Patent Publication Number: US-2019186008-A1

Title: Process for forming compositionally-graded thin films

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of prior U.S. patent application Ser. No. 15/846,56, filed Dec. 19, 2017, entitled “Thin-film optical device with varying layer composition”, by L. Tutt, which is incorporated herein by reference in its entirety. Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. ______ (Docket K002221US03), entitled “System for forming compositionally-graded thin films”, by L. Tutt et al., which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to the field of thin films with a composition gradient and more particularly to compositionally-graded thin films fabricated using a spatial atomic layer deposition process. 
     BACKGROUND OF THE INVENTION 
     Thin films having composition gradients are useful for their mechanical, physical, electrical and optical properties. Functionally graded materials have a continuous distribution of materials, and have been used in designing electronic, biological and optical materials. Compositionally-graded inorganic thin films are useful as encapsulants, providing improved mechanical properties over inorganic thin films of a uniform composition. 
     Rugate filters, also known as gradient index reflection filters, are an example of an optical application of compositionally-graded thin films. Rugate filters differ from discrete stacked filters in that the index of refraction varies as a function of the height (i.e., the z-location) within the deposited film. Typically, the optical thickness of the refractive index period determines the reflection band position, and the amplitude of the variation of the index of refraction determines the reflection bandwidth. As generally known, multiple reflection bands can be generated by serially depositing individual index of refraction profiles for each reflection band or, alternatively, by superimposing multiple index of refraction profiles and depositing the bands in parallel. The use of superposition allows for increased film complexity without adding to the mechanical thickness of the deposited film. In instances where superimposed indices exceed the material indices or result in excessively high slew rates of the material sources, both serial and parallel techniques can be used. 
     The typical technique for forming compositionally-graded films is the co-sputtering of multiple materials and changing the ratio of the deposited materials. This technique is relatively fast but involves the use of a vacuum chamber and produces a large amount of waste due to the need to position the sample at a sufficient distance to achieve relatively good uniformity. Rotation of the sample to achieve uniformity, which is effective, means that the deposition must be slow enough that the changes in the ratio of deposited material are still applied uniformly during the rotations. Other vacuum processes have been used for fabricating compositionally-graded thin films including pulsed laser deposition (PLD), sputtering, chemical vapor deposition (CVD), and atomic layer deposition (ALD), which all suffer from similar issues to co-sputtering. Recently, sol-gel processes have been used, and while an atmospheric process precise control of the gradient within the thin film is difficult to achieve. 
     Atomic layer deposition (ALD) is known to yield extremely precise layer thicknesses and uniformity. ALD is a film deposition technology that can provide improved compositional control as a function of thickness (i.e., compositional resolution) and conformal capabilities, compared to other deposition techniques. The ALD process segments the conventional thin-film deposition process into single atomic-layer deposition steps. Advantageously, ALD steps are self-terminating and can deposit one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers, with typical dimensions on the order of no more than a few Angstroms. In ALD, deposition of an atomic layer is the outcome of a physical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the absence of the other precursor or precursors of the reaction. In practice, in any system it is difficult to avoid some direct reaction of the different precursors leading to a small amount of vapor phase nucleation and reaction. 
     The basic ALD process requires alternating, in sequence, the flux of chemicals to the substrate. The representative ALD process, as discussed above, is a cycle having four different operational stages: 
     1. ML x  reaction; 
     2. ML x  purge; 
     3. AH y  reaction; and 
     4. AH y  purge, and then back to stage  1 . 
     This repeated sequence of alternating surface reactions and precursor-removal that restores the substrate surface to its initial reactive state, with intervening purge operations, is a typical ALD deposition cycle. A key feature of ALD operation is the restoration of the substrate to its initial surface chemistry condition. Using this repeated set of steps, a film can be layered onto the substrate in equal metered layers that are all alike in chemical kinetics, deposition per cycle, composition, and thickness. 
     As noted above, ALD has been used for making reflective interference filters having graded compositions. See for example the article “Introducing atomic layer epitaxy for the deposition of optical thin films” by D. Riihelä et al. (Thin Solid Films, Vol. 289, pp. 250-255, 1996). In this case, each layer is pure but since each layer is only about one molecular layer thick an average refractive index of a subset of layers can be made to be between two extremes by alternating high and low index layers. This approach requires a vacuum system, is relatively slow due to flushing the chamber after each pulse (each pulse depositing &lt;1 nm), and does not yield intermediate refractive index layers, only when averaged over multiple layers. When multiple precursors are mixed and introduced there is competition for service sites and depletion of the more reactive material as the gases move farther from their introduction orifice into the chamber. The net result is a spatially varying material. If the precursors have separate orifices this only exacerbates the non-uniformity. 
     Spatial ALD is an embodiment of ALD where the two or more reactive precursors are separated in space and all or a portion of the sample is moved from one precursor zone to another, rather than pulsing the precursors in time as in traditional temporal ALD. In some Spatial ALD configurations, the substrate is flat and forms a wall of a micro chamber with a gas distribution head. Each reactive gas is separated from the others by an inert gas. To apply layers, the sample is moved from one reactive chamber to the next. Usually this is accomplished by moving the sample in an oscillatory or rotational fashion. In some cases, if there are enough alternating orifices of reactive and inert gases the sample can make a single pass. This would be especially beneficial to web-based deposition of very thin films. 
     A number of prior art references have described methods for fabricating compositionally-graded thin films for a number of different applications. U.S. Patent Application Publication 2009/0258237 to Choi et al., entitled “Graded composition encapsulation thin film comprising anchoring layer and method of fabricating the same,” discloses the use of graded composition layers for the encapsulation of organic material layers, such as those that are used in organic light emitting devices (OLEDs). Choi et al. employ physical vapor deposition (PVD) processes for the formation of the thin-film layer. Broadly, PVD process are those that transfer a material from a source to a substrate by vaporizing the source material. These are typically vacuum processes and include sputtering, pulsed laser deposition (PLD), ion beam deposition (IBD), and ion beam assisted deposition (IBAD). These processes suffer from the short comings noted above—namely high materials waste, difficulty in precise gradient control, and vacuum processing. As such, it is desired to have a process to form compositionally-graded thin-film layers for encapsulation, that can deliver precise control of the composition but avoids vacuum processing and has low material waste. 
