Patent Grant
9484504
1. Field
The present invention relates to micro LED devices. More particularly embodiments of the present invention relate to a method and structure for integrating micro LED devices on a substrate with a tunable color emission spectrum.
2. Background Information
Quantum dots are semiconductor nanocrystals that can be tuned to emit light throughout the visible and infrared spectrum. Due to the small size of 1 to 100 nm, more typically 1 to 20 nm, quantum dots display unique optical properties that are different from those of the corresponding bulk material. The wavelength, and hence color, of the photo emission is strongly dependent on the size of a quantum dot. For an exemplary cadmium selenide (CdSe) quantum dot, light emission can be gradually tuned from red for a 5 nm diameter quantum dot, to the violet region for a 1.5 nm quantum dot. There are generally two types of schemes for quantum dot (QD) excitation. One uses photo excitation, and the other uses direct electrical excitation.
One proposed implementation for quantum dots is integration into the backlighting of a liquid crystal display (LCD) panel. Current white light emitting diode (LED) backlight technology for LCD panels utilizes a cerium doped YAG:Ce (yttrium aluminum garnet) down-conversion phosphor layer over a plurality of blue emitting LED chips. The combination of blue light from the LED chips and a broad yellow emission from the YAG:Ce phosphor results in a near white light. It has been proposed to replace the YAG:Ce phosphor with a blend of quantum dots to achieve the white backlighting.
Another proposed implementation for quantum dots is a quantum dot light emitting diode (QD-LED). The structure of the QD-LED is similar to that of the organic light emitting diode (OLED), with the major difference being that the organic emission layer from the OLED technology is replaced with a quantum dot film between a hole transporter layer and an electron transporter layer. Once an electric field is applied, the electrons and holes move into the quantum dot film, where they are captured in the quantum dots and recombine, emitting photons.
Light emitting devices for lighting or display applications are disclosed. In an embodiment, a light emitting device including a bottom electrode, a micro LED device bonded to the bottom electrode, a top electrode in electrical contact with the micro LED device, and a wavelength conversion layer around the micro LED device. The wavelength conversion layer includes phosphor particles. In an embodiment, the phosphor particles are dispersed within a matrix, such as glass or polymer. In an embodiment, phosphor particles are quantum dots whose emission properties are related to particle size and shape. In an embodiment, the phosphor particles exhibit luminescence due to their composition. In an embodiment, the wavelength conversion layer is dome shaped.
The micro LED device may have a maximum width of 1 μm-100 μm in some embodiments. In an embodiment the micro LED device comprise a quantum well layer within a p-n diode. The micro LED device can be designed to emit at specific wavelengths in the ultraviolet (UV) or visible spectrum. In some embodiments, the micro LED device emits a primarily blue light and is formed of a semiconductor material such as gallium nitride (GaN), indium gallium nitride (InGaN), or zinc selenide (ZnSe).
In an embodiment a light distribution layer is formed between the micro LED device and the wavelength conversion layer. The light distribution layer may be transparent. The light distribution layer may be dome shaped. In an embodiment, the light distribution layer is formed of a material such as glass, epoxy, silicone, and acrylic. The wavelength conversion layer may also be formed of a material such as glass, epoxy, silicone, and acrylic, and the two layers may be formed of the same material. In an embodiment, the refractive index of the light distribution layer and the wavelength conversion layer match. In another embodiment, the difference in refractive index is within 0.3, or more specifically within 0.1. In an embodiment, the light distribution layer is dome shaped.
An oxygen barrier film may be formed over the wavelength conversion layer. For example, the oxygen barrier film may include a material such as Al2O3, SiO2, SiNx, and spin on glass. Color filters can also be included in the light emitting devices. In one embodiment, a color filter is above the wavelength conversion layer. In another embodiment, a color filter such as a pigment or dye is dispersed within the wavelength conversion layer. The micro LED devices may also be placed within reflective bank structures. In an embodiment, the wavelength conversion layer is wider than the reflective bank structure in order to cover the sidewalls of the reflective bank structure.
The lighting and display applications in accordance with embodiments of the invention may include a plurality of micro LED devices, and a plurality of wavelength conversion layers around the plurality of micro LED devices, with each wavelength conversion layer comprising phosphor particles, for example, dispersed within a glass or polymer matrix. In some embodiments, each micro LED device is design to emit the same emission spectrum (e.g. visible spectrum or UV spectrum). In one embodiment, each wavelength conversion layer is designed to emit the same color emission spectrum. In another embodiment, there are multiple groups of wavelength conversion layers, with each group designed to emit a different color emission spectrum. For example, the different groups may be arranged into pixels, with each pixel comprising at least one micro LED device from each group. In an embodiment, each pixel is capable of emitting white light.
