Patent Document

[0001]    The instant application claims priority to application Ser. No. 13/154,419, filed Jun. 6, 2011, which claims priority to provisional application No. 61/439,816, filed Feb. 5, 2011; the instant application also claims priority to application Ser. No. 12/139,409, filed Jun. 13, 2008, which itself claims priority to provisional application No. 60/944,000, filed Jun. 16, 2007; the disclosure of all the identified applications are incorporated herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The disclosure relates to printheads for evaporative printing of materials for organic light emitting device or diode (OLED). More specifically, the disclosure relates to a MEMS printhead fabricated from silicon material and assembled using backside support structure so as to define a modular printhead for ease of construction, attachment and assembly. 
         [0004]    2. Description of Related Art 
         [0005]    Organic optoelectronic devices, such as organic light emitting diodes (OLEDs) used for flat-panel displays, are fabricated by depositing layers of organic film onto a target substrate and coupling the top and bottom of the film stack to electrodes. Using advanced techniques, film layer thicknesses on the order of 100 nanometers can be achieved. 
         [0006]    One such technique deposits OLED film layers onto substrate by thermal evaporation of the organic material from a thermal printhead. The organic ink material is first dissolved in a liquid carrier to form a liquid ink. The ink is transferred to the printhead, and the target substrate and printhead are drawn into close proximity. The ink is then heated in stages. The first stage evaporates the solvent. During the second stage, the ink is heated rapidly above its sublimation temperature until the organic ink materials evaporate to cause condensation of the organic vapor onto the target substrate. The process may be repeated until a desired film layer thickness is achieved. The composition of ink may be varied to achieve different colors and to optimize other properties such as viscosity and sublimation temperature. 
         [0007]    In printing such films it also is important to deposit a dry film onto a surface so that the material being deposited forms a substantially solid film upon contact with the substrate. The solid film must have a uniform thickness. This is in contrast with ink printing where wet ink is deposited onto the surface and the ink then dries to form a solid film. Because ink printing deposits a wet film, it is commonly referred to as a wet printing method. 
         [0008]    Wet printing methods have several significant disadvantages. First, as ink dries, the solid content of the ink may not be deposited uniformly over the deposited area. That is, as the solvent evaporates, the film uniformity and thickness varies substantially. For applications requiring precise uniformity and film thickness, such variations in uniformity and thickness are not acceptable. Second, the wet ink may interact with the underlying substrate. The interaction is particularly problematic when the underlying substrate is pre-coated with a delicate film. Finally, the surface of the printed film can be uneven. An application in which these problems are resolved is critical to OLED deposition. 
         [0009]    The problem with wet printing can be partially resolved by using a dry transfer printing technique. In transfer printing techniques in general, the material to be deposited is first coated onto a transfer sheet and then the sheet is brought into contact with the surface onto which the material is to be transferred. This is the principle behind dye sublimation printing, in which dyes are sublimated from a ribbon in contact with the surface onto which the material will be transferred. This is also the principle behind carbon paper. However, the dry printing approach introduces new problems. Because contact is required between the transfer sheet and the target surface, if the target surface is delicate it may be damaged by contact. Furthermore, the transfer may be negatively impacted by the presence of small quantities of particles on either the transfer sheet or the target surface. Such particles will create a region of poor contact that impedes transfer. 
         [0010]    The particle problem is especially acute in cases where the transfer region consists of a large area, as is typically employed in the processing of large area electronics such as flat panel televisions. In addition, conventional dry transfer techniques utilize only a portion of the material on the transfer medium, resulting in low material utilization and significant waste. Film material utilization is important when the film material is very expensive. 
         [0011]    In addition, high resolution OLED displays may require pixel characteristic dimensions on the order of 100 microns or less. To achieve this degree of quality control, the printhead gap, that is, the gap between the printhead and the target substrate should be specified on an order of magnitude commensurate with the desired pixel characteristic dimensions. MEMS technology has been proposed for fabricating thermal printheads for evaporative deposition having this level of precision. One of the problems to be solved with this approach, and which is addressed by the present disclosure, is how to deliver thermal energy to the printing surface of a MEMS thermal printhead while enabling a sufficiently small print gap. 
