Abstract:
A method of sealing a glass package comprising providing a first glass substrate, the first substrate having first and second alignment marks. A second glass substrate having third and fourth alignment marks is aligned to the first substrate by translating the first substrate relative to the second substrate, and aligning the second and fourth alignment marks by rotating the first substrate relative to the second substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/773399 filed on 14 Feb. 2006, the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to sealing a glass envelope, and particularly to sealing glass substrates with a frit. 
         [0004]    2. Technical Background 
         [0005]    Organic light emitting diode displays offer an attractive alternative to LCD and plasma based display technologies. However, the organic light emitting elements of the display devices are susceptible to damage from exposure to moisture (humidity) and oxygen, and therefore must be hermetically sealed within an appropriate envelope to prevent exposure to such environmental hazards. The hermetic seal must be robust enough to protect the organic material over the lifetime of the device incorporating the display (e.g. television, cell phone, etc.). 
         [0006]    Glass envelopes are an ideal container in which to encase the OLED device. Such envelope may be sealed using an epoxy, or other adhesive material, but more recently, sealing via a glass frit disposed between the substrates comprising the display has proven to be a desirable alternative, owing at least in part to the hermetic nature of the glass seal formed by the frit. Nevertheless, as the applications for OLED displays increase in size, from camera and cell phone screens to larger devices, such as televisions, the size of the substrates used to manufacture such devices must also increase in size to provide the necessary economies of scale. That is, equipment manufacturers typically deposit a plurality of display devices between two substrate, seal the substrates, then separate (divide) the sealed substrates into a plurality of finished display devices. Hermetically sealing a plurality of OLED display devices between two substrates repeatedly, with a high degree of precision, has proven challenging. 
         [0007]    As described above, because the organic layers of the OLED display device are susceptible to damage from heat, the frit used to form the seal between the glass substrates cannot be heated in an oven, since the heat would be applied equally to the frit and to the organic materials. Sealing the frit by traversing the frit with a laser, thereby heating the frit and forming the hermetic seal is the desired approach. 
         [0008]    A current method of laser sealing alignment is to align the laser to each cell (each frit frame) manually prior to sealing, which is time consuming and has potential for human error. This manual process also relies on the edges of the substrates as markers for alignment. The lateral and rotational tolerance of the cells containing the OLED devices with respect to the substrate edges and fritted cover sheets is not repeatable enough for the glass edges to be used for alignment of large size substrates containing a multiplicity of OLED devices. As the size and/or number of cells per substrate increases, and hence the size of the sheets, the need to align the entire substrate to the fritted cover sheet becomes critical for efficiency, precision and yield. The alignment of the OLED devices to the frit forming each cell is critical to ensure the hermeticity of the seal 
         [0009]    In some processes magnets have been used as a method to apply a force between the OLED-containing substrate and the fritted cover sheet during laser sealing. However, magnets are not practical for large substrate sizes. In addition, magnets have been identified as an enhancer of Newton&#39;s Rings within sealed substrates due to their non-uniform force on the substrates. A process and equipment that applies a uniform repeatable force over the entire substrate, and which is capable of repeatable, precise alignment of the substrate components is therefore desirable. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with an embodiment of the present invention, a method of sealing a glass envelope is disclosed comprising a first glass substrate having first and second alignment marks. A second glass substrate having third and fourth alignment marks, and comprising at least one frit wall disposed thereon, is aligned to the first substrate by translating the first substrate relative to the second substrate, and aligning the second and fourth alignment marks by rotating the first substrate relative to the second substrate. The translating is accomplished along one or both of orthogonal axes in the plane of the first substrate. The resulting stack is then sealed by heating and melting the at least one frit wall with a laser. 
         [0011]    In another embodiment, a method of sealing a glass envelope is provided comprising positioning a first substrate comprising first and second alignment marks on an alignment table having an axis of rotation such that the axis of rotation passes through the first alignment mark, positioning a second substrate comprising third and fourth alignment marks over the first substrate, the second substrate including a frit deposited thereon, aligning the first alignment mark with the third alignment mark, rotating the first substrate about the axis of rotation to align the second alignment mark with the fourth alignment mark, and heating the frit with a laser beam to melt the frit and form a hermetic seal between the first and second substrates 
         [0012]    In still another embodiment, an apparatus for assembling and sealing a glass envelope is disclosed comprising a rotatably mounted alignment table for receiving a substrate, the alignment table being movable along x and y directions in a plane, the x and y directions being mutually orthogonal, a substrate transporter for transporting the first substrate to the alignment table and a laser sealing system for sealing a second substrate having a frit deposited thereon to the first substrate. 
