Patent Publication Number: US-2023155097-A1

Title: Electronic device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of pending U.S. Ser. No. 16/924,447, filed on Jul. 9, 2020, which is a Continuation of application Ser. No. 16/222,136, filed on Dec. 17, 2018 (now U.S. Pat. No. 10,749,090, issued Aug. 18, 2020), which is a Continuation of application Ser. No. 15/855,062, filed on Dec. 27, 2017 (now U.S. Pat. No. 10,193,042, issued Jan. 29, 2019), the entirety of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a display device. The disclosure in particular relates to a protective layer of the display device. 
     Description of the Related Art 
     Electronic products that come with a display panel, such as smartphones, tablets, notebooks, monitors, and TVs, have become indispensable necessities in modern society. With the flourishing development of such portable electronic products, consumers have higher expectations regarding the quality, the functionality, and the price of such products. The development of next-generation display devices has been focused on techniques that are energy saving and environmentally friendly. 
     Light-emitting diodes (LEDs) based upon gallium nitride (GaN) are expected to be used in future high-efficiency lighting applications, replacing incandescent and fluorescent lighting lamps. Current GaN-based LED devices are prepared by heteroepitaxial growth techniques on substrate materials. A typical wafer level LED device structure may include a lower n-doped GaN layer formed over a sapphire substrate, a single quantum well (SQW) or multiple quantum well (MWQ), and an upper p-doped GaN layer. 
     Micro-LED technology is an emerging flat panel display technology. Micro LED displays drives an array of addressed micro LEDs. In the current manufacturing method, micro LEDs are formed and diced into several micro LED dies (e.g., micro-lighting dies). The driving circuits and related circuits are formed on the glass substrate to provide an array substrate (e.g., TFT array substrate), and the micro LED dies are then mounted on the array substrate. Bare dies are commonly used in micro LEDs, wherein the bare dies are surrounded by a protective layer such as an anisotropic conductive film (ACF) layer. An ACF layer may serve as a conductive route between the electrode of micro LED and the TFT array substrate. Typically, the top surface of an ACF layer is level with that of a micro LED so as to provide protection. However, this results in a waste of ACF material and it limits the space for filling the light conversion layer. In addition, conductive particles having varying sizes in the ACF layer may also lead to poor conductivity or poor reflectivity. 
     Accordingly, it is desirable to develop a design that employs protective layers, which can effectively maintain or improve the performance of LED structures. 
     SUMMARY 
     In accordance with some embodiments of the present disclosure, an electronic device is provided. The electronic device includes a substrate, a driving circuit, a diode and a light shielding element. The driving circuit is disposed on the substrate. The diode is electrically connected to the driving circuit. The light shielding element overlaps the substrate. A surface of the light shielding element has a first width. A cross-sectional-surface of a portion of the light shielding element has a second width. In addition, the second width is greater than the first width in a cross-sectional view, and the surface is closer to the substrate than the cross-sectional surface of the portion. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS.  1 A- 1 C  illustrate the cross-sectional views of the display device in accordance with some embodiments of the present disclosure. 
         FIGS.  2 A- 2 C  illustrate the cross-sectional views of the display device in accordance with some embodiments of the present disclosure. 
         FIGS.  3 A- 3 B  illustrate the cross-sectional views of the display device in accordance with some embodiments of the present disclosure. 
         FIG.  4    illustrates the cross-sectional view of the display device in accordance with some embodiments of the present disclosure. 
         FIGS.  5 A- 5 C  illustrate the cross-sectional views of the display device in accordance with some embodiments of the present disclosure. 
         FIGS.  6 A- 6 B  illustrate the cross-sectional views of the display device in accordance with some embodiments of the present disclosure. 
         FIG.  7    illustrates the cross-sectional view of the display device in accordance with some embodiments of the present disclosure. 
         FIG.  8    illustrates the cross-sectional view of the display device in accordance with some embodiments of the present disclosure. 
         FIG.  9 A  illustrates a partially enlarged portion of the display device as shown in  FIG.  5 A . 
         FIGS.  9 B- 9 E  illustrate the cross-sectional views of the first conductive element in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The display device of the present disclosure and the manufacturing method thereof are described in detail in the following description. In the following detailed description, for purposes of explanation, numerous specific details and embodiments are set forth in order to provide a thorough understanding of the present disclosure. The specific elements and configurations described in the following detailed description are set forth in order to clearly describe the present disclosure. It will be apparent, however, that the exemplary embodiments set forth herein are used merely for the purpose of illustration, and the inventive concept may be embodied in various forms without being limited to those exemplary embodiments. In addition, the drawings of different embodiments may use like and/or corresponding numerals to denote like and/or corresponding elements in order to clearly describe the present disclosure. However, the use of like and/or corresponding numerals in the drawings of different embodiments does not suggest any correlation between different embodiments. In addition, in this specification, expressions such as “first material layer disposed on/over a second material layer”, may indicate the direct contact of the first material layer and the second material layer, or it may indicate a non-contact state with one or more intermediate layers between the first material layer and the second material layer. In the above situation, the first material layer may not be in direct contact with the second material layer. 
     It should be noted that the elements or devices in the drawings of the present disclosure may be present in any form or configuration known to those with ordinary skill in the art. In addition, the expressions “a layer overlying another layer”, “a layer is disposed above another layer”, “a layer is disposed on another layer” and “a layer is disposed over another layer” may indicate that the layer is in direct contact with the other layer, or that the layer is not in direct contact with the other layer, there being one or more intermediate layers disposed between the layer and the other layer. 
     In addition, in this specification, relative expressions are used. For example, “lower”, “bottom”, “higher” or “top” are used to describe the position of one element relative to another. It should be appreciated that if a device is flipped upside down, an element that is “lower” will become an element that is “higher”. 
     It should be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers, portions and/or sections, these elements, components, regions, layers, portions and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, portion or section from another region, layer or section. Thus, a first element, component, region, layer, portion or section discussed below could be termed a second element, component, region, layer, portion or section without departing from the teachings of the present disclosure. 
     It should be understood that this description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. The drawings are not drawn to scale. In addition, structures and devices are shown schematically in order to simplify the drawing. 
     The terms “about” and “substantially” typically mean +/−20% of the stated value, more typically +/−10% of the stated value, more typically +/−5% of the stated value, more typically +/−3% of the stated value, more typically +/−2% of the stated value, more typically +/−1% of the stated value and even more typically +/−0.5% of the stated value. The stated value of the present disclosure is an approximate value. When there is no specific description, the stated value includes the meaning of “about” or “substantially”. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should be appreciated that, in each case, the term, which is defined in a commonly used dictionary, should be interpreted as having a meaning that conforms to the relative skills of the present disclosure and the background or the context of the present disclosure, and should not be interpreted in an idealized or overly formal manner unless so defined. 
