U.S. patent application number 09/795812 was filed with the patent office on 2001-11-01 for micromachined two dimensional array of piezoelectrically actuated flextensional transducers.
This patent application is currently assigned to The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Khuri-Yakub, Butrus Thomas, Percin, Gokhan.
Application Number | 20010035700 09/795812 |
Document ID | / |
Family ID | 24115524 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035700 |
Kind Code |
A1 |
Percin, Gokhan ; et
al. |
November 1, 2001 |
Micromachined two dimensional array of piezoelectrically actuated
flextensional transducers
Abstract
A transducer suitable for ultrasonic applications, fluid drop
ejection and scanning force microscopy. The transducer comprises a
thin piezoelectric ring bonded to a thin fully supported clamped
membrane. Voltages applied to said piezoelectric ring excite
axisymmetric resonant modes in the clamped membrane.
Inventors: |
Percin, Gokhan; (Los Altos,
CA) ; Khuri-Yakub, Butrus Thomas; (Palo Alto,
CA) |
Correspondence
Address: |
Aldo J. Test
FLEHR HOHBACH TEST ALBRITTON & HERBERT LLP
Suite 3400
Four Embarcadero Center
San Francisco
CA
94111-4187
US
|
Assignee: |
The Board of Trustees of the Leland
Stanford Junior University
|
Family ID: |
24115524 |
Appl. No.: |
09/795812 |
Filed: |
February 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09795812 |
Feb 27, 2001 |
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09098011 |
Jun 15, 1998 |
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09098011 |
Jun 15, 1998 |
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08530919 |
Sep 20, 1995 |
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5828394 |
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Current U.S.
Class: |
310/324 ;
310/328 |
Current CPC
Class: |
B41J 2/14201 20130101;
B41J 2/1634 20130101; B05B 17/0646 20130101; Y10S 977/869 20130101;
Y10S 977/887 20130101; B41J 2/1628 20130101; B41J 2202/15 20130101;
Y10S 977/86 20130101; B05B 17/0607 20130101; B41J 2/1607 20130101;
Y10S 977/837 20130101; Y10S 977/872 20130101; B41J 2/1635 20130101;
B41J 2002/1437 20130101; B41J 2/1629 20130101 |
Class at
Publication: |
310/324 ;
310/328 |
International
Class: |
H01L 041/04 |
Goverment Interests
[0002] The research leading to this invention was supported by the
Defense Advanced Research Projects Agency of the Department of
Defense, and was monitored by the Air Force Office of Scientific
Research under Grant No. F49620-95-1-0525.
Claims
What is claimed is:
1. A two-dimensional array of piezoelectrically actuated
flextensional fluid drop ejectors comprising: a plurality of
membranes having a selected area, said membranes including one or
more apertures, a support structure engaging the outer edges of
each of said membranes to flexibly support the membranes, said
support structure and said membranes configured to form fluid
reservoirs so that fluid to be ejected is in contact with said
membranes, a piezoelectric transducer carried on one surface of
each of said membranes surrounding said aperture, said transducer
including a body of piezoelectric material having first and second
spaced opposite surfaces, conductive contacts on the opposite
surfaces of said body of piezoelectric material for each of said
transducers for applying a voltage across said piezoelectric
material to cause flextensional movement of said body of
piezoelectric material whereby the associated membrane flexes
responsive to applied voltage whereby the application of an ac
voltage of predetermined frequency causes said membrane to
resonate, and conductive means for applying said voltages across
selected piezoelectric transducer to selectively bring membranes
into resonance to selectively eject droplets perpendicular to the
surface of said membranes through said orifice.
2. A piezoelectrically actuated flextensional transducer as in
claim 1 in which the membranes are silicon nitride.
3. A piezoelectrically actuated flextensional transducer as in
claim 1 in which said membranes are polysilicon.
4. A piezoelectrically actuated flextensional transducer as in
claim 1 in which said support structure is silicon oxide.
5. A piezoelectrically actuated flextensional transducer as in
claim 1 in which said membranes are circular and said piezoelectric
transducers are annular.
6. A piezoelectrically actuated flextensional transducer as in
claim 1 in which the membranes merge to form a single membrane with
multiple piezoelectric transducers.
