Patent Application
20240021757
This application claims the benefit of priority to Japanese Patent Application No. 2022-113186, filed on Jul. 14, 2022, the entire contents of which are incorporated herein by reference.
An embodiment of the present invention relates to a display device in which light emitting diodes (LEDs) are arranged on a substrate, a method for manufacturing a display device, and method for inspecting a display device to check its quality.
A micro-LED display is known in which micro light-emitting diodes, called micro-LEDs, are mounted on pixels arranged in a matrix. The micro-LED display has a structure in which micro-LEDs individualized from a wafer, or the like, are mounted on a substrate on which circuits, called a backplane, are formed.
As a method of mounting a micro-LED on a substrate, it is disclosed that an anisotropic conductive film (ACF) is used to adhere an anode and cathode of the micro-LED to an anode pad and a cathode pad on the substrate. Also disclosed is a method of forming alignment marks by using aligned ACF and agglomerating the ACF. A method of connecting and fixing flip chip LEDs having a step between the anode and cathode to electrodes on a substrate by using bumps of different sizes is disclosed.
In recent years, a method of mounting flip-chip micro-LEDs on a substrate has been adopted in which the electrode on the micro-LED chip is in direct contact with the electrode on the substrate, and is fixed by heating and crimping in the presence of a resin such as NCF (Non Conductive Film). In this case, it is necessary to confirm that the electrical connection between the electrodes on the micro-LED side and the electrodes on the substrate side is sufficiently secured. One way to evaluate such a connection is to observe the indentation caused by the micro-LED electrodes being pressed into the substrate electrode. Therefore, the connection between the electrodes on the micro-LED side and the electrodes on the substrate side must be such that the indentation is clearly created.
A display device in an embodiment according to the present invention includes a substrate, a first electrode and a second electrode on the substrate, and an LED chip disposed on the first electrode and the second electrode and having an n-side pad electrode and a p-side pad electrode. The n-side pad electrode has a first protruding portion, the first protruding portion protruding toward the substrate and in contact with the first electrode, and the p-side pad electrode has a second protruding portion, the second protruding portion protruding toward the substrate and in contact with the second electrode.
A method for manufacturing a display device in an embodiment according to the present invention includes pressing a first protruding portion which protrudes toward a substrate of an n-side pad electrode of an LED chip against a first electrode disposed on the substrate, and pressing a second protruding portion which protrudes toward the substrate of a p-side pad electrode of the LED chip against a second electrode disposed on the substrate.
A method for inspecting a display device in an embodiment according to the present invention includes observing indentations formed on a first electrode and a second electrode on a substrate after an LED chip having a n-side pad electrode with a first protruding portion and a p-side pad electrode with a second protruding portion is pressed against the first electrode and the second electrode on the substrate, respectively, and determining a state of electrical connection between the LED chip and the first electrode and the second electrode from a state of the indentations.
Hereinafter, embodiments of the present invention are described with reference to the drawings. However, the present invention can be implemented in many different aspects, and should not be construed as being limited to the description of the following embodiments. For the sake of clarifying the explanation, the drawings may be expressed schematically with respect to the width, thickness, shape, and the like of each part compared to the actual aspect, but this is only an example and does not limit the interpretation of the present invention. For this specification and each drawing, elements similar to those described previously with respect to previous drawings may be given the same reference sign (or a number followed by a, b, etc.) and a detailed description may be omitted as appropriate. The terms “first” and “second” appended to each element are a convenience sign used to distinguish them and have no further meaning except as otherwise explained.
As used herein, where a member or region is “on” (or “below”) another member or region, this includes cases where it is not only directly on (or just under) the other member or region but also above (or below) the other member or region, unless otherwise specified. That is, it includes the case where another component is included in between above (or below) other members or regions. In the following description, unless otherwise noted, the direction in which the first and second electrodes are disposed with respect to the substrate in a cross-sectional view is referred to as “up”, “upper”, “top surface”, or “top side”, and vice versa as “down”, “lower”, “bottom surface”, or “bottom side”.
A structure of a display device according to an embodiment of the present invention will be described. In the following description, the structure of a connection part between an LED chip and electrodes disposed on the substrate will mainly be described.
Although not shown in detail in
The LED chip 102 is described using a micro-LED as an example, where the chip size is more than a few micrometers and less than 100 μm. However, an embodiment of the present invention can use LEDs of any size, and can be used according to the application and form of the display device.
As shown in
In order to make an area of the light-emitting region (area of the active layer) of the LED chip 102 as large as possible, an area forming the cathode electrode 106 is smaller than an area forming the anode electrode 108.
The LED chip 102 may have a structure where the n-side pad electrode 110 is disposed beyond a step 104 of the LED chip 102 to embed the step 104, as shown in
The structures of the n-side pad electrode 110 and p-side pad electrode 112 are described in detail.
As shown in
In the following description, the n-side pad electrode 110 includes a body portion, the first protruding portion 132, and other portions that connect to the cathode electrode 106, and the p-side pad electrode 112 includes a body portion, the second protruding portion 134, and other portions that connect to the anode electrode 108. In other words, the first protruding portion 132 is a part in the n-side pad electrode 110 and the second protruding portion 134 is a part in the p-side pad electrode 112. The body portion and the first protruding portion 132 of the n-side pad electrode 110 and the body portion and the second protruding portion 134 of the p-side pad electrode 112 may be configured by one continuous member (material) or by combining different members (materials).
The upper portions 110a and 112a of the n-side pad electrode 110 and A-side pad electrode 112 in the side view shown in
The n-side pad electrode 110 has the first protruding portion 132 having a shape protruding downward (in the direction toward the substrate) in the side view, and this portion contacts the first electrode 116 on the substrate 120. Similarly, the p-side pad electrode 112 has the second protruding portion 134 having a downward (toward the substrate) shape in the side view, and this portion is in contact with the first electrode 116 on the substrate 120.
The contact surfaces of the first protruding portion 132 and the second protruding portion 134 with the electrode on the substrate side are rectangular, as shown in the shaded area in the bottom view in
The contact area of the first protruding portion 132 with the electrode on the substrate side in the bottom view should be sufficiently smaller than the maximum cross-sectional area of the n-side pad electrode 110, but it is more preferable that it is 70% or less of the maximum cross-sectional area of the n-side pad electrode 110. Similarly, the contact area of the second protruding portion 134 in the bottom view should be sufficiently smaller than the maximum cross-sectional area of the p-side pad electrode 112, but it is more preferable that it is 50% or less of the maximum cross-sectional area of the p-side pad electrode 112.
The maximum cross-sectional area of the pad electrode indicates the area of the outermost figure of each pad electrode in the bottom view, for example, the outermost rectangular cross-sectional area in the bottom view in
The height of the first protruding portion 132 in a lateral view is preferably 10% or more and more preferably 25% or more of the height of the entire n-side pad electrode 110. Similarly, the height of the second protruding portion 134 in a lateral view is preferably 10% or more and more preferably 25% or more of the overall height of the n-side pad electrode 110.
The first protruding portion 132 and the second protruding portion 134 may be plural in the n-side pad electrode 110 and the p-side pad electrode, respectively. The number of first protruding portions 132 and second protruding portions 134 may not be the same.
The n-side pad electrode 110 and p-side pad electrode 112 can be made of known electrode materials. As an example, metallic materials such as aluminum (Al), gold (Au), silver (Ag), palladium (Pd), and indium (In), and low temperature softening materials such as solder and conductive paste can be used as electrode materials. The forming method of the n-side pad electrode 110 and the p-side pad electrode 112 is not particularly limited, and known plating methods and deposition methods such as sputtering can be used.
The first protruding portion 132 and the second protruding portion 134 can also be made of known electrode materials. As an example, metallic materials such as aluminum (Al), gold (Au), silver (Ag), palladium (Pd), and indium (In), or low temperature softening materials such as solder or conductive paste can be used as electrode materials. A known plating method or a known deposition method such as sputtering can be used to form the first protruding portion 132 and the second protruding portion 134.
The first protruding portion 132 may be configured of the same material as other areas of the n-side pad electrode 110 or of a different material. Similarly, the second protruding portion 134 may be configured of the same material as other areas of the p-side pad electrode 112 or of a different material. For example, only the first protruding portion 132 and the second protruding portion 134 may be configured with gold in the n-side pad electrode 110 and the p-side pad electrode 112, while the other regions may be configured with copper.
