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Claims  |
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We claim:
1. An electro-mechanical capacitive device comprising:
(a) a first substrate of crystalline silicon having opposite sides, a
portion of one side having boron diffused therein to a selected depth
having a layer between 1 and 10 microns in thickness with boron atom
concentrations sufficient to render the layer conductive and resistant to
etching by selected silicon etchants, a cavity formed in the opposite side
of said substrate bottoming on the boron diffused etchant resistant layer
to define a diaphragm in said layer under said cavity having a thickness
of between approximately 1 and 10 microns, lateral dimensions between
approximately 1 millimeter and 2 centimeters, and internal tension of the
order of magnitude of 10.sup.5 dynes per centimeter; and
(b) a second electrode spaced away and insulated from the boron diffused
layer side of said diaphragm at a distance selected such that mechanical
deflection of said diaphragm will measurably change the capacitance
between said diaphragm and said second electrode, whereby changes in
pressure across said diaphragm which deflect said diaphragm can be
measured by measuring the change in capacitance between said diaphragm and
second electrode, and whereby an oscillating electrical signal applied
between said diaphragm and said second electrode will induce mechanical
vibration of said diaphragm to provide an effective resonant impedance to
the electrical signal.
2. The device of claim 1 wherein said second electrode is spaced away from
said diaphragm a distance between 1 and 25 microns.
3. The device of claim 1 wherein said second electrode is comprised of a
layer of conducting silicon, and wherein said device includes a layer of
silicon dioxide interposed between the etchant resistant layer side of
said silicon substrate and said second electrode and provides electrical
insulation thereof, and wherein a chamber is formed in said silicon
dioxide layer between said diaphragm and said conductive silicon electrode
to allow free deflection of said diaphragm.
4. The device of claim 1 wherein the space between said diaphragm and said
second electrode is sealed off from the side of said silicon substrate
having said cavity therein, such that changes in ambient pressure will
result in differentials in pressure across said diaphragm to deflect said
diaphragm and change the value of the capacitance between said diaphragm
and said second electrode;
5. The device of claim 1 wherein said etchant resistant layer is formed
intermediate the sides of said silicon substrate and wherein a second
cavity is formed in said silicon substrate on the side opposite said first
cavity such that a diaphragm is formed in said etchant resistant layer
between said first and second cavities.
6. The device of claim 1 wherein said second electrode comprises a layer of
electrically insulating material spaced away from said diaphgram and a
layer of conductive metal deposited on the surface of the layer of
insulating material adjacent to said diaphragm.
7. The device of claim 6 wherein said insulating material is boro-silicate
glass having a cavity formed therein under said diaphragm, and wherein
said layer of conducting metal is deposited on the bottom of said cavity.
8. The device of claim 1 wherein the side of said diaphragm opposite to the
cavity in said silicon substrate is covered by a layer of boro-silicate
glass less than approximately 1,300 Angstroms thick, said boro-silicate
glass layer being structurally weakened such that it exerts substantially
no tension on the underlying boron doped silicon diaphragm.
9. An electro-mechanical resonant circuit comprising:
(a) a first substrate of crystalline silicon having opposite sides, a
portion of one side having an etchant resistant and electrically
conductive layer formed therein to a selected depth, a cavity formed in
the opposite side of said substrate bottoming on said etchant resistant
layer to define a diaphragm in said layer under said cavity, the thickness
of the etchant resistant layer forming said diaphragm and the lateral
dimensions of said diaphragm being selected such that said diaphragm
exhibits substantial physical deflection and mechanical resonance in
response to electrically induced forces on said diaphragm;
(b) a second electrode spaced away and insulated from the etchant resistant
layer side of said diaphragm at a distance selected such that electric
charge on said second electrode will cause mechanical deflections of said
diaphragm when oppositely charged, whereby said diaphragm and said
conducting electrode form two plates of a capacitor having a capacitance
varying with the physical deflection of said diaphragm; and
(c) signal source means for applying an oscillating electrical signal at a
selected frequency between said diaphragm and said second electrode so as
to cause mechanical vibrations of said diaphragm in response to the
varying electric field between said second electrode and said diaphragm.
