Patent Publication Number: US-2010130091-A1

Title: Method and apparatus for sealing a photonic assembly

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to a method of sealing a photonic assembly, and in particular a method and apparatus for applying a force on the photonic assembly to facilitate sealing the assembly to form a photonic device. The device may be, for example, an organic light emitting diode device comprising sealed glass substrate plates. 
     2. Technical Background 
     Photonic devices may be comprised of several sealed glass substrates including a photonic material disposed between the substrates. By photonic material what is meant is a material that either produces light in response to an applied electric current and/or voltage or produces an electric current in response to exposure to light. For example, organic light emitting devices typically comprise two glass substrates and an organic light emitting material positioned between the substrates. The assembly is sealed with a sealing material that surrounds the organic electroluminescent (EL) material and joins the substrates, thus sealing out contaminants that may degrade the organic EL material. The sealing material may be for example a glass-based frit, or the sealing material may be an adhesive, such as an organic adhesive. The sealing material is disposed about a perimeter of the device and, in conjunction with the glass plates, form an encapsulating glass package for the photonic material (e.g. EL material). The photonic device may be, for example, a display device useful for cell phones, computers, personal data assistants (PDAs), or a lighting panel. In other applications, the photonic device may comprise a photovoltaic (PV) device, such as a PV panel, for converting light into electrical energy. 
     In the case of organic light emitting diode devices in particular, care must be taken to ensure that the seal between the substrate plates is sufficiently hermetic to protect the organic electroluminescent material for a suitable length of time based on the particular application and need. This is true because the organic materials used in the manufacture of organic light emitting diode devices are sensitive to oxygen and moisture, and can quickly degrade when exposed to the atmosphere. Proper seals are often formed by applying a force against at least one of the substrates to ensure appropriate contact between the substrate plates and the sealing material using small magnets that are placed on top of the assembly when a steel plate is used to support the assembly. However, such approaches may result in uneven force around the perimeter of the seal, and debris from handing of the magnets may contaminate the seal. Thus, a sealing method that does not contribute contaminates to the resulting photonic device, and which are capable of applying a uniform force around the perimeter of the assembly would be beneficial. 
     SUMMARY 
     In one embodiment, a method of sealing a photonic assembly is disclosed comprising positioning a frame over and about a perimeter of a photonic assembly comprising a first substrate plate, a second substrate plate and a first sealing material disposed between the first and second substrate plates, a first gasket being positioned between the frame and the photonic assembly and a second gasket being positioned between the frame and a support plate supporting the photonic assembly, reducing a pressure in a free space region between the frame and the photonic assembly below an ambient pressure external to the frame, thereby causing a force to be exerted on the frame; curing the first sealing material to form a first seal between the first and second substrate plates; and removing the frame from the sealed assembly. 
     In another embodiment, an apparatus for sealing a photonic assembly is described comprising a frame formed as a closed loop encircling an open area, a first gasket positioned proximate an outside perimeter of the frame, a second gasket positioned proximate an inside perimeter of the frame and wherein the frame is adapted so that the first and second gaskets contact a support plate and the photonic assembly, respectively, when the frame is positioned over and about the photonic assembly. 
     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 
         FIG. 1  is a cross sectional view of an exemplary photonic assembly; 
         FIG. 2  is a perspective view of an apparatus for sealing a photonic assembly according to an embodiment of the present invention; 
         FIG. 3  is a cross sectional side view of a portion of the apparatus of  FIG. 2 ; 
         FIG. 4  is a top down view of the apparatus of  FIG. 2  shown holding down a photonic assembly comprising a plurality of photonic devices. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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 “component” includes aspects having two or more such components, unless the context clearly indicates otherwise. 
     Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. For example, the phrase “optionally substituted component” means that the component can or can not be substituted and that the description includes both unsubstituted and substituted aspects of the invention. 
     Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes 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. 
