Abstract:
A system and method for continuous atomic layer deposition. The system and method includes a housing, a moving bed which passes through the housing, a plurality of precursor gases and associated input ports and the amount of precursor gases, position of the input ports, and relative velocity of the moving bed and carrier gases enabling exhaustion of the precursor gases at available reaction sites.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/857,798 filed Jul. 24, 2013, which is incorporated by reference herein in its entirety. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The U.S. Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Government and the University of Chicago and/or pursuant to DE-AC-02-06 CH11357 between the U.S. Government and the UChicago Argonne, LLC representing Argonne National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to an improved system and method for atomic layer deposition (“ALD” hereafter). In particular, the invention relates to a new method and system for ALD deposition processing and manufacture of end product by using self-limiting reaction condition to avoid need of enclosures and purge areas for separation of two or more reacting gases. Consequently, there is no need for dosing valves since precursor flows continuously, and there is 100% material utilization as part of the subject ALD method. Further, a low vapor pressure precursor can be used with a fast moving conveyance system; and there is no need for isolation of different reaction zones nor for use of system components having strict tolerance features either chemically or mechanically. 
     BACKGROUND OF THE INVENTION 
     Among the techniques used to deposit thin films, chemical vapor deposition (“CVD” hereinafter) is a method that exposes substrates to one or more reacting gases. The non-volatile decomposition product is a solid material that accumulates on the substrate in the form of a thin film. 
     A variant of CVD is atomic layer deposition (“ALD” as previously noted). In this technique, surfaces are exposed to gaseous species that exhibit limited reactivity, that is, that the reaction and therefore the growth shuts itself down after all surface reacting sites are consumed. Conventional ALD is carried out by sequentially exposing surfaces to two or more different gaseous species. The growth during each exposure is self-limited, and this leads to homogeneous films. 
     In a more recent innovation of the ALD method, these sequential exposures mentioned above are separated not in time, but in space. In one configuration, a surface is moved across different enclosures, each of them containing a different gaseous species. In a second configuration, the surface is at rest, and a movable head containing two or more different enclosures is moved over the surface, resulting in alternate exposures to the reacting gases. Combinations of these two configurations also form part of the prior art. 
     All these prior art designs share a common feature: the need to physically isolate one gas from the other to avoid gas-phase reactions between the two species; and this feature would effectively kill the self-limited nature of ALD. Since there must be relative motion between the surface and the enclosure wherein the gas is dosed, it is not possible to have an airtight sealing of the gaseous species. Instead, a fraction of the species will diffuse out of the enclosure through the space between the moving substrate and the enclosure walls. Moreover, both the excess gas that does not react with the surface and the gas that escapes from the enclosure need to be removed from the system. This methodology requires the implementation of different purge strategies, including purge regions separating enclosures with two different kinds of gases to quickly remove the gas before it leads to undesired gas phase reactions or the use of high pressure gas to act as effective barriers between the different enclosures. Therefore, such prior art systems and methods require construction of complicated structures and highly demanding chemical processing conditions which make difficult and impractical the use of ALD for most commercial applications. 
     SUMMARY OF THE INVENTION 
     The method and system of the invention most preferably includes use of a fixed precursor gas being injected over a moving bed or conveyance coordinated such that the amount of precursor molecules released is less than the available reaction sites presented on a reaction surface by the moving bed/web incident to the flow. This methodology leads to the complete exhaustion of precursor gas with no need for removal ports, no precursor waste, and no purge stages being needed. The spacing of these injection ports is sufficient such that all precursor is adsorbed onto the moving bed/web surface before the next precursor injection port is encountered, and thus no purge stage is required. For multiple ALD layers, the bed/web can be made long enough to accommodate multiple precursor injection ports with sufficient spacing between them, or the bed/web can move back and forth under the fixed or even adjustable position injection ports. 
     Fixed, saturation ALD will lead to about 99%+coverage (dependent on dwell time), while this invention, based on precursor exhaustion, will result in 95% coverage (with only random “holes” in the coverage). However, since operational ALD requires at least several passes, these coverage differences will be likely filled in or “washed out.” Since this invention&#39;s approach is so much simpler than present “spatial ALD” solutions, the method and system will result in much cheaper equipment to be used (no gas bearings, no purge system, no pump-out zones, no complicated injector timing, etc.). In addition, there will be much less need for precision tolerances for system set-up and maintenance, increased equipment up-time, lower equipment maintenance, higher throughput, elimination of expensive wasted precursor, elimination of waste precursor separation or disposal requirements, much tighter coverage distribution and thus much higher quality product with a much more easily controlled process. 