In an exemplary embodiment, a light emitting device includes an array of pixels with each pixel comprising a plurality of subpixels designed for different color emission spectra. A first subpixel may include a first micro LED device bonded to a first bottom electrode, a first top electrode in electrical contact with the first micro LED device, and a first wavelength conversion layer around the micro LED device, the first wavelength conversion layer comprising a first phosphor particles, for example dispersed within a glass or polymer matrix. A second subpixel may include a second micro LED device bonded to a second bottom electrode, a second top electrode in electrical contact with the second micro LED device, and a second wavelength conversion layer around the micro LED device, the second wavelength conversion layer comprising a second phosphor particles, for example dispersed within a glass or polymer matrix. In such an embodiment, the first and second micro LED devices have the same composition for the same emission spectrum, and the first and second phosphor particles are designed for different color emission spectra. In an embodiment, the first and second top electrodes are physically and electrically connected.
In an exemplary embodiment a first subpixel may include a first micro LED device bonded to a first bottom electrode, a first top electrode in electrical contact with the first micro LED device, and a first wavelength conversion layer around the micro LED device, the first wavelength conversion layer comprising a first phosphor particles, for example dispersed within a glass or polymer matrix. A second subpixel may include a second micro LED device bonded to a second bottom electrode, a second top electrode in electrical contact with the second micro LED device, wherein a wavelength conversion layer including phosphor particles is not formed around the second micro LED device.
Embodiments of the present invention describe light emitting devices which incorporate a wavelength conversion layer around a micro LED device. The light emitting devices in accordance with embodiments of the invention may include a plurality of micro LED devices and a plurality of wavelength conversion layers. A variety of color emission spectra and patterns can be accomplished by selection of emission spectrum combinations for the micro LED devices and the wavelength conversion layers, where present, in the light emitting devices. In an embodiment, the wavelength conversion layer includes phosphor particles (e.g. quantum dots that exhibit luminescence due to their size and shape in addition to their composition, or particles that exhibit luminescence due to their composition). In this manner, the light emission can be accurately tuned to specific colors in the color spectrum, with improved color gamut.
In some embodiments, the wavelength conversion layer is a portion of a micro lens formed around a micro LED device. Each micro lens structure may include a variety of configurations and optionally include a number of different layers such as a light distribution layer, matching layer, oxygen barrier, and color filter. In some embodiments, the micro lens or specific layers forming the micro lens can be dome shaped. The dome shape profile may be hemispherical, flattened, or narrowed. For example, a hemispherical micro lens profile may improve light extraction and create a Lambertian emission pattern. Flattening or narrowing of the dome profile can be used to adjust viewing angle for the light emitting device. In accordance with embodiments of the invention, the thickness and profile the layers forming the micro lens structure can be adjusted in order to change the light emission beam profile from the micro LED device, as well as color over angle characteristics of the light emitting device which can be related to edge effects.
In one aspect, the incorporation of micro LED devices in accordance with embodiments of the invention can be used to combine the performance, efficiency, and reliability of wafer-based LED devices with the high yield, low cost, mixed materials of thin film electronics, for both lighting and display applications. The term “micro” LED device as used herein may refer to the descriptive size scale of 1 to 100 μm. For example, each micro LED device may have a maximum width of 1 to 100 μm, with smaller micro LED devices consuming less power. In some embodiments, the micro LED devices may have a maximum width of 20 μm, 10 μm, or 5 μm. In some embodiments, the micro LED devices have a maximum height of less than 20 μm, 10 μm, or 5 μm. Exemplary micro LED devices which may be utilized with some embodiments of the invention are described in U.S. Pat. No. 8,426,227, U.S. patent application Ser. No. 13/436,260, U.S. patent application Ser. No. 13/458,932, U.S. patent application Ser. No. 13/711,554, and U.S. patent application Ser. No. 13/749,647. The light emitting devices in accordance with embodiments of the invention may be highly efficient at light emission and consume very little power (e.g., 250 mW for a 10 inch diagonal display compared to 5-10 watts for a 10 inch diagonal LCD or OLED display), enabling reduction of power consumption of an exemplary display or lighting application incorporating the micro LED devices and wavelength conversion layers.
