Abstract:
A sealing device and method are described herein that can be used to manufacture a hermetically sealed glass package. In one embodiment, the hermetically sealed glass package is suitable to protect thin film devices which are sensitive to the ambient environment (e.g., oxygen, moisture). Some examples of such glass packages are organic emitting light diode (OLED) displays, sensors, and other optical devices. The present invention is demonstrated using an OLED display as an example.

Description:
CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/748,131 filed on Dec. 6, 2005 and entitled “Method for Frit Sealing Glass Packages”. The contents of this document are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method for hermetically sealing glass packages that are suitable to protect thin film devices which are sensitive to the ambient environment (e.g., oxygen, moisture). Some examples of such glass packages are organic emitting light diode (OLED) displays, sensors, and other optical devices. The present invention is demonstrated using an OLED display as an example. 
       BACKGROUND 
       [0003]    OLEDs have been the subject of a considerable amount of research in recent years because of their use and potential use in a wide variety of electroluminescent devices. For instance, a single OLED can be used in a discrete light emitting device or an array of OLEDs can be used in lighting applications or flat-panel display applications (e.g., OLED displays). OLED displays are known to be very bright and to have a good color contrast and wide viewing angle. However, OLED displays and in particular the electrodes and organic layers located therein are susceptible to degradation resulting from interaction with oxygen and moisture which leak into the OLED display from the ambient environment. It is well known that the life of the OLED display can be significantly increased if the electrodes and organic layers located therein are hermetically sealed from the ambient environment. Unfortunately, it has been difficult to develop a sealing process to hermetically seal the OLED display. Some of the factors that can make it difficult for one to properly seal the OLED display are briefly mentioned below:
       The hermetic seal should provide a barrier for oxygen (10 −3  cc/m 2 /day) and water (10 −6  g/m 2 /day).   The size of the hermetic seal should be minimal (e.g., &lt;2 mm) so it does not have an adverse effect on size of the OLED display.   The temperature generated during the sealing process should not damage the materials (e.g., electrodes and organic layers) within the OLED display. For instance, the first pixels of OLEDs which are located about 1-2 mm from the seal in the OLED display should not be heated to more than 100° C. during the sealing process.   The gases released during sealing process should not contaminate the materials within the OLED display.   The hermetic seal should enable electrical connections (e.g., thin-film chromium electrodes) to enter the OLED display.       
 
         [0009]    Today one way to seal the OLED display is to form a hermetic seal by heating and softening a frit that is doped with a material which is highly absorbent at a specific wavelength of light. In particular, a high power laser is used to heat-up and soften the frit to form a hermetic seal between a first substrate plate (cover glass) which has the frit located thereon and a second substrate plate (substrate glass) which has the OLEDs located thereon. The frit is typically ˜1 mm wide and ˜6-100 um thick. If the absorption and thickness of the frit is uniform, then the sealing can be done at a constant laser energy and a constant speed which causes a uniform temperature rise within the frit. However, when the frit is relatively thin, then 100% of the laser energy is not absorbed by the frit and instead some of the laser energy is absorbed or reflected by metal electrodes which cross the frit at certain points and are attached to the OLEDs. Since, it is often desirable to use thin frits and the metal electrodes often have different reflectivity and absorption properties as well as different thermal conductivities than the bare substrate glass, this can create an uneven temperature distribution within the frit during the sealing process. The uneven temperature distribution within the frit can lead to problematical cracks, residual stress and/or delamination which prevents/degrades a hermetic connection between the cover glass and the substrate glass. 
         [0010]    To address this sealing problem, the assignee of the present invention has developed several different sealing techniques which have been disclosed in U.S. patent application Ser. No. 11/095,144 filed on Mar. 30, 2005 and entitled “Method for Backside Sealing Organic Light Emitting Diode (OLED) Displays” (the contents of this document are incorporated herein by reference). Although these sealing techniques work well there is still a desire to develop new and improved sealing techniques which can be used to hermetically seal an OLED display (or glass package). This particular need and other needs have been satisfied by the sealing device and the sealing method of the present invention. 
       SUMMARY 
       [0011]    The present invention includes a sealing device and a sealing method for manufacturing a hermetically sealed OLED display. In one embodiment, the sealing method includes the following steps: (a) using a laser to direct a laser beam towards a frit which is located within an unsealed OLED display; (b) moving the laser (or the unsealed OLED display) such that the laser beam moves at a substantially constant speed on a sealing line along which there is the frit, electrode free regions, and electrode occupied regions that are located between two substrate plates; (c) modulating a power of the laser while the laser beam is moving at the substantially constant speed such that the laser beam imparts proper laser energy to the frit to cause the frit to melt and form a hermetic seal which connects the two substrate plates; (d) synchronizing the moving step and the modulating step such that the power of the laser can be controlled to change as needed at predetermined points along the sealing line depending on whether or not the laser beam is currently located over the electrodes that are present between the two substrate plates and depending on whether or not the laser beam is currently located over a curved portion of the sealing line on the two substrate plates; and (e) if the power of the laser is maximized or minimized at any point during the modulating step, then the speed that the laser beam is moved along the sealing line on the two substrate plates can be changed so that the laser beam is able to continue imparting proper laser energy to the frit so as to cause the frit to melt and form the hermetic seal which connects the two substrate plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A more complete understanding of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
           [0013]      FIGS. 1A and 1B  respectively show a cross-sectional side view and a top view that illustrate the basic components of a sealing device and a hermetically sealed OLED display in accordance with a first embodiment of the present invention; 
           [0014]      FIG. 2  is a flowchart illustrating the steps of a preferred sealing method for manufacturing the hermetically sealed OLED display shown in  FIGS. 1A and 1B  in accordance with the first embodiment of the present invention; 
           [0015]      FIGS. 3A-3B  are two diagrams which are used to help explain an additional feature that can be used to enhance the moving step associated with the sealing method shown in  FIG. 2  in accordance with the first embodiment of the present invention; 
           [0016]      FIGS. 4A-4D  there is shown an exemplary OLED display along with several different graphs which are used to help explain in more detail some of the different features associated with the sealing method shown in  FIG. 2  in accordance with the first embodiment of the present invention; 
           [0017]      FIGS. 5A-5E  there are illustrated six drawings/graphs that are associated with a couple of experiments (and their results) which were conducted to test how well the sealing method shown in  FIG. 2  functions to hermetically seal two glass substrates (which did not contain OLEDs and electrodes) in accordance with the first embodiment of the present invention; and 
           [0018]      FIG. 6  is a cross-sectional side view that illustrates the basic components of a sealing device which can be used to hermetically seal an OLED display in accordance with a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring to  FIGS. 1-5 , there are several diagrams which are used to help explain how a sealing device  100  and a sealing method  200  can be used to hermetically seal an OLED display  102  in accordance with a first embodiment of the present invention. Although the sealing device  100  and the sealing method  200  are discussed below with respect to the manufacturing of a hermetically sealed OLED display  102 , it should be understood that the same sealing device  100  and sealing method  200  can also be used in other applications which require the sealing of two glass plates. Accordingly, the sealing device  100  and the sealing method  200  of the present invention should not be construed in a limited, manner. 
         [0020]    Referring to  FIGS. 1A and 1B , there are respectively shown a cross-sectional side view and a top view that illustrate the basic components of the sealing device  100  and the hermetically sealed OLED display  102  in accordance with the present invention. As shown in  FIG. 1A , the sealing device  100  includes a computer  104  (which processes instructions stored therein to implement the sealing method  200 ), a laser  106  (or other heat source  106 ), and a platform  108  (on which an unsealed OLED display  102  is placed). In one embodiment, the laser  106  can be a semiconductor laser with an un-polarized, multi-mode diode array source operating at a wavelength of 810 nm. The exemplary semiconductor laser  106  used in the experiments described herein was capable of delivering multiple power levels of up to ˜60 watts in a spot size of ˜0.3 mm diameter. 
