Note: Descriptions are shown in the official language in which they were submitted.
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PRINTHEAD HAVING POLYSILSESQUIOXANE COATING ON INK EJECTION
FACE
Field of the Invention
The present invention relates to the field of printers and particularly inkjet
printheads. It has been developed primarily to improve print quality and
printhead
maintenance in high resolution printheads.
Background of the Invention
Many different types of printing have been invented, a large number of which
are
presently in use. The known forms of print have a variety of methods for
marking the print
media with a relevant marking media. Commonly used forms of printing include
offset
printing, laser printing and copying devices, dot matrix type impact printers,
thermal paper
printers, film recorders, thermal wax printers, dye sublimation printers and
ink jet printers
both of the drop on demand and continuous flow type. Each type of printer has
its own
advantages and problems when considering cost, speed, quality, reliability,
simplicity of
construction and operation etc.
In recent years, the field of ink jet printing, wherein each individual pixel
of ink is
derived from one or more ink nozzles has become increasingly popular primarily
due to its
inexpensive and versatile nature.
Many different techniques on ink jet printing have been invented. For a survey
of
the field, reference is made to an article by J Moore, "Non-Impact Printing:
Introduction
and Historical Perspective", Output Hard Copy Devices, Editors R Dubeck and S
Sherr,
pages 207 - 220 (1988).
Ink Jet printers themselves come in many different types. The utilization of a
continuous stream of ink in ink jet printing appears to date back to at least
1929 wherein
US Patent No. 1941001 by Hansell discloses a simple form of continuous stream
electro-
static ink jet printing.
US Patent 3596275 by Sweet also discloses a process of a continuous ink jet
printing including the step wherein the ink jet stream is modulated by a high
frequency
electro-static field so as to cause drop separation. This technique is still
utilized by several
manufacturers including Elmjet and Scitex (see also US Patent No. 3373437 by
Sweet et
al)
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Piezoelectric ink jet printers are also one form of commonly utilized ink jet
printing
device. Piezoelectric systems are disclosed by Kyser et. al. in US Patent No.
3946398
(1970) which utilizes a diaphragm mode of operation, by Zolten in US Patent
3683212
(1970) which discloses a squeeze mode of operation of a piezoelectric crystal,
Stemme in
US Patent No. 3747120 (1972) discloses a bend mode of piezoelectric operation,
Howkins
in US Patent No. 4459601 discloses a piezoelectric push mode actuation of the
ink jet
stream and Fischbeck in US 4584590 which discloses a shear mode type of
piezoelectric
transducer element.'
Recently, thermal ink jet printing has become an extremely popular form of ink
jet
printing. The ink jet printing techniques include those disclosed by Endo et
al in GB
2007162 (1979) and Vaught et al in US Patent 4490728. Both the aforementioned
references disclosed ink jet printing techniques that rely upon the activation
of an
electrothermal actuator which results in the creation of a bubble in a
constricted space,
such as a nozzle, which thereby causes the ejection of ink from an aperture
connected to
the confined space onto a relevant print media. Printing devices utilizing the
electro-
thermal actuator are manufactured by manufacturers such as Canon and Hewlett
Packard.
As can be seen from the foregoing, many different types of printing
technologies
are available. Ideally, a printing technology should have a number of
desirable attributes.
These include inexpensive construction and operation, high speed operation,
safe and
continuous long term operation etc. Each technology may have its own
advantages and
disadvantages in the areas of cost, speed, quality, reliability, power usage,
simplicity of
construction operation, durability and consumables.
In the construction of any inkjet printing system, there are a considerable
number of
important factors which must be traded off against one another especially as
large scale
printheads are constructed, especially those of a pagewidth type. A number of
these
factors are outlined below.
Firstly, inkjet printheads are normally constructed utilizing micro-
electromechanical systems (MEMS) techniques. As such, they tend to rely upon
standard
integrated circuit construction/fabrication techniques of depositing planar
layers on a
silicon wafer and etching certain portions of the planar layers. Within
silicon circuit
fabrication technology, certain techniques are better known than others. For
example, the
techniques associated with the creation of CMOS circuits are likely to be more
readily used
than those associated with the creation of exotic circuits including
ferroelectrics, gallium
arsenide etc. Hence, it is desirable, in any MEMS constructions, to utilize
well proven
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semi-conductor fabrication techniques which do not require any "exotic"
processes or
materials. Of course, a certain degree of trade off will be undertaken in that
if the
advantages of using the exotic material far out weighs its disadvantages then
it may
become desirable to utilize the material anyway. However, if it is possible to
achieve the
same, or similar, properties using more common materials, the problems of
exotic
materials can be avoided.
A desirable characteristic of inkjet printheads would be a hydrophobic ink
ejection
face ("front face" or "nozzle face"), preferably in combination with
hydrophilic nozzle
chambers and ink supply channels. Hydrophilic nozzle chambers and ink supply
channels
to provide a capillary action and are therefore optimal for priming and for
re-supply of ink to
nozzle chambers after each drop ejection. A hydrophobic front face minimizes
the propensity
for ink to flood across the front face of the printhead. With a hydrophobic
front face, the
aqueous inkjet ink is less likely to flood sideways out of the nozzle
openings. Furthermore,
any ink which does flood from nozzle openings is less likely to spread across
the face and mix
on the front face ¨ they will instead form discrete spherical microdroplets
which can be
managed more easily by suitable maintenance operations.
