Force transducer etched from silicon, and method of producing the same

Abstract: An accelerometer fabricated by silicon etching techniques. A first suspended beam (14) is formed having a first conductive portion (24), the beam being deflectable in response to an acceleration force. A second beam (36) having second and third conductive portions (42,44) is suspended over the first beam (14). Phased lock loop circuitry (32) oscillates the second beam (36) at resonance and provides an electrical signal proportional to the acceleration force.

Description: The invention relates to force transducers wherein an applied force, such as an acceleration force or a fluid pressure force, is converted into an electrical signal. In particular, the invention relates to force transducers and methods for producing such transducers by etching silicon substrates. Force transducers are known having a suspended mass, such as a pendulum or cantilever, which deflects in response to an applied force. Typically, capacitive plates are coupled to surfaces of both the suspended mass and an adjacent structure. As the suspended mass deflects, the resulting change in capacitance provides an electrical indication of the applied force. An example of an accelerometer is disclosed in US-A-4,679,434 issued to Stewart et al. A cantilever is formed by etching a substantially U- shaped cavity through a silicon wafer. Two Pyrex ™ planar surfaces are then attached to opposing planar surfaces of the substrate such that the cantilever is suspended therebetween. Conductive plates are bonded to both Pyrex ™ surfaces and the suspended mass. A differential amplifier applies a voltage to the plates in response to detected deflection of the suspended mass for restoring the suspended mass to its null position. The applied voltage is, allegedly, proportional to the applied force. US-A-4 805 456 discloses an accelerometer in which an acceleration causes deflection of a mass located in an opening in a silicon substrate and is connected to it by two pairs of beams on opposite sides bridging the gaps in the x-and y-directions. Components of acceleration in these directions are detected by measuring the differential changes in the resonant frequency of these beams caused by the axial stretching and compression of opposite beams of the respective pairs resulting from the deflection. Acceleration in the z-direction is determined by measurement of the resulting deflection in this direction of the mass, or of an additional beam connected thereto, relative to wafers of Pyrex ™ or silicon opposite it in the z-direction by capacitative or null electrostatic methods. The inventor herein has recognised numerous disadvantages of the prior approaches. In motor vehicle applications, for example, accelerometers are deployed in air bag systems wherein false triggering due to noise is intolerable. When the motor vehicle traverses rough road surfaces, there may be deflection of the suspended mass due to noise resulting in erroneous interpretation as a collision. Since the prior approaches appear to have the ability of sensing displacement only in the time frame of motion of the suspended mass, their ability to discriminate against vehicular noise is limited. An additional disadvantage is that only a portion of the structure disclosed may be fabricated by silicon processing technology. Thus, the potential advantages of batch processing technology are not fully utilised. An additional disadvantage is that deflection of the suspended mass is essentially arcuate. Accordingly, capacitive changes and corresponding electrical measurements are nonlinear with respect to the applied force. It is an object of the present invention to provide a force transducer fabricated by silicon processing technology having high noise immunity and linearity. According to the present invention there is provided an apparatus for sensing an applied force as hereinafter set forth in Claim 1. Preferably, the power means includes a phased lock loop having feedback from the sensing means for oscillating the second suspended beam at its resonant frequency. Preferably, a pair of flexing means is included which are symmetrically positioned around the first suspended beam for providing linear deflection thereof in response to the acceleration force. By oscillating the second beam, deflection of the first suspended beam is sampled at a rate faster than the deflection thereby providing the advantage of higher immunity to noise than heretofore possible. Thus, false triggering such as when traversing a rough road surface, is substantially eliminated. Further, by providing a first suspended beam coupled at opposing ends, beam deflection is substantially linear with respect to an applied force. An advantage is thereby obtained of accurate sensing of an applied force. In another aspect of the invention, a method as set forth in claim 9 is provided for fabricating an