Technical Field
[0001] This invention relates generally to thermal ink jet (TIJ) printing and more particularly
to a new and improved process for fabricating thermal ink jet printheads and printhead
structures produced thereby.
Background Art
[0002] In the art of thermal ink jet printing, it is known to provide thin film resistor
(TFR) type printheads for generating thermal energy which is applied or transfered
to a plurality of ink reservoirs during an ink jet printing operation. Typically these
reservoirs will be aligned with the individual heater resistors of the thin film resistor
substrate. The printhead will typically include an underlying silicon substrate member
upon which a thin passivation layer of silicon dioxide is deposited, and a resistor
material such as tantalum aluminum is then deposited on the silicon dioxide layer
to serve as the resistive heater material for the device structure. Traces of a conductive
material, such as aluminum, are then formed on the resistive layer in a predefined
pattern which defines the length and the width of the individual resistive heater
elements.
[0003] A protective inert barrier layer material, such as silicon carbide, is deposited
atop the conductive pattern in order to protect the underlying materials from ink
corrosion and cavitation wear. Such corrosion and wear is caused by the collapsing
vapor bubble which would otherwise be transmitted from the ink reservoirs defined
on top of the protective barrier layer in preestablished geometries. This type of
structure is generally well known in the art and is described, for example, in the
Hewlett-Packard Journal, Vol. 36, No. 5, May 1985, incorporated herein by reference.
[0004] In the past, one process for defining these ink reservoirs involved forming a pattern
in a polymer film disposed on the surface of the silicon carbide barrier layer so
as to define individual and separated reservoirs vertically aligned with the underlying
resistive heater elements. Using this process, a photoresist polymer film was both
ultraviolet (U. V.) and thermally cured on the surface of the inert barrier layer,
and then a separate adhesive system was used to secure a nozzle plate to the top surface
of the polymer film. Typical polymer materials suitable for this ink reservoir-defining
film are sold by the Dupont Company of Wilmington, Delaware under the trade names
"RISTON" and " VACREL".
[0005] While the above process has proven generally satisfactory in many respects, there
have nevertheless been observed flatness variations in both the nozzle plate and the
barrier film. These flatness variations produce air gaps between these two members
and the intermediate polymer layer using the above prior art process, and these air
gaps may produce mechanical failure due to breakdown of the adhesive bond at these
gaps. Furthermore, these gaps also reduce the frequency response of printhead operation.
These air gaps are located across the surface of the printhead and cause an irregular
volume of ink to be ejected therefrom, and the gaps also tend to interrupt or destroy
the flow pattern of ink created by the third wall of the polymer barrier layer. This
fact in turn shortens resistor life and increases cavitation damage. Additionally,
the attachment of the nozzle plate, as mentioned above, required the utilization of
a separate adhesive system.
Disclosure of Invention
[0006] The general purpose of this invention is to provide a new and improved TIJ printhead
fabrication process and structure produced thereby which eliminates the above gaps
between the polymer barrier layer and the nozzle plate attached thereto and accomplishes
the same. while simultaneously eliminating the need for a separate adhesive system
for attaching the nozzle plate to the polymer barrier layer defining the individual
ink reservoirs.
[0007] To accomplish this purpose, we have discovered and developed a new and improved process
which includes providing a thin film resistor (TFR) structure having a plurality of
resistive heater elements therein, and thereafter forming an ultraviolet ("UV") cured
(polymerized), but thermally uncured (ie no molecular cross-linking), pattern of photoresist
on the upper surface of the thin film resistor structure. This photoresist polymer
film is patterned to define a plurality of ink reservoirs disposed above the plurality
of resistive heater elements, respectively. Then, a nozzle plate having a plurality
of ink ejection orifices therein is aligned with the photoresist barrier layer, with
the individual orifices in the nozzle plate being aligned with the ink reservoirs
in the barrier layer.
[0008] Next, a predetermined amount of heat and pressure is applied via a heat staker or
laminator to the nozzle plate to thereby produce a partial thermal curing of the photoresist
barrier layer. This thermal curing produces excellent initial adhesion between the
photoresist film and both the nozzle plate and the thin film resistor substrate structure,
and the pressure applied to the structure during this process eliminates air gaps
resulting from uneveness of adjacent layers . This curing eliminates air gaps between
the nozzle plate and the polymer film and it further eliminates the need for a separate
adhesive material for securing the above adjacent members one to another. Finally,
the printhead is removed from the heat staker and transferred to an oven where the
thermal curing process is completed for a predetermined curing temperature and time.
