Abstract:
A solid compacted pellet of organic material for use in a vapor deposition source, and a method of making same is disclosed. The solid compacted pellet includes: a support member for permitting rotation of the pellet, or transmitting cooling fluid, or both; and a compacted solid core of organic material molded onto and about the support member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 09/898,369 filed Jul. 3, 2001 entitled “Method of Compacting Organic Material in Making An Organic Light-Emitting Device” by Van Slyke et al; U.S. patent application Ser. No. 10/073,690 filed Feb. 11, 2002, entitled “Using Organic Materials in Making An Organic Light-Emitting Device” by Ghosh et al, and U.S. patent application Ser. No. 10/195,947 filed Jul. 16, 2002, entitled “Compacting Moisture-Sensitive Organic Material in Making An Organic Light-Emitting Device” by Ghosh et al, the teachings of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an improved method of forming a solid compacted pellet from powders including organic materials wherein such solid compacted pellet is used in physical vapor deposition to make an organic layer on a structure which will form part of an organic light emitting diode (OLED) display. More particularly, this invention relates to a solid compacted pellet of organic materials comprising a support member for permitting rotation of the pellet or transmitting cooling fluid during the vapor deposition process. 
     BACKGROUND OF THE INVENTION 
     An organic light-emitting diode (OLED), also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes. 
     In a passive-matrix OLED of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by physical vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10 −3  Torr. A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes. 
     Such conventional passive-matrix OLED displays are operated by applying an electrical potential (also referred to as a drive voltage) between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate. 
     In an active-matrix OLED, an array of anodes are provided as first electrodes by thin-film transistors (TFTs) which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in commonly-assigned U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference. 
     Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference. 
     Organic materials useful in making OLED displays, for example organic hole-transporting materials, organic light-emitting materials predoped with an organic dopant, and organic electron-transporting materials can have relatively complex molecular structures with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition. 
     The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders, flakes or granules have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a substrate to provide an organic layer thereon. 
     Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition: 
     (i) powders, flakes, or granules are difficult to handle because they can acquire electrostatic charges via a process referred to as triboelectric charging; 
     (ii) powders, flakes, or granules of organic materials generally have a relatively low physical density (expressed in terms of weight per unit volume) in a range from about 0.05 to about 0.2 g/cm 3 , compared to a physical density of an idealized solid organic material of about 1 g/cm 3 ; 
     (iii) powders, flakes, or granules of organic materials have an undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10 −6  Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; and 
     (iv) powders, flakes, or granules can have a relatively high ratio of surface area/volume of such particles and a correspondingly high propensity to entrap air and/or moisture between particles under ambient conditions. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source which is disposed in a chamber must be thoroughly outgased by preheating the source once the chamber has been evacuated to a reduced pressure. If outgasing is omitted or is incomplete, particles can be ejected from the source together with a vapor stream during physical vapor-depositing an organic layer on a structure. An OLED, having multiple organic layers, can be or can become functionally inoperative if such layers include particles or particulates. 
     Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially nonuniform sublimation or vaporization of organic material and can, therefore, result in potentially nonuniform vapor-deposited organic layers formed on a structure. 
     The design and performance of linear evaporation source for organic materials are described by Steven VanSlyke et al, SID 2002 Digest, pp. 886-889, 2002. The organic material in powder form is placed inside a quartz boat and heated simultaneously by bottom and top heaters. The bottom heater is used primarily to degas the powder and the top heater is operated at a temperature sufficient to vaporize the upper surface of the organic powder by radiative heating. The linear source provides a significant advantages over the conventional point source, especially thickness uniformity over a large surface area of deposition. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of compacting organic material adaptable for making an organic layer on a structure which will form part of an OLED display. 