Hitherto, the present Applicant has described the use of PDMS
(polydimethylsiloxane) for coating the front face of a printhead and providing
a
hydrophobic surface. However, whilst PDMS has excellent hydrophobic properties
and can
be readily incorporated into a printhead MEMS fabrication process, it has
relatively poor
wear-resistance and may be scratched or otherwise damaged by a wiper blade
used for
printhead maintenance (see, for example, US Patent No. 7,758,149). It would
therefore be
desirable to provide a printhead having a hydrophobic ink ejection face, which
can be
readily produced by a MEMS fabrication process and which has good wear-
resistance.
Summary of the Invention
In a first aspect, there is provided a printhead having an ink ejection face,
wherein
at least part of the ink ejection face is coated with a hydrophobic polymeric
material, the
polymeric material being comprised of a polysilsesquioxane. Printheads
according to the
present invention have excellent durability and wear-resistance making them
compatible
with various printhead maintenance operation involving contact with the ink
ejection face
(e.g. wiping). Moreover, the polysilsesquioxane can be deposited in a thin
layer (0.5 to 2
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microns) by a spin-on process, which is readily incorporated into a MEMS
printhead
fabrication process.
Optionally, the polysilsesquioxane is selected from the group consisting of:
poly(alkylsilsesquioxanes) and poly(arylsilsesquioxanes)
Optionally, the polysilsesquioxane is selected from the group consisting of:
poly(methylsilsesquioxane) and poly(phenylsilsesquioxane).
Optionally, the polymeric material is deposited and hardbaked onto a nozzle
plate
of the printhead during MEMS printhead fabrication.
Optionally, the printhead comprises a plurality of nozzle assemblies formed on
a
1() substrate, each nozzle assembly comprising: a nozzle chamber, a nozzle
opening defined in
a roof of the nozzle chamber and an actuator for ejecting ink through the
nozzle opening.
Optionally, the polymeric material is coated on a nozzle plate of the
printhead, the
nozzle plate being at least partially defined by the roof of each nozzle
chamber.
Optionally, each roof has a hydrophobic outside surface relative to the inside
surfaces of each nozzle chamber by virtue of the hydrophobic coating.
Optionally, each nozzle chamber comprises a roof and sidewalls comprised of a
ceramic material.
Optionally, the ceramic material is selected from the group consisting of:
silicon
nitride, silicon oxide and silicon oxynitride.
Optionally, the roof is spaced apart from the substrate, such that sidewalls
of each
nozzle chamber extend between the nozzle plate and the substrate.
Optionally, the actuator is a heater element configured for heating ink in the
chamber so as to form a gas bubble, thereby forcing a droplet of ink through
the nozzle
opening.
Optionally, the heater element is suspended in the nozzle chamber.
Optionally, the actuator is a thermal bend actuator comprising:
a first active element for connection to drive circuitry; and
a second passive element mechanically cooperating with the first element,
such that when a current is passed through the first element, the first
element
expands relative to the second element, resulting in bending of the actuator.
Optionally, the thermal bend actuator defines at least part of a roof of each
nozzle
chamber, whereby actuation of the actuator moves said a moving portion of the
roof
towards a floor of said nozzle chamber.
Optionally, the nozzle opening is defined in said moving portion of the roof.
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Optionally, nozzle opening is defined in a stationary portion of the roof
Optionally, the polymeric material defines a mechanical seal between the
moving
portion and a stationary portion of the roof, thereby minimizing ink leakage
during
actuation of the actuator.
5 In a second aspect, there is provided a printhead having an ink ejection
face,
wherein at least part of the ink ejection face is coated with a polymeric
material, said
polymeric material being comprised of a polymerized siloxane incorporating
nanoparticles.
In accordance with the second aspect, the nanoparticles impart desirable
properties to the
polymeric coating, such as durability, wear-resistance, fatigue-resistance,
hydrophobicity,
hydrophilicity etc.
Optionally, the polymerized siloxane is selected from the group consisting of:
poly(alkylsilsesquioxanes), poly(arylsilsesquioxanes) and polydialkylsiloxanes
Optionally, the polymerized siloxane is selected from the group consisting of:
poly(methylsilsesquioxane), poly(phenylsilsesquioxane) and
polydimethylsiloxane.
Optionally, the nanoparticles are selected from the group consisting of:
inorganic
nanoparticles and organic nanoparticles.
Optionally, the inorganic nanoparticles are selected from the group consisting
of:
metal oxides, metal carbonates and metal sulfates.
Optionally, the inorganic nanoparticles are selected from the group consisting
of:
silica, zirconium oxide, titanium oxide, aluminium oxide, calcium carbonate,
tin oxide, zinc
oxide, copper oxide, chromium oxide, calcium oxide, tungsten oxide, iron
oxide, cobalt
oxide and barium sulfate.
Optionally, the organic nanoparticles are selected from the group consisting
of:
cross-linked silicone resin particles, cross-linked polyolefin resin
particles, cross-linked
acryl resin particles, cross-linked styrene-acryl resin particles, cross-
linked polyester
particles, polyimide particles, melamine resin particles and carbon nanotubes.
Optionally, the nanoparticles are incorporated in the polymerized siloxane in
an
amount ranging from 1 to 70 wt.%.
Optionally, the nanoparticles have an average particle size in the range of 1
to
100nm.
Optionally, the printhead compriss a plurality of nozzle assemblies formed on
a
substrate, each nozzle assembly comprising: a nozzle chamber, a nozzle opening
defined in
a roof of the nozzle chamber and an actuator for ejecting ink through the
nozzle opening.
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Optionally, the polymeric material is coated on a nozzle plate of the
printhead, the
nozzle plate being at least partially defined by the roof of each nozzle
chamber.
