Any and all applications, if any, for which a foreign or domestic priority claim is identified in the Application Data Sheet of the present application are hereby incorporated by reference under 37 CFR 1.57.
This invention generally relates to device assembly on a printed circuit board (PCB) and, more particularly, to a fluidic assembly method for the fabrication of emissive panels on PCBs using negative pressure features.
The fluidic transfer of microfabricated electronic devices, optoelectronic devices, and sub-systems from a donor substrate/wafer to a large area and/or unconventional substrate provides a new opportunity to extend the application range of electronic and optoelectronic devices. For example, display pixel size light emitting diode (LED) micro structures, such as rods, fins, or disks, can be first fabricated on small size wafers and then be transferred to large panel substrate to make a direct emitting display. One conventional means of transferring these LED microstructures is through a pick-and-place process. However, with a display comprising millions of elements, such a process may take several hours to complete and is therefore inefficient.
The fluidic self-assembly of electronic devices, such as LEDs and photovoltaics, is often performed by surface energy minimization at molten solder capillary interfaces so that both mechanical and electrical connections can be made to an electrode during assembly, as demonstrated in U.S. Pat. No. 7,774,929. In one aspect, electronic devices are captured in shape-matched well structures, followed by electrical integration processes, as demonstrated in U.S. Pat. No. 6,316,278.
Some problems yet to be addressed with conventional fluidic assembly processes are related to the distribution method over large scales, the integration of microcomponents to drive circuitry over large areas, and the potential mechanisms for the repair of defective microcomponents. Over large scales, conventional fluidic assembly into wells is challenged by the dual requirements of maximum velocities for microcomponent capture and minimum distribution velocities for high-speed array assembly. Similarly, achieving the microcomponent dispensing scheme and flow velocity uniformity necessary for a high yield over the whole assembly substrate becomes very challenging over greater-than-centimeter scales.
An LED is considered as a spot light source with light radiating from a point, much like a point source. A spot light, however, limits the illumination to light within a specified cone or beam of light only. In many applications, it is desirable to use LEDs for uniform illumination over an area. It is common to use an array of LEDs to make such an area light source. For example, in an LED direct back lit liquid crystal display (LCD), an array of LEDs is placed behind an LCD display. Diffuser films or brightness enhancing film can be inserted between the LED array and LCD display to improve the backlight uniformity.
Micro-LED (μLED) process and devices have been disclosed previously in an application entitled, DISPLAY WITH SURFACE MOUNT EMISSIVE ELEMENTS, invented by Schuele et al., filed Jan. 19, 2017, Ser. No. 15/410,001, and in an application entitled, SYSTEM AND METHOD FOR THE FLUIDIC ASSEMBLY OF EMISSIVE DISPLAYS, invented by Sasaki et al, Ser. No. 15/412,731, filed Jan. 23, 2017, which are incorporated herein by reference. Although various substrates can be used for μLED devices, glass is typically the default substrate. This makes sense since it is industrial standard to fabricate thin film transistor (TFT) circuitry on a glass substrate. However, some μLED applications, such as a backlight unit for LCD displays (BLU) and flat panel lighting features, can be configured as passive matrix arrays, which do not need TFT circuits to drive the μLEDs. For these applications, a glass substrate is not required and there would be the advantages of lower cost and increased flexibility if alternate types of substrates could be used.
It would be advantageous if the substrates could be designed with features to enhance the capture of emissive elements using a negative pressure.
Disclosed herein is a light emitting diode (LED) array that is fabricated on a printed circuit board (PCB) substrate, which may be flexible. The substrate simply requires two layers of metal wiring to enable the LED array, one on the front surface and the other on the back surface of the substrate. A via hole is made by laser drilling prior the metal formation. During the metal formation process, the via hole is partially filled with the metal to enable electrical interconnection between front surface metal and back surface metal. The via holes also serve as pressure channels through the substrate, which enables control of gas or fluid flow by applying a pressure differential across the substrate.
