Abstract:
An insulated strain gage including a layer of semiconductive material and a layer of insulating material, where a side of the first insulating layer is disposed adjacent to a side of the semiconductive layer. A method of manufacturing the insulated strain gage includes the steps of forming an insulating layer of insulating material, and depositing a semiconductive layer of semiconductive material on top of the first insulating layer. The bottom side of the semiconductive layer is adjacent to a top side of the insulating layer. The insulated strain gage may be part of an apparatus for measuring strain on an object. The apparatus measures the strain on an object by translating deformations of the object resulting from an applied force into electrical signals. The apparatus includes a sensor, a insulated strain gage, and a circuit. The insulated strain gage includes an insulating layer and is bonded to the mechanical sensor by an adhesive. The circuit is connected to the insulated strain gage and receives signals indicating an electrical value of the insulated strain gage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of prior filed provisional application entitled, “Silicon On Insulator Strain Gage,” Ser. No. 60/075,135, filed Feb. 18, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of strain measurement. More particularly, the present invention relates to using semiconductive strain gages to measure strain on an object. 
     2. Discussion of the Related Art 
     When force is applied to an object, this results in stress on the object. Stress is the force per unit area acting on the object. When an object experiences stress, the object will experience deformation, which is the change in shape of the object in any dimension. Strain is a measurement of the intensity of this deformation. More specifically, strain is the deformation per unit length of the object in any dimension resulting from stress. 
     Devices employing a variety of techniques are available to measure strain on an object. Typically, these devices translate the mechanical strain on an object into an electrical signal. Strain gages are commonly used in such devices. 
     One type of conventional strain gage consists of a monolith of conductive or semiconductive material whose resistance changes when the gage deforms. Typically, this type of conventional strain gage is mechanically bonded to a mechanical sensor with an adhesive to form a strain gage assembly. Specifically, as illustrated in FIG. 1, an adhesive is applied to the mechanical sensor  2  to form an adhesive layer  3 , the strain gage  4  is pressed against the adhesive layer  3 , and the adhesive layer  3  is allowed time to cure. It should be noted that FIG. 1 is not drawn to scale. The adhesive may be an epoxy, paste or other suitable bonding compound or agent. 
     In operation, when a force impacts on the mechanical sensor  2 , the resultant deformation of the mechanical sensor  2  causes the strain gage  4  to similarly deform, with a resultant change in electrical resistance. This change in resistance is measured and used to provide a measurement of the strain on the mechanical sensor  2 . This change in resistance may also be used to determine the stress or pressure on the mechanical sensor  2 . Force cells, loads cells, pressure transducers and accelerometers are typical devices that make use of this principle. 
     The material of the strain gage  4  can be a conductive metal or a semiconductive material. Semiconductive materials have the advantage of providing a larger change in resistance for a given change in strain than do conductive metals. In a strain gage assembly  1 , attaching a strain gage  4  of semiconductive material to a mechanical sensor  2  of metal material may result in electrical shorts or electrical leakage during operation. Specifically, shorts and leakage result when the adhesive layer  3  is too thin or unevenly applied. Subsequently, when the strain gage  3  is pressed against the mechanical member  2  and the adhesive is allowed to cure, the strain gage  4  may actually contact the mechanical member  2  at points, resulting in a short circuit between the mechanical sensor  2  and the strain gage  4 . 
     If the adhesive layer  3  is too thin at a certain point, current will leak across the adhesive layer  3  when a sufficient voltage potential exists between the mechanical sensor  2  and the strain gage  4 . The voltage at which leakage will occur is the dielectric breakdown voltage of the adhesive layer  4 . 
     To prevent electrical shorts and electrical leakage, the strain gage  4  must be better insulated from the mechanical sensor  2 . Initially, this involves choosing an insulating adhesive as opposed to a non-insulating adhesive. Conventionally, to achieve improved insulation, a filled adhesive is chosen to make the adhesive layer. The filler of a filled adhesive is typically a granular substance such as a fine powder. The purpose of using a filled adhesive is to increase the thickness of the adhesive layer  3 . Increasing the thickness of the adhesive layer  3  produces a higher dielectric breakdown voltage of the adhesive layer  3 . Since the breakdown voltage is higher, there is less likelihood of electrical leakage across the adhesive layer  3 . 
     Although using a filled adhesive produces a higher dielectric breakdown voltage, the application of only one coat of filled adhesive does not provide a high enough dielectric breakdown voltage in the adhesive layer to avoid electrical shorts and electrical leakage. 
     Conventionally, to further improve insulation, two coats of the filled adhesive are applied to produce the adhesive layer  3 . FIG. 2 is a perspective side view of a section  9  of the strain gage assembly  1  of FIG. 1 that shows in further detail the adhesive layer  3 . The adhesive layer  3  includes a pre-coat  5  and a gage coat  6 . During manufacturing, the pre-coat  5  of filled adhesive is applied to the mechanical sensor  2  and allowed time to cure. Next, the gage coat  6  of filled adhesive is applied to the pre-coat  5 , the strain gage  4  is pressed against the gage coat  6 , and the gage coat  6  is allowed time to cure. Adding the extra coat assures that the adhesive layer  3  is sufficiently thick to void electrical shorts and electrical leakage. Typically, the pre-coat  5  and gage coat  6  are of the same or similar adhesive material. The thickness of the adhesive layer  3  is represented in FIG. 2 by t a . 
     A conventional strain gage assembly  1  of FIG. 2 that uses a strain gage  3  of semiconductive material has an adhesive layer  3  with a thickness t a  of approximately 1.0 mil (25.4 μm). The dielectric strength of a material is the voltage potential at which dielectric breakdown will occur per unit length of the material. For filled adhesives typically used for the strain gage assembly  3  of FIG. 2, the dielectric breakdown of the filled adhesive is approximately 250 Volts per mil (250 V/mil; 9.84 V/μm). Therefore, the typical dielectric breakdown voltage of the adhesive layer  3  is approximately 250 Volts (250 V/mil×1.0 mil). 
     Using a filled adhesive and adding a second coat of adhesive increases the thickness of the adhesive layer  3 . Although increasing the thickness produces a higher dielectric breakdown of the adhesive layer  3 , as the thickness of the adhesive layer  3  increases, mechanical performance can decrease. Furthermore, the fillers of filled adhesives can reduce the strength of the adhesive. The filler within the adhesive may have inconsistent granule size and this can make it more difficult for the adhesive layer  3  to bond the strain gage  4  to the mechanical member  2 , create high stress points in the strain gage  4 , and introduce possible voids between the strain gage  4  and the adhesive layer  3 . 
     When choosing an adhesive, one wants an adhesive with the best combination of performance parameters, for example, highest strength, highest dielectric breakdown, and broadest temperature range. The need to use a filled adhesive, however, limits the choices of adhesives for use in the adhesive layer  3  of a strain gage assembly  1 . Furthermore, as discussed above, as thickness of the adhesive layer  3  increases, mechanical performance of the adhesive layer  3  can decrease. Consequently, when choosing an adhesive, tradeoffs are made between the adhesive&#39;s strength, temperature range, and dielectric breakdown. 
     Therefore, the benefits of increasing the thickness of the adhesive layer  3  must be weighed against the drawbacks caused by such an increase. This results in a tradeoff between the electrical insulation provided by the adhesive layer  3  and the mechanical performance of the adhesive layer  3 . The filled adhesive, the amount and granule sizes of the filler in the filled adhesive, and the thickness of the adhesive layer  3  are chosen in light of these tradeoffs. Typically, the adhesive layer  3  used in the strain gage assembly  1  of FIG. 2 has a shear strength of approximately 3,000 p.s.i. and an operating temperature range from approximately −60° F. to 250° F. 
     Thus, for the strain gage assembly  1  where the strain gage  4  is made of semiconductive material, it is desirable to eliminate the need for a filled adhesive and a pre-coat  5  in order to improve the mechanical performance of the adhesive layer  3 , while at the same time providing sufficient electrical insulation between a strain gage  4  and the mechanical sensor  2 . Furthermore, it is desirable to eliminate the need for the pre-coat  5  to save time and labor costs associated with the extra step of applying the pre-coat  5 . 
     A conventional technique for manufacturing the strain gage  4  of semiconductive material involves mechanically or chemically cutting a small bar of semiconductor material into the appropriate shape. A diamond saw is often used for initial cutting, which results in a rough cut which must be refined by further mechanical or chemical means. Chemical cutting or shaping may involve dipping the cut pieces into a chemical pool or similar methods. Typically, several steps are required to refine the initial rough cut of the semiconductive material into the final size that also meets electrical requirements. These manual cutting and refining processes are inefficient and imprecise in comparison to the automated processes used in today&#39;s technologies. 
     Extracting the finished strain gages from the semiconductor bar is a costly and time consuming process. Extracting is commonly done manually, which may involve a person extracting the finished gages with the aid of tweezers and a magnifying device. The labor costs and inherent human error associated with this manual extraction process introduce more cost and inefficiency to the manufacturing of conventional strain gages. 
     For measuring the resistance of the strain gage  4 , wires for electrical connection may be attached directly to the semiconductive material. Alternatively, contact pads may be manufactured and affixed to the strain gage as part of the gage-making process, with the wires then connected to the contact pads. Although connecting the wires directly involves fewer manufacturing steps than using contact pads, it is more difficult and costly to connect directly to silicon than to connect to a contact pad, and contact pads provide a more reliable electrical contact to the semiconductive material. 
     It is desirable to reduce the imprecision and costs associated with the conventional manual processes described above for manufacturing and extracting the strain gage  4 . 
     With a mechanical sensor  2  having a thickness of approximately 0.010 in (254 μm), the conventional strain gage assembly  1  manufactured using the above techniques uses a strain gage  4  with thickness of approximately 0.0005 in (12.7 μm). As described above, a typical adhesive layer  3  has a thickness of approximately 0.0010 in (25.4 μm). Therefore, for a conventional strain gage assembly  1 , the combined thickness of the strain gage  4  and the adhesive layer  3  is approximately 0.0015 in (38.1 μm) 
     It is desirable to reduce the thickness of the strain gage  4  and the adhesive layer  3  so as to improve the mechanical performance of the strain gage assembly  1 . 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention is an insulated strain gage that includes an insulating layer, where the insulated strain gage is manufactured using conventional semiconductor manufacturing techniques. 
     One embodiment of the invention is an insulated strain gage comprising a layer of semiconductive material and a layer of insulating material, where a side of the first insulating layer is adjacent to a side of the semiconductive layer. 
     Another embodiment of the invention is an apparatus for measuring the strain on an object by translating deformations of the object resulting from an applied force into electrical signals, where the apparatus comprises a mechanical sensor, at least one insulated strain gage, and a circuit. The insulated strain gage includes an insulating layer and is bonded to the mechanical sensor. The circuit is connected to the insulated strain gage to receive signals indicating an electrical value of the insulated strain gage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     It should be understood that the drawings are provided for the purpose of illustration only and are not intended to define the limits of the invention. The foregoing and other objects and advantages of the present invention will become apparent with reference to the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side view of a conventional strain gage assembly; 
     FIG. 2 is a perspective side view of a portion of the conventional strain gage assembly of FIG. 1; 
     FIG. 3 is a perspective side view of an embodiment of an insulated strain gage according to the invention; 
     FIG. 4 a  is a perspective side view of another embodiment of an insulated strain gage according to the invention; 
     FIG. 4 b  is a cross-sectional side view of the insulated strain gage of FIG. 4 a ; 
     FIG. 5 is a side view of a strain gage assembly including the insulated strain gage of FIGS.  4   a  and  4   b  and an adhesive layer; 
     FIG. 6 a  is a side view of a strain gage assembly according to the invention at rest; 
     FIG. 6 b  is a side view of a strain gage assembly according to the invention under strain; and 
     FIG. 7 is a schematic diagram illustrating a Wheatstone Bridge circuit with two insulated strain gages coupled to an amplifier circuit according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3 illustrates an embodiment of an insulated strain gage  22  according to the invention. The insulated strain gage  22  includes an insulating layer  32  of insulating material adjacent to and in contact with a layer  33  of semiconductive material. Within this disclosure the term insulated strain gage means a monolithically integrated combination of a layer of semiconductive material disposed on an insulating substrate. 
     In the embodiment of FIG. 3, the insulating material may comprise SiO 2 , but other insulating materials known by those skilled in the art to have the same or similar physical and insulating properties adequate for use in an insulated strain gage may be used. Also, in the embodiment of FIG. 3, insulating layer  32  has a thickness t il  of approximately 2.0 μm and a dielectric strength of approximately 500 V/μm. These parameters produce a breakdown voltage of approximately 1000 V. The insulating layer  32  can serve as an integral insulator for the insulated strain gage  22  when the insulated strain gage  22  is part of a strain gage assembly as will be described in more detail below. 
     In the embodiment of FIG. 3, the semiconductive material is silicon because the electrical properties of silicon are well known to those of skill in the art, and because conventional fabrication techniques have developed around the use of silicon. Alternatively, germanium or some other suitable semiconductive material known to those of skill in the art may be used as the semiconductive material. In the embodiment of FIG. 3, semiconductive layer  33  has a thickness t s  of approximately 2.0 μm. Thus, in the embodiment of FIG. 3, the thickness of the insulated strain gage  22  is the combined thickness of the insulating layer  32  and the semiconductive layer  33 , which is approximately 4.0 μm. Alternatively, semiconductive layer  33  could be thinner than 2.0 μm. For example, layer  33  could have a thickness t s  of approximately 1.0 μm. At this thickness, however, more careful calibration of semiconductor fabricating machines may be required so as to have more precise machine tolerances. For example, a typical semiconductor fabricating machine tolerance of ±0.5 μm would be 50% of 1.0 μm, and a more careful calibration of the machine would probably be desired. 
     In the embodiment of FIG. 3, semiconductive layer  33  is doped with Boron ions. Alternatively, other impurities of a p-type conductivity may be used as the dopant. Also, n-type dopants may be used depending on the desired polarities, resistive, and thermal properties of semiconductive layer  33 , as will be understood by those of skill in the art of semiconductor manufacturing. The quantity of impurity used will impact the resistivity, or conversely the conductivity, of the semiconductive layer. In the embodiment of FIG. 3, the semiconductive material is doped with a quantity of impurities that produce a resistivity of approximately 0.12 Ω-cm, although the amount of doping may be varied according to the desired electrical characteristics for the insulated strain gage  22 . 
     FIGS. 4 a  and  4   b  illustrate an embodiment of an insulated strain gage  23  according to the invention. FIG. 4 b  illustrates a cross-sectional side view along line a—a of FIG. 4 a . Insulated strain gage  23  includes the insulated strain gage  22  with the addition of another insulating layer  30  of insulating material and contact pads  31  of conducting material. As can be seen in FIGS. 4 a  and  4   b , insulating layer  30  is adjacent to the sides and the top of semiconductive layer  33 . Further, contact pads  31  extend from the top side of insulating layer  30  to a bottom side of insulating layer  30  where contact pads  31  contact semiconductive layer  33 . 
     In the embodiment of FIG. 4 b , insulating layer  30  comprises SiO 2 , which is the same material as insulating layer  32 , but other insulating materials known by those skilled in the art to have the same or similar physical and insulating properties adequate for use in an insulated strain gage  23  may be used. Also, in the embodiment of FIG. 4 b , insulating layer  32  has both a top side thickness t 12  and lateral side thickness t 13  of approximately 2.0 μm. Thus, in the embodiment of FIG. 4 b , the thickness of the insulated strain gage  23  is the combined thickness of insulating layer  32 , semiconductive layer  33 , and insulating layer  30 , which is approximately 6.0 μm. 
