Patent Publication Number: US-2023147310-A1

Title: Wafer detecting apparatus and wafer delivery apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wafer detecting apparatus for detecting whether or not there is a wafer in a cassette that has opposite side-walls in which a plurality of pairs of horizontal slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other, a wafer delivery apparatus for delivering a wafer from the cassette and delivering a wafer into the cassette, a method of determining whether or not there is an irregularity concerning a wafer stored in the cassette, a program for enabling a computer to carry out the method, and a non-transitory recording medium that records the program therein. 
     Description of the Related Art 
     Device chips including integrated circuits (ICs), large-scale-integration (LSI) circuits, etc., are indispensable components in electronic equipment, typically mobile phones and personal computers. Such device chips are produced by demarcating a plurality of areas on a face side of a wafer with projected dicing lines such that each area includes a device, and dividing the wafer along the projected dicing lines. 
     Before being divided into individual device chips, wafers with devices formed thereon are often thinned down for the purpose of producing smaller and lighter device chips. According to one known method of thinning down wafers, a wafer is ground by a grinding apparatus that has a chuck table for holding a face side, i.e., a lower surface, of the wafer under suction thereon and a grinding wheel being disposed above the chuck table and having a circular array of grindstones mounted at circumferentially spaced intervals on a lower surface of the grinding wheel. 
     In order to grind wafers on a grinding apparatus, it is necessary to perform a prerequisite step for introducing those wafers into the grinding apparatus. For example, according to the general chip fabrication procedure, one lot of wafers, e.g., approximately 25 wafers, are stored in a cassette, and the cassette is then placed onto a cassette table in the grinding apparatus. 
     The cassette generally has opposite side-walls in which pairs of slots, e.g., 25 slots, are defined at corresponding heights in confronting relation to each other, the pairs of slots being vertically spaced from one another. The wafers are retained by the respective pairs of slots. The grinding apparatus generally has a wafer detecting apparatus having a non-contact-type sensor and a wafer delivery apparatus having a robot hand for holding one at a time the wafers, in the vicinity of the cassette table. 
     The wafer detecting apparatus is combined with a mechanism for opening a lid on a front side of the cassette, for example. The wafer detecting apparatus detects the height of a pair of slots that are retaining a wafer, with the non-contact-type sensor. The wafer delivery apparatus is provided as one of the components of the grinding apparatus, for example. Using the robot hand, the wafer delivery apparatus takes a wafer to be ground from the cassette and stores a ground wafer back into the cassette (see, for example, JP 2001-284303A). 
     When a wafer is to be removed from the cassette placed on the cassette table of the grinding apparatus, the robot hand of the wafer delivery apparatus is inserted into the cassette while referring to the height of the pair of slots as detected by the non-contact-type sensor of the wafer detecting apparatus. The robot hand thus positionally controlled makes it possible to increase the efficiency with which to carry the wafer out of the cassette. 
     SUMMARY OF THE INVENTION 
     However, the wafer retained by a pair of slots defined in the opposite side-walls of the cassette tends to flex downwardly. In particular, if a wafer is thin or has residual stresses left therein due to devices formed thereon, the wafer is often liable to warp. When a warped wafer is stored in the cassette, the wafer is likely to flex largely downwardly. 
     Specifically, when a wafer stored in the cassette flexes downwardly, the height of a portion of the wafer that is remote from the side-walls of the cassette, i.e., a central portion of the wafer, is likely to differ largely from the height of portions of the wafer that are close to the side-walls of the cassette, i.e., opposite end portions of the wafer. If the difference between these heights exceeds an allowable range or tolerance, when the robot hand of the wafer delivery apparatus is inserted into the cassette, it is highly probable that the robot hand will collide with the central portion of the wafer, damaging the wafer. 
     It is therefore an object of the present invention to prevent wafers from being damaged when they are taken out of a cassette. 
     In accordance with an aspect of the present invention, there is provided a wafer detecting apparatus for detecting whether or not there is a wafer in a cassette that has opposite side-walls in which a plurality of pairs of slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other. The wafer detecting apparatus includes a detecting unit having a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer in the cassette that has an open front end, a lifting and lowering unit for selectively lifting and lowering the detecting unit vertically, an output unit for outputting information regarding the wafer stored in the cassette, a controller for controlling operation of the detecting unit, the lifting and lowering unit, and the output unit, and an input unit for entering information regarding the wafer stored in the cassette into the controller. The controller includes a storage section for storing a value of an actual thickness of the wafer stored in the cassette, the value being entered from the input unit, and a threshold value representing an allowable range for a difference between a value of an apparent thickness of the wafer and the value of the actual thickness of the wafer, a determining unit for determining whether or not the difference between the value of the apparent thickness of the wafer and the value of the actual thickness of the wafer that has been stored in the storage section exceeds the threshold value, the value of the apparent thickness of the wafer being obtained by operating, while actuating the lifting and lowering unit to selectively lift and lower the detecting unit, the non-contact-type sensor to detect whether or not there is the wafer in the cassette, and a warning section for operating the output unit to issue a warning about an irregularity concerning the wafer stored in the cassette, if the determining section determines that the difference exceeds the threshold value. 
