METHOD FOR FORMING A DRY GLASS-BASED FRIT

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of U.S. Provisional Application Serial No. 61/106730 filed on October 20, 2008, and U.S. Application No. 12/504276, filed July 16, 2009, the contents of which are relied upon and incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present invention relates to a method for forming a dry frit based on an inorganic glass. More particularly, the present invention relates to a method for forming a dry inorganic glass-based frit suitable for use as a sealing medium for glass packages.

BACKGROUND

[0003] Electroluminescent (EL) devices, such as organic light emitting diode devices, are typically manufactured by forming multiple devices in a single assembly using large master (mother) sheets of glass. That is, the devices are encapsulated between two large glass sheets or plates to form a composite assembly, after which individual devices are cut from the composite assembly. Each device of the composite assembly includes a seal surrounding the organic light emitting diodes of the individual device that seals the top and bottom plates together, and protects the organic light emitting diodes disposed within, since some devices, particularly organic light emitting diodes, degrade in the presence of oxygen and moisture that can be found in the ambient atmosphere. The EL devices may be sealed using an adhesive, e.g. epoxy, or more recently, using a glass frit that is heated to melt the frit and form the seal between the two plates.

[0004] Frit sealed devices exhibit certain advantages over adhesive-sealed devices, not least of which is the superior hermeticity without the need for getters sealed within the device to scavenge contaminants. Thus, frit sealed devices are able to provide for a longer lived device than has been achievable with adhesive seals. Nevertheless, it has been found that frit sealed devices may succumb to deterioration due to moisture contained in and released by the frit into the cavity housing the organic light emitting material during the sealing process.

SUMMARY

[0005] Methods are disclosed for forming a dry glass-based frit suitable for sealing electronic devices, and in particular electronic devices comprising organic materials, such as organic light emitting diode displays, organic light emitting diode lighting panels, and certain classes of organic-based photovoltaic devices.

[0006] In one embodiment, a method of forming a dry glass frit is disclosed comprising forming a batch material comprising vanadium and phosphorous, heating the batch material in a conditioning step to a temperature of between about 450 ⁰ C and 55O ⁰ C for at least about 1 hour, melting the batch material after the conditioning step to form a glass melt, cooling the glass melt to form a glass wherein an OH content of the glass is equal to or less than about 20 ppm as measured by direct insertion probe mass spectrometry.

[0007] In another embodiment a glass powder for forming a glass-based frit is disclosed wherein the glass powder comprises vanadium, phosphorous and a metal halide. [0008] In still another embodiment, a glass powder for forming a glass-based frit is disclosed wherein the glass powder comprises V ₂ Os, P ₂ Os and a metal halide. [0009] In yet another embodiment, a method of forming a glass frit is disclosed comprising forming a batch material comprising V ₂ Os, P ₂ O ₅ and a metal halide heating the batch material in a conditioning step to a temperature of between about 450 ⁰ C and 550 ⁰ C for at least about 1 hour, melting the batch material after the conditioning step to form a glass melt, cooling the glass melt to form a glass and wherein an OH content of the glass is equal to or less than about 20 ppm.

[0010] The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures. It is intended that all such additional systems, methods features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a cross sectional view of an exemplary glass package comprising an organic material.

[0012] FIG. 2 is plot of percent transmittance as a function of wavenumber illustrating a typical measurement for β-OH.

[0013] FIG. 3 is a schematic diagram of a DIP-MS apparatus for measuring outgassed water vapor.

[0014] FIG. 4 is a graphical representation of a Standard heating schedule according to embodiments of the present invention.

[0015] FIG. 5 is a graphical representation of a compressed heating schedule according to embodiments of the present invention

[0016] FIG. 6 is a plot showing the results of a DIP-MS measurement conducted on a coarse hand-ground sample of frit composition C ₁ , showing the extracted ion chromatogram for water.

