WO2005064697A1

WO2005064697A1 - Light emitting diode with vertical electrode structure and manufacturing method of the same

Info

Publication number: WO2005064697A1
Application number: PCT/KR2004/003021
Authority: WO
Inventors: Seong-Jin Kim; Chang-Yeon Kim; Yong-Seok Choi; Young-Heon Han; Don-Soo Kim; Soon-Jae Yu
Original assignee: Itswell Co. Ltd.
Priority date: 2003-12-30
Filing date: 2004-11-22
Publication date: 2005-07-14
Also published as: KR100613273B1; KR20050013047A

Abstract

A light conversion LED is fabricated by a method comprising: forming a buffer layer, an n-contact layer, an n-cladding layer, an active layer, a p-cladding layer, a p-contact layer, a conductive transparent electrode and a light reflecting layer in their order on a sapphire substrate; forming first and second receptor contact layers on both surfaces of a receptor substrate; forming a junction layer on at least one of the light reflecting layer and the second receptor contact layer; arranging the sapphire, substrate and the receptor substrate so that the light reflecting layer of the sapphire substrate is opposed to the second receptor contact layer of the receptor substrate and thermally compressing a resultant structure; removing the sapphire substrate; forming a second electrode on the n-contact layer and a first electrode on the first receptor contact layer; and spin-coating a phosphor layer on the n-contact layer and the second electrode.

Description

LIGHT EMITTING DIODE WITH VERTICAL ELECTRODE STRUCTURE AND MANUFACTURING METHOD OF THE SAME

Technical Field The present invention relates to a Light Emitting Diode (LED) and a fabrication method thereof. Light conversion in an LED is realized generally by three schemes: Yellow YAG phosphor is applied to a blue LED to convert blue light into white light. Red, green and blue phosphors are applied to a UV LED to convert UV ray into white light. In addition, red, green and blue LEDs can be combined together to mix their rays into white light. In this case, LEDs are used as light sources for exciting phosphor, and phosphor acts as a medium for wavelength conversion that absorbs light and generates another color of light. The first scheme of applying yellow YAG phosphor to a blue LED advantageously provides a simple fabrication method but its color rendering is limited. On the other hand, the second scheme of adopting red, green and blue phosphor and UV ray has a problem of high fabrication cost in spite of excellent color rendering. In particular, these schemes show difficulties in fabrication of high brightness LEDs designed to convert color. Thus, a combination of red, green and blue LEDs designed to convert color is also promoted.

Background Art Now the structure of a conventional LED will be described with reference a drawing. FIG. 1 is a cross- sectional view illustrating a conventional LED. The LED includes an LED chip 200 die-bonded on a lead frame 101 by a conductive paste 500. The LED chip 200 is connected at a pair of electrodes to lead frames 101 and 102 via wires 301 and 302. Phosphor mixture 400 covers the chip 200, and an epoxy 600 is formed to surround the lead frames. According to conventional fabrication methods of phosphor lamps, phosphor mixture is dispensed over a chip die-bonded to lead frames to cover the chip and then transparent epoxy is molded surrounding the chip and the lead frames or phosphor-containing epoxy is molded surrounding the whole part of the lead frames. However, dispensed phosphor is ununiformly distributed causing variation in Color Rendering Index (CRI) so that LED products show different values in the dispensing quantity, thickness and mixing ratio of phosphor even though they are fabricated by the same process and time. Thus, it is difficult to fabricate uniform LEDs designed to convert color. In case that an LED adopts a nitride-based light emitting source, a substrate is made of sapphire in order to reduce crystal defect in epitaxial growth since sapphire has a lattice constant and a crystal structure similar to those of nitride-based semiconductor. However, since sapphire is an insulator, both of first and second electrodes should be formed on the same growth surface of an epitaxial layer. Forming both electrodes on the same side requires that the electrodes use a predetermined area for wire bonding so that the LED diode has at least a predetermined chip size. This as a result limits chip yield per wafer. In addition, the substrate made of an insulator can hardly discharge external static elasticity, which may potentially cause defects. Also, this may degrade the reliability of a resultant LED as well as create various restrictions in a packaging process. In addition, since sapphire is a poor heat conductor, the LED cannot efficiently dissipate heat to the outside in actuation. So, high voltage application for high power output is limited thereby obstructing the fabrication of a high brightness LED.

Disclosure of the Invention Technical Object The present invention has been made to solve the above problems, and it is therefore an object of the invention to provide an LED having a vertical electrode structure and a fabrication method thereof. It is another object of the invention to provide an LED having uniform phosphor distribution and a fabrication method thereof.