     An article by Y.-H. Choi et al., entitled “Design and fabrication of compositionally graded inorganic oxide thin films: Mechanical, optical and permeation characteristics” (Acta Materialia, Vol. 50, pp. 6595-6503, 2010) similarly discloses co-sputtering as a method to form compositionally-graded films for encapsulation. Similarly, an article by Y. Wang et al., entitled “On the novel biaxial strain relaxation mechanism in epitaxial composition graded La 1-x Sr x MnO 3  thin film synthesized by RF magnetron sputtering” (Coatings, Vol. 5, pp. 802-815, 2015) also discloses the use of co-sputtering to form compositionally-graded films. 
     U.S. Pat. No. 9,056,331 to Bulliard et al., entitled “Thin layer having composition gradient and production method thereof,” also discloses the use of graded composition layers for the encapsulation of organic material layers. Lee et al. use a sol-gel process to form the compositionally-graded thin film encapsulation layer, alone or in combination with PVD. The sol-gel process is limited in ability to precisely control the composition of the graded film, while changes in the composition-gradient are possible with different sol-gel conditions, there is no ability to design an exact profile or to create complex gradients in a single process. As such, there remains a need for a high-precision process that can form complex compositional profiles. 
     An article by G. Riveros et al., entitled “Electrodeposition and characterization of composition-graded CdS x Se (1-x)  multilayer thin film structures” (Journal of Alloys and Compounds, Vol. 686, pp. 235-244, 2016) discloses the formation of composition-graded stacked layer samples by electrodeposition. As disclosed, the graded thin films are deposited using a time consuming wet chemistry technique which requires that the thin film be at least semi-conducting. This approach does not address the need for a fast, easy process to deposit compositionally-graded thin films. Additionally, there are applications in electronics for using compositionally-graded thin films to control the bandgap of semiconductor materials layers. 
     There remains a need for a process which enables control of the composition gradient, without requiring the flushing of gases or the concomitant waste of materials. Furthermore, there is a need for a technique capable of rapid deposition of compositionally-graded thin films on a wide range of substrates without the use of a vacuum. 
     SUMMARY OF THE INVENTION 
     The present invention represents a process of making a compositionally-graded thin film including: 
     providing a substrate having a substrate surface; 
     providing a spatial atomic layer deposition system having a deposition unit, wherein the deposition unit includes a first reactive gas zone and a second reactive gas zone; 
     providing a mixing system to mix a controllable ratio of a first reactive gaseous species and a second reactive gaseous species to provide a first reactive gaseous material; 
     supplying the first reactive gaseous material to the first reactive gas zone of the deposition unit; 
     supplying a second reactive gaseous material including a third reactive gaseous species to the second reactive gas zone of the deposition unit; 
     wherein the first reactive gaseous species and the second reactive gaseous species are each capable of reacting with the second reactive gaseous material but do not react with each other under a specified set of operating conditions; 
     causing a relative motion between the deposition unit and the substrate according to a specified motion profile to cause the substrate surface to be sequentially exposed to the first and second reactive gaseous materials in the first and second reactive gas zones, respectively, thereby depositing material on the substrate; and 
     controlling the mixing system to vary the ratio of the first and second reactive gaseous species during the relative motion such that the combination of the controlling the mixing system and the motion profile causes the deposition of a compositionally-graded thin film having a variable composition as a function of height above the substrate surface. 
     This invention has the advantage that it enables the formation of a thin film having a variable composition as a function of height above the substrate without the use a vacuum chamber. 
     It has the additional advantage that the variable composition thin film is rapidly generated with molecular layer precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic block diagram showing the functional elements of an SALD deposition system; 
         FIGS. 2A-2C  are cross-sectional side views of SALD deposition heads useful in the present invention having a single ALD cycle; 
         FIG. 3A  is a cross-sectional side view of an alternative embodiment of an SALD deposition head having 1.5 ALD cycles; 
         FIG. 3B  is a plan view of the SALD head of  FIG. 3A ; 
         FIG. 4  is a high-level diagram showing the components of a system for deposition of a gradient index optical filter according to an embodiment of the present invention; 
         FIG. 5  is a graph showing a refractive index profile specifying the refractive index versus height above the substrate for a two-band reflector; 
         FIG. 6  is a graph showing the calculated reflection spectrum of a thin-film interference filter generated from the refractive index profile of  FIG. 5 ; 
         FIG. 7  is a graph showing measured refractive index as a function of the percentage of the first reactive precursor in the homogeneous gaseous mixture; 
         FIG. 8  is a graph showing measured growth per oscillation as a function of the percentage of the first reactive precursor in the homogeneous gaseous mixture; 
         FIG. 9  is a graph showing the experimental reflection spectrum of a thin-film interference filter generated according to the refractive index profile of  FIG. 5 ; and 
         FIG. 10  is a graph showing the experimental reflection spectrum of a thin-film interference filter generated based on the refractive index profile of  FIG. 5  where the heights above the substrate are stretched by 15%; 
     
    
    
     It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” Additionally, directional terms such as “on,” “over,” “top,” “bottom,” “left,” and “right” are used with reference to the orientation of the figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense. The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are generally not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense. Even though specific embodiments of the invention have been described herein, it should be noted that the present invention is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. The features of the different embodiments can be exchanged, where compatible. 
     It is to be understood that elements not specifically shown, labeled, or described can take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. It is to be understood that elements and components can be referred to in singular or plural form, as appropriate, without limiting the scope of the invention. 
     The example embodiments of the present invention are illustrated schematically and are not to scale for the sake of clarity. One of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention. Therefore, the provided figures are not drawn to scale but are intended to show overall function and the structural arrangement of some embodiments of the present invention. 
     The embodiments of the present invention relate components for systems useful for thin-film deposition. In preferred embodiments, the thin-film deposition is done using a spatial atomic layer deposition (SALD) process. For the description that follows, the term “gas” or “gaseous material” is used in a broad sense to encompass any of a range of vaporized or gaseous elements, compounds, materials or mixtures. Other terms used herein, such as: reactant, precursor, vacuum, and inert gas, for example, all have their conventional meanings as would be well understood by those skilled in the materials deposition art. Reactant gas flows can include one or more reactive species together with an inert gas. In some embodiments, the reactive gases can include a reactive plasma, such as supplied by a remote plasma source. One type of remote plasma source that can be used includes a surface dielectric barrier discharge source. As such, plasma-enhanced spatial ALD (PE-SALD) arrangements are considered to be useful in some embodiments. 
     Unless otherwise explicitly noted or required by context (for example, by the specified relationship between the orientation of certain components and gravity), the term “over” generally refers to the relative position of an element to another and is insensitive to orientation, such that if one element is over another it is still functionally over if the entire stack is flipped upside down. As such, the terms “over”, “under”, and “on” are functionally equivalent and do not require the elements to be in contact, and additionally do not prohibit the existence of intervening layers within a structure. The term “adjacent” is used herein in a broad sense to mean an element next to or adjoining another element. The figures provided are not drawn to scale but are intended to show overall function and the structural arrangement of some embodiments of the present invention. 
