the present invention has particularly beneficial utility in monitoring the z - axis position of a transfer blade on a wafer transfer robot in an integrated cluster tool used in the processing of semiconductors . however , the invention is not so limited in application , and while references may be made to such integrated cluster tools , the invention is more generally applicable to monitoring the vertical or z - axis position of objects in a variety of industrial and product applications . referring to fig3 - 5 , an integrated cluster tool in implementation of the present invention is generally indicated by reference numeral 36 in fig3 and typically includes a pair of loadlock chambers 38 each having a chamber interior 39 that houses a cassette stage 40 for receiving a wafer - supporting cassette ( not illustrated ). the chamber interior 39 is closed by a loadlock door 42 . the loadlock chambers 38 are located adjacent to a transfer chamber 44 closed by a removable transfer chamber lid 46 having multiple viewing ports 47 for viewing the interior of the transfer chamber 44 . a transfer robot 48 is contained in the transfer chamber 44 and is fitted with a transfer blade 50 ( fig4 ). the transfer robot 48 is operated to remove wafers ( not shown ) from the loadlock chambers 38 and among an orientation chamber ( not shown ), multiple process chambers ( not shown ) and a cooldown chamber ( not shown ) positioned around the transfer chamber 44 , and the transfer robot 48 places the wafers back in the loadlock chamber 38 after processing . the z - axis monitoring apparatus of the present invention is generally indicated by reference numeral 34 in the schematic of fig5 and includes a ccd sensor housing 52 ( fig4 ) which , as illustrated in fig3 is mounted directly above a viewing port 47 a in the transfer chamber lid 46 adjacent to the loadlock chamber 38 . as illustrated in fig4 the ccd sensor housing 52 contains a ccd ( charge - coupled device ) displacement sensor 58 , which may be a ccd sensor manufactured and sold by the keyence co ., ltd . a laser diode 54 is included in the ccd sensor housing 52 , and a pair of spaced - apart optical lenses 56 is positioned directly beneath the laser diode 54 . the laser diode 54 and optical lenses 56 are located along a common vertical axis 55 . a set of condenser lenses 57 , which may be three in number , is provided in adjacent relationship to the ccd displacement sensor 58 . the condenser lenses 57 and ccd displacement sensor 58 are located along a common reflection axis 59 that is disposed at an angle with respect to the vertical axis 55 . the vertical axis 55 indicates the path of laser light emitted from the laser diode 54 and through the optical lenses 56 and the viewing port 47 a ( fig3 ) in the transfer chamber lid 46 , to the transfer blade 50 , in application of the present invention as hereinafter further described . some of the laser light is reflected from the transfer blade 50 , back through the viewing port 47 a and through the condenser lenses 57 to the ccd displacement sensor 58 , respectively , along the path indicated by the reflection axis 59 . as illustrated in fig5 the ccd sensor 58 is connected to a ccd laser sensor controller 66 , which receives an operational current of typically 24 volts from a dc power supply 64 . the dc power supply 64 may receive 110 volts of ac current . a “ signal out ” port of the ccd laser sensor controller 66 is connected to a “ signal in ” port of an rd analog controller 60 , which receives an operational current of 24 volts from the dc power supply 64 . the analog controller 60 may include an lcd display 63 for displaying the height of the transfer blade 50 , as hereinafter further described . in one embodiment of the present invention , the analog controller 60 includes five signal ports 61 for purposes hereinafter described . the signal ports 61 are connected to a “ signal high ” port and a “ signal low ” port on an interface pcb 62 which receives an operational current of 24 volts from the dc power supply 64 . a “ signal out ” port on the interface pcb 62 is connected to a wafer sensor signal ( w / s ) port on an amat system controller 68 which controls the various components of the integrated cluster tool 36 , including the robot 48 . an electrical schematic for the interface pcb is shown in fig6 . as illustrated