In another aspect, embodiments of the invention provide for configurations that allow phosphor particles of different emission spectra to be separated from one another while still providing good color mixing of the light as perceived by the viewer. Separating the phosphor particles from each other in each subpixel can prevent secondary absorption of light emitted from a phosphor particle emitting a different spectrum (e.g. absorption of green light emitted from a green emitting phosphor particle by a red emitting phosphor particle). This may increase efficiency and reduce unintended color shift. However, if phosphor particles are separated in a non-micro LED devices system, the different color emitting areas may be visible and result in spatially non-uniform color of the light source. In the micro LED devices systems in accordance with embodiments of the invention the spatial color separation can be small enough (e.g. approximately 100 μm or less) that it will not be perceived by the human eye. In this manner, the “micro” LED device scale enables the arrangement of micro LED devices and wavelength conversion layers including phosphor particles with small enough pitch (e.g. approximately 100 μm or less) between adjacent micro LED devices or subpixels that the spatial color separation is not perceived by the human eye.
In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the present invention. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments.
The terms “spanning”, “over”, “to”, “between” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “spanning”, “over” or “on” another layer or bonded “to” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers.
Referring now to
In accordance with embodiments of the invention, the term “phosphor” may refer to any type of wavelength converting material that will absorb light at one wavelength and emit light at another wavelength. One type of phosphor particle is a quantum dot. Quantum dots are semiconductor materials where the size of the structure is small enough (less than tens of nanometers) that the electrical and optical characteristics differ from the bulk properties due to quantum confinement effects. For example, the emission properties of quantum dots are related to their size and shape in addition to their composition. Fluorescence of quantum dots is a result of exciting a valence electron by absorbing a certain wavelength, followed by the emission of lower energy in the form of photons as the excited electrons return to the ground state. Quantum confinement causes the energy difference between the valence and conduction bands to change based on size and shape of the quantum dot meaning that the energy and wavelength of the emitted photons is determined by the size and shape of the quantum dot. The larger the quantum dot, the lower the energy of its fluorescence spectrum. Accordingly, smaller quantum dots emit bluer light (higher energy) and larger quantum dots emit redder light (lower energy). This allows size-dependent tuning of the semiconductor photoluminescence emission wavelength throughout the visible spectrum, with a sharp emission spectrum and high quantum efficiency.
Examples of quantum dot materials include, but are not limited to, groups II-VI, III-V, IV-VI semiconductor materials. Some exemplary compound semiconductors include CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb. Some exemplary alloyed semiconductors include InGaP, ZnSeTe, ZnCdS, ZnCdSe, and CdSeS. Multi-core structures are also possible. Exemplary multi core configurations may include a semiconductor core material, a thin metal layer to protect the core from oxidation and to aid lattice matching, and a shell to enhance the luminescence properties. The core and shell layers may be formed of the same material, and may be formed of any of the exemplary compound semiconductors or alloyed semiconductors listed above. The metal layer often comprises Zn or Cd.
In accordance with embodiments of the invention, one type of phosphor particle is a particle that exhibits luminescence due to its composition. Some exemplary phosphor particles that exhibit luminescence due to their composition include sulfides, aluminates, oxides, silicates, nitrides, YAG (optionally doped with cerium), and terbium aluminum garnet (TAG) based materials. Other exemplary materials include yellow-green emitting phosphors: (Ca,Sr,Ba)Al2O4:Eu (green), (Lu, Y)3Al5O12:Ce3+(LuAG, YAG) (yellow-green), Tb3Al5O12:Ce3+(TAG) (yellow-green); orange-red emitting phosphors: BaMgAl10O17:Eu2+(Mn2+), Ca2Si5N8:Eu2+ (orange-red), (Zn,Mg)S:Mn (green, red), (Ca,Sr,Ba)S:Eu2+ (red); uv-deep blue absorbing phosphors for blue and yellow-green emission: (Mg,Ca,Sr,Ba)2SiO4:Eu2+ (uv-blue excitation, yellow emission), (Mg,Ca,Sr,Ba)3Si2O7:Eu2+ (uv-deep blue excitation, blue-green emission), Ca8Mg(SiO4)4Cl2:Eu2+ (uv-deep blue excitation, blue emission); and phosphors that can emit over the full visible spectrum depending on composition and processing (Sr,Ca,Ba)SixOyNz:Eu2+ (y>0 green, y=0 red), Y2O2S:Eu3+ (blue-green), (Ca,Mg,Y)vSiwAlxOyNz:Eu2 (yellow-green-red). In some embodiments the particle size for such phosphor particles may be from 1 μm to 20 μm. In other embodiments, the particles size for such phosphor particles can be nanoparticles from 100 nm to 1 μm. The phosphor particles can also include a blend of the 1 μm to 20 μm particles and 100 nm to 1 μm nanoparticles. Nanoparticles may be useful, for example, to reduce the amount of settling when dispersed within a matrix material of a wavelength conversion layer prior to curing or solvent removal, which may result in more even distribution of the nanoparticles and light emission of the light emitting device.
