Patent ID: 12199213

DESCRIPTION OF THE EMBODIMENTS

Related Art and Preliminary Configuration Examples

Prior to a specific description of an embodiment of the present disclosure, the related art and preliminary configuration examples will be summarized below.

The display device in the related art which is described in Japanese Unexamined Patent Application Publication No. 2002-141492 has already been practically used in a case that an organic EL (electroluminescence) is deposited as a light-emitting layer on a silicon substrate having a drive circuit thereon. However, there is room for improvement in terms of luminance and durability.

In the liquid crystal display device in the related art described in International Publication No. 2010/143461, a structure in which a gap between adjacent phosphors and color filters is filled with a black matrix, and the black matrix includes reflective bodies covering sidewalls and an absorbing body has been disclosed. This technology is based on a liquid crystal display device, and the target of the technology is a direct-view, large display device.

The target of the related art described in International Publication No. 2017/130607 is a light source for illumination and is not an image display device.

In the compact projection-type display device used in the glasses-type terminal for AR or the like, it is necessary to reduce the pixel size to about several micrometers in order to realize high resolution. On the other hand, in order to cause large portion of excitation light to be absorbed by a wavelength conversion layer to perform wavelength conversion, the thickness of the wavelength conversion layer is in the range of from several micrometers to more than 10 μm, resulting in an increase in an aspect ratio (ratio of height/width) of the wavelength conversion layer (for example, 2 or more), and thus it is very difficult to form a pattern of the wavelength conversion layer.

In order to facilitate the formation of a pattern of the wavelength conversion layer, it is preferable to reduce the thickness of the wavelength conversion layer. For this reason, the inventors of the present disclosure have studied an improvement in the conversion efficiency by disposing a layer (hereinafter referred to as a reflection layer) that transmits excitation light and that reflects long-wavelength light whose wavelength has been down-converted on the excitation light incident side of a wavelength conversion layer, and disposing a layer (hereinafter referred to as a transmission layer) that reflects the excitation light and that transmits the long-wavelength light whose wavelength has been down-converted on the emission side of the wavelength conversion layer to efficiently emit the long-wavelength light and to confine the excitation light in the wavelength conversion layer.

However, steps of forming the reflection layer and the transmission layer must be newly added, resulting in an increase in the number of production steps. In addition, such a transmission layer and a reflection layer are usually formed of dielectric multilayer films, and such dielectric multilayer films have a problem in that the films tend to degrade due to, for example, moisture absorption after deposition.

In particular, in the case of the reflection layer, it is necessary to perform a step of forming a wavelength conversion layer after deposition. Consequently, a plurality of wet steps are performed, and degradation of the reflection layer tends to occur in the production process. Furthermore, the reflection layer needs to efficiently reflect both red light and green light, and has a more complex structure than the transmission layer. A structure for producing such a complex reflection layer more simply and stably has been required. In particular, realization of a device structure capable of realizing a reflection layer having high durability is an important issue.

In view of the above, an image display device that is easily produced with a relatively small number of production steps is realized by the configuration of an image display device described in each embodiment below.

In the description of embodiments below, a detailed description relating to a drive circuit substrate50will be omitted. This is because an example of the drive circuit substrate50is a silicon substrate (semiconductor substrate) on which LSIs are formed, and can be produced by a known technique. In addition, a micro LED element (hereinafter also referred to as a micro LED or a micro light-emitting element) may have various planar shapes such as a rectangular shape, a polygonal shape, a circular shape, or an elliptical shape, and the longest length of the planar shape is assumed to be 60 μm or less. It is assumed that an image display device200includes 3,000 or more micro light-emitting elements integrated in a pixel region1.

In the following, a description will be made of only a case where a micro LED element100is formed of a nitride semiconductor that emits light in a wavelength range from ultraviolet light to green. However, the micro LED element100can be replaced with an AlInGaP-based element that emits light in a wavelength range from yellowish green to red, or an AlGaAs-based or GaAs-based element that emits light in a wavelength range from red to infrared light.

In addition, a description will be made of only a configuration in which an N-side layer11is disposed on the light emission side of a nitride semiconductor layer14that forms the micro LED element100. Alternatively, a configuration in which a P-side layer13is disposed on the light emission side may also be used. Each of the N-side layer11, a light emission layer12, and the P-side layer13is usually not a single layer but is optimized by including a plurality of layers. However, such structures do not directly relate to the configuration of the present disclosure, and thus the detailed structure of each layer is not described. While the light emission layer is generally disposed between an N-type layer and a P-type layer, the N-type layer and the P-type layer may include non-doped layers or layers having dopants with opposite conductivity. Therefore, the layers that sandwich the light emission layer therebetween are hereinafter described as an N-side layer and a P-side layer.

First Embodiment

Hereinafter, an image display device200according to a first embodiment of the present disclosure, the image display device200having micro LED elements100as a light source thereon, will be described with reference toFIGS.1to6B.FIG.1is a sectional view of an image display device200including a plurality of micro LED elements100.FIG.2is a top view of a pixel region of the image display device200.FIGS.3A to6Bare sectional views illustrating a process for producing the micro LED elements100and the image display device200.

Overall Configuration

As illustrated inFIG.1, an image display device200includes a pixel region1, a common interconnection region2, a dummy region3, and a peripheral region4. In the pixel region1, pixels5are arranged in an array, and each of the pixels5includes a blue sub-pixel6, a red sub-pixel7, and a green sub-pixel8as illustrated inFIG.2. The blue sub-pixel6, the red sub-pixel7, and the green sub-pixel8emit blue light, red light, and green light, respectively, and light having various colors can be emitted as the pixel5by adjusting the intensities of the respective types of light.FIG.1illustrates a sectional view taken along line I-I inFIG.2. The blue sub-pixel6, the red sub-pixel7, and the green sub-pixel8include micro LEDs100B,100R, and100G, respectively. The micro LEDs100B,100R, and100G have the same structure and emit blue light (excitation light). Hereinafter, in the case where all the micro LEDs100B,100R, and100G are collectively denoted, the micro LEDs100B,100R, and100G are referred to as micro LEDs100. InFIG.2, the pixels each have a square shape, and the sub-pixels each have a rectangular shape with longer sides having the same length as the pixels. However, the shape of the pixels and the shape of the sub-pixels may be other shapes, and the sub-pixels are not limited to the three types of blue, red, and green.

The micro LED elements100B,100R, and100G each include a nitride semiconductor layer14, a P-electrode19P (first electrode), and a common N-electrode56(second electrode). The common N-electrode56is disposed on the light emission surface side, and the P-electrode19P is disposed on the drive circuit substrate50side. The P-electrode19P is connected to a P-drive electrode51on the drive circuit substrate50. The common N-electrode56is connected to an N-drive electrode52on the drive circuit substrate50through a plug55in the common interconnection region2. The micro LED elements100are supplied with a current from the corresponding P-drive electrodes51to emit light. The light is emitted in a direction opposite to the direction toward the drive circuit substrate50, that is, in the direction toward the common N-electrode56side. The micro LEDs100B,100R, and100G individually divided by a pixel isolation trench15, and the pixel isolation trench15is filled with a filling material20. The isolation of the micro LED elements100is desirable from the viewpoint of preventing light crosstalk between pixels. When the nitride semiconductor layers14is connected between adjacent micro LED elements100, light generated in any of the micro LED elements100is emitted from the adjacent pixel to the outside through the nitride semiconductor layer14(light crosstalk). Light crosstalk decreases the contrast and color purity of a display image, and thus is not preferable. The filling material20prevents light crosstalk and planarizes the surface to facilitate the formation of the common N-electrode56and wavelength conversion portions and a light convergence portion on the common N-electrode56.

The peripheral region4defines the outer edge of the image display device200and includes a scribe region for cutting the image display device200into individual pieces and a connection portion for connecting to an external circuit, such as a wire-bonding pad. In the peripheral region4, the nitride semiconductor layer14has been removed. The dummy region3is a region other than the pixel region1, the common interconnection region2, and the peripheral region4of the image display device200. Although no light is emitted, the nitride semiconductor layer14is disposed in this region so as to ensure flatness of the surface.

