Patent Publication Number: US-6664730-B2

Title: Electrode structure of el device

Description:
The subject matter of this application is related to concurrently pending patent application Ser. Nos. 09/802,976 and 09/802,977, which are incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor fabrication, and more particularly to the use of a bus line in devices fabricated on a flexible substrate. 
     BACKGROUND OF THE INVENTION 
     Organic light emitting devices (OLEDs), which make use of thin films that emit light when excited by electric current, are becoming an increasingly popular technology for applications such as flat panel displays. Popular OLED configurations include double heterostructure, single heterostructure, and single layer, and a wide variety of organic materials, some of which are described in U.S. Pat. No. 5,707,745, which is incorporated herein by reference. As used herein, the term “organic material” includes polymers as wells as small molecule materials that may be used to fabricate OLEDs. 
     Flat panel displays typically include an array of picture elements, or pixels, deposited and patterned on a substrate. Such a pixel array is typically a matrix of rows and columns. In an OLED display, each OLED pixel includes an organic light emitting diode that is situated at the intersection of each column and row line. The first OLED displays, like the first LCD (Liquid Crystal Displays), have typically been addressed as a passive matrix (PM) display. This means that to cause a particular pixel to luminesce, electrical signals are applied to the row and column lines of that particular pixel. The more current that is pumped through each pixel diode, the brighter the pixel appears visually. One method of providing grayscale to the display is to vary the current level of the pixel. 
     In practice, a voltage is applied to a single row line, and a path for current flow is selectively provided at individual columns. This provides current flow through selected pixels on the single row, thus allowing current to flow causing each pixel in that row line to luminesce at the desired brightness. The next row line is then addressed, and once again, all the pixels on that row line are energized to produce the required brightness. The display continuously scans all the row lines sequentially, typically completing at least 60 scans of the overall display each second. In this way, flicker is not seen since the display is addressed fast enough, for typical observation conditions, that the pixels cannot be seen to be continuously turning on and off Preferably, the magnitude of current flow through each column can be controlled, such that the brightness of the pixels can be controlled. 
     For OLEDs from which the light emission is only out of the bottom of the device, that is, only through the substrate side of the device, a transparent anode material such as indium tin oxide (ITO) may be used as the bottom electrode. Since the top electrode of such a device does not need to be transparent, such a top electrode, which is typically a cathode, may be comprised of a thick and reflective metal layer having a high electrical conductivity. In contrast, for transparent or top-emitting OLEDs, a transparent cathode such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 may be used. As distinct from a transparent or bottom-emitting OLED, a top-emitting OLED is one which may have an opaque and/or reflective substrate, such that light is produced only out of the top of the device and not through the substrate, or can be a fully transparent OLED that may emit from both the top and the bottom. 
     The transparent cathode that is used in such a transparent or top-emitting device preferably has optical transmission characteristics such that the OLED has an optical transmission of at least about 50%, although lower optical transmissions may be used. More preferably, the transparent cathode has optical transmission characteristics that permit the OLED to have an optical transmission of at least about 70%, still more preferably, at least about 85%. These requirements place significant limitations on the materials and thicknesses of the transparent cathode. 
     The transparent cathodes as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745 typically comprise a thin layer of metal such as Mg:Ag with a thickness, for example, that is less than about 100 angstroms. The Mg:Ag layer is coated with a transparent, electrically-conductive, sputter-deposited, ITO layer. Such cathodes may be referred to as compound cathodes or as TOLED (“Transparent-OLED”) cathodes. The thickness of the Mg:Ag and ITO layers in such compound cathodes may each be adjusted to produce the desired combination of both high optical transmission and high electrical conductivity, for example, an electrical conductivity as reflected by an overall cathode resistivity of about 30-100 Ω/□ (ohms per square). However, even though such a relatively low resistivity may be acceptable for certain types of applications, such a resistivity may still be somewhat too high for a passive matrix array of OLED pixels in which the current that powers each pixel needs to be conducted across the entire array through the narrow strips of the compound cathode. 
     It is expected that new materials may be developed that can be used as transparent electrodes having resistivities less than 30 ohms per square, and that the present invention may be used in conjunction with such materials. Also, resistivities greater than 100 ohms per square may be acceptable for some applications, and the present invention may also be used in such applications. 
     It is known to use bus lines to mitigate limitations on the electrical conductivity of a transparent electrode. In the context of a passive matrix array of OLEDs, the bus line is a thick electrically conductive strip that runs parallel to a transparent electrode, and which provides electrical conductivity in the direction of the electrode. For example, U.S. Pat. No. 6,016,033 to Jones et al. discloses the use of a bus line in an array of OLEDs. Because the bus line is made of a thick electrically conductive material, it does not transmit light, and unfavorably results in an inactive area on the array of OLEDs. Because it is desirable to maximize the active area of an OLED display, it is desirable to minimize the area of the bus line. The active area may be quantified by a “fill-factor,” which is the percentage of the area of an array that is active or that emits light. Because of the enhanced electrical conductivity that is provided by a bus line, a bus line may be used notwithstanding the disadvantageous inactive area. 
     The organic materials of an OLED are very sensitive, and may be damaged by conventional semiconductor processing. For example, any exposure to high temperature or chemical processing may damage the organic layers and adversely affect device reliability. As a result, the processes conventionally used to fabricate a thick metal feature such as a bus line may damage any organic layers that are already present. 
     One technique that may be used to protect the delicate organic layers of an OLED is an integrated mask through which layers may be selectively deposited during fabrication. The mask is “integrated” because it is left in place after fabrication, thus being integrated into the final device. Using an integrated mask is particularly desirable where the steps used to pattern material or to remove a mask have the potential to damage the device. Even where the integrated mask does not cover the delicate organic layers, the integrated mask protects the delicate organic layers by providing a patterning mechanism that does not require the patterning or removal of a mask once the organic layers are in place, i.e., the potentially damaging processes used to form the integrated mask are performed before the organic layers are present, and the potentially damaging processes used to remove a mask are not performed at all because the integrated mask is left in place. It is known to use an integrated mask to fabricate the top electrodes of an array of OLEDs, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al. 