Technical Solution In order to realize the above objects, the present invention proposes an LED as follows: According to an aspect of the invention, there is provided an LED comprising: a conductive receptor substrate having top and bottom surfaces; a first electrode formed on the bottom surface of the receptor substrate; a junction layer formed on the top surface of the receptor substrate, and having conductivity and ohmic characteristics; a light reflecting layer formed on the junction layer; a first conductive cladding layer formed on the light reflecting layer; an active layer formed on the first conductive cladding layer; a second conductive cladding layer formed on the active layer; a second electrode formed on the second conductive cladding layer; and a phosphor layer coated on the second conductive cladding layer and the second electrode, and having a contact opening for exposing the second electrode. Preferably, the buffer layer, the first conductive contact layer, the first cladding layer, the active layer, the second cladding layer and the second conductive contact layer are made of In_x(Ga_yAlι-y)N (0<x<l, 0<y<l, x+y>l). The LED may further comprise: a first receptor contact layer formed between the first electrode and the receptor substrate; a second receptor contact layer formed between the receptor substrate and the junction layer; a first conductive contact layer and a conductive electrode formed between the light reflecting layer and the first conductive cladding layer; and a second conductive contact layer formed between the second conductive cladding layer and the second electrode. Preferably, the first conductive layers are p-type and the second conductive layers are n-type, the phosphor layer contains red, green and blue phosphors and resin. In addition, the phosphor layer may contain YAG phosphor and resin. According to another aspect of the invention, there is provided an LED comprising: an insulator substrate having first and second surfaces; a buffer layer formed on the first surface of the insulator substrate; a first conductive contact layer formed on the buffer layer; a first conductive cladding layer formed on the first conductive contact layer; an active layer formed on the first conductive cladding layer; a second conductive cladding layer formed on the active layer; a second conductive contact layer formed on the second conductive cladding layer; a light reflecting layer formed on the second conductive contact layer; a first electrode formed on the light reflecting layer; a second electrode formed on the first conductive contact layer; and a phosphor layer formed on the second layer of the insulator substrate. Preferably, the buffer layer, the first conductive contact layer, the first cladding layer, the active layer, the second cladding layer and the second conductive contact layer are made of In_x(Ga_yAlι-_y)N (0<x<l, 0<y<l, x+ y>l). The LED may further comprise a conductive transparent electrode layer formed between the light reflecting layer and the second conductive contact layer. Preferably, the first conductive layers are n-type and the second conductive layers are p-type, and the phosphor layer contains red, green and blue phosphors and resin. In addition, the phosphor layer may contain YAG phosphor and resin. According to further another aspect of the invention, such an LED is fabricated according to a following method. The method comprises following steps of: forming a buffer layer, an n-contact layer, an n-cladding layer, an active layer, a p-cladding layer, a p-contact layer, a conductive transparent electrode and a light reflecting layer in their order on a sapphire substrate; forming first and second receptor contact layers on both surfaces of a receptor substrate; forming a junction layer on at least one of the light reflecting layer and the second receptor contact layer! arranging the sapphire substrate and the receptor substrate so that the light reflecting layer of the sapphire substrate is opposed to the second receptor contact layer of the receptor substrate and thermally compressing a resultant structure; removing the sapphire substrate; forming a second electrode on the n-contact layer and a first electrode on the first receptor contact layer; and spin-coating a phosphor layer on the n-contact layer and the second electrode. The method may further comprise a step of removing the buffer layer after the sapphire substrate-removing step, in which the sapphire substrate-removing step and the buffer layer-removing step comprise performing at least one selected from a group consisting of wet etching, dry etching of Chemical Mechanical Polishing (CMP) or ICP/RIE and mechanical polishing, wherein the wet etching uses an etching solution containing at least one of HC1, H₂SO₄, H3PO₄, HNO₃, KOH, NaOH and 4 H₃PO₄+ 4CH3COOH+ HN0₃+ H₂0 (aluetch) and mixtures thereof, and wherein the sapphire substrate-removing step and the buffer layer-removing step adopt the wet etching and the dry etching together, wherein the wet etching is adapted to etch the substrate and the dry etching is adapted to etch the buffer layer. In addition, the sapphire substrate-removing step may comprise backside-polishing the sapphire substrate via mechanical polishing and etching the backside-polished sapphire substrate, and preferably further comprise a step of, after the phosphor layer spin-coating step, selectively etching the phosphor layer to form a contact opening thereby exposing the second electrode. Preferably, the receptor substrate comprises one selected from a group consisting of a semiconductor substrate, a conductive oxide substrate and a metal substrate, wherein the semiconductor substrate is made of one of Si, GaAs, InP and InAs, the conductive oxide substrate is made of one of Indium Tin Oxide (ITO), ZrB and ZnO, and the metal substrate is made of one of CuW, Mo, Au, Al, Au and Ag. The thermal compression step may comprise depositing at least one selected from a group consisting of Ti, Ni, Au, Sn, In, Pd, Ag, Sn, Ag and Rh and thermally compressing a resultant structure in order to achieve ohmic characteristics, the sapphire substrate -removing step may be performed by at least one selected from a group consisting of mechanical polishing, wet etching and dry etching, and the first electrode may preferably contain at least one selected from a group consisting of ITO, InSnO, Ti/Ni/Au, Ti, Al, Rd, Pt, Ta, Ni, Cr, Au and Pd. According to yet another aspect of the invention, such an LED is fabricated according to a following method. The method comprises following steps of: forming a buffer layer, an n-contact layer, an n-cladding layer, an active layer, a p-cladding layer, a p-contact layer, a conductive transparent electrode layer, a light reflecting layer and a first electrode layer in their order on a first surface of a sapphire substrate; selectively etching the first electrode layer, the light reflecting layer, the conductive transparent electrode layer, the p-contact layer, the p-cladding layer, the active layer, the n-cladding layer and the n-cladding layer to expose a partial area of the n-contact layer; forming a second electrode on the exposed area of the n-contact layer; spin-coating a phosphor layer on a second surface of the sapphire substrate; dividing the sapphire substrate into respective chips; and flip-chip bonding the respective chips to lead frames. The method may further comprise a step of partially cutting the sapphire substrate in thickness direction before the phosphor layer spin-coating step and a step of selectively etching the phosphor layer to partition the sapphire substrate according to the respective chips before the sapphire substrate-dividing step. In addition, the method may further comprise a step of heat treating a resultant structure at a temperature of about 300 to 700 °C in a furnace of an atmosphere containing nitrogen or oxygen after the first or second electrode is formed.

