GB2126447A - Real-time Fourier transforming transducer and applications thereof - Google Patents

Abstract

A transducer 2 comprises a plurality of cantilever beams 11 arranged to have different mechanical resonances and each supporting or having integrated therein a strain- sensitive element R1-R2, and means for monitoring the condition of said elements when the or some of the cantilever beams come into resonance or are deflected. The device may be used in audio spectrum analysis. <IMAGE>

Description

SPECIFICATION Real-time Fourier transforming transducer and applications thereof Field of the Invention This invention relates to a real-time Fourier transforming transducer and to applications of the same.

Background of the Invention Semi-conductor materials, in particular single-crystal silicon, have found high volume usages in the electronics industry. More recently, the techniques evolved for the fabrication of miniaturized semi-conductor devices have been applied to the production of miniaturized components whose function is mechanical, rather than electronic. For example, commercially available pressure transducers based on miniaturized single-crystal silicon elements have been available since early seventies.

Other mechanical devices fabricated in mono-crystalline silicon include micromechanical light modulator arrays and inkjet nozzles.

Examples of the latter are described, for example, in U.S. Patent 3,921,916 to Bassous and U.S. Patent 4,007,464 to Bassous et al.

The use of silicon as a mechanical material has been reviewed by Kurt E. Petersen in "Proceedings of the IEEE", Volume 70, No.

5, May 1982 at pages 420-457.

Summary of the Invention According to the present invention, there is provided a real-time Fourier transforming transducer which comprises a plurality of cantilever beams arranged to have different mechanical resonances and each supporting or having integrated therein a strain-sensitive element, and means for monitoring the condition of said elements when the or some of the cantilever beams come into resonance or are deflected.

According to another aspect of the invention, there is provided an integrated circuit comprising an integrated circuit element having thereon a parameter which varies with strain, said element being provided in or on a freely resonant cantilever portion of the integrated circuit.

In a third aspect, the invention provides a transducer comprising a plurality of freely resonant cantilevers each having a different resonant frequency, and each of said cantilevers carrying or incorporating a strain-sensitive element which is connected to or capable of connection to means for determining the response of the respective elements to strain induced by resonant vibration of the cantilever.

In a fourth aspect, the invention provides a microphone comprising a membrane supporting an integrated circuit chip including (i) a plurality of cantilevers capable of free resonance and arranged to resonate at different frequencies, and (ii) circuit means for providing an electrical output for each of the cantilevers, each output being dependent upon the resonant conditions of the cantilever with which it is associated.

In a further aspect, the invention provides a method of audio spectrum analysis, which comprises positioning a device or an integrated circuit as defined above in the area where the audio field is to be analysed, and measuring the response of the strain-sensitive elements on the cantilevers of the device or integrated circuit.

It is to be understood that the terms "sound" and "audio", as used herein, include within their ambit infra-sound and ultrasound as well as sound of those frequencies to which the human ear is ordinarily responsive.

Description of the preferred embodiments Apparatus in accordance with the invention preferably has a large number, and more preferably from 10 to 100, cantilevers. The cantilevers are preferably beams, i.e. elongate rectangular lamellae, but other geometries may be adopted when desired or convenient.

The cantilevers may be arranged side-by-side in, for example, a single component (e.g. on or from a wafer of single-crystal silicon) or they may be arranged in any other convenient fashion.

The preferred material for construction of a device in accordance with this invention is a single-crystal silicon. Integrated circuit fabrication techniques are preferably employed for the construction of the device. The invention is not limited, however, to micro-miniature structures produced by integrated circuit fabrication techniques; thus a device in accordance with the invention may be fabricated on a miniature, rather than micro-miniature, scale.

The strain-responsive element is preferably an electrical resistance which can be applied to, or formed in the body of, the cantilever.

When the invention is embodied in an integrated circuit or in a device fabricated with integrated circuit techniques, the strain-responsive parameter is preferably a diffused resistor, although an insulated gate field effect device which is connected circuit-wise to operate as a resistance could also be used.

The means for measuring or monitoring the strain-sensitive parameter carried by a freely resonant cantilever in a device of this invention can be integrated onto the cantilever or to a region adjacent the cantilever. In a presently preferred embodiment, four diffused resistances are fabricated so that two of the resistances are located in an area of the cantilever which will be subjected to stress when the cantilever resonates, and the other two of the resistances being located adjacent the cantilever proper. The four resistances are interconnected as a bridge circuit.

