US20060202219A1

Movatterモバイル変換

Info

Publication number: US20060202219A1
Application number: US11/143,993
Authority: US
Inventors: Kenichi Ohashi; Yasuharu Sugawara; Shuji Itonaga; Yasuhiko Akaike
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 2005-03-09
Filing date: 2005-06-03
Publication date: 2006-09-14
Also published as: US20070145883A1; US7667224B2; JP2006253298A

Abstract

A semiconductor light emitting device comprises: a substrate; a semiconductor stacked structure; a first electrode; a second electrode; and a reflective film. The substrate has a top face and a rear face electrode forming portion opposed thereto, and is translucent to light in a first wavelength band. The rear face electrode forming portion is surrounded by a rough surface. The semiconductor stacked structure is provided on the top face of the substrate and includes an active layer that emits light in the first wavelength band. The first electrode is provided on the semiconductor stacked structure, and the second electrode is provided on the rear face electrode forming portion. The reflective film is coated on at least a portion of the rough surface.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-065589, filed on Mar. 9, 2005; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor light emitting device and a semiconductor light emitting apparatus, and more particularly to a semiconductor light emitting device and a semiconductor light emitting apparatus having improved extraction efficiency for light emitted from the active layer.

Semiconductor light emitting devices such as LEDs (light emitting diodes) and LDs (laser diodes) provide various emission wavelengths, high emission efficiency, and long lifetime while being compact in size. For this reason, they are widely used for display, lighting, communication, sensor, and other devices.

In such a semiconductor light emitting device, a semiconductor multilayer film including an n-type cladding layer, active layer, p-type cladding layer, and the like is formed on a substrate of GaAs or sapphire by direct epitaxial growth, or by lamination with a heterogeneous substrate. Electrodes are further formed on the n-type and p-type layers, respectively (e.g., Japanese Laid-Open Patent Applications 2002-353502 and 2001-217467).

However, this type of semiconductor light emitting device does not have sufficiently high extraction efficiency for light emitted from the active layer.

More specifically, the light emitted downward from the active layer is incident on the electrode provided under the substrate. However, the substrate has an alloyed region formed with the electrode material near the interface with the electrode. This causes a problem that the light emitted from the active layer is prone to absorption, which leads to a certain loss inside the chip.

In addition, the light reflected from the lower electrode is attenuated by optical absorption in passing through the active layer. This causes a problem that the reflected light cannot be fully exploited.

There is another problem that total reflection is likely to occur at the side face and the like of the chip. More specifically, the above-described LED is typically processed into a rectangular parallelepiped shape having six smooth faces by cleavage and dicing, and covered with mold resin or the like. However, due to the large difference between a high refractive index of the semiconductor crystal (about 3.5) and a low refractive index of the mold resin (about 1.5), total reflection is likely to occur at the interface therebetween. This decreases the probability that the light emitted inside the chip is extracted outside the chip.

An approach to improving the decrease of light extraction efficiency is to roughen the surface by wet etching or the like to form asperities (e.g., Japanese Laid-Open Patent Application 2001-217467). However, surface roughening is not effective for extracting light emitted toward the bottom face inside the chip mounted on a packaging member.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a semiconductor light emitting device comprising:

a substrate having a top face and a rear face electrode forming portion opposed thereto, the substrate being translucent to light in a first wavelength band, the rear face electrode forming portion being surrounded by a rough surface;

a semiconductor stacked structure provided on the top face of the substrate and including an active layer that emits light in the first wavelength band;

a first electrode provided on the semiconductor stacked structure;

a second electrode provided on the rear face electrode forming portion; and

a reflective film coated on at least a portion of the rough surface.

According to other aspect of the invention, there is provided a semiconductor light emitting device comprising:

a substrate being translucent to light in a first wavelength band; and

a semiconductor stacked structure provided on a major surface of the substrate and including an active layer that emits light in the first wavelength band,

the substrate having a recess on a mounting surface opposed to the major surface.

a substrate having first and second major surfaces and being translucent to light in a first wavelength band;

a semiconductor stacked structure provided on the first major surface of the substrate and including an active layer that emits light in the first wavelength band, at least a portion of the semiconductor stacked structure having a first rough surface formed thereon;

a dielectric film provided on the first rough surface;

a bonding pad provided on the dielectric film;

a thin line electrode portion provided on the semiconductor stacked structure and electrically connected to the semiconductor stacked structure and the bonding pad; and

an electrode provided on the second major surface of the substrate.

According to other aspect of the invention, there is provided a semiconductor light emitting apparatus comprising:

a packaging member;

a semiconductor light emitting device mounted on the packaging member; and

a wire connected to the semiconductor light emitting device,

the semiconductor light emitting device including:

- a semiconductor stacked structure including an active layer that emits light; and
- a bonding pad provided on the semiconductor stacked structure and connected to a fusion bonding portion for the wire, the bonding pad having a smaller pattern than the fusion bonding portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device according to a first embodiment of the invention;

FIG. 2 is a schematic view illustrating extraction paths of light emitted from theactive layer3;

FIGS. 3A to3C and4A to4C are process cross-sectional views showing part of a process of manufacturing a semiconductor light emitting device according to the embodiment of the invention;

FIG. 5 is a micrograph showing therough surface9 formed on the rear face of theGaP substrate1;

FIGS.6 to9 are schematic views illustrating the configuration of the rear face of thesubstrate1 in the embodiment of the invention;