In the pixel region1of the drive circuit substrate50, pixel driving circuits for respective pixels are arranged. Mainly in the dummy region3, a row selection circuit, a column signal output circuit, an image processing circuit, an input-output circuit, and other circuits are arranged. A dummy-drive electrode53on the drive circuit substrate50is disposed to fix the nitride semiconductor layer14and keep light from entering these circuits.

Configuration of Pixels

The blue sub-pixel6has, on the common N-electrode56, a transparent portion21formed of a transparent resin pattern that contains scattering particles, and the emission direction of blue light emitted from the micro LED100B is broadened by the scattering particles. However, the blue sub-pixel6emits the blue light to the outside as it is without performing wavelength conversion. The transparent portion21need not contain scattering particles. The transparent portion21increases the blue light emission compared with the case without the transparent portion21. If the transparent portion21is not there, the blue light goes to air directly from the nitride semiconductor layer14so that the blue light emission decreases due to larger difference of refractive index than the case that the transparent portion21exists. The red sub-pixel7has a red wavelength conversion portion22which is a resin pattern containing a material that performs wavelength conversion from the blue light emitted from the micro LED100R to red light (long-wavelength light), and emits red light. The green sub-pixel8has a green wavelength conversion portion23which is a resin pattern containing a material that performs wavelength conversion from the blue light emitted from the micro LED100G to green light (long-wavelength light), and emits green light.

The micro LEDs100B,100R, and100G include the nitride semiconductor layer14. The nitride semiconductor layer14includes, from the light emission surface side, an N-side layer11, a light emission layer12, and a P-side layer13in that order, and a reflection layer10is included inside the N-side layer11. The reflection layer10may be disposed on an end portion of the N-side layer11, and such a case is also included in the case where “the reflection layer10is included inside the N-side layer11”. In the present embodiment, the reflection layer10is included inside the nitride semiconductor layer14and is disposed on the wavelength conversion layer side with respect to the light emission layer12.

According to the above configuration, since the reflection layer10is included inside the N-side layer11, the reflection layer10can be easily produced. The reason for this is as follows. For example, even when micro LEDs100that do not include a reflection layer10are formed on the drive circuit substrate50, and a reflection layer10is then formed on the micro LEDs100, the same effects as those of the present embodiment can be achieved. In this case, however, a step of depositing the reflection layer10and a step of dividing the reflection layer10for each micro LEDs100are additionally required. In addition, a step of filling gaps between divided portions of the reflection layer10is also necessary. These steps are not necessary when the reflection layer10is included inside the N-side layer11. Accordingly, the reflection layer10is preferably included inside the N-side layer11. However, for example, in small-volume production in which the increase in the number of steps does not cause a problem, the reflection layer10may be formed after the formation of the micro LEDs100.

As described above, the image display device200is an image display device200including a drive circuit substrate50, micro LED elements100, and a red wavelength conversion portion22and a green wavelength conversion portion23(wavelength conversion layers) that convert light emitted from the micro LED elements100and that emit converted light to a side opposite to the drive circuit substrate50, the micro LED elements100, the red wavelength conversion portion22, and the green wavelength conversion portion23(wavelength conversion layers) being sequentially stacked on the drive circuit substrate50. The micro LED elements100include a reflection layer (first multilayer film)10that reflects the light down-converted by the wavelength conversion layers.

According to the above configuration, since the reflection layer10is made of a nitride semiconductor, the reflection layer10is very stable, and degradation in the subsequent process is suppressed. In addition, the reflection layer10can be easily formed. Accordingly, the reflection layer10can be easily and stably produced. This configuration also has an advantage that light crosstalk is not increased. In the case where a reflection layer is disposed between the micro LEDs100and the wavelength conversion portions22and23without dividing the reflection layer for each micro LED100, light crosstalk is generated through the reflection layer. However, since the reflection layer10is formed inside the micro LEDs100in this configuration, additional light crosstalk is not generated.

As described above, in the image display device200, the micro LED elements100emit blue light, and the red wavelength conversion portion22and the green wavelength conversion portion23(wavelength conversion layers) convert the blue light into long-wavelength light (red light and green light).

According to the above configuration, the image display device200emits, for example, blue light as an example of excitation light and can further convert the blue light into long-wavelength light such as red light and green light with the wavelength conversion layers.

The reflection layer10(first multilayer film) is formed of a multilayer structure of nitride semiconductor materials and has a characteristic of transmitting blue light (excitation light) and reflecting light (long-wavelength light) having longer wavelengths than blue light.

According to the above configuration, since the reflection layer10includes nitride semiconductor materials, the reflection layer10is very stable, and degradation of the reflection layer10does not occur in the subsequent process.

The reflection layer10(first multilayer film) has high reflection properties at least in the green region (for example, wavelength: 520 nm±15 nm) and the red region (for example, wavelength: 630 nm±15 nm). In the red sub-pixel7, part of red light generated by the red wavelength conversion portion22is incident on the micro LED100R, but is reflected at the reflection layer10, transmitted again through the red wavelength conversion portion22, and emitted to the outside. In the case where the reflection layer10is not provided, the red light incident on the micro LED100R is repeatedly reflected at the interface between the P-electrode19P and the P-side layer13and at sidewalls of the nitride semiconductor layer14, and considerable portion (25% or more) of the red light is absorbed inside the micro LED100R. The reflectance of visible light at a nitride semiconductor/metal electrode interface is generally low, and thus the loss is large. Only when the metal electrode is made of silver, the reflectance of visible light is 90% or more. However, it is difficult to establish an ohmic contact with the P-layer, and silver tends to cause failures due to migration. Thus, it is difficult to apply silver to the structure as illustrated inFIG.1. In the case where palladium, which easily establishes an ohmic contact, is used as the P-electrode19P, the reflectance is only approximately 50%. Even when a composite layer of Ni/ITO is used as the P-electrode19P, the reflectance is about 50% or less. In the case where a light-absorbing material is used for the filling material20in order to keep light from entering to adjacent micro LEDs100, light is strongly absorbed by the sidewalls of the nitride semiconductor layer14, and red-light absorption inside the micro LED100R further increases. Accordingly, in reality, it is necessary to increase the reflectance by using the reflection layer10formed of a multilayer film as described above. As a result, the red-light extraction efficiency can be improved, and the red-light emission efficiency can be enhanced. This also applies to the green sub-pixel8.

It is not necessary that the reflection layer10have a high reflectance over the entire region of a wavelength that is longer than that of blue light, and, in some cases, the reflection layer10preferably has peak reflectances in the green region and the red region. In some cases, the red wavelength conversion portion22and the green wavelength conversion portion23have broad light emission peaks. In such a case, the spectra of long-wavelength light emitted from the respective wavelength conversion portions are formed to be sharp by strongly reflecting light in the green region and the red region. As a result, the color purity can be enhanced. The peak value of the reflectance is preferably 70% or more.

In the red sub-pixel7and the green sub-pixel8, a transmission layer25is disposed on the red wavelength conversion portion22and the green wavelength conversion portion23. The transmission layer25has a characteristic of reflecting blue light (excitation light) and transmitting light (long-wavelength light) having a longer wavelength than blue light.

The transmission layer25is formed of, for example, a dielectric multilayer film including a titanium oxide thin film and a silicon dioxide thin film. In the red sub-pixel7, red light generated by the red wavelength conversion portion22is transmitted through the transmission layer25and emitted to the outside. However, blue light is reflected at the transmission layer25and returned to the red wavelength conversion portion22, and thus is absorbed again in the red wavelength conversion portion22. Light that has traveled to the micro LED100R side without being absorbed by the red wavelength conversion portion22is transmitted through the reflection layer10and incident on the P-electrode19P/P-side layer13interface. Accordingly, since the blue light is confined between the transmission layer25and the P-electrode19P/P-side layer13interface, the amount of blue light emitted to the outside is extremely small. In addition, while blue light passes through the red wavelength conversion portion22a number of times, wavelength conversion proceeds, and the conversion efficiency increases. Thus, the emission of blue light to the outside can be reduced by providing the transmission layer25to enhance the conversion efficiency in the red wavelength conversion portion22. The thickness of the red wavelength conversion portion22can be further reduced by using this effect. This also applies to the green sub-pixel8.