     In the wafer detecting apparatus according to the aspect of the present invention, the non-contact-type sensor should preferably be a light-transmissive sensor that includes a light emitter for emitting light and a light detector for detecting the light emitted from the light emitter. 
     In accordance with another aspect of the present invention, there is provided a wafer delivery apparatus for taking a wafer out of and into a cassette that has opposite side-walls in which a plurality of pairs of slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other. The wafer delivery apparatus includes a robot hand for holding the wafer, the robot hand having a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer in the cassette that has an open front end, a lifting and lowering unit for selectively lifting and lowering the robot hand vertically, an output unit for outputting information regarding the wafer stored in the cassette, a controller for controlling operation of the robot hand, the lifting and lowering unit, and the output unit, and an input unit for entering information regarding the wafer stored in the cassette into the controller. The controller includes a storage section for storing a value of an actual thickness of the wafer stored in the cassette, the value being entered from the input unit, and a threshold value representing an allowable range for a difference between a value of an apparent thickness of the wafer and the value of the actual thickness of the wafer, a determining unit for determining whether or not the difference between the value of the apparent thickness of the wafer and the value of the actual thickness of the wafer that has been stored in the storage section exceeds the threshold value, the value of the apparent thickness of the wafer being obtained by operating, while actuating the lifting and lowering unit to selectively lift and lower the robot hand, the non-contact-type sensor to detect whether or not there is the wafer in the cassette, and a warning section for operating the output unit to issue a warning about an irregularity concerning the wafer stored in the cassette, if the determining section determines that the difference exceeds the threshold value. 
     In the wafer delivery apparatus according to the other aspect of the present invention, the non-contact-type sensor should preferably be a light-transmissive sensor that includes a light emitter for emitting light and a light detector for detecting the light emitted from the light emitter. 
     In the wafer delivery apparatus according to the other aspect of the present invention, the robot hand should preferably be a robot hand for holding the wafer under suction forces generated by a suction source, or a robot hand for holding the wafer under suction forces produced by gas supplied from a gas supply source, on the basis the Bernoulli effect. 
     In accordance with a further aspect of the present invention, there is provided a method of determining whether or not there is an irregularity concerning a wafer stored in a cassette that has opposite side-walls in which a plurality of pairs of slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other. The method includes a storing step of storing a value of an actual thickness of the wafer stored in the cassette and a threshold value representing an allowable range for a difference between a value of an apparent thickness of the wafer and the value of the actual thickness of the wafer, a determining step of determining whether or not the difference between the value of the apparent thickness of the wafer and the value of the actual thickness of the wafer that has been stored in the storing step exceeds the threshold value, the value of the apparent thickness of the wafer being obtained by operating, while selectively lifting and lowering a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer in the cassette that has an open front end, the non-contact-type sensor to detect whether or not there is the wafer in the cassette, and a warning step of issuing a warning about an irregularity concerning the wafer stored in the cassette, if the determining step determines that the difference exceeds the threshold value. 
     In accordance with a still further aspect of the present invention, there is provided a program for a computer that includes an output unit for outputting information regarding a wafer stored in a cassette, a controller for controlling operation of the output unit, and an input unit for entering information regarding the wafer stored in the cassette into the controller, the program causing the computer to carry out a method of determining whether or not there is an irregularity concerning the wafer stored in the cassette that has opposite side-walls in which a plurality of pairs of slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other. The method includes a storing step of storing, in a storage section of the controller, a value of an actual thickness of the wafer stored in the cassette, the value being entered from the input unit, and a threshold value representing an allowable range for a difference between a value of an apparent thickness of the wafer and the value of the actual thickness of the wafer, a determining step of determining, by a central processing unit of the controller, whether or not the difference between the value of the apparent thickness of the wafer and the value of the actual thickness of the wafer that has been stored in the storage section exceeds the threshold value, the value of the apparent thickness of the wafer being obtained by operating, while selectively lifting and lowering a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer in the cassette that has an open front end, the non-contact-type sensor to detect whether or not there is the wafer in the cassette, and a warning step of operating, by the central processing unit, the output unit to issue a warning about an irregularity concerning the wafer stored in the cassette, if the central processing unit determines that the difference exceeds the threshold value. 
     In accordance with a yet still further aspect of the present invention, there is provided a non-transitory recording medium storing therein a program for a computer that includes an output unit for outputting information regarding a wafer stored in a cassette, a controller for controlling operation of the output unit, and an input unit for entering information regarding the wafer stored in the cassette into the controller, the program causing the computer to carry out a method of determining whether or not there is an irregularity concerning the wafer stored in the cassette that has opposite side-walls in which a plurality of pairs of slots vertically spaced from one another are defined and that is capable of storing wafers in such a manner that each of the wafers is retained by a corresponding one of the pairs of slots, the pair of slots being positioned at the same height in confronting relation to each other. The method includes a storing step of storing, in a storage section of the controller, a value of an actual thickness of the wafer stored in the cassette, the value being entered from the input unit, and a threshold value representing an allowable range for a difference between a value of an apparent thickness of the wafer and the value of the actual thickness of the wafer, a determining step of determining, by a central processing unit of the controller, whether or not the difference between the value of the apparent thickness of the wafer and the value of the actual thickness of the wafer that has been stored in the storage section exceeds the threshold value, the value of the apparent thickness of the wafer being obtained by operating, while selectively lifting and lowering a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer in the cassette that has an open front end, the non-contact-type sensor to detect whether or not there is the wafer in the cassette, and a warning step of operating, by the central processing unit, the output unit to issue a warning about an irregularity concerning the wafer stored in the cassette, if the central processing unit determines that the difference exceeds the threshold value. 
     According to the present invention, a warning is issued about an irregularity concerning a wafer stored in the cassette if the difference between the value of an apparent thickness of the wafer and the previously obtained value of an actual thickness of the wafer exceeds a threshold value, the value of the apparent thickness of the wafer being obtained by a non-contact-type sensor for detecting a front portion of the wafer laterally of the wafer. Consequently, the wafer is prevented from being damaged when it is taken out of the cassette. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view schematically illustrating a wafer detecting apparatus by way of example; 
         FIG.  2    is a fragmentary front elevational view schematically illustrating the manner in which the wafer detecting apparatus obtains the values of apparent thicknesses of wafers; 
         FIG.  3    is a block diagram schematically illustrating hardware components included in a computer; 
         FIG.  4    is a block diagram schematically illustrating functional sections included in a controller; 
         FIG.  5    is a flowchart schematically illustrating by way of example a sequence of a method of determining whether or not there is an irregularity concerning a wafer stored in a cassette; 
         FIG.  6    is a perspective view schematically illustrating a wafer delivery apparatus by way of example; 
         FIG.  7 A  is a fragmentary front elevational view schematically illustrating the manner in which the wafer detecting apparatus obtains the values of apparent thicknesses of wafers; and 
         FIG.  7 B  is a fragmentary front elevational view schematically illustrating the manner in which the wafer detecting apparatus obtains the values of apparent thicknesses of wafers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG.  1    schematically illustrates in perspective a wafer detecting apparatus for detecting whether or not there is a wafer in a cassette for storing wafers therein.  FIG.  1    also schematically illustrates by way of example a wafer to be detected by the wafer detecting apparatus and the cassette for storing wafers therein. The wafer and the cassette will first be described below, and then, the wafer detecting apparatus will be described below. 
     The wafer, denoted by  1  in  FIG.  1   , is of a circular shape and may be made of a semiconductor material such as a silicon (Si). The wafer  1  has a face side on which a plurality of areas are demarcated by a plurality of intersecting projected dicing lines, and a plurality of devices such as ICs or LSI circuits are formed in the respective areas. The wafer  1  is not limited to any particular materials, shapes, structures, sizes, etc. The wafer  1  may be a substrate made of, for example, any of other semiconductor materials, ceramic, resin, metal, or the like. 
     Similarly, the devices on the wafer  1  are not limited to any particular kinds, numbers, shapes, structures, sizes, layouts, etc. A film-like tape having substantially the same diameter as the wafer  1  may be affixed to the face side of the wafer  1 . The tape is made of resin, for example, and protects the devices on the wafer  1  from shocks imposed on the face side of the wafer  1  when a reverse side of the wafer  1  is ground. 
     The cassette, denoted by  2  in  FIG.  1   , has a flat rectangular upper plate  2   a.  The upper plate  2   a  is of such a shape that a pair of adjacent two of its four corners are beveled and that the remaining pair of two corners are left unbeveled. The cassette  2  also has a rear-wall, not illustrated, extending vertically and perpendicularly to the upper plate  2   a  and having an upper end fixed to the lower side of a rear end of the upper plate  2   a  that is positioned between the two beveled edges. 
     The cassette  2  further includes two side-walls  2   b  and  2   c  extending vertically and perpendicularly to the upper plate  2   a  and having respective upper ends fixed to the lower sides of two ends of the upper plate  2   a  that are positioned between the beveled edges and the unbeveled corners. The cassette  2  is free of a front wall on the lower side of a front end of the upper plate  2   a  that is positioned between the unbeveled corners. In other words, the cassette  2  has an open front end. 