Optionally, each nozzle chamber comprises a roof and sidewalls comprised of a
ceramic material selected from the group consisting of: silicon nitride,
silicon oxide and
silicon oxynitride.
Optionally, the roof is spaced apart from the substrate, such that sidewalls
of each
nozzle chamber extend between the nozzle plate and the substrate.
Optionally, the actuator is a heater element configured for heating ink in the
chamber so as to form a gas bubble, thereby forcing a droplet of ink through
the nozzle
opening.
Optionally, the heater element is suspended in the nozzle chamber.
Optionally, the actuator is a thermal bend actuator comprising:
a first active element for connection to drive circuitry; and
a second passive element mechanically cooperating with the first element,
such that when a current is passed through the first element, the first
element
expands relative to the second element, resulting in bending of the actuator.
Optionally, the thermal bend actuator defines at least part of a roof of each
nozzle
chamber, whereby actuation of said actuator moves said a moving portion of
said roof
towards a floor of said nozzle chamber.
Optionally, the nozzle opening is defined in either one of: said moving
portion of
said roof; or a stationary portion of said roof.
Optionally, the polymeric material defines a mechanical seal between said
moving
portion and a stationary portion of said roof, thereby minimizing ink leakage
during
actuation of said actuator.
In a third aspect, there is provided an inkjet printhead for ejection of an
ejectable
fluid, the printhead having an ink ejection face coated with a polymeric
material
incorporating nanoparticles, wherein the nanoparticles impart one or more
predetermined
characteristics to the ink ejection face, the predetermined characteristics
complementing at
least one of:
an inherent property of the ejectable fluid;
a printhead maintenance regime associated with the printhead; and
a type of nozzle actuator.
The invention according to the third aspect, enables the surface
characteristics of
the ink ejection face to be tuned to a predetermined characteristic of the
printer. For
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example, printhead maintenance may be prioritized in some printers, whereas
optimal fluid
ejection may be prioritized in other printers. Alternatively, the
nanoparticles may be
selected to provide a compromise of printer characteristics.
Optionally, the one or more predetermined characteristics are selected from
the
group consisting of: hydrophilicity; hydrophobicity; wear-resistance; and
fatigue-
resistance.
Optionally, the one or more predetermined characteristics are imparted by one
or
more of: surface energy characteristics of the nanoparticles; size of the
nanoparticles;
amount of the nanoparticles; and wearability of the nanoparticles.
Optionally, the nanoparticles are selected from the group consisting of:
inorganic
nanoparticles and organic nanoparticles.
Optionally, the inorganic nanoparticles are selected from the group consisting
of:
silica, zirconium oxide, titanium oxide, aluminium oxide, calcium carbonate,
tin oxide, zinc
oxide, copper oxide, chromium oxide, calcium oxide, tungsten oxide, iron
oxide, cobalt
oxide and barium sulfate.
Optionally, the organic nanoparticles are selected from the group consisting
of:
cross-linked silicone resin particles, cross-linked polyolefin resin
particles, cross-linked
acryl resin particles, cross-linked styrene-acryl resin particles, cross-
linked polyester
particles, polyimide particles, melamine resin particles and carbon nanotubes.
Optionally, the the inherent property of the ejectable fluid is selected from
the
group consisting of: hydrophilicity; hydrophobicity; viscosity; surface
tension; and boiling
point.
Optionally, the ejectable fluid is selected from the group consisting of:
aqueous
fluids and non-aqueous fluids.
Optionally, the printhead maintenance regime comprises one or more operations
selected from the group consisting of: printhead capping; printhead wiping;
printhead
flooding; and non-contact ink removal.
Optionally, the the polymeric material is a comprised of a polymerized
siloxane.
Optionally, the polymerized siloxane is selected from the group consisting of:
poly(alkylsilsesquioxanes), poly(arylsilsesquioxanes) and polydialkylsiloxanes
Optionally, the polymerized siloxane is selected from the group consisting of:
poly(methylsilsesquioxane), poly(phenylsilsesquioxane) and
polydimethylsiloxane.
Other optional embodiments of the printhead according to the third aspect
mirror
those optional embodiments according to the first and second aspects.