Accordingly, a fluidic assembly method is provided for the fabrication of emissive panels. The method provides an emissive substrate having an insulating layer with a top surface and a back surface, and a dielectric layer overlying the insulating layer top surface patterned to form a plurality of wells. Each well has a bottom surface formed on the insulating layer top surface with a first electrical interface electrically connected to a first conductive pressure channel. The conductive pressure channels are each made up of a pressure via with sidewalls formed between the well bottom surface and the insulating layer back surface. A metal layer coats the sidewalls, and a medium flow passage formed interior to the metal layer. Each pressure via has a minimum cross-sectional area, and the medium flow passage has a minimum cross-sectional area greater than 50% of the pressure via minimum cross-sectional area.
The first electrical interface is directly or indirectly connected through intervening elements to a conductive first matrix trace. The emissive substrate also includes a conductive second matrix of traces. The first matrix traces may be formed on either the insulating layer top surface or the insulating layer bottom surface, with the second matrix traces being formed on the opposite insulating layer surface. The first and second matrices are control lines used to selectively enable individual emissive elements or groups of emissive elements.
The method flows a liquid suspension of emissive elements across the dielectric layer, and applies a negative pressure, from the insulating layer back surface to the wells, via the first conductive pressure channels. The emissive elements have a first electrical contact formed on their top surfaces and, optionally, a post connected to, and extending from their bottom surfaces. The emissive elements are captured in the wells in response to the negative pressure and liquid suspension flow, with their top surfaces facing the well bottom surfaces. After capture, the emissive substrate is annealed. In response to the annealing, the first electrical contact of each emissive element is connected to the first electrical interface of a corresponding well. In one aspect, prior to annealing the emissive substrate, solder flux is introduced to each first electrical interface through a corresponding first conductive pressure channel.
In one type of emissive substrate, each well bottom surface includes a second electrical interface electrically connected to a second conductive pressure channel formed between the insulating layer top and back surfaces. The second matrix traces are directly or indirectly connected to corresponding second conductive pressure channels. In this aspect the emissive elements have a top surface with both the first electrical contact and a second electrical contact, and the second electrical contact becomes connected to the second electrical interface when the emissive substrate is annealed.
In another type of emissive substrate, the first matrix traces are formed on the insulating layer bottom surface, and each well bottom surface additionally includes a second electrical interface electrically connected by a conductive intralevel trace, formed on the insulating layer top surface, to a corresponding second matrix trace. The emissive elements have a top surface with the first electrical contact and a second electrical contact, and the second electrical contact becomes connected to the second electrical interface when the emissive substrate is annealed.
In yet another type of emissive substrate, the first matrix traces are formed on the insulating layer bottom surface, and the dielectric layer has an intersection via associated with each well, exposing a corresponding second matrix trace on the insulating layer top surface. In this version the emissive elements have a top surface with the first electrical contact and a bottom surface with a second electrical contact. Subsequent to annealing the emissive substrate, when the first electrical contact is connected to the first electrical interface, a local interconnect is from the second electrical contact of each emissive element to the corresponding second matrix trace on the insulating layer top surface through a corresponding intersection via.
After connecting the emissive elements electrical contacts to the emissive substrate, a color modifier may be formed overlying the top surface of the dielectric and, optionally, a liquid crystal display (LCD) panel may be formed overlying the top surface of the color modifier.
Additional details of the above-described method and an emissive panel apparatus are provided below.
A control matrix comprising a conductive first matrix of traces 122 is formed on the insulating layer top surface 106 (
Surface mount emissive elements (SMEEs) 126 populate the wells 112. Each emissive element 126 comprises a top surface 128 overlying a corresponding well bottom surface 114, and a bottom surface 130. A first electrical contact 132 is formed on the emissive element top surface 128 and is connected to a corresponding well first electrical interface 118. In one aspect as shown, each SMEE 126 has a post 134 extending from, and connected to its bottom surface 130. In one aspect as shown in
The emissive elements described above may be light emitting diodes (LEDs) or micro LEDs (μLEDs). Since a μLED is a two-terminal device, making a μLED array requires two layers of metal wiring (e.g., 2 control matrices). It is common to use monolithic integration processes in a thin-film transistor (TFT) LCD fabrication facility (fab) to make two layers of metal with an insulating layer between them. For this process, the two layers of metal and one layer of insulator are all deposited on top of a substrate. The substrate can be glass, ceramic, polyimide (PI) film, or similar.