     In addition, in some embodiments of the present disclosure, terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. 
     The term “elevation” used herein means the distance from a substrate to a target surface. In particular, the term “elevation” may refer to the distance from a substantially planar region of a substrate to a target surface. For example, in accordance with some embodiments illustrated herein, an evaluation may refer to the distance from the bottom surface of a substrate to a target surface. 
     The display device provided in the present disclosure includes a protective layer having the elevation that is lower than the elevation of the upper semiconductor layer of the light-emitting unit (e.g., LED, micro LED and so on). In this case, less material is required for the protective layer compared to general display devices where the elevation of the protective layer is level with that of the upper semiconductor layer. In addition, there will be more space for the wavelength conversion layer, which is disposed over the protective layer, to fill in. In accordance with some embodiments of the present disclosure, the display device includes the protective layer having the elevation that is higher than the elevation of the quantum well of the light-emitting unit so as to prevent moisture and oxygen from damaging the quantum well. Furthermore, the protective layer of such a design may also prevent shorts or increase the reflectivity. Moreover, in accordance with some embodiments of the present disclosure, the display device includes a buffer layer disposed between the light emitting unit and the wavelength conversion layer so that the wavelength conversion layer may be unaffected by the current or heat produced by the light emitting-unit. 
       FIG.  1 A  illustrates a cross-sectional view of the display device  10  in accordance with some embodiments of the present disclosure. It should be understood that additional features may be added to the display device in some embodiments of the present disclosure. In another embodiment of the present disclosure, some of the features described below may be replaced or eliminated. 
     Referring to  FIG.  1 A , the display device  10  may include a driving substrate  100 , a light-emitting unit  200  and a first protective layer  300 . The driving substrate  100  may include a substrate  102 , a driving circuit  104 , a gate dielectric layer  106 , a first insulating layer  108  and a second insulating layer  110 . The driving substrate  100  may serve as a switch of the light-emitting unit  200 . As shown in  FIG.  1 A , the driving circuit  104  is disposed on the substrate  102 . In some embodiments of the present disclosure, the substrate  102  may include, but is not limited to, glass, quartz, sapphire, polycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET), rubbers, glass fibers, other polymer materials, any other suitable substrate material, or a combination thereof. In some other embodiments of the present disclosure, the substrate  102  may be made of a metal-glass fiber composite plate, a metal-ceramic composite plate, a printed circuit board, or any other suitable material, but it is not limited thereto. It should be understood that although the driving circuit  104  in some embodiments as illustrated in figures is an active driving circuit including thin-film transistors (TFT), the driving circuit  104  may be a passive driving circuit in accordance with another embodiment. In some embodiments, the driving circuit  104  may be controlled by an IC or a microchip. For example, in this embodiment, the driving circuit  104  may include the conductive layer, the insulating layer and the active layer, which serve as a TFT. The active layer may include semiconductor materials such as amorphous silicon, polysilicon or metal oxide. The active layer may include a pair of source/drain regions doped with suitable dopants and an undoped channel region formed between the source/drain regions. 
     The gate dielectric layer  106 , the first insulating layer  108  and the second insulating layer  110  are sequentially disposed on the substrate  102 . The driving circuit  104  may be surrounded by the gate dielectric layer  106 , the first insulating layer  108  and the second insulating layer  110 . In some embodiments of the present disclosure, the material of the gate dielectric layer  106  may include silicon oxide, silicon nitride, silicon oxynitride, high-k dielectric material, any other suitable dielectric material, or a combination thereof. The high-k dielectric material may include, but is not limited to, metal oxide, metal nitride, metal silicide, transition metal oxide, transition metal nitride, transition metal silicide, metal oxynitride, metal aluminate, zirconium silicate, zirconium aluminate. In some embodiments of the present disclosure, the materials of the first insulating layer  108  or the second insulating layer  110  may be formed of an organic material, an inorganic material or a combination thereof. The organic material may include, but is not limited to, an acrylic or methacrylic organic compound, isoprene compound, phenol-formaldehyde resin, benzocyclobutene (BCB), PECB (perfluorocyclobutane) or a combination thereof. The inorganic material may include, but is not limited to, silicon nitride, silicon oxide, or silicon oxynitride or a combination thereof. 
     In some embodiments of the present disclosure, the gate dielectric layer  106 , the first insulating layer  108  or the second insulating layer  110  may be formed by using chemical vapor deposition or spin-on coating. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     Still referring to  FIG.  1 A , the light-emitting unit  200  may be disposed on the driving substrate  100 . The light-emitting unit  200  may be disposed on the driving circuit  104  and electrically connected to the driving circuit  104 . Specifically, the light-emitting unit  200  may be coupled to the driving circuit  104  through the vias and the pads. The light-emitting unit  200  may include a first semiconductor layer  202 , a quantum well layer  204  disposed on the first semiconductor layer  202  and a second semiconductor layer  206  disposed on the quantum well layer  204 . The light-emitting unit  202  may include LED or micro LED. In accordance with some embodiments of the present disclosure, the cross-sectional area of the light emitting unit  200  may have a length of about 1 μm to about 150 μm and may have a width ranging from about 1 μm to about 150 μm. In some embodiments, the light emitting unit  200  may have a size ranging from about 1 μm × 1 μm × 1 μm to about 150 μm × 150 μm × 150 μm. 
     In some embodiments of the present disclosure, the first semiconductor layer  202  may be formed of the III-V compounds having dopants of the first conductivity type, e.g. gallium nitride having p-type conductivity (p-GaN). In some embodiments of the present disclosure, the quantum well layer  204  may include a homogeneous interface, a heterogeneous interface, a single quantum well (SQW) or a multiple quantum well (MQW). The material of the quantum well layer  204  may include, but is not limited to, indium gallium nitride, a gallium nitride or a combination thereof. In some embodiments of the present disclosure, the second semiconductor layer  206  may be formed of the III-V compounds having dopants of the second conductivity type, e.g. gallium nitride having n-type conductivity (n-GaN). In addition, the above III-V compounds may include, but is not limited to, indium nitride (InN), aluminum nitride (AlN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), aluminum indium gallium nitride (AlGaInN) or a combination thereof. 
     In some embodiments of the present disclosure, the first semiconductor layer  202 , the quantum well layer  204  or the second semiconductor layer  206  may be formed by using an epitaxial growth process. For example, metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HYPE), liquid phase epitaxy (LPE), or another suitable process may be used to form the first semiconductor layer  202 , the quantum well layer  204  or the second semiconductor layer  206 . 