10. The resonant circuit of claim 9 including means for applying a DC
voltage bias between said diaphragm and said second electrode to displace
siad diaphgram toward said second electrode and vary the effective
capacitance between the same in response to the DC voltage bias.
11. The resonant circuit of claim 9 wherein said second electrode is spaced
away from said diaphragm a distance between 1 and 25 microns.
12. The resonant circuit of claim 9 wherein said second electrode is
comprised of a layer of conducting silicon, and a layer of silicon dioxide
interposed between the etchant resistant layer side of said silicon
substrate and said layer of conducting silicon to provide electrical
insulation thereof, and wherein a chamber is formed in said silicon
dioxide layer between said diaphragm and said conductive silicon layer to
allow free deflection of said diaphragm.
13. The resonant circuit of claim 9 wherein the space between said
diaphragm and said second electrode is sealed off from the side of sid
silicon substrate having said cavity therein, such that changes in ambient
pressure will result in differentials in pressure across said diaphragm to
deflect said diaphragm and change the value of the capacitance between
said diaphragm and said second electrode.
14. The resonant circuit of claim 9 wherein said etchant resistant layer is
formed intermediate the sides of said silicon substrate, and wherein a
second cavity is formed in said silicon substrate on the side opposite
said first cavity such that a diaphragm is formed in said etchant
resistant layer between said first and second cavities.
15. The resonant circuit of claim 9 wherein said second electrode comprises
a layer of electrically insulating material spaced away from said
diaphragm and a layer of conductive metal deposited on the surface of the
layer of insulating material adjacent to said diaphragm.
16. The resonant circuit of claim 15 wherein said insulating material is
boro-silicate glass having a cavity formed therein under said diaphragm,
and wherein said layer of conducting metal is deposited on the bottom of
said cavity.
17. The resonant circuit of claim 9 wherein said etchant resistant layer
comprises silicon with boron diffused therein in an amount sufficient to
render the layer conductive and resistant to selected silicon etchants,
and wherein the side of said diaphragm opposite to the cavity in said
silicon substrate is covered by a layer of boro-silicate glass less than
approximately 1,300 Angstroms thick, said boro-silicate glass layer being
structurally weakened such that it exerts substantially no tension on the
underlying boron doped silicon diaphragm.
18. An electro-mechanical capacitive device comprising:
(a) a first substrate of crystalline silicon having opposite sides, a
portion of one side having an etchant resistant layer formed therein to a
selected depth, a cavity formed in the opposite side of said substrate
bottoming on said etchant resistant layer to define a diaphragm in said
layer under said cavity, said diaphragm being of a selected thickness and
having selected lateral dimensions whereby it is capable of substantial
deflection when exposed to external forces and exhibits mechanical
resonance;
(b) a second conductive silicon electrode spaced away from said etchant
resistant layer side of said diaphragm at a selected distance whereby when
electric charge is placed on said conducting electrode it will cause
mechanical deflection of said diaphragm when oppositely charged, and
whereby said diaphragm and said conducting electrode form two plates of a
capacitor having a capacitance varying with the physical deflection of
said diaphragm;
(c) insulating material separating and electrically insulating said
conductive electrode from said diaphragm, a chamber formed in said
insulating material between said diaphragm and said silicon electrode, a
channel formed through said silicon electrode leading to an orifice in
said chamber; and
(d) a cap mounted to said diaphragm in position to close said orifice when
a selected electrostatic attraction is applied between said diaphragm and
said second silicon electrode.
19. an electro-mechanical device comprising:
(a) a first substrate of crystalline silicon having opposite sides, a
portion of one side having an etchant resistant layer formed therein to a
selected depth, a cavity formed in the opposite side to said substrate
bottoming on said etchant resistant layer to define a diaphragm in said
layer under said cavity, said diaphragm being of a selected thickness and
having selected lateral dimensions whereby it is capable of substantial
deflection when exposed to external forces and exhibits mechanical
resonance;
(b) a second conductive silicon electrode spaced away from said etchant
resistant layer side of said diaphragm at a selected distance whereby when
electric charge is placed on said conducting electrode it will cause
mechanical deflections of said diaphragm when oppositely charged, and
whereby said diaphragm and said conducting electrode form two plates of a
capacitor having a capacitance varying with the physical deflection of
said diaphragm; and
(c) a substantially flat optically transparent layer formed on the side of
said silicon substrate opposite that facing said conducting electrode and
extending over said cavity, whereby displacements of said diaphragm cause
visible changes by constructive interference of light coming in through
said transparent layer and light reflected from the surface of said
diaphragm facing said transparent layer.