FIG. 10 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of the embodiment of the invention;

FIG. 11 is a schematic view for describing light extraction at therough surface9;

FIG. 12 is a schematic view illustrating light extraction paths in the variation of the embodiment of the invention;

FIG. 13 is a schematic view showing a semiconductor light emitting device according to another variation of the embodiment of the invention;

FIG. 14 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device of the embodiment of the invention which is mounted on a packaging member;

FIGS. 15A to15C and16A to16C are process cross-sectional views illustrating a formation process by dry etching;

FIG. 17 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of the embodiment of the invention;

FIG. 18 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device according to the embodiment of the invention;

FIG. 19 is a plan view illustrating an electrode pattern formed on the surface of the semiconductor light emitting device;

FIG. 20 is a schematic cross-sectional view of a semiconductor light emitting device investigated by the inventors in the course of reaching the invention;

FIG. 21 is a schematic view illustrating a situation where the light scattered below thebonding pad7A is reflected toward the side face of the device and extracted outside;

FIG. 22 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of the embodiment of the invention;

FIG. 23 is a schematic cross-sectional view showing a semiconductor light emitting device according to a second variation of the embodiment of the invention;

FIG. 24 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device of the embodiment of the invention;

FIG. 25 is an enlarged view of a bonding pad portion of the semiconductor light emitting device;

FIG. 26 is a schematic view illustrating a situation where part of the light emitted below thefusion bonding portion80 is extracted outside through a gap between theextended electrode portions7D;

FIG. 27 is a schematic view illustrating an electrode pattern in the embodiment of the invention;

FIG. 28 is a schematic view showing another example electrode pattern in the embodiment of the invention;

FIGS. 29A and 29B are schematic views showing a semiconductor light emitting device according to a variation of the embodiment of the invention;

FIG. 30 is a schematic cross-sectional view showing a semiconductor light emitting device according to another variation of the embodiment of the invention;

FIG. 31 is a schematic cross-sectional view showing a semiconductor light emitting device according to still another variation of the embodiment of the invention;

FIG. 32 is a schematic cross-sectional view showing a semiconductor light emitting apparatus of the embodiment of the invention;

FIG. 33 is a schematic cross-sectional view showing another example of the semiconductor light emitting apparatus; and

FIGS.34 to36 are schematic cross-sectional views showing still another example of the semiconductor light emitting apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to the drawings.

First Embodiment

FIG. 1 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device according to a first embodiment of the invention.

More specifically, the semiconductor light emitting device of this example has a structure comprising asubstrate1 on which acladding layer2,active layer3,cladding layer4, andcurrent diffusion layer5 are stacked in this order. Anelectrode7 is provided on thecurrent diffusion layer5 via a contact layer (not shown). On the other hand, anelectrode8 is formed on part of the rear side of thesubstrate1. The remaining portion is formed into arough surface9 with asperities, the surface of which is coated with areflective film10.

Thesubstrate1 is translucent to light emitted from theactive layer3. For example, thesubstrate1 is made of p-type GaP. Thecladding layer2 can be formed from p-type InAlP, theactive layer3 from InGaAlP, thecladding layer4 from n-type InAlP, and thecurrent diffusion layer5 from n-type InGaAlP. In this case, the contact layer provided between thecurrent diffusion layer5 and theelectrode7 may be made of n-type GaAs.

Epitaxial growth of an InGaAlP-based compound semiconductor layer directly on theGaP substrate1 is difficult. For this reason, astacked structure6 of InGaAlP-based compound semiconductor is first epitaxially grown on a GaAs substrate. The p-type GaP substrate1 is laminated thereon by wafer bonding technology. The GaAs substrate can then be removed by etching or the like to form the stacked structure of the present example.

Thereflective film10 may be made of, for example, metal such as gold (Au), or dielectric. In this respect, for example, silicon oxide or silicon nitride may be used to form the so-called “HR (High Reflectance) coating” in which the relationship between the refractive index and the thickness yields a maximal reflectance. Alternatively, thereflective film10 may be made of a DBR (Distributed Bragg Reflector) in which two kinds of semiconductor layers having different refractive indices are alternately stacked.

In this embodiment, the extraction efficiency for light emitted downward from theactive layer3 can be improved by providing therough surface9 and thereflective film10 on the rear face of thesubstrate1.

FIG. 2 is a schematic view illustrating extraction paths of light emitted from theactive layer3.

The light emitted from theactive layer3 is scattered toward theside face1S of thesubstrate1 by therough surface9 and thereflective film10 as shown by arrows in this figure. Since the scattered light is incident on theside face1S of thesubstrate1 at a relatively small angle (i.e., at a nearly perpendicular angle with respect to theside face1S), it is emitted outside at theside face1S without total reflection. As described above, this type of semiconductor light emitting device is typically sealed with translucent resin having a refractive index of about 1.5. Therefore the light emitted from inside the chip is susceptible to total reflection at the interface between the semiconductor layer and the resin. On the contrary, in the present embodiment, the light can be scattered by therough surface9 and thereflective film10 and made incident on theside face1S of thesubstrate1 at a small angle. Therefore the light can be extracted outside without total reflection.

If a flat reflective film is provided without therough surface9 on the rear face of thesubstrate1, the light emitted downward from theactive layer3 is reflected upward by this reflective film. In this case, however, the reflected light passes through theactive layer3, leading to a certain loss due to reabsorption. On the contrary, in the present embodiment, therough surface9 serves to scatter the light toward theside face1S, reducing the loss due to absorption.