       SUMMARY 
       [0012]    The disclosure generally relates to a modular printhead configured for print gaps less than 50 micrometers. In one embodiment, the disclosure is directed to an integrated printhead, comprising: a printhead die supporting a plurality of micropores thereon; a support structure for supporting the printhead die; a heater interposed between the printhead die and the support structure; and an electrical trace connecting the heater to a supply source. The support structure accommodates the electrical trace through an electrical via formed within it so as to form a solid state printhead containing all of the connections within and providing easily replaceable printhead. 
         [0013]    A method for constructing a printhead comprises the steps of: forming a plurality of micropores on a distal surface of a printhead die; forming a heater on the proximal surface of the printhead die; forming at least one electrical trace on a support structure; and connecting the support structure to one of the electrical heater or the printhead die through a connection joint. The support structure provides a solid state connection between an electrical supply source and the heater and wherein the connection joint provides a path for said connection. The printhead can define an integrated, solid state, device. 
         [0014]    Another embodiment of the disclosure relates to a printhead module for printing OLED material. The printhead includes a printhead die having a proximal surface and a distal surface, the distal surface defining a plurality of micropores. A heater is in thermal communication with the proximal surface of the printhead die. A support structure is also provided to receive and support and the printhead die. The support structure provides a trace for connecting the heater to a supply source. The printhead die in combination with the heater and the support structure form a printhead. In one embodiment, the distal surface of the printhead die defines a flatness tolerance of less than 20 micron. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other embodiments of the disclosure will be discussed with reference to the following exemplary and non-limiting illustrations, in which like elements are numbered similarly, and where: 
           [0016]      FIG. 1  schematically shows a substrate containing an OLED film; 
           [0017]      FIG. 2  is a schematic representation of a conventional open printhead; 
           [0018]      FIG. 3  schematically shows OLED printing using a conventional open printhead; 
           [0019]      FIG. 4A  illustrates a blind OLED printhead; 
           [0020]      FIG. 4B  schematically shows the dispensing process; 
           [0021]      FIG. 5  schematically shows a cross sectional view of an embodiment of the disclosure; 
           [0022]      FIGS. 6A and 6B  show a printing process according to an embodiment of the disclosure; 
           [0023]      FIG. 7A  shows an integrated assembly according to one embodiment of the disclosure; 
           [0024]      FIG. 7B  shows the printhead die according to one embodiment of the disclosure; 
           [0025]      FIG. 8A  shows an integrated printhead support structure according to an embodiment of the disclosure; 
           [0026]      FIG. 8B  is an exploded representation of the integrated printhead support structure of  FIG. 8A ; 
           [0027]      FIG. 8C  shows the backside of the integrated printhead assembly of  FIG. 8A ; 
           [0028]      FIG. 9  is an exemplary process for implementing an embodiment of the disclosure; and 
           [0029]      FIGS. 10A-10C  schematically represent printing an OLED film according to one embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Application of OLED material requires producing substantially flat OLED layers on the substrate. Variation in thickness can result in undesirable outcomes and post-manufacturing failures. 
         [0031]      FIG. 1  schematically shows a substrate supporting an OLED film. Specifically,  FIG. 1  shows substrate  100  having OLED film  110  thereon. As stated, it is desired to deposit the OLED film with a uniform thickness. In the schematic representation of  FIG. 1 , OLED film  110  has a flat surface. 
         [0032]      FIG. 2  is a schematic representation of a conventional OLED printhead. The OLED Printhead  210  of  FIG. 2  is positioned over substrate  200 . Printhead  210  receives OLED material  230  in liquid form. In one embodiment, OLED material  230  is defined by solid OLED particles which are suspended and/or dissolved in a carrier liquid. Conventional carrier liquids include solvents that may have low vapor pressure. 
