Patent Application
20050262943
The scientific and