         [0013]    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. 
         [0014]    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 an exemplary embodiment of the invention, and together with the description serve to explain the principles and operations of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of an apparatus for assembling and sealing glass substrates to form a glass envelope according to an embodiment of the present invention; 
           [0016]      FIG. 2  is an top-down illustration of the movement (translation and rotation) of a first substrate relative to a second substrate using alignment marks to align the first and second substrates. 
           [0017]      FIG. 3  is a top view of first and second substrates, showing exemplary positioning of OLED devices and frit walls, and exemplary positioning of alignment marks. 
           [0018]      FIG. 4  is a cross sectional view of a first substrate having OLED devices disposed thereon, and it&#39;s positioning relative to a second substrate having frit walls disposed thereon, prior to final assembly and sealing. 
           [0019]      FIG. 5  is a perspective view of an apparatus for assembly and sealing of glass substrates to form a glass envelope 
           [0020]      FIG. 6  is a partial perspective view of the apparatus of  FIG. 5  showing the rail and sealing systems. 
           [0021]      FIG. 7  is a diagrammatic view of an assembly and sealing system in accordance with an embodiment of the present invention 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. 
         [0023]    Organic light emitting diode (OLED) display devices typically comprise at least a first substrate including organic light emitting layers disposed thereon. This first substrate is often referred to as the backplane. A second, transparent, substrate is then placed overtop the backplane, with a sealing material disposed between the first and second substrates to create a hermetically sealed package with the OLEDs disposed therein. The substrates themselves are typically less than about 1 mm in thickness, but more generally less than about 0.7 mm in thickness. The seal may be an epoxy seal, or, in some cases, the seal may be a glass seal formed by a frit material disposed between the two substrates. In the instance of a frit seal, a frit paste is first deposited onto the cover substrate in a pre-determined pattern matching generally the pattern of deposited OLED devices on the first substrate, and then pre-sintered by heating the frit to a sintering temperature, to sinter the frit and burn off organic binders in frit. To improve manufacturing efficiencies, the backplane may include a plurality of individual OLED display devices disposed thereon in an array. When the substrates are assembled, each OLED display device of the array of display devices is encapsulated between the backplane, the cover and a frame-shaped wall of frit. After the substrates are sealed, the individual OLED display devices may be cut from the sealed parent substrates to form separate OLED displays. 
         [0024]    In some instances where frit sealing is performed, a temporary mask having transparent regions coinciding with the frit patterns on the cover substrate may be placed over the cover substrate to ensure that the laser used to heat and melt the frit and seal the first substrate to the second substrate does not accidentally also heat any of the organic materials used to form the OLED device. Such organic materials are intolerant of the high temperatures used to melt the frit, and may be damaged or destroyed if contacted by the laser light. If used, the mask must also be aligned precisely to the cover substrate to ensure a proper hermetic seal is made around each individual OLED display device. 
         [0025]    In accordance with an embodiment of the present invention illustrated in  FIG. 1 , an assembly and sealing apparatus is provided, referred to generally by the reference numeral  10 . Assembly and sealing apparatus  10  comprises alignment table  12 , further comprising a vacuum chuck  14  having a plurality of orifices  16  (see  FIG. 6 ) opening on surface  17  and connected to a vacuum source (not shown). Orifices  16  have been omitted from  FIG. 1  so as not to obscure other details. Vacuum chuck  14  may be an integral part of alignment table  12 , or vacuum chuck  14  may be mounted on alignment table  12 . In any event, alignment table  12  and vacuum chuck  14  are preferably secured one to the other and move as a unit. Surface  17  of the vacuum chuck includes an alignment mark  18  to facilitate placement of a backplane substrate thereon. As depicted in the figures, the alignment marks are circles and crosshairs, but may be other shapes, such as dots. Alignment table  12  is adapted such that the alignment is capable of movement in both the “x” and “y” directions, where the x and y directions define a plane parallel with surface  17  of vacuum chuck  14 . Alignment table  12  is also capable of rotation about a z axis  19  within the xy plane, z axis  19  being also perpendicular to the xy plane. Preferably, the vacuum chuck alignment mark  18  is coincident with the z rotational axis  19  of alignment table  12 . 