     Embodiments of the present invention are illustrated and described with a particular orientation for convenience; and unless indicated specifically, such as by discussion of gravity or weight vectors, no general orientation with respect to gravity should be assumed. For convenience, the following coordinate system is used: the z-axis is perpendicular to the output face of the deposition head, the x-axis is parallel to the primary motion direction (in the plane of the output face), and the y-axis is perpendicular to the primary motion axis (in the plane of the output face). 
     As employed herein the term “optical coating” is intended to encompass rugate coatings that are used with radiation within the visible spectrum of wavelengths, and also coatings that are used with radiation within other wavelength bands, such as the ultraviolet (UV) and infrared (IR) spectrums. 
     An atomic layer deposition (ALD) process accomplishes thin-film growth on a substrate by the alternating exposure of two or more reactive materials, commonly referred to as precursors, either in time or space. A first precursor is applied to react with the substrate. The excess of the first precursor is removed and a second precursor is then applied to react with the substrate surface. The excess of the second precursor is then removed, and the process is repeated. In all ALD processes, the substrate is exposed sequentially to a series of reactants that react with the substrate. The thickness of the ALD (and SALD) deposited thin films is controlled by the number of ALD cycles to which the substrate is exposed, where a cycle is defined by the exposure to the minimum required reactant and purge gas flows to form the desired thin-film composition. For example, in a simple design, a single cycle can provide one application of a first reactant gaseous material G 1  and one application of second reactant gaseous material G 2 . In order to effectively achieve repeated cycles, SALD requires either motion of the substrate past the deposition head or the development of complex equipment such that the delivery head with its gas connections, can be moved relative to the substrate. Thin films of appreciable thickness can be accomplished by either: 1) using a deposition head containing a sufficient number of gas distribution cycles and moving the substrate (or the deposition head) in a unidirectional motion relative to the deposition head (or substrate), or 2) using a deposition head with a limited number of cycles and using relative reciprocating motion. 
     In order to effectively use an SALD deposition head for thin-film deposition, it is commonly employed within a larger SALD system, or apparatus. Typically, such systems are specifically designed to deposit thin films on a particular type of substrate (for example, either rigid or flexible). Furthermore, SALD systems typically utilize a singular motion profile type that is chosen as a result of the design of the deposition head and the type of substrate being coated. In many cases, SALD systems are further designed for a specific application, and as such are configured to coat a single material at a given thickness on a substrate having a particular form factor. 
       FIGS. 1, 2A-2C and 3A-3B  are adapted from commonly assigned, co-pending U.S. Patent Application Publication 2018/0265978 by Spath et al., entitled “Deposition system with repeating motion profile,” which is incorporated herein by reference. Additional details regarding various SALD configurations that can be adapted for use with the present invention can be found in commonly assigned, co-pending U.S. Patent Application Publication 2018/0265976, entitled “Modular thin film deposition system,” by Spath et al.; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265977, entitled “Deposition system with vacuum pre-loaded deposition head,” by Spath et al.; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265969, entitled “Dual gas bearing substrate positioning system,” by Spath; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265982, entitled “Deposition system with moveable-position web guides,” by Spath et al.; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265979, entitled “Deposition system with modular deposition heads,” by Spath et al.; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265970, entitled “Porous gas-bearing backer,” by Spath; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265980, entitled “Deposition system with interlocking deposition heads,” by Tutt et al.; to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265971, entitled “Vertical system with vacuum pre-loaded deposition head,” by Spath et al.; and to commonly assigned, co-pending U.S. Patent Application Publication 2018/0265981, entitled “Heated gas-bearing backer,” by Spath; each of which is incorporated herein by reference. 
     As known by one skilled in the art, each SALD system requires at least three functional elements in order to effectively deposit a thin film, namely a deposition unit, a substrate positioner and a means of relative motion. As shown in schematic block diagram of  FIG. 1 , SALD system  200  of the present invention is preferably one in which a substrate  97  is moved relative to a fixed deposition unit  210 . As such, substrate  97  is positioned over the output face  134  of a deposition unit  210  by substrate positioner module  280 , and relative motion between the substrate  97  and the deposition unit  210  is accomplished by motion of the substrate positioner module  280  using relative motion system  270 , which can also be referred to as a motion controller or a motion control means. The deposition unit  210 , substrate positioner module  280  and relative motion system  270  are functional elements of deposition subsystem  205  of SALD system  200 . In various embodiments of the present invention, the deposition unit  210  can be a single deposition head or can be a deposition unit that include an array of deposition heads. The relative motion system  270  interacts with the substrate positioner module  280  to move the substrate  97  relative to the deposition unit  210 . 
     Many types of substrates can be coated with the SALD system  200 . The substrates  97  used in the present invention can be any material that acts as a mechanical support for the subsequently coated layers and is reactive with at least one of the reactants. The substrate  97  can include a rigid material such as glass, silicon, or metals. The substrate can also include a flexible material such as a polymer film or paper. Useful substrate materials include organic or inorganic materials. For example, the substrate can include inorganic glasses, ceramic foils, and polymeric materials. The thickness of substrate  97  can vary, typically from about 25 μm to about 1 cm. Using a flexible substrate  97  allows for roll processing, which can be continuous, providing economy of scale and economy of manufacturing relative to flat or rigid supports. 
     In some exemplary embodiments, the substrate  97  can include a temporary support or support material layer, for example, when additional structural support is desired for a temporary purpose, e.g., manufacturing, transport, testing, or storage. In these example embodiments, the substrate  97  can be detachably adhered or mechanically affixed to the temporary support. For example, a flexible polymeric support can be temporarily adhered to a rigid glass support to provide added structural rigidity during the deposition process. The glass support can be removed from the flexible polymeric support after completion of the manufacturing process. The substrate  97  can be bare indicating that it contains no substantial materials on its surface other the material from which it is composed. The substrate  97  can include various layers and patterned materials on the surface. 
     The substrate positioner module  280  is designed to position the substrate  97  in the x- and y-directions relative to the output face  134  of the deposition unit  210 . The SALD system  200  may also include a secondary substrate positioner (not shown) which is designed to control the position of the substrate  97  in the z-direction. 