     Each of the side-walls  2   b  and  2   c  has a plurality of horizontal slots  2   d  defined in an inner side surface thereof and vertically spaced from one another. Specifically, the slots  2   d  defined in the inner side surface of the side-wall  2   b  horizontally confront the respective slots  2   d  defined in the inner side surface of the side-wall  2   c  at corresponding heights. The wafer  1  can be retained in the cassette  2  by a pair of horizontally confronting slots  2   d  at a corresponding height. 
     A joint bar  2   e  interconnecting the side-walls  2   b  and  2   c  extend between a central portion of the lower end of the inner side surface of the side-wall  2   b  and a central portion of the lower end of the inner side surface of the side-wall  2   c.  The joint bar  2   e  has a width, as measured along the slots  2   d,  that is smaller than the corresponding width of the side-walls  2   b  and  2   c.  The cassette  2  is free of any other structures than the joint bar  2   e  between the lower ends of the side-walls  2   b  and  2   c,  so that the cassette  2  is open in its lower end in front of and behind the joint bar  2   e.    
     There is no limitation on the number of the slots  2   d  defined in the inner side surfaces of the side-walls  2   b  and  2   c.  For example, each of the side-walls  2   b  and  2   c  may have as many slots  2   d  as the number of one lot of wafers  1 , e.g., approximately  25  wafers  1 . The cassette  2  may be in the form of a cassette having a front lid that can be closed to tightly seal an inner space thereof, known as a front-opening unified pod (FOUP). 
     The wafer detecting apparatus  4  illustrated in  FIG.  1    has a U-shaped detecting unit  6 . The detecting unit  6  includes a front connector  6   a  shaped as a square pole and extending laterally and a pair of arms  6   b  and  6   c  shaped as a square pole and extending rearwardly from the respective ends of the front connector  6   a.  The detecting unit  6  also has a light emitter  8   a  and a light detector  8   b  mounted on the distal ends, i.e., rear ends, of the arms  6   b  and  6   c,  respectively. The light emitter  8   a  and the light detector  8   b  jointly make up a light-transmissive sensor, i.e., a non-contact-type sensor,  8 . 
     The light emitter  8   a  has a light emission surface facing the light detector  8   b,  and emits light from the light emission surface toward the light detector  8   b.  The light detector  8   b  has a light detection surface facing the light emitter  8   a,  and detects light emitted from the light emitter  8   a  with the light detection surface. The light emitted from the light emitter  8   a  is of a wavelength that is not transmittable through the wafer  1 . 
     Therefore, when the wafer  1  is present between the light emitter  8   a  and the light detector  8   b,  the light emitted from the light emitter  8   a  does not reach the light detector  8   b  and hence is not detected by the light detector  8   b.  The non-contact-type sensor  8  detects whether or not the wafer  1  is present between the light emitter  8   a  and the light detector  8   b,  on the basis of whether or not the light detector  8   b  detects light while the light emitter  8   a  is emitting light toward the light detector  8   b.    
     The detecting unit  6  is coupled to a lifting and lowering unit  10  that selectively lifts and lowers the detecting unit  6  vertically. The lifting and lowering unit  10  has an air cylinder including a piston rod  10   a  whose upper end is fixed to the lower side of a central portion of the front connector  6   a.  When the air cylinder is actuated, the piston rod  10   a  is lifted or lowered, lifting or lowering the detecting unit  6 . 
     The distance between the light emitter  8   a  and the light detector  8   b  of the non-contact-type sensor  8  is smaller than the distance between the inner side surface of the side-wall  2   b  and the inner side surface of the side-wall  2   c  of the cassette  2 . The light emitter  8   a  and the light detector  8   b  are disposed one on each side of a front portion of the wafer  1  in the cassette  2  as viewed in plan. 
     Consequently, while the lifting and lowering unit  10  is operated to lift or lower the detecting unit  6 , the non-contact-type sensor  8  is energized to detect the front portion of the wafer  1  in the cassette  2  laterally of the wafer  1 , so that the position, i.e., height, of the wafer  1  can be obtained. 
     Further, the wafer detecting apparatus  4  is able to obtain the apparent thickness of the wafer  1  when the non-contact-type sensor  8  operates as described above.  FIG.  2    schematically illustrates in fragmentary front elevation the manner in which the wafer detecting apparatus  4  obtains the apparent thicknesses of wafers  1 . As illustrated in  FIG.  2   , the cassette  2  stores a wafer  1   a  that is not flexing, a wafer  1   b  that is slightly flexing, and a wafer  1   c  that is largely flexing, in such a manner that these wafers are arrayed successively upwardly in the order named. 
