Abstract:
A wafer grinder withholds a lateral force and adjusts a tilt of worktable with precision. The wafer grinder is used to grind a wafer; that is, the tilt angle of wafer can be adjusted so that wafer is ground with precision. The wafer grinder has a housing module, a rotary worktable module, air pressure spindle module and an adjustment module.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a wafer grinder, and in particular to a wafer grinder that withholds lateral force and accurately makes adjustment to a worktable. 
   2. Description of Related Art 
   As is widely known in semiconductor industries, how to improve a total thickness variation (TTV) of a wafer grinder is a current topic of interest. The related technique includes a feedback circuit and an improved mechanism suitable for implementation in wafer grinders with high precision. Thus, it meets requirement of wafer grinding in the modern era. Utility and cost-cutting of the wafer grinder are widely recognized. 
   Generally, the wafer grinder includes an air pressure worktable and an air pressure spindle and is used to machine or mill brittle material. A vacuum disk is made of porous ceramic and is positioned above the air pressure worktable. A plurality of air holes are positioned within the air pressure worktable and used as the air pressure spindle. When milling or machining wafers with diamond wheels, the spindles of the diamond wheels are bent by a reaction force and are tilted. In addition, temperature of the diamond wheels rises so that the position of the diamond wheels is not in alignment with the wafers. 
   In the prior art, U.S. Pat. No. 5,567,199 discloses a conventional wafer grinder. Referring to  FIG. 1 , it illustrates a schematic drawing of a conventional wafer grinder. A displacement meter  31   a  is positioned on a worktable  22   a  and is used to measure displacement of a piezoelectric actuator  3   a . Then, the displacement meter  31   a  transmits measurement data to a central computing unit (CPU)  32   a , and the piezoelectric actuator  3   a  adjusts its displacement in response to measurement data of the displacement meter  31   a . Thus, a wafer  2   a  is held by a vacuum disk  21   a  and rotated in a certain rotational direction. The wafer  2   a  is ground by a grinding wheel  1   a  due to relative movement between the wafer  2   a  and the grinding wheel  1   a . However, the piezoelectric actuator  3   a  is subjected to a lateral force during the grinding process, which has a significant bad impact on the precision of grinding. The &#39;199 patent discloses two features. Firstly, a feedback system is additionally positioned at the displacement meter  31   a  and is used to measure the displacement of the piezoelectric actuator  3   a . However, a measuring device and an object measured are not in the same geometrical position, so the measurement is prone to error. Secondly, the piezoelectric actuator  3   a  is used to adjust the worktable  22   a  and is positioned below the worktable  22   a . Because the piezoelectric actuator  3   a  is subject to the weight of the worktable  22   a , the piezoelectric actuator  3   a  is prone to damage. 
   Additionally, U.S. Pat. No. 5,816,895 is shown in  FIG. 2 . The conventional wafer grinder utilizes four piezoelectric actuators  3   b  to adjust a tilt angle of a worktable  22   b . The four piezoelectric actuators  3   b  are positioned below the worktable  22   b  and separated by 90 degrees. Alternately, four piezoelectric actuators  12   b  are positioned at a spindle of a wafer grinder  1   b  to adjust the tilt of the spindle of the wafer grinder  1   b . Further, three displacement meters  31   b  are arranged over the wafer (not numbered) and transmit a thickness of the wafer to a central computing unit (CPU)  32   b  during the grinding process. A piezoelectric device  34   b  is used to control the piezoelectric actuators  12   b  and  3   b  so that the wafer  2   b  is ground by the wafer grinder  1   b  because the worktable  22   b  is adjusted by the piezoelectric device  34   b . However, because the piezoelectric actuator  3   a  is subject to the weight of the worktable  22   a , it has a significant impact on the precision of the grinding process. The &#39;985 patent discloses two features. Firstly, a feedback system is additionally positioned with the displacement meters  31   b  and is used to measure the thickness of the wafer  2   b . Thus, a measuring device and an object measured are not in the same geometrical position, so the measurement is prone to error. Secondly, the piezoelectric actuator  3   b  is used to adjust the worktable  22   b  and is positioned below the worktable  22   b . It is thus prone to damage. 