[0017] FIG. 7 is a plot showing the results of a control DIP-MS measurement as conducted in FIG. 6, but without a sample, indicating that the events (spikes) shown in FIG. 6 are related to outgassing of structural water species during the 400-700 ⁰ C temperature ramp. [0018] FIG. 8 is a plot showing the results of a DBP-MS measurement conducted on coarse hand-ground samples of control (non-dry) frit composition C ₁ , showing outgassing of structural water species during the 400-700 ⁰ C temperature ramp compared to results for sample C ₂ showing no peaks.

[0019] FIG. 9 is a photograph that shows fused quartz crucibles of the control batch composition following 485° calcination (left) and 600 ⁰ C calcination (right).

DETAILED DESCRIPTION

[0020] In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements.

[0021] Hermetically sealed glass packages may be used for a variety of uses, including such photonic devices as optical displays (e.g. flat panel television, cell phones displays, camera displays) and photovoltaic devices (e.g. solar cells). While epoxy seals have been used extensively for certain components, such as liquid crystal displays (LCDs), more recent work is being done on encapsulated organic materials that may be used for similar purposes. For example, organic light emitting diodes are finding application in both display devices and lighting. Certain organic materials are also finding use in the field of photo voltaics, wherein organic solar cells are showing promise.

[0022] While organic materials provide some benefit, the organic materials comprising the devices are susceptible to high temperature, oxygen and moisture exposure. That is, when exposed to temperatures in excess of about 100 ⁰ C, or oxygen or water, the organic material can quickly degrade. For this reason, great care must be taken to ensure devices employing organic materials are hermetically sealed. One such method includes sealing the organic material between glass plates. Inorganic glasses are uniquely suited as containers for housing an organic material. They are substantially environmentally stable, and highly impervious to diffusion of moisture and oxygen. However, the resulting package is only as good as the material that forms the seal between the plates.

[0023] Prior art devices have often employed epoxy adhesives as a sealing medium between glass plates. The manufacture of LCD displays is one such example. However, the degree of long term hermeticity required by certain organic materials suitable for use in electronic devices such as the previously mentioned displays, lighting panels and photovoltaic devices is better met by a glass seal between the plates. Thus, the use of an inorganic glass-based frit has become the sealing medium of choice for organic electronic devices. [0024] By way of example and not limitation, an exemplary frit sealing method for organic light emitting diode display 10 (FIG. 1) may comprise forming a photonic element 12 on a first (backplane) glass substrate 14. Photonic element 12 typically includes an anode and cathode electrodes (not shown) and one or more layers of the photonic material (e.g. organic light emitting material) positioned between the two electrodes. A frit 16 is positioned between the backplane substrate and a second glass (cover) substrate 18. The frit may, for example, be first dispensed onto the cover substrate. In some embodiments, the frit is first dispensed as a paste onto cover substrate 18, then heated to sinter the frit and adhere it to the

cover substrate. The sintering may be performed in an oven. Cover substrate 18 is then positioned in at least partial overlying registration with the backplane substrate, and the frit heated by an irradiation source 20, such as laser 20 that emits laser beam 22 to soften the frit and form a hermetic seal between the cover substrate and the backplane substrate, thereby producing a hermetic glass package containing the OLED.

[0025] In general, water present in glasses can be grouped into two broad categories: structural water where the water atoms (generally present as hydroxyl or OH ions) attach to the glass-forming polyhedra molecular structure during the melting process and become a basic part of the glass network; and surface water where, for example, water molecules present during ball-milling of a glass to produce a frit attach themselves during the milling to unsatisfied valence sites on the frit particle's surface created by broken bonds. Typically, surface water can be removed by a simple drying process, such as by heating the glass of the frit, whereas structural water is much more tenaciously bound, and can persist in the glass during any drying step.

[0026] Although the presence of water in a glass does not necessarily degrade glass properties (save for increased mid-ER. absorbance), its release (outgassing) during subsequent heating in a frit sealing process may have implications for commercial use of the glass. One particular application affected by water outgassing involves the use of glass frits for sealing OLED devices, which are extremely susceptible to even ppm levels of water. Water, as used herein, may take the form of a vapor phase (such as during outgassing, or as a hydroxyl ion, OH).

