Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
1997-05-30
2004-05-11
Wille, Douglas (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S094000
Reexamination Certificate
active
06734468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction, a Group III nitride compound semiconductor device having this type of electrode pads, and methods of forming these electrode pads. In particular, the invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction with improved adhesive strength and luminous efficiency. Further, the invention relates to a method for forming the electrode pads and preventing the side etching of a protection film when forming a window through the protection film by etching.
2. Description of Background Information
Conventionally, gold (Au) deposited on a surface of a p-type conductive gallium nitride (GaN) layer is used as an electrode. Because the adhesive strength between the Au layer and the GaN layer is weak, an electrode layer made of Au peels easily away from the GaN layer during alloying processes performed on the GaN layer.
As a means to improve the adhesive strength, an electrode with a double layer structure is employed interposing a nickel (Ni) layer between the Au electrode layer and the GaN layer.
The electrode made of the Ni layer and the Au layer formed thereon also functions as an electrode pad for wire bonding. As shown in
FIG. 5A
, the semiconductor device is uniformly covered with a protection layer
20
to protect the surface of the device. As shown in
FIGS. 5A and 5B
, mask layers
21
are formed exposing some part of the protection layer
20
over the Au layer. The exposed portion of the protection layer
20
is removed by etching to form a window
20
a
. A lead wire is bounded to the electrode pad of the window
20
a.
However, because the adhesive strength between the Au layer and the protection layer
20
is weak, etching liquid penetrates between them. Thus, some part of the protection layer
20
, even a part which is under the mask layers
21
are removed as shown in FIG.
5
B. As a result, forming the window
20
a
as planned is difficult.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to obtain an electrode pad strongly adhered to respective the Group III nitride compound semiconductor and the protection film.
Another object of the present invention is to improve ohmic characteristic of the electrode layer. The ohmic characteristic is defined as figure of volt-ampere (VI) characteristic. Improved ohmic characteristic includes fine linearity of VI characteristic and small contact resistance.
Further object of the present invention is to improve adhesive strength between the electrode layer and the Group III nitride compound semiconductor.
Further object of the present invention is to improve luminous intensity achieved by improvement in effective current density.
The Group III nitride compound semiconductor satisfies the formula Al
x
Ga
y
In
1−x−y
N, wherein 0≦x≦1, 0≦y≦1, and 0≦x+y≦1.
A first aspect of the present invention is directed to an electrode pad for a Group III nitride compound semiconductor having a p-type conduction. The electrode pad successively includes a first metal layer formed on one of the semiconductor layer and an electrode layer, a second metal layer formed on the first metal layer, and a third metal layer formed on the second metal layer. A protection film is formed covering over the surface of the device and exposing a central portion of the third metal layer. The second metal layer is made of gold (Au). A composite element of the first metal layer has an ionization potential lower than gold (Au), and a composite element of the third metal layer has adhesiveness to the protection film stronger than gold (Au).
The composite element of the first metal layer should preferably be at least one of nickel (Ni), Iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta), vanadium (V), manganese (Mn), aluminum (Al), and silver (Ag). The composite element of the third metal layer should preferably be at least one of aluminum (Al), nickel (Ni), and titanium (Ti). Further, the protection film should preferably be made of silicon oxide eg., SiO, SiO
2
, and Si
2
O
S3
or silicon nitride e.g., SiN
2
and Si
3
N
4
. The most preferable combination of the composite metals for the electrode pad is the first metal layer comprising nickel (Ni), the second metal layer comprising gold (Au), and the third metal layer comprising aluminum (Al).
The electrode pad with the above identified structure may be formed directly on the semiconductor layer or on the electrode layer formed on the semiconductor layer depending on purposes. An LED, for example, requires an electric current to flow through a semiconductor layer vertically and uniformly for increasing an emission area. In this case, an electrode layer formed on the semiconductor layer is fairly wider than the electrode pad.
In order to improve an ohmic characteristic including lowering contact resistance, and Increase the adhesive strength between the electrode layer and the semiconductor layer, the electrode layer should preferably have a multi-layer structure at least comprising a first electrode layer formed on the semiconductor layer and a second electrode layer formed on the first electrode layer. The first electrode layer comprises an element that has an ionization potential that is lower than that of the second electrode layer and the second electrode layer comprises an element which has an ohmic characteristic to the semiconductor layer better than that of the first electrode layer. Heat treatment for alloying process of the semiconductor makes the element of the second electrode layer distributed more deeply into the semiconductor layer than that of the first electrode layer. Namely, the order of distribution of the elements is reversed by heat treatment. Before heat treatment, the element of the second electrode layer exists on that of the first electrode layer. After heat treatment, however, the element of the second electrode layer exists below that of the first electrode layer. The discovery of this phenomena is another aspect of the present invention.
The reversal of element distribution, however, does not occur at some portion of the electrode layer which is covered with an electrode pad. Accordingly, the portion of the electrode layer under the electrode pad has a poor ohmic characteristic i.e., a high contact resistance, and thus, flow of electric current detours around this portion This structure contributes to an improved luminous efficiency. Since emitted light under non-transparent electrode pad cannot be emitted outside, utilizing this structure enables the electric current to flow to effective portions of a device so that emitted light can extend outside. Consequently, enlarging effective current density improves luminous efficiency. In view of the improved luminous efficiency, the protection film is not necessary and the electrode pad may be formed In a double or a single layer structure.
The first electrode layer should preferably be made of at least one of nickel (Ni), iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta), vanadium (V), manganese (Mn), aluminum (Al), and silver (Ag), and the second electrode layer be at least one of palladium (Pd), gold (Au), iridium (Ir), and platinum (Pt). The most preferable combination of the composite metals of the electrode layer is the first electrode layer comprising nickel (Ni) and the second electrode layer comprising gold (AU). In this case, heat treatment reverses the relative positions of Ni and Au. Gold (Au) moves deeper with respect to the semiconductor layer than nickel (Ni). Heat treatment may preferably be carried out at a temperature generally ranging from 400° C. to 700° C.
As another aspect of the present invention, the Group III nitride compound semiconductor device using the above described electrode pads includes one of a light-emitting diode (LED), a laser diode (LD), and a transistor.
Because the composite metal of the first metal layer has an ionization pote
Horiuchi Shigemi
Noiri Shizuyo
Shibata Naoki
Uemura Toshiya
McGinn & Gibb PLLC
Toyoda Gosei Co,., Ltd.
Wille Douglas
LandOfFree
Devices related to electrode pads for p-type group III... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Devices related to electrode pads for p-type group III..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Devices related to electrode pads for p-type group III... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3269625