     In various configurations, the substrate  97  can be attached to a backer device during deposition, which also serves as a substrate support mechanism. The backer device can be used as heat source for the substrate, or to stiffen otherwise flexible substrates. A backer that is temporarily attached to the substrate, by vacuum for example, is intended to move with the substrate during relative motion between the substrate and a fixed deposition head. The backer attachment can provide greatly increased rigidity and flatness to flexible substrates. A substrate support mechanism useful in the present invention can be larger than the substrate, as might be used to stabilize piece-parts of flexible substrate or approximately the same size as the substrate, or significantly smaller than the substrate when the substrate is rigid and self-supporting. 
     The deposition unit  210  can use any type of SALD deposition head that is known in the art.  FIGS. 2A-2C  illustrate deposition heads  30  that are configured to simultaneously supply a plurality of gaseous materials from the output face in different gas zones within a deposition zone  305 . In all three figures, the deposition zone  305  contains the necessary gas zones for a single two-step ALD deposition cycle. Moving from left to right within the deposition zone  305 , there is a first reactive gas zone  313  (G 1 ) followed by an inert gas purge zone  314  (P), and a second reactive gas zone  315  (G 2 ). As the relative motion system  270  ( FIG. 1 ) moves the substrate  97  relative to the deposition head  30  (the x-direction being the primary motion direction as indicated by motion arrow  98 ), a particular location on the substrate  97  sees the above sequence of gases which results in ALD deposition. Deposition heads  30  of the present can include a deposition zone  305  with gas zones for any number of ALD deposition cycles, the single ALD cycle illustrated is for clarity of understanding. 
     The SALD systems of the present invention can use any deposition unit  210  so long it has the required gas delivery process and geometry to form first and second reactive gas zones that are separated by a purge zone. These zones may be arranged in a linear fashion as illustrated in  FIGS. 1-3 , or can be arranged in other geometries such as in pie-shaped sections around a central axis, or any other appropriate configuration. 
     In exemplary embodiments, the deposition unit has a deposition head  30  with an output face  134  for providing the gases, forming gas zones between the deposition head  30  and the substrate  97  in the required order to accomplish an ALD cycle, as illustrated by the simplified deposition head  30  of  FIG. 2A . In preferred embodiments, the reactive gases (G 1  and G 2 , for example) have little or no intermixing to avoid a vapor phase reaction component during film deposition or gas phase reactions. The purge zone  314  (P) serves to separate the reactive gases G 1 , G 2  and allows for the removal of any reaction byproducts from the substrate surface as it moves through the purge zone  314 . 
     A single deposition cycle (moving from left to right) is defined by an inert gas flow I, followed by a first reactive gas flow G 1 , followed by an inert purge gas flow P, and lastly by a second reactive gas flow G 2 . The deposition zone  305  has a deposition zone length that spans the distance from the start of the first reactive gas zone to the end of the last reactive gas zone (e.g., from the first reactive gas zone  313  to the second reactive gas zone  315  in  FIG. 2A ). 
     The deposition heads  30  illustrated in  FIGS. 2A-2C , have extended inert zones  308 ,  309  on either side of the deposition zone  305  where the substrate is exposed to an inert gas (I). One of the advantages of the configuration of deposition head  30  and SALD system  200  containing is that it can be used to coat on substrates  97  whose length is much larger than the length of the deposition zone  305 . A further advantage of some embodiments is the ability to control the environment of the region of the substrate being actively coated during deposition. Additionally, the relatively small deposition head size allows for lower cost manufacturing of the deposition head. 
     The deposition head  30  of  FIG. 2B  illustrates an embodiment where one or more of the gas zones use a transverse arrangement, such as that disclosed in commonly-assigned U.S. Pat. No. 7,456,429 to Levy et al., entitled “Apparatus for atomic layer deposition,” which is incorporated herein by reference. In a transverse flow arrangement, the flow of gases during deposition is orthogonal, or transverse, to the direction of substrate motion and is exhausted either out the edges of the deposition head  30 , or into exhaust slots along the perimeter of the deposition head  30 . As illustrated, the deposition head  30  has gas slots  110  (i.e., output slots  112 ) that are configured to supply the gases into their corresponding gas zones. In other embodiments, the deposition head  30  provides gas to the elongated parallel gas zones through an array of orifices, rather than through the illustrated output slots  112  (elongated channels). 
     The deposition head  30  of  FIG. 2C  illustrates a preferred gas bearing deposition head  30  configuration. The principles and design of gas bearing deposition heads  30  has been described in detail in commonly-assigned U.S. Patent Application Publication 2009/0130858 to Levy, entitled “Deposition system and method using a delivery head separated from a substrate by gas pressure,” as well as in commonly-assigned U.S. Pat. No. 7,572,686 to Levy et al., entitled “System for thin film deposition utilizing compensating forces,” both of which are incorporated herein by reference. As shown in  FIG. 2C , an exemplary deposition unit  210  includes a deposition head  30  that operates on a vacuum-preloaded gas bearing principle having an output face  134  (facing upward) having gas slots  110  which provide gases into the gas zones and exhaust gases from the gas zones. Gases are provided into the gas zones by spatially separated elongated output slots  112  (extending in the y-direction). Each gas zone includes a corresponding output slot  112 . Adjacent exhaust slots  114  remove (or exhaust) gas from the gas zones. The exhaust slots  114  are positioned to define the boundaries of the various gas zones. As illustrated, the gas zones are equivalent to those of  FIGS. 2A and 2B . 
     In these preferred arrangements wherein the deposition head  30  operates using a gas bearing principle the substrate  97  is positioned above the output face  134  of the deposition head  30  and is maintained in close proximity to the output face  134  by an equilibrium between the pull of gravity, the flow of the gases supplied to the output face  134  through the output slots  112 , and a slight amount of vacuum at the exhaust slots  114 . While the gas openings in this example are gas slots  110  (also referred to as gas channels) that extend in the y-direction, one skilled in the art will recognize that the gas openings could also have other geometries, such as a row of nozzles or circular orifices, so long as the proper gases are delivered into and exhausted from the gas zones between the deposition head and the substrate. 
     As shown in  FIG. 2C , the gases are introduced and exhausted in alternating output slots  112  and exhaust slots  114  in the output face  134  of the deposition head  30 . The flow of gases between the output slots  112  during deposition is primarily in the direction of substrate travel (forward and backward) toward the adjacent exhaust slots  114 . As discussed earlier, the region that spans the reactive gas zones can be referred to as the deposition zone  305 , which is preferably surrounded by two inert zones  308 ,  309 . The individual gas zones within the deposition zone  305 , where the substrate  97  is exposed to each gas, generally extend outward from the corresponding output slot  112  to the two adjacent exhaust slots  114  as illustrated for the first reactive gas zone  313 , the purge zone  314 , and the second reactive gas zone  315 . In the illustrated configuration, the extended inert zones  308 ,  309  extend from the inert gas output slots  112  to the edges of the deposition head  30 . In alternative embodiments, the extended inert zones  308 ,  309  can include additional output slots  112  or other gas supply features. Additionally, the extended inert zones  308 ,  309  can include exhaust slots  114 , or other exhaust features, to provide additional protection/separation from the external environment  15 . 