     The light-emitting unit  200  may further include a first electrode  208  and a second electrode  210 . In accordance with some embodiments of the present disclosure, the first electrode  208  and the second electrode  210  may serve as the n-electrode and p-electrode of the light-emitting unit  200 . In some embodiments, the first electrode  208  and/or the second electrode  210  may be formed of metallic conductive materials, transparent conductive materials or a combination thereof. The metallic conductive material may include, but is not limited to, copper, aluminum, tungsten, titanium, gold, platinum, nickel, copper alloys, aluminum alloys, tungsten alloys, titanium alloys, gold alloys, platinum alloys, nickel alloys, any other suitable metallic conductive materials, or a combination thereof. The transparent conductive material may include transparent conductive oxides (TCO). For example, the transparent conductive material may include, but is not limited to, indium tin oxide (ITO), tin oxide (SnO), zinc oxide (ZnO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), indium tin oxide (ITZO), antimony tin oxide (ATO), antimony zinc oxide (AZO), any other suitable transparent conductive materials, or a combination thereof. In some embodiments of the present disclosure, the first electrode  208  and the second electrode  210  may be formed by, but is not limited to, chemical vapor deposition, physical vapor deposition, electroplating process, electroless plating process, any other suitable processes, or a combination thereof. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. The physical vapor deposition may include, but is not limited to, sputtering, evaporation, pulsed laser deposition (PLD), or any other suitable method. 
     Still referring to  FIG.  1 A , the first protective layer  300  is disposed on the driving substrate  100  and adjacent to light-emitting unit  200 . In other words, the light-emitting unit  200  is surrounded by the first protective layer  300 . The first protective layer  300  may prevent moisture or oxygen from damaging the quantum well layer  204  of the light-emitting unit  200 . In some embodiments of the present disclosure, the first protective layer  300  may be transparent or semi-transparent to the visible wavelength so as to not significantly degrade the light extraction efficiency of the display device. The first protective layer  300  may be formed of organic materials or inorganic materials. In some embodiments, the inorganic material may include, but is not limited to, silicon nitride, silicon oxide, silicon oxynitride, aluminum oxide, any other suitable protective materials, or a combination thereof. In some embodiments, the organic material may include, but is not limited to, epoxy resins, acrylic resins such as polymethylmetacrylate (PMMA), benzocyclobutene (BCB), polyimide, and polyester, polydimethylsiloxane (PDMS), any other suitable protective materials, or a combination thereof. 
     In some embodiments of the present disclosure, the first protective layer  300  may be formed by using chemical vapor deposition (CVD), spin-on coating or printing. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     In addition, the first protective layer  300  may further include a plurality of conductive elements  302  formed therein. As shown in  FIG.  1 A , some of the conductive elements  302  may be dispersed in the first protective layer  300 , and some of the conductive elements  302  may be formed on the second insulating layer  110  in accordance with some embodiments of the present disclosure. In particular, the conductive elements  302  may further include the first conductive elements  302   a  and the second conductive elements  302   b . The first conductive elements  302   a  are disposed underneath a first terminal S 1  of the light-emitting unit  200 . The first terminal S 1  is opposed to a second terminal S 2 . In some embodiments, the first terminal S 1  and the second terminal S 2  may refer to the bottom and the top of the light-emitting unit  200  respectively. 
     The first conductive elements  302   a  may be disposed between the light-emitting unit  200  and the driving substrate  100 . Specifically, the first conductive elements  302   a  may be disposed between the first electrode  208  and the second insulating layer  110  or the second electrode  210  and the second insulating layer  110 . The first conductive elements  302   a  may be disposed between the first electrode  208  and the contact pads on the second insulating layer  110  or the second electrode  210  and the contact pads on the second insulating layer  110 . In addition, the first conductive elements  302   a  may electrically connect the first electrode  208  or the second electrode  210  with the driving circuit  104 . On the other hand, the second conductive elements  302   b  may be disposed in the region out of the light-emitting unit  200 . The second conductive elements  302   b  may be dispersed in the first protective layer  300 . The second conductive elements  302   b  also may be disposed at the bottom of the first protective layer  300 . 
     The conductive elements  302  may be formed of conductive materials to serve as an electrical contact of the light-emitting unit  200 . The conductive elements  302  may also serve as reflective particles to reflect the light emitted by light-emitting unit  200 . In some embodiments of the present disclosure, the conductive elements  302  may be formed of high reflective conductive materials. In some embodiments, the material of the conductive element  302  may include, but is not limited to, gold, platinum, silver, copper, iron, nickel, tin, aluminum, magnesium, palladium, iridium, rhodium, ruthenium, zinc, gold alloys, platinum alloys, silver alloys, copper alloys, iron alloys, nickel alloys, tin alloys, aluminum alloys, magnesium alloys, palladium alloys, iridium alloys, rhodium alloys, ruthenium alloys, zinc alloys, any other suitable conductive materials, or a combination thereof. In addition, further details regarding the conductive elements  302  will be discussed later. 
     As shown in  FIG.  1 A , the second semiconductor layer  206  of the light-emitting unit  200  includes a top surface  206   a . The first protective layer  300  includes a top surface  300   a . In some embodiments of the present disclosure, the elevation E 1  of the top surface  206   a  of the second semiconductor layer  206  is higher than the elevation E 2  of the top surface  300   a  of the first protective layer  300 . It should be noted that the term “elevation” used herein refers to the distance from the substrate  102  to a target surface. Specifically, the term “elevation” may refer to the distance from the bottom surface the substrate  102  to a target surface. For example, the elevation E 1  of the top surface  206   a  is defined as the distance from the substrate  102  to the top surface  206   a.    
     As described above, the elevation E 1  of the top surface  206   a  of the second semiconductor layer  206  is higher than the elevation E 2  of the top surface  300   a  of the first protective layer  300 . In this way, less material is required to form the protective layer  300  so that the material may be saved, compared with conventional display devices where the elevation of the protective layer is substantially level with that of the upper semiconductor layer (e.g., the second semiconductor layer  206 ). In addition, there will be more space for the wavelength conversion layer  304  to fill in so that the optical performance of the display device may be improved. In some embodiments of the present disclosure, the difference between the elevation E 1  of the second semiconductor layer  206  and the elevation E 2  of the first protective layer  300  ranges from about 0.02 μm to about 5 μm, or from about 0.2 μm to about 2 It should be noted that the difference between the elevation E 1  and the elevation E 2  should not be too small, or the space where the additional portions  304 ′ may be filled will be reduced and thus the illumination efficiency will be decreased and the benefit of material saving may not be achieved; and the difference between the elevation E 1  and the elevation E 2  should not be too great, or the protecting efficiency of the first protective layer  300  will be reduced and the light-emitting unit  200  may become easily affected by the environment. 
     Moreover, as shown in  FIG.  1 A , the quantum well layer  204  of the light-emitting unit  200  includes a top surface  204   a . In some embodiments of the present disclosure, the elevation E 2  of the top surface  300   a  of the first protective layer  300  is higher than the elevation E 3  of the top surface  204   a  of the quantum well layer  204 . In other words, the quantum well layer  204  is embedded in the first protective layer  300 . In this way, quantum well layer  204  of the light-emitting unit  200  may be fully protected by the first protective layer  300  so as to prevent moisture and oxygen from affecting or damaging the quantum well layer  204 . In some embodiments of the present disclosure, the difference between the elevation E 2  of the first protective layer  300  and the elevation E 3  of the quantum well layer  204  ranges from about 0.1 μm to about 10 or from about 1 μm to about 5 It should be noted that the difference between the elevation E 2  and the elevation E 3  should not be too small, or the protecting efficiency of the first protective layer  300  will be reduced and the light-emitting unit  200  may become easily affected by the environment; and the difference between the elevation E 2  and the elevation E 3  should not be too great, or the heat capacity of the light-emitting unit  200  will be too great so that heat may be trapped in the first protective layer  300  and may result in damages to the light-emitting unit  200  or the wavelength conversion layer  304  formed thereon. In addition, if the difference between the elevation E 2  and the elevation E 3  is too great, the benefit of material saving also may not be achieved. 
     In addition, it should be understood that, although the display device  10  include the wavelength conversion layer  304  disposed on the light-emitting unit  200  in the embodiments illustrated in  FIG.  1 A , the wavelength conversion layer  304  may be simply replaced with a transparent material without the function of wavelength conversion (e.g., without phosphor particles or quantum dot materials). For example, the transparent material may include, but is not limited to, a polymer or glass matrix. 
     In some embodiments of the present disclosure, the top surface  300   a  of the first protective layer  300  may be disposed at any suitable position between the top surface  204   a  of the quantum well layer  204  and the top surface  206   a  of the second semiconductor layer  206  as long as the quantum well layer  204  is covered by the first protective layer  300 . 
     Next, still referring to  FIG.  1 A , the display device  10  may further include the wavelength conversion layer  304  disposed on the light-emitting unit  200  and the first protective layer  300 , and a light shielding layer  306  disposed on the first protective layer  300 . The wavelength conversion layer  304  may be disposed between the light shielding layers  306  and cover the light-emitting unit  200 . The light shielding layer  306  may define a subpixel region in the display device  10 . Each subpixel may correspond to a light emitting unit  200 . In some embodiments, each subpixel may correspond to more than one light emitting units  200 . 
     In some embodiments of the present disclosure, the wavelength conversion layer  304  includes a portion  304 ′ that covers a portion of the sidewall  206   s  of the second semiconductor layer  206 . As described above, the additional portions  304 ′ of the wavelength conversion layer  304  may further improve the optical performance of the display device, as compared with the conventional display devices where the top surface of the protective layer is substantially level with that of the upper semiconductor layer (i.e., without the additional wavelength conversion portions). 
     The wavelength conversion layer  304  may include phosphors for converting the wavelength of light generated from the light emitting unit  200 . In some embodiments of the present disclosure, the wavelength conversion layer  304  may include a polymer or glass matrix and a dispersion of phosphor particles within the matrix. The light emission from the light emitting unit  200  may be tuned to specific colors in the color spectrum. For example, the wavelength conversion layer  304  includes the phosphors for converting the light emitted from the light emitting unit  200  into red light, green light, blue light or the light of any other suitable color. In some other embodiments, the wavelength conversion layer  304  includes quantum dot materials. The quantum dot material may have a core-shell structure. The core may include, but is not limited to, CdSe, CdTe, CdS, ZnS, ZnSe, ZnO, ZnTe, InAs, InP, GaP, or any other suitable materials, or a combination thereof. The shell may include, but is not limited to, ZnS, ZnSe, GaN, GaP, or any other suitable materials, or a combination thereof. In addition, it should be understood that although the wavelength conversion layer  304  as illustrated in  FIG.  1 A  appears to have a convex top surface, the wavelength conversion layer  304  may have any other suitable shapes according to needs. Similarly, the configuration of the light-shielding layer  306  is not limited to that as illustrated in  FIG.  1 A . The light-shielding layer  306  may also have any other suitable configurations according to needs. 
     The light-shielding layer  306  disposed adjacent to the wavelength conversion layer  304  may enhance the contrast of luminance. In some embodiments of the present disclosure, the light shielding layer  306  is formed of an opaque material such as a black matrix material. The black matrix material may include, but is not limited to, organic resins, glass pastes, and resins or pastes including black pigments, metallic particles such as nickel, aluminum, molybdenum, and alloys thereof, metal oxide particles (e.g. chromium oxide), or metal nitride particles (e.g. chromium nitride), or any other suitable materials. 
     In some embodiments of the present disclosure, the wavelength conversion layer  304  and the light shielding layer  306  may be formed by using chemical vapor deposition (CVD), spin-on coating or printing. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     As shown in  FIG.  1 A , the display device  10  may further include a second protective layer  308  covering the wavelength conversion layer  304  and the light shielding layer  306 . The second protective layer  308  may prevent the wavelength conversion layer  304  and the light shielding layer  306  from being affected by the outer environment. The second protective layer  308  may be formed of organic materials or inorganic materials. In some embodiments, the inorganic material may include, but is not limited to, silicon nitride, silicon oxide, silicon oxynitride, aluminum oxide, any other suitable protective materials, or a combination thereof. In some embodiments, the organic material may include, but is not limited to, epoxy resins, acrylic resins such as polymethylmetacrylate (PMMA), benzocyclobutene (BCB), polyimide, and polyester, polydimethylsiloxane (PDMS), any other suitable protective materials, or a combination thereof. 
     In some embodiments of the present disclosure, the second protective layer  308  may be formed by using chemical vapor deposition (CVD), spin-on coating or printing. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     In addition, the display device  10  may further include an adhesive layer  310  and a cover substrate  312 . The adhesive layer  310  may be disposed between the second protective layer  308  and the cover substrate  312  to affix the cover substrate  312  to the second protective layer  308 . The adhesive layer  310  may be formed of any suitable adhesive material. On the other hand, the material of the cover substrate  312  may include, but is not limited to, glass, quartz, sapphire, polycarbonate (PC), polyimide (PI), polyethylene terephthalate (PET), any other suitable substrate material, or a combination thereof. 