[0009] The above advantages and features of this invention will become more readily apparent
in the following description of the accompanying drawings.
Brief Description of Drawings
[0010]
Figure 1 is a process flow chart indicating the individual process steps and sequence
thereof utilized in a preferred process embodiment of the invention.
Figure 2 is a cross section view of a thin film resistor substructure which is typical
of state-of-the-art multiple heater devices used in thermal ink jet printheads and
particularly adapted for use in practicing the present invention.
Figures 3A through 3C illustrate the heat staking operation utilized in a preferred
process embodiment of the invention.
Best Mode for Carrying Out the Invention
[0011] Referring now to Figure 1, there is shown an eight step process of carrying out the
invention and includes the provision of a thin film resistor (
TFR) substrate which has been fabricated using state of the art semiconductor processing
techniques. A dry polymer film is then laminated on the upper surface of the TFR substrate,
and this upper surface will typically be an inert barrier layer of either SiC or Si
3N
4. This polymer film is then partially polymerized with UV light and selectively developed
to form an ink reservoir barrier mask on the TFR substrate. Then the barrier mask
is subjected to additional ultraviolet light to provide a further cure thereof.
[0012] Next, a nozzle plate (also referred to sometimes as an orifice plate) is aligned
with and placed upon the barrier mask in preparation for a heat stake operation described
in more detail below with reference to Figure 3. Finally, and after the completion
of the heat stake operation, the printhead structure is transferred to an oven for
final curing at a predetermined time and elevated temperature.
[0013] Referring now to Figure 2, a thin film resistor structure which may be used in carrying
out the present invention will typically consist of a silicon substrate member 10
upon which a thin silicon dioxide ,Si02, surface barrier layer 12 is deposited using
known thermal oxidation techniques. Then, a resistive layer 14 of tantalum aluminum,
TaAl, is sputtered deposited on top of the Si02 barrier layer 12, and thereafter a
metalization pattern 16 which will typically be aluminum is formed as shown on the
surface of the tantalum aluminum layer 14. The metalization pattern 16 will have openings
therein defining the lateral dimensions of the individual resistors in the TFR structure.
[0014] Next, another, outer surface passivation layer 18 is deposited on the outer surface
of the conductive pattern 16 and will typically consist of either silicon carbide,
SiC, or silicon nitride, Si
3N
4. These latter materials are highly inert and are thus protective of the underlying
materials from both ink corrosion and cavitation wear produced by the ink and ink
ejections respectively during a thermal ink jet printing operation. The processing
details used in producing a thin film resistor substrate structure of the type shown
in Figure 2 are generally well known to those skilled in this art and more fully described
in the above' identified Hewlett-Packard Journal published May 1985 and incorporated
herein by reference.
[0015] Referring now to Figure 3A through 3C, the TFR substrate of Figure 2 is illustrated
only schematically as a single member in Figure 3A without showing the individual
layers therein. However, Figures 3A - 3C still show the location of the four tantalum-aluminum
heater resistors 20 which are subsequently aligned with the openings 22 in the polymer
ink reservoir barrier layer 24. This barrier layer 24 is laminated on the TFR substrate
10 as a dry film of a material such as VACREL or RISTON which are trade names of photoresist
polymer materials commercially available from the Dupont Company of Wilmington, Delaware.
Thus, the polymer film 24 is processed using conventional photolithographic masking,
ultraviolet exposure and etching techniques in order to form the plurality of openings
22 therein which define the boundaries of four individual ink reservoirs. These reservoirs
are disposed immediately above the four resistive heater elements 20 of the thin film
substrate 10 which supports it. Once the barrier layer 24 is developed to remove the
unwanted selected portions of the dry film and create openings 22 therein, the structure
of Figure 3A is exposed to some additional ultraviolet light to further UV cure the
barrier layer 24 in accordance with the processing detail given below.
[0016] . In Figure 3B, an orifice or nozzle plate 26 having a plurality of openings 28 therein
is aligned on the barrier layer 24 as shown, so that the orifices 28 are precisely
centered with the ink reservoirs 22 in the barrier layer 24. With the nozzle plate
26 thus in place, a hot chuck 30 of a heat staker apparatus is moved down into thermal
and pressure contact with the top surface of the nozzle plate 26 and held there at
a predetermined pressure and for a predetermined time to partially thermally cure
the barrier layer material 24. This step provides a good initial interface adherance
and good surface contour match at the barrier layer-nozzle plate interface, as well
as at the barrier
layer-TFR substrate interface.
