Abstract:
A device grinding method comprising the steps of holding the undersurface of a protective member which supports a plurality of devices by affixing their front surfaces onto the top surface of the protective member, on the chuck table of a grinding machine and grinding the rear surfaces of the plurality of devices held on the chuck table through the protective member by a grinding means while the chuck table is rotated, to form the thicknesses of the plurality of the devices to have a predetermined value, wherein the metering portion of a non-contact thickness metering equipment is brought to a position right above the rotating rotation locus of a predetermined device out of the plurality of devices supported on the chuck table through the protective member, the rear surfaces of the plurality of devices are ground by the grinding means while the thickness of the rotating predetermined device is measured with the non-contact thickness metering equipment, and the grinding by the grinding means is terminated when the thickness of the device measured with the non-contact thickness metering equipment reaches a predetermined value.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method of grinding the rear surfaces of a plurality of devices formed on a wafer such as a semiconductor wafer until their thicknesses become a predetermined value after it is divided into the plurality of devices. 
   DESCRIPTION OF THE PRIOR ART 
   In the production process of a semiconductor device, a device such as IC or LSI is formed in a plurality of areas sectioned by streets (dividing lines) which are formed in a lattice pattern on the front surface of a substantially disk-like wafer, and individual devices are manufactured by dividing this semiconductor wafer into the areas each having a device formed thereon, along the streets. In general, the rear surface of the wafer is ground to a predetermined thickness by a grinding machine before it is divided into individual chips. 
   Meanwhile, a production method in which the devices are rated by a quality checking with a tester before the wafer in which the plurality of devices have been formed, is divided into individual devices and the obtained individual devices of the same rating are ground to a thickness required for each application purpose is carried out. It is difficult to measure the thickness of each device directly in order to grind the rear surface of each device until its thickness becomes a predetermined value after the wafer is divided into individual devices. 
   To solve the above problem, a ring-shaped measurement frame having a thickness larger than the finish thickness and substantially the same thickness as that of each device before grinding is affixed onto a protective tape and a plurality of devices are affixed to an area surrounded by the ring-shaped measurement frame in the protective tape to combine the measurement frame with the devices through the protective tape. JP-A 2001-351890 discloses a grinding method in which the above measurement frame and the devices combined together through the protective tape are held on the chuck table of a grinding machine and ground at the same time while the thickness of the ring-shaped measurement frame is measured. 
   The grinding method disclosed by the above patent document has a problem with productivity because the ring-shaped measurement frame must be prepared and is not economical because the ring-shaped measurement frame is an article of consumption. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a device grinding method capable of grinding individual devices to a predetermined thickness without using an article of consumption such as a measurement frame. 
   To attain the above object, according to the present invention, there is provided a device grinding method comprising the steps of holding the undersurface of a protective member which supports a plurality of devices by affixing their front surfaces onto the top surfaces of the protective member, on the chuck table of a grinding machine and grinding the rear surfaces of the plurality of devices held on the chuck table through the protective member by a grinding means while the chuck table is rotated, to form the thicknesses of the plurality of devices to have a predetermined value, wherein 
   the metering portion of a non-contact thickness metering equipment is brought to a position right above the rotating rotation locus of a predetermined device out of the plurality of devices held on the chuck table through the protective member, the rear surfaces of the plurality of devices are ground by the grinding means while the thickness of the rotating predetermined device is measured with the non-contact thickness metering equipment, and the grinding by the grinding means is terminated when the thickness of the device measured with the non-contact thickness metering equipment reaches a predetermined value. 