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Brief Description of the Drawings
Optional embodiments of the present invention will now be described by way of
example only with reference to the accompanying drawings, in which:
Figure 1 is a partial perspective view of an array of nozzle assemblies of a
thermal
inkjet printhead;
Figure 2 is a side view of a nozzle assembly unit cell shown in Figure 1;
Figure 3 is a perspective of the nozzle assembly shown in Figure 2;
Figure 4 shows a partially-formed nozzle assembly after deposition of side
walls
and roof material onto a sacrificial photoresist layer;
Figure 5 is a perspective of the nozzle assembly shown in Figure 4;
Figure 6 is the mask associated with the nozzle rim etch shown in Figure 7;
Figure 7 shows the etch of the roof layer to form the nozzle opening rim;
Figure 8 is a perspective of the nozzle assembly shown in Figure 7;
Figure 9 is the mask associated with the nozzle opening etch shown in Figure
10;
Figure 10 shows the etch of the roof material to form the elliptical nozzle
openings;
Figure 11 is a perspective of the nozzle assembly shown in Figure 10;
Figure 12 shows the oxygen plasma ashing of the first and second sacrificial
layers;
Figure 13 is a perspective of the nozzle assembly shown in Figure 12;
Figure 14 shows the nozzle assembly after the ashing, as well as the opposing
side
of the wafer;
Figure 15 is a perspective of the nozzle assembly shown in Figure 14;
Figure 16 is the mask associated with the backside etch shown in Figure 17;
Figure 17 shows the backside etch of the ink supply channel into the wafer;
Figure 18 is a perspective of the nozzle assembly shown in Figure 17;
Figure 19 shows the nozzle assembly of Figure 7 after deposition of a
hydrophobic
polymeric coating;
Figure 20 is a perspective of the nozzle assembly shown in Figure 19;
Figure 21 shows the nozzle assembly of Figure 19 after deposition of a
protective
metal film; and
Figure 22 shows the nozzle assembly of Figure 21 after etching through the
protective metal film, the polymeric coating and the nozzle roof;
Figure 23 shows the completed nozzle assembly after backside MEMS processing
and removal of photoresist;
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Figure 24 is a perspective of the nozzle assembly shown in Figure 23;
Figure 25 is a side-sectional view of a partially-fabricated alternative
inkjet nozzle
assembly after a first sequence of steps in which nozzle chamber sidewalls are
formed;
Figure 26 is a perspective view of the partially-fabricated inkjet nozzle
assembly
shown in Figure 25;
Figure 27 is a side-sectional view of a partially-fabricated inkjet nozzle
assembly
after a second sequence of steps in which the nozzle chamber is filled with
polyimide;
Figure 28 is a perspective view of the partially-fabricated inkjet nozzle
assembly
shown in Figure 27;
Figure 29 is a side-sectional view of a partially-fabricated inkjet nozzle
assembly
after a third sequence of steps in which connector posts are formed up to a
chamber roof;
Figure 30 is a perspective view of the partially-fabricated inkjet nozzle
assembly
shown in Figure 29;
Figure 31 is a side-sectional view of a partially-fabricated inkjet nozzle
assembly
after a fourth sequence of steps in which conductive metal plates are formed;
Figure 32 is a perspective view of the partially-fabricated inkjet nozzle
assembly
shown in Figure 31;
Figure 33 is a side-sectional view of a partially-fabricated inkjet nozzle
assembly
after a fifth sequence of steps in which an active beam member of a thermal
bend actuator
is formed;
Figure 34 is a perspective view of the partially-fabricated inkjet nozzle
assembly
shown in Figure 33;
Figure 35 is a side-sectional view of a partially-fabricated inkjet nozzle
assembly
after a sixth sequence of steps after coating with a polymeric layer,
protecting with a metal
layer and etching a nozzle opening;
Figure 36 is a side-sectional view of completed inkjet nozzle assembly, after
backside MEMS processing and removal of photoresist; and
Figure 37 is a cutaway perspective view of the inkjet nozzle assembly shown in
Figure 36.
Description of Optional Embodiments
The present invention may be used with any type of printhead. The present
Applicant
has previously described a plethora of inkjet printheads. It is not necessary
to describe all
such printheads here for an understanding of the present invention. However,
the present
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invention will now be described in connection with a thermal bubble-forming
inkjet
printhead and a mechanical thermal bend actuated inkjet printhead. Advantages
of the
present invention will be readily apparent from the discussion that follows.
5 Thermal Bubble-Forming Inkjet Printhead
Referring to Figure 1, there is shown a part of printhead comprising a
plurality of
nozzle assemblies. Figures 2 and 3 show one of these nozzle assemblies in side-
section and
cutaway perspective views.
Each nozzle assembly comprises a nozzle chamber 24 formed by MEMS fabrication
10 techniques on a silicon wafer substrate 2. The nozzle chamber 24 is
defined by a roof 21 and
sidewalls 22 which extend from the roof 21 to the silicon substrate 2. As
shown in Figure 1,
each roof is defined by part of a nozzle surface 56, which spans across an
ejection face of the
printhead. The nozzle surface 56 and sidewalls 22 are formed of the same
material, which is
deposited by PECVD over a sacrificial scaffold of photoresist during MEMS
fabrication.
Typically, the nozzle surface 56 and sidewalls 22 are formed of a ceramic
material, such as
silicon dioxide or silicon nitride. These hard materials have excellent
properties for printhead
robustness, and their inherently hydrophilic nature is advantageous for
supplying ink to the
nozzle chambers 24 by capillary action. However, the exterior (ink ejection)
surface of the
nozzle surface 56 is also hydrophilic, which causes any flooded ink on the
surface to spread.
Returning to the details of the nozzle chamber 24, it will be seen that a
nozzle
opening 26 is defined in a roof of each nozzle chamber 24. Each nozzle opening
26 is
generally elliptical and has an associated nozzle rim 25. The nozzle rim 25
assists with drop
directionality during printing as well as reducing, at least to some extent,
ink flooding from
the nozzle opening 26. The actuator for ejecting ink from the nozzle chamber
24 is a heater
element 29 positioned beneath the nozzle opening 26 and suspended across a pit
8. Current is
supplied to the heater element 29 via electrodes 9 connected to drive
circuitry in underlying
CMOS layers 5 of the substrate 2. When a current is passed through the heater
element 29, it
rapidly superheats surrounding ink to form a gas bubble, which forces ink
through the nozzle
opening. By suspending the heater element 29, it is completely immersed in ink
when the
nozzle chamber 24 is primed. This improves printhead efficiency, because less
heat
dissipates into the underlying substrate 2 and more input energy is used to
generate a bubble.
As seen most clearly in Figure 1, the nozzles are arranged in rows and an ink
supply
channel 27 extending longitudinally along the row supplies ink to each nozzle
in the row.
The ink supply channel 27 delivers ink to an ink inlet passage 15 for each
nozzle, which
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supplies ink from the side of the nozzle opening 26 via an ink conduit 23 in
the nozzle
chamber 24.
