Ink jet printing device

An ink jet printhead has improved resistance to the corrosive effects of ink by coating ink sensitive areas with a photo-imageable benzocyclobutene (BCB) polymer. The BCB can be patterned so as to provide a protective coating over selected areas while leaving other areas uncoated. In one described embodiment, a thermal ink jet printer is formed by bonding together a channel plate and a heater plate. Resistors and electrical connections are formed in the surface of the heater plate. A BCB layer is formed so as to overlie the heater plate to protect the electrical elements while providing pit structure for the heater and for ink flow bypass.

BACKGROUND AND MATERIAL DISCLOSURE STATEMENT 
This invention relates to an ink jet printing device which uses energy to 
cause ink droplets contained within channels formed internally to the 
printhead to be expelled from an orifice onto a recording material. More 
particularly, the invention relates to an ink jet printhead having 
improved protection from the corrosive effects of ink on ink sensitive 
areas of the printhead. 
In the ink jet printing art, a printhead is provided having one or more ink 
filled channels communicating with an ink supply chamber, the channels 
having one end formed as a nozzle orifice. The ink forms a meniscus at the 
nozzle prior to being expelled. Energy is applied to the ink channels in 
the form of heat created by pulsing heating resistors or by a 
piezoelectrically applied force to the channel walls to cause an ink 
droplet to be expelled from the nozzle onto the recording material. After 
a droplet is expelled, additional ink replenishes the channel and reforms 
the meniscus. 
The ink must flow in such a manner that the energy generator, either the 
resistor heater element in a thermal ink jet printer or piezoelectric 
plates in the piezo printer are in sufficient contact to transfer energy 
to the ink. Because of the corrosive nature of the ink used, the 
electronic circuitry associated with the energy generators must be 
protected by a protective coating which must also be photo-imageable to 
pattern and expose the energy generating elements during the fabrication 
process. Preferred photo-imageable materials used extensively in the prior 
art for passivation and other purposes are polyimide and dry film solder 
mask polymers (PMMA). For example, as disclosed in U.S. Pat. Nos. 
4,774,530 and 4,829,324, the top surface of a heater wafer is patterned 
with a relatively thick pattern layer of polyimide which is applied in a 
curing process and passivates the underlying electronic circuitry while 
also placing the heater surface at the bottom of a pit structure to 
improve ink ejection characteristics. U.S. Pat. No. 5,113,203 discloses a 
printhead in FIG. 1 where electrodes 209, 210 are protected by a spin 
coated photosensitive polyimide layer 213, 214. Polymers have also been 
used to form a nozzle plate for a printhead as disclosed in U.S. Pat. No. 
5,291,226. 
The use of polyimides and PMMA's in ink jet printheads has certain 
disadvantages. One is the poor resistance to inks having strong bases and 
polar solvents, resulting in corrosion of the polyimide coating. Another 
disadvantage is sensitivity to high temperatures. Another disadvantage of 
the PMMA composition is sensitivity to high temperatures. A still further 
disadvantage is shrinkage during the curing process lessening control over 
the planarity of the polyimide layer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an ink jet printing 
device wherein an improved photo-imageable formulation is used to provide 
passivation and protective coatings to internal components of the 
printhead which are susceptible to the corrosive effects of the ink. 
The improved photo-imageable material is provided by forming the coating as 
a layer of a cured photo-imageable benzocyclobutene (BCB) polymer. 
More particularly, the present invention relates to a printhead for 
ejecting a recording liquid onto a recording medium, the printhead having 
an internal structure which includes at least a channel for holding said 
recording liquid, 
at least one nozzle for ejecting liquid onto the recording medium, 
channel means providing a liquid flow path between said chamber and said 
nozzle, 
an energy generator for introducing energy into the liquid contained in 
said channel, 
means for selectively energizing said energy generating means so as to 
cause periodic ejections of said liquid through said nozzle onto said 
recording medium and 
a cured photo-imageable benzocyclobutene (BCB) polymer formed in at least a 
portion of said internal structure.

DESCRIPTION OF THE INVENTION 
The invention will be described in conjunction with a thermal ink jet 
printer of the type disclosed in U.S. Re. 32,572 to Hawkins et al., U.S. 
Pat. No. 5,010,355 to Hawkins et al. and U.S. Pat. No. 4,851,371 to Fisher 
et al, the disclosures of all of which are hereby incorporated by 
reference. It is understood that the invention has utility and other types 
of printhead structures as will be seen. As disclosed in these patents, 
thermal ink jet printheads are generated in batches by aligning and 
adhesively bonding an anisotropically etched channel wafer to a heater 
wafer followed by a dicing step to separate the bonded wafer into 
individual printheads. The prior art interposed a patterned thick film 
insulative layer between the channel wafer and the heater wafer. A 
preferred insulation material has been polyimide. According to the 
invention, and as shown in FIG. 1, the polyimide layer has been replaced 
by a layer 18 of a photo-imageable benzocyclobutene resin. FIG. 1 shows a 
cross-sectional view of a printhead. Printhead 10 comprises an 
anisotropically etched channel plate 11 aligned and bonded to heater plate 
12. The printhead is fixedly attached to a daughter board 19 having 
electrodes 13 thereon which connect to a drive circuit and power supply 
(not shown). The channel plate 11 has a through etched reservoir 14 with 
its open bend serving as inlet 15 and a plurality of channels 16 
anisotropically etched therein. Ends of the channels 16 open through 
nozzle face 29 and terminate at slanted ends 21. The open ends of the 
channels serve as nozzles 8. The heater plate has an array of heating 
elements 25 and addressing electrodes 22 formed on the surface of the 
heater plate 12 which confront the channel plate. The heating elements and 
electrodes are formed on an insulative layer 27 and are passivated by an 
insulative layer 28. A protective layer 9, such as tantalum, is deposited 
over the heating elements. Thick film insulative layer 18, in a preferred 
embodiment, is a 10 micron thick photosensitive BCB interposed between the 
heater plate and the channel plate. Layer 18 is patterned to expose the 
heating elements, thereby placing them in separate pits 26 and to form ink 
flow bypass pits 24 between the reservoir 14 and the ink channel 16. Ink 
thus flows from reservoir 14 to channels 16 around the closed end of the 
channels 21 as shown by arrow 23. The addressing electrodes of the 
printhead is connected to the daughter board electrodes 13 by wire bonds 
30 which are subsequently passivated (not shown). The anisotropically 
etched channels 16 have a triangular cross-sectional area and the 
materials surrounding the nozzle at the nozzle face 29 is silicon on two 
sides of the triangular shaped nozzle and thick film layer material layer 
on the third side. 
In a preferred embodiment, layer 18 comprises a photo-imageable 
Benzocyclobutene (BCB) formulated to be photosensitive to ultraviolet 
light and curable in a single step. In a general formulation: 
##STR1## 
wherein each R is independently: hydrogen, aliphatic including alkyls, 
halogens, such as Cl, Br, and the like, and SiH.sub.3, Si(OH).sub.3, and 
COOH. In a specific embodiment, the BCB contains by weight 35-63% 
divinylsiloxane-bis benzocyclobutene isomers, and an aromatic solvent such 
as mesitylene, the formulation being: 
##STR2## 
The BCB formulation further comprises photocrosslinkers m-azidophenyl 
sulfone, (0.2-3.0%) and 2,6-bis 
(4-azidobenzylidene)-4-methylcyclo-hexanone (0.25-3.0%) and an antioxidant 
polymerized 1,2-dihydro-2,2,4-trimethylquinoline (0.2-3.0%). The 
orientation of the polymer is a three-dimensional chain expressed by the 
following chemical reaction, which may be continued indefinitely: 
##STR3## 
The BCB layer is cured in one embodiment by the following process. The cure 
takes place in an oxygen-free environment and can be by oven, hot plate, 
RTC, e-beam. For an oven curing, the cure cycles were as follows: 
soft cure: 5 min. ramp to 50.degree. C., 5 min. soak; 15 min. ramp to 
100.degree. C., 15 min. soak; 15 min. ramp to 150.degree. C., 15 min soak; 
30 min. ramp to 210.degree. C., 30 min. soak followed by natural cool 
down; 
hard cure: 5 min. ramp to 50.degree. C., 5 min. soak; 15 min. ramp to 
100.degree. C., 15 min. soak; 15 min. ramp to 150.degree. C., 15 min soak; 
60 min. ramp to 250.degree. C., 60 min. soak followed by natural cool 
down. 
