Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-10-04
2001-09-11
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
Reexamination Certificate
active
06287882
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light emitting diode (LED) with a metal-coated reflective permanent substrate and the method for manufacturing the same.
BACKGROUND OF THE INVENTION
A cross sectional view of a conventional light emitting diode is illustrated in FIG.
1
. The light emitting diode
100
includes a semiconductor substrate
102
, a first ohmic contact electrode
101
formed on the rear side of the semiconductor substrate
102
, a light generating region
103
formed on the semiconductor substrate
102
, and a second ohmic contact electrode
106
formed on the light generating region
103
. The light generating region
103
is formed by a P-type region and an N-type region, and grown on the gallium arsenide (GaAs) substrate
102
. Because of the current crowding effect, limited emitting angle of the light and light absorption of the substrate, the illumination in this light emitting diode is not strong.
The crystal lattice constants in most of the light generating region
103
are matched with that of the gallium arsenide substrate. Namely, the light emitting diode of visible light is directly fabricated on the gallium arsenide substrate
102
. However, since the energy gap of gallium arsenide is 1.43 eV which is smaller than that of the visible light and the light emitted from the diode is isotropic, part of the light enters the substrate and is absorbed by the gallium arsenide substrate.
U.S. Pat. Nos. 5,008,718 and 5,233,204 disclose a transparent window layer structure for increasing the output light of a light emitting diode. Referring to
FIG. 2
, the structure of the light emitting diode
200
is formed by a transparent window layer
204
grown on the light emitting diode
100
shown in FIG.
1
. By means of the transparent window layer, the current crowding effect in a conventional light emitting diode is reduced and the current spread to emit light is increased. As a result, the illumination of the light emitting diode is enhanced.
The proper material for the transparent window layer
204
includes GaP, GaAsP, and AlGaAs, etc., whose energy gap is larger than those of the materials in the AlGaInP light generating region. Under this condition, the critical angle of the emitted light can be increased and the current crowding effect is reduced so as to enhance the illumination of the light emitting diode. However, in the electric property, since the materials on the uppermost layer of the transparent window layer
204
and the AlGaInP light generating region have a hetero junction, the energy gap difference causes the positive forward bias voltage V
f
of the light emitting diode to increase. As a result, the power consumption of the light emitting diode is increased.
U.S. Pat. Nos. 5.237,581 and 4,570,172 disclose a light emitting diode
300
with a multilayer reflecting structure, as shown in FIG.
3
. The structure of
FIG. 3
includes a semiconductor substrate
302
, a lower multilayer reflector
305
formed on the semiconductor substrate
302
, a light generating region
303
formed on the lower multilayer reflector
305
, an upper multilayer reflector
304
formed on the light generating region
303
, a first ohmic contact electrode
306
on the upper multilayer reflector
304
, and a second ohmic contact electrode
301
formed on the rear side of the semiconductor substrate
302
. By means of the semiconductor multilayer reflector, namely, a distributed Bragg reflector (DBR), the light transmitting to the substrate is reflected backwards so as to be emitted out of the light emitting diode. Accordingly, the light illumination is increased.
In this prior art light emitting diode, the lower multilayer reflector
305
serves to reflect 90% of the light emitted from the light generating region to the light absorption substrate, while the upper multilayer reflector serves to guide light to the upper surface of the light emitting diode. Therefore, the problem of light absorption by the substrate is alleviated, and the problem related to limited critical angle is also improved. However, since the multilayer reflector has many hetero junctions, the effect of energy gap difference is enlarged and hence the forward bias V
f
is increased. As a consequence, although the DBR structure disclosed in U.S. Pat. Nos. 5,237,581 and 4,570,172 can reflect the light impinging on the substrate, the DBR structure has a high reflectivity only for normal incident light (shown in D
1
of FIG.
3
). For oblique incident light (such as D
2
, D
3
, and D
4
shown in
FIG. 3
) the reflectivity is decreased. Thus the improvement to the illumination of a light emitting diode in visible light band is limited, whereas the DBR structure increases difficulty in growing the thin film epitaxial layer and thus the fabrication cost.
U.S. Pat. No. 5,376,580 discloses a light emitting diode with wafer bonding, wherein a gallium arsenide substrate serves as a temporary substrate to grow a light emitting diode structure including a confinement layer, an active layer and another confinement layer. Then the light emitting diode structure is adhered to a transparent substrate, and the GaAs substrate is removed. Therefore, the problem of light absorption by a substrate can be solved completely. Because the transparent substrate disclosed in the prior art is made of GaP, a thick GaP window layer is needed to handle thin epitaxial layers properly. Consequently, the LED device is relatively thin after the GaAs substrate is removed because of the thick window layer. The light emitting diode becomes easier to break and more difficult to fabricate. Furthermore, the wafer bonding needs to be processed in high temperature for a long period of time (about 600~700° C. for at least one hour), which results in degraded p-n junction and inferior diode performance.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a method for manufacturing a light emitting diode with a metal-coated reflective permanent substrate. The optical properties of this permanent substrate are not relevant because light is reflected before reaching the substrate. The reflective mirror on the permanent substrate is made of metal and employed as a bonding agent. An LED element is adhered to the permanent substrate with this reflective mirror. After being adhered, an etching agent is used to remove the GaAs substrate. Therefore, the problem of light absorption by the substrate is reduced, and the drawback of the p-n junction being affected by high temperature and long process duration is eliminated completely. The illumination can thus be greatly enhanced.
Another object of the present invention is to provide a light emitting diode structure combined with a metal-coated reflective permanent substrate. The light emitting region of the light emitting diode can be any conventional structure. For example, it may be a light emitting region with a double hetero structure including an upper cladding layer, an active layer and a lower cladding layer, a light emitting region with a single hetero structure, or a light emitting region of a homostructure. The permanent substrate with a metal-coated reflective mirror of the present invention can be applied to all kinds of conventional light emitting regions and thus it has wide applications.
A further object of the present invention is to provide a process for manufacturing a light emitting diode with a metal-coated reflective permanent substrate. The process comprises the steps of selecting a temporary substrate to grow a light emitting region and form an LED element; selecting a permanent substrate to bond the LED element to the permanent substrate by a reflective metal bonding agent, removing the temporary substrate adhered by the permanent substrate/LED element by mechanic grinding or chemical etching; fabricating a plane LED element with the permanent substrate, and forming ohmic contact electrodes on the plane LED element. According to the present invention, the illumination of a light emitting diode is enhanced.
It is also an object of
Chang Kuo-Hsiung
Horng Ray-Hua
Huang Man-Fang
Lin Kun-Chuan
Wei Sun-Chin
Christianson Keith
Visual Photonics Epitaxy Co., Ltd.
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