     Next,  FIG.  1 B  illustrates a cross-sectional view of the display device  10  in accordance with other embodiments of the present disclosure. It should be noted that the same or similar elements or layers in above and below contexts are represented by the same or similar reference numerals. The materials, manufacturing methods and functions of these elements or layers are the same or similar to those described above, and thus will not be repeated herein. The difference between the embodiments shown in  FIG.  1 B  and  FIG.  1 A  is that the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  1 B  has a concave shape while the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  1 A  is substantially planar. 
     As shown in  FIG.  1 B , the wavelength conversion layer  304  located above the light-emitting unit  200  has a first thickness T 1 , and the wavelength conversion layer  304  including the additional portions  304 ′ located above the protective layer  300  has a second thickness T 2 . In some embodiments of the present disclosure, the first thickness T 1  may be defined as the maximum thickness of the wavelength conversion layer  304  that is located above the light-emitting unit  200 . In some embodiments of the present disclosure, the second thickness T 2  may be defined as the maximum thickness of the wavelength conversion layer  304  that is located above the protective layer  300 . In this embodiment, the difference between the first thickness T 1  and the second thickness T 2  in one subpixel may be smaller due to the concave shape of the top surface  300   a , as compared with that of the substantially planar top surface  300   a  (as shown in  FIG.  1 A ). In addition, in some embodiments of the present disclosure, the third thickness T 3  of the additional portions  304 ′ that is closer to the light-emitting unit  200  may be smaller than the fourth thickness T 4  of the additional portions  304 ′ that is farther from the light-emitting unit  200  due to the concave shape of the top surface  300   a.    
     In some embodiments of the present disclosure, the concave shape of the top surface  300   a  may be formed due to the hydrophobic properties of the chosen materials of the first protective layer  300 . In some embodiments of the present disclosure, the concave shape of the top surface  300   a  may be formed by a patterning process. The patterning process may include a photolithography process and an etching process such as a selective etching process. The photolithography process may include, but is not limited to, photoresist coating (e.g., spin-on coating), soft baking, hard baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying, and other suitable processes. The etching process may include dry etching process or wet etching process. 
     Next,  FIG.  1 C  illustrates a cross-sectional view of the display device  10  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  1 C  and  FIG.  1 A  is that the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  1 C  has a convex shape while the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  1 A  is substantially planar. 
     As shown in  FIG.  1 C , in this embodiment, the fifth thickness T 5  of the additional portions  304 ′ that is closer to the light-emitting unit  200  may be greater than the sixth thickness T 6  of the additional portions  304 ′ that is farther from the light-emitting unit  200  due to the convex shape of the top surface  300   a . In addition, the reflected light L may be concentrated to increase the illumination efficiency due to the convex surface of the top surface  300   a.    
     In some embodiments of the present disclosure, the convex shape of the top surface  300   a  may be formed due to the hydrophilic properties of the chosen materials of the first protective layer  300 . In some embodiments of the present disclosure, the convex shape of the top surface  300   a  may be formed by a patterning process. The patterning process may include a photolithography process and an etching process such as a selective etching process. The photolithography process may include, but is not limited to, photoresist coating (e.g., spin-on coating), soft baking, hard baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying, and other suitable processes. The etching process may include dry etching process or wet etching process. 
     Next,  FIG.  2 A  illustrates a cross-sectional view of the display device  20  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  2 A  and  FIG.  1 A  is that the light-emitting unit  200 ′ in the embodiment shown in  FIG.  2 A  is a vertical chip type light-emitting diode while the light-emitting unit  200  in the embodiment shown in  FIG.  1 A  is a flip chip type light-emitting diode. 
     As shown in  FIG.  2 A , the light-emitting unit  200 ′ may be disposed on the driving circuit  104  and electrically connected to the driving circuit  104 . The first electrode  208  of the light-emitting unit  200 ′ is disposed on the conductive elements  302  and may serve as a bottom electrode of the light-emitting unit  200 ′. The first semiconductor layer  202 , the quantum well layer  204  and the second semiconductor layer  206  are sequentially stacked on the first electrode  208 . The second electrode  210  is disposed on the second semiconductor layer  206  and may serve as a top electrode of the light-emitting unit  200 . In addition, in this embodiment, the light-emitting unit  200 ′ may further include a contact layer  212  disposed on the second electrode  210  and the first protective layer  300 . The contact layer  212  may serve as an electrical contact of the light-emitting unit  200 ′. In some embodiments of the present disclosure, the contact layer  212  may be conformally formed over the second electrode  210  and the first protective layer  300 . The contact layer  212  may couple to the circuit from different array or to the circuit outside the panel. 
     In some embodiments of the present disclosure, the material of the contact layer  212  may include transparent conductive oxides (TCO). For example, the transparent conductive material may include, but is not limited to, indium tin oxide (ITO), tin oxide (SnO), zinc oxide (ZnO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), indium tin oxide (ITZO), antimony tin oxide (ATO), antimony zinc oxide (AZO), any other suitable transparent conductive materials, or a combination thereof. 
     In addition, the contact layer  212  may be formed by using chemical vapor deposition (CVD) or spin-on coating. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     Similar to the display device  10  in  FIG.  1 A , the display device  20  of the embodiment as shown in  FIG.  2 A , the elevation E 1  of the top surface  206   a  of the second semiconductor layer  206  is higher than the elevation E 2  of the top surface  300   a  of the first protective layer  300 . In some embodiments of the present disclosure, the difference between the elevation E 1  of the second semiconductor layer  206  and the elevation E 2  of the first protective layer  300  ranges from about 0.02 μm to about 5 μm, or from about 0.2 μm to about 2 In addition, the elevation E 2  of the top surface  300   a  of the first protective layer  300  is higher than the elevation E 3  of the top surface  204   a  of the quantum well layer  204 . In some embodiments of the present disclosure, the difference between the elevation E 2  of the first protective layer  300  and the elevation E 3  of the quantum well layer  204  ranges from about 0.1 μm to about 10 μm, or from about 1 μm to about 5 μm. 
     Furthermore, in the display device  20  of the embodiment as shown in  FIG.  2 A , the second electrode  210  of the light-emitting unit  200 ′ includes a top surface  210   a . The elevation E 4  of the top surface  210   a  of the second electrode  210  is also higher than the elevation E 2  of the top surface  300   a  of the first protective layer  300 . In some embodiments of the present disclosure, the difference between the elevation E 4  of the second electrode  210  and the elevation E 2  of the first protective layer  300  ranges from about 0.02 μm to about 5 μm, or from about 0.2 μm to about 2 μm. 