     These and other advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one embodiment of a method and system for continuous ALD; 
         FIG. 2  Illustrates systematically two chemical operating regimens for low pressure, fast web flow and high vapor pressure, slow web flow; 
         FIG. 3  illustrates characteristic decay length for a precursor versus reaction probability in the ALD reactor system; 
         FIG. 4A  illustrates a section of the reaction system with precursor A and precursor B input ports separated by distance L; and  FIG. 4B  illustrates characteristic decay length versus reaction probability for the system of  4 A and having reaction width d; 
         FIG. 5  illustrates adaptation of the method of the system of the invention for coating of high surface area materials, such as catalyst supports wherein particles are disposed on a moving web before being subjected to the ALD system and process; 
         FIG. 6  illustrates another ALD system embodiment similar to  FIG. 1  but with a controller and precursor feed system or reservoir; 
         FIG. 7  illustrates a schematic flow diagram in one embodiment of operating one ALD method and system of the invention; 
         FIG. 8  illustrates another embodiment of the method and system of the invention with a plasma discharge downstream of an injector of precursor; and 
         FIG. 9A  illustrates a top view of yet another embodiment of the invention having a moving surface over which is a rotating head containing feed injectors disposed above the moving surface; and  FIG. 9B  is a side view of  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a preferred example embodiment of one form of a continuous ALD system  10 . The deposition system  10  includes an enclosure body or housing  12  that encapsulates the deposition system  10  from the exterior. This enclosure body  12  can sustain a differential pressure with respect to the outside. A moving substrate  14  enters and exits through  18  and  20 . If the moving substrate  14  is a flexible web the substrate  14  can be processed outside the system  10 , for instance as part of a roll-to-roll system. Two gaseous precursors contained in  22  and  24  are dosed into the system  10  through input  26  and input  28 , respectively. The system  10  contains no means to prevent the mixing of the precursors A and B inside the system  10 , including no physical separation or flow-induced separation between the inputs  26  and  28 . Further, for the system  10  the insertion inputs  26  and  28  can be any design able to insert precursors inside the system  10 , for instance nozzles, tubes, slits, vaporizers or any of the means known in the prior art. Various carrier gas  29  can be inserted as part of the process of the invention; and flow control can be achieved by a pump  31 . It should be noted the method can be performed without need of isolation from ambient. 
     While the inputs  26  and  28  can include any valves and dosing mechanisms known in the prior art, including the ability to pulse the reactants, the embodiment of  FIG. 1  works without having to pulse the insertion of reactants, that is, it can work under constant dose of the precursors A and/or B. The system and method avoids the presence of gas-phase reaction under self-limiting conditions by controlling the amount of the precursor dosed through system  26  and  28  in such a way that 1) the rate of consumption is faster than the rate of precursor insertion and 2) precursor A is completely consumed before reaching the input  28 . This aspect of the invention can be estimated in ways that will be explained hereinafter. In the case of the design of  FIG. 1 , the dimensions of the system  10 , the distance between the inputs  26  and  28 , velocity of the substrate or the web  14  and average flow velocity are all important parameters determining the amount of precursor dosed. In addition to this, the reactivity of the precursors will also condition the amount of the precursors A and B that can be inserted in the system  10  without negatively affecting the quality of the product material. 
     Some advantages of this approach are: 
     1. 100% materials utilization: all the precursor is consumed as part of the ALD process. 
     2. Continuous operation: no need to pulse precursors. 
     3. Suitable for low vapor pressure precursors. 
     4. Reasonable reactor sizes, no small tolerances, robust against web vibration and other perturbations of a moving wall system. 
     5. Reactor size decreases with higher reaction probabilities: suitable for large surface area substrates. 
     A variety of operating regimes can exist for the system  10  illustrated in  FIG. 1  and variations on this embodiment can function in a pure ALD mode with self-limited chemistry which can be deduced from models of precursor reactor and transport under ALD conditions. These models described hereinafter can produce good agreement under conventional ALD operations. 
     Considering the steady state equations of a precursor flowing into the system  10  with moving walls and where the surface chemistry is given by a first-order irreversible Langmuir kinetics, two regimes can be distinguished. These two regimes depend on an excess number, defined as the number of molecules per surface site, and the ratio of the flow and web velocity: 
     