     The light emitter  8   a  and the light detector  8   b  of the non-contact-type sensor  8  are positioned below the wafers  1   a,    1   b,  and  1   c,  and are disposed one on each side of front portions of these wafers in the cassette  2  as viewed in plan. 
     When the light emitter  8   a  emits light L toward the light detector  8   b  while the non-contact-type sensor  8  is being lifted, the light detector  8   b  does not detect the light L if any one of the wafers  1   a,    1   b,  and  1   c  is present between a pair of broken lines DL illustrated in  FIG.  2   . 
     In this case, therefore, intervals I 1 , I 2 , and I 3  between the upper and lower ends of the wafers  1   a,    1   b,  and  1   c  that are present between the broken lines DL can be obtained as the respective values of the apparent thicknesses of the wafers  1   a,    1   b,  and  1   c.  The larger the flexing of the wafers  1   a,    1   b,  and  1   c  is, the larger the apparent thicknesses of the wafers  1   a,    1   b,  and  1   c  are. 
     The wafer detecting apparatus  4  further includes a computer.  FIG.  3    schematically illustrates in block form hardware components included in the computer. The computer, denoted by  12  in  FIG.  3   , includes an output unit  16 , a controller  18 , an input unit  20 , and a communication interface  22  that are interconnected by a bus  14  in such a manner as to send signals to and receive signals from each other. 
     The output unit  16  outputs information, etc., regarding the wafers  1  stored in the cassette  2 . The output unit  16  includes at least one of a display, a printer, a speaker, and a warning lamp, i.e., a pilot lamp. The output unit  16  is used, for example, to issue a warning about an irregularity concerning one or more of the wafers  1  in the cassette  2 . 
     The controller  18  controls the above components of the wafer detecting apparatus  4 , i.e., the non-contact-type sensor  8 , the lifting and lowering unit  10 , and the output unit  16 . The controller  18  includes a central processing unit (CPU)  24  and a storage device  26 . The storage device  26  includes a main storage unit  26   a  having a high rewriting speed and an auxiliary storage unit  26   b  having a large storage capacity. 
     The main storage unit  26   a  stores data that is frequently rewritten by the CPU  24 . The auxiliary storage unit  26   b  stores various programs for enabling the CPU  24  to perform certain processing operations, and also stores data, etc., to be used in the processing operations performed by the CPU  24 . For example, the auxiliary storage unit  26   b  stores data to be used in determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2 . 
     The step of determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2  is carried out by comparing the values of the apparent thicknesses of the wafers  1  that are obtained as described above and the values of actual thicknesses of the wafers  1  that are measured by a known measuring instrument or the like. The data to be used in the determining step includes the values of actual thicknesses of the wafers  1  and a threshold value representing an allowable range or tolerance for the differences between the values of the apparent thicknesses of the wafers  1  and the values of actual thicknesses of the wafers  1 , for example. 
     The CPU  24  reads a program from the auxiliary storage unit  26   b  and executes the read program. For example, the CPU  24  reads, from the auxiliary storage unit  26   b,  a program for determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2 , and issuing a warning if there is a wafer irregularity, and executes the read program. 
     For example, the CPU  24  determines whether or not the difference between the value of the apparent thickness of a wafer  1  in the cassette  2  and the value of the actual thickness of the wafer  1  exceeds the threshold value. If the difference exceeds the threshold value, then the CPU  24  controls the output unit  16  to issue a warning about an irregularity concerning the wafer  1 . 
     The main storage unit  26   a  includes a volatile memory such as a dynamic random access memory (DRAM) or a static random access memory (SRAM), for example. The auxiliary storage unit  26   b  includes a non-volatile memory such as a solid state drive (SSD), i.e., a not-and (NAND) flash memory, or a hard disk drive (HDD), i.e., a magnetic storage unit, for example. 
     The input unit  20  enters information regarding the wafers  1  stored in the cassette  2  into the controller  18 . The input unit  20  includes at least one of a keyboard, a mouse, a touch pad, and a microphone, and enters data to be used in determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2 , into the controller  18 , for example. 
     The communication interface  22  includes at least one of communication circuits, i.e., an analog-to-digital conversion circuit (ADC) and a digital-to-analog conversion circuit (DAC), etc., input/output (I/O) ports, etc. The communication interface  22  is used to enter programs recorded in a non-transitory recording medium such as a universal serial bus (USB) flash drive into the controller  18 , for example. 
     The non-transitory recording medium stores therein a program for determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2 , and issuing a warning if there is a wafer irregularity, for example. 
       FIG.  4    schematically illustrates in block form functional sections included in the controller  18 .  FIG.  4    also illustrates components of the wafer detecting apparatus  4  that are associated with the functional sections of the controller  18 . As illustrated in  FIG.  4   , the controller  18  has a storage section  18   a,  a determining section  18   b,  and a warning section  18   c.    
