Method of making auxiliary electrode layer for common electrode pattern in thermal printhead

The present invention provides a method of making an auxiliary electrode layer for a common electrode pattern in a thermal printhead. The method of the present invention includes the steps of: preparing a master substrate (1') which has an obverse surface provided with a common electrode pattern (4) and corresponds to a plurality of head substrates; forming at least one slit (9) in the master substrate (1') where the slit extends along the common electrode pattern (4); and forming an auxiliary electrode layer (6) on a reverse surface of the master substrate (1') so that the auxiliary electrode layer (6) extends via the slit (9) for electrical connection to the common electrode pattern (4). The slit has a width of no less than 0.5 mm for example, particularly no less than 0.8 mm for controlling to provide a proper turnover (R) of the auxiliary electrode layer (6).

TECHNICAL FIELD 
The present invention relates to a method of making an auxiliary electrode 
layer for a common electrode pattern in a thermal printhead. 
BACKGROUND ART 
Thermal printheads have been widely used for a printer of an office 
automation apparatus such as a facsimile machine, a printer of a ticket 
vending machine and a label printer. As is commonly known, a thermal 
printhead selectively provides heat to a printing medium such as 
thermosensitive paper or thermal-transfer ink ribbon to form needed image 
information. 
Thermal printheads are divided mainly into thin film-type thermal 
printheads and thick film-type thermal printheads, depending upon methods 
of forming their heating resistors (heating dots) and electrode conductor 
layers for example. In a thin film-type thermal printhead, a heating 
resistor and an electrode conductor layer are made in the form of a thin 
film on a substrate or a glass glaze layer by sputtering for example. On 
the other hand, in a thick film-type thermal printhead, at least the 
heating resistor is made in the form of a thick film through such steps as 
screen printing and sintering. 
In general, for thermal printheads, a series of linear heating dots are 
formed preferably adjacent to a longitudinal edge of the head substrate. 
This is because the arrangement of disposing the series of heating dots 
adjacent to a longitudinal edge of the head substrate advantageously makes 
it possible to avoid interference with the printing medium as well as to 
increase degrees of positioning freedom and improve printing quality by 
holding the head substrate relative to the platen at a certain angle. 
However, when the series of heating dots are adjacent to a longitudinal 
edge of the head substrate, space for formation of the common electrode 
pattern is correspondingly reduced, thereby failing to provide a 
sufficient current capacity (current passage) necessary for heat 
generation. As a result, the resistance of the common electrode pattern 
will cause a problem of irregular heat generation at the heating dots due 
to a voltage drop along the series of heating dots, which results in 
deterioration of printing quality. Particularly for color printing, which 
has been coming into wider use recently, it is highly required to ensure a 
large current capacity since all of the heating dots are frequently heated 
simultaneously to perform so-called "solid printing." 
To meet such a requirement, International Publication WO 95/32867 discloses 
a thermal printhead with the arrangement shown in FIGS. 5 and 6 of the 
attached drawings of the present application. (Note that the above 
international publication was published on Dec. 7, 1995, which is later 
than the priority date of the present application, Jun. 13, 1995, so that 
the international publication is not to be regarded as prior art against 
the present application.) The above-mentioned thermal printhead will be 
described below. 
The thermal printhead illustrated in FIGS. 5 and 6 includes a head 
substrate 11 of an insulating material such as alumina-ceramic for 
example. The head substrate 11, which is rectangular in cross section, 
includes an obverse surface 11a, a reverse surface 11b opposite to the 
obverse surface 11a, a first longitudinal edge surface 11c and a second 
longitudinal edge surface 11d opposite to the first longitudinal edge 
surface 11c. The obverse surface 11a of the head substrate 11 is formed 
with a glass glaze layer 12 as a heat reservoir. The glass glaze layer 12 
includes a convex portion 12a, adjacent to the first longitudinal edge 
surface 11c of the head substrate 11, which has a curved cross section. 
The surface of the glaze layer 12 is formed with a thin film of a resistor 
layer 13. The resistor layer 13 is divided by slits S (see FIG. 6) at a 
predetermined pitch to extend transversely of the head substrate 11 (that 
is, perpendicularly of the longitudinal edge surfaces 11c, 11d of the head 
substrate 11). 
The surface of the resistor layer 13 is formed with a common electrode 
pattern 14 adjacent to the first longitudinal edge surface 11c of the head 
substrate 11, and individual electrodes 15 which are spaced from the 
common electrode pattern 14 and extend from the convex portion 12a of the 
glaze layer 12 toward the second longitudinal edge surface 11d of the head 
substrate 11. 
The slits S extending to the common electrode pattern 14 electrically 
insulate the individual electrodes 15 from each other. 
As described above, the individual electrodes 15 are spaced from the common 
electrode pattern 14. Thus, the resistor layer 13 is exposed between the 
common electrode pattern 14 and the individual electrodes 15, and the 
exposed portions function as heating dots (heating regions) 13a linearly 
extending along the first longitudinal edge surface 11c of the head 
substrate 11. 
The heating regions (heating dots) 13a of the resistor layer 13, the common 
electrode pattern 14 and the individual electrodes 15 are covered with a 
protecting layer 20. Due to the protecting layer 20, the common electrode 
pattern 14 and the individual electrodes 15 are prevented from getting 
oxidized through contact with the air and worn out through contact with a 
printing medium (not shown). 
The common electrode pattern 14 is electrically connected, at an end closer 
to the first longitudinal edge surface 11c of the head substrate 11, to an 
auxiliary electrode layer 16 made of a metal such as aluminum for example. 
Thus, every portion of the common electrode pattern 14 is electrically 
connected to each other via the auxiliary electrode layer 16, thereby 
being kept at a same electrical potential. In other words, the auxiliary 
electrode layer 16 functions as a member commonly connecting all portions 
of the common electrode pattern 14. 
The auxiliary electrode layer 16 covers the first longitudinal edge surface 
11c of the head substrate 11, the reverse surface 11b and the second 
longitudinal edge surface 11d. Thus, the auxiliary electrode layer 16 has 
a large area to allow increased electrical passage so that the voltage 
drop which might otherwise be caused longitudinally of the thermal 
printhead is substantially eliminated. As a result, a large amount of 
current is provided even for an instance where all heating dots 13a are 
simultaneously heated (that is, even for solid printing), thereby 
preventing deterioration of printing quality. 
The thermal printhead with the above arrangement may be formed by a method 
illustrated by FIGS. 7a-7j for example. 
First, as shown in FIG. 7a, an alumina-ceramic master substrate 11' 
dimensionally corresponding to a plurality of head substrates is prepared. 
The master substrate 11' will be divided later along longitudinal division 
lines DL1 and transverse division lines DL2 to provide the plurality of 
head substrates. 
Then, as shown in FIG. 7b, a master glaze layer 121 is formed by sintering 
a glass paste which is applied over the master substrate 11'. 
Then, as shown in FIG. 7c, a groove 17 extending into the thickness of the 
master substrate 11' is formed by using a dicing cutter (not shown) which 
cuts through the master glaze layer 12' along a predetermined longitudinal 
division line DL1. Thus, the master glaze layer 12' is divided into 
separate glaze layers 12. 
Then, as shown in FIG. 7d, the glaze layer 12 is formed with a convex 
portion 12a adjacent to the groove 17 by heating the master substrate 11' 
at a temperature of about 850.degree. C. for about 20 minutes. The 
formation of the convex portion 12a is realized under the influence of the 
surface tension of the glass material which is in a liquidized state by 
the heating. 
Then, as shown in FIG. 7e, a resistor layer 13 including tantalum nitride 
as the main component is made in the form of a thin film over the glaze 
layer 12 by reactive sputtering. 
Then, as shown in FIG. 7f, a conductive layer 18 of e.g. aluminum is formed 
on the resistor layer 13 by sputtering. 
Then, as shown in FIG. 7g, after forming the slits S (see FIG. 6) by 
etching the resistor layer 13 and the conductive layer 18, only the 
conductor layer 18 is partially removed by etching for exposure of 
portions of the resistor layer 13 to form heating dots 13a. Thus, the 
conductor layer 18 is divided into the common electrode pattern 14 and the 
individual electrodes 15. 
Then, as shown in FIG. 7h, the master substrate 11' is divided by a dicing 
cutter (not shown) along the respective division lines DL1, DL2 to provide 
separate head substrates 11. 
Then, as shown in FIG. 7i, while each head substrate 11 is being moved in 
the direction of arrow X, conductive metal is sputtered from below to be 
fixed on the first longitudinal edge surface 11c, the reverse surface 11b 
and the second longitudinal edge surface 11d for forming a proper 
thickness of the auxiliary electrode layer 16 made of aluminum for 
example. 
Finally, as shown in FIG. 7j, a protecting layer 20 is formed to cover the 
common electrode pattern 14, the individual electrodes 15 and the heating 
dots 13a or the exposed regions of the resistor layer 13. 
In the method described above, the formation of the auxiliary electrode 
layer 16 is performed after the division of the master substrate 11' into 
the separate head substrates 11 (see FIGS. 7h and 7i). However, it has 
been found that the following problems will occur by the method of making 
the auxiliary electrode layer 16 described above. 
First, since the auxiliary electrode layers 16 are formed after the master 
substrate 11' is divided into the plurality of separate head substrates 
11, specially adjusted magazines and tools are needed to deal with the 
plurality of head substrates 11 individually, which results in a larger 
equipment cost. Further, the process of making an auxiliary electrode 
layer 16 individually for each of a plurality of head substrates 11 will 
make the production rate low. Such a factor, together with the increased 
equipment cost, will increase the production cost. 
Secondly, when the auxiliary electrode layer 16 is formed for each separate 
head substrate 11, the conductive metal to be sputtered can easily reach 
the obverse surface of the head substrate 11, going beyond the common 
electrode pattern, and may further extend to the heating dots 13a or the 
exposed portions of the resistor layer 13. As a result, the auxiliary 
electrode layer 16 partially or wholly covers the heating dots 13a to 
prevent the heat generation at the heating dots 13a. 
Thirdly, when the auxiliary electrode layers 16 are formed after dividing 
the master substrate 11' into the plurality of separate head substrates 
11, apparatus for carrying and supporting the separate head substrates 11 
will come into direct contact with the head substrates 11, thereby 
possibly causing a secondary damage to the obtained thermal printheads. On 
the other hand, when the master substrate 11' is not divided, the carrying 
and supporting of the master substrate 11' can be performed with the use 
of the marginal portions thereof. Thus, there are much less possibilities 
of causing a damage to the head substrates 11 which will be separated 
afterward. 
DISCLOSURE OF THE INVENTION 
Therefore, the object of the present invention is to provide a method by 
which an auxiliary electrode layer for a common electrode pattern is 
formed effectively and at a low cost for respective thermal printheads, 
while providing an easier control of the condition of electrical 
connection between the common electrode pattern and the auxiliary 
electrode layer. 
To attain the above object, the present invention provides a method of 
making an auxiliary electrode layer for a common electrode pattern in a 
thermal printhead. The method includes the steps of: 
preparing a master substrate which has an obverse surface provided with a 
common electrode pattern and corresponds to a plurality of head 
substrates; 
forming at least one slit in the master substrate, the slit extending along 
the common electrode pattern; and 
forming an auxiliary electrode layer on a reverse surface of the master 
substrate so that the auxiliary electrode layer extends via the slit into 
electrical connection to the common electrode pattern. 
The slit may preferably have a width of no less than 0.5 mm or particularly 
no less than 0.8 mm for providing good electrical connection between the 
auxiliary electrode layer and the common electrode pattern. 
According to a preferred embodiment of the present invention, the master 
substrate may have at least one groove extending along the common 
electrode pattern and the common electrode pattern extends into the 
groove. A step portion is provided by forming the slit in the groove so 
that the slit is smaller in width than the groove. The auxiliary electrode 
layer is arranged to extend onto the step portion into electrical 
connection to the common electrode pattern. 
Other objects, features and advantages of the present invention will be 
clearer from the detailed explanation of the embodiment described below 
with reference to the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
A preferred embodiment of the present invention will be described below 
with reference to the accompanying drawings. 
FIGS. 1 and 2 show an example of thermal printhead made by the method 
according to the present invention. The thermal printhead includes an 
elongate head substrate 1 which is made of an insulating material such as 
alumina-ceramic for example and has a thickness of about 0.6-0.7 mm for 
example. The head substrate 1 has a generally rectangular cross section 
and includes an obverse surface 1a, a reverse surface 1b opposite to the 
surface 1a, a first longitudinal edge surface 1c and a second longitudinal 
edge surface (not shown) opposite to the first longitudinal edge surface 
1c. 
The obverse surface la of the head substrate 1 is formed with a glass glaze 
layer 2 as a heat reservoir with a thickness of about 100 .mu.m. The glaze 
layer 2 includes a curved edge portion 2a adjacent to the first 
longitudinal edge surface 1c of the head substrate 1. 
The surface of the glaze layer 2 is formed with a thin film of resistor 
layer 3. The resistor layer 3 is divided into individual strips which are 
mutually spaced by slits S (see FIG. 2) at a predetermined pitch and 
extend transversely of the head substrate 1 (that is, perpendicularly to 
the first longitudinal edge surface 1c of the head substrate 1). 
The surface of the resistor layer 3 is formed with a common electrode 
pattern 4 adjacent to the first longitudinal edge surface 1c of the head 
substrate 1 and also with individual electrodes 5 which are spaced from 
the common electrode pattern 4 and extend from the curved edge portion 2a 
of the glaze layer 2 toward the second longitudinal edge surface (not 
shown) of the head substrate 1. The slits S, which extend into the common 
electrode pattern 4, electrically insulate the individual electrodes 5 
from each other. 
As described above, the individual electrodes 5 are spaced from the common 
electrode pattern 4. Thus, the resistor layer 3 is exposed between the 
common electrode pattern 4 and the individual electrodes 5 so that these 
exposed portions provide heating dots (heating regions) 3a arranged in a 
line along the first longitudinal edge surface 1c of the head substrate 1. 
In the illustrated embodiment, the first longitudinal edge surface 1c of 
the head substrate 1 is formed with a step portion 1d onto which the 
resistor layer 3 and the common electrode pattern 4 extend. The portion of 
the common electrode pattern 4 extending onto the step portion 1d from the 
obverse surface is electrically connected to an auxiliary electrode 6 
extending from the reverse surface onto the step portion 1d. The auxiliary 
electrode 6 entirely covers the reverse surface 1b of the head substrate 
1, and this provides a largely covered area. As a result, current passage 
is enlarged enough to substantially prevent a voltage drop longitudinally 
of the head substrate 1. 
Though not illustrated, the heating regions (heating dots) 3a of the 
resistor layer 3, the common electrode pattern 4 and the individual 
electrodes 5 may be covered with a protecting layer made of SiO.sub.2 
and/or Ta.sub.2 O.sub.5. Such a protecting layer serves to prevent the 
heating regions 3a of the resistor layer 3, the common electrode pattern 4 
and the individual electrodes 5 from being oxidized by the air or being 
worn out by contacting with a printing medium (not shown). 
Though not illustrated either, the auxiliary electrode 6 may be formed to 
cover not only the first longitudinal edge surface 1c but also the 
entirety of the second longitudinal edge surface (not shown) opposite to 
the first longitudinal edge surface. In this way, much enlarged current 
passage is obtained. 
The thermal printhead with the above-described arrangement may be 
advantageously made by the following method. 
First, as shown in FIG. 3a, an alumina-ceramic master substrate 1' is 
prepared which is large enough to provide a plurality of head substrates 
after separation along longitudinal division lines DL1 and transverse 
division lines DL2. In the illustrated embodiment, the master substrate 1' 
has dimensions corresponding to two rows of three head substrates disposed 
longitudinally. 
Then, as shown in FIG. 3b, the obverse surface of the master substrate 1' 
is provided with a master glaze layer 2' which is formed by sintering 
glass paste applied to the surface. 
Then, as shown in FIG. 3c, a groove 7 extending through a master glaze 
layer 2' into the master substrate 1' is formed with the use of a dicing 
cutter (not shown) along a longitudinal center division line DL1. As a 
result, the master glaze layer 2' is divided into separate glaze layers 2. 
The groove 7 is utilized in a latter process for making a step portion 1d. 
Then, as shown in FIG. 3c again, a curved edge portion 2a is formed 
adjacent to the groove 7 of the glaze layer 2 by heating the master 
substrate 1' at a temperature of about 850.degree. C. for about 20 
minutes. The formation of such a curved edge portion 2a is due to the 
surface tension of the glass material, which is in a liquidized state by 
the heating. 
Then, as shown in FIG. 3d, a thin film-like resistor layer 3 is formed with 
a thickness of e.g. about 0.1 .mu.m by sputtering TaSiO.sub.2 over the 
glaze layer 2 and the master substrate 1'. As a result, the resistor layer 
3 is formed to extend into the groove 7 of the master substrate 1'. The 
resistor layer 3 may be formed by reactive sputtering with the use of a 
material having tantalum nitride as the main component. 
Then, as shown in FIG. 3e, a conductor layer 8 is formed over the resistor 
layer 3 by sputtering. The conductor layer 8 also extends into the groove 
7 of the master substrate 1'. Typically, the conductor layer 8 is made of 
aluminum (Al), but it may be made of copper (Cu) or gold (Au). 
Then, as shown in FIG. 3f, upon formation of slits S (see FIG. 2) by 
etching the resistor layer 3 and the conductor layer 8, the conductor 
layer 8 is partially removed by etching for exposure of portions of the 
resistor layer 3 to provide heating dots 3a. As a result, the conductor 
layer 8 is divided into the common electrode pattern 4 and the individual 
electrodes 5. 
Then, as shown in FIG. 3g, a slit 9 is formed along the groove 7. The width 
W and the length L of the slit 9 (see FIG. 3g and FIG. 3a) are smaller 
than those of the groove 7. Thus, the groove 7 and the slit 9 provides a 
step portion 1d. At this stage, however, since the master substrate 1' is 
not divided into unit head substrates 1 (see FIG. 1) yet, subsequent steps 
can be effectively performed to the master substrate it (that is, a 
plurality of unit head substrates 1). The slit 9 may be formed by a 
dicing, laser or water-jet method for example. 
The cutting manner shown in FIG. 3g, in which the width W of the slit 9 is 
smaller than that of the groove 7, is referred to as step cut. On the 
other hand, a cutting manner in which the slit 9 and the groove 7 are 
arranged to have the same width is referred to as full cut. In the present 
invention, it may be possible to provide a full cut instead of a step cut. 
Then, as shown in FIG. 3h, while moving the master substrate 1' in the 
direction of arrow X, conductive metal (aluminum or copper for example) is 
sputtered from below to form a proper thickness (about 2 .mu.m for 
example) of auxiliary electrode layer 6 over the reverse surface of the 
master substrate 1'. Thus, the auxiliary electrode layer 6 will extend 
into the slit 9 of the master substrate 1' and further turn over onto the 
step portion 1d for electrical conduction with the common electrode 
pattern 4. 
Advantageously, the film thickness of the auxiliary electrode layer 6 in 
the slit 9 and the turnover of the auxiliary electrode layer 6 into the 
step portion 1d can be controlled by the width of the slit 9. 
Finally, though not illustrated, upon forming a protecting layer for the 
resistor layer 3, the common electrode pattern 4 and the individual 
electrodes 5, the master substrate 1' is divided along the respective 
division lines DL1, DL2 (FIG. 3a) to provide separate thermal printheads 
(see FIGS. 1 and 2). 
According to the method described above, the formation of the auxiliary 
electrode 6 can be performed with a non-divided master substrate 1' and 
there is no need to deal with the plurality of head substrates separately, 
thereby remarkably improving the production efficiency and reducing the 
production cost. Further, there is no need to install magagines or tools 
exclusively used for dealing with the plurality of head substrates, 
thereby reducing the cost of equipment. Still further, carrying or 
supporting the master substrate 1' can be performed with the use of the 
marginal portions of the substrate. Therefore, it is possible to eliminate 
a secondary damage which might be caused on the individual head substrates 
due to direct contact with the apparatus in transportation and support. 
The condition of electrical conduction between the auxiliary electrode 6 
and the common electrode pattern 4 is determined by the turnover R (FIG. 
3h) of the auxiliary layer 6 for the common electrode pattern 4. As 
already described, the turnover R of the auxiliary layer 6 is determined 
by the width W of the slit 9. Therefore, selecting the width W of the slit 
9 can control the condition of electrical conduction between the auxiliary 
layer 6 and the common electrode pattern 4. Some description about this 
will be given hereinafter by referring to FIG. 4. 
FIG. 4 is a graph showing how the turnover R of the auxiliary electrode 6 
changes and also how the electric resistance between auxiliary electrode 
layer 6 and common electrode pattern 4 changes, depending on the width W 
of the slit 9. The abscissa of FIG. 4 is assigned to the width W (mm) of 
the slit. The left-side ordinate in FIG. 4 is assigned to the electrical 
resistance between the auxiliary electrode layer 6 and the common 
electrode pattern 4 in natural logarithm (ln.OMEGA.), whereas the 
right-side ordinate is assigned to the turnover R (.mu.m) of the auxiliary 
electrode layer 6. The resistance between the auxiliary electrode layer 6 
and the common electrode pattern 4 was measured between a position on the 
common electrode pattern 4 spaced by about 0.1-0.2 mm from the surface of 
the glaze layer 2 formed on the head substrate 1 and a position on the 
auxiliary electrode layer 6 spaced by 250 mm from the former position. 
The curve A in FIG. 4 shows the relation between the slit width W and the 
resistance measured between the auxiliary electrode layer 6 and the common 
electrode pattern 4, where a step cut is performed to the slit 9. The 
curve B shows the relation between the slit width W and the resistance 
measured between the auxiliary electrode layer 6 and the common electrode 
pattern 4, where a full cut is performed to the slit 9. The curve C shows 
the relation between the slit width W and the turnover R. 
As can be seen from FIG. 4, where the slit width W is no greater than 0.3 
mm, the auxiliary electrode layer 6 can hardly extend onto the common 
electrode pattern 4 (that is, the turnover R is almost zero, and the 
auxiliary electrode layer 6 hardly contacts or overlaps the common 
electrode pattern 4), and the resistance between the auxiliary electrode 
layer 6 and the common electrode pattern 4 is rendered remarkably high or 
about 11 M.OMEGA.. Alternatively, where the slit width W is within a range 
of 0.3-0.5 mm (0.3 mm and 0.5 mm not included), the auxiliary electrode 
layer 6 can gradually extend onto the common electrode pattern 4, whereas 
the resistance between the auxiliary electrode layer 6 and the common 
electrode pattern 4 rapidly decreases. Further, where the slit width W is 
no less than 0.5 mm, the turnover R of the auxiliary electrode layer 6 
onto the common electrode pattern 4 is rendered no less than 20 82 m, and 
the resistance remains in a range of no greater than 22.OMEGA.. This 
follows that the condition of the electrical conduction between the 
auxiliary electrode layer 6 and the common electrode pattern 4 can be kept 
within an allowable range by making the slit width W no less than 0.5 mm. 
Particularly, where the slit width W is no less than 0.8 mm, the turnover 
R of the auxiliary electrode layer 6 onto the common electrode pattern 4 
is no less than 50 .mu.m, thereby realizing a good electrical connection 
therebetween. 
As described above, in the method of the present invention in which the 
master substrate 1' is formed with a slit 9, the turnover R of the 
auxiliary electrode layer 6 onto the common electrode pattern 4 is 
controlled by adjusting the slit width W. Thus, the electrical resistance 
between the auxiliary electrode layer 6 and the common electrode pattern 4 
can be determined to fulfill a desired object. 
The preferred embodiment according to the present invention being thus 
described, the present invention is not limited to the embodiment. For 
example, the method of making a resistor layer, a common electrode 
pattern, individual electrodes and an auxiliary electrode layer is not 
limited to sputtering, but other methods such as CVD method are also 
applicable. Further, materials and configurations of a head substrate and 
other constituting elements are not restricted by the embodiment. Still 
further, the method according to the present invention is applicable to 
production of a thin film-type thermal printhead as well as a thick 
film-type thermal printhead.