Patent Publication Number: US-2019186012-A1

Title: Thin-film optical device with varying layer composition

Description:
FIELD OF THE INVENTION 
     This invention pertains to the field of optical filters and more particularly to optical reflection filters fabricated using an atomic layer deposition process. 
     BACKGROUND OF THE INVENTION 
     Rugate filters, also known as gradient index reflection filters, are a type of optical reflection filter. Rugate filters differ from discrete stacked filters in that the index of refraction varies as a function of the height with 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 technique to apply the film is typically by 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 to 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. 
     A technique which yields extremely precise layer thicknesses and uniformity is atomic layer deposition (ALD). Atomic layer deposition (“ALD”) is a film deposition technology that can provide improved thickness resolution and conformal capabilities, compared to sputtering of the materials. 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 chemical vapor deposition reaction. The goal of any system claiming to perform ALD is to obtain device performance and attributes commensurate with an ALD system. 
     In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. For example, a metal precursor molecule, ML x , comprises a metal element, M that is bonded to an atomic or molecular ligand, L. For example, M could be, but would not be restricted to, Al, W, Ta, Si, Zn, etc. The metal precursor reacts with the substrate when the substrate surface is prepared to react directly with the molecular precursor. For example, the substrate surface typically is prepared to include hydrogen-containing ligands, AH or the like, that are reactive with the metal precursor. Sulfur (S), oxygen (O), and Nitrogen (N) are some typical A species. The gaseous metal precursor molecule effectively reacts with all of the ligands on the substrate surface, resulting in deposition of a single atomic layer of the metal: 
       substrate-AH+ML x →substrate-AML x-1 +HL   (1)
 
     where HL is a reaction by-product. During the reaction, the initial surface ligands, AH, are consumed, and the surface becomes covered with L ligands, which cannot further react with metal precursor ML x . Therefore, the reaction self-terminates when all of the initial AH ligands on the surface are replaced with AML x-1  species. The reaction stage is typically followed by an inert-gas purge stage that eliminates the excess metal precursor from the chamber prior to the separate introduction of a second reactive gaseous precursor material. 
     The second molecular precursor is then used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and redepositing AH ligands. In this case, the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H 2 O, NH 3 , H 2 S). The next reaction is as follows: 
       substrate-A-ML+AH y →substrate-A-M-AH+HL   (2)
 
     This converts the surface back to its AH-covered state. (Here, for the sake of simplicity, the chemical reactions are not balanced.) The desired additional element, A, is incorporated into the film and the undesired ligands, L, are eliminated as volatile by-products. Once again, the reaction consumes the reactive sites (this time, the L terminated sites) and self-terminates when the reactive sites on the substrate are entirely depleted. The second molecular precursor then is removed from the deposition chamber by flowing inert purge-gas in a second purge stage. 
     In summary, then, 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.
       