         [0026]    Backplane substrate  20 , comprising a plurality of OLED devices  21  disposed thereon, is positioned over vacuum chuck  14 , and vacuum chuck alignment mark  18  aligned with a first alignment mark  22  on backplane substrate  20 . This may be conveniently accomplished by moving alignment table  12  in either or both of the “x” and “y” directions until alignment mark  18  on vacuum chuck  14  is aligned with alignment mark  22  on backplane substrate  20 . After alignment of backplane substrate alignment mark  22  with vacuum chuck alignment mark  18 , backplane substrate  20  and vacuum chuck  14  are brought into contact, and a vacuum is applied to the vacuum chuck, securing backplane substrate  22  to chuck  14 . 
         [0027]    Following placement of backplane substrate  20  onto vacuum chuck  14 , cover substrate  24 , comprising a plurality of frame-shaped frit walls  25  (see  FIGS. 3 ,  4 ), is positioned over the backplane substrate. Alignment table  12  is translated in either or both of the x and y directions until a first alignment mark  26  on cover substrate  24  is aligned with alignment mark  22  on backplane substrate  20 . Once alignment marks  22  and  26  are aligned, alignment table  12  is rotated about axis  19  until a second alignment mark  28  on backplane substrate  20  is aligned with a corresponding second alignment mark  30  on cover substrate  24 . This process is illustrated in  FIG. 2 , indicating translation of backplane substrate  20  according to the “x” and “y” arrows, until alignment marks  22 ,  26  are aligned, then rotation of backplane substrate  20  (via alignment table  12 ) through an angle θ until alignment marks  28 ,  30  are centered one with the other. Note that the pattern of OLED devices and frit patterns on the backplane and cover substrates, respectively, have been omitted so as not to obscure other details of  FIG. 2 . After backplane substrate  20  and cover substrate  24  are aligned and brought into contact, OLED devices  21  are circumscribed by the frit walls  25 . This is depicted more clearly in  FIGS. 3-4 .  FIG. 3  shows a top view of cover substrate  24  overlayed on backplane substrate  20  after alignment of the substrates and depicts OLED devices  21  within the boundaries of frit walls  25 .  FIG. 4  illustrates a partial cross section of backplane substrate  20  and cover substrate  24 , and also illustrates OLED devices  21  and frit walls  25 , just prior to bringing the fritted cover substrate into contact with the backplane substrate. 
         [0028]    Once alignment mark pairs  22 ,  26  and  28 ,  30  are aligned, fritted cover substrate  24  may be brought into contact with backplane substrate  20  and secured in place, such as by clamping. Clamping may be accomplished by any appropriate means that will not interfere with the sealing process, and which does not damage the OLED devices disposed between the backplane and cover substrates. 
         [0029]    While not necessary, vacuum chuck  14  may include second alignment mark  32  such that backplane substrate  20  may be aligned with vacuum chuck  14  in a manner as disclosed above for cover substrate  24 . That is, prior to securing backplane substrate  20  to vacuum chuck  14 , but after moving alignment table  12  in one or both of the x and y directions to align alignment mark  18  with alignment mark  22 , rotating alignment table  12  until alignment mark  32  on vacuum chuck  14  is alignment with alignment mark  28  on backplane substrate  20 . 
         [0030]    If optional sealing laser mask  34  is to be used, mask  34  is positioned and aligned similarly to the process described above. Mask  34  is positioned over the stacked substrates  20 ,  24 . Alignment table  12  is then translated in either or both of the x and y directions until alignment marks  22 ,  26  are aligned with a first alignment mark  36  on the mask. Alignment table  12  is then rotated until alignment marks  28 ,  30  are aligned with a second alignment mark  38  on mask  34 . Mask  34  is then brought into contact with cover substrate  24 , and secured in place. 
         [0031]    To ensure proper sealing, pressure plate  40  may also be applied to the stack of aligned substrates. The pressure plate may be a simple, thick, substrate transparent to the wavelength of light from the sealing laser, which is placed overtop the aligned stack, and exerts a substantially uniform pressure on the stack through the action of gravity. By transparent what is meant is that the pressure plate will not absorb laser energy in an amount which impedes the sealing process. Since pressure plate  40  will not become a permanent part of the glass package, nor play a part in the sealing process beyond exerting pressure on the aligned stack, critical alignment of the pressure plate is not necessary, but may, if desired, be accomplished in the manner described above, if appropriate alignment marks are included on pressure plate  40 . 
         [0032]    Once the stack is assembled, the first, backplane substrate, and the second, cover substrate may be sealed by traversing the frit disposed between the backplane and cover substrates with a laser beam to heat the frit. The heated frit melts and forms a hermetic seal between the backplane and cover substrates. The laser beam is directed at the frit through the cover substrate, the optional mask, if used, and the pressure plate. Consequently, it is desirable that the cover substrate, and the pressure plate are substantially transparent to the wavelength of light emitted by the laser. Those portions of the mask conforming to the frit placement should also be substantially transparent to the wavelength of the laser beam. 
         [0033]    Is it preferable that alignment of one alignment mark to another alignment mark is accomplished to within ±20 μm, i.e. the distance between the center of one alignment mark and the center of another alignment mark aligned to the preceding alignment mark should not exceed 20 μm. To that end, it should be appreciated that the steps described supra may be automated such that the method may be carried out quickly, rapidly and precisely. For example,  FIG. 5  illustrates additional elements which may be used with exemplary apparatus  10  for carrying out the assembly and sealing method described above. Apparatus  10  may further include a preparation and sealing chamber  42  comprising a preparation portion  44  and a sealing portion  46 , the preparation portion and the sealing portion being joined such that substrates prepared in the preparation portion may be freely transported between the preparation portion and the sealing portion. Preferably the preparation and sealing chamber  42  is capable of being hermetically sealed such that an atmosphere appropriate to the sealing process may be maintained within the chambers. For example, the preparation and sealing chamber atmosphere may be comprised primarily of an inert or noble gas such as nitrogen or argon. 
         [0034]    Apparatus  10  may further include a substrate transporter system  48  and a laser sealing system  50 , in addition to alignment table  12 . Substrate transporter system  48  may utilize any known method of transporting thin substrate sheets from one location to another. As illustrated in  FIG. 6 , substrate transporter system  48  may include a raised rail system  52  which supports a vacuum-assisted substrate carrier assembly  54  which is attached to the rail system. In one particular embodiment, rail system  52  comprises extendable cantilevered arms  56  that allow the rail system to extend into sealing portion  46  from preparation portion  44 . After delivering a component (e.g. a substrate) to the sealing chamber, the substrate carrier assembly attached to extendable arms  56  is retrieved back into preparation portion  44  by retracting extendable arms  56  from the sealing portion. Thus, it is intended that transporter system  48  will not interfere with laser sealing system  50 . 
         [0035]    Vacuum assistance on substrate carrier assembly  54  may be used to hold a substrate component which is to be added to the stack, such as the first (backplane) or second (cover) substrate, the pressure plate, etc. For example, the substrate carrier assembly may include one or more vacuum chucks  58  which may be used to secure the component to the substrate carrier assembly. When the first, backplane substrate is being secured to the substrate carrier assembly, it is desirable that the one or more vacuum chucks  58  do not contact any of the OLED devices disposed on the surface of the backplane substrate. Since the backplane substrate is typically the first component of the stack and the OLED devices will be disposed between the backplane substrate and the subsequent cover substrate, the OLED devices will be on the surface of the backplane substrate facing the substrate carrier assembly and vacuum chucks. Consequently, the one or more vacuum chucks  58  may be adjusted in position to avoid contact with the OLED devices. As shown in  FIG. 6 , vacuum chucks  58  are attached to support beams  60 , which are in turn slidably connected to slide bars  62 . For example, support beams  60  may be connected to slide bars  62  by bushings (not shown). This allows support beams  60  to be moved in a direction orthogonal to the direction of travel of extension arms  56  by sliding on slide bars  62 , thereby allowing transporter system  48  to handle substrates having a variety of widths and/or OLED device patterns. 
         [0036]    In contrast to the backplane substrate, the cover substrate will be oriented such that the surface of the cover substrate on which the frit is deposited will be facing away from the transport system. Thus, the vacuum chucks may contact the surface of the cover substrate opposite the frit-deposited surface, without concern for damaging the frit. 
         [0037]    In accordance with the present embodiment, the backplane substrate will be moved by transporter system  48  from preparation portion  44  of chamber  42  to a position over the alignment table  12  in sealing portion  46  of chamber  42  with the OLED material facing up, avoiding contact with the OLED material. The transfer of the backplane substrate is facilitated by cantilevered rail system  52  to move the substrates via substrate carrier assembly  54  from preparation portion  44  of chamber  42  to sealing portion  46  of chamber  42 . First alignment mark  22  on backplane substrate  20  is then aligned with alignment mark  18  on vacuum chuck  14 . 
