Abstract:
Apparatus and methods of automated wafer-grinding using grinding surface position monitoring. In one embodiment, an apparatus for grinding a working surface includes a grinding surface engageable with at least a portion of the working surface, and a feed mechanism that controllably adjusts a position of the grinding surface. The apparatus further includes a position sensor that senses a position of the grinding surface along an axis approximately normal to the working surface and a controller that receives a position signal from the position sensor and transmits a control signal to the feed mechanism in response to the position signal. In alternate embodiments, the position sensor may be an acoustic sensor, an optical sensor, or another type of sensor. The grinding surface may include a grinding material suspended in a binder, the grinding material being worn during grinding. In an alternate embodiment, an apparatus further includes a supplemental sensor that senses an operating characteristic and outputs a characteristic signal. The controller receives the characteristic signal and transmits the control signal to the feed mechanism based on at least one of the position signal or the characteristic signal. In alternate embodiments, the characteristic signal may include a pressure of the grinding surface on the working surface, a shaft speed of a drive shaft, or a current drawn by a drive motor.

Description:
TECHNICAL FIELD 
     The present invention relates to apparatus and methods of automated wafer-grinding of semiconductor wafers, and more particularly to automatically grinding by monitoring a grinding parameter. 
     BACKGROUND OF THE INVENTION 
     The source material for manufacturing semiconductor chips is usually a relatively large wafer of silicon. Such wafers may be produced by slicing a silicon crystal ingot to a suitable thickness to obtain a number of nearly disk-shaped semiconductor wafers. Both surfaces of each wafer are subjected to abrasive machining, and then etched in a suitable mixed acid solution. One surface of each wafer is then polished to obtain a mirror surface. Circuits are fabricated in the mirror surface of the resulting semiconductor wafer by known processing steps, such as, for example, printing, etching, diffusion, or doping. 
     When the silicon wafers are sliced from the crystal ingot, the thickness of the wafers is usually greater than desirable for a finished integrated circuit product so as to provide a more robust wafer to stand up to the rigors of the integrated circuit fabrication process. Relatively thick silicon wafers may be necessary, for example, during certain integrated circuit fabrication steps to prevent warpage and breakage of the wafer as a result of heating, handling, and other circuit fabrication processes. Because the thickness of the wafer after the circuit fabrication process is usually greater than desirable for device packaging restrictions, it is typically necessary to grind a backside surface of the wafer opposite from the surface on which the integrated circuits are formed to reduce the wafer thickness. 
     Automated grinding machines for grinding the backside surfaces of wafers are known. Conventional grinding machines generally include a plurality of chuck tables that secure a plurality of wafers to be ground by one or more grinding wheels. A conventional grinding wheel typically includes a plurality of diamonds embedded in a resinous binder, with some of the diamonds exposed and some unexposed. As the grinding progresses, the exposed diamonds wear down to the level of the binder. The binder is selected to erode during grinding to expose fresh diamonds. The rate of wear of the grinding wheel may be dependent on the composition of the binder, the grinding rate, or other factors, as described more fully below. 
     FIG. 1 is a side elevational view of an automated grinding machine  10  for grinding a backside surface  25  of a wafer  12  in accordance with the prior art. The grinding machine  10  includes a spindle housing  14  disposed about a spindle  16  having a rotatable grinding shaft  18 . A grinding wheel  20  is rigidly secured to the end of the shaft  18 . A spindle motor  22  rotates the shaft  18  and the grinding wheel  20  at conventional speeds of 2400-3200 RPM during the grinding process, causing the grinding wheel  20  to grind away semiconductor material from the backside surface  25  of the wafer  12 . The spindle housing  14  is coupled to a feed mechanism  26  that allows the placement and the feed rate of the grinding wheel  20  to be adjusted relative to the wafer  14  to provide, for example, different grinding rates. 
