Abstract:
A method and apparatus for comparing a force to a signal, or comparing two signals, through mechanical movement of capacitive plates in a transducer. The transducer plates are separated by d, which in one embodiment is preferably a linear function of a pressure or force F. In that embodiment, application of a signal i(t+τ) to the plates will cause a voltage representing a correlation between F and i to appear between the plates. In another embodiment, instead of an external mechanical force or pressure, an electrical signal V related to a signal S may drive the transducer plates to achieve a voltage indicating a correlation between S and the signal input i(t+τ). Transducers to practice the invention may be microelectromechanical devices fabricated using integrated circuit techniques to permit small size and low cost.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to capacitive pressure or force transducers, and in particular to the correlation of two signals with each other by means of such transducers. 
     BACKGROUND 
     It is useful to match one time-varying pressure signal with another. Pressure waves in fluids, such as audible sounds or SONAR responses, can be compared with previously recorded pressure wave signals in order to recognize or identify their signature. However, comparison of signal outputs from known sensors in real time, i.e. as the signal is produced, is not readily done except by converting the signals to digital representations and performing intense computational algorithms on the representative data. It is desirable to perform signal comparisons without a need for such computation. 
     Pressure or force transducers are known which rely upon a variation in capacitance between conductive plates which move with respect to each other under the influence of force or pressure. It is known to fabricate pressure transducers on substrates, similarly as an integrated circuit, in order to take advantage of batch processing capabilities, microminiaturization, and compatibility with integrated circuit manufacture. For example, U.S. Pat. No. 5,888,845 to Bashir, et al. describes a capacitive pressure transducer utilizing a membrane of heavily doped semiconductor crystal as a flexible, electrically conductive plate which shifts position with respect to a fixed perforated metallization layer. Pressure waves in such a device must be conducted through perforations in the metallization. 
     It is known to provide micromachined piezoelectric cantilevers to sense sound wave pressure. For example, U.S. Pat. No. 5,633,552 to Lee, et al. describes a pressure sensor micromachined from piezoelectric material for use as microphones and as microspeakers. However, piezoelectric material senses only change in position, and thus the output is related to the derivative of the position caused by the force, rather than being directly related to the position caused by the force. Moreover, the piezoelectric material is not compatible with some integrated circuit fabrication techniques. 
     The existing art provides transducers, but does not solve the problem of comparing signals using such transducers to reduce computation. Thus, a need exists for a method of comparing signals using a device which might function as a transducer. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-noted needs and provides further benefits. The invention provides a method of comparing signals with each other by using physical movement of capacitive plates. The comparison output preferably represents a correlation function of the input signals. In one aspect, the invention compares a time-varying pressure with a second signal, the second signal representing, for example, a previously recorded time-varying pressure. In another aspect, the invention compares two signals to each other. In another aspect the invention provides devices which are useful to provide such comparison of signals by correlation function. 
     One embodiment of a sensor to practice the force comparison method of the present invention has a particularly useful relationship between applied pressure (force) and resulting capacitive plate spacing, such that an applied time-varying pressure can be correlated with an analog signal which may, for example, be derived from a reference time-varying pressure. In this embodiment, the distance d between two capacitive plates varies substantially as a linear function of an applied pressure p, i.e. Δd=k(p). An I(t) signal applied to the plates will therefore provide an output voltage V O (t) which is the correlation function of I(t) with p(t). Thus, if I(t) represents a reference pressure signal P 0 (t), which has perhaps been previously recorded, the presently applied pressure signal p(t) can be effectively correlated to P 0 (t). Also, since a time offset between I(t) and p(t) could cause a phase difference which would neutralize any correlation, a variable delay factor τ should be added to the signal I(t) so that it becomes I(t+τ). 
     Any sensor constructed to yield a linear change in capacitive plate displacement as a function of pressure may be used with the inventive method of correlation taught herein. However, a preferred sensor is a microelectromechanical device constructed by semiconductor processing methods, in which regions of deposited metal provide the capacitive plates. Preferably, one of the metal regions is deposited upon a semiconductor substrate while the other metal region is supported by a beam, this plate-supporting beam being separated from the substrate by etching, and supported from the substrate on one or two ends. The plate-supporting beam is positioned under a diaphragm, which preferably contacts the plate-supporting beam through a contact point. In this embodiment, the area under the diaphragm is preferably evacuated, and then the diaphragm is sealed around the plate. 
     In one aspect, the invention permits real-time comparison of two electrical signals, which may accordingly represent any signal source. The distance d between two capacitive plates can be made to vary as a function of an applied voltage v(t) which corresponds to a first of the two signals. Then, a current corresponding to a second of the two signals may be applied to the capacitive plates, yielding a voltage which is indicative of the correlation function between the two signals . In this case, if V is the voltage applied to cause variation in the plate separation distance d, d=k 1 (V) ⅔ , and therefore the applied voltage must be preprocessed from the signal S(t) with which correlation is sought, such that the driving voltage V(t)=k 2 (S {fraction (3/2)} (t)). 
