Flip-chip GaN LED fabrication method

A flip-chip LED fabrication method includes the steps of (a) providing a GaN epitaxial wafer, (b) forming a first groove in the GaN epitaxial layer, (c) forming a second groove in the GaN epitaxial layer to expose a part of the N-type GaN ohmic contact layer of the GaN epitaxial layer, (d) forming a translucent conducting layer on the epitaxial layer, (e) forming a P-type electrode pad and an N-type electrode pad on the translucent conducting layer, (f) forming a first isolation protection layer on the P-type electrode pad, the N-type electrode pad, the first groove and the second groove, (g) forming a metallic reflection layer on the first isolation protection layer, (h) forming a second isolation protection layer on the first isolation protection layer and the metallic reflection layer, (i) forming a third groove to expose one lateral side of the N-type electrode pad, (j) separating the processed GaN epitaxial wafer into individual GaN LED chips, and (k) bonding at least one individual GaN LED chip thus obtained to a thermal substrate with a conducting material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to LED fabrication technology and more particularly, to a flip-chip GaN-based LED fabrication method, which enables a high reliable GaN LED chip to be bonded to a thermal substrate having a large heat dissipation area with a conducting adhesive, facilitating heat dissipation, enhancing the luminous efficiency of the GaN LED chip and prolonging its lifespan.

2. Description of the Related Art

Many different types of light emitting apparatus are commercially available. Following the trend of the next generation for green energy, LED (light emitting diode), more particularly, white LED has been intensively used in street lamps, tunnel lamps, hand lamps, sign boards, home lightings and backlight modules for LCD panel for the advantages of power-saving, small size, high stability and high reliability characteristics.

Commercial white light emitting devices commonly use blue GaN-based LED chips to match with yellow, green or red phosphor. However, a sapphire substrate has the drawback of poor heat transfer characteristic. Temperature will affect he reliability and lifespan of sapphire substrate-based LED chips. According to the luminous efficiency of regular LED devices, about 50%˜60% of the power will be turned into heat. Further, according to conventional LED module fabrication methods, phosphor is mixed with epoxy resin or silicon adhesive subject to a predetermined ration and then the mixture thus prepared is coated on LED chips. During operation of a conventional LED device, waste heat will be accumulated in the inside of the LED device, lowering the luminous efficiency and shortening the lifespan. An overheat may result in device burnout. Therefore, the factor of heat dissipation efficiency has been greatly emphasized. If waste heat cannot be quickly dissipated during operation of a LED package product, the life cycle and reliability of the product will be badly affected. To facilitate quick dissipation of waste heat, heat pipe, heat-transfer block, heat sink and/or radiation fin may be used with a LED module. However, the use of these attached devices relatively increases the product size, reducing the advantages of small size and light weight of LED products.

FIG. 12illustrate a GaN LED chip according to the prior art, which comprises a support frame A8, a sapphire substrate A1supported on the support frame A8and fixedly bonded thereto with an adhesive, an N-type GaN ohmic contact layer A2formed on the sapphire substrate A1, a light-emitting layer A3formed on the N-type GaN ohmic contact layer A2, a P-type GaN ohmic contact layer A4formed on the light-emitting layer A3, a translucent conducting layer A5formed on the P-type GaN ohmic contact layer A4for distributing electric current and enhancing luminous efficiency, a P-type electrode pad A6and an N-type electrode pad A7respectively formed on the translucent conducting layer AS and the N-type GaN ohmic contact layer A2, and gold or aluminum wires A9electrically connecting the P-type electrode pad A6and an N-type electrode pad A7to external contacts A10. According to this design, a part of the light emitted by the light-emitting layer A3goes through the P-type electrode pad A6outside the translucent conducting layer A5to the phosphor in the packaged shell. Because the P-type electrode pad A6blocks a part of the light emitted by the light-emitting layer A3, the luminous efficiency of the LED package is lowered.

To eliminate the aforesaid electrode pad light-blocking problem, U.S. Pat. No. 5,557,115 discloses an improved LED design, entitled “Light emitting semiconductor device with sub-mount”, as shown inFIG. 13, which employs flip chip technology to increase the effective light-emitting area. According to this design, the light emitting semiconductor device comprises a sapphire substrate B1, a buffer layer B2and an N-type GaN ohmic contact layer B3formed in proper order on the sapphire substrate B1, a light-emitting layer B4and a P-type GaN ohmic contact layer B5formed in proper order on the center area of the N-type GaN ohmic contact layer B3, a P-type electrode pad B6connecting the P-type GaN ohmic contact layer B5to an external heat-transfer substrate, N-type electrodes B7disposed at two opposite lateral sides relative to the light-emitting layer B4, and an N-type electrode pad B8connecting one N-type electrode B7to the external heat-transfer substrate. Thus, the light-emitting surface B11is fully opened for emitting light efficiently. However, the use of the metallic P-type electrode pad B6and N-type electrode pad B8to reflect light tends to cause an increase of the forward voltage, lowering the luminous efficiency.

FIG. 14illustrates still another prior art LED design disclosed in U.S. Pat. No. 6,514,782, entitled “Method of making an III-nitride light-emitting device with increased light generating capability”. According to this design, LED dies (chips) C1are electrically connected to solder contacts C11; C21of a circuit board C2by means of heat-transfer blocks C3, such as gold balls or gold-tin solder bumps. This method has the drawback of high manufacturing cost. As the electrode pads in the flip-chip LED device are electrically connected to the circuit board and capable of reflecting the light emitted by the light-emitting layer toward the sapphire substrate, the metallic property of the electrode pads tends to cause an increase in the forward voltage, thereby lowering the luminous efficiency. This prior art design discloses the formation of a metallic reflection layer on the circuit board, however, the far distance between the metallic reflection layer and the light-emitting layer causes an attenuation of the emitted light.

FIG. 15illustrates still another prior art LED design disclosed in U.S. Pat. No. 6,514,782, which comprises a sapphire substrate D1, an N-type GaN ohmic contact layer D2, a light-emitting layer D3, a P-type GaN ohmic contact layer D4, a translucent conducting layer D5and conducting metallic reflection layer D6. Further, the N-type GaN ohmic contact layer D2and the translucent conducting layer D5are respectively and electrically connected to the conducting metallic reflection layer D6and a circuit board D8by electrodes D7. Further, in order to prevent conduction between the electrodes D7and the conducting metallic reflection layer D6and to protect the conducting metallic reflection layer D6against current leakage, a polyimide insulation layer D9is filled in the gaps. However, it is difficult to form the polyimide insulation layer D9without affecting the relative surface elevation between the electrodes D7and the conducting metallic reflection layer D6. A significant elevational difference between the electrodes D7and the conducting metallic reflection layer D6will lead to a connection error between the LED and the circuit board, lowering the yield rate.

FIG. 16illustrates a flip-chip GaN-based LED design according to Taiwan Patent M350824. According to this design, an etching technique is employed to divide an epitaxial layer into a first epitaxial layer portion El and a second epitaxial layer portion E2so that the P-type electrode pad E3and the N-type electrode pad E4can be approximately maintained at the same elevation. However, in order to reduce the contact impedance between the N-type electrode pad E4and the second epitaxial layer portion E2, the N-type electrode pad E4must be extended to form an ohmic contact with an N-type GaN ohmic contact layer E21. This designs enables the he P-type electrode pad E3and the N-type electrode pad E4to be approximately maintained at the same elevation, facilitating circuit board connection. However, the metallic reflection layer E5round the conducting layer E6tends to increase the forward voltage. Further, this design does not teach any measure to protect the surface of the edge of the grooves in the LED chip.

Further, the aforesaid prior art designs cannot eliminate accumulation of waste heat to lower the luminous efficiency due to high consumption of power. The use of polyimide insulation layer to minimize power consumption relatively lowers heat dissipation performance. Further, if the P-type electrode pad and the N-type electrode pad are not maintained at the same elevation, circuit board connection may fail, lowering the yield rate.

Therefore, it is desirable to provide a flip-chip LED, which eliminates the drawbacks of the aforesaid prior art designs.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the circumstances in view. It is one object of the present invention to provide a flip-chip LED fabrication method, which improves the product quality and the yield rate, facilitates heat dissipation, enhances the luminous efficiency and prolongs the product lifespan.

