Patent Publication Number: US-2012028011-A1

Title: Self-passivating mechanically stable hermetic thin film

Description:
This application claims the benefit of U.S. Provisional Application No. 61/368,011, filed Jul. 27, 2010, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND AND SUMMARY 
     The present disclosure relates generally to hermetic barrier layers, and more particularly to self-passivating, inorganic, mechanically stable hermetic thin films. 
     Recent research has shown that single-layer thin film inorganic oxides, at or near room temperature, typically contain nanoscale porosity, pinholes and/or defects that preclude or challenge their successful use as hermetic barrier layers. In order to address the apparent deficiencies associated with single-layer films, multi-layer encapsulation schemes have been adopted. The use of multiple layers can minimize or alleviate defect-enabled diffusion and substantially inhibit ambient moisture and oxygen permeation. Multiple layer approaches generally involve alternating inorganic and polymer layers, where an inorganic layer is typically formed both immediately adjacent the substrate or workpiece to be protected and as the terminal or topmost layer in the multi-layer stack. Because multiple layer approaches are generally complex and costly, economical thin film hermetic layers and methods for forming them are highly desirable. 
     Hermetic barrier layers formed according to the present disclosure comprise a single deposited inorganic layer that during and/or after its formation reacts with inward diffusing moisture or oxygen to form a self-passivating, mechanically stable hermetic thin film. The reaction product between moisture or oxygen and the first inorganic layer forms a second inorganic layer at the deposited layer-ambient interface. The first and second inorganic layers cooperate to isolate and protect an underlying substrate or workpiece. 
     In embodiments, the first inorganic layer can be formed on a surface of a workpiece by room temperature sputtering from a suitable target material. As deposited, the first inorganic layer can be substantially amorphous. The workpiece can be, for example, an organic electronic device such as an organic light emitting diode. Reactivity of the first inorganic layer with moisture or oxygen is sufficiently compressive and cooperative that a self-sealing structure is formed having mechanical integrity substantially devoid of film buckling, delamination or spalling. 
     According to one embodiment, a hermetic thin film comprises a first inorganic layer formed over a substrate, and a second inorganic layer contiguous with the first inorganic layer. The first inorganic layer and the second inorganic layer comprise substantially equivalent elemental constituents, while a molar volume of the second inorganic layer is from about −1% to 15% greater than a molar volume of the first inorganic layer. An equilibrium thickness of the second inorganic layer, which is formed via oxidation of the first inorganic layer, is at least 10% of, but less than, an initial thickness of the first inorganic layer. The second inorganic layer according to embodiments has a crystalline microstructure. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a single chamber sputter tool for forming self-passivating, mechanically stable hermetic thin films; 
         FIG. 2  is an illustration of a calcium-patch test sample for accelerated evaluation of hermeticity; 
         FIG. 3  shows test results for non-hermetically sealed (left) and hermetically sealed (right) calcium patches following accelerated testing; 
         FIG. 4  shows glancing angle (A, C) and thin film (B, D) x-ray diffraction (XRD) spectra for a hermetic film-forming material (top series) and a non-hermetic film forming material (bottom series); 
         FIG. 5  is a series of glancing angle XRD spectra for hermetic (top) and non-hermetic (bottom) films following accelerated testing; and 
         FIGS. 6A-6I  show a series of glancing angle XRD spectra for hermetic thin films following accelerated testing. 
     
    
    