20. An electro-mechanical capacitive device comprising:
(a) a first substrate of crystalline silicon having opposite sides, a
portion of one side having an etchant resistant layer formed therein to a
selected depth, a cavity formed in the opposite side of said substrate
bottoming on said etchant resistant layer to define a diaphragm in said
layer under said cavity, said diaphragm being of a selected thickness and
having selected lateral dimensions whereby it is capable of substantial
deflection when exposed to external forces and exhibits mechanical
resonance;
(b) a second conductive silicon electrode spaced away from said etchant
resistant layer side of said diaphragm at a selected distance whereby when
electric charge is placed on said conducting electrode it will cause
mechanical deflections of said diaphragm when oppositely charged, and
whereby said diaphragm and said conducting electrode form two plates of a
capacitor having a capacitance varying with the physical deflection of
said diaphragm; and
(c) a plurality of electrically conductive pyramidal prominences formed in
association with said conducting electrode in position to be individually
contacted by said diaphragm when said diaphragm is deflected a selected
distance toward said conducting electrode, each said electrically
conductive prominence being capable of being separately provided with
electric charge to attract said diaphragm toward contact therewith. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The invention relates to monocrystalline silicon diaphragms which by virtue
of their electrostatic deformation capability are applicable in a wide
range of uses.
In the manufacture of integrated circuits and integrated electronic devices
wherein a substrate of semiconductor material such as silicon is utilized,
devices which behave as inductors which are compatible with the substrate
material have long been sought. No satisfactory method has previously been
achieved, thus requiring the use of large scale or discrete components in
conjunction with integrated circuits. The elimination of such discrete
components would therefore be valuable in both the reduction of the size
as well as the weight of circuits requiring inductive behaving devices.
The high cost of manufacturing such hybrid circuits is a result of the
manufacturing step of adding or attaching the discrete components to the
integrated circuits which have already been mechanized by integrated
circuit techniques; the elimination of such a step may therefore
considerably reduce the cost of manufacturing such circuits.
Silicon and other semiconductor membranes of thin section have been known
in the art, but it has not been heretofore discovered that the
electrostatic deformation of such membranes having certain dimensions
enable them to be utilized as a variable capacitance, as an
electromechanical resonator (by means of the superimposition of an AC
voltage upon a DC bias for creation of the electrostatic force) or for
other applications wherein a small controllable or resonant movement of a
diaphragm is useful.
In the prior art, for example, thin silicon diaphragms have been used as
pressure sensors, but such devices have generally been manufactured in
such a fashion as to form a configuration of strain gauge elements. Such
an application is taught in the U.S. Pat. No. 3,697,918 to Orth et al.
U.S. Pat. No. 3,814,998 to Thoma et al. shows the silicon membranes which
are utilized to form a sandwich with a dielectric core; the decrease of
the thickness of the inner dielectric core changes the capacitance of the
sandwich. However, in none of the prior art which has contemplated the use
of silicon membranes has it been recognized that the application of
electrostatic attraction forces between a thin silicon membrane formed in
a silicon wafer and an electrode of opposite polarity can induce both
movement and electromechanical resonance.
The present invention, however, contemplates structures which are capable
of exhibiting resonant behavior as well as controlled deflection in
response to external forces. These structures enable the construction of
improved pressure transducers, electro-optical display devices,
electromechanical resonant devices such as tank circuits, and inductive
devices, all of which are capable of mechanization on the same substrate
as integrated circuits as conventionally manufactured and by techniques
which are compatible with present integrated circuit manufacture.