       
         
           
             
               
                 
                   
                     S 
                     0 
                   
                   ⁢ 
                   
                     n 
                     0 
                   
                   ⁢ 
                   V 
                 
                 S 
               
               ⁢ 
               
                 u 
                 υ 
               
             
             = 
             
               γ 
               ⁢ 
               
                 u 
                 υ 
               
             
           
         
       
     
     Where S 0  is the average area of a surface site, n 0  is the precursor density at the entrance of the inputs  26  and  28 , V and S are the volume and the surface of the system  10 , u is the average flow velocity and υ is the surface (or web) velocity. 
     These regions of reaction are shown substantially in  FIG. 2 . When γu/υ&gt;1, complete saturation is achieved; and the remaining precursor keeps flowing downstream. This region requires the isolation of the two precursors, and it is unsuitable for the one embodiment given in  FIG. 1 . This region correspond to the operating conditions of the prior art. However, when γu/υ&lt;1, 100% of the precursor is consumed. A self-extinguishing pulse under ALD conditions, has been achieved under these conditions; and a system  10  is established such as that defined in the current invention. 
     Note that, while in conventional CVD methods, complete consumption of the precursor is expected after a long enough distance of travel in a reactor, the existence of such region is not guaranteed under ALD conditions. Only when the precursor flows are carefully chosen to ensure the self-extinguishing condition, it is then possible to run the system  10  such as that depicted in  FIG. 1  to carry out the highly desirable form of ALD described herein. Also note that two different operating conditions are possible, one in which the flow and the reactor surface of the web  15  move in the same direction and another one in which they move in opposite directions. 
     The characteristic length for precursor decay can be determined and which is given by: 
               L   ⁡     (   ɛ   )       =         4   ⁢           ⁢   ud         υ   th     ⁢     β   ⁡     (     1   -     c     0   ⁢                 )           ⁢     log   ⁡     (       1   -       c   0     ⁡     (     1   -   ɛ     )         ɛ     )               
here u is the average flow velocity, c 0  is the final coverage after all the precursor is consumed, υ th  is the average thermal velocity, β is the bare reaction probability, d is the vertical gap of the reactor (distance from injector to moving web or bed), and epsilon is the tolerance for precursor depletion. This expressions is obtained under the assumption of a first order irreversible Langmuir kinetics to represent ALD&#39;s self-limited chemistry.
 