     A controller  27 , such as a computer, is electrically connected to the grinding wheel  20  by electrical conductor  29  to receive feedback signals, and to a feed rate motor  31  by electrical conductor  33  to send control signals thereto. The controller  27  is also connected to a shaft speed sensor  19  by electrical conductor  35 , to a spindle motor current detector  21  by electrical conductor  37 , and to the spindle motor  22  by electrical conductor  23 . The wafer  12  is secured to a chuck table platform  30  of a chuck table  28  by a suitable securing mechanism, such as vacuum suction, with the front side of the wafer  12  that includes the integrated circuits positioned against the chuck table platform  30 . The chuck table platform  30  is secured to a shaft  32  which is driven by a chuck table motor (not shown) at conventional speeds of between 50-300 RPM. 
     FIG. 2 is a bottom plan view of the grinding wheel  20  of the grinding machine  10  of FIG.  1 . FIG. 3 is a partial cross-sectional radial view of the grinding wheel  20  of FIG.  2 . As shown in FIGS. 2 and 3, the grinding wheel  20  includes a disk portion  40  and an annular shoulder  42  depending downwardly from the peripheral edge  41  of the disk portion  40 . The annular shoulder  42  includes a lower surface  47 . A plurality of cylindrical cavities  44  are formed in the lower surface  47  of the annular shoulder  42  and a cylindrical grinding tooth  46  is disposed in each cavity  44 . Each cavity  44  is connected to a central shaft-receiving bore  43  by a pressure signal transmission pathway  45 . 
     As best shown in FIG. 3, each grinding tooth  46  includes a body  48  having a first end  50 , which includes a grinding surface  24 , and a second end  52 . The second end  52  is disposed in the cavity  44 . A pressure sensor  54  is disposed in the cavity  44  between the second end  52  and the disk portion  40 . The pressure sensors  54  may include, for example, a piezoelectric element  60  that produces an electrical voltage when it is squeezed. Thus, the pressure sensor  54  may convert mechanical pressure on the grinding teeth  46  into an electrical signal, the strength of which increases or decreases with the pressure exerted by the grinding wheel  20  against the backside surface  25  of the wafer  12 . The grinding surface  24  may include a plurality of diamonds suspended in a resinous binder. As disclosed, for example, in U.S. Pat. No. 5,827,112 to Ball, incorporated herein by reference, the binder may be selected to be reactive with wheel dressing and to dissolve, either mechanically, or chemically or both. As the binder dissolves, the dull diamonds from the grinding surface  24  are released and washed away, leaving freshly exposed sharp diamonds. 
     The controller  27  may receive input signals from the pressure sensors  54  to indicate the pressure exerted by the grinding wheel  20  against the wafer  12 . The controller  27  may also receive input signals from the speed sensor  19  indicative of the rotational speed of the shaft  18 , and input signals from the current detector  21  which indicate the amount of current being drawn by the spindle motor  22 . Based on these input signals, the controller  27  may adjustably control various operating parameters of the automated grinding machine  10 , including, for example, the feed rate of the feed rate motor  31 , the rotational speed of the spindle motor  22 , or the release of wheel dressing for sharpening the grinding wheel  20 . 
     FIG. 4 is a schematic view of a typical grind recipe  80  of a grinding machine  10  in accordance with the prior art. During the grinding process shown in FIG. 4, the grinding wheel  20  descends along a z-axis as a function of time t (shown as the horizontal axis in FIG.  4 ), allowing the grinding teeth  46  to grind away the backside surface  25  of the wafer  12 . During a first or “rapid descent” phase  82 , the grinding wheel  20  maintains a relatively high rate of descent between times t 0  and t 1 . During a second or “F 1  removal” phase  84 , the rate of descent of the grinding wheel  20  is decreased (typically 40 microns per minute) between times t 1  and t 2 . Finally, during a third or “F 2  removal” phase  86 , the rate of descent of the grinding wheel  20  is further decreased (typically 20 microns per minute) between times t 2  and t 3 . Thus, in the representative grind recipe  80 , the time required to remove a wafer layer of thickness z 0 -z 3  is the time t 3 -t 0 . The times t 1 , t 2 , and t 3  are typically selected to avoid stress cracks or other defects in the wafer  12 . 
     In addition to descent rate of the grinding wheel, other operating conditions of the grinding machine  10  may be varied during the phases  82 ,  84 ,  86 . For example, the rotational rate of the grinding wheel may be varied, or different grinding wheels having grinding surfaces with different diamond sizes may be used. Grinding machines  10  having grind recipes of the type shown in FIG. 4 typically process approximately  35  wafers per hour. 
