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TECHNICAL FIELD 
     The present invention relates to insulating glass units (IGUs) having a low emissivity (low-E) coating stack for films that are suspended and tensioned in the IGUs, with particular emphasis upon both the quality of the infrared reflecting layer formed in the coating stack and the resistance of the low-E coating stack to cracking or crazing. 
     BACKGROUND ART 
     U.S. Pat. No. 4,335,166 to Lizardo et al. describes an insulating glass unit (IGU) comprising a frame with spacers that support a heat-shrinkable plastic sheet between a pair of spaced apart, but substantially parallel, glass panes to provide an integral unit. 
     Heating the assembled unit causes the plastic sheet to shrink so as to become taut and wrinkle-free. The plastic sheet may be a polyethylene terephthalate (PET) film that can be coated on one or both sides with an infrared reflective material. 
     U.S. Pat. No. 4,799,745 to Meyer et al. describes visually transparent, infrared (IR) reflecting composite films useful in IGUs like that described in the aforementioned Lizardo patent. A transparent support can be selected from among rigid and non-rigid but minimally stretchable solids, including glass and various polymers (including PET). A layer stack of 5 or 7 alternating dielectric and metal layers is sputter-deposited onto one surface of the support. The dielectric layers can be composed of an inorganic metal or semimetal oxide or salt having a refractive index between 1.75 and 2.25, such as indium oxide, tin oxide, titanium dioxide, silicon dioxide, bismuth oxide, chromium oxide, zinc sulfide, magnesium fluoride, or mixtures thereof. Polymer dielectrics are also disclosed. The metal layers can be composed of silver, gold, platinum, palladium, aluminum, copper, nickel, or alloys thereof (e.g., silver alloyed with up to 25% gold). Spacer dielectric layers between the two or three metal layers have thicknesses between 40-200 nm, preferably 50-110 nm, and especially 70-100 nm. Boundary dielectric layers on the outside of the stack have thicknesses between 20-150 nm, preferably 25-90 nm, and especially 30-70 nm. (These thicknesses are for the inorganic dielectric materials. Polymer dielectric layers with their lower refractive index are disclosed to be somewhat thicker.) The metal layers have a combined total thickness between 12-80 nm, with each metal layer having a thickness between 4-40 nm, preferably 4-17 nm, especially 5-13 nm, with 10-12 nm each indicated for two-metal-layer stacks and 5-10 nm each for three-metal-layer stacks. 
     A variety of window assemblies have a film coating laminated to or deposited directly onto one or more glass substrates, rather than suspend a sheet in a space between pairs of glass panes. 
     U.S. Pat. No. 6,503,636 to Le Masson et al. describes a transparent polymer (e.g. polyester) substrate that is provided with a stack of layers including at least one silver layer reflecting thermal radiation. The stack is constructed to prevent stresses from causing it to delaminate or curl up. In particular, the presence of an AIN layer under tensile stress compensates for the compressive stresses in a less than 15 nm thick ZnO layer contiguous with the silver layer, so that the film will lie flat when laminated. 
     U.S. Reissued Patent RE 37,446 and U.S. Pat. No. 5,532,062, both to Miyazaki et al., describe low emissivity films comprising a glass substrate coated with a stack of alternating oxide and metallic films. The oxide film furthest from the substrate has an internal stress not more than 1.1×10 10  dyne/cm 2  in order to prevent exfoliation of that surface film from the underlying metal layer due to moisture damage, with consequent turbidity or haze. In order to achieve this internal stress reduction, the 20-70 nm thick, outermost ZnO film is doped with at least one of Si, B, Ti, Mg, Cr, Sn or Ga in a total of up to 10 atomic %, and preferably 2 to 6 atomic %, with respect to the total quantity including Zn. The other oxide layers closer to the substrate may be selected from ZnO, SfO 2 , ZnO—SnO 2  multi-layers, or a doped ZnO like the outermost oxide layer. At least one of the metal film layers may be an IR reflecting layer composed of Ag, or an alloy whose major component is Ag including at least one of Au, Cu and Pd. 
     Zinc oxide is a well-known seed layer for the growth of silver. The thicker the ZnO seed layer, the better the epitaxial growth of silver on the seed. This results in higher quality silver and consequently a lower emissivity for a given area-specific amount of silver. However, in contexts where a film layer is suspended in tension between windowpanes rather than directly coated onto a windowpane, the brittleness of the highly crystalline zinc oxide becomes a problem. Shrinking or tensioning of the film tends to cause zinc oxide layers to experience crazing, forming a network of myriad visible cracks. Too much shrinking (≧≈1.0%) results in cracked film. However, too little shrinking (≦≈0.5%) results in sagging or wrinkled film that is also visible as image distortions reflected from the film within the window. The distortion from low film tension is exaggerated when the IGU is exposed to elevated ambient temperatures since the thermal expansion coefficient of the film is higher than that of the glass panes. 