     The storage section  18   a  stores data that is entered from the input unit  20  and that is to be used in determining whether or not there is an irregularity concerning the wafers  1  stored in the cassette  2 . For example, providing the input unit  20  includes a keyboard, the storage section  18   a  stores data entered from the keyboard when it is operated by an operator. 
     In order to obtain the values of the apparent thicknesses of the wafers  1 , while actuating the lifting and lowering unit  10  to lift or lower the detecting unit  6 , the determining section  18   b  energizes the non-contact-type sensor  8  to detect whether or not there is a wafer  1 . Further, the determining section  18   b  determines whether or not the differences between the values of the apparent thicknesses of the wafers  1 , which are obtained by actuating the lifting and lowering unit  10  and energizing the non-contact-type sensor  8 , and the values of the actual thicknesses of the wafers  1 , which are stored in the storage section  18   a,  exceed the threshold value stored in the storage section  18   a.    
     If the determining section  18   b  determines that the differences between the values of the apparent thicknesses of the wafers  1  and the values of the actual thicknesses of the wafers  1  exceed the threshold value, then the warning section  18   c  operates the output unit  16  to issue a warning about an irregularity concerning a wafer or wafers  1  stored in the cassette  2 . Providing the output unit  16  includes a display and a warning lamp, the warning section  18   c  displays a message indicating the wafer irregularity on the display and also turns on or flashes on and off the warning lamp. 
       FIG.  5    schematically illustrates by way of example a sequence of a method of determining whether or not there is an irregularity concerning a wafer  1  stored in the cassette  2 , using the wafer detecting apparatus  4 . According to the method, the value of the actual thickness of the wafer  1  and the threshold value representing an allowable range or tolerance for the difference between the value of the apparent thickness of the wafer  1  and the value of the actual thickness of the wafer  1  are stored (storing step S 1 ). 
     Specifically, the operator operates the input unit  20  to enter, into the controller  18 , the value of the actual thickness of the wafer  1  that is measured by the known measuring instrument or the like and the threshold value that is established in view of the manner in which the wafer  1  is taken out of the cassette  2 . When the value of the actual thickness of the wafer  1  and the threshold value are entered into the controller  18 , the CPU  24  stores these entered values into the auxiliary storage unit  26   b  of the storage device  26 . In other words, the storage section  18   a  of the controller  18  stores the value of the actual thickness of the wafer  1  and the threshold value therein. 
     Then, while the non-contact-type sensor  8  is being lifted or lowered, the non-contact-type sensor  8  is energized to detect whether or not there is a wafer  1  in the cassette  2  (detecting step S 2 ). Specifically, the CPU  24 , i.e., the determining section  18   b,  actuates the lifting and lowering unit  10  to lift or lower the detecting unit  6 , while at the same time energizing the non-contact-type sensor  8  to detect whether or not there is a wafer  1  in the cassette  2 . 
     Therefore, the CPU  24 , i.e., the determining section  18   b,  obtains the value of the apparent thickness of the wafer  1 . Then, by referring to the value of the apparent thickness of the wafer  1  and the value of the actual thickness of the wafer  1  and the threshold value that are stored in the auxiliary storage unit  26   b,  i.e., the storage section  18   a,  the CPU  24 , i.e., the determining section  18   b,  determines whether or not the difference between the value of the apparent thickness of the wafer  1  and the value of the actual thickness of the wafer  1  exceeds the threshold value (determining step S 3 ). 
     If the difference is determined as exceeding the threshold value in determining step S 3  (S 3 : YES), then a warning is issued about an irregularity concerning the wafer  1  stored in the cassette  2  (warning step S 4 ). Specifically, the CPU  24 , i.e., the warning section  18   c,  operates the output unit  16  to issue a warning about an irregularity concerning the wafer  1  stored in the cassette  2 . 
     As described above, the wafer detecting apparatus  4  is able to issue a warning about an irregularity concerning the wafer  1  stored in the cassette  2 , if the difference between the value of the apparent thickness of the wafer  1 , which is obtained by using the non-contact-type sensor  8  that detects the front portion of the wafer  1  laterally of the wafer  1 , and the previously obtained value of the actual thickness of the wafer  1  is in excess of the threshold value. Consequently, the wafer  1  is prevented from being damaged when it is taken out of the cassette  2 . 
       FIG.  6    schematically illustrates in perspective by way of example a wafer delivery apparatus for removing a wafer  1  from the cassette  2  and bringing a wafer  1  into the cassette  2 . In  FIG.  6   , the wafer  1  and the cassette  2  are omitted from illustration for the sake of convenience. 