In other words, since the transmission layer25can reflect blue light and transmit light having a longer wavelength than the blue light, emission of the blue light from the red sub-pixel7and the green sub-pixel8is prevented, and the blue light can be efficiently subjected to wavelength conversion. As a result, color purities of the red sub-pixel7and the green sub-pixel8improve, and the light emission efficiency of the image display device200can be improved. Furthermore, the reduction in the thickness of the wavelength conversion layer facilitates the production.

Since the dielectric multilayer film constituting the transmission layer25has high hygroscopicity and easily degrades, the whole of the dielectric multilayer film is preferably covered with a passivation film26. The passivation film26may be a CVD film such as a silicon nitride film or made of a resin material such as a silicone resin.

Production Method

Next, an example of a method for producing micro LED elements100will be described with reference toFIGS.3A to6B.

As illustrated inFIG.3A, an N-side layer11, a light emission layer12, and a P-side layer13are deposited on a growth substrate9in that order to form a nitride semiconductor layer14. The N-side layer11includes a reflection layer10. As the growth substrate9, for example, a (111) plane silicon substrate can be used. In particular, the growth substrate9preferably has the same size as a drive circuit substrate50. The growth substrate9may be made of, for example, sapphire (Al2O3) or SiC. As a material of the nitride semiconductor layer14, for example, a GaN-based semiconductor can be used. As an apparatus for growing the nitride semiconductor layer14on the growth substrate9, for example, an MOCVD apparatus can be used. The growth substrate9may have a surface with an uneven structure. In the case where the growth substrate9has a surface with an uneven structure, preferably, the surface is once planarized by epitaxial growth, and the reflection layer10is then grown. Preferably, the N-side layer11does not include a high-resistance layer therein because it is necessary to allow a current to flow in a thickness direction thereof, and the N-side layer11is preferably an N-type good conductor across the layer thickness direction. The reflection layer10in this configuration is included in the N-side layer and thus has N-type conductivity. In addition, the warp of the growth substrate9is preferably small at a stage at which the nitride semiconductor layer14is formed on the growth substrate9and the temperature of the resulting growth substrate9is returned to room temperature. In the case of an 8-inch wafer, the warp is preferably 35 μm or less in order to facilitate bonding to the drive circuit substrate50(step illustrated inFIG.3Cdescribed later). Such a reduction in the warp can be realized by providing a suitable buffer layer in the N-side layer11.

The light emission layer12includes a multi-quantum well layer including an InGaN layer and a GaN layer. The N-side layer11and the P-side layer13each have a multilayer structure including various layers. In the present embodiment, specific configurations of the N-side layer11, the light emission layer12, and the P-side layer13are not particularly limited, and, for example, configurations of an N-side layer, a light emission layer, and a P-side layer used in existing LED elements can be appropriately employed. Accordingly, in the present embodiment, the description of the specific configurations of the N-side layer11, the light emission layer12, and the P-side layer13is omitted.

The reflection layer10can be formed by, for example, stacking a plurality of pairs of an AlxGa(1-x)N layer and a GaN layer as illustrated inFIG.22. The total number of the AlxGa(1-x)N layers is 36, and the thickness of each of the layers is about 57 nm to 122 nm. Thirty five GaN layers are included between the AlxGa(1-x)N layers, and the thickness of each of the layers is about 53 nm to 114 nm. The reflection layer10had a total film thickness of about 5.2 m. With this structure, a reflectance of 65% or more was ensured at a wavelength of 520 nm, and a reflectance of 80% or more was ensured at a wavelength of 630 nm.

A thickness to of the N-side layer11is generally m or less and is about 5 μm±2 μm in many cases. A thickness tmqwof the light emission layer12is generally 10 nm or more and 200 nm or less and is about 50 nm or more and 100 nm or less in many cases. A thickness tpof the P-side layer13is generally 50 nm or more and 1,000 nm or less and is about 100 nm or more and 300 nm or less in many cases.

As illustrated inFIG.3B, a P-electrode layer19is formed over the entire surface of the P-side layer13. At this stage, the P-electrode layer19is formed over the entire surface of the wafer serving as the growth substrate9and is not patterned. Examples of the suitable P-electrode layer19include a metal thin film such as a palladium film, which easily establishes an ohmic contact with the P-side layer; a metal multilayer film including an aluminum thin film and a palladium film that has a thickness of about 5 nm and that is disposed at the interface; and a multilayer film including an ITO (indium tin oxide) film serving as a transparent electrode, nickel, and aluminum, on the P-side layer side. It is also preferable for the P-electrode layer19to have a gold or copper layer on the surface side, which is suitable for connecting to a P-drive electrode51on the drive circuit substrate50.

As illustrated inFIG.3C, the growth substrate9in which the P-electrode layer19is formed on the nitride semiconductor layer14is bonded to a drive circuit substrate50such that the surface of the P-electrode layer19side faces the drive circuit substrate50. A driving circuit of the image display device200is formed in the drive circuit substrate50. The driving circuit substrate50includes electric circuits such as a pixel driving circuit configured to drive each of the micro LED elements100, a row selection circuit configured to select a specific row of pixels arranged in a two-dimensional array, a column signal output circuit configured to output a light emission intensity signal in a specific column, and an image processing circuit. On the surface of the drive circuit substrate50, P-drive electrodes51configured to supply a current to the respective micro LED elements100are exposed in the pixel region1, an N-drive electrode52is exposed in the common interconnection region2, a dummy-drive electrode53is exposed in the dummy region3, and an I/O-electrode (external connection electrode)54is exposed in the peripheral region4.FIGS.3A to3Eeach show a schematic sectional view of one image display device200, but the steps are actually performed in a state of a substrate on which a plurality of image display devices200are arranged. The drive circuit substrate50is, for example, an 8-inch silicon substrate, and several hundred driving circuits of the image display devices200are arranged on the substrate. By this bonding, the P-drive electrode51, the N-drive electrode52, the dummy-drive electrode53, and the I/O-electrode54on the drive circuit substrate50are connected to the P-electrode layer19. In this case, the bonding may be performed by connecting the metal electrodes (for example, copper) together either directly or with metal nanoparticles serving as an adhesive layer therebetween. In this bonding step, precise alignment is not necessary. In the case of bonding wafers to each other, it is enough that the wafers overlap with each other. In order to avoid stress caused by expansion and contraction due to heating and cooling at the time of bonding, the growth substrate9and the drive circuit substrate50are preferably made of the same material. In particular, the materials of the growth substrate9and the driving circuit substrate50are each preferably silicon.

Next, in the step inFIG.3D, the growth substrate9is separated. In the case of a silicon substrate, the growth substrate9can be removed by a combination of, for example, grinding, polishing, plasma etching, and wet etching. In the step illustrated inFIG.3C, the nitride semiconductor layer14is bonded to the drive circuit substrate50in the state where the growth substrate9remains. Alternatively, the nitride semiconductor layer14may be bonded to the drive circuit substrate50after the nitride semiconductor layer14is temporarily transferred onto another substrate (transfer substrate), and the transfer substrate may be separated.

Subsequently, a pixel isolation trench15is formed as illustrated inFIG.3E. The pixel isolation trench15is a trench that is formed by etching at least layers from the nitride semiconductor layer14to the P-electrode layer19and that divides the layers. In the pixel region1, the micro LED elements100are individually divided by the pixel isolation trench15. The reflection layer10is also divided by the pixel isolation trench15for each micro LED element100. The P-electrode layer19in the pixel region1serves as a P-electrode19P that is connected to the P-side layer13of the corresponding micro LED element100. A boundary trench15B is simultaneously formed at each boundary between the pixel region1and the common interconnection region2, and between the common interconnection region2and the dummy region3. The common interconnection region2and the dummy region3may further be divided into small pieces by the boundary trench15B. The P-electrode layer19in the common interconnection region2serves as an N-electrode19N that is connected to the N-drive electrode52, and the P-electrode layer19in the dummy region3serves as a dummy P-electrode19D. In the peripheral region4, the nitride semiconductor layer14and the P-electrode layer19are removed (exposed region150), and the I/O-electrode54is exposed. In the common interconnection region2, a common electrode contact hole15H is formed on the N-drive electrode52.