   As described in U.S. Pat. Nos. 5,567,199 and 5,816,895, the adjustment mechanism of the worktable is positioned below the worktable to support the weight of the worktable and has no pre-compression device. As described in the &#39;199 patent, a detection system is positioned near the spindle of the grinding wheel and is prone to contamination by machine oil. According to the &#39;199 patent, the detection system of the piezoelectric actuator is a strain gauge attached to the surface of the piezoelectric actuator. Because the strain gauge and the piezoelectric actuator are in the same position, the displacement is regarded as an input value. 
     FIG. 3A  illustrates how a wafer  2   c  is ground by a grinding wheel  1   c . The wafer  2   c  is ground by friction between the wafer  2   c  and the grinding wheel  1   c  during the grinding process. Further referring to  FIG. 3B , the predetermined position  12   d  of the grinding wheel  1   c  does not coincide with the practical position  11   d  of the grinding wheel  1   c . Thus, it renders non-uniform the thickness of the wafer  2   d.    
   Thus, there is need to develop a mechanism for adjusting a machining angle of a wafer. 
   SUMMARY OF THE DISCLOSURE 
   It is an object of the present invention to provide a wafer grinder. 
   In order to accomplish one object of the present invention, the present invention provides a wafer grinder. The present invention utilizes a static air pressure spindle to absorb the lateral force, and a piezoelectric actuator is concentrically positioned with a displacement meter. 
   The present invention includes a housing module, a rotary worktable module, an air pressure spindle module and an adjustment module. The housing module is fixedly positioned on the wafer grinder. The rotary worktable module includes a worktable body and a spindle, which are rotatably positioned to the wafer grinder. The rotary worktable module also has a wafer holding sub-module. The air pressure spindle module is positioned on the wafer grinder and has an air channel. The air channel is used to direct air with certain pressure to the housing module and the rotary worktable module to act as an air cushion spindle to support the worktable body and the spindle. The adjustment module is positioned at the housing module and has a piezoelectric actuator and a displacement meter. The longitudinal rotation between the spindle and the worktable body has an air cushion to offset the lateral force during the grinding process. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention can be fully understood from the following detailed description and preferred embodiment with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic drawing of a conventional wafer grinder; 
       FIG. 2  is another schematic drawing of a conventional wafer grinder; 
       FIG. 3A  is a schematic drawing illustrating how a wafer is ground by a grinding wheel; 
       FIG. 3B  is another schematic drawing illustrating how a wafer is ground by a grinding wheel; 
       FIG. 4  is a cross-sectional view of a wafer grinder of the present invention; 
       FIG. 5  is another cross-sectional view of a wafer grinder of the present invention; 
       FIG. 6  is a top planar view of a wafer grinder of the present invention; 
       FIG. 7  is another top planar view of a wafer grinder of the present invention; and 
       FIG. 8  is a chart illustrating a relationship between displacement of a piezoelectric actuator and input signals according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following detailed description is of the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating general principles of embodiments of the invention. The scope of the invention is best defined by the appended claims. 
   Referring to  FIG. 4 , a wafer grinder of the present invention includes a rotary worktable  1 , an air pressure protection bearing  2 , a housing  3  and an adjustment module  4 . The rotary worktable  1  includes a spindle  16  positioned at a center of the air pressure protection bearing  2  and a housing  3 . The rotary worktable  1  includes a rotary worktable body  19 , which has a vacuum nozzle  17  and a pump hose  18 . The adjustment module  4  is positioned at perimeter of the air pressure protection bearing  2  and the housing  3  and is equally separated by 120 degrees, as shown in  FIG. 6 . The rotary worktable  1  includes a vacuum disk  11  made of porous material  15 . As shown in  FIG. 4 , a support plate  12  is defined below the vacuum disk  11 . Further referring to  FIG. 7 , an adjusting screw  121  is provided in the support plate  12  so as to adjust the vacuum disk  11  to hold a wafer of 4 inches, 6 inches or 8 inches. The rotary worktable  1  also includes a coupling  13  made of rubber. The coupling  13  connects with a timing plate belt pulley  14 , so motor power is delivered by the timing plate belt pulley  14 . The air pressure protection bearing  2  includes an air inlet  21  and an air outlet  22 , as shown in  FIG. 5 . An air inlet hose  311  and an air outlet hose  321  are used to deliver static pressure air to the air pressure protection bearing  2 . As shown in  FIG. 4 , the adjustment module  4  includes a piezoelectric actuator  41  to adjust a tilt angle of the rotary worktable  1 . A pre-compression bolt  42 , a hexagonal socket screw  43 , a disc spring  44  and a locking screw  45  generate a pre-compressive force on the piezoelectric actuator  41 . The result is an improved rigidity of the rotary worktable  1 . A displacement meter is positioned within the adjustment module  4  so as to measure the displacement of the rotary worktable  1 . The displacement meter and the piezoelectric actuator  41  are placed in the same geometrical position.  FIG. 8  illustrates a relationship between the displacement of the piezoelectric actuator  41  and corresponding input signal. Most of the relationship is linear. An auxiliary groove  33  is integrally formed with a holding body  34  to facilitate machining the airflow hose (the air inlet hose  311  and the air outlet hose  321 ). 