     A preferred embodiment of a device to practice this aspect of the present invention provides a micromechanical cantilevered beam supporting a conductive plate to which a voltage is applied with respect to an anchor plate. The voltage, which corresponds to the first signal, thereby applies a force to move the cantilevered beam in accordance with the first signal. The conductive plate is thus moved with respect to a capacitive sensing plate, so that a voltage, which is produced on the capacitive sensing plate by an applied current corresponding to the second signal, indicates a correlation of the first and second signals. 
     Thus, devices according to the present invention provide an ability to compare analog signals in real time, without digital conversions and intense computation, and indicate a correlation between the analog signals by a resultant voltage. In one aspect one of the signals is a physical force or pressure signal, but an immediate physical signal is not necessary to practice the comparison method of the invention. 
     Preferred embodiments of devices suitable for practicing the invention may be fabricated using batch integrated circuit manufacturing techniques. These embodiments perform the comparison function in a small region, permitting comparison of physically small force loci such as fluid pressure waves, including audible sounds, and also due to their small size permit a substantial frequency range of operation. Moreover, these embodiments lend themselves to formation of arrays of such sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a top view of a capacitive diaphragm pressure transducer. 
     FIG. 1 b  is a section view of the transducer of FIG. 1 a.    
     FIG. 2 a  is a top view of a diaphragm transducer using a beam supported plate. 
     FIG. 2 b  is a section view of the transducer of FIG. 2 a.    
     FIG. 3 a  is a top view of a different diaphragm transducer arrangement. 
     FIG. 3 b  is a section view of the transducer of FIG. 3 a.    
     FIG. 4 schematically represents a circuit for use with the present invention. 
     FIGS. 5 a, b  show the transducer of FIG. 2 b  without and with pressure applied. 
     FIGS. 6 a-d  show a section view of fabrication steps for MEM transducer plates. 
     FIG. 7 is a top view of the transducer of FIGS. 61 a-d.    
     FIGS. 8 a, b  show a section and top view of a cantilever transducer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to compare two signals according to the present invention, a physical capacitive device is required. According to a preferred embodiment, such a device preferably has capacitive plates which are separated by a distance d which varies as a linear function of an applied force or pressure. FIGS. 1 a  and  1   b  show a top and side view, respectively, of a device which accomplishes such a relationship to a good approximation. Substrate  4  supports a capacitive plate  2  which is separated from diaphragm  6  by a distance, which is d. The region  20  enclosed by diaphragm  6  is defined by circular wall  8 , and is preferably evacuated. In that case, diaphragm  6  is uniformly loaded, edge-clamped, and circular, and the pressure dependence of the diaphragm is: y(x)=−3(r 2 −x 2 ) 2 (1−μ 2 )p/(16Y t 3 ), where x is the radial distance from the center, r is the radius, and t is the thickness for the diaphragm, and Y and μ are Young&#39;s modulus and Poisson&#39;s ratio, respectively. The center displacement, at x=0, is y(0)=−3 r4 (1−μ 2 )p/(16 Y t 3 ), which for r={fraction (5/64)} inch, t=0.004 inches, and for a stainless steel having μ=0.305 and Y=27.6×10 6  psi, the displacement is (4×10 6 ) p inches. For p=100 psi, y(0)=4×10 −4  inches ≈10 μm. Finally, the spacing d between the conductive diaphragm  6  and capacitive plate  2  is expected to be a good approximation of the distance at the center, x=0, as long as plate  2  is very small compared to the diameter of the diaphragm. 
     An alternative way to assure linearity of the capacitive plate spacing response to the diaphragm center displacement is to use the center of the diaphragm to actuate a separate transducer element as shown in top and side views in FIGS. 2 a  and  2   b , respectively. A comparable diaphragm  6 , substrate  4 , circular wall  8 , plate  2 , and region as in FIGS. 1 a  and  1   b  is shown, but now upper plate  18  is supported by a beam  14  which connects the plate to substrate  4  at left beam end pad  10  or right beam end pad  12 , or both. Pip  16 , centered on the diaphragm and on the upper plate, may be used to ensure that upper plate  18  moves according to the position of the center of diaphragm  6 . Under some circumstances upper plate  18  might cease to press against diaphragm  6 , and for such circumstances pip  16  is preferably adhesive in nature. For linearity, the resistance of the beam-supported upper plate  18  must be negligible compared to the diaphragm resistance. 