     As used herein, a “frit” or “frit composition,” unless specifically stated to the contrary, refers to mixture of base and absorbing components, and optionally a filler material such as an inert filler for adjusting a coefficient of thermal expansion of the frit. The term “frit” or “frit composition” can refer to any physical form of a frit, including a powder, a paste, an extruded bead, and can also refer to an attached or unattached frit deposited on a substrate. 
     As used herein, a “loop”, in reference to the frit or adhesive location, refers to a line of a material that forms a bounded region. The loop line can, for example, intersect with one or more portions of the line forming the bounded region (a closed loop), or can be a continuous line having no beginning or end and also forming a bounded region. A loop can have curved portions, straight portions, and/or corners, and no specific geometry is intended. 
     As used herein, a “perimeter” can refer to either the outer edge of a device or a location at or near the outer edge of a device. For example, a material positioned around the perimeter of a substrate can mean that the material is positioned either on the edge of the substrate or on a surface of the substrate at or near the edge. 
     As used herein, a photonic assembly is any assembly having components that utilize photons in the functioning of a device formed from the assembly. For example, a photonic assembly may be used to convert electrical energy into light, or light into electrical energy. Photonic assemblies may, for example, be included in electroluminescent displays, electroluminescent lighting panels, or photovoltaic (PV) devices for converting light into electrical energy (e.g. solar cells). For the purposes of discussion and not limitation, the following disclosure will be presented in the context of a photonic assembly used to fabricate an electroluminescent device such as an organic light emitting diode (OLED) display device suitable for use as a television or computer display, with the understanding that the invention may be similarly used to seal other photonic assemblies. 
     An exemplary photonic assembly  10 , illustrated in  FIG. 1 , comprises first substrate  12 , second substrate  14 , and sealing material  16  disposed between the two substrates. Sealing material  16  is preferably formed as a closed loop positioned inside of the perimeter of the first and second substrates. Photonic assembly  10  further comprises a photonic material  18  positioned between the two substrates within the area defined by the encircling sealing material  16 . Photonic material  18  may be, for example, an organic light emitting material. When sealing material  16  is appropriately conditioned or processed, a seal is formed between the first and second substrates  12 ,  14  that connects the substrates and forms an encapsulating package that protects the photonic material disposed therein. That is, the photonic material is protected between the two substrate plates  12 ,  14  and the encircling sealing material  16 . Photonic assembly  10  may further comprise electrical leads, anodes, cathodes, or other electrical connection members (not shown) that may pass through sealing material  16  as appropriate to the particular device. 
     As used herein, processing or conditioning of the sealing material to form the seal will be referred to as “curing” of the sealing material, where curing shall be understood to mean heating, irradiating, or any other method of applying energy to the sealing material that results in the formation of a seal that joins the first and second substrates. Thus, for example, the sealing material may be a ultraviolet (UV) or infrared (IR) light curable adhesive, such as a polymer resin that undergoes cross linking when exposed to the irradiating light, a heat curable resin, or the curing may comprise irradiating a glass-based frit with an infrared light sufficient to melt the glass-based frit that, upon cooling, forms a seal between the substrates. 
     First and second substrates  12 ,  14  may be glass, plastic or any other material suitable as a substrate for a photonic device. At least one of the substrates should be transparent, preferably at visible wavelengths, for transmitting or receiving light through the substrate. In some embodiments both substrates may be transparent. The composition of the substrates may dictate the choice of sealing material. For example, an adhesive or a glass based frit may be used to join glass substrates. On the other hand, if substrates  12 ,  14  are plastic, a glass based frit may be potentially inappropriate if a curing temperature that exceeds a temperature that can be tolerated by the plastic is involved. In some embodiments, both a glass based frit and an adhesive may be used to form a plurality of assembly seals. Such seals may be formed, for example, utilizing an inner, glass frit-based seal formed from a glass frit, and an adhesive seal (e.g. an epoxy seal) formed outside of (e.g. concentric with) the inner frit seal. Dual seals combine a hermetic inner glass seal with a resilient outer seal for increased mechanical integrity. 