   As shown in  FIG. 4 , the wafer grinder according to present invention includes a housing module having a holding body  34  and being fixedly positioned on the wafer grinder. The rotary worktable module includes a worktable body  19  and spindle  16  that are rotatably positioned on the holding body  34  of wafer grinder. The rotary worktable module also has a wafer holding sub-module. The air pressure spindle module is positioned on the holding body  34  of the wafer grinder and has an air channel (the air inlet hose  311  and the air outlet hose  321 ). The air channel is used to direct air with a certain pressure to the housing module and the rotary worktable module to act as an air cushion spindle to support the worktable body  19  and the spindle  16 . The adjustment module is positioned on the holding body  34  and has a piezoelectric actuator  41  and a displacement meter. The longitudinal rotation between the spindle  16  and the worktable body has an air padding to offset the lateral force during the grinding process. 
   The description of the present invention is as follows. The holding body  34  includes the auxiliary groove  33  to facilitate drilling of the airflow grooves. The base structure  50  of the wafer grinder is usually a conventional civil structure for setting up a machine to ensure stability. The wafer holding sub-module is provided on the worktable body  19  and also includes the vacuum nozzle  17  and the pump hose  18  so that a wafer is held steadily by a vacuum. In general, the spindle  16  is connected to the worktable body  19  to rotate the worktable body  19  so that it is more convenient for the configuration of all components. The rotary worktable module further includes an adjustment sub-module positioned within the worktable body  19 . The adjustment sub-module includes the adjusting screw  121  to block longitudinally the vacuum nozzle  17  to adjust to wafers of different sizes. To make measurements accurate, the piezoelectric actuator  41  and the displacement meter are in the same geometrical position. To make displacement uniform, three pairs of piezoelectric actuator  41  and displacement meter are positioned at the bottom of the worktable body  19  by the same separation to adjust a tilt angle of the worktable body  19 . To make movement accurate, the spindle  16  is driven by a flexible belt-like structure to prevent shock from being transmitted to the spindle  16 . To make design of the grinder easy, the spindle  16  further includes the coupling  13  and the timing plate belt pulley  14 . The coupling  13  connects with a timing plate belt pulley  14 , so motor power is delivered by the timing plate belt pulley  14 . To keep rigidity of the wafer grinder, the disc spring  44  generates a pre-compressive force on the piezoelectric actuator  41 . 
   The advantages of the present invention can be summarized as follows: 
   1. The present invention provides an air pressure worktable that can adjust a machining angle with greater precision. In particular, the spindle  16  is subjected to a lateral cutting force so that cutting precision and lifetime of grinder can be extended. The air pressure protection bearing  2  helps to rotate the rotary worktable  1  with precision. The air pressure protection bearing  2  experiences the displacement of the piezoelectric actuator  41  and generates the tilt angle. 
   2. The present invention provides the capability of measuring tilt of a rotary worktable  1  because the piezoelectric actuator  41  and a displacement meter are in the same geometrical position. The disc spring  44  and a locking screw  45  produce a pre-compressive force on the piezoelectric actuator  41  so that rigidity of the rotary worktable  1  is increased and displacement is controlled with much higher precision. 
   3. The present invention provides a rotary worktable  1  to hold wafers of 4, 6 or 8 inches. Four adjusting screws  121  are used to adjust a groove within the support plate  12  and make changes to the holding surface. 
   While the invention has been described with reference to the preferred embodiments, the description is not intended to be construed in a limiting sense. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents.