Wafer surface machining method and apparatus

Abstract: A surface machining apparatus (10) comprises a suction transfer device that is capable of transferring a ground thin wafer (26) held thereon without damaging it. After the wafer (26) is ground at two different grinding stages (18, 20), the suction transfer device transfers the wafer (26) from a chuck table (48, 52, 54) to a cleaning stage (22). A holding surface (68A) of a suction disc (68) in the suction transfer device is composed of a porous member (73) having substantially the same diameter as the wafer (26), so that the entire surface of the wafer (26) can be held on the holding surface (68A) of the suction disc (68).

Description: The present invention relates generally to a wafer surface machining method and a wafer surface machining apparatus. The preambles of claim 1 and 2 reflect the state of art known from JP 09193006. A surface machining apparatus comprises a housing stage for containing the wafers, a grinding stage for grinding the wafers, and a suction transfer device for transferring the wafers between the housing stage and the grinding stage. The conventional suction transfer device has a plurality of suction pads on a holding surface of a suction disc, and holds the wafers on the suction pads by suction. Recently, there has been a desire for thin semiconductor wafers as ICs have become increasingly integrated. For this reason, the surface machining apparatus grinds the wafer thin. The surface machining apparatus is provided with a chuck table, a grindstone, and a reverse surface cleaning device. The obverse surface of the semiconductor wafer is held on the chuck table and the grindstone is pressed against the reverse surface of the semiconductor wafer. The chuck table and the grindstone are rotated to grind the reverse surface of the wafer. On completion of the grinding, the wafer is removed from the chuck table, and then the reverse surface cleaning device cleans the reverse surface of the wafer. Grinding the wafer thin decreases the strength of the wafer. In the case of the conventional suction disc that holds the wafer thereon with a plurality of suction pads, the peripheral part of the wafer easily chips or breaks during the transfer. In view of the foregoing, it is an object of the present invention to provide a wafer surface machining method and a wafer surface machining apparatus that has a suction transfer device capable of transferring the thin wafers without damaging them, and that prevents the wafers from being damaged at the protection film separation stage. According to the present invention there is provided a wafer surface machining method as set out in claim 1 and a wafer surface machining apparatus as set out in claim 2. Further preferred embodiements are subject matters of the dependent claims 3 to 6 BRIEF DESCRIPTION OF THE DRAWINGS The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: Fig. 1 is a perspective view illustrating the entire structure of a surface machining apparatus for semiconductor wafers according to preferred embodiments of the present invention; Fig. 2 is a plan view of the surface machining apparatus in Fig. 1; Fig. 3 is a sectional view of assistance in explaining a suction disc according to the first embodiment; Fig. 4 is a bottom view of the suction disc in Fig. 3; and Fig. 5 is a sectional view of assistance in explaining a suction disc according to the second embodiment; This invention will be described in further detail by way of example with reference to the accompanying drawings. Fig. 1 is a perspective view showing a surface machining apparatus for grinding semiconductor wafers according to an embodiment of the present invention, and Fig. 2 is a plan view thereof. As shown in Fig. 1, a cassette housing stage 14, an alignment stage 16, a rough grinding stage 18, a finish grinding stage 20 and a cleaning stage 22 are arranged on a body 12 of the surface machining apparatus 10. In the following description a transfer robot (28) is not part of the invention. Two cassettes 24 are detachably mounted in the cassette housing stage 14. A number of wafers 26 are stored in the cassettes 24 before their reverse surfaces are ground. A transfer robot 28 captures the wafers 26 one by one and sequentially transfers them to the alignment stage 16. The transfer robot 28 is supported on a beam 30, which stands on the body 12, through a lifting unit 32. The lifting unit 32 connects to a feed screw (not shown), which is built in the beam 30. When the feed screw feeds the lifting unit 32, the transfer robot 28 reciprocates in directions indicted by arrows A and B in Figs. 1 and 2 along the beam 30. The transfer robot 28 sequentially transfers the wafers 26 on a predetermined route within the