Other materials may also be dispersed within the wavelength conversion layer. For example, the other materials may be dispersed within the matrix material, such as glass or polymer matrix of the wavelength conversion layer. In an embodiment, a light scattering agent such as a TiO2 or Al2O3 particles are dispersed within the wavelength conversion layer. Such light scattering agents may have the effect of increasing the phosphor particle efficiency by increasing scattered light within the wavelength conversion layer. Such light scattering agents may additionally have the effect of reduced bleeding of the micro LED device emitted light through the wavelength conversion layer. In an embodiment, a pigment or dye may be dispersed within the wavelength conversion layer 110. This may have the effect of incorporating a color filter into the wavelength conversion layer. In an embodiment, the pigment or dye may have a color similar to the emission wavelength of the phosphor particle. In this manner, the pigment or die can absorb wavelengths other than those being emitted from the phosphor particle, further sharpening the emission spectrum of the assembly. For example, in a particular embodiment, the micro LED device 100 is a gallium nitride (GaN) based material, and emits a blue (e.g. 450 nm-495 nm) or deep blue (e.g. 420 nm-450 nm) light. Quantum dots designed for red emission may be dispersed in the wavelength conversion layer 110 in order to absorb the blue or deep blue emission from the micro LED device 100 and convert the emission wavelength to red. In such an embodiment, a red pigment or dye may also be dispersed within the wavelength conversion layer 110 to also absorb colors other than red. In this manner, the red pigment or dye may absorb additional blue or deep blue light, thereby reducing bleeding of the unconverted blue or deep blue light. Exemplary pigments include lithol rubine (Red), B-copper thalocyanine (Blue), and diarylide yellow (Yellow). It is to be appreciated that a blue micro LED device and red phosphor particles with red pigment or dye is exemplary and a variety of emission spectrum configurations for the micro LED devices and wavelength conversion layers, where present, are possible.
In accordance with embodiments of the invention, the light emitting device configuration including the micro LED device and wavelength conversion layer can be incorporated into a variety of lighting or display devices. A plurality of micro LED devices can be bonded to bottom electrodes on a substrate and a plurality of wavelength conversion layers formed around the plurality of micro LED devices. The wavelength conversion layers can be designed to all emit the same color emission spectrum, or the wavelength conversion layers can be divided into multiple groups of wavelength conversion layers, with each group designed to emit a different color emission spectrum. In this manner, the light emitting devices can emit any color or patterns of colors depending upon the arrangement and content of the micro LED devices and wavelength conversion layers. In one embodiment, white light can be generated by incorporating red (e.g. 620 nm-750 nm) and green (e.g. 495 nm-570 nm) emitting phosphor particles in a wavelength conversion layer positioned around a blue emitting (e.g. 450 nm-495 nm) micro LED device. In another embodiment, white light can be generated by incorporating multiple micro LED devices into a pixel, with each micro LED device designed to emit the same emission spectrum (e.g. visible spectrum or UV spectrum), and different wavelength conversion layers designed to covert color emission. In this manner, by including phosphor particles of a single color emission spectrum in each wavelength conversion layer, secondary absorption of light emitted from different emission spectra of different phosphor particles is avoided. This may increase efficiency and reduce unintended color shift. For example, a pixel may contain 3 micro LED devices all designed to emit blue light, with one red emitting wavelength conversion layer around one micro LED device, one green emitting wavelength conversion layer around a second micro LED device, and the third micro LED device either not including a wavelength conversion layer around it or including a blue emitting wavelength conversion layer around it. In one embodiment, white light can be generated by incorporating multiple micro LED devices into a pixel, with each micro LED device designed to emit UV light, with one red emitting conversion layer around one micro LED device, one green emitting wavelength conversion layer around a second micro LED device, and one blue emitting wavelength conversion layer around a third micro LED device. In another embodiment, white light can be generated by incorporating combinations of micro LED devices designed for different emission spectrum and different wavelength conversion layers, or no wavelength conversion layers. In another exemplary embodiment, white light can be generated with a micro LED device designed for red emission with no overlying wavelength conversion layer, a micro LED device designed for blue emission with an overlying wavelength conversion layer designed for green emission, and a micro LED device designed for blue emission with no overlying wavelength conversion layer.