         [0033]    Printhead  210  may include one or more resistive heaters. In  FIG. 2 , resistive heaters  220  are shown on the distal surface of printhead  210 . While the schematic representation of  FIG. 2  shows resistive elements  220  on the distal (bottom) surface of printhead  210 , it is possible to place the heater on the proximal (top) surface. It is also possible to include resistive heaters on both the proximal and the distal surfaces of the printhead. In an operative embodiment, liquid material  230  is received at the proximal surface of printhead  210  and is drawn into micropores  212 . Resistive heaters  220  remove substantially all the carrier fluid from OLED material  230  and evaporate the solid OLED material prior to dispensing. This process is schematically shown in  FIG. 3  where OLED vapor  330  is dispensed from printhead  310 . The OLED particles can be vaporized prior to dispensing. The sublimated particles are received at substrate  300  which is cooler than the distal surface of printhead  310  (and heaters  320 ). The difference in temperature causes OLED material  330  to condense on substrate  300 , thereby forming OLED film  340 . 
         [0034]      FIG. 4A  illustrates a conventional blind OLED printhead. As compared to the printhead shown in  FIG. 3 , the blind printhead has micropores that are closed at one end. In  FIG. 4A , micropores  412  are formed as blind micropores in printhead  410 . Heaters  420  are positioned on the proximal surface of printhead  410 . Heaters  420  communicate via wire-bond line  440  with voltage source  450 . 
         [0035]    In operation, blind pores  412  of printhead  410  receive OLED material  460  which includes a quantity of solid particles suspended and/or dissolved in a carrier liquid. OLED material  460  is drawn into micropores  412  through the capillary action of the micropores. Heaters  420  are then activated and printhead  410  is then rotated about an axis (or positioned relative to the substrate) to face substrate  400  as shown by the arrow.  FIG. 4B  schematically shows the dispensing process from blind pores of printhead  410 . Here, heaters  420  have evaporated the carrier fluid from the OLED material  460 . In addition, the solid particles have been substantially evaporated. Upon facing substrate  400 , vaporized OLED material  460  dispenses from the blind micropores and condenses on substrate  400  to form OLED film  470 . 
         [0036]    As stated, optimal manufacturing requires OLED film  470  to have a uniform thickness and sufficient print resolution. OLED material  460  ejects from printhead  410  and spreads laterally as it approaches the substrate. The proximity of printhead  420  to substrate  400  can directly affect the thickness uniformity and resolution of OLED film  470 . It is therefore desired to minimize the print gap to distances that are comparable to the edge spreading of acceptable printed features which may be approximately 20-30 micrometers for typical applications. Print gaps of less than 50 micrometers preclude the use of wire-bonds or other protruding features from the distal surface of the printhead. 
         [0037]      FIG. 5  schematically shows one embodiment of the disclosure. In contrast with a conventional printhead, the embodiment of  FIG. 5  defines an integrated device which can be readily assembled (or disassembled) to a large array of printers without having wire-bonds protruding from the printhead surface. Wire-bonds also preclude the printing of OLED material at small gaps as required to attain sufficient feature resolution and thickness uniformity. In  FIG. 5 , printhead  510  is shown with blind pores  512  on its distal surface. Printhead  510  can be formed from a silicon die. Resistive heaters  550  are connected to the proximal surface of the printhead. Support structure  520  is coupled to the printhead/heaters through a series of interconnects, including, under-bump metal  526 , solder bump  524  and package pad  522 . The interconnects provide electrical connection between resistive heaters  550  and power supply  560  through connection lines  562  and  564 . Support structure  520  can comprise one or more of ceramic or semiconductor material. Support structure  520  includes internal electrical traces  542  and  544  which communicate with connection lines  562  and  564 , respectively. 
