Abstract:
A method for metering powdered or granular material onto a heated surface to vaporize such material. The method comprises providing a rotatable auger d for receiving powdered or granular material and as the rotatable auger rotates, such rotatable auger translates such powdered or granular material along a feed path to a feeding location. The method also providing at least one opening at the feeding location such that the pressure produced by the rotating rotatable auger at the feeding location causes the powdered or granular material to be forced through the opening onto the heated surface in a controllable manner. The material is agitated or fluidized proximate to the feeding location.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. Pat. No. 7,232,588 issued Jun. 19, 2007, by Michael Long et al, entitled “Device and Method for Vaporizing Temperature Sensitive Materials”, U.S. Pat. No. 7,288,285 issued Oct. 30, 2007, by Michael Long et al, entitled “Delivering Organic Powder to a Vaporization Zone”, U.S. Pat. No. 7,288,286 issued Oct. 30, 2007, by Michael Long et al, entitled “Delivering Organic Power to a Vaporization Zone”, U.S. Patent Publication No. 2006/0177576 published Aug. 10, 2006, by Michael Long et al, entitled “Controllably Feeding Organic Material in Making OLEDS”, and U.S. Pat. No. 7,213,347 issued May 8, 2007, by Michael Long et al, entitled Metering Material To Promote Rapid Vaporization” the disclosures of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to making devices by vaporizing material and more particularly to controllably feeding material to a heated surface. 
     BACKGROUND OF THE INVENTION 
     An organic light emitting diode (OLED) device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211. 
     Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate dependant vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties. 
     To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and they are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. In the prior art, it has been necessary to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber and degas the just-introduced organic material over several hours before resuming operation. The low deposition rate and the frequent and time consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities. 
     A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the dopant is very close to that of the host material. This is generally not the case and as a result, prior art devices frequently require the use of separate sources to co-deposit host and dopant materials. 
     A consequence of using single component sources is that many sources are required in order to produce films containing a host and multiple dopants. These sources are arrayed one next to the other with the outer sources angled toward the center to approximate a co-deposition condition. In practice, the number of linear sources used to co-deposit different materials has been limited to three. This restriction has imposed a substantial limitation on the architecture of OLED devices, increases the necessary size and cost of the vacuum deposition chamber and decreases the reliability of the system. 
     Additionally, the use of separate sources creates a gradient effect in the deposited film where the material in the source closest to the advancing substrate is over represented in the initial film immediately adjacent the substrate while the material in the last source is over represented in the final film surface. This gradient co-deposition is unavoidable in prior art sources where a single material is vaporized from each of multiple sources. The gradient in the deposited film is especially evident when the contribution of either of the end sources is more than a few percent of the central source, such as when a co-host is used. 
     A further limitation of prior art sources is that the geometry of the interior of the vapor manifold changes as the organic material charge is consumed. This change requires that the heater temperature change to maintain a constant vaporization rate and it is observed that the overall plume shape of the vapor exiting the orifices can change as a function of the organic material thickness and distribution in the source, particularly when the conductance to vapor flow in the source with a full charge of material is low enough to sustain pressure gradients from non-uniform vaporization within the source. In this case, as the material charge is consumed, the conductance increases and the pressure distribution and hence overall plume shape improve. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an effective way to vaporize powders. 
     This object is achieved in a method for metering powdered or granular material onto or in close proximity to a heated surface to vaporize such material, comprising: 
     (a) providing a rotatable auger for receiving powdered or granular material and as the rotatable auger rotates, such rotating rotatable auger translates such powdered or granular material along a feed path to a feeding location; 
     (b) providing at least one opening at the feeding location such that the pressure produced by the rotating rotatable auger at the feeding location causes the powdered or granular material to be forced through the opening onto the heated surface in a controllable manner, and 
     (c) agitating or fluidizing the powdered or granular material in proximity to the feeding location in cooperation with the rotatable auger so as to facilitate the flow of powdered or granular material through the opening(s) to the heated surface where the powdered or granular material is vaporized. 
     An advantage of this invention is that it provides controlled delivery of powdered or granular material with reduced expenditures of power. Feed uniformity is substantially improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of one embodiment of the invention; 
         FIG. 2  is a block diagram of a closed-loop control for the invention; 
         FIGS. 3A and 3B  show detail cross-sectional perspectives of an alternative embodiment of the invention; 
         FIG. 4  is a detail cross-section perspective of another alternative embodiment of the invention; and 
         FIG. 5  is a detail cross-sectional perspective of still another alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , an apparatus  5  for metering powdered or granular material  10  such as organic material into a heated surface  40  is shown. The apparatus  5  is includes a container  15  which holds material  10 . Material  10  can have one or more components and can be powdered or granular. A rotatable auger  20  is disposed in an auger enclosure  22  which in turn is disposed in a material receiving relationship with the container  15 . The auger enclosure  22  has openings  24  for receiving material  10  from the container  5 . The rotatable auger  20  moves material  10  along a feed path  25  to a feeding location  30 . Rotation of the rotatable auger  20  causes the material  10  to be subject to pressure at the feeding location  30 . This pressure forces the material  10  through one or more openings  35  formed in a member  36 . Member,  36  can be attached to the rotatable auger  20  so that the member  36  rotates with the rotatable auger  20 , and carries material  10  into contact with a heated surface  40  where the material  10  is flash evaporated. The rotation of member  36  provides agitation or fluidization of material  10  in the proximity to the openings  35 , reducing the tendency of the material  10  to compact into an agglomerated solid inside the auger enclosure  22  or heat sink  42  that would restrict material flow. The proximity of the feeding location  30  to the heated surface  40  can cause the feeding location to be heated by radiation and the auger enclosure  22  by conduction from the feeding location  30 . It can be desirable to coat the feeding location  30  and the openings  35  in member  36  with a thermally insulating layer such as anodization or a thin layer of glass or mica. Additionally, the feeding location  30  can be made of a material of high thermal conductivity and provided with a thermally conductive path to a heat sink  42 . The heat sink  42  can be a passive device that depends on radiation or convection to a fluid, or it can be an active cooling device such as a Peltier effect chiller. Insulating the feeding location  30  can reduce condensation of vaporized material in the feeding location  30 , especially around the openings  35 . Providing a conductive path to heat sink  42 , reduces thermal exposure of material  10 , and thereby improves material lifetime within the auger enclosure  22 . 
     The apparatus  5  can operate in a closed-loop control mode, in which case a sensor  50  is utilized to measure the vaporization rate of the material  10  as it is evaporated at the heated surface  40 . The sensor  50  can also be used in measuring the material vaporization rate on a substrate either directly or indirectly. For example, a laser can be directed through the plume of evaporated material to directly measure the local concentration of vaporized material. Alternatively, crystal rate monitors indirectly measure the vaporization rate by measuring the rate of deposition of the vaporized material on the crystal surface. These two approaches represent only two of the many well-known methods for sensing the vaporization rate. 
     Turning now to  FIG. 2 , the apparatus  5  can be operated under closed-loop control which is represented by block diagram. In a close-loop control system, the sensor  50  provides data to a controller  55 , which in turn determines the rate of revolution of a motor  45 . The closed loop control can take many forms. In a particularly preferred embodiment, the controller  55  is a programmable digital logic device, such as a microcontroller, that reads the input of the sensor  50 , which can be either analog input or direct digital input. The controller  55  is operated by an algorithm that utilizes the sensor input as well as internal or externally derived information about the motor  45  rotational speed and the temperature of the heated surface  40  to determine a new commanded speed for the rotatable auger  20  and a new commanded temperature for the heated surface  40 . There are many known classes of algorithm, such as proportional integral differential control, proportional control, differential control, that can be adapted for use suited to control the apparatus  5 . The control strategy can employ feedback as well as feedforward. Alternatively, the control circuit can be implemented as an analog control device, which can implement many of the same classes of algorithm as the digital device. 
       FIGS. 3A and 3B  show different perspectives of the detail of an alternative embodiment. The portion of the embodiment not shown are essentially the same as those of  FIG. 1 . This embodiment differs in how the material  10  at the end of the rotatable auger  20  is fluidized or agitated. A clockwork spring  60  is attached to the rotatable auger  20  so that it rotates with the rotatable auger  20 , agitating or fluidized material  10  in the vicinity of the member  36  containing the openings  35 . The member  36  may be rigidly affixed to the auger enclosure  22  or may instead be constrained to rotate with the rotatable auger  20 . By maintaining an agitated or fluidized region of material  10  in the immediate proximity of the member  36 , the tendency of the material  10  to compact into an agglomerated solid inside the auger enclosure  22  is reduced. 
       FIG. 4  shows a detail view of yet another embodiment of the invention. In this embodiment, the rotatable auger  20  terminates in a spreader  65  which rotates with the rotatable auger  20 . The spreader  65  is a cone-shaped member that spreads the material  10  away from the shaft of the rotatable auger  20  towards the opening  35 . The single opening  35  is in the form of an annulus and is formed between the spreader  65  on the inside and heat sink  42 . Heat sink  42 , is rigidly attached to the auger enclosure  22 . The rotation of the spreader  65  within the heat sink  42 , sets up a shear in the material, causing agitation and reducing the tendency of the material  10  to compact into an agglomerated solid inside the auger enclosure  22  or the heat sink  42 . 
       FIG. 5  shows a detail of another embodiment of the invention. In this embodiment, the openings are provided by a fine screen  75 . A vibratory actuator  70  imparts vibrational energy to the screen  75  agitating or fluidizing the material  10  in the feeding location  30 . The direction of the vibration may be co-axial to the rotatable auger  20 , perpendicular to the axis of the rotatable auger  20 , or both co-axial or perpendicular. Fluidized material  10  is forced through the screen  75  by the rotation of the rotatable auger  20 . Material  10  passing through the screen  75  then encounters the heated surface  40  which is spaced a short distance from the screen  75 . This distance is typically on the order of 50-100 microns, but could be larger or smaller depending on particle size of the material being fed, the size of the openings in the screen  75 , and other factors. 
     It is understood by those of ordinary skill in the art that although the invention is motivated by the need to reduce the time organic materials spend at elevated temperature and is described in the context of vaporization of organic materials, the invention is suitable for vaporization of any powdered or granular material. 
     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 
     
         
           5  Apparatus 
           10  Organic material 
           15  Container 
           20  Rotatable auger 
           22  Auger enclosure 
           24  Auger enclosure opening 
           30  Feeding location 
           35  Opening 
           36  Member 
           40  Heated surface 
           42  Heat sink 
           45  Motor 
           50  Sensor 
           55  Controller 
           60  Clockwork spring 
           65  Spreader 
           70  Vibratory actuator 
           75  Screen