Abstract:
A generally flexible strain gage comprising a strain sensing element, and a generally flexible substrate supporting the strain sensing element. The strain sensing element is made of single crystal or polycrystalline semiconducting material. The invention also includes a method for forming a generally flexible strain gage comprising the step of selecting a wafer having a portion of a base material and portion of a single crystal or polycrystalline semiconducting material located thereon. The method further comprises the steps of etching a strain sensing element out of the semiconducting material and forming a generally flexible substrate onto said sensing element.

Description:
The present application claims priority to U.S. Provisional Application No. 60/094,358 filed Jul. 28, 1998 and to U.S. Provisional Application No. 60/105,250 filed Oct. 22, 1998. 
    
    
     This invention was made with U.S. Government support under Contract No. N66001-97-C-8614; Contract No. DAAL 01-97-C-0054; and Contract No. DAAL 01-98-C-0036. The U.S. Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to the strain gage arts. It finds particular application to a generally flexible strain gage having a semiconducting stain sensing element and will be described with particular reference thereto. 
     BACKGROUND OF THE INVENTION 
     Strain gages are often used to sense strain in a subject material. The strain gage has a strain sensing element that is attached or adhered to the subject material. When the subject material is strained, the resistance of the sensing element changes in proportion to the strain it experiences. The change in resistance in the sensing element as it is compressed or elongated is measured and used to calculate strain in the subject material. Foil strain gages, which have a metal sensing element, are often used to measure strain. However, metal sensing elements have a relatively low gage factor, which reduces the sensitivity of the gage. The use of a semiconductor material, such as doped single crystal silicon or polycrystalline silicon, as the sensing element increases the gage factor of the strain gage dramatically. However, because most semiconductor materials are generally fragile, a semiconducting sensing element is prone to fracture. In order to protect the semiconducting strain sensing element, it is typically mounted upon a rigid backing to provide support (termed a “backed” gage). This rigid backing prohibits use of the gage on curved surfaces. Alternately, an unbacked gage may be used wherein the sensing element is directly adhered to the material. However, unbacked gages are difficult to mount, and the sensing element is exposed and thus still prone to fracture. The unbacked gages are too fragile to mount on curved surfaces. Accordingly, there is a need for a strain gage having a semiconducting strain sensing element, wherein the strain gage has a generally flexible substrate and/or sensing element to provide ease of handling and enable the gage to be used on curved or irregular surfaces. 
     Many difficulties arise when trying to form a single crystal semiconducting material or polycrystalline material on a flexible substrate, such as polyimide. It is known that amorphous silicon may be adhered to a flexible substrate using glow-discharge decomposition, but this process can not be used with other forms of silicon. Instead, single crystal or polycrystalline silicon must be deposited or grown by different methods, such as epitaxial growth. However, epitaxial growth requires temperatures of around 950° C., and a polyimide substrate decomposes around 550-580° C. Polycrystalline silicon deposition also occurs at temperatures above 500° C. Thus, epitaxial growth is not feasible for use with polyimide substrates. Furthermore, epitaxial growth can only deposit silicon on an already-existing layer of single crystal silicon, and therefore this process cannot be used to deposit single crystal or polycrystalline silicon on a flexible substrate. 
     The present invention is also directed to a method of manufacturing a sensor, and more particularly, to a method of manufacturing a generally flexible strain gage. When manufacturing a backed sensor, generally the sensor or sensing element is oriented on the substrate or backing, and the sensor is then fixed to the substrate. This process requires high precision instruments or a trained individual to locate the sensing element in the desired location and orientation. The sensing element is quite difficult to handle due to its brittleness and small size. Accordingly, there is a need for a method of forming a sensor which minimizes or avoids having to handle, locate or attach the sensing element. 
