Edge-type printhead with contact pads

An edge-type printhead and method of fabricating the same, eliminates the need for precision grinding, lapping, and polishing of a substrate, avoids the need for precision etching of electrode patterns, avoids the use of highly refined etchable thick film pastes, and avoids the need for precision glaze application in the construction thereof. Contact pads are provided on the printhead writing surface. The contact pads facilitate accurately and inexpensively delineated resistor lengths, provide resistor current spreading for full dot width printing and control resistor row straightness. Contact pads permit the use of standard wet or chemical etching with wide spacing between electrodes while facilitating full width printed dots with narrow spacing. The contact pads are applied to the writing edge after the edge-type substrate is laminated, sectioned and the writing surface is polished.

FIELD OF THE INVENTION 
The present invention relates to edge-type thermal printheads and in 
particular to laminated edge-type thermal printheads. 
BACKGROUND OF THE INVENTION 
Thermal printheads are known which are laminated structures comprising an 
alumina substrate having alternating conductive and insulating layers (see 
for instance, U.S. Pat. No. 4,651,168 to Terajima et al.). Such prior art 
printheads, as illustrated in FIGS. 1A through 1E typically comprise an 
alumina substrate (10) having a metallic layer disposed thereon which may 
be patterned to provide a plurality of selectable electrodes (12). An 
insulating layer (14) of glaze is usually disposed upon the selectable 
electrodes (12) and subsequently has disposed thereon another metallic 
layer which provides a common electrode (16). A protective insulating 
material (17) may be disposed on the common electrode (16). The depth or 
amount of insulating glaze (14) disposed on the plurality of selectable 
electrodes (12) typically determines the length of heating elements or 
thin film resistors (18) disposed between respective selectable electrodes 
(12) and the common electrode (16). 
Print quality is effectively a function of the resistors (18) and the 
characteristics of the insulative layer (14) upon which the resistors (18) 
are disposed. Certain characteristics of the resistors (18), such as the 
length determined by the insulative layer (14), significantly influence 
print quality, especially in long, high resolution printheads. Width of 
the resistors (18) is also a critical characteristic, because resistance 
value of a particular resistor is determined by first dividing the length 
of the resistor by its width to determine a number of "squares" of 
resistive material. The number of squares is then multiplied by the sheet 
resistance (Ohms per square) of the particular resistive material to 
determine the total resistance of each resistor. Total resistance 
determines the amount of heat generated for thermal printing. Thus, the 
length and width, i.e. resistance, of these resistors (18) must be 
accurately controlled to achieve high quality printing. 
Ideally uniform print quality from resistor to resistor would require, as 
illustrated in FIG. 1A, an ideally uniformly planar substrate (10), 
perfectly regularly shaped selectable electrode (12) geometries, an 
ideally uniformly applied insulative layer (14), and an ideally uniformly 
planar second metallic or common electrode layer (16). However, as 
illustrated somewhat exaggeratedly in FIGS. 1B through 1E, various 
imperfections and irregularities occur in the fabrication of such 
laminated edge-type thermal printheads. Imperfections and irregularities 
effect resistor dimensioning, ultimately negatively impacting print 
quality. 
Imperfections or unevenness in the alumina substrate (10), as illustrated 
in FIG. 1B, may be perpetuated throughout the various layers of the 
printhead. An uneven substrate (10) results in subsequently applied uneven 
and irregular selectable electrodes (12). Further, a similarly unevenly 
applied insulative glaze layer (14) will be disposed upon the electrodes 
(12) and substrate (10) and result in a correspondingly uneven common 
electrode layer (16). 
Significantly costly mechanical processes may be undertaken, such as 
lapping and polishing of the substrate (10) to assure an even substrate 
(10), such as illustrated in FIG. 1C. However, lapping and polishing of 
the substrate (10) will not assure precision etched electrodes (12). 
Standard photolithographic techniques may not be adequate to uniformly 
meet the dimensional requirements of an electrode thickness in the order 
of 5 microns, necessary to achieve good electrical connection to the 
resistor and may result in irregularly shaped electrodes. Further, close 
spacings of electrodes (10-15 microns). necessary in high resolution 
(greater than 200 dpi) heads and required for complete electrode/resistor 
contact, are difficult to achieve with standard photolithographic 
techniques because of increased likelihood of bridging and shorting. The 
resulting electrodes, overetched to reduce the likelihood of shorts, may 
lack full resistor contact, such as illustrated in FIGS. 1C and 1D. Full 
dot width printing may be precluded because current from the electrode 
(12) will not spread adequately throughout the resistor to heat the entire 
resistor surface area. Thus, a precision etching technique, such as ion 
milling, would be necessary to make the widest possible electrodes with 
narrow spacing between them as required for high resolution heads. 
However, precision etching techniques add additional and expensive 
processing steps and cannot absolutely preclude bridging and shorting 
between electrodes that may result never-the-less from lumpy, high 
granularity etchable thick film gold paste used in the electrode 
fabrication process. Greater precision and quality may require highly 
refined pastes. 
Although precision ion milling of the selectable electrodes fabricated from 
highly refined pastes, permits greater control of the electrode geometry 
that can be fabricated on a precision ground or lapped substrate, resistor 
length and consequently print quality may still be negatively impacted by 
application of a non-uniform insulative glaze layer (14), such as 
illustrated in FIG. 1E. Elimination of imperfections in the insulative 
layer further requires surface finishing, such as precision grinding or 
lapping in order to avoid irregularities resulting from laminating the 
common electrode (16) on top of insulative layer (14) imperfections. 
Precision grinding or lapping of the insulative layer must also be highly 
controlled so as to avoid irregularities in the polished insulative layer, 
such as a wedged, uneven grinding as illustrated in FIG. 1F. 
SUMMARY OF THE INVENTION 
The present invention is an edge-type printhead and method of fabricating 
the same, which eliminates the need for precision grinding, lapping, and 
polishing of a substrate, avoids the need for precision etching of 
electrode patterns, avoids the use of highly refined etchable thick film 
pastes, and avoids the need for precision glaze application in the 
construction of a laminated thermal printhead. 
According to the invention, contact pads are provided on a thermal 
printhead writing surface. The contact pads facilitate accurately and 
inexpensively delineated resistor lengths. The contact pads according to 
the invention provide resistor current spreading for full dot width 
printing and control resistor row straightness. Uniformity created by the 
contact pads results in substantially uniform thermal characteristics, 
which simplifies hysteresis control in smart heads. The contact pads 
according to the invention are applied to the writing edge after the 
edge-type substrate is laminated, sectioned and the writing surface is 
polished. High resolution (greater than 200 dpi) long thermal printheads 
are fabricated using standard thick film materials and processes, while 
contact pads applied with high resolution, .high accuracy thin film 
techniques are implemented on the writing surface to control accuracy and 
precision of the resistors for optimum print quality. 
Features of the invention include the ability to use "as-fired" alumina 
substrates instead of precision lapped and polished substrates, resulting 
in a significant cost saving advantage. Contact pads according to the 
invention permit the use of relatively inexpensive standard wet or 
chemical etching with wide spacing between electrodes while facilitating 
full width printed dots with narrow spacing. Print dot row straightness is 
achieved. Stringent cleaning of precision lapped substrates and laminated 
layers is avoided.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 2 and 4, structure edge-type thermal printhead is 
constructed by laminating a first layer of metal onto an alumina substrate 
(10). Typically, the first metal layer is laminated as a conductive paste 
which is patterned and etched to form a plurality of selectable electrodes 
(12) as discussed hereinbefore with respect to the prior art. However, the 
first metal layer may also be deposited as a unitary conductive layer that 
will form a common electrode layer. Following the first metal layer, an 
insulative glaze layer (14), such as the Johnson Matthey JM300 or JM600 
series of dielectric pastes for thermal printheads, or the like, is 
deposited on the first metal layer and fired. It is significant to note 
that according to the invention only minor consideration need be given to 
the planarity of the alumina substrate and the subsequently deposited 
glaze, because imperfections of the planar surfaces will be accommodated 
by the deposition of contact pads as discussed hereinafter. A second metal 
layer is deposited onto the insulative glaze (14). In this illustrative 
embodiment the second metal layer is deposited as a common electrode (16). 
A protective glaze layer (20) may be deposited over the common electrode 
as desired. The printhead, subsequent to constructing the various layers, 
is sectioned exposing a writing surface (22) complete with imperfections 
as discussed hereinbefore. The writing surface (22) is polished to prepare 
the writing surface for application of subsequently applied resistive 
elements (18), positioned between respective selectable electrodes (12) 
and the common electrode (16). 
A printhead according to the present invention, further comprises a 
longitudinal contact pad (24) corresponding to and substantially aligned 
with the common electrode (16). A plurality of individual contact pads 
(26), correspond to respective individual selectable electrodes (12). The 
longitudinal contact pad (24) and the individual contact pads (26) are 
positioned so that the individual contact pads (26) are in a straight row 
and the longitudinal contact pad (24) is substantially parallel thereto. 
The contact pads are dimensioned to accommodate imperfections and can be 
of any dimension reasonable and practical for a particular application, as 
appreciated by one of ordinary skill in the art. The primary consideration 
is that the contact pads be dimensioned to define resistors with uniform 
length in a straight row. 
As illustrated in FIGS. 4 and 3 selectable electrodes (12) and the common 
electrode (16), have an irregular edge caused by surface imperfections on 
the applicable laminae or introduced by the imprecision of the etching 
technique or due to the texture of the material used to form the 
metalization. While the irregularity is an undesirable condition, the 
longitudinal (24) and, individual (26) contact pads superimposed over the 
termination edge effectively emulate perfect electrodes. Application of 
resistors to the writing surface, with the contact pads in place results 
in the individual contact pads (26) acting as current spreaders dispersing 
current to promote full width dots and consequently higher quality, 
substantially more uniform print dots. 
A cross-sectional view as shown in FIG. 5 illustrates the relationship of 
the contact pads vis-a-vis the resistor. The process of fabrication is 
illustrated in FIG. 6. 
After the writing surface is sectioned (50), it is polished and cleaned. 
Subsequently, the individual and longitudinal contact pads are applied 
proximate to the respective underlying electrodes. Cleaning (60), to 
achieve intimate contact pad metal to electrode conductivity, involves an 
ion beam or sputter etch glow discharge process which removes surface 
contaminants and oxides prior to contact pad metal disposition. It should 
be noted that this cleaning step does not represent significant additional 
processing, as the same cleaning is necessary prior to resistor 
application in typical laminated structure edge-type printhead 
fabrication. 
After cleaning (60), a contact pad metal is deposited on the appropriate 
Sites. preferrably, metals having high conductivities are used to 
facilitate current spreading. Refractory metals such as tungsten and 
molybdenum are preferred. Precious and semi-precious metals such as gold, 
palladium, ruthenium, platinum, rhodium or their alloys could be used, 
however, such metals do not adhere well and tend to flake when deposited 
directly on a substrate. 
In the case where precious and semi-precious metals are to be used, an 
adhesive metal such as chromium or titanium-tungsten must first be 
deposited (70), illustrated in FIG. 6 as an optional step. The adhesive 
metal bonds tenaciously to the substrate and will readily bond with a 
subsequently applied precious or semi-Precious metal. However, adhesive 
metals tend to be relatively poor conductors, thus, their use is not 
preferred. 
The contact pad metal is deposited (80), on the cleaned writing surface or 
optionally on the previously deposited adhesive metal, by a thin film 
process which effects electrical continuity with the underlying 
electrodes. The thin film process permits high accuracy deposition of the 
metal so that contact pads can be closely spaced to enhance the resolution 
of the printhead. Either subtractive etch processes or additive lift-off 
stenciling processes are suitable for sputtering or evaporation deposition 
of the selected metal in a vacuum system. Preferrably, a sputter 
deposition is performed to blanket metalize the contact pad area. The 
metal is then patterned for precision etching by ion beam milling. 
Chemical etching may be suitable depending upon the precision required by 
the application. 
When the contact pads are in place, and prior to the resistors being 
applied (90), the writing surface is again cleaned (85) using a glow 
discharge cleaning process to remove contaminants. 
Standard patterning and application techniques are used to put down the 
resistors on the writing surface subsequent to the contact pad deposition. 
A resistor material as known in the art, such as titanium silicide or 
tantalum carbide, is deposited in a thin film sputter deposition. The 
writing surface having the sputtered resistive material disposed thereon 
is then patterned and subjected to a subtractive etch process. Similar to 
the process of depositing the contact pads, the resistive element 
deposition can involve either ion beam milling of the patterned resistive 
material or a less precise chemical etch depending upon the degree of 
precision desired. 
Optionally, and in most cases preferrably, a wear layer (30) is applied 
(95) over the resistive elements and contact pads. The wear layer must be 
a material which has high abrasion resistance and suitable thermal 
conductivity and shock resistance properties while functioning to prohibit 
oxidation of the resistive elements. Preferrably, tantalum pentoxide is 
sputter deposited over the resistors and contact pads to provide such a 
wear layer. Silicon nitride may also be suitable. 
Although the illustrative embodiment disclosed herein describes the contact 
pads and resistors as being sputtered and etched, one of ordinary skill in 
the art can appreciate that the contact pads and and resistive elements 
can be deposited by patterning a resist and subsequently blanket 
depositing the contact pad or resistive elements and performing a lift off 
process removal of the resist so that the desired structures remain 
thereafter. 
While the illustrative embodiment employed a first metal layer as selective 
electrodes and a second metal layer as a common electrode it will be 
appreciated that the first layer deposited could be the common layer with 
the selectable electrodes deposited thereafter. Further, a plurality of 
metal layers could be laminated to fabricate a printhead according to the 
invention having a plurality of rows of selectable electrodes and/or a 
plurality of common electrodes. 
Although the invention has been shown and described with respect to an 
exemplary embodiment thereof, various other changes, omission and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.