[0017] The hot chuck 30 of the heat staker is brought into contact with the nozzle plate
28 under sufficient pressure to allow the barrier material 24 to plastically deform
and adhere and conform to the contour of the upper surface of the nozzle plate 28.
Then, the chuck 30 is released and the ink jet printhead of Figure 3C is transferred
to a hot oven to fully cure the dry barrier film 24 in accordance with the processing
schedule below.
[0018] The present invention allows batch processing of parts during a nozzle plate attachment
operation and thereby results in quick bonding of the nozzle plate 26 to the barrier
layer 24 as indicated. This process deforms the dry film barrier layer 24 into the
shape of the nozzle plate 28 to thereby fill any gaps therein, and in addition, further
prevents the printhead substrate 10 from overheating during the nozzle plate attachment
process. Furthermore, this process allows the batch processing of parts without the
requirement for clamping, which has been a frequent requirement in prior dry film-adhesive
cures. And, as mentioned above, the present process eliminates the need for separate
adhesives and adhesive bonding operations during the assembly process. Finally, the
present process imparts long lasting structural integrity to the printhead structures
being fabricated and insures permanent (plastic) deformation of the dry film barrier
layer material.
[0019] Various modifications may be made to the above described process without departing
from the scope of the present invention. For example, the barrier layer materials
are not limited to the particular VACREL and RISTON polymers sold by the Dupont Company
and may instead employ other suitable polymer materials.
[0020] The following table of values includes those processing parameters which have been
used successfully in the reduction to practice of the present invention.
Industrial Applicability
[0021] The present invention is useful in the manufacture of thin film resistor type printheads
used in thermal ink jet printers.
1. A process for bonding a nozzle plate in place with respect to a printhead substrate
which comprises:
a. forming a polymer barrier layer which is not fully thermally cured on the surface
of said substrate,
b. aligning and placing a nozzle plate on the surface of said barrier layer,
c. applying measured heat and pressure to said nozzle plate for a predetermined time
and temperature sufficient to cause said barrier layer to deform and conform to contours
in said nozzle plate, and
d. further heating said substrate, barrier layer and nozzle plate at a predetermined
elevated temperature to thermally cure said barrier layer, whereby said barrier layer
is firmly secured to said substrate and nozzle plate without the requirement for separate
adhesives and adhesive bonding operations.
2. A process for fabricating a thermal ink jet printhead comprising:
a. providing a thin film resistor structure having a plurality of resistive heater
elements therein,
b. forming a pattern in a UV cured but thermally uncured or at least partially thermally
uncured pattern in a polymer film on the upper surface of said thin film resistor
structure to define a plurality of ink reservoirs disposed above said plurality of
resistive heater elements, respectively,
c. aligning orifices of a nozzle plate with said ink reservoirs and placing said nozzle
plate on the top surface of said barrier layer,
d. applying a predetermined heat and pressure to said nozzle plate to cause said barrier
layer to deform and conform to surface contours in said nozzle plate, and
e. further thermally curing said barrier layer at a predetermined elevated temperature
and for a predetermined time sufficient to fully cure said barrier layer and produce
good adhesion between said barrier layer and both said nozzle plate and said thin
film resistor structure, whereby said process requires no individual adhesive or glueing
steps.
3. A process for bonding a nozzle plate to a printhead substrate which includes the
steps of:
a. providing a polymer barrier layer material which is not fully cured thermally between
and in intimate contact with aligned major facing surfaces of said substrate and nozzle
plate, and
b. thermally curing said barrier layer under conditions of predetermined heat and
pressure and for a predetermined time sufficient to produce a good adhesive bond between
said barrier layer and both said substrate and nozzle plate.
4. The invention defined in claim 3 wherein the thermal cure of said barrier layer
is achieved in a two step process of first exposing said composite substrate-barrier
layer-nozzle plate structure to predetermined heat and pressure to produce partial
thermal curing of said barrier layer and plastic deformation thereof and initial adherance
to both said nozzle plate and said substrate, and secondly heating said composite
structure at a further elevated temperature and for a time sufficient to fully thermally
cure said barrier layer and also to produce optimum adherance to said substrate and
to said nozzle plate.
5. A composite thin film resistor substrate - barrier layer - nozzle plate structure
which is manufactured by the steps of:
a. providing a thermally uncured or at least partially thermally uncured polymer barrier
layer material between and in intimate contact with aligned major facing surfaces
of said substrate and nozzle plate, and
b. thermally curing said barrier layer under conditions of predetermined heat and
pressure and for a predetermined time sufficient to produce a good adhesive bond between
said barrier layer and both said substrate and nozzle plate.