     Traditionally this has not been a problem because In 2 O 3  has been used as the seed layer material, since In 2 O 3  has a more amorphous or glassy structure in comparison and is therefore less subject to crazing. However, In 2 O 3  is not as good a seed for the deposition of high quality (lower emissivity) silver. 
     SUMMARY DISCLOSURE 
     An IGU is provided wherein the suspended and tensioned coated film has a ZnO seed layer that is at most 15 nm thick. The thinner ZnO is better able to withstand the strain of a tensioned film without crazing, while still able to serve as an adequate seed for high quality silver deposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are perspective views of corner portions of two insulating glass unit (IGU) embodiments in accord with the present invention installed within a frame. The IGU in  FIG. 1A  has a single suspended film, while the IGU in  FIG. 1   b  is provided with two suspended films. 
         FIG. 2  is a side sectional view of the IGU in  FIG. 1A . 
         FIG. 3  is a side sectional view of a first coated film embodiment in accord with the present invention and usable in the IGU embodiments of  FIGS. 1A and 1B . 
         FIGS. 4A through 4D  are side sectional views corresponding to  FIG. 2  that illustrate the steps of assembling an IGU with suspended tensioned film. 
         FIGS. 5 and 6  are side sectional views of second and third coated film embodiments in accord with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1A , an IGU  11  is shown, here as installed within an optional frame  13 . The IGU  11  itself includes a pair of glass panes  15  and  17 , a pair of spacers  19  and  21 , and a coated sheet  23  suspended between the panes  15  and  17 . The spacers  19  and  21  support the panes  15  and  17  and sheet  23  in a spaced apart and substantially parallel relation. The coated sheet  23  is transparent to visible light, but reflective of infrared (or thermal) light due to the low emissivity coating. Additionally, the sheet  23  embodies certain improvements in crack resistance while maintaining a desired low emissive property. 
     An alternative embodiment is seen in  FIG. 1B , wherein an IGU  31  includes a pair of glass panes  35  and  37 , three spacers  39 - 41 , and a pair of coated sheets  43  and  45  suspended between the pains  35  and  37 . As in the first embodiment, the spacers  39 - 41  support the panes  35  and  37  and the pair of sheets  43  and  45  in mutually spaced apart and substantially parallel relation to one another. Both sheets are transparent and resistant to cracking under tension. At least one, and preferably both, of the sheets  43  and  45  exhibit the infrared reflectivity, low emissivity properties of sheet  23 . 
     Again, the IGU  31  is shown installed in an optional frame  33 . Frames  13  or  33 , not part of the invention itself, may be provided by secondary window manufacturers who purchase IGUs  11  or  31  from a primary manufacturer of the IGUs themselves, e.g. to supply decorative features to the windows they sell directly to consumers. 
     With reference to  FIG. 2 , a sectional view of  FIG. 1A , shows that the spacers  19  and  21  are located only the perimeter or edges of the respective panes  15  and  17  and sheet(s)  23 . The panes  15  and  17  and sheet  23  may be bonded to the spacers  19  and  21  using an adhesive sealant (not shown), which could be a poly-isobutylene (PIB) adhesive. A secondary sealant  25 , e.g. of polyurethane or silicone, ensures that the interior of the IGU is sealed from moisture. Further, the spacers  19  and  21  may be filled with a desiccant material to remove any residual moisture between the panes to prevent fogging of the IGU. 
     With reference to  FIG. 3 , the sheet  23  (and likewise, at least one of the sheets  43  and  45  in  FIG. 1B ), is a visually transparent, infrared reflecting, composite film in which a series of layers  53 - 59  are coated onto a surface of a polymer substrate  51 . In particular, the sheet  23  may be a polyethylene terephthalate (PET) film  51  coated with a stack of dielectric and metal layers  53 - 59 . Varieties of PET film are available with heat shrink properties that allow the film to be tensioned (made taut) after assembly. This substrate is typically from 25 to over 125 micrometers thick. 
     The first layer  53  immediately adjacent to the polymer substrate  51  may be an amorphous dielectric, such as indium oxide (In 2 O 3 ). it is typically about 20 to 80 nm thick. 