In the above exemplary embodiments, a red-green-blue (RGB) subpixel arrangement is obtained, and each pixel includes three subpixels that emit red, green and blue lights, respectively. It is to be appreciated that the RGB arrangement is exemplary and that embodiments are not so limited. Examples of other subpixel arrangements that can be utilized include, but are not limited to, red-green-blue-yellow (RGBY), red-green-blue-yellow-cyan (RGBYC), or red-green-blue-white (RGBW), or other subpixel matrix schemes where the pixels may have different number of subpixels, such as the displays manufactured under the trademark name PenTile®.
Referring now to
In accordance with embodiments of the invention, the light emitting device configuration including the micro LED device and wavelength conversion layer can be incorporated into a variety of lighting or display devices. Exemplary lighting applications include interior or exterior lighting applications, such as billboard lighting, building lighting, street lighting, light bulbs, and lamps. Exemplary display applications include passive matrix display and active matrix displays, such as, display signage, display panels, televisions, tablets, phones, laptops, computer monitors, kiosks, digital cameras, handheld game consoles, media displays, ebook displays, or large area signage display.
Referring now to
In the following description, specific examples are described and illustrated for integrating micro LED devices with wavelength conversion layers into lighting or display devices. It is to be appreciated, however, that the following embodiments are exemplary and are not intended to exclusive of one another, and that the following embodiments may be combined in certain situations.
Referring now to
Substrate 201 may be a variety of substrates such as, but not limited to, a display substrate, a lighting substrate, a substrate with functional devices such as transistors or integrated circuits (ICs), or a substrate with metal redistribution lines. Depending upon the particular application, substrate 201 may be opaque, transparent, or semi-transparent to the visible wavelength spectrum (e.g. 380-750 nm wavelength), and substrate 201 may be rigid or flexible. For example, substrate 201 may be formed of glass, metal foil, metal foil covered with dielectric, or a polymer such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulphone (PES), aromatic fluorine-containing polyarylates (PAR), polycyclic olefin (PCO), and polyimide (PI). In an embodiment, the substrate 201 includes working circuitry 210. For example substrate 201 may be an active matrix backplane including working circuitry 210 such as a driving transistor, switching transistor, and capacitor. In an embodiment, substrate 201 is a thin film transistor (TFT) substrate including working circuitry 210.
Conductive electrodes or electrode lines 310, 330 can be formed on, within, or over substrate 201. For example, the electrodes or electrode lines 310, 330 function as an anode, cathode or ground, or an electrical line to anode, cathode, or ground. In interests of clarity, 310 is referred to as a bottom electrode or electrode line and 330 is referred to as a ground line in the remainder of the description. However, it is to be appreciated that this is one embodiment, and other configurations are possible. While the remainder of the description is made with regard to this designation, it is understood that this is not the sole embodiment.
Bottom electrode 310 and ground line 330 can be formed of a variety of materials, and either may be opaque, transparent, or semi-transparent to the visible wavelength spectrum. Exemplary transparent conductive materials include amorphous silicon, transparent conductive oxides (TCO) such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), carbon nanotube film, or a transparent conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, polyacetylene, polypyrrole, and polythiophene. In an embodiment bottom electrode 310 is approximately 100 nm-200 nm thick ITO. In an embodiment, the bottom electrode 310 includes nanoparticles such as silver, gold, aluminum, molybdenum, titanium, tungsten, ITO, and IZO. The bottom electrode 310 or ground line 330 may also be reflective to the visible wavelength. In an embodiment, a bottom electrode 310 or ground line 330 comprises a reflective metallic film such as aluminum, molybdenum, titanium, titanium-tungsten, silver, or gold, or alloys thereof.
A bonding layer 314 may optionally be formed between the micro LED device 100 and the bottom electrode 310 to facilitate bonding of the bottom contact 104 of micro LED device 100 to the bottom electrode 310 on substrate 201. In an embodiment, bonding layer 314 includes a material such as indium, gold, silver, molybdenum, tin, aluminum, silicon, or an alloy or alloys thereof.