Furthermore, if theelectrode8 is formed entirely on the rear face of thesubstrate1, an alloyed region is formed at the interface between thesubstrate1 and theelectrode8, and absorbs the light emitted from theactive layer3, leading to a certain loss. In contrast, according to the present embodiment, no alloyed region is present on therough surface9, which reflects the light in conjunction with thereflective film10 with high efficiency. As a result, the loss due to absorption is reduced.

Next, the present embodiment will be described with reference to a method of manufacturing an InGaAlP-based light emitting device by way of example.

FIGS. 3A to3C and4A to4C are process cross-sectional views showing part of a process of manufacturing a semiconductor light emitting device according to this embodiment.

First, as shown inFIG. 3A, an InAlPetch stop layer94,GaAs contact layer26, InGaAlPcurrent diffusion layer5, n-typeInAlP cladding layer4, InGaAlPactive layer3, p-typeInAlP cladding layer2,InGaP bonding layer34, andInAlP cover layer96 are grown on an n-type GaAs substrate92. The n-type GaAs substrate92 may be a mirror-finished substrate having a diameter of 3 inches and a thickness of 350 μm, and doped with silicon (Si) at a carrier concentration of about 1×10¹⁸/cm³.

Theetch stop layer94 may have a thickness of 0.2 μm. TheGaAs contact layer26 has a thickness of 0.02 μm and a carrier concentration of 1×10¹⁸/cm³. The InGaAlPcurrent diffusion layer5 is made of InGaAlP and may have a thickness of 1.5 μm. The n-type cladding layer4 is made of InAlP and may have a thickness of 0.6 μm. Theactive layer3 is made of InGaAlP and may have a thickness of 0.4 μm. The p-type cladding layer2 is made of InAlP and may have a thickness of 0.6 μm. TheInGaP bonding layer34 may have a thickness of 0.1 μm, and theInAlP cover layer96 may have a thickness of 0.15 μm.

Next, this epitaxial wafer is washed with surfactant, immersed in a mixture of ammonia and hydrogen peroxide solution with a volume ratio of 1:15 to etch the rear side of theGaAs substrate92, thereby removing any reaction products and the like produced in the epitaxial growth and attached to the rear face of the epitaxial wafer.

Next, the epitaxial wafer is washed again with surfactant. The topmostInAlP cover layer96 is then removed with phosphoric acid to expose theInGaP bonding layer34.

Subsequently, as shown inFIG. 3B, aGaP substrate1 is laminated. In the following, a process of direct lamination will be described.

TheGaP substrate1 may be, for example, a mirror-finished, (100)-oriented p-type substrate having a diameter of 3 inches and a thickness of 300 μm. A high concentration layer may be formed on the surface of theGaP substrate1 to lower the electric resistance at the bonding interface. As a preprocess for direct bonding, theGaP substrate1 is washed with surfactant, immersed in dilute hydrofluoric acid to remove natural oxidation film on the surface, washed with water, and then dried using a spinner. With regard to the epitaxial wafer, after thecover layer96 on the surface thereof is removed, it is treated with dilute hydrofluoric acid, washed with water, and spin-dried, in the same way as for theGaP substrate1. Preferably, these preprocesses are entirely performed under a clean atmosphere in a clean room.

Next, the preprocessed epitaxial wafer is placed with theInGaP bonding layer34 turned up, on which theGaP substrate1 is mounted with its mirror surface turned down, and closely contacted at room temperature.

Next, as a final step of direct bonding, the wafers contacted at room temperature are set up in a line on a quartz boat, and placed in a diffusion oven for heat treatment. The heat treatment may be performed at a temperature of 800° C. for a duration of one hour in an atmosphere of argon containing 10% hydrogen. This heat treatment integrates theGaP substrate1 with theInGaP bonding layer34, thereby completing the bonding.

Next, as shown inFIG. 3C, theGaAs substrate92 of the epitaxial wafer is removed. More specifically, the bonded wafer is immersed in a mixture of ammonia and hydrogen peroxide solution to selectively etch theGaAs substrate92. This etching step stops at the InAlPetch stop layer94. Next, etching is performed with phosphoric acid at 70° C. to selectively remove the InAlPetch stop layer94.

The foregoing process results in a bonded substrate for LED in which the GaPtransparent substrate1 is bonded to thestacked structure6 of InGaAlP-based semiconductor.

Next, as shown inFIG. 4A, an n-side electrode7 is formed on theGaAs contact layer26, and a p-side electrode8 is formed on the rear face of theGaP substrate1.

Thecontact layer26 surrounding the n-side electrode7 is etched away in order to avoid absorption by theGaAs contact layer26.

The n-side electrode7 may be a stacked structure of, for example, AuGe (250 nm)/Mo (150 nm)/AuGe (250 nm)/Au (300 nm) from thecontact layer26 side. The p-side electrode8 may be made of, for example, metal containing gold (Au) with 5% zinc (Zn). In addition, a eutectic solder layer such as AuSn (1000 nm) may be provided via Au (100 nm) on the surface of the p-side electrode8.

Next, as shown inFIG. 4B, arough surface9 is formed on the rear face of thesubstrate1.

First,protection films11 are formed on the n-side electrode7 and the p-side electrode8, respectively. Theprotection film11 may be made of material such as resist, silicon oxide, or silicon nitride, for example.