In the sectional view ofFIG.3E, the sectional shape of the pixel isolation trench15is preferably a forward tapered shape formed by sidewalls of each of the micro LED elements100. This is because the pixel isolation trench15is easily filled in a step of forming a filling material20in the subsequent process. If the sectional shape is a reverse tapered shape, bubbles or voids tend to remain on the sidewalls, and variations in the light output tend to occur. However, in the case where the taper angle greatly deviates from 90 degrees, the area of the light emission layer12is decreased. Thus, the taper angle is preferably in the range of from 70 degrees to 110 degrees.

The processing in steps subsequent to this step is performed on the drive circuit substrate50, and each patterning is performed with precise alignment with respect to the drive circuit substrate50. In this process, the step of forming the pixel isolation trench15and the boundary trench15B, and the step of forming the exposed region150and the common electrode contact hole15H may be separately performed.

Subsequently, as illustrated inFIG.4A, the pixel isolation trench15is filled with a filling material20, and the nitride semiconductor layer14is exposed. At the same time, the boundary trench15B, the common electrode contact hole15H, and the exposed region150are also filled with the filling material20. The filling material20is a layer having a main object of planarizing the surface for formation of a common N-electrode in the subsequent process. The filling material20may be a resin material or a CVD film. The filling material20may be a resin to which a pigment that absorbs light, carbon black, or the like is added so as to prevent light from leaking to adjacent pixels. Alternatively, the filling material20may be a resin to which a white pigment serving as a reflective material or scattering particles are added so that the reflection is reinforced to improve the light output of the micro LED elements100. Alternatively, leakage of light to adjacent pixels may be prevented by disposing a layered structure that includes a transparent insulating film and a metal film having a high reflectance on a sidewall of the pixel isolation trench15.

Subsequently, as illustrated inFIG.4B, the filling material20in the common electrode contact hole15H is removed, and as illustrated inFIG.4C, the common electrode contact hole15H is filled with a plug55. The plug55may be made of a material such as tungsten. Furthermore, as illustrated inFIG.4D, a common N-electrode56is formed. The common N-electrode56may be a transparent conductive film, such as ITO, a metal mesh electrode having an opening corresponding to a large portion of the nitride semiconductor layer14and a metal thin film pattern disposed above the pixel isolation trench15, or a combination of these. In the case of the mesh electrode, the mesh electrode may also function as a planarization portion24described below. The common N-electrode56is connected to the N-side layer11of the micro LED elements100, and is connected to the N-drive electrode52through the plug55in the common interconnection region.

Next, as illustrated inFIG.4E, a green wavelength conversion portion23is formed on a green sub-pixel8. This step can be performed by a photolithography technique using a negative resist mixed with wavelength conversion particles. Alternatively, the green wavelength conversion portion23can be formed by forming a mold using a positive resist, applying thereon a resin mixed with wavelength conversion particles and scattering particles to fill the recess, removing the resin material remaining on the flat portion, and further removing the positive resist material. The wavelength conversion particles may be phosphor particles, quantum dots, or quantum rods.

Similarly, as illustrated inFIGS.5A and5B, a red wavelength conversion portion22and a transparent portion21are formed on a red sub-pixel7and a blue sub-pixel6, respectively. The green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21preferably have substantially the same thickness. This is because when the thicknesses of the wavelength conversion portions and the transparent portion in the sub-pixels are different from each other, the difference in light-emission-distribution characteristics among the sub-pixels increases, resulting in a problem in that the color varies depending on the viewing direction. In addition, there is also an advantage of ensuring flatness of the surface and facilitating the formation of a transmission layer25and a passivation film26in the subsequent process.

Subsequently, as illustrated inFIG.5C, a planarization portion24is formed. The planarization portion24is formed for the purpose of filling gaps between the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21in the pixel region1to planarize the surface and planarizing regions other than the pixel region1to facilitate the formation of a transmission layer25and a passivation film26in the subsequent process. The planarization portion24is formed of a resin material and may be formed of a resin to which a pigment that absorbs light, carbon black, or the like is added so as to prevent light from leaking to adjacent sub-pixels. In contrast, the planarization portion24may be a resin to which a white pigment serving as a reflective material or scattering particles are added so that the reflection is reinforced to improve the light output of the sub-pixels. That is, the planarization portion24is a light-shielding material that prevents light from leaking to adjacent sub-pixels by light absorption or reflection.

In the present embodiment, after the formation of the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21, the planarization portion24is formed. Alternatively, the mold described above may be formed, the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21may be subsequently formed, and the mold may be left to function as the planarization portion24. In this case, after the formation of the planarization portion24, recesses are formed in regions where the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21are to be formed, and the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21are subsequently formed in the recesses. The planarization portion24may be formed of a transparent resin material whose surface is covered with a metal film or a metal material besides the resin material described above. In such a case, the metal film or the metal material is preferably silver or aluminum, which has a high reflectance.

The planarization portion24is preferably disposed above the filling material20as illustrated inFIG.1. In other words, preferably, the transparent portion21and the wavelength conversion portions22and23completely cover the light emission surfaces of the micro LED elements100. This is because if the bottom surface of the planarization portion24overlaps the light emission surfaces of the micro LED elements100, the light extraction efficiency from the micro LED elements100to the transparent portion21and the wavelength conversion portions22and23decreases. The side surfaces of the planarization portion24are preferably inclined in order to improve the light extraction efficiency from the transparent portion21and the wavelength conversion portions22and23. In a range of 90 degrees or less, the smaller the angle of inclination, the better. However, preferably, the bottom surface of the planarization portion24does not overlap the light emission surfaces of the micro LED elements100as described above.

Furthermore, as illustrated inFIG.5D, a transmission layer25is formed. The transmission layer25is formed by depositing a dielectric multilayer film, and the dielectric multilayer film is left only on the red sub-pixel7and the green sub-pixel8and removed from regions other than these sub-pixels. The dielectric multilayer film can be processed by patterning with a typical lithography technique. As illustrated inFIG.24, the dielectric multilayer film is, for example, a film in which eight titanium oxide (TiO2) layers from 16 nm to 78 nm and seven silicon oxide (SiO2) layers from 11 nm to 90 nm are alternately deposited and has a total film thickness of 780 nm. For blue light (around 460 nm), the transmission layer25has a high reflectance of 90% or more at an angle of incidence of 20 degrees or less. In contrast, for green light and red light, the transmission layer25has a reflectance of 5% or less and has a high transmittance. Prior to the formation of the transmission layer25, a transparent resin layer may be formed in order to planarize the surface.

As illustrated inFIG.6A, a passivation film26is deposited on the transmission layer25. The passivation film26is preferably provided as a protective film because the quality of the transmission layer25tends to change due to, for example, moisture absorption. The passivation film is, for example, a silicon nitride film formed by a plasma CVD method or the like or a silicone resin.

Subsequently, the passivation film26, the planarization portion24, and the filling material20in the peripheral region4are removed, and an I/O-electrode54is exposed at the surface. Lastly, image display devices200formed on the drive circuit substrate50are individually cut and each mounted in a package.

When the common electrode56, the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21are formed, the surface of the image display device200is preferably flat over the entire surface as illustrated inFIG.4C. Since a resin layer is applied in many cases, unless the surface is flat, there may be problems in that non-uniformity such as striation is generated during the application and that a uniform wavelength conversion layer cannot be formed. In the case where the nitride semiconductor layer14is not provided in the dummy region3, a difference in height of several micrometers, which corresponds to the thickness of the nitride semiconductor layer14, is generated, and thus such flatness is not ensured, resulting in a serious problem. Accordingly, the nitride semiconductor layer14in the dummy region3is necessary, and the dummy-drive electrode53for fixing the nitride semiconductor layer14is also necessary.