     Various grinding machines have been disclosed to control the forces applied to the wafer. For example, U.S. Pat. No. 5,035,087 to Nishiguchi et al discloses a grinding machine that compares the shaft motor current and a rotation speed of the shaft with predetermined values to derive actual and desired grinding resistance values. The shaft speed is adjusted to bring the actual grinding resistance value closer to the desired value. U.S. Pat. No. 5,545,076 to Yun et al discloses an apparatus for removing dust from a wafer during the grinding process includes a controller for controlling the grinding device and cleaning device. U.S. Pat. No. 5,607,341 to Leach discloses an apparatus for polishing the wafer having a plurality of blocks that move up and down in a grinding wheel. A magnetic fluid is contained in the grinding wheel and cooperates with a magnet disposed below the wafer to apply a force to the blocks. Thus, various methods are known for controlling the grinding force exerted by the grinding wheel  20  on the wafer  12 , thereby controlling the grinding rate. 
     Prior to commencing a grinding procedure, a calibration may be performed with the wafer  12  removed from the chuck table platform  30 . The feed mechanism  26  may lower the grinding wheel  20  until the grinding surfaces  24  (FIG. 3) of the grinding wheel  20  contact the chuck table platform  30 , providing a “zero” or reference position along the z axis (FIG. 1) which may be stored, for example, in a memory of the controller  27 . As the grinding wheel  20  is raised, a series of measurements of the distance between the grinding surfaces  24  and the chuck table platform  30  may be made and entered into the controller  27  to create a database of measured calibration data in the memory of the controller  27 . Thus, based on a given position of the feed mechanism  26 , the controller  27  may determine a “predicted” position of the grinding surfaces  24  of the grinding wheel  20  based on the measured calibration database. 
     Because the grinding surfaces  24  wear during the grinding process, the predicted position of the grinding surfaces  24  based on the measured calibration data may not accurately reflect the true position of the grinding surfaces  24 , particularly after the grinding surfaces  24  have been used for an extended period of time. Generally, the longer the grinding wheel  20  is used, the greater may be the discrepancy between the predicted position of the grinding surfaces  24  determined from the measured calibration data, and the actual position of the grinding surfaces  24 . The discrepancy between the predicted and actual positions of the grinding surfaces  24  results in uncertainty over the true thickness of the wafer  12  during the grinding process. For thick wafers, however, the uncertainty over the true thickness of the wafer  12  may be negligible. Alternately, the grinding process may be repeatedly interrupted to manually measure the actual thickness of the wafer  12  until a desired wafer thickness is achieved. 
     Although desirable results have been achieved using the above-described grinding machines and grinding procedures, the ever-increasing demands of the semiconductor industry for reducing the size of semiconductor chip assemblies are placing unprecedented demands on such machines and procedures to be more accurate. For example, decreasing the size of semiconductor chip assemblies requires decreasing the thickness of the wafer. As wafer thickness is reduced, increased requirements are placed on the grinding machine to more accurately determine the thickness of the wafer and to more accurately control the grinding rate of the grinding wheel  20  against the backside surface  25  of the wafer  12 . As wafer thickness is decreased, extra care must be taken to ensure that the wafer is not over-ground or made too thin. 
     Furthermore, because thinner wafers are more prone to stress cracking or breakage due to the pressure from the grinding wheel, the descent rate of the grinding wheel must be more carefully controlled to avoid damaging thinner wafers. The uncertainty over the actual thickness of the wafer due to the wear of the grinding surfaces may become more important as the wafer thickness is decreased, and may require more frequent interruptions of the wafer grinding process to measure the actual thickness of the wafer. The grinding process is thereby slowed, and the throughput of the manufacturing process is reduced. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to apparatus and methods of automated wafer-grinding using grinding surface position monitoring. In various aspects of the invention, grinding surface position monitoring may include, for example, monitoring acoustic or optical signals reflected (or through-beam or electrically or magnetically coupled) from the grinding surface, and may be used in combination with monitoring of other operating characteristics, such as grind pressure, shaft speed, or current drawn by a drive motor. Apparatus and methods according to the invention provide improved accuracy and increased throughput of the grinding process. 