     ALD has been used for making reflective interference filters. 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 similar to ALD in that there are two or more reactive precursors, but the difference is that the gases are not pulsed but instead free flowing and a portion of the sample is moved from one effective chamber to another. In the simplest Spatial ALD configuration, the substrate is flat and forms a wall of a micro chamber. 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 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. 
     There is a need therefore for a process which does not require the flushing of gases, the concomitant waste of materials and a much more rapid deposition of intermediate refractive index layers. 
     SUMMARY OF THE INVENTION 
     The present invention represents a process of making a thin-film optical device including: 
     providing a substrate; 
     providing a plurality of gaseous material sources including a first gaseous source providing a first reactive gaseous material, a second gaseous source providing a second reactive gaseous material, a third gaseous source providing a third reactive gaseous material, and an inert gaseous material source providing an inert gaseous material, wherein the first reactive gaseous material and the second reactive gaseous material each react with the third reactive gaseous material but do not react with each other under a specified set of operating conditions; 
     providing a mixing system to mix a controllable ratio of the first and second reactive gaseous materials to provide a homogeneous gaseous mixture; 
     providing a delivery head in fluid communication with the mixing system, the third gaseous material source and the inert gaseous material source through a plurality of inlet ports, the mixing system being connected to a first inlet port, the third gaseous material source being connected to a second inlet port, and the inert gaseous material source being connected to a third inlet port, the delivery head including an output face having a first plurality of elongated substantially parallel output channels connected in fluid communication with the first inlet port, a second plurality of elongated substantially parallel output channels connected to a second inlet port, and a third plurality of elongated substantially parallel output channels connected to a third inlet port, wherein at least some of the third elongated output channels are positioned to separate at the first elongated output channels and the second elongated output channels; 
     simultaneously directing the homogeneous gaseous mixture, the third reactive gaseous material, and the inert gaseous material to flow through the first elongated output channels, the second elongated output channels, and the third elongated output channels, respectively, of the delivery head toward the substrate; 
     causing an oscillating relative motion between the delivery head and the substrate to cause the third reactive gaseous material to react with a portion of the substrate that has been treated with the homogeneous gaseous mixture thereby forming thin film layers of deposited material; and 
     controlling the mixing system to change the ratio of the first and second reactive gaseous materials as a function of time such that the thin film layers of deposited material have a varying composition. 
     This invention has the advantage that it enables the formation of an gradient index optical interference filter without the use a vacuum chamber. 
     It has the additional advantage that a gradient index is rapidly generated with molecular layer precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of one embodiment of a delivery device for atomic layer deposition according to the present invention; 
         FIG. 2  is a cross-sectional side view of one embodiment of a delivery device for atomic layer deposition with a more detailed view of the distributions of the gases for the present invention; 
         FIG. 3  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. 4  is a graph showing a refractive index profile specifying the refractive index versus height above the substrate for a two-band reflector; 
         FIG. 5  is a graph showing the calculated reflection spectrum of a thin film interference filter generated from the refractive index profile of  FIG. 4 ; 
         FIG. 6  is a graph showing measured refractive index as a function of the percentage of the first reactive precursor in the homogeneous gaseous mixture; 
         FIG. 7  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. 8  is a graph showing the experimental reflection spectrum of a thin film interference filter generated according to the refractive index profile of  FIG. 4 ; and 
         FIG. 9  is a graph showing the experimental reflection spectrum of a thin film interference filter generated based on the refractive index profile of  FIG. 4  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 
     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 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. 
     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, or materials. 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. 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. 
     For the description that follows, superposition has its conventional meaning, wherein elements are laid atop or against one another in such manner that parts of one element align with corresponding parts of another and that their perimeters generally coincide. The terms “upstream” and “downstream” have their conventional meanings as relates to the direction of gas flow. 
     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. 