[0027] In a typical frit manufacturing process, a glass is formed by conventional glass forming methods e.g. sol-gel or by heating granular batch materials (sands). The resulting glass can then be melted, made into thin ribbon, and then ball-milled to a desired particle size. For example, a mean particle size of 3 μm is suitable for use in the manufacture of OLED devices. Following ball-milling, the powdered frit glass may be blended with a filler to obtain a predetermined coefficient of thermal expansion of the frit blend. For example, a suitable coefficient of thermal expansion filler is beta eucryptite. Once the blend has been made and predried, such as heating the blend in an oven, a paste is prepared by mixing the frit glass (or blended frit as the case may be) with an organic vehicle (e.g. texanol), an organic binder (e.g. ethylcellulose), and various dispersants and surfactants as needed. The frit paste

is then dispensed into a specific pattern (for example a loop or frame-like pattern) on a glass substrate, heated in air to burn-out the organics, and thereafter exposed to a subsequent heating to 400 ⁰ C in N ₂ to presenter the frit. As the term implies, the step of presintering consolidates the frit and adheres the frit to the (cover) substrate. Laser-sealing the pre- sintered substrate to a mating substrate (backplane substrate) of one or more OLED devices is typically accomplished using a laser that traverses the consolidated frit, heats and softens the frit and whereupon a seal is formed between the cover substrate and the backplane substrate when the frit cools and solidifies. During laser sealing the frit seal is heated above 400 ⁰ C for at least a few tenths of a second, causing structural water (i.e. OH) in the frit to be released, and possibly degrading the OLED.

[0028] The effort to eliminate water outgassing in the glass during subsequent heating of the frit to 700 ⁰ C has focused on reducing the OH content of the glass. Two approaches were utilized: (1.) composition changes to the glass, and (2.) physical changes to the melting process. Measuring the amount of water was accomplished according to two methods: measuring β-OH (essentially measuring the mid-IR absorbance peak of the OH ^' ion), and DIP-MS (direct insertion probe mass spectrometry). In accordance with the present invention, a dry glass (and resulting dry frit) is defined as possessing a β-OH value equal to or less than about 0.3 mm ^" , or alternatively an OH content equal to or less than about 20 ppm when measured by direct insertion probe mass spectrometry. Preferable, the glass comprises a β-OH value equal to or less than about 0.3 mm ^" 1 and an OH content equal to or less than about 20 ppm when measured by direct insertion probe mass spectrometry. Preferably the glass exhibits no water detectable out-gassing by DEP-MS when reheated to 700 ⁰ C either as a coarse hand-ground powder, or as a fine (3μm) ball-milled powder.

[0029] The β-OH measurements were made on annealed pieces of glass that had been ground and then polished to a thickness of 0.1 - 0.4 mm. β-OH measurements provide data on the total concentration of hydroxyl ions in the glass, not just on those hydroxyls that will de- absorb over a specific temperature region. As shown in FIG. 2 and equation 1 below, β-OH is a ratio of baseline transmittance to transmittance at the OH ^" absorption peak, and is directly proportional to hydroxyl ion concentration for glasses identical, or very similar, to each other in composition.

β-OH = log(ref%T/OH%T)/(thk) 1

where ref %T is the transmittance level at a nearby non-OH absorbing region, OH%T is the transmittance level at the base of the OH peak (~3380 cm-1) and thk is the sample thickness (mm).

[0030] β-OH is directly proportional to the hydroxyl ion concentration for glasses identical, or very similar, to each other in composition. β-OH measurements provide the relative hydroxyl (OH) absorption coefficient for all hydroxyl ions in the glass, not just on those hydroxyls which will de-absorb over a specific temperature region. Any conventional infrared spectroscopy technique can be utilized for the measurements, such as Fourier transform infrared spectroscopy.