     The second layer  55  may be the seed layer, composed of a more crystalline dielectric than the indium oxide layer  53 . In particular, a seed layer  55  in accord with present invention is a zinc-based oxide layer that is a most 15 nm, and typically 5 to 10 nm thick. The zinc-based oxide layer is typically selected from any of a variety of silver-seeding layers including ZnO, aluminum-doped zinc oxide (with up to about 2% Al) (commonly known as ZAO), gallium-doped zinc oxide (with up to about 2% Ga)(commonly known as ZGO), ZnO/SnO 2  (with the Sn content between 1% and 10% of the total zinc and tin content), and ZnO/In 2 O 3  (with the In content being approximately 10% of the total zinc and indium content). The selected zinc-based oxide material may be sputtered from a ceramic or metallic target. The thinness of this ZnO layer  55  gives it the ability to withstand the strain of the tensioned sheet without cracking. A minimum thickness of 5 nm ensures that the outer surface of the ZnO layer  55  can serve as an adequate seed for high quality silver deposition. 
     The third coating layer  57  is the metallic infrared reflective low emissivity coating, which may be composed of silver or of a silver alloy that includes palladium, copper and/or gold. The thickness of the metallic layer  57  is typically 5 to 60 nm, giving it adequate visible light transmission. 
     A very thin (&lt;5 nm) cap layer (not shown), such as nichrome (NiCr), Ti, ZAO or nichrome nitride (NiCrN x ), may be coated on top of the silver layer to preserve the silver quality during the deposition of the outer dielectric. 
     An outer dielectric layer  59  is formed on the metallic layer  57 . This may be composed of indium oxide, and is typically 20 to 50 nm thick. The choice of indium oxide for dielectric layers  53  and  59  is motivated by its crack resistance due to its amorphous quality, while zinc oxide is used for the seed layer to ensure high quality silver deposition for low emissivity. But the zinc oxide seed layer is kept thin enough to minimize its susceptibility to cracking under stress. 
     As seen in  FIG. 4A , assembly of an IGU begins by bonding a window pane  17  to one of the spacers  21  using an adhesive sealant. Likewise, window pane  15  is bonded to the other spacer  19 . The sheet  23  is bonded to both spacers  19  and  21 , leaving the structure seen in  FIG. 4B , but generally will not be sufficiently taut to remove all wrinkles  23   b . In  FIG. 4C , the assembled unit is subject to a heat treatment  49  causes the PET substrate of sheet  23  to shrink. This removes any wrinkles  23   b , leaving a generally planar sheet  23 , suspended in substantially parallel relation to the panes  15  and  17 , as seen in  FIG. 4D . Although heating the assembled unit to cause the plastic sheet to shrink so as to become taut and wrinkle-free is one way to tension the sheet  23 , other tensioning techniques could be used. In any case, despite the strain, the coating materials, including the zinc oxide seed layer  55 , are resistant to cracking. 
     With reference to  FIG. 5 , an alternative embodiment of the suspended sheet has the polymer substrate  61  coated on both of its surfaces. As in  FIG. 3 , the coating begins on both surfaces with generally amorphous dielectric coatings  62  and  63 , e.g. of In 2 O 3 , typically 20 to 80 nm thick. Seed layers  64  and  65  are composed of thin ZnO of at most 15 nm thickness. Metallic IR reflecting layers  66  and  67 , typically of silver or a silver alloy, also 5 to 60 nm thick, are deposited on the respective seed layers. The use of zinc oxide ensures high quality deposition of the silver, giving the sheet its markedly lower emissivity. Finally, another amorphous dielectric coating  68  and  69 , e.g. of 20 to 60 nm In 2 O 3 , serves as protective outer coat on the silver. 
     With reference to  FIG. 6 , yet another embodiment of the suspended film sheet has a thicker stack with multiple IR reflecting layers  77  and  87 . Thus, a PET substrate  71  is coated with a first set of amorphous dielectric, crystalline seed dielectric, metallic IR reflecting, and amorphous dielectric layers  73 - 79 , followed by yet another sequence of seed dielectric layer  85 , metallic IR reflecting layer  87 , and amorphous outer dielectric layer  89 . This can be repeated any number of times, provided that the cumulative thickness of all of the metallic layers does not exceed 60 nm, in order that there be adequate visible transparency through the IGU. As before, the amorphous dielectric may be chosen to be In 2 O 3 , while the various seed layers are zinc oxide, each not exceeding 15 nm in thickness for adequate crack resistance.

Summary:
A low-e insulating glass unit has a suspended, coated IR reflecting polymer sheet under tension, e.g. from heat shrinkage. The polymer sheet is coated with a multilayer stack of dielectric and metallic layers, including at least one silver layer deposited upon a zinc oxide seed layer that is at most 15 nm thickness. The use of zinc oxide ensures good seeding for high quality silver layer growth, thereby providing low emissivity. The thinness of the zinc oxide ensures that it resists cracking when the polymer sheet is tensioned.