Thermal ink-jet printhead with an optimized fluid flow channel in each ejector

A thermal ink-jet ejector having a fluid flow channel extending between an ink inlet and a nozzle for the ejection of liquid ink therefrom, includes a rear channel diffuser disposed between the heating element and the inlet, and/or a front channel diffuser disposed between the heating element and the nozzle. Each diffuser includes an arrangement of tapers which decrease the flow impedance of liquid ink flowing toward the nozzle, and increase the flow impedance of liquid ink flowing toward the inlet. The arrangement increases the kinetic energy of droplets being ejected, and also increases the speed of re-fill of the channel with liquid ink following ejection.

The present invention relates to a printhead for a thermal ink-jet printer, 
in which the fluid flow channel of each ejector is specially shaped with 
impedance-controlling tapers, for optimal performance. 
In thermal ink-jet printing, droplets of ink are selectably ejected from a 
plurality of drop ejectors in a printhead. The ejectors are operated in 
accordance with digital instructions to create a desired image on a print 
sheet moving past the printhead. The printhead may move back and forth 
relative to the sheet in a typewriter fashion, or the linear array may be 
of a size extending across the entire width of a sheet, to place the image 
on a sheet in a single pass. 
The ejectors typically comprise capillary channels, or other ink 
passageways, which are connected to one or more common ink supply 
manifolds. Ink is retained within each channel until, in response to an 
appropriate digital signal, the ink in the channel is rapidly heated by a 
heating element disposed on a surface within the channel. This rapid 
vaporization of the ink adjacent the channel creates a bubble which causes 
a quantity of liquid ink to be ejected through an opening associated with 
the channel to the print sheet. The process of rapid vaporization creating 
a bubble is generally known as "nucleation." One patent showing the 
general configuration of a typical ink-jet printhead is U.S. Pat. No. 
4,774,530, assigned to the assignee in the present application. 
In most designs of ejectors in ink-jet printheads currently in common use, 
the capillary channel which retains the liquid ink immediately prior to 
ejection is typically a simple tube of a uniform cross-section along its 
entire effective length. The channel may be round, square, or triangular 
in cross-section, but the cross-section does not vary at different points 
along the axis of the capillary channel. When a vapor bubble of liquid ink 
nucleates in such a channel, by the nature of the physics of nucleation, 
the expanding vapor bubble expands in all available directions. As a 
practical matter, such nucleation not only causes liquid ink disposed in 
the channel between the heating element and the nozzle to be pushed out of 
the nozzle, but also presents a force to liquid ink which is disposed 
between the heating element and the inlet to the capillary channel. In 
other words, in a standard-design ejector, nucleation pushes some ink out 
of the channel, but equally pushes a considerable quantity of ink 
"backwards" into the ink supply. 
This backward flow of liquid ink is a source of many practical 
disadvantages. First, the fact that one-half of the kinetic energy 
provided by the heating element is not used to eject toward the print 
sheet represents a waste of energy and a loss of drop velocity and drop 
volume. Further, the fact that liquid ink is pushed back into the ink 
supply with every ejection causes a requirement of more time for the 
capillary channel to re-fill with liquid ink, and therefore puts a 
significant constraint on the operating frequency of an individual 
ejector. In brief, this two-direction flow of ink with every ejection in 
the standard ejector introduces a trade-off between drop velocity and/or 
drop volume on one hand and re-fill speed on the other hand. 
The present invention proposes a design of an ink-jet ejector having a flow 
rectifier which minimizes the ratio of "backward" versus "forward" flow of 
liquid ink with each ejection. 
In the prior art, the article by Stemme and Stemme, "A Novel Piezoelectric 
Valveless Fluid Pump," The Seventh Intemational Conference on Process 
Transducers, Yokohama, Japan (1993) pp. 110-113, which relates to PCT 
application WO-A-94/19609, discloses a diaphragm-type piezoelectric pump 
wherein fluid inlets and outlets include a constricting element having a 
larger pressure drop in one flow direction than in the opposite flow 
direction. 
U.S. Pat. No. 4,368,477 discloses an ink-jet printhead in which individual 
ejectors are each provided with a diagonally-extending ink duct. The 
downstream end of each duct is formed with a wedge-shaped tapered portion, 
each having a leading edge wall carrying a discharge orifice for ink 
droplets. 
U.S. Pat. No. 4,550,326 discloses a nozzle plate for a "roofshooter" 
printhead in which, as shown in FIGS. 8A and 8B, the orifices are tapered 
in front of the ink meniscus. 
U.S. Pat. No. 4,675,693 discloses an ink-jet printhead in which the minimum 
cross-sectional area of a "discharge port" is optimized with respect to 
the volume of the droplets intended to be discharged. 
U.S. Pat. No. 5,041,844 discloses a thermal ink-jet printhead having a 
channel geometry that controls the location of the bubble collapse on the 
heating elements. In one embodiment, the heating elements are located in a 
pit, and the channel portion upstream from the heating element has a 
length and cross-sectional flow area that is adjusted relative to the 
channel portion downstream from the heating element, so that the upstream 
and downstream portions of the channel have substantially equal ink flow 
impedances. 
U.S. Pat. No. 5,278,585 discloses a thermal ink-jet printhead including a 
flow-directing one-way valve for reducing back-flow forces generated by 
the droplet ejecting ink vapor bubbles, so that most of the bubble 
generated forces are used to eject ink droplets from the printhead 
nozzles. A movable flap is located within the capillary channel, to 
restrict backflow. 
According to one embodiment of the present invention, there is provided a 
thermal ink-jet printhead comprising at least one ejector. The ejector 
comprises a structure defining a fluid flow channel for passage of liquid 
ink therethrough. The fluid flow channel is defined along an axis 
extending from an inlet to a nozzle. A heating element is exposed within 
the fluid flow channel between the inlet and the nozzle. The fluid flow 
channel defines a first taper in at least one dimension along the axis, 
the first taper being disposed between the heating element and the inlet 
and opening toward the nozzle. 
According to another embodiment of the present invention, there is provided 
a thermal ink-jet printhead comprising at least one ejector. The ejector 
comprises a structure defining a fluid flow channel for passage of liquid 
ink therethrough, the fluid flow channel being defined along an axis from 
an inlet to a nozzle. A heating element is exposed within the fluid flow 
channel between the inlet and the nozzle. The fluid flow channel defines a 
rear channel diffuser between the heating element and the inlet. The rear 
channel diffuser comprises a forward taper opening toward the nozzle and a 
rearward taper opening toward the inlet. A cone angle of each of the 
forward taper and rearward taper is selected so that flow impedance of 
liquid ink flowing through the rear channel diffuser toward the inlet is 
greater than flow impedance of liquid ink flowing through the rear channel 
diffuser toward the nozzle. According to another aspect of the invention, 
there is provided within the fluid flow channel a front channel diffuser 
between the heating element and the nozzle, the front channel diffuser 
comprising a forward taper opening toward the nozzle and a rearward taper 
opening toward the inlet, a cone angle of each of the forward taper and 
the rearward taper providing flow impedance of liquid ink flowing through 
the front channel diffuser toward the inlet greater than flow impedance of 
liquid ink flowing through the front channel diffuser toward the nozzle.

FIG. 1 is a plan view of a single ejector as would be found in a thermal 
ink-jet printhead incorporating the present invention. As is well known, 
it is typical for ink-jet printheads to include 100 or more such ejectors, 
spaced from 300 to 600 ejectors to the linear inch. Also as is well known, 
each printhead is typically formed in a largely silicon structure, such as 
a silicon chip, having various voids etched therein to form capillary 
channels for the flow of liquid ink therethrough. 
With reference to FIG. 1, a portion of a printhead chip, here indicated as 
10, defines therein a fluid flow channel generally indicated as 12, which 
is aligned along an axis 14. The fluid flow channel 12 extends from an 
inlet port 16 to a nozzle 18. As is known in the art of thermal ink-jet 
printheads, liquid ink from an external supply (not shown) is introduced 
into fluid flow channel 12 through inlet 16, where it is retained largely 
by capillary force within the channel 12 until it is ejected through 
nozzle 18 and directed onto a print sheet. 
The source of energy for ejecting liquid ink retained in channel 12 through 
nozzle 18 onto a print sheet is a heating element 20. In common designs of 
thermal ink-jet printheads, heating element 20 is in the form of an area 
of polysilicon which has been doped to a specific resistivity and which is 
covered with various protective passivation layers (not shown). The 
heating element 20 is connected by conductive leads (not shown) to a 
voltage source, which is activated when it is desired to eject a droplet 
of ink at a particular moment. Heating element 20 thus serves as a 
resistance heater which, when activated by a voltage, nucleates liquid ink 
which is immediately adjacent the surface thereof. This nucleation creates 
a vapor bubble which begins directly on the surface of heating element 20, 
and then expands as vaporization continues, and effectively pushes out 
liquid ink retained in the channel 12 between heating element 20 and 
nozzle 18 until the vapor bubble collapses. 
As mentioned above, when heating element 20 creates a vapor bubble of 
liquid ink immediately adjacent thereto, not only will the expanding 
bubble created by heating element 20 push out liquid ink which is retained 
between the heating element 20 and nozzle 18, but by virtue of the 
equilibrium of pressure around the surface of a bubble, also push against 
liquid ink disposed between heating element 20 and inlet 16. When this ink 
is pushed against by the bubble, it follows that the ink will be pushed 
out of the inlet 16 and back into the ink supply. In order to minimize 
this undesirable back flow of liquid ink, the present invention proposes 
various flow-rectifying structures which influence the relative impedance 
along axis 14 to favor the flow of ink toward nozzle 18 as oppose to 
toward inlet 16. 
In order to perform this adjustment of impedance, the present invention 
provides various tapers in the cross-section of channel 12 along axis 14. 
According to the present invention, the channel 12 defines a rear channel 
diffuser 30 and a front channel diffuser 32. With reference to rear 
channel diffuser 30, it can be seen that diffuser 30 comprises a first 
taper 40 and a second taper 42; with reference to front channel diffuser 
32, it can be seen that the diffuser comprises a third taper 44 and a 
fourth taper 46. 
For each of the rear channel diffuser 30 and the front channel diffuser 32, 
the intention of the two tapers is that the relatively gradual taper 
toward the direction of the nozzle, and the relatively sharp tapers toward 
the direction of the inlet, have the function of creating a high impedance 
of ink flow in the direction toward the inlet 16, and a relatively low 
impedance for the flow of ink toward the direction of the nozzle 18. Thus, 
the rear channel diffuser 30 has a high impedance during the ejection of a 
droplet of liquid ink through nozzle 18, and a low impedance for ink 
entering the channel 12 through inlet 16 during re-fill. With respect to 
front channel diffuser 32, it will be seen that there will be a low 
impedance for ink being pushed through the diffuser toward the nozzle 18, 
but a higher impedance for any ink being drawn inward from nozzle 18, 
which may occur in a manner to be described in detail below. 
In one practical embodiment of the present invention, the preferred angles 
for the high-impedance tapers such as 40, 44 is not more than 30 degrees 
in total "cone angle," that is, from one wall of channel 12 to the other. 
In general, in the context of ink-jet printing, 30 degrees has been found 
to be above the critical angle for the desired impedance effect, this 
being the angle at which the liquid ink releases from the wall of channel 
12 at a given velocity. Under commonly-expected conditions of ink 
composition and ejection frequency, an optimum cone angle has been found 
to be about 10 degrees for the forward-facing tapers. With respect to the 
tapers 42 and 46, the preferred cone angles for these tapers should be 
greater than 30 degrees but may be as high as 90 degrees or more. (As used 
in the claims herein, it will be understood that the "cone angle" refers 
to a taper of the fluid flow channel in at least one dimension, in the 
case of a fluid flow channel of rectangular cross-section; it will be 
understood that such a cone angle concept can apply equally to a 
semicircular or circular cross-section as well. Further, in certain of the 
claims, each of the rear channel diffuser 30 and front channel diffuser 32 
are described as having forward facing and rearward facing tapers, forward 
facing tapers opening toward the nozzle and rearward-facing tapers opening 
toward the inlet.) 
Thus, for a nucleating bubble of vaporized ink originating from heating 
element 20, the liquid ink being pushed out from this bubble will face a 
high impedance from taper 40, and a relatively low impedance from taper 
46. This lower impedance through front channel diffuser 32 will cause more 
ink to be pushed through nozzle 18 than backwards towards inlet 16, in the 
finite time of ejection before the vapor bubble collapses. In this way, 
the back flow toward inlet 16 is reduced with every ejection. 
After the ejection of liquid ink from nozzle 18, a new supply of liquid ink 
must be loaded into channel 12 through inlet 16. The nature of taper 42 of 
rear diffuser 30 creates a low-impedance flow into the bulk of channel 12. 
During the vapor bubble collapse, the high-impedance property of taper 44 
presents a high impedance for liquid ink to flow from the space in 
channels 12 between front channel diffuser 32 and nozzle 18, hence 
maximizing the re-use of bubble collapse energy for refill of the fluid 
flow channel through inlet 16 and diffuser 30. It follows that less liquid 
ink needs to be supplied by slow capillary refill action through inlet 16, 
hence reducing the refill time and increasing the maximum print speed. 
According to a preferred embodiment of the present invention, there is 
further provided within channel 12 an extended portion generally indicated 
as 50, between the taper 44 of front channel diffuser 32 and nozzle 18. 
Following the ejection of a droplet of liquid ink through nozzle 18, the 
presence of extension 50 will cause a small quantity of liquid ink to 
remain in channel 12 even after ejection. This small quantity of liquid 
ink which will remain generally in the area of extended portion 50 can 
serve as a liquid seal to enhance the speed and efficiency of the re-fill 
of liquid ink from inlet 16. The small remainder of liquid ink facilitated 
by extended portion 50 also prevents the undesirable intake of air during 
the re-fill stage; if any air is sucked back during the re-fill stage 
beyond front channel diffuser 32, the presence of this stray air bubble 
before ejection will have an undesirable effect on the amount of ink 
ejected in the next ejection, and may also damage the printhead, if in the 
next ejection the heating element 20 has no liquid ink thereagainst to 
absorb heat energy. The extent of extended portion 50 relative to the rest 
of the channel 12 will vary by specific design, but as a general 
guideline, it is desirable that the extra volume to channel 12 provided by 
extended portion 50 be approximately equal to one-half the volume 
encompassed between heating element 20 and taper 46. As a practical 
matter, what is important is that extended portion 50 be long enough to 
cause a "bridge" of liquid ink, effectively sealing nozzle 18, to remain 
therein after each ejection. 
With the channel design of the present invention, two key advantages are 
obtained: first, more ink is ejected through nozzle 18 than through inlet 
16 with every ejection, and the flow of liquid ink to re-fill the channel 
12 after an ejection is enhanced. In the ongoing operation of a particular 
ejector, these two advantages have the effects of (a) increasing the 
kinetic energy of each droplet emitted through the nozzle; and (b) 
increasing the speed of re-fill, thereby increasing the maximum possible 
frequency of operation, which is the time between ejections. 
The various trade-offs involved in designing a specific version of the 
ejector of the present invention can be summarized by the following 
equation: 
##EQU1## 
where P.sub.max =maximum kinetic power (kinetic energy per unit time)of an 
ejected droplet; m=mass of an ejected droplet; v=velocity of an ejected 
droplet; and f.sub.max =maximum frequency of ejection (i.e., the inverse 
of the ejection plus refill time). 
In general, it has been found that the design trade-off between droplet 
volume and droplet velocity summarized by the above equation can be 
manifest by the selection of neck width between the forward- and 
rearward-facing tapers for each diffuser. The presence of a front channel 
diffuser such as 32 may have the effect of decreasing the size of an 
ejected droplet relative to a straight-sided channel 12 of similar 
dimensions. However, in some contexts, the emission of a smaller droplet 
of ink may be desirable from a standpoint of ink absorption by paper. 
FIG. 2 is a perspective view, not to scale, of the channel 12 formed in 
section 10 as shown in the plan view of FIG. 1. It will be noted that, 
according to presently-practical techniques of fabrication of ink-jet 
printheads, that the channel of the present invention is formed in the 
surface of a substrate, such as a silicon chip, leading to a channel 12 
having a rectangular cross-section. Although it may be preferable to 
provide a nozzle having circular cross-section or semicircular 
cross-sections, the use of a rectangular cross-section as shown in FIG. 2 
is effective at obtaining the desired impedances. The cross-sectional area 
of the flow path through fluid flow channel 12 can be kept constant 
despite the constrictions of channel diffusers 30 and 32, by using deeper 
channels with a rectangular cross-section. 
In order to obtain the desired profile of the fluid flow channels 12 
according to the present invention, it is preferred to use dry-etching 
techniques, such as reactive ion etching, on silicon or other materials. 
Channels can be formed in the surface of a silicon chip, as shown in FIG. 
2, and then another layer can be added over the main surface 60 of the 
chip as shown in FIG. 2, in order to enclose the channel 12. An alternate 
technique is to form the desired profiles of channels 12 in a layer of 
polyimide, and sandwich this layer of polyimide between two silicon chips, 
one or both of which may include a heating element 20 defined therein in 
an appropriate place. 
While the invention has been described with reference to the structure 
disclosed, it is not confined to the details set forth, but is intended to 
cover such modifications or changes as may come within the scope of the 
following claims.