     There are two types of spatial ALD heads. In one the gasses flow down the open channel or slot perpendicular to the transport direction. This type is exemplified in U.S. Pat. No. 7,456,429, which is incorporated herein by reference. This type will suffer from the same issues that standard ALD does in that the when two gases are mixed, the more reactive one will deposit at the entrance and the ratio will change as the gas flow moves to the exhaust. 
     The second type of deposition head is exemplified in U.S. Pat. Nos. 7,572,686 and 8,182,608, which are incorporated herein by reference. This deposition head is the type shown in  FIG. 1  wherein the gases impinge on the sample substantially perpendicularly and flow substantially parallel to the transport direction. This type of deposition head is preferred for use in the method of the present invention. 
     Referring to  FIG. 1 , there is shown a cross-sectional side view of an exemplary embodiment of a delivery head  10  for atomic layer deposition onto a substrate  20  in accordance with the present invention. Delivery head  10  has a gas inlet conduit  14  that serves as an inlet port for accepting a first gaseous material, a gas inlet conduit  16  for an inlet port that accepts a second gaseous material, and a gas inlet conduit  18  for an inlet port that accepts a third gaseous material. These gases are emitted at a depositing output face  36  via output channels  12 , having a structural arrangement that may include a diffuser. 
     The dashed line arrows in  FIG. 1  and subsequent  FIG. 2  refer to the delivery of gases to substrate  20  from delivery head  10 . In  FIG. 1 , dotted line arrows also indicate paths for gas exhaust (shown directed upwards in this figure) through exhaust channels  22 , in communication with an exhaust conduit  24  that provides an exhaust port. (For simplicity of description, gas exhaust is not indicated in  FIG. 2 .) 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, it is recognized that the delivery head  10  may contain several independent exhaust conduits  24 . The pressure generated by the flow of the gases through the output channels  12  create a gas fluid bearing that maintains a substantially uniform distance between the output face  36  of the delivery head  10  and the substrate  20 . 
     In an exemplary embodiment, gas inlet conduits  14  and  16  are adapted to accept first and second gases that react sequentially on the substrate surface to effect ALD deposition, and gas inlet conduit  18  receives a purge gas that is inert with respect to the first and second gases. Delivery head  10  is spaced a distance D from substrate  20 , which may be provided on a substrate support  96 , as described in more detail subsequently. 
     Reciprocating motion can be provided between the substrate  20  and the delivery head  10 , either by movement of the substrate  20 , by movement of the delivery head  10 , or by movement of both the substrate  20  and the delivery head  10 . In the exemplary embodiment shown in  FIG. 1 , substrate  20  is moved by a substrate support  96  across the output face  36  in a reciprocating fashion, as indicated by the arrow A and by phantom outlines to the right and left of substrate  20  in  FIG. 1 . It should be noted that reciprocating motion is not always required for thin-film deposition using delivery head  10 . Other types of relative motion between substrate  20  and delivery head  10  could also be provided, such as movement of either substrate  20  or delivery head  10  in one or more directions. 
     The cross-sectional view of  FIG. 2  shows gas flows emitted over a portion of the output face  36  of delivery head  10  (with the exhaust path omitted as noted earlier). In this particular arrangement, each output channel  12  is in gaseous flow communication with one of gas inlet conduits  14 ,  16  or  18  seen in  FIG. 1 . Each output channel  12  delivers typically a first reactive gaseous material M, or a second reactive gaseous material O, or a third inert gaseous material I. 
     The configuration of  FIG. 2  shows a relatively basic or simple arrangement of gases. It is envisioned that a plurality of non-metal deposition precursors (like material O) or a plurality of metal-containing precursor materials (like material M) may be delivered sequentially at various ports in a single thin-film deposition. Alternately, a mixture of reactive gases, for example, a mixture of metal precursor materials or a mixture of metal and non-metal precursors may be applied at a single output channel when making complex thin film materials, for example, having alternate layers of metals or having lesser amounts of dopants admixed in a metal oxide material. Significantly, an inter-stream labeled I for an inert gas, also termed a purge gas, separates any reactive gas channels in which the gases are likely to react with each other. First and second reactive gaseous materials M and O react with each other to effect ALD deposition, but neither reactive gaseous material M nor O reacts with the inert gaseous material I. 
     The nomenclature used in  FIG. 2  and following suggests some typical types of reactive gases. For example, first reactive gaseous material M can be a metal-containing compound, such as a material containing zinc; and second reactive gaseous material O can be an oxygen or chalcogenide containing gaseous material. The inert gaseous material I can be nitrogen, argon, helium, or other gases commonly used as purge gases in ALD systems. Inert gaseous material I is inert with respect to first and second reactive gaseous materials M and O. Reaction between the first and second reactive gaseous materials would form a metal oxide or other binary compound, such as zinc oxide ZnO or ZnS, used in semiconductors, in one embodiment. 
     The delivery head  10  may have one or more sets of reactive precursors. A set of reactive precursors includes both M and O reactants, along with the concomitant inert gas slots and exhaust slots. If there are more than one set, then more layers are deposited per pass of the substrate  20  under the delivery head  10 . 