To achieve these and other objects of the present invention, a flip-chip LED fabrication method includes the steps of: (a) providing a GaN (Gallium Nitride) epitaxial wafer comprising a substrate and a GaN epitaxial layer arranged on the substrate, the GaN epitaxial layer comprising a N-type GaN ohmic contact layer, a light-emitting layer and a P-type semiconductor layer; (b) forming a first groove in the GaN epitaxial layer at a predetermined location by etching to expose a part of the substrate of the GaN epitaxial wafer; (c) forming a second groove in the GaN epitaxial layer at a predetermined location adjacent to the first groove by etching to expose a part of the N-type GaN ohmic contact layer of the GaN epitaxial layer and to divide the GaN epitaxial layer into two epitaxial layer portions; (d) forming a translucent conducting layer on the epitaxial layer portions; (e) forming a P-type electrode pad and an N-type electrode pad on the translucent conducting layer corresponding to the epitaxial layer portions respectively; (f) forming a first isolation protection layer on the P-type electrode pad, the N-type electrode pad, the first groove and the second groove; (g) forming a metallic reflection layer on the first isolation protection layer adjacent to the P-type electrode pad and corresponding to the translucent conducting layer; (h) forming a second isolation protection layer on the first isolation protection layer and the metallic reflection layer; (i) etching the first isolation protection layer and the second isolation protection layer to form a third groove adjacent to the metallic reflection layer and to expose one lateral side of the N-type electrode pad; (j) employ grinding, laser wafer scribing, chipping and die sorting techniques to separate the processed GaN (Gallium Nitride) epitaxial wafer into multiple individual GaN LED chips; and (k) bonding at least one individual GaN LED chip thus obtained to a thermal substrate with a conducting material to electrically connect the P-type electrode pad and N-type electrode pad of each individual GaN LED chip to the positive solder pad and negative solder pad of the thermal substrate by the conducting adhesive and to let the conducting adhesive climb over the N-type electrode pad into the third groove and be accumulated and cured therein.

As the first isolation protection layer and the metallic reflection layer are respectively formed at two opposite sides relative to the isolation protection layer, the translucent conducting layer and the metallic reflection layer are electrically isolated to minimize forward voltage and power consumption during electricity conduction operation of the metallic reflection layer, avoiding interference with the light-emitting efficiency of the GaN LED chip. Further, the metallic reflection layer is directly mounted inside the GaN LED chip for direct reflection, avoiding light loss.

Further, subject to the formation of the first groove and the second groove in the GaN epitaxial layer by etching to expose a part of the N-type GaN ohmic contact layer of the GaN epitaxial layer and to divide the GaN epitaxial layer into two epitaxial layer portions, the posteriorly formed P-type electrode pad and N-type electrode pad have the same elevation, stabilizing electric connection between the GaN LED chip and the thermal substrate and improving LED module yield rate.

Further, the first isolation protection layer and the second isolation protection layer are formed on the two epitaxial layer portions at two opposite sides relative to the second groove, and therefore, the two epitaxial layer portions are well isolated and protected, avoiding a short circuit and enhancing operation reliability.

Further, the third groove is formed in the first isolation protection layer and the second isolation protection layer adjacent to the metallic reflection layer by etching to expose one lateral side of the N-type electrode pad so that the applied conducting material can climb over the N-type electrode pad into the third groove and be accumulated and cured therein, enhancing the binding strength and the strength of solidification and the connection stability between the N-type electrode pad of the GaN LED chip and the negative solder pad of the thermal substrate.

Further, the GaN LED chip is electrically connected with the thermal substrate by means of the large area conducting material, and the insulative adhesive is bonded to the thermal substrate between the positive solder pad and the negative solder pad so that the second isolation protection layer can be fixedly secured to the thermal substrate between the positive solder pad and the negative solder pad by means of the conducting material, saving the manufacturing cost, facilitating heat dissipation, improving the luminous efficiency of the GaN LED chip and its lifespan.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIGS. 1˜4, a flip-chip LED fabrication method in accordance with the present invention includes the steps of:(100) Provide a GaN (Gallium Nitride) epitaxial wafer consisting of a GaN epitaxial layer2, which comprises a N-type GaN ohmic contact layer11, a light-emitting layer12and a P-type semiconductor layer13and is arranged on a substrate100(seeFIG. 2).(101) Form a first groove101in the GaN epitaxial layer2at a predetermined location by etching to expose a part of the substrate100of the GaN epitaxial wafer (seeFIG. 3).(102) Form a second groove102in the GaN epitaxial layer2at a predetermined location adjacent to the first groove101by etching to expose a part of the N-type GaN ohmic contact layer11of the GaN epitaxial layer2so that the GaN epitaxial layer2is divided by the second groove102into two epitaxial layer portions2A;2B (seeFIG. 4).(103) Form a translucent conducting layer14on the surface of the epitaxial layer portions2A;2B (seeFIG. 5).(104) Form a P-type electrode pad15and an N-type electrode pad16on the translucent conducting layer14(seeFIG. 5) corresponding to the epitaxial layer portions2A;2B respectively.(105) Form a first isolation protection layer17on the P-type electrode pad15, the N-type electrode pad16, the first groove101and the second groove102(seeFIG. 6).(106) Form a metallic reflection layer18on the surface of the first isolation protection layer17adjacent to the P-type electrode pad15and corresponding to the translucent conducting layer14(seeFIG. 6).(107) Form a second isolation protection layer19on the surface of the first isolation protection layer17and the metallic reflection layer18(seeFIG. 7).(108) Etch the first isolation protection layer17and the second isolation protection layer19to form a third groove103adjacent to the metallic reflection layer18and to expose one lateral side of the N-type electrode pad16(seeFIG. 8).(109) Employ a series of steps including grinding, laser wafer scribing, chipping and die sorting to obtain individual GaN LED chips1(seeFIG. 9).(110) Bond each individual GaN LED chip1to a thermal substrate3with a conducting adhesive4(seeFIG. 10). Referring toFIGS. 2˜9, a N-type GaN ohmic contact layer11, a light-emitting layer12and a P-type semiconductor layer13are formed on a substrate100in proper order, and then a photo mask is designed at a selected location and then exposing and developing techniques are employed to remove the photoresist from the GaN epitaxial layer2corresponding a predetermined area for first groove101. Thereafter, an etching technique (dry etching or wet etching technique) is employed to remove a part of the GaN epitaxial layer2from the substrate100, thereby forming a first groove101to expose a part of the substrate100of the GaN epitaxial wafer. Thereafter, employ exposing and developing techniques to remove the photoresist from the GaN epitaxial layer2corresponding a predetermined area for second groove102, and then employ an etching technique to remove a part of the GaN epitaxial layer2from the substrate100, thereby forming a second groove102to expose a part of the N-type GaN ohmic contact layer11of the GaN epitaxial layer2. The etching depth of the first groove101is deeper than that of the second groove102.

Thereafter, employing a vapor deposition or sputter deposition technique to form a translucent conducting layer14on the surface of the P-type semiconductor layer13, and then employ exposing, developing and lift-off techniques to form a P-type electrode pad15and an N-type electrode pad16on the translucent conducting layer14, enabling the N-type electrode pad16to extend to the surface of the N-type GaN ohmic contact layer11. Thereafter, form a first isolation protection layer17on the P-type electrode pad15, the N-type electrode pad16, the first groove101and the second groove102, enabling the first groove101and the second groove102to be completely covered by the first isolation protection layer17. Thereafter, employ exposing developing and etching techniques to expose the surface of the -type electrode pad15and the surface of the N-type electrode pad16, and then employ exposing, developing and lift-off techniques to form a metallic reflection layer18on the surface of the first isolation protection layer17adjacent to the P-type electrode pad15and corresponding to the translucent conducting layer14, and then form a second isolation protection layer19on the surface of the first isolation protection layer17and the surface of the metallic reflection layer18, and then employ exposing, developing and etching techniques to form a third groove103adjacent to the metallic reflection layer18and to expose one lateral side of the N-type electrode pad16, and then employ exposing, developing and etching techniques to expose the surface of the P-type electrode pad15and the surface of the N-type electrode pad16. Thereafter, grind the substrate100to a thickness below 100 μm, and then employ laser wafer scribing, chipping and die sorting techniques to separate the processed wafer into individual GaN LED chips1.

Referring toFIG. 10, an individual GaN LED chip1can be electrically connected to a thermal substrate3by means of flip-chip packaging. The thermal substrate3can be prepared by aluminum, copper, ceramics or any suitable thermally conductive material, carrying a printed circuit having electrically insulated positive solder pad31and negative solder pad32and a conducting material4(silver adhesive, solder balls or solder paste) formed on the positive solder pad31and the negative solder pad32and/or the thermal substrate3between the positive solder pad31and the negative solder pad32. During flip-chip packaging, the P-type electrode pad15and N-type electrode pad16of the GaN LED chip1are respectively electrically connected to the positive solder pad31and negative solder pad32of the thermal substrate3through the conducting material4, and the second isolation protection layer19is bonded to the thermal substrate3between the positive solder pad31and the negative solder pad32with an insulative adhesive5that can be UV light curable adhesive, heat curable adhesive or anaerobic adhesive applied by means of spot gluing or dipping techniques. Thus, the conducting material4and the thermal substrate3provide a large heat dissipation surface area for quick dissipation of waste heat from the GaN LED chip1, eliminating the drawbacks of limited heat-transfer surface are of the prior art designs that use metal wires or metal bumps for conduction, and simplifying the LED fabrication process and time.

When curing the individual GaN LED chip1and the thermal substrate3, the conducting material4can climb over the N-type electrode pad16into the third groove103and be accumulated and cured therein, avoiding poor wetting or insufficient solder and enhancing the bonding strength. As the third groove103is concealed in the individual GaN LED chip1, electric connection between the N-type electrode pad16of the individual GaN LED chip1and the negative solder pad32of the thermal substrate3is enhanced and well protected, avoiding friction or impact damage.

Further, the substrate100of the GaN LED chip1can be selected from the group of sapphire, silicon carbon (SiC), zinc oxide (ZnO), magnesium oxide (MgO), gallium oxide (Ga2O3) aluminum gallium nitride (AlGaN), gallium lithium oxide (GaLiO), aluminum lithium oxide (AlliO) and Spinel. Further, the translucent conducting layer14can be selected from the group of indium oxide (In2O3), tin oxide (SnO2), IMO (indium molybdenum oxide), zinc oxide (ZnO), indium zinc oxide (IZO), cellium indium oxide (CeIn2O3), ITO (indium tin oxide), metallic bi-layer of N/Au, metallic bi-layer of Pt/Au and metallic bi-layer of Be/Au. The metallic reflection layer18can be selected from the group of silver (Ag), aluminum (Al), rhodium (Rh) and their composite with nickel (Ni), platinum (Pt), beryllium (Be), titanium (Ti) or chrome (Cr).

Further, the first isolation protection layer17and the second isolation protection layer19can be selected from the group of silicon oxide (SiO2), silicon nitride (Si3N4), liquid glass, Teflon, polyimide (PI), aluminum oxide (Al2O3), titanium oxide (TiO), tantalum oxide (Ta2O5), yttrium oxide (Y2O3) and diamond thin film and their alloys. Further, the P-type electrode pad15and the N-type electrode pad16can be selected from the group of titanium-gold alloy, titanium-aluminum alloy, chrome-gold alloy and chrome-aluminum alloy.

Referring toFIG. 11andFIG. 10again, the first groove101, the second groove102and the third groove103can be U-grooves, V-grooves, or grooves configured subject to any other shapes.

In conclusion, the invention provides a clip-chip GaN LED fabrication method, which has the advantages and features as follows:1. The first isolation protection layer17and the metallic reflection layer18are respectively formed at two opposite sides relative to the second isolation protection layer19so that the translucent conducting layer14and the metallic reflection layer18are electrically isolated to minimize forward voltage and power consumption during electricity conduction operation of the metallic reflection layer18, avoiding interference with the light-emitting efficiency of the individual GaN LED chip1. Further, the metallic reflection layer18is directly mounted inside the individual GaN LED chip1for direct reflection, avoiding light loss.2. Subject to the formation of the first groove101and the second groove102in the GaN epitaxial layer2by etching to expose a part of the N-type GaN ohmic contact layer11of the GaN epitaxial layer2and to divide the GaN epitaxial layer2into two epitaxial layer portions2A;2B, the posteriorly formed P-type electrode pad15and N-type electrode pad16have the same elevation, stabilizing electric connection between the individual GaN LED chip1and the thermal substrate3and improving LED module yield rate.3. The first isolation protection layer17and the second isolation protection layer19are formed on the two epitaxial layer portions2A;2B at two opposite sides relative to the second groove102, and therefore, the two epitaxial layer portions2A;2B are well isolated and protected, avoiding a short circuit and enhancing operation reliability.4. The third groove103is formed in the first isolation protection layer17and the second isolation protection layer19adjacent to the metallic reflection layer18by etching to expose one lateral side of the N-type electrode pad16so that the applied conducting material4can climb over the N-type electrode pad16into the third groove103and be accumulated and cured therein, enhancing the binding strength and the strength of solidification and the connection stability between the N-type electrode pad16of the individual GaN LED chip1and the negative solder pad32of the thermal substrate3.5. The individual GaN LED chip1is electrically connected with the thermal substrate3by means of the large area conducting material4, and the insulative adhesive5is bonded to the thermal substrate3between the positive solder pad31and the negative solder pad32so that the second isolation protection layer19can be fixedly secured to the thermal substrate3between the positive solder pad31and the negative solder pad by means of the conducting material4, saving the manufacturing cost, facilitating heat dissipation, improving the luminous efficiency of the individual GaN LED chip1and its lifespan.