surface machining apparatus 10. Since the transfer robot 28 is supported on the beam 30, there is only a short distance between the cassette 14 and the alignment stage 16. This reduces the size of the surface machining apparatus 10. If the transfer robot 28 is placed at the top of the body 12, the transfer robot 28 must be disposed in a space between the cassette housing stage 14 and the alignment stage 16 to transfer the wafers 26. This increases the size of the surface machining apparatus 10. This embodiment alleviates such a problem by mounting and moving the transfer robot 28 in the space above the body 12, and this reduces the size of the surface machining apparatus 10. The transfer robot 28 is a general industrial robot, which comprises a U-shaped arm 34 and three links 36, 38 & 40. Suction pads 35 for holding the wafer 26 are attached to both ends of the arm 34. The transfer robot 28 generates sucking force at the suction pads 35 by drawing air through the suction pads 35. The arm 34 is rotatably supported by the link 36, and a motor (not shown) rotates the arm 34 about the axis of the link 36. The link 36 rotatably connects to the link 38 through a shaft 42, and a motor (not shown) rotates the link 36 about the shaft 42. The link 38 rotatably connects to the link 40 through a shaft 44, and a motor (not shown) rotates the link 38 about the shaft 44. The link 40 also connects to an output shaft of a motor (not shown) through a shaft 46. Running the motor rotates the link 40 about the shaft 46. The motor connects to a lifting rod (not shown) of the lifting unit 32. By controlling the arm 34 and the three links 36, 38 & 40 with the motors and controlling the contraction of the lifting rod of the lifting unit 32, the robot 28 can hold the wafer on the suction pads 35 and take out the wafer 26 from the cassette 24. The robot 28 transfers the wafer 26 to the alignment stage 16. At the alignment stage 16, the wafer 26 transferred from the cassette 24 is aligned to a predetermined position. The aligned wafer 26 is held on the suction pads 35 of the transfer robot 28. Then, the wafer 26 is transferred to a chuck table 48, and it is held at a preset position on the chuck table 48. The chuck table 48 is placed on a turn table 50, and chuck tables 52 & 54 having the same function as the chuck table 48 are placed at a predetermined interval on the turn table 50. The chuck table 52 is located at the rough grinding stage 18, and the wafer 26 held on the chuck table 52 is roughly ground. The chuck table 54 is located at the finish grinding stage 20, and the wafer 26 held on the chuck table 54 is finish-ground (undergoes precision grinding and spark-out grinding). Although not shown in Figs. 1 and 2, the bottoms of the chuck tables 48, 52 & 54 connect to spindles of motors. The motors rotate the chuck tables 48, 52 & 54. A gauge (not shown) measures the thickness of the wafer 26 held on the chuck table 48. The measured wafer 26 is shifted to the rough grinding stage 18 due to the turn of the turn table 50 in a direction of an arrow C in Figs. 1 and 2. At the rough grinding stage 18, a cup-shaped grindstone 56 roughly grinds the reverse surface of the wafer 26. As shown in Fig. 1, the cup-shaped grindstone 56 connects to an output shaft (not shown) of a motor 58, and is attached to a grindstone feeding unit 62 through a support casing 60 of the motor 58. The grindstone feeding unit 62 moves the cup-shaped grindstone 56 with the motor 58 up and down, and the downward movement presses the cup-shaped grindstone 56 against the reverse surface of the wafer 26. Consequently, the reverse surface of the wafer 26 is roughly ground. The downward moving amount of the cup-shaped grindstone 56, in other words, the thickness of the material removed by the cup-shaped grindstone 56 is determined according to a previously-registered reference position of the cup-shaped grindstone 56 and the thickness of the wafer 26. After the cup-shaped grindstone 56 moves away from the wafer 26, the gauge (not shown) measures the thickness of the wafer 26, whose reverse surface has been ground roughly at the rough grinding stage 18. The measured wafer 26 is shifted to the finish grinding stage 20 due to the turn of the turn table 50 in the direction of the arrow C. At the finish grinding stage 20, a cup-shaped grindstone 64 in Fig. 2 performs the precision grinding and the spark-out grinding for the wafer 26. The structure of the finish grinding stage 20 will not be described here since it has substantially the same structure as the rough grinding stage 18. On completion of the finish grinding at the finish grinding stage 20, the cup-shaped grindstone 64 moves away from the wafer 26, and the wafer 26 is transferred to the position of the