In some embodiments, it is preferred that each cantilever has a frequency response such that the response curves of adjacent cantilevers overlap at the 3 dB level. In this way, a device of the present invention can have a generally flat frequency response and can effectively cover a wide frequency band.

Devices in accordance with the invention when fabricated as integrated circuits or when fabricated using integrated circuit technology have high Q values. It is preferred to operate the devices by subjecting them to the audio spectrum which is to be analysed, and mechanically locking the cantilevers at a predetermined frequency; the desired measurement is then taken each time the cantilevers are released or at the instant prior to them being locked. Conveniently, the cantilevers can be locked at a low frequency, preferably between 25 and 100 Hz. The particular frequency adopted will depend upon the function to be fulfilled by the device.

With micro-miniature cantilevers such as those fabricated by integrated circuit techniques, the resonant frequencies of the cantilevers will normally be in ultrasonic range.

The resonances of the cantilevers can be shifted by interposing a region containing a relatively high density fluid between the sound source which is to be analysed and the device of the invention. The resonant frequencies can also be adjusted by forming a mass at the free end of the cantilever. The optimum frequency response of the cantilevers can also be determined by selective positioning of the diffused resistors.

The method of audio spectrum analysis in accordance with this invention has many applications. In one embodiment, the transducer functions as a microphone. In this embodiment, a microphonic membrane carries a chip which is fabricated to have a plurality of adjacent cantilevers dimensioned and arranged so that their resonant frequencies cover the audio spectrum. The cantilevers are preferably constructed to have incorporated therein diffused resistors.

In a further development of the present invention, a device of the type defined above is associated with a micro-processor or a computer programmed to respond to one or more predetermined frequency distributions. This aspect of the invention finds application in several areas, some of which are discussed briefly below.

Where the audio input is a human voice, the device can function as a voice detector thus facilitating positive identification of individuals whose voice characteristics have been previously recorded. Examples are as follows: signature-by-telephone, logging in on computers, access to security premises, credit card verification and voice-operated locks. The microphone device and a micro-processor can. in such an application, be incorporated in a telephone handset.

The device can be used in acoustic fault detection-for example, where the acoustic input is the sound generated by an internal combustion engine or by fluid flowing through a section of a pipeline. In this application, the instantaneous audio frequency spectrum can be compared with standard samples held in a memory bank and the result of this comparison is indicative of the current conditions of the appliance under test.

Devices of this invention can be incorporated into security systems. For example, the devices may be used in intruder detecting systems, e.g. those located in a building or around a secure area. For example, they can form part of perimeter detection systems.

Such system are ordinarily subject to sonic interference caused, for example, by wind or the movement of small mammals or birds.

Intrusive actions have characteristic sound patterns, and a microphone incorporating a device of this invention in association with a micro-processor or computer can be used to analyse the sound patterns at one or more perimeter stations and to give a warning indication only when the analysed patterns correspond to those indicative of human intrusion or other event which is to be detected.

In a further aspect, the method of this invention is applied to ultrasonic testing. The spectral analysis obtained by a device of this invention at any given instant can be compared with spectra held in a computer memory bank to determine whether or not the item under test is satisfactory. In a development of this aspect, the method of the invention can be used to effect hardness measurements.

The method of the invention can also be applied in seismic testing, where the response of underlying strata to a shock wave generated at or close to the surface is measured.

Generally, the underlying strata gives rise to a reflected wave which is detected by an array of geophones placed over the area of interest.

The geophone output is then subjected to spectral analysis. A device in accordance with the present invention can be incorporated into a geophone to give a real-time, Fourier transformed output. Such a device may similarly be incorporated in a hydrophone such as is used in underwater seismic testing.

The method of the invention may also be applied in determining the acoustic response of cavities of a greater or lesser size. It can be important to have quantitative data on the acoustic properties of a room or hall, and the method of the present invention enables such data to be derived directly.

The nature of the measurement devices of this invention is particularly suited for their use in measurement of ultrasonic frequencies.

The method of the invention, accordingly, can be used in conjunction with kinetic proximity detectors based upon the Doppler effect and to guidance systems, safety devices (e.g. for automobiles) and navigation aids which incorporate such proximity detectors. Doppler shift measurements may also be used for automatic focussing of a camera on a moving object.

