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Researchers Submit Patent Application, "Low-Profile MEMS Thermal Printhead Die Having Backside Electrical Connections", for Approval

February 13, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Golda, Dariusz (Menlo Park, CA); Kim, Hyeun-Su (Palo Alto, CA); Gassend, Valerie (San Carlos, CA), filed on September 25, 2013, was made available online on January 30, 2014.

The patent's assignee is Kateeva, Inc.

News editors obtained the following quote from the background information supplied by the inventors: "The present disclosure relates generally to printheads for evaporative printing of organic materials. More specifically, the disclosure relates to a MEMS printhead fabricated from an SOI material and assembled using backside solder connections for layering organic thin film onto a substrate by sublimation of ink.

"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.

"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.

"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 electrical energy to the printing surface of a MEMS thermal printhead while enabling a sufficiently small print gap."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "The present disclosure provides a thermal, non-contact printhead die that can be positioned to within 10-100 microns of a substrate receiving ink from the printhead. The reduced print gap enables layers of organic LED compounds to be printed onto the substrate for superior film morphology, uniformity, and feature resolution.

"In one embodiment, a thermal printhead die according to the disclosure is formed on an SOI structure having a printing surface, a buried oxide layer, and a mounting surface opposite the printing surface. A plurality of ink delivery sites are formed on the printing surface, each site having an ink-receiving and ink-dispensing structure, and an ohmic heater formed adjacent to the ink-receiving and ink-dispensing structure. At least one under-bump metallization ('UBM') pad is formed on the mounting surface and electrically connected to the ohmic heater. Ink received by the ink-delivery site and heated by the ohmic heater may then be delivered to a substrate by sublimation.

"The ohmic heater may be electrically coupled to the UBM pad through the buried oxide layer by means of a through-silicon via ('TSV') plug. The TSV plug may be formed by a doping process. An interconnect metal, such as a titanium-tungsten-aluminum layer, may be formed to couple the TSV plug to the UBM pad and to the ohmic heater. Each ink delivery site may include an electrical conduction cavity formed through the printing surface, so that the interconnect metal connects the ohmic heater to the TSV plug along a sidewall of the conduction cavity and through an opening in the buried oxide layer.

"A method according to the disclosure prescribes steps for manufacturing a MEMS thermal printhead die on an SOI structure having a top surface, a buried oxide layer, and a mounting surface opposite the top surface. In one embodiment, the method provides steps for: (a) forming a plurality of ink-dispensing pores on the top surface of the SOI structure, each pore comprising a recessed area, (b) forming an ohmic heater adjacent to at least one of the ink-dispensing pores, forming at least one UBM pad on the mounting surface, and (d) forming at least one TSV plug through the buried oxide layer, the TSV plug electrically coupling the ohmic heater to the UBM pad through the buried oxide layer. The method may further include: forming through the mounting surface a plurality of ink-receiving cavities, each ink-receiving cavity corresponding to a plurality of the ink-dispensing pores, that allow flow of ink between each ink-receiving cavity and the printing surface, or forming a conduction cavity through the printing surface so that one or more layers of interconnect metal may connect the ohmic heater to the TSV plug along a sidewall of the conduction cavity.

"Another embodiment of a method according to the disclosure enables manufacturing of a MEMS thermal printing system. The manufacturing steps include: (a) forming a plurality of ink-dispensing pores on a top surface of an SOI structure, (b) forming a plurality of ohmic heaters, each ohmic heater adjacent to a corresponding plurality of ink-dispensing pores and an ink-receiving cavity, forming one or more UBM pads on the mounting surface, (d) forming a plurality of TSV plugs through a buried oxide layer of the SOI structure, each TSV plug electrically coupling one of the ohmic heaters to the one or more UBM pads, (e) dispensing a solder ball onto each of the UBM pads, (f) reflowing the solder balls, (g) contacting the reflowed solder balls to complementary pads on the mounting substrate, and (h) curing the solder balls so that the cured solder provides mechanical connection between the printhead die and the mounting substrate, and electrical connection from each complementary pad to one of the ohmic heaters through the mounting surface of the SOI structure.

BRIEF DESCRIPTION OF THE DRAWINGS

"Other systems, methods, features and advantages of the disclosure will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the disclosure, and be protected by the accompanying claims. Component parts shown in the drawings are not necessarily to scale, and may be exaggerated to better illustrate the important features of the disclosure. In the drawings, like reference numerals may designate like parts throughout the different views, wherein:

"FIG. 1 is a cross-sectional view of a portion of a thermal printhead die including an ink-delivery site according to one embodiment of the disclosure;

"FIG. 2 is a magnified cross-sectional view of the thermal printhead die of FIG. 1 showing backside electrical connections, according to another embodiment of the disclosure;

"FIG. 3 is a process flow diagram illustrating one embodiment of a method according to the disclosure for manufacturing a MEMS thermal printhead die;

"FIG. 4 is a magnified cross-sectional view of a thermal printhead die illustrating a fabrication step in which solder balls are placed onto the backside of UBM pads;

"FIG. 5 is a magnified cross-sectional view of a thermal printhead die illustrating a fabrication step after jetting, in which the solder balls are reflowed on the UBM pads;

"FIG. 6 is a magnified cross-sectional view of a thermal printhead die illustrating another fabrication step in which the printhead die is flipped and aligned with a mounting substrate;

"FIG. 7 is a magnified cross-sectional view of a thermal printhead die illustrating another fabrication step in which the die is brought into contact with the mounting substrate and the solder is reflowed a second time;

"FIG. 8 is a process flow diagram illustrating another embodiment of a method for manufacturing a MEMS thermal printhead die;

"FIG. 9 is a schematic diagram illustrating an apparatus for precision attachment of a MEMS thermal printhead die to a mounting substrate according to the disclosure; and

"FIG. 10 is a process flow diagram illustrating another embodiment of a method according to the disclosure for manufacturing a MEMS thermal printhead die."

For additional information on this patent application, see: Golda, Dariusz; Kim, Hyeun-Su; Gassend, Valerie. Low-Profile MEMS Thermal Printhead Die Having Backside Electrical Connections. Filed September 25, 2013 and posted January 30, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=2129&p=43&f=G&l=50&d=PG01&S1=20140123.PD.&OS=PD/20140123&RS=PD/20140123

Keywords for this news article include: Kateeva Inc.

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Source: Politics & Government Week


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