in fig3 the rd analog controller 60 and the interface pcb may be mounted on the top of the loadlock chamber 38 . referring next to fig4 and 5 , in typical application the z - axis monitoring apparatus 34 of the present invention is capable of monitoring the z - axis position of the transfer blade 50 inside the transfer chamber 44 of the integrated cluster tool 36 in order to prevent inaccurate insertion of the transfer blade 50 in a wafer cassette ( not shown ) contained in the loadlock chamber 38 and scratching wafers ( not shown ) upon removal of the wafers from the cassette . to this end , the ccd displacement sensor 58 is capable of detecting a z - axis distortion of the transfer blade 50 to within about 1 μm . in operation , the transfer robot 48 initially extends the transfer blade 50 toward the loadlock chamber 38 preparatory to removing a wafer from the cassette in the loadlock chamber 38 . as it moves toward the loadlock chamber 38 , the transfer blade 50 passes beneath the viewing port 47 a in the transfer chamber lid 46 . simultaneously , the laser diode 54 in the ccd sensor housing 52 emits a laser beam having a path indicated by the vertical axis 55 in fig4 and 4a , which laser beam passes through the vertical optical lenses 56 and through the viewing port 47 a , respectively , and strikes the transfer blade 50 . some of the laser light is reflected from the transfer blade 50 along the path indicated by the reflection axis 59 , and passes through the condenser lenses 57 which condense the reflected laser light onto the ccd displacement sensor 58 . the reflected laser light forms an image on the ccd displacement sensor 58 , and the image of the reflected light varies depending on the z - axis position of the transfer blade 50 . the ccd displacement sensor 58 may be calibrated to detect variations in the reflected image corresponding to z - axis positions of the transfer blade 50 throughout a range of typically about 10 . 0 mm . depending on the z - axis position of the transfer blade 50 , the ccd displacement sensor 58 generates an analog voltage signal typically in the range of from + 5 volts to − 5 volts . for example , an ideal z - axis position of the transfer blade 50 for wafer transfer may correspond to 0 volts , whereas a “ lo ” position of the transfer blade 50 , as illustrated in fig5 may correspond to − 3 volts and an “ ll ” ( low - low ) position of the transfer blade 50 may correspond to − 5 volts . conversely , a “ hi ” position of the transfer blade 50 may correspond to + 3 volts , and an “ hh ” ( high high ) position of the transfer blade 50 may correspond to + 5 volts . this analog voltage signal is transmitted from the ccd sensor 58 to the laser sensor controller 66 and from the “ signal out ” port of the laser sensor controller 66 to the “ signal in ” port of the analog controller 60 . the analog controller 60 converts the analog voltage signal to a z - axis position of the transfer blade 50 , typically in millimeters , and may display this z - axis position of the transfer blade 50 in millimeters on the lcd display 63 of the analog controller 60 . in the embodiment of the apparatus 34 in which the analog controller 60 is connected to the system controller 68 through the interface pcb 62 , the analog controller 60 sends the analog voltage signal to the appropriate signal port 61 . the signal ports 61 may range from “ high high ” ( hh ), “ high ” ( hi ), “ low ” ( lo ), or “ low low ” ( ll ). in the event that the transfer blade 50 is at the ideal height for the wafer transferring operation and no wafer is supported on the transfer blade 50 , the analog voltage signal sent from the laser sensor controller 66 to the analog controller 60 is 0 volts . in that case , no analog voltage signal is transmitted from the signal ports 61 on the analog controller 60 to the “ signal high ” port or the “ signal low ” port on the pcb interface 62 . the wafer sensor signal transmitted from the “ signal out ” port of the interface pcb 62 to the w / s port on the system controller 68 is 24 volts . accordingly , the transfer robot 48 continues the wafer - transfer operation as the transfer blade 50 is inserted in the chamber interior 39 of the loadlock chamber 38 prior to receiving a wafer ( not shown ) from a wafer cassette ( not shown ) in the chamber interior 39 . after the transfer