However, there is a cheaper way to make substrates with multiple layer wiring using printed circuit board (PCB) technology. PCBs are commonly used for mechanical support and electrical connections between electronic components using conductive tracks, pads, and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs can be single sided (one copper layer), double sided (two copper layers), or multi-layer (outer and inner layers). Conductors on different layers are connected by vias between layers. Some exemplary PCB materials include polychlorinated biphenyl, polyimide, polyether ether ketone (PEEK), polyester, polyethylene terephthalate (PET) and any other materials commonly used for printed circuit board (PCB) and flex printed circuits board (FPC).
The dimension of a typical μLED is in the range of 3-150 microns (μm), and the dimensions of the metal interconnects are usually in a similar range to arrange the substrate electrodes that form contacts to the μLED. The required metal dimensions can be easily formed using monolithic photolithography processes in integrated circuit (IC) and LCD fabs. It is an advantage if μLEDs can be assembled on the PCB.
The via hole that connects the PCB front surface metal and back surface metal is also a common PCB feature. However, the via hole is usually filled with metal (the most common metal being copper (Cu)) and the front surface metal and back surface metal thickness are in the range of 5 μm to 20 μm. To enable the μLED array described above, conventional PCB processes are modified so that the via is partially filled by metal, with an opening in each well.
In
In
In
As an alternative to
Using the emissive substrate of
After the fluidic assembly, clean-off and drying steps are performed, and the electrical connections between the μLED electrodes and substrate electrodes (emissive panel electrical interfaces) are formed by a process similar to soldering. The substrate electrodes are conductive metals, which may be gold, molybdenum, titanium, tungsten, silver, indium, tin, or copper, including layered and alloyed combinations. Similarly, the electrodes on the μLED anode and cathode may be composed of gold, molybdenum, titanium, tungsten, silver, indium, tin, or copper, including layered and alloyed combinations. The choice of metals on each component is chosen for conductivity and manufacturability, but mainly for formation of a bond and an electrical connection between the μLED and the substrate.
As noted above, the mass of the μLED is very small so the downward force causing intimate contact between the two surfaces to be bonded is very low. In conventional surface mount soldering the component is much larger and heavier than the μLED 126, so the conventional component weight is sufficient to establish physical contact leading to soldering when the two electrodes are heated. The use of differential pressure to force flow and apply downward force on the μLED, as shown in
Step 1902 provides an emissive substrate comprising an insulating layer with a top surface and a back surface, and a dielectric layer overlying the insulating layer top surface patterned to form a plurality of wells. Each well comprises a bottom surface formed on the insulating layer top surface with a first electrical interface electrically connected to a first conductive pressure channel, which is operatively connected to a conductive first matrix trace. As noted above, each conductive pressure channel comprises a pressure via with sidewalls formed between the well bottom surface and the insulating layer back surface. A metal layer coats the sidewalls and a medium flow passage is formed interior to the metal layer. Each pressure via has a minimum cross-sectional area, and the medium flow passage has a minimum cross-sectional area greater than 50% of the pressure via minimum cross-sectional area.
The emissive substrate further comprises a conductive second matrix of traces, where first matrix traces are formed on either the insulating layer top surface or the insulating layer bottom surface, and the second matrix traces are formed on the opposite insulating layer surface.
Step 1904 flows a liquid suspension of emissive elements across the dielectric layer. The liquid may, for example, be one of a number of types of alcohols, polyols, ketones, halocarbons, or water. In one aspect the emissive elements may be indium gallium nitride (InGaN), GaN, aluminum gallium indium phosphide (AlGaInP), or aluminum gallium nitride (AlGaN) LEDs. In another aspect, flowing the liquid suspension across the emissive substrate top surface includes engaging an auxiliary mechanism for distributing the emissive elements, such as a brush (rotating or non-rotating), wiper, rotating cylinder, or mechanical vibration. In another aspect, the emissive elements have a post, or more than one post, extending from, and connected to the emissive element bottom surface.