     One problem that has been observed with conventional integrated masks is the shorting of adjacent electrode layers across the mask. It is known to use integrated masks having an overhang to mitigate shorting across the mask, as disclosed in U.S. Pat. No. 5,701,055 to Nagayama et. al. However, even with a conventional overhang, the process used to deposit the electrode must meet certain criteria to avoid shorting problems. 
     First, in order to avoid shorting problems, the “footprint” of the deposited material, defined as the surface area onto which significant material is deposited, should be sharply limited to those surfaces having a direct line of sight, unobstructed by the mask, to the source of material being deposited. The footprint should not extend onto surfaces that do not have a direct line of sight to the source of deposited material. Whether deposition is limited in this way is dependent upon the deposition process. Processes such as low energy deposition of metals by thermal evaporation, generally result in footprint relatively sharply limited to surfaces having a direct line of sight to the source of material. In contrast, processes such as chemical vapor deposition may result in a significantly larger footprint, such that there is significant deposition onto surfaces not having a direct line of sight to the source of material. Processes such as sputter depositing have a footprint between that of thermal evaporation and that of chemical vapor deposition. One mechanism that may lead to a larger footprint is collisions between atoms or molecules of the material being deposited during transit to the substrate, and the resultant scattering. A low “sticking efficiency” of the atoms or molecules being deposited is another such mechanism. These mechanisms may lead to the side walls of a conventional integrated mask being coated with substantial quantities of the material being deposited even though such surfaces may not be within a direct line of sight from the source material. 
     Second, in order to avoid shorting problems, deposition that is significantly off-axis should be avoided, even for processes that have a limited footprint. Off-axis deposition is deposition from an angle not perpendicular to the substrate. Off-axis deposition may result in surfaces losing their protection from a direct line of sight to the source of material being deposited, such that material may be deposited into the recessed area under an overhang. Moreover, off-axis deposition often involves deposition from a variety of different angles, such that even more surface area loses protection from a direct line of sight during the process. Off-axis deposition may occur for many reasons. For example, the geometry of the substrate, the source, and their relative locations may lead to significant off-axis deposition, and may even lead to significant variations in the angle of deposition at different points on the substrate. The substrate may be placed on a moving conveyor belt during deposition, which inherently leads to off-axis deposition when the substrate is not directly beneath the source of material, and may lead to very large angle off-axis deposition when the substrate is at the edge of the deposition chamber. The substrate may also be rotated during deposition. Even processes that have a somewhat limited footprint, such as sputter depositing, may lead to shorting if there is significant off-axis deposition. It is believed that sputter depositing through a conventional integrated mask leads to shorting when off-axis deposition from angles of about 30 degrees or greater is present, although there may be shorting at smaller angles depending upon the exact process parameters. 
     It may be desirable, or even necessary in some cases, to use processes having a large footprint, and/or off-axis deposition, to fabricate certain types of layers that are used in an OLED. However, such processes may cause deposition to occur in undesirable regions of the device. For example, an ITO layer typically needs to be deposited using a high energy vacuum sputtering process that produces substantial scattering. Thus, using conventional integrated masks, it may not be possible to fabricate desirable but previously unattainable structures such as transparent or top-emitting OLEDs using a compound Mg:Ag/ITO cathode in a passive matrix OLED array. The use of such a conventional mask may result in the ITO layer causing harmful shorting across adjacent compound cathode strips. 
     FIG. 1 (prior art) illustrates the type of shorting of an electrode layer that may occur across an integrated mask due to the use of a deposition process that is not perfectly unidirectional. Mask  110  is fabricated on top of substrate  100 . A metal layer  120  is deposited over mask  110 , with the goal of fabricating electrodes  120   a  and  120   b  that are electrically separated. If the vapor deposition process is highly unidirectional, electrodes  120   a  and  120   b , and residual layer  120   d , are deposited. Any residual layer  120   c  that forms would not be continuous or thick enough to have significant electrical conductivity, because the vapor deposition process is highly unidirectional and there is no direct line of sight between the metal deposition source and residual layer  120   c . However, if the vapor deposition process produces too much scattering and is, thus, insufficiently unidirectional, residual layer  120   c  may be continuous and have significant electrical conductivity. In this case, residual layers  120   c  and  120   d  may form a short between electrodes  120   a  and  120   b.    
     It would be desirable to be able to fabricate transparent or top-emitting OLEDs that exploit the high optical transmission of compound cathodes, such as Mg:Ag/ITO, in a passive matrix display, but without having such devices limited by the lower electrical conductivity of such compound cathodes. Furthermore, it would be desirable to be able to vapor deposit electrically conductive materials without encountering the shorting problems that may be experienced whenever such electrically conductive materials undergo substantial scattering during the deposition process. 
     OLED technology offers the potential to be highly functional and durable in a flexible format. OLEDs fabricated on flexible substrates may offer several advantages over OLEDs fabricated on conventional substrates, such as glass. These potential advantages include impact resistance, light weight, thinness, in-use flexibility and conformability. Flexible OLEDs are discussed in more detail in Weaver et al., “Flexible Organic LED Displays,” 2001 Soc. Vac. Coaters 505/856-7188, 44th Annual Technical Conf Proc. (2001) ISSN 0737-5921 (“Weaver et al.”), which is incorporated by reference in its entirety. 