         [0038]    Controller  590  can comprise one or more processor circuits (not shown) and one or more memory circuits (not shown) for controlling voltage source  560 . Controller  590  controls heating of one or a plurality of printheads. For this purpose, the memory circuit can contain instructions for the processor to heat the OLED material to substantially remove all of the carrier fluid, and to activate the heater to vaporize and eject solid OLED particles from micropores  512  onto the substrate. In an exemplary embodiment, the memory circuit may also contain instructions to bring the printhead into close proximity with the print substrate. Such instructions can be implemented with the aid of a processor circuit and one or more actuation systems in which one or both of the substrate and/or the printhead are positioned (aligned) relative to each other prior to printing. 
         [0039]      FIGS. 6A and 6B  schematically show a printing process according to an embodiment of the disclosure. In  FIG. 6A , printhead die  610  is positioned to receive OLED material  605  at micropores  612 . OLED material  605  includes solid and dissolved OLED particles in a carrier solution. Integrated system  601  includes printhead die  610 , support structure  620 , heater  650 , UMB metals  626 , solder balls  624 , package pads  622  and electrical contacts  630 . Support structure  620  includes traces  640  for connecting heater  650  to a supply source (not shown) and/or a controller (not shown). Upon receiving OLED material  605 , resistive heaters  650  can be activated to evaporate substantially all of the carrier liquid from the distal surface of the printhead. 
         [0040]    Gaps  613  are formed in printhead die  610  to form an isolated structure and to reduce the thermal mass of the heated portions of the printhead. Thus, the ink receiving portion of the printhead can be heated quickly and efficiently. It should be noted that gaps  613  appear only in the profile of the structure of  FIG. 6 . A planar view of the integrated printhead will be discussed below. 
         [0041]    UMB metal  626 , solder ball  624  and package pad  622  can be made conductive in order to communicate electrical power to resistive heaters  650 . In accordance with an embodiment of the disclosure, integrated system  601  defines a solid state system which excludes wire-bonds. 
         [0042]    Referring to  FIG. 6B , the integrated printhead is now positioned and aligned relative to the substrate such that the distal surface of the printhead is facing substrate  600 . In one embodiment of the disclosure, after the alignment, resistive heaters can be used to evaporate any OLED material remaining in the micropores. The evaporated OLED material  607  condenses on substrate  600  to form OLED film  609 . 
         [0043]    As stated, it is highly desirable to deposit OLED film  607  with uniform thickness and sufficient feature definition. Applicant has discovered that the thickness uniformity and print resolution of the OLED film  607  is directly related to the gap  611  spanning between the distal surface of printhead die  610  and substrate  600 . In an embodiment of the disclosure, gap  611  is configured to be less than 30 micron. In a preferred embodiment, the gap is in the range of 25-30 microns. In another preferred embodiment, the gap is less than 20 microns. In an exemplary embodiment, the distal surface of the printhead die defines a flatness tolerance of less than 20 micron. In another exemplary embodiment, the distal tolerance is in the range of 5-10 microns. 
         [0044]      FIG. 7A  shows an integrated assembly according to one embodiment of the disclosure. The integrated assembly  700  includes support structure  760 , solder bump joints  750 , suspension structure  720  and suspended platform  710 . As stated, support structure  760  can be formed from ceramic material. Insulation trenches  770  are also shown in  FIG. 7A , corresponding to the gaps  513  and  613  shown in  FIGS. 5 and 6 , respectively. The suspended structure  720  enables thermally insulating the micropores in the printhead from the remaining portions of the solid mass. 
         [0045]      FIG. 7B  shows the printhead die according to one embodiment of the disclosure. Printhead die  705  shows suspended platform  710 , suspension structure  720 , heater metal  730 , and UBM pad  740 . It can be readily seen from  FIGS. 7A and 7B  that the assembly of printhead die  705  onto a support structure  760  will include coupling the two parts and reflowing solder bump joints  750 . Thus, the disclosed embodiment results in a significant manufacturing efficiency. 