     In the embodiment of FIG. 4 a , the conducting material of contact pads  31  is aluminum. Alternatively, other conducting materials, particularly metals, with suitably high electrical conductivity and thermal resistance may be used, as is well known to those of skill in the art of semiconductor manufacturing. These contact pads  31  can be used to connect the insulated strain gage  23  to an electrical circuit with, for example, wire bonds. 
     The resistance of the insulated strain gage  23  is r=(σ×1 a )/(w×t), where σ is the resistivity of the semiconductive layer  33 ,  1   a  is the active length of the semiconductive layer  33 , w is the width of the semiconductive layer  33 , and t s  is the thickness of the semiconductive layer  33 . The active length is the length of the semiconductive layer  33  between the contact pads  31 . In the embodiment of FIG. 4 a  the semiconductive layer  33  has a resistance of 5,000 Ω, thickness t s =2.0 μm, and resistivity σ of 0.12 Ω-cm, where the width w and the active length 1 a  of the semiconductive layer  33  can be designed to achieve the resistance of 5000 Ω. Of course, the manufacturing process can be altered to change any of these parameters to produce an insulated strain gage having a desired size and shape as well as desired electrical and mechanical properties. 
     One aspect of the invention is that conventional semiconductor manufacturing techniques can be used to make the insulated strain gage  22  or  23 . Specifically, conventional methods known to those of skill in the art of semiconductor manufacturing can be used to form the insulating layers  30  and  32  and the semiconductive layer  33 , dope the semiconductive layer  33 , etch openings in insulating layer  30 , and fill the openings with conducting material to form contact pads  31 . 
     The precision of an automated semiconductor fabrication process produces very even and precise insulation layers  32  and  30  and semiconductive layer  33  in comparison to the manual processes of conventional strain gage manufacturing used to produce the conventional strain gage  4  and adhesive layer  3 . Furthermore, using automated semiconductor fabrication reduces the cost of making a strain gage by eliminating conventional manufacturing labor costs associated with manually refining a silicon block and extracting individual strain gages from the block. 
     FIG. 5 illustrates an embodiment of a portion of a strain gage assembly  20  including the insulated strain gage  23 , or, alternatively, insulated strain gage  22  according to the invention. Strain gage assembly  20  includes a mechanical sensor  2  that is the same or similar to the mechanical sensor  2  of the conventional strain gage assembly  1  illustrated in FIG.  1 . The strain gage assembly  20  also includes an adhesive layer  21  of adhesive and the insulated strain gage  23 . The integral insulating layer  32  of the insulated strain gage  23  produces a strain gage assembly  20  with several advantages over the conventional strain gage assembly  1  of FIG. 1, as will be discussed below. 
     In the strain gage assembly  20 , the insulating layer  32  insulates the insulated strain gage  23  from the mechanical member  2 . As discussed above, insulating layer  32  has a breakdown voltage of approximately 1000 V. This is an improvement over the breakdown voltage of 500 V of the adhesive layer  3 , which provided insulation for the conventional strain gage assembly  1 . 
     In the conventional strain gage assembly  1 , the adhesive layer  3  insulated the strain gage  4  from the mechanical member  2 . According to the invention, however, the insulating layer  32  insulates the semiconductive layer  33  of the insulated strain gage  23  from the mechanical member  2 . Consequently, the adhesive layer  21  no longer needs to provide insulation. Therefore, the adhesive layer  21  may be chosen solely for its mechanical performance, without concern for its insulating properties. Further, the high breakdown voltage of the insulator  32  eliminates the need to use a filled adhesive to increase the breakdown voltage of the adhesive layer  21 , which, as discussed above, impaired the bonding properties and consistency of the adhesive layer  3 . Thus, the chosen adhesive for adhesive layer  21  may be an unfilled adhesive, thereby improving the bonding properties of the adhesive layer  21 . 
     Also, because of the high breakdown voltage and even application of the insulating layer  32 , a pre-coat is no longer needed to prevent electrical shorts and electrical leaks. Since, as discussed above, the pre-coat  5  further impairs the mechanical performance of the adhesive layer  3 , eliminating the need for the pre-coat  5  further improves the mechanical performance of the adhesive layer  21 . 