     In this invention, two or more precursor gas streams are combined before exhausting from a single output channel  12  in the delivery head  10 . This is preferably obtained by joining the gas flows external to the delivery head  10  but may occur in the delivery head  10  if the gases are sufficiently mixed before impingement on the surface of the substrate  20 . Two precursors are required (e.g., reactants M 1  and M 2 ) which do not chemically interact, at least at the temperature of the delivery head  10 . Both precursors will react with 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 surface of 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 of the films are maximally different. 
     Preferred examples of M 1 O and M 2 O are TiO 2  and Al 2 O 3  which can be generated as films by spatial ALD having refractive indexes of 2.4 and 1.6, respectively. 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. Other example high refractive index materials are ZnO, ZrO 2 , HfO 2 . A 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. 
     For visible wavelength filters, exemplary materials for substrate  20  are BK-7, fused silica, and sapphire. For IR wavelength filters, exemplary materials for substrate  20  are sapphire, zinc selenide, and germanium. 
     A diagram illustrating a deposition system  100  is shown in  FIG. 3 . A controller  300  (which can also be referred to as a computer) controls the gas flows through delivery head  10  to form a thin-film optical device on substrate  20 , such that the layers of deposited material have a varying composition. The controller 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  20  over the delivery head  10  in order to form a thin film coating having a specified refractive index 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  320  to control mass flow controllers  110  in sync with the motion of the substrate  20 . 
     The controller  300  also controls the motion of the substrate  20  by communicating signals to a motor  360  (or to a motor controller) through wiring  330 . Typically, the substrate  20  is oscillated back and forth on a stage  350  with an acceleration at each reversal of direction and an intervening interval of constant velocity. Movement of the substrate  20  in a forward direction and then back in a reverse direction is considered to be an “oscillation,” or equivalently a “cycle.” 
     The delivery head  10  and stage  350  are preferably heated by a thermal heater, radiant heater, or any other method known to those skilled in the art. The stage  350  in  FIG. 3  is shown moving the substrate  20  below the deposition head but it could be in any orientation (e.g., in an inverted orientation or a vertical orientation). 
     Bubblers  205  and  215  are fed by separate inert gas conduits  115  controlled by corresponding mass flow controllers  110  which receive an inert gas flow from an inert gas source  120 . The bubblers  205  and  215  contain first and second reactive precursors (e.g., metal precursors M 1  and M 2 ), respectively. The output of the bubbler  205  is a gas flow including the first reactive gaseous material (e.g., the metal precursor M 1 ) flowing through a first reactive gas conduit  200 . Likewise, the output of the bubbler  215  is a gas flow including the second reactive gaseous material (e.g., the metal precursor M 2 ) flowing through a second reactive gas conduit  210 . The bubbler  205 , together with the first reactive gas conduit  200 , the inert gas source  120 , and the corresponding mass flow controller  110  and inert gas conduit  115  can be considered to be a first gaseous source which provides a gas flow of the first reactive gaseous material. Likewise, the bubbler  215 , together with the first reactive gas conduit  210 , the inert gas source  120 , and the corresponding mass flow controller  110  and inert gas conduit  115  can be considered to be a second gaseous source which provides a gas flow of the second reactive gaseous material 
     The gas flows of the first and second reactive gaseous materials are combined in a mixing system  340 , together with an inert gas flow in an inert gas conduit  230 , to provide a homogeneous gaseous mixture to gas inlet conduit  14 . In an exemplary embodiment, the mixing system  340  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 a homogeneous gaseous mixture. Other types of mixing systems  340  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  340 . 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  300  controls the concentrations and the ratio of the first and second reactive gaseous materials in the homogeneous gaseous mixture, together with the total gas flow through the gas inlet conduit  14 , by controlling the corresponding mass flow controllers  110 . 
     Similarly, bubbler  225  is fed by an inert gas conduits  115  controlled by a corresponding mass flow controllers  110  which receives an inert gas flow from the inert gas source  120 . The bubbler  225  contains a third reactive precursors (e.g., reactant O). The output of the bubbler  225  is a gas flow including the third reactive gaseous material (e.g., the reactant O) flowing through a third reactive gas conduit  220 . This gas flow is combined with an inert gas flow in an inert gas conduit  240  using a mixing system  345  to provide a gas flow including the third reactive gaseous material through gas inlet conduit  16 . The controller  300  controls the concentration of the third reactive gaseous, together with the total gas flow provided through the gas inlet conduit  16 , by controlling the corresponding mass flow controllers  110 . The bubbler  225 , together with the third reactive gas conduit  220 , the inert gas source  120 , and the corresponding mass flow controller  110  and inert gas conduit  115  can be considered to be a third gaseous source which provides a gas flow of the third reactive gaseous material. 