blade 50 is positioned in the chamber interior 39 of the loadlock chamber 38 and as the wafer is moved from the loadlock chamber 38 onto the transfer blade 50 , the ccd sensor 58 transmits the analog voltage signal corresponding to the “ hi ” z - axis position to the “ signal in ” port of the analog controller 60 through the “ signal out ” port of the laser sensor controller 66 . the analog controller 60 , in turn , sends the analog voltage signal to the hi signal port 61 thereof . the hi signal port 61 then sends the analog voltage signal to the interface pcb 62 at the “ signal high ” port thereof , in which case the wafer sensor signal sent from the “ signal out ” port of the interface pcb 62 to the wis port of the system controller 68 is 0 volts . this prompts the system controller 68 to continue operation of the transfer robot 48 and movement of the transfer blade 50 and wafer supported thereon to the appropriate chambers ( not illustrated ) of the integrated cluster tool 36 . in the event that a wafer supported on the transfer blade 50 inadvertently slides in and out of a wafer pocket ( not illustrated ) on the transfer blade 50 during the wafer - transfer operation , the analog voltage signal corresponding to the highest limit ( hh ) of the transfer blade 50 is sent from the ccd sensor 58 to the hh port 61 of the analog controller 60 , and from the hh port 61 to the “ signal high ” port on the interface pcb 62 . this causes the wafer sensor signal transmitted from the “ signal out ” port of the interface pcb 62 to the w / s port on the system controller 68 to increase from 0 volts to 24 volts . consequently , the system controller 68 displays a “ no wafer on blade ” alarm message and terminates further operation of the transfer robot 48 and wafer - transferring movement of the transfer blade 50 . in the event that the transfer blade 50 is distorted and the reflective portion of the transfer blade 50 is located at a z - axis position which is higher than the ideal position , the ccd sensor transmits the analog voltage signal corresponding to the “ hi ” or “ hh ” z - axis position to the “ signal in ” port of the analog controller 60 through the “ signal out ” port of the laser sensor controller 66 . the analog controller 60 , in turn , sends the analog voltage signal to the hi signal port 61 or hh signal port 61 thereof , as appropriate . the hi or hh signal port 61 then sends the analog voltage signal to the interface pcb 62 at the “ signal high ” port thereof , in which case the wafer sensor signal sent from the “ signal out ” port of the interface pcb to the w / s port of the system controller 68 decreases from 24 volts to 0 volts . this prompts the system controller 68 to display a “ wafer on blade ” alarm message and terminate operation of the transfer robot 48 and movement of the transfer blade 50 until the transfer blade 50 can be adjusted , fixed or replaced . in the event that the transfer blade 50 is distorted and the reflective portion of the transfer blade 50 is located at a z - axis position which is lower than the ideal position , the analog controller 60 sends the analog voltage signal from the ccd sensor 58 and laser sensor controller 66 to the ll signal port 61 or lo signal port 61 thereof , as appropriate . the ll or lo signal port 61 then sends the analog voltage signal to the interface pcb 62 at the “ signal low ” port thereof , in which case the wafer sensor signal sent from the “ signal out ” port of the interface pcb to the wis port of the system controller 68 decreases from 24 volts to 0 volts . this prompts the system controller 68 to display a “ wafer on blade ” alarm message and terminate operation of the transfer robot 48 and movement of the transfer blade 50 until the transfer blade 50 can be adjusted , fixed or replaced . table i below summarizes the wafer sensor signal ( w / s ) and alarm message displayed by the system controller 68 for each status of the transfer blade 50 . table i wafer sensor signal alarm status ( w / s ) message wafer sliding 0 v − − − − − − & gt ; + 24 v no wafer on blade wafer on blade 0 v n / a no wafer on + 24 v n / a blade no wafer on + 24 v − − − − − − & gt ; 0 v wafer on blade and blade blade distorted while the preferred embodiments of the invention have been described above , it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention .