Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~IE INVENTION
The invention relates to monocrystalline silicon dia-
phragms 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 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 previousiy 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 co~t of manufacturing such hydrid 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.
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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 guage elements. Such an application
is taught in the ~.S. Patent 3,697,918 to Orth et al. U.S.
Patent 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. Ilowever, in none
of the prior art which has comtemplated the use of silicon
membranes has it recognized that the application of electro- ~F
static 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 disclosure, 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, electro-
mechanical 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. ~j
T~in membranes constructed on
the order of a micron in thickness can be produced by selec~
tively etching the surfaces of silicon wafers. Such membranes
are capable of physical deflection in response to the appli-
c~tion of electrostatic forces o them.
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More particularly in accordance with the invention there is
provided 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 dlaphragm 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. The second electrode
may have an insulating material separating and electrically
insulating it from the diaphragm with a chamber formed in the
insulating material between the diaphragm and the second
electrode and a channel formed through the second electrode
leading to an orifice in the chamber. A cap mounted on the
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diaphragm opens and closes the orifice under selected electro-
static attraction between the diaphragm and the second electrode.
A flat optically transparent layer may be formed on the side of
the silicon substrate opposite to that facing the second
electrode so that displacement of the diaphragm causes visible
changes by constructive interference of incoming light between
the optically transparent layer and the surface of the diaphragm
facing the transparent layer. A plurality of electrically
conductive pyramidal prominences may be formed in association
with the second electrode in position to be individually
contacted by the diaphragm, each prominence being capable of
being separately provided with electric charge to attract the
diaphragm toward contact therewith.
Specific embodiments of the invention will now be described
having reference to the accompanying drawings in which:
Figure 1 shows a generalized configuration of a diaphragm
capable of electrostatic deflection and electromechanical
resonance.
Figure 2 shows a particular embodiment of a valve utilizing a
thin silicon diaphragm.
Figure 3 shows a particular embodiment utilizing a diaphragm
for the formation of a selectably tunable tank circuit.
Figure 4 illustrates a construction utilized to determine
the deformation of a diaphragm.
Figure 5 illustrates a construction utilized for exhibiting
electrostatic deformation of a diaphragm.
Figure 6 shows electro-optical display cell utilizing a thin
silicon diaphragm.
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DESCRIPTION OF PREFERRED EM~3ODIMENTS
The new silicon membranes or thin diaphragms 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 to form substrates
for integrated circuits. In a preferred method, wafers can r
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 devise~d by con- r
ventional 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 r
diffusion of the boron in the wafer. Although not restricted f
to the use of boron, boron or another suitable substance may ~r
be useful for retarding the particular etch used and may be
introduced into the silicon wafer by other well known tech-
niques 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 L
silicon diaphragm according to the invention as one plate ~
of a capacitive device. Although many other forms of con- ~
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ductors may be used to form a second plate or second electrode,
an embodiment is shown in Figure 1 which utilizes a second
silicon wafer as the second conductor or plate of a capacitor.
In the structure shown in Figure 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 Figure 1 creates a capacit~
ance 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 Figure 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 dia-
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phragm 50 may be utilized as a force, temperature ~f pressure
transducer wherein the measured quantity may be related to
capacitance changes. With the particular structure shown in
Figure 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 Figure 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 attrac-
tion 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 Figure 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 Figure 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
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on the plates, the device may be utilized as an extremely ac-
curate micropositioner.
A particular advantage to the structure shown in Figure
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 here-tofore described mechanical de-
formations 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 Figure 1 as a voltage between layers 10
and 20 is therefore capable of causing and sustaining mecha~-
ical 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 re-
sonant behavior is analogous to that of a quartz crystal and
the macroscopic behavior of the device of the invention ex-
hibits circuit behavior at its terminals substantially as
does an electrical capacitive-inductive resonant tank circuit.
- Because the process of manufacturing the new diaphragm
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 under alternating current excitation is shown in
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Figure 3. Figure 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 in-
dividual 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 Figure 3 between the upper 310 and lower
320 silicon layers, causes resonance of the diaphragm 350 to
be preselected and the device ean thus act as a "tuned" tank
at partieular frequeneies.
Beeause the resonant displacement ofsuch electrostatical-
ly eharged diaphragms involves the movement of an electrical
eharge pattern at a partieular frequeney, the diaphragms, and
partieularly oseillating diaphragms, may be useful for sourees
for the radiation and propagation of eleetromagnetie signals.
