GB1596982A - Mechanical resonator arrangements - Google Patents

Description

(54) MECHANICAL RESONATOR ARRANGEMENTS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London W.C.2., England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to acoustic resonators and strain gauges fabricated from monolithic silicon by selective etching techniques.

Our published UK Specification No.

1,211,499 describes a method of manufacturing a semiconductor device, including the steps of providing a silicon substrate having a p-n junction therein, masking the surface of the n-type layer to expose that area or those areas thereof to be etched, and etching the exposed area or areas with an etch solution of a diamine, water and either catechol or catechol derivatives which form a complex with silicon, which solution is selective such that it does not act upon the p-type layer, the maximum depth of the etched layers being limited by the p-n junction.

It was previously thought that the etch inhibiting effect was due to electrochemical effects at the p-n junction. Further work has indicated however that the inhibiting effect is due mainly to the concentration of a particular dopant being greater than a minimum level. Thus, for example, doping with boron to a level of at least 4x 10'9 atoms/cc produces this effect. In this way, by area and level control of p-doping of a silicon body, a device can be fashioned from the body by selective etching away of the undoped regions.

According to the invention there is provided a semiconductor acoustic resonator, including a body fabricated from single crystal silicon and having one or more integral flexible portions which one or more flexible portions are responsive each to an applied mechanical vibration at its resonant frequency, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

According to the invention there is further provided a semiconductor acoustic resonator, including a substantially laminar rectangular silicon member and supported from a surrounding silicon body by contiguous filament extending from the silicon member at regions which, when the member is vibrated at its resonant frequency, are located at vibrational nodes, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification in which: Fig. 1 is a plan view of a strain gauge of the vibrating type; Fig. 2 shows an accelerometer using a vibrational strain gauge of the type of Fig. 1; and Fig. 3 shows a mechanical resonator arrangement; and to the accompanying drawing in which Fig. 4 shows an alternative resonator arrangement.

Referring to Fig. 1, the strain gauge arrangement includes a silicon filament 11 having integral silicon mounting pads 12 at its ends, the filament being formed by etching from a selectively doped silicon chip.

The filament is stretched between supports 13 to which the mounting pads 12 are secured.

The natural vibrational frequency of the taut silicon filament is a function of the filament cross section and the tension. Thus measurement of the vibrational frequency of the filament gives an indication of the strain applied to the filament.

Fig. 2 shows an accelerometer employing a pair of silicon filaments as the sensing elements. A machined metal block 21 has an extending leg 22 which leg provides the inertial element of the accelerometer. The leg 22 is flanked by short side legs 23 and 24 separated from the central leg 22 by keyhole slots 25 and 26 respectively. The legs 22, 23 and 24 are bridged by a silicon transducer assembly 26 comprising three silicon pads 27, one secured to each leg, supporting silicon filaments 28 under tension therebetween.

The filaments 28 should preferably be matched in cross section and tension in the undisplaced configuration of the device. Acceleration of the device causes consequent displacement of the centre leg 22 with respect to the side legs 23 and 24 producing a mismatch between the vibrational frequencies of the two filaments 28. A measure of the acceleration may be determined from the resultant beat frequency obtained from the filament 28.

Fig. 3 shows a multi-element cantilever type resonator. The device is formed from a silicon crystal 31 which is selectively etched to provide a transverse valley 32 and a comblike array of thin, e.g. 2-10 microns, silicon cantilevers 33 extending from one end of the crystal 31, the cantilever being graded in length across the width of the crystal. The arrangement is mounted on a rigid clamp 34.

Excitation of the resonator by a mechanical input drive applied between the valley 32 and the cantilevers 33, and comprising either a single frequency or several frequencies, causes the corresponding cantilever or cantilevers 32 to resonate. Resonant vibration of the cantilevers may advantageously be detected optically.

Fig. 4 shows a single frequency resonator in which the resonant element is a rectangular silicon plate 41 supported in a frame 42 via filaments or bridge members 43. The bridges 43 are positioned relative to the plate member 41 such that, when the plate member is resonating at its resonant frequency, modal regions coincide with the points at which the bridges are attached to the plate.

As before the device is formed by etching from a selectively doped single crystal silicon chip, The resonator arrangements described herein may be mounted in an evacuated enclosure thus avoiding the damping effect of air. In some applications the resonator Qfactor may be controlled by varying the gas pressure within the enclosure.