Furthermore, when the transmission layer25is formed, the flatness is similarly required, and the planarization portion24is preferably provided. The step of exposing the peripheral region including the I/O-electrode54is preferably performed after the formation of the common electrode56, the green wavelength conversion portion23, the red wavelength conversion portion22, the transparent portion21, and the transmission layer25.

Effects of Reflection Layer and Transmission Layer

The green wavelength conversion portion23and the red wavelength conversion portion22are each formed by using quantum dots as wavelength conversion particles, dispersing the quantum dots in a negative resist, and patterning the resist by a photolithography technique. The film thickness of each of the portions after patterning was 8 m. The amounts of quantum dots dispersed were each adjusted such that the transmission intensity of blue light (peak wavelength: 450 nm, peak half-width: 17 nm) serving as excitation light was 1%. The peak wavelength of light emitted from the green wavelength conversion portion23is 530 nm and the half-width of the peak is 30 nm. The peak wavelength of light emitted from the red wavelength conversion portion22is 630 nm and the half-width of the peak is 32 nm.

The dielectric multilayer film constituting the transmission layer25is formed by stacking seven pairs of a TiO2thin film (thickness: 35.8 nm) and a SiO2thin film (thickness: 76.8 nm) by an ion-beam vapor deposition method. After the stacking, a resist pattern was left only in portions of the red sub-pixel7and the green sub-pixel8by a photolithography technique, and a portion of the dielectric multilayer film, the portion being other than the portions of the red sub-pixel7and the green sub-pixel8, was removed by a dry etching technique to form the transmission layer25. A silicone resin was used as the passivation film26. When the transmission layer25is not provided in the portions of the red sub-pixel7and the green sub-pixel8, the green wavelength conversion portion23and the red wavelength conversion portion22need to have a thickness of 8 μm in order that the amount of leakage of blue light (amount of energy) in the red sub-pixel7and the green sub-pixel8is 1/100 relative to red light and green light, respectively. In contrast, when the transmission layer25is provided, the film thickness of the green wavelength conversion portion23and the red wavelength conversion portion22could be reduced to 4.2 m and 4.0 m, respectively. Accordingly, the thickness of each of the wavelength conversion portions could be reduced to about half by the transmission layer25. This enables the aspect ratio of each of the wavelength conversion portions to be significantly reduced, and thus miniaturization of the pixels can be easily realized.

Meanwhile, by providing the reflection layer10in the nitride semiconductor layer14, the amount of red light emitted from the red sub-pixel7was improved by about 8% compared with the case where the reflection layer10was not provided. The reason for this is as follows. In the case where the reflection layer10is not provided, about half of red light generated in the red wavelength conversion portion22travels to the micro LED100side, is lastly reflected inside the micro LED100, and returns from the micro LED100to the red wavelength conversion portion22. However, a large loss is generated because the reflectance of the reflection is low. In contrast, a larger amount of red light is returned from the micro LED100to the red wavelength conversion portion22by the reflection layer10to enhance the red-light extraction efficiency. Presumably, the amount of light emission can be further improved by further improving the reflectance of the reflection layer10.

Regarding green light, the amount of light emission was improved by about 10% compared with the case where the reflection layer10was not provided. The mechanism of the improvement is the same as that in the case of red. The factor of the difference in degree of the improvement is considered to be one or both of the following. (1) Since the reflectance of green light at the P-side layer13/P-electrode19P interface is lower than that of red light, the improvement effect seems to be higher. (2) Regarding characteristics of the reflection layer10, the reflectance to green light became higher than the reflectance to red light.

As described above, the thicknesses of the green wavelength conversion portion23and the red wavelength conversion portion22can be reduced and light emission efficiencies can be improved by providing the transmission layer25on the green wavelength conversion portion23and the red wavelength conversion portion22(light emission side) and providing the reflection layer10under the green wavelength conversion portion23and the red wavelength conversion portion22(excitation light source side). Miniaturization is facilitated, and the amounts of expensive wavelength conversion materials used are reduced. Thus, an effect of reducing the production cost is also achieved.

Second Embodiment

The present embodiment differs from the first embodiment in that the transmission layer25is not provided. Other configurations of the present embodiment are the same as those of the first embodiment.

The transmission layer25can be omitted when the micro LEDs100each have a relatively large size, and the thicknesses of the green wavelength conversion portion23and the red wavelength conversion portion22can be made large enough to prevent blue light from leaking. This is because an increase in the number of steps is thereby suppressed and equipment such as an apparatus for forming a dielectric multilayer film can be reduced. Even when the transmission layer25is omitted, the effect of improving the light extraction, the effect being achieved by the reflection layer10, is useful. In addition, once the formation of the reflection layer10is incorporated in the step of forming the nitride semiconductor layer14, an increase in the cost due to the reflection layer10is slight.

An image display device200ainFIG.7Acan be produced through the steps up toFIG.5C. The passivation film26may be added to this structure. An image display device200binFIG.7Bhas a structure in which a blue-light absorbing filter (filter layer that absorbs excitation light)29that absorbs blue light is further provided on a red sub-pixel7and a green sub-pixel8of the image display device200a. When the output of the micro LEDs100has surplus power, the output of the micro LEDs100may be increased in some cases in order to obtain desired red light/green light. In such a case, blue light which is excitation light leaking from the green wavelength conversion portion23and the red wavelength conversion portion22is absorbed by the blue-light absorbing filter29to thereby prevent color purities of the red light/green light from decreasing. According to the present embodiment, the light output can be improved in the image display device200aand the image display device200bwhile an increase in the cost is reduced to the minimum.

Third Embodiment

The present embodiment differs from the second embodiment in that none of the green wavelength conversion portion23, the red wavelength conversion portion22, and the transparent portion21is provided, a yellow wavelength conversion portion30is provided over the pixels, and color filters of blue, green, and red are provided. Other configurations of the present embodiment are the same as those of the second embodiment.

As illustrated in an image display device200cinFIG.8, a yellow wavelength conversion portion30is formed over the pixel region1. The yellow wavelength conversion portion30is excited by blue light and emits yellow light, and white light is emitted as a whole. A blue color filter31, a red color filter32, and a green color filter33are disposed on a blue sub-pixel6, a red sub-pixel7, and a green sub-pixel8, respectively, and the blue sub-pixel6, the red sub-pixel7, and the green sub-pixel8emit blue light, red light, and green light, respectively. That is, the red color filter32transmits only red light and does not transmit blue light (excitation light) and green light (part of long-wavelength light). The green color filter33transmits only green light and does not transmit blue light (excitation light) and red light (part of long-wavelength light). The blue color filter31transmits blue light and does not transmit red light and green light (long-wavelength light). InFIG.8, the blue color filter31, the red color filter32, and the green color filter33are disposed to be separated from each other, but may be disposed to be in close contact with each other. The yellow wavelength conversion portion30is disposed to extend over a plurality of pixels. Alternatively, the yellow wavelength conversion portion30may be divided by the planarization portion24for each sib-pixel, as illustrated inFIG.1.

YAG phosphor fine particles can be used in the yellow wavelength conversion portion30. YAG phosphors have higher stability than quantum dots and can be used at relatively high temperatures. Accordingly, the image display device200ccan be operated at higher power than devices that use quantum dots or other phosphor materials. Thus, this configuration is useful when a high light output is required.

The process for producing this configuration is very simple because the yellow wavelength conversion portion30is merely formed as the wavelength conversion portion. The wavelength conversion portion need not be processed for each pixel, and a commonly used color filter technology can be employed. Thus, the production process is easily performed in terms of technology. Meanwhile, the effect of improving the light extraction, the effect being achieved by the reflection layer10, is useful. In addition, once the formation of the reflection layer10is incorporated in the step of forming the nitride semiconductor layer14, an increase in the cost due to the reflection layer10is slight.

According to the present embodiment, the light output can be improved in the image display device200cwhile an increase in the cost is reduced to the minimum.

Fourth Embodiment

The present embodiment differs from the first embodiment in the method for producing the transmission layer25. Other configurations of the present embodiment are the same as those of the first embodiment.