     In one aspect, an apparatus for grinding a working surface includes a grinding surface engageable with at least a portion of the working surface, and a feed mechanism that controllably adjusts a position of the grinding surface. The apparatus further includes a position sensor that senses a position of the grinding surface along an axis approximately normal to the working surface and a controller that receives a position signal from the position sensor and transmits a control signal to the feed mechanism in response to the position signal. In alternate aspects, the position sensor may be an acoustic sensor, an optical sensor, or another type of sensor. The grinding surface may include a grinding material suspended in a binder, the grinding material being worn during grinding. 
     In an alternate aspect, an apparatus further includes a supplemental sensor that senses an operating characteristic and outputs a characteristic signal. The controller receives the characteristic signal and transmits the control signal to the feed mechanism based on at least one of the position signal or the characteristic signal. In alternate aspects, the characteristic signal may include a pressure of the grinding surface on the working surface, a shaft speed of a drive shaft, a current drawn by a drive motor, or some other parameter. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of an automated grinding machine in accordance with the prior art. 
     FIG. 2 is a bottom plan view of a grinding wheel of the grinding machine of FIG.  1 . 
     FIG. 3 is an enlarged, partial cross-sectional radial view of the grinding wheel of FIG.  2 . 
     FIG. 4 is a schematic view of a typical grind recipe of a grinding machine in accordance with the prior art. 
     FIG. 5 is a side elevational view of an automated grinding machine having an acoustic sensor in accordance with an embodiment of the invention. 
     FIG. 6 is an enlarged, partial cross-sectional radial view of the grinding wheel and the acoustic sensor of the grinding machine of FIG.  5 . 
     FIG. 7 is a schematic view of a grind recipe of the grinding machine of FIG. 6 compared with the typical grind recipe of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is generally directed to apparatus and methods of automated wafer-grinding using grinding surface position monitoring. Grinding surface position monitoring may include, for example, monitoring acoustic or optical signals reflected from the grinding surface, and may be used in combination with monitoring of other operating characteristics, such as grind pressure, shaft speed, or current drawn by a drive motor. Apparatus and methods according to the disclosed embodiment of the invention provide improved accuracy and increased throughput of the grinding process. 
     Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 5-7 to provide a thorough understanding of such embodiments. One skilled in the art will understand, however, that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description. 
     Unless otherwise stated, the construction and operation of various components of the embodiments described below may be of conventional design. Such components will be referred to using the same names and designation numbers as were used in the preceding discussion. For the sake of brevity, such components will not be described in further detail herein, as these components are within the understanding of those skilled in the relevant art. 
     FIG. 5 is a side elevational view of an automated grinding machine  100  having an acoustic sensor  170  in accordance with an embodiment of the invention. The acoustic sensor  170  is positioned proximate the grinding wheel  20  and is coupled to the controller  27  by a signal lead  172 . As shown in FIG. 5, the acoustic sensor  170  transmits one or more acoustic signals  174  toward the grinding wheel  20 . 
     FIG. 6 is an enlarged, partial cross-sectional radial view of the grinding wheel  20  and the acoustic sensor  170  of the grinding machine  100  of FIG.  5 . In this embodiment, the acoustic sensor  170  includes an acoustic source  176  that transmits the acoustic signals  174 , and an acoustic receiver  178  that receives reflected acoustic signals  180  from the grinding wheel  20 . The reflected acoustic signals  180  may include first reflected signals  182  that reflect from the grinding surfaces  24  of the grinding teeth  46 , and second reflected signals  184  that reflect from the lower surface  47  of the grinding wheel  20  at the base of the grinding teeth  46 . 
     The acoustic sensor  170  may be any suitable type of acoustic sensor that determines position of an object based on transmitted and reflected acoustic signals. For example, the acoustic sensor  170  may be one of the sensor types disclosed in U.S. Pat. No. 5,852,232 issued to Samsavar et al, U.S. Pat. No. 4,285,053 issued to Kren et al, U.S. Pat. No. 4,175,441 issued to Urbanek et al, U.S. Pat. No. 3,918,296 issued to Kitada, or U.S. Pat. No. 3,694,800 issued to Frank, which patents are incorporated herein by reference. Generally, acoustic position sensors may transmit an acoustic signal toward an object and receive a reflected acoustic signal from the object, and may determine a distance to the object based on a time measured between the transmitted and received acoustic signals and a known or assumed speed of sound. Alternately, the distance may be inferred from measured interference patterns in the transmitted and received acoustic waves, or by other suitable means, as disclosed, for example, in the above-referenced patents. 
