Abstract:
The present invention pertains generally to a high-index film deposited on a substrate, the film comprising a layer of a prescribed seed material and an overlaying layer of titanium dioxide (TiO 2 ). The seed material has a prescribed, uniform inter-atomic spacing adapted to cause the overlaying TiO 2  to have a high-index phase. The present invention also pertains generally to a method for forming a high-index film, comprising the steps of first forming a layer of a seed material having the prescribed, uniform inter-atomic spacing, and then forming a layer of TiO 2  atop the seed material, such that the TiO 2  has the high-index phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Priority is claimed to U.S. Provisional Application Ser. No. 61/061,080, filed on Jun. 12, 2008, the contents of which are incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to optical coatings and, more particularly, to optical coatings incorporating films of high-index titanium dioxide (TiO 2 ) and to methods for making such films and coatings. 
         [0003]    Dielectric coatings for optical interference filters generally comprise alternating layers of a material having a high refractive index and a material having a low refractive index, the alternating layers deposited on a substrate such as glass. It is desirable to have as large a difference as possible between the high and low refractive index values to make an effective filter and to minimize the thickness and production cost of a coating having a desired spectral performance. It is also desirable to use materials that exhibit as little absorption and scattering as possible in the wavelength range of interest in order to optimize transmission and reflection. 
         [0004]    Filters have been produced using atomic layer deposition (ALD) for a limited number of optical applications that require relatively thick coatings. ALD is a slow and expensive process for thick coatings, but ALD is useful if precise layer thickness and minimal defects are required. TiO 2  has been used in ALD optical filters, because TiO 2  has a high index of refraction (typically about 2.40 when deposited at about 300° C. from titanium tetrachloride (TiCl 4 ) and H 2 O precursors). However, because TiO 2  tends to crystallize readily above 150° C., and consequently exhibits greater scattering and absorption, TiO 2  is often laminated with other materials, such as aluminum oxide (Al 2 O 3 ), to limit crystal size and reduce scattering (see U.S. Patent Application Publication No. 2006/0134433 A1). 
         [0005]    Prior art work based upon lamination of thin TiO 2  layers takes advantage of the property of TiO 2  to remain amorphous for relatively thin layers when grown on a “randomly” ordered surface or on a surface having a significantly different crystal lattice. If the deposited TiO 2  layer thickness exceeds 10 to 20 nanometers (nm), however, the film starts to form a polycrystalline phase having a grain structure. The grains scatter light propagating though the film and lead to optical losses. If the TiO 2  is kept amorphous by limiting layer thickness with nano-lamination, the polycrystalline phase will not form, and the films will retain optical transparency. 
         [0006]    Unfortunately, there are two problems with the nano-lamination approach. First, the laminating material reduces the average refractive index of the high-index layer in which the laminating material is incorporated, since the laminating material has a relatively low refractive index. For example, Al 2 O 3  has a refractive index of only about 1.644 at 633 nm. Second, an amorphous film has a lower packing density, higher coefficient of thermal expansion (CTE), and lower index of refraction than a mono-crystalline film comprising the same molecules. The nano-laminated TiO 2  that is used for ALD optical filters is generally deposited on substrates at temperatures in the range of 270 to 350° C. These temperatures produce films that are primarily amorphous and that have a moderate density and a moderate composite index of refraction. TiO 2  films deposited by ALD at temperatures less than about 150° C. tend to have a low packing density, a low index of refraction, and high tensile stress. 
         [0007]    There is thus a need for a high-index material for use in an interference filter, the high-index material having a high index value (n), a low absorption coefficient (k), and low scattering. There is also a need for a method for producing such a high-index material. The present invention provides such a high-index material and a method for producing it. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention pertains generally to a thin film and optical interference filter incorporating a high-index titanium dioxide material. The film comprises a layer of a seed material having a prescribed, uniform inter-atomic spacing and a layer of TiO 2  deposited on the layer of seed material. The seed material has a prescribed, uniform inter-atomic spacing adapted to cause the overlaying TiO 2  to have a high-index phase. The present invention also pertains generally to a method for forming a high-index film, the method comprising forming a layer of a seed material having the prescribed, uniform inter-atomic spacing and forming over the layer of seed material a layer of TiO 2  in the high-index phase. 