     Therefore, being able to choose a non-insulating, unfilled adhesive and not needing a pre-coat results in an adhesive layer  21  with improved strength over the adhesive layer  3  of the conventional strain gage assembly  1 . In the embodiment of FIG. 5, the adhesive layer  21  has a shear strength between 3,000 and 5,000 p.s.i. and has an operating temperature range from approximately −60° F. to 257° F. 
     The ability to use a non-insulating, unfilled adhesive substantially reduces the thickness of the adhesive layer  21 . In the embodiment of FIG. 5, the adhesive layer  21  has a thickness of approximately 6.0 μm, as opposed to a thickness of approximately 25.4 μm for the adhesive layer  3  of the conventional strain gage assembly  1 . Also in the preferred embodiment of FIG. 5, the insulated semiconductor strain gage  22  or  23  has a thickness of approximately 4.0 μm or 6.0 μm, respectively. Thus, the combined thickness of the insulated strain gage  22  or  23  and the adhesive layer  21  of approximately 10.0 μm or approximately 12.0 μm, respectively, represents over a 66% reduction from the combined thickness of the conventional strain gage  4  and adhesive layer  3  of 38.1 μm. This reduced thickness results in an improved mechanical performance of the strain gage assembly  20 . 
     The insulated strain gage  22  or  23  can be used as a part of variety of devices known to those of skill in the art, such as force cells, loads cells, pressure transducers and accelerometers. Combinations of insulated strain gages  22  and  23  can be used in combination with electrical circuitry to measure forces acting on a mechanical device. 
     FIGS. 6 a  and  6   b  illustrate an embodiment of a pair of insulated strain gages  41  and  42  being used in combination with a mechanical sensor or beam  40 . The strain gages  41  and  42  each may be either insulated semiconductor strain gage  22  or  23 . FIG. 6 a  shows the combination at rest. FIG.  6   b  shows a force F applied to beam  40 . Force F causes beam  40  to bend causing a strain on beam  40 . This bend causes strain gage  42  to stretch or be in tension, and causes strain gage  41  to compress or be in compression. For this reason, strain gage  41  is called the compression gage and strain gage  42  is called the tension gage. 
     In the embodiment of FIG. 6 a , two wire bounds  41 A and  41 B are each connected to an end of compression gage  41 , and two wire bounds  42 A and  42 B are each connected to an end of tension gage  42 . If the insulated strain gage  41  or  42  is of the embodiment of insulated strain gage  23 , the wire bounds connect to contact pads  43 . The signals on the wire bounds indicate the change in resistance across each strain gage. In FIG. 6 a , the resistance across compression gage  41  will decrease and the resistance across tension gage  42  will increase. These wire bounds can be a part of the circuitry for calculating the strain on beam  40  as a function of the change in resistance. 
     FIG. 7 illustrates one example of such circuitry that includes a Wheatstone Bridge configuration  50 . Wheatstone bridges are ideal for accurately measuring small changes in resistance. In the first embodiment, strain gage  41  and  42  are connected in series with respect to an input potential Vi and in parallel with respect to an output potential Vo. An excitation signal produces an input voltage Vi of the order of several volts. This configuration could be part of a pressure transducer or other such mechanical-electrical translating devices. 
     As discussed above in the embodiment of FIG. 4, the resistance of the insulated strain gage  23 , or, alternatively, the insulated strain gage  22 , is approximately 5,000 Ω. Thus, the resistance of each of the strain gages  41  and  42  is approximately 5,000 Ω. With such a high impedance and an input potential of several volts, the resulting output potential Vo will be of the order of millivolts. In the embodiment of FIG. 7, amplification circuitry  51  is coupled to the output signal Vo to produce an output potential Va of the desired range. 
     The use of conventional semiconductor wafer technology makes the insulated strain gage  23  especially useful within integrated circuits, for example, an application specific integrated circuit (ASIC). Accordingly, the circuit of FIG. 7 can be manufactured as an ASIC. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.