Patent Application
20120028011
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.
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
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
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 (<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.
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/m2 per day at 60° C. and 90% relative humidity.
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.
Referring still to
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 SnO2 and the resulting composite layer exhibits good hermeticity. When SnO2 is deposited as the first inorganic layer, however, the resulting film is not hermetic.
As seen with reference to
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 SnF2, 15 to 25 mole percent P2O5, and 1.5 to 3 mole percent of a dopant oxide such as WO3 and/or Nb2O5.
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.
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 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.
†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.
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 cm3/m2/day (e.g., less than about 10−3 cm3/m2/day), and limit the transpiration (diffusion) of water to about 10−2 g/m2/day (e.g., less than about 10−3, 104, 10−5 or 10−6 g/m2/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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61368011 | Jul 2010 | US |