In the bottom view shown in
As described below, the connection between the n-side pad electrode 110 and p-side pad electrode 112 on the LED chip 102 and the first electrode 116 and second electrode 118 is made by indenting the substrate 120 and the LED chip 102. In this case, the smaller the contact area between the first protruding portion 132 and the second protruding portion 134 and the electrodes on the substrate, the easier it is for an indentation to occur on the electrode on the substrate 120 side. When an indentation can be clearly confirmed during the quality evaluation described below, it can be determined that the electrical connection between the LED chip 102 and the substrate 120 is maintained.
The process of forming the n-side pad electrode 110 and p-side pad electrode 112 on the LED chip 102 is described with reference to
As mentioned above, the LED chip 102 has a structure in which the n-type semiconductor layer, active layer, and p-type semiconductor layer (not shown in the figure) are stacked, and the active layer and p-type semiconductor layer are removed from the top surface of the n-type semiconductor layer, and the passivation film 114 is provided on the surface. In such a chip, the opening 115a for bonding the n-type semiconductor layer to the n-side pad electrode 110 and the opening 115b for bonding the p-type semiconductor layer to the p-side pad electrode 112 are formed to partially expose the cathode electrode 106 and anode electrode 108 on the LED chip 102, and the cathode electrode 106 and the anode electrode 108 are partially exposed on the LED chip 102.
The n-side pad electrode 110 and the p-side pad electrode 112 are connected to the first electrode 116 and the second electrode 118 on the substrate 120 to mount the LED chip 102. At this time, the thickness of the n-side pad electrode 110 and p-side pad electrode 112 is adjusted so that the substrate 120 and LED chip 102 are in a horizontal position.
As mentioned above, the surface of the substrate 120 side of the n-side pad electrode 110 has the first protruding portion 132 formed with one portion protruding toward the substrate, and this first protruding portion 132 is in contact with the first electrode 116 on the substrate 120 side. Similarly, the surface on the substrate 120 side of the p-side pad electrode 112 has the second protruding portion 134 formed with one portion protruding toward the substrate, and the second protruding portion 134 is in contact with the second electrode 118 on the substrate 120 side. It is possible to reduce the area of the portion where the electrodes are in contact with each other by having the n-side pad electrode 110 and the p-side pad electrode 112 contact the electrode on the substrate side in such a form.
The substrate 120 on which the LED chip 102 is mounted must be observed from the back side to observe the contact state of the electrodes, as described below. Therefore, it is preferable to use a material with good light transmittance for the substrate, for example, a substrate configured with glass, quartz, or sapphire can be used.
For example, a film of NCF or similar material is cut and scattered over the top surface of the substrate, and these are heated to near their melting temperature. Once the NCF is heated and softened, the LED chip is placed from the arrow direction shown in
The NCF used for adhering the LED chip 102 to the substrate 120 can be, for example, a polyimide thermosetting adhesive film or an adhesive using epoxy or acrylic resin. In addition to NCF, conductive adhesives, anisotropic conductive films (ACF), and other known adhesive materials can be used to adhere the LED chip 102 to the substrate 120. However, when a conductive adhesive or anisotropic conductive film is used instead of NCF, the adhesive should be conductive only in the areas where the first protruding portion 132 and the first electrode 116 and the second protruding portion 134 and the second electrode 118 are connected, so that the first electrode 116 and the second electrode 118 do not short, and the conductive particles of the ACF are assumed to be present to prevent shorts.
As described above, the electrical connection between the LED chip 102 and the substrate 120 is made by pressing the n-side pad electrode 110 with the first protruding portion 132 and the p-side pad electrode 112 with the second protruding portion 134 on the LED chip 102 side against the first electrode 116 and the second electrode 118 on the substrate 120 side and then hardening the resin such as NCF by curing using NCF. In this process, electrical conductivity between the substrate 120 and the LED chip 102 is ensured by the first protruding portion 132 and the second protruding portion 134 on the LED chip 102 side being sufficiently pressed against the electrodes on the substrate side. Therefore, when manufacturing the display device, a process is required to inspect whether or not the above electrodes are sufficiently press-fitted.
