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.
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 device cannot be deteriorated with lapse of time.
And the present invention also contains the following invention in this specification.
The aspect of the present invention is a light-emitting device which has a substrate and layers made of gallium nitride group compound semiconductor laminated on the substrate, and 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 and transmits the light in the region other than the predetermined region.
Then light of the predetermined region of wavelength, which is emitted from the substrate side of the device, can be reflected by the reflection film, resulting in improving the luminous intensity of the device. And because light whose wavelength is out of the predetermined region passes the reflection film, the substrate can be recognized from the reflection film side of the device and separating process becomes easier.
Another aspect of the present invention is to form the reflection film in which a film whose refractive index is smaller than 1.5 and a film whose refractive index is larger than 1.8 are laminated.
This enables the reflection film to reflect light efficiently even if there is lapse of time.
Another aspect of the present invention is to form the reflection film in which a layer whose refractive index is smaller than 1.5 and a layer whose refractive index is larger than 1.8 are laminated in sequent.
By forming a layer made of a material, whose refractive index is small, directly on the substrate, dependence of reflection light on an incident angle can be lowered. As a result, the device can reflect light more effectively.
Another aspect of the present invention is that the layer having refractive index lower than 1.5 comprises at least one of SiO2, MgF2, CaF2, LiF, and AlF3, and the layer having refractive index larger than 1.8 comprising at least one of TiO2, Y2O3, ZrO2, CeO2, HfO2, and Ta2O5.
Then the reflection film becomes to have a large refractive index in the predetermined wavelength region. And in the other region of wavelength, the reflection film has large transmittivity. Accordingly the reflection film has large wavelength selectivity.
Another aspect of the present invention is to form the reflection film in which a silicon oxide (SiO2) layer and a titanium oxide (TiO2) layer are laminated in sequent.
As a result, the reflection film becomes to have a larger reflectivity in the predetermined wavelength region and larger transmittivity in the other region of wavelength and larger wavelength selectivity as a result.
And another aspect of the present invention is to form the reflection film in which at least 2 pairs of a silicon oxide (SiO2) layer having a thickness about 125 nm and a titanium oxide (TiO2) layer having a thickness about 125 nm are laminated alternately.
As a result, the reflection film having a large reflectivity in the wavelength region of 450 nm to 560 nm and a large transmittivity in the wavelength region of 640 nm to 780 nm can be obtained.