7  EP 2 000 008 B1 8  precursor zones via pump 58 or another source of suc- tion. Optionally, pump 58 may be coupled to all zones, withflowfromthe various zones beingcontrolled tomain- tainthe pressure differential. Themigration of precursors from the precursor zones 14, 16 into the isolation zone 20 may also be prevented or limited by controlling both the relative flow rates of gases into the zones and pump- ingspeedsfromthezones, through the use offlowcontrol valves or other flow control devices. A control system (not shown) responsive to pressure sensors in the vari- ous zones may also be utilized to control gas injection and exhaust flow rates to help maintain a desired pres- sure differential. [0025] In one example, isolation zone 20 operates at a pressure ofapproximately(5 millitorr)(i.e., the inert gas injection pressure maybe0,67Pa (5 millitorr)), andpres- sure differentialsof approximately0.013Pa (0.1millitorr)) are maintained between isolation zone 20 and each of theprecursor zones14, 16, so thatan operatingpressure of approximately (0.65 Pa) (4.9 millitorr) is maintained in precursor zones 14, 16 by way of suction applied to pre- cursor zones 14, 16 by pump 58. Lower and significantly higher pressure differentials may also be used in some embodiments. The necessary pressure differential will be affected by the geometry of passageways 54, 56 (in- cluding height, width, and tunnel length, if applicable), the headroom and margins around substrate 12 within passageways 54, 56, the transport speed of substrate 12, the surface roughness of substrate 12 and passage- ways 54, 56, and the location at which inert gas is inject- ed, such as direct injection into passageways 54, 56 or generally into inert gas chamber 50. Other factors, such as operating temperature, pressure, precursor species, and substrate type, may also affect the amount of pres- sure differential necessary to inhibit or prevent migration of precursor gases through passageways. [0026] In some ALD processes, precursor gases hav- ing a very low vapor pressure are utilized. To facilitate pumping and diffusion control, inert gas may be mixed with such precursor gases, either before or after intro- duction of the precursor gases into the system 10, to control the pressure within precursor zones 14, 16. [0027] In some embodiments, it may be desirable to equalize the pressures, or to deliberately mismatch the pressures in two or more precursor zones to optimize growth conditions, or improve utilization of precursor ma- terials. It may also be desirable to pump two or more of the zones separately, and introduce inert gas into the precursor zones separately to further reduce zone mi- gration; for instance, a cross- flow condition may be used to flowprecursor ina direction orthogonal to the passage- ways54, 56 (betweenfirst and secondends72, 84). Inert gas may be introduced locally within or near passage- ways 54, 56, to inhibit gases from either adjacent zone fromcrossingthrough passageways54, 56. If further iso- lation is necessary, multiple differentially- pumped and purgedzones may beusedin series, with flow- restricting passagewaysorwipervalve isolation between zonesand exhaust paths from each of the zones. [0028] As described above, the precursor zones 14, 16 may be pumped to achieve an isolating pressure dif- ferential between the isolation zone and the precursor zones. In one configuration(not shown), separate pumps couldbeusedfor eachofthe zones 14, 16, 20, preventing mixing of precursor gases in the pump stack and the at- tendant growth of material or reaction byproducts in any of the pumping lines, therebypreventingpowderandres- idue from accumulating and clogging the pump stack. Another way to inhibit undesirable material deposits in thepump stackisto trap exhaust precursors usinga pre- cursor trap 59, such as a simple inline liquid nitrogen cooled trap, for example model TLR4XI150QF sold by Kurt J. Lesker Company (www.lesker.com). Similar pre- cursor traps may be placed in each of the precursor ex- haust lines upstream of their junction before the pump 58. By using inert gases and precursor materials having different vapor pressures at a given temperature, it may be possible to trap andreclaimup to approximately100% of exhaust precursor gases, while passing inert gases to the pump stack. And because different precursors are notmixedinthe zones,theprecursor purityismaintained, enabling up to 100% utilization of precursor materials. Oncefilled, traps 59may beturned into precursor sourc- es by replacing the liquid nitrogen with a heated liquid or by activating heating elements outside the trap, then re- versing the pumping direction or closing an isolation valve (not shown) between pump 58 and trap 59. The particular operating temperatureof trap/ sourcewould de- pend on the precursor being trapped and its vapor pres- sure. A liquid nitrogen trap, for example, may operate at lower than 100° Kelvin. Additional trap/source configu- rations are described below with reference to FIG. 4. [0029] A substrate transport mechanism 60 of system 10 includes multiple turning guides for guiding substrate 12, including a set of firstturning guides 64 spacedapart along first precursor zone 14 and a second set of turning guides66 spaced apart alongsecond precursor zone 16. Turning guides 64, 66 cooperate to define an undulating transport path of substrate 12 as it advances through system 10. The substrate transport mechanism 60 may include a payout spool 72 for paying out substrate 12 from a first coil (input roll 74) for receipt at a first end 76 of isolation zone 20, vessel 30, or one of the precursor zones 14, 16. The substrate transport mechanism 60 may further include a take-up spool 82 for receiving the coated substrate 12 from a second end 84 of isolation zone 20, vessel 30, or one of the precursor zones 14, 16 opposite first end 76, and coiling the substrate 12 into a take-up roll 86 or second coil. Payout spool 72 and/or take-up spool 82 may be located within vessel 30, such as within isolation zone 20, as depicted in FIGS. 1-2. Alternatively, payout and take-up spools 72, 82 may be locatedoutsideofvessel 30(i.e.,outside ofisolation zone 20 and first and second precursor zones 14, 16), as de- picted in FIGS. 3 and 4. Input and take- up rolls 74, 86 will change diameter during operation of system 10, and 5