     Next,  FIG.  2 B  illustrates a cross-sectional view of the display device  20  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  2 B  and  FIG.  2 A  is that the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  2 B  has a concave shape while the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  2 A  is substantially planar. Moreover, the top surface  212   a  of the contact layer  212  may also has a concave shape. 
     As shown in  FIG.  2 B , the wavelength conversion layer  304  located above the light-emitting unit  200 ′ has a seventh thickness T 7 , and the wavelength conversion layer  304  including the additional portions  304 ′ located above the protective layer  300  has an eighth thickness T 8 . In some embodiments of the present disclosure, the seventh thickness T 7  may be defined as the maximum thickness of the wavelength conversion layer  304  that is located above both the light-emitting unit  200 ′ and the contact layer  212 . In some embodiments of the present disclosure, the eighth thickness T 8  may be defined as the maximum thickness of the wavelength conversion layer  304  that is located both above the protective layer  300  and the contact layer  212 . In this embodiment, the difference between the seventh thickness T 7  and the eighth thickness T 8  in one subpixel may be smaller due to the concave shape of the top surface  300   a  and the top surface  212   a , as compared with that of the substantially planar top surface  300   a  and the top surface  212   a  (as shown in  FIG.  2 A ). In addition, in some embodiments of the present disclosure, the ninth thickness T 9  of the additional portions  304 ′ that is closer to the light-emitting unit  200 ′ may be smaller than the tenth thickness T 10  of the additional portions  304 ′ that is farther from the light-emitting unit  200 ′ due to the concave shape of the top surface  300   a  and the top surface  212   a.    
     Next,  FIG.  2 C  illustrates a cross-sectional view of the display device  20  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  2 C  and  FIG.  2 A  is that the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  2 C  has a convex shape while the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  2 A  is substantially planar. Moreover, the top surface  212   a  of the contact layer  212  may also has a convex shape. 
     As shown in  FIG.  2 C , in this embodiment, the eleventh thickness T 11  of the additional portions  304 ′ that is closer to the light-emitting unit  200 ′ may be greater than the twelfth thickness T 12  of the additional portions  304 ′ that is farther from the light-emitting unit  200 ′ due to the convex shape of the top surface  300   a  and the top surface  212   a . In addition, the reflected light L may be concentrated to increase the illumination efficiency due to the convex surface of the top surface  300   a  and the top surface  212   a.    
     Next,  FIG.  3 A  illustrates a cross-sectional view of the display device  30  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  3 A  and  FIG.  1 A  is that the second insulating layer  110 ′ in the embodiment shown in  FIG.  3 A  further includes a bank portion  110   b.    
     As shown in  FIG.  3 A , the bank portion  110   b  of the second insulating layer  110 ′ protrudes toward the light shielding layer  306 . The bank portion  110   b  of the second insulating layer  110 ′ includes a top surface  110   ba . In some embodiments of the present disclosure, the elevation E 5  of the top surface  110   ba  of the second insulating layer  110 ′ is lower than the elevation E 2  of the top surface  300   a  of the first protective layer  300 . In such a configuration, the materials of the first protective layer  300  may be conserved since the bank portions  110   b  of the second insulating layer  110 ′ occupies some of the spaces that are originally to be filled in with the first protective layer  300 . On the other hand, the elevation E 5  of the top surface  110   ba  of the second insulating layer  110 ′ may be lower or higher than the elevation E 3  of the top surface  204   a  of the quantum well layer  204 . 
     In some embodiments of the present disclosure, since the second insulating layer  110 ′ includes the bank portions  110   b , the light-emitting units  200  may be disposed in the trench or the cavity defined by the bank portions  110   b . In some embodiments, a plurality of light-emitting units  200  are disposed in the same trench defined by the bank portions  110   b . In other embodiments, each of the light-emitting unit  200  is disposed in a cavity defined by the bank portions  110   b  separately. In addition, each cavity includes a plurality of light-emitting units  200  disposed therein in accordance with some embodiments. 
     In addition, the bank portion  110   b  may be formed by performing a patterning process to the second insulating layer  110 ′. The patterning process may include a photolithography process and an etching process such as a selective etching process. The photolithography process may include, but is not limited to, photoresist coating (e.g., spin-on coating), soft baking, hard baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying, and other suitable processes. The etching process may include dry etching process or wet etching process. 
     In accordance with some embodiments of the present disclosure, the elevation E 5  of the top surface  110   ba  of the second insulating layer  110 ′ is equal to the elevation E 2  of the top surface  300   a  of the first protective layer  300 . In accordance with other embodiments of the present disclosure, the elevation E 5  of the top surface  110   ba  of the second insulating layer  110 ′ is higher than the elevation E 2  of the top surface  300   a  of the first protective layer  300  (as shown in  FIG.  3 B ). In addition, the second insulating layer  110 ′ may be a two-layered structure or multi-layers stack structure in accordance with some embodiments. 
     Next,  FIG.  4    illustrates a cross-sectional view of the display device  40  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  4    and  FIG.  1 A  is that the top surface  300   a  of the first protective layer  300  in the embodiment shown in  FIG.  4    includes a plurality of recesses  314 . In other words, the top surface  300   a  of the first protective layer  300  has a pothole structure. 
     In some embodiments of the present disclosure, the recesses  314  of the top surface  300   a  of the first protective layer  300  may be randomly distributed. In some embodiments of the present disclosure, the size of the recess  314  may range from about 1 nm to about 10 um or from about 100 nm to about 2 um. In addition, in such a configuration, the recesses  314  of the top surface  300   a  of the first protective layer  300  may prevent the reflected light from being trapped in the first protective layer  300  due to the total reflection. Accordingly, the recesses  314  of the top surface  300   a  may increase or improve the illumination efficiency of the display device. 
     In some embodiments of the present disclosure, the recesses  314  of the top surface  300   a  may be formed by a patterning process. The patterning process may include a photolithography process and an etching process such as a selective etching process. The photolithography process may include, but is not limited to, photoresist coating (e.g., spin-on coating), soft baking, hard baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying, and other suitable processes. The etching process may include dry etching process or wet etching process. 
     Next,  FIG.  5 A  illustrates a cross-sectional view of the display device  50  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  5 A  and  FIG.  1 A  is that the display device  50  further includes a buffer layer  316  disposed on the light-emitting unit  200 . 