Devices of the invention may also be used in conjunction with vehicle guidance systems such as are used for effecting automatic navigation of vehicles within factories such as are used for transporting components from one site within the factory to another.

The method of the present invention may also be applied in the aerodynamics art. For example, the frequency spectrum of turbulent air can be measured directly by acoustic measurements over a given time scale at a fixed point, or at a series of points.

Devices in accordance with the present invention can be used to effect the digitisation of a sound signal. Noise is limited because the detection of the acoustic signal and its spectral analysis occur simultaneously in a single unit.

Devices of the invention may also find application in automatic ornithological observation stations, since the song or flight of birds generates an acoustic spectrum which is species-specific.

The invention may also find application in the control of noise pollution. It is known that sounds can be rendered less offensive and more environmentally acceptable if compensated by "anti-noise" To this end, intrusive noises are subjected to rapid frequency and phase analysis and a sound signal is generated which corresponds to the intrusive noise in frequency, but is 180 out of phase at each frequency. Devices in accordance with this invention may be used in phase analysis as well as frequency analysis since at a given frequency, one cantilever will be at its resonant frequency, while adjacent cantilevers will be close to resonance and will thus vibrate at lower amplitude than, and progressively out of phase with, the resonant cantilever.

The invention may also be applied to the process control of ultrasonic processing equipment, for examples ultrasonic wire bonders, ultrasonic welding equipment and ultrasonic agitation devices. If there is a change in geometry of the article being treated by such processing equipment, the ultrasonic load is altered, and the optimisation of the ultrasonic input is lost. Closed-loop feedback techniques can thus be employed in conjunction with a device of this invention to optimise the ultrasonic input.

The invention may also find application in encryption techniques. The spectral analysis of a sound as determined by a device of the present invention may be subjected to an algorithm such as spectral transformation, e.g.

scrambling, prior to its transmission along a communications network. At the receiving end of the network, the reverse algorithm is applied at a decryption station.

It will be appreciated that a device in accordance with the invention may be used as a sound generator as well as a sound detector.

Thus the application of appropriate electrical signals to the cantilevers of a device of this invention enables the device to act as a miniature loudspeaker.

In a somewhat similar field, the invention may be used in the measurement of fluid flow by measurement of a Doppler frequency shift caused by the velocity of the fluid.

Although the fabrication techniques which may be employed to produce a device in accordance with the present invention do not themselves form part of the invention, nevertheless a convenient technique for producing a chip containing a plurality of cantilevers will now be described in general terms. Firstly, the areas of the silicon wafer which are to become the cantilevers are heavily doped with boron.

Boron atoms enter the silicon lattice substitutionally. These boron-doped areas serve as an etch-stop layer during subsequent an isotropic etching. An epitaxial layer is then grown on the wafer to the desired thickness, and thereafter an insulating layer is deposited or grown over the epitaxial layer. The insulating layer is then patterned to correspond with the cantilevers which are to be produced, and finally the exposed silicon is etched using an anisotropic etching medium such as EDP (ethylene diamine, pyrocatechol and water) to undercut the insulator thus freeing the cantilevers, e.g.

in the form of beams, which then overhang an etched well the basin of which is formed by the boron-doped layer. The electronic components required to be fabricated in or on the cantilevers and adjacent areas of the wafer are produced at the appropriate fabrication stages by known means. Typically, the cantilevers can be from 10 to 500 microns long and 0.1 to 1.5 microns in thickness. The resonant frequency of the cantilevers can be adjusted from their natural values by provision of a mass at the free end of the cantilevers. Such a mass can be deposited in the appropriate region by conventional techniques prior to the anisotropic etching step. In general, all fabrication steps need to be completed prior to the anisotropic etch.

If the patterns on the insulating layer are oriented in the (110) direction of the silicon lattice, the anisotropic etching step is effective in causing undercutting only in the region of the cantilever beams.

It will be appreciated that the cantilevers in a device of the present invention can be produced by a range of fabrication techniques known in the art, in addition to that described above.

The invention will now be described in greater detail with reference to one embodiment thereof which is illustrated in the accompanying drawings.