In the first embodiment, after the formation of the wavelength conversion portions, the transmission layer25is formed on the wavelength conversion portions. Accordingly, since the dielectric multilayer film constituting the transmission layer25is formed on a resin layer, there is an upper limit to the formation temperature, and stability of the dielectric multilayer film is limited. In order to form a more stable transmission layer25, a transmission layer25fis obtained by forming a dielectric multilayer film over the entire surface of a transparent substrate34, and removing a portion of the dielectric multilayer film located in a portion corresponding to a blue sub-pixel6. As illustrated inFIG.9A, the transparent substrate34and the transmission layer25fobtained as described above are bonded to the drive circuit substrate50prepared in the step inFIG.5C. Thus, an image display device200dillustrated inFIG.9Bis formed. A transparent adhesive may be used in the bonding. As the transparent substrate, quartz, sapphire, heat-resistant glass, or the like can be used, and the transmission layer25fcan be formed at a high temperature. Therefore, a more stable member is obtained.

This configuration also achieves the same effects as those in the first embodiment. Furthermore, the transmission layer25fis formed at a high temperature and thus has durability. Since the pixel region1of the image display device200dis sealed with the transparent substrate34, durability can be improved.

Fifth Embodiment

The present embodiment differs from the first embodiment in the configuration of the reflection layer10. Other configurations of the present embodiment are the same as those of the first embodiment.

The present embodiment differs from the first embodiment in that the reflection layer10is provided in the N-side layer11in the first embodiment whereas a reflection layer is provided in a P-side layer13in micro LED elements100e(collectively referring to a micro LED element B100Be, a micro LED element R100Re, and a micro LED element G100Ge) illustrated inFIG.10. Even when a reflection layer10eis disposed in the P-side layer13, reflection properties for green light and red light generated in the green wavelength conversion portion23and the red wavelength conversion portion22, respectively, do not significantly change. This is because the light emission layer12hardly absorbs green light and red light. The reflection layer10eis disposed in the P-side layer13and thus has P-type conductivity. In the present embodiment, the reflection layer10eis included inside the nitride semiconductor layer14eand disposed on the drive circuit substrate50side with respect to the light emission layer12. The reflection layer10eis also divided by the pixel isolation trenches15for each micro LED element100e, and therefore it is effective in reducing the light crosstalk.

Furthermore, the light output can be improved by adding a high reflectance for blue light (excitation light) in addition to red light/green light (long-wavelength light) in the reflection layer10e.

As described above, in an image display device200e, the reflection layer10eis disposed on the drive circuit substrate50side with respect to the light emission layer12in a nitride semiconductor layer14ethat forms the micro LED elements100eand also reflects blue light (excitation light).

The reflection layer10ehas high reflection properties at least in the green region (for example, wavelength: 520 nm±15 nm), the red region (for example, wavelength: 630 nm±15 nm), and the blue region (for example, wavelength: 460 nm±15 nm). This is because since a high reflectance for blue light which is excitation light can also be realized on the P-side layer13side, the light output of the micro LED element B100Be, the micro LED element R100Re, and the micro LED element G100Ge can be improved. Accordingly, the light output of a blue sub-pixel6improves, the light output of a red sub-pixel7and a green sub-pixel8also improves, and the light emission efficiency of the entire image display device200ecan be improved.

According to the present embodiment, the light output can be improved in the image display device200e.

Sixth Embodiment

The present embodiment differs from the first embodiment in micro LED elements100f. Other configurations of the present embodiment are the same as those of the first embodiment. The micro LED elements100of the first embodiment are so-called upper/lower electrode-type elements that include the P-electrode19P on the drive circuit substrate50side and the common N-electrode56on the light emission side. The micro LED elements100fof the present embodiment has a configuration in which a P-electrode and an N-electrode are each disposed on one side.

As illustrated inFIG.11, micro LED elements100f(collectively referring to a micro LED element B100Bf, a micro LED element R100Rf, and a micro LED element G100Gf) each have a P-electrode19fP and an N-electrode19fN on the drive circuit substrate50fside. The drive circuit substrate50fhas a P-drive electrode51fand an N-drive electrode52ffor each sub-pixel thereon, and the P-drive electrode51fand the N-drive electrode52fare connected to the P-electrode19fP and the N-electrode19fN, respectively. The drive circuit substrate50fsupplies a predetermined current to the micro LED elements100fand controls light emission. This configuration is advantageous in that the step of producing the common N-electrode56can be omitted in the process of producing an image display device200f, and the image display device200fcan be easily produced. On the other hand, both the P-electrode and the N-electrode need to be disposed on one side of the micro LED elements100f, it is more difficult to miniaturize the elements. This configuration is suitable for use in a head-up display or a projector that requires high power. In the present embodiment, the reflection layer10is included inside the nitride semiconductor layer14and disposed on the wavelength conversion layer side with respect to the light emission layer12. Although the reflection layer10is formed of a nitride semiconductor, it is not necessary to allow a current to flow in the up-down direction as in the first embodiment, and the reflection layer10may have a higher resistivity than a portion of the N-side layer11(resistivity: 1.0 to 10 mΩcm) other than the reflection layer10. The high resistivity of the reflection layer10enables crystallinity of the nitride semiconductor layer14to be improved and enables the light output of the micro LED elements100fto be improved. InFIG.11, the N-electrode19fN is provided for each sub-pixel. However, the N-electrode19fN need not be provided for each of the micro LED elements100f, and a plurality of micro LED elements100fmay share a single N-electrode19fN.

From the viewpoint of the image display device200f, the effects achieved by the reflection layer10and the transmission layer25are the same as those in the first embodiment although the arrangement of the electrodes of the micro LED elements100fis different. The thicknesses of the green wavelength conversion portion23and the red wavelength conversion portion22can be reduced, and the light emission efficiency can be improved by providing the transmission layer25on the green wavelength conversion portion23and the red wavelength conversion portion22(on the light emission side) and providing the reflection layer10under the green wavelength conversion portion23and the red wavelength conversion portion22(on the excitation light source side). Miniaturization is facilitated, and the amounts of expensive wavelength conversion materials used are reduced. Thus, an effect of reducing the production cost is also achieved.

Production Method

Next, an example of a method for producing micro LED elements100fwill be described with reference toFIGS.12A to14C. Descriptions regarding the same steps as those inFIGS.3A to6Bwill be omitted. The significant difference fromFIGS.3A to6Blies in that, besides the arrangement of the electrodes, the production method includes forming micro LED elements100fon a growth substrate9f, cutting the resulting growth substrate9finto individual pieces in units of image display devices200f, and subsequently bonding the divided unit of the LED elements100fonto a drive circuit substrate50fin units of pieces.

The present embodiment is the same as the first embodiment in that a nitride semiconductor layer14including a reflection layer10is formed on a growth substrate9fas illustrated inFIG.12A. However, for example, a (0001) plane sapphire substrate can be used as the growth substrate9fin the present embodiment.

As illustrated inFIG.12B, portions of a P-side layer13, a light emission layer12, and an N-side layer11are etched to form a mesa16, and the resulting surface is then covered with a protective film17as illustrated inFIG.12C. The protective film17is formed of, for example, silicon dioxide (SiO2). Subsequently, as illustrated inFIG.12D, a P-side contact hole18P is formed on the P-side layer13in a top portion of the mesa16, and an N-side contact hole18N is formed in an exposed portion of the N-side layer11in the bottom portion of the mesa16. A P-electrode19fP and an N-electrode19fN are formed in the P-side contact hole18P and the N-side contact hole18N, respectively, as illustrated inFIG.12E. Subsequently, as illustrated inFIG.13A, the protective film17and the nitride semiconductor layer14are etched to form a pixel isolation trench15fand to separate micro LED elements100ffrom each other. The reflection layer10is also divided by the pixel isolation trenches15ffor each micro LED element100f.

In the present configuration, since the processing is performed from the light emission layer12side by a dry etching technique, as illustrated inFIGS.12C and13A, it is easy to cover the light emission layer12with the inclined side surfaces of the mesa16and to incline the side surfaces of the N-side layer11of the micro LED elements100f. Each of the side surfaces is inclined to be open with respect to the light emission direction. Thus, the light extraction efficiency of the micro LED elements100fcan be enhanced. Furthermore, by covering the sidewalls of the pixel isolation trench15with a highly reflective metal film, leakage of light from the side surfaces of the micro LED elements100fis prevented, and the light extraction efficiency in the light emission direction can be enhanced. By disposing a transparent insulating film between the side surface of the N-side layer11and the metal film, the light extraction efficiency of the micro LED elements100fcan be further enhanced.