The MEMS fabrication process for manufacturing such printheads was described
in
detail in our previously filed US Patent No. 7,303,930. The latter stages of
this fabrication
process are briefly revisited here for the sake of clarity.
Figures 4 and 5 show a partially-fabricated printhead comprising a nozzle
chamber
24 encapsulating sacrificial photoresist 10 ("SAC1") and 16 ("SAC2"). The SAC1
photoresist 10 was used as a scaffold for deposition of heater material to
form the suspended
heater element 29. The SAC2 photoresist 16 was used as a scaffold for
deposition of the
to sidewalls 22 and roof 21 (which defines part of the nozzle surface 56).
In the prior art process, and referring to Figures 6 to 8, the next stage of
MEMS
fabrication defines the elliptical nozzle rim 25 in the roof 21 by etching
away 2 microns of
roof material 20. This etch is defined using a layer of photoresist (not
shown) exposed by
the dark tone rim mask shown in Figure 6. The elliptical rim 25 comprises two
coaxial rim
lips 25a and 25b, positioned over their respective thermal actuator 29.
Referring to Figures 9 to 11, the next stage defines an elliptical nozzle
aperture 26
in the roof 21 by etching all the way through the remaining roof material,
which is
bounded by the rim 25. This etch is defined using a layer of photoresist (not
shown)
exposed by the dark tone roof mask shown in Figure 9. The elliptical nozzle
aperture 26 is
positioned over the thermal actuator 29, as shown in Figure 11.
With all the MEMS nozzle features now fully formed, the next stage removes the
SAC1 and SAC2 photoresist layers 10 and 16 by 02 plasma ashing (Figures 12 and
13).
Figures 14 and 15 show the entire thickness (150 microns) of the silicon wafer
2 after
ashing the SAC1 and SAC2 photoresist layers 10 and 16.
Referring to Figures 16 to 18, once frontside MEMS processing of the wafer is
completed, ink supply channels 27 are etched from the backside of the wafer to
meet with
the ink inlets 15 using a standard anisotropic DRIE. This backside etch is
defined using a
layer of photoresist (not shown) exposed by the dark tone mask shown in Figure
16. The
ink supply channel 27 makes a fluidic connection between the backside of the
wafer and
the ink inlets 15.
Finally, and referring to Figures 2 and 3, the wafer is thinned to about 135
microns
by backside etching. Figure 1 shows three adjacent rows of nozzles in a
cutaway
perspective view of a completed printhead integrated circuit. Each row of
nozzles has a
respective ink supply channel 27 extending along its length and supplying ink
to a plurality
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of ink inlets 15 in each row. The ink inlets, in turn, supply ink to the ink
conduit 23 for
each row, with each nozzle chamber receiving ink from a common ink conduit for
that
row.
As already discussed above, this prior art MEMS fabrication process inevitably
leaves a hydrophilic ink ejection face by virtue of the nozzle surface 56
being formed of
ceramic materials, such as silicon dioxide, silicon nitride, silicon
oxynitride, aluminium
nitride etc.
In a preferred process for hydrophobizing the nozzle surface 56 (and as
described in
US 2009/0139961), the wafer is coated with a hydrophobic polymer 80
immediately after the
to nozzle rim etch at the stage exemplified in Figures 7 and 8.
A thin layer (about 1 to 2 microns) of the hydrophobic polymer 100 is spun
onto the
wafer and hardbaked to provide the partially-fabricated printhead shown in
Figures 19 and
20.
Referring now to Figure 21, a protective metal film 90 (ca. 100 nm thickness)
is then
deposited onto the polymer layer 80. The metal film is typically comprised of
titanium or
aluminium and protects the hydrophobic polymer 80 from late-stage oxygen
ashing
conditions. Hence, the polymer layer 80 is not exposed to aggressive ashing
conditions and
retains its hydrophobic characteristics throughout the MEMS processing steps.
Figure 22 shows the wafer after etching the nozzle opening 26 through the
metal
film 110, the polymer layer 80 and the nozzle roof 21. This etching step
utilizes a
conventional patterned photoresist layer (not shown) as a common mask for all
nozzle
etching steps. In a typical etching sequence, the metal film 90 is first
etched, either by
standard dry metal-etching (e.g. BC13/C12) or wet metal-etching (e.g. H202 or
HF). A
second dry etch is then used to etch through the polymer layer 80 and the
nozzle roof 21.
Typically, the second etch step is a dry etch employing 02 and a fluorinated
etching gas
(e.g. SF6 or CEO.
Once the nozzle opening 26 is defined as shown in Figure 22, backside MEMS
processing steps (e.g. etching ink supply channels, wafer thinning etc) and
late-stage
ashing of photoresist can proceed in accordance with known protocols,
analogous to the
steps described above in connection with Figures 14 to 18. Final removal of
the metal film
90 using a H202 or HF rinse yields the completed nozzle assembly shown in
Figures 23 and
24, having the hydrophobic polymer layer 80.
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Thermal Bend Actuator Printhead
From the foregoing, it will be appreciated that any type of printhead may be
hydrophobized in an analogous manner. However, the polymeric coatings are
particularly
advantageous for use in the Applicant's thermal bend actuator nozzle
assemblies, because
the polymer layer acts as a mechanical seal between a moving roof portion and
a stationary
body of the printhead. These advantages are discussed in greater detail in the
Applicant's US
Publication No. 2008/0225076.
Figures 25 to 37 shows a sequence of MEMS fabrication steps for an inkjet
nozzle
to assembly 100 described in our earlier US Publication No. US
2008/0309728. The
completed inkjet nozzle assembly 100 shown in Figures 36 and 37 utilizes
thermal bend
actuation, whereby a moving portion of a roof bends towards a substrate
resulting in ink
ejection.