     The wafer delivery apparatus, denoted by  28  in  FIG.  6   , has a first actuating unit  30  shaped as a cylindrical post extending vertically. The first actuating unit  30  includes a lifting and lowering unit, not illustrated, and a rotary actuator, not illustrated. The lifting and lowering unit has an air cylinder including a piston rod extending through an opening defined in an upper wall of the first actuating unit  30 . When actuated, the lifting and lowering unit selectively lifts and lowers a robot hand  34 , etc., to be described later. 
     The rotary actuator rotates the piston rod about a rotational axis that extends vertically. The piston rod has an upper end coupled to a delivery arm assembly  32 . The delivery arm assembly  32  is a robot arm assembly having a plurality of joints. 
     Specifically, the delivery arm assembly  32  includes a plate-shaped first arm  32   a  extending horizontally. The first arm  32   a  has an end whose lower side is coupled to an upper end of the piston rod for movement and rotation in unison with the piston rod. The first arm  32   a  has an opposite end whose upper side is coupled to a lower end of a first joint, not illustrated, shaped as a cylindrical post extending vertically. 
     The first joint has an upper end coupled to a plate-shaped second arm  32   b  extending horizontally. The second arm  32   b  has an end whose lower side is coupled to an upper side of the opposite end of the first arm  32   a  by the first joint in such a manner that the second arm  32   b  rotates about a vertically extending rotational axis. The second arm  32   b  has an opposite end whose upper side is coupled to a lower end of a second joint  32   c  shaped as a cylindrical post extending vertically. 
     The second joint  32   c  has an upper end coupled to a second actuating unit  32   d  shaped as a rectangular parallelepiped extending horizontally. The second actuating unit  32   d  has an end whose lower side is coupled to the upper side of the opposite end of the second arm  32   b  by the second joint  32   c  in such a manner that the second actuating unit  32   d  rotates about a vertically extending rotational axis. 
     The second actuating unit  32   d  includes an electric motor, not illustrated, for rotating a spindle  32   e  about a horizontal rotational axis. The spindle  32   e  extends through an opening defined in a side-wall at an opposite end of the second actuating unit  32   d  and has a distal end portion exposed out of the second actuating unit  32   d.  The distal end portion of the spindle  32   e  is coupled to a proximal end, which is shaped as a rectangular parallelepiped, of the robot hand  34  by a plate-shaped joint member  32   f.    
     The robot hand  34  has a holding plate  34   a  that is shaped as a circular plate and that is integrally joined to the proximal end of the robot hand  34 . The holding plate  34   a  has a number of suction ports  34   b  defined in an upper surface thereof acting as a holding surface of the robot hand  34 . The suction ports  34   b  are held in fluid communication with a suction source, not illustrated, such as an ejector through a fluid channel and a fluid control valve that are housed in the robot hand  34 , for example. 
     When the suction source is actuated while the fluid control valve is being open, the suction source generates a negative pressure that is transmitted as suction forces to a space in the robot hand  34  near the suction ports  34   b.  The wafer delivery apparatus  28  can hold a wafer  1  on the holding surface of the robot hand  34  under the suction forces acting through the suction ports  34   b  on the wafer  1  when the suction source is in operation. 
     The robot hand  34  may be replaced with a robot hand that is capable of holding a wafer  1  out of contact therewith. The alternative robot hand is able to hold a wafer under suction forces produced by gas supplied from a gas supply source, on the basis the Bernoulli effect. 
     Specifically, the robot hand based on the Bernoulli effect has a groove that is defined in a surface thereof and that is supplied with the gas from the gas supply source. When the gas supplied from the gas supply source flows in the groove, it generates a negative pressure in a space near the groove, holding a wafer  1  under suction out of contact with the robot hand. 
     As illustrated in  FIG.  6   , the robot hand  34  supports a pair of arms  36   a  and  36   b  each shaped as a square pole extending horizontally, on a reverse side thereof that is opposite the holding surface of the robot hand  34 . A wafer detecting apparatus includes a light emitter  38   a  and a light detector  38   b  that are mounted on the respective distal ends of the arms  36   a  and  36   b.  The light emitter  38   a  and the light detector  38   b  jointly make up a light-transmissive sensor, i.e., a non-contact-type sensor,  38 . 
     Since the light emitter  38   a  and the light detector  38   b  of the non-contact-type sensor  38  are identical respectively to the light emitter  8   a  and the light detector  8   b  of the non-contact-type sensor  8  illustrated in  FIG.  1   , they will not be described in detail below. The wafer delivery apparatus  28  includes a computer, not illustrated. The computer of the wafer delivery apparatus  28  includes the same hardware components as those illustrated in  FIG.  3   , and includes a controller having the same functional sections as those illustrated in  FIG.  4   . Therefore, those functional sections will be omitted from description below. 
