Patent Publication Number: US-6662657-B2

Title: Force-sensing transducers, accelerometers, rate sensors, methods of forming force-sensing transducers, and methods of forming vibrating-beam force transducers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority from U.S. patent application Ser. No. 09/702,372 filed Oct. 30, 2000 now U.S. Pat. No. 6,435,029, which in turn is a divisional of and claims priority from U.S. patent application Ser. No. 09/104,844 filed Jun. 25, 1998, now U.S. Pat. No. 6,161,440, which in turn claims priority from U.S. Provisional Application No. 60/055,646, filed Aug. 14, 1997. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to force-sensing transducers, accelerometers, rate sensors, methods of forming force-sensing transducers, and to methods of forming vibrating-beam force transducers. 
     BACKGROUND OF INVENTION 
     Force-sensing transducers can be used to measure force, acceleration, pressure, and the like. One type of force-sensing transducer is a resonating force transducer. Exemplary transducers are described in U.S. Pat. Nos. 5,367,217, 5,339,698, and 5,331,242, the disclosures of which are incorporated by reference. Another type of force-sensing transducer is an accelerometer. Exemplary accelerometers are described in U.S. Pat. Nos. 5,594,170 5,501,103, 5,379,639, 5,377,545, 5,456,111, 5,456,110, and 5,005,413, which are incorporated by reference herein. Other types of force-sensing transducers can be used as rate sensors. Exemplary rate sensors are described in U.S. Pat. Nos. 5,717,140, 5,376,217, 5,696,323, 5,691,472, and 5,668,329, which are hereby incorporated by reference. Yet other force-sensing transducers can be used as acceleration and rate sensors. Exemplary sensors are described in U.S. Pat. Nos. 5,627,314, 5,557,046, 5,341,682, 5,331,853, 5,331,854, and 5,319,976, the disclosures of which are incorporated by reference herein. 
     Force-sensing transducers such as those incorporated by reference above, can experience problems associated with metalization which can adversely affect the transducer&#39;s performance. In particular, bias performance can be adversely affected when a conductor having a thermal coefficient of expansion different from that of the substrate, is deposited and used during operation. Specifically, metal conductors can be deposited at high temperatures and, because of the difference in thermal coefficient of expansion with the substrate, the deposited metal conductor can “creep” because of high thermal stress developed between the metal conductor and the substrate. Metal creep occurs when the deposited metal yields during the application of some external stimulant and does not return to its initial condition. The change in condition results in a change in bias operating point. 
     A preferred conductor material is gold. Gold is typically used in forming metal conductors on such substrates because it exhibits high conductivity and other traits normally associated with noble metals. However, forming the metal conductors using gold exacerbates the adverse effects on bias performance because gold has a very high thermal expansion coefficient relative to typical substrate materials such as quartz and silicon. Additionally, gold has a very low yield strength. 
     Traditional methods of reducing metal creep include using metal alloys with higher yield strength than pure gold, using alloys with thermal coefficients of expansion closely matched to the particular substrate material, removing metalization layers all together, compensating the metal creep effects by matching metal conductors on opposing surfaces, and designing a very low spring rate support structure to counter the effects of creep. 
     This invention arose out of concerns associated with providing improved force-sensing transducers, accelerometers, and rate sensors. This invention also arose out of concerns associated with providing improved methods of forming force-sensing transducers such as those mentioned above. 
     SUMMARY OF INVENTION 
     Force-sensing transducers, accelerometers, rate sensors, and methods of forming force-sending transducers are described. 
     In one embodiment, a substrate includes a force-sensing element. An adhesion layer is disposed over less than an entirety of the force-sensing element, and a conductive layer is disposed over the force-sensing element and supported in a bonded relationship therewith through the adhesion layer. 
     In another embodiment, a substrate includes a proof mass and a vibratable assembly connected therewith and configured to detect an acceleration force. An adhesion layer is disposed over less than an entirety of the vibratable assembly, and a conductive path is disposed over the vibratable assembly and fixedly bonded therewith through the adhesion layer. 
     In another embodiment, a Coriolis rate sensor includes a substrate having a vibratable assembly connected therewith. An adhesion layer is disposed over less than an entirety of the vibratable assembly and a conductive path is disposed over the vibratable assembly and fixedly bonded therewith through the adhesion layer. 
     In yet another embodiment, a substrate is provided having a force-sensing element defining an area over and within which a conductive layer of material is to be formed. A patterned adhesion layer is formed over less than an entirety of the area. A conductive layer is formed over the area and bonded to the substrate through the patterned adhesion layer. 