     However, due to the limitations on the materials and thicknesses of conventional electrodes used in OLEDs, such electrodes may not be well-suited for use in OLEDs fabricated on a flexible substrate. In particular, the flexing of the substrate may cause such electrodes to crack, delaminate, or otherwise sustain damage that could adversely affect reliability. It would be desirable to achieve electrodes that could be reliably used on OLEDs fabricated on a flexible substrate. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the invention, an organic light emitting device is provided. The device has a first electrode, an insulating strip disposed over a portion of the first electrode, and a bus line disposed on top of the insulating strip, such that the bus line is electrically insulated from the first electrode by the insulating strip. An integrated mask is disposed over the bus line, such that a portion of the bus line remains exposed vis-a-vis the insulating strip and the integrated mask. An organic layer is disposed over the first electrode, such that the organic layer is electrically connected to the first electrode. A second electrode is disposed over the organic layer, such that the second electrode is electrically connected to the organic layer, and such that the second electrode is electrically connected to the exposed portion of the bus line. A method of fabricating the device is also provided, which involves depositing the organic layer and the second electrode through the integrated mask, such that the second electrode is in electrical contact with the bus line. 
     The bus line may be disposed completely under the recessed area and completely under the overhang on one side of the base such that none of the bus line contributes to a loss in the fill factor of the OLED array. The recessed area on at least one side of the base has an aspect ratio sufficiently large such that no vapor deposition may occur in the furthermost interior, distal, depths of the recessed area even when highly scattered materials are vapor deposited. However, the outermost, proximal, exposed portion of the bus line is sufficiently close to the outermost extension of the overhang so as to allow vapor deposition of a highly scattered second electrode material to make electrical contact with at least a proximal portion of the exposed portion of the bus line. 
     In an embodiment of the invention, a photoresist mask is provided, having a central region fabricated on an underlying layer. An overhang supported by the central region is separated from the underlying layer by a recessed area. The recessed area has an aspect ratio of at least about 1.5. The mask may be advantageously used to pattern electrodes deposited through the mask by chemical vapor deposition or sputtering, such that there is no significant conductivity across the mask between the patterned electrodes. 
     In an embodiment of the invention, an array of OLEDs is provided. The array includes a first electrode that has a plurality of physically separate pieces that are electrically connected by a bus line. The array further includes a second electrode, and an organic layer disposed between the first and second electrodes. The organic layer is electrically connected to the second electrode, and to at least one of the physically separate pieces of the first electrode. The embodiment may be used to fabricate an array of OLEDs on a flexible substrate. 
     In an embodiment of the invention, an electrode having a plurality of physically separate pieces is provided. A bus line electrically connects the plurality of physically separate pieces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (prior art) illustrates the shorting of an electrode layer across an integrated mask due to the use of a deposition process that is not perfectly unidirectional. 
     FIG. 2 shows an cross-sectional illustration of a mask in accordance with an embodiment of the invention. 
     FIG. 3 shows a cross-sectional photograph of an actual mask fabricated in accordance with the present invention, similar to the mask schematically illustrated in FIG.  2 . 
     FIG. 4 shows a cross sectional illustration of a device incorporating an integrated mask in accordance with an embodiment of the present invention. 
     FIG. 5 shows the device of FIG. 4 from a top view. 
     FIG. 6 shows a cross section of an OLED having a barrier layer. 
     FIG. 7 (prior art) shows a top view of a schematic diagram of a passive matrix display. 
     FIG. 8 shows a top view of electrodes having physically separate pieces, where the electrode spans a substrate in one direction. 
     FIG. 9 (prior art) shows a top view of a large, physically contiguous electrode fabricated on a substrate. 
     FIG. 10 shows a top view of an electrode that covers most of a substrate, where the electrode includes physically separate pieces. 
     FIG. 11 shows a top view of a physically contiguous electrode that covers most of a substrate, where bus lines running in two directions electrically connect various regions of the electrode. 
     FIG. 12 shows a top view of physically contiguous electrodes that span the length of a substrate in one direction, but are relatively narrow in the other direction, where bus lines running parallel to the electrodes electrically connect various regions of the electrodes. 
     FIG. 13 shows an electrode similar to the electrode of FIG. 10, and shows devices that each incorporate several of the physically separate pieces of the electrode. 
     FIG. 14 shows an electrode similar to the electrode of FIG. 10, and shows multiple devices sharing a single physically separate piece of an electrode. 
    
    
     DETAILED DESCRIPTION 
     The present invention will be described with reference to the illustrative embodiments in the following processes and drawing figures. 
     One embodiment of the present invention provides a photoresist mask having an overhang larger than those available in the prior art. This large overhang enables the use of processes that were not previously available, such as depositing electrodes through an integrated mask using chemical vapor deposition (CVD), or using sputter deposition with significant off-axis deposition. For example, this embodiment may enable the use of sputter deposition where there is off-angle deposition from angles greater than 30 degrees—angles up to 45 degrees, 60 degrees, or even approaching 90 degrees. These electrodes may be transparent if they are made of the appropriate materials, such as indium tin oxide (ITO), MgAg, aluminum, or Al:LiF, fabricated to the appropriate thickness. 
     The large overhang may also enhance the yield of processes that are presently practiced with conventional masks, and allow such processes to be reliably used with less a stringent controls. In addition, even highly directional deposition processes generally have an angle of deposition that varies significantly over a few centimeters. An integrated mask with a large overhang may be used to increase the area over which processes sensitive to angle of deposition is important, such as processes previously practices with shadow masks or integrated masks with smaller overhangs. 
     Preferably, the mask is monolithic, where monolithic means that the mask is fabricated from a single material, such as photoresist. A monolithic mask avoids disadvantages associated with multi-layer masks, such as additional processing steps. 
     Another embodiment of the present invention provides a method of using an integrated mask to fabricate a passive matrix array of OLEDs. 
     FIG. 2 shows a cross sectional illustration of a mask  200  in accordance with an embodiment of the invention. Mask  200  is disposed on a substrate  210 . Mask  200  has a central region  220  and an overhang  230 . In central region  220 , the mask material is in contact with underlying layers such as substrate  210 . The underlying layer may also comprise layers other than the substrate that have been previously fabricated. Overhang  230 , by way of contrast, is not in contact with underlying layers. Instead, overhang  230  is separated from underlying layers by a recessed area  240  in which mask material is not present. Mask  200  may be fabricated entirely of a single layer of photoresist, avoiding the additional steps and increased processing expenses associated with masks that include multiple layers of photoresist and/or other materials. 
     Mask  200  has a total height represented by line  205 , and a top width represented by line  250 . Central region  220  has a base width represented by line  270 . Overhang  230  and recessed area  240  have a depth represented by line  260 . At the outer edge of mask  200 , overhang  230  has a thickness represented by line  235 , and recessed area  240  has a height represented by line  245 . 
     Mask  200  is fabricated with an overhang having dimensions and aspect ratios unprecedented in the prior art. One feature of the invention is the “aspect ratio” of recessed area  240 . This aspect ratio is defined as the ratio of depth  260  to height  245 . In an embodiment of the invention, the aspect ratio is at least 1.5. More preferably, the aspect ratio is at least 2.0. Most preferably, the aspect ratio is at least 2.2. It is believed that these aspect ratios are sufficiently large to minimize the deposition of electrode material into the depths of recessed area  240  to the point that there is no substantial conductivity across mask  200  due to such electrode material. 
     In one embodiment of the invention, recessed area  240  preferably has a depth  260  of about 9.5 microns to 13.5 microns, more preferably about 11 to 12 microns, and most preferably about 11.5 microns. Larger depths may result in an overhang that is not rigidly supported, and smaller depths may reduce the functionality of the overhang. The height  245  of recessed area  240  may be about 2 to 6 microns, more preferably about 3 to 5 microns, and most preferably about 4 microns. Lower heights may result in the overhang undesirably touching the substrate at points, or in the overhang coming so close to the substrate that a subsequently deposited layer may undesirably bridge the gap between the overhang and the substrate. Higher heights may reduce the functionality of the overhang by allowing subsequently deposited material to readily enter too deeply into the depths of recessed area  240 . However, depending upon the particular process that is used to subsequently deposit material, dimensions outside of these preferred ranges may be used. Central region  120  preferably has a width of about 5-9 microns, more preferably about 6-8 microns, and most preferably about 7 microns. Smaller widths may result in a structurally unstable mask. Larger widths may result in the mask having an undesirably large footprint. However, for applications where mask footprint is not critical, larger widths may be readily used. 
     In one embodiment of the invention, mask  200  does not have a “foot.” A foot is a region of the mask near the underlying layer where the central region becomes wider as it nears the underlying layer. For example, FIG. 1 illustrates a mask  110  having a foot. By way of contrast, mask  200  of FIG. 2 does not have a foot, because central region  220  becomes narrower as it nears underlying substrate  210 . 
     In some embodiments of the invention, it is desirable to use planarizing or encapsulation techniques after mask  200  has been used to fabricate a device. Many of the conventional planarizing or encapsulating techniques work best when applied to a device having height variation of less than about 3.0 microns, and more preferably less than about 1.5 microns. It is therefore desirable in some embodiments of the invention to keep the total height  205  less than about 2.5 microns, and more preferably less than about 1.5 microns. 
     FIG. 3 shows a cross sectional photograph of an actual mask fabricated in accordance with the present invention, similar to the mask schematically illustrated in FIG.  2 . 
     FIG. 4 shows a cross sectional illustration of a device  400  incorporating an integrated mask  450  in accordance with an embodiment of the present invention. Device  400  may be fabricated as follows. A first electrode  420  is deposited and patterned onto a substrate  410  using conventional techniques. First electrode  420  and substrate  410  may be made of conventional materials having conventional dimensions. If device  400  is a top-emitting OLED, first electrode  420  and substrate  410  are preferably selected to maximize back reflection. 
     An insulating strip  430  is then fabricated using conventional techniques. Insulating strip may be made of any suitable non-electrically conducting material, such as SiN x , Al 2 O 3 , photoresist, or polyimide, and is preferably at least about 1000 Å thick, such that insulating strip electrically reliably insulates first electrode  420  from bus line  440 . The width of insulating strip  430  depends upon the type of device manufactured. For a passive matrix display manufactured using presently available equipment, the array may have a pitch of about 50-400 microns. Subsequent technological developments may enable a broader range of pitches. Preferably, the insulating strip has a width that is about 10 to 45 percent of the pitch, and more preferably about 20 to 30 percent of the pitch. A smaller width may not adequately separate adjacent pixels. A larger width may unnecessarily increase the inactive area of the array with a corresponding undesirable decrease in fill-factor. It is desirable to maximize the fill-factor, or the percentage of the area of an emissive array that actually emits light. In the embodiment of FIG. 4, the fill-factor is the percentage of the area not covered by insulating strip  430 . In other embodiments, other features may further reduce fill-factor. 
     A bus line  440  is then fabricated over insulating strip  430  using conventional techniques. Bus line  440  may be made of any suitable metal or other electrically conductive material, such as gold, silver, aluminum or copper, or any suitable alloy. The dimensions of bus line  440  are preferably chosen such that bus line  440  has a resistivity appropriate for the device being fabricated. The appropriate resistivity is generally based on the overall size of the display, and may be readily determined by one of skill in the art. For example, in one embodiment, a resistivity of about 0.3 ohms per square is preferred for a 2 inch square display with 80 dots per inch (dpi). A resistivity that is too large may reduce the usefulness of the bus line, although any resistivity significantly lower than that of second electrode  470  has at least some usefulness. A resistivity that is too small may not result in any additional functionality, and may require an unnecessarily large bus line  440 . 
     A mask  450  is then fabricated over insulating strip  430  and bus line  440 , having dimensions consistent with the embodiment of FIG.  2 . Mask  450  is fabricated such that a portion of bus line  440  remains exposed vis-a-vis the insulating strip and the integrated mask, so that a subsequently deposited second electrode  470  may be electrically connected to bus line  440 . Mask  450  is preferably fabricated such that only one side of bus line  440  is exposed, such that the second electrodes  440  of two adjacent devices  400  separated by mask  450  may not contact the same bus line  440 . 
     One or more organic layers  460  are then deposited through mask  450 , such that organic layers  460  are electrically connected to first electrode  420 . Two layers can be “electrically connected” without being in physical contact. For example, an intermediate layer, such as an injection enhancement layer, may be present between an electrode and an organic layer that are electrically connected. Organic layers  460  may include any conventional OLED organic materials. Organic layers  460  may be a single layer, or may include the multiple layers of conventional OLED structures, such as a single or double heterostructure, or one or more layers containing a mixture of OLED organic materials. 
     A second electrode  470  is then deposited through mask  450  and over organic layers  460 , such that second electrode is electrically connected to organic layers  460 . The dimensions of mask  450  allow second electrode  470  to be fabricated without bridging across mask  450 , using processes that would lead to bridging across a conventional mask. Second electrode  470  may be, for example, a sputter deposited electrode made of ITO, aluminum, or LiF:Al. These materials and other may be advantageously used to fabricate a very thin second electrode  470  which is transparent, such that light emitted from organic layer  460  may pass through second electrode  470  to a viewer, in which case device  400  would be a top-emitting OLED. 
     Note that residual organic and metal layers (not shown) are deposited on top of mask  450  during the deposition of organic layers  460  and second electrode  470 , respectively. 
     Organic layers  460  and second electrode  470  are deposited such that second electrode  470  makes electrical contact with bus line  440 , without significant interference from organic layers  460 . In order to achieve this electrical contact, second electrode  470  should extend past organic layers  460  onto bus line  440 . This extension may occur because second electrode  470  is deposited using a technique that is less unidirectional than that used to deposit organic layers  460 . Various process parameters, such as angle of deposition, may also be controlled to ensure that second electrode  470  makes electrical contact with bus line  440 . 
     Depositing bus line  440  over insulating strip  430  advantageously avoids the creation of any additional inactive area on substrate  410 . Using the device configuration shown in FIGS. 4 and 5, it is expected that a “fill-factor” or active area of at least 70% should be possible. 
     Mask  450  is preferably deposited over bus line  440  such that one side of bus line  440  is completely covered by mask  450 . This complete coverage on one side prevents a second electrode from an adjacent device from contacting bus line  440 . 
     In one embodiment of the invention, the top width of mask  450  is preferably about 100 microns, and the width of insulating strip  430  is preferably about 150 microns. Preferably, the ratio of the width of insulating strip  430  to the top width of mask  450  is about 1.5:1. Preferably, the width of bus line  440  is about 30-40 microns. Preferably, the outermost exposed edge of bus line  440  is within about 10 microns of the edge of recess  455 , i.e., preferably the distance represented by line  457  is about +10 microns to about −10 microns. If bus line  440  does not extend past recess  455 , and is too far within recess  455 , it may be difficult to achieve good electrical contact between bus line  440  and second electrode  470 . It is understood that the invention is not limited to these specific dimensions. 
     Device  400  emits light when a current is passed between first electrode  420  and second electrode  470 , through organic layer  460 . Second electrode  470  is electrically connected to bus line  440 , and current may reach second electrode  470  through bus line  440 , in addition to running along the length of second electrode  470 . 
     Although FIG. 4 shows only a single device  400 , the invention may be used with multiple devices  400 . 
     FIG. 5 shows the device of FIG. 4 from a top view. FIG. 4 is a cross section of FIG. 5 across line  4 ′. Mask  450  is not shown. Bus line  440  is disposed on top of insulating strip  430 , such that there is no electrical contact between bus line  440  and underlying first electrode  420 . Bus line  440  allows current to pass readily in the direction of second electrode  470 , effectively lowering the resistance of second electrode  470 . 
     The embodiment of FIGS. 4 and 5 is primarily intended for use as a top-emitting OLED, such that light is emitted through second electrode  470 . However, the embodiment may also be used as a regular bottom-emitting OLED, such that light is emitted through first electrode  420  and substrate  410 . 
     It is to be understood that the present invention may be used to fabricate much larger arrays of organic devices than those specifically described herein. Although FIG. 5 shows a 1×2 array of devices, much larger arrays may be fabricated. Moreover, a multi-color display may be fabricated by depositing various down-conversion layers known to the art, or using different organic materials in different devices. For example, down-conversion layers may be patterned such that three adjacent individual devices  400 —one with no down conversion layer that emits blue, one with a blue-to-green down conversion layer that emits green, and one with a blue-to-red down conversion layer that emits red—form a single multi-color pixel. An array of these multi-color pixels may be fabricated to form a multi-color display. A multi-color array may also be fabricated by a number of other methods, such as using an array of white-emitting OLEDs in combination with color filters or a distributed Bragg reflector. 
     It is also to be understood that the present invention is not limited to the specific passive-matrix embodiment illustrated in FIGS. 4 and 5, and may be used to in a wide variety of other embodiments. For example, the present invention may be used to fabricate an active matrix display. Each device in an active matrix array generally has one electrode that is individually controlled for that device using a transistor. The other electrode is generally shared with all of the devices in the array. In one embodiment of the present invention, an integrated mask having the novel dimensions disclosed herein may be used to enable the fabrication of an active matrix display having, for example, a common second electrode. Such an array would be similar in appearance to the embodiment of FIGS. 4 and 5, but the first electrode of each device (pixel) in the array would be individually controlled by thin film transistors embedded in the substrate. These first electrodes would not be connected in rows or columns as they are in a passive matrix display. All of the devices in the array would share a common second electrode, similar in appearance to that of FIGS. 4 and 5. Although the integrated mask may locally separate the second electrode into strips, these strips could be electrically connected to each other at the periphery of the array. This approach advantageously allows the second electrode to benefit from the bus lines embedded in the integrated mask, while avoiding the problems associated with fabricating a bus line after the delicate organic layer is present. 
     Flexible substrates have several advantages that may make them preferable to other substrates in certain circumstances. First, flexible substrates may be used to fabricate arrays that can be flexed during the lifetime of the array—for example, a display screen that may be rolled up for storage. Second, flexible substrates may be used to fabricate curved displays by flexing the substrate once and fixing it in position—for example, a display screen may be attached to the curved windshield of a car or the curved surface of a building. Third, flexible substrates may enable the use of particular processes during manufacturing, irrespective of whether the array is to be flexed later—for example, the substrate may be stored or processed on rollers. Fourth, flexible substrates may be advantageous for reasons unrelated to the flexibility, such as cost, in which case the substrate may be inadvertently flexed during or after fabrication of the array of OLEDs. Flexible substrates may also be advantageous for other reasons. The term “flexible” as used herein means that the substrate may be bent such that the radius of curvature is 1 meter or less without snapping or sustaining other permanent damage. However, some applications may use substrates that may be bent to much smaller radii of curvature, for example 5 mm, without sustaining damage. 
     Of the materials that are presently commercially available, preferred flexible substrates include polyethylene terephthalate (PET), poly-ethersulphone (PES), polycarbonate (PC), polyethylenenaphthalate (PEN) and polyimide (PI). Each of these materials has advantages and disadvantages that are more fully described in Weaver et al. It is expected that chemical companies will develop new materials that are better suited for use as a flexible substrate for the fabrication of OLED displays. It is also expected that various embodiments of the present invention may be practiced with such substrates when they become available. 
     OLEDs are sensitive to environmental factors, and may rapidly degrade if exposed to moisture or oxygen. Many of the preferred flexible substrates are permeable to moisture and oxygen, such that an OLED fabricated thereon may have a reduced lifetime. In order to address this problem, a barrier layer is preferably deposited onto the flexible substrate prior to the fabrication of the OLED. A preferred barrier layer includes multiple sublayers of a high density dielectric, for example SiO 2 , Si x N y , Al 2 O 3 , or indium-tin oxide (ITO), alternating with sublayers of a polymer. A preferred barrier layer is Barix®, available from Vitex Systems, Inc. of Sunnyvale, Calif. It is believed that a Barix® barrier layer is made up of alternating sublayers of polyacrylate and Al 2 O 3 . Preferably, the barrier layer has a total thickness of about 1 to 10 microns, and includes 1 to 10 sublayers of high density dielectric and polymer layers. The barrier layer may also be used to planarize the substrate, i. e., to fill in any variations in the surface to provide a planar surface on which to fabricate the OLEDs. Other preferred barrier layers are disclosed in U.S. Pat. Nos. 5,686,360, 5,757,126, 5,771,562, 5,811,177 and 5,874,804, which are incorporated by reference. 
     Flexing a flexible substrate may cause the electrodes of the OLEDs to crack, delaminate, fracture or otherwise sustain damage which can adversely affect device reliability. The flexible substrate itself may also be damaged. It is believed that such damage is caused by the stresses imposed on the electrodes and the substrate by such flexing. It is also believed that the stresses are greater in larger electrodes, and for smaller radii of curvature. Yet, it is often desirable to fabricate electrodes that span the entire array of devices, and it may also be desirable to flex the substrate to a small radius of curvature. 
     For example, the pixels of the passive matrix display are organized by columns and rows. Each device in a particular column share a common top electrode, which forms a strip running the length of the array. Similarly, each device in a particular row shares a common bottom electrode, which forms a strip running the length of the array in a direction perpendicular to that of the top electrodes. As a result, the electrodes of a passive matrix display may be quite long—on a screen having a seventeen inch width, the electrodes corresponding to the rows are about seventeen inches long, and are conventionally formed of a single strip of a thin conductive metal or metal oxide. The terms “row” and “column” are used herein interchangeably to refer to a line of pixels that spans an array, and do not require any particular direction or association with a top or bottom electrode. 
     By way of further example, the devices of a conventional active matrix display generally share a common top (or bottom) electrode, and have individually controllable bottom (or top) electrodes. As a result, the top (or bottom) electrode spans the entire length and width of the display, and is conventionally formed of a single sheet of a thin conductive metal or oxide. As used herein, the term “bottom electrode” refers to the electrode which is closest to the substrate, and is generally the first electrode fabricated, and the term “top electrode” refers to the electrode which is furthest from the substrate, and is generally the last electrode fabricated. 
     In one embodiment of the invention, large electrodes are physically separated into a group of smaller distinct pieces, where each piece serves as an electrode or part of an electrode for one or more devices. A bus line is used to preserve the electrical continuity of the group of smaller distinct pieces, in order to preserve the functionality of the original, larger electrode. The smaller electrode pieces are less susceptible to damage when the substrate is flexed. The bus line, which is not subject to the same constraints as the electrode, may be fabricated to withstand flexing better than the electrodes. For example, at least one electrode of an OLED is generally transparent, which means that the electrode must be very thin and fabricated from particular materials, which may make the electrode susceptible to damage when flexed. By way of contrast, a bus line does not cover the entire active surface of the device, and need not be transparent. As a result, the bus line may be fabricated to a greater thickness and from a wider variety of materials. 
     FIG. 6 shows a cross section of an OLED  600 . The device is fabricated on a flexible substrate  610 . A barrier layer  620  is deposited on substrate  610  to block any moisture or oxygen that might permeate substrate  610 . A bottom electrode  630 , an organic layer  640 , and a top electrode  650  are deposited, in that order, over the barrier layer. Preferably, at least one of the electrodes, bottom electrode  630  or top electrode  650 , is transparent such that light may pass through the electrode from organic layer  640  to a viewer. However, some OLED applications may involve side-emitting OLEDs, in which case both electrodes may be non-transparent. Although the electrodes, organic layer and barrier layer are described and illustrated as single layers, they may each include various sublayers as known to the art. For example, the organic layer may include the sublayers of a single or double heterostructure OLED, as described in U.S. Pat. No. 5,707,745, which is incorporated herein by reference. In addition, device  600  may include additional layers known to the art that are not illustrated, such as a hole injection enhancement layer or a protective top layer. 
     The transparent electrodes, if any, of an OLED are preferably fabricated from a conductive metal oxide. Preferred materials include indium tin oxide. Preferred non-transparent electrode materials include LiF:Al. Conductive polymers such as polyaniline and poly (3,4-ethylenedioxythiophene)/poly (styrenesulphonate) (PEDOT/PS) may also be used. 
     FIG. 7 (prior art) shows a top view of a schematic diagram of a passive matrix display  700 . Bottom electrodes  720 , a blanket organic layer (not shown to allow better illustration of bottom electrodes  720 ), and top electrodes  730  have been deposited on substrate  710 , in that order. FIG. 7 illustrates four bottom electrodes  720 , each of which spans the length of substrate  710  in the horizontal direction. Similarly, the five top electrodes  730  each span the length of substrate  710  in the vertical direction. Pixels  740  are defined by the regions where bottom electrodes  720  and top electrodes  730  overlap, with the organic layer in between. FIG. 7 illustrates a 5×4 array of pixels  740 , although only one is labeled. In a conventional passive matrix OLED display, substrate  710  is an inflexible material such as glass, ceramic or Si, which provides a stable base for the display and acts as a barrier to moisture and oxygen. 
     FIG. 8 shows a top view of electrodes  805  fabricated on a substrate  810 . FIG. 8 illustrates four electrodes  805 , each of which spans the length of substrate  810  in the horizontal direction. Electrodes  805  include physically separate pieces  820 . The physically separate pieces  820  of each electrode  805  are electrically connected by bus lines  830 . Electrodes  805  may be used, for example, to fabricate a passive matrix display similar to that of FIG. 7 on a flexible substrate, where electrodes  805  replace bottom electrodes  720  of FIG. 7 to avoid undesirable cracking of bottom electrodes  720  due to the flexing or other stressing of substrate  710 . 
     For example, an 80 dots per inch (dpi) passive matrix display could be fabricated using the structure illustrated in FIG.  8 . In one example of such a display, physically separate pieces  820  are 280 microns by 280 microns, and are separated from the other physically separate pieces  820  by 40 microns on all sides. Bus lines  830  have a width of 40 microns. In another example of such a display, physically separate pieces  820  are 260 microns by 260 microns, and are separated from the other physically separate pieces  820  by 60 microns on all sides. Bus lines  830  have a width of 40 microns. In yet another example of such a display, physically separate pieces  820  are 300 microns by 300 microns, and are separated from the other physically separate pieces  820  by 20 microns on all sides. Bus lines  830  have a width of 40 microns. 
     By way of further example, a 60 dots per inch (dpi) passive matrix display could be fabricated using the structure illustrated in FIG.  8 . In one example of such a display, physically separate pieces  820  are 380 microns by 380 microns, and are separated from the other physically separate pieces  820  by 40 microns on all sides. Bus lines  830  have a width of 40 microns. 
     In each of these examples, physically separate pieces  820  extend slightly under the bus line  830  to which it is electrically connected, such that there is no electrical contact between a physically separate piece  820  and the bus line  830  of an adjacent electrode  805 . 
     A conventional passive matrix OLED display, such as that illustrated in FIG. 7, has been strained to about 3-4%. While there was a negligible effect on device performance, similar experiments performed on rows of ITO lined with metal bus lines revealed significant cracking in both the ITO and bus line regions. It is expected that such cracking could lead to reliability problems, particularly over the long lifetime that is required of display devices. The examples of dimensions that could be used to fabricate the structure of FIG. 8 are preferred dimensions that are expected to mitigate cracking at pixel dimensions that are presently commercially desirable. However, the invention may be practiced with a much wider range of dimensions. 
     FIG. 9 (prior art) shows a top view of a large, physically contiguous electrode  905  fabricated on a substrate  910 . Electrode  905  would be suitable, for example, for use as the common electrode of a conventional active matrix display. 
     FIG. 10 shows a top view of an electrode  1005  fabricated on a substrate  1010 . Electrode  1005  includes physically separate pieces  1020 . The physically separate pieces  1020  of electrode  1005  are electrically connected by bus lines  1030  and  1040 . As a result, electrode  1005  covers most of substrate  1000 . Electrode  1005  may be used, for example, to replace a large physically contiguous electrode that spans an entire active matrix display, such as electrode  905  of FIG. 9, to avoid undesirable cracking of such a large electrode when the substrate is flexed or otherwise subjected to stress. 
     In one embodiment of the invention, large electrodes that may be susceptible to cracking are not separated into smaller pieces to avoid such cracking. Instead, bus lines are used to reinforce the electrical connection between different regions of the large electrode. If the electrode cracks, current could still flow unimpeded through the bus lines to all regions of the electrode. 
     FIG. 11 shows a top view of a physically contiguous electrode  1105  that covers most of substrate  1110 . For example, electrode  1105  would be suitable for use as the common electrode shared by all devices of an active matrix display. Bus lines  1130  and  1140  are disposed over, and are electrically connected to, electrode  1105 . In the event that electrode  1105  cracks, current may flow through bus lines  1130  and  1140  past the crack. Depending upon the flexing or other stress that is expected to be put on electrode  105 , a variety of bus line configurations may be used. For example, bus lines  1140  may be omitted if any cracking is expected to be parallel to bus lines  1140 . 
     FIG. 12 shows a top view of physically contiguous electrodes  1220  that span the length of substrate  1210  in one direction, but are relatively narrow in the other direction. Bus lines  1230  are disposed over, and are electrically connected to, electrodes  1220 . In the event that electrodes  1220  cracks, current may flow through bus lines  1230  past the crack. Because of the small dimension of electrodes  1220  in a direction perpendicular to bus lines  1230 , it is likely that any cracking would occur approximately perpendicular to bus lines  1230 , and not parallel to bus lines  1230 . Electrodes  1220  may be used, for example, to fabricate a passive matrix display similar to that of FIG. 7 on a flexible substrate, where electrodes  1220  replace bottom electrodes  720  of FIG. 7 to mitigate any undesirable cracking of bottom electrodes  720  due to the flexing of substrate  710 . 
     In an embodiment of the invention, electrodes having physically separate pieces may be used in the individual devices of an OLED display in a variety of ways. For example, each physically separate piece of the electrode may correspond to exactly one device. With reference to FIG. 10, for example, if a single device is fabricated over each physically separate piece  1020 , then this one-to-one correspondence is established. 
     In an embodiment of the invention, each device may include a plurality of physically separate pieces of an electrode. FIG. 13 shows a common electrode  1305  similar to common electrode  1005  of FIG.  10 . Substrate  1310 , physically separate pieces  1320 , bus lines  1330 , and bus lines  1340  of FIG. 13 correspond to substrate  1010 , physically separate pieces  1020 , bus lines  1030 , and bus lines  1040 , respectively, of FIG.  10 . Through appropriate placement of the other electrodes and organic layers using conventional knowledge, devices  1350  may be defined. Each device  1350  includes a plurality, four in this example, of the physically separate pieces of electrode  1305 . The embodiment of FIG. 13 may be useful, for example, where a physically contiguous electrode as large as the pixel would be susceptible to cracking. 
     In an embodiment of the invention, each physically separate piece of an electrode may be a part of multiple devices. FIG. 14 shows a common electrode  1405  similar to common electrode  905  of FIG.  9 . Substrate  1410 , physically separate pieces  1420 , bus lines  1430 , and bus lines  1440  of FIG. 14 correspond to substrate  1010 , physically separate pieces  1020 , bus lines  1030 , and bus lines  1040 , respectively, of FIG.  10 . Through appropriate placement of the other electrodes and organic layers using conventional knowledge, devices  1450  may be defined. A plurality of devices  1450 , four in this example, share each of the physically separate pieces of electrode  1405 . Even though each physically separate piece  1420  may support several devices  1450 , the devices are shown on only one physically separate piece  1420  for clarity of illustration. The embodiment of FIG. 14 may be useful, for example, where the size at which a physically contiguous electrode becomes susceptible to cracking is larger than the size of an individual pixel, but smaller than the size of the array. 
     In an embodiment of the invention, the various electrode configurations described above with respect to flexible substrates may also be fabricated on rigid substrates. The advantageous properties conveyed by the electrode configurations may be useful for displays fabricated on rigid substrates in a number of circumstances. For example, thermal cycling of the display may lead to reliability problems if the coefficient of thermal expansion of the substrate differs significantly from that of the various layers in the device, such as the electrodes. Separating the electrodes into physically separate pieces may alleviate this problem. 
     Although many of the embodiments show a bottom electrode having physically separate pieces that are electrically connected by a bus line, the invention may also be practiced on a top electrode. Although it is expected that the mechanisms that lead to the cracking or other failure of a top electrode may differ from those that lead to the cracking or other failure of a bottom electrode, the division of the top electrode into smaller, physically separate pieces may be useful. For example, many active matrix displays have a common top electrode, and the present invention may be used to separate such an electrode into physically separate pieces. Similarly, in a stacked OLED having more than two electrodes per stacked device, the invention may be practiced on any or all of the electrodes. 
     Although many of the embodiments are not specifically described with respect to multicolor displays, it is understood that all embodiments may be readily adapted for use in a multicolor display by one of ordinary skill in the art. 
     Although many embodiments illustrate a bus line deposited over an electrode, the invention is not limited to such a configuration. For example, the electrode may be deposited over the bus line. 
     The present invention may be used to fabricate a number of products, including flat panel displays, photodetectors and transistors. The present invention may be used to fabricate opto-electronic devices as well, such as arrays of photovoltaic cells or photodiodes. 
     EXAMPLES 
     Photoresist masks having an overhang consistent with the present invention were fabricated using the following technique. 
     NR7-6000-PY photoresist was obtained from Futurexx, located in Franklin, N.J. Although the exact formulation of this photoresist is not known to the inventors, it was developed at their request and is presently commercially available. NR7-6000-PY is a negative photoresist, because patterning the photoresist involves selective exposure to ultraviolet radiation, and the subsequent removal of the unexposed portions. The use of a negative resist is preferred for integrated masks, because negative resists can be extensively baked to minimize subsequent outgassing. Such outgassing may damage the organic layers of an OLED, or other sensitive device features, thereby undesirably reducing the lifetime of the OLED. 
     The photoresist was spin deposited at 2000 RPM for about 40 seconds. The photoresist was then baked at 120° C. for 5 minutes. Next, the photoresist was exposed to ultraviolet radiation from a high pressure mercury lamp operated at a power of 13.5 mW/cm 2 , for 150 seconds through a mask, such that only the portions of the photoresist that were to remain after developing were exposed. Then, the photoresist was baked at 100° C. for 5 minutes. The photoresist was then developed in RD6, a developer commercially available from Futurexx. 
     The resultant mask had an appearance similar to that of the mask illustrated in FIG.  2  and photographed in FIG.  3 . The total width of the mask, represented by line  250  in FIG. 2, was approximately the same as the width of photoresist that was exposed to ultraviolet radiation. 
     While the present invention is described with respect to particular examples and preferred embodiments, it is understood that the present invention is not limited to these examples and embodiments. In particular, the present invention is not limited to OLEDs, and may be applied to a wide variety of electronic devices. In addition, with respect to OLEDs, the present invention is not limited to the particular examples and embodiments described. The present invention as claimed therefore includes variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art.