High output light emitting diode and method for fabricating the same

0001This application claims the benefit of <patcit id="pcit0001" dnum="KR20040117766"><text>Korean Patent Application No. 2004-0117766</text></patcit>, filed on December 31, 2004.

<u>BACKGROUND OF THE INVENTION</u>

<u>Field of the Invention</u>

0002The present invention relates to a high output light emitting diode (LED) and a method for fabricating the same wherein a sidewall of the LED is inclined and a reflective film is formed on the inclined sidewall to allow light emitted outside of the sidewall to reflect from the reflective film and to emit upward of the device, thereby enabling to improve a light output and dispensing with an additional passivation process.

<u>Discussion of the Related Art</u>

0003Generally, an LED is widely used in applied fields such as color bulletin boards, traffic lights, keypad light sources for mobile telephones, light sources for illumination and LCD backlights.

0004<figref idref="f0001">FIG.1</figref> is a cross-sectional view of a conventional LED. The LED includes a substrate (10) sequentially stacked thereon with an N-type semiconductor layer (11), an active layer (12) and a P-type semiconductor layer (13), mesa etched from the P-type semiconductor layer (13) to portions of the N-type semiconductor layer (11), a transparent electrode (14) formed on the P-type semiconductor layer (13), an N electrode (15) formed on the mesa-etched N-type semiconductor layer (11) and a P electrode (16) formed on the transparent electrode (14).

0005f current flows in the N electrode (15) and the P electrode (16) in the LED, light is generated from the active layer (12) and emitted outside of the device. Many attempts have been made to increase the light output of the LED, and a device manufacturing of flip chip structure is one of the attempts.

0006<figref idref="f0001">FIG.2</figref> is a mimetic cross-sectional view of a flip chip LED bonded to a sub mount substrate according to the prior art. Referring to the LED structure of <figref idref="f0001">FIG.1</figref>, the flip chip LED is structured in such a manner that the P-type semiconductor layer (13) is stacked thereon with a reflective film (16) for P electrode, the reflective film (16) and the N electrode (15) are bonded to a sub mount substrate (30) by conductive bonding materials such as solders (20a. 20b), and light is emitted toward the substrate (10).

0007In other words, the flip chip type light device is such that the light emitted from the active layer (12) is reflected from the reflective film (16) to be emitted outside of the device via the substrate (10).

0008However, the light emitted from the flip chip type light device is reflected from an entire surface of the device to the disadvantage of wasting the light emitted from the sidewall of the device.

0009Particularly, in the case of a cleavage plane of the device, part of light emitted from a sidewall of the device is reflected from the cleavage plane, confined inside the device and cannot be emitted therefrom, thereby resulting in decreased light emission from the device. S. Illek et al. "Scability of buried microreflector light-emitting diodes for high-current applications", Light-Emitting Diodes: Research, Manufacturing and Applications VII, 27-29 Jan 2003, San Jose, discloses the manufacturing of high-brightness, substrate-less LED in the AlGaInP material by a combination of wafer soldering using meta layers and the introduction of buried micro-reflector structures. The <patcit id="pcit0002" dnum="GB2311413A"><text>patent application GB 2 311 413</text></patcit> discloses an internal or external interface of a LED textured with a regular periodic structure extending in one or more directions to enhance light extraction from the device. The device has a p-n junction region sandwiched between epitaxial regions on a substrate. A two -dimensional periodic pattern of protuberances or indentations is provided on the top, bottom and/or side surfaces of the device. The patent application <patcit id="pcit0003" dnum="US20010000209A"><text>US2001/0000209</text></patcit> discloses a method for designing semiconductor LEDs such that the side surfaces are formed at preferred angles relative to vertical to improve light extraction efficiency.

<u>SUMMARY OF THE INVENTION</u>

0010The object of the present invention is fulfilled by a device as defined in claim 1 and a method as defined in claim 5. Preferred embodiments are defined in the dependent claims. The present invention is directed to solve the afore-mentioned problems, and it is an object of the invention to provide a high output LED and a method for fabricating the same wherein a sidewall of the LED is inclined and a reflective film is formed on the inclined sidewall to allow light emitted outside of the sidewall to reflect from the reflective film and to emit upward of the device, thereby enabling to improve a light output and dispensing with an additional passivation process.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0011FIG. is a cross-sectional view of an LED according to the prior art.

0012<figref idref="f0001">FIG.2</figref> is a mimetic cross-sectional view of a flip chip LED bonded to a sub mount substrate according to the prior art.

0013<figref idref="f0002">FIGS. 3a, 3b, 3c</figref>, <figref idref="f0003">3d, and 3e</figref> are cross-sectional views illustrating and explaining a fabricating process of a high output LED according to a first example useful for the understanding of the present invention.

0014<figref idref="f0003">FIG. 4</figref> is a mimetic cross-sectional view illustrating and explaining a state where the high output LED according to the first example useful for the understanding of the present invention is bonded to a reflective plate to emit light.

0015<figref idref="f0004">FIGS. 5a, 5b, 5c</figref>, <figref idref="f0005">5d and 5e</figref> are cross-sectional views illustrating and explaining a fabricating process of a high output LED according to a second example useful for the understanding of the present invention.

0016<figref idref="f0005">FIG. 6</figref> is a mimetic cross-sectional view illustrating and explaining a state where light is emitted from the high output LED according to the second example useful for the understanding of the present invention.

0017<figref idref="f0006">FIGS. 7a and 7b</figref> are perspective views explaining a fabricating process of a high output LED according to an embodiment of the present invention.

0018<figref idref="f0007">FIGS. 8a and 8b</figref> are plan views of examples of mask layers according to an embodiment of the present invention.

0019<figref idref="f0008">FIGS. 9a and 9b</figref> are perspective views explaining a fabricating process of a high output LED according to a third example useful for the understanding of the present invention.

0020<figref idref="f0009">FIGS. 10a and 10b</figref> are perspective views explaining a process of forming an electrode and a reflective film on each LED according to the third example useful for the understanding of the present invention.

0021<figref idref="f0010">FIGS. 11a and 11b</figref> are schematic comparative diagrams in which a current flows in an LED having a square upper surface and an LED having a round upper surface according to the third example useful for the understanding of the present invention.

<u>DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS</u>

0022<figref idref="f0002 f0003">FIGS. 3a through 3e</figref> are cross-sectional views explaining a fabricating process of a high output LED according to a first example useful for the understanding of the present invention.

0023A substrate (100) is sequentially stacked thereon with an N-type semiconductor layer (110), an active layer (120), and a P-type semiconductor layer (130) (<figref idref="f0002">FIG.3a</figref>), and mesa-etching is carried out from the P-type semiconductor layer (130) to portions of the N-type semiconductor layer (110) so that the sidewall thereof is inclined (<figref idref="f0002">FIG.3b</figref>).

0024In other words, in the process of <figref idref="f0002">FIG. 3b</figref>, the sidewall is etched at a predetermined angle (α1).

0025In order to etch the sidewall having an inclination, a mask layer is formed on the upper surface of the P-type semiconductor (130) except for a region to be etched, and the P-type semiconductor (130) is masked by the mask layer and dry-etched to obtain an inclined sidewall.

0026Successively, remaining sidewall of the N-type semiconductor layer (110), the active layer (120) and the P-type semiconductor layer (130) except for the mesa-etched region is etched to form an inclined sidewall (<figref idref="f0002">FIG.3c</figref>).

0027At this time, the inclined angle (α2) of the remaining sidewall may be the same as the angle (α1) of the mesa-etched sidewall, or may be different. Preferably, the inclined angles (α1 and α2) are in the range of 30~70 degrees.

0028Then, the inclined sidewall is evaporated with a reflective film (140) (<figref idref="f0003">FIG.3d</figref>). The inclined sidewall facilitates the evaporation of the reflective film (140). Preferably, the reflective film (140) is an anti-reflection (AR) film, a film stacked with reflective layers, or a high reflection (HR) film.

0029The HR film is an insulating film having high reflection, and serves per se to reflect the light emitted from a sidewall of an LED and functions to passivate the device as well.

0030Furthermore, the AR film per se is an insulating film having a high transmission factor. Although the AR film does not act as a reflective film, it functions as an excellent reflective material and serves to protect the device if formed with a reflective film at an external side thereof because it has a high optical transmission factor.

0031Preferably, the reflective film is made of metal such as Ag or Al.

0032As a last step, the N electrode (150) is formed on the mesa-etched N-type semiconductor layer (110) and the P electrode (160) is formed on the P-type semiconductor layer (<figref idref="f0003">FIG.3e</figref>). As illustrated in <figref idref="f0003">FIG.3e</figref>, the afore-mentioned process enables to fabricate an LED according to the first example useful for the understanding of the present invention.

0033<figref idref="f0003">FIG. 4</figref> is a mimetic cross-sectional view illustrating and explaining a state where the high output LED according to the first example useful for the understanding of the present invention is bonded to a reflective plate to emit light.

0034The high output LED according to the first example useful for the understanding of the present invention fabricated via the processes from <figref idref="f0002 f0003">FIG.3a through 3e</figref>, the high output LED is formed in such a manner that a substrate (100) is sequentially stacked thereon with an N-type semiconductor layer (110), an active layer (120) and a P-type semiconductor layer (130), mesa-etching is performed from the P-type semiconductor layer (130) to portions of the N-type semiconductor layer (110), a mesa-etched sidewall and sidewalls of the P-type semiconductor layer (130), the active layer (120) and N-type semiconductor layer (110) are all inclined, the inclined sidewalls are formed with a reflective film (140), an N electrode (150) is formed on the mesa-etched N-type semiconductor layer (110), and a P electrode (160) is formed on the P-type semiconductor layer (130).

0035The LED (170) thus fabricated is bonded to an upper surface of a reflective plate (190) by bonding material (180). The light emitted from the active layer (120) is reflected by the reflective film (140) formed at a sidewall of the LED to be emitted upward of the P-type semiconductor layer (130) of the device. Consequently, the light is not irradiated to the sidewall but advances straight to the upward of the device, enabling to improve the light output.

0036<figref idref="f0004 f0005">FIGS. 5a through 5e</figref> are cross-sectional views illustrating a fabricating process of a high output LED according to a second example useful for the understanding of the present invention.

0037A substrate (200) is sequentially stacked with an N-type semiconductor layer (210), an active layer (220) and a P-type semiconductor layer (230) (<figref idref="f0004">FIG.5a</figref>). Etching is carried out from the P-type semiconductor layer (230) to the N-type semiconductor layer (210) to form an etched sidewall (<figref idref="f0004">FIG.5b</figref>). The etched sidewall has a predetermined angle (α3). The substrate (200) is detached from the N-type semiconductor layer (210) (<figref idref="f0004">FIG.5c</figref>). Successively, the P-type semiconductor layer (230) is formed thereunder with a reflecting P electrode (240) (<figref idref="f0005">FIG.5d</figref>). The inclined sidewall is formed with a reflective film (250) and the N-type semiconductor layer (210) is formed thereon with an N electrode (260) (<figref idref="f0005">FIG.5e</figref>). Preferably, a width (W1) of the P-type semiconductor layer (230) is smaller than a width (W2) of the N-type semiconductor layer (210).

0038<figref idref="f0005">FIG.6</figref> is a mimetic cross-sectional view explaining a state where light is emitted from the high output LED according to the second example useful for the understanding of the present invention.

0039The high output LED according to the second example useful for the understanding of the present invention fabricated by the process illustrated in <figref idref="f0004 f0005">FIGS. 5a through 5e</figref> is formed in such a manner that a P-type semiconductor layer (230) is sequentially stacked thereon with an active layer (220) and an N-type semiconductor layer, sidewalls of the P-type, semiconductor layer (230), the active layer (220) and the N-type semiconductor layer are inclined, the inclined sidewalls are formed with a reflective film (250), the P-type semiconductor layer (230) is formed thereunder with a reflective P electrode (240) and the N-type semiconductor layer is formed thereon with an N electrode.

0040In the LED thus constructed, the light emitted from the active layer (220) is reflected from the reflective film (250) formed on a sidewall of the LED and the reflective N electrode (240) and emitted to the outside of the N-type semiconductor layer (210) on the device.

0041Consequently, the light is not emitted to the sidewall in the LED of the second example useful for the understanding of the present invention, and the light advances straight and is emitted upward of the device, enabling to improve the light output.

0042<figref idref="f0006">FIGS. 7a and 7b</figref> are perspective views explaining a fabricating process of a high output LED according to an embodiment of the present invention.

0043A substrate (300) is sequentially stacked thereon with a N-type semiconductor layer (310), an active layer (320) and a P-type semiconductor layer (330), and the P-type semiconductor layer (330) is formed thereon with a mask layer (340) formed at a sidewall thereof with indents (341) (<figref idref="f0006">FIG.7a</figref>).

0044The P-type semiconductor layer (330) is masked by the mask layer (340), and etching is carried out from the P-type semiconductor layer (330) to the N-type semiconductor layer (310). The etched sidewall is formed with inclinations and indents (<figref idref="f0006">FIG. 7b</figref>).

0045Successively, the substrate (300) is detached from the N-type semiconductor layer (310). The reflective P electrode is formed under or underneath the P-type semiconductor layer (330). The inclined sidewall is formed with a reflective film and N electrode is formed on the N-type semiconductor layer (310).

0046There is an advantage in the high output LED thus fabricated according to an embodiment of the present invention in that the inclined sidewall is formed with indents to increase a light emitting area and a light output.

0047In other words, the sidewall of the light emitting structure is formed with indents to allow the sidewall of the active layer to be also formed with indents, thereby increasing the light emitting area.

0048<figref idref="f0007">FIGS. 8a and 8b</figref> are plan views of an example of a mask layer according to aspects relating to a an embodiment of the present invention.

0049Referring to <figref idref="f0007">FIG.8a</figref>, the mask layer is a square layer or a layer whose margin is interjected with removed regions each spaced at a predetermined distance apart. Referring to <figref idref="f0007">FIG.8b</figref>, the mask layer is a square layer or a layer, one margin of which is removed of a region.

0050The mask layer may be multi-angled, or formed at a marginal sidewall thereof with indents combined of plain or curved surfaces. The indents of the sidewalls of the LED formed by use of the mask may be formed with plain surface and curved surface.

0051<figref idref="f0008">FIGS. 9a and 9b</figref> are perspective views explaining a fabricating process of a high output LED according to a third example useful for the understanding of the present invention.

0052A substrate (400) is sequentially stacked with a N-type semiconductor layer (410), an active layer. (420) and a P-type semiconductor layer (430). The P-type semiconductor layer (430) is formed thereon with a plurality of disc-shaped mask layers (440) each spaced a predetermined distance apart (<figref idref="f0008">FIG.9a</figref>).

0053Successively, the P-type semiconductor layer (430) is masked by the mask layer (440) and etching is executed from the P-type semiconductor layer (430) to the N-type semiconductor layer (410). The etched sidewall is formed with an inclination (<figref idref="f0008">FIG.9b</figref>).

0054By the process thus described according to <figref idref="f0008">FIG.9b</figref>, the substrate (400) may be formed thereon with a plurality of cylindrical light emitting structures, each structure having a wider lower portion and a smaller upper portion.

0055<figref idref="f0009">FIGS. 10a and 10b</figref> are perspective views explaining a process of forming an electrode and a reflective film on each LED according to the third example useful for the understanding of the present invention.

0056Following the process of <figref idref="f0008">FIG.9b</figref>, the substrate (400) is detached from the N-type semiconductor layer (410), and reversely positioned to obtain a light emitting structure (450) stacked with the P-type semiconductor layer (430), the active layer (420) and the N-type semiconductor layer (410) as illustrated in <figref idref="f0009">FIG.10a</figref>.

0057The light emitting structure (450) has a cylindrical shape where a diameter of the P-type semiconductor layer (430) is smaller than that of the N-type semiconductor layer (410).

0058Now, referring to <figref idref="f0009">FIG.10b</figref>, a sidewall of the light emitting structure (450) is formed with a reflective film (480). A lower portion of the P-type semiconductor layer (460) is formed with a P electrode (460) and an upper portion of the N-type semiconductor layer (410) is formed with an N electrode (470).

0059This process completes a fabrication of a cylindrical LED. The cylindrical LED can allow the current to flow uniformly to make the light strength on a light emitting surface even such that color sensitivity becomes excellent when an LED is applied to a light source and a display.

0060<figref idref="f0010">FIGS. 11a and 11b</figref> are schematic comparative diagrams in which a current flows in an LED having a square upper surface and an LED having a round upper surface according to aspects relating to the third example useful for the understanding of the present invention.

0061An LED having a square upper surface as shown in <figref idref="f0010">FIG.11</figref> a allows each distance of a current reaching from the P electrode (470a) to a margin of the P-type semiconductor layer (431) to differ, resulting in an uneven light strength. Meanwhile, an LED having a round upper surface as illustrated in <figref idref="f0010">FIG.11b</figref> allows each distance of a current reaching from the P electrode to a margin of the P-type semiconductor layer (430) to be identical, resulting in an even light strength.

0062Consequently, the LED according to the third example useful for the understanding of the present invention enables the light strength from a light emitting surface to be even.

0063As apparent from the foregoing, there is an advantage in the preferred embodiment of the present invention thus described in that a sidewall of an LED is inclined and a reflective film is formed on the inclined sidewall to allow light emitted outside of the sidewall to reflect from the reflective film and to emit upward of the device, thereby enabling to improve a light output and dispensing with an additional passivation process.

0064Another advantage is that indents are formed on a sidewall of a light emitting structure to thereby increase a light emitting surface and to improve a light output.

0065Still another advantage is that a cylindrical LED is embodied to allow light from a light emitting surface to be emitted in an even strength by flowing current uniformly.

0066The present invention has been described above in varied detail by reference to particular exemplary embodiments and figures. The above disclosure should be construed as limited only by the appended claims.