Patent Publication Number: US-2006006137-A1

Title: Micro-fabricated sensor

Description:
RELATED APPLICATION  
      This patent application claims the benefit of U.S. Provisional Application No. 60/541,864 filed on Feb. 3, 2004 pursuant to 35 USC § 119(e) and incorporates the provisional application by reference in its entirety. 
    
    
     FIELD OF INVENTION  
      This invention pertains generally to a sensor, and in particular to a micro-fabricated sensor.  
     BACKGROUND  
      Every year, various companies, governments, and individuals incur high cost due to corrosion of structures. For example, the health of structures such as aircraft, ships, bridges, automobiles, pipelines, and power line towers are subject to fatigue and failure through various means, including corrosion and stress related cracking. To counteract this cost, scheduled examination and maintenance on vehicles and structures are put in place. However, the examination and maintenance does not always guarantee structural integrity. For example, a structural failure could occur between two maintenance sessions because corrosion was not bad enough to alert the examiner at the time of the first maintenance but rapid corrosion took place after the examination. Another way to counteract the loss from corrosion is Condition Based Monitoring (CBM), which entails evaluating the extent to which a structure is corroded and approximating the time at which preventative measures need to be implemented. While CBM is generally more effective than regular scheduled maintenance, it has the problem of higher cost. Once the cost of CBM becomes closer to or exceeds the cost of damages caused by corrosion itself, implementing CBM is not cost-justified.  
      Thus, search still continues for a sensor device that is cost effective and lends itself to easy integration with monitoring or failure prediction systems. For easy integration with existing systems, the sensor device would have to include a means for reading and logging the sensor measurements and an interface to the rest of the system without becoming too large in size.  
     SUMMARY  
      In one aspect, the invention is a micro-fabricated sensor device useful for monitoring deterioration of a structure. The device includes a first electrode having a first finger and a second electrode having a second finger. The second finger is positioned about apart from the first finger by about 1 mm or less. The current flow between the first electrode and the second electrode correlates with a degree of deterioration of the electrodes.  
      In another aspect, the invention is a system for monitoring corrosion in a structure. The system includes a plurality of LPR sensors, an electronic controller, a multiplexing network, and electronic components attached to a polyimide flex circuit carrier. Each of the LPR sensors includes a working electrode and a reference electrode. The electronic controller is programmed to read measurements from each of the LPR sensors. The multiplexing network enables the controller to address each of the LPR sensors, and electronic components match the LPR sensors to the controller. The polyimide flex circuit carrier has passivated metal interconnects and bond pads onto which the LPR sensors, the electronic components, the electronic controller, and the multiplexing network are attached.  
      In yet another aspect, the invention is a method of preparing a sensor device. The method includes providing an electrically conductive material having a first surface and a second surface that are in substantially parallel planes with respect to each other. The first surface is photolithographically patterned with a first electrode having first fingers and a second electrode having second fingers, and the patterned electrodes are etched partway between the first surface and the second surface. The partly-etched electrically conductive material is mounted on a carrier with the first surface contacting the carrier and heated to be bonded. The second surface is photolithographically patterned with the first and the second electrodes that are aligned with the etched portions of the first surface, and the patterns are etched until the first and the second electrodes are formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a micro-fabricated linear polarization resistance (LPR) sensor in accordance with the invention.  
       FIG. 2  is a schematic illustration of a flex circuit carrier that may be coupled with the sensor of  FIG. 1 .  
       FIGS. 3A and 3B  are flow diagrams depicting a method of combining the sensor with the flex circuit carrier to produce the sensor device.  
       FIGS. 4A and 4B  are schematic illustrations of a sensor device that includes the sensor and a flex circuit carrier.  
       FIGS. 5 and 6  are DC and AC instrumentation amplifier circuits that may be used in the sensor device.  
       FIG. 7  shows an exemplary set of measurements for a single sensor sweep.  
       FIG. 8  illustrates the effective resistance of the sensor as a function of time exposed to a humidified atmosphere.  
       FIG. 9  is a diagram of a circuit for controlling the potential applied to the first and the sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Embodiments of the invention are described herein in the context of a dual-electrode corrosion sensor. However, it is to be understood that the embodiments provided herein are just preferred embodiments, and the scope of the invention is not limited to the applications or the embodiments disclosed herein. For example, the invention may be adapted for other types of corrosion sensors, or sensors that measure stress, humidity, pH, temperature, and chemical ion detection.  
      In one aspect, the present invention describes a miniature or micro corrosion sensor and its method of fabrication. Other aspects of the invention describe a system platform in which at least one or more corrosion sensors are integrated on a common substrate.  
      As a material corrodes and forms an oxide, the oxide creates an anodic cell. If the corroding material has two sections, a. potential and resistance can be measured between the two sections. The potential and resistance values can be used to compute the effective mass loss of the device. The device is most useful if the sensor material is matched to the properties of the structure that is being monitored, so that it is possible to establish a corrosion rate of the structure. To form a sensor device that can be manufactured cost effectively and can be easily integrated into monitoring systems, the two-section sensor is integrated with microfabricated peripheral electronics.  
       FIG. 1  is a schematic diagram of a micro-fabricated linear polarization resistance (LPR) sensor  10  in accordance with the invention. As shown, the LPR sensor has a first electrode  12  and a second electrode  14  that are patterned in an interlaced manner. The first electrode  12  has first fingers  16  and the second electrode  14  has second fingers  18  that are interlaced with the first fingers  16 . The first fingers  16  and the second fingers  18  do not touch each other, but are spaced apart by a gap d. The distance d between the first fingers  16  and the second fingers  18  determines the sensitivity of the sensor  10 . The thickness of the sensor  10  (which is measured into the page on  FIG. 1 ) determines how long the sensor  10  will be operational. The thicker the material, the more mass there is to corrode and the longer it will take for the sensor  10  to reach its operational limit.  
      To form an exemplary embodiment of the sensor  10 , shim stock of the particular material having a thickness less than 75 μm (e.g., 50 μm) is photolithographically patterned to form the interlaced electrode array. The widths (w 1 , w 2 ) of the first and second fingers  16 ,  18  may be in the range of 10-200 μm, with lengths (l) of 0.1-20 mm, although the invention is not limited to these dimensions. The gap (d) between the first fingers  16  and the second fingers  18  may be any size up to 1 mm, and can be selected according to the desired sensitivity level. In the exemplary embodiment, the gap d is set at 150 μm, the first fingers  16  have a width w 1  of 450 μm, and the second fingers  18  have a width w 2  of 150 μm. The overall sensor length L is 9.5 mm and the overall sensor width W is 20 mm in this exemplary embodiment. The sensor  10  is preferably made of the same material as the as the structure that is being monitored or a shim stock matched to the material of the structure (e.g., Aluminum 1020, stainless steel). More specifically, the sensor  10  may be made from the same material as the structure that is being monitored.  
      As already mentioned, the LPR sensor  10  is made of a material that corrodes, such as the material of the structure being monitored. Corrosion occurs when a metal or alloy (herein collectively referred to as “metal”) is exposed to a fluid of sufficient oxidizing power. At the interface between the metal and the fluid, metal ions escape from the metal surface, leaving a surplus of electrons. The excess electrons flow from the anodic sites on the metal surface to cathodic sites where they are consumed, creating a corrosion current. A corrosion current, thus, is a measure of the loss of the metal from the metal surface. The corrosion current (I corr ) can be calculated from the linear polarization resistance and used to estimate the corrosion rate. However, as the anodic and cathodic sites continually shift and change their positions, I corr  cannot be directly measured from the metal surface. Hence, a small potential drop (ΔE) is applied externally to induce a measurable current flow (ΔI) at the corroding surface. At a given value of ΔE, I corr  is directly proportional to the induced current ΔI, as shown by Equation 1:  
                 Δ   ⁢           ⁢   E       Δ   ⁢           ⁢   I       =         β   a     ×     β   b         2.303   ×     I   corr     ×     (       β   a     +     β   b       )                 (     Equation   ⁢           ⁢   1     )             
 