     DETAILED DESCRIPTION 
     A method of forming a self-passivating, mechanically stable hermetic thin film comprises forming a first inorganic layer over a substrate, and exposing a free-surface of the first inorganic layer to oxygen to form a second inorganic layer contiguous with the first inorganic layer, wherein a molar volume of the second inorganic layer is from about −1% to 15% greater than a molar volume of the first inorganic layer, and an equilibrium thickness of the second inorganic layer is at least 10% of but less than an initial thickness of the first inorganic layer. The first inorganic layer can be amorphous, while the second inorganic layer can be at least partially crystalline. 
     In embodiments, the molar volume change (e.g., increase) manifests as a compressive force within the layers that contributes to a self-sealing phenomenon. Because the second inorganic layer is formed as the spontaneous reaction product of the first inorganic layer with oxygen, as-deposited layers (first inorganic layers) that successfully form hermetic films are less thermodynamically stable than the corresponding second inorganic layers. Thermodynamically stability is reflected in the respective Gibbs free energies of formation. 
     Self-passivating, mechanically stable hermetic thin films can be formed by physical vapor deposition (e.g., sputter deposition or laser ablation) or thermal evaporation of a suitable starting material onto a workpiece or test piece. A single-chamber sputter deposition apparatus  100  for forming such thin films is illustrated schematically in  FIG. 1 . 
     The apparatus  100  includes a vacuum chamber  105  having a substrate stage  110  onto which one or more substrates  112  can be mounted, and a mask stage  120 , which can be used to mount shadow masks  122  for patterned deposition of different layers onto the substrates. The chamber  105  is equipped with a vacuum port  140  for controlling the interior pressure, as well as a water cooling port  150  and a gas inlet port  160 . The vacuum chamber can be cryo-pumped (CTI-8200/Helix; MA, USA) and is capable of operating at pressures suitable for both evaporation processes (˜10 −6  Torr) and RF sputter deposition processes (˜10 −3  Torr). 
     As shown in  FIG. 1 , multiple evaporation fixtures  180 , each having an optional corresponding shadow mask  122  for evaporating material onto a substrate  112  are connected via conductive leads  182  to a respective power supply  190 . A starting material  200  to be evaporated can be placed into each fixture  180 . Thickness monitors  186  can be integrated into a feedback control loop including a controller  193  and a control station  195  in order to affect control of the amount of material deposited. 
     In an example system, each of the evaporation fixtures  180  are outfitted with a pair of copper leads  182  to provide DC current at an operational power of about 80-180 Watts. The effective fixture resistance will generally be a function of its geometry, which will determine the precise current and wattage. 
     An RF sputter gun  300  having a sputter target  310  is also provided for forming a layer of inorganic oxide on a substrate. The RF sputter gun  300  is connected to a control station  395  via an RF power supply  390  and feedback controller  393 . For sputtering inorganic, mechanically stable hermetic thin films, a water-cooled cylindrical RF sputtering gun (Onyx-3™, Angstrom Sciences, Pa) can be positioned within the chamber  105 . Suitable RF deposition conditions include 50-150 W forward power (&lt;1 W reflected power), which corresponds to a typical deposition rate of about ˜5 Å/second (Advanced Energy, Co, USA). In embodiments, an initial thickness (i.e., as-deposited thickness) of the first inorganic layer is less than 50 microns (e.g., about 45, 40, 35, 30, 25, 20, 15 or 10 microns). Formation of the second inorganic layer can occur when the first inorganic layer is exposed to oxygen, which can be in the form of ambient air, a water bath, or steam. 
     To evaluate the hermeticity of the hermetic barrier layers, calcium patch test samples were prepared using the single-chamber sputter deposition apparatus  100 . In a first step, calcium shot (Stock #10127; Alfa Aesar) was evaporated through a shadow mask  122  to form 25 calcium dots (0.25 inch diameter, 100 nm thick) distributed in a 5×5 array on a 2.5 inch square glass substrate. For calcium evaporation, the chamber pressure was reduced to about 10 −6  Torr. During an initial pre-soak step, power to the evaporation fixtures  180  was controlled at about 20 W for approximately 10 minutes, followed by a deposition step where the power was increased to 80-125 W to deposit about 100 nm thick calcium patterns on each substrate. 