apparatus for detecting an applied force, in particular an accelerometer. More specifically, the method comprises the steps of: forming a sacrificial layer of an etchant material over one planar surface of a silicon substrate; forming a layer of polysilicon over the sacrificial layer; forming a first suspended beam by selectively etching the substrate to form cavities defining the beam; forming a second beam by selectively etching the polysilicon layer; etching away the sacrificial layer to suspend the second beam over the first suspended beam; forming conductive portions on both the first and the second beams; coupling an electrical oscillator between conductive portions on the first and second beams for oscillating the second beam; and coupling sensing circuitry between conductive portions on the first and second beams, for sensing the applied force by sensing capacitive changes between the conductive portions. The above aspect of the invention provides an advantage of fabricating the entire sensing apparatus by silicon processing technology. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a perspective view of an embodiment which utilises the invention to advantage; Figure 2 is a cross-sectional view taken along line 2-2 of Figure 1; Figure 3 is a bottom view of a portion of the embodiment shown in Figure 1; Figure 4 is a block diagram of electronic circuitry coupled to the embodiment shown in Figures 1- 3; Figure 5a is a cross-sectional view taken along lines 5a-5a in Figure 1; and Figures 5b-5h illustrate various processing steps in manufacturing the embodiment shown in Figure 1 as viewed along lines 5a-5a in Figure 1. Referring to Figure 1, in general terms which are described in greater detail hereinafter, force transducer 10 is shown in this example fabricated from silicon substrate 12, {100} silicon in this example. Suspended beam 14 is shown integrally formed from substrate 12 and connected at its opposing ends to supporting frame 16 via respective flexing members 18 and 20 which are also integrally formed from silicon substrate 12. For the particular example presented herein, force transducer 10 is shown as an accelerometer wherein beam 14 deflects in response to an acceleration force. With suitable modification, such as addition of a diaphragm, beam 14 may also be responsive to other applied forces such as fluid pressure. In the later example, force transducer 10 may be utilised to advantage as a pressure transducer. Flexing members 18 and 20 are shown symmetrically positioned around beam 14 such that beam deflection is rectilinear in response to the applied force. The mass of beam 14 and dimensions of flexing members 18 and 20 are chosen to provide a desired deflection and response time for a particular application. For example, when used as an accelerometer in an air-bag system, it was found desirable to sense up to 50g's of acceleration in 20ms. A corresponding deflection of 2µm in 20ms was also found desirable. This deflection can be accommodated by a silicon sensing mass of .0012 grams supported by a flexing member having dimensions of 40µm x 5735µm x 897µm. Suspended beam 14 includes conductive portion 24 (Figures 2 and 5a) which forms a capacitive plate. In this example, conductive portion 24 is formed by doping a pattern of ions on top surface 26 of beam 14 as described in more detail later herein. Other ways of producing a similar conductive portion are readily apparent to those skilled in the art including vapour deposition of a conductive layer such as nickel or platinum. The conductive portion may also be formed by sequential vapour deposition of layers of chromium, gold, and chromium. Conductive portion 24 is electrically connected to conductive tab 30 for coupling to electronic circuitry 32. It is noted that conductive portion 24 is connected to tab 30 by conventionally forming a hole into a doped region of substrate 12 and depositing a conductive material therein. This connection may also be accomplished by depositing conventional conductive traces on substrate 12 between conductive portion 24 and tab 30. Continuing with Figure 1, and also referring to the cross-sectional view shown in Figure 2, suspended beam 36 is shown suspended over beam 14 and connected to supporting frame 16. As described in greater detail hereinafter with particular reference to Figures 5a-5h, beam 36 is fabricated by growing, and appropriately etching, a layer of polysilicon over substrate 12. Those skilled in the art will recognise that beam 36 may also be formed by other materials such as silicon oxide, or silicon nitride, or a hybrid combination of both. Further, beam 36 may be formed by bonding a silicon beam to substrate 12 utilizing a conventional anodic bonding process. Beam 36 is dimensioned such that it is electronically driven into oscillation at a desired frequency as described in greater detail hereinafter. For the particular example presented