0058] The present invention is not limited to the above preferred embodiment, but various modifications may be made. The size and shape of each part shown in the accompanying drawings are not limitative, but may be appropriately changed within the scope of the effect of the present invention. Furthermore, the above preferred embodiment may be appropriately modified without departing from the scope of the object of the present invention.

[0059] For example, although the grinding and polishing apparatus 1 in the above preferred embodiment is configured to perform either the first wafer inspection process or the second wafer inspection process, the grinding and polishing apparatus 1 may selectively perform the first and second wafer inspection processes, or appropriate combinations of the first and second wafer inspection processes. In the latter case, the polishing deficiency is determined depending on whether or not the amplitude sum at each frequency of the image data after Fourier transformation is greater than the predetermined amplitude sum. Further, the frequency distribution corresponding to the amplitude sum greater than the predetermined amplitude sum is extracted by filtering, and the frequency distribution extracted as above is inversely Fourier transformed.Furthermore, the polishing deficiency is again determined based on whether the amplitude after the inverse Fourier transform is less than or equal to the predetermined limit. This method allows the polishing deficiency to be determined in two steps.

[0060] Furthermore, in the preferred embodiment described above, the wafer W is polished again when the defective polishing of the wafer W is detected. As a modification, the wafer W may be discarded without being polished again when the defective polishing is detected.

[0061] As described above, the present invention has the effect of removing saw marks on the polished surface of the wafer to reduce polishing defects. In particular, the present invention is applicable to a fully automatic grinding and polishing apparatus capable of fully automatically performing grinding and polishing operations, and is also applicable to a wafer inspection method to be performed by this grinding and polishing apparatus.

[0062] The present invention is not limited to the details of the preferred embodiment described above. The scope of the invention is defined by the accompanying claims.

Claims

[1] A wafer inspection method for imaging a polished surface of a wafer (W) to check the polishing result of the wafer, the wafer inspection method comprising: a polishing step for polishing an upper surface of the wafer held on a chuck table (42) and ground by grinding elements (48, 49); an imaging step for imaging at least a predetermined area of ​​the polished surface of the wafer (W) along 360° around the wafer center to thereby obtain image data; and a determination step of Fourier transforming the image data to thereby extract a frequency distribution (Fd) corresponding to the saw marks (99), then inverse Fourier transforming the frequency distribution previously extracted to obtain an amplitude of each saw mark (99), and then determining defective polishing of the wafer in the case where the amplitude is greater than a predetermined limit. [2] A wafer inspection method for imaging a polished surface of a wafer (W) to check the polishing result of the wafer, the wafer inspection method comprising: a polishing step for polishing an upper surface of the wafer held on a chuck table (42) and ground by grinding elements (48, 49); an imaging step of imaging at least a predetermined area of ​​the polished surface of the wafer along 360° around the wafer center to thereby obtain image data; and a determination step for Fourier transforming the image data to generate a frequency distribution, and then determining defective polishing of the wafer (W) in the case where the amplitude sum at a part of the frequencies in the frequency distribution is greater than a predetermined amplitude sum (D2), and in the event that defective polishing is detected, a step of determining whether or not the polishing step of the wafer (W) is to be carried out again depending on whether the thickness of the wafer (W) falls within a tolerance or not. [3] A grinding and polishing apparatus (1) configured to carry out the wafer testing method according to claim 1 or 2, comprising: a chuck table (42) having a holding surface (43) for holding a lower surface of a wafer (W); a rotating means for rotating the clamping table (42); a grinding means (46, 51) having grinding elements (48, 49) arranged in a ring shape and adapted to abut against an upper surface of the wafer (W) held on the chuck table (42), thereby grinding the upper surface of the wafer (W); a polishing agent (56) for polishing the upper surface of the wafer (W) ground by the abrasive; and a wafer inspection mechanism (91) for inspecting a polished surface of the wafer (W), which is the upper surface of the wafer polished by the polishing agent (56); wherein the wafer testing mechanism (91) comprises: an imaging camera (92) for imaging the polished surface of the wafer from the upper side thereof to obtain image data; a light source for horizontally applying light to a predetermined area of ​​the polished surface of the wafer to be imaged by the imaging camera; and a detecting means (93) for detecting a defective polishing of the wafer from the image data. [4] The grinding and polishing apparatus (1) according to claim 3, wherein the grinding and polishing apparatus (1) is configured to polish the upper surface of the wafer (W) again by the polishing agent (56) when the defective polishing is detected by the detecting means (93) of the wafer inspection mechanism (91).

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