     The separation between the inputs  26  and  28  depends on the velocity of the web  15 , the vertical dimension of the reactor zone of the system  10 , the mean thermal velocity, the reaction probability, the coverage and the tolerance that is required for the process. Characteristic values are shown in  FIG. 3 , showing how separations of the order of ten centimeters can be achieved under selected optimal conditions. 
     This formula above can further be used to estimate the distance between the inputs  26  and  28 . Also, from the results obtained it is clear that one critical parameter in the design feature is the bare reaction probability of the precursor. Therefore, it is important to understand the chemistry of the precursor in order to adapt the experimental setup to a particular one of the system  10 . Likewise, the design of the system  10  also can affect the distance between injectors. In  FIG. 4A  is shown the influence of width d on the characteristic length of precursor consumption under preferred ALD conditions. 
     The distances shown in  FIGS. 3 and 4  have to be understood as upper boundaries with respect to a real system, since they are calculated using a one dimensional model, which is equivalent to assuming an injection slit. If the system  10  uses point inputs  26  and  28 , the distance is reduced due to the influence of diffusion perpendicular to the direction of the flow. 
     While more complex simulations can be used to simulate the interaction between the flow and the moving walls of the web  14 , the formula as presented above captures the main features of the system  10 , and the ratio u/υ can be chosen from more accurate, and well known, computational fluid dynamic simulations. 
     The results show that a high reaction probability affects positively the distance between the inputs  26  and  28 . This makes the method ideal to coat high surface area materials, since the effective reaction probability on high surface area materials is larger than that on planar substrates. In  FIG. 5  is shown how the method and the system  10  can be adapted to the coating of high surface area materials, for instance a catalyst support  30 . In  FIG. 5  we have the system  10  being similar to that presented in  FIG. 1 , except that prior to its treatment in the ALD system  10 , particles  32  are deposited onto the moving surface of the web  14 . Therefore, the moving surface acts as a conveyor belt for the particles  32 , allowing their treatment in a continuous operation. Examples of this method can include methods to synthesize catalyst in a continuous operation by the ALD or to modify electrode materials for energy storage applications, such as batteries. 
     In another embodiment, since the results above show a correlation between the dosing and the velocity of the surface of the web  14 , in  FIG. 6  an embodiment is shown which is similar to that of  FIG. 1 , with an enclosure body or housing  34  in which a moving surface  36  is fed and coated by one or more inputs  38  and  40  connecting the housing  34  to a precursor reservoir or feed system  46 . In this embodiment, there is included a flow controller  48  that establishes a feedback between the velocity of the web  14  and the amount of precursor dosed. Through use of the flow controller  48 , the precursor flow is controlled as a function of the velocity of the web  14 , and the velocity of the web  14  is controlled as a function of the flow. Therefore a bi-directional feedback between velocity and reactant flow is established. Through the use of the controller  48  the system  10  ensures its operation under the self-extinguishing regime as described hereinafter. One possible mode of operation of this embodiment of the system  10  is shown schematically in  FIG. 7 , through which the system  10  periodically measures the velocity of the web  14  and adjusts the pressure to ensure that the system  10  works under self-extinguishing conditions. 
     In yet another embodiment shown in  FIG. 8 , the system  10  is similar to that described in the previous figures, and includes a plasma discharge  50  generated downstream of the input  52 . This plasma discharge  50  can be used to activate precursor A or otherwise incorporate a treatment process to the moving surface of the web  14  is added downstream of the precursor dose. As an example of an application of this embodiment, one could carry out a plasma etching process can be done under conditions in which the precursor is unreactive to a particular gas that is fed into the housing  12  of the system  10  but that, when activated through a plasma discharge, generates localized species that alter a deposition surface  54  inducing, for instance, the etching of organic substances. 
     In a further embodiment, shown in  FIGS. 9A and 9B , the system  10  includes the moving surface  54  for the web  14  fed at a certain velocity. Over the moving surface  54 , there is a rotating head  56 . This head  56  contains inputs  58  and  60  that feed reactants into a chamber  62  directly over the moving surface  54 . The inputs  58  and  60  can be single point injectors or slides. The dosing is controlled through an axle  64 , either by feeding a signal that opens a reservoir inside the head  56  or by directly feeding the gases into the inputs  58  and  60 . Conventional prior art mechanisms can be included in the spinning form of the head  56  to purge the gases from the system  10  and to control the gap between the surface  54  and the rotating head  56  through the use of a high flow of gas (a gas bearing). In the embodiment, the rotating head has no mechanisms for purging the excess gas or otherwise to maintain a gap or isolate the chambers based on the use of large flows. Instead the system  10  described in  FIG. 9  relies on the control of the flow to achieve advantageous self-extinguishing ALD conditions described above. 
     In additional embodiments reasonable generalizations of the systems  10  described above, include, but are not limited to, the use of more than two channels  70  of the system  10 , the variation of the spacing between the different channels  70 , the use of more than one moving surfaces  54  of the web  15  and the adaptation of the reactor geometry to curved surfaces that could be used to treat surfaces directly on a roll. 
     The method and system adaptation described herein can be applied to any method or arrangement able to operate in an ALD mode, (thermal, plasma and radical assisted, and UV-assisted) and can be used for applications such as catalysis, photovoltaics, transparent electronics, energy storage, barrier coatings for organic photovoltaics and organic light-emitting diodes, and transparent conducting oxide materials. This method is particularly well suited for the coating of high surface area materials, for instance catalyst supports, and the coating of high-cost precursors where achieving a 100% materials utilization offers significant advantages in terms of the cost of the process. 
     The methodology of the invention therefore eliminates the stringent tight tolerances required in many existing spatial ALD approaches to avoid the cross-talk between different precursors in the ALD process. Instead, the method herein relies on the self-extinguishing nature of the pulse to eliminate the cross talk. 
     In another aspect of the methodology ALD can be used under continuous deposition of particles. The fact that the effective reaction or sticking probability becomes much larger in the presence of particles is extremely convenient for the instant invention since the speed of the continuous process relative to the size of the chamber is determined by the sticking probability. 
     However, the fact that the instant method does not require tight tolerances at the points where the web  15  or belt  30  crosses through the reaction embodiment means that the coating of the particles  32 , as a form of the substrate in the ALD process, is enabled in the presence of mechanical agitation. This agitation greatly impacts the speed of the process by reducing the time required to achieve saturation and ensures that the particles  32  are coated homogenously. Examples of methods for increasing the mixing of the particles  32  would include including a device  80  (see  FIG. 5 ) for vibration and/or periodic oscillations in the web  15  to fluidize the particles  32  in the gas phase inside the chamber, or they could be applied to the external operation of the moving substrate, like vibrations intrinsic from the operation of the belt  30 . The mechanical agitation allows for much improved mixing of the particles  32  on the conveyor belt  30 , and the ALD process is characterized by an effective sticking probability can be determined from the size, packing density and total volume of the particles  32  beforehand, and these can be used to determine the throughput of the process and tune it to the particle loading. 
     The present invention has been described herein with reference to the preferred embodiments and accompanying drawings. These embodiments and drawings do not serve to limit the invention, but are set forth for illustrative purposes. The scope of the invention is defined by the claims that follow. Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.