WHAT WE CLAIM IS: 1. A semiconductor acoustic resonator, including a body fabricated from single crystal silicon and having one or more integral flexible portions, which one or more flexible portions are responsive each to an applied mechanical vibration at its resonant frequency, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

2. A semiconductor acoustic resonator, including a substantially laminar rectangular silicon member and supported from a surrounding silicon body by contiguous filaments extending from the silicon member at regions which, when the member is vibrated at its resonant frequency, are located at vibrational nodes, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

3. A multi-element semiconductor mechanical resonator, including a substantially rectangular single crystal silicon body having a transverse valley across its width, and a comb-like array of flexible silicon cantilevers integral with the body and extending from one end thereof, wherein each said cantilever is responsive to a different applied vibrational frequency, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

4. A semiconductor acoustic resonator device substantially as described herein with reference to Figs. 1 and 2 or to Fig. 3 of the drawing accompanying the Provisional Specification, or to Fig. 4 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. measurement of the vibrational frequency of the filament gives an indication of the strain applied to the filament. Fig. 2 shows an accelerometer employing a pair of silicon filaments as the sensing elements. A machined metal block 21 has an extending leg 22 which leg provides the inertial element of the accelerometer. The leg 22 is flanked by short side legs 23 and 24 separated from the central leg 22 by keyhole slots 25 and 26 respectively. The legs 22, 23 and 24 are bridged by a silicon transducer assembly 26 comprising three silicon pads 27, one secured to each leg, supporting silicon filaments 28 under tension therebetween. The filaments 28 should preferably be matched in cross section and tension in the undisplaced configuration of the device. Acceleration of the device causes consequent displacement of the centre leg 22 with respect to the side legs 23 and 24 producing a mismatch between the vibrational frequencies of the two filaments 28. A measure of the acceleration may be determined from the resultant beat frequency obtained from the filament 28. Fig. 3 shows a multi-element cantilever type resonator. The device is formed from a silicon crystal 31 which is selectively etched to provide a transverse valley 32 and a comblike array of thin, e.g. 2-10 microns, silicon cantilevers 33 extending from one end of the crystal 31, the cantilever being graded in length across the width of the crystal. The arrangement is mounted on a rigid clamp 34. Excitation of the resonator by a mechanical input drive applied between the valley 32 and the cantilevers 33, and comprising either a single frequency or several frequencies, causes the corresponding cantilever or cantilevers 32 to resonate. Resonant vibration of the cantilevers may advantageously be detected optically. Fig. 4 shows a single frequency resonator in which the resonant element is a rectangular silicon plate 41 supported in a frame 42 via filaments or bridge members 43. The bridges 43 are positioned relative to the plate member 41 such that, when the plate member is resonating at its resonant frequency, modal regions coincide with the points at which the bridges are attached to the plate. As before the device is formed by etching from a selectively doped single crystal silicon chip, The resonator arrangements described herein may be mounted in an evacuated enclosure thus avoiding the damping effect of air. In some applications the resonator Qfactor may be controlled by varying the gas pressure within the enclosure. WHAT WE CLAIM IS:

1. A semiconductor acoustic resonator, including a body fabricated from single crystal silicon and having one or more integral flexible portions, which one or more flexible portions are responsive each to an applied mechanical vibration at its resonant frequency, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

2. A semiconductor acoustic resonator, including a substantially laminar rectangular silicon member and supported from a surrounding silicon body by contiguous filaments extending from the silicon member at regions which, when the member is vibrated at its resonant frequency, are located at vibrational nodes, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

3. A multi-element semiconductor mechanical resonator, including a substantially rectangular single crystal silicon body having a transverse valley across its width, and a comb-like array of flexible silicon cantilevers integral with the body and extending from one end thereof, wherein each said cantilever is responsive to a different applied vibrational frequency, and wherein said resonator is formed by boron doping a silicon crystal such that the doping profile corresponds to the device geometry followed by removal of the undoped portions of the crystal with a chemically selective etch.

4. A semiconductor acoustic resonator device substantially as described herein with reference to Figs. 1 and 2 or to Fig. 3 of the drawing accompanying the Provisional Specification, or to Fig. 4 of the accompanying drawing.