[0001] This invention relates generally to the art and technology of thermal ink jet printing
and more particularly to a new and improved thin film resistor (TFR) printhead architecture
and geometry which is used in the manufacture of disposable thermal inkjet (TIJ) pens.
[0002] In the design of the thin film resistor printheads used in the manufacture of thermal
inkjet pens, it has been a common practice to photolithographically define and electrically
interconnect a plurality of heater resistors, such as those made of tantalum aluminum,
on a thin film substrate and then construct a corresponding plurality of aligned firing
chambers and associated orifice openings above and adjacent to the heater resistors.
These firing chambers and orifice openings are used in ejecting inkfrom a region within
the firing chambers and above the heater resistors and onto a print medium. As is
well known, these firing chambers have commonly been constructed of a selected polymer
material disposed on the TFR substrate and on top of which an orifice plate such as
a gold plated nickel material is disposed and aligned with respect to the firing chambers.
The polymer barrier layer is also photolithographically defined so as to have a predetermined
firing chamber geometry and pattern adjacent to which an ink feed channel or port
is used to fluidically connect each firing chamber with a source of ink supply.
[0003] In operation, electrical drive pulses are selectively applied to conductive traces
leading into the various heater resistors situated in the bottom of each firing chamber
to thereby heat the ink to boiling in each firing chamber and above each heater resistor.
This resistor firing in turn produces a vapor bubble and a corresponding pressure
field within the firing chamber used for thermally ejecting ink onto an adjacent print
medium.
[0004] In the past, the cross-sectional geometry of the firing chambers defined by the walls
of the polymer barrier located between the thin film resistor substrate and the orifice
plate was partially rectangular in shape and typically of three sided wall construction.
This construction defined the firing chamber areas surrounding the heater resistors
on three sides thereof. These firing chambers and ink flow ports connected thereto
serve not only to define an inkfiow path and ink firing chamber for each heater resistor,
but this architecture also serves to fluidically isolate adjacent heater resistors
and thereby minimize undesirable crosstalk therebetween.
[0005] Examples of the above three sided rectangular shaped barrier layer geometries are
t hose used in the three color disposable pen adapted for use in Hewlett Packard's
PaintJet thermal inkjet printer. This disposable pen and the PaintJet thermal ink
jet printer in which it has been successfully used are described in further detail
in the Hewlett Packard Journal, Volume 39, No. 4, August 1988, incorporated herein
by reference. The general architecture of the orifice plate and ink feed geometry
for the above PaintJet pen is also described in U.S. Patent No. 4,771,295, issued
to Jef- frey P. Baker et al., assigned to the present assignee and also incorporated
hereby by reference. In addition, both two-sided and three-sided barrier layer and
firing chamber geometries have been used previously in other types of thermal ink
jet pens such as those disclosed, for example, in U.S. Patent Nos. 4,542,389 and 4,550,326,
issued to Ross R. Allen, and U.S. Patent No. 4,794,410, issued to Howard H. Taub,
all assigned to the present assignee and also incorporated herein by reference.
[0006] Whereas the above Hewlett Packard thermal ink jet pen designs of three-sided barrier
layer and firing chamber construction have performed quite satisfactorily under most
conditions of operation, there are nevertheless certain situations where the above
three-sided rectangular-shaped barrier layer designs have not been totally suitable
for producing acceptably uniform ink drop volumes, printed dot and line uniformity
and a corresponding acceptable print quality, particularly during the high frequency
operation of the thermal inkjet pen when operating in a graphics mode. It is the solution
to this problem to which the present invention is directed.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, it has been discovered that a continuously
curved concave firing chamber wall precisely aligned with respect to the geometry
of the heater resistor produces a significant improvement in the uniformity and consistency
of ink drop volumes being ejected from these firing chambers and associated orifice
openings. This in turn results in a significant improvement in overall print quality.
It is believed that the above previously unacceptable variations in printed dot size
and corresponding drop volume produced by the earlier described thermal inkjet pens
resulted from the fact that residual air from the vaporized fluid unnecessarily accumulated
in both the rectangular corner and in the gaps between the barrier layer walls and
the resistor edges of the earlier designed firing chambers.
[0008] When a thermal inkjet drop generator design allows the residual air and gases from
previous printing cycles to accumulate on or near the heater resistor surface, this
air and gas will provide low temperature nucleation sites on the heater resistor that
will alter the time into the drive pulse width that ink vaporization begins. This
alteration in turn will vary the pressure delivered to the ink being ejected from
the printhead. Because ink drop volume surging within an ink firing chamber diminishes
as the thermal ink jet firing frequency is reduced, it has been concluded that this
alteration results from some time dependent process that diminishes after drop ejection,
and the re-dissolution process of the residual air left over from the bubble vaporization
process is such a time dependent process.
[0009] Accordingly, the general purpose and principal object of the present invention is
to significantly improve the uniformity of ink drop volumes and corresponding dot
and line sizes during thermal ink jet printing in both the text and graphics modes
in order to improve the overall print quality of the hardcopy output. This purpose
and object are achieved and accomplished by, among otherthings, the disposition of
a novel-architecture barrier layer and ink firing chamber over a thin film resistor
substrate having a plurality of polygon shaped heater resistors formed thereon. The
barrier layer defines a plurality of ink firing chambers each formed by a continuously
curved or arcuate concave wall that is laterally within the boundary of the region
defined by the resistor and a 10 micrometer wide margin around the resistor. In this
manner, the firing chamber wall is close to the sides of the resistor without having
an excessive amount of the vertex portions of the resistor underlying the barrier
layer.
[0010] An orifice plate is disposed on top of the barrier layer and has a corresponding
plurality of orifice openings, with one orifice opening being aligned, respectively,
with each firing chamber for ejecting uniform-volume ink drops therefrom during an
ink jet printing operation.
[0011] Another object of this invention is to provide a new and improved thermal ink jet
printhead of the type described wherein significant improvements in high frequency
performance and resulting print quality can be achieved using as a minimum of process
and design modifications to existing thermal ink jet printhead manufacturing processes
and TIJ pen designs.
[0012] Another object is to provide a new and improved thermal inkjet printhead of the type
described wherein the drop ejection stability and drop-to-drop consistency has been
significantly improved with respect to known prior art TIJ pen designs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The advantages and features of the disclosed invention will readily be appreciated
by persons skilled in the art from the following detailed description when read in
conjunction with the drawing wherein:
FIG. 1 is a partially cut away isometric view showing a thermal inkjet printhead that
includes an improved firing chamber construction in accordance with the invention.
FIG. 2 is an abbreviated plan view of the firing chamber configuration of the thermal
ink jet printhead of FIG. 1.
FIG. 3 is an abbreviated plan view of a further firing chamber configuration in accordance
with the invention.
FIG. 4 is an abbreviated plan view of another firing chamber configuration in accordance
with the invention.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0014] In the following detailed description and in the several figures of the drawing,
like elements are identified with like reference numerals.
[0015] Referring now to FIG. 1, the thermal inkjet printhead shown therein is designated
generally as 10 and includes a substrate member 12 upon which a polymer barrier layer
14 is disposed and configured in the geometry shown. The substrate 12 will typically
be constructed of either glass or silicon or some other suitable insulating or semiconductor
material which has been surface-oxidized and upon which a plurality of polygon shaped
heater resistors 26 are photolithographically defined, for example in a layer of resistive
material such as tantalum-aluminum. These heater resistors 26 are electrically connected
by conductive trace patterns (not shown) which are used for supplying drive current
pulses to these heater resistors during a thermal ink jet printing operation. In addition,
there is also provided surface passivation and protection insulating layers (not shown)
between the overlying polymer barrier layer 14 and the underlying heater resistors
26 and conductive trace patterns, and the details of this TIJ printhead construction
are given in the above-identified Hewlett Packard Journal, Volume 39, No. 4, and also
in the Hewlett Packard Journal, Volume 36, No. 5, May 1985, also incorporated herein
by reference.
[0016] The polymer barrier layer 14 will typically be photo-defined from a well-known polymeric
material such as Vacrel available from the DuPont Company of Wil- mington, Delaware,
and using known photolithographic masking and etching processes which are used to
define the geometry of the firing chamber 18. The firing chamber 18 is defined by
a continuously curved or arcuate concave wall 16 that extends upwardly from the resistor
and which has ends near the perimeter or boundary of the resistor. The ends of the
continuously curved concave wall 16 form a single ink conveying opening into the firing
chamber 18, and are connected to the sides of a rectangularly shaped ink feed channel
28 which extends as shown to receive ink at the slanted or angled lead-in end sections
30 that define an ink entry port of the polymer barrier layer 14. Thus, the firing
chamber 18 is integrally joined to the rectangularly shaped ink feed channel 28 and
associated inkflow entry port 30 which are operative to supply ink to the firing chamber
18 during drop ejection of inkfrom the thermal inkjet printhead.
[0017] The continuously curved concave firing chamber wall 16 overlies the resistor and
is configured so that it remains within boundary of the area defined by the polygon
shaped resistor and a 10 micrometer wide margin surrounding the polygon shaped resistor,
whereby the continuously curved wall 16 remains close to the sides of the polygon
shaped resistor. In otherwords, the continuously curved concave wall 16 is contained
within the upward extension or projection of the perimeter of the region defined by
the polygon shaped resistor and a 10 micrometer wide margin surrounding the resistor.
Depending upon the implementation, the continuously curved concave wall 16 can pass
inside of some of the vertices of the polygon shaped resistor. Also, the continuously
curved wall can be completely within the boundary of the resistor, for example such
that portions of the wall are tangential to some of the sides of the polygon shaped
resistor.
[0018] By way of illustrative example, FIG. 1 schematically illustrates an implementation
of the invention with a square resistor 26 and a chamber wall 16 that is in the form
of part of a circular cylinder, and FIG. 2 sets forth a schematic plan view thereof.
The resistor 26 has a linear side dimension of L, for example in the range of about
35 to 60 micrometers, and the cylinder axis CA of the partial circular cylinder firing
chamber wall 16 is substantially orthogonal to the resistor 26 (and to the plane of
FIG. 2) and passes through the center of the square shape of the resistor 126. The
barrier wall is a partial circular cylinder in the sense that the portion of the cylinder
between the wall ends 16a has been removed. As illustrated in FIG. 2, a cross section
of t he partial circular cyl inder comprises the major arc, on the circle that includes
the cross section, between end points that are on the wall ends 16a which are adjacent
to the perimeter of the resistor 26 and, by way of example, are spaced apart by less
than the resistor side dimension L. The term "major" refers to the longer path between
such end points on the circle that includes the cross section of the partially cylindrical
wall. The cross section of the smaller portion of the cylinder that has been removed
to provide an opening in the wall would be the minor arc between such end points.
[0019] Viewed another way, the partial circular cylinder wall is the larger portion of a
circular cylinder that remains after the cylinder is sliced by a plane parallel to
the cylinder axis CA and containing the wall ends 16a.
[0020] The partial circular cylinder wait 16 has a cylinder diameter D that is selected
such that the wall remains within the boundary of the area defined by the square resistor
26 and a 10 micrometer wide margin M surrounding the resistor. In other words, to
the extent that any portion of the chamber wall 16 is outside the resistor boundary,
the radial distance S between the partial circular cylinder wall 16 and each of the
resistor sides comprises the maximum spacing between a resistor side and an outboard
portion of the partially cylindrical wall, and such radial distance S must be less
that 10 micrometers. Thus, in accordance with the invention the cylinder diameter
D is selected to be less than L + 20 micrometers, and FIG. 2 illustrates the example
wherein the cylinder diameter is less than L+20 micrometers and greater than L, whereby
the wall 16 passes inside of the vertexes of the resistor.
[0021] FIG. 3 illustrates, by way of further illustrative example, a square resistor having
a side dimension L and an associated partial circularly cylindrical firing chamber
wall having a cylinder diameter D that is substantially equal to L.
[0022] FIG. 4 illustrates, by way of another example, a rectangular resistor and an associated
firing chamber formed of a partial elliptical cylindrical barrier wall which is a
partial ellipse in cross section.
[0023] Referring again to FIG. 1, an orifice plate 32 of conventional construction and fabricated
typically of gold plated nickel is disposed as shown on the upper surface of the polymer
barrier layer 14, and the orifice plate 32 has a convergently contoured orifice opening
34 therein which is typically aligned with the center of the heater resistor 26. However,
in some cases the orifice opening 34 may be slightly offset with respect to the center
of the heater resistor in order to control the directionality of the ejected ink drops
in a desired manner.
[0024] In accordance with the present invention, it has been discovered that considerable
improvements in ink drop generation stability and drop-to-drop volume consistency
for enhancing overall print quality is obtained with the foregoing relation between
the continuously curved concave wall 16 and the underlying square resistor26. With
such configuration, the residual air and gases from previous printing cycles are not
allowed to significantly accumulate on or near the heater resistor surface and thereby
produce undesirable pressure variants in the firing chamber 16. Such undesirable pressure
variations would otherwise be delivered to the fluid being ejected and contribute
to the overall detriment of drop-to-drop volume consistency and drop ejection stability.
The improved performance of this novel architecture manifests itself in a clearly
visible improvement in the clarity, uniformity and print quality of the printed media.
[0025] Although the foregoing has been a description and illustration of specific embodiments
of the invention, various modifications and changes thereto can be made by persons
skilled in the art without departing from the scope and spirit of the invention as
defined by the following claims.
1. A thermal inkjet printhead comprising:
a thin film substrate (12) with a plurality of thin film resistors (26) each substantially
polygon shaped in plan view;
a barrier layer (14) overlying the substrate; and
respective firing chambers (18) formed in said barrier layer for each of the resistors;
each firing chamber formed by a continuously arcuate concave barrier wall (16) that
is within the boundary of an area defined by the resistor and a 10 micrometer margin
(M) around the resistor, said barrier wall having ends adjacent the perimeter of the
associated resistor and forming a single opening into said firing chamber.
2. The thermal inkjet printhead of Claim 1 wherein portions of said barrier wall are
inside of the perimeter of the associated resistor.
3. The thermal inkjet printhead of Claim 1 wherein portions of said barrier wall are
outside of the perimeter of the associated resistor.
4. The thermal inkjet printhead of Claim 1 wherein each of said resistors is square,
and wherein each of said firing chambers is formed of a barrier wall that is a partial
circular cylinder.
5. The thermal inkjet printhead of Claim 4 wherein said barrier wall passes on the
inside of the vertexes of the associated resistor.
6. The thermal inkjet printhead of Claim 1 wherein each of said resistors is rectangular,
and wherein each of said firing chambers is formed of a wall that is a partial elliptical
cylinder.
7. The thermal inkjet printhead of Claim 6 wherein said barrier wall passes on the
inside of the vertexes of the associated resistor.