[0031] DEP-MS measurements were made on either coarse hand ground (-200M/+100M, or approximately 75-150 μm), or fine ball-milled (equal to or less than an average particle size of 3μm) powder. Unlike the vacuum furnace mass spectroscopy technique used for many standard mass spec studies, the DIP-MS arrangement, shown diagrammatically in FIG. 3, makes use of a heated probe 28 containing the sample to be tested 30 that is placed directly within the ionization region (electron impact ionizer 32 ) of the mass spectrometer 34. In addition to the above components, the exemplary DEP-MS arrangement if FIG. 3 further includes quadrapole ion analyzer 36 and detector 38. Wavy line 40 represents an ion path from sample 30 to detector 38. Unlike a vacuum furnace mass spectrometry measurement, there is no need for a quartz transfer tube and associated problems of deposition of chemical species, or permeability of the tube at high temperatures. Thus, the DEP-MS measurement lends itself to more reliable quantitative analysis of chemical species. [0032] Two different heating schedules were used for the DEP-MS measurements: a) a Standard Cycle (FIG.4) where the sample was heated to 400 ⁰ C, held for 5 hrs to remove any surface water, and then heated to 700 ⁰ C at a rate of 10°C/min., and b) a Compressed Schedule (FIG. 5) which utilizes the same temperature ramp-up to 400 ⁰ C as the Standard Schedule, but includes a shorter hold time at 400 ⁰ C (2hrs), and utilizes a faster heat-up ramp to 700 ⁰ C (50°C/min.). All samples were heated in vacuum throughout the entire DEP-MS run. [0033] Shown in FIG. 6 are the results of a DEP-MS measurement conducted on a coarse hand-ground sample of a frit glass composition suitable for laser sealing of an OLED device showing the extracted ion chromatogram for water (and plotted a nano-Amperes as a function of. time in minutes). The run was made on the Standard Schedule. A small amount of water

outgassing from surface water was recorded in the first few minutes of the run as the sample was heated to 400 ⁰ C. During the 4 hr hold at 400 ⁰ C (from 20 min to 260 min), no additional water out-gassing events were recorded, confirming that the initial water evolution was related to surface water. Once sample heating resumed, several discrete events related to water evolution are observed beginning at approximately 550 ⁰ C.

[0034] Note that these discrete events are not observed when a control measurement is run without a sample (FIG. 7), further indicating that the events are related to outgassing of structural water species during the 400-700 ⁰ C temperature excursion. Only a broad undefined shallow peak is observed as a characteristic of the general background signal of the instrument during the control measurement.

[0035] As noted in Table I, the use of halide compounds was found to be particularly effective for reducing structural water levels, as indicated by both the significantly-lowered β-OH levels of the halide-containing compositions, as well as by the complete absence of detectable water outgassing during the 400-700° heating ramp as detected by the DDP-MS measurement. Table I provides a summary of the results for 4 compositions (C ₂ - C ₄ ) compared to a control composition (Ci) without a halide. Shown in FIG. 8 is a comparison of the high temperature portion of a DEP-MS scan for the non-halide containing Ci sample, and the substantially identical C ₂ sample with all Al ₂ O ₃ replaced by AlF ₃ . Both materials were coarse, hand-ground glass powders. The scan for the fluorine-containing glass (C ₂ ) represented by curve 42 shows a featureless pattern with no distinct events. By contrast, the scan for the Ci sample represented by curve 44 shows several discrete water evolution events occurring in the approximately 550-650 ⁰ C range. The β-OH value for the C5 sample was higher that expected, and out of line with the other halide results, and is believed to be a result of poor sample preparation (as the β-OH measurement is sensitive to surface cleanliness of the sample). DEP-MS measurements for sample C3 and C4 were not conducted.

Table I

[0036] In addition to the including halides in the frit, additional trials were conducted independently of halide incorporation where the melting process was modified to produce glasses with low β-OH values and which did not exhibit structural water outgassing during subsequent DEP-MS analysis.