     SUMMARY OF THE INVENTION 
     The present invention is a generally flexible strain gage incorporating a relatively thin semiconducting sensing element mounted to a generally flexible substrate. The strain gage of the present invention has a flexible backing, which makes it easier to mount and enables the gage to be conformed to curved surfaces. The strain sensing element is thin enough to be flexible, and is made from a semiconducting material, such as doped single crystal silicon, which provides a high gage factor relative metal foil, or amorphous silicon as in Uchida U.S. Pat. No.; 4,658,233. In a preferred embodiment, the invention is a generally flexible strain gage comprising a semiconducting single crystal strain sensing element or a semiconducting polycrystalline strain sensing element, and a generally flexible substrate supporting the strain sensing element. 
     The present invention is also directed to a method for manufacturing a sensor or sensing element mounted on a flexible substrate. In the present method, the sensing element is formed on a wafer, and the flexible substrate is then formed about the sensing element. Once the flexible substrate is cured, the wafer is etched to a precise depth to remove the bulk of the silicon substrate and expose the sensing element. The etch is preferably performed using dry etching techniques, since wet etching can damage sensing elements on the unetched side of the wafer. Furthermore, Reactive Ion Etching (RIE), and preferably Deep Reactive Ion Etching (DRIE) is the dry etch process of choice because it offers high etch rates and high selectivity to etch stop materials. 
     Because the sensing element is formed directly on the wafer, the sensing element is anchored in the desired location. The substrate can then be formed about the sensing element. In this manner, the sensing element also need not be directly handled or located. In a preferred embodiment, the invention is a method for forming a generally flexible strain gage comprising the step of selecting a wafer having a portion of a base material and portion of a single crystal semiconducting material or polycrystalline semiconducting material located thereon. The method further comprises the steps of etching a strain sensing element out of the semiconducting material and forming a generally flexible substrate onto said sensing element. 
     Other features and advantages of the present invention will be apparent from the following description, with reference to the accompanying drawings and claims, which form a part of the specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a top view of one embodiment of the strain gage of the present invention; 
     FIG. 2 is a top view of an alternate embodiment of the strain gage of the present invention; 
     FIG. 3 is a cross sectional side view of the strain gage of FIG. 2; 
     FIGS. 4-12 are cross sectional side views showing the steps of the preferred method for forming the strain gage of FIG. 2; 
     FIG. 13 is a cross sectional side view showing the strain gage of FIG. 12 attached to subject material; 
     FIG. 14 is a top view of an array of strain gages; 
     FIG. 15 is a top view of an array of strain gages connected to a processing chip; and 
     FIGS. 16-27 are cross sectional side views showing the steps of a method for forming an alternate embodiment of the strain gage of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the present invention is a strain gage  10  including a strain sensing element  12 . The strain sensing element  12  may be any suitable semiconducting material, such as single crystal silicon, doped single crystal silicon, germanium, amorphous silicon, polycrystalline silicon, and the like. However, single crystal semiconducting materials are preferred materials for the sensing element  12 . The strain sensing element  12  includes a first end  14  and a second end  16 , and the sensing element  12  is preferably thin enough to be generally flexible. The first end  14  is connected to a first electrical lead  18  that couples the first end  14  to the first metal output pad  20 . Similarly, the second end  16  is electrically connected to the second output pad  24  by the second lead  22 . In a preferred embodiment, each output pad  20 ,  24  is formed unitarily with its associated lead  18 ,  22 . The output pads  20 ,  24  and leads  18 ,  22  may be any electrically conductive material, preferably aluminum, nickel or copper. 
     An alternate embodiment of the strain gage  10  is shown in FIG.  2  and includes generally the same arrangement of the sensing element  12 , leads  18 ,  22 , output pads  20 ,  24 , and substrate  26 . The output pads  20 ,  24  are on opposing sides of the sensing element  12 . In both FIG.  1  and FIG. 2, the sensing element  12 , leads  18 ,  22  and output pads  20 ,  24  are carried on a generally flexible substrate  26 , such as polyimide. An oxide layer  28  is carried on top of the substrate  26 , and is located on top of the sensing element  12 , leads  18 ,  22 , and substrate  26 . In a preferred embodiment the output pads  20 ,  24  extend upwardly through the oxide layer  28 , and the leads  18 ,  22  and sensing element  12  are “submerged” below the oxide layer  28 . FIG. 3 illustrates the oxide layer  28  that is located on top of the sensing element  12 , leads  18 ,  22  and substrate  26 . 
     A preferred method for forming the strain gage  10  of the present invention is shown in FIGS. 4-12. However, it should be understood that the gage of the present invention may be formed by other, alternate methods. Furthermore, the method described below may be used for manufacturing a wide variety of sensors, and is not limited to the specific type of strain gage sensor described below. For example, the sensors manufactured with the process may include, but are not limited to, heat flux sensors, pressure sensors, accelerometers, temperature sensors, rate sensors, gas sensors, and flow rate sensors. 
     The process is begun with a wafer  30  which includes a base material  32 , oxide layer  34  and doped silicon layer  36  located on top of the oxide  34 . The base material  32  is preferably single crystal silicon or polysilicon, and the oxide  34  is preferably silicon dioxide. The base material  32  has a thickness sufficient to lend strength and stiffness to the wafer  30 , which facilitates handling the wafer  30 . If a flexible sensor is desired, the thickness of the doped silicon layer  36  is less than 20 microns, preferably between about 5 and 20 microns, and further preferably between about 7 and 10 microns. Of course, the thickness can be less than 5 microns if desired, depending on its application. In the described embodiment, the silicon layer  36  will be formed as a strain sensing element, so the silicon layer  36  is doped to the desired sheet resistance, typically ranging from about 10 ohms per square to about 1000 ohms per square and above, depending upon the desired resistance of the gage. These sheet resistances will yield a sensing element having a resistance from about 100 to 10,000 ohms, depending upon the shape and thickness of the sensing element. The silicon layer  36  may be p-type, n-type, or intrinsic. 
     It should be understood that the doped silicon layer  36  may be replaced with any material from which it is desired to form a sensor. The silicon layer  36  may also be patterned or formed into a sensor other than a strain gage. For example, if a gas sensor is to be formed, the silicon layer  36  is etched to form a comb-finger capacitor. Furthermore, when forming a strain gage, the silicon layer  36  may be replaced with other suitable materials, such as germanium or amorphous silicon, to form a strain gage having a strain sensing element formed from such materials. 
     Once the wafer  30  is selected, a mask  38  is placed on top of the doped silicon layer  36 . The strain sensing element  12  is then formed using standard photolithography and etching which removes the undesired portion of the silicon layer  36 , and leaves behind the desired portion of the layer  36  as the strain sensing element  12 . (see FIG. 5) If a sensor or sensing element other than a strain gage is to be manufactured, it is formed in place of the strain sensing element  12 . This formation of the sensor may involve multiple steps and treatments to the layer  36  beyond those specifically discussed herein. Additionally, multiple layers of materials may be used in place of the single layer  36 . The formation of the sensor may also involve the addition of other materials and/or additional dry or wet etching. 
     Returning to the illustrated example, FIG. 5 shows the wafer  30  after the strain sensing element  12  has been formed. Next, as shown in FIG. 6, a mask pattern  31  is placed on top of the sensing element  12  and the oxide layer  34 . A pair of windows  44  are formed on either side of the strain sensing element  12 . The windows  44  are cutouts in the mask pattern  31  that leave portions of the oxide layer  34  exposed. A second pair of windows  46  are located on the first end  14  and second end  16  of the strain sensing element  12 , and the windows  46  leave the ends  14 ,  16  exposed. During the next step, the oxide layer  34  is etched through the windows  44 . The oxide layer  34  is etched through a portion of its thickness to form indention  48  (FIG.  7 ). Next, the first end  14  and second end  16  of the sensing element  12  are exposed to a means of doping, such as diffusion or implant, to further dope the ends  14 ,  16  of the sensing element  12 . The additionally doped areas of the sensing element  12  are illustrated with different shading in the accompanying figures. This additional doping of the strain sensing element  12  may be practiced to improve the conductivity between the sensing element  12  and the leads  18 ,  22 , but is not essential to the invention. The additional doping step may be desired if n-type silicon is being used for the strain sensing element  12 . 