     Using any of the embodiments of deposition head  30  of  FIGS. 2A-2C , an SALD deposition process can be accomplished by oscillating the position of the substrate  97  across the deposition head  30  (in the in-track direction indicated by the motion arrow  98 ) for the number of cycles necessary to obtain a uniform deposited film of the desired thickness for the given application. 
       FIG. 3A  is a cross-sectional view of a deposition head  30  illustrating an exemplary configuration where the deposition zone  305  is arranged to be symmetric, so that as the substrate  97  is moved relative to the deposition head  30  a position can “see” a full cycle exposure in either a forward or reverse direction.  FIG. 3B  illustrates a plan view corresponding to the cross-sectional view of  FIG. 3A , where the cross-sectional view is taken along the line A-A′ of the plan view. In common parlance, the deposition head  30  illustrated in  FIG. 3A-3B  can be referred to a “one-and-a-half cycle head” or a “1.5 cycle head.” Moving from left-to-right through the deposition zone  305 , the substrate  97  is exposed to (in order) a first reactive gas zone  313  where the substrate is exposed to a first reactive gas G 1 , an inert purge zone  314  where the substrate is exposed to an inert purge gas P, a second reactive gas zone  315  where the substrate is exposed to a second reactive G 2 , another inert purge zone  314  where the substrate is exposed to the purge gas P, and another first reactive gas zone  313  where the substrate is exposed to the first reactive gas G 1 . Moving in the reverse direction from right-to-left through the deposition zone  305 , the substrate  97  is exposed to the same sequence of gases as in the forward (left-to-right) direction, namely the first reactive gas G 1 , the inert purge gas P, the second reactive gas G 2 , the inert purge gas P, and the first reactive gas G 1 . The advantage of this symmetry is that feeding the substrate  97  from left-to-right or right-to-left results in equivalent exposure, and entrance and exit sides of the deposition head  30  depend of the direction of relative motion of the substrate  97  not the design of the deposition head  30 . 
     As with the previous embodiments, the gas zones (or regions) are between the substrate  97  and the deposition head  30 . The labels in  FIG. 3A  are placed above the substrate for clarity and to further emphasize the small working distance  94  between the process-side of substrate  97  and the output face  134  of the deposition head  30  enabled by the use of a vacuum-preloaded gas bearing deposition head  30 . As illustrated in the plan-view of  FIG. 3B , in addition to the output slots  112  (shown as black lines) and the exhaust slots  114  (shown as gray lines) in the deposition zone  305  (shown as a shaded area), there are additional output slots  320  orthogonal to the gas slots  110  in the deposition zone  305 . The additional gas output slots  320  provide inert gas to the cross-track edge region of the deposition head  30 , providing further isolation of the deposition zone  305  from the external environment  15 . 
     The exemplary gas bearing deposition head  30  of  FIG. 3A  has gas slots  110  corresponding to 1.5 ALD cycles to provide the proper sequence of gas exposure in the forward and reverse directions. As the substrate  97  is oscillated back and forth over the deposition head  30 , it will provide only a single ALD cycle (one G 1  and one G 2  exposure) per single direction pass over the deposition head  30 , therefore a round trip oscillation provides two ALD cycles. Furthermore, when the second precursor G 2  is reactive with the external environment, while the first precursor G 1  is not, this arrangement provides additional protection against unwanted reactions involving G 2 . An example of a precursor pair that would benefit from this arrangement is water and trimethylaluminum (TMA), where water is the non-reactive precursor G 1  and TMA is the highly reactive precursor G 2 . 
     An SALD deposition head operating as a vacuum-preloaded gas bearing has been described in the aforementioned U.S. Pat. No. 7,572,686 (Levy et al.). As noted, the use of a vacuum-preloaded gas bearing can provide efficiency of materials utilization, freedom from gas intermixing, and fast reaction kinetics due to the very small gap between the substrate  97  (deposition side) and the output face  134  of the deposition head  30 . 
     Inlet ports (not shown in  FIGS. 3A-3B ) are used to supply the various gasses to the deposition head  30 . When there are multiple gas slots  110  that supply the same gas, a single inlet port is typically used to supply the gas, and a manifold is used to distribute the gas to the appropriate gas slots  110 . For example, a first inlet port can provide a supply of the first reactive gas G 1  to the deposition head  30 , and a first manifold can direct the gas to the output slots  112  in the first reactive gas zones  313 . A second inlet port can provide a supply of the second reactive gas G 2  to the deposition head  30 , and a second manifold can direct the gas to the output slot  112  in the second reactive gas zones  315 . A third inlet port can provide a supply of the purge gas P to the deposition head  30 , and a third manifold can direct the gas two the output slots  112  in the purge zones  314 . Typically, the purge gas G and the inert gas I are the same gas. In this case, the third manifold can also direct the purge gas to the output slots  112  in the inert zones  308 ,  309 . Otherwise, a separate inlet port can be provided for the inert gas. Likewise, the exhaust slots  114  are connected to one or more exhaust ports using corresponding gas manifolds. Because the exhaust gases may still contain quantities of unreacted precursors, it may be undesirable to allow an exhaust flow predominantly containing one reactive species to mix with one predominantly containing another species. As such, the deposition head  30  may contain several independent exhaust conduits. 
     Depending on the relative sizes of the deposition head  30 , the first and second reactive gas zones  313 ,  315 , the purge zones  314  and the substrate  97 , some parts of the surface of the substrate  97  can be simultaneously exposed to the first and second reactive gases G 1 , G 2 . For example, if the substrate  97  is larger than the deposition head  30 , or if the purge zone  314  is smaller than the substrate  97  a first portion of the substrate  97  will be exposed to the first reactive gas G 1  in the first reactive gas zone  313  at the same time a second portion of the substrate  97  is being exposed to the second reactive gas G 2  in the second reactive gas zone  315 . On the other hand, if the purge zone  314  is larger than the substrate  97 , then the entire substrate will be exposed to the first reactive gas G 1  prior to being exposed to the second reactive gas G 2 . 