Known procedures for forming thin membranes in silicon substrates require a
lengthy deposition diffusion of an impurity into a portion of the
substrate to a desired depth. For example, boron is known to provide
etchant resistant properties when diffused into silicon above certain
concentration levels. The typical procedure for forming an etchant
resistant layer with this type of impurity is to diffuse boron from a gas
ambient into a portion of the silicon substrate. To achieve an etchant
resistant layer 1 to 3 microns thick having sufficient boron concentration
to resist specific silicon etchants, it has been necessary to retain the
silicon substrate in the diffusion furnace for long periods of time, in
the range of 3 hours or more. A surface phase is inevitably formed on the
surface of the substrate which is primarily a boro-silicate glass, i.e., a
mixture of boron oxide and silicon dioxide. The boro-silicate glass is
generally removed before further processing is performed on the substrate.
Such procedures are incompatible with the formation of further integrated
circuit components on the silicon slab, since the long deposition
diffusion time required allows the boron to penetrate an oxide mask in
other areas of the substrate. As a result, there will exist unwanted
levels of boron impurities which can interfere with the construction of
semi-conductor devices on the substrate.
SUMMARY OF THE INVENTION
In accordance with the present invention, and particularly in accordance
with the preferred embodiments illustrated in the following specification,
thin membranes constructed on the order of a micron in thickness can be
produced by selectively etching the surfaces of silicon wafers. Such
membranes are capable of physical deflection in response to the
application of electrostatic forces on them.
Thin membranes are formed in a silicon substrate in a manner which
minimizes the introduction of unwanted impurities in areas of the
substrate other than that in which the membrane is to be formed. The
process involves a short and concentrated deposition diffusion of boron
into one of the surfaces of the substrate, followed by rapid and
substantially oxygen and water vapor free transfer of the substrate to a
drive furnace in which diffusion to the selected depth takes place over a
controlled period of time. Oxygen and water vapor are substantially
excluded from the drive furnace. We have determined that only a relatively
thin borosilicate glass layer is formed on the substrate during the
deposition diffusion, and that, under the conditions mentioned above, this
glass layer deteriorates during the drive diffusion such that it does not
exert appreciable strain on the underlying silicon membrane. With such
processing, membranes in larger silicon substrates may be formed to
precisely controlled thicknesses with nearly 100% yield, while leaving the
remainder of the substrate substantially free of boron impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the invention, especially various
features, objects and advantages thereof, may be obtained from the
following detailed description when taken in conjunction with the drawings
in which:
FIG. 1 shows a generalized configuration of a diaphragm made in accordance
with the present invention capable of electrostatic deflection and
electromechanical resonance.
FIG. 2 shows a particular embodiment of a valve utilizing a thin silicon
diaphragm constructed in accordance with the present invention.
FIG. 3 shows a particular embodiment utilizing the diaphragm according to
the invention for the formation of a selectably tunable tank circuit.
FIG. 4 illustrates a construction utilized to determine the deformation of
a diaphragm.
FIG. 5 illustrates a construction of the invention utilized for exhibiting
electrostatic deformation of a diaphragm.
FIG. 6 shows an electro-optical display cell utilizing a thin silicon
diaphragm made in accordance with the invention.
FIG. 7 is a cross-sectional view of another embodiment of a device in
accordance with the invention, having a membrane formed in a silicon
substrate over a cavity formed in a glass base.
FIG. 8 is a top view of the device of FIG. 7.
FIG. 9 is an expanded cross-sectional view taken along the line 9--9 of
FIG. 7.
FIG. 10 is a schematic view of a circuit employing the device of FIG. 7 as
an element thereof.
FIG. 11 is an illustrative graph showing the relationship between resonant
frequency and quality factor versus bias voltage for the device of FIG. 7.