   Since the non-contact thickness metering equipment is used in the device grinding method of the present invention, the rear surfaces of the plurality of devices can be ground by the grinding means while the thicknesses of the individually divided devices are directly measured. Therefore, the ring-shaped measurement frame for measuring the thickness of each device indirectly does not need to be manufactured, thereby improving productivity and the thickness accuracy of each device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a grinding machine for carrying out the device grinding method of the present invention; 
       FIG. 2  is a constitutional block diagram of a non-contact thickness metering equipment provided in the grinding machine shown in  FIG. 1 ; 
       FIG. 3  is a perspective view showing a state of a plurality of devices being affixed onto the front surface of a protective member; 
       FIG. 4  is an explanatory diagram showing the relationship between the plurality of devices held on the chuck table of the grinding machine shown in  FIG. 1  and a grinding wheel; and 
       FIG. 5  is an enlarged sectional view of the metering portion of the non-contact thickness metering equipment provided in the grinding machine shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described in more detail with reference to the accompanying drawings. 
     FIG. 1  is a perspective view of a grinding machine  1  for carrying out the device grinding method of the present invention. The grinding machine  1  shown in  FIG. 1  comprises a housing generally denoted by  2 . This machine housing  2  has a rectangular parallelepiped main body  21  and an upright wall  22  which projects upward substantially vertically and is mounted on the rear end portion (upper right end portion in  FIG. 1 ) of the main body  21 . A pair of guide rails  221  and  221  extending in the vertical direction is installed on the front side of the upright wall  22 . A grinding unit  3  as a grinding means is mounted on the pair of guide rails  221  and  221  in such a manner that it can move in the vertical direction. 
   The grinding unit  3  comprises a movable base  31  and a spindle unit  4  attached to the movable base  31 . The movable base  31  has a pair of leg portions  311  and  311  extending in the vertical direction on both side portions of the rear surface, and to-be-guided grooves  312  and  312  to be slidably fitted to the pair of guide rails  221  and  211  are formed in the pair of leg portions  311  and  311 , respectively. A support portion  313  projecting forward is provided on the front side of the movable base  31  which is slidably mounted on the pair of guide rails  221  and  221  on the upright wall  22 . The spindle unit  4  as a grinding means is installed in the support portion  313 . 
   The spindle unit  4  as a grinding means comprises a spindle housing  41  mounted on the support portion  313 , a rotary spindle  42  rotatably mounted in the spindle housing  41 , and a servo motor  43  as a drive source for rotary-driving the rotary spindle  42 . The rotary spindle  42  rotatably supported in the spindle housing  41  has one end portion (lower end portion in  FIG. 1 ) which projects from the lower end of the spindle housing  41 , and a wheel mount  44  is attached to the one end (lower end in  FIG. 1 ) of that portion. A grinding wheel  5  is mounted on the undersurface of this wheel mount  44 . This grinding wheel  5  is constituted by an annular grindstone base  51  and a plurality of segments, each composed of a grindstone  52  mounted on the undersurface of the grindstone base  51 , and the grindstone base  51  is mounted on the wheel mount  44  by fastening screws  53 . The above servo motor  43  is controlled by a control means  9  which will be described later. 
   The illustrated grinding machine  1  has a grinding unit feed mechanism  6  for moving the above grinding unit  3  in the vertical direction (direction perpendicular to the holding surface of a chuck table which will be described later) along the pair of guide rails  221  and  221 . This grinding unit feed mechanism  6  has a male screw rod  61  which is mounted on the front side of the upright wall  22  and extends substantially vertically. The upper end portion and lower end portion of this male screw rod  61  are rotatably supported by bearing members  62  and  63  mounted on the upright wall  22 , respectively. A pulse motor  64  as a drive source for rotary-driving the male screw rod  61  is installed on the upper bearing member  62  and the output shaft of the pulse motor  64  is transmission-coupled to the male screw rod  61 . On the rear side of the movable base  31 , there is provided a coupling section (not shown) projecting backward from the center portion in its width direction. A threaded through-hole (not shown) extending in the vertical direction is formed in this coupling section, and the male screw rod  61  is screwed into this threaded through-hole. Therefore, when the pulse motor  64  is rotated in the normal direction, the movable base  31 , that is, the grinding unit  3  is moved down, namely, advanced, and when the pulse motor  64  is rotated in the adverse direction, the movable base  31 , that is, the grinding unit  3  is moved up, namely, backed up. The pulse motor  64  is controlled by the control means  9  that is described later. 