         [0009]    In one embodiment, the present invention encompasses a high-index film comprising a layer of a seed material and a layer of TiO 2  deposited on the layer of the seed material, wherein the film has a refractive index of at least 2.55 and an absorption coefficient of at most 1×10 −4 , at a wavelength of 633 nm. The present invention also encompasses a method for forming high-index film having a refractive index of at least 2.55 and an absorption coefficient of at most 1×10 −4 , at a wavelength of 633 nm, the method comprising forming a layer of a seed material and forming a layer of TiO 2  on the layer of the seed material. The seed material preferably is selected from the group consisting of zirconium dioxide (ZrO 2 ) and hafnium dioxide (HfO 2 ). 
         [0010]    In one particular embodiment, the present invention pertains to a film comprising TiO 2  in a primarily mono-crystalline (rutile) phase with minimum threading dislocations and crystal defects (which lead to optical losses). The present invention also pertains to a method for growing TiO 2  on an arbitrary starting material surface in a primarily rutile phase with minimum threading dislocations and crystal defects. 
         [0011]    In another embodiment, the present invention pertains to an optical filter comprising a plurality of layers having a low refractive index interleaved with a plurality of layers having a high refractive index deposited onto a substrate. Each of the plurality of the high-index layers comprises a layer of seed material and a layer of titanium dioxide deposited on the layer of seed material. The seed material has a prescribed, uniform inter-atomic spacing adapted to cause the overlaying layer of titanium dioxide to be deposited in a primarily rutile phase. In the optical filter of the invention, each of the plurality of high refractive index layers preferably has a refractive index of at least 2.55 and an absorption coefficient of at most 1×10 −4 , at a wavelength of 633 nm. 
         [0012]    The present invention also pertains to a method for forming such an optical filter, comprising the steps of providing a substrate and depositing a thin film on the substrate, including a plurality of steps of depositing a layer of material having a low refractive index interleaved with a plurality of steps of depositing a layer of material having a high refractive index. Each of the plurality of steps of depositing a layer of material having a high refractive index comprises the steps of depositing a layer of a seed material and depositing a layer of titanium dioxide onto the layer of seed material. The layer of seed material and the layer of titanium, together, comprise the layer of high refractive index material. The layer of seed material has a prescribed, uniform inter-atomic spacing adapted to cause the overlaying layer of titanium dioxide to be deposited in a primarily rutile phase. 
         [0013]    In more detailed features of the invention, the number of ALD cycles used to deposit each ZrO 2  seed layer preferably is more than seven, more preferably is in the range of seven to 28, and most preferably is in the range of about 14 to about 18. In addition, the TiO 2  layer preferably has a thickness less than 80 nm, or more preferably less than 20 nm, and most preferably less than 10 nm. 
         [0014]    Other features and advantages of the present invention should become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1A  is a side sectional view of a high-index TiO 2 /ZrO 2  film, in accordance with an embodiment of the present invention. 
           [0016]      FIG. 1B  is a side sectional view of an optical filter comprising a high-index TiO 2 /ZrO 2  film, in accordance with an embodiment of the present invention. 
           [0017]      FIG. 2  is a graph depicting the refractive index as a function of wavelength of two TiO 2 /ZrO 2  films in accordance with embodiments of the present invention, one film having ZrO 2  layers that are seven ALD cycles thick and the other film having ZrO 2  layers that are 14 ALD cycles thick. 
           [0018]      FIG. 3  is a graph depicting the refractive index as a function of wavelength of two films, one film having 1400 ALD cycles of TiO 2  and the other film having 1400 ALD cycles of TiO 2  atop a seed layer of 14 ALD cycles of ZrO 2  in accordance with an embodiment of the present invention. 
           [0019]      FIG. 4A  is a table showing measured and calculated data for several coatings incorporating TiO 2  and ZrO 2 , deposited at a temperature of 475° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k TZ ); and the percentage change in peak optical transmission of the combined ZrO 2  and TiO 2  layers after baking for 70 hours at 950° C. (ΔT). 
