CA1094229A - Electrostatically deformable thin silicon membranes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to thin silicon membranes formed in layers of silicon such as are normally utilized as substrates in the manufacture of integrated electronic circuits. The thin membranes are capable of deformation by electrostatic forces and are applicable to a wide range of uses including the manufacture of solid state pressure sensors, resonant, and antenna structures, as well as electro-optical display elements.

Description

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.

-2-lOg~2~9 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.

~o~zz9 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 ~ .

~O~Z?~9 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.

- 4a -B

~`^ 10~42Z9 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-J~,0~?4~?~

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 10~42Z9 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 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 `~:

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-- 10. -10~4;ZZ9 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~"

~ lOa -10~ 2Z9 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.

la~4z~9 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 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-' .

1094Z~9 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 - 13a .. . .. : .
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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.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS;

1. An electro-mechanical resonant circuit comprising:
(a) a first substrate of crystalline silicon having opposite sides, a portion of one side having an etchant re-sistant 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 con-ducting 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 dia-phragm 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 dia-phragm.

2. The resonant circuit of Claim 1 including means for applying a DC voltage bias between said diaphragm and said second electrode to displace said diaphragm toward said second electrode and vary the effective capacitance between the same in response to the DC voltage bias.

3. The resonant circuit of Claim 1 wherein said second electrode is spaced away from said diaphragm a distance be-tween 1 and 25 microns.

4. The resonant circuit of Claim 1 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 silicion to provide electrical insulation there-of, and wherein a chamber is formed in said silicon dioxide layer between said diaphragm and said conductive silicion layer to allow free deflection of said diaphragm.

5. The resonant circuit 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 be-tween said diaphragm and said second electrode.

6. The resonant circuit 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.

7. The resonant circuit of Claim 1 wherein said second electrode comprises a layer of electrically insulating mater-ial 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.

8. The device of Claim 1 wherein said second electrode is formed of a second layer of conducting silicon, and in-cluding insulating material separating and electrically in-sulating said second electrode from said diaphragm, a chamber formed in said insulating material between said diaphragm and said second electrode, a channel formed through said second electrode leading to an orifice in said chamber, and further including a cap mounted to said diaphragm in position to be biased to open and close said orifice under a selected electrostatic attraction between said diaphragm and said second electrode.

9. The device of Claim 1 including a substantially flat optically transparent layer formed on the side of said sil-icon substrate opposite that facing said second electrode and extending over said cavity, the displacement of said diaphragm thereby causing visible changes by constructive interference of incoming light between said optically transparent layer and the surface of said diaphragm facing said transparent layer.

10. The device of Claim 1 including a plurality of electrically conductive pyramidal prominences formed in as-sociation with said second electrode in position to be in-dividually contacted by said diaphragm when it is deflected toward said second electrode, each said electrically con-ductive prominence being capable of being separately pro-vided with electric charge to attract said diaphragm toward contact therewith.

11. The device of Claim 1 wherein the thickness of said diaphragm is between 1 and 4 microns.

12. The device of Claim 11 wherein said etchant resistant layer forming said diaphragm consists essentially of crystalline silicon with boron interspersed therein in an amount sufficient to make said layer resistant to etching by selected silicon etchants.

13. The device of Claim 1 wherein said etchant resistant layer side of said diaphragm has a coating of electrically conductive metal thereon.