Step 1906 applies a negative pressure, from the insulating layer back surface to the wells, via the first conductive pressure channels. Step 1906 may be enabled by locating the insulating layer back surface overlying a porous support substrate and applying the negative pressure through the support substrate. Alternatively, the emissive substrate may be attached to a frame perimeter with a center opening, and the negative pressure applied through the frame opening. Step 1908 captures the emissive elements in the wells. In one aspect, the emissive elements are captured in response to the combination of the negative pressure and the suspension flow. Step 1910 anneals the emissive substrate, and in response to the annealing, Step 1912 electrically connects the first electrical contact of each emissive element to the first electrical interface of a corresponding well. In one aspect, Step 1909a removes solvent from the liquid suspension and dries the emissive substrate in response to negative pressure. In another variation, prior to annealing the emissive substrate in Step 1910, Step 1909b introduces a solder flux to each first electrical interface through a corresponding first conductive pressure channel. The fluxing agent may be a dimethylammonium chloride, diethanolamine, and glycerol solution dissolved in isopropanol, an organic acid, or a rosin-type flux, for example.
In one aspect, Step 1902 provides an emissive substrate where each well bottom surface additionally comprises a second electrical interface electrically connected to a second conductive pressure channel formed between the insulating layer top and back surfaces, with the second matrix traces operatively connected to corresponding second conductive pressure channels (see
More explicitly, providing the emissive substrate in Step 1902 may include the following substeps. Step 1902a provides the insulating layer. Step 1902b forms an array of pressure vias from the insulating layer top surface to the insulating layer back surface. Step 1902c deposits metal overlying the insulating layer top surface, insulating layer back surface, and pressure vias, wherein the pressure vias are coated with metal, leaving a medium flow passage. Step 1902d patterns the insulating layer top surface metal layer to expose well bottom surfaces and form the first matrix of traces, and patterns the insulating layer back surface to form the second matrix of traces. Step 1902e forms the patterned dielectric layer overlying the insulating layer top surface exposing the well bottom surfaces.
In alternative substeps, Step 1902f forms a photo-sensitive (PR) material overlying the insulating layer top surface and back surface, patterned to expose the pressure vias, first matrix trace regions, and second matrix trace regions. Step 1902g deposits metal overlying the insulating layer top surface, insulating layer back surface, and pressure vias. The pressure vias are coated with metal, leaving a medium flow passage.
In another aspect, Step 1902 provides an emissive substrate where the first matrix traces are formed on the insulating layer bottom surface and electrically connected to the first CPC, and each well bottom surface additionally comprises a second electrical interface electrically connected by a conductive intralevel trace, formed on the insulating layer top surface to a corresponding second matrix trace (see
In another variation, Step 1902 provides an emissive substrate with the first matrix traces being formed on the insulating layer bottom surface, and the dielectric layer has an intersection via associated with each well, exposing a corresponding second matrix trace. Flowing the liquid suspension of emissive elements in Step 1904 includes flowing vertical emissive elements having a top surface with the first electrical contact and a bottom surface with a second electrical contact. Subsequent to annealing the emissive substrate in Step 1910, Step 1914 forms a local interconnect from the second electrical contact of each emissive element to the corresponding second matrix trace on the insulating layer top surface through a corresponding intersection via (see
In one aspect, subsequent to electrically connecting the first electrical contacts to the first electrical interfaces in Step 1912, Step 1916 forms a color modifier overlying the emissive elements. As another option, subsequent to forming the color modifier in Step 1916, Step 1918 forms a liquid crystal display (LCD) panel overlying a top surface of the color modifier.
An emissive panel and associated emissive panel assembly processes have been presented. Examples of particular materials, dimensions, and circuit layouts have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.
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