herein, beam 36 is oscillated at approximately 60kHz which is achieved by beam dimensions of approximately 1.6um x 2952um x 5795um. In view of deflection by beam 14, and oscillating amplitude of beam 36, a spacing between the beams of 2um was found to be desirable in this particular example. Reference is now made to the bottom view of beam 36 shown in Figure 3. Sensing plate 40, driving plates 42 a-d , and guard plates 44 a-b are shown formed on bottom surface 46 of beam 36. Plates 40, 42 a-d , and 44 a-b are formed by etching a doped polysilicon layer as described later herein. It is noted however that alternate methods of forming these plates are utilised to advantage such as doping beam 36 in an appropriate pattern, or vapour depositing conductive layers such as chromium-gold-chromium on bottom surface 46. Forming the plates on a top or an intermediate surface of beam 36 may also be used to advantage. Sensing plate 40 is shown coupled to conductive tab 64 on top surface 70 of substrate 12 by conductive trace 50. Driving plates 42 a-d are shown interconnected and connected to conductive tab 66 on top surface 70 by conductive trace 52. Guard plates 44 a-b are ohmically coupled through beam 36 to conductive tab 56 formed thereon. Tab 56, and accordingly guard plates 44 a-b , are coupled to conductive tab 68 on top surface 70 via conductive trace 58. Tabs 64, 66, and 68 interconnect electronic circuitry 32 with plates 40, 42 a-d , and 44 a-b . As described in greater detail hereinafter, conductive traces 50, 52, and 58, and conductive tabs 58, 64, and 66, are formed by vapour depositing layers of chromium-gold-chromium such that the traces extend along bottom surface 46 of beam 36 and top surface 70 of substrate 12. Force transducer 10 is hermetically sealed in a casing (not shown) filled with a gas at sufficient pressure to produce a desired damping of mechanical oscillations. A description of electronic circuitry 32 is now provided with reference to the electronic schematic shown in Figure 4. Sensing capacitor 40' is a representation of the capacitance between sensing plate 40 and conductive portion 24. Similarly, driving capacitor 42' represents the electrical equivalent of the capacitance between driving plates 42 a-d and conductive portion 24. Guard capacitor 44' represents the capacitance between guard plates 44 a-b and conductive portion 24. Voltage source V s is shown connected in series through resistor 70 to an input of isolation amplifier 72. Voltage source V s is also shown connected in series through resistor 70 and resistor 74 to one plate of capacitor 40', the other plate being connected to ground. Isolation amplifier 72 is shown as an operational amplifier having feedback to an input terminal for stability. The output of amplifier 72 is shown coupled to one plate of guard capacitor 44' which has its other plate connected to ground. Oscillator 81 is shown coupled across driving capacitor 42' and responsive to feedback signal f d from detector 83 for driving beam 36 into oscillation at its resonant frequency. Detector 83 is shown coupled across resistor 74 for providing output signal M d and feedback signal f d which are proportional to deflection of beam 14'. The voltage applied to sensing capacitor 40' through the series interconnection of resistors 70 and 74 results in a current flow through resistor 74. Detector 83 detects changes in this current flow in response to capacitive changes in sensing capacitor 40'. Circuitry 32 forms a phased lock loop wherein beam 36 is driven at resonance and the amplitude of deflection is controlled by feedback to maintain a constant amplitude of deflection. As beam 14 deflects in response to an acceleration force, there is corresponding change in feedback signal f d for maintaining constant amplitude of deflection and constant frequency. There is also a corresponding change in output signal M d which is proportional to deflection of beam 14 and, accordingly, the force applied thereto due to the acceleration. In one particular application, beam 36 is oscillated at 60kHz and beam 14 is dimensioned to respond at 12.5Hz. Thus, the phased lock loop provides, in this example, approximately 4800 samples per cycle of deflection thereby achieving greater noise immunity than heretofore possible. An example of fabricating accelerometer 10 utilizing conventional silicon processing techniques is now described with reference to Figures 5a-5h. Referring first to Figure 5b, a {100} silicon wafer 12 is shown having a conductive portion 24 patterned by conventionally doping ions, such as boron ions, through top surface 70. After the doping process, layer of silicon oxide 72 is grown over top surface 70 and layer of silicon oxide 74 is grown over bottom surface 76. The process steps for suspending beam 14 from frame 16 and forming portions of flexing members 18 and 20 is now described with reference to Figure 5c and Figure 1. A