     As shown in  FIG.  5 A , the buffer layer  316  may be disposed on the light-emitting unit  200  and the first protective layer  300 . The buffer layer  316  may be disposed between the light-emitting unit  200  and the wavelength conversion layer  304 . In some embodiments of the present disclosure, the buffer layer  316  may be conformally formed over the light-emitting unit  200  and the first protective layer  300 . In addition, the buffer layer  316  may cover the sidewall  206   s  of the second semiconductor layer  206 . As described above, the buffer layer  316  may be disposed between the light-emitting unit  200  and the wavelength conversion layer  304  so that the direct contact between the light-emitting unit  200  and the wavelength conversion layer  304  may be avoided. Thus, the wavelength conversion layer  304  may be unaffected by the current or heat produced by the light-emitting unit  200 . As described above, although the display device  50  include the wavelength conversion layer  304  disposed on the light-emitting unit  200  in the embodiments illustrated in  FIG.  5 A , the wavelength conversion layer  304  may be simply replaced with a transparent material without the function of wavelength conversion (e.g., without phosphor particles or quantum dot materials). For example, the transparent material may include, but is not limited to, a polymer or glass matrix. 
     In some embodiments of the present disclosure, the buffer layer  316  may be an insulator. In some embodiments of the present disclosure, the buffer layer  316  may include organic materials and/or inorganic materials. The buffer layer  316  may be formed of organic insulating materials. The organic insulating material may include, but is not limited to, polyamide, polyethylene, polystyrene, polypropylene, polyester, polyimide, polyurethane, silicones, polyacrylate, benzo-cyclo-butene (BCB), polyvinylpyrrolidone (PVP), polyvinylfluoride (PVF), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polymethylmetacrylate (PMMA), polydimethylsiloxane (PDMS), any other suitable organic insulating materials, or a combination thereof. The inorganic insulating material may include, but is not limited to, SiO x , SiN x , AlO x , any other suitable inorganic insulating materials, or a combination thereof. 
     In addition, the buffer layer  316  may be formed by using chemical vapor deposition (CVD) or spin-on coating. The chemical vapor deposition may include, but is not limited to, low-pressure chemical vapor deposition (LPCVD), low-temperature chemical vapor deposition (LTCVD), rapid thermal chemical vapor deposition (RTCVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), or any other suitable method. 
     In some embodiments of the present disclosure, the thickness of the buffer layer  316  may not be uniform. As shown in  FIG.  5 A , the buffer layer  316  located above the first protective layer  300  may have a thirteenth thickness T 13 , and the buffer layer  316  located above the light-emitting unit  200  has a fourteenth thickness T 14 . In some embodiments of the present disclosure, the thirteenth thickness T 13  of the buffer layer  316  is greater than the fourteenth thickness T 14  of the buffer layer  316 . That is, the buffer layer  316  disposed directly on the light-emitting unit  200  may be thinner than the buffer layer  316  directly disposed on the first protective layer  300 . In such a configuration, the intensity of the light emitted from the light-emitting unit  200  will not be greatly decreased since the buffer layer  316  disposed on the light-emitting unit  200  is thinner. In some embodiments of the present disclosure, the difference between the thirteenth thickness T 13  and the fourteenth thickness T 14  may range from about 0.001 um to about 5 um or from about 0.05 um to about 2 um. In some embodiments of the present disclosure, the thirteenth thickness T 13  of the buffer layer  316  may range from about 0.02 μm to about 5 μm or from about 0.1 um to about 2 um. In some embodiments of the present disclosure, the fourteenth thickness T 14  of the buffer layer  316  may range from about 0.02 μm to about 5 μm or from about 0.1 um to about 2 um. 
     Next,  FIG.  5 B  illustrates a cross-sectional view of the display device  50  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  5 B  and  FIG.  5 A  is that the top surface  316   a  of the buffer layer  316  on the first protective layer  300  in the embodiment shown in  FIG.  5 B  has a concave shape while the top surface  316   a  of the buffer layer  316  on the first protective layer  300  in the embodiment shown in  FIG.  5 A  is substantially planar. 
     In addition, as shown in  FIG.  5 C , the top surface  316   a  of the buffer layer  316  on the first protective layer  300  may have a convex shape in accordance with other embodiments of the present disclosure. 
     Next,  FIG.  6 A  illustrates a cross-sectional view of the display device  60  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  6 A  and  FIG.  5 A  is that the buffer layer  316 ′ in the embodiment shown in  FIG.  6 A  is a discontinuous structure while the buffer layer  316  in the embodiment shown in  FIG.  5 A  is a continuous structure. 
     As shown in  FIG.  6 A , the buffer layers  316 ′ within different subpixels may be discontinuous. In other words, the buffer layers  316 ′ from different subpixels may be separated. In this embodiment, a portion of the buffer layers  316 ′ may be disposed over the light-emitting unit  200  while another portion of the buffer layers  316 ′ may be disposed over the first protective layer  300 . In particular, the buffer layers  316 ′ may entirely cover the top surface  206   a  of the second semiconductor layer  206 , and partially cover the top surface  300   a  of the first protective layer  300 . In some embodiments of the present disclosure, the buffer layer  316 ′ extends from the light-emitting unit  200  through the wavelength conversion layer  304  and to the light shielding layer  306 . 
     In some embodiments of the present disclosure, the buffer layers  316 ′ may be formed by a patterning process. The patterning process may include a photolithography process and an etching process such as a selective etching process. The photolithography process may include, but is not limited to, photoresist coating (e.g., spin-on coating), soft baking, hard baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying, and other suitable processes. The etching process may include dry etching process or wet etching process. 
     Next,  FIG.  6 B  illustrates a cross-sectional view of the display device  60  in accordance with other embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  6 B  and  FIG.  6 A  is that the buffer layer  316 ″ in the embodiment shown in  FIG.  6 B  does not extend to the light shielding layer  306 . 
     As shown in  FIG.  6 B , the buffer layer  316 ″ may entirely cover the top surface  206   a  of the second semiconductor layer  206 , and partially cover the sidewall  206   s  of the second semiconductor layer  206 . The buffer layer  316 ″ does not extend along the top surface  300   a  of the protective layer  300 . Similarly, the buffer layers  316 ″ may be formed by the patterning process as described above. 
     Next,  FIG.  7 A  illustrates a cross-sectional view of the display device  70  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  7 A  and  FIG.  2 A  is that the display device  70  further includes a buffer layer  316  disposed on the contact layer  212 . 
     As shown in  FIG.  7   , the buffer layer  316  may be disposed between the contact layer  212  and the wavelength conversion layer  310 . In some embodiments of the present disclosure, the buffer layer  316  may be conformally formed over the contact layer  212 . As described above, the buffer layer  316  may be disposed between the contact layer  212  and the wavelength conversion layer  310  so that the direct contact between the contact layer  212  and the wavelength conversion layer  310  may be avoided. Thus, the wavelength conversion layer  310  may be unaffected by the current or heat produced by the light-emitting unit  200  including the contact layer  212 . 