7. A piezoelectrically actuated flextensional transducer as in
claims 1, 2, 3, 4, or 5 in which the apertures are spaced apart a
distance less than 100 .mu.m.
8. A piezoelectrically actuated flextensional transducer as in
claims 1, 2, 3, 4, or 5 in which the apertures are spaced apart a
distance between 50 and 100 .mu.m.
Description
RELATED APPLICATION
[0001] This application is a continuation of co-pending application
Ser. No. 09/098,011 filed Jun. 15, 1998, which is a
continuation-in-part of application Ser. No. 08/530,919 filed Sep.
20, 1995, now U.S. Pat. No. 5,828,394 issued Oct. 27, 1998.
BRIEF SUMMARY OF THE INVENTION
[0003] This invention relates generally to piezoelectrically
actuated flex-tensional transducer arrays and method of
manufacture, and more particularly to such transducer arrays which
can be used as ultrasonic transducers, fluid drop ejectors and in
scanning force microscopes.
BACKGROUND OF THE INVENTION
[0004] Fluid drop ejectors have been developed for inkjet printing.
Nozzles which allow the formation and control of small ink droplets
permit high resolution, resulting in printing sharper characters
and improved tonal resolution. Drop-on-demand inkjet printing heads
are generally used for high-resolution printers. In general,
drop-on-demand technology uses some type of pulse generator to form
and eject drops. In one example, a chamber having a nozzle orifice
is fitted with a piezoelectric wall which is deformed when a
voltage is applied. As a result of the deformation, the fluid is
forced out of the nozzle orifice and impinges directly on an
associated printing surface. Another type of printer uses bubbles
formed by heat pulses to force fluid out of the nozzle orifice.
[0005] There is a need for an improved fluid drop ejector for use
not only in printing, but also, for photoresist deposition in the
semiconductor and flat panel display industries, drug and
biological sample delivery, delivery of multiple chemicals for
chemical reactions, DNA sequences, and delivery of drugs and
biological materials for interaction studies and assaying. There is
also need for a fluid ejector that can cover large areas with
little or no mechanical scanning.
[0006] Various types of ultrasonic transducers have been developed
for transmitting and receiving ultrasound waves. These transducers
are commonly used for biochemical imaging, non-destructive
evaluation of materials, sonar, communication, proximity sensors
and the like. Two-dimensional arrays of ultrasound transducers are
desirable for imaging applications. Making arrays of transducers by
dicing and connecting individual piezoelectric elements is fraught
with difficulty and expense, not to mention the large input
impedance mismatch problem that such elements present to
transmit/receiving electronics.
[0007] Scanning force microscopes have been applied to many kinds
of samples which cannot be imaged by the other scanning probe
microscopes. Indeed, they have the advantage of being applicable to
the biological science field where, in order to image living
biological samples, the development of scanning force microscopes
in liquid with minimum heat production specification is needed. In
addition, non-contact scanning force microscopes operating in
liquid would permit imaging soft and sensitive probe lithography
and high density data storage. Two dimensional arrays of atomic
force probes with self-exciting piezoelectric sensing would provide
a scanning force microscope which would meet the identified
needs.
OBJECTS AND SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide a
flextensional piezoelectric transducer array for use in ultrasonic
transducers, droplet ejectors and scanning force microscopes.
[0009] It is another object of the invention to provide a fluid
drop ejector having an array of piezoelectrically actuated
flextensional transducers in which the drop size, drop velocity,
ejection rate and number of drops can be easily controlled.
[0010] It is another object of the invention to provide a
micromachined flextensional membrane array with each membrane
having a piezoelectric transducer which is selectively
addressed.
[0011] It is a further object of the invention to provide a fluid
drop ejector in which a membrane including a nozzle is actuated to
eject droplets of fluid, at or away from the mechanical resonance
of the membrane.
[0012] It is another object of the present invention to provide an
array of piezoelectric flextensional transducers which can be used
for sending and receiving sound, and which can be selectively
addressed for ultrasonic imaging.
[0013] It is a further object of the present invention to provide
an array of flextensional piezoelectrically actuated membranes
which are electrostatically positioned.