Advantageous Effects As described above, when the light (color) conversion LED is fabricated using a vertical electrode LED and red, green, blue and white phosphors, the second and first electrodes are formed separately on top and bottom surfaces of the chip. This can reduce the chip surface area thereby increasing chip yield per wafer. Also, the color conversion LED fabricated from a vertical electrode LED or a flip-chip LED can efficiently discharge heat and static electricity thereby making a high brightness color conversion LED. Also, the invention removes the sapphire substrate via backside polishing, dry etching or wet etching thereby improving productivity significantly. In case of laser lift-off, this can protect epitaxial layers from heat damage. Furthermore, the phosphor layer can be formed at a uniform thickness via spin-coating, and thus color conversion LEDs can be stably produced with little variation in CRI.

Brief Description of the Drawings FIG. 1 is a cross-sectional view illustrating a conventional LED; FIG. 2 is a cross-sectional view illustrating an LED according to a first embodiment of the invention; FIGS. 3 to 8 are cross-sectional views illustrating an intermediate process for fabricating the LED according to the first embodiment of the invention; FIG. 9 is a graph comparing the etch rate of sapphire with that of GaN by ICP/RIE dry etching; FIG. 10 is a graph comparing the etch rate of sapphire with that of GaN by a mixed solution of sulfuric acid (H₂S0₄) and phosphoric acid (H₃PO₄); FIG. 11 is a photograph illustrating the surface of a nitride semiconductor buffer layer in which a sapphire substrate is removed by wet etching; FIG. 12 is a graph illustrating the current- oltage curve of a nitride semiconductor layer measured with a probe in which a sapphire substrate is removed by wet etching; FIG. 13 is a cross-sectional view illustrating an LED according to a second embodiment of the invention; FIGS. 14 to 17 are cross-sectional views illustrating an intermediate process for fabricating the LED according to the first embodiment of the invention; and FIG. 18 is a spectrum graph of a white LED fabricated according to an embodiment of the invention.

Best Mode for Carrying out the Invention The thickness is exaggerated in order to clearly indicate several layers and areas in the drawings, and the same reference signs are used to designate the same or similar parts throughout the specification. When it is mentioned that a specific component such as a layer, film, area and sheet is arranged "above" a second component, this is not to be understood that the specific component is located "directly above" the second component, but a third component may intermediate between the first and second components. On the other hand, when it is mentioned that a specific component is located "directly on" a second component, this means that there is no other component intermediating between the first and second components. The following detailed description will discuss preferred embodiments of a laser diode having a vertical electrode structure according to the present invention in conjunction with the accompanying drawings. FIG. 2 is a cross-sectional view illustrating an LED according to a first embodiment of the invention. The LED according to the first embodiment of the invention includes lead frames 101 and

102, a chip 100 bonded to the lead frame 101 and a wire 21 connecting one electrode of the chip 100 to the lead frame 102. The chip 100 includes a first electrode 16, a first receptor contact layer 14, a receptor substrate 13, a second receptor contact layer 12, a receptor-side junction metal layer 11, an epitaxy-side junction metal layer 10, a light reflecting layer 9, a conductive transparent electrode 8, a p-contact layer 7, a p-cladding layer 6, an active layer 5, an n-cladding layer 4 and an n-contact layer 3, which are formed in their order one atop another. The n-contact layer 3 supports a second electrode 15 formed thereon. On the n-contact layer 3, a phosphor layer 17 is formed to convert the wavelength of light. The phosphor layer 17 has a contact opening that exposes the second electrode 15 so that the wire 21 is bonded to the second electrode 15 through the contact opening. The phosphor layer 17 is formed as follow: Red, green and blue phosphors are mixed into resin such as PE (polyethylene), PP (polypropylene) and acryl together with anti-precipitation solution. A resultant phosphor mixture is agitated at a room or higher temperature to melt into liquid. Then, the liquid phosphor mixture is spin-coated. The receptor substrate 13 may be of a semiconductor substrate of for example Si, GaAs and SiC or a metal substrate of for example Au, Al, CuW, Mo and W. The receptor-side junction metal layer 11 and the epitaxy-side junction metal layer 10 are made of low temperature metal such as AuSn, In, Pd, Ag, Au, Sn, InPd and Agin. The two junction metal layers 11 and 10 are welded together via thermal compression so that the receptor substrate