   A chuck table mechanism  7  is installed in the main body  21  of the above machine housing  2 . The chuck table mechanism  7  comprises a chuck table  71 , a cover member  72  for covering a portion around the chuck table  71 , and bellows means  73  and  74  installed in the front and the rear of the cover member  72 . The chuck table  71  is designed to be rotated by a rotary-drive means (not shown) and to suction-hold a wafer as a workpiece on the front surface of the chuck table  71  by activating a suction means that is not shown. The chuck table  71  is moved between a workpiece mounting area  70   a  shown in  FIG. 1  and a grinding area  70   b  opposed to the grinding wheel  5  constituting the above spindle unit  4  by a chuck table moving means that is not shown. The bellows means  73  and  74  may be made from a suitable material such as canvas cloth. The front end of the bellows means  73  is fixed to the front end wall of the main body  21  and the rear end of the bellows means  73  is fixed to the front end wall of the cover member  72 . The front end of the bellows means  74  is fixed to the rear end wall of the cover member  72  and the rear end of the bellows means  74  is fixed to the front side of the upright wall  22  of the machine housing  2 . When the chuck table  71  is moved in the direction indicated by the arrow  71   a , the bellows means  73  is expanded and the bellows means  74  is contracted, and when the chuck table  71  is moved in the direction indicated by the arrow  71   b , the bellows means  73  is contracted and the bellows means  74  is expanded. 
   The illustrated grinding machine  1  comprises a non-contact thickness metering equipment  8  for measuring the thickness of each device which is held on the chuck table  71  mounted on the above cover member  72 . As this non-contact thickness metering equipment  8  may be used a thickness metering equipment disclosed by JP-A 2006-38744 applied for a patent by the present applicant. 
   The non-contact thickness metering equipment  8  will be described with reference to  FIG. 2 . 
   The non-contact thickness metering equipment  8  shown in  FIG. 2  comprises a cylindrical metering case  81  installed in the above cover member  72 . This cylindrical metering case  81  has a support portion  811  which is installed upright and has an opening  811   a  at the lower end, a horizontal portion  812  which extends horizontally from the upper end of the support portion  811 , and a metering portion  813  which extends downward from the end of the horizontal portion  812  and has an opening  813   a  at the lower end. The support portion  811  is supported to the above cover member  72  in such a manner that it can turn. The support portion  811  is designed to be turned by a turn-drive means that is not shown. Therefore, the metering portion  813  of the cylindrical metering case  81  is swung with the support portion  811  as a center thereof by causing to turn the support portion  811  by means of the turn-drive means that is not shown. 
   An ultrasonic wave transmitter  82  and a reflected wave receiver  83  are installed in the metering portion  813  of the above cylindrical metering case  81 . The ultrasonic wave transmitter  82  is connected to an ultrasonic oscillation means  85  through an ultrasonic propagation means  84 . The reflected wave receiver  83  is connected to a reflected wave receiving means  87  through an ultrasonic propagation means  86 . This reflected wave receiving means  87  supplies a received signal to the control means  9 . The non-contact thickness metering equipment  8  in the illustrated embodiment comprises a fluid supply means  88  for supplying a fluid to the metering portion  813  of the cylindrical metering case  81 . This fluid supply means  88  comprises, for example, a pure water supply means  881  for supplying pure water and an electromagnetic on-off valve  883  installed in a pipe  882  for connecting the pure water supply means  881  to the opening  811   a  of the above support portion  811 . The above control means  9  controls the ultrasonic wave transmitter  82 , the electromagnetic on-off valve  883  and also the servo motor  43  of the spindle unit  4  as the grinding means and the pulse motor  64 , based on a received signal from the reflected wave receiving means  87 . 
   The illustrated grinding machine  1  is constituted as described above, and the method of grinding the plurality of devices by means of the above grinding machine  1  will be described hereinbelow. 