     As mentioned earlier, the gas zones in the SALD deposition can have any geometry. In addition to the linear gas zone arrangement that has been described, another class of SALD deposition geometries includes gas zones arranged circularly around a central axis. An example of such an SALD system useful for wafer processing is illustrated U.S. Pat. No. 9,514,933 to Lei et al., entitled “Film deposition using spatial atomic layer deposition or pulsed chemical vapor deposition.” In this case, the deposition unit provides a series of gas zones in a radial configuration, and the substrate is transported through the gas zones on a rotating platen in a circular motion profile. Other useful SALD configurations are discussed in the article “Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition” by P. Poodt et al. (Journal of Vacuum Science Technology A, Vol. 30, 010802, January/February 2012). It should be understood to one skilled in the art that any of these SALD configurations have deposition units with the necessary first and second reactive gas zones, and can be used in accordance with the present invention by providing time-varying gas mixtures to one or both of the reactive gas zones by using a controller  600  to control appropriate mixing systems  640 ,  645 . 
     In some SALD configurations, a portion of the substrate may be exposed with each gas zone (as illustrated in  FIGS. 1-3 ) or alternatively, the entire substrate can be exposed within each reactive gas zone. In embodiments where the entire substrate is exposed to the first reactive gaseous material prior to being exposed to the second reactive gaseous material, the substrate is moved from zone to zone similarly to the illustrated SALD system however the gas zones are large enough to accommodate an entire substrate. When depositing layers on substrates of finite size, such as wafers or glass parts, this can be advantageous. 
     In typical SALD configuration, the first reactive gaseous material G 1  can be an oxygen or chalcogenide containing gaseous material ( 0 ); and the second reactive gaseous material G 2  can be a metal-containing compound (M), such as a material containing zinc. The inert gaseous material I (which in an exemplary embodiment will be assumed to be the same as the purge gas P) can be gases such as nitrogen, argon or helium. The inert gaseous material I is inert with respect to first and second reactive gaseous materials M and O. 
     While the operation of the SALD configurations has been described with respect to a first reactive gaseous material G 1  and a second reactive gaseous material G 2 , it should be understood that the first and second reactive gas zones  313  and  315  can contain additional gases. For example, in some configurations the first reactive gas zone  313  can contain a mixture of first reactive gaseous material G 1  and an inert gas, and similarly the second reactive gas zone  315  can contain a mixture of the second reactive gaseous material G 2  and an inert gas. In the present invention, both the first reactive gas zone  313  and the second reactive gas zone  315  can contain a mixture of multiple reactive species, so long as they meet the requirements necessary to accomplish ALD film growth. Commonly-assigned U.S. Pat. No. 8,361,544 to Fedorovskaya et al., entitled “Thin film electronic device fabrication process,” which is incorporated herein by reference, discloses the use a mixture of reactant gases, for example, a mixture of metal precursor materials or a mixture of metal and non-metal precursors which may be applied at a single output channel. 
     In accordance with the present invention, two or more reactive precursor gases are present in first reactive gas zone  313 . In an exemplary embodiment using delivery head  30 , gas streams from gas sources for the two or more reactive precursor gases are combined before exhausting from an output slot  112  in the delivery head  30 . This is preferably obtained by joining the gas flows external to the delivery head  30  but may occur in the delivery head  30  if the gases are sufficiently mixed before impingement on the surface of the substrate  97 . Two precursors are required (e.g., reactants M 1  and M 2 ) that do not chemically interact with each other, at least at the temperature of the deposition head  30 . Both precursors will react with a third precursor (e.g., reactant O). The reactions yield M 1 O and M 2 O respectively. When the M 1  and M 2  precursors are mixed and react with the substrate surface which is terminated with O, a mix of M 1 O and M 2 O results. In a preferred configuration, the M 1 O and M 2 O precursors are chosen such that the respective refractive indexes (or other material properties) of the films are substantially different. It should be understood that M 1 O and M 2 O are abbreviations for the compounds formed and should not limit the present invention to metal-oxide. Furthermore, it should be understood that the exact stoichiometry of the thin-film materials will depend on the precursors used in the process. 
     For fabricating compositionally-graded thin films, examples of useful M 1 O and M 2 O compounds are TiO 2  and Al 2 O 3 , which can be deposited by spatial ALD, and have refractive indexes of 2.4 and 1.6, respectively. Generally, M 1 O and M 2 O can be chosen to be any materials whose properties are complementary, whose precursors have the attributes mentioned, and that can provide the desired properties in the compositionally-graded thin films. In the fabrication of Rugate filters, for example, any high refractive index materials may be used when coupled with a low refractive material as long as the precursors have the attributes mentioned and are not significantly absorbing at the desired wavelengths. Example high refractive index materials include ZnO, ZrO 2 , HfO 2 . An example low refractive index material is SiO 2 . 
     Two metal precursors which do not react until higher than 200° C. are trimethyl aluminum and titanium tetrachloride. When used with water as the co-reactant, it is possible to deposit a film of any refractive index between 1.6 and 2.4. The ability to obtain a specific refractive index is directly attributable to the thin-film composition. In fact, refractive index is a useful measurable property to determine the composition of functionally-graded thin films for use in applications other than optics. 
     For visible wavelength filters, exemplary materials for substrate  97  are BK-7, fused silica, and sapphire. For IR wavelength filters, exemplary materials for substrate  97  are sapphire, zinc selenide, and germanium. 
     A diagram illustrating a deposition system  400  is shown in  FIG. 4 . In an exemplary embodiment, controller  600  (which can also be referred to as a computer) controls the gas flows through deposition head  30  to form a compositionally-graded thin film on substrate  97 , such that the layers of deposited material have a uniform composition in the plane of the substrate, and vary as a function of thickness in the z-direction (i.e., height above the substrate). The controller  600  has access to information necessary to control the gas flows in order to achieve a layer of deposited material having a specified refractive index for each oscillation of the substrate  97  over the deposition head  30  in order to form a thin-film coating having a specified refractive index, or other material property, as a function of height. This information is generated from data characterizing the refractive index and the growth rate of the deposited thin film as a function of the ratio of the M 1  and M 2  precursors. The information for the gas flows is transmitted along wiring  620  to control mass flow controllers  410  in sync with the motion of the substrate  97 . 
     The controller  600  also controls the motion of the substrate  97  by communicating signals to a motor  660  (or to a motor controller) through wiring  630 . Typically, the substrate  97  is oscillated back and forth on a stage  650  with an acceleration at each reversal of direction and an intervening interval of constant velocity. Movement of the substrate  97  in a forward direction and then back in a reverse direction is considered to be an “oscillation.” The oscillation of the substrate is one method of achieving relative motion between the deposition unit and the substrate surface, alternatively other types of motion profiles are possible, including moving the deposition head  30  over a stationary substrate  97 , or moving both the deposition head  30  and the substrate  97 . In alternative system designs, the substrate  97  rotates at a continuous velocity through gas zones positioned around a central axis. Alternate motion profiles are also within the scope of the present invention, including variations in velocity as a function of film height or other such variations. 