chuck table 48 in Fig. 1 due to the turn of the turn table 50 in the direction of the arrow C. The wafer 26 is held on a suction disc 68 provided at the end of a moving part or a transfer arm 66. The suction disc 68 has a disc-shaped porous suction surface 68A, which has substantially the same diameter as the wafer 26. Then, the wafer 26 is transferred to the cleaning stage 22 due to the turn of the transfer arm 66 in the direction of an arrow D in Fig. 1. Fig. 3 is a sectional view showing the first embodiment of the suction disc 68 provided at the end of the transfer arm 66, and Fig. 4 is a bottom view showing the suction disc 68. As shown in Figs. 3 and 4, the suction disc 68 is constructed in such a way that a disc-shaped porous body 73 of substantially the same diameter as the wafer 26 is fitted in a reverse cup-shaped frame 71, in which a suction pipe 69 is formed. The suction pipe 69 connects to a suction device (not shown) through a suction route formed in the transfer arm 66. The operation of the suction device causes the suction surface 68A of the suction disc 68, which is formed by the porous body 73, to generate sucking force. Consequently, the entire surface of the wafer 26 can be held on the suction surface 68A. The porous body 73, for example, is made of porous ceramic, sintered metal or porous resin. The porous body 73 is not necessarily fitted in the frame 71, but the suction surface 68A of the suction disc 68 must be formed by the porous body 73. As stated above, the thin wafer 26 after the rough grinding and the finish grinding is held on the suction disc 68 with the suction surface 68A, which is formed by the disc-shaped porous body 73 of substantially the same diameter as the wafer 26. Consequently, the entire surface of the wafer 26 can be held on the suction surface 68A as if the entire surface of the wafer 26 were united with the entire suction surface 68A of the suction disc 68. Thus, the suction surface 68A serves as a stiffening plate for the wafer 26. This prevents the peripheral edge of the wafer 26 from chipping or breaking when the thin wafer 26 is held or transported. The wafer 26 is cleaned at the cleaning stage 22, and it is stored on a predetermined shelf of the predetermined cassette 24. This completes the wafer processing procedure of the surface machining apparatus 10 according to the present invention. In this embodiment, only the transfer arm 66 for transferring the ground wafer 26 is provided with the suction disc 68, which has the suction surface 68A formed by the porous body 73 of substantially the same diameter as the wafer 26, but it is preferable to also provide the transfer robot 28 with the same suction disc as the suction disc 68. Fig. 5 is a sectional view showing a suction disc 80 according to the second embodiment. Parts similar to those of the suction disc 68 according to the first embodiment described with reference to Fig. 3 are denoted by the same reference numerals, and they will not be described. In Fig. 5, the suction disc 80 connects to the transfer arm 66 through a joint or an air cylinder 82. The air cylinder 82 comprises a cylinder case 84, a rod 86 and a suction pump 90. The cylinder case 84 is fixed to the transfer arm 66, and the rod 86 is supported by the cylinder case 84 so that the rod 86 can move vertically. The suction disc 80 is attached to the rod 86 across a ball 92. The ball 92 is rotatably supported between the rod 86 and a supporting member 96, which is fixed to the bottom of the rod 86 through a screw 94 and is made of hard rubber. Thus, the suction disc 80, which is fixed to the ball 92 through a screw 98, is capable of tilting with respect to the transfer arm 66 in conformity with the inclination of the surface of the wafer 26 held on the chuck table 48. This enables the entire surface of the wafer 26 to be uniformly held on the suction disc 80. A sleeve 104 is inserted in the cylinder case 84, and a spring 102 is arranged between the sleeve 104 and a top flange 87 of the rod 86. The spring 102 presses the rod 86 upwardly. Running the suction pump 90 draws air from a ball housing chamber 106 enclosed by the rod 86 and the supporting member 96. The negative pressure resulting from the drawing deforms the supporting member 96 elastically in such a direction as to reduce the space in the ball housing chamber 106. This causes the ball 92 to be fixed between the rod 86 and the supporting member 96, so that the ball 92 can not rotate. Opening a vacuum breaker 107 returns the pressure in the ball housing chamber 106 to the atmospheric pressure. Therefore, the support member 96 returns to the original form, the ball 92 becomes capable of rotating, and the suction disc 80 becomes capable of tilting again. The suction disc 80 