Metal fusion bonding – Process – Alternative bonding
Reexamination Certificate
2000-03-31
2001-10-09
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Alternative bonding
C228S255000, C228S179100
Reexamination Certificate
active
06299056
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a horizontal type light-emitting diode (referred to as an LED hereinafter) to be used as connected to an electric wiring board in a state in which its pn junction plane is perpendicular to the wiring surface of the electric wiring board, a method for manufacturing the LED and a method for connecting the LED to the electric wiring board.
A great many LEDs are used for self-light-emitting type display devices that can be seen clearly even in a dark place or as back illuminators of display devices. In particular, the LEDs are considered most promising as small-size thin type display devices and the back illuminators thereof for use in narrow spaces of letter display sections or number buttons of portable telephone units, finders of cameras and the like.
The LED chip itself has a small side dimension of not greater than 300 &mgr;m. In order to make good use of the properties of the LED, the prior art reference of Japanese Patent Laid-Open Publication No. SHO 57-49284 discloses a horizontal type LED that needs no connection of a metal wire and connects its pn junction plane perpendicular to the wiring surface of an electric wiring board.
FIG. 9
is a perspective view showing a state in which a conventional LED is mounted horizontally on an electric wiring board. In
FIG. 9
are shown an n-type semiconductor layer
401
, a p-type semiconductor layer
402
, a negative electrode
403
and a positive electrode
404
. The layers
401
through
404
constitute the conventional LED
400
. When connecting the LED
400
to an electric wiring board
408
, there have been the processes of temporarily fixing the LED
400
onto the electric wiring board
408
with an adhesive
410
that has an electrical insulating property, connecting the negative electrode
403
to a wiring section
409
a
of the electric wiring board
408
by means of solder
405
that serves as a brazing material and connecting the positive electrode
404
to a wiring section
409
b
of the electric wiring board
408
by means of solder
406
that serves as a brazing material. The adhesive
410
is an insulator, and if a pn junction plane
420
is covered with the adhesive
410
, then there is the effect of preventing the LED
400
from being damaged through short circuit in the soldering stage.
However, the LED
400
is formed by laminating the n-type semiconductor layer
401
and the p-type semiconductor layer
402
on a semiconductor wafer (not shown) that has a circular shape of a diameter of about 50 mm (or a rectangular shape of a side dimension of about 10 mm) and dicing the semiconductor wafer into squares of a size of about 300 &mgr;m.
Therefore, if the LED
400
is used in a horizontal posture, light is taken out from a side surface
407
of the LED
400
, the side surface
407
being brought in contact with a dicing blade. The n-type semiconductor layer
401
and the p-type semiconductor layer
402
of the LED
400
have high refractive indices. Therefore, light generated from the pn junction plane
420
does not go outward since it is reflected inside except for light roughly perpendicularly incident on the side surfaces
407
a,
407
b
and
407
c
that are the surfaces from which light is taken out.
The above phenomenon will be described with reference to FIG.
10
. As shown in
FIG. 10
, if light A generated in a portion that belongs to the pn junction plane
420
and is located apart from the side surface
407
is emitted in a direction inclined at an angle of not smaller than &PHgr; relative to the pn junction plane
420
, then the light is incident on the positive electrode
404
and disappears through dispersion, absorption and the like. For example, assuming that the pn junction plane
420
has a size D of 300 &mgr;m and the p-type semiconductor layer
402
being transparent to the emitted light has a thickness t of 2 &mgr;m, then the light that can go out of the side surface
407
without being incident on the positive electrode
404
is only the light that is emitted at an angle of not greater than 0.4° relative to the pn junction plane
420
.
Conversely, the light that is not incident on the positive electrode
404
even when inclined at an angle &thgr; of 10° relative to the pn junction plane
420
is the light generated at a point that belongs to the pn junction plane
420
and located within a distance d of not greater than about 10 &mgr;m from the side surface
407
. The light emitted at the inclination angle of 10° relative to the pn junction plane
420
is to be inclined at an angle of about 35° relative to the junction plane when radiated outward as refracted at the side surface
407
, the angle of about 35° being approximately equal to the angle of radiation of the LED
400
. In other words, it can be appreciated that the light can be efficiently taken out when it is generated at a depth d of not greater than about 10 &mgr;m from the side surface
407
.
On the other hand, the light emission efficiency of the pn junction plane
420
in the vicinity of the side surface
407
of the LED
400
is lowered due to mechanical damage in the dicing stage. For this reason, if the conventional LED is used in the horizontal posture, then the light emission efficiency is apparently reduced. Therefore, it is proper to etch the portion in the vicinity of the mechanically damaged side surface
407
by immersing the semiconductor in an acid liquid that dissolves the semiconductor. However, there has been the problem that the negative electrode
403
and the positive electrode
404
are also dissolved for the reasons described later.
If the LED is used in the horizontal posture, both the positive electrode and the negative electrode are connected to the wiring sections of the electric wiring board by means of silver paste or solder. Particularly in the case of an LED to be used in a place that may have an elevated temperature, the solder should preferably be used for the connection.
When performing connection by means of solder, the positive electrode
404
and the negative electrode
403
should preferably be provided by a metal such as gold (Au) that has a good affinity with solder and is not eroded by acid. However, it is also well known that, if the positive electrode
404
and the negative electrode
403
are made of only Au, the metal is totally dissolved in the solder, resulting in failed soldering. For this reason, it is a usual practice to employ nickel (Ni) that has a good affinity with solder and cover its surface with Au for the prevention of the oxidation of Ni. However, Ni is eroded by acid. Accordingly, there has been the problem that Ni exposed on the side surfaces is dissolved by the acid when the wafer is divided into small pieces of individual LEDs by dicing and the side surfaces are etched by being immersed in an acid liquid and then falls off together with Au located on Ni.
As shown in
FIG. 9
, in order to prevent the n-type semiconductor layer
401
and the p-type semiconductor layer
402
of the LED
400
from being damaged through short circuit when connecting the wiring sections
409
a
and
409
b
with the positive electrode
404
and the negative electrode
403
of the LED
400
with solders
406
and
405
, respectively, the pn junction plane
420
is covered with the adhesive
410
that is an insulator. However, if the quantity of adhesive
410
is small, the portion that belongs to the pn junction plane
420
and is exposed on the side surface
407
cannot be completely covered, frequently causing damage and short circuit of the LED
400
. The pn junction plane
420
of the LED
400
is normally located several micrometers to several tens of micrometers apart from the surface of the negative electrode
403
or the positive electrode
404
. Therefore, if the quantity of adhesive
410
is increased so as to ensure the covering, then the adhesive
410
adheres to the wiring section
409
a
or the wiring section
409
b,
resulting in the failed adhesion of the brazing material. This also leads to the problem that the pn junct
Dunn Tom
Johnson Jonathan
Nixon & Vanderhye PC
Sharp Kabushiki Kaisha
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