The following are two sample patterning processes for the different 
thicknesses of material: 
SAMPLE 1 
1. Deposit 2-3 gm Dow BCB type #93006-28; 
2. Spin at 500 RPM for 20 sec., 2500 for 30 sec.; 
3. Bake for 20 min. at 75.degree.; 
4. Expose in contact with 250 mJoules; 
5. Puddle develop for 50 sec. with Stoddard solvent; 
6. Rinse for 20 sec. in proponal; 
7. Cure (final thickness 5.3 microns). 
SAMPLE 2 
1. Deposit 2-3 gm Dow BCB type #93005-83; 
2. Spin at 500 RPM for 20 sec., 2700 for 30 sec.; 
3. Bake for 40 min. at 75.degree.; 
4. Expose in contact with 360 mJoules; 
5. Puddle develop for 120 sec. with Stoddard solvent; 
6. Rinse for 20 sec. in proponal; 
7. Cure (final thickness 8.6 microns). 
The BCB layer is a polymer structure with a three-dimensional chain making 
it extremely chemically resistant. Further, because the polymerization 
reaction is just a re-orientation of the monomers, there is no shrinkage 
during curing, and the coating has good planerization properties. This 
characteristic is unlike the polyimide of the prior art where hydrolysis 
must take place resulting in significant shrinking during cure and a 
result in lack of planerity. 
The above description provides an example of using a photo-imageable BCB 
coating to form a pit layer in a specific thermal ink jet printhead 
fabrication. In the same printhead, another use of the BCB coating would 
be to create the adhesive bond between the channel plate 11 and the heater 
12. As disclosed in U.S. Pat. No. 4,678,529, hereby incorporated by 
reference, a printhead is disclosed wherein the channel plate is bonded to 
the heater plate by applying a diluted EPON.RTM. thermoplastic adhesive to 
the channel plate by a spin coating process. The photo-imageable BCB 
formulation of the present invention can be used instead of the EPON.RTM. 
formulation. 
The efficacy of the BCB formulation is not limited only to the structure 
disclosed above but is equally useful in any of the printheads discussed 
supra. For example, the nozzle member disclosed in U.S. Pat. No. 5,291,226 
is described as formed of a polymer material section. The BCB coating can 
be used to provide a section through which the nozzles can be formed, the 
section providing enhanced protection from the effects of ink on both 
sides of the nozzle plate. As another example, and as disclosed in U.S. 
Pat. No. 5,008,689, a printhead includes an orifice plate and an ink 
channel, the ink in the channel heated by a resistor. The resistor is 
formed on a plastic layer. The present BCB polymer formulation can be used 
to form the plastic layer. The contents of this patent are hereby 
incorporated by reference. 
In sum, the cured photo-imageable BCB polymer of the present invention can 
be used as a protective layer, or as a base layer to provide a structure 
such as a pit layer for any regions of an ink jet printhead which would be 
effected by the corrosiveness of the particular ink being used. 
While the embodiment disclosed herein is preferred it will be appreciated 
from this teaching that various alternative modifications, variations or 
improvements therein may be made by those skilled in the art which are 
intended to be encompassed by the following claims: