Ceramic piezoelectric ink jet print heads

Details Ceramic piezoelectric ink jet print heads Ceramic piezoelectric ink jet print heads

1998-01-13

2002-07-02

Fuller, Benjamin R. (Department: 2855)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06412924

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to improvements in or relating to ceramic piezoelectric ink jet print heads of the kind having an ink channel for connection to an ink ejection nozzle and to a reservoir for the ink, and a piezoelectric wall actuator which forms part of the channel and is displaceable in response to a voltage pulse thereby generating a pulse in liquid ink in the channel due to a change of pressure therein which causes ejection of a liquid droplet from the channel. Such print heads are referred to hereafter as piezoelectric ceramic ink jet print heads.
BRIEF DESCRIPTION OF THE DRAWINGS
Examples of such print heads as described, for example, in EP-A-277703, EP-A-278590 and EP-A-364136 are shown in
FIGS. 1-3
.
FIGS. 1 and 2
are different sectional views of the same ink jet printhead, and
FIG. 3
is a view similar to that of
FIG. 1
showing another form of printhead.
An enlarged view of a channel construction for which the teachings of the invention are particularly well suited is shown in
FIG. 4. A
portion of the channel of
FIG. 4
is illustrated in
FIGS. 5-8
including various examples of multi-layer coatings after being deposited according to the teachings of the present invention.
One form of ink jet printhead
10
comprises a multiplicity of parallel ink channels
12
forming an array in which the channels are mutually spaced in an array direction perpendicular to the length of the channels. The channels are formed at a density of two or more channels per mm. in a sheet
14
of piezoelectric material, suitably PZT, poled in the direction of arrows
15
and are defined each by side walls
16
and a bottom surface
18
, the thickness of the PZT being greater than the channel depth. The side walls
16
are generally at an angle of no more than 10° from the normal to the bottom wall. The channels
12
are open topped and in the printhead are closed by a top sheet
20
of insulating material which is thermally matched to the sheet
14
and is disposed parallel to the surfaces
18
and bonded by a bonding layer
21
to the tops
22
of the walls
16
. The channels
12
on their side wall surfaces are lined with a metallised electrode layer
34
. It will be apparent therefore that when a potential difference of similar magnitude but opposite sign is applied to the electrodes on opposite faces of each of two adjacent walls
16
, the walls will be subject to electric fields in opposite senses normal to the poling direction
15
. The walls are in consequence deflected in shear mode.
Referring now to
FIG. 2
, the channels
12
therein are provided on facing walls
16
thereof with metallised electrodes
34
which extend from the edges of the tops
16
of the walls down the walls to a location well short of the bottom surface
18
of the channels. There is an optimum metallisation depth which gives maximum wall displacement at about the mid-height of the walls depending on the distribution of wall rigidity. In this form the walls are of the so-called cantilever type.
In
FIG. 2
, it will be seen that the channels
12
comprise a forward part
36
of uniform depth which is closed at its forward end by a nozzle plate
38
having formed therein a nozzle
40
from which droplets of ink in the channel are expelled by activation of the facing actuator walls
16
of the channel. The channel
12
rearwardly of the forward part
36
also has a part
42
of lesser depth extending from the tops
22
of the walls
16
than the forward part
36
. The metallised plating
34
which is on opposed surfaces of the walls
16
occupies a depth approximately one half that of the channel side walls but greater than the depth of the channel part
42
so that when plating takes place the side walls
16
and bottom surface
18
of the channel part
42
are fully covered whilst the side walls in the forward part
36
of the channel are covered to approximately one half the channel depth in that part. One suitable electrode metal used is an alloy of nickel and chromium, i.e. nichrome. Alternatively, aluminium provides a high conductivity electrode and the metal track in the part
42
is suitable for applying a wire bond connection. Aluminium in particular requires to be coated with a layer of passivation to inhibit electrolysis and bubble formation or corrosion which could occur if the electrode is in direct contact with the ink.
It will be noted that a droplet liquid manifold
46
is formed in the top sheet
20
transversely to the parallel channels
12
which communicates with each of the channels
12
and with a duct
48
which leads to a droplet liquid supply (not shown).
In the arrangement shown in
FIG. 3
, wherein elements common with the embodiment of
FIGS. 1 and 2
are identified by the same reference numerals as in
FIGS. 1 and 2
, a sheet
14
is employed therein having upper and lower regions poled in opposite senses as indicated by the arrows
15
. A sheet
50
′ of glass or other insulating material is employed as a stiffening means for the sheet
14
of piezo-electric material. The electrodes
34
are deposited so as to cover the facing channel side walls from the tops thereof down to a short distance from the bottoms of the channels so that a region of each side wall extending from the top of the channel and poled in one sense and a substantial part of a lower region of the side wall poled in the reverse sense are covered by the relevant electrode. Thus, it will be appreciated that the arrangement described operates to deflect the channel side walls into chevron form. Other forms of ink jet printhead having an array of ink channels separated by piezoelectric wall actuators described in the art are also suitable for the application of the process of this invention.
The invention is concerned with passivation of the walls of the channels; that is, the deposition of a protective layer on the walls by coating. The purpose of the passivation is to provide a coating acting as an electron or ion or ink barrier and therefore to protect the channel walls from attack by the ink and/or to protect the ink from the channel walls. Protection of the channel walls from the ink is particularly desirable where the ink is aqueous or otherwise electrically conductive.
Where—as is the case in the example given above—the channel includes opposed walls comprising piezoelectric ceramic material and is provided with electrodes for connection to voltage pulse generating means, passivation is particularly desirable to protect the electrodes from the ink and also to insulate the ink from the electrodes, and more particularly the fields generated by the electrodes, especially where the ink is a dispersion. In one embodiment of this form of ink jet print head, the channels are formed with opposed side walls and a bottom wall all of piezoelectric ceramic material, e.g. by cutting or machining an open channel from a block of the material, and a top wall which closes the channel. In this embodiment, in general the side walls and bottom wall are passivated.
IBM Technical Disclosure Bulletin, Vol. 23, No. 6, November 1980, page 2520 discloses a method for passivation of an ink jet silicon nozzle plate whereby a first overcoat of thermal SiO
2
is applied to a silicon substrate followed by a second overcoat of glow discharge silicon carbon. Formation of the first overcoat generally entails substrate temperatures of the order of 900° C.
EP-A-0 221 724 discloses an ink jet printer nozzle having a substrate of silicon or glass and a coating resistant to corrosion by aqueous and non-aqueous inks. The coating comprises respective layers of silicon nitride, silicon nitride with aluminium nitride, and aluminium nitride. Sputtering, Chemical Vapour Deposition (CVD) and evaporation are given as suitable techniques for forming the coating. Typical substrate temperatures are given as 700-800° C. and, as described, ion-assisted deposition is a line-of-sight coating process.
U.S. Pat. No. 4,678,680 discloses the use of an ion beam implanting device to implant ions in the aperture plate of an ink

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