To confirm that the electrodes are sufficiently pressed together, the indentation marks made by the first protruding portion 132 and the second protruding portion 134 on the LED chip 102 side on the substrate-side first electrode 116 and second electrode 118 when they are pressed to the substrate are observed. It is possible to evaluate that the electrical conductivity between the LED chip 102 and the substrate 120 is good if these indentation marks can be clearly observed.
The indentation can be confirmed by observing the unevenness on the first electrode 116 and the second electrode 118 on the substrate 120 from the back side of the substrate 120. In other words, this is done by transmitting light from the back side of the substrate 120, observing using a microscope such as a differential interference microscope, capturing images of the indentations visible under the microscope through an imaging device, and checking the image data. Therefore, at least between the substrate 120 and the first electrode 116 and the second electrode 118, there should be no light-shielding metallic material other than the first electrode 116 and the second electrode 118 in the area overlapping the first protruding portion 132 and the second protruding portion 134.
The indentation is not limited to the form mentioned above. For example, when ACF is used to bond the LED chip 102 to the substrate 120, the unevenness on the grains created when the conductive particles in the ACF are squeezed is observed on the electrode, resulting in a different form of indentation than when NCF is used.
The evaluation of electrical continuity between the LED chip 102 and substrate 120 may be done by visual judgment by an inspector, alternatively, the shape of the indentation and the electrical continuity between the LED chip 102 and the substrate 120 may be evaluated using AI (Artificial Intelligence) that has been trained on the shape of the indentation and the electrical continuity between the LED chip 102 and the substrate 120.
The configuration of a display device according to an embodiment of the present invention is shown below. The display device of the present embodiment has a structure in which an LED chip is provided in a pixel. The pixel has a structure in which the LED chip is connected to the first electrode 116 and the second electrode 118 formed on the circuit board by the n-side pad electrode 110 and the p-side pad electrode 112 on the LED chip, as shown in
The display part 202 is arranged with scanning signal lines 206 for inputting scanning signals to the pixels 204 and data signal lines 208 for inputting video signals. The scanning signal lines 206 and data signal lines 208 are arranged to intersect. An input terminal part 210a for the scanning signal lines 206 and an input terminal part 210b for the data signal lines 208 are arranged on the periphery of the substrate 120. The input terminal parts 210a and 210b are connected to a flexible printed wiring substrate 212. A driver IC 214 may be mounted on the flexible printed wiring substrate 212.
The first electrode 116 and the second electrode 118 are disposed on the third insulating layer 148. The first electrode 116 is electrically connected to the scanning signal line 206 through the second insulating layer 146 and also the first contact hole 158a through the second insulating layer 146, and the second electrode 118 is electrically connected to the data signal line 208 through the second contact hole 158b through the third insulating layer 148. The fourth insulating layer 150 is disposed on the upper layer side of the first electrode 116 and the second electrode 118. The first electrode 116 and the second electrode 118 are exposed by openings formed in the fourth insulating layer 150 at the positions where they are to be in contact with the n-side pad electrode 110 and the p-side pad electrode 112 on the LED chip 102 side.
The LED chip 102 is disposed on the first electrode 116 and the second electrode 118. The LED chip 102 is electrically connected to the first electrode 116 via the protruding portion 132 of the n-side pad electrode 110 and to the second electrode 118 via the protruding portion 134 of the p-side pad electrode 112. A base metal film 126 may be deposited between the first protruding portion and the first electrode 116 and between the second protruding portion and the second electrode 118.
As shown in
The structure of the pixel, which can be designed and changed as appropriate by a person skilled in the art based on the pixel structure of the display device described above as one embodiment of the invention, also belongs to the technical scope of the invention as long as it encompasses the gist of the invention.
Within the scope of the idea of the invention, a person skilled in the art can conceive of various examples of changes and modifications, and these changes and modifications also fall within the technical scope of the invention. For example, in one embodiment of the invention described above, any addition, deletion, or modification made by a person skilled in the art, as well as any addition or omission of a process or any modification of conditions, belongs to the technical scope of the invention, as long as it does not depart from the gist of the invention.
It is understood that the advantageous effects brought about by the aspects described in one embodiment of the invention, which are obvious from the description herein and which may be conceived by a person skilled in the art from time to time, are naturally brought about by the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-113186 | Jul 2022 | JP | national |