     The deposition head  30  and the stage  650  (i.e., the substrate support mechanism) are preferably heated by a thermal heater, radiant heater, or any other method known to those skilled in the art. The stage  650  in  FIG. 4  is shown moving the substrate  97  below the deposition head but it could be in any orientation (e.g., in an inverted orientation or a vertical orientation). Bubblers  505  and  515  are fed by separate inert gas conduits  415  controlled by corresponding mass flow controllers  410  which receive an inert gas flow from an inert gas source  420 . The bubblers  505  and  515  contain first and second reactive precursors (e.g., metal precursors M 1  and M 2 ), respectively. The output of the bubbler  505  is a gas flow including a first reactive gaseous species (e.g., the metal precursor M 1 ) flowing through a first reactive gas species conduit  500 . Likewise, the output of the bubbler  515  is a gas flow including a second reactive gaseous species (e.g., the metal precursor M 2 ) flowing through a second reactive gas species conduit  510 . The bubbler  505 , together with the first reactive gas species conduit  500 , the inert gas source  420 , and the corresponding mass flow controller  410  and inert gas conduit  415  can be considered to be a first gaseous source which provides a gas flow of the first reactive gaseous species M 1 . Likewise, the bubbler  515 , together with the first reactive gas conduit  510 , the inert gas source  420 , and the corresponding mass flow controller  410  and inert gas conduit  415  can be considered to be a second gaseous source which provides a gas flow of the second reactive gaseous species M 2 . 
     The gas flows of the first and second reactive gaseous species are combined in a mixing system  640 , together with an inert gas flow in an inert gas conduit  530 , to provide a first reactive gaseous material having a homogeneous gaseous mixture of the first and second reactive gaseous species and the inert gas to gas inlet conduit  434 . In an exemplary embodiment, the mixing system  640  is simply a series of conduit joints where the gas flows through the individual conduits are merged into a combined gas flow. With this arrangement, the gaseous elements in the individual gas flows will mix together to provide the homogeneous gaseous mixture. Other types of mixing systems  640  can also be used including those which include active or passive mixing devices which can be used to speed the formation of the homogeneous mixture. An example of an active mixing device would be a stirring device which stirs the gas flow as it passes through the mixing system  640 . An example of a passive mixing device would be a series of baffles which the gas flow passes through to redirect the gas flow. The controller  600  controls the concentrations and the ratio of the first and second reactive gaseous species in the first reactive gaseous material, together with the total gas flow through the gas inlet conduit  434 , by controlling the corresponding mass flow controllers  410 . 
     Similarly, bubbler  525  is fed by inert gas conduit  415  and controlled by a corresponding mass flow controller  410  which receives an inert gas flow from the inert gas source  420 . The bubbler  525  contains a third reactive precursor (e.g., reactant O). The output of the bubbler  525  is a gas flow including the third reactive gaseous species (e.g., the reactant O) flowing through a third reactive gas species conduit  520 . The bubbler  525 , together with the third reactive gas species conduit  520 , the inert gas source  420 , and the corresponding mass flow controller  410  and inert gas conduit  415  can be considered to be a third gaseous source which provides a gas flow of the third reactive gaseous species. This gas flow is combined with an inert gas flow in an inert gas conduit  540  using a mixing system  645  to provide a gas flow of a second reactive gaseous material including the third reactive gaseous species through gas inlet conduit  436 . The controller  600  controls the concentration of the third reactive gaseous species, together with the total gas flow provided through the gas inlet conduit  436 , by controlling the corresponding mass flow controllers  410 . 
     The first reactive gaseous material including the homogeneous gaseous mixture of the first and second reactive gaseous species (e.g., reactants M 1  and M 2 ) flowing through the gas inlet conduit  434 , and the second reactive gaseous material including the third reactive gaseous species (e.g., reactant O) flowing through the gas inlet conduit  436  enter the deposition head  30 , together with an inert purge gas flowing through gas inlet conduit  438 . Gas manifolds in the deposition head  30  are used to direct the gas flows to the appropriate reactive gas zones (e.g., through the output slots  112  in the geometry illustrated in  FIG. 3A ) in order to provide the desired ALD process onto the substrate  97  as it is moved relative to the gas zones  313 ,  314 ,  315 . Gases are continuously exhausted from the deposition head  30  illustrated in  FIG. 3A  through exhaust slots  114  and then through one or more exhaust conduits  424 , which are generally connected to corresponding vacuum systems. The components used to supply the gaseous materials to the gas zones of the deposition head  30  can collectively be referred to as a gas delivery system. 
     It may be appreciated that the bubblers  505 ,  515 ,  525  are only used for cases where the reactive precursors are liquids. In other embodiments, one or more of the reactive precursors can be gaseous materials. In this case, the reactive gaseous materials can be supplied by a corresponding gas source and controlled directly with an associated mass flow controller. 
     To generate a compositionally-graded thin-film, it is necessary to know both the refractive index (or other material property) and the growth rate for different ratios of the first and second reactive gaseous species in the homogeneous gaseous mixture of the first reactive gaseous material. Usually this can be obtained by direct measurements of uniform thick films. With that information, the appropriate gas flows needed to provide the appropriate ratios for each oscillation of the substrate  97  can be calculated. A table of the required gas flows can be predetermined, or the information can be generated by the system using the calibration information and desired film properties. This information can then be used to control the mass flow controllers  410  in sync with the motion of the substrate  97  relative to the deposition head  30 . For use in the exemplary oscillating system, the syncing can be closed loop where after each oscillation a signal is sent to the controller  600  to transmit the information to the mass flow controllers  410 . The syncing can also be open loop where the length of time for each ALD cycles within the motion profile is known and the information is transmitted to the mass flow controllers  410  at the appropriate time. 
     Example #1 
     The open source program Openfilters was used to generate a target refractive index versus height above substrate profile for a two-band reflection filter having bands centered on 580 nm and 710 nm.  FIG. 5  is a graph  700  showing the resulting target refractive index versus height from substrate, and  FIG. 6  is a graph  710  showing the corresponding calculated reflection spectrum. This reflection filter is an example of an optical interference filter or rugate filter composed of a compositionally-graded thin film. 