         [0021]    As shown in  FIGS. 1A-1B , the OLED display  102  includes a multilayer sandwich of a cover plate  110  (e.g., first glass plate  110 ), a frit  116 , one or more OLEDs  112 /electrodes  114 , and a substrate plate  118  (e.g., second glass plate  118 ). The OLED display  102  has a hermetic seal  120  (formed from the frit  116 ) which protects the OLEDs  112  located between the two plates  110  and  118 . The hermetic seal  120  is typically located just inside the outer edges of the OLED display  102 . And, the OLEDs  112  are located within a perimeter of the hermetic seal  120 . As can be seen, the electrodes  114  which are connected the OLEDs  112  pass/extend through the hermetic seal  120  so they can be connected to an external device (not shown). It is the presence of these electrodes  114  (e.g., non-transparent metal electrodes  114 ) that can make it difficult to form the hermetic seal  120  between the two plates  110  and  118 . This is because the electrodes  114  have different patterns and different optical/thermal properties such that some of the laser energy from the laser  106  is either absorbed or reflected by the electrodes  114  which creates an uneven temperature distribution in the frit  108  during the sealing process which can lead to a non-hermetic connection between the two plates  110  and  118 . How this problem is solved by using the sealing device  100  and the sealing method  200  of the present invention is described below with respect to  FIGS. 2-5 . 
         [0022]    Referring to  FIG. 2 , there is a flowchart illustrating the steps of the preferred method  200  for manufacturing the hermetically sealed OLED display  102 . Beginning at step  202 , the sealing device  100  uses the laser  106  to direct a laser beam  122  towards the sealing line  124  on the unsealed OLED display  102  and in particular towards the frit  116  which is located between the two substrate plates  110  and  118 . At step  204 , the sealing device  100  moves the laser  106  (or the unsealed OLED display  102 ) such that the laser beam  122  moves at a substantially constant speed on the sealing line  124  along which there is the frit  116 , regions free of electrodes  114 , and regions occupied by electrodes  114  which are connected to the OLEDs  112  located between the two glass plates  110  and  118 . At step  206 , the sealing device  100  modulates the power of the laser  106  while the laser beam  122  is moved at the substantially constant speed along the sealing line  124  such that the laser beam  122  is able to impart sufficient laser energy (not too much or too little laser energy) to the frit  116  which causes the frit  116  to melt and form the hermetic seal  120  which connects the two glass plates  110  and  118 . At step  208 , the sealing device  100  synchronizes both the moving step  204  and the modulating step  206  so that the power of the laser  106  can be controlled to change in a linear or non-linear fashion as is needed at predetermined points along the sealing line  124  depending on whether or not the laser beam  122  is currently located over the electrodes  114  that are present between the two substrate plates  110  and  118  and depending on whether or not the laser beam  122  is currently located over a curved portion of the frit  116  (or sealing line  124 ) between the two substrate plates  110  and  118 . At step  210 , the sealing device  100  may have to dynamically change the speed which the laser beam  122  is moving along the sealing line  124  on the two substrates  110  and  118  if the laser  106  reaches its maximum or minimum power capacity at any point during the modulation step  206  so that the laser beam  122  can continue to impart sufficient laser energy (not too much or too little laser energy) to the frit  116  to cause the frit  116  to melt and form the hermetic seal  120  between the two substrate plates  110  and  118 . A detailed discussion about these features and some additional features associated with the sealing method  200  are discussed next with respect to  FIGS. 3-5 . 
         [0023]    Referring to  FIGS. 3A-3B , there are two diagrams which are used to help explain an additional feature that can be used to enhance the moving step  204  of the sealing method  200  in accordance with the present invention. The moving step  204  can be enhanced by implementing an overlapping scheme which helps prevent the creation of a start/stop defect that could adversely affect the formation of the hermetic seal  120  in the OLED display  102  (see also the experimental results discussed below with respect to  FIGS. 5A-5E ). To implement the overlapping scheme, the sealing device  100  would move the laser  106  in a clockwise fashion (or counter-clockwise fashion) such that the laser beam  122  is first directed at a start point “a” on the sealing line  124  of two substrate plates  110  and  118  and passed over the sealing line  124  around an entire perimeter of the two substrate plates  110  and  118  and then passed over the start point “a” and an overlapping portion “l” of the sealing line  124  again until it reaches a stop point “b”. This overlapping scheme helps prevent the creation of start/stop defects in the hermetic seal  120  by ensuring that sufficient laser energy has been imparted to the frit  116  to cause the frit  116  to melt and form the hermetic seal  120  between the two substrate plates  110  and  118 . The actual power output from the laser  106  on the overlapping portion “l” can be experimentally determined and would not normally exceed the maximum power of the laser  106  but if it did then the speed can be decreased as needed so the hermetic seal  120  can still be formed between the two substrate plates  110  and  118 . An exemplary laser power profile using the sealing method  200  and the overlapping moving step  204  is discussed next with respect to  FIG. 3B . 
         [0024]    As shown in  FIG. 3B , the laser  106  is first powered and the sealing starts on the start point “a”. Then, the laser  106  has its power ramped-up to full power before the laser beam  122  reaches the end of the overlapping portion “l”. After this point, the power level of the laser  106  would be modulated such that the laser beam  122  would have a lower power at or near the nine different electrodes  114  and at or near the four curves in the OLED display  102 . Once, the laser beam  122  reaches the start point “a” again then the laser  106  would have its power ramped-down to no power while the laser beam  122  is moved for a second time over the overlapping portion “l” towards the stop point “b”. As can be seen, the overlapping portion “l” is used to first ramp-up the power during the first pass and then it is used to ramp-down the power during the second pass of the laser beam  122 . The distance of the overlapping portion “l” could be determined if the laser power level used was known and if the sealing speed used in the sealing process was also known. For instance, if the sealing speed is 20 mm/s and the laser power is reduced from 35 W (full power level) to 0 W at a slope of −50 W/s then this would result in a 14 mm long overlapping portion “l”. Of course, it is not necessary for the whole overlapping portion “l” to be overlapped by the laser beam  122  nor is it necessary to ramp-up/ramp-down the laser power to/from full power while in the overlapping portion “l”. 
         [0025]    In the example shown in  FIGS. 3A-3B , the sealing pattern used was a rounded rectangular pattern and the laser  106  was enabled/powered so the sealing started directly on the frit  116  at the 0° tangential point (in the x-direction). Alternatively, the laser  106  could have been enabled/powered so the sealing started directly on the frit  116  at the 90° tangential point (in the y-direction). As shown, the laser  106  is enabled/powered such that the sealing is started directly on the sealing line  124  (frit  116 ) which by itself is a substantial improvement over the traditional sealing techniques in which the laser  106  was enabled/powered and moved outside the sealing line  124  (frit  116 ) before the laser beam  122  would actually be directed towards the sealing line  124  (frit  116 ) on the two substrates  110  and  118 . Another OLED display  102 ′ is discussed next which can be sealed by utilizing the sealing method  200  and the overlapping moving step  204  in accordance with the present invention. 
         [0026]    Referring to  FIGS. 4A-4D , there is shown an exemplary OLED display  102 ′ along with several different graphs which are used to help explain in more detail some of the different features associated with the sealing method  200  in accordance with the present invention. This exemplary OLED display  102 ′ has four electrodes  114   a ′,  114   b ′,  114   c ′ and  114   d ′ and four corners  115   a ′,  115   b ′,  115   c ′ and  115   d ′ at which the power of the laser  106  is modulated in both a linear fashion and a non-linear fashion to ensure that sufficient laser energy is imparted to the frit  116 ′ to cause the frit  116 ′ to melt and form the hermetic seal  120 ′ between the two substrate plates  110 ′ and  118 ′. In particular, the different materials associated with the glass-frit-glass areas and the glass-frit-electrode-glass areas required that the perimeter be segmented into 16 parts which was then used to help form a power profile (see  FIGS. 4A-4B ). In  FIG. 4B , the power profile was generated under the assumption that the sealing was to occur in a clockwise fashion. The laser power modulation can be achieved by using a direct analog control over the laser  106  where a control signal is sent to laser  106  from an analog output port in an input/output board within the computer  104  (see  FIG. 1A ). 
         [0027]    To seal the OLED display  102 ′, a relative motion has to be realized between the laser  106  and the sealing line  124 ′ on the OLED display  102 ′ (see steps  204 ,  206  and  208  in  FIG. 2 ). This can be achieved by either moving the laser  106  or by moving the two substrate plates  110 ′ and  118 ′ but in either case the laser power control needs to be synchronized with the motion such that the laser power changes only where it is suppose to change at different points along the sealing line  124 ′ on the OLED display  102 ′. There are several different methods that the sealing device  100  can use to synchronize the motion and the power control over the laser  106 . For instance, the sealing device  100  can synchronize the motion and power control over the laser  106  by using an encoder/timing method  400   a  or a field-programmable gate array (FPGA) method  400   b  (see  FIG. 1A ). 
         [0028]    In the encoder/timing method  400   a , the sealing device  100  includes a motion system encoder  402  (e.g., attached to platform  108  as shown in  FIG. 1A ) that functions at the laser start point “l” to send a pulse to the computer  104  to start a motion clock  404   a  used for motion control and to start a laser clock  404   b  used for laser power control (see  FIGS. 4C-4D ). Assuming, the clocks  404   a  and  404   b  have accurate clock rates, then the encoder/timing method  400   a  would be able to synchronize the motion of the laser  106 /OLED device  102 ′ with the modulation of the power of the laser  106 . The encoder/timing method  400   a  also enables the laser power to be changed at a spatial resolution which is much higher than the motion resolution of the laser  106 /OLED display  102 ′ (see  FIG. 4D ). The finer spatial resolution enables the laser power to be changed much faster than the motion control by interpolating multiple power changing points during each stage move from one point to another point (e.g., five laser power changes can be made during one motion change in  FIG. 4D ). 
         [0029]    The spatial resolution-motion resolution relationship can also be represented in the following equation: Clock duration of laser analog control=(Motion update duration)×(required spatial resolution of laser power control)/(Motion resolution) where the rate of laser analog control would normally be rounded to the nearest integer. For instance, assume the OLED display  102 ′ is sealed at a rate of 20 mm/s and that the motion update rate (from one point to another one) is 10 ms. Then, the motion resolution=sealing speed (20 mm/s)* motion update rate (10 ms)=0.2 mm=200 um. And, if the spatial resolution of laser power control is 10 um, then the laser analog control clock duration=10 ms*10 um/200 um=0.5 ms. Thus, the laser control rate=1/0.5 ms=2 KHz. Of course, it should be appreciated that the spatial resolution of the laser power modulation control could also be independent of the sealing speed. 
         [0030]    The encoder/timing method  400   a  can provide a great deal of flexibility when sealing different types of OLED displays (which have different electrode layouts/configurations). For instance, the encoder signal of motion start is used to synchronize the start of the laser power control and the motion control. And, then during the sealing, the control of the motion and the power of the laser  106  follow individual predetermined motion and power control profiles (where the power control profile can be determined experimentally). However, if there is a product line which continually uses the same frit layout in the OLED display, then the FPGA method  400   b  may be a desirable way for synchronizing the motion and the analog power control over the laser  106 . The FPGA method  400   b  involves the use of specially configured input/output (I/O) hardware. Basically, the specially configured I/O hardware is built specifically for sealing one type of OLED display  102 . With this specially configured hardware, it may be desirable to have the ability to monitor two encoder signals (associated with the x-axis and the y-axis) to determine that a fixed spatial interval has been traversed (on either an individual axis or between two axes, or multiple axes i.e., that a incremental vector length has been traversed) so as to be able to synchronize the laser output power. In this way, the laser  106  may be synchronized with absolute position and/or velocity and various advanced control schemes may be pursued to implement the sealing method  200 . The FPGA method  400   b  is desirable due to its speed where the “algorithms” are actually implemented in “hardware” as opposed to software and as such the “algorithms” can be executed in a single clock cycle (e.g., 80 MHz). 
         [0031]    Alternatively, there is a laser/robot motion control system currently available which has an analog power update rate that is equal to the motion update rate. As such, the use of this type of laser/robot motion control system in the present invention would mean that there would be no need for the external synchronization of the laser power control with the motion control since the motion and analog power updates would be bundled. However, because the motion update rate is usually at the ms level, then the laser spatial resolution would be considered relatively low especially when sealing at a relatively fast speed. For example, if the sealing speed is at 100 mm/s and the motion and analog power control where updated together at 1 ms level, then the laser spatial resolution could not exceed the motion spatial resolution. This may not be desirable. 
         [0032]    Referring to  FIGS. 5A-5E , there are illustrated six drawings/graphs which are associated with (several experiments (and their results) that were conducted to test how well the sealing method  200  (including the overlapping moving step  204  and the encoder/timing method  400   a ) hermetically seals two glass substrates (without the OLEDs  112  and electrodes  114 ).  FIG. 5A  (PRIOR ART) is a diagram illustrating the traditional laser start/stop process with a fixed laser power that was used to seal two glass substrates. The resulting sealed glass packages underwent a stress analysis test utilizing a polarimeter which indicated a high localized stress at the point “p1” where the laser started/stopped. Plus, a mechanical anticlastic bending analysis test demonstrated that the “traditional” laser start/stop location “p1” was the predominant source for failure. In thirty-nine different tests, 95% of the failures originated at the laser start/stop point “p1” and 5% of the failures originated at the dispense start/stop of the frit  116 . 
         [0033]    In contrast,  FIG. 5B  is a diagram illustrating the new variable laser power start/stop process with a modulated laser power (see also  FIG. 2 ). Basically, the variable laser power can distribute residual stress over a broad area rather than have the stress occur at a point source “p2”. To accomplish this, the laser power is slowly ramped-up and then ramped-down to form a laser start/stop point “p2”, rather than abruptly entering and exiting the same frit location with full sealing power as was done at point “p1” in the traditional laser start/stop process (see  FIG. 5A ). The glass plates which were sealed by the variable power laser start/stop process had a ˜25% improvement in mechanical integrity when compared to the glass plates sealed by the traditional laser start/stop process. In particular, the polarimetry stress analysis tests indicated considerably lower stress levels at or near the laser start/stop “p2” when compared to the “traditional” laser start/stop “p1”. And, the anticlastic bending mechanical analysis tests demonstrated that the laser start/stop “p2” was no longer the predominant source for failure. In thirty-nine different tests, 93% of the failures originated from the frit dispense start/stop point and 7% of the other failures originated from other locations along the sealing line  124  (not the laser start/stop point “p2”). 
         [0034]    Moreover, an additional experiment was conducted that indicated where the variable laser power start/stop process increased the peak load capabilities of the sealed glass substrates (see  FIG. 5C ). In  FIG. 5C , the ▪ represents tested samples (Corning Inc. glass sold under name EAGLE®) which had one frit pattern using the full power variable laser power start/stop process (where mean=19.8, std.dev.=1.19), the represents tested samples which had one frit pattern using the traditional laser power start/stop process (where mean=15.9, std.dev.=1.53) and the Δ represents tested scored samples which had multiple frit patterns using the traditional laser power start/stop process (where mean=15.3, std.dev.=1.27). These experimental results demonstrate that the variable power laser start/stop process is also a good way to increase the mechanical integrity of a sealed glass package (e.g., OLED display). 
         [0035]    Furthermore, another experiment was conducted which showed that the sealing method  200  should reduce the laser power at corners to reduce the potential for thermal damage to OLEDs  112  that are located near the corners of the two substrates  110  and  118 .  FIG. 5D  is a photo which illustrates that the sealing temperatures around a corner can be ˜200° C. higher than the sealing temperature along the linear edges when the laser power and the laser speed are not varied. Thus, these results recommend that the sealing method  200  should reduce the laser power (e.g., ˜3-5 W for 38 W sealing power) when sealing at the corners of the two substrates  110  and  118 . For instance,  FIG. 5E  illustrates the segments a, b, c and d of a corner at which the laser power should be reduced (if desired the laser power could be increased back to the original level at some distance after point “d”). In operation, the computer  104  and software allows the laser power level to be specified at a number of positions along the radius of the curve such as at “a” and “b” by linearly interpolating the power level between “a” and “d”. If desired, the amount that the laser power could be reduced around the corner can be calculated by using some sort of equation like 1-w/2/R that would also enable one to keep the same laser residence time at the corner (in this experiment w=1 mm and R=1.5 mm). An advantage of reducing the power level at the corners is that this should help increase the mechanical strength of the hermetic seal  120  and also prevent thermal damage of the OLEDs  112 . 
         [0036]    Lastly, an experiment was conducted that demonstrated where temperatures over the frit  116 /electrode  114  areas are typically higher than the temperatures over the frit  116 /non-electrode areas when applying a constant laser power. Since, electrodes  114  are usually good thermal conductors, OLEDs  112  near the electrodes  114  can be thermally affected. Thus, if the laser power is reduced quickly over the electrodes  114  and resumed higher over the non-electrode areas, then the temperatures along the sealing line  124  and hence nearby to the OLEDs  112  can be controlled. 
         [0037]    Referring to  FIG. 6 , there is a cross-sectional side view that illustrates the basic components of a sealing device  100 ′ which can be used to hermetically seal an OLED display  102  in accordance with a second embodiment of the present invention. In this embodiment, the sealing device  100 ′ has the same components as the aforementioned sealing device  100  except that the new sealing device  100 ′ has a laser  106  with a laser focusing unit  600  attached thereto which is used to change the shape and density of the laser beam  122 ′. For instance, the laser focusing unit  600  can have an aperture that is controlled to change the shape and density of the laser beam  122 ′ by focusing or de-focusing the laser beam  122 ′ at the frit  116  (a de-focused laser beam  122 ′ is shown in  FIG. 6 ). The sealing device  100 ′ also implements the sealing method  200  described above but in this embodiment instead of performing step  210  (dynamically changing the speed that the laser beam  122  moves along the sealing line  124  when the laser,  106  reaches its maximum or minimum power capacity) the laser focusing unit  600  would dynamically adjust the shape and power density of the laser beam  122 ′ so that the laser beam  122 ′ continues to impart sufficient laser energy (not too much or too little laser energy) to the frit  116  to cause the frit  116  to melt and form the hermetic seal  120  which connects the two substrate plates  110  and  118 . 
         [0038]    From the foregoing, it should be appreciated by those skilled in the art that the present invention relates to a sealing method  200  where the sealing device  100  and  100 ′ synchronizes the laser power and the substantially constant motion of the laser/OLED display  102  so as to change the laser power timely and precisely at predetermined locations on the OLED display  102  and  102 ′. This variable power sealing method  200  has several other features including (for example): (1) ramping up/down and overlapping the laser power over the start/stop area to reduce residual stress at the laser start/stop area; (2) reducing the laser power at corners to reduce the chance for thermal damage to the OLEDs  112  which are located near the corners; and (3) reducing the laser power over electrodes  114  themselves to reduce the chance for thermal damage to the electrodes  114  and to reduce the chance for a problematical heat transfer from the electrodes  114  to the OLEDs  112 . In addition, the variable power sealing method  200  has many other advantages and features some of which are as follows:
       The laser power modulation scheme can be used to control laser power for sealing applications which involve materials other than glass-frit-glass and glass-frit-electrode-glass.   The laser analog power control is sufficiently fast therefore the laser power modulation possesses a good spatial resolution. For instance, some lasers  106  can be used that have a power change delay which is less than 100 micron seconds this means it is possible to have a spatial resolution better than 0.5 micron when the sealing speed is 5 mm/s.   The laser power modulation scheme requires the synchronization between the laser power and the position of the OLED display so that one can change a laser power level precisely at target  2 D sealing positions. For OLED sealing, a power profile can be experimentally obtained before performing the actual sealing operation in a manufacturing environment. The sealing system  100  and  100 ′ would follow the power control profile and if desired could also use a feedback mechanism (e.g., temperature feedback mechanism) to help meet different laser power requirements over the glass-frit-glass areas and glass-frit-electrode-glass areas. The power control profile (without the feedback mechanism) can be used to implement an open-loop sealing method  200  which generally results in repeatable and robust sealing results.   The laser power modulation scheme is a better sealing solution than a method that uses a mask because it can be very difficult to properly place/align a mask over the electrodes  114  within the OLED display  102  and  102 ′.   The laser power modulation scheme is a better sealing solution than a method that uses a variable speed because a mechanical system can not change speeds instantaneously. For instance, the variable speed method can require a long transition time, which can result in unwanted transition areas. However, the variable speed method is still used in the present invention whenever the laser  106  reaches its maximum output power limit or its minimum output power limit (see step  210  in  FIG. 2 ).   The laser power modulation scheme can also speed-up the OLED frit sealing process to meet a fast “tactical time” completion requirement (which is commonly in the 4-minute range regardless of the size of the OLED displays  102  and  102 ′). In addition, the laser power modulation scheme facilitates a standby mode which occurs when the laser  106  moves from one frit pattern (in one OLED display) to another frit pattern (in another OLED display). Plus, the laser power modulation scheme features minimal delay in the time when laser  106  is switched back to the ON mode from the standby mode, which keeps the laser  106  from experiencing a thermal shock as well as helps reduce the pattern indexing time for large substrate OLED frit sealing.   The sealing method  200  can be used to seal two glass sheets together without the aid of a frit. In this case, one of the glass sheets may be doped with the same material used to dope the frit such that the doped glass sheet can now absorb the heat from the laser  106 .   The glass plates  102  and  110  which can be sealed to one another can be Code 1737 glass plates and EAGLE 2000™ glass plates which are manufactured by Corning Inc. Or, the glass plates  102  and  110  could be made by companies such as Asahi Glass Company (e.g., OA10 glass and OA21 glass), Nippon Electric Glass Company, NHTechno and Samsung Corning Precision Glass Company (for example). In the OLED application, it is highly desirable that the two substrate plates  110  and  118  have the same or similar CTE, coefficient of thermal expansion.   The electrodes  114  can be non-transparent, reflective, absorptive, transmissive or any combination thereof.   The frit  116  can be a low temperature glass frit that contains one or more absorbing ions chosen from the group including iron, copper, vanadium, and neodymium (for example). The frit  116  may also be doped with a filler (e.g., inversion filler, additive filler) which lowers the coefficient of thermal expansion of the frit  116  so that it matches or substantially matches the coefficient of thermal expansions of the two glass plates  110  and  118 . For a more detailed description about the compositions of some exemplary frits  116  that could be used in this application reference is made to U.S. patent application Ser. Nos. 10/823,331 and 10/414,653 (the contents of these documents are incorporated by reference herein).       
 
         [0049]    Although two embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.