A MICROMECHANICAL ACCELEROMETER

Abstract: There is disclosed an accelerometer comprising a substrate, a proof mass mounted to the substrate for movement in a first direction perpendicular to the plane of the substrate, the proof mass carrying a first sensing electrode, a second sensing electrode mounted relative to the substrate, wherein the first and second sensing electrodes comprise surfaces extending at an angle relative to the substrate and defining between them a sensing gap the width of which varies with movement of the proof mass in the first direction.

Description: Title of the Invention A MICROMECHANICAL ACCELERO ETER Technical Field of the Invention This invention relates to a micromechanical accelerometer, that is an accelerometer constructed, preferably from silicon, using micromechanical techniques. Background of the invention Silicon microfabricated accelerometers are being intensively developed because of the possibility of low cost batch fabrication. Such accelerometers include a seismic or proof mass which is anchored to a silicon substrate by a micromechanical suspension system. In one commercially available chip (ADXL50), the position of the proof mass is detected by a differential capacitance measurement. Using a force-balance configuration, the proof mass deflects in response to acceleration to which the chip is subjected, a sense signal arising from the differential capacitance measurement is amplified and fed back to electrostatically deflect the proof mass and counteract the effect of the acceleration. The output from the accelerometer is the required feedback voltage to counteract the effect of the acceleration. Tunnel based accelerometers have also been proposed, in which the sense signal is derived from a tunnel current between a sensing tip and a counter electrode. Such a tunnel based sensor has been proposed in a paper entitled "A miniature high sensitivity broad-band accelerometer based on electron tunnelling transducers", authored by Howard K. Rockstad et al and published at the 7th International Conference on Solid State Sensors and Actuators, pages 836 to 839. Such tunnel based accelerometers have a greater sensitivity than capacitive accelerometers, but they require more than one silicon wafer for their manufacture. In the arrangement described in the preceding article by Rockstad et al, three wafers are required to make the accelerometer. Each wafer includes components of the accelerometer which must be carefully aligned when the wafers are secured together. Manufacturing costs of such accelerometers are high because of the need to produce components within three wafers and then to carefully align the wafers during construction of the accelerometer. It is an object of the present invention to provide an accelerometer which combines high sensitivity and low cost manufacture. Summary of the Invention According to one aspect of the present invention there is provided an accelerometer comprising: a substrate; a proof mass mounted to the substrate for movement in a first direction perpendicular to the plane of the substrate, the proof mass carrying a first sensing electrode; a second sensing electrode mounted relative to the substrate; wherein the first and second sensing electrodes comprise surfaces extending at an angle relative to the substrate and defining between them a sensing gap the width of which varies with movement of the proof mass in the first direction. It will be apparent that the second sensing electrode will generally be mounted relative to the substrate in a manner which is relatively stiff compared to the mounting of the proof mass to the substrate. In the described embodiment, the proof mass is mounted in a cantilevered fashion, that is with one end of a cantilever fixed to the substrate and the other end of the cantilever free to move in the first direction. In one embodiment, the cantilever itself can constitute the proof mass. In another embodiment, the cantilever carries a region of larger cross-section which constitutes the proof mass. In a still further embodiment, a region of larger cross-section constituting the proof mass is mounted relative to the substrate using two cantilevers on either side of the proof mass. This arrangement is stiffer in a plane parallel to the plane of the substrate. The accelerometer can comprise circuitry for sensing variations in the width of the sensing gap. In one arrangement, the sensing circuitry detects changes in a tunnelling current between the first and second sensing electrodes. In this arrangement, a voltage is applied between the proof mass and the substrate. Arrangements in which the proof mass is provided by an area of larger cross-section are advantageous in this respect because they allow more easy application of a voltage sufficient to generate the required electric field to cause the sensing gap to adopt a width where a tunnelling current will result. In a particularly preferred embodiment, the second sensing electrode is mounted to the substrate at a location internally of a surrounding proof mass. The proof mass can comprise a perforated diaphragm suspended for movement in the first direction relative to the substrate. An accelerometer in accordance with the present invention can be much more easily and cheaply manufactured than accelerometers according to the prior art because the proof mass, the first sensing electrode and the second