           [0020]      FIG. 4B  is a table showing measured and calculated data for several coatings incorporating TiO 2  and ZrO 2 , deposited at a temperature of 475° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); and the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k TZ ). 
           [0021]      FIG. 5  is a table showing measured and calculated data for coatings deposited on various substrates. Each coating includes eight layers of TiO 2  and ZrO 2 , with each layer having 14 ALD cycles of ZrO 2  followed by 165 ALD cycles of TiO 2 , deposited at a temperature of 520° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); and the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k 633 ). 
           [0022]      FIG. 6  is a graph showing optical transmission as a function of wavelength of 1400 ALD cycles of TiO 2 , after deposition at 475° C. (AD) and after baking for 70 hours at 950° C. 
           [0023]      FIG. 7  is a graph showing optical transmission as a function of wavelength of 1400 ALD cycles of TiO 2  and 14 ALD cycles of ZrO 2 , after deposition at 475° C. (AD) and after baking for 70 hours at 950° C. 
           [0024]      FIG. 8  is a schematic diagram depicting the crystal structure of the rutile phase of TiO 2 . 
           [0025]      FIG. 9  is a graph showing the x-ray diffraction pattern of a sample of 1400 ALD cycles of TiO 2  deposited on 14 ALD cycles of ZrO 2 , in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    With reference now to the accompanying drawings, and particularly to  FIG. 1A , there is shown a side sectional view of a high-index TiO 2 /ZrO 2  film  10  in accordance with one preferred embodiment of the present invention, the film incorporating a plurality of ZrO 2  layers ( 14   a - 14   n ) interleaved with a plurality of TiO 2  layers ( 16   a - 16   n ). The first ZrO 2  layer  14   a  is formed atop a low index substrate layer  12 , and the first TiO 2  layer  16   a  is formed atop the first ZrO 2  layer  14   a.  Subsequent alternating layers of ZrO 2  and TiO 2  may be formed atop the first ZrO 2  and TiO 2  layers, culminating in the final ZrO 2  layer  14   n  and final TiO 2  layer  16   n.  The present invention encompasses any number of alternating ZrO 2  layers and TiO 2  layers, including only one ZrO 2  layer and only one TiO 2  layer. 
         [0027]    The ZrO 2  layers  14   a - 14   n  and TiO 2  layers  16   a - 16   n  all are deposited using atomic layer deposition (ALD). The ZrO 2  layers preferably are substantially thinner than are the TiO 2  layers. The TiO 2  layers  16   a - 16   n  are grown using titanium chloride (TiCl 4 ) and H 2 O precursors, at substrate temperatures in the temperature range of about 450 to 500° C. on the thin ZrO 2  seed layers  14   a - 14   n.  In this way, a high index of refraction and low absorption coefficient can be achieved. 
         [0028]    With reference now to  FIG. 1B , there is shown a side sectional view of an optical interference filter  18  comprising a plurality of layers having a high refractive index ( 10   a - 10   n ) interleaved with a plurality of layers having a low refractive index ( 12   a - 12   n ) deposited on a substrate  20 . Each of the plurality of high-index layers comprises a TiO 2 /ZrO 2  film  10  like that depicted in  FIG. 1A . 
         [0029]      FIG. 2  is graph depicting the refractive index as a function of wavelength of two TiO 2 /ZrO 2  films in accordance with the present invention, deposited using ALD. One film includes ZrO 2  layers that are each 7 ALD cycles thick, and the other film includes ZrO 2  layers that are 14 ALD cycles thick. The film that includes 14-cycle ZrO 2  layers has a higher composite refractive index than does the film having 7-cycle ZrO 2  layers, despite the fact that ZrO 2  generally has a lower index of refraction than does TiO 2 . 
         [0030]      FIG. 3  is a graph depicting the refractive index as a function of wavelength of two TiO 2 /ZrO 2  films, deposited using ALD. One film includes 1400 ALD cycles of TiO 2 , and the other film includes 1400 ALD cycles of TiO 2  atop a seed layer of 14 ALD cycles of ZrO 2 . The film that includes the seed layer of 14 cycles of ZrO 2  has a higher composite refractive index that does the film lacking the ZrO 2  seed layer. 