In the following embodiments description is made with regard to a specific vertical micro LED device 100 structure. It is to be appreciated, that the specific micro LED device 100 illustrated is exemplary and that embodiments of the invention are not limited. In the particular embodiment illustrated, the micro LED devices 100 include a micro p-n diode between a bottom contact 104 and top contact 102. In an embodiment, the micro p-n diode is several micros thick, such as 30 μm or less, or even 5 μm or less, with the top and bottom contacts 104, 102 being 0.1 μm-2 μm thick. The micro p-n diode may include a n-doped layer 109, a p-doped layer 105, and one or more quantum well layers 108 between the n-doped layer and p-doped layer. In the particular embodiment illustrated in
Referring again to
In an embodiment the sidewall passivation layer 316 at least partially covers the bottom electrode 310. In the embodiments illustrated, the sidewall passivation layer completely covers the bottom electrode 310, however, this is not required. Any combination of other insulating layers can be used to electrically insulate the bottom electrode 310 from other electrically conductive layers. In accordance with embodiments of the invention, a sidewall passivation layer 316 may not be required where a conformal dielectric barrier layer 107 is present along sidewalls of the micro LED devices 100. Alternatively, a sidewall passivation layer 316 may be formed in combination with an existing conformal dielectric barrier layer 107.
Following the formation of optional sidewall passivation layer 316, a top electrode layer 318 is formed on the micro LED device 100 and in electrical contact with the top contact 102 and ground line 330. Depending upon the particular application in the following description, top electrode layer 318 may be opaque, reflective, transparent, or semi-transparent to the visible wavelength. Exemplary transparent conductive materials include amorphous silicon, transparent conductive oxides (TCO) such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), carbon nanotube film, or a transparent conductive polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, polyacetylene, polypyrrole, and polythiophene. In an embodiment top electrode layer 318 is approximately 50 nm-1 μm thick ITO-silver-ITO stack, with the silver layer thin enough to be transparent to the visible wavelength spectrum. In a particular embodiment, the top electrode layer 318 is formed by ink jet printing. In an embodiment top electrode layer 318 is approximately 50 nm-1 μm thick PEDOT. Other methods of formation may include chemical vapor deposition (CVD), physical vapor deposition (PVD), or spin coating depending upon the desired area to be coated and any thermal constraints. In accordance with embodiments of the present invention, the top electrode layer 318 may be formed over a plurality of the micro LED devices 100 on substrate 201, electrically connecting the plurality of the micro LED devices 100 to ground line 330.
Following the formation of top electrode layer 318, a light distribution layer 320 may optionally be formed over the micro LED device 100. In an embodiment, light distribution layer 320 is dome shaped. In an embodiment, a dome shape may create radial spreading of the light emitted from the micro LED device. In an embodiment, the dome shape profile is hemispherical. In other embodiments the dome shape profile can be flattened or narrowed. In an embodiment, the thickness and profile of the light distribution layer provides a base structure upon which a micro lens structure is formed in order to change the light emission beam profile from the micro LED device 100, as well as color over angle characteristics of the light emitting device which can be related to edge effects. Light distribution layer 320 may be formed of a variety of transparent materials such as epoxy, silicone, and acrylic, which have the following reported refractive indices (n) at nominal 590 nm wavelength: n=1.51-1.57 (epoxy), n=1.38-1.58 (silicone), n=1.49 (acrylic). In an embodiment, light distribution layer 320 is formed by ink jet printing. In an embodiment, the light distribution layer 320 is formed by application of a molten glass droplet. Glass compositions can range from a variety of compositions ranging from acrylic grass, crown glass, flint glass, and borosilicate glasses that possess indices of refraction that can be matched to those of epoxy, silicone, or acrylic above. The light distribution layer 320 may be thicker than the micro LED device 100. In an embodiment, the light distribution layer 320 is 1 μm-100 μm thick.
In accordance with embodiments of the invention, the light distribution layer 320 may allow the light emitted from the micro LED device 100 to spread out prior to entering the wavelength conversion layer 110, and also decrease the optical intensity of light entering the wavelength conversion layer (and color filter). In one aspect, the spread out light may result in more even emission from the wavelength conversion layer 110 to be formed over the transparent light distribution layer 320. In another aspect, reduction of the optical density may reduce thermal degradation of the phosphor particles in wavelength conversion layer, prolonging lifetime of the light emitting device. In yet another aspect, spreading out of the light and reduction of the optical intensity may reduce the amount of back reflection from the wavelength conversion being that is reabsorbed by the micro LED device 100. In accordance with embodiments of the invention, inclusion of the light distribution layer 320 may increase total light emission, increase emission uniformity, and increase sharpness of the color spectrum for the light emitting device. The thickness and profile of the light distribution layer may also provide a base structure upon which a micro lens structure is formed in order to change the light emission beam profile from the micro LED device 100, as well as color over angle characteristics of the light emitting device which can be related to edge effects.
Following the formation of the optional light distribution layer 320, a matching layer 322 may optionally be formed over the light distribution layer 320 prior to forming the wavelength conversion layer 110. The matching layer 322 may function to match the indices of refraction for the light distribution layer 320 and wavelength conversion layer 110 to reduce back reflection of light. For example, where layers 320, 110 are formed of, for example, an epoxy, silicone, acrylic, or glass having different indices of refraction, the matching layer 322 is formed of an epoxy, silicone, acrylic, or glass having an index of refraction between that of layers 320, 110. In accordance with embodiments of the invention, the polymer matrix forming layers 320, 110 is the same, and layers 320, 110 have an identical index of refraction. In another embodiment, the index of refraction for layers 320, 110 is within 0.3, or more particularly within 0.1. In an embodiment, matching layer is 2 μm or less in thickness. In an embodiment, curing of the matching layer 322 may be thermal or UV.
In accordance with embodiments of the invention, a wavelength conversion layer 110 is formed around the micro LED device 100, and around the optional light distribution and matching layers, if present. In an embodiment, the wavelength conversion layer 110 is formed of phosphor particles. For example, the wavelength conversion layer is formed of a spray deposition method followed by removal of solvents. In an embodiment, the wavelength conversion layer includes a dispersion of phosphor particles in a matrix material such as a polymer or glass matrix material. Other filler materials such as pigment, dye, or scattering particles may also be dispersed within the matrix, or among the phosphor particles themselves if no matrix material is present. In an embodiment, wavelength conversion layer 110 is formed by ink jet printing, and UV cured. In an embodiment, the wavelength conversion layer 110 is formed by application of a molten glass droplet, where the fillers are thermally and chemically stable within the molten glass droplet. The thickness of the wavelength conversion layer 110, as well a concentration of fillers, e.g. phosphor particles, pigment, dye, or light scattering particles are tuned to achieve the requisite color spectrum. For example, minimized color bleeding from the micro LED device through the wavelength conversion layer, and maximized emission from the phosphor particles. Thickness of the wavelength conversion layer 110 (as well as optional light distribution layer 320) may also be partly determined by the spacing between micro LED devices. For example, micro LED devices may be spaced more closely together in high resolution display applications compared to lighting applications. In an embodiment, the wavelength conversion layer 110 is 5 μm-100 μm thick, or more specifically 30 μm-50 μm thick for an exemplary 5 μm wide and 3.5 μm tall micro LED device 100. In some embodiments, the thickness of the wavelength conversion layer and concentration of fillers may be designed to allow some light from the micro LED device 100 to pass through resulting a mix of the micro LED device light spectrum and the converted light spectrum to achieve a blended emission spectrum, for example, white light. Concentration of the color converting materials (e.g. phosphor particles, pigment, dye) as well as thickness of the layers can depend upon the particular application of the light emitting device, for example, if full color conversion (e.g. from blue to red, or blue to green, etc.) is to occur, of leakage or bleeding of light from the underlying micro LED device is to occur, or if a mixture of converting materials is employed. In an embodiment where full color conversion (e.g. from blue to red, or blue to green, etc.) occurs a volume loading percent of greater than 50% color converting materials may be included in the wavelength conversion layer. In an embodiment, the wavelength conversion layer includes greater than 50% volume loading of phosphor particles.
In accordance with some embodiments of the invention, the polymer matrix forming the wavelength conversion layer 110 may be permeable to oxygen or moisture. In an embodiment, following the formation of the wavelength conversion layer 110, an oxygen barrier film 324 may optionally be formed in order to protect the wavelength conversion layer 110 from oxygen or moisture absorption. For example, where wavelength conversion layer 110 includes quantum dots, the oxygen barrier film 324 can act as a barrier to oxygen or moisture absorption by the quantum dots, thereby prolonging the lifetime of the quantum dots in the lighting or display device. Suitable materials for the oxygen barrier film 324 include, but are not limited to, Al2O3, SiO2, SiNx, and glass. The deposition method for oxygen barrier film 324 may be a low temperature method in order to not thermally degrade the quantum dots or other fillers. Exemplary conformal deposition methods include atomic layer deposition (ALD), sputtering, spin on, and lamination. The oxygen barrier film may also be blanket deposited over the entire substrate, or over all of the micro LED devices. In an embodiment, an Al2O3 oxygen barrier film is deposited by atomic layer deposition (ALD).