     Next, as shown in FIG. 8, a layer of metal  50  is deposited on top of the oxide layer  34  and the sensing element  12 , and the metal layer  50  fills in the indentions  48 . The metal  50  will eventually form the leads  18 ,  22  and output pads  20 ,  24 . The metal is preferably aluminum, and is sp uttered onto the wafer  30 . However, nearly any desired method of forming the leads and connecting pads may be utilized. As shown in FIG. 9, a mask  52  is subsequently located on top of the metal layer  50  to protect those portions of the metal layer  50  which are not to be removed in the next processing step. 
     As the next step, the areas of the metal layer  50  that are not covered by the mask  52  are removed by etching, leaving behind the output pads  20 ,  24  and the leads  18 ,  22  (FIG.  10 ). Finally, the generally flexible substrate  26  is spun on the oxide layer  34 , pads  20 ,  24 , leads  18 ,  22  and sensing element  12  (FIG.  11 ). The flexible substrate  26  is preferably polyimide. Once the polyimide is cured, the base material  32  is removed, and the oxide layer  34  is etched to a sufficient depth until the pads  20 ,  24  are exposed (FIG.  12 ). The base material  32  is preferably removed by deep reactive ion etching (DRIE), and the oxide layer  34  is preferably dry etched, exposing the output pads for contacts. Plating can be used to increase metal thickness on the output pads. After it is manufactured, the gage can be mounted to a test specimen  56  with strain gage adhesive, and the output pads  20 ,  24  provide a surface upon which wires  58 ,  60  may be bonded or soldered (FIG.  13 ). The output wires  58 ,  60  couple the strain gage  10  to an electronic component or components that may calculate the strain measured by the strain gage  10 , or further process the output signal. Alternately, the output wires  58 ,  60  may be connected directly to the ends  14 ,  16  of the strain sensing element  12 , and the leads  18 ,  22  and output pads  20 ,  24  may not be formed on the strain gage  10 . The output pads merely provide a surface to improve the convenience of electrically connecting the output wires  58 ,  60  to the ends  14 ,  16  of the strain sensing element  12 . 
     As can be seen, using the method of the present invention the sensing element  12  is first formed on the wafer  30 . After the pads  20 ,  24  and leads  18 ,  22  are formed, the substrate  26  is formed about the components of the strain gage  10 . Finally, the bulk portion of the wafer  30  is removed, preferably by etching, leaving sensor or strain gage on the substrate  26 . In this manner, the strain sensing element  12  need not be handled and/or located on the substrate  26 , but the substrate  26  is in stead formed about the strain sensing element  12 . This enables easier, faster, and more consistent manufacturing of strain gages. 
     Although the preferred method of forming the strain gage of the present invention has been described, many other methods may be used to fabricate the flexible sensors of the present invention. For example, a semiconducting strain sensing element m ay be directly placed upon a substrate, in stead of forming a substrate about the strain sensing element. The ends of the strain sensing element may then be connected to a pair of output pads or directly connected to the output wires. 
     The strain gage of the present invention may be formed on a wafer using batch procedures wherein a plurality of strain gages are formed upon a single sheet of flexible material. After manufacturing the gages, each gage may be trimmed from the other surrounding gages for use. Furthermore, a plurality of strain gages may be formed on a wafer or substrate, and some or all of the gages may be electrically connected to an output area or common component. For example, a strain gage array  66  is shown in FIG.  14 . The strain gage array  66  illustrated therein includes a plurality of strain gages  10 . The leads  18 ,  22  of each individual strain gage  10  may be patterned to terminate at a single location  68 . In this manner, the ends of all of the leads  18 ,  22  are collected in a common location  68  for ease of connecting to the output wires. In the alternate embodiment of the strain gage array  66 ′ shown in FIG. 15, the leads  18 ,  22  are commonly routed to an electronic component  70  on the substrate, such as a data acquisition IC. The component  70  may receive the leads  18 ,  22 , and process the signal provided by the leads  18 ,  22  of each of the individual strain gages  10 . For example, the electronic component  70  may provide amplification, bridge completion, interface electronics, A/D conversion, multiplexing, storage and/or telemetry operation. The electronic component  70  has a plurality of terminals  72  for providing data input/output (I/O) or power to the electronic component. By locating the electronic component  70  on the substrate, the array  66 ′ can perform processing steps, thereby making it much more powerful and adaptable than existing strain arrays. Furthermore, because the electronic component  70  is located close to the sensors  10 , the sensor output travels a minimal distance before it is processed. This reduces the loss of signal strength and minimizes exposure to interference. 