[0037] Shown in Table Et is a listing of the various process change experiments and the structural water level measured (β-OH) and/or the quantity of structural water evolved (DIP- MS). As may be seen, these various experiments involved determining the effect of thermal cycling during melting (Experiment 1), air-calcining of the batch material with N ₂ melting (Experiment 2), air-calcining of the batch material (either 485° or 600 ⁰ C) combined followed by air-melting (Experiments 3 and 4) of the batch material; melting all but the V ₂ Os component of the basic glass, then re-melting with V ₂ Os (Experiment 5); and re-melting standard cullet in an induction furnace and bubbling O ₂ or N ₂ /O ₂ through the melt during re- melting (Experiments 6 and 7). Most of these approaches resulted in a substantially lower β- OH value and/or no structural water outgassing detected by DIP-MS measurement relative to the standard process, with the exception of high-to-low-to-high thermal cycling during melting (Experiment 1); and 600° calcining plus standard 1000 ⁰ C melting (Experiment 5).

Table II

[0038] An interesting feature of the results is the effect of calcining temperature. Calcining was selected as a potential means to reduce structural water since it would permit water present as a constituent of any raw materials of the frit blend to escape from the batch before being accommodated into the melt structure. Interestingly, 485 ⁰ C air-calcining/1000°C air- melting (Experiment 3) had a substantial effect in lowering the amount of structural water (β- OH = 0.205), but 600 ⁰ C air-calcining/1000°C air-melting (Experiment 5) was relatively ineffective (β-OH = 0.433). A possible explanation is provided by FIG. 9, which shows fused quartz crucibles 46 and 48 of the control batch composition following 485° calcination and 600 ⁰ C calcination, respectively. The calcined 485°C batch is essentially a loose, porous, unconsolidated powder, whereas substantial melting occurred with the 600 ⁰ C calcined batch,

since the melting point of one of the key batch components (phosphorus pentoxide) is 563°C. One possible explanation is that the liquid phase generated at 600 ⁰ C sealed off many escape passages for evolved water, leading to greater incorporation of water in the melt structure than with the lower temperature 485°C calcining. The combination of 485 ⁰ C air- calcining/1000°C with N ₂ -melting (Experiment 2) produced the lowest β-OH of all approaches. It is believe this occurred because the effect of 485 ⁰ C air-calcining was combined with the ability of N ₂ to sweep through the glass melt and carry off water species. The beneficial effect of N ₂ was also observed for Experiments 6 and 7, where conventionally- melted cullet was re-melted in a N ₂ atmosphere with either O ₂ or N ₂ /O ₂ bubbled through the melt.

[0039] Following the completion of the physical experiments in Table H, three approaches were selected for repeat testing to determine reproducibility of the water-free results. These were: halide replacement OfAl ₂ O ₃ (e.g. AlF ₃ ); 485 ⁰ C calcining in air for 2 hr. followed by melting at 1000° in air; and 485°C calcining in air for 2hr followed by melting at 1000° in a N ₂ atmosphere. A comparison of these techniques are shown in Table EI with respect to β- OH and water outgassing results. The three approaches which produced a dry glass in the initial experiments produced dry glass in the repeat work.

Table HI

[0040] The absence of structural water out-gassing seen above for several of the approaches was also seen in a fine-ground (equal to an less than about 3μm particle size) ball-milled powder, as well as for frit blend pastes made of the fine-ground powders after a 400 ⁰ C presintering treatment as indicated by the DBP-MS results provided in IV.

Table IV

[0041] The several techniques described above for producing dry glass and frits appear to have relevance to vanadium and phosphate containing glasses in general, rather than to just the Sb ₂ O ₃ vanadium phosphate glasses currently used for OLED frit sealing. Shown below in Table V are β-OH values for an Sb-free, Fe ₂ O ₃ -V ₂ Os-P ₂ Os glass according to an embodiment of the present invention.

Table V

[0042] It should be emphasized that the above-described embodiments of the present invention, particularly any "preferred" embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.