     Following evaporation of the calcium, the patterned calcium patches were encapsulated using comparative inorganic oxide materials as well as hermetic inorganic oxide materials according to various embodiments. The inorganic oxide materials were deposited using room temperature RF sputtering of pressed powder sputter targets. The pressed powder targets were prepared separately using a manual heated bench-top hydraulic press (Carver Press, Model 4386, Wabash, Ind., USA). The press was typically operated at 20,000 psi for 2 hours and 200° C. 
     The RF power supply  390  and feedback control  393  (Advanced Energy, Co, USA) were used to form first inorganic oxide layers over the calcium having a thickness of about 2 micrometers. No post-deposition heat treatment was used. Chamber pressure during RF sputtering was about 1 milliTorr. The formation of a second inorganic layer over the first inorganic layer was initiated by ambient exposure of the test samples to room temperature and atmospheric pressure prior to testing. 
       FIG. 2  is a cross-sectional view of a test sample comprising a glass substrate  400 , a patterned calcium patch (˜100 nm)  402 , and an inorganic oxide film (˜2 μm)  404 . Following ambient exposure, the inorganic oxide film  404  comprises a first inorganic layer  404 A and a second inorganic layer  404 B. In order to evaluate the hermeticity of the inorganic oxide film, calcium patch test samples were placed into an oven and subjected to accelerated environmental aging at a fixed temperature and humidity, typically 85° C. and 85% relative humidity (“85/85 testing”). 
     The hermeticity test optically monitors the appearance of the vacuum-deposited calcium layers. As-deposited, each calcium patch has a highly reflective metallic appearance. Upon exposure to water and/or oxygen, the calcium reacts and the reaction product is opaque, white and flaky. Survival of the calcium patch in the 85/85 oven over 1000 hours is equivalent to the encapsulated film surviving 5-10 years of ambient operation. The detection limit of the test is approximately 10 −7  g/m 2  per day at 60° C. and 90% relative humidity. 
       FIG. 3  illustrates behavior typical of non-hermetically sealed and hermetically sealed calcium patches after exposure to the 85/85 accelerated aging test. In  FIG. 3 , the left column shows non-hermetic encapsulation behavior for Cu 2 O films formed directly over the patches. All of the Cu 2 O-coated samples failed the accelerated testing, with catastrophic delamination of the calcium dot patches evidencing moisture penetration through the Cu 2 O layer. The right column shows positive test results for nearly 50% of the samples comprising a CuO-deposited hermetic layer. In the right column of samples, the metallic finish of 34 intact calcium dots (out of 75 test samples) is evident. 
     Both glancing angle x-ray diffraction (GIXRD) and traditional powder x-ray diffraction were used to evaluate the near surface and entire oxide layer, respectively, for both non-hermetic and hermetic deposited layers.  FIG. 4  shows GIXRD data (plots A and C) and traditional powder reflections (plots B and D) for both hermetic CuO-deposited layers (plots A and B) and non-hermetic Cu 2 O-deposited layers (plots C and D). Typically, the 1 degree glancing angle used to generate the GIXRD scans of  FIGS. 4A and 4C  probes a near-surface depth of approximately 50-300 nanometers. 
     Referring still to  FIG. 4 , the hermetic CuO-deposited film (plot A) exhibits near surface reflections that index to the phase paramelaconite (Cu 4 O 3 ), though the interior of the deposited film (plot B) exhibits reflections consistent with a significant amorphous copper oxide content. The paramelaconite layer corresponds to the second inorganic layer, which formed via oxidation of the first inorganic layer (CuO) that was formed directly over the calcium patches. In contrast, the non-hermetic Cu 2 O-deposted layer exhibits x-ray reflections in both scans consistent with Cu 2 O. 