         [0031]    With reference now to  FIG. 4A , there is shown a table setting forth measured and calculated data for several TiO 2 /ZrO 2  coatings in accordance with the present invention, deposited on a fused silica substrate at a temperature of 475° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k TZ ); and the percentage change in peak optical transmission of the combined ZrO 2  and TiO 2  layers after baking for 70 hours at 950° C. (ΔT). 
         [0032]    As shown in  FIG. 4A , the calculated refractive index of the TiO 2  increases from 2.485 for the coating incorporating seven ALD cycles of ZrO 2  to 2.604 for the coating incorporating 14 ALD cycles of ZrO 2 . When the number of cycles of ZrO 2  is increased to 28, however, the calculated refractive index of the TiO 2  is observed to decrease substantially. Also as shown in  FIG. 4A , the refractive index of 1400 ALD cycles of pure TiO 2 , deposited at 475° C., is 2.545 at 633 nm. This refractive index increases to 2.675 when the 1400 ALD cycles of TiO 2  are grown on a seed layer of 14 ALD cycles of ZrO 2 . Thus, despite the fact that ZrO 2  generally has a lower index of refraction than that of TiO 2 , the presence of the seed layer of 14 ALD cycles of ZrO 2  can raise the refractive index of a film containing TiO 2 . 
         [0033]    The data provided in  FIG. 4A  indicate that the ZrO 2  grows crystalline at a temperature of about 450 to 500° C. and that its lattice achieves appropriate regularity and an appropriate lattice constant at a thickness of about 1.0 nm. This crystal lattice promotes the growth of a preferentially-ordered, high-density layer of TiO 2  atop the layer of ZrO 2 . X-ray diffraction data collected in a grazing incidence mode show that TiO 2 , grown at temperatures of in the range of about 450 to 500° C. on a ZrO 2  seed layer, consists primarily of material in the rutile phase ( FIG. 8 ). 
         [0034]    With reference now to  FIG. 8 , there is shown a schematic diagram depicting the crystal structure of the rutile phase of TiO 2 . The rutile phase of TiO 2  has a high packing density (4.274 g/cm 3 ) and, consequently, the highest theoretically possible refractive index for TiO 2 . The high packing density also reduces the tensile stress of the resulting film after it has cooled, when the film is deposited on a substrate having a lower coefficient of thermal expansion (CTE) than that of the film. 
         [0035]    With reference now to  FIG. 4B , there is shown a table setting forth measured and calculated data for several TiO 2 /ZrO 2  coatings in accordance with the present invention, deposited on various substrates at a temperature of 475° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); and the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k TZ ). 
         [0036]    The data from Run  374  provided in  FIG. 4B  indicate that a ZrO 2  seed layer of nine or more ALD cycles produces a TiO 2  layer having a high index of refraction, regardless of whether the TiO 2 /ZrO 2  coating is deposited on a fused silica substrate (GE124), an aluminosilicate substrate (Corning 1737), or a D263 glass substrate. This indicates that ZrO 2  can be used as a seed layer for growing TiO 2  in the rutile phase on an arbitrary starting surface. 
         [0037]    The data provided in  FIG. 4B  also indicate that the calculated refractive index of the TiO 2  layers at 633 nm may decrease as the thickness of the TiO 2  layers increases significantly beyond 80 nm. For example, in run  375  (where the thickness of the TiO 2  layers is about 150 nm), the calculated refractive index of the TiO 2  layers at 633 nm is less than it is in other runs (where the thickness of the TiO 2  layers is less than about 80 nm). The data thus indicate that, at thicknesses significantly beyond 80 nm, a less than optimal percentage of the TiO 2  layers is being deposited in the rutile phase. Each TiO 2  layer, therefore, preferably has a thickness less than 80 nm, or more preferably less than 20 nm, and most preferably less than 10 nm. One suitable approach for obtaining a greater TiO2 thickness while maintaining a high refractive index is suggested by run  379 , wherein the TiO 2  layers are laminated at regular intervals with 15 ALD cycles of ZrO 2 . 