     The homogeneous gaseous mixture including the first and second reactive gaseous materials (e.g., reactants M 1  and M 2 ) flowing through the gas inlet conduit  14 , and the third reactive gaseous material (e.g., reactant O) flowing through the gas inlet conduit  16  enter the delivery head  10 , together with an inert purge gas flowing through gas inlet conduit  18 . A gas manifold in the delivery head  10  is used to direct the gas flows to the appropriate output channels  12  ( FIG. 1 ) in order to provide the desired ALD process onto the substrate  20  as it is moved past the output channels  12 . Exhaust gases from exhaust channels  22  ( FIG. 1 ) are exhausted from the delivery head  10  through one or more exhaust conduits  24 , which are generally connected to corresponding vacuum systems. 
     It may be appreciated that the bubblers  205 ,  215 ,  225  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 the gradient index filter, it is necessary to know both the refractive index and the growth rate for different ratios of the first and second reactive gaseous materials in the homogeneous gaseous mixture. 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  20  can be calculated. A table of the required gas flows can be predetermined, or the information can be generated on the fly. This information can then be used to control the mass flow controllers  110  in sync with the oscillations. The syncing can be closed loop where after each oscillation a signal is sent to the controller  300  to transmit the information to the mass flow controllers  110 . The syncing can also be open loop where the length of time for each oscillation is known and the information is transmitted to the mass flow controllers  110  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. 4  is a graph  400  showing the resulting target refractive index versus height from substrate, and  FIG. 5  is a graph  410  showing the corresponding calculated reflection spectrum. This reflection filter is an example of an optical interference filter or rugate filter. 
     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 gases were controlled by mass flow controllers  110  passing dry nitrogen through bubblers  205 ,  215  containing the reactive precursors. The total gas flow of nitrogen passing through the bubblers  205 ,  215  always totaled 25 sccm. They were mixed with a 1500 sccm dilution of dry nitrogen through inert gas conduit  230  and provided to the metal output channels  12  ( FIG. 1 ) on the delivery head  10 . 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  240  and provided to the oxygen output channels  12 . The metal and oxygen output channels  12  were separated by purge gas output channels  12  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  20 . The resulting first calibration function is shown in graph  420  of  FIG. 6 . 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  430  of  FIG. 7 . The delivery head  10  was a double outlet head giving four deposition layers per oscillation. The refractive index profile of  FIG. 4  was used in conjunction with the calibration functions of  FIGS. 6-7  to determine a table of gas flows versus oscillation number required to form an interference filter having the reflection spectrum shown in  FIG. 4 . 
     A BK-7 glass substrate  20  was placed under the delivery head  10 . 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 layers 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  20  was 36 mm. This gives an oscillation time of 0.74 sec. The gas flow rates were therefore adjusted every 0.74 sec. 
     The delivery head  10  and the substrate  20  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  440  of  FIG. 8 . 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. 5 , 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. 6 , the x-axis of target refractive index profile in  FIG. 4  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  450  of  FIG. 8 . It can be observed that the peak wavelengths of the reflection bands have both been shifted in the red direction as expected. 
     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 only two examples are given it will be obvious to one skilled in the art how to achieve optical interference filters with other reflection or transmission spectra shapes. Furthermore, while the exemplary embodiments described here have related to the formation of 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 optical devices. The invention is particularly well-suited to thin-film optical devices which require depositing layers having different compositions to provide different indices of refraction (e.g., a gradient index profile). Other examples of thin-film optical devices that can be fabricated using the process of the present invention would include waveguides that are useful for guiding light on the surface of a substrate. 
     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 
       10  delivery head
 
 12  output channel
 
 14  gas inlet conduit
 
 16  gas inlet conduit
 
 18  gas inlet conduit
 
 20  substrate
 
 22  exhaust channel
 
 24  exhaust conduit
 
 36  output face
 
 96  substrate support
 
 100  deposition system
 
 110  mass flow controller
 
 115  inert gas conduit
 
 120  inert gas source
 
 200  first reactive gas conduit
 
 205  bubbler
 
 210  second reactive gas conduit
 
 215  bubbler
 
 220  third reactive gas conduit
 
 225  bubbler
 
 230  inert gas conduit
 
 240  inert gas conduit
 
 300  controller
 
 320  wiring
 
 330  wiring
 
 340  mixing system
 
 345  mixing system
 
 350  stage
 
 360  motor
 
 400  graph
 
 410  graph
 
 420  graph
 
 430  graph
 
 440  graph
 
 450  graph