     The XRD results suggests that hermetic films exhibit a significant and cooperative reaction of the sputtered (as-deposited) material with moisture in the near surface region only, while non-hermetic films react with moisture in their entirety yielding significant diffusion channels which preclude effective hermeticity. For the copper oxide system, the hermetic film data (deposited CuO) suggest that paramelaconite crystallite layer forms atop an amorphous base of un-reacted sputtered CuO, thus forming a mechanically stable and hermetic composite layer. 
     In embodiments of the present disclosure, a hermetic thin film is formed by first depositing a first inorganic layer on a workpiece. The first inorganic layer is exposed to moisture and/or oxygen to oxidize a near surface region of the first inorganic layer to form a second inorganic layer. The resulting hermetic thin film is thus a composite of the as-deposited first inorganic layer and a second inorganic layer, which forms contiguous with the first as the reaction product of the first layer with moisture and/or oxygen. 
     A survey of several binary oxide systems reveals other materials capable of forming self-passivating hermetic thin films In the tin oxide system, for example, as-deposited amorphous SnO reacts with moisture/oxygen to form crystalline SnO 2  and the resulting composite layer exhibits good hermeticity. When SnO 2  is deposited as the first inorganic layer, however, the resulting film is not hermetic. 
     As seen with reference to  FIG. 5 , which shows GIXRD spectra for SnO (top) and SnO 2 -deposited films (bottom) after 85/85 exposure, the hermetic film (top) exhibits a crystalline SnO 2  (passivation) layer that has formed over the deposited amorphous SnO layer, while the non-hermetic film exhibits a pure crystalline morphology. 
     According to further embodiments, the choice of the hermetic thin film material(s) and the processing conditions for incorporating the hermetic thin film materials are sufficiently flexible that the workpiece is not adversely affected by formation of the hermetic thin film Exemplary hermetic thin film materials can include copper oxide, tin oxide, silicon oxide, tin phosphate, tin fluorophosphate, chalcogenide glass, tellurite glass, borate glass, as well as combinations thereof. Optionally, the hermetic thin film can include one or more dopants, including but not limited to tungsten and niobium. 
     A composition of a doped tin fluorophosphate starting material suitable for forming a first inorganic comprises 35 to 50 mole percent SnO, 30 to 40 mole percent SnF 2 , 15 to 25 mole percent P 2 O 5 , and 1.5 to 3 mole percent of a dopant oxide such as WO 3  and/or Nb 2 O 5 . 
     In embodiments, the thin film can be derived from room temperature sputtering of one or more of the foregoing materials or precursors for these materials, though other thin film deposition techniques can be used. In order to accommodate various workpiece architectures, deposition masks can be used to produce a suitably patterned hermetic thin film. Alternatively, conventional lithography and etching techniques can be used to form a patterned hermetic thin film from a uniform layer. 
     Additional aspects of suitable hermetic thin film materials are disclosed in commonly-owned U.S. Application No. 61/130,506 and U.S. Patent Application Publication Nos. 2007/0252526 and 2007/0040501, the entire contents of which are hereby incorporated herein by reference in their entirety. 
       FIGS. 6A-6H  show a series of GIXRD plots, and  FIG. 61  shows a Bragg XRD spectrum for a CuO-deposited hermetic thin film following accelerated testing. Bragg diffraction from the entire film volume has an amorphous character, with the paramelaconite phase present at/near the film&#39;s surface. Using a CuO density of 6.31 g/cm 3 , a mass attenuation coefficient of 44.65 cm 2 /g, and an attenuation coefficient of 281.761 cm −1 , the paramelaconite depth was estimated from the GIXRD plots of  FIG. 6 . In  FIGS. 6A-6H , successive glancing incident x-ray diffraction spectra obtained at respective incident angles of 1°, 1.5°, 2°, 2.5°, 3.0°, 3.5°, 4°, and 4.5° show the oxidized surface (paramelaconite) comprises between 31% (619 nm) and 46% (929 nm) of the original 2 microns of sputtered CuO after exposure to 85° C. and 85% relative humidity for 1092 hours. A summary of the calculated surface depth (probed depth) for each GIXRD angle is shown in Table 1. 
     In embodiments, an equilibrium thickness of the second inorganic layer is at least 10% (e.g., at least 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 75%) of the initial thickness of the first inorganic layer. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Paramelaconite depth profile 
               
            
           
           
               
               
               
               
            
               
                   
                 FIG. 
                 GIXRD angle (degrees) 
                 Probed Depth (nm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 6A 
                 1 
                 300 
               
               
                   
                 6B 
                 1.5 
                 465 
               
               
                   
                 6C 
                 2 
                 619 
               
               
                   
                 6D 
                 2.5 
                 774 
               
               
                   
                 6E 
                 3 
                 929 
               
               
                   
                 6F 
                 3.5 
                 1083 
               
               
                   
                 6G 
                 4 
                 1238 
               
               
                   
                 6H 
                 4.5 
                 1392 
               
               
                   
                 6I 
                 n/a 
                 2000 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 highlights the impact of volume change about the central metal ion on the contribution to film stress of the surface hydration products. It has been discovered that a narrow band corresponding to an approximate 15% or less increase in the molar volume change contributes to a hermetically-effective compressive force. In embodiments, a molar volume of the second inorganic layer is from about −1% to 15% (i.e., −1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15%) greater than a molar volume of the first inorganic layer. The resulting self-sealing behavior (i.e., hermeticity) appears related to the volume expansion. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Calculated Molar Volume Change for Various Materials 
               
            
           
           
               
               
               
               
            
               
                 Sputter Target 
                   
                 Δ Molar 
                   
               
               
                 Material/First 
                   
                 Volume 
                 Hermetic 
               
               
                 Inorganic Layer 
                 Second Inorganic Layer 
                 [%] 
                 Layer? 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 SnO 
                 SnO 2   
                 5.34 
                 yes 
               
               
                 FeO 
                 Fe 2 O 3   †   
                 27.01 
                 no 
               
               
                 Sb 2 O 3   
                 Sb 2 O 5   †   
                 63.10 
                 no 
               
               
                 (senarmonitite) 
               
               
                 Sb 2 O 3   
                 Sb 2 O 5   †   
                 67.05 
                 no 
               
               
                 (valentinite) 
               
               
                 Sb 2 O 3   
                 Sb + 3Sb + 5O 4  (cervantite) 
                 −9.61 
                 no 
               
               
                 (valentinite) 
               
               
                 Sb 2 O 3   
                 Sb 3 O 6 (OH) (stibiconite) †   
                 −14.80 
                 no 
               
               
                 (valentinite) 
               
               
                 TiO 3   
                 TiO 2   †   
                 17.76 
                 no 
               
               
                 SiO 
                 SiO 2  (β-quartz) †   
                 12.21 
                 yes 
               
               
                 SiO 
                 SiO 2  (vitreous) †   
                 35.30 
                 no 
               
               
                 Cu 2 O 
                 Cu +   2 Cu 2+   2 O 3 (paramelaconite) †   
                 12.30 
                 no 
               
               
                 CuO 
                 Cu +   2 Cu 2+   2 O 3  (paramelaconite) 
                 0.97 
                 yes 
               
               
                   
               
               
                   † estimate 
               
            
           
         
       
     
     Table 3 shows the hermetic-film-forming inorganic oxide was always the least thermodynamically stable oxide, as reflected in its Gibbs free energy of formation, for a given elemental pair. This suggests that as-deposited inorganic oxide films are metastable and thus reactive towards hydrolysis and/or oxidation. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Gibbs Formation Free Energy (ΔG° formation ) of Various Oxides 
               
            
           
           
               
               
               
               
            
               
                   
                 Target Material 
                 ΔG° formation  [kJ/mol] 
                 Hermetic Layer 
               
               
                   
                   
               
               
                   
                 SnO 
                 −251.9 
                 yes 
               
               
                   
                 Sn 2 O 
                 −515.8 
                 no 
               
               
                   
                 SiO 
                 −405.5 
                 yes 
               
               
                   
                 SiO 2   
                 −850.9 
                 no 
               
               
                   
                 CuO 
                 −129.7 
                 yes 
               
               
                   
                 Cu 2 O 
                 −146.0 
                 no 
               
               
                   
                   
               
            
           
         
       
     
     A hermetic layer is a layer which, for practical purposes, is considered substantially airtight and substantially impervious to moisture. By way of example, the hermetic thin film can be configured to limit the transpiration (diffusion) of oxygen to less than about 10 −2  cm 3 /m 2 /day (e.g., less than about 10 −3  cm 3 /m 2 /day), and limit the transpiration (diffusion) of water to about 10 −2  g/m 2 /day (e.g., less than about 10 −3 , 10 4 , 10 −5  or 10 −6  g/m 2 /day). In embodiments, the hermetic thin film substantially inhibits air and water from contacting an underlying workpiece. 
     As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “layer” includes examples having two or more such “layers” unless the context clearly indicates otherwise. 
     Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, examples include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred. 
     It is also noted that recitations herein refer to a component of the present invention being “configured” or “adapted to” function in a particular way. In this respect, such a component is “configured” or “adapted to” embody a particular property, or function in a particular manner, where such recitations are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” or “adapted to” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.