Radiant energy – Photocells; circuits and apparatus – Signal isolator
Reexamination Certificate
2002-07-02
2004-08-17
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Signal isolator
C257S080000, C438S025000
Reexamination Certificate
active
06777703
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating an optically coupled device having an assembly of a photoemitter that emits light of a predetermined level according to a flow of input current and a photodetector that receives the light from the photoemitter to provide output current.
2. Description of the Background Art
An optically coupled device is conventionally fabricated using a photoemitter and a photodetector. In the fabrication of an optically coupled device, there is variation in the characteristics of the photoemitter as well as in the photodetector. By the synergistic effect of the variation in both the characteristics of the photoemitter and the photodetector, the variation in the characteristics of the optically coupled device is significantly increased.
A plurality of photoemitters are fabricated from one semiconductor wafer. Also, a plurality of photodetectors are fabricated from one semiconductor wafer. The characteristics of the plurality of photoemitters are tested for every one wafer in preparation for the fabrication of an optically coupled device. Also, the characteristics of the plurality of photodetectors are tested for every one wafer in preparation for the fabrication of an optically coupled device.
Only a photoemitter whose variation range in characteristics is smaller than the entire variation permissible range of characteristics of a photoemitter based on the characteristic test of each wafer is used in the fabrication of an optically coupled device.
Also, only a photodetector whose variation range in characteristics is smaller than the entire variation permissible range of characteristics of a photodetector based on the characteristic test of each wafer is used in the fabrication of an optically coupled device.
Thus, measures are taken so that the variation range in characteristics of an optically coupled device is below a desired level as to optically coupled devices whose variation range in characteristics is increased by the synergistic effect of the variation in characteristics of both the photoemitter and the photodetector.
In the fabrication of a conventional optically coupled device, various processes such as the photoemitter and photodetector fabrication process, the photoemitter and photodetector test process, the optically coupled device assembly process (die-bonding, wire-bonding, precoating, molding and the like), the optically coupled device test process, and the packaging process are respectively controlled by an independent computer.
Due to the synergistic effect of variation in characteristics of a photoemitter and a photodetector, the variation in characteristics of an optically coupled device will become greater than the entire variation in characteristics of a photoemitter and the entire variation in characteristics of a photodetector.
In the case where a certain photoemitter and a certain photodetector are combined, there is a possibility of the assembled optically coupled device having a value of characteristics outside the permissible range despite each certain photoemitter unit and photodetector unit having a characteristic variation within the permissible range. In such a case, the yield of the optically coupled device is degraded.
Furthermore, in order to improve the yield of the optically coupled device, it is necessary to set the variation in the characteristics of the photoemitter unit and the photodetector unit to be with in an extremely small range taking into account the synergistic effect of the variation in characteristics of both the photoemitter and the photodetector.
However, if the respective variations in characteristics of the photoemitter unit and the photodetector unit are set to be within an extremely small range, there is a disadvantage that the respective yields of the photoemitter and the photodetector will be degraded.
A specific example of this disadvantage will be described based on a photocoupler including an infrared-emitting diode and i phototransistor.
Referring to
FIG. 7
, a photocoupler
1
has an infrared-emitting diode
2
die-bonded to a frame
4
, and a phototransistor
3
die-bonded to a frame
5
. Infrared-emitting diode
2
is molded by a precoat resin
6
.
Infrared-emitting diode
2
and phototransistor
3
are disposed facing each other. Frames
4
and
5
, infrared-emitting diode
2
and phototransistor
3
are molded by a first mold resin
7
and a second mold resin
8
.
When a current is input (referred to as “IF” hereinafter: unit [A]) to infrared-emitting diode
2
from a first circuit at the input side as shown in
FIG. 8
in photocoupler
1
, infrared light is output from infrared-emitting diode
2
.
This infrared light is received by phototransistor
3
. Upon receiving infrared light, phototransistor
3
conducts a flow of an output current (referred to as “IC” hereinafter: unit [A]) at a predetermined amplification factor (referred to as “hFE” hereinafter) to a second circuit at the output side.
By the above-described mechanism, photocoupler
1
can transmit an electrical signal from the first circuit to the second circuit with the first circuit at the input side insulated from the second circuit; at the output side. In photocoupler
1
, the ratio of IC to IF, i.e., IC/IF×100 is called the current transmission rate (referred to as “CTR” hereinafter: unit [%]).
When an electronic circuit is fabricated using photocoupler
1
, the circuitry must be designed taking into account the change in the CTR due to temperature and over time. The designing of circuitry employing photocoupler
1
will become easier as the range of change in CTR is smaller.
The CTR of photocoupler
1
is generally determined by the amount of light (quantity of light) arriving at phototransistor
3
(quantity of light referred to as “PO” hereinafter) among the light output from infrared-emitting diode
2
and the hFE of phototransistor
3
.
This means that the variation range of CTR is extremely increased due to the synergistic effect of the variation of PO and the variation of hFE. As a result, the yield of photocoupler
1
with respect to CTR will be degraded.
Infrared-emitting diode
2
is fabricated by epitaxial growth for every one batch formed of n wafers. However, the quantity of light of infrared-emitting diode
2
in each wafer of one batch is actually variable, as shown in FIG.
9
. This means that the variation in the PO distribution of each of the n wafers is not equal even in the case where epitaxial growth is conducted in the same one batch.
Furthermore, the actual quantity of light of infrared-emitting diode
2
per one batch varies as shown in FIG.
10
. The variation in the PO distribution per 1 batch becomes greater than the quantity of light variation of each wafer in one batch.
Therefore, the total variation of the PO distribution of infrared-emitting diode
2
that is used by the manufacturer of photocoupler
1
will become further greater than the quantity of light variation of each wafer in one batch and the quantity of light variation per one batch, as shown in FIG.
11
.
The same can be said for the fabrication of a phototransistor. The total variation in the hFE distribution shown in
FIG. 14
is greater than the hFE distribution variation of each wafer shown in FIG.
12
and the hFE distribution variation for one batch shown in FIG.
13
.
In the present state of affairs, the hFE range of the phototransistor can be specified during fabrication of a photocoupler. However, the PO range of the infrared-emitting diode cannot be specified.
Therefore, a photocoupler
1
having a combination of a phototransistor
3
of a large hFE and an infrared-emitting diode
2
of a large PO will exhibit an extremely large CTR. In contrast, a photocoupler
1
having a combination of a phototransistor
3
of a small hFE and an infrared-emitting diode
2
of a small PO will exhibit in an extremely small CTR.
By the synergistic effect of the variation in characteristics of an infrared-emitting diode and varia
Birch & Stewart Kolasch & Birch, LLP
Pyo Kevin
Sharp Kabushiki Kaisha
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