[0014] The foregoing and other objects are achieved by an array of
flextensional membranes, each provided with a piezoelectric
transducer which can activate the membrane and/or provide a signal
representing membrane displacement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The foregoing and other objects of the invention will be
more fully understood from the following description read in
connection with the accompanying drawings, wherein:
[0016] FIG. 1 is a sectional view of a piezoelectrically actuated
transducer in accordance with the invention.
[0017] FIG. 2 is a top plan view of the ejector shown in FIG.
1.
[0018] FIG. 3 is a sectional view of a drop-on-demand fluid drop
ejector using a piezoelectrically actuated transducer in accordance
with the invention.
[0019] FIGS. 4A-4C show the ac voltage applied to the piezoelectric
transducer of the piezoelectrically actuated transducers of FIGS. 1
and 2, the mechanical oscillation of the membrane, and continuous
ejection of fluid drops.
[0020] FIGS. 5A-5C show the application of ac voltage pulses to the
piezoelectric transducer of the piezoelectrically actuated
transducer of FIGS. 1 and 2, the mechanical oscillation of the
membrane and the drop-on-demand ejection of drops.
[0021] FIGS. 6A-6C show the first three mechanical resonant modes
of a membrane as examples among all the modes of superior order in
accordance with the invention.
[0022] FIGS. 7A-7D show the deflection of the membrane responsive
to the application of an excitation ac voltage to the piezoelectric
transducer and the ejection of droplets in response thereto.
[0023] FIGS. 8A-8D show the steps in the fabrication of a matrix of
piezoelectrically actuated flextensional transducers of the type
shown in FIGS. 1 and 2.
[0024] FIG. 9 is a top plan view of a matrix fluid drop ejector
formed in accordance with the process of FIGS. 8A-8D.
[0025] FIG. 10 shows another embodiment of a matrix fluid drop
ejector.
[0026] FIGS. 11A-11E show the steps for the fabrication of a matrix
of piezoelectrically actuated flextensional transducer in
accordance with another procedure.
[0027] FIG. 12 shows the real part of the input impedance of the
transducer matrix of FIG. 11 as a function of frequency.
[0028] FIG. 13 shows the change in the real part of the input
impedance of the transducer matrix of FIG. 11 in air and vacuum as
a function of frequency.
[0029] FIG. 14 shows the transmission of ultrasound in air in the
transducer matrix of FIG. 11.
[0030] FIGS. 15A-15H show the steps in fabricating a
piezoelectrically actuated flextensional transducer matrix in
accordance with a back process.
[0031] FIG. 16 shows an atomic force microscope probe mounted on
the membrane of a piezoelectrically actuated flextensional
transducer.
[0032] FIGS. 17A-17H show the steps in forming a matrix of
transducers of the type shown in FIG. 16.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] A piezoelectrically actuated flextensional transducer
according to one embodiment of this invention is shown in FIGS. 1
and 2. The transducer includes a support body or substrate 11 which
can have apertures for the supply of fluid if it is used as a
droplet ejector as will be presently described. A cylindrical wall
12 supports and clamps an elastic membrane 13. The support 11, wall
12 and membrane 13 define a reservoir 14. When the transducer is
used as a droplet ejector, an aperture 16 may be formed in the wall
12 to permit continuous supply of fluid into the reservoir to
replenish fluid which is ejected, as will be presently described.
The fluid supply passage could be formed in the support body or
substrate 11. A piezoelectric annular transducer 17 is attached to
or formed on the upper surface of the membrane 13. The transducer
17 includes conductive contact films 18 and 19. The piezoelectric
film can also be formed on the bottom surface of the membrane, or
can itself be the membrane.
[0034] When the piezoelectrically actuated transducer is used as an
ultrasound transmitter or receiver, or as a fluid droplet ejector,
or in a scanning force microscope, the clamped membrane is driven
by the piezoelectric transducer so that it mechanically oscillates
preferably into resonance. This is illustrated in FIGS. 4 through
6. FIG. 4A shows a sine wave excitation voltage which is applied to
the piezoelectric transducer. The transducer applies forces to the
membrane responsive to the applied voltage. FIG. 4B shows the
amplitude of deflection at the center of the membrane responsive to
the applied forces. It is noted that when the power is first
applied, the membrane is only slightly deflected by the first power
cycle, as shown at 22, FIG. 4B. The deflection increases, whereby,
in the present example, at the third cycle, the membrane is in
maximum deflection, as shown at 23, FIG. 4B. At this point, its
deflection cyclically continues at maximum deflection with the
application of each cycle of the applied voltage. When the
transducer is used as a droplet ejector, it permits the ejection of
each corresponding drop, as shown in FIG. 4C. When the power is
turned off, the membrane deflection decays as shown at 24, FIG. 4B.
The frequency at which the membrane resonates is dependent on the
membrane material, its elasticity, thickness, shape and size. The
shape of the membrane is preferentially circular; however, the
other shapes, such as square, rectangular, etc., can be made to
resonate and eject fluid drops. In particular, an elliptic membrane
can eject two drops from its focal points at resonance. The amount
of deflection depends on the magnitude of the applied power. FIG. 6
shows, for a circular membrane, that the membrane may have
different modes of resonant deflection. FIG. 6A shows deflection at
its fundamental frequency; FIG. 6B at the first harmonic and FIG.
6C at the second harmonic.
[0035] The action of the membrane to eject drops of fluid is
illustrated in FIGS. 7A-7D. These figures represent the deflection
at the fundamental resonance frequency. FIG. 7A shows the membrane
deflected out of the reservoir, with the liquid in contact with the
membrane. FIG. 7B shows the membrane returning to its undeflected
position, and forming an elongated bulb of fluid 26 at the orifice
nozzle. FIG. 7C shows the membrane extending into the reservoir and
achieving sufficient velocity for the bulb 26 to cause it to break
away from the body of fluid and form a droplet 27 which travels in
a straight line away from the membrane and nozzle toward an
associated surface such as a printing surface. FIG. 7D represents
the end of the cycle and the shape of the fluid bulb 26 at that
point.
[0036] Referring to FIG. 4C, it is seen that the membrane reaches
maximum deflection upon application of the third cycle of the
applied voltage. It then ejects drops with each cycle of the
applied voltage as long as the applied voltage continues. FIGS.
5A-5C show the application of excitation pulses. At 29, FIG. 5A, a
four-cycle pulse is shown applied, causing maximum deflection and
ejection of two single drops, FIG. 5C. The oscillation then decays
and no additional drops are ejected. At 30, three cycles of power
are applied, ejecting one drop, FIG. 5C. It is apparent that drops
can be produced on demand. The drop rate is equal to the frequency
of the applied excitation voltage. The drop size is dependent on
the size of the orifice and the magnitude of the applied voltage.
The fluid is preferably fed into the reservoir at constant pressure
to maintain the meniscus of the fluid at the orifice in a constant
concave, flat, or convex shape, as desired. The fluid must not
contain any air bubbles, since it would interfere with operation of
the ejector.
[0037] FIG. 3 shows a fluid drop ejector which has an open
reservoir 14a. The weight of the fluid keeps the fluid in contact
with the membrane. The bulb 26a is ejected by deflection of the
membrane 13 as described above.
[0038] A fluid drop ejector of the type shown in FIG. 3 was
constructed and tested. More particularly, the resonant membrane
comprised a circular membrane of steel (0.05 mm in thickness; 25 mm
in diameter, having a central hole of 150 .mu.m in diameter). This
membrane was supported by a housing composed of a brass cylinder
with an outside diameter of 25 mm and an inside diameter of 22.5
mm. The membrane was actuated by an annular piezoelectric plate
bonded on its bottom and on axis to the circular membrane. The
annular piezoelectric plate had an outside diameter of 23.5 mm and
an inside diameter of 18.8 mm. Its thickness was 0.5 mm. The
reservoir was formed by the walls of the housing and the top was
left open to permit refilling with fluid. The device so constructed
ejected drops of approximately 150 .mu.m in diameter. The ejection
occurred when applying an alternative voltage of 15 V peak to the
piezoelectric plate at a frequency of 15.5 KHz (with 0.3 KHz
tolerance of bandwidth), which corresponded to the resonant
frequency of the liquid loaded membrane. This provided a bending
motion of the membrane with large displacements at the center.
Thousands of identical drops were ejected in one second with the
same direction and velocity. The level of liquid varied from 1-5 mm
with continuous ejection while applying a slight change in
frequency to adapt to the change in the resonant frequency of the
composite membrane due to different liquid loading. When the level
of liquid remained constant, the frequency of drop formation
remained relatively constant. The excitation was sinusoidal,
although square waves and triangular waveforms were used as
harmonic signals and also gave continuous drop ejection as the
piezoelectric material was excited to cause flextensional vibration
of the membrane.