masking pattern is first etched into silicon oxide layer 74 utilizing conventional photolithographic techniques to form openings for cavity 80, cavity 82, and pits 84, 86, 88, and 90. An anisotropic etchant, such as aqueous potassium hydroxide, is then applied to the masked openings in silicon oxide layer 74. The etchant acts against the nominal {100} planes at a rate approximately 40 to 100 times greater than the intersecting {100} planes thereby forming pits 84, 86, 88, and 90. Concurrently, the etchant acts against the {100} planes to form cavities 80 and 82 extending from bottom surface 76 through top surface 70 along {111} planes. Thus, the outer surface of the walls defining cavities 80 and 82 follow the {111} planes intersecting top surface 70 at an angle of approximately 54,7°. Footings 92 (Figure 2), composed of silicon nitride, are formed into silicon oxide layer 72 for isolating beam 36. In a conventional manner, footing openings are patterned and etched into silicon oxide layer 72. A silicon nitride layer is then formed over top surface 70 and subsequently etched such that only footings 92 remain of the silicon nitride. Referring to Figure 5d, layer of silicon oxide 94 is grown over bottom surface 76 forming a passivation layer over pits 84, 86, 88, and 90. A first layer of polysilicon is then formed over silicon oxide layer 72 and appropriately etched to form sensing plate 40, driving plates 42 a-d and guard plates 44 a-b utilizing conventional photolithographic and etching techniques. The plates are also doped with ions, such as boron ions, such that they are conductive. As shown in Figure 5e, second layer of polysilicon 96 is then grown over top surface 70 covering plates 40, 42 a-d , and 44 a-b . Layer of polysilicon 96 is subsequently etched utilizing conventional photolithographic and etching techniques to form beam 36 as shown in Figure 5f. Referring to Figure 5g, silicon oxide layer 98 is then grown over top surface 70 to form a passivation layer over beam 36. Now referring to Figure 5h, top layers of silicon oxide 72 and 98 are appropriately masked using conventional photolithographic techniques and etched to form etchant patterns on top surface 70 aligned with the desired location of flexing members 18 and 20. An anisotropic etchant is then applied to form etchant pits 102 and 104 through top surface 70 thereby completing flexing members 18 and 20. It is noted that cavities 80 and 82 may also be formed by partial etching from bottom surface 76 and subsequent etching from top surface 70 when forming etchant pits 102 and 104. In this manner, a more compact accelerometer 10 is achieved. The silicon oxide layers are then etched, in a manner known as sacrificial etching, such that beam 36 becomes suspended over beam 14 as shown in Figure 5a. An alternate fabricating process is now described, with continued reference to Figures 5a-h wherein plates 40, 42 a- d , and 44 a-b are formed from conductive layers rather than doped polysilicon. The process steps previously described herein with reference to Figures 5b-5c are repeated. With reference to Figure 5d, plates 40, 42 a-d , and 44 a-b , are formed by vapour deposition of sequential layers of chromium, gold and chromium over silicon oxide layer 72. These layers are conventionally patterned and etched to define plates 40, 42 a-d , and 44 a-b . In addition, traces 50, 52, and 58, and tabs 56, 64, 66 and 68 (Figures 1 and 2) are also defined during this step. The remaining process steps are the same as those previously described herein with reference to Figures 5e-5h. Those skilled in the art will recognise that there are other processes utilizing silicon processing technology which may be utilised to advantage to produce a force transducer embodying the invention such as the one illustrated in Figure 1. For example, conductive portion 24 could be formed by vapour deposition of a conductive material over top surface 70. Similarly, plates 40, 42 a-d , and 44 a-b could be formed by vapour deposition of a conductive material over the top surface of beam 36. Conductive plates 40, 42 a-d , and 44 a-b could also be formed by appropriately doping suspended beam 36. It is also noted that suspended beam 36 may comprise other materials such as silicon oxide, silicon nitride, or a hybrid combination of both or other similar materials. In configurations utilizing a hybrid combination, plates 40, 42 a-d , and 44 a-b could be formed between layers. Suspended beam 36 may also be formed separately and bonded to substrate 12. For example, in another alternate embodiment, beam 36 is formed by etching a separate silicon wafer and bonding the resultant beam to supporting frame 16 by conventional anodic bonding techniques. In this later example, conductive plates 40, 42 a-d , and 44 a-b are formed by either doping silicon beam 36 or depositing conductive material, such as nickel or platinum, on the underside of beam 36.

Claims: An apparatus for measuring an applied force, comprising a first suspended beam (14) deflectable in response to the applied force, a second suspended beam (36) positioned opposite said first suspended beam, and electrical detecting means (40, 83) for sensing the applied force by sensing the deflection of said first suspended beam in response to said applied force, relative to said second suspended beam, characterised by an oscillator (81) for applying electrical power between said first and second beams to deflect and oscillate said second suspended beam (36). An apparatus as claimed in claim 1, wherein said oscillator is connected in a phased lock loop having feedback from said detecting means for oscillating said second suspended beam at a predetermined frequency. An apparatus as claimed in claim 1 or claim 2, wherein said second suspended beam is arranged to resonate at its resonant frequency. An apparatus as claimed in any one of the preceding claims, wherein said first suspended beam is etched from a silicon substrate (12) and includes a first conductive portion (24) and said second suspended beam includes a second conductive portion (42) and a third conductive portion (40), both being positioned opposite said first conductive portion, said first and second conductive portions being coupled to said oscillator (81) and said first and third conductive portions being coupled to said electrical detecting means (83). An apparatus according to claim 4, wherein said electrical detecting means (83) comprises sensing circuitry for sensing capacitative changes between said first and third conductive portions. An apparatus according to claim 5, wherein said first conductive portion (24) is formed by doping a portion of said first suspended beam (14) or by coupling a conductive layer to a portion of said first suspended beam. An apparatus according to one of the preceding claims, wherein said second suspended beam (36) is comprised of polysilicon, silicon oxide, silicon nitride, conductive metal material, doped polysilicon material or layers of silicon oxide and silicon nitride. An apparatus as claimed in any one of claims 4 to 7, wherein said first suspended beam is defined by a portion of a silicon substrate (12) between a pair of cavities (80, 82) and is connected to said substrate by a pair of flexing means (18, 20) symmetrically positioned around said first suspended beam and integrally formed from said substrate for enabling deflection of said first suspended beam in response to said applied force normal to the plane of said substrate. A method for fabricating an accelerometer from silicon, comprising the steps of etching a pair of cavities (80, 82) through a silicon substrate (12) to define a first suspended beam (14) therebetween, said first suspended beam being deflectable in response to acceleration, forming a first conductive portion (24) on said first suspended beam, coupling a second suspended beam (36) to said substrate such that said second suspended beam is suspended opposite said first suspended beam, forming second and third conductive portions (42 and 40 respectively) on said second suspended beam opposite said first conductive portion (24), coupling an electrical oscillator circuit between said first and second conductive portions for deflecting and oscillating said second suspended beam, and coupling an electrical detector circuit between said first and third conductive portions for sensing the deflection of said first suspended beam relative to said second suspended beam. A method as claimed in claim 9, wherein said step of forming said first conductive portion comprises doping said silicon substrate and wherein said step of forming said second and third conductive portions comprises doping polysilicon. A method according to claim 9 or claim 10, wherein said first and second suspended beams are formed by the steps of forming a sacrificial layer (72) of an etchant material over one planar surface (70) of a silicon substrate (12), forming a layer (98) of polysilicon over said sacrificial layer, selectively etching said substrate to form said cavities (80, 82) defining said first suspended beam, selectively etching away said polysilicon layer (98) to form said second beam and etching away said sacrificial layer (72) to suspend said second beam (36) over said first suspended beam (14). A method as claimed in claim 11, further comprising the step of forming flexible members (18, 20) between said first suspended beam and said substrate. A method according to any one of claims 9 to 12 wherein said electrical detector circuit comprises sensing circuitry for sensing capacitative changes between said second and third conductive portions.

Inventors: SICKAFUS EDWARD NATHAN, SICKAFUS, EDWARD NATHAN
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1990
Main classification: G01L1
IPC codes: G01P15