     FIGS. 3 a  and  3   b  represent one of the ways in which a beam-supported capacitive plate  18  actuated by diaphragm  6 , preferably via pip  16 , can be utilized with smaller diaphragm areas. In this case, support wall  9  surrounds upper plate  18  more closely, such that upper plate  18  is distinctly more predictably parallel to capacitive plate  2  than would be a diaphragm of any significant bending. FIG. 3 b  shows a connection to upper plate  18  through left beam end pad  10 , but right beam end pad  12  is omitted. In this embodiment, the right end may be allowed to float rather than being anchored to the substrate. The float of the beam support serves two purposes: first, the beam resistance to movement is reduced, making any nonlinearity of the beam force-displacement function more negligible compared to the dominant diaphragm force-displacement function; and second, the ease with which the unanchored beam support permits flexing of the beam reduces a tendency of upper plate  18  to bow at extremes of travel due to tension in beam  14 . Both of these effects tend beneficially to preserve response linearity. However, since in many instances neither effect will be significant, floating the right beam end is not called for in all instances. 
     FIG. 4 shows a schematic representation of a circuit used with the present invention. Upper plate  42  is driven with respect to lower capacitive plate  44  by current source  46 , which can be constructed for particular applications by any method, or which many are presently known in the art d is the distance between upper and lower plates  42  and  44 , and d varies as a function of time d(t) with F, which is also a function of time F(t). Preferably, d(t)=d 0 −(k 2 F(t)) over the time and force of interest. The voltage of upper plate  42  is represented by A and that of lower plate  44  is represented by B, while current source  46  supplies a comparison signal i(t) along with a delay factor τ, that is, i(t+τ). Then:            v   AB          (   t   )       =       ∫   0   t              i        (     t   +   τ     )       /     C        (   t   )                 t                                
     We have said that F(t)=k 1  d(t); and for capacitive area A, C(t)=ε 0  A/d(t); and hence,          V   AB     =       1   /     (       k   1          ɛ   0        A     )              ∫   0   t            F        (   t   )            i        (     t   +   τ     )               t                                  
     As can be seen from the last equation, V AB  is proportional to the correlation between F(t) and i(t), with alignment delay τ. 
     FIGS. 5 a  and  5   b  show the effect of plate area  18  as diaphragm  6  flexes. Because upper plate area  18  is much wider than supporting beam  14 , beam  14  does almost all of the bending when pip  16  presses against upper plate  18 . As mentioned with regard to FIG. 3 b , flexing of upper plate  18  may be reduced even farther, if necessary, by allowing one end of supporting beam  14  to float rather than being anchored to substrate  4 . 
     Approximate dimensions for a preferred structure as shown in FIGS. 5 a  and  5   b  are as follows: upper plate  18  is square, and approximately 0.33 mm on a side. Beam  14  is approximately 0.35 mm from plate  18  to interconnect anchor  24  (FIG.  7 ). Region  20  of the diaphragm structure is evacuated such that, in the ambient pressure of choice, the spacing between upper plate  18  and capacitive plate  2  is about 2 microns, and the dynamic range is about +/−1 micron for a diaphragm similar to that described with respect to FIGS. 1 a  and  1   b , which yields a dynamic pressure range of about +/−10 psi. Of course, the smaller diaphragm shown in FIGS. 3 a  and  3   b  could be readily fabricated to provide a similar dynamic range by using thinner or weaker materials. It is desirable for some applications that pip  16  be adhesively placed, or indeed be a bit of adhesive material; but in some instances upward pressure from upper plate  18  may keep pip  16  in compression so that upper plate  18  follows the movement of diaphragm  6 . 
     With a spacing between upper plate  18  and capacitive plate  2  of 2 microns, and plate area of about 0.1 mm 2 , the capacitance will be about 0.5 picofarad, and a signal current of 0.1 μA applied for 100 μs will yield about 20V. 
     Devices for use in this embodiment of the present invention lend themselves to fabrication using common integrated circuit techniques, as shown by the fabrication sequence depicted in FIGS. 6 a  through  6   d . First, a suitable substrate  4  has metallization patterned thereon to form pads  10  and  12 , and capacitive plate  2 . Interconnect metallization would generally also be patterned at this time, though it is not shown; any technique may be employed to provide the patterned metallization, including for example lithographic resist lift-off, resist definition and metal etch, or less common techniques. This metallization is preferably begun with about 250-500 Å of Ti to ensure adhesion to the substrate, followed by about 1000 Å of Pt to protect the Ti from diffusion of Au, and about 2000 Å of Au; however, aluminum or other metals are satisfactory for many purposes. 
     FIG. 6 b  shows a sacrificial layer  52  which has been disposed. In the present example, the thickness of this layer is typically 2 to 20 microns, but the desired thickness will depend upon application. The material is preferably silicon dioxide, but any compatible material may be used which is readily removed both vertically and laterally without excessively affecting other layers. After layer  52  is deposited by any compatible technique, such as plasma enhanced chemical vapor deposition (PECVD) or sputtering, the sacrificial layer is patterned and selectively etched to create features  50  and  51 . One or both of these will be used to interconnect the substrate to upper plate  18 . Those skilled in the art will recognize that similar features in sacrificial layer  52  will permit other features, such as  22  (FIG. 7) which need not include metallization, to provide a support for beam  14  which is anchored to substrate  4 . 
     FIG. 6 c  shows that insulating and structural layer  54  has been deposited and patterned by any compatible technique. Layer  54  is preferably silicon nitride or other material with a significantly different etch rate than the sacrificial layer  52 . The pattern includes access through layer  54  such that metallization layer  56 , disposed next, can interconnect first metallization layer pads  10 ,  12  and form the metallization of beam  14 . This metallization, typically sputter deposited, is preferably 200 Å of Ti followed by 1000 Å of Au (thinner than the metallization mentioned above), but of course alternative metals and thicknesses may be selected. Following the deposition and patterning of metallization layer  56 , second dielectric layer  58  is preferably disposed thereover. It is generally preferred that insulating layers  54  and  58  are of the same material, for example silicon nitride, and the same thickness, for example 500 Å, so as to balance stresses in the beam. Such balancing is not required for all applications. 
     FIG. 6 d  shows the section view of the beam-supported capacitive-plate transducer after a further step of etching away the sacrificial layer  52 , leaving the beam  14  and plate  18  separated from substrate  4 . 
     The dimensions of such a structure may be varied widely depending upon application. A preferred embodiment, as shown in top view in FIG. 7, has dimensions approximately as follows: plate  18  is 330 microns square; beams  14  are 30 microns wide and 350 microns between support/anchor structure  22 ,  24  and plate  18 ; dielectric layers  54  and  58  depend upon the insulation thickness required between plates, and, along with the metallization thickness, upon the structural strength desired for the supporting beam. 
     A variation in processing is needed to accommodate the beam structure shown in FIGS. 3 a  and  3   b , which has the right end free from anchoring. One such variation is to omit pad  12  during the metallization patterning step of FIG. 6 a . Then, when forming features  51  and  50 , feature  51  can be made somewhat differently than shown in FIG. 6 b . A timed etch can remove sacrificial layer  52  above pad  10  to form feature  50  as usual, but leave some of sacrificial layer  52  above substrate  4  when forming feature  51 . The steps in FIG. 6 c  would then be completed in the usual manner, leaving a thin part of sacrificial layer  52  below the supporting part of the beam structure. When the sacrificial layer  52  is removed, the beam will be freed on the right end, reducing the resistance of the transducer to bring plate  18  closer to capacitive plate  2 . This would reduce bending of plate  18  from beam tension, and make the transducer bending resistance more negligible compared to the diaphragm actuator, so as to help ensure linearity of response. 
     FIGS. 8 a, b  show a section and top view respectively of another variation on a transducer for use with the present invention, in which the upper plate is reference item  72 . Plate  72  is supported from pad  10  as a cantilever structure, with beam  14  supporting plate  72  and held in turn by means of interconnect  24  and anchor structures  22  from substrate  4 . 
     Pad  70  and capacitive plate  2  may be joined as a single capacitive plate to form a capacitive transducer for use as shown for the beam-supported structures described above. In that event, cantilever beam  14  is preferably attached to an actuating item such as diaphragm  6  of FIG. 3 b , and beam  14  may be narrowed to about 60 microns wide, as necessary, to help ensure that the resistance of the beam is negligible to the forces moving the diaphragm and transducer together. 
     However, if pad  70  and capacitive plate  2  are separated as shown, then pad  70  can be used to convert an input signal into an electrostatic force which can move transducer plate  72 , instead of the plate being actuated by pressure or force. In this event it is preferred that cantilever beam  14  be as wide as capacitive plate  2 , that is approximately 333 microns, in order to enhance the stiffness of the transducer. The distance d between plates  2  and  72  may be controlled in this configuration by voltage V between plates  72  and  70 , as long as the voltage between plate  2  and plate  72  is negligible by comparison to V. In this event, the relationship will be: d=do+k V ⅔ . Therefore, if V=S {fraction (3/2)} /k, a correlation of signal S with input current i(t+τ) will be obtained in the way described above, without need for a diaphragm or other means to direct an mechanical force to the capacitive transducer. 
     The invention has been described in exemplary embodiments, but is not to be limited thereto. Rather, it is defined only by the claims which follow.