     It will be understood that the acoustic sensor  170  may be replaced with any suitable position sensing apparatus, such as optical or electromagnetic sensors, including those which sense the position of an object using visible, ultraviolet, or infrared light. For example, the acoustic sensor  170  may be replaced by one of the optical sensor types disclosed in U.S. Pat. No. 5,825,481 issued to Alofs et al, U.S. Pat. No. 5,131,740 issued to Maekawa, U.S. Pat. No. 5,056,913 issued to Tanaka et al, U.S. Pat. No. 4,865,443 issued to Howe et al, U.S. Pat. No. 4,639,140 issued to Lerat, U.S. Pat. No. 4,673,817 issued to Oomen, U.S. Pat. No. 4,657,382 issued to Busujima et al, U.S. Pat. No. 3,899,251 issued to Frenk et al, U.S. Pat. No. 3,885,872 issued to Howe et al, or U.S. Pat. No. 3,815,994 issued to Peckham, which patents are incorporated herein by reference. In the following discussion, for the sake of brevity, the position sensor will be described as an acoustic sensor  170  although it will be understood that any type of position sensing apparatus may be employed. 
     The automated grinding machine  100  having the acoustic sensor  170  may be operated in a variety of ways to provide desirable results, including to provide improved grinding accuracy, increased throughput, and to monitor the wear of the grinding surfaces  24  during operation of the machine. For example, in one embodiment, a method of operating the grinding machine  100  includes performing a calibration procedure with the wafer  12  removed from the chuck table platform  30  prior to commencing a grinding procedure. The feed mechanism  26  may lower the grinding wheel  20  until the grinding surfaces  24  (FIG. 3) of the grinding wheel  20  contact the chuck table platform  30 , providing a “zero” or reference position along the z axis (FIG. 5) which may be stored, for example, in a memory of the controller  27 . As the grinding wheel  20  is raised, a series of measurements of the distance between the grinding surfaces  24  and the chuck table platform  30  may be made and entered into the controller  27  to create a database of measured calibration data in the memory of the controller  27 . Thus, based on a given position of the feed mechanism  26 , the controller  27  may determine a “predicted” position of the grinding surfaces  24  of the grinding wheel  20  based on the measured calibration database. 
     Alternately, in the same or another calibration procedure, a different set of “predicted” grinding surface positions may be created using the acoustic sensor  170 . For example, the acoustic sensor  170  may be operated to transmit acoustic signals  174  toward the grinding wheel  20  and may receive the reflected acoustic signals  180  (either the first or second reflected signals  182 ,  184 , or both). Based on the first reflected signals  182 , and using known acoustic signal processing techniques, a series of first position measurements of the grinding surfaces  24  may be determined by the acoustic sensor  170  and may be entered into the controller  27  to form a first calibration database. Similarly, based on the second reflected signals  184 , a series of second position measurements of the lower surface  47  of the grinding wheel  20  may be determined by the acoustic sensor  170  and may be entered into the controller  27  to form a second calibration database. 
     In operation, the grinding wheel  20  of the grinding machine  100  may be raised to a starting position and the wafer  12  may be positioned on the chuck table platform  30  for grinding. As the rotating grinding wheel  20  descends toward the wafer  12 , the acoustic sensor  170  may be used to transmit acoustic signals  174  onto the grinding surfaces  24  and to receive the first reflected acoustic signals  182 . Based on known signal processing techniques (described in the above-referenced patents), an “actual” position of the grinding surfaces  24  during the grinding operation may be determined. 
     As described more fully below, by determining the “actual” position of the grinding surfaces  24  during operation of the grinding machine  100 , the wafer thickness t w  during the grinding process may be accurately determined and controlled. Also, by comparing the “actual” position with the “predicted” position of the grinding surfaces  24 , the wear of the grinding surfaces  24  may be monitored during the grinding operation. Finally, because the wear of the grinding surfaces  24  may be monitored during operation, downtime of the grinding machine  100  may be reduced and the throughput of the grinding process may be improved. 
