Liquid level control in an acoustic droplet emitter

Details Liquid level control in an acoustic droplet emitter Liquid level control in an acoustic droplet emitter

1998-10-13

2001-10-16

Barlow, John (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06302524

ABSTRACT:

BACKGROUND
This invention relates generally to acoustic droplet emission and more particularly concerns a capping structure which provides liquid level control and meniscus placement for an acoustic droplet emitter.
Turning now to
FIG. 1
a device which generates liquid droplets using focussed acoustic energy is shown. Such devices are known in the art for use in printing applications. Detailed descriptions of acoustic droplet formation and acoustic printing can be found in the following U.S. patent applications Ser. No. 4,308,507 titled “Liquid Drop Emitter” by Lovelady et al., issued Dec. 29
th
, 1981; U.S. patent application Ser. No. 4,697,195 titled “Nozzleless Liquid Droplet Ejectors”, by Quate et. al., issued Sep. 29
th
, 1987; U.S. patent application Ser. No. 5,041,849 titled “Multi-Discrete-Phase Fresnel Acoustic Lenses And Their Application To Acoustic Ink Printing” to Quate et al., issued Aug. 20
th
, 1991; U.S. patent application Ser. No. 5,121,141 titled “Acoustic Ink Printhead With Integrated Liquid Level Control Layer” to Hadimioglu et al., issued Jun. 9
th
, 1992; U.S. patent application Ser. No. 5,608,433 titled “Fluid Application Device And Method Of Operation” by Quate, issued Mar. 4
th
, 1997, all herein incorporated by reference, as well as other patents.
The most important feature of the device shown in
FIG. 1
is that it does not use nozzles and is therefore unlikely to clog, especially when compared to other methods of forming and ejecting small, controlled droplets. The device can be manufactured using photolithographic techniques to provide groups of densely packed emitters each of which can eject carefully controlled droplets. Furthermore, it is known that such devices can eject a wide variety of materials, U.S. Pat. No. 5,591,490 titled “Acoustic Deposition Of Material Layers” by Quate, issued Jan. 7
th
, 1997 and herein incorporated by reference, describes a method for using an array of such acoustic droplet emitters to form a uniform layer of resist, U.S. Pat. No. 5,565,113 titled
“Lithographically Defined Ejection Units” by Hadimioglu et al., issued Oct. 15
th
1996, and herein incorporated by reference, states that the principles of Acoustic Ink Printing(AIP) are suitable for ejection of materials other than marking fluids, such as mylar catalysts, molten solder, hot melt waxes, color filter materials, resists, chemical compounds, and biological compounds. U.S. Pat. No. 5,520,715 titled “Directional Electrostatic Accretion Process Employing Acoustic Droplet Formation” by Oeftering, issued May 28
th
, 1996, and herein incorporated by reference describes using focussed acoustic energy to emit droplets of liquid metal.
With the above concepts firmly in mind, the operation of an exemplary acoustic droplet emitter will now be described. There are many variations in acoustic droplet emitters and the description of a particular droplet emitter is not intended to limit the disclosure but to merely provide an example from which the principles of acoustic droplet generation can be applied in the context of this invention.
FIG. 1
shows an acoustic droplet emitter
10
shortly after emitting of a droplet
12
of a liquid
14
and before a mound
16
on a free surface
18
of the liquid
14
has relaxed. The forming of the mound
16
and the subsequent ejection of the droplet
12
is the result of pressure exerted by acoustic forces created by a ZnO transducer
20
. To generate the acoustic pressure, RF energy is applied to the ZnO transducer
20
from an RF source
22
via a bottom electrode
24
and a top electrode
26
. The acoustic energy from the transducer
20
passes through a base
28
into an acoustic lens
30
. The acoustic lens
30
focuses its received acoustic energy into a small focal area which is at or very near the free surface
18
of the liquid
14
. It should be noted that while the acoustic lens
30
is depicted as a fresnel lens, that other lenses are also possible. For example, concave acoustic beam forming devices such as that shown in U.S. Pat. No. 4,751,529, titled “Microlenses For Acoustic Printing”, by Elrod et al., issued Jun. 14
th
, 1988 have also been used. Provided the energy of the acoustic beam is sufficient and properly focused relative to the free surface
18
of the liquid
14
, a mound
16
is formed and a droplet
12
is subsequently emitted on a trajectory T.
The liquid is contained by a plate
34
which has a opening
32
in which the free surface
18
of the liquid
14
is present and from which the droplet
12
is emitted. The liquid
14
flows through a channel defined by sidewalls
36
and the top surface
38
of base
28
and past the acoustic lens
30
without disturbing the free surface
18
. Although the sidewalls
36
are depicted as inwardly sloping, resulting in a channel that is narrower at the opening
32
than at the surface
38
of the base
28
, this need not be so. Examples of other channel configurations are shown in U.S. Pat. No. 5,121,141, issued Jun. 9
th
, 1992, by Hadimioglu et al., and titled, “Acoustic Ink Printhead With Integrated Liquid Level Control Layer” and U.S. Pat. No. 5,450,107, issued Sep. 12
th
, 1995, by Rawson and titled “Surface Ripple Wave Suppression By Anti-Reflection In Apertured Free Ink Surface Level Controllers For Acoustic Ink Printers”, both herein incorporated by reference. The width W of the opening
32
is many times larger than the droplet
12
which is emitted such that the width W of the opening has no effect on the size of the droplet
12
thereby greatly reducing clogging of the opening, especially as compared to other droplet ejection technologies. It is this feature of the droplet emitter
10
which makes its use desirable for emitting droplets of a wide variety of materials. Also important to the invention is the fact that droplet size of acoustically generated and emitted droplets can be precisely controlled. Drop diameters can be as small as
16
microns allowing for the deposition of very small amounts of material.
However, the free surface
18
of the liquid
14
must be a precise focal distance d from the acoustic lens
30
so that the acoustic energy focussed by the acoustic lens
30
can be focussed at or very near to the free surface
18
. Variations in the distance d will cause the acoustic energy generated by the transducer
20
to be misfocused by the acoustic lens
30
and often results in misfired droplets
12
. Many techniques have been used to control the placement of the free surface
18
relative to acoustic lens
30
.
Most commonly, surface tension, fluid pressure, and the edge of an orifice opening are relied upon to place the free surface
18
at the appropriate distance d. If the liquid
14
is supplied at the correct pressure then the surface tension will hold the free surface
18
in place with a meniscus extending between the sidewalls
36
, as shown in FIG.
1
. If the pressure is increased the liquid
14
will spill through the opening, if the pressure is decreased the free surface
18
of the liquid
14
will slip down the sidewalls
36
of the plate
34
instead of being adjacent to the top surface
40
of the plate
34
as shown in FIG.
1
.
This method requires uniformity of the pressure of liquid
14
and is dependent on variations in the thickness of the plate
34
. In the case of an acoustic droplet emitter which has a single emitter or a small number of emitters, pressure uniformity can often be sufficiently maintained. However, as the number of emitters disposed in a single channel grow larger, maintaining uniformity can be problematic. Furthermore, the free surface may not be maintained by the sidewalls of the channel but by the sidewalls of a relatively short capping structure as shown in any of U.S. Pat. No. 5,121,141 titled “Acoustic In Printhead With Integrated Liquid Level Control Layer” to Hadimioglu et al., issued Jun. 9
th
, 1992, U.S. Pat. No. 5,450,107, titled “Surface Ripple Wave Suppression By Anti-Reflection In Apertured Free Ink Surface Level Controllers For Acoustic Ink Printers”, by Rawson, issued Sep. 12

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