sensing electrode can be formed as components in a single wafer. The proof mass, the first sensing electrode and the second sensing electrode can be formed in single crystal silicon. The invention also provides a method of making an accelerometer comprising the steps of: forming a component supported at opposed end regions with respect to a substrate; and cutting a gap in the component at an angle incident to the plane of the substrate to form two angled surfaces which extend at an angle relative to the substrate and which constitute first and second sensing electrodes defining between them a sensing gap. By using these steps, the accelerometer can be formed in a single composite wafer comprising a silicon substrate, a layer of silicon dioxide and a top layer of silicon. While this provides particular advantages, it will readily be appreciated that the accelerometer could be formed in a conventional wafer on top of which has been deposited a layer of silicon dioxide and a layer of silicon. The cutting step can be carried out using a focused high energy ion beam or a laser. These techniques provide an angled cut in a simple and cost effective manner. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings. Brief Description of the Drawings Figures la and lb are a plan view and a side view of sensing electrodes in an accelerometer; Figures 2a and 2b are a plan view and a side view of a proof mass with sensing electrodes in an accelerometer; Figures 3a and 3b are a plan view and a side view of an alternative embodiment of a proof mass with sensing electrodes in an accelerometer; Figure 4 is a plan view of an alternative arrangement of a proof mass in an accelerometer; Figure 5 is a further alternative arrangement of a proof mass in an accelerometer; Figures 6a and 6b are a plan view and a side view of an alternative embodiment of sensing electrodes in an accelerometer; Figure 7 is a circuit diagram of sense circuitry for an accelerometer; and Figures 8a to 8e are sections through a wafer during various steps in the manufacture of an accelerometer. Description of the Preferred Embodiments Figures la and lb illustrate the concept underlying the accelerometers of the present invention. In Figures la and lb, reference numeral 2 denotes a component connected to a proof mass and which has a first sensing electrode in the form of a sensing tip 4. Reference numeral 6 denotes a component which is mounted to the substrate and which has a second sensing electrode in the form of a counter electrode 8. In Figure la, the components 2 and 6 are formed in a plane parallel to the plane of a substrate. Thus it can be seen from Figure lb that the sensing tip 4 and counter electrode 8 each comprise a surface extending at an angle relative to the substrate. The surfaces of the sensing tip 4 and counter electrode 8 define between them a sensing gap 10, the width of which varies when the component 2 moves with respect to the component 6. The angle of the surfaces of the sensing tip 4 and counter electrode 8 can lie for example between 30° and 60° with respect to the plane of the substrate and in a particularly advantageous embodiment, the angle may lie bbeettwweeeenn 53° and 55° to align with the crystal (111) direction. Figures 2a and 2b illustrate in plan view and side view respectively one embodiment of an accelerometer using a sensing tip and counter electrode with angled surfaces as described above with reference to Figures la and lb. In Figure 2a, reference numeral 21 denotes a substrate to which is mounted on a cantilever 25 a proof mass 23. The proof mass 23 carries a sensing tip 24 which comprises a surface angled with respect to the substrate in the same manner as sensing tip 4 in Figure lb. The accelerometer includes a component 26 which is mounted to the substrate 21 and which is relatively stiff compared to the cantilever 25 carrying the proof mass. The component 26 carries a counter electrode 28 which also has an angled surface in a similar manner as the counter electrode 8 of Figure lb. When the accelerometer of Figures 2a and 2b is subject to an accelerating force, the proof mass moves in a direction substantially perpendicular to the plane of the substrate (that is into and out of the plane of the paper in Figure 2a) . This movement causes the width of a sensing gap 30 defined between the sensing tip 24 and the counter electrode 28 to vary. This variation is sensed in a manner to be described more fully later. Figures 3 " a and 3b illustrate an alternative embodiment of an accelerometer in which the mounting of the proof mass provides greater stiffness parallel to the plane of the substrate than the embodiment shown in Figure 2a. In Figure 3a, reference numeral 33 denotes the proof mass which is mounted in a cantilevered fashion on two cantilevers 35a,35b relative to the substrate 31 (Figure 3b) . The proof mass 33 carries a sensing tip 34. A component 36 mounted to the substrate and relatively stiff with respect to the proof mass mounting carries a counter electrode 38. As described hereinabove in relation to Figures la and lb and Figures 2a and 2b, the sensing tip 34 and counter electrode 38 comprise surfaces angled with respect to the substrate and which define therebetween a sensing gap 40. In the arrangement of Figures 2a and 2b and Figures 3a and 3b, the relatively stiff component 26,36 is arranged externally of the proof mass 23,33. Figure 4 