Although not shown in the figure, the growth substrate9fon which the micro LED elements100fare formed is polished, cut in units of image display devices200f, and divided into pieces. The growth substrate9fis bonded onto a drive circuit substrate50fin the state of a divided piece, as illustrated inFIG.13B. The drive circuit substrate50fmay be in a wafer state or in a state of a chip divided in units of image display devices200f. Hereinafter, a description will be made of a case in a wafer state.

Subsequently, the growth substrate9fis separated as illustrated inFIG.13C. Desirably, the bonding state inFIG.13Bis temporary adhesion, and the actual connection is formed after the separation of the growth substrate9finFIG.13C. The reason for this is as follows. In the case where thermal expansion is different between the growth substrate9fand the drive circuit substrate50f, it is difficult to perform a process accompanying a significant increase in temperature in a state where the growth substrate9fis present. Therefore, the actual connection accompanying an increase in temperature is preferably formed after the separation of the growth substrate9f. In this configuration, each of the micro LED elements100fcan emit light in the temporary adhesion state or the actual connection state under control of the drive circuit substrate50f. Accordingly, characteristics of the respective micro LED elements100fcan be tested. Therefore, in the case where a defective micro LED element100fis found, the repair can be performed by removing the defective micro LED element100fand bonding a normal product. The present embodiment is also advantageous in that such repair can be easily performed because electrical connection of the micro LED elements100fis present only on the drive circuit substrate50fside.

The subsequent steps are illustrated inFIGS.13D to14C.FIG.13Dillustrates a step of forming a filling material20as inFIG.4A, andFIGS.14A to14Cillustrate the same steps as those inFIGS.4E to6B. Accordingly, descriptions of these steps are omitted.

Seventh Embodiment

The present embodiment differs from the sixth embodiment in micro LED elements100g. Other configurations of the present embodiment are the same as those of the sixth embodiment. In the micro LED elements100fof the sixth embodiment, the reflection layer10is formed of a nitride semiconductor layer. In micro LED elements100gof the present embodiment, a dielectric multilayer film is used as a reflection layer10g. Therefore, a method for growing a nitride semiconductor layer14gis changed, however, points other than the reflection layer10gare the same as those in the sixth embodiment.

FIG.15is a schematic sectional view of an image display device200gof the present embodiment. The image display device200gdiffers from the image display device200fin that micro LED elements100ghave a reflection layer10gformed of a dielectric multilayer film, and that the reflection layer10ghas through portions42in part thereof. In the present embodiment, the reflection layer10gis included inside the nitride semiconductor layer14gand disposed on the wavelength conversion layer side with respect to the light emission layer12.

As illustrated inFIG.15, in the image display device200g, the reflection layer10gis a dielectric multilayer film, the nitride semiconductor layer14gthat forms the micro LED elements100ghas the through portions42in part of the reflection layer10g, and the through portions42are provided for each of the micro LED elements100g. The through portions42are filled with a nitride semiconductor such as GaN. In this configuration, since a current does not need to be supplied to the through portions42, the nitride semiconductor in the through portions42may have a higher resistivity than other portions of the N-side layer11g.

According to the configuration described above, since the through portions42are provided for each of the micro LED elements100g, variations in characteristics among the micro LED elements100gare reduced, and the reflectance of the reflection layer10gis improved by using the dielectric multilayer film and thus the light output can be improved.

FIGS.16A to16Dillustrates steps of forming a nitride semiconductor layer14gthat forms micro LED elements100g. As illustrated inFIG.16A, a seed layer40is formed on a growth substrate9g, and a dielectric multilayer film is formed on the seed layer40. The seed layer40is, for example, a GaN layer. The seed layer40can be omitted in some cases depending on the type of the growth substrate9g.

The dielectric multilayer film must be a film that is stable at a high temperature. For example, a combination of silicon dioxide (SiO2) and a silicon nitride film (Si3N4) formed by a CVD method is preferred. The dielectric multilayer film can be formed by stacking a plurality of pairs of a SiO2layer and a Si3N4layer. For example, six pairs of a SiO2layer having a thickness of 89 nm and a Si3N4layer having a thickness of 65 nm, the pairs each having a total thickness of 154 nm, are deposited, and six pairs of a SiO2layer having a thickness of 108 nm and a Si3N4layer having a thickness of 79 nm, the pairs each having a total thickness of 187 nm, are formed thereon. The total number of the pairs is 12, and the reflection layer10ghas a total film thickness of about 2 μm. With this structure, a reflectance of 80% can be ensured at a wavelength of 520 nm and a wavelength of 630 nm.

Next, as illustrated inFIG.16B, openings41are formed in the reflection layer10gto expose the seed layer40at bottom portions thereof. The openings can be formed by a typical photolithography technique and a typical dry etching technique. The arrangement period of the openings41is preferably at least the same as the arrangement period of pixels or a reciprocal of an integer multiple of the arrangement period of pixels. This is a condition necessary for arranging the same number of openings41for each of the micro LED elements100g. Although the area of the openings41is smaller than the entire area and the optical influence of the openings41is not large, the effect of the reflection layer10gis weakened with an increase in the number of the openings41. Accordingly, it is preferable to arrange the openings for each of the micro LED elements100gin the same manner.

Subsequently, as illustrated inFIG.16C, an N-side layer11gis grown. In the initial growth, the openings41are first filled with a GaN film by using a selective growth technique of a GaN film. Subsequently, a GaN layer is extended on the reflection layer10gby growing the GaN film in the lateral direction, the surface is planarized, and the film thickness is increased to form the N-side layer11g.

Furthermore, as illustrated inFIG.16D, a light emission layer12and a P-side layer13are grown on the N-side layer11gto form a nitride semiconductor layer14g. The steps of growing the light emission layer12and the P-side layer13are the same as those of the sixth embodiment.

The step of forming micro LED elements100gand the step of producing an image display device200gafter the formation of the nitride semiconductor layer14gare the same as those of the sixth embodiment.

In this configuration, since the reflection layer10gis formed by stacking two types of dielectric films having refractive indices that are significantly different from each other, the reflectance of the reflection layer10gcan be improved. Accordingly, the emission efficiency of red light and green light can be further improved. In the present embodiment, the transparent portion21and the wavelength conversion portions22and23each having an upper surface with substantially the same size as the corresponding micro LED element100gare disposed as illustrated inFIG.15. Alternatively, a transparent portion21and wavelength conversion portions22and23each having an upper surface with a larger size than the corresponding micro LED element100gmay be disposed as in the first embodiment.

Modification

FIG.17illustrates a modification of the seventh embodiment. This modification is an embodiment in which the nitride semiconductor layer14gdescribed in the seventh embodiment is applied to the micro LED elements100of the first embodiment.

As illustrated inFIG.17, micro LED elements100h(collectively referring to a micro LED element B100Bh, a micro LED element R100Rh, and a micro LED element G100Gh) have a reflection layer10hand through portions42. Other structures are the same as those of the micro LED elements100of the first embodiment. In this configuration, it is necessary to cause a current to flow through the through portions42, and thus the GaN layer constituting the seed layer40and the through portions42is doped to be N-type and has conductivity. That is, the through portions42are filled with a conductive nitride semiconductor.

As described above, for the upper/lower electrode-type micro LED elements100h, a reflection layer formed of a dielectric multilayer film is similarly disposed in the nitride semiconductor layer14h, and the light output of the image display device200hcan be improved.

