Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2001-08-23
2004-04-27
Smith, Matthew (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S088000, C257S098000, C257S163000, C257S431000, C438S022000, C438S047000
Reexamination Certificate
active
06727518
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting device using gallium nitride (GaN) group compound semiconductor in which electrodes are formed on the same side to a substrate. Especially, the present invention relates to a light-emitting device whose light ejection quantity from the electrode side of the device is improved.
And the present invention relates to a method for manufacturing a light-emitting device using gallium nitride (GaN) group compound semiconductor device. Especially, the present invention relates to a method for forming a metal layer so as to cover entire back of the substrate.
2. Description of the Related Art
A conventional light-emitting device using GaN group compound semiconductor, which has semiconductor layers laminated on an insulation sapphire substrate and has a positive and a negative electrodes formed on the same side to the substrate, has been known.
FIG. 8
illustrates a sectional view of a conventional light-emitting device
30
which is installed on a lead frame
31
. The light-emitting device
30
has an emission layer
34
which emits light of a certain wavelength. The positive and the negative electrodes
35
and
36
are formed at the upper side the substrate
33
. The entire back of the substrate
33
is die-bonded to the lead frame
31
by using a paste
32
made of resin. Each of the electrodes
35
and
36
is connected to the device at a predetermined portion electrically so that light can be emitted from the electrodes side.
There is not, however, selectivity of direction of the emitted light in the conventional device
30
having a structure shown in FIG.
8
. Because the direction of light emitted by the emission layer
34
cannot be selected, light reflected by the back surface of the substrate
33
largely contributes to the quantity of light ejected from the upper surface of the device on which the electrodes
35
and
36
are formed. But the paste
32
which is formed to cover the entire back of the substrate
33
absorbs light, resulting in degrading reflectivity of the substrate. As a result, luminous intensity of the conventional light-emitting device
30
becomes smaller. And because the paste
32
with lapse of time deteriorates (or discolors to be yellow) in the atmosphere or the heat which is generated by driving the device
30
, the quantity of reflected light decreases and luminous intensity degrades with lapse of time. Therefore, inventors of the present invention formed a metal layer so as to cover the entire back of the substrate
33
, which results in improving the reflectivity and the luminous intensity.
Because the substrate
33
is hard, in order to separate a wafer into each device, the substrate
33
is ordinary polished until it becomes a lamella and then a scribing process and a breaking process are carried out to the back surface of the substrate
33
. Thus in a conventional method a metal layer is formed after polishing the substrate. When a metal layer as a reflection layer is formed on the back surface of the substrate
33
before carrying out a scribing process, positioning for scribing the wafer becomes difficult. And when a metal layer is formed after the scribing process, washing the back surface of the wafer becomes difficult and the wafer cannot be heated when the metal layer is formed, because of an adhesive sheet adhered on the side of the wafer where the electrode
35
and
36
are formed. That results in degraded adhesiveness between the metal layer and the back surface of the substrate
33
. An oxide film as a reflection film can be formed in place of the metal layer, but there are some difficulties in manufacturing process, e.g., controlling the thickness of the oxide film.
An object of the present invention is, therefore, to improve luminous efficiency of a light-emitting device using gallium nitride group compound semiconductor.
Another object of the present invention is to form a metal layer on a back surface of a substrate. As a result, reflection of light is improved with lapse of time and a quantity of light ejected from the electrode side becomes larger.
Another object of the present invention is to effectively obtain light reflected by the back surface of a sapphire substrate and to improve a quantity of light ejected from the electrode side of the device with lapse of time.
And another object of the present invention is to obtain a light-emitting device which can be manufactured easily.
Each of these objects is aimed at each characteristics of the present invention, so it is not necessary for the present invention to achieve all the objects at one time.
SUMMARY OF THE INVENTION
To achieve the above objects, a first aspect of the present invention is that a light-emitting device, in which at least an n-type nitride group compound semiconductor layer and a p-type gallium nitride group compound semiconductor layer are laminated on a substrate, can emit light in a predetermined region of wavelength. The light-emitting device has a positive and a negative electrodes both formed on the same side to the substrate, and a reflection film formed on the opposite side to the substrate, which reflects light in the predetermined region of wavelength.
The second aspect of the present invention is that the reflection film is a metal layer.
The third aspect of the present invention is that the metal layer is made of aluminum (Al).
The fourth aspect of the present invention is to fix the reflection film on a lead frame by a paste made of resin.
The fifth aspect of the present invention is that the reflection film consists of multiple layers.
And the inventors of the present invention also invented following methods for manufacturing such a device.
A method for manufacturing the light-emitting device using gallium nitride group compound semiconductor comprises the following 6 processes: forming a first groove by cutting a wafer, having a substrate on which a gallium nitride group compound semiconductor layer is formed, from its back surface to a predetermined depth (a first depth) (process 1); cutting at a portion of the upper surface of the wafer, which is corresponding to the portion of the back surface where the first groove is formed, to a predetermined depth (a second depth) (process 2); polishing the back surface of the substrate until the substrate become a lamella having only a trace of the first groove (process 3); forming a metal layer so as to cover the entire back of the substrate (process 4); scribing the metal layer along the first groove (process 5); and breaking the wafer to separate into each devices (process 6).
Through process 1, a wafer having a substrate and gallium nitride group compound semiconductor formed on the substrate is cut on the back surface at a predetermined depth (a first depth). Accordingly, a first groove is obtained. Through process 2, the wafer is cut on a portion of the upper surface, which corresponds to where the first groove is formed, at a predetermined depth (a second depth). Accordingly, a second groove is obtained. Here process 2 may be carried out in advance of process 1. Through process 3, the back surface of the substrate is polished until the substrate becomes to have only a trace of the first groove. Through process 4, a metal layer is formed on the back surface of the substrate. Then, through process 5, a scribing process is carried out to the metal layer along the first groove, and through process 6, the wafer is separated into each device by breaking.
As described above, although the metal layer is formed so as to cover the back surface of the substrate, positioning for scribing on the metal layer is easy because of the trace of the first groove which can be recognized through the metal layer. Because of the metal layer formed on the back surface of the substrate, light output from the device to the substrate side is reflected by the metal layer and luminous intensity of the device can be highly improved. And because the reflection does not depend on materials such as resin, emission of the
Oshio Takahide
Uemura Toshiya
Luu Chuong A
McGinn & Gibb PLLC
Smith Matthew
Toyoda Gosei Co,., Ltd.
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