     In some embodiments of the present disclosure, the thickness of the buffer layer  316  may not be uniform. As shown in  FIG.  7   , the buffer layer  316  disposed directly above the light-emitting unit  200  may be thinner than the buffer layer  316  directly disposed above the first protective layer  300  in accordance with some embodiments of the present disclosure. In such a configuration, the intensity of the light emitted from the light-emitting unit  200  will not be greatly decreased since the buffer layer  316  disposed on the light-emitting unit  200  is thinner. 
     Next,  FIG.  8    illustrates a cross-sectional view of the display device  80  in accordance with some embodiments of the present disclosure. The difference between the embodiments shown in  FIG.  8    and  FIG.  5 A  is that the top surface  316   a ″ of the buffer layer  316  on the light-emitting unit  200  in the embodiment shown in  FIG.  8    is rougher than the top surface  316   a ′ of the buffer layer  316  on the first protective layer  300 . 
     As shown in  FIG.  8   , in this embodiment, the top surface  316   a  of the buffer layer  316  may further include the top surface  316   a ″ that is disposed above the light-emitting unit  200  and the top surface  316   a ′ that is disposed above the first protective layer  300  and out of the light-emitting unit  200 . As describe above, the top surface  316   a ″ may be rougher than the top surface  316   a ′. In some embodiments, the surface roughness of the top surface  316   a ′ may range from about 2 nm to about 30 nm. In some embodiments, the surface roughness of the top surface  316   a ″ may range from about 5 nm to about 100 nm. In some embodiments, the difference of the surface roughness between the top surface  316   a ″ and the top surface  316   a ′ may range from about 3 nm to about 100 nm. In such a configuration, the light from the light-emitting unit  200  may be emitted more uniformly so that the conversion efficiency of the wavelength conversion layer  304  may be increased. 
     In some embodiments of the present disclosure, the rough top surface  316   a ″ may be formed by an etching process. The etching process may include dry etching process or wet etching process. 
     Next,  FIG.  9 A  illustrates a partially enlarged portion of the display device  50  in  FIG.  5 A . As shown in  FIG.  9 A , the conductive elements  302  may include the first conductive elements  302   a  and the second conductive elements  302   b . The first conductive elements  302   a  may be disposed underneath the first terminal S 1  of the light-emitting unit  200 . The first conductive elements  302   a  may be disposed between the light-emitting unit  200  and the second insulating layer  110 . In particular, some of the first conductive elements  302   a  may be disposed between the first electrode  208  and the contact structures (e.g., the conductive pads) on the second insulating layer  110 ; and some of the first conductive elements  302   a  may be disposed between the second electrode  210  and the contact structures (e.g., the conductive pads) on the second insulating layer  110 . On the other hand, the second conductive elements  302   b  may be disposed in the region out of the light-emitting unit  200 . The second conductive elements  302   b  may be dispersed in the first protective layer  300  or disposed at the top surface  110   a  of the second insulating layer  110 . 
     In accordance with some embodiments of the present disclosure, a height-to-width ratio of the first conductive element  302   a  ranges from about 0.25 to about 0.75 or from about 0.4 to about 0.6. However, it should be noted that the height-to-width ratio of the first conductive element  302   a  should not be too small, or the contact yield will dramatically decrease; and the height-to-width ratio of the first conductive element  302   a  should not be too great, or the contact resistance will be too high due to the contact area of a single conductive element  302   a  is too low. 
     In accordance with some embodiments of the present disclosure, a height-to-width ratio of the second conductive element  302   b  ranges from about 0.7 to about 1.3 or from about 0.8 to about 1.2. However, it should be noted that the height-to-width ratio of the second conductive element  302   b  should not be too small, or the light being reflected by the second conductive element  302   b  will be nonuniform; and the height-to-width ratio of the first conductive element  302   a  should not be too great, or the light being reflected by the second conductive element  302   b  will dramatically decrease. 
     It should be noted that the height-to-width ratio used herein is measured from the cross-sectional structure obtained from the conductive element  302 . In particular, the variation of height-to-width ratio may be from about 0% to about 5% due to the process for obtaining the cross-sectional structure. In addition, the “height” of the height-to-width ratio is defined as the maximum height along a first direction of a cross-sectional structure obtained from the conductive element  302 . The “width” of the height-to-width ratio is defined as the maximum width along a second direction of a cross-sectional structure obtained from the conductive element  302 . The above first direction and the second direction are orthogonal to each other. 
     For example,  FIG.  9 B  and  FIG.  9 C  illustrate the cross-sectional views of the first conductive elements  302   a  in accordance with some embodiments of the present disclosure. Referring to  FIG.  9 B  and  FIG.  9 C , the cross-sectional structure of the exemplary first conductive element  302   a  has a maximum height H 1  along a first direction A and a maximum width W 1  along a second direction B. The first direction A is orthogonal to the second direction B. In these examples, the height-to-width ratio of the first conductive element  302   a  is H 1 /W 1 . Moreover, as shown in  FIG.  9 B  and  FIG.  9 C , the cross-sectional structure of the first conductive element  302   a  may have, but is not limited to, an ellipse shape or an ellipse-like shape. 
     Next,  FIG.  9 D  and  FIG.  9 E  illustrate the cross-sectional views of the second conductive elements  302   b  in accordance with some embodiments of the present disclosure. As shown in  FIG.  9 D  and  FIG.  9 E , the cross-sectional structure of the exemplary second conductive element  302   b  has a maximum height H 2  along a first direction A and a maximum width W 2  along a second direction B. The first direction A is orthogonal to the second direction B. In these examples, the height-to-width ratio of the second conductive element  302   b  is H 2 /W 2 . Moreover, the cross-sectional structure of the second conductive element  302   b  may have, but is not limited to, a circular shape or a circular-like shape. 
     To summarize the above, the display device provided in the present disclosure includes a protective layer having the elevation that is lower than the elevation of the upper semiconductor layer of the light-emitting unit. In such a configuration, less material is required for the protective layer compared to general display devices where the elevation of the protective layer is level with that of the upper semiconductor layer. In addition, there will be more space for the wavelength conversion layer, which is disposed over the protective layer, to fill in. In accordance with some embodiments of the present disclosure, the display device includes the protective layer having the elevation that is higher than the elevation of the quantum well of the light-emitting unit so as to prevent moisture and oxygen from damaging the quantum well. Furthermore, the protective layer of such a design may also prevent shorts or increase the reflectivity. 
     In addition, in accordance with some embodiments of the present disclosure, the display device includes a buffer layer disposed between the light emitting unit and the wavelength conversion layer so that the wavelength conversion layer may be unaffected by the current or heat produced by the light emitting-unit. 
     Although some embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by one of ordinary skill in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.