Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-08-18
2004-01-13
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06677970
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a light-emitting diode array and an optical print head provided with such a light-emitting diode array.
BACKGROUND ART
Conventionally, light-emitting diodes (LEDs) are widely used as display devices for their advantages of emitting clear light, operating from low voltages, needing simple peripheral circuits, and for other reasons.
Nowadays, optical print heads employing light-emitting diode arrays are much studied for the purpose of finding their practical applications as light sources in optical printers or the like based on electrophotography.
In an optical printer employing as a light source a light-emitting diode array, which emits light by itself, the individual dots of the light-emitting diode array are made to emit light in accordance with an image signal, and the thus emitted light is shone, through a unit-magnification imaging device such as a gradient-index lens, onto a photoconductive drum to form an electrostatic latent image thereon. Then, toner is selectively deposited on the photoconductive drum by a developer unit, and the thus deposited toner is transferred onto plane paper or the like, thereby achieving printing.
A print head employing a light-emitting diode array offers advantages of (1) being free of movable parts and composed of a small number of components, and thus capable of being made compact, and (2) permitting as many array chips as required to be connected together so as to be readily formed to have a desired total length.
In a light-emitting diode array having a light-emitting region density of, for example, 600 dpi (dots per inch), the light-emitting regions are arranged with a pitch of 42 to 43 &mgr;m. Thus, considering the width of non-light-emitting regions secured between the light-emitting regions, the width of the light-emitting regions themselves is smaller than that pitch. On the light-emitting regions, electrodes each having a width smaller than 42 to 43 &mgr;m are formed with a pitch of 42 to 43 &mgr;m. These electrodes formed on the light-emitting regions are connected to wiring patterns that are formed to be so fine as to have a width smaller than 42 to 43 &mgr;m each and arranged with a pitch of 42 to 43 &mgr;m or smaller. These wiring patterns are in turn connected to wider bonding electrodes (pads). This light-emitting diode array, having electrodes and wiring formed as described above, needs to be electrically connected, by wire bonding or the like, to, for example, a driving device for driving the light-emitting diode array.
Even stitch bonding, which permits bonding with the smallest width at present, requires a bonding width of 40 &mgr;m, to which it is inevitable to add a further 20 &mgr;m considering the positioning accuracy of bonding equipment. That is, as the width of a bonding electrode, it is necessary to secure about 60 &mgr;m at least.
Accordingly, in a high-density light-emitting diode array having a density of 600 dpi or higher, arranging bonding electrodes in a single row makes wire bonding difficult; for this reason, it is customary to arrange them in a zigzag in two rows to secure a greater electrode pitch. Even then, however, it is possible to secure 60 &mgr;m at best as the width of a bonding electrode. This has recently been making extremely difficult to electrically connect a light-emitting diode array and a driving device for it together by wire bonding.
Conventional light-emitting diodes and light-emitting diode arrays of this type designed for use in optical printers are disclosed, for example, in Japanese Utility Model Application Published No. H7-36754 and Japanese Patent Applications Laid-Open Nos. H5-347430 and H5-155063.
FIG. 4
is a plan view of the light-emitting diode array for use in an optical printer disclosed in the above-mentioned Japanese Utility Model and Patent Applications, and
FIG. 5
is a sectional view taken along line A-A′ shown in FIG.
4
.
In these figures, reference numeral
1
represents a compound semiconductor, made of GaP, GaAsP, GaAlAs, GaAs, or the like, that has a plurality of light-emitting regions
11
formed on the top surface thereof by selective diffusion so as to be arranged in a single row or in a zigzag in two rows. Reference numerals
2
,
3
, and
4
represent insulating films, made of Si
3
N
4
, SiO
2
, AlO
3
, or the like, that are laid over one another on the top surface of the compound semiconductor
1
. These insulating layers
2
,
3
, and
4
are laid in a plurality of layers to serve as the selectively diffused film of the light-emitting regions, as a protective film for protecting the top surface of the compound semiconductor
1
, as a pinhole-prevention film, as a wiring-reinforcement/primary-coating film, and as a light-extraction/brightness-adjustment film. Reference numeral
5
represents an electrode layer, made of Al or the like, that is laid over the insulating films
2
and
3
so as to have ohmic contacts with the light-emitting regions and that serves as electrodes connected to the light-emitting regions, as bonding electrodes, and as wiring patterns that connect those electrodes together. Reference numeral
6
represents a common electrode, made of Au or the like, that is laid on the bottom surface of the compound semiconductor
1
.
Here, in the case of a high-density light-emitting diode array having a density of 600 dpi or higher, applying wire bonding directly to the bonding electrodes formed on the top surface of the light-emitting diode array causes the following problems.
To obtain a higher wiring density, it is necessary to reduce the width of the wiring patterns that are connected to the bonding electrodes and reduce the intervals between adjacent wiring patterns. However, if they are reduced extremely, when the wiring patterns are deformed by external force that may be applied thereto during the array cleaning or head assembling process, there is the risk of deformed wiring patterns readily making contact with the adjacent wiring patterns. To prevent this, it is necessary to increase the thickness of the protective films that are formed on the individual wiring patterns to prevent corrosion and the like. For example, in a light-emitting diode array having a density of about 300 dpi, forming 0.2 &mgr;m thick inorganic protective films (made of SiO
2
, SiN, or the like) will suffice; by contrast, in a high-density light-emitting diode array having a density of 600 dpi or higher, such thin films do not provide sufficient protection, and thus a material of the polyimide family is tentatively used as the insulating film
4
.
FIGS. 6
to
8
are diagrams illustrating how wire bonding is performed on a light-emitting diode array of which the insulating film
4
is made of, for example, a material of the polyimide family.
As shown in
FIG. 6
, when the insulating film
4
is made of, for example, a material of the polyimide family, first, a thick-film solution of a material of the polyimide family is spun on the electrode layer
5
to form the insulating film
4
serving as a protective film, and the upper portion of the insulating film
4
is patterned by etching to form openings
4
a
that permit connection of the wiring patterns. These openings
4
a
are shaped, for example, as indicated by hatching in
FIG. 7
, so that comparatively wide connection regions
5
a
are formed on the electrode layer
5
.
Then, as shown in
FIG. 8
, ball bonding is performed on the connection regions
5
a
of the electrode layer
5
by using a bonding wire
13
made of gold or the like and dispensed through a capillary
12
, and then the thus ball-bonded connection regions
5
a
are individually connected to the output terminals of the driving device by the bonding wire
13
. Alternatively, first, ball bonding is performed on the output terminals of the driving device by using the bonding wire
13
, and then the thus ball-bonded output terminals of the driving device are individually connected to the connection regions
5
a
. In this case, there exist no ball-like portions on the connection regions
5
a
that
Arent Fox Kintner & Plotkin & Kahn, PLLC
Pham Hai
Sanyo Electric Co,. Ltd.
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