     An apparatus  20  for sealing a photonic assembly in accordance with an embodiment of the invention is shown in  FIG. 2 . Apparatus  20  comprises frame  22  in the form of a closed loop encircling or enclosing an open area  23 . For example, the overall shape of frame  22  may be a rectangle. Frame  22  is preferably formed from a light weight metal such as aluminum, although other materials may be used (e.g. stainless steel). It is preferable to use a corrosion resistant material to prevent buildup of potential contaminants on the surface of the frame. As best shown in  FIG. 3 , frame  22  includes a generally vertical leg portion  24  and a generally horizontal arm portion  26 , giving the frame an “L” shaped cross section. In this context, the terms “vertical” and “horizontal” are terms relative to the orientation of the frame, and a reference plane. That is, when frame  22  is placed over and about assembly  10 , leg  24  is generally vertical relative to a plane of the substrates of assembly  10 . The term “generally” in the present context is intended to imply that the leg portion and arm portions may have other angular orientations relative to each other, and need not be precisely orthogonal, but that frame  20  includes a leg portion that is positionable about an outside perimeter of assembly  10 , while at the same time an arm portion extends over a portion of the assembly  10  perimeter. 
     Apparatus  20  may further comprise support plate  28  for supporting frame  22  and photonic assembly  10 . Support plate  28  is preferably planar, and can be manufactured from a variety of materials, including but not limited to a ceramic, aluminum or stainless steel. The plane of support plate  28  is parallel to the plane of at least one of the major surfaces of substrates  12  or  14 , and preferably parallel to the major surfaces of both substrates. 
     Frame  22  may further comprise first and second gaskets  30 ,  32  ( FIG. 3 ) formed from a compliant material that preferably does not emit volatile constituents such as plasticizers, particularly if heated, and does not leave a stain or residue on the assembly substrates or support plate. As illustrated in the cross section of  FIG. 3 , first gasket  30  is positioned on the underside surface  34  of leg portion  24  proximate exterior perimeter or surface  36 , while second gasket  32  is positioned on underside surface  38  of arm portion  26  proximate interior perimeter (surface)  40 . Alternatively, first gasket  30  may be included as a portion of support plate  28 . For example, support plate  28  may comprise a groove into which gasket  30  may be held. Alternatively, gasket  30  may be loosely positioned between the support plate and the frame. 
     Frame  22  may further define one or more passages  42  to which a vacuum may be applied. As shown in  FIG. 3 , frame  22  is supported by support plate  28  and positioned around and over assembly  10  such that first gasket  30  is in contact with support plate  28  and second gasket  32  is in contact with first substrate  12  (or second substrate  14 , recognizing that assembly  20  may be flipped and placed on support plate in a reversed position). The resulting free space region  44  is thus formed between frame  22 , assembly  10  and support plate  28 . Accordingly, passages  42  may extend from an outside surface of frame  22  (e.g. surface  36  and/or surface  37 ) to a surface of frame  22  within free space region  44  (e.g. surface  38 ) such that when a vacuum is applied to passage  42  the atmosphere within free space region  44  is removed and the pressure in the free space region is reduced. As an example,  FIG. 3  illustrates passage  42  extending from exterior surface  36  to interior surface  38 . Passage  42  could just as easily be a straight passage through arm portion  26  from surface  37  to surface  38 . Preferably, the atmosphere disposed between first and second substrates  12 ,  14  is also removed, as indicated by arrow  46 , thereby reducing the pressure of the assembly interior atmosphere. 