     It is a further object of the invention to provide a method of making an organic layer from a solid compacted pellet of organic material and on a structure which will form part of an OLED display. 
     In one aspect, the present invention provides a method of forming a solid compacted pellet of organic materials adaptable for making an organic layer on a structure which will form part of an OLED display, comprising the steps of: 
     (a) providing a die defining a cavity and a first and second punches in opposite sides of the cavity and movable into such cavity, the first and second punches each having corresponding openings for receiving a cooling tube; 
     (b) placing organic material in a powder form inside the die cavity over the first punch and around the metal tube; and 
     (c) applying sufficient heat by a heating source and pressure by the punches to the organic material in the die cavity to compact the organic material into a solid compacted pellet molded about the cooling tube. comprising a metal tube at the center extending through the linear source. 
     In another aspect, the present invention provides a method of making a solid compacted pellet of organic material for use in a vapor deposition source, comprising the steps of: 
     (a) a support member for permitting rotation of the pellet, or transmitting cooling fluid, or both; and 
     (b) a compacted solid core of organic material molded onto and about the support member. 
     In another aspect, the present invention provides a method of making an organic layer from a solid compacted pellet of organic material on a structure, which will form part of an OLED display comprising the steps of: 
     (a) providing a solid compacted pellet of organic material comprising at least one organic host and one organic dopant; 
     (b) placing such a solid compacted pellet of organic material inside a heating source disposed in a physical vapor deposition chamber, circulating cold water through a cooling tube, and rotating the solid compacted pellet of organic material to exposing an area of the surface to heat for a short duration; 
     (c) positioning the substrate in the chamber and in a spaced relationship with respect to the heating source; 
     (d) evacuating the chamber to a reduced pressure; and 
     (e) applying heat to the top surface of the solid compacted pellet of organic material to cause at least a portion to sublime to provide a vapor of the organic materials which form the organic layer on the substrate 
     A feature of the present invention is that a solid compacted pellet of organic material can be compacted by the method of the invention wherein at least one organic host material and at least one organic dopant material are mixed prior to the compaction process. 
     Another feature of the present invention is that the method of compacting organic powder into a solid compacted pellet can be accomplished with relatively simple tools and at a location remote from a location of use of such linear source in a physical vapor deposition apparatus. 
     A further feature of the present invention is that the method of compacting organic powder into a solid compacted pellet substantially facilitates transfer or shipping of organic material in and between different locations. 
     Another feature of the present invention is that a solid compacted pellet of an OLED material can be made by the method of the present invention wherein a powder of at least one OLED host material and a powder of at least one organic dopant material are mixed or blended to provide a mixture prior to compacting the mixture into a solid compacted pellet of organic material. 
     Another feature of the present invention is that a solid compacted pellet of organic material comprising a host and one or more organic dopants eliminates the need of co-evaporation which require more than one evaporation source disposed inside a vacuum chamber. 
     Another beneficial feature of the present invention is that the solid compacted pellet of organic material is cooled at the center and a small fraction of the rotating surface is heated at a given time thereby causing uniform deposition and homogeneous composition of the organic layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a cross-sectional view of a linear source for deposition of organic layers for OLED displays of the prior art; 
     FIG. 2 depicts a perspective of the FIG. 1 of the prior art; 
     FIG. 3 shows an exploded view of a solid compacted pellet of organic materials according to the invention placed inside a heating source; 
     FIGS. 4A-4E depict cross-sectional views of the process sequence for compacting the solid compacted pellet of organic material, wherein; 
     FIG. 4A shows the cross-sectional view of a die-punch set for compacting organic materials; 
     FIG. 4B shows the step of filling the die cavity with organic material; 
     FIG. 4C shows the final step of compacting to form a solid compacted pellet of organic material; 
     FIG. 4D shows the step of ejecting the compacted solid compacted pellet of organic material from the die cavity; 
     FIG. 4E shows the solid compacted pellet of organic material having a cooling tube extending through its center; and 
     FIG. 5 depicts the schematic representation of a vacuum deposition chamber for OLED displays using the solid compacted pellet of organic material as an evaporation source according to the present invention. 
    
    
     The term “powder” is used herein to denote a quantity of individual particles, which can be flakes, granules, or mixtures of varied particles and shapes including single or plurality of molecular species. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The organic layers of an OLED include an organic or organo-metallic material that produces light, known as electroluminescence (EL), as a result of electron-hole recombination in the layer. Hereinafter, the term “organic” will be taken to include both purely organic as well as organo-metallic materials. Turning to FIG. 1, a cross-sectional view of a linear source  10  of the prior art is shown wherein organic material is heated from the top surface to deposit organic layers for an OLED display. According to the prior art described by S. VanSlyke et al, SID 2002 Digest, pp. 886-889, 2002, which is incorporated herein for reference, organic material  20  in powder form is placed in the quartz boat  30  and heated by a bottom heater  40  and a top heater  50  including tantalum. The top heater  50  is operated at a temperature sufficient to vaporize the upper surface of the organic powder by radiative heating, avoiding the necessity to heat the entire source charge to the vaporization temperature. The purpose of the bottom heater  40  is to degas and heat the organic material  20  to a temperature far below the vaporization temperature. A baffle  60  made also from tantalum is also incorporated in the top heater  50  to prevent spitting of the solid organic particles during the deposition process. Source design for thermal physical vapor-deposition of organic materials useful in constructing OLED displays, is described in commonly-assigned U.S. Pat. No. 6,237,529, the disclosure of which is herein incorporated by reference. 
     Turning to FIG. 2, there is shown the perspective of the FIG. 1 of the linear source  100 , which has been described in detail hereinbefore. For the purpose of clarity, the bottom heater  40  (FIG. 1) is not shown. The top heater  110  has an opening  120  through which the organic vapor escapes and deposits on the substrate. The quartz boat  130  is generally secured to the base plate of the vacuum deposition chamber. 
     Turning to FIG. 3, there is shown an exploded view of the solid compacted pellet assembly  200  according to the present invention. A solid compacted pellet  210  of organic material including a cooling tube  220  located at the center of the solid compacted pellet  210  of organic material which is nested inside a refractory boat  230 . A method of making the solid compacted pellet  210  of organic material by the process of compaction will be described later. A rectangular slot  240  is provided at both ends of the refractory boat  230  to accommodate the cooling tube  220  of the solid compacted pellet  210  of organic material. The refractory boat  230  can be made from electrically insulating materials which can withstand high temperature and thermal shock due to rapid heating or rapid cooling. The preferred material for the refractory boat  230  is quartz or fused silica. Other high temperature glasses or ceramics can also be used. Electrically conductive heating element  250  in the form of a rod or wire is attached to a lid  260  through an orifice  262  located on both ends of the lid  260 . Preferred material for the heating element  250  is tantalum and a plurality of heating elements  250  can also be used depending on the rate of evaporation and the physical dimension of the solid compacted pellet  210 . A plurality of slits  270  are provided around the center of the top surface  264  of the lid  260  so that organic vapor can exit from the solid compacted pellet  210  of organic material during the deposition process. The deposition condition might dictate the slit  270  configuration to be a single linear opening instead of plurality of openings. 
     Turning to FIGS. 4A-4E, there are shown cross-sectional views of the compaction sequence for compacting organic materials to form a solid compacted pellet of organic material including a support member for permitting rotation of the solid compacted pellet, or transmitting cooling fluid, or both wherein a die-punch assembly  300  is placed in a press (not shown) to complete the compaction process steps. 
     In FIG. 4A, a first punch  310  is placed inside a die cavity  320  of a die  330 . One end (upper) of the first punch  310  is provided with a seat  340  for accommodating a cooling tube  350  (FIG. 4B) which will be used as a supporting member to mold a solid compacted solid core of organic material onto and about the cooling tube  350 . The other (bottom) end of the first punch  310  is provided with a ledge  314  so that the upward travel of the first punch  310  along a first direction  322  is restricted to a certain distance as the ledge  314  hits the recess  324  of the die  320 . The top surface  312  of the first punch  310  is allowed to rest against the vertical die wall  316  of the die  320  so that virtually there is no air gap between the die wall  316  and the outer surface  318  of the first punch  310 . A plurality of heating cartridges  332  are embedded inside the die  330  and connected to an electrical power supply (not shown). 
     Turning to FIG. 4B now, a cooling tube  350  is placed upright on the seat  340  of the first punch  310 , and a known amount of organic powder  360  is poured around the cooling tube  350  and filled the die cavity  320  of the die  330 . The cooling tube  350  is preferably made from stainless steel for the purpose of strength and vacuum compatibility. The organic powder  360  can be composed of a single organic molecule or can be a mixture of plurality of organic molecules. The die  330  is then heated by energizing the heating cartridges  332  at a temperature well below Tg (the lowest Tg if the organic powder  360  is a mixture of plurality of organic molecules) of the organic powder  360  as a part of the compaction process. A second punch  370  is then located above the die  330  for the next sequence. 
     FIG. 4C illustrates the compaction step of the process, wherein the second punch  370  is moved by the upper ram of a press (not shown) in a second direction  372 , which is opposite to the first direction  322  of the first punch  310 . The second punch  370  has a bore  374  so that the cooling tube  350  has enough room to travel during the compaction process. After the die  330  has reached the steady state of a set temperature, a known amount of pressure ranging between 2,000 and 10,000 psi is applied in the second direction  372  to the second punch  370  to complete the compaction process. 
     Turning to FIG. 4D, the first punch  310  is then pressurized to move in the first direction  322  in order to remove the compacted solid compacted pellet of organic material  380  from the die cavity  320  including the tube  350  as an integral part of the solid compacted pellet of organic material  380 . FIG. 4E shows the cross-sectional view of the solid compacted pellet of organic material  380  according to the present invention which is stored in a vacuum container until further use. The shape of the solid compacted pellet of organic material  380  can be varied by selecting a die and corresponding first and second punches so that outer surface can have a chosen profile. 
     Referring to FIG. 5, there is shown a schematic of the physical vapor deposition chamber  400  for OLED displays including a bell jar  500  which is kept under high vacuum, wherein a solid compacted pellet assembly  410  of organic material including a solid compacted pellet  420  of organic material is anchored to a base plate  451  of the deposition chamber  400 . A cooling tube  430  extending through the center of the solid compacted pellet  420  of organic material is connected to an inlet tube  432  and an outlet tube  434  on both ends for the purpose of transmitting cold water through the cooling tube  430  in order to keep the core of the solid compacted pellet of organic material  420  cooler with respect to the outer surface  422 . A heating element  440  of the solid compacted pellet assembly  410  of organic material is electrically connected to a power supply  450 . Provisions are also made to rotate the solid compacted pellet  420  of organic material by connecting the cooling tube  430  to a mechanical or electrical rotator  460  located outside the physical vapor deposition chamber  400 . A substrate  470  is anchored to a fixture spaced apart from the solid compacted pellet of organic material  420 . Deposition thickness of organic layers on the substrate  470  is monitored by a crystal  480  which is electrically connected to a deposition rate monitor  484 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 linear source 
               
               
                 20 
                 organic material 
               
               
                 30 
                 quartz boat 
               
               
                 40 
                 bottom heater 
               
               
                 50 
                 top heater 
               
               
                 60 
                 baffle 
               
               
                 100 
                 linear source 
               
               
                 110 
                 top heater 
               
               
                 120 
                 opening 
               
               
                 130 
                 quartz boat 
               
               
                 200 
                 solid compacted pellet assembly 
               
               
                 210 
                 solid compacted pellet 
               
               
                 220 
                 cooling tube 
               
               
                 230 
                 refractory boat 
               
               
                 240 
                 rectangular slot 
               
               
                 250 
                 heating element 
               
               
                 260 
                 lid 
               
               
                 262 
                 orifice 
               
               
                 264 
                 top surface 
               
               
                 270 
                 slits 
               
               
                 300 
                 die-punch assembly 
               
               
                 310 
                 first punch 
               
               
                 312 
                 top surface 
               
               
                 314 
                 ledge 
               
               
                 316 
                 die wall 
               
               
                 318 
                 outer surface 
               
               
                 320 
                 die cavity 
               
               
                 322 
                 first direction 
               
               
                 324 
                 recess 
               
               
                 330 
                 die 
               
               
                 332 
                 heating cartridge 
               
               
                 340 
                 seat 
               
               
                 350 
                 cooling tube 
               
               
                 360 
                 organic powder 
               
               
                 370 
                 second punch 
               
               
                 372 
                 second direction 
               
               
                 374 
                 bore 
               
               
                 380 
                 solid compacted pellet 
               
               
                 400 
                 physical vapor deposition chamber 
               
               
                 410 
                 solid compacted pellet assembly 
               
               
                 420 
                 solid compacted pellet 
               
               
                 422 
                 outer surface 
               
               
                 430 
                 cooling tube 
               
               
                 432 
                 inlet tube 
               
               
                 434 
                 outlet tube 
               
               
                 440 
                 heating element 
               
               
                 450 
                 power supply 
               
               
                 451 
                 base plate 
               
               
                 460 
                 rotator 
               
               
                 470 
                 substrate 
               
               
                 480 
                 crystal 
               
               
                 484 
                 deposition rate monitor 
               
               
                 500 
                 bell jar