Subsequently, arough surface9 is formed by etching the rear face of theGaP substrate1 exposed around the periphery of the p-side electrode8. The etching condition may be, for example, immersion in concentrated hydrofluoric acid for about 10 minutes.

FIG. 5 is an electron micrograph showing therough surface9 formed on the rear face of theGaP substrate1 according to this process. As a result of hydrofluoric acid etching, the rear face of thesubstrate1 is covered with pyramids having a width and height of generally 1 micrometer. Therough surface9 composed of a collection of such pyramids provides a high scattering effect on the light emitted downward from theactive layer3.

Subsequently, as shown inFIG. 4C, therough surface9 is coated with areflective film10.

More specifically, for example, thereflective film10 can be formed by depositing gold (Au) using vacuum deposition. Subsequently, theprotection films11 provided on both sides of the wafer are removed. Chips are separated by dicing or otherwise to result in a semiconductor light emitting device of the present embodiment.

When metal is used for the material of thereflective film10, alloying with thesubstrate1 decreases the reflectance and leads to a certain loss. For this reason, when heat treatment (sinter) is needed to lower the contact resistance of the n-side electrode7 and the p-side electrode8, thereflective film10 is formed preferably after this heat treatment.

Alternatively, ohmic metal can be used for the material of thereflective film10. More specifically, when light absorption due to alloying with thesubstrate1 is not substantial, ohmic metal may be used for the material of thereflective film10.

As described above, in this embodiment, the rear face of theGaP substrate1 is etched by hydrofluoric acid to form arough surface9 that provides a high scattering effect, which allows improvement of light extraction efficiency.

FIGS.6 to9 are schematic views illustrating the configuration of the rear face of thesubstrate1 in this embodiment.

More specifically, the p-side electrode8 may be formed near the center of the rear face of thesubstrate1 in a circular shape as shown inFIG. 6, or in a square shape as shown inFIG. 7. In addition, as shown inFIG. 8, the p-side electrode8 may be divided into a plurality of portions. Division of the electrode into a plurality of portions serves to alleviate concentration of current and to uniformly inject current into theactive layer3, which leads to light emission in a wide region.

Alternatively, as shown inFIG. 9, afirst portion8A provided near the center of the rear face of thesubstrate1 may be connected to asecond portion8B shaped in a thin line extending around the periphery. This can also result in uniform injection of current into theactive layer3 and light emission in a wide region.

It is to be understood that FIGS.6 to9 are illustrative only. For example, the p-side electrode8 may have a pattern of polygon, ellipse, or any other shapes. Similarly, the number and arrangement thereof may be varied. Such variations are encompassed within the scope of the invention.

FIG. 10 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of this embodiment. With regard to this figure, the elements similar to those described above with reference to FIGS.1 to9 are marked with the same reference numerals and will not be described in detail.

In this variation, theside face1S of thesubstrate1 is tapered, and thus thesubstrate1 is shaped like a truncated pyramid. Arough surface9 is formed on theside face1S. Furthermore, areflective film10 is provided on therough surface9 in a region extending from the bottom face of thesubstrate1, that is, the lower face with a p-side electrode8 provided thereon, to halfway theside face1S.

Therough surface9 not covered with thereflective film10 has an effect of increasing light extraction efficiency.

FIG. 11 is a schematic view for describing light extraction at therough surface9.

More specifically, therough surface9 made of pyramids is formed on theside face1S of thesubstrate1. The light traveling inside thesubstrate1 along the arrow A is totally reflected along the arrow B when the light is incident on therough surface9 at an angle greater than the critical angle. However, this reflected light is incident on the opposedrough surface9 at an angle less than the critical angle and can be extracted outside from thesubstrate1. In this manner, when the light traveling inside thesubstrate1 enters a salient portion of therough surface9, it is subjected to one or more total reflections and can be extracted outside as shown by the arrow C.

FIG. 12 is a schematic view illustrating light extraction paths in this variation.

As described above with reference toFIG. 11, light can be extracted with high efficiency at therough surface9 not covered with thereflective film10.

However, when this semiconductor light emitting device is mounted with an adhesive30 such as silver paste or solder, the adhesive30 may climb up on the side face of the chip as shown. Light cannot be extracted in the portion where the adhesive30 climbed up in this manner. On the contrary, in this variation, light extraction is facilitated by coating therough surface9 with thereflective film10 near the mounting surface of the chip. More specifically, in the region where the adhesive30 climbs up, the light inside the chip is reflected by thereflective film10 to allow external extraction. As a result, the light extraction efficiency can be improved.

The taperedside face1S of thesubstrate1 in this variation can be formed, for example, by dicing. More specifically, a dicing blade having a V-shaped cross section can be used to dice thesubstrate1 from the rear side for forming a V-shaped groove. Alternatively, the V-shaped groove may be formed by etching. Chips are separated along the V-shaped groove thus formed to result in the taperedside face1S. In this case, therough surface9 can be formed by applying the roughening treatment as described above with reference toFIG. 4B when the V-shaped grooves have been formed, or after the chips are separated.

FIG. 13 is a schematic view showing a semiconductor light emitting device according to another variation of this embodiment.

More specifically, in this variation, only the lower portion of thesubstrate1 is tapered. Arough surface9 is formed on theside face1S of thesubstrate1. Therough surface9 in the tapered portion is coated with areflective film10.

This can avoid shielding light due to climbing up of the adhesive30, and simultaneously help the light reflected by thereflective film10 be incident on thevertical side face1S as illustrated by the arrow A. As a result, the light extraction efficiency can be further increased.

The semiconductor light emitting device of this variation can be manufactured by adjusting the depth of the groove when the V-shaped grooves are formed on the rear face of thesubstrate1 by using a dicing blade having a V-shaped cross section or by etching. After the V-shaped grooves are formed, a scriber or a thin dicing blade is used to cut off the remaining portion. In this way, the side face of the V-shaped groove becomes a tapered portion, and the remaining portion becomes the vertical side face.

Second Embodiment

Next, as a second embodiment of the invention, a semiconductor light emitting device having a recess on the rear face of the chip will be described.

FIG. 14 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device of this embodiment which is mounted on a packaging member.

More specifically, the semiconductor light emitting device of this embodiment also has asubstrate1 and a semiconductor stackedstructure6. The semiconductor stackedstructure6 includes an active layer and cladding layers as appropriate, and emits light in response to injection of current via

electrodes

7 and8. The semiconductor light emitting device is mounted on apackaging member28 such as a lead frame or mounting board with an adhesive30.

In this embodiment, a pyramidal orconical recess20 is provided on the rear face of the semiconductor light emitting device so as not to overlap theside face1S of the substrate. Therecess20 may be shaped as a pyramid or a circular cone. Theelectrode8 is provided, for example, near the center of the recess. Such arecess20 can increase the light extraction efficiency. This point will be described with reference to a comparative example.

More specifically, consider a comparative example of the semiconductor light emitting device having a flat rear face where an electrode is provided near the center. When such a semiconductor light emitting device is mounted on a packaging member, an adhesive such as silver paste or solder may run off around the device and climb up on the side face of the device. It is thus impossible to extract light in the portion where the adhesive climbed up. On the other hand, the light emitted downward from the active layer is reflected by the flat rear face of the device, and the reflected light is absorbed in the active layer, which leads to a certain loss.

On the contrary, in the present embodiment, a pyramidal orconical recess20 is provided on the rear face of the semiconductor light emitting device. Therefore, as shown inFIG. 14, the light emitted from the active layer can be reflected toward theside face1S of the substrate and extracted outside without passing through the active layer. That is, the loss due to absorption by the active layer can be reduced.

Furthermore, therecess20 absorbs any excess of the adhesive30. Thus the adhesive30 can be prevented from climbing up on theside face1S of the device. Therefore the light reflected from therecess20 toward theside face1S is extracted outside without being shielded by the adhesive30.

The side face of therecess20 in the semiconductor light emitting device of this embodiment has an oblique angle of, for example, about 25 to 45 degrees relative to the mounting surface of the device. Such arecess20 can be formed by, for example, dry etching or laser processing.

FIGS. 15A to15C and16A to16C are process cross-sectional views illustrating a formation process by dry etching.

More specifically, first, as shown inFIG. 15A, amask layer40 made of relatively soft material such as resist is formed on the rear face of thesubstrate1 where a recess is to be formed.

Next, as shown inFIG. 15B, apress42 is forced on themask layer40. Thepress42 has protrusions42P each corresponding to therecess20 to be formed.

Forced by thepress42, as shown inFIG. 15C, recesses44 corresponding to the protrusions42P are formed on themask layer40.

Next, as shown inFIG. 16A, anisotropic etching such as ion milling or RIE (reactive ion etching) is used to etch themask layer40 from above. The etching pattern of themask layer40 is then transferred to theunderlying substrate1. Etching of themask layer40 proceeds as shown inFIG. 16B. When themask layer40 is completely etched as shown inFIG. 16C, therecesses20 have been formed on the surface of theunderlying substrate1.

As an alternative to the process described above, for example, laser processing may be used to form arecess20 on the rear face of thesubstrate1. In this case, the rear face of thesubstrate1 is irradiated with a scanned laser beam to successively etch a certain amount. A pyramidal orconical recess20 can be formed by gradually reducing the scanning field of the laser beam.

FIG. 17 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of the present embodiment.

In this variation, the portion of therecess20 outside theelectrode8 is coated with areflective film10. Thereflective film10 may be any one of the various films described above with reference to the first embodiment.

Thereflective film10 can further increase light reflectance at therecess20. As a result, the light emitted downward from the active layer can be reflected with high efficiency and extracted outside via theside face1S.

Furthermore, in this embodiment, a rough surface as described above with reference toFIG. 11 may be provided on theside face1S.

Third Embodiment

Next, as a third embodiment of the invention, a semiconductor light emitting device having a reduced loss of light below the bonding pad will be described.

FIG. 18 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device according to this embodiment.

FIG. 19 is a plan view illustrating an electrode pattern formed on the surface of this semiconductor light emitting device.

With regard to these figures again, the elements similar to those described above with reference to FIGS.1 to17 are marked with the same reference numerals and will not be described in detail.

In this embodiment, theelectrode7 formed on the semiconductor stackedstructure6 has abonding pad7A and a thinline electrode portion7B connected thereto. Thebonding pad7A is a connecting portion for gold wire or the like that is connected to an external circuit (not shown). The thinline electrode portion7B is a portion for electrical contact with the semiconductor layer via anohmic GaAs layer26. The chip may measure generally 200 micrometers to 1 millimeter per side. Thebonding pad7A may have a diameter of generally 100 to 150 micrometers. The thinline electrode portion7B may have a line width of generally 2 to 10 micrometers.

In this embodiment, arough surface9 is formed on the surface of the semiconductor stackedstructure6 below thebonding pad7A, and adielectric layer50 is provided thereon. Therough surface9 may be similar to that described above with reference to the first embodiment. Thedielectric layer50 may be formed by, for example, SOG (spin on glass). Such structure below thebonding pad7A can improve the extraction efficiency for light from the semiconductor light emitting device. This point will be described with reference to a comparative example.

FIG. 20 is a schematic cross-sectional view of a semiconductor light emitting device investigated by the inventors in the course of reaching the invention.

In this comparative example, the semiconductor stackedstructure6 has a flat surface, on which acurrent block layer52 made of semiconductor is provided. For example, when the semiconductor stackedstructure6 is made of InGaAlP-based compound semiconductor that emits red light, thecurrent block layer52 may be made of non-doped InGaP or the like. Thecurrent block layer52 serves to block the injection of current from thebonding pad7A into the underlying semiconductor layer. That is, it is difficult to extract externally the light emitted below thebonding pad7A because it is shielded by thebonding pad7A. For this reason, thecurrent block layer52 is provided to turn the portion below thebonding pad7A into a non-emitting region NE.

However, the structure of this comparative example has a problem that, when the light emitted by current injection from the thinline electrode region7B is directed below thebonding pad7A as shown by the arrow A, it is absorbed by theGaAs contact layer26 to result in a certain loss. In addition, the light reflected below thebonding pad7A travels toward theopposed electrode8 as shown by the arrow B, and is absorbed in the alloyed region formed in the vicinity of theelectrode8, which leads to another loss. Furthermore, since the light emitted below thethin line electrode7B is incident on theside face1S of thesubstrate1 at a relatively large incident angle, it is prone to total reflection at theside face1S. This causes another problem of decreasing light extraction efficiency.

On the contrary, in the present embodiment, first, adielectric layer50 is provided below thebonding pad7A, which has a current blocking effect and an effect of increasing reflectance. More specifically, since thedielectric layer50 is insulator, it can definitely block current and ensure that light emission below thebonding pad7A is reduced.

Furthermore, thedielectric layer50 serves to reflect the light emitted from theactive layer3 with high efficiency. For example, if thedielectric layer50 is made of silicon oxide, and assuming that the underlying InGaAlP layer has a refractive index of n=3.2 and silicon oxide has a refractive index of n=1.45, then the critical angle for total reflection at the interface therebetween is as small as about 27 degrees. That is, of the light emitted from the active layer and being incident on thedielectric layer50, the light having an incident angle above 27 degrees is totally reflected. Additionally, in this case, the light having an incident angle below 27 degrees is also subjected to about 14% reflection. In this way, thedielectric layer50 serves to reflect the light emitted from theactive layer3 with high efficiency.

Furthermore, according to this embodiment, arough surface9 can be provided on the surface of the semiconductor stackedstructure6 to scatter light. As a result, as shown inFIG. 21 by the arrow A, the light scattered below thebonding pad7A can be reflected toward theside face1S of the device and extracted outside.

FIG. 22 is a schematic cross-sectional view showing a semiconductor light emitting device according to a variation of this embodiment.

More specifically, in this variation, arough surface9 is provided on theside face1S of thesubstrate1. Such arough surface9 serves to increase the light extraction efficiency by taking advantage of multiple reflections as described above with reference toFIG. 11. That is, the light emitted below the thinline electrode portion7B or the light scattered at therough surface9 below thebonding pad7A can be extracted via theside face1S with high efficiency.

FIG. 23 is a schematic cross-sectional view showing a semiconductor light emitting device according to a second variation of this embodiment.

More specifically, in this variation, theside face1S of thesubstrate1 is tapered. This enables the light emitted below the thinline electrode portion7B or the light reflected below thebonding pad7A to be incident on theside face1S at a smaller incident angle. As a result, total reflection at theside face1S can be reduced to further increase the light extraction efficiency. Additionally, in this variation, arough surface9 similar to that shown inFIG. 22 may be provided on theside face1S.

Fourth Embodiment

Next, as a fourth embodiment of the invention, a semiconductor light emitting device having an improved extraction efficiency for light from below the bonding pad will be described.

FIG. 24 is a schematic view illustrating the cross-sectional structure of a semiconductor light emitting device of this embodiment.

FIG. 25 is an enlarged view of a bonding pad portion of this semiconductor light emitting device. With regard to these figures, the elements similar to those described above with reference to FIGS.1 to23 are marked with the same reference numerals and will not be described in detail.

In this embodiment, theelectrode7 formed on the top face of the device is composed of abonding pad7C andextended electrode portions7D. However, thebonding pad7C has a smaller pattern area than afusion bonding portion80 for gold (Au) or other wire to be connected thereto. For example, when a gold wire having a diameter of about 20 to 30 micrometers is ball bonded, thefusion bonding portion80 will have a generally circular shape having a diameter of about 80 to 120 micrometers. In contrast, the diameter of thebonding pad7C of the light emitting device of this embodiment is set to, for example, about 40 to 70 micrometers. In addition,extended electrode portions7D are extended from thebonding pad7C in order to secure strength against wire bonding and to diffuse current over a wide range. The structure below thebonding pad7C and theextended electrode portions7D is made to allow current injection via a contact layer or the like (not shown).

FIG. 26 is a schematic view illustrating a situation where part of the light emitted below thefusion bonding portion80 is extracted outside through a gap between theextended electrodes7D.

Typically, the light emitted below thebonding pad7C is shielded by thebonding pad7C and cannot be directly extracted outside. In addition, in a structure where current is injected into a semiconductor layer below thebonding pad7C, an alloyedregion18 of metal and semiconductor is formed below thebonding pad7C. Absorption of light emission by this alloyedregion18 leads to a certain loss. Therefore thebonding pad7C formed larger than the size of the wirefusion bonding portion80 decreases the light extraction efficiency.

In contrast, according to this embodiment, the size of thebonding pad7C is made smaller than the wirefusion bonding portion80. As shown inFIG. 26, this enables part of the light emitted below thefusion bonding portion80 to be extracted outside through a gap between theextended electrode portions7D. Therefore, in this embodiment, thesubstrate1 does not necessarily need to be transparent to the light emitted from theactive layer3. Of course, this embodiment has a similar advantageous effect when applied to a semiconductor light emitting device having atransparent substrate1.

FIG. 27 is a schematic view illustrating an electrode pattern in this embodiment.

More specifically, abonding pad7C smaller than the wirefusion bonding portion80 is provided.Extended electrode portions7F having a narrow width are radially connected to thebonding pad7C. The light emitted below thefusion bonding portion80 can be extracted outside between theextended electrode portions7F. In addition, thinline electrode portions7E having an even narrower width can be extended to the periphery of the chip to uniformly inject current over a wide range and produce light emission.

FIG. 28 is a schematic view showing another example electrode pattern in this embodiment.

More specifically, in this example,extended electrode portions7D having a wider width are formed below thefusion bonding portion80, andextended electrode portions7F having a narrower width are formed otherwise. Formation ofextended electrode portions7D having a wider width below thefusion bonding portion80 facilitates increasing strength against wire bonding. That is, semiconductor layers can be protected more definitely against pressure, ultrasonic waves, and the like applied during wire bonding. In addition, formation ofextended electrode portions7F having a narrow width and thinline electrode portions7E having an even narrower width outside thefusion bonding portion80 serves to uniformly inject current over a wide range and to extract light emission at high efficiency without shielding.

FIG. 29A is an enlarged schematic plan view showing theelectrode7 of the semiconductor light emitting device according to a variation of this embodiment, andFIG. 29B is a schematic cross-sectional view thereof.

More specifically, in this variation, the surface of the semiconductor layer below the fusion bonding portion80 (e.g., thefusion bonding portion80 shown by a dot-dashed line inFIGS. 27 and 28) outside the electrode7 (extended

electrode portions

7D and7F,bonding pad7C, etc.) is covered with atransparent film21 being translucent to light emission. Such atransparent film21 serves to increase strength against wire bonding. It also serves to protect the semiconductor layer when the semiconductor light emitting device is sealed with resin. Furthermore, thetransparent film21 allows part of the light emitted below thebonding pad7C to be extracted outside more efficiently. That is, as shown inFIG. 29B by the arrow A, the light emitted below thebonding pad7C can be made incident on thetransparent film21 and reflected at the surface of thetransparent film21 to propagate in thetransparent film21. In this way, the light emitted below thefusion bonding portion80 can be extracted by propagating in thetransparent film21.

In this case, thetransparent film21 is preferably formed from material having a smaller refractive index than the transparent resin (having a refractive index of about 1.5) for sealing the light emitting device. Such atransparent film21 can be formed by, for example, the SOG (Spin On Glass) method. In the SOG method, liquid SOG raw material based on, for example, inorganic silicates or organic silicates such as methyl siloxanes is applied to the surface of a wafer using the spin coating method. Subsequently, a transparent silicon oxide film can be obtained by, for example, applying heat treatment at 300 to 400° C. The silicon oxide film thus obtained has a refractive index of 1.4 or less, which can be used as atransparent film21 in this variation.

Furthermore, the strength against wire bonding can be increased when thetransparent film21 and theelectrode7 have a comparable thickness. However, the advantageous effect of light extraction is achieved even when thetransparent film21 has a smaller thickness than theelectrode7.

Additionally, in this variation again, the light extraction efficiency can be further improved by forming a rough surface of asperities on the rear face of the translucent substrate as described above with reference to the first embodiment, or by providing a recess on the rear face of the translucent substrate as described above with reference to the second embodiment.

FIG. 30 is a schematic cross-sectional view showing a semiconductor light emitting device according to another variation of this embodiment.

More specifically, in this variation, arough surface9 is formed on the surface of the semiconductor layer. Formation of therough surface9 serves to increase the light extraction efficiency by taking advantage of multiple reflections as described above with reference toFIG. 11. That is, the light emitted from theactive layer3 can be extracted with high efficiency whether the light is emitted below thefusion bonding portion80 or in other light emitting regions.

FIG. 31 is a schematic cross-sectional view showing a semiconductor light emitting device according to still another variation of this embodiment.

More specifically, this variation has a combined structure of the variations shown inFIGS. 29 and 30. Thetransparent film21 and therough surface9 provided below thefusion bonding portion80 facilitate reflecting and scattering effects, which allow the light emitted below thefusion bonding portion80 to be extracted outside with higher efficiency.

Fifth Embodiment

Next, as a fifth embodiment of the invention, a semiconductor light emitting apparatus equipped with the semiconductor light emitting device of the embodiment of the invention will be described. More specifically, a semiconductor light emitting apparatus with high brightness can be obtained by packaging the semiconductor light emitting device described above with reference to the first to fourth embodiments on a lead frame, mounting board, or the like.

FIG. 32 is a schematic cross-sectional view showing a semiconductor light emitting apparatus of this embodiment. The semiconductor light emitting apparatus of this example is a resin-sealed semiconductor light emitting apparatus called the “bullet-shaped” type.

A cup portion102C is provided on top of alead102. The semiconductorlight emitting device101 is mounted on the bottom face of the cup portion102C with an adhesive or the like. It is connected to anotherlead103 using awire104. The inner wall of the cup portion102C constitutes alight reflecting surface102R, which reflects the light emitted from the semiconductorlight emitting device101 and allows the light to be extracted above. In this example, in particular, the light emitted from the side face and the like of the transparent substrate of the semiconductorlight emitting device101 can be reflected by thelight reflecting surface102R and extracted above.

The periphery of the cup portion102C is sealed withtranslucent resin107. Thelight extraction surface107E of theresin107 forms a condensing surface, which can condense the light emitted from the semiconductorlight emitting device101 as appropriate to achieve a predetermined light distribution.

FIG. 33 is a schematic cross-sectional view showing another example of the semiconductor light emitting apparatus. More specifically, in this example, theresin107 sealing the semiconductorlight emitting device101 has rotational symmetry about itsoptical axis107C. It is shaped as set back and converged toward the semiconductorlight emitting device101 at the center. Theresin107 of such shape results in light distribution characteristics where light is scattered at wide angles.

FIG. 34 is a schematic cross-sectional view showing still another example of the semiconductor light emitting apparatus. More specifically, this example is called the “surface mounted” type. The semiconductorlight emitting device101 is mounted on alead102, and connected to anotherlead103 using awire104. These leads102 and103 are molded infirst resin109. The semiconductorlight emitting device101 is sealed with secondtranslucent resin107. Thefirst resin109 has an enhanced light reflectivity by dispersing fine particles of titanium oxide, for example. Itsinner wall109R acts as a light reflecting surface to guide the light emitted from the semiconductorlight emitting device101 to the outside. That is, the light emitted from the side face and the like of the semiconductorlight emitting device101 can be extracted above.

FIG. 35 is a schematic cross-sectional view showing still another example of the semiconductor light emitting apparatus. More specifically, this example is also what is called the “surface mounted” type. The semiconductorlight emitting device101 is mounted on alead102, and connected to anotherlead103 using awire104. The tips of these

leads

102 and103, together with the semiconductorlight emitting device101, are molded intranslucent resin107.

FIG. 36 is a schematic cross-sectional view showing still another example of the semiconductor light emitting apparatus. In this example, a structure similar to that described above with reference toFIG. 32 is used. In addition, the semiconductorlight emitting device101 is covered withphosphor108. Thephosphor108 serves to absorb the light emitted from the semiconductorlight emitting device101 and convert its wavelength. For example, ultraviolet or blue primary light is emitted from the semiconductorlight emitting device101. Thephosphor108 absorbs this primary light and emits secondary light having different wavelengths such as red and green. For example, three kinds of phosphor may be mixed, and thephosphor108 may absorb ultraviolet radiation emitted from the semiconductorlight emitting device101 to emit white light composed of blue, green, and red light.

Thephosphor108 may be applied to the surface of the semiconductorlight emitting device101, or may be contained in theresin107.

In any semiconductor light emitting apparatus shown in FIGS.32 to36, a semiconductor light emitting apparatus with high brightness can be offered by providing the semiconductor light emitting device described above with reference to the first to fourth embodiments to extract light from the top and/or side faces of the semiconductorlight emitting device101 with high efficiency.

Embodiments of the invention have been described with reference to specific examples. However, the invention is not limited to the specific examples. For example, various variations of the semiconductor light emitting device and the semiconductor light emitting apparatus with respect to their structure and the like are also encompassed within the scope of the invention.

For example, any details of the layered structure constituting the semiconductor light emitting device modified as appropriate by those skilled in the art are also encompassed within the scope of the invention, as long as they comprise the feature of the invention. For instance, the active layer may be made of various materials in addition to InGaAlP-based material, including Ga_xIn_1-xAs_yN_1-y-based (0≦x≦1, 0≦y≦1), AlGaAs-based, and InGaAsP-based materials. Similarly, the cladding layers and optical guide layer may also be made of various materials.

In addition, the wafer bonding described as a typical example of the method of manufacturing a LED having a light-transmitting substrate may also be applied to conventionally known LEDs such as AlGaAs-based LEDs in which the transparent substrate is obtained by thick epitaxial growth.

Any shape and size of the semiconductor light emitting device modified as appropriate by those skilled in the art are also encompassed within the scope of the invention, as long as they comprise the feature of the invention.

Furthermore, a semiconductor light emitting device and a semiconductor light emitting apparatus obtained from any combination of two or more of the embodiments of the invention are also encompassed within the scope of the invention. More specifically, for example, a semiconductor light emitting device and a semiconductor light emitting apparatus obtained by combining the first embodiment of the invention with any of the second to fourth embodiments of the invention are also encompassed within the scope of the invention. The third and the fourth embodiments may also be combined. Any other combinations that are technically feasible are also encompassed within the scope of the invention.

Any other semiconductor light emitting devices and semiconductor light emitting apparatuses that can be modified and implemented as appropriate by those skilled in the art on the basis of the semiconductor light emitting devices and semiconductor light emitting apparatuses described above as the embodiments of the invention also belong to the scope of the invention.