13 is bonded with an epitaxial layer. The junction metal layers 11 and 10 can be replaced with a conductive epoxy film. A buffer layer 2, the n-contact layer 3, the n-cladding layer 4, the active layer 5, the p-cladding layer 6 and the p-contact layer are made of for example In_x(Ga_yAlι-_y)N (x>0, y>0). The light reflecting layer 9 is made of single or multiple layers containing at least one of Ti, Ni, Al, Ag, Au, Rh and Pd to have excellent reflectivity. The first electrode 16 of the chip 100 is bonded to the lead frame 101 via the conductive paste 500 and the second electrode 15 of the chip 100 is connected to another electrode of the lead frame 102. In the LED of this structure, the second electrode 15 and the first electrode 16 are formed separately on both sides (i.e., top and underside) of the chip thereby reducing the area of the chip. Therefore, this can improve chip yield per wafer. In addition, since the receptor substrate 13 of excellent electric conductivity is used as a structure of the chip, heat and/or static electricity can be efficiently discharged. Furthermore, current can uniformly flow through the entire cross-section of the chip thereby allowing the LED to operate at high voltage. So, high optical power can be realized from a unit device. In addition, since the phosphor layer 17 can be formed uniformly, it is possible to fabricate an LED having little variation in color rendering. Now a fabrication method for the LED having the above structure will be described as follows: FIGS. 3 to 8 are cross-sectional views illustrating an intermediate process for fabricating the LED according to the first embodiment of the invention. First, as shown in FIG. 3, a buffer layer 2, an n-contact layer 3, an n-cladding layer 4, an active layer 5, a p-cladding layer 6 and a p-contact layer 7 are formed in their order on a sapphire (AI2O₃) substrate 1 via Metal Organic Chemical Vapor Deposition (MOCVD), Liquid Phase Epitaxy (LPE), Molecular Beam Epitaxy (MBE) and so on. Then, as shown in FIG. 4, a conductive transparent electrode 8 and a light reflecting layer 9 are formed on the p-contact layer 7. On the light reflecting layer 9, an epitaxy- side junction metal layer 10 is formed for the purpose of welding or junction. The light reflecting layer 9 and the conductive transparent electrode 8 are deposited for example via E-Beam, Thermal Evaporation or Sputtering. A first receptor contact layer 14 is formed on the top area of a receptor substrate 13 made of semiconductor or metal, and a second receptor contact layer 12 and a receptor-side junction metal layer 11 are formed on the underside of the receptor substrate 13. The epitaxy-side junction metal layer 10 and the receptor-side junction metal layer 11 are contacted to each other and applied with a pressure of about 1 to 6MPa at a temperature of about 200 to 500 °C for about 3 minutes to 1 hour to weld the junction metal layers 10 and 11 together. In case that Au is used as eutectic metal, this process is preferably performed under a pressure of about 2.5MP at a temperature of about 300°C for a welding time of about 10 minutes. In order to prevent the respective layers from oxidation under high temperature, this thermal compression can be performed in a gas atmosphere containing any of Ar, He, Kr, Xe, Rn and so on or an atmosphere containing N2, halogen or air (including 0₂) so that the contact layers can overcome the energy gap between metal and semiconductor to reduce contact resistance. Alternatively, instead of the junction metal layers 10 and 11, for example an epoxy film can be used to attach the receptor to the epitaxial layer. Then, as shown in FIG. 5, the sapphire substrate 1 is removed via at least one of mechanical polishing, wet etching and dry etching. The buffer layer 2 and in part the n-contact layer 3 are removed together with the sapphire substrate 1. The buffer layer 2 is grown at a relatively lower temperature compared to other layers to have an amorphous structure. Since this amorphous structure has a poor crystal structure and absorbs short wavelength in vicinity of 365nm, it is removed to enhance light efficiency. Alternatively, the buffer layer 2 may not be removed in the fabrication of an LED that emits long wavelength of 400nm or more. The n-contact layer 3 shows film quality variation in which a later formed area shows higher quality than an earlier formed area. Therefore, in the n-contact layer 3, a lower area of poor quality is partially removed since an upper area shows higher quality compared to a lower area. Now a method of removing the sapphire substrate 1, the buffer layer 2 and in part the n-contact layer 3 will be described in detail. First, after a protective layer of for example Spin-on Glass (SOG), SiN_x or Si0₂ is deposited at l m to protect the semiconductor surface or the receptor substrate from being etched or damaged in wet etching, the sapphire substrate 1 is lapped, and a lapped area is mirror polished to have a smooth surface. The sapphire substrate 1 is lapped via Chemical Mechanical Polishing (CMP), ICP/RIE dry etching, mechanical polishing with alumina (AI₂O₃) powder or wet etching. The wet etching is performed with an etching solution containing at least one of HCl, H₂SO₄, H₃PO₄, HNO₃, KOH, NaOH and Aluetch (4 H₃PO₄+ 4CH3COOH+ HN0₃+ H₂0). In this case, it is more desirable for the sapphire substrate 1 to have a smaller thickness. However, since extremely small thickness may cause damage to the nitride semiconductor layer, the sapphire substrate 1 preferably has a thickness of about 5 to 300^m, and preferably of about 20 to 150μna. It is also preferred that the mirror-polished sapphire substrate 1 has a surface roughness of 1 an or less. Otherwise, the surface roughness of the sapphire substrate 1 may be transferred to the n-contact layer 2 to potentially damage a resultant nitride semiconductor structure when the sapphire substrate 1 and the buffer layer 2 are etched. After the lapping and polishing, the sapphire substrate 1 is etched via wet etching and/or dry etching. In the sapphire etching, the dry etching may be performed first. On the other hand, the wet etching may be performed earlier. The dry etching preferably adopts ICP/RIE or RIE. The wet etching preferably adopts a mixed etching solution containing at least one of HCl, H₂SO₄, H₃PO₄, HNO₃, KOH, NaOH and 4 H₃P0₄+ 4CH3COOH+ HN0₃+ H₂0 (aluetch). ICP and RIE power may be raised as high as possible to etch the sapphire substrate 1 at a higher rate in the dry etching, whereas attention should be given since high temperature may damage the epitaxial layer of nitride semiconductor. The sapphire substrate 1 is wet etched as follows: The etch rate of the sapphire substrate 1 by the mixed etching solution containing at least one of HCl, H₂SO„, H₃PO₄, HN0₃, KOH, NaOH and 4 H₃PO₄+ CH3COOH+ HN0₃+ H₂0 from a test sapphire substrate. Then, the sapphire substrate 1 is immersed into the etching solution for a time period in which it can be etched for 110 to 120% in its thickness. The etching is performed for the time period corresponding 110 to 120% thickness in order to minimize any problems that may cause thickness unevenness to the sapphire substrate 1 after lapping. In this case, the buffer layer 2 shows an etch rate that is 1/50 or less compared to that of the sapphire substrate 1. That is, the buffer layer 2 has an etch selectivity of 50 or more with respect to the sapphire substrate 1. Therefore, although the etching is performed for a time period that is sufficient to completely etch the sapphire substrate 1, there is no risk of damaging the layers underlying the buffer layer 2 since the buffer layer 2 is etched slowly enough. In addition, the temperature of the etching solution is preferably maintained at a temperature of 100 °C or more in order to shorten process time.