     A filter design was determined where the first reactive precursor M 1  was titanium tetrachloride (TiCl 4 ) and the second reactive precursor M 2  was trimethyl aluminum (TMA). The flow rates of the first and second reactive gaseous species were controlled by mass flow controllers  410  passing dry nitrogen through bubblers  505 ,  515  containing the reactive precursors. The total gas flow of nitrogen passing through the bubblers  505 ,  515  always totaled 25 sccm. They were mixed with a 1500 sccm dilution of dry nitrogen through inert gas conduit  530  and provided to the metal output slots  112  ( FIG. 1 ) on the deposition head  30 . Water was used as the oxygen source using 40 sccm through bubbler  225  and was mixed with a 2250 sccm dilution of dry nitrogen through inert gas conduit  540  and provided to the oxygen output slots  112 . The metal and oxygen output slots  112  were separated by purge gas output slots  112  supplied by dry nitrogen at 3000 sccm. 
     The refractive index as a function of the percentage of TiCL 4  in the homogeneous gaseous mixture was determined experimentally using the deposition system  100  ( FIG. 3 ) to deposit thin films on a BK-7 glass substrate  97 . The resulting first calibration function is shown in graph  720  of  FIG. 7 . The growth per oscillation as a function of the percentage of TiCL 4  in the homogeneous gaseous mixture was also determined and the resulting second calibration function is shown in graph  730  of  FIG. 8 . The deposition head  30  included 2.5 cycles, arranged similarly to the 1.5 cycle configuration of  FIG. 3B , providing four ALD cycles per oscillation. The refractive index profile of  FIG. 5  was used in conjunction with the calibration functions of  FIGS. 7-8  to determine a table of gas flows versus oscillation number required to form an interference filter having the reflection spectrum shown in  FIG. 5 . 
     A BK-7 glass substrate  97  was placed over the deposition head  30 . The controller  300  was loaded with the data for the flow rates versus oscillation number. A total of 10,436 oscillations were used where four ALD cycles were deposited per oscillation. The acceleration/deceleration was set to 1920 mm/sec 2  and the substrate velocity was 101.6 mm/sec. The total distance traveled by the substrate  97  was 36 mm. This gives an oscillation time of 0.74 sec. The gas flow rates were therefore adjusted every 0.74 sec. 
     The deposition head  30  and the substrate  97  were heated to 180° C. and the program was run, finishing in 129 minutes. The resulting reflection spectrum measured at 6 degrees off axis is shown in the graph  740  of  FIG. 9 . Note that two reflection bands were obtained as expected. It can be seen that the peak wavelengths of the reflection bands are slightly blue shifted relative to the theoretical reflection spectrum shown in  FIG. 6 , presumably due to small calibration errors of the growth per oscillation. 
     Example #2 
     To demonstrate that the reflection bands can easily be shifted, and that errors in the laydown calibrations were likely responsible for the shifts observed in  FIG. 9 , the x-axis of target refractive index profile in  FIG. 5  was stretched by 15% and a new table of gas flows versus oscillation number was determined. The total number of oscillation now became  12 , 001  and required 149 minutes to complete. The measured reflection spectrum of the resulting thin-film filter is shown in the graph  750  of  FIG. 10 . It can be observed that the peak wavelengths of the reflection bands have both been shifted in the red direction as expected. 
     In the exemplary embodiment illustrated in  FIG. 4 , the first reactive gaseous material is a homogeneous mixture of first and second reactive precursors (i.e., metal precursors M 1  and M 2 ) and the second reactive gaseous material contains a third reactive precursor (i.e., reactant O). However, it will be obvious to one skilled in the art that this configuration can be generalized so that the first and second reactive gaseous materials can include different numbers of reactive precursors. For example, the first reactive gaseous material can include a mixture of more than two reactive precursors. Similarly, the second reactive gaseous material can include a mixture of a plurality of reactive precursors. The mixing systems  640 ,  645  will provide a homogeneous mixture of the associated reactive precursors. 
     It is clear that other refractive index profiles can be obtained by this method in a relatively short period of time compared to conventional ALD and with better layer thickness and composition control than are obtainable using sputtering processes. While the exemplary compositionally-graded thin films described here have related to the formation of compositionally-graded thin film optical interference filters (e.g., rugate filters), it will be obvious to one skilled in the art that the same fabrication processes can be used to form other types of thin-film devices including compositionally-graded thin films. The invention is particularly well-suited to thin-film devices which require depositing layers having different compositions to provide different optical, electrical or mechanical properties. 
     Other examples of thin-film optical devices that can be fabricated using the process of the present invention would include optical waveguides that are useful for guiding light on the surface of a substrate. Such devices require varying the refractive index as a function of height above the substrate and can be fabricated using the method described earlier. 
     Encapsulation layers can be formed from compositionally-graded thin-film layers such that the permeability to oxygen and water vapor is improved over single component thin films. In semiconductor devices, compositionally-graded thin films are useful to control the electrical properties at critical interfaces and within the bulk of the material layer. For instance, it can be useful to controlling the band-gap and energy in semiconductor layers. Multiple applications benefit from using compositionally-graded films to tune the mechanical stress and strain properties of thin-film devices. Compositionally-graded thin-films of Al 2 O 3  and SiO 2  are useful as encapsulation layers and can be formed using the process of the present invention using TMA and SiCl 3 H as M 1  and M 2  in the homogenous gas mixture in first reactive gas. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
           15  external environment 
           30  deposition head 
           94  working distance 
           97  substrate 
           98  motion arrow 
           110  gas slot 
           112  output slot 
           114  exhaust slot 
           134  output face 
           200  SALD system 
           205  deposition subsystem 
           210  deposition unit 
           270  relative motion system 
           280  substrate positioner module 
           305  deposition zone 
           308  inert zone 
           309  inert zone 
           313  first reactive gas zone 
           314  purge zone 
           315  second reactive gas zone 
           320  output slot 
           400  deposition system 
           410  mass flow controller 
           415  inert gas conduit 
           420  inert gas source 
           424  exhaust conduit 
           434  gas inlet conduit 
           436  gas inlet conduit 
           438  gas inlet conduit 
           500  first reactive gas species conduit 
           505  bubbler 
           510  second reactive gas species conduit 
           515  bubbler 
           520  third reactive gas species conduit 
           525  bubbler 
           530  inert gas conduit 
           540  inert gas conduit 
           600  controller 
           620  wiring 
           630  wiring 
           640  mixing system 
           645  mixing system 
           650  stage 
           660  motor 
           700  graph 
           710  graph 
           720  graph 
           730  graph 
           740  graph 
           750  graph