Brief description of the drawings In the drawings: Figure 1 illustrates the arrangement of a microphonic membrane incorporating a device of this invention; Figure 2 shows, on a greatly enlarged scale, the integrated circuit component in accordance with the invention used in the arrangement of Fig. 1; Figure 3a shows in diagrammatic form the mode of operation of the transducer; Figure 3b is an electrical schematic of the arrangement illustrated in Fig. 3a; and Figure 4 is an electrical schematic of a switching arrangement operative in conjunction with the circuit elements of Fig. 3b.

Referring now to Figs. 1 and 2 of the drawings, a microphonic membrane 1 carries an integrated circuit chip 2 fabricated in the manner illustrated in Fig. 2. Conductor tracks 3 are deposited on the membrane 1 and are connected to the chip 2 through bond wires 4.

As shown more specifically in Fig. 2, the chip 2 comprises a plurality of cantilever beams 11 three of which (11a, 11band 11c) are shown. The cantilevers 11 overhang a depression 1 2 formed in the body 1 3 of a silicon crystal. The chip 2 is constructed using controlled an isotropic undercut eteching with an etchant such as EDP. Each cantilever is dimensioned differently from the remainder, with the result that each cantilever has a selective passband to mechanical vibration, and can be considered to be tuned to different frequencies according to its length dimensions and/or according to the mass (not shown) deposited at the free end of the cantilever.

Each cantilever 11 has two diffused resistor devices formed therein and two further diffused resistors formed adjacent to the stem of the cantilever. The arrangement employed is illustrated diagrammatically in Fig. 3a. The insulated gate field effect devices are operated as resistances, and are designated R1, R2, R3 and R4. The resistances are connected as shown in a bridge configuration by evaporated metal tracks such as 1 4. A schematic representation of the bridge circuit thus produced is given in Fig. 3b.

The resistors R1 and R2 are located in an area of cantilever 11 which will be strained when the cantilever resonates in response to an acoustic signal. The resistances of resistors R, and R2 thus will vary in response to an applied acoustical signal. Resistors R2 and R4 are connected to a constant supply voltage V +, while resistors R, and R3 are held at a constant supply voltage V - . An electrical output is taken between S + and S - . This output is proportional to the constant supply voltage (V + )-(V -) and to the deformation amplitude. This amplitude will be frequency selective, since the cantilever 11 acts as a high Q mechanical filter.

The arrangement of resistors as illustrated in Fig. 3a also has the advantage of providing temperature-compensated measurements.

The chip 2 has a built-in switching network between different frequency components. An electrical schematic of the switching network is shown in Fig. 4. The switch gate signals such as F, and F2 may be set in turn through an on-chip decoder, or may be clocked in sequentially in a shift register, or provided for by any on-chip logic. The switches and any associated on-chip logic can be implemented through MOS technology.

Depending on the logic circuit driving the switch gate signals, any kind of filter can be implemented and the filter can be programmed by digital signals.

In the application of Fourier filtering, the output is advantageously switched at a low frequency, e.g. 25 Mz, through the consecutive cantilever signals. The amplitude detection may be done off chip using a separate operational amplifier, or on-chip using a builtin MOS amplifier, or using dynamic MOS techniques.

Claims (5)

1. A real-time Fourier transforming transducer which comprises a plurality of cantilever beams arranged to have different mechanical resonances and each supporting or having integrated therein a strain-sensitive element, and means for monitoring the condition of said elements when the or some of the cantilever beams come into resonance.

2. An integrated circuit comprising an integrated circuit element having thereon a parameter which varies with strain, said element being provided in or on a freely resonant cantilever portion of the integrated circuit.

3. A transducer comprising a plurality of freely resonant cantilevers each having a different resonant frequency, and each of said cantilevers carrying or incorporating a strainsensitive element which is connected to or capable of connection to means for determining the response of the respective elements to strain induced by resonant vibration of the cantilever.

4. A microphone comprising a membrane supporting an integrated circuit chip including (i) a plurality of cantilevers capable of free resonance and arranged to resonate at different frequencies, and (ii) circuit means for providing an electrical output for each of the cantilevers, each output being dependent upon the resonant condition of the cantilever with which it is associated.

5. A method of audio spectrum analysis, which comprises positioning a device or an integrated circuit as defined in the area where the audio field is to be analysed, and measuring the response of the strain-sensitive elements on the cantilevers of the device or integrated circuit.