illustrates in plan view an embodiment where the proof mass designated by reference numeral 43 is constructed symmetrically around a component designated by numeral 46, which is mounted to the substrate so as to be relatively stiff with respect to the mounting of the proof mass. In the same manner as has already been discussed, the relatively stiff component 46 carries a counter electrode 48 and the proof mass 43 carries a sensing tip 44. The sensing tip 44 and the counter electrode 48 define between them a sensing gap 50. The arrangement of Figure 4 has a higher in-plane stiffness than that of Figures 2a and 2b and Figures 3a and 3b. Figure 5 illustrates in plan view an arrangement wherein the proof mass, denoted by reference numeral 53 is formed as a continuous diaphragm supported with respect to the substrate. A component 56 is again arranged in the middle of the diaphragm mounted to the substrate in a manner which is relatively stiff compared to the mounting of the proof mass and carries a counter electrode 58 which defines with a sensing tip 54 provided on the proof mass 53 a sensing gap 60. In the arrangement of Figures 4 and 5, the sensing tip 44 and 54 and counter electrode 48,58 have angled surfaces as already discussed above with reference to Figures la,lb,2a,2b and 3a,3b. Figures 6a and 6b are a plan view and side view respectively of a different embodiment of sensing electrodes in which the sensing tip comprises a point 74 having an angled surface 74a when seen in side view and the counter electrode 78 has an angled plane surface 78a. The sharpness of the point may be adjusted and multiple points may be used. The sensing tip 74 and counter electrode 78 of Figures 6a and 6b can be used in place of the plane angled surfaces of the preceding figures. Figure 7 is a block diagram of a feedback loop constituting the circuitry used to sense changes in the width of the sensing gap of the preceding arrangements. A voltage is applied between the structure constituting the proof mass and cantilever and the substrate to bring the width of the sensing gap to a distance where a tunnelling current will pass between the sensing tip and counter electrode. For example, a voltage of about 8V can be applied to hold the sensing gap at a width of about 5nm. When the accelerometer is subjected to an accelerating force, the proof mass moves in a direction perpendicular to the plane of the substrate thus changing the width of the sensing gap. This change in width changes the magnitude of the tunnelling current and hence alters the voltage applied between the substrate and the sensing tip. The change in voltage alters the electric field in a direction such as to restore the gap to its original width. The output of the sensor is the change in magnitude of the voltage as a result of changing the tunnelling current, which is proportional to the square root of the accelerating force. In Figure 7, g(s) represents the accelerating force function applied to the proof mass 23. The resulting force on the proof mass F (s) is input to a feedback comparator 80 which also receives a feedback force F Q (s). The resulting force Fs(s) is converted to a displacement y(s) dependent on the stiffness of the cantilever 25 used to support the proof mass 23. The displacement y(s) causes a corresponding change in the tunnel current i(s) which is fed to a current to voltage converter 82 the output of which v(s) is the output of the accelerometer. The output v(s) also acts through the activation arm of the feedback loop to generate the feedback force F Q (s) . As described above, the difference in voltage applied between the structure constituting the cantilever and the proof mass and the substrate causes a local electric field which acts to hold the gap to about 5nm in the absence of acceleration. This is so-called electrostatic pulling. Electrostatic pulling could be replaced by other forms of activation, such as piezoelectric. The output voltage would be fed back into a piezoelectric material and the force generated by that material would restore the gap width in place of forces from the electric field. Other activation methods may also be used. Overload protection for the accelerometer is provided by physical end stops. The silicon substrate 21 itself prevents excessive downward motion. A further glass plate, for example denoted 27 in Figure 2b essentially a second substrate, although not necessarily single crystal, can be provided to limit upward motion. Although a second substrate may be provided to limit upward motion, there is no need for any components to be formed within the second substrate and thus there is no need for careful alignment of the second substrate with the substrate carrying the accelerometer components. Further protection against the possibility of an overload damaging the sensing electrodes can be provided by decreasing the spring constant of the extremity of the cantilever 25 in Figure 2a closest to the sensing tip 24 in Figure 2(a) such that it bends should the proof mass component 23 crash into the substrate. Similarly this may also be done for the extension of the proof mass 33 which carries the sensing tip 34 in Figure 3(a) . Further protection against the possibility of an overload damaging the sensing electrodes can be provided by decreasing the spring constant of component 26 in Figure 2(a) such that it bends should the proof mass component 23 crash into the substrate. Similarly this can also be done for component 36 in Figure 3(a) . Reference will now be made to Figures 8a to 8e to describe a method of manufacture of an accelerometer in accordance with one embodiment of the invention. In Figure 8a reference numeral 100 denotes a single composite wafer comprising a silicon substrate 102, a layer of silicon dioxide on top of the substrate 104 and a top layer of silicon 106. The thickness of the substrate in the described example is 500um <100>Si, the thickness of the oxide layer 104 is 2um and the thickness of the top silicon layer 106 is 7 um. Composite wafers having this structure are commercially available. The top layer 106 could be single crystal silicon or polysilicon. Single crystal silicon can be produced by depositing silicon and recrystallising it after deposition or by bonding two oxidized wafers and thinning one side. A layer 108 of silicon nitride is placed on top of the top silicon layer 106. Photoresist 110 is used to define a pattern to be etched from the silicon, for example that shown in plan view in Figure 2a. The silicon nitride layer 108 is etched using the photoresist 110 as a mask to give the structure shown in Figure 8b. The silicon nitride layer is then used as an etch stop for a subsequent etch through the top silicon layer which will define the shape of the proof mass and supporting cantilevers and fixed contact components. In section, the structure is as shown in Figure 8c. A wet etch using buffered hydrofloric acid is then used to remove the oxide layer from beneath the top silicon layer to provide silicon components supported in a cantilevered fashion and spaced from the substrate by an undercut region 103. This is shown in section in Figure 8d. Figure 8a to 8d are sections taken along a line running up and down the paper in Figures 2a and 2b. Figure 8e is a section taken transversely of these sections and which extends through the cantilevered and fixed components before a cut is made to define the tip and counter electrode. The location and angle of the cut is illustrated for example by arrow A. The cut is made using a focused high energy ion beam or a laser to define two angled parallel surfaces constituting the sensing electrodes of the accelerometer. The initial gap may have a width of about 200nm and is closed as discussed above by application of an electric field to a width which will allow a tunnelling current to flow, for example about 5nm.

Claims: CLAIMS : 1. An accelerometer comprising: a substrate; a proof mass mounted to the substrate for movement in a first direction perpendicular to the plane of the substrate, the proof mass carrying a first sensing electrode; a second sensing electrode mounted relative to the substrate; wherein the first and second sensing electrodes comprise surfaces extending at an angle relative to the substrate and defining between them a sensing gap the width of which varies with movement of the proof mass in the first direction. 2. An accelerometer according to claim 1 wherein the proof mass and the first and second sensing electrodes are formed as components in a single wafer. 3. An accelerometer according to claim 1 or 2 which is formed in single crystal silicon. 4. An accelerometer according to any preceding claim which comprises circuitry for sensing variations in the width of the sensing gap. 5. An accelerometer according to claim 4 wherein the sensing circuitry detects changes in a tunnelling current between the first and second sensing electrodes. 6. An accelerometer according to any preceding claim wherein the second sensing electrode is mounted to the substrate at a location internally of a surrounding proof mass. 7. An accelerometer according to claim 6 wherein the proof mass comprises a perforated diaphragm suspended for movement in the first direction relative to the substrate. 8. An accelerometer according to any preceding claim which comprises a limit plate arranged to limit movement of the proof mass away from the substrate. 9. A method of making an accelerometer comprising the steps of: forming a component supported at opposed end regions with respect to a substrate; and cutting a gap in the component at an angle incident to the plane of the substrate to form two angled surfaces which extend at an angle relative to the substrate and which constitute first and second sensing electrodes defining between them a sensing gap. 10. A method according to claim 9 wherein the accelerometer is formed in a single composite wafer comprising a silicon substrate, a layer of silicon dioxide and a top layer of silicon. 11. A method according to claim 9 or 10 wherein the cutting step comprises cutting using a focused high energy ion beam. 12. A method according to claim 9 or 10 wherein the cutting step comprises cutting using a laser.

Inventors: BURGESS STUART CLYDE, CHIANG HIANG-SWEE, KIRIYAMA TAKASHI, KLAUBERT HEATHER, MOORE DAVID FRANK, SHIBAIKE NARITO, BURGESS, STUART, CLYDE, CHIANG, HIANG-SWEE, KIRIYAMA, TAKASHI, KLAUBERT, HEATHER, MOORE, DAVID, FRANK, SHIBAIKE, NARITO
Applicants: BURGESS STUART CLYDE, CHIANG HIANG SWEE, KIRIYAMA TAKASHI, KLAUBERT HEATHER, LYNXVALE LTD, MOORE DAVID FRANK, SHIBAIKE NARITO, BURGESS, STUART, CLYDE, CHIANG, HIANG-SWEE, KIRIYAMA, TAKASHI, KLAUBERT, HEATHER, LYNXVALE LIMITED, MOORE, DAVID, FRANK, SHIBAIKE, NARITO
Application year: 1995
Main classification: G01P15
IPC codes: G01P15, H01L29