     The wafer delivery apparatus  28  can operate according to the method illustrated in  FIG.  5   . Specifically, the wafer delivery apparatus  28  can issue a warning about an irregularity concerning a wafer  1  stored in the cassette  2 , if the difference between the value of an apparent thickness of the wafer  1 , which is obtained by using the non-contact-type sensor  38  that detects the front portion of the wafer  1  laterally of the wafer  1 , and the previously obtained value of an actual thickness of the wafer  1  is in excess of a threshold value. Consequently, the wafer  1  is prevented from being damaged when it is taken out of the cassette  2 . 
     The wafer delivery apparatus  28  can also tilt the non-contact-type sensor  38 , i.e., can change the direction of light emitted from the light emitter  38   a  toward the light detector  38   b,  by turning the spindle  32   e  about its horizontal rotational axis. The wafer delivery apparatus  28  is capable of recognizing more specifically an irregularity concerning wafers  1  stored in the cassette  2 , by obtaining the values of apparent thicknesses of the wafers  1  with the non-contact-type sensor  38  that is selectively set in differently tilted states. 
       FIGS.  7 A and  7 B  schematically illustrate in plan the manner in which the wafer detecting apparatus incorporated in the wafer delivery apparatus  28  obtains the values of apparent thicknesses of wafers  1 . As illustrated in  FIGS.  7 A and  7 B , the cassette  2  stores therein a wafer  1   a  that is not flexing, a wafer  1   d  that is not flexing but skewed, i.e., that is retained by a pair of slots  2   d  at different heights, and a wafer  1   c  that is largely flexing, in such a manner that these wafers are arrayed successively upwardly in the order named. 
     The light emitter  38   a  and the light detector  38   b  of the non-contact-type sensor  38  illustrated in  FIGS.  7 A and  7 B  are positioned below the wafers  1   a,    1   d,  and  1   c,  and are disposed one on each side of front portions of these wafers in the cassette  2  as viewed in plan. 
     In  FIG.  7 A , the non-contact-type sensor  38  is not tilted with respect to horizontal directions, i.e., is aligned with horizontal directions. Therefore, light emitted from the light emitter  38   a  toward the light detector  38   b  travels in a direction parallel to the horizontal directions. In  FIG.  7 B , the non-contact-type sensor  38  is tilted with respect to horizontal directions, i.e., is not aligned with horizontal directions. Therefore, light emitted from the light emitter  38   a  toward the light detector  38   b  travels in a direction not parallel to the horizontal directions. 
     When the light emitter  38   a  emits light L toward the light detector  38   b  while the non-contact-type sensor  38  is being lifted, the light detector  38   b  does not detect the light L if any one of the wafers  1   a,    1   d,  and  1   c  is present between a pair of broken lines DL 1  illustrated in  FIG.  7 A  or between a pair of broken lines DL 2  illustrated in  7 B. 
     In this case, therefore, intervals I 11 , I 12 , and I 13  between the upper and lower ends of the wafers  1   a,    1   d,  and  1   c  that are present between the pair of broken lines DL 1  and intervals I 21 , I 22 , and I 23  between the upper and lower ends of the wafers  1   a,    1   d,  and  1   c  that are present between the pair of broken lines DL 2  can be obtained as the values of apparent thicknesses of the wafers  1   a,    1   d,  and  1   c.    
     Specifically, the values of the apparent thicknesses of the wafers  1   a  and  1   c  that are retained by the pairs of slots  2   d  positioned at the corresponding heights are increased by tilting the non-contact-type sensor  38 . On the other hand, the value of the apparent thickness of the wafer  1   d  retained by the pair of slots  2   d  at different heights is reduced by tilting the non-contact-type sensor  38 . 
     Stated otherwise, it can be recognized whether or not the wafer  1   d  is unduly skewed in the cassette  2 , on the basis of whether or not the value of the apparent thickness of the wafer  1   d  increases when the tilt of the non-contact-type sensor  38  increases. The value of the apparent thickness of the wafer  1   d  may be obtained while the non-contact-type sensor  38  is tilted such that the light emitter  38   a  is higher than the light detector  38   b  as illustrated in  FIG.  7 B  or while the non-contact-type sensor  38  is tilted such that the light emitter  38   a  is lower than the light detector  38   b.    
     Alternatively, the value of the apparent thickness of the wafer  1   d  may be obtained while the non-contact-type sensor  38  is in one of the three positional states: the non-contact-type sensor  38  is in the state as illustrated in FIF.  7 A, i.e., the non-contact-type sensor  38  is aligned with the horizontal directions; the non-contact-type sensor  38  is in the state as illustrated in  FIG.  7 B , i.e., the non-contact-type sensor  38  is tilted such that the light emitter  38   a  is higher than the light detector  38   b;  or the non-contact-type sensor  38  is tilted such that the light emitter  38   a  is lower than the light detector  38   b.    
     The structural details and method according to the above embodiment and modifications may be changed or modified without departing from the scope of the present invention. 
     The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.