FIG. 12 is an illustrative graph showing the change in equivalent circuit
parameters for the device of FIG. 7 as a function of bias voltage.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Silicon membranes or thin diaphragms manufactured and utilized in
accordance with the present invention are typically on the order of a
micron in thickness and may be produced by selectively etching one or more
surfaces of a thin wafer of silicon of the type generally utilized for
substrates of integrated circuits. In a preferred method, wafers can be
prepared by means of diffusing boron into one surface thereof to a depth
corresponding to the thickness desired for the particular diaphragm or
membrane. The other side of the wafer may then be etched away in a pattern
devised by conventional integrated circuit preparation and manufacturing
techniques. It has been found that the diffused boron in the silicon forms
a barrier to the etching process which enables the thickness of the
diaphragm to correspond to the depth of diffusion of the boron in the
wafer. Although not restricted to the use of boron, boron or another
suitable substance may be useful for retarding the particular etch used
and may be introduced into the silicon wafer by other well known
techniques such as epitaxial growth or ion implantation. Many conventional
techniques exist for the selective etching away of silicon material to a
desired depth, and are appropriate. One that has been utilized and found
suitable will be described in an example of an experimental embodiment
below.
Diaphragms or membranes produced in the above described manner are useful
for the creation of several forms of devices. One form of device which has
a large range of uses employs a silicon diaphragm according to the
invention as one plate of a capacitive device. Although many other forms
of conductors may be used to form a second plate or second electrode, an
embodiment is shown in FIG. 1 which utilizes a second silicon wafer as the
second conductor or plate of a capacitor. In the structure shown in FIG.
1, a one micron thick silicon dioxide layer 30 is sandwiched between two
silicon wafers 10 and 20. The first or upper silicon layer 10 has formed
in it a one micron thick diaphragm 50 in accordance with the techniques
already suggested. Immediately below the diaphragm a portion of the
silicon dioxide layer has been selectively removed so as to form a cavity
or chamber 40 between the diaphragm 50 and the second or lower silicon
wafer 20. Such a cavity is of course necessary so that the proper and
desired range of movement of the diaphragm 50 may be effected. The
diaphragm of the first silicon wafer 10 and the second silicon wafer 20
separated by the oxide layer 30 form a capacitor across the chamber. Such
a structure of the diaphragm 50 and the second or lower wafer 30 form two
plates of the capacitor.
Since the structure shown in FIG. 1 creates a capacitance and since the
thin membrane 50 is deformable under the application of electrostatic and
other forces, the particular structure may be utilized as a sensor. For
example, in response to forces on the diaphragm in FIG. 1, the diaphragm
50 may itself be deformed, causing a change in the spacing of the
diaphragm 50 in relation to the lower silicon wafer or layer 20. In this
manner, the relative spacing of the "plates" causes a change in the
overall capacitance of the device. Deforming forces may be generated by
the expansion of a gas under thermally changing conditions, as a response
to the pressure of acoustic waves or other forces sufficient to cause the
deformation of the diaphragm 50. Thus, the diaphragm 50 may be utilized as
a force, temperature or pressure transducer wherein the measured quantity
may be related to capacitance changes. With the particular structure shown
in FIG. 1, many force-generating phenomena under investigation may be
quantized in terms of capacitance variation.
In addition to the above-described direct mechanical transducive aspect of
the structure shown in FIG. 1, the particular structure illustrated also
exhibits electrostatic phenomena capable of broad utilization. By placing
a DC bias potential across both the upper (10) and lower (20) silicon
wafers, a charge pattern is caused to form on the lower surface of the
diaphragm 50 and the upper surface of the lower silicon wafer exposed to
the chamber 40 etched in the silicon dioxide layer 30. Such a charge
pattern causes electrostatic attraction between the diaphragm 50 and the
lower silicon wafer 20. In certain voltage ranges such electrostatic
attraction will be sufficient to cause a measurable and substantially
linear physical deformation of the diaphragm.
This mechanical motion may be exploited in the creation of valves such as
shown in FIG. 2 that may be opened or closed in response to the
application of a DC voltage between a diaphragm 250 and a lower silicon
layer 220. In the structure of FIG. 2, a diaphragm 250 has formed integral
with it a cap or valve 260, which in response to deformation of the
silicon membrane 250, is caused to seal an orifice 270 formed in lower
silicon layer 220. Also, because deflections are extremely controllable by
means of variation in bias voltage as the accompanying table has shown, by
use of suitable means in controlling the electrostatic charge on the
plates, the device may be utilized as an extremely accurate
micropositioner.
A particular advantage to the structure shown in FIG. 1 is that it is
capable of exhibiting resonant behavior if, in addition to a DC bias, the
diaphragm 50 is excited by an AC voltage. Because the heretofore described
mechanical deformations induced by the electrostatic charge formed on the
surface of the layer 20 and diaphragm 50 can induce mechanical resonance
of the diaphragm, electrical energy applied to the structure shown in FIG.
1 as a voltage between layers 10 and 20 is therefore capable of causing
and sustaining mechanical resonance of the diaphragm 50. At such
frequencies the resonating mechanical diaphragm acts as an energy storage
element for electrical energy like a tuned circuit. Such resonant behavior
is analogous to that of a quartz crystal and the macroscopic behavior of
the device of the invention exhibits circuit behavior at its terminals
substantially as does an electrical capacitive-inductive resonant tank
circuit.
Because the process of manufacturing a diaphragm in accordance with the
present invention is compatible with integrated circuit manufacturing
techniques applied to the same silicon wafer, the manufacture of filters,
oscillators and tuners which embody or require resonant tank circuits is
therefore possible on an integrated circuit basis. One example of a use
exploiting the resonant behavior of the diaphragm according to the
invention under alternating current excitation is shown in FIG. 3. FIG. 3
shows an arrangement made from silicon wafers or wafer segments of a solid
state silicon crystal; and which is capable of being tuned to
predetermined multiple frequencies. The device thus forms an integral
solid state tuner. This device, as shown, incorporates individually doped
pyramids of silicon 380 displaced at various lengths along a lower silicon
wafer 320. Voltages placed on an individual doped silicon pyramid may be
useful in causing the electrostatic deflection of an upper silicon
diaphragm 350 in an upper silicon layer 310 into contact with a particular
energized pyramid 380. Of course, the layers 310 and 320 are separated by
a silicon dioxide layer 330. By selectively energizing particular
individual pyramid structures 380, an appropriate resonant length of the
diaphragm 350 may be selected. Also, imposition of an AC excitation on the
structure shown in FIG. 3 between the upper 310 and lower 320 silicon
layers, causes resonance of the diaphragm 350 to be preselected and the
device can thus act as a "tuned" tank at particular frequencies.
Because the resonant displacement of electrostatically charged diaphragms
in accordance with the invention involves the movement of an electrical
charge pattern at a particular frequency, the diaphragms, and particularly
oscillating diaphragms, may be useful for sources for the radiation and
propagation of electromagnetic signals. The diaphragm structures may
therefore be extremely useful in the creation of small scale antennas
which may be formed in integrated circuits along with associated
circuitry. It is also apparent that the frequency at which the diaphragm
may resonate is a function of external forces acting on the diaphragm.
Thus, a resonating device according to the invention can therefore be used
to monitor or transduce such forces. The resonance of such structrues can
be controlled externally by the application of external direct current
energy allowing for the creation of the DC tunable filter and that the
resonance of the device may be altered by a particular DC bias level on
the diaphragm. Forces acting on the diaphragm other than the electrostatic
force may also include forces generated by pressure, forces by temperature
change of a gas, or by the acceleration of a mass allowing the creation of
devices such as accelerometers, temperature and pressure transducers. Of
course, each of the structures indicated above are compatible with
integrated circuit processing techniques.
In order to guide those skilled in the art in the fabrication and use of
the diaphragms according to this invention, the following description sets
forth exemplary methods and techniques for construction of the diaphragms
together with examples of devices which have been produced experimentally
in accordance with the present invention.
Diaphragms in accordance with the invention are constructed from thin
silicon wafers of the type which are generally utilized to provide
substrate material in the manufacture of integrated circuits. The
diaphragms are prepared by first diffusing an etchant-retarding substance
such as boron into the wafer to a depth corresponding to the thickness
desired for the resulting diaphragm. The surface of the wafer opposite
that into which the diffusion of etchant-retardant has been effected may
be subsequently masked for the application of etchant so that a diaphragm
of appropriate size may be constructed. An etching process found to be
suitable is described in Volume MAG-11, IEEE Transactions on Magnetics,
Mar. 2, 1975, in an article entitled "Single Crystal Silicon Barrier
Josephson Junctions," p. 766, by C. L. Huang and T. Van Duzer.
Typical diaphragm configurations are on the order of 0.8 cm square, i.e.,
0.8 cm on a side and of a thickness of 2 to 4.mu. (microns). Experimental
testing has revealed that such diaphragms respond to force in a
substantially linear manner. For example, in one experiment, a diaphragm
of the general configuration (0.8 cm square, 2.4.mu. thick) described was
wax mounted on a polished steel plate such that a hole cut in the plate
was aligned directly under the diaphragm as shown in FIG. 4. By connecting
a tubing fitting to the side of steel plate 455 opposite the diaphragm
450, it was possible to use a water manometer to apply static pressure to
the diaphragm by means of aperture 460 in plate 455. This static pressure,
applied to the back side of diaphragm 450, was able to cause transverse
deflection of the diaphragm. This deflection was measured by placing the
assembly of plate 455 and diaphragm 450 under microscope observation and
using the change in focus at 400X magnification with a dial guage on the
fine focus adjustment. The dial gauge was graduated in division of 0.0001
in., and a maximum transverse deflection to an applied pressure
differential ratio of 0.5.mu. per 100 dyne/cm.sup.2 was measured with
linearity being maintained for deflections up to 8.mu.. Deflections over
15.mu. with pressures over 3000 dyne/cm.sup.2 were applied without
rupturing the diaphragm. The deflections for applied pressure differential
is given in Table A below for this experiment.
TABLE A
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Applied pressure differential
Deflections in
in cm H.sub.2 O mils
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0 0
0.20 0.15
0.30 0.30
0.40 0.40
0.55 0.50
0.70 0.55
0.85 0.65
1.10 0.75
1.20 0.80
1.35 0.85
1.55 0.95
1.85 1.00
2.05 1.05
2.35 1.15
2.75 1.25
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In another experiment, a square silicon diaphragm 0.8 cm on a side was
recessed 19.mu. from the polished surface of the silicon wafer by relief
etching of the silicon, and was mounted on a second silicon wafer on which
aluminum had been evaporated, as is illustrated in FIG. 5. In the drawing,
number 550 indicates the diaphragm, 510 the silicon layer in which
diaphragm 550 is formed, 520 the lower wafer, 525 the aluminum layer
evaporated on wafer 520, 530 a SiO.sub.2 layer separating the upper wafer
510 from aluminum layer 525, and 560 a relief port etched in wafer 520.
Relief port 560 is approximately 1 mm. square, providing gas relief to
chamber 540, formed between diaphragm 550 and aluminum layer 525, so that
no pressure differential would exist across diaphragm 550.
A bias voltage of 50 v was applied between layers 510 and 520, resulting in
a maximum transverse deflection of approximately 2.mu.. Application of a
40 v DC bias with an 80 v peak-to-peak sinusoidal voltage superimposed at
1 Hz resulted in transverse deflection changes of a least 6.mu.. The
application of about 20 v peak-to-peak AC sine wave voltages at higher (1
Hz to 100 kHz) frequencies failed to result in any noticable resonance
peak characteristics because of the effect of air damping, but at
frequencies of about 4 kHz, the motion of the diaphragm produced an
audible acoustic wave with the intensity of the sound increasing sharply
with the simultaneous application of a DC bias voltage of about 40 v. The
acoustic signal from the diaphragm was audible for frequencies up to the
human hearing limit at about 18 kHz.
FIG. 6 shows an additional embodiment of the invention wherein a diaphragm
in accordance with the invention is utilized in the creation of optical
display elements. The device shown in FIG. 6 employs a diaphragm 650
formed in a layer 610 of silicon. Layer 610 is separated from a second
silicon layer 620 by a silicon dioxide (SiO.sub.2) layer 630, partially
etched to provide an open chamber 640 immediately beneath diaphragm 650.
Additionally, a transparent layer 670 is bonded to the upper surface of
layer 610 so as to create a second open chamber 660 between diaphragm 650
and layer 670. The layer 670 may be of any transparent solid material; one
such suitable material may be Pyrex.
The display element illustrated in FIG. 5 may be operated by placing a
voltage between layers 610 and 620 sufficient to cause deformation of the
diaphragm 650. This deformation changes the distance or relative spacing
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