         [0038]    Thus, together,  FIGS. 4A and 4B  show that the number of ALD cycles for the ZrO 2  seed layer preferably is more than seven, more preferably is in the range of seven to 28, and most preferably is in the range of about 14 to about 18. It will be appreciated, however, that the invention also encompasses ZrO 2  seed layers formed using a number of ALD cycles outside these preferred ranges, if other process parameters are appropriately varied. The data set forth in  FIGS. 4A and 4B  apply to depositions performed using different ALD deposition tools, and it will be appreciated that the data do not precisely correlate with each other. Those skilled in the art, therefore, will appreciate that an optimum number of ALD cycles must be empirically determined based on the equipment and process parameters that are available. 
         [0039]    With reference now to  FIG. 5 , there is shown a table setting forth measured and calculated data for several coatings deposited on various substrates. Each coating includes eight layers of TiO 2  and ZrO 2 , with each layer having 14 ALD cycles of ZrO 2  followed by 165 ALD cycles of TiO 2 , deposited at a temperature of 520° C. For each coating, the data includes the combined thickness in nanometers of the ZrO 2  layers (t Z ); the calculated refractive index of the ZrO 2  layers at 633 nm (n Z ); the combined thickness in nanometers of the TiO 2  layers (t T ); the calculated refractive index of the TiO 2  layers at 633 nm (n T ); the combined thickness in nanometers of the ZrO 2  and TiO 2  layers (t TZ ); the composite refractive index of the ZrO 2  and TiO 2  layers at 633 nm (n TZ ); and the absorption coefficient of the combined ZrO 2  and TiO 2  layers (k 633 ). 
         [0040]    As shown in  FIG. 5 , the calculated refractive index of the TiO 2  layers is generally lower when the TiO 2 /ZrO 2  film is deposited at a temperature of 520° C. than it is when the TiO 2 /ZrO 2  film is deposited at a temperature of 475° C. This indicates that the optimal deposition temperature for this set of process conditions is less than 520° C. It is believed, however, that the TiO 2 /ZrO 2  film can be grown at temperatures as low as 400° C. and as high as 550° C. 
         [0041]    The data set forth in  FIGS. 4A ,  4 B and  5  are for depositions produced in either a P400 tool or a P800 tool manufactured by Planar Oy (now Beneq Oy), of Espoo, Finland. The process conditions for producing the ZrO 2  layers included a repetition of the following cycle: a dose of H 2 O followed by a nitrogen purge, and one or more successive doses of ZrCl 4 , followed by a nitrogen purge. The ZrCl 4  was preheated to 250° C. The process conditions for producing the TiO 2  layers included a repetition of the following cycle: a dose of H 2 O, a nitrogen purge of 1.5 seconds, a dose of TiCl 4 , and a second nitrogen purge of 1.5 seconds. The TiCl 4  was kept at 23° C. A single dose of H 2 O was supplied between the ZrO 2  and TiO 2  layers to provide full saturation of the surface with H 2 O. The process may be expressed by the following formula: 
         [0000]      N*(X*(H 2 O+2*ZrCl 4 )+H 2 O+Y*(H 2 O+TiCl 4 )),       where N is the number of layers of TiO 2  and ZrO 2 ,   X is the number of cycles of ZrO 2  in each layer, and   Y is the number of cycles of TiO 2  in each layer.
 
For example, the process for the depositions represented by the data set forth in  FIG. 5  may be expressed by the following formula:
         
         [0000]      8*(14*(H 2 O+2*ZrCl 4 )+H 2 O+165*(H 2 O+TiCl 4 )). 
         [0045]    As shown in  FIGS. 4A and 4B , the preferentially-ordered, rutile phase of the TiO 2  produced at a temperature of about 475° C., in conjunction with a ZrO 2  seed layer having more than 10 ALD cycles, exhibits low absorption and scattering. Absorption coefficients (k TZ ) from about 3.3×10 −9  to about 7.2×10 −9  were achieved for coatings having thicknesses of about 80 nm. The absorption coefficient was reduced by six orders of magnitude when the ZrO 2  seed layer was increased from seven ALD cycles (k TZ =2.30×10 −3 ) to 14 ALD cycles (k TZ =3.30×10 −9 ) in combination with 165 ALD cycles of TiO 2  on a fused silica substrate ( FIG. 4A ). The absorption coefficient for 1400 ALD cycles of TiO 2  with the addition of a seed layer of 14 ALD cycles of ZrO 2  (k TZ =2.13×10 −9 ) also was six orders of magnitude smaller than the absorption coefficient of 1400 ALD cycles of pure TiO 2  (k TZ =3.26×10 −3 ) ( FIG. 4A ). 
         [0046]    An additional benefit of using ZrO 2  as a seed layer in place of other lamination materials such as Al 2 O 3  is the relatively high refractive index of ZrO 2  (about 2.2 at 633 nm). Because ZrO 2  has a much higher refractive index than those of other lamination materials such as Al 2 O 3  (about 1.644 at 633 nm), ZrO 2  is believed to have a less deleterious effect on the composite refractive index of the high-index layers of the resulting film. 
         [0047]    ZrO 2 , produced from ZrCl 4  and H 2 O precursors, also has the advantage of being completely free of carbon contamination. Carbon contamination is often found in materials that are produced using metal-organic precursors, such as Al 2 O 3 , which can be produced from trimethylaluminium (Al 2 (CH 3 ) 6 ) and H 2 O precursors. Carbon can adversely affect a coating&#39;s absorption coefficient and the ability of the coating to operate at elevated temperatures. 
         [0048]    The high-density rutile phase of TiO 2 , which is produced according to the present invention, also exhibits good thermal stability. Good thermal stability can be important in some applications, such as infrared-reflective coatings for energy efficient halogen lamps. 
         [0049]    With reference now to  FIG. 6 , there is shown a graph depicting the optical transmission as a function of wavelength of 1400 ALD cycles of TiO 2  after deposition at 475° C. and after baking for 70 hours at 950° C. Similarly,  FIG. 7  is a graph showing the optical transmission as a function of wavelength of 1400 ALD cycles of TiO 2  and 14 ALD cycles of ZrO 2  after deposition at 475° C. and after baking for 70 hours at 950° C. 
         [0050]    As shown in  FIGS. 6 and 7 , the optical transmission at 450 nm of the pure TiO 2  decreases about 15.3 percent after baking for 70 hours at 950° C. In comparison, the optical transmission at 450 nm of the TiO 2  grown atop the ZrO 2  seed layer decreases only about 1.3 percent after baking for 70 hours at 950° C. 
         [0051]    The rightmost column of  FIG. 4A  shows the percentage change in peak optical transmission after 70 hours of baking at 950° C. (ΔT). This percentage change is a metric for the thermal stability of the various depositions reflected in  FIG. 4A .  FIG. 4A  shows that the best thermal stability coincides with the highest refractive index for TiO 2  and that this occurs in a deposition having 14 ALD cycles of ZrO 2 . The worst thermal stability occurs in the pure TiO 2  deposition, which has no ALD cycles of ZrO 2 . The optical losses in the pure TiO 2  are believed to result from scattering due to the growth of disordered crystalline structures. A TiO 2  film grown on a ZrO 2  seed layer according to the present invention has superior stability at elevated temperatures. 
         [0052]    Other materials, such as hafnium dioxide (HfO 2 ), that produce a highly-ordered seed layer may be used in place of ZrO 2 . HfO 2 , like ZrO 2 , has a valence state of +4 and can be deposited via ALD using hafnium tetrachloride (HfCl 4 ) and H 2 O as precursors. 
         [0053]    The present invention has been described above in terms of presently preferred embodiments so that an understanding of the present invention can be conveyed. However, there are other embodiments not specifically described herein for which the present invention is applicable. Therefore, the present invention should not to be seen as limited to the forms shown, which is to be considered illustrative rather than restrictive.