Abstract:
A method of making an encapsulated plasma sensitive device. The method comprises: providing a plasma sensitive device adjacent to a substrate; depositing a plasma protective layer on the plasma sensitive device using a process selected from non-plasma based processes, or modified sputtering processes; and depositing at least one barrier stack adjacent to the plasma protective layer, the at least one barrier stack comprising at least one decoupling layer and at least one barrier layer, the plasma sensitive device being encapsulated between the substrate and the at least one barrier stack, wherein the decoupling layer, the barrier layer, or both are deposited using a plasma process, the encapsulated plasma sensitive device having a reduced amount of damage caused by the plasma compared to an encapsulated plasma sensitive device made without the plasma protective layer. An encapsulated plasma sensitive device is also described.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 11/112880, filed Apr. 22, 2005, entitled Apparatus for Depositing a Multilayer Coating on Discrete Sheets, which is a continuation-in-part of application Ser. No. 10/412133, filed Apr. 11, 2003, entitled Apparatus for Depositing a Multilayer Coating on Discrete Sheets. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to encapsulated devices, and more particularly to encapsulated plasma sensitive devices, and to methods of making encapsulated plasma sensitive devices. 
     Many devices are subject to degradation caused by permeation of environmental gases or liquids, such as oxygen and water vapor in the atmosphere or chemicals used in the processing of the electronic product. The devices are usually encapsulated in order to prevent degradation. 
     Various types of encapsulated devices are known. For example, U.S. Pat. No. 6,268,695, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Jul. 31, 2001; U.S. Pat. No. 6,522,067, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Feb. 18, 2003; and U.S. Pat. No. 6,570,325, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making”, issued May 27, 2003, all of which are incorporated herein by reference, describe encapsulated organic light emitting devices (OLEDs). U.S. Pat. No. 6,573,652, entitled “Encapsulated Display Devices”, issued Jun. 3, 2003, which is incorporated herein by reference, describes encapsulated liquid crystal displays (LCDs), light emitting diodes (LEDs), light emitting polymers (LEPs), electronic signage using electrophoretic inks, electroluminescent devices (EDs), and phosphorescent devices. U.S. Pat. No. 6,548,912, entitled “Semiconductor Passivation Using Barrier Coatings,” issued Apr. 15, 2003, which is incorporated herein by reference, describes encapsulated microelectronic devices, including integrated circuits, charge coupled devices, light emitting diodes, light emitting polymers, organic light emitting devices, metal sensor pads, micro-disk lasers, electrochromic devices, photochromic devices, microelectromechanical systems, and solar cells. 
     Generally, encapsulated devices can be made by depositing barrier stacks adjacent to one or both sides of the device. The barrier stacks typically include at least one barrier layer and at least one decoupling layer. There could be one decoupling layer and one barrier layer, there could be multiple decoupling layers on one side of one or more barrier layers, or there could be one or more decoupling layers on both sides of one or more barrier layers. The important feature is that the barrier stack has at least one decoupling layer and at least one barrier layer. 
     One embodiment of an encapsulated display device is shown in  FIG. 1 . The encapsulated display device  100  includes a substrate  105 , a display device  110 , and a barrier stack  115 . The barrier stack  115  includes a barrier layer  120  and a decoupling layer  125 . The barrier stack  115  encapsulates the display device  110 , preventing environmental oxygen and water vapor from degrading the display device. 
     The barrier layers and decoupling layers in the barrier stack can be made of the same material or of a different material. The barrier layers are typically about 100-400 Å thick, and the decoupling layers are typically about 1000-10,000 Å thick. 
     Although only one barrier stack is shown in  FIG. 1 , the number of barrier stacks is not limited. The number of barrier stacks needed depends on the level of water vapor and oxygen permeation resistance needed for the particular application. One or two barrier stacks should provide sufficient barrier properties for some applications. The most stringent applications may require five or more barrier stacks. 
     The barrier layers can be deposited using a vacuum process, such as sputtering, chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), evaporation, sublimation, electron cyclotron resonance-plasma enhanced vapor deposition (ECR-PECVD), and combinations thereof. Suitable barrier materials include, but are not limited to, metals, metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof. 
     The decoupling layers can be deposited using a vacuum process, such as flash evaporation with in situ polymerization under vacuum, or plasma deposition and polymerization, or atmospheric processes, such as spin coating, ink jet printing, screen printing, or spraying. Suitable materials for the decoupling layer include, but are not limited to, organic polymers, inorganic polymers, organometallic polymers, hybrid organic/inorganic polymer systems, and silicates. 
     As an example, an OLED can be encapsulated with a barrier stack including one or more polymeric decoupling layers and one or more barrier layers. The polymeric decoupling layers can be formed from acrylate functional precursors which are deposited using flash evaporation and polymerized by ultraviolet (UV) exposure. The barrier layers can be reactively sputtered aluminum oxide. 
     Depositing multi-layer barrier stacks on relatively insensitive substrates such polymer films does not typically result in damage to the substrate. In fact, several patents disclose the use of plasma treatment to improve properties for a multi-layer barrier on a substrate. U.S. Pat. No. 6,083,628 discloses plasma treatment of polymeric film substrates and polymeric layers from acrylates deposited using a flash evaporation process as a means of improving properties. U.S. Pat. No. 5,440,466 similarly discusses plasma treatment of substrates and acrylate layers to improve properties. 
     However, we have found that some of the devices being encapsulated have been damaged by the plasma used in depositing the barrier and/or decoupling layers. Plasma damage has occurred when a substrate with a plasma sensitive device on it, such as an OLED, is encapsulated with a multi-layer barrier stack in which a plasma based and/or assisted process is used to deposit a barrier layer or decoupling layer. For example, plasma damage has occurred when reactively sputtering a barrier layer of AlO x  under conditions suitable for achieving barrier properties, sputtering a barrier layer of AlO x  onto the top surface of a plasma sensitive device, and/or sputtering a barrier layer of AlO x  on a vacuum deposited, acrylate based polymeric layer. The damage observed when depositing a barrier layer onto a previously deposited decoupling layer is distinct, and is the subject of co-pending application Ser. No. 60/711,136 (VIT 0062 MA). 
     Plasma damage associated with deposition of a barrier layer, a decoupling layer, or another layer essentially has a negative impact on the electrical and/or luminescent characteristics of a device resulting from encapsulation. The effects will vary by the type of device, the manufacturer of the device, and the wavelength of the light emitted. It is important to note that plasma damage is dependent on the design of the device to be encapsulated. For example, OLEDs made by some manufacturers show little to no plasma damage while OLEDs made by other manufacturers show significant plasma damage under the same deposition conditions. This suggests that that there are features within the device that affect its sensitivity to plasma exposure. 
     One way to detect plasma damage is to measure the voltage needed to achieve a specified level of luminescence. Another way is to measure the intensity of the luminescence. Plasma damage results in higher voltage requirements to achieve the same level of luminescence (typically 0.2 to 0.5 V higher for an OLED), and/or lower luminescence. 
     Although not wishing to be bound by theory, plasma damage that is observed when a decoupling layer employing plasma, a sputtered AlO x , or another layer employing plasma is formed (deposited) directly on an OLED or other sensitive device is believed to be due to an adverse interaction with one or more components of the plasma, including charged or neutral species, UV radiation, and high thermal input. 
     Thus, there is a need for a method of preventing the damage caused by processes utilizing plasma in the encapsulation of various devices. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need by providing a method of making an encapsulated plasma sensitive device comprising: providing a plasma sensitive device adjacent to a substrate; depositing a plasma protective layer on the plasma sensitive device using a process selected from non-plasma based processes, or modified sputtering processes; and depositing at least one barrier stack adjacent to the plasma protective layer, the at least one barrier stack comprising at least one decoupling layer and at least one barrier layer, the plasma sensitive device being encapsulated between the substrate and the at least one barrier stack, wherein the decoupling layer, the barrier layer, or both are deposited using a plasma process, the encapsulated plasma sensitive device having a reduced amount of damage caused by the plasma compared to an encapsulated plasma sensitive device made without the plasma protective layer. 
     Another aspect of the invention is an encapsulated plasma sensitive device. The encapsulated plasma sensitive device includes a substrate; a plasma sensitive device adjacent to a substrate; a plasma protective layer on the plasma sensitive device, the plasma protective layer deposited using a process selected from non-plasma based processes, or modified sputtering processes; and at least one barrier stack adjacent to the protective layer, the at least one barrier stack comprising at least one decoupling layer and at least one barrier layer, the plasma sensitive device being encapsulated between the substrate and the at least one barrier stack, wherein the encapsulated plasma sensitive device has a reduced amount of damage caused by a plasma compared to an encapsulated plasma sensitive device made without the plasma protective layer. By “adjacent,” we mean next to, but not necessarily directly next to. There can be additional layers intervening between the substrate and the barrier stacks. By “on,” we mean deposited directly on the previous layer without any intervening layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of a portion of one embodiment of an encapsulated display device. 
         FIG. 2  is a diagram of one embodiment of a modified sputtering process according to the present invention. 
         FIG. 3  is a diagram of another embodiment of a modified sputtering process according to the present invention. 
         FIG. 4  are graphs showing a comparison of the voltage shift with and without a plasma protective layer. 
         FIG. 5  are graphs showing a comparison of the leakage current with and without a plasma protective layer. 
         FIG. 6  are graphs showing a comparison of the light output with and without a plasma protective layer. 
         FIG. 7  is a graph showing the voltage shift as a function of the thickness of a plasma protective layer deposited using thermal evaporation. 
         FIG. 8  is a graph showing the voltage shift as a function of DF (irradiation time x discharge power). 
         FIG. 9  is a graph showing the voltage shift as a function of the thickness of a plasma protective layer deposited using a modified sputtering process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The addition of a layer to shield the underlying device from exposure to the plasma (from deposition of the barrier layer, the decoupling layer, or both) has been shown to reduce or avoid plasma damage. 
     One method involves the deposition of a plasma protective layer using a non-plasma based process. Suitable non-plasma based processes include both vacuum processes and atmospheric processes. Suitable vacuum processes include, but are not limited to, thermal evaporation, electron beam evaporation, chemical vapor deposition (CVD), and metalorganic chemical vapor deposition (MOCVD), catalytic chemical vapor deposition, laser thermal transfer, or evaporation or chemical vapor deposition followed by ion assisted densification. Suitable atmospheric processes include, but are not limited to, spin coating, ink jet printing, screen printing, spraying, gravure printing, offset printing, and laser thermal transfer. With atmospheric processes, the working gases should be free of O 2  and H 2 O content. 
     The plasma protective layer can be made of inorganic and organic materials. Suitable inorganic materials include, but are not limited to, metal halides, such as LiF 2 , MgF 2 , CaF 2 , and SiO x . Suitable organic materials include, but are not limited to, aluminum tris 8-8-hydroxyquinoline, phthalocyanines, naphthalocyanines, and similar polycyclic aromatics. 
     Another method involves depositing the plasma protective layer using a modified sputtering process. Modified sputtering processes include, but are not limited to, modified reactive sputtering processes. By changing the sputtering configuration and/or the process conditions of the sputtering, a less energetic process in terms of the impact on the receiving surface can be obtained. This expands the range of plasma protective layers to include a wider range of inorganic compounds, e.g., AlO x  and SiO x  based layers, which have advantages including being dielectrics and chemically inert. However, the changes impact the physical and, to a lesser degree, the chemical properties of the deposited layers. For example, the density (increased porosity), stress, and grain size can be altered. One result of this can be the loss of barrier properties, despite the demonstrated ability to shield the underlying OLED from plasma damage. For example, a layer of AlO x  could be deposited under conditions that avoid plasma damage, and a second layer of AlO x  could be deposited as a barrier layer when the encapsulation is designed with the barrier layer first. 
     One modification of the sputtering process involves the use of a screen placed between the target cathode (the source of at least a part of the material to be deposited) and the substrate with the device to be sputter coated. A diagram of this process is shown in  FIG. 2 . The cathode  210  is on one side of the screen  215 , while the substrate  220  with the OLED  225  is on the other side. The inert sputtering gas  230  is introduced on the cathode side, and the sputtering plasma  235  is also on that side. The reactive gas  240  is fed on the substrate/device side. The presence of the screen  215  reduces the reaction of the reactive gas with the surface of the cathode  210 . 
     Another modification of the sputtering process involves off-axis sputtering. A diagram of one embodiment of this process is shown in  FIG. 3 . The substrate/device receiving the sputtered material is placed at a position removed from (and oblique to) the sputtering plasma.  FIG. 3  shows a “facing target” or “facing cathode” arrangement. There are two cathodes  250 ,  255  facing each other. The sputtering plasma  260  is positioned between the two cathodes  250 ,  255 . The substrate/device  265  is located off to the side and perpendicular to the two cathodes  250 ,  255 . This arrangement eliminates a direct path from the cathodes  250 ,  255  to the substrate/device  265 . The sputtered species will reach the substrate only after multiple collisions and lose most of their energy along the way. This will reduce radical generation in the decoupling polymer layer. Another advantage of the off-axis sputtering process is the mitigation of the thermal impact of sputter deposition on thermally sensitive substrates, i.e., the indirect path of less energetic species results in less heating of receiving surfaces. There are other variations of off-axis sputtering which result in similar deposition schemes and which could be used. 
     Instead of changing the sputtering configuration, or in addition to it, it is also possible to vary the process parameters for the sputtering, including the exposure time or the energy/power of the plasma. Experimental results have shown that the longer the device is exposed to the plasma, the greater the plasma damage. This has led to increasing the process speed in order to reduce or eliminate the plasma damage. 
     Typically, for a sputtering configuration that has been determined to deposit a satisfactory barrier layer (cathode, magnet placements, spacing, gas feeds, etc.), barrier layers are deposited at a track speed of about 30 cm/min at a power of about 2000 watts. One modified process involves increasing the track speed to about 90 cm/min (about three times standard track speed) and increasing the power to 2500 watts. The increase in track speed offsets the higher power resulting in decreased overall exposure to the plasma. Alternatively, the track speed can be decreased to about 20 cm/min and the power decreased to about 500 watts. The power reduction offsets the slower speed, resulting in lower exposure to the plasma. 
     EXAMPLE 1 
     OLEDs made by two manufacturers were tested for voltage shift and light decrease. The OLEDs were supplied by the manufacturers on glass substrates. They were then encapsulated. The first layer was a thick layer (1000 Å) of AlO x  followed by 4 acrylate polymer (0.5 microns)/AlO x  (300 Å) pairs. The oxide layers were sputtered without a screen (Configuration I). 
     The results are shown in Table 1. A blue OLED made by manufacturer 1 showed a voltage shift of 0.5-0.8 V, and a moderate light decrease. A green OLED made by manufacturer 3 showed a voltage shift of 1 V, with a strong light decrease. 
     Voltage shift and light decrease for OLEDs encapsulated by sputtering the oxide layer with a screen were measured (Configuration II). The OLEDs were encapsulated with a first thick layer (1000 Å) of AlO x  followed by either 4 or 6 acrylate polymer (0.5 microns)/AlO x  (300 Å) pairs. The OLEDs were processed at the standard track speed of 30 cm/min and the standard power of 2000 watts. 
     The results are shown in Table 1. OLEDs from different manufacturers showed varying amounts of voltage shift and light decrease. Furthermore, different colored OLEDs from the same manufacturer showed different amounts of voltage shift and light decrease. This confirms that there is a variation in the plasma damage for OLEDs from different manufacturers, and for different colored OLEDs. 
     
       
         
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Configuration II 
               
             
          
           
               
                   
                 Configuration I 
                 Standard 
               
             
          
           
               
                   
                 Voltage 
                 Light 
                 Voltage 
                 Light 
               
               
                 OLED 
                 Shift 
                 Decrease 
                 Shift 
                 Decrease 
               
               
                   
               
             
          
           
               
                 Mfr 1 
                 blue 
                 0.5-0.8 
                 moderate 
                   0-0.3 
                 no 
               
               
                 Mfr 2 
                 blue 
                   
                   
                 0.2 
                 moderate 
               
               
                   
                 green 
                   
                   
                 0.5 
                 strong 
               
               
                   
                 red 
                   
                   
                 0.3-0.4 
                 strong 
               
               
                   
                 red 1 
               
               
                 Mfr 3 
                 green 
                 1 
                 strong 
                 0.6 
                 moderate 
               
               
                 Mfr 4 
                 green 
                   
                   
                 0.2 
                 no 
               
               
                 Mfr 5 
                   
                   
                   
                 0.1-0.5 
                 no 
               
               
                 Mfr 6 
                 green 
                   
                   
                   
                 1.8 
               
               
                 Mfr 7 
                 yellow 
                   
                   
                 1   
                 strong 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 2 
     Several encapsulated OLEDs were made with a plasma protective layer of 300 Å of LiF. The LiF was deposited using a thermal evaporation process. The OLEDs had a thick layer (1000 Å) of AlO x  followed by either 4 or 6 acrylate polymer (0.5 microns)/AlO x  (300 Å) pairs. 
     The OLEDs were tested for voltage shift, leakage current, and lightout. For comparison, several OLEDs were made without the LiF protective layer. The results are shown in  FIGS. 4-6  and Table 2. The LiF protective layer eliminated the voltage shift induced by exposure to the plasma during sputtering deposition of the thick barrier layer. 
       FIG. 7  shows the voltage shift for various thicknesses of LiF. A layer of LiF was deposited on the OLEDs followed by a layer of AlO x  as a barrier layer. The results suggest that a thickness of at least about 300 Å may be needed to eliminate the voltage shift. 
     
       
         
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Configuration II 
               
             
          
           
               
                   
                 Standard 
                 LiF protection 
               
             
          
           
               
                   
                 Voltage 
                 Light 
                 Voltage 
                 Light 
               
               
                 OLED 
                 shift 
                 Decrease 
                 shift 
                 Decrease 
               
               
                   
               
             
          
           
               
                 Mfr 1 
                 blue 
                   0-0.3 
                 no 
                 0 
                 no 
               
               
                 Mfr 2 
                 blue 
                 0.2 
                 moderate 
                 na 
                 na 
               
               
                   
                 green 
                 0.5 
                 strong 
                 0 
                 no 
               
               
                   
                 red 
                 0.3-0.4 
                 strong 
                 na 
                 na 
               
               
                   
                 red 1 
               
               
                 Mfr 3 
                 green 
                 0.6 
                 moderate 
                 0.1-0.2 
                 no 
               
               
                 Mfr 4 
                 green 
                 0.2 
                 no 
                 0 
                 no 
               
               
                 Mfr 5 
                   
                 0.1-0.5 
                 no 
                   
               
               
                 Mfr 6 
                 green 
                   
                 1.8 
                   
               
               
                 Mfr 7 
                 yellow 
                 1   
                 strong 
                 0 
                 no 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 3 
     The effect of process parameters on voltage shift was evaluated. Table 3 shows a comparison of the effect of exposure time on plasma damage. The modified conditions involved increasing the track speed to 90 cm/min (about three time standard track speed) and increasing the power to 2500 watts, and decreasing track speed to 20 cm/min and the power to 500 watts. The voltage shift and light decrease of the OLEDs made using Configuration II and standard sputtering conditions for the barrier (power of 2000 watts, track speed of 30 cm/min) are included for comparison. The voltage shift and light decrease are reduced or eliminated when the exposure time is reduced. 
       FIG. 8  shows the voltage shift as a function of discharge factor. Discharge factor is an approach to quantifying the exposure of a receiving surface, such as a polymer, an OLED, etc., to reactive species present in sputtering plasmas. It is related to the irradiation dose (energy/area) and is calculated by multiplying irradiation time by discharge power. Voltage shift increased with increasing discharge factor, as shown by  FIG. 8 . A comparable voltage shift can be obtained under different deposition conditions, e.g., power or track speed. The dominant factor is exposition time, with power having a lesser influence. This suggests that decreasing the exposition time should decrease the voltage shift. 
     The voltage shift was also measured as a function of aluminum oxide protective layer thickness. By changing the process conditions (increasing the track speed to 90 cm/min (about three time standard track speed) and increasing the power to 2500 watts, and decreasing track speed to 20 cm/min and the power to 500 watts.), the thickness of the aluminum oxide protective layer was varied. As shown in  FIG. 9 , there is a minimum thickness of about 100 Å above which no further voltage shift is induced by the plasma process. 
     
       
         
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Configuration II 
               
             
          
           
               
                   
                 Standard 
                 Modified 
               
             
          
           
               
                   
                 Voltage 
                 Light 
                 Voltage 
                 Light 
               
               
                 OLED 
                 shift 
                 Decrease 
                 shift 
                 Decrease 
               
               
                   
               
             
          
           
               
                 Mfr 1 
                 blue 
                   0-0.3 
                 no 
                 0   
                 no 
               
               
                 Mfr 2 
                 blue 
                 0.2 
                 moderate 
                 0.1 
                 no 
               
               
                   
                 green 
                 0.5 
                 strong 
                 0.15-.35  
                 no 
               
               
                   
                 red 
                 0.3-0.4 
                 strong 
                  0.1/0.4* 
                 no 
               
               
                   
                 red 1 
                   
                   
                 0.1-0.3 
                 no 
               
               
                 Mfr 3 
                 green 
                 0.6 
                 moderate 
                   
               
               
                 Mfr 4 
                 green 
                 0.2 
                 no 
                   
               
               
                 Mfr 5 
                   
                 0.1-0.5 
                 no 
                   
               
               
                 Mfr 6 
                 green 
                   
                 1.8 
                 no 
                   
               
               
                 Mfr 7 
                 yellow 
                 1   
                 strong 
               
               
                   
               
             
          
         
       
     
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and methods disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.