 In Equation 1, β a  and β b  are Tafel constants that can be obtained from a well-known Tafel plots for the system under consideration. 
 
      A potentiostat may be used to adjust the potential (ΔE) on the metal surface in a controlled manner so that the corresponding current values can be measured as a function of the potential. The relationship between ΔE and ΔI is linear at values of ΔE close to that of the equilibrium potential, which is assumed by the metal in the absence of any induced potential ΔE. The slope of this line has the value ΔE/ΔI and has the units of resistance. The slope is therefore called “polarization resistance.” The value of polarization resistance obtained from a potential sweep over a predetermined range can then be used to determine I corr  by using the relationship of Equation 1. Furthermore, the rate of corrosion (CR) may be calculated by using I corr  and the relationship of Equation 2:  
               C   ⁢           ⁢   R     =         I   corr     ×   k   ×   E   ⁢           ⁢   W       d   ×   A               (     Equation   ⁢           ⁢   2     )             
 
 where EW=equivalent weight of the material in grams/equivalent, 
          k=a constant,     d=density of the corroding material, and     A=area of the sample.        

      While the sensor  10  described in this invention is a micro-LPR sensor, other sensors that lend themselves to miniaturization (e.g., sensors for strain, pH, humidity, and temperature) may also be used in the sensor device described herein.  
       FIG. 2  is a schematic illustration of a flex circuit carrier  22  that may be coupled with the sensor  10 . The flex circuit  22  is a commercially available component such as a polymer/polyimide flexicircuit carrier (e.g., Flexi907 made by 3M) and provides a platform on which one or more of the sensors  10  is mounted. The flex circuit carrier  22  includes bond pads  21  for electronics that support the sensor  10 , such as electronics for interrogating the sensor  10  to acquire data, electronics for power management, electronics for wireless transmission, and a controller. These electronics are shown in  FIG. 4B  below. Furthermore, there are electrical leads in the flex circuit  22  that are embedded in the flexible polymer/polyimide substrate (e.g., a buffer circuit for each sensor  10 ). The electrical leads may be about 50-200 μm thick, and can be formed as a sheet or even as a roll of tape. The electrical leads are copper traces, for example, which are patterned to form routing interconnects between the system controller and the sensor array. The copper is electrodeposited to thicknesses of approximately 20 μm in order to provide very low resistance, thereby minimizing signal losses. As shown, the flex circuit carrier  22  has at least one bond pad  21  for mounting the sensor  10  or the supporting electronic components. The bond pad  21  of the flexi circuit carrier  22  provides a platform that is corrosion resistant and electrically nonconductive onto which one or more sensors  10  or electronic components can be mounted. The patterned leads are coated with another layer of polyimide or a silicone agent to electrically passivate the connection. However, the bond pads  21  remain uncoated.  
       FIGS. 3A and 3B  are flow diagrams  30 ,  40  depicting a method of combining the sensor  10  with the flex circuit carrier  22  to produce a sensor device. The sensor  10  may be made by first providing the sensor material (steps  31  and  41 ) and etching the desired pattern by using techniques such as chemical, electrochemical, laser, or other etching/machining techniques known in the art (steps  32  and  42 ). For example, the electrode pattern may be chemically etched from one side to approximately half way through the thickness of the shim stock material. The etching is done using ferric chloride, for example. Depending on the particular material making up the sensor, e.g., aluminum, nickel, stainless steel, copper, etc., other chemical etchants (e.g., various acids such as phosphoric, hydroflouric, nitric or hydrochloric acid) may be used instead of ferric chloride. The etched side is then cleaned of the photoresist (steps  33  and  43 ) and mounted face down onto a polymer or a polyimide carrier (steps  34  and  44 ). Bonding is achieved by heating the material such that the polyimide flows to surround the surfaces of the electrode pattern already etched (steps  35  and  45 ). An elevated pressure may be applied during the heating. Electrodes on the top (unetched) side of the shim stock material are then aligned and patterned photolithographically (steps  36  and  46 ). The electrodes are formed by completing the chemical etch the remainder of the way through the sensor material (steps  37  and  47 ).  
      This method depicted in the flow diagrams  30 ,  40  enables handling and further packaging of the sensor  10  by mounting it on a carrier. The polymer/polyimide coating that surrounds the fingers  16 ,  18  of the sensor  10  protects the sidewalls of the etched electrodes, such that the corrosion resistance remains linear over a greater range of material corrosion and is thus more accurate in predicting the amount of corrosion. Specific components of the system can be protected from the surrounding environment by encapsulation techniques, such as coating with a layer of silicon adhesive to hermetically seal and protect the electronics from the surrounding environment, while the sensor  10  is appropriately exposed to the same environment which the structure being monitored experiences. In fabricating the sensor device, care must be taken to expose only the top surface to the atmosphere, with a substantial portion of the side surfaces being encapsulated in the corrosion resistant material.  
      Alternatively, for certain materials, electroplating or vacuum depositing of predefined patterns will also form the sensor device.  
       FIG. 4A and 4B  are schematic illustrations of a sensor device  50 , which includes the sensor  10  and the flex circuit carrier  22 . The sensor  10  is directly attached to the flex circuit carrier  22  with the electrodes aligned and attached to the exposed bondpads  21  (see  FIG. 2 ).  FIG. 4B  illustrates how a plurality of sensors  10  can be multiplexed and connected to a microcontroller  52  (e.g., TI MSP 430). The microcontroller  52  interrogates the sensors  10  and collects readings. A wireless unit  54 , which is also coupled to the microcontroller  52 , handles power and transmits the collected readings to an appropriate data logger, for example via an antenna  56 . Although  FIG. 4B  depicts one possible embodiment of the sensor device  50 , different embodiments may be used as appropriate for particular applications. For example, in some embodiments, the connection between the sensor device  50  and the data logger may not be wireless. In other embodiments, there may be a memory coupled to the microcontroller  52  that locally logs the collected readings.  
      The flex circuit carrier  22  may be provided with an appropriate adhesive which allows the entire system to be directly attached to the structure being monitored.  
       FIG. 5  is a DC instrumentation amplifier circuit  60  that may be used in the sensor device  50 . The DC circuit  60  may be implemented with LPC660 op-amps, which provide high impedance and low input currents. In operation, a known current is supplied to the electrodes  12 ,  14  and the voltage developed across the electrodes  12 ,  14  is measured. The reverse current is then supplied and the voltage measured. These two voltage-current measurements give the resistance. Care is taken to ensure that ΔV from the rest potential does not exceed +/−20 mV. Various inductors and capacitors are used to reduce RF noise. The amount of noise that is picked up by the circuit  60  is further reduced by placing the LPR sensor  10  in direct contact with the flex circuit carrier  22  and encapsulating the circuit  60  in a polymer to ensure that it was not exposed to the corrosive environment.  
       FIG. 6  is an AC amplifier circuit  70  that may be used in the sensor device  20 . It may be preferable to use an AC current to excite the sensor  10  than to use a DC current because any reaction caused in one half cycle would be reversed in the other, having no net effect, and thus allowing bigger signals to be used. Also, the AC circuit  70  is immune to DC potentials in the sensor  10 . The first op-amp is used to make the oscillator, which feeds the sensor in series with a capacitor to block any DC current. The second op-amp produces an output proportional to the conductance of the sensor. The third op-amp turns the AC signal to DC to feed into the data acquisition system, be it microcontroller or computer. The RC on the output reduces ripple, however, it requires a few seconds charge-up time.  
     EXAMPLE 1  
     Micro-LPR Device Fabrication and Test  
      A device is made using the same material (“the source material”) as the structure to be monitored. The device can be made from the actual source material, or from shim stock matched to the source material. For the initial device fabrication, original source material (1.6 mm thick) was milled into 25 mm squares, then ground down to 200 μm using the double disk grinding technique. The squares were lapped to further reduce the material down to 50 μm and 100 μm thickness. In conjunction with this process, standard shim stock (50 μm and 100 μm thick) of the same material type was used. Using this sensor, the effect on sensor sensitivity due to material property fluctuation could be monitored. The sensor was mounted on a non-conducting platform that includes DuPont&#39;s Kapton®, which is not susceptible to corrosion and is not electrically conductive. The device was then fabricated so that only its top surface would be exposed to the atmosphere while the rest is encapsulated in the polymer. This selective exposure of the top surface is achieved by attaching the sensor material directly to the polyimide carrier. A photolithographic pattern is then formed in a photoresist coating applied to the top surface of the sensor material, providing the layout of the electrodes. Chemical etching is performed to remove the material exposed by the pattern, leaving behind the electrode fingers and interconnects.  
      Matching electronics are integrated with the fingers of the sensor electrodes. The matching circuit consists of appropriate capacitance, resistance, and operational amplifiers such that the signal being sent by the digital data acquisition device is converted to analog and back in a well-known fashion, so as to be accurately stored for later evaluation. The sensor testing is done utilizing an electronic controller chip which is programmed to periodically interrogate specific sensors and store the data. The controller first conducts a baseline calibration to determine the zero-current voltage, then sweeps the voltage from −100 mV to +100 mV from the baseline voltage. At each voltage increment, which may be 1-10 mV, the current through the sensor is measured and stored.  FIG. 7  shows an exemplary set of measurements for a single sensor sweep. The results are subsequently compared to the previously measured data for that sensor or a complete histogram in order to evaluate changes occurring over time.  
       FIG. 8  illustrates the effective resistance of the sensor as a function of time exposed to a humidified atmosphere. The increase in resistance as a function of time indicates the effects of corrosion due to mass loss of the sensor electrodes. These results can also provide mass loss rates due to corrosion, which further assist in prediction of structural failure. The controller can be interfaced to a wireless or hard-wire communication link, so as to transmit the data after each measurement, or other scheduled periods which make sense.  
      In practice, a plurality of sensors 10 can be incorporated onto a sheet of flexible circuit material as described previously, and multiplexed via the copper interconnects. The array of sensors can be controlled by a single data acquisition device, and can reasonably be distributed over areas on the order of a few meters square. In order to monitor large structures, it is expected that numerous sensor systems would be applied to provide accurate monitoring over a large percentage of the area of the structure.  
       FIG. 9  is a diagram of a circuit  80  for controlling the potential drop (ΔE) applied to the first and the second electrodes  12 ,  14  of the sensor  10  (see  FIG. 1 ). The voltage divider formed by R1 and R15 sets the inverted input voltage for a first op-amp  82  and a second op-amp  84 . By varying the voltage on the input of a DAC  86  between 0 and 1.5 V with R16, the potential on the sensor  10  is stepped down. The resistances are chosen so that the value of R16 is greater than that of R17 according to the drop in the potential. By using field effect transistor (FET) op-amps, the current that passes into the v+ and v− terminals are kept at a low level or eliminated. A dramatic increase in the output amplification can be achieved by increasing the resistance value of R20.  
      Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention.