Abstract:
Organic light emitting diode (OLED) backplanes and fine metal masks (FMMs) are described. In an embodiment, an OLED backplane includes an array of raised bottom electrodes, and an FMM includes an array of pixel openings and an array of recesses. The FMM can be positioned over the backplane such that the pixel openings are over the raised bottom electrodes onto which a layer is to be evaporated, and the recesses are over the raised bottom electrodes that are to be protected from the evaporated species.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority from U.S. Provisional Application No. 62/269,677, filed on Dec. 18, 2015, which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Field 
         [0003]    Embodiments described herein relate to organic light emitting diode (OLED) displays and fine metal masks (FMMs) used on OLED production. 
         [0004]    Background Information 
         [0005]    State of the art displays for phones, tablets, computers, and televisions utilize glass substrates with thin film transistor (TFTs) to control transmission of backlight though pixels based on liquid crystals. More recently emissive displays such as those based on organic light emitting diodes (OLEDs) have been introduced as being more power efficient, and allowing each pixel to be turned off completely when displaying black. 
         [0006]    An OLED display includes a matrix of pixels including multiple layers of organic films. For example, each OLED may include an electron transport layer, a hole transport layer, and an organic emission layer between the electron transport layer and the hole transport layer. The multiple layers may additionally include an electron injection layer and hole injection layer. Typically different organic emission layers are deposited for different color emission. A fine metal mask (FMM) is commonly used as a shadow mask during vapor deposition of the organic emission layers within the subpixels of an OLED display. 
       SUMMARY 
       [0007]    OLED backplanes, FMMs, and methods of OLED production are described. In an embodiment, a backplane used for OLED fabrication includes a substrate, an array of raised bottom electrodes on the substrate, and a pixel defining layer (PDL) on the substrate. In an embodiment, the PDL includes an array of openings over the array of raised bottom electrodes, and each raised bottom electrode in the array of raised bottom electrodes is thicker than the PDL. 
         [0008]    In an embodiment, a FMM includes a frame with top side and a bottom surface, an array of pixel openings in the frame, and an array of recesses in the bottom surface of the frame laterally between the pixel openings. Each recess may include sidewalls and a top surface. 
         [0009]    In an embodiment, a FMM is positioned over the display substrate such that an array of recesses in the FMM is directly over a first array of raised ground contacts, and an array of pixel openings in the FMM is directly over a second array of raised ground contacts. An array of organic emission layers can then be evaporated on the second array of raised contacts, or an intervening layer such as a hole transport layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic top view illustration of a display backplane including spacers. 
           [0011]      FIG. 2  is a schematic close-up cross-sectional side view illustration of an FMM positioned over a display backplane including spacers. 
           [0012]      FIG. 3  is a schematic close-up cross-sectional side view illustration of an array of organic emission layers deposited on a display backplane including spacers. 
           [0013]      FIG. 4  is a schematic close-up cross-sectional side view illustration of an FMM positioned over a display backplane including raised electrodes in accordance with an embodiment. 
           [0014]      FIG. 5  is a perspective view of the bottom side of an FMM in accordance with an embodiment. 
           [0015]      FIG. 6  is a perspective view of the bottom side of an FMM including spacers in accordance with an embodiment. 
           [0016]      FIG. 7  is a schematic close-up cross-sectional side view illustration of an FMM including a spacer positioned over a display backplane including raised electrodes in accordance with an embodiment. 
           [0017]      FIG. 8  is a schematic close-up cross-sectional side view illustration of an array of organic emission layers deposited on a display backplane including raised electrodes in accordance with an embodiment. 
           [0018]      FIG. 9  is a schematic close-up cross-sectional side view illustration of multiple layer stack OLEDs in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments describe OLED displays, FMMs, and methods of fabricating OLED displays. In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0020]    The terms “top”, “bottom”, “over”, “to”, “between”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over”, “spanning” or “on” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
         [0021]    In one aspect, embodiments describe display backplanes and FMMs that may be used to mitigate color mixing between adjacent subpixels, and increase pixel density (pixels per inch, PPI) or aperture ratio. In an embodiment, a backplane includes a pixel defining layer (PDL) and an array of raised bottom electrodes (e.g anodes). The raised bottom electrodes may be thicker than the PDL. In an embodiment, an FMM includes an array of recesses in the bottom surface of the FMM frame laterally between the pixel openings. In operation, the FMM may be positioned over the display backplane so that the array of recesses are positioned over the array of raised electrodes. In such an arrangement, the bottom surface of the FMM can be located below the top surfaces of the raised electrodes (or the top surfaces of any intervening layers on the raised electrodes) during evaporation of the organic emission layers. This may inhibit the migration of the evaporated organic emission layers, and reduce potential color mixing even in the case of FMM misalignment. 
         [0022]    Referring now to  FIG. 1  a cross-sectional side view illustration is provided of a display backplane  100 . As illustrated, a display backplane  100  may include an array of pixels  104 , and an array of spacers  110  formed on a substrate  102 . Each pixel  102  may have a plurality of subpixels  106 , each designed for a different color emission. The particular pixel  104  illustrated in  FIG. 1  includes a blue-emitting subpixel  106 B, a green-emitting subpixel  106 G, and a red-emitting subpixel  106 R for an RGB color arrangement, though this is exemplary and embodiments are not limited to a specific color arrangement. 
         [0023]      FIG. 2  is a schematic close-up cross-sectional side view illustration of an FMM  200  positioned over a display backplane  100  including spacers  142 . As shown, the display backplane  100  may include a substrate  102 . For example, substrate  102  may include a TFT substrate  120  and optionally a planarization layer  122 . An array of electrodes  130  (e.g. anodes) may be formed on the planarization layer  122 . A dielectric layer  124  may optionally separate the electrodes  130 . A top surface of the electrodes  130  and dielectric layer  124  may optionally be planarized. A pixel defining layer (PDL)  140  including openings  144  is formed over the array of electrodes  130 , and a plurality of spacers  142  may be formed on the PDL  140  or as part of the PDL. For example, the spacers  142  and PDL  140  may be formed of the same layer using a half tone lithography mask. 
         [0024]    Still referring to  FIG. 2 , the FMM  200  includes a frame  200  and an array of pixel openings  204  in the frame. The pixel openings  204  may include sloped sidewalls  222 , and a step  220  near a bottom surface  210  of the frame  200 . As shown, the bottom surface  210  of the frame  200  may rest upon the plurality of spacers  142  during a deposition operation. During such a deposition operation (e.g. evaporation), the deposited species may potentially migrate underneath the bottom surface  210  of the FMM  200  and deposit as a shadow zone near the bottom electrodes  130  that are intended to be covered by the FMM  200 , potentially causing color mixing. This may be attributed to the total thickness (T) of the PDL  140 , spacers  142 , and FMM step  220  height (S).  FIG. 3  is a schematic close-up cross-sectional side view illustration of an array of organic emission layers  150  deposited on a display backplane  100  including spacers  142 . As shown, edges  152  of the organic emission layers  150  may creep through the gaps between the FMM  200  and top surface of the PDL  140 . 
         [0025]    Referring now to  FIG. 4 , in accordance with embodiments an FMM  200  including recesses  235  is positioned over a display backplane  100  including raised bottom electrodes  130  (e.g. anodes). In the embodiment illustrated, the backplane  100  may include a substrate  102 , an array of raised bottom electrodes  130  on the substrate  102 , and a PDL  140  on the substrate  102 . For example, substrate  102  may include a TFT substrate  120  and optionally a planarization layer  122 . The PDL  140  includes an array of openings  144  over the array of raised bottom electrodes  130 . Each raised bottom electrode  130  may correspond to a subpixel  106  (e.g.  106 R,  106 G,  106 B), and be independently addressable. In the embodiment illustrated, each raised bottom electrode  130  in the array of raised bottom electrodes is thicker than the PDL  140 . 
         [0026]    The raised bottom electrodes  130  may be formed of a single layer, or a layer stack. For example, the raised bottom electrodes  130  may be formed of metals, conductive oxides, conductive polymers, and combinations thereof. For example, the raised bottom electrodes  130  may be formed of indium-tin-oxide (ITO), refractory metal, silver (Ag), or combinations thereof. In an embodiment, the raised bottom electrode  130  comprise layer stacks of ITO/Ag, ITO/Ag/ITO, ITO/Ag alloy/ITO. These combinations are exemplary, and embodiments are not so limited. For example, ITO may have a uniform work function so that the hold injection barrier is small to the OLED. Ag may function as a mirror layer. 
         [0027]    In an embodiment, the raised bottom electrodes  130  are at least  2 μm thick, such as  3 μm thick. The raised bottom electrodes  130  may include top surfaces  132  and sidewalls  134 . In an embodiment, the raised bottom electrodes  130  are formed by evaporation or sputtering, or a combination of evaporation or sputtering multiple layers. A PDL  140  may then be formed over the substrate  102  and patterned to form openings  144  over the top surfaces  132  of the raised bottom electrodes  130 . The openings  144  may create injection region boundaries for the OLEDs that are formed on the raised bottom electrodes  130 . In an embodiment, the PDL  140  is formed of a polymer, such as polyimide (PI), acrylic, or benzocyclobutene (BCB). In an embodiment, the PDL  140  is formed using a technique such as spin coating, though other techniques may be used. As shown, the coated PDL  140  is formed on the top surfaces  132  and sidewalls  134  of the raised bottom electrodes  130  and patterned to form openings  144 . In accordance with embodiments, the PDL  140  is thinner than the raised bottom electrodes  130 . For example, the PDL may be less than 1.5 μm thick, such as 1.0 μm thick. Due to the difference in thicknesses of the raised bottom electrodes  130  and the PDL  140 , the PDL includes lip regions  146  around the raised bottom electrodes  130  on the top surfaces  132  of the raised bottom electrodes  130 , and well regions  148  laterally between adjacent raised bottom electrodes  130 . In an embodiment, a top surface  149  of the well regions  148  is below a top surface  132  of each raised bottom electrode  130 . 
         [0028]    Still referring to  FIG. 4 , in accordance with an embodiment an FMM  200  is positioned over the display backplane  100  with the recesses  235  in the FMM positioned directly over the raised bottom electrodes  130 . The particular embodiment illustrated in  FIG. 4  illustrates an FMM  200  positioned over the display backplane  100  prior to deposition of any of the OLED layers. In application, the formation of the OLED layers may include a number of different FMMs  200  and deposition operations. For example, the formation of an exemplary OLED may include the deposition of around 10-15 layers. Thus, while the specific embodiment illustrated in  FIG. 4  shows the bottom surface  240  of the FMM residing on the top surface  149  of the PDL  140  well portions  148 , this is exemplary and one or more layers may already be formed on top of the underlying structure. 
         [0029]    In an embodiment, an FMM  200  includes a frame  202  with a top side  223  and a bottom surface  240 . An array of pixel openings  204  are formed in the frame  202 . For example, the pixel openings  204  extend between the top side and the bottom surface  240 . In accordance with embodiments, an array of recesses  235  is formed in the bottom surface  240  of the frame  202  laterally between the pixel openings  204 . Each recess  235  may include sidewalls  232  and a top surface  234 . The sidewalls  232  for each recess  235  may form a complete loop. Each of the pixel openings  204  may include tapered sidewalls  222  so that the pixel openings  204  are wider at the top side  223  of the frame than at the bottom surface  240  of the frame  202 . The tapered sidewalls  222  for each pixel openings  204  may additionally span directly over a plurality of recesses  235  in the bottom surface  240  of the frame  202 . 
         [0030]    In an embodiment, the FMM  200  additionally includes legs  230  (e.g. ridges) between the array of pixel openings  204  and the array of recesses  235 . In an embodiment, the legs  230  (e.g. ridges) include sidewalls  220  that define the pixel openings  204  at the bottom surface  240 . Alternatively, sidewalls  222  may extend all the way to the bottom surface  240 . During use, the recesses  235  can be positioned directly over the raised bottom electrodes  130  with the legs  230  (e.g. ridges) laterally surrounding (e.g. completely laterally surrounding) the top surfaces  132  of the respective raised bottom electrodes  130  (or top surfaces of any overlying layers that may have already been deposited over the raised bottom electrodes  130 ). 
         [0031]      FIG. 5  is a perspective view of the bottom side of an FMM  200  in accordance with an embodiment. In the embodiment illustrated in  FIG. 5 , the bottom surface  240  of the frame  202  (e.g. bottom surface of the legs/ridges  230 ) may be a planar bottom surface in a grid pattern that is defined by the array of pixel openings  204  and the array of recesses  235 . A close up illustration is additionally provided in  FIG. 5  of a pixel area of the frame  202  including a recess  235 B to be positioned over a raised bottom electrode  130  of a blue-emitting subpixel  106 B, a recess  235 G to be positioned over a raised bottom electrode  130  of a green-emitting subpixel  106 B, and a pixel opening  204 R to be positioned over a raised bottom electrode  130  of a red-emitting subpixel  106 R. As shown, the arrays of recesses may include a first array of recesses (e.g.  235 B) with a first top surface  234  with a first area, and a second array of recesses (e.g.  235 G) with a second top surface  234  with a second area that is different from the first area. Similarly, the areas of the corresponding top surfaces  132  of the raised bottom electrodes  130  may be different. 
         [0032]    Referring now to  FIGS. 6-7 , in some embodiments the FMM  200  may include an array of spacers  242  protruding from the bottom surface  240  in a direction opposite of the array of recesses  235 . The spacers  242  may be formed separately from the frame  202 , or as part of the frame  202 .  FIG. 6  is a perspective view of the bottom side of an FMM  200  including spacers  242  in accordance with an embodiment.  FIG. 7  is a schematic close-up cross-sectional side view illustration of an FMM  200  including a spacer  242  positioned over a display backplane  100  including raised electrodes  130  in accordance with an embodiment. In accordance with embodiments, the spacers  242  can be dispersed across the (e.g. planar) bottom surface  240  of the FMM  200 . In one embodiment, the thickness of the spacers  242  is less than thickness of the raised bottom contacts  130 . In such an arrangement, while a separation distance may be created between the bottom surface  240  of the FMM and underlying structure on the display backplane  100 , the relative heights may create a substantial barrier to migration of the evaporated species. 
         [0033]    Referring now to  FIG. 8  a schematic close-up cross-sectional side view illustration is provided of an array of organic emission layers  150  deposited on a display backplane  100  including raised electrodes  130  in accordance with an embodiment. The particular embodiment illustrated in  FIG. 8  illustrates the formation of the organic emission layers  150  without other corresponding layers within an OLED. Accordingly, the organic emission layers  150  illustrated in  FIG. 8  are illustrative of any layers formed using an FMM  200  in accordance with embodiments. In the particular embodiment illustrate the deposited organic emission layers  150  may be formed over the top surfaces  132  of the raised bottom electrodes  130 , over the lip regions  146  and well regions  148  of the PDL  140 . 
         [0034]      FIG. 9  is a schematic close-up cross-sectional side view illustration of multiple layer stack OLEDs in accordance with an embodiment. The particular cross-section illustrated in  FIG. 9  is exemplary of a cross-section taken along the red-emitting and green-emitting subpixels. As shown, arrays of organic emission layers  150 R,  150 G are formed over the array of bottom electrodes  130 . While only red and green-emitting organic emission layers are illustrated, in an exemplary RGB system, the array of organic emission layers includes a first array of red-emitting organic emission layers, a second array of green-emitting organic emission layers, and a third array of blue-emitting organic emission layers. 
         [0035]    In an embodiment, a common hole injection layer followed by common hole transport layer (illustrated together as  162 ) are formed over each of the raised bottom electrodes  130 . Separate organic emission layers  150  are the specific raised bottom electrodes  130  for the corresponding subpixels. A common electron transport layer followed by common electron injection layer (illustrated together as  164 ) are then formed over the arrays of organic emission layers  150 . A common top electrode layer (e.g. cathode) may then be formed over the common electron injection layer. The top electrode layer may be formed a transparent conductive oxide, such as ITO, or a transparent conductive polymer. 
         [0036]    In an embodiment, a method of forming an OLED display comprises positioning an FMM over a display substrate that includes a first array of raised ground contacts and a second array of raised ground contacts. The FMM may include a frame, an array of pixel openings in the frame, and an array of recesses in the bottom surface of the frame laterally between the pixel openings. In the embodiment, positioning the FMM over the display substrate includes positioning the array of recesses directly over the first array of raised ground contacts, and positioning the array of pixel openings directly over the second array of raised ground contacts. An array of organic emission layers is then evaporated on the second array of raised ground contacts. 
         [0037]    Prior to positioning the FMM over the display substrate, a PDL may be formed on the display substrate and patterned to form an opening over each raised ground contact. Each raised ground contact may be thicker than the PDL. In an embodiment, the FMM may include an array of spacers on a bottom surface of the FMM. Each raised ground contact may be thicker than each spacer. One or more layers may have already been formed over the arrays of raised ground contacts prior to evaporating the array of organic emission layers. In an embodiment, positioning the FMM over the display substrate includes resting the FMM on a hole transport layer. 
         [0038]    In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a display backplane and FMM. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration.