3  EP 2 000 008 B1 4  andatleast someofthe secondturning guidesareadapt- ed to support thesubstrate during a change ina direction of travel of the substrate toward the first precursor zone. In some embodiments, more than two precursor zones are provided, allprecursor zonesbeing isolated fromone another. The substrate transport mechanism may in- clude a payout spool and a take-up spool for roll-to-roll processing of the substrate. [0008] A method of thin film deposition, according to one embodiment, includes introducing an inert gas into an isolation zonethat isinterposedbetween first andsec- ond precursor zones, introducing first and second pre- cursor gases into the respective first and second precur- sor zones,andthenguiding a flexiblesubstrate backand forth between the first and second precursor zones and through a series of flow-restricting passageways of the isolation zone, so that the substrate transits through the first and second precursor zones multiple times. The methodfurtherincludesgenerating pressuredifferentials between the isolation zone and the first precursor zone andbetweentheisolation zone and the second precursor zone, the pressure differentials being sufficient to inhibit migration of the first and second precursor gases out of therespectivefirstand second precursorzones andmix- ing of the first and second precursor gases within one of thezones, thereby essentiallypreventingreactionswith- in the zones between nonadsorbed amounts of the first and second precursor gases. The pressure differential may be achieved, for example, by differential injection of gases into the various zones or by differential pumping or throttling of exhaust gases from the various zones. In some embodiments, aninert gas isinjectedintosome or allof the passageways.As the substrate transitsthrough thefirst precursor zoneamonolayer of the first precursor gas is adsorbed to the surface of the substrate, and on a subsequent transit ofthe substrate through thesecond precursor zone thesecond precursor gas reacts with the adsorbed first precursor at the surface of the substrate, to thereby deposit a thin film on the substrate. Many lay- ersof materialmaybedepositedbyguidingthesubstrate along a serpentine path that traverses between the first and second precursor zones many times. [0009] In some embodiments of the method and sys- tem, the substrate is transported through three or more precursor zones, all isolated from one another by the isolation zone. One or more of the turning guides, pre- cursors, precursor zones, isolationfluid,or isolation zone may be heated. [0010] In some embodiments, the isolation and pre- cursor zones may be operated at approximately atmos- phericpressures,whilein others the pressuremay range from relatively low vacuum pressures (e.g. 0,13 Pa (1 millitorr)) to positive pressures of 667 to 2000 hPa (500 to 1500 Torr (approx. 1-2 atmospheres)). [0011] In some embodiments of the method and sys- tem,the flexiblesubstrate maybe advancedcontinuously along a serpentine path in a first direction to complete a first pass, and subsequently rewound along the serpen- [0019] Amongother possiblebenefits, certain systems and methods disclosed herein may facilitate the deposi- tion of barrier layers and transparent conductors on flex- tine path in a second direction opposite the first direction to complete a second pass. [0012] Embodiments of the method may also include the steps of switching precursors during or between passes, introducing dopants into one or more precursor zones, and/or introducing a radical into one or more of the precursor zones. A length or duration of some of the transits through the precursor zonesmay be adjusted, in someembodiments,bymovably mountedturningguides or zone dividers. [0013] Systemsand methodsfor trappingexhaust pre- cursor gases for disposal, recycling, or reclaim are also disclosed. [0014] FIG. 1isaschematiccross-sectional viewillus- trating a system and method for ALD on a flexible sub- strate, in accordance with a first embodiment; [0015] FIG. 2isaschematiccross-sectional viewillus- trating a system and method utilizing ALD for coating layers of different materials onto a flexible substrate, in accordance with a second embodiment; [0016] FIG. 3isaschematiccross-sectional viewillus- tratinga systemandmethod for ALDin which asubstrate is moved through a linear multi- stage ALD reactor, in accordance with a third embodiment; and [0017] FIG. 4isaschematiccross-sectional viewillus- trating a system and method for ALD on a flexible sub- strate according to a fourth embodiment, including a pre- cursor recovery and recycling system. [0018] In accordance with embodiments described herein, a flexible substrate, such as a plastic or metallic web or filament, for example, is threaded between adja- cent zones each having a different precursor chemical or isolation fluid present therein. As the substrate is ad- vanced, each segment of the substrate ispreferablyres- ident in the precursor zones long enough to accomplish the requisite adsorption and reaction of precursor chem- icals on the substrate surface. An isolation zone inter- posed between the precursor zones prevents mixing of the different precursor gases. The substrate is moved throughthe zonestoachieve a thinfilmcoatingconsistent with the coatings deposited by conventional ALD proc- esses. In addition to enabling the deposition of a highly conformalthinfilmcoating onweb materialsandonother flexible elongate substrates, systems and methods ac- cording to the embodiments described herein may avoid the need to deliver into a common reaction chamber a sequence of precursor and purge gas pulses in alternat- ing succession, as is done in a conventional traveling wave-type ALD reactor. Detailed Description of Preferred Embodiments Brief Description of the Drawings 3