referring now to the drawings , fig1 depicts a conventional spin chuck that holds a wafer w thereon in a generally horizontal orientation . the spin chuck includes a rotary base body 1 to which upper plate 2 is connected . a series of pin assemblies 4 project upwardly through openings formed in the upper plate 2 . as shown in greater detail in fig2 , each pin assembly 4 comprises an enlarged based equipped with gear teeth 6 , which mesh with the gear teeth 7 of a common ring gear 3 that drives all of the pin assemblies 4 in unison . because the contact region 5 of each pin assembly 4 is eccentric to the rotational axis of the pin assembly , that rotation displaces the contact region 5 radially of the spin chuck , as described for example in u . s . pat . nos . 4 , 903 , 717 and 5 , 513 , 668 . the correct positioning of the pin assemblies 4 is aided by respective needle bearings 8 and associated springs 9 . however , as discussed above , in order to remove any given pin assembly 4 for repair or maintenance or replacement , the entire chuck must be disassembled , including removing the upper plate 2 from the chuck base body 1 . referring now to fig3 , in a first embodiment according to the invention , a rotary chuck 12 is mounted in a process module 11 for single wafer wet processing of semiconductor wafers . each chuck 12 holds a single wafer , but there may be a plurality of such chuck mounted in a given process module 11 , with the chucks being controlled by common control mechanisms and supplied by common wafer handling equipment . pin assemblies 40 are positioned so as to hold a wafer w of a predetermined diameter , for example , 300 mm or 450 mm . rotary chuck 12 comprises a chuck base body 10 and an upper plate 20 secured thereto . an infra - red heating assembly 60 comprising one or more ir lamps 68 is positioned above the upper plate 20 of the chuck 12 . as described more fully in commonly - owned copending application u . s . 2013 / 0061873 , the heating assembly 60 is mounted on a stationary post that passes through the center of chuck 12 , so that the heating assembly 60 remains stationary as the chuck 12 rotates . furthermore , the heating assembly , although above the upper plate , is below the level at which a wafer w is held by pin assemblies 40 , so that the heating assembly 60 heats a wafer w from the underside . the stationary structure also accommodates fluid supply nozzles 62 , 64 , 66 , which supply respectively different gases and / or liquids to the underside of a wafer w . as shown in fig4 , each pin assembly 40 projects upwardly from the upper plate 20 , passing through a respective opening 26 that traverses the upper plate 20 . unlike in the spin chuck of fig1 and 2 , in the present embodiment the opening 26 is sufficiently wide to permit the entire pin assembly , including the enlarged base 42 thereof , to be removed through the opening 26 . also positioned within the opening 26 of upper plate 20 , and surround the main shaft of pin assembly 40 , is an insert 50 . an o - ring 22 is seated in a groove formed in opening 26 , to prevent liquid from entering the chuck 12 between the insert 50 and the upper plate 20 . insert 50 includes an opening 56 which , like opening 26 , is preferably cylindrical . the width ( or diameter ) of opening 56 is however less than that of opening 26 , so that the main shaft of the pin assembly 40 can pass through the opening 56 , but the enlarged base portion 42 cannot pass through opening 56 . also visible in fig4 are a ring gear 30 that meshes via teeth 37 with corresponding teeth formed on the enlarged base portion 42 of each pin assembly 40 , and a needle bearing 18 and associated spring 19 , for positioning the respective pin assembly 40 . turning now to fig5 - 7 , it can be seen that each insert 50 is secured to the upper plate 20 by a pair of screws 52 . in particular , each insert 50 is received in a corresponding recess 24 formed in the upper plate 20 . recess 24 includes the opening 26 that traverses the upper plate 20 , and a pair of recessed , non - traversing regions in which bores 28 are formed for receiving the screws 52 . thus , to remove a pin assembly 40 from the chuck 12 of fig3 - 7 , it is necessary only to unscrew the screws 52 , whereafter the pin assembly can be grasped , for example at the contact region 45 thereof , and lifted out of engagement with teeth 37 of ring gear 30 , up and out of the chuck through opening 26 in the upper plate 20 , along with the insert 50 that will remain captive on the pin assembly 40 . it will be appreciated that the above operation is much quicker and simpler than in the prior art , as the chuck 12 need not be disassembled or removed from its drive and elevating unit , or , indeed , even removed from its surrounding process module 11 . fig8 shows a pin assembly 40 more clearly , including the eccentric contact region 45 that bears against a wafer edge in use , and also provides subjacent support to the wafer so that it is maintained at a predetermined distance above the upper plate 20 . also shown is the enlarged base portion 42 , which includes gear teeth 46 formed integrally therewith , the gear teeth 42 of each pin assembly in use being in simultaneous meshing engagement with the gear teeth 37 of common ring gear 30 . pin assembly 40 also preferably includes a registration or “ timing ” mark , such as the hole 47 formed in the enlarged base portion 42 , which is formed at the same angular position on each of the pin assemblies 40 , so that each pin assembly 40 can be more readily inserted into the chuck through opening 26 and into meshing engagement with the gear teeth 37 of gear ring 30 in its correct angular orientation relative to the other pin assemblies 40 . fig9 shows an insert 50 more clearly , including the central opening 56 through which the main shaft of pin assembly 40 may pass , but which does not permit passage of the enlarged base portion 42 , and a pair of openings 54 that receive the screws 52 in this embodiment so that the flat heads of the screws are flush mounted with the upper surface of the insert 50 . in fig1 , the insert 50 is shown captive on the pin assembly 40 , as it would be in use and also when the screws 52 are removed and the pin assembly 40 is extracted from the chuck 12 by lifting the pin assembly 40 and insert 50 out of the upper plate 20 of the chuck 12 . fig1 shows another embodiment of an insert 51 suitable for use in preferred embodiments according to the present invention . the insert 51 of fig1 is in general symmetrical about the rotational axis of pin assembly 40 , and is secured to the upper plate 21 not by separate screws 52 but instead by its own screw threading 53 that mates with corresponding threading formed in the threaded bore 27 of plate 21 . the part 55 of insert 51 that projects above the surface of upper plate 21 is preferably nut - shaped , so as to be able to receive a deep socket that has clearance for the pin assembly 40 and allows the insert 51 to be screwed into the threaded bore 27 of upper plate 21 . an o - ring 23 is seated in a groove of the bore 27 above the threaded part thereof , to prevent liquid from seeping between the insert 51 and the upper plate 21 . alternatively or in addition , the insert 21 may be formed with a lip seal 57 that creates a friction fit between the insert 51 and the upper plate 21 . lastly , in fig1 , yet another embodiment of an insert 58 is illustrated , which is like the insert 50 described in connection with fig5 - 7 , but in which the screws 52 are captive within the insert 58 , by virtue of collars 59 , which permit the screws 52 to be screwed and unscrewed from the bores 28 in recess 24 , but which prevent the screws 52 from being removed from the insert 58 . this embodiment therefore further simplifies the assembly and disassembly of pin assemblies from the upper plate , as it eliminates any risk of the screws 52 falling into the chuck 12 or elsewhere in the process module 11 . while the present invention has been described in connection with various preferred embodiments thereof , it is to be understood that those embodiments are provided merely to illustrate the invention , and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims .