Barrier structure for thermal ink-jet printheads

A three-sided barrier structure (22), comprising three walls (24a-c), is provided in conjunction with a resistor (10) used in a thermal ink-jet printhead. Placement of the structure less than about 25 .mu.m from the resistor results in longer resistor life and an improvement in the static bubble purging ability of the printhead.

TECHNICAL FIELD 
The present invention relates to ink-jet printers, and, more particularly, 
to improved thermal ink-jet printheads employed in such printers. 
BACKGROUND ART 
In thermal ink-jet printheads, thin film resistors are employed as heaters 
to form a bubble of ink over the resistor surface. The growth and collapse 
of the bubble causes an ink droplet to be ejected from an orifice 
associated with the resistor. The ejected droplet of ink is directed 
toward a medium, such as paper. 
At a predetermined time, as determined by a signal sent to the printer 
from, say a computer, the resistor is heated (by I.sup.2 R heating) to a 
temperature sufficient to vaporize a thin layer of ink directly over the 
resistor, which rapidly expands into a bubble. This expansion, in turn, 
causes part of the ink remaining between the resistor and the orifice to 
be expelled through the orifice toward the medium. In present use, the 
resistor is heated to provide a surface temperature of a few hundred 
degrees, at repetition frequencies up to 50 kHz and above. However, 
heating of the resistor itself lasts less than about 10 .mu.sec. 
The presence of wall-like structures, commonly called "barriers", in the 
immediate vicinity of a thermal ink-jet resistor has significant effects 
on the performance of the device. 
When a vapor bubble collapses over a resistor which has no barrier 
structure in its immediate vicinity (barriers which are several mils away 
have little effect), the event approximately has axial symmetry with the 
final collapse point at the center of the resistor. In this case, fluid 
can flow freely from all directions as the bubble collapses. 
When a wall or barrier is placed near the resistor, refill cannot occur 
from this direction, thus the bubble appears to be pushed towards the wall 
by fluid filling from all other directions. A single-sided barrier 
structure for an array of resistors is impractical to implement, since it 
would not actually isolate adjacent resistors, which is the original 
function of the barrier. A two-sided barrier configuration causes refill 
to occur from two directions; the final stages of bubble collapse occurs 
in an approximate line across the center of the resistor. Thus, the single 
collapse point (which in practice may be a small area) is spread into a 
line which reduces the rate or magnitude of impacting at any one point on 
the line. However, the bubble collapse attained does permit bubble 
collapse on the resistor and does permit refill to occur from more than 
one direction. 
Three-sided barriers have been shown, but due to their configuration, have 
not resulted in improving resistor life or expulsion of static bubbles. 
See, for example, U.S. Pats. Nos. 4,502,060; 4,503,444; 4,542,389; and 
4,550,326. 
DISCLOSURE OF INVENTION 
In accordance with the invention, a three-sided barrier structure adjacent 
a resistor in a thermal ink-jet printhead can provide a number of 
advantages if placed within certain critical distances. Placement of such 
barriers less than about 25 .mu.m from such resistors can provide (1) an 
increase in the life of a resistor by helping to sweep away the collapsing 
bubble from the center of the resistor and (2) an improvement in the 
self-purging by the printhead of static bubbles. 
A two-sided barrier structure, if placed less than about 25 .mu.m from the 
resistor, also provides an increase in the life of the resistor. However, 
the self-purging of static bubbles is not as readily attained as for the 
three-sided barrier structure.

BEST MODES FOR CARRYING OUT THE INVENTION 
Referring now to the drawings wherein like numerals of reference designate 
like elements throughout, a resistor 10 is depicted. In the following 
description, in each case, the ink droplet is ejected normal to the plane 
of the resistor. This is in contrast to configurations, in which the ink 
droplet is ejected parallel to the plane of the resistor. 
FIG. 1A illustrates a top plan view of a resistor 10 with no neighboring 
barrier structure. FIGS. 1B-D are line drawings of a portion of a 
photographic sequence showing how a vapor bubble 12 collapses near the 
center of the resistor 10. The lifetime of the resistor 10 is typically 
less than about 20.times.10.sup.6 firings. 
FIG. 2A illustrates a top plan view of a resistor 10 with a two-sided 
barrier structure 14 comprising two walls 16A, 16B, FIGS. 2B-D are line 
drawings of a portion of a photographic sequence showing a bubble 18 
elongating across the width of the resistor 10 as it collapses, finally 
breaking up into several bubble fragments before vanishing completely. 
It is seen that for the two-sided barrier configuration depicted, the 
bubble collapses in a band across the center of the resistor 10. Such 
bubble collapse is attained so long as the distance from the edge of the 
resistor 10 to the wall 16 is less than about 25 .mu.m, as discussed below 
in connection with the three-sided barrier structure. 
In configurations with distances greater than about 25 .mu.m, the bubble 
collapse is similar to that attained with no barrier structure. Thus, the 
bubble collapse band is an improvement over an essentially bubble collapse 
point, and accordingly, lifetime of the resistor is increased. For 
example, the lifetime of the resistor 10 where the walls 16 are greater 
than about 25 .mu.m from the resistor is typically less than about 
20.times.10.sup.6 firings, while the lifetime of the resistor where the 
walls are less than about 25 .mu.m from the resistor may range up to about 
100.times.10.sup.6 firings. 
However, the bubble does not move off the resistor 10 unless the barriers 
are offset, that is, closer on one side than on the other. An offset 
two-sided barrier may, therefore, be acceptable. 
While a parallel configuration is depicted, it will be appreciated that 
non-parallel configurations, as well as variations of parallel 
configurations, e.g., a "bracket" shape, may also be employed in the 
practice of the invention. 
Finally, static bubble elimination, achieved with the three-sided barrier 
structure, as described below, is not attained with the two-sided barrier 
structure 14, even within the indicated distance separation. Nonetheless, 
since resistor lifetime improvement is attained, this configuration is 
considered to fall within the scope of the invention. 
FIG. 3A illustrates a top plan view of a resistor 10 with a three-sided 
barrier structure 22 in accordance with the invention. The barrier 
structure comprises three walls 24A, 24B, 24C. FIGS. 3B-D are line 
drawings of a portion of a photographic sequence showing a collapsing 
bubble 26 which is shifted toward the third side 24C of the barrier 
structure 22 by the refilling liquid (not shown) which enters from the 
open side of the barrier structure, as indicated by arrow 28. The final 
stages of bubble collapse take place off the resistor 10, forming bubble 
fragments 30 along the rear wall 24C. 
The three-sided barrier structure 22 of the invention may comprise, for 
example, a block U-shaped configuration, with the resistor 10 placed in 
the bight of the U, as depicted in FIG. 3A, or variants thereof, so long 
as one side remains open for entry of ink, indicated by arrow 28, from an 
ink reservoir (not shown). 
It should be noted that the photographs upon which the line drawings of 
FIGS. 1B-D, 2B-D and 3B-D are based were for a pond test and that the 
details of the collapsing bubbles in a completely assembled printhead 
(with an orifice plate--not shown) may be somewhat different. However, the 
basic principles would remain the same. 
The three-sided barrier structure 22 of the invention should be placed such 
that none of the walls 24A-C are no further than about 25 .mu.m from the 
resistor 10. Such placement provides an increase in the life of the 
resistor 10 by helping to sweep away the collapsing bubble from the center 
of the resistor, as shown in FIGS. 3B-D. For example, the lifetime of the 
resistor 10 where the walls 24 are greater than about 25 .mu.m from the 
resistor is typically less than about 20.times.10.sup.6 firings, while the 
lifetime of the resistor where the walls are less than about 25 .mu.m from 
the resistor may range up to about 200.times.10.sup.6 firings. Where the 
walls 24 are less than about 10 .mu.m from the resistor 10, the lifetime 
may exceed 200.times.10.sup.6 firings. 
Sweeping the collapsing bubble from the center of the resistor 10 increases 
the life of the resistor, since cavitation, which is a problem with 
structures of less than three sides, is greatly reduced. Such cavitation 
results in a shock wave which strikes the same area (typically the central 
area) on the resistor 10 each time the resistor is pulsed to fire a 
bubble. The cavitation effect leads to erosion of the bubble collapse area 
and concomitant early failure of the resistor. This problem is further 
exacerbated by the fact that the center of the resistor 10 is also the 
hottest region, and the coincidence of the bubble collapse area with the 
center of the resistor results in additional erosion. 
Use of the three-sided barrier structure 22 of the invention and placement 
thereof less than about 25 .mu.m from the resistor 10 also provides an 
improvement in the self-purging by the printhead of static bubbles. Static 
bubbles (not shown) contain gases rather than vaporized ink vehicle and 
enter the head by a variety of mechanisms. Their "collapse", by dissolving 
back into the ink, can take from about 10 to 10.sup.9 times longer than 
vapor bubbles, depending on their size. 
Preferably, the barrier 22 should be within about 10 .mu.m of the resistor 
10, and most preferably within about 5 .mu.m, in order to fully realize 
the benefits of the sweeping effect. Also, accumulation of microbubbles 
and growth thereof on the walls 24A-C of the barrier 22 is minimized as 
the walls are moved closer to the resistor, especially in the range of 
less than about 10 .mu.m. 
Asymmetrical placement of the barrier structure 22 about the resistor 10 is 
not critical, so long as the maximum distance listed above is not exceeded 
on any of the three sides adjacent a barrier wall 24. It appears that the 
smallest distance between the resistor 10 and the wall 24 controls where 
the bubble will move to. However, it will be remembered that static 
bubbles tend to be stored in large spaces, so that while some misalignment 
between the resistor 10 and the barrier structure 22 is acceptable, such 
misalignment should be minimized. 
The barrier structure 22 may comprise suitable polymeric or metallic 
materials. Examples of such materials include dry film resists, such as 
Vacrel and Riston, available from E. I. duPont de Nemours (Wilmington, 
Del.), polyimide compositions, plated nickel, and the like. 
The three-sided barrier structure 22 of the invention, with walls 24 within 
the critical distance of the resistor 10, afford several advantages over 
one- and two-barrier configurations. First, because refill is from one 
direction, the collapsing bubble 26 is swept off the resistor toward the 
"back" barrier wall 24C. There is also a tendency for the bubble 26 to 
divide into several components 30, which weakens the collapse energy at 
any given point. 
Further, the barrier structure 22 assists the purging of static bubbles 
which may have several origins: (1) air trapped in the printhead when it 
is first filled with ink; (2) gases dissolved in the ink which come out of 
solution; (3) air gulped in from outside during operation due to a 
meniscus folding back on itself; (4) gaseous products of chemical 
corrosion; and (5) agglomeration of microbubbles. 
With other prior art approaches, when a static bubble resides in the 
immediate neighborhood of the resistor 10, it receives a strong impulse 
force every time a vapor bubble exposion occurs; this moves the static 
bubble to another location. With the three-sided barrier structure 20 of 
the invention, the bubble is confined to remain in the immediate vicinity 
of the resistor by three physical walls 24A-C and one virtual wall, which 
is the refill flow from the fourth direction, shown by arrow 28 in FIG. 
3A. 
It is also possible for the static bubble to be moved into the fluid region 
directly above the resistor, in which case it may be ejected from the 
printhead with the next drop. In fact, this may be expected to happen 
eventually after some number of impulses. 
For one- or two-sided barriers, the static bubble may move away from the 
resistor to a region where the vapor explosion force cannot influence it 
(although the static bubble may have a large effect on device operation). 
It should be noted that this problem is likely to occur with placement of 
the three-sided barrier 22 at a distance much greater than about 25 .mu.m 
from the resistor 10, since the bubble can be trapped between the resistor 
and the barrier wall and not be influenced by vapor bubble explosions. 
INDUSTRIAL APPLICABILITY 
Two- and three-sided barrier wall configurations associated with resistors 
used in thermal ink-jet printers, spaced less than about 25 .mu.m from 
such resistors, are expected to find use in printers to improve resistor 
life and, in the case of three-sided barrier structures, static bubble 
purging ability of the printhead. 
Thus, two- and three-sided barrier wall configurations, to be used in 
association with a resistor employed in a thermal ink-jet printhead and 
spaced no more than about 25 .mu.m from the resistor, have been disclosed. 
Placement of such barriers within the critical distance from the resistor 
results in longer resistor life and, in the case of three-sided 
configurations, an improvement in the static bubble purging ability of the 
printhead. Many modifications and changes of an obvious nature will make 
themselve apparent to those of ordinary skill in the art, and all such 
modifications and changes are deemed to fall within the scope of the 
invention, as defined by the appended claims.