     FIG. 3  is a perspective view of the plurality of devices  10  affixed to the surface of a protective member  11 . The device  10  is manufactured by cutting the plurality of the devices formed on the front surface of a silicon wafer having a thickness of, for example, 700 μm and rated the same based on the results of a quality checking by a tester. The front surfaces having circuits formed thereon, of the plurality of devices  10  are affixed to the top surface of the protective member  11  (device supporting step). Therefore, the rear surfaces  101  of the plurality of devices  10  face up. 
   The undersurface of the protective member  11  having the plurality of devices  10  affixed onto its top surface is placed on the chuck table  71  positioned at the workpiece mounting area  70   a  of the grinding machine  1  as shown in  FIG. 1 . Therefore, the rear surfaces  101  of the plurality of devices  10  affixed to the top surface of the protective member  11  face up. The plurality of devices  10  placed on the chuck table  71  are suction-held on the chuck table  71  by the suction means (not shown) through the protective member  11  (device holding step). After the plurality of devices  10  are suction-held on the chuck table  71 , the control means  9  activates the chuck table moving means (not shown) to move the chuck table  71  in the direction indicated by the arrow  71   a  to bring the chuck table  71  to the grinding area  70   b  , and specifically, beneath the peripheries of the plurality of grindstones  52  of the grinding wheel  5  such that the grindstones  52  pass over the center of rotation of the chuck table  71 . Then, the control means  9  activates the turn-drive means (not shown) to turn the support portion  811  constituting the cylindrical metering case  81  of the non-contact thickness metering equipment  8  to bring the metering portion  813  to a position right above the predetermined rotation locus of a predetermined device  10   a  out of the plurality of devices  10  held on the chuck table  71  through the protective member  11  (metering position setting step). 
   After the grinding wheel  5  and the plurality of devices  10  held on the chuck table  71  are set at the predetermined position relationship and the metering portion  813  constituting the cylindrical metering case  81  of the non-contact thickness metering equipment  8  is located at a metering position, the control means  9  drives the rotary-drive means (not shown) to rotate the chuck table  71  in the direction indicated by the arrow A in  FIG. 4  at a revolution of, for example, 300 rpm and drives the above servo motor  43  to rotate the grinding wheel  5  in the direction indicated by the arrow B at a revolution of, for example, 6,000 rpm. Then, the control means  9  drives the pulse motor  64  of the grinding unit feed mechanism  6  in the normal direction to lower the grinding wheel  5  (for grinding) so as to press the plurality of grindstones  52  against the rear surfaces  101  (surface to be ground) which are the top surfaces of the plurality of devices  10 , at a predetermined pressure. As a result, the rear surfaces  101  (surface to be ground) of the plurality of devices  10  are ground (grinding step). 
   In the above grinding step, the thickness of the predetermined device  10   a  which rotates along the predetermined rotation locus is measured by the non-contact thickness metering equipment  8 . The step of metering the thickness of the device  10   a  will be described below. 
   Since the chuck table  71  holding the plurality of devices  10  is rotated at a revolution of 300 rpm as described above, it makes 5 revolutions in one second. Therefore, the predetermined device  10   a  which rotates along the predetermined rotation locus passes right below the metering portion  813  constituting the cylindrical metering case  81  of the non-contact thickness metering equipment  8  five times in one second. So, the control means  9  activates the ultrasonic oscillation means  85  and the reflected wave receiving means  87  constituting the non-contact thickness metering equipment  8  to oscillate a pulse ultrasonic wave from the ultrasonic oscillating means  85  once in one second. Therefore, the control means  9  reads a received signal received by the reflected wave receiving means  87  once each time the chuck table  71  makes 5 revolutions. 
