a wide variety of gases in metered fashion to some or all of the defined gas zones either alternately or in combination this unique capability allows manufacturers to easily fine tune layer uniformity in process to achieve optimum and repeatable layer uniformity over simple and complex topologies upper coil assembly 9 has a plurality of gas supply passages 17 passing through an upper plate surface each supply passage 17 feeds to one of multiple gas zones defined by a plurality of radial channels provided within assembly 11 shown in further figs and descriptions below gas supply tubes and fittings adapted to conduct gases to passages 17 are not shown here for simplicity coolant delivery tubes 21 an inlet and an outlet are provided on the upper surface of coil assembly 11 and are adapted to allow coolant to circulate through coolant channels in diffuser 9 more detail about diffuser 9 and internal components is provided below 0029 fig2 is a section view of diffuser 9 of fig1 taken along the section line aa upper channel assembly 11 has a plurality of radial gas zones that are of differing diameters and are positioned in spaced concentric fashion in this example there are a total of thirteen zones 23 however there may be more or fewer zones 23 without departing from the spirit and scope of the present invention each zone 23 is an independent circular channel and is supplied by one gas supply passage 17 four of which are shown in this section view by this arrangement different gases may be supplied to different gas zones 23 independently with no gas mixing or crosstalk from one zone to.