[0039] As will be presently described, the fluid drop ejector can
be implemented using micro-machining semiconductive materials
employing semiconductor processing technologies. The housing could
be silicon and silicon oxide, the membrane could be silicon
nitride, and the piezoelectric transducer could be a deposited thin
film such as zinc oxide. In this manner, the dimensions of an
ejector could be no more than 100 microns and the orifice could be
anywhere from a few to tens of microns in diameter. Two-dimensional
matrices can be easily implemented for printing at high speed with
little or no relative motion between the fluid drop ejector and
object upon which the fluid is to be deposited.
[0040] It is apparent that the piezoelectrically actuated
flextensional membranes can be vibrated to generate sound in air or
water by driving the piezoelectric transducer at the proper
frequency. The individual piezoelectrically actuated transducers
forming the array are designed to have a maximum displacement at
the center of the membrane at the resonant frequency. The
complexity of the structure and the fact that the piezoelectric
transducer is a ring rather than a full disk, necessitates the use
of finite element analysis to determine the resonant frequencies of
the composite structure, the input impedance of the piezoelectric
transducer, and the normal displacement of the surface.
[0041] It is well know that the transverse displacement .xi. of a
simple membrane of uniform thickness, in vacuum, obeys the
following differential equation: 1 4 + D 2 t 2 = 0 ( 1 )
[0042] The axisymmetric free vibration frequencies for an
edge-clamped circular membrane are given by 2 = 2 a 2 / D ( 2 )
[0043] where .lambda. represents the eigenvalues of Eq. (1),
.alpha. is the radius of the membrane, .rho. is the mass per unit
area of the membrane, and 3 D = Eh 3 12 ( 1 - v 2 ) ( 3 )
[0044] where E is Young's modulus, h is the membrane thickness, and
v is Poisson's ratio. The above equations suggest that the resonant
frequency is directly proportional to the thickness of the membrane
and inversely proportional to the square of the radius. However, it
is also known that the resonant frequency will be decreased by
fluid loading on one or both sides of the membrane. The shift in
the fluid loaded resonant frequency of a simple membrane is 4 f w =
f a 1 + ( 4 )
[0045] where .beta.=.rho..sub.wa/.rho..sub.mh is a thickness
correction factor, .rho..sub.w is the density of the liquid,
.rho..sub.m is the mass density of the circular membrane, and
.GAMMA. is the non-dimensional added virtual mass incremental
(NAVMI) factor, which is determined by boundary conditions and mode
shape. For the first order axisymmetric mode and for water loading
on one side of the membrane, .gamma. is 0.75. The resonant
frequency can be expected to shift down by about 63%.
[0046] The foregoing membrane analysis is also applicable to the
droplet ejector application of the piezoelectrically actuated
flextensional transducer and the resonant frequency of the membrane
will be shifted down as discussed above.
[0047] Referring to FIGS. 8A-8D, the steps of fabricating a matrix
of piezoelectrically actuated transducers of the type shown in
FIGS. 1 and 2 from semiconductor material are shown for a typical
process. By well-known semiconductor film or layer-growing
techniques, a silicon substrate 41 is provided with successive
layers of silicon oxide 42, silicon nitride 43, metal 44,
piezoelectric material 45 and metal 46. The next steps, shown in
FIG. 8B, are to mask and etch the metal film 46 to form disk-shaped
contacts 48 having a central aperture 49 and interconnected along a
line 50, FIG. 9. The next step is to etch the piezoelectric layer
in the same pattern to form transducers 51. The next step, FIG. 8C,
is to mask and etch the metal film 44 to form disk-shaped contacts
52 having central apertures 53 and interconnected along columns 55,
FIG. 9. The next steps, FIG. 8D, are to mask and etch orifices 54
in the silicon nitride layer 43. This is followed by selectively
etching the silicon oxide layer 42 through the orifices 54 to form
a fluid reservoir 56. The silicon nitride membrane 43 is supported
by silicon oxide posts 57.
[0048] FIG. 9 is a top plan view of the matrix shown in FIGS.
8A-8D. The dotted outline shows the extent of the fluid reservoir.
It is seen that the membrane is supported by the spaced posts 57.
The upper contacts of the piezoelectric members in the horizontal
rows are interconnected along the lines 50 as shown and the lower
contacts of the piezoelectric members in the columns are
interconnected along lines 55 as shown, thereby giving a matrix in
which the individual membranes can be excited, thereby ejecting
selected patterns of drops or to direct ultrasound.
[0049] By micro-machining, closely spaced patterns of orifices or
nozzles can be achieved. If the spacing between orifices is 100
.mu.m, the matrix will be capable of simultaneously depositing a
resolution of 254 dots per inch. If the spacing between orifices is
50 .mu.m, the matrix will be capable of simultaneously depositing a
resolution of 508 dots per inch. Such resolution would be
sufficient to permit the printing of lines or pages of text without
the necessity of relative movement between the print head and the
printing surface.
[0050] The invention has been described in connection with the
ejection of a single fluid as, for example, for printing a single
color or delivering a single biological material or chemical. It is
apparent that ejectors can be formed for ejecting two or more
fluids for color printing and chemical or biological reactions. The
spacing of the apertures and the size and location of the
associated membranes can be selected to provide isolated reservoirs
or isolated columns or rows of interconnected reservoirs. Adjacent
rows or columns or reservoirs can be provided with different
fluids. An example of matrix of fluid ejectors having isolated rows
of fluid reservoirs is shown in FIG. 10. The fluid reservoirs 56a
are interconnected along rows 71. The rows are isolated from one
another by the walls 57a. Thus, each of the rows of reservoirs can
be supplied with a different fluid. Individual ejectors are
energized by applying voltages to the interconnections 50a and 55a.
The illustrated embodiment is formed in the same manner as the
embodiment of FIG. 9 by controlling the spacing of the apertures
and/or the length of sacrificial etching. The processing of the
fluid drop ejector assembly can be controlled so that there are
individual fluid reservoirs with individual isolated membranes. The
spacing and location of apertures and etching can be controlled to
provide ultrasonic transducers having individual or combined
transmitting membranes.
[0051] The preferred fabrication process for micromachined two
dimensional array flextensional transducers is given in FIGS.
11A-G. The process starts with growing a sacrificial layer, chosen
to be silicon oxide. A membrane layer of low-pressure chemical
vapor deposition silicon nitride is grown on top of the sacrificial
layer. The bottom Ti/Au electrode layer for the piezoelectric
transducers is deposited on the membrane by e-beam evaporation. The
bottom metal layer is patterned by wet etch, and access holes for
sacrificial layer etching are drilled in the membrane layer by
plasma etch, FIG. 11B. A piezoelectric ZnO layer is deposited on
top of the bottom electrode by dc planar magnetron reactive
sputtering. The ZnO layer is patterned by masking and wet etching,
FIG. 11C. The top Cr/Au electrode layer is then formed by e-beam
evaporation at room temperature and patterned by liftoff, FIG. 11D.
The last step is etching the sacrificial layer by wet etch, FIG.
11E, and this concludes the front surface micromachining of the
piezoelectrically actuated flextensional array of transducers.
[0052] FIG. 12 shows the real part of the electrical input
impedance of only one row of 60 elements of devices formed in
accordance with the above which on center are spaced 150 .mu.m
apart. The silicon nitride membrane was 0.3 .mu.m thick and had a
diameter of 90 .mu.m. Operating in air, the transducers had a
resonant frequency of 3.0 MHz and a fractional bandwidth of about
1.5%. The real part of the electrical input impedance was a 280
.OMEGA. base value. It was determined by SPICE simulation that this
base value is caused by the bias lines connecting the individual
array elements. This can be avoided by using electroplating to
increase the thickness of the bias lines. FIG. 12 also shows the
existence of acoustical activity in the device, and an acoustic
radiation resistance R.sub.a of 150 .OMEGA.. FIG. 13 presents the
change of the electrical input impedance in vacuum of a device
consisting of one row of 60 3.07 MHz in vacuum (at 50 mTorr). This
result is in accordance with expectations, since the resonant
frequency and the real part of electrical input impedance at
resonance should increase in vacuum. FIG. 14 shows the result of an
air transmission experiment where an acoustic signal is received
following the electromagnetic feedthrough. The insertion loss is
112 dBs. In the transmit/receive experiment, the receiver had one
row of 60 elements, and the transmitter had two rows of 120
elements. Loss due to electrical mismatches was 34.6 dBs. Other
important loss sources are alignment of receiver and transmitter,
and structural losses.
[0053] An alternative micromachining fabrication process can be
employed to manufacture micromachined two dimensional array
flextensional ultrasonic transducers and droplet ejectors by using
a back process concept. FIGS. 15A-15J illustrate the process flow
for this embodiment of the invention. A sacrificial layer and
membrane are grown on a relatively thin, i.e. 200 .mu.m double side
polished silicon wafer. The silicon oxide and silicon nitride on
the back surface are patterned to have access openings from the
back side to the silicon by dry plasma etch, FIG. 15B. The silicon
is etched until enough silicon is left to support subsequent
process steps, FIG. 15C. Bottom metal electrode layer is deposited
on the upper surface and patterned, FIG. 15D. A Piezoelectric layer
is deposited and patterned, FIG. 15E. And top metal electrode layer
is formed by the liftoff method, FIG. 15F. At this step,
lithography can be used to form orifices for droplet ejectors;
however, this is not shown. Later, isotropic or anisotropic silicon
wet etchant is used to remove the remaining supporting silicon,
FIG. 15G. At this step, the front surface of the wafer is protected
by a mechanical fixture or protective polymer film. After removing
the remaining silicon, the sacrificial layer is etched by wet etch,
FIG. 15H. Note that, depending on the size of holes etched from the
back, sacrificial layer may not be needed at all.
[0054] Orifices for droplet ejectors may be drilled by dry plasma
etching. The structure can be bounded to glass or other kind of
support. This will provide access for liquid in case of droplet
ejectors, and an ability of changing back pressure and boundary
conditions, i.e., different back load impedance by filling
different liquids in the back of the membrane, in ultrasonic
transducers.
[0055] The flextensional piezoelectric transducer array can be used
in a two dimensional scanning force microscope both for force
sensing and nanometer scale lithography applications. Referring to
FIG. 16, an individual probe 60 is shown on a deflected membrane 61
of a flextensional piezoelectric transducer having piezoelectric
transducer 62. An array of individual probes mounted on individual
membranes can be fabricated by micromachining in the vacuum
previously described. An ac voltage is applied across the
piezoelectric material to set the compound membrane into vibration.
At the resonant frequencies of the compound membrane, the
displacement of the probe tip is large. The tip sample spacing is
controlled for each array element as by electrostatically
deflecting the membrane applying a dc voltage to the piezoelectric
transducer. A transducer array with electrostatic deflection of the
membrane will be presently described.
[0056] In dynamic scanning force microscopy applications, the
spring in the probe support is a critical component, the maximum
deflection for a given force is needed. This requires a spring that
is as soft as possible. At the same time, a stiff spring with high
resonant frequency is necessary in order to minimize response time.
On the other hand, we need the minimum number of passes of the
probe tip and the maximum force that could be applied by a probe on
a photoresist to achieve the desired patterning of the photoresist
by the tip. This case requires a bigger spring constant and higher
resonant frequency. Polysilicon membrane can be used to obtain
higher spring constant values, whereas silicon nitride membrane can
be used to obtain smaller spring constant values.
[0057] In scanning force microscopy, the probe dynamically scans
across the sample surface. The dynamic mode is commonly divided
into two modes, the non-contact mode and the cyclic-contact
(tapping) mode. In the cyclic-contact mode, a raster probe vibrates
at its resonant frequency and gradually approaches the sample until
the probe tip taps the surface at the bottom of each vibration
cycle. The cyclic-contact becomes the prevailing operation mode in
air, because an SFM operated in this mode offers as high a
resolution as an SFM operated in a contact mode. A cyclic-contact
SFM does not damage the surface of soft samples as much as the
contact SFM.
[0058] In the contact mode a feedback loop maintains the atomic
force between the tip and the sample constant by adjusting the
tip-sample spacing by electrostatic actuation or by piezoelectric
actuation in case of individual addressing for each array element.