Heating for maintaining the etching solution at 100°C or more can be performed directly or indirectly. In the direct heating, the etching solution is placed on a heater or directly contacted by the heater. In the indirect heating, optical absorption is adopted to heat the etching solution. In addition, pressure may be boosted in order to raise the temperature of the etching solution over its boiling temperature. At the wet etching, the sapphire substrate 1 was etched for 22.16,um for 20 minutes, showing an etch rate of l.l m/min. This etch rate is interesting enough to be compared with dry etch rate, and satisfactory in view of chip mass production. The wet etching has more advantages than any other approaches in view of mass production since it is not restricted by the productivity of equipments. When this invention is applied to mass production, there is an important factor to ensure process conditions capable of raising the etch selectivity between the nitride semiconductor buffer layer 2 and the sapphire substrate 1. In particular, the buffer layer 2 can be effectively utilized as an etch stop for sapphire. Although not shown, the sapphire etch stop layer may adopt a protective layer of for example SiN or SiO or an In_x(Ga_yAlι-_y)N layer (0<x<l, 0<y<l, x+ y>l).

Preferably, it is effective to increase Al ratio. In addition, a protective layer of for example SOG,

SiN or Si0₂ may be deposited on the receptor layer 12 to protect the receptor substrate 13 from damage in the wet etching. Alternatively, the receptor substrate 13 may be so formed to contain at least one of Au, Pt, Rh and Pd, which are not damaged by an etching solution. According to an experiment result, the thin film made of a specific metal such as Pt and Au or another material such as SiN and Siθ₂ is rarely etched by a mixture solution containing at least one of HCl, H₂S0₄, H₃P0₄, HN0₃, KOH, NaOH and 4 H3PO₄+ 4CH3COOH+ HNO3+ H₂0. In addition, the thin film shows high corrosion resistance against the dry etching such as ICP/RIE. So, it is to be understood that the thin film has a wide application range. FIG. 9 is a graph comparing the etch rate of sapphire with that of GaN by ICP/RIE dry etching. As shown in FIG. 9, the etch rates of sapphire and GaN semiconductor increase in proportion to ICP and RIE power, but the etch selectivity of sapphire with respect to nitride semiconductor decreases. Regarding this result, when the sapphire substrate 1 is etched via dry etch of ICP/RIE, the etching is rarely stopped at the buffer layer 2 of nitride semiconductor. So, an approach such as optical analysis and residual gas analysis has to be applied in order to stop etching at the buffer layer 2. Even though such an analysis technique is used, etching is successfully stopped at a very low ratio. However, wet etching can utilize the buffer layer 2 as an etch stop layer thereby achieving process margin that is essential to mass production. FIG. 10 is a graph comparing the etch rate of sapphire with that of GaN by a mixed solution of sulfuric acid (H₂SO₄) and phosphoric acid (H₃PO₄); As seen in FIG. 10, the etch selectivity of sapphire with respect to nitride semiconductor by the mixed solution of H₂S0₄ and H₃PO₄ can be 100 or more at a specific temperature. This result reports that the buffer layer 2 can be effectively used as an etch stop layer for the sapphire substrate 1. A high etch selectivity of 100 or more was obtained at a high temperature of 100 °C or more. In particular, since the etch rate of sapphire reaches or exceeds ljt min at a specific temperature, it is apparent that the fabrication method proposed by this invention is more advantageous than any conventional methods in view of production cost, productivity and process stabilization. FIG. 11 is a photograph illustrating the surface of a nitride semiconductor buffer layer in which a sapphire substrate is removed by wet etching. As shown in FIG. 11 even after the sapphire substrate 1 was removed, substantially no breaks or damages were found from the nitride semiconductor buffer layer, which had a very clean surface state. FIG. 12 is a graph illustrating the current-voltage curve of a nitride semiconductor layer measured with a probe in which a sapphire substrate is removed by wet etching. As shown in FIG. 12, no current flows until the sapphire substrate 1 is removed, and a current of lpA flows at IV after the sapphire substrate 1 is removed. When the buffer layer 2 of nitride semiconductor is removed via ICP/RIE or RIE, current level is raised sharply up to 40pA.

