Gas separation: apparatus – Solid sorbent apparatus
Reexamination Certificate
2001-08-08
2004-05-25
Lawrence, Frank M. (Department: 1724)
Gas separation: apparatus
Solid sorbent apparatus
C096S147000, C055S385600, C252S194000, C206S204000, C502S400000
Reexamination Certificate
active
06740145
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to control of moisture inside a packaged electronic device and relates particularly to an improved desiccant and desiccant package which desiccates highly moisture-sensitive electronic devices to prevent premature device failure or premature degradation of device performance.
BACKGROUND OF THE INVENTION
Various microelectronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant package within a cover. Desiccant packages include a container for receiving solid water absorbing particles (a desiccant) or providing such particles into a binder. Examples of solid water absorbing particles include molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials which are used to maintain the humidity level within the above range.
Particular microelectronic devices, for example, organic light-emitting devices (OLED) or panels, polymer light-emitting devices, charge-coupled device (CCD) sensors, and micro-electro-mechanical sensors (MEMS) require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm. Such low levels are not achievable with desiccants of silica gel materials and of Drierite materials. Molecular sieve materials can achieve humidity levels below 1000 ppm within an enclosure if dried at a relatively high temperature. However, molecular sieve materials have a relatively low moisture capacity at humidity levels at or below 1000 ppm, and the minimum achievable humidity level of molecular sieve materials is a function of temperature within an enclosure: moisture absorbed, for example, at room temperature, can be released into the enclosure or package during temperature cycling to higher temperature, such, as, for example, to a temperature of 100° C. Solid water absorbing particles used within such packaged devices include 0.2 to 200 &mgr;m particle size powders of metal oxides, alkaline earth metal oxides, sulfates, metal halides, or perchlorates, i.e. materials having desirably relatively low values of equilibrium minimum humidity and high moisture capacity. However, such materials even when finely divided into powders of 0.2 to 200 &mgr;m particle size often chemically absorb moisture relatively slowly compared to the above-mentioned molecular sieve, silica gel, or Drierite materials. Such relatively slow reaction with water vapor leads to a measurable degree of device degradation of performance following the sealing of the desiccant inside a device cover due to, for example, moisture absorbed on the inside of a device, moisture vapor present within the sealed device, and moisture permeating through the seal between the device and the cover from the outside ambient.
Some solid water absorbing particles, particularly molecular sieve materials which entrain moisture by physical absorption within microscopic pores, require a dehydrating step at substantially elevated temperature prior to use within a device enclosure, thus increasing the number of process steps and calling for additional apparatus, such as, for example, a controllable furnace to achieve substantial dehydration.
Selection of solid water absorbing particles and the method of applying selected particles to an inner portion of a device enclosure prior to sealing the device within or by the enclosure is governed by the type of device to be protected from moisture. For example, highly moisture-sensitive organic light-emitting devices or polymer light-emitting devices require the selection of particular solid water absorbing particles and methods of application, since organic materials or organic layers are integral constituents of such devices. The presence of organic materials or layers may, for example, preclude the use of certain solvents or fluids in the application of a solid water absorbing particles dispersed in a fluid to organic-based devices. Furthermore, a thermal treatment, if required, of a desiccant contained within a sealed device enclosure, needs to be tailored to the constraints imposed by thermal properties of the organic constituents or layers of the device. At any rate, release of solvent vapors during a thermal treatment of a desiccant disposed within a sealed device enclosure must be avoided or minimized if solvent vapors can adversely affect organic constituents of organic-based electronic devices. The aforementioned considerations pertaining to organic-based electronic devices may not be as important if the electronic device to be desiccated is strictly an inorganic or metallic device such as, for example, a MEMS device or a CCD sensor without an organic color filter overlay.
For highly moisture sensitive electronic devices, such as organic light-emitting devices or polymer light-emitting devices, VanSlyke, U.S. Pat. No. 5,047,687 teaches the use of a protective layer comprised of a mixture of at least one organic component of the organic electroluminiescent medium and at least one metal having a work function in the range of from 4.0 to 4.5 eV capable of being oxidized in the presence of ambient moisture. The metal in the protective layer is described by VanSlyke as being sufficiently reactive to be oxidized by ambient atmospheric moisture over an extended period of time when incorporated into the organic EL device. In this use the metal is used as solid water absorbing particles for moisture in the protective layer. That neither a coated layer of metal film alone nor successively coated layers of the metal and organic films were effective in preventing the dark spot growth due to ambient moisture was attributed to the slow oxidation of the bulk metal. VanSlyke, therefore, teaches that the oxidation susceptibility of reactive metals that can be oxidized by ambient moisture is enhanced by the higher surface to volume ratios achieved by co-deposition of the metal into a mixed layer of metal and an organic medium. However, VanSlyke does not teach the required metal desiccant particle size for optimal moisture absorption protection nor does he teach the effect of metal particle size on performance in protecting organic EL devices.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. For example, Kawami et al., European Patent Application EP 0 776 147 A1 disclose an organic EL element enclosed in an airtight container which contains a drying substance comprised of a solid compound for chemically absorbing moisture. The drying substance is spaced from the organic EL element, and the drying substance is consolidated in a predetermined shape by vacuum vapor deposition, sputtering, or spin-coating. Kawami et al. teach the use of the following solid water absorbing particles: alkali metal oxides, alkali earth metal oxides, sulfates, metal halides, and perchlorates. Kawami et al., however, do not teach the effect of particle size of these solid water absorbing particles on their performance.
Shores, U.S. Pat. No. 5,304,419 discloses a moisture and particle getter for enclosures which enclose an electronic device. A portion of an inner surface of the enclosure is coated with a pressure sensitive adhesive containing a solid desiccant with average particle size usually 0.2 to 100 &mgr;m and preferably 0.5 to 10 &mgr;m.
Shores, U.S. Pat. No. 5,401,536 describes a method of providing a moisture-free enclosure for an electronic device, the enclosure containing a coating or adhesive with desiccant properties. The coating or adhesive comprises a protonated alumina silicate powder with average particle size 0.2 to 100 &mgr;m, preferably 1 to 10 &mgr;m, dispersed in a polymer.
Shores, U.S. Pat. No. 5,591,379 discloses a moisture gettering composition for h
Bessey Peter G.
Boroson Michael L.
Irvin Glen C.
Kaminsky Cheryl J.
Rowley Lawrence A.
Eastman Kodak Company
Lawrence Frank M.
Owens Raymond L.
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