The starting point for MEMS fabrication is a standard CMOS wafer having CMOS
drive circuitry formed in an upper portion of a silicon wafer. At the end of
the MEMS
fabrication process, this wafer is diced into individual printhead integrated
circuits (ICs),
with each IC comprising drive circuitry and plurality of nozzle assemblies.
As shown in Figures 25 and 26, a substrate 101 has an electrode 102 formed in
an
upper portion thereof. The electrode 102 is one of a pair of adjacent
electrodes (positive
and earth) for supplying power to an actuator of the inkjet nozzle 100. The
electrodes
receive power from CMOS drive circuitry (not shown) in upper layers of the
substrate 101.
The other electrode 103 shown in Figures 25 and 26 is for supplying power to
an
adjacent inkjet nozzle. In general, the drawings shows MEMS fabrication steps
for a
nozzle assembly, which is one of an array of nozzle assemblies. The following
description
focuses on fabrication steps for one of these nozzle assemblies. However, it
will of course
be appreciated that corresponding steps are being performed simultaneously for
all nozzle
assemblies that are being formed on the wafer. Where an adjacent nozzle
assembly is
partially shown in the drawings, this can be ignored for the present purposes.
Accordingly,
the electrode 103 and all features of the adjacent nozzle assembly will not be
described in
detail herein. Indeed, in the interests of clarity, some MEMS fabrication
steps will not be
shown on adjacent nozzle assemblies.
In the sequence of steps shown in Figures 25 and 26, an 8 micron layer of
silicon
dioxide is initially deposited onto the substrate 101. The depth of silicon
dioxide defines
the depth of a nozzle chamber 105 for the inkjet nozzle. After deposition of
the Si02 layer,
CA 02760206 2013-08-06
14
it is etched to define walls 104, which will become sidewalls of the nozzle
chamber 105,
shown most clearly in Figure 26.
As shown in Figures 27 and 28, the nozzle chamber 105 is then filled with
photoresist or polyimide 106, which acts as a sacrificial scaffold for
subsequent deposition
steps. The polyimide 106 is spun onto the wafer using standard techniques, UV
cured
and/or hardbaked, and then subjected to chemical mechanical planarization
(CMP)
stopping at the top surface of the Si02 wall 104.
In Figures 29 and 30, a roof member 107 of the nozzle chamber 105 is formed as
well as highly conductive connector posts 108 extending down to the electrodes
102.
Initially, a 1.7 micron layer of Si02 is deposited onto the polyimide 106 and
wall 104. This
layer of Si02 defines a roof 107 of the nozzle chamber 105. Next, a pair of
vias are formed
in the wall 104 down to the electrodes 102 using a standard anisotropic DRIE.
This etch
exposes the pair of electrodes 102 through respective vias. Next, the vias are
filled with a
highly conductive metal, such as copper, using electroless plating. The
deposited copper
posts 108 are subjected to CMP, stopping on the Si02 roof member 107 to
provide a planar
structure. It can be seen that the copper connector posts 108, formed during
the electroless
copper plating, meet with respective electrodes 102 to provide a linear
conductive path up
to the roof member 107.
In Figures 31 and 32, metal pads 109 are formed by initially depositing a 0.3
micron layer of aluminium onto the roof member 107 and connector posts 108.
Any highly
conductive metal (e.g. aluminium, titanium etc.) may be used and should be
deposited with
a thickness of about 0.5 microns or less so as not to impact too severely on
the overall
planarity of the nozzle assembly. The metal pads 109 are positioned over the
connector
posts 108 and on the roof member 107 in predetermined 'bend regions' of the
thermoelastic active beam member.
In Figures 33 and 34, a thermoelastic active beam member 110 is formed over
the
Si02 roof 107. By virtue of being fused to the active beam member 110, part of
the Si02
roof member 107 functions as a lower passive beam member 116 of a mechanical
thermal
bend actuator, which is defined by the active beam 110 and the passive beam
116. The
thermoelastic active beam member 110 may be comprised of any suitable
thermoelastic
material, such as titanium nitride, titanium aluminium nitride and aluminium
alloys. As
explained in the Applicant's earlier US Publication No. 2008/0129793, vanadium-
aluminium alloys are a preferred
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material, because they combine the advantageous properties of high thermal
expansion,
low density and high Young's modulus.
To form the active beam member 110, a 1.5 micron layer of active beam material
is
initially deposited by standard PECVD. The beam material is then etched using
a standard
5 metal etch to define the active beam member 110. After completion of the
metal etch and
as shown in Figures 33 and 34, the active beam member 110 comprises a partial
nozzle
opening 111 and a beam element 112, which is electrically connected at each
end to
positive and ground electrodes 102 via the connector posts 108. The planar
beam element
112 extends from a top of a first (positive) connector post and bends around
180 degrees to
10 return to a top of a second (ground) connector post.
Still referring to Figures 33 and 34, the metal pads 109 are positioned to
facilitate
current flow in regions of potentially higher resistance. One metal pad 109 is
positioned at
a bend region of the beam element 112, and is sandwiched between the active
beam
member 110 and the passive beam member 116. The other metal pads 109 are
positioned
15 between the top of the connector posts 108 and the ends of the beam
element 112.
Referring to Figure 35, a hydrophobic polymer layer 80 is deposited onto the
wafer
and covered with a protective metal layer 90 (e.g. 100 nm aluminum). After
suitable
masking, the metal layer 90, the polymer layer 80 and the 5i02 roof member 107
are then
etched to define fully a nozzle opening 113 and a moving portion 114 of the
roof The etch
is typically a two-stage etch process as described above in connection with
Figure 22.
