Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2000-04-06
2001-12-11
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
Reexamination Certificate
active
06330014
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermal head, particularly relates to a thermal head wherein a conductive layer electrically connected to a common electrode is formed on a heat insulating layer and a heater element is laminated on the conductive layer via a layer insulating layer.
2. Description of the Prior Art
Heretofore, a general thermal head as a recording head mounted in a thermal printer and others records on thermosensitive recording paper in colors by arraying plural heater elements composed of a heating resistor in one or plural columns on a substrate and selectively energizing and heating each heater element according to record information or prints on various record media by melting ink of an ink ribbon and transferring it on a record medium such as plain paper and paper for an overhead projector (OHP).
Such a conventional type thermal head is provided with a substrate
124
made of a ceramic or silicon and others and having an insulating layer as shown in
FIGS. 24 and 25
, and a convex portion
124
a
the section of which is approximately trapezoidal is formed in the vicinity of the right end of the upper surface of the substrate
124
shown in FIG.
25
.
Also, a heat insulating layer
125
is formed on the upper surface of the substrate
124
and plural heater elements
126
formed in a desired pattern by executing plasma etching by gas acquired by mixing fluoric gas and gaseous oxygen after a heating resistor made of cermet such as Ta—SiO
2
is formed by sputtering and others and hereby, arrayed in a row according to the number of dots corresponding to desired resolution are provided on the upper surface of the heat insulating layer
125
on the convex portion
124
a.
Further, a common electrode
127
is formed on the right side of each heater element
126
shown in FIG.
25
and is formed in a U shape along the outside margin of a thermal head
123
as shown in FIG.
24
.
A common electrode terminal
127
a
is formed at two ends of the right and left outside margins of the common electrode
127
. A terminal plating layer
127
b
is applied to the upper surface of the common electrode terminal
127
a.
In the meantime, plural separate electrodes
128
for independently energizing each heater element
126
are patterned above the left side of the heater element
126
shown in
FIG. 25. A
connecting electrode
129
on the separate electrode side for electrically connecting the separate electrode
128
and the heater element
126
is formed between the separate electrode
128
and the heater element
126
. The left end of the separate electrode
128
shown in
FIG. 25
is equivalent to a separate electrode terminal
128
a
and a terminal plating layer
128
b
is applied to the upper surface of the separate electrode terminal
128
a.
The common electrode
127
and the separate electrode
128
are made of Al or Cu and others, are formed in a predetermined pattern by etching and others after Al or Cu is deposited on the upper surface of the heater element
126
and the heat insulating layer
125
by sputtering and others until Al or Cu is approximately 2 &mgr;m thick, and a part between the common electrode
127
and the separate electrode
128
of each heater element
126
is equivalent to a heating part
126
a.
Also, a passivation layer
130
for preventing oxidation and abrasion is formed on/over the upper surface of the heat insulating layer
125
, the heater element
126
, the common electrode
127
and the separate electrode
128
. The passivation layer
130
is made of SIALON (Si—Al—O—N compound solid solution) excellent in oxidation resistance and abrasion resistance and is laminated by sputtering so that any upper surface except the separate electrode terminal
128
a
and the common electrode terminal
127
a
is coated.
In the conventional type thermal head
123
having configuration described above, predetermined recording is performed by selectively heating the heater element
126
by selectively energizing between the common electrode
127
and the separate electrode
128
, melting ink of an ink ribbon touched to the part and transferring it on recording paper or directly coloring thermosensitive recording paper.
Also, in the conventional type thermal head
123
, as described above, the convex heat insulating layer
125
is provided near to the end of the substrate
124
with the layer biased, the heating part
126
a
is formed on the convex heat insulating layer
125
and the quality of recording is enhanced by using such a thermal head
123
.
However, the conventional type thermal head
123
has a problem that as the common electrode
127
is extremely elongated in a narrow part between the end of the substrate
124
and the heater element
126
because the respective heater elements
126
are formed near the end of the substrate
124
as shown in
FIG. 24
, the resistance value of the electrode is increased even if the common electrode
127
is made of copper or aluminum, so-called voltage drop occurs in the common electrode
127
and dispersion occurs in exoergic temperature among the heater elements
126
.
Further, the length of an energizing path from the heater element
126
to the common electrode terminal
127
a
differs every heater element
126
and the heater element
126
near the center of the row is the farthest from the common electrode terminal
127
a
. Therefore, the quantity of voltage drop in the common electrode
127
connected to the heater element
126
near the center of the row is increased and the exoergic temperature of the heater element
126
near the center of the row sometimes lowers.
In thermal imprint printing for example when voltage drop occurs in such a thermal head and the exoergic temperature of the heater element
126
lowers, there is a problem that the quality of printing is deteriorated such as the unevenness of density in printing is increased.
For a countermeasure against voltage drop, it is conceivable to reduce the quantity of voltage drop by thickening the thickness of the common electrode
127
, however, when the thickness of the common electrode
127
is thickened, it becomes difficult to laminate the passivation layer
130
on the common electrode
127
, in addition, in printing, the pressure of the thermal head
123
and a platen of a printer not shown concentrates at the heating part
126
a
and the common electrode
127
adjacent to the heating part
126
a
and large shearing stress is applied to the common electrode
127
.
Therefore, there is a defect that material fatigue (metal fatigue) occurs in the common electrode
127
made of soft metallic material such as Al and Cu, the passivation layer
130
is peeled or destroyed early, disconnection due to abrasion and corrosion occurs in the common electrode
127
and the life is short.
Also, voltage drop is inhibited and the difference of exoergic temperature in the position of each heater element
126
can be removed by using a well-known thermal head called an alternate lead head adjacent to the row of each heater element
126
and provided with plural adjacent common electrodes, however, in this case, there is also a defect that as the adjacent common electrode is formed between adjacent heater elements, a dimension between heater elements is increased, a dimension between dots of a printed image is increased and high definition printing is difficult.
SUMMARY OF THE INVENTION
The object of the invention is to provide a thermal head wherein voltage drop in a common electrode of the thermal head is properly corrected, a separate heater element is heated at uniform temperature and a satisfactory image free of unevenness in printing density can be acquired.
That is, the thermal head according to the invention is characterized in that it is provided with a heat insulating layer formed on the upper surface of a substrate, plural heater elements formed in a row over the heat insulating layer, plural separate electrodes respectively electrically connected to one and the other of the plural heater el
Arauchi Susumu
Terao Hirotoshi
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Tran Huan
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