         [0046]      FIG. 8A  shows an integrated printhead support structure according to an embodiment of the disclosure. The integrated printhead support structure  800  is shown with flanges  810  at both ends thereof which provide a retention system. Printhead support structure  800  is also shown with bond pads  820  which can be made from gold, silver or any other electrically conductive material that may be wetted by solder. 
         [0047]      FIG. 8B  is an exploded representation of the printhead support structure of  FIG. 8A . Here, bond pads  820  are shown on the top layer of the integrated printhead assembly, followed by a first set of electric vias  830 , internal routing traces  840  and a second set of electric vias  850 . As seen from  FIGS. 8A and 8B  the modular design can provide significant manufacturing efficiency. The layered design can be made with alumina and tungsten electrical traces. In an exemplary embodiment, the layers are screen-printed. Jetted solder balls or electroplated solder can be used for eutectic reflow die attach. In one embodiment, solder balls with an approximate composition of 95 wt. % tin, less than 5 wt. % of silver, and less than 1 wt. % tin were used to provide electrical, thermal and mechanical connections. The solder balls can be designed to provide a separation in a range of about 100-400 μm gap between the printhead and the support structure. In another embodiment, the gap is in a range of about 200-300 μm. 
         [0048]      FIG. 8C  shows the underside of the support structure shown in  FIG. 8A . Specifically,  FIG. 8C  shows electrical contact pads  870  which allow controller ( 590 ,  FIG. 5 ) and/or power supply ( 560 ,  FIG. 5 ) to communicate with the printhead. Flange  810  is also shown in  FIG. 8 . Heat generated by heater  730  is transferred from the integrated printhead to a heat sink through thermal contact area  880 . 
         [0049]      FIG. 9  is an exemplary process for implementing an embodiment of the disclosure. The process of  FIG. 9  starts with step  910  in which one or more micropores are formed in a die. In one embodiment the micropore is a blind micropore. In another embodiment, the micropore defines a cavity which connects the top and the bottom surfaces of the die. In still another embodiment, a series of blind and complete micropores are formed on a silicon die. In step  920 , a resistive heater is formed on one or both surfaces of the die. The resistive heater can be made from conventional material. In step  930 , a support structure is formed using a ceramic material in which the support structure is configured with electrical traces for communicating with the resistive heater. In step  940 , the support structure is coupled to the printhead using one or more connection joints. The connection joints can be formed by providing an under-bump metal on the heater (UBM), depositing a solder bump on the UBM and interposing a package pad between the solder bump and the support structure. When joined, the printhead and the support structure form a solid state, integrated unit without extending wire-bonds In addition, because the printhead provides a solid state unit, there are no wires on the distal end of the printhead; this enables positioning the printhead in an exceptionally close proximity to the substrate, which in turn allows printing significantly improved flat OLED films. 
         [0050]      FIGS. 10A-10C  schematically represent printing an OLED film according to one embodiment of the disclosure. In  FIG. 10A-10C , the printhead is a stationary relative to the OLED supply and the substrate. Referring to  FIG. 10A , OLED supply  1010  dispenses OLED droplet  1015  onto a surface of printhead  1030 . While not shown, the printhead includes one or more micropores and resistive heaters (including a control circuit) as described above. Printhead  1030  is held stationary by chucks  1020 . In  FIG. 10B , glass panel substrate  1050  is positioned above the printhead. In addition, the heaters associated with printhead  1030  are activated so as to evaporate substantially all of the carrier liquid contained in OLED droplet  1015  ( FIG. 1 ). In  FIG. 10C , resistive heaters cause evaporation of the OLED material from printhead  1030  and its condensation on substrate  1050 . During the process shown in  FIGS. 10A-10C , printhead  1030  remains stationary while OLED supply  1010  and substrate  1050  are moved and aligned relative to the printhead. Film  1055  which forms on printhead  1050  defines an OLED film with desired thickness and surface topography. 
         [0051]    While the principles of the disclosure have been illustrated in relation to the exemplary embodiments shown herein, the principles of the disclosure are not limited thereto and include any modification, variation or permutation thereof.

Technology Category: h