     One may note that the first and second calibration databases need not be created, and the acoustic sensor  170  may simply be operated without calibration data during a grinding procedure to determine the distance from the acoustic sensor  170  to the grinding wheel  20  (either distance to the grinding surfaces  24  or to the lower surface  47 , or both). If the acoustic sensor  170  is not positioned at the reference position (i.e. at the same plane as the chuck table platform  300 ), then a reference distance d as shown in FIG. 6 may be determined, such as during a calibration procedure, and stored, for example, in the controller  27 . 
     In yet another alternate method of operation, the grinding wheel  20  of the grinding machine  100  may be raised to a starting position and the wafer  12  may be positioned on the chuck table platform  30  for grinding. As the rotating grinding wheel  20  descends toward the wafer  12 , the controller  27  may monitor a first characteristic of the grinding machine  100 . The first characteristic may include, for example, a pressure signal from the pressure sensors  54 , a shaft speed signal from the shaft speed sensor  19 , a current drawn by the drive motor  22 , or some other operating characteristic of the grinding machine  100 . Similarly, the acoustic sensor  170  transmits acoustic signals  174  and receives reflected acoustic signals  180  which may be received by the acoustic sensor  170  and processed by the acoustic sensor  170  or the controller  27  to provide an actual position of the grinding surfaces  24  of the grinding wheel  20 . The grinding wheel  20  continues to descend until the grinding surfaces  24  of the grinding teeth  46  engage with the backside surface  25  of the wafer  12 . 
     Based on the monitored first characteristic, the controller  27  may determine the point at which the grinding teeth  46  engage the backside surface  25 . For example, if the first characteristic is a pressure signal from the pressure sensors  54 , the controller  27  may detect an increase in the pressure signal when the grinding surfaces  24  engage the wafer  12 . Similarly, if the first characteristic is a current signal indicating a current drawn by the drive motor  22 , the controller  27  may detect an increase in the current drawn by the drive motor  22  when the grinding surfaces  24  engage the wafer  12  as the drive motor  22  draws more current to maintain the rotational rate of the grinding wheel  20 . If the first characteristic is a shaft speed signal, the controller  27  may detect a decrease in the shaft speed as the grinding surfaces  24  engage the wafer  12 . 
     During a grinding operation, the grinding surfaces  24  wear down, decreasing the distance between the grinding surfaces  24  and the lower surface  47 , denoted as tooth height h t  in FIG.  6 . To monitor the tooth height h t , the acoustic sensor  170  transmits acoustic signals  174  toward the grinding wheel  20  and receives the first reflected signals  182  (which reflect from the grinding surfaces  24 ) and the second reflected signals  184  (which reflect from the lower surface  47 ). The acoustic sensor  170  may then process the first and second reflected signals  182 ,  184  to determine the distances between the acoustic sensor  170  and the grinding and lower surfaces  24 ,  47 , respectively. From this information, the acoustic sensor  170  may determine the tooth height h t . Alternately, the acoustic sensor  170  may simply receive the first and second reflected signals  182 ,  184  and may transmit signals indicative of having received the first and second reflected signals  182 ,  184  to the controller  27 . The controller  27  may then perform the necessary processing to determine the height h t  of the grinding teeth  46 . 
     One may note that in alternate embodiments, grinding surface position monitoring may be accomplished by varying the above-described methods. For example, the acoustic or optical signals which are monitored to determine the position of the grinding surface need not be reflected signals, but rather, by proper orientation of the sensor (or the use of additional sensors), position sensing may be accomplished by through-beam sensing, or may be accomplished via electrical or magnetic coupling. 
     The acoustic sensor  170  advantageously permits the grinding machine  100  to monitor tooth height h t  during grinding operations, the actual position of the grinding surfaces  24  may be determined at all times during the grinding process. This reduces or eliminates the need to shut down the grinding machine  100  to manually measure and determine the wear of the grinding teeth  46  until the grinding surfaces  24  are worn out. Because measurement of the tooth height h t  may be performed rapidly and accurately using the acoustic sensor  170 , the need for labor-intensive manual measurement of the tooth height h t  may be eliminated, and down time of the grinding machine  100  may be reduced. Also, because accurate information regarding the tooth height h t  may be constantly available during the grinding process, the life of the grinding wheel  20  may be optimized. 
     Because the actual position of the grinding surfaces  24  is determined using the acoustic sensor  170 , the actual wafer thickness t w  during the grinding process may be determined. The controller  27  may also utilize the reference distance d (FIG. 6) in determining the actual position of the grinding surfaces  24 , and thus, the actual wafer thickness t w . Based on the actual position of the grinding surfaces  24 , the controller  27  may adjustably control the feed mechanism  26  to accurately grind the wafer  12  to a desired wafer thickness t w . 
     Because the acoustic sensor  170  may be used during the grinding process to determine the actual position of the grinding surfaces  24 , the grinding apparatus  100  may provide improved control over the wafer thickness t w . Thus, over-grinding of the wafer  12  may be avoided. Also, the descent rate of the grinding wheel  20  may be more carefully controlled as the wafer thickness t w  decreases to avoid causing stress fractures within the wafer  12 . 
     Another advantage of the grinding machine  100  having the acoustic sensor  170  is that the grinding recipe may be more optimally designed. For example, FIG. 7 is a schematic view of a grind recipe  180  of the grinding machine  100  compared with the typical grind recipe  80  of FIG.  4 . As shown in FIG. 7, the grind recipe  180  includes a rapid descent phase  182 , an F 1  removal phase  184 , and an F 2  removal phase  186 . In addition to descent rate, other operating conditions of the grinding machine  100  may be varied during the phases  182 ,  184 ,  186 . For example, the rotational rate of the grinding wheel  20  may be varied, or different grinding wheels having grinding surfaces with different diamond sizes may be used. 
     Because the acoustic sensor  170  allows the wafer thickness t w  to be accurately monitored during the grinding process, the grinding machine  100  may employ a more aggressive grind recipe  180  compared with the typical grind recipe  80  of the prior art. Thus, in the grind recipe  180  shown in FIG. 7, the rates of descent of the grinding wheel  20  during the phases  182 ,  184 ,  186  are greater than the comparable rates of descent of the prior art grind recipe  80 . Because the grind recipe  180  of the grinding machine  100  may be more aggressive (i.e. faster descent rates) than the prior art grind recipe  80 , the time required to remove a wafer layer of thickness z 0 -z 3  is the time t 6 -t 0 , which may be substantially shorter than the time required (t 3 -t 0 ) using the prior art grind recipe  80 . Thus, the grinding machine  100  having the acoustic sensor  170  may advantageously reduce the grinding time cycle, and may desirably increase the throughput of the manufacturing process. For example, the grinding machine  100  operating according to the grinding recipe  180  may produce approximately 50 wafers per hour, or more. 
     It may be noted that the above-described apparatus and methods of automated wafer-grinding using grinding surface position monitoring may be used to accurately grind a variety of semiconductor components and materials, and not just the silicon wafer materials specifically described above. The inventive apparatus and methods disclosed herein may be applied to automated grinding processes for grinding a variety of materials and components in which accurate control of material thickness is desired, such as other semiconductor substrates, metallic layers, insulative layers and the like. Furthermore, embodiments of the invention are not limited to grinding devices having rotatable grinding surfaces, but may with equal success have other grinding surface motion, including reciprocating grinding surfaces such as those disclosed, for example, in U.S. Pat. No. 5,643,059 issued to Chen, and U.S. Pat. No. 3,643,045 issued to Beck, which patents are incorporated herein by reference. Therefore, the apparatus and methods disclosed herein should not be limited to the particular embodiments or to the particular application of grinding silicon wafers described above. 
     The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part to create additional embodiments within the scope and teachings of the invention. 
     Thus, although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings provided herein can be applied to other apparatus and methods of directed to apparatus and methods of automated wafer-grinding using grinding surface position monitoring, and not just to the embodiments described above and shown in the accompanying figures. Accordingly, the scope of the invention should be determined from the following claims.