     The reduced pressure in free space volume  44  results in a pressure differential between free space volume  44  and the ambient atmosphere surrounding apparatus  20 , that in turn results in a force F being applied to the top surface  37  of frame  22 . Force F a) causes frame  22  to be forced downward toward support plate  28  and a temporary seal to form between first and second gaskets  30 ,  32  and support plate  28  and assembly  10 , respectively, and b) results in increased contact between substrates  12 ,  14  and sealing material  16 . The increased contact between substrates  12 ,  14  and sealing material  16  facilitates an improved seal between the substrates when sealing material  16  is cured. In some embodiments, at least a portion of the atmosphere contained between the substrate plates and within a perimeter of the sealing material, e.g. surrounding photonic material  18 , may also be removed (as indicated by arrow  46 ) in response to the vacuum applied to passage  42  so that a differential pressure is also produced across first substrate plate  12  (or substrate  14 ), and thus additional force may be applied against first sealing material  16  by the ambient pressure external to substrate plate  12  (or substrate  14 ). 
     In some embodiments, a vacuum may be applied through passages  52  in support plate  28  that open into free space region  44 . Passages  52  may be in addition to passages  42 . That is, both passages  42  and passages  52  may be used, but the use of both is not necessary. In other embodiments, additional passages  50  may be formed in support plate  28  that are positioned underneath assembly  10  such that when a vacuum is applied to the additional passages  50 , at least plate  14  (or plate  12 ) may be held firmly against support plate  28 . In the event that plate  28  includes passages  50 , a third gasket  54  positioned between bottom substrate plate  14  and support plate  28  may optionally be used to seal free space region  44  from passages  50 . The vacuum applied in each of the foregoing embodiments may be applied using conventional methods, such as a vacuum pump and other associated hardware (e.g. an accumulator bottle for reducing surging from the pump, valves, piping, etc.). The vacuum applied to passages  42 , and/or  50 , and/or  52  may be independently controlled via one or more control valves (not shown) to control the differential pressure and therefore the force applied to assembly  10  by frame  22 . Preferably, the vacuum (reduced pressure) is controlled within 50 mbar or less. 
     Once frame  22  is positioned over and biased against assembly  10  and support plate  18  via a pressure differential across frame  22 , sealing material  16  is cured by a method appropriate to the sealing material. For example,  FIG. 3  illustrates a glass based frit as the sealing material. The glass based frit is cured by irradiating the frit through substrate  12  with a laser beam  56  produced by laser  58 . Laser beam  56  is traversed over the length of seal material  16  until the seal material is properly cured. In this case, the laser beam heats the frit until the frit softens and forms a seal between substrates  12 ,  14 . A fuller description of an exemplary method for laser sealing a glass package can be found in U.S. Pat. No. 6,998,776, the contents of which are incorporated herein in their entirety by reference. 
       FIG. 4  depicts a top-down view of apparatus  20  positioned over and about assembly  10 , wherein assembly  10  comprises a plurality of closed loops of sealing material  16  positioned between substrates  12 ,  14 . According to the embodiment of  FIG. 4 , passages  42  extending through frame  22  to free space region  44  are disposed in a top surface of the frame. Each loop of sealing material  16  represents a future photonic device: after sealing material  16  is cured, each subsequently sealed device is separated from the parent composite structure between the individual device seals. 
     Use of the present invention may:
         reduce apparatus and assembly set up times compared to the use of discrete force applicators, such as magnets.   reduce the risk of damaging a lens associated with an irradiating light source with smoke when irradiating the sealing material if a rubberized magnet is inadvertently burned with, for example, a laser used in the sealing process.   apply a uniform force on the entire assembly, and minimize the risk that a substrate plate is not driven into the photonic material, thus physically damaging the EL material and rendering the device unusable. A tunable vacuum level can be applied with appropriate valving and metering control to provide an evenly distributed and proper level of contact between the sealing material and the substrates while avoiding substrate-to-photonic material contact.   secure the substrate plate(s) in place, thus preserving proper alignment of the substrates and/or the sealing material.   eliminate the need for large vacuum chambers.   enable fast vacuum cycle times compared to the time required to cycle a large volume vacuum chamber.   eliminate obstructions between, for example, a laser and target device because the contact force is already applied.       

     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.