The moving portion 114 comprises a thermal bend actuator 115, which is itself
comprised of the active beam member 110 and the underlying passive beam member
116.
The nozzle opening 113 is defined in the moving portion 114 of the roof so
that the nozzle
opening moves with the actuator during actuation. Configurations whereby the
nozzle
opening 113 is stationary with respect to the moving portion 114, as described
in US
Publication No. 2008/0129793, are also possible and within the ambit of the
present
invention.
A perimeter space or gap 117 around the moving portion 114 of the roof
separates
the moving portion from a stationary portion 118 of the roof. This gap 117
allows the
moving portion 114 to bend into the nozzle chamber 105 and towards the
substrate 101
upon actuation of the actuator 115. The hydrophobic polymer layer 80 fills the
gap 117 to
provide a mechanical seal between the moving portion 114 and stationary
portion 118 of
the roof 107. The polymer has a sufficiently low Young's modulus to allow the
actuator to
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16
bend towards the substrate 101, whilst preventing ink from escaping through
the gap 117
during actuation.
In the final MEMS processing steps, and as shown in Figures 36 and 37, an ink
supply channel 120 is etched through to the nozzle chamber 105 from a backside
of the
substrate 101. Although the ink supply channel 120 is shown aligned with the
nozzle
opening 113 in Figures 36 and 37, it could, of course, be positioned offset
from the nozzle
opening.
Following the ink supply channel etch, the polyimide 106, which filled the
nozzle
chamber 105, is removed by ashing in an oxidizing plasma and the metal film 90
is
removed by an HF or H202 rinse to provide the nozzle assembly 100.
Polymer Layer Comprising MSQ
The hydrophobic polymer layer 80 has proven to be an important feature of the
Applicant's printheads. Not only does it hydrophobize the front face of the
printhead, which
helps to improve overall print quality, it also assists with printhead
maintenance by
presenting a planar hydrophobic surface for a printhead maintenance means
(e.g. wiper
blade) employed to maintain the printhead in an operable condition. Of course,
in the case of
the thermal bend-actuated printheads 100 described above, the polymer 80
provides the
additional function of mechanically sealing the moving part of the nozzle from
the body of
the printhead.
Hitherto, the Applicant has proposed the use of polydimethylsiloxane (PDMS).
This
material can be readily incorporated in MEMS fabrication processes, has
excellent
hydrophobicity and a Young's modulus which allows efficient thermal bend
actuation.
However, PDMS has relatively poor wear-resistance and can be scratched or
otherwise
damaged by repeated contact with, for example, a wiper blade.
The Applicant has now found that polysilsesquioxanes provide superior wear-
resistance to PDMS whilst still maintaining all the advantages of PDMS.
Polysilsesquioxanes belong to the general class of polymers known as
polymerized siloxanes
or silicones, and have the empirical formula (RSi015)n, where R is hydrogen or
an organic
group and n is an integer representing the length of the polymer chain. The
organic group
may be C142 alkyl (e.g. methyl), C1_10 aryl (e.g. phenyl) or C1_16 arylalkyl
(e.g. benzyl). The
polymer chain may be of any length known in the art (e.g. n is from 2 to
10,000).
Poyl(alkylsilsesquioxanes) and poly(arylsilsesquioxanes), such as
poly(methylsilsesquioxane) and poly(phenylsilsesquioxane) have been shown to
have
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17
excellent hydrophobicity, durability and wear-resistance when used as the
polymer layer 80
in the Applicant's printheads. For example, printheads coated with MSQ or PSQ
could be
wiped clean without damage, even after ink and paper fibres were baked onto
the printhead
for 1 hour.
Poly(methylsilsesquioxane) is also known in the art as methylsilsequioxane,
MSQ,
MSSQ, PMSQ and PMSSQ. Poly(phenylsilsesquioxane) is also known in the art as
phenylsilsequioxane, PSQ, PSSQ, PPSQ and PPSSQ. For the sake of brevity, the
Applicant
shall hereinafter refer to poly(methylsilsesquioxane) as MSQ and refer to
poly(phenylsilsesquioxane) as PSQ.
MSQ has a low dielectric constant (k = 2.7) and has been used previously as an
insulating material. However, the use of MSQ as a hydrophobic coating for MEMS
inkjet
printheads was not previously known.
MSQ or PSQ may be incorporated into printheads as the polymer layer 80 by the
MEMS fabrication process described above. A MSQ or PSQ solution is spun on to
the wafer
to a depth of about 0.5 to 5 microns (e.g. 1 micron) and then hardbaked to
promote adhesion
to the nozzle plate and to provide a durable ink ejection face for the
printhead. Hardbaking
may include a UV curing step. For example, a typical hardbaking process may
comprise the
following steps:
1. Contact bake @ 110 C for 2 min immediately after coating
2. Contact bake @ 300 C for 6.5 min
3. UV expose for 130sec (-1300mJ)
4. Oven cure for 1 hour (starting @ 180 C and ramping up @ ¨4 C/min)
Although the Applicant's hardbaking process described above provides MSQ-
coated
or PSQ-coated printheads having excellent durability, it will be appreciated
that hardbaking
may follow any conventional procedure.
MSQ and PSQ each have a Young's modulus of about 3 GPa, which is somewhat
higher than that of PDMS. However, the Applicant has found that thermal bend-
actuated
printheads still operate efficiently when the polymer layer 80 is comprised of
MSQ or PSQ,
notwithstanding its higher Young's modulus. Moreover, the overall robustness
of MSQ and
PSQ usually outweighs any downsides arising from their higher Young's moduli.