On the other hand, pneumatic actuation can be used for tip-sample
spacing without individual addressing. In case of tapping mode, the
piezoelectric layer is utilized for exciting the membrane and
detecting the membrane displacement, whereas electrostatic
actuation is utilized to control the tip-sample spacing. By
utilizing the admittance spectrum of the piezoelectric layer, the
dynamic SFM can be easily constructed. In tapping mode, the peak
height of the piezoelectric resonance spectrum (admittance)
decreases by the tip-sample spacing. In addition, when the
composite membrane operates in the tapping mode of the
piezoelectric SFM, piezoelectric charge output detection may be
used for the force sensing method.
[0059] The fabrication process for micromachined two dimensional
array of electrostatically deflected flextensional
piezoelectrically actuated SFM probes is shown in FIG. 17A. The
process starts with high resistivity silicon substrate. A thermal
oxide layer used for masking in ion implantation is grown on the
substrate, and patterned by wet etch in order to define the bottom
electrode for electrostatic actuation, FIG. 17A. Dopant atoms are
then implanted to form a conductive region which serves as the
bottom electrode for electrostatic actuation of the flextensional
membrane, FIG. 17B. After stripping of the masking oxide, a silicon
oxide sacrificial layer is grown. The sacrificial layer can be
patterned by lithography to define the lateral dimension of the
individual array element. A membrane layer of LPCVD silicon nitride
is grown on top of the sacrificial layer. Polysilicon can be used
as membrane to obtain higher spring constant. The bottom Ti/Au
electrode layer for a piezoelectric transducer is deposited on the
membrane by e-beam evaporation, FIG. 17C. The bottom electrode
layer is patterned by wet etch, and a piezoelectric ZnO layer is
deposited on top of the bottom electrode, FIG. 17D. After
patterning the ZnO layer by wet etch, the top Cr/Au electrode layer
is formed by e-beam evaporation and patterned by liftoff, FIG. 17E.
A Spindt tip or probe is formed at the center of the membrane by
allowing holes defined in a sacrificial photoresist template layer
to be self-occluded by evaporated Cr/Au layer, forming very sharp
tips. Holes are etched in the back side by deep reactive ion
etching thru the silicon substrate. These thru holes are not only
used to remove the sacrificial layer, but also can be used for
pneumatic actuation of the membrane to control the tip-sample
spacing. The last step is etching the sacrificial layer by wet etch
or by HF vapor plasmaless-gas-phase etch, FIG. 17H.
[0060] Micromachined two dimensional array flextensional
transducers and droplet ejectors have common advantages over
existing designs. First of all, they are micromachined in two
dimensional arrays by using conventional integrated circuit
manufacturing processes. They have piezoelectric actuation, that
means AC signals drive the devices. The devices have optimized
dimensions for specific materials.
[0061] For ultrasonic applications, devices can be broadband by
utilizing different diameter of devices on the same die. Two
dimensional array can be focused by appropriate addressing. Also,
if the back process is used, the devices will have already sealed
membranes, thus, they can be used as immersion transducers.
[0062] Micromachined two dimensional array flextensional
piezoelectrically actuated droplet ejectors can eject any liquid as
long as compatible membrane material is chosen. The device eject
without any waste. They can be operated both in the drop-on-demand
and the continuous mode. They may also eject small solid particles
such as talc or photoresist. They can be used for ejecting
expensive biological, chemical materials in small amounts.
[0063] The micromachined two dimensional array of flextensional
transducers can be used in scanning atomic force microscopy. The
array elements can be individually addressed for scanning. The
array elements use self-excited piezoelectric sensing and
electrostatic actuation. The device is capable of operating in
high-vacuum, air, or liquid. Moreover, on-board driving, sensing,
and addressing circuitries can be combined with the array.
[0064] Different materials can be used as sacrificial layer.
Various materials can be used as membrane as long as they are
compatible with sacrificial layer etch. In the back process,
depending on the size of holes etched from back, sacrificial layer
may not be needed at all. Other kinds of piezoelectric thin films,
such as sputtered PZT and PVDF can be used instead of zinc oxide.
Other metal thin films can be used instead of gold, since they are
not exposed to any subsequent wet etch of other materials.
Dimensions of devices can be optimized depending on where they will
be used and what kinds of materials will be used in their
fabrication.
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