     The sensor arrays  66 ,  66 ′ may be configured such that they are custom manufactured for use upon a known specimen. For example, the flexible substrate may be sized to fit around the specimen&#39;s surface, and the strain gages may be manufactured such that they are located in precise, desired locations that correspond to areas of particular interest to be measured on the specimen when the substrate is fitted onto the specimen. In this manner, when the flexible sheet is mounted to the specimen, the strain gages can measure the strain the specimen experiences at its critical points. Again, the leads of the various strain gages, other sensors and/or electronic component(s) may all run to a single location which is convenient for bonding, or to data acquisition, storage, and/or telemetry circuitry located on the flexible sheet. 
     A method for forming a sensor having microelectronic circuitry monolithically integrated thereon is shown in FIGS. 16-27. Although only a single microelectronic component and a single sensor are shown, a plurality of these components and sensors may be incorporated into the finished product. The method described below and shown in FIGS. 16-27 is generally the same as the method described above in conjunction with FIGS. 4-12, with the primary difference being the addition of microelectronic circuitry into the finished product. As shown in FIG. 16, the process begins with a wafer  30  having a silicon layer  35 . The wafer  30  includes an oxide layer  34  and base material  32 . Next, as shown in FIG. 17, microelectronic circuitry  74  is fabricated into the wafer  30  by conventional methods. Alternately, a wafer  30  having microelectronic circuitry  74  already formed therein may be purchased from a vendor. The microelectronic circuitry  74  is preferably fabricated using any of a variety of technologies, including complimentary metal oxide semiconductor (CMOS), other metal oxide semiconductor technologies such as DMOS, UMOS, VMOS, LDMOS and the like, bipolar processes, hybrid technologies such as BiCMOS, or other technologies. The microelectronic circuitry  74  typically processes, conditions, or otherwise treats the output from the sensor and/or performs other functions required by the end user&#39;s system. 
     As shown in FIG. 18, the microelectronic circuitry  74  is next protected by a mask  76 , and the exposed silicon  35  is doped. Alternately, the step shown in FIG. 18 may be omitted, and the layer  35  may be doped before the circuitry  74  is fabricated in silicon layer  35 . Next, a mask  78  is located over the circuitry  74  and part of the layer  35  (FIG.  19 ). The exposed silicon layer  35  is then selectively removed, leaving behind the sensing element  12  shown in FIG. 20. A mask  80  is then placed over the circuitry  74 , sensing element  12  and part of the oxide layer  34  (FIG.  21 ). The exposed portion of the oxide layer  34  is then etched through part of its thickness to form indentations  48  (FIG.  22 ). 
     A layer of metal or other conductive material  82  is then deposited over the entire assembly (FIG.  23 ), and another mask  84  is located over the metal layer  82  (FIG.  24 ). The exposed portions of the metal layer  82  are removed, leaving behind the pads  86 ,  88 , as well as leads  90 ,  92 ,  94  (FIG.  25 ). Alternately, a “lift-off” technique, as know to those skilled in the art, may be employed to pattern the metal layer  82 . Finally, the flexible layer  26  is spun on the entire assembly (FIG. 26) and the base material  32  is removed, resulting in the structure shown in FIG.  27 . The resultant assembly  96  is a strain gage and microelectronic circuitry mounted on a flexible substrate. The assembly can be mounted to non-flat components, and the microelectronic circuitry  74  provides processing capability to the assembly  96 . Of course, other sensors besides strain gages may be formed using the above-described method. 
     The preferred form of the invention has been described above. However, with the present disclosure in mind it is believed that obvious alterations to the preferred embodiments, to achieve comparable features and advantages, will become apparent to those of ordinary skill in the art.