         [0038]    In accordance with the present embodiment, alignment table  12  may also be capable of translation in the Z direction, i.e. parallel to the axis of rotation of the table. Consequently, once a substrate component, such as the backplane substrate, is properly positioned above alignment table  12 , the alignment table is translated along the Z direction until the alignment table is in contact with the substrate. A vacuum is applied to the component substrate from vacuum chuck  14  mounted on alignment table  12  to secure the component substrate to the alignment vacuum chuck, and the vacuum to vacuum chucks  58  on transfer substrate carrier assembly  54  are released. 
         [0039]    Assuming the transferred substrate described supra is the backplane substrate, the fritted cover substrate is secured to the retracted substrate carrier assembly and then moved from the preparation portion  44  of chamber  42  to a position over the backplane substrate in the sealing portion  46  with the frit material facing downward. 
         [0040]    The fritted cover substrate is aligned with the backplane substrate, first by translating the backplane substrate via the alignment table in either or both of the x and y directions such that alignment mark  22  on backplane substrate  20  is aligned with alignment mark  26  on cover substrate  24 . Alignment mark  18  preferably has the same center as the axis of rotation  19  of alignment table  12  so that once alignment mark  22  is aligned with alignment mark  26 , subsequent alignment requires only a rotation of alignment table  12 . Thus, once alignment marks  22 ,  26  are aligned, alignment table  12  is rotated until alignment mark  28  is aligned with alignment mark  30 . Obviously, the distance between alignment marks  22  and  28  are the same as the distance between alignment marks  26  and  30 , and alignment marks  26 ,  30  are arranged on cover substrate  24  such that when alignment marks  26  and  30  are aligned with alignment marks  22  and  28 , respectively, cover substrate  24  is appropriately aligned with backplane substrate  20 . Once cover substrate  24  is aligned with backplane substrate  20 , cover substrate  24  is brought into contact with backplane substrate  20 , and suitably clamped to backplane substrate  20 . Cover substrate  24  is released from substrate carrier assembly  54  and the substrate carrier assembly and extension arms  56  are retracted from the sealing portion of chamber  42  into the preparation portion. 
         [0041]    If mask  34  is to be used during the sealing process, the mask is positioned and aligned to the substrate stack in a manner consistent with the description above. The mask may consist of a substrate which is generally non-transmissive to the laser light used to seal the substrates, but which comprises transparent regions coinciding with the frit array on the cover substrate. For example, the mask may be formed by conventional vapor deposition methods. The mask may then be clamped into place on the stack. This is followed by positioning and alignment of the pressure plate with respect to the stack. In some embodiments, the pressure plate and mask may be a unitary structure, e.g. a thick plate of silica with appropriate transparent regions to facilitate sealing of the frit. That is, the pressure plate may include a photolithographic pattern which allows only selected transmission of an incident laser beam. 
         [0042]    The entire stack, comprising backplane substrate  20  (with OLED display devices  21 ), fritted cover substrate  24 , pressure plate  40  and optionally mask  34  is then aligned with laser sealing system  50  by rotating the alignment table until alignment mark  38  aligns with a reticle (not shown) in an alignment lens of the laser sealing system. 
         [0043]    As depicted in  FIG. 7 , laser sealing system  50  includes laser  62 , laser transport system  64 , optical guidance system  66  and control computer  68 . Laser  62  emits a laser light having a wavelength which is readily absorbed by the frit, but conversely easily transmitted by the cover substrate, the pressure plate, and the transparent regions of the optional mask substrate. For example, lasers emitting at a wavelength of 810 nm are commercially readily available and may be used in the sealing process. Laser transport system  64  is adapted such that laser  62  may be translated in both the “x” and “y” directions of a plane parallel with surface  17  of vacuum chuck  14 , and the stacked substrates. For example, a conventional gantry system may be used. As shown in  FIG. 7 , an optical guidance system is connected with the computer and adapted via conventional machine vision software to move in reference to predetermined landmarks on the substrates to be sealed. For example, the optical guidance system may include a camera wherein the camera, in conjunction with computer  68 , guides laser transport system  64  so that movement of laser  62  can be precisely controlled in accordance with a predetermined set of instructions programmed into the computer. This may be accomplished by focusing the camera on a predetermined point on the substrate stack, for example, an alignment mark, which may then serve as a reference point for all further movement of the laser. The computer then relays the appropriate instructions to the laser transport system to drive the laser in a predetermined pattern appropriate for a given frit pattern on cover substrate  24 . The computer may further turn laser  62  on or off, or otherwise adjust the power of laser  62  as necessary to accomplish proper melting of the frit and sealing of the backplane substrate to the cover substrate. 
         [0044]    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. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.