     In still another embodiment, a substrate is provided and etched sufficiently to form a plurality of vibratable beams arranged in a force-sensing configuration. An insulative layer of material is formed over the vibratable beams and an adhesion layer pattern is formed over the vibratable beams. The pattern comprises a plurality of spaced-apart pattern components, with each beam having three pattern components spaced apart along its length. Conductive material is formed over the vibratable beams and the adhesion layer pattern. The conductive materials is more fixedly attached to the adhesion layer pattern than to vibratable beam portions not having the adhesion layer pattern thereover. The substrate is temperature cycled effective to weaken the attachment between the conductive material and the vibratable beam portions not having the adhesion layer pattern thereover. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a force-sensing transducer which can be utilized in connection with one or more embodiments of the invention. 
     FIG. 2 is a top plan view of a different force-sensing transducer which can be utilized in connection with one or more embodiments of the present invention. 
     FIG. 3 is a top plan view of an accelerometer which can be utilized in connection with one or more embodiments of the present invention. 
     FIG. 4 is a top plan view of an acceleration and rate sensor which can be utilized in connection with one or more embodiments of the present invention. 
     FIG. 5 is a top plan view of a force-sensing transducer in accordance with one embodiment of the present invention. 
     FIG. 6 is an exaggerated elevational view of a vibrating beam clamped on both ends, and shown in an excited state. 
     FIG. 7 is a view of the FIG. 5 force-sensing transducer at a processing step which is subsequent to that which is shown in FIG.  5 . 
     FIG. 8 is a view which is taken along line  8 — 8  in FIG. 7, and shows a portion of the illustrated force-sensing transducer at a processing step which is subsequent to that which is shown in FIG.  7 . 
     FIG. 9 is a view of the FIG. 8 force-sensing transducer portion at a processing step which is subsequent to that which is shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4 show various embodiments of force-sensing transducers which can be utilized in connection with one or more embodiments of the invention described below. Transducer  10  includes a substrate  12  having a force-sensing element  14  which is preferably etched from the substrate material. In the illustrated and preferred embodiment, force-sensing element  14  comprises a vibratable assembly having a plurality of vibratable beams  16 ,  18 ,  20 , and  22 . Of course, more or less beams can be provided. In a preferred embodiment, the force-sensing element comprises an integral material, with crystalline silicon being preferred. The specific force-sensing transducer shown in FIG. 1 is described in more detail in U.S. Pat. No. 5,367,217, incorporated by reference above. 
     FIG. 2 shows a different force-sensing transducer generally at  10   a . Like numerals from the above-described embodiment have been utilized where appropriate, with differences being indicated by the suffix “a”, or with different materials. Force-sensing transducer  10   a  is configured as an angular rate sensor or Coriolis rate sensor. Vibratable assembly  14   a  includes a plurality of vibrating beams  24 ,  26 ,  28  and  30  defining an outer pair of vibrating beams  24 ,  28  are mechanically coupled to the inner beams  26 ,  30  by way of interconnecting members  32 ,  34 . Interconnecting members  32 ,  34  are formed generally at the midpoint of outer beams  24 ,  28  and inner beams  26 ,  30 . Beams  24 ,  26 ,  28 , and  30 , as well as interconnecting members  32 ,  34  are formed by forming slots (not specifically designated) between the beams. The operation of force-sensing transducer  10   a  is described in detail in U.S. Pat. No. 5,717,140 incorporated by reference above. 
     FIG. 3 shows a force-sensing transducer in the form of an accelerometer generally at  10   b , which can be configured in accordance with various embodiments of the present invention. Like numerals from the above-described embodiments are utilized where appropriate, with differences being indicated by the suffix “b” or with different numerals. 
     Accelerometer  10   b  is preferably formed from a single wafer of silicon through micromachining techniques. Accelerometers such as this and others are described in U.S. Pat. No. 5,005,413, which is incorporated by reference above. Accelerometer  10   b  is configured as a force-sensing transducer which senses an acceleration force which acts upon the accelerometer. In this example, substrate  12   b  includes a proof mass  36  and a vibratable assembly  14   b  connected therewith and configured to detect the acceleration force. Proof mass  36  is supported by a pair of flexures  38 , 40 . In this example, force-sensing transducer  10   b  comprises a vibrating beam accelerometer whose vibratable assembly  14   b  comprises a plurality of vibratable beams  42 ,  44  and  46 ,  48 . The beams are arranged, together with proof mass  36 , in a configuration which develops different vibratory frequencies responsive to movement or deflection of proof mass  36  in accordance with an acceleration applied thereto. The various frequencies at which the beams vibrate give an indication of the acceleration force acting upon the proof mass. The acceleration-sensing operation of this force-sensing transducer is described in U.S. Pat. No. 5,379,639 incorporated by reference above. 
     Briefly, vibratable assembly  14   b  is configured into a push-pull configuration such that when proof mass  36  experiences an acceleration, it is moved or deflected along an input axis either generally into or out of the plane of the page upon which FIG. 1 appears. Such movement or deflection causes one of the pairs of beams  42 ,  44  or  46 ,  48  to go into tension, and the other of the pairs of beams to go into compression. As this compression and tension occurs, the respective frequencies at which the beams vibrate change in a known and measurable manner. 
     FIG. 4 shows a force-sensing transducer  10   c  in the form of an acceleration and angular rate sensing device. In the illustrated example, the device includes a first accelerometer  50  having a proof mass  52  connected to substrate  12   c  by a pair of flexures  54 . The flexures define a hinge axis HA 1  for proof mass  52 . A second accelerometer  56  is provided and includes a proof mass  58  and a pair of flexures  60  connecting the proof mass to substrate  12   c . Flexures  60  define a second hinge axis HA 2  for proof mass  58 . A pair of force-sensing elements  14   c  are provided in the form of a vibratable assembly similar to the one described below in FIG. 5. A link  62  is provided and connects accelerometers  50 ,  56  together and acts as a rocker arm to ensure that the accelerometers can be dithered 180° out of phase. A discussion of various aspects of the dynamics of dithering as such pertains to accelerometers and rate sensors can be found in many of the references above. The specific structure and operation of the FIG. 4 acceleration and rate sensor is described in U.S. patent application Ser. No. 08/949,883, the disclosure of which is incorporated by reference herein. 
     A plurality of contact pads T 1 , G 1 , R 1 , P 1 , D 1 , and D 2 , P 2 , R 2 , G 2 , and T 2,  are disposed over the substrate and provide contacts through which desirable electrical connection can be made to outside-world circuitry for the dither drive, dither pick-off, beam or tine drives, and beam or tine pick-offs. Conductive material, such as gold, is disposed over the contact pads and various other areas of the substrate including vibratable assemblies  14   c.    
     In the devices described above, each uses a vibratable assembly to sense a force such as acceleration or angular rate. Aspects of the invention described just below are well suited for implementation in connection with these and other devices. 
     For illustrative purposes only, an exemplary force-sensing transducer is shown in FIG. 5 generally at  10   d . Like numerals from the above-described embodiment have been utilized where appropriate, with differences being indicated with the suffix “d” or with different numerals. A force-sensing element or vibratable assembly  14   d  is provided and includes a plurality of beams  64 ,  66 , which are preferably vibratable. A non-vibratable assembly  68  is provided for supporting a reference resistor which will ultimately be formed when conductive material is provided over the vibratable and non-vibratable assemblies  14   d ,  68  respectively. Non-vibratable assembly  68  includes a pair of beams  70 ,  72  which are tied together by a plurality of pins  74 ,  76 , and  78 . Beams  70 ,  72  do not vibrate and are provided to match the electrical path resistance of beams  64 ,  66  when conductive material is disposed thereover. When an electrical current is provided over a conductive path over vibrating beams  64 ,  66 , a voltage drop develops relative to those beams. Similarly, a current provided over beams  70 ,  72  develops a generally identical voltage drop such that the two voltage drops can be subtracted from one another and cancel. This assists outside circuitry in its discrimination of the feedback signals from the vibrating beams. Support structures  80  are provided and tie the ends of both the vibratable and non-vibratable beams together. Beams  64 ,  66 ,  70 , and  72  extend along respective long axes and define individual respective areas over and within which a conductive layer of material is to be formed. 
     FIG. 6 shows an exemplary schematic of a beam B with neutral inflection points P 1  and P 2 . A discussion of neutral inflection points occurs in U.S. Pat. No. 5,717,140 incorporated by reference above. 
     Referring to FIGS. 7 and 8, an insulative material layer  82  is formed over the beam assemblies. An exemplary material is silicon dioxide which can be grown to a thickness of 5,000 Angstrom. An adhesion layer pattern  84  is formed over substrate  12   d.  In the illustrated example, adhesion layer pattern  84  is formed over both the vibratable assembly  14   d  and the non-vibratable assembly  68 . In addition, the adhesion layer pattern is formed over support structures  80  as shown. The adhesion layer pattern is preferably disposed over less than an entirety of force-sensing element or vibratable assembly  14   d.  In one embodiment, adhesion layer pattern  84  comprises a plurality of discrete layers which are spaced apart over vibratable assembly  14   d.  In the illustrated example, each beam  64 ,  66 ,  70 , and  72  includes three discrete layers of adhesion material intermittently disposed over and in contact with less than an entirety of force-sensing element  14 , i.e., beams  64 ,  66 ,  70 , and  72 . Accordingly, the individual discrete layers define a plurality of spaced-apart pattern components which are disposed along each beam&#39;s length. 
     In a preferred embodiment, each vibratable beam has at least one neutral inflection point such as P 1  and P 2  (FIG. 6) and one of the discrete adhesion layers is disposed over the one neutral inflection point. In the illustrated example, beams  64  and  66  have two neutral inflection points which are disposed on either side of the beam&#39;s centermost point. As viewed in FIG. 8, the leftmost and rightmost adhesion layer patterns  84  are disposed over the neutral inflection points of beam  64 . Accordingly, each vibratable beam has at least one neutral inflection point having a discrete adhesion layer disposed thereover. In a most preferred embodiment, each beam has a discrete layer disposed over its centermost point, as well as each neutral inflection point. In this example, each beam has no more than three discrete adhesion layers disposed thereover and spaced therealong. 
     A preferred material for the adhesion layer pattern is chrome which can be patterned onto the beams through shadow masking techniques. Other manners of providing a patterned adhesion layer can be used. An exemplary thickness for the patterned adhesion layer is 100 Angstrom. In this example, the preferred chrome is deposited at both ends of a vibrating beam in 67-micron-wide sections. Limiting the chrome deposition to support structures  80  and the above-described three spaced-apart locations, reduces the total area of chrome deposition from about 1,000 microns to about 200 microns which results in a 5-to-1 reduction in the adhesion area. 
     Accordingly, application of the present invention to a vibrating beam provides advantages which help to reduce metal creep. By defining the chrome deposition areas at the neutral bending or inflection points, e.g. approximately 22.4% of the length from where the beam attaches to support structures  80 , the stress in the conductor, in this case gold, is further reduced. Defining a chrome deposition area at the midpoint of the beam tends to stress the deposited metal less than elsewhere, though the midpoint is not at a neutral inflection point. Thus, the gold is bonded to the beam through the chrome interface at three points along the length of the conductive path, as well as at both ends. The number of points of attachment and the area of adhesion can be varied along the width of the beam as well as along its length, for example, in a checkerboard pattern. While there are innumerable number of pattern variations, the spirit of the invention remains the same in reducing metal creep effects by reducing the total adhesion area. 
     FIG. 8 shows a conductive material layer or path  86  formed over vibratable beam  64  and the adhesion layer pattern  84 . Conductive layer  86  is more fixedly attached to the adhesion layer pattern than to the vibratable beam portions  88  which do not have adhesion layer pattern  84  thereover. 
     FIG. 9 shows vibrating beam  64  after the beam has been temperature cycled to effectively weaken the attachment between conductive material  86  and vibratable beam portions  88 . By temperature cycling the finished beam over a greater range of temperatures than expected during actual use, for example temperature cycling from −65. degree. C. to 135. degree. C. for use from −55. degree. C. to 125. degree. C., the conductive material  86 , e.g. gold-to-oxide, adhesion bond is broken in the oxide area but the conductive material-to-chrome bond remains intact. Particularly, the excess cold cycle stresses the gold past its yield point which helps activate the reduction in metal creep. Such temperature cycling causes a portion of conductive layer  86  which is not bonded with the patterned adhesion layer to separate from the underlying substrate material. Thus, adhesion layer  84  is intermittently disposed over and in contact with less than an entirety of force-sensing element  14 , i.e., beams  64 ,  66 ,  70 , and  72 ; and conductive layer  86  is disposed over force-sensing element  14  and intermittently supported in a bonded relationship therewith through adhesion layer  84 . 
     Some of the advantages achieved by the present invention are that adverse effects of metalization on bias performance in force transducers can be reduced by providing a large reduction in metal creep while using standard conductors such as gold. Reductions in metal creep are achieved through the provision of reduced metalization adhesion areas. In vibrating beam force sensors, additional bias performance enhancement can be provided by forming the metalization adhesion areas at a beam&#39;s neutral inflection point(s). Such advantages can be achieved without meaningful increases in cost by using traditional metalization materials, for example, gold, and by taking advantage of readily available supplies, masks, and processing procedures. 
     The invention has been described in compliance with the applicable statutes. Variations and modifications will be readily apparent to those of skill in the art. It is therefore to be understood that the invention is not limited to the specific features shown and described, since the disclosure comprises preferred forms of putting the invention into effect. 
     The invention is, therefore, to be interpreted in light of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.