The diaphragm structures may therefore be extremely useful in
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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 constructed as here des-
cribed can therefore be used to monitor or transduce such
forces. The resonance of such structures can be controlled
externally by the application of external direct current en-
ergy allowing for the creation of the DC tunable filter and
that the resonance of the device may be altered by a particu-
lar 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 the devices such as acceleratometers, temperature and pres-
sure 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 fabri-
cation and use of the new diaphragms, the following description
sets forth exempiary methods and techniques for construction of
the diaphragms together with examples of devices which have
been produced experimentally.
Diaphragms are constructed from thin silicon wafers of
the type which are generally utilized to provide substrate mat-
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erial in the manufacture of integrated circuits. The dia-
phragms are prepared by fi`rst diffusing an etchant-retarding
substance such as boron into the wafer to a depth correspond-
ing to the thickness desired for the resulting diaphragm.
The surface of the wafer opposite that into which the dif-
fusion of etchant-retardant has been effected may be sub-
sequently 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-ll,
IEEE Transactions on Magnetics, March 2, 1975, in an article
entitled "Single Crystal Silicon Barrier Josephson Junctions~"
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p. 766, by C.L. Huange and T. Van Duser.
Typical diaphragm configurations are on the order of .8
cm square, i.e., .8 cm on a side and of a thickness of 2 to
4~M. 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
(O.8 cm square, 2-4~MM 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 Figure
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~ M per 100 dyne/cm2 was
measured with linearity being maintained for deflections up
to 8~ M. Deflections of up to 15~ M with pressures over
3000 dyne/cm2 were applied without rupturing the diaphragm.
The deflections for applied pressure differential is given
in Table A below for this experiment.
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TAsLE A
Applied pressure differential Deflection in
in cm H20 mils
O O
0.20 0.15
0.30 0.30
0.40 0.40
0.55 0.50
0.70 0.55
10 0.85 0.65
1.10 0.75
1.20 0.80
1.35 0.85
1.55 0.95
15 1.85 1.00
2.05 1.05
2.35 1.15
2.75 1.25
In another experiment, a square silicon diaphragm 0.8
cm on a side was recessed 19~ M 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 Figure 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 SiO2
layer separating the upper wafer 510 from aluminum layer 525,
and 560 a relief port etched in wafer 520. Relief port 560 is
approximately lmm 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 ~ . Application of a 40 v DC bias with an
80 v peak-to-peak AC sinusoidal voltage superimposed at 1 Hz
resulted in transverse deflection changes of at least 6~ M.
The application of about 20 v peak-to-peak AC sine wave
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voltages at higher (1 H to ]00 kH ) frequencies failed to
result in any noticeable resonance characteristics, but at
frequencies of about 4 kHz, the-motion of the diaphragm pro-
duced 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 KH .
Figure 6 shows an additional embodiment of the invention
wherein a diaphragm is utilized in the creation of optical
display elements. The device shown in Figure 6 employs a
diaphragm 650 formed in a layer 610 of silicon. Layer 610
is separated from a second silicion layer 620 by a silicon
dioxide (SiO2) 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 chamber660 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 Figure 5 may be oper-
ated by placing a voltage between layers 610 and 620 suf-
ficient to cause deformation of the diaphragm 650. This de-
formation changes the distance or relative spacing between
diaphragm 650 and Pyrex layer 670, changing the optical con-
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structive interference of incoming radiation and thus chang-
ing the optical characteristics of the device in a visually
detectable manner. An array of such devices may thus form an
electrostatically controllable display, wherein the
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degree of constructive interference for each diaphragm is
readily controllable by variation of the electrostatic
deformation of the diaphragms. Multicolored displays
utilizing such devices are thus possible.
In all of the embodiments shown, it is necessary that
an even, flat bond be effected between the silicon layer in
which the diaphragm is formed and the silicon dioxide layer
which separates the silicon layers or wafers. One technique
which may be suitable is described in the Journal of Applied
Physics, V. 40, No~ 10, 1969, p. 3946, "Field Assisted
Glass-Metal Sealing," by G. Wallis and D.I. Pomerantz.
While having shown and described particular embodiments
of the present invention, it is to be understood that additional
embodiments, applications and modifications of such embodiments
will be apparent to those skilled in the art which may be
included within the spirit and scope of the invention as
defined in the following claims.