In the present modification, the nitride semiconductor layer14gof the seventh embodiment is combined with the (upper/lower electrode-type) micro LED elements100of the first embodiment. Alternatively, an upper/lower electrode-type micro LED elements can be formed by the production process described in the seventh embodiment. In this case, it is easy to surround the light emission layer12by inclined side surfaces and to incline the side surfaces of the N-side layer11of the micro light-emitting elements as in the sixth embodiment and the seventh embodiment. By inclining each of the side surfaces to be open with respect to the light emission direction, the light extraction efficiency of the micro light-emitting elements can be enhanced. Furthermore, by covering the sidewalls of the pixel isolation trench15with a highly reflective metal film, leakage of light from the side surfaces of the micro light-emitting elements is prevented, and the light extraction efficiency in the light emission direction can be enhanced. By disposing a transparent insulating film between the side surface of the N-side layer11and the metal film, the light extraction efficiency of the micro light-emitting elements can be further enhanced.

Eighth Embodiment

The present embodiment differs from the first embodiment in micro LED elements100i. Other configurations of the present embodiment are the same as those of the first embodiment. The micro LED elements100of the first embodiment have, inside the nitride semiconductor layer14, a reflection layer10including a nitride semiconductor. A reflection layer10iof this configuration is formed of a dielectric multilayer film and disposed outside a P-side layer13. That is, the reflection layer10iis disposed on the drive circuit substrate50side with respect to the light emission layer12.

As illustrated inFIG.18, micro LEDs100i(collectively referring to a micro LED element B100Bi, a micro LED element R100Ri, and a micro LED element G100Gi) have a transparent electrode layer44and a reflection layer10ion the drive circuit substrate50side of a P-side layer13, and the transparent electrode layer44is connected to a P-drive electrode51with a P-electrode19iP therebetween. A nitride semiconductor layer14ithat forms the micro LEDs100iincludes an N-side layer11i, a light emission layer12, and the P-side layer13and need not include a reflection layer. Other configurations are the same as those of the first embodiment.

In this configuration, a dielectric multilayer film is used as the reflection layer10i. The reflection layer10ipreferably has a high reflectance for blue light (excitation light) besides red light and green light (long-wavelength light). As a result, the same effect as that in the fifth embodiment can be generated. Specifically, the light output can be improved by adding a high reflectance for blue light in addition to red light/green light in the reflection layer10i. This is because since a high reflectance for blue light which is excitation light can also be realized on the P-side layer13side, the light output of the micro LED element B100Bi, the micro LED element R100Ri, and the micro LED element G100Gi can be improved. Accordingly, the light output of a blue sub-pixel6improves, the light output of a red sub-pixel7and a green sub-pixel8also improves, and the light emission efficiency of the entire image display device200ican be improved.

Furthermore, in the present embodiment, a multilayer film that is formed at a relatively high temperature, that is stable, and that uses pairs of dielectric films having refractive indices that are significantly different from each other is easily used as the reflection layer10i, and a reflection layer10ihaving high reflectances for blue light, red light, and green light can be formed as a relatively thin layer. Consequently, an increase in the cost due to an improvement in light output characteristics can be reduced to the minimum.

As illustrated inFIG.18, the reflection layer10iis a dielectric multilayer film and is disposed on the drive circuit substrate50side with respect to the nitride semiconductor layer14ithat forms the micro LED elements100i, and the drive circuit substrate50and the nitride semiconductor layer14iare connected to each other with an electrode therebetween, the electrode extending through the reflection layer10i. According to the configuration described above, in the image display device200i, the drive circuit substrate50and the nitride semiconductor layer14ican be connected to each other with an electrode therebetween, the electrode extending through the reflection layer10i.

Production Method

As illustrated inFIG.19A, a nitride semiconductor layer14iincluding an N-side layer11i, a light emission layer12, and a P-side layer13is grown on a growth substrate9, and a transparent electrode layer44and a reflection layer10iare then deposited. The deposition temperature of the reflection layer10imay be in a temperature range in which the transparent electrode layer44does not degrade and may be 600° C. or lower. Thus, the reflection layer10ican be deposited at a relatively high temperature, and a stable, good film can be obtained. The transparent electrode layer44is formed of, for example, ITO (indium-tin-oxide) and has a thickness of about 50 nm to 600 nm. The reflection layer10iwas formed by stacking 17 pairs of a TiO2thin film and a SiO2thin film as illustrated inFIG.23. The thickness of the TiO2thin film is in the range of from 8 nm to 75 nm, and the film thickness of the SiO2thin film is in the range of from 8 nm to 171 nm. The film thickness was optimized for each layer. The film thickness was optimized so that the reflectance was 80% or more on average at an angle of incidence of 25 degrees or less in the wavelength range of from 440 nm to 650 nm. The thin films are stacked at a substrate temperature of 300° C. by an ion-beam vapor deposition method. The total thickness was 2.85 μm.

As illustrated inFIG.19B, after the stacking of the reflection layer10i, openings45are formed by a photolithography technique and a dry etching technique. The transparent electrode layer44appears at the bottom of the openings45. Subsequently, as illustrated inFIG.19C, a P-electrode layer19iis formed. The P-electrode layer19ipreferably has plug portions that fill the openings45. A flat film portion that covers planar portions is preferably present but may be omitted. The subsequent steps of bonding the growth substrate9to a drive circuit substrate50as illustrated inFIG.19Dand separating the growth substrate9as illustrated inFIG.19Eare the same as those in the first embodiment.

In the formation of a pixel isolation trench15iillustrated inFIG.20A, the nitride semiconductor layer14i, the transparent electrode layer44, the reflection layer10i, and the P-electrode layer19iare sequentially etched. Other configurations are the same as those in the first embodiment. The subsequent steps are the same as those in the first embodiment. Thus, the image display device200iinFIG.18is formed.

Ninth Embodiment

The present embodiment significantly differs from the first embodiment in that the emission wavelength of micro LED elements100jcorresponds to bluish-purple light (peak wavelength: 410 nm±15 nm), which is near-ultraviolet light, and the transparent portion21is replaced with a blue wavelength conversion portion21j. Accordingly, the layer configuration of a reflection layer10jis changed so as to reflect blue light (peak wavelength: 460±15 nm) in addition to red and green. In addition, a transmission layer25jcovers the entire pixel region1including a blue sub-pixel6, transmits the entire visible region from blue to red, and reflects only bluish-purple light. Other configurations are the same as those in the first embodiment.

As illustrated inFIG.21, the configuration of an image display device200jof this configuration is not significantly different from that of the first embodiment. The quantum well layers of a light emission layer12jare changed so that the micro LED elements100jemit bluish-purple light. Mainly, the indium (In) concentration of the quantum well layers can be reduced. The layer configuration of the reflection layer10jis changed so as to reflect blue light in addition to red and green. These are changes concerning a nitride semiconductor layer14j.

On the blue sub-pixel6, the transparent portion21is disposed in the first embodiment, whereas the blue wavelength conversion portion21jis disposed in this configuration. The blue wavelength conversion portion21jcan be formed by dispersing wavelength conversion particles such as a phosphor, quantum dots, or quantum rods in a resin as in the red wavelength conversion portion and the green wavelength conversion portion. The transmission layer25jis disposed not only on a red sub-pixel7and a green sub-pixel8but also on a blue sub-pixel6. The film configuration of the transmission layer25jis also changed so as to transmit blue light in addition to red and green and to reflect bluish-purple light.

As described above, according to this configuration, the emission wavelength of excitation light of the micro LED elements is not limited to blue but may be a wavelength of near-ultraviolet light or ultraviolet light or another wavelength. When near-ultraviolet light or ultraviolet light is used as excitation light, the thicknesses of the blue wavelength conversion portion21j, the green wavelength conversion portion23, and the red wavelength conversion portion22can be reduced and the light emission efficiency can be improved by providing the reflection layer10junder the blue wavelength conversion portion21j, the green wavelength conversion portion23, and the red wavelength conversion portion22(on the excitation light source side). Miniaturization is facilitated, and the amounts of expensive wavelength conversion materials used are reduced. Thus, an effect of reducing the production cost is also achieved.

The present disclosure is not limited to each embodiment described above, and various modifications can be made thereto within the scope of the claims. Embodiments based on appropriate combinations of technical methods disclosed in different embodiments are also encompassed in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining technical methods disclosed in the respective embodiments.

The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2018-038038 filed in the Japan Patent Office on Mar. 2, 2018, the entire contents of which are hereby incorporated by reference.

It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.