Referring briefly back to
A color filter layer 328 may optionally be formed over the wavelength conversion layer 110 to filter out colors emitting through the wavelength conversion layer 110 other than those desired and sharpen the emission spectrum of the light emitting device. By way of example, a red color filter layer 328 may be placed over a wavelength conversion layer 110 including red emitting phosphor particles in order to filter out colors other than red, a green color filter layer 328 may be placed over a wavelength conversion layer 110 including green emitting phosphor particles in order to filter out colors other than green, and a blue color filter layer 328 may be placed over a wavelength conversion layer 110 including blue emitting phosphor particles in order to filter out colors other than blue. Referring back to
Referring now to
Up until this point the passivation layer 316 and wavelength conversion layer 110 have been described as separate layers. However, in some embodiments the wavelength conversion layer 110 can also function as a passivation layer. Referring now to
It is to be appreciated that while
Referring now to
In an embodiment, patterned bank layer 304 is formed of an insulating material and may be formed by a variety of techniques such as lamination, spin coating, CVD, and PVD. Patterned bank layer 304 may be may be opaque, transparent, or semi-transparent to the visible wavelength. Patterned bank layer 304 may be formed of a variety of materials such as, but not limited to, photodefinable acrylic, photoresist, silicon oxide (SiO2), silicon nitride (SiNx), poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polyimide, acrylate, epoxy, and polyester. In an embodiment, patterned bank layer is formed of an opaque material such as a black matrix material. The patterned bank layer openings may be formed using a suitable technique such as lithography, and may expose the bottom electrode 310.
A reflective layer 312 is then formed over the patterned bank layer 304 and within the openings spanning the sidewalls and bottom surface of each of the openings. The reflective layer may be electrically conducting. In an embodiment, the reflective layer 312 functions as the bottom electrode and a separate bottom electrode 310 is not required. The reflective layer 312 may be formed of a number of conductive and reflective materials, and may include more than one layer. In an embodiment, a reflective layer 312 comprises a metallic film such as aluminum, molybdenum, titanium, titanium-tungsten, silver, or gold, or alloys thereof. The reflective layer 312 may also include a conductive material which is not necessarily reflective, such as amorphous silicon, transparent conductive oxides (TCO) such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO), carbon nanotube film, or a transparent conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, polyacetylene, polypyrrole, and polythiophene. In an embodiment, the reflective layer includes a stack of a conductive material and a reflective conductive material. In an embodiment, the reflective layer includes a 3-layer stack including top and bottom layers and a reflective middle layer wherein one or both of the top and bottom layers are transparent. In an embodiment, the reflective layer includes a conductive oxide-reflective metal-conductive oxide 3-layer stack. The conductive oxide layers may be transparent. For example, the reflective layer 312 may include an ITO-silver-ITO layer stack. In such a configuration, the top and bottom ITO layers may prevent diffusion and/or oxidation of the reflective metal (silver) layer. In an embodiment, the reflective layer includes a Ti—Al—Ti stack. In an embodiment, the reflective layer includes an ITO-Ti—ITO stack. In an embodiment, the reflective layer includes an ITO-Ti—Al—Ti—ITO stack. In an embodiment, the reflective layer is 1 μm or less in thickness. The reflective layer may be deposited using a suitable technique such as, but not limited to, PVD.
Referring to
Referring now to the light distribution layer 320, in the embodiment illustrated in
Similar to
Referring now to
Referring now to
Referring now to
Referring to
Referring to
In some embodiments, the display 1230 includes one or more micro LED devices 100 that are formed in accordance with embodiments of the invention described above. For example, the display 1230 may include a plurality of micro LED devices and a plurality of wavelength conversion layers around the micro LED devices.
Depending on its applications, the display system 1200 may include other components. These other components include, but are not limited to, memory, a touch-screen controller, and a battery. In various implementations, the display system 1200 may be a television, tablet, phone, laptop, computer monitor, kiosk, digital camera, handheld game console, media display, ebook display, or large area signage display.
In utilizing the various aspects of this invention, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for integrating micro LED devices and wavelength conversion layers into lighting and display application. Although the present invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed invention useful for illustrating the present invention.
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