ICP/RIE and RIE use a mixed gas containing at least one of BC13, C12, HBr and Ar as their etching gas. This result reports that the wet etching and the dry etching are adapted to effectively etch the sapphire substrate 1 and the buffer layer 2 of nitride semiconductor to expose the n-contact layer 3 of nitride semiconductor. This feature is a very important result indicating that electric properties of the exposed surface can be detected with a probe station at every process step to effectively monitor etching procedures. Then, as shown in FIG. 6, one of Ti, Al, Rd, Pt, Ta, Ni, Cr, Au and their alloys capable of forming a transparent electrode, a transmissive electrode or an ohmic contact an such as ITO, InSnO and Ti/Ni/Au is deposited on the n-contact layer 3 and lifted off, and a resultant structure is heat-treated at a temperature of about 300 to 700 °C in an atmosphere containing nitrogen or oxygen to form a second electrode 15 that forms an ohmic contact with the n-contact layer 3. Then, a first electrode 16 is formed on the first receptor contact layer 14. Then, as shown in FIG. 7, a phosphor mixture is spin-coated on the n-contact layer 3 and the second electrode 15 to coat a phosphor layer 17. In this case, the phosphor mixture is obtained by mixing red, green and blue phosphors into resin such as PE (polyethylene), PP (polypropylene) and acryl together with anti-precipitation solution and then agitating the mixed substances at a room or higher temperature to melt into liquid. Then, the phosphor mixture is coated with a spin coater using centrifugal force. Next, as shown in FIG. 8, the phosphor layer 17 is etched via photolithography to form a contact opening exposing the second electrode 15, and an LED wafer having respective layers is diced or sawn into respective chips. After that, a chip is mounted on a lead frame 101 with a conductive paste 500 such as Ag paste and its second electrode 15 is connected to a lead frame 102 by bonding a wire 21 as shown in FIG. 2. As described above, this process can remove the sapphire substrate 1 via backside polishing and dry or wet etching so as to remarkably improve productivity while protecting the epitaxial layer from thermal damage in case of Laser Lift- Off (LLO). As the sapphire substrate is removed via backside polishing and etching, the surface of the n-contact layer 3 forms a microscopic ridge-valley structure to concentrate light. In addition, the phosphor layer 17 is formed via spin coating such that the phosphor layer 17 is formed at a uniform thickness. Therefore, color conversion LEDs can be stably fabricated with little variation in color rendering. Although the above first embodiment uses backside polishing and etching to remove the sapphire substrate 1, LLO can be adopted instead. Each of the first and second electrodes 16 and 15 can be formed of single or multiple layers. For example, the second electrode 15 can be formed of single or multiple layers containing at least one of Ti, Al, Ni, Au, Rh, Pd, Pt and so on. The first electrode 16 can be made of single metal or alloy containing at least one of Ni, Au, Ti, Rh, Pd, Al, Cr, Pt, Ta and so on. Then, the metal layers can be converted into alloys via for example heat treatment.

Second Embodiment A second embodiment of the invention will be described as follows: FIG. 13 is a cross-sectional view illustrating an LED according to a second embodiment of the invention. The LED according to the second embodiment of the invention includes a lead frame 30 and a chip flip-chip bonded to the lead frame 30. The chip is structured as follows: The chip has a buffer layer 2, an n-contact layer 3, an n-cladding layer 4, an active layer 5, a p-cladding layer 6, a p-contact layer 7, a conductive transparent electrode 8, a light reflecting layer 9 and a first electrode 16 formed in their order on a sapphire substrate 1. A second electrode 15 is formed on an partial area of the n-contact layer 3 that is exposed by partially removing the first electrode 16, the light reflecting layer 9, the conductive transparent electrode 8, the p-contact layer 7, the p-cladding layer 6, the active layer 5, the n-cladding layer 4 and the n-contact layer 3. On the underside of the sapphire substrate 1, a phosphor layer 17 for converting the wavelength of light is formed. The phosphor layer 17 is formed as follow: Red, green and blue phosphors are mixed into resin such as PE, PP and acryl together with anti-precipitation solution. A resultant phosphor mixture is agitated at a room or higher temperature to melt into liquid. Then, the liquid phosphor mixture is spin-coated. Each of the first and second electrodes 16 and 15 can be formed of single or multiple layers. For example, the second electrode 15 can be formed of single or multiple layers containing at least one of Ti, Al, Ni, Au, Rh, Pd, Pt and so on. The first electrode 16 can be made of single metal or alloy containing at least one of Ni, Au, Ti, Rh, Pd, Al, Cr, Pt, Ta and so on. The chip is overturned with the phosphor layer 17 positioned atop and then bonded to the lead frame 30. The chip is bonded to the lead frame 30 via conductive pastes 24 and 25 such as Ag paste. The LED of this structure shows little variation in color rendering since the phosphor layer

17 is formed uniform. A fabrication method of such an LED will be described as follows: FIGS. 14 to 17 are cross-sectional views illustrating an intermediate process for fabricating the LED according to the first embodiment of the invention. First, a buffer layer 2, an n-contact layer 3, an n-cladding layer 4, an active layer 5, a p-cladding layer 6 and a p-contact layer 7 are formed in their order on a sapphire (AI₂O₃) substrate 1 via MOCVD, LPE, MBE and so on. The buffer layer 2, the n- and p-cladding layers 4 and 6 and the active layer 5 are made of In_x(Ga_yAlι-_y)N. In succession, a conductive transparent electrode 8 and a light reflecting layer 9 are formed on the p-contact layer 7, and a first electrode 16 is made of metal such as Au on the light reflecting layer 9. The light reflecting layer 9 and the conductive transparent layer 8 are deposited via E-Beam, thermal evaporation, sputtering and so on. Then, as shown in FIG. 14, the first electrode 16, the light reflecting layer 9, the conductive transparent electrode 8, the p-contact layer 7, the p-cladding layer 6, the active layer 5, the n-cladding layer 4 and the n-contact layer 3 are selectively etched to expose a partial area of the n-contact layer 3. Next a second electrode 15 is formed on the n-contact layer 3. Then, the sapphire substrate 1 is cut partially in thickness direction via etching. Alternatively, the sapphire substrate 1 may be etched after lapping. Then, as shown in FIG. 15, a phosphor mixture is spin-coated on the sapphire substrate 1 to form a phosphor layer 17. The phosphor mixture is obtained by mixing red, green and blue phosphors into resin such as PE, PP and acryl together with anti-precipitation solution and then agitating the mixed substances at a room or higher temperature to melt into liquid. Then, the phosphor mixture is coated with a spin coater using centrifugal force. In this case, chips of exciting light may be of a UV ray or blue light source. Then, as shown in FIG. 16, the phosphor layer 17 is selectively etched to partition an LED wafer into individual chip areas. Then, as shown in FIG. 17, the LED wafer having the respective layers is diced or sawn into individual chips. Next, as shown in FIG. 13, the individual chips are flip chip bonded. FIG. 18 is a spectrum graph of a white LED fabricated according to an embodiment of the invention. A white LED is fabricated by mixing red, green and blue non-YAG phosphors together and then spin-coating a resultant mixture. In the while LED, CRI indicates x=0.31 and y=0.33 and color rendering is 85 or more.

Industrial Applicability According to the present invention as described hereinbefore, since the phosphor layer 17 is formed at a uniform thickness via spin coating, LEDs having little variation in CRI can be stably fabricated. Furthermore, a high bright exciting light source of the invention allows the fabrication of a high bright color conversion LED. Therefore, it is expected that the LED of the invention will be applied as a future illumination source. While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A Light Emitting Diode (LED) comprising: a conductive receptor substrate having top and bottom surfaces; a first electrode formed on the bottom surface of the receptor substrate; a junction layer formed on the top surface of the receptor substrate, and having conductivity and ohmic characteristics; a light reflecting layer formed on the junction layer; a first conductive cladding layer formed on the light reflecting layer; an active layer formed on the first conductive cladding layer; a second conductive cladding layer formed on the active layer; a second electrode formed on the second conductive cladding layer; and a phosphor layer coated on the second conductive cladding layer and the second electrode, and having a contact opening for exposing the second electrode.

2. The LED according to claim 1, wherein the buffer layer, the first conductive contact layer, the first cladding layer, the active layer, the second cladding layer and the second conductive contact layer are made of In_x(Ga_yAlι-_y)N (0<x<l, 0<y<l, x+ y>l).

3. The LED according to claim 1, further comprising: a first receptor contact layer formed between the first electrode and the receptor substrate; a second receptor contact layer formed between the receptor substrate and the junction layer; a first conductive contact layer and a conductive electrode formed between the light reflecting layer and the first conductive cladding layer; and a second conductive contact layer formed between the second conductive cladding layer and the second electrode.

4. The LED according to claim 2, wherein the first conductive layers are p-type and the second conductive layers are n-type.

5. The LED according to claim 1, wherein the phosphor layer contains red, green and blue phosphors and resin.

6. The LED according to claim 1, wherein the phosphor layer contains YAG phosphor and resin.

7. A Light Emitting Diode (LED) comprising: an insulator substrate having first and second surfaces; a buffer layer formed on the first surface of the insulator substrate; a first conductive contact layer formed on the buffer layer; a first conductive cladding layer formed on the first conductive contact layer; an active layer formed on the first conductive cladding layer; a second conductive cladding layer formed on the active layer; a second conductive contact layer formed on the second conductive cladding layer; a light reflecting layer formed on the second conductive contact layer; a first electrode formed on the light reflecting layer; a second electrode formed on the first conductive contact layer; and a phosphor layer formed on the second layer of the insulator substrate.

8. The LED according to claim 7, wherein the buffer layer, the first conductive contact layer, the first cladding layer, the active layer, the second cladding layer and the second conductive contact layer are made of In_x(Ga_yAlι-_y)N (0<x<l, 0<y<l, x+ y>l).

9. The LED according to claim 7, further comprising a conductive transparent electrode layer formed between the light reflecting layer and the second conductive contact layer.

10. The LED according to claim 7, wherein the first conductive layers are n-type and the second conductive layers are p-type.

11. The LED according to claim 7, wherein the phosphor layer contains red, green and blue phosphors and resin.

12. The LED according to claim 7, wherein the phosphor layer contains YAG phosphor and resin.

13. A fabrication method of Light Emitting Diodes (LEDs) comprising following steps of: forming a buffer layer, an n-contact layer, an n-cladding layer, an active layer, a p-cladding layer, a p-contact layer, a conductive transparent electrode and a light reflecting layer in their order on a sapphire substrate; forming first and second receptor contact layers on both surfaces of a receptor substrate; forming a junction layer on at least one of the light reflecting layer and the second receptor contact layer; arranging the sapphire substrate and the receptor substrate so that the light reflecting layer of the sapphire substrate is opposed to the second receptor contact layer of the receptor substrate and thermally compressing a resultant structure; removing the, sapphire substrate; forming a second electrode on the n-contact layer and a first electrode on the first receptor contact layer; and spin-coating a phosphor layer on the n-contact layer and the second electrode.

14. The fabrication method according to claim 13, further comprising a step of removing the buffer layer after the sapphire substrate-removing step.

15. The fabrication method according to claim 14, the sapphire substrate-removing step and the buffer layer-removing step comprise performing at least one selected from a group consisting of wet etching, dry etching of Chemical Mechanical Polishing (CMP) or ICP/RIE and mechanical polishing, wherein the wet etching uses an etching solution containing at least one of HCl, H₂S0₄, H3PO₄, HNO₃, KOH, NaOH and 4 H₃PO₄+ CH3COOH+ HN0₃+ H₂0 (aluetch) and mixtures thereof.

16. The fabrication method according to claim 15, wherein the sapphire substrate-removing step and the buffer layer-removing step adopt the wet etching and the dry etching together, wherein the wet etching is adapted to etch the substrate and the dry etching is adapted to etch the buffer layer.

17. The fabrication method according to claim 13, wherein the sapphire substrate-removing step comprises backside-polishing the sapphire substrate via mechanical polishing and etching the backside-polished sapphire substrate.

18. The fabrication method according to claim 13, further comprising a step of, after the phosphor layer spin-coating step, selectively etching the phosphor layer to form a contact opening thereby exposing the second electrode.

19. The fabrication method according to claim 13, wherein the receptor substrate comprises one selected from a group consisting of a semiconductor substrate, a conductive oxide substrate and a metal substrate, wherein the semiconductor substrate is made of one of Si, GaAs, InP and InAs, the conductive oxide substrate is made of one of Indium Tin Oxide (ITO), ZrB and ZnO, and the metal substrate is made of one of CuW, Mo, Au, Al, Au and Ag.

20. The fabrication method according to claim 13, wherein the thermal compression step comprises depositing at least one selected from a group consisting of Ti, Ni, Au, Sn, In, Pd, Ag, Sn, Ag and Rh and thermally compressing a resultant structure in order to achieve ohmic characteristics.

21. The fabrication method according to claim 13, wherein the sapphire substrate-removing step is performed by at least one selected from a group consisting of mechanical polishing, wet etching and dry etching.

22. The fabrication method according to claim 13, wherein the first electrode contains at least one selected from a group consisting of ITO, InSnO, Ti/Ni/Au, Ti, Al, Rd, Pt, Ta, Ni, Cr, Au and Pd.

23. A fabrication method of Light Emitting Diodes (LEDs) comprising following steps of: forming a buffer layer, an n-contact layer, an n-cladding layer, an active layer, a p-cladding layer, a p-contact layer, a conductive transparent electrode layer, a light reflecting layer and a first electrode layer in their order on a first surface of a sapphire substrate; selectively etching the first electrode layer, the light reflecting layer, the conductive transparent electrode layer, the p-contact layer, the p-cladding layer, the active layer, the n-cladding layer and the n-cladding layer to expose a partial area of the n-contact layer; forming a second electrode on the exposed area of the n-contact layer; spin-coating a phosphor layer on a second surface of the sapphire substrate! dividing the sapphire substrate into respective chips; and flip-chip bonding the respective chips to lead frames.

24. The fabrication method according to claim 23, further comprising a step of partially cutting the sapphire substrate in thickness direction before the phosphor layer spin-coating step.

25. The fabrication method according to claim 23, further comprising a step of selectively etching the phosphor layer to partition the sapphire substrate according to the respective chips before the sapphire substrate-dividing step.

26. The fabrication method according to claim 13 or 23, further comprising a step of heat treating a resultant structure at a temperature of about 300 to 700 °C in a furnace of an atmosphere containing nitrogen or oxygen after the first or second electrode is formed.