Of course,
in thermal bubble-forming printheads where there are no moving parts, the
Young's modulus
of the polymeric layer 80 is irrelevant to nozzle actuation.
The present inventors consider that the use of MSQ or PSQ represents a
significant
breakthrough in inkjet printhead technology. Hydrophobizing inkjet printheads,
especially
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18
those manufactured by a MEMS fabrication process, was seen as a very
significant challenge
for all industry players. The present Applicant has demonstrated that MSQ or
PSQ may be
incorporated into a MEMS fabrication process and provides a hydrophobic ink
ejection face
having excellent durability and wear-resistance. This desirable combination of
features had
not been achieved previously in the art.
Polymer Laver Containing Nanopartieles
Although, as described above, MSQ and PSQ have significant advantages over
PDMS for use as a polymer coating, there may be some instances where PDMS is
still the
material of choice. For example, in low-powered thermally bend-actuated
printheads, the
lower Young's modulus of PDMS may be advantageous for minimizing drop ejection
energies. It would be desirable to improve, for example, the wear-resistance
characteristics of
PDMS without comprising its low Young's modulus.
In other scenarios, the polymer coating of a printhead may have properties
which
do not suit the particular fluid being ejected from the printhead. It should
be noted that
thermal bend-actuated printheads may eject both aqueous and non-aqueous
liquids (e.g.
polymers for printing OLEDs), and the ink ejection face of the printhead may
have
characteristics which complement the inherent properties of the fluid being
ejected. These
properties may include, for example, the fluid's hydrophilicity,
hydrophobicity, viscosity,
surface tension and/or boiling point.
Alternatively, the ink ejection face may have characteristics which complement
a
particular type of printhead maintenance regime employed (e.g. printhead
capping/wiping as
described in US Patent No. 7,758,149; or printhead flooding/non-contact
maintenance as
described in US 7,401,886). For example, wear-resistance is important for
printhead
maintenance regimes involving contact with the printhead, but less important
for non-contact
maintenance regimes.
Alternatively, the ink ejection face may have characteristics which complement
a
particular type of nozzle actuator. For example, fatigue-resistance is
important for thermal
bend-actuators where the polymeric material seals a moving portion of the
nozzle to the body
of the printhead. However, fatigue-resistance is less important in non-moving
nozzles, such
as the thermal bubble-forming nozzles described above.
The ability to 'tune' the characteristics of the ink ejection face without
changing
fundamentally the MEMS fabrication process would be highly desirable. This
'tuning' may
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improve, for example, the toughness, wear-resistance, fatigue-resistance
and/or the surface
energy characteristics of the ink ejection face. As a trivial example, when
printing
hydrophobic liquids such as polymers, the ink ejection face should preferably
be relatively
hydrophilic rather than hydrophobic (in contrast with printing aqueous inks).
The availability of silicone polymers incorporating nanoparticles (sometimes
known
in the art as "fillers") means that the characteristics of the silicone
polymer (e.g. PDMS,
MSQ, PSQ) may be modified by changing the nanoparticles incorporated therein.
The use of
different nanoparticles will correspondingly 'tune' the characteristics of the
ink ejection face
defined by the polymer layer 80.
Of course, the nanoparticles may be of any suitable type, size and shape
depending
on the particular application. The nanoparticles may comprise inorganic
particles, organic
particles or a combination of both. Some examples of inorganic nanoparticles
are metal
oxides, metal carbonates and metal sulfates. More specifically, the inorganic
nanoparticles
may be, for example, silica (including colloidal silica), zirconium oxide,
titanium oxide,
aluminium oxide, calcium carbonate, tin oxide, zinc oxide, copper oxide,
chromium oxide,
calcium oxide, tungsten oxide, iron oxide, cobalt oxide, barium sulfate etc.
Some examples
of organic nanoparticles are cross-linked silicone resin particles (e.g. PDMS,
MSQ, PSQ),
cross-linked polyolefin resin particles (e.g. polystyrene, polyethylene,
polypropylene), cross-
linked acryl resin particles, cross-linked styrene-acryl resin particles,
cross-linked polyester
particles, polyimide particles, melamine resin particles, carbon nanotubes
etc.
As used herein, the term "nanoparticles" refers to particles have an average
particle
size in the range of 1 to 1000 nm, more usually 1 to 100 nm, and more usually
1 to 50 nm.
Average particles sizes of about 20 nm are generally preferred. The particles
may be
monodisperse or polydisperse.
The nanoparticles may be present in an amount ranging from 1 to 70 wt.%,
optionally 5 to 60 wt.%, optionally 10 to 50 wt.%. The amount of nanoparticles
present will
depend on the requisite characteristics of the polymer film.
The nanoparticles may be incorporated into the polymer by any suitable
process,
such as the sol-gel process, which is well known to the person skilled in the
art. The resulting
polymer may be deposited by any suitable process, such as a spin-on process
followed by
hardbaking.
PDMS polymers incorporating silica nanoparticles are known in the art and such
polymers may be used as the polymer layer 80 in the present invention. The
silica
nanoparticles impart the desirable characteristics of wear-resistance and
fatigue-resistance to
-
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the PDMS polymer. Depending on the amount of silica particles present, the
PDMS may
also have a relatively hydrophilic surface which is useful in some
applications.
It will be appreciated by ordinary workers in this field that numerous
variations
and/or modifications may be made to the present invention as shown in the
specific
5 embodiments. The present embodiments are, therefore, to be considered
in all respects to be
illustrative and not restrictive.