   The procedure of measuring the thickness of the device  10   a  with the non-contact thickness metering equipment  8  is described below. To measure the thickness of the device  10   a  with the non-contact thickness metering equipment  8 , the control means  9  turns on the electromagnetic on-off valve  883  to open it. As a result, pure water is supplied from the pure water supply means  881  into the cylindrical metering case  81  through the pipe  882 . The pure water supplied into the cylindrical metering case  81  flows over the plurality of devices  10  held on the chuck table  71  from the opening  813   a  of the metering portion  813  as shown in  FIG. 5 , thereby forming a fluid film  884  between the top surface (rear surface  101 ) of the device  10  and the opening  813   a  and filling a portion below the metering portion  813  (that is, portion below the wave transmitting portion of the ultrasonic wave transmitter  82  and the wave receiving portion of the reflected wave receiver  83 ). Then, the control means  9  activates the ultrasonic oscillation means  85  and the reflected wave receiving means  87  constituting the non-contact thickness metering equipment  8 . As a result, a pulse ultrasonic wave  820  having a frequency of, for example, about 30 MHz is oscillated from the ultrasonic wave transmitter  82 . The ultrasonic wave  820  oscillated from the ultrasonic wave transmitter  82  is reflected on the top surface (rear surface  101 ) of the device  10  and the undersurface (front surface) of the device  10 . A first reflected wave  821  reflected on the top surface (rear surface  101 ) of the device  10  and a second reflected wave reflected on the undersurface (front surface) of the device  10  are received by the reflected wave receiver  83  and propagated to the reflected wave receiving means  87  through the ultrasonic propagation means  86 . Thus, the reflected wave receiving means  87  which has received the first reflected wave  821  and the second reflected wave  822  supplies a received signal to the control means  9 . Since a fluid (pure water) is filled between the wave transmitting portion of the ultrasonic wave transmitter  82  and the wave receiving portion of the reflected wave receiver  83  and the device  10 , an ultrasonic wave propagates well. 
   The control means  9  calculates the thickness of the device  10  based on the received signal from the reflected wave receiving means  87 . That is, the thickness of the device.  10  can be obtained by calculating the difference between the time period from the time when a pulse ultrasonic wave is oscillated from the ultrasonic oscillation means  85  to the time when the first reflected wave  821  from the top surface (rear surface  101 ) of the device  10  is received by the reflected wave receiving means  87  and the time period from the time when a pulse ultrasonic wave is oscillated from the ultrasonic oscillation means  85  to the time when the second reflected wave  822  from the undersurface (front surface) of the device  10  is received by the reflected wave receiving means  87 . Stated more specifically, when the time period from the time when a pulse ultrasonic wave is oscillated from the ultrasonic oscillation means  85  to the time when the first reflected wave  821  from the top surface (rear surface  101 ) of the device  10  is received by the reflected wave receiving means  87  is represented by T 1 , the time period from the time when a pulse ultrasonic wave is oscillated from the ultrasonic oscillation means  85  to the time when the second reflected wave  822  from the undersurface (front surface) of the device  10  is received by the reflected wave receiving means  87  is represented by T 2 , the sound speed in the inside of the device  10  is represented by V, and the incident angle and reflection angle of the ultrasonic wave  820  are represented by θ, the thickness W of the device  10  can be obtained from the following equation:
 
 W=V ×( T 2 −T 1)×cos θ÷2
 
   The above grinding step is carried out while the thickness W of the device  10  is measured with the non-contact thickness metering equipment  8  as described above. When the thickness W of the device  10  measured with the non-contact thickness metering equipment  8  becomes a set value (for example, 300 μm), the control means  9  drives the pulse motor  64  of the grinding unit feed mechanism  6  in the opposite direction to lift the grinding wheel  5 . As a result, the grinding function of the grinding wheel  5  ends. 
   Since the non-contact thickness metering equipment  8  is used in the device grinding method of the present invention as described above, the rear surfaces of the plurality of devices  10  can be ground by the grinding means while the thicknesses of the individual devices  10  are directly measured, thereby eliminating the need for manufacturing a ring-shaped measurement frame for measuring the thickness of each device  10  indirectly. Therefore, productivity and the thickness accuracy of each device are improved. 
   While the invention has been described based on the embodiment shown in the accompanying drawings, it is to be understood that the invention is not limited thereto and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof. For example, although a thickness metering equipment using an ultrasonic wave is used as the non-contact thickness metering equipment in the illustrated embodiment, a non-contact thickness metering equipment using a laser beam may be used.