connects to a suction pump 110 through a vacuum breaker 108. Closing the vacuum breaker 108 and running the suction pump 110 drives the suction disc 80 in vacuum. The suction disc 80 connects to a vacuum gauge 112, which measures the pressure of the suction disc 80. The chuck table 48 connects to a suction pump 116 through a vacuum breaker 114. Opening the vacuum breaker 114 stops the suction of the chuck table 48 for the wafer 26. The chuck tables 52 & 54 are constructed in the same manner as the chuck table 48. The suction disc 80 of the second embodiment has been developed to eliminate the disadvantages as described below. In a conventional method for removing the wafer from the chuck table and holding the wafer on the suction disc, after the suction of the chuck table is stopped, a liquid, gas or bubble is supplied from the suction surface of the chuck table to blow up the wafer. Then, the lifted wafer is held on the suction pad. In the conventional method, however, the extremely thin wafer is damaged due to the pressure, which is generated when the wafer is blown up, or the wafer is out of position on the suction disc. Moreover, if the suction disc is fixed to the transfer arm in a state where the suction disc is incapable of tilting, the sucking force of the suction disc for the wafer is reduced. Thus, the wafer may be left on the chuck table. Furthermore, if the wafer is held on the suction disc and the transfer arm is turned without lifting the wafer from the chuck table, the obverse of the wafer facing downward is rubbed against the chuck table. This may damage the obverse, where the microchips are formed, of the wafer. If the wafer is attempted to remove from the chuck table without taking into consideration an influence of the surface tension of the liquid between the chuck table and the wafer, the wafer is sometimes left on the chuck table since the surface tension operates to draw the wafer onto the chuck table. A description will now be given of the operation of the suction disc 80 according to the second embodiment for eliminating the above-mentioned disadvantages. As shown in Fig. 5, the transfer arm 66 positions the suction disc 80 above the wafer 26 held on the chuck table 48. At this time, the vacuum breaker 107 is open, and thus, the ball 92 is rotatable and the suction disc 80 is tiltable with respect to the transfer arm 66. The distance L between the suction disc 80 and the wafer 26 is determined so that the suction disc 80 can move to the wafer 26 with its sucking force. The distance L is also determined in such a way that the influence of the surface tension of the liquid between the chuck table 48 and the wafer 26 does not reach the maximum (i.e., the liquid is kept from splitting) when the suction disc 80 removes the wafer 26 from the chuck table 48. Then, the vacuum breaker 108 is closed, and the suction pump 110 is run. This causes the suction disc 80 to move down to the wafer 26 against the force of the spring 102 with its sucking force to hold the wafer 26 thereon. At this time, the suction disc 80 can tilt freely with respect to the transfer arm 66. The suction disc 80 tilts in conformity with the surface of the wafer 26 so as to hold the entire surface of the wafer 26. If the vacuum gauge 112 detects that the pressure in the suction disc 80 has reached an optimum value, it means that the suction disc 80 holds the wafer 26 steadily. Then, the vacuum breaker 107 is closed and the suction pump 90 is run. This causes the ball 92 to be fixed between the rod 86 and the supporting member 96 so that the ball 92 can not rotate. Thus, the suction disc 80 is fixed to the transfer arm 66 in such a way that the suction disc 80 does not tilt any further. Then, the vacuum breaker 114 is opened to cancel the suction of the wafer 26 on the chuck table 48 for the wafer 26. Consequently, the force of the spring 102 moves up the rod 86, and the suction disc 80 moves up to the original position. At this time, the suction disc 80 is located in such a way that the surface tension of the liquid between the chuck table 48 does not reach the maximum and the liquid is kept from splitting. This prevents the wafer 26 from being left on the chuck table 48. Then, the transfer arm 66 is moved horizontally to transfer the wafer 26, which is held on the suction disc 80, to the cleaning stage 22. Since the wafer 26 is moved away from the chuck table 48 in advance, the obverse surface (where the microchips are formed) of the wafer 26 is not rubbed against the chuck table 48. Thus, the obverse surface of the wafer 26 will not be damaged by the chuck table 48. For the reasons stated above, the use of the suction disc 80 in the second embodiment eliminates the disadvantages in the conventional method.

Claims: A wafer surface machining method using a wafer surface machining apparatus (10) comprising: a table (48, 52, 54) for holding a first surface (26A) of a wafer (26) by suction; a machining part (18, 20) for machining a second surface (26B) of the wafer (26) held on the table (48, 52, 54) by the first surface (26A), the second surface (26B) being opposed to the first surface (26A); and a suction transfer device for transferring the wafer (26) from the table (48, 52, 54) to a predetermined stage (22), the suction transfer device holding the second surface (26B) of the wafer (26) held on the table (48, 52, 54) by the first surface (26A); characterised by the steps the suction transfer device positions a suction disc (80) over the wafer (26) held on the table (48, 52, 54) by a moving part (66); driving the suction disc (80) to suck air whereby the suction disc (80) with a supporting part (82) is moved downward to the wafer (26) on the table (48, 52. 54) by sucking force of the suction disc (80) to thereby hold the wafer (26) on the suction disc (80); that the suction of the table (48, 52, 54) is stopped and the wafer (26) is so released from the table (48, 52, 54) to thereby hold the wafer (26) only on the suction disc (80); the wafer (26) is moved upwards from the table (48, 52, 54) by a predetermined amount and horizontally transferred by the moving part (66) to the predetermined stage (22). A wafer surface machining apparatus (10) comprising: a table (48, 52, 54) for holding a first surface (26A) of a wafer (26) by suction; a machining part (18, 20) for machining a second surface (26B) of the wafer (26) held on the table (48, 52, 54) by the first surface (26A), the second surface (26B) being opposed to the first surface (26A); and a suction transfer device for transferring the wafer (26) from the table (48, 52, 54) to a predetermined stage (22), the suction transfer device holding the second surface (26B) of the wafer (26) held on the table (48, 52, 54) by the first surface (26A); characterised in that the suction transfer device comprises a suction disc (80) having a holding surface composed of a porous member (73) of substantially the same diameter as the wafer (26); a moving part (66) for reciprocating the suction disc (80) between the table (48, 52, 54) and the predetermined stage (22); and a supporting part (82) for supporting the suction disc (80) such that the suction disc (80) is moved freely by the sucking force of the suction disc (80) toward the table (48, 52, 54) with respect to the moving part (66), in such a manner that the wafer (26) held on the table (48, 52, 54) is also held by the sucking force of the suction disc (80). The wafer surface machining apparatus (10) as defined in claim 2, further characterized by a spring (102) arranged in the supporting part (82). The wafer surface machining apparatus (10) as defined in claim 3, further characterized by a suction pump (110) that sucks air to move the suction disc (80) to the wafer (26) against the force of the spring (102) with its sucking force to hold the wafer (26) thereon. The wafer surface machining apparatus (10) as defined in claim 1, wherein the supporting part (82) of the suction transfer device comprises a joint (86, 92, 96) capable of supporting the suction disc. The wafer surface machining apparatus (10) as defined in claim 5, wherein the joint (86, 92, 96) comprises a mechanism capable of fixing the suction disc (80) against the moving part (66).

Inventors: ISHIKAWA TOSHIHIKO, KATAGIRI YASUSHI, ISHIKAWA, TOSHIHIKO, KATAGIRI, YASUSHI
Applicants: TOKYO SEIMITSU CO LTD, TOKYO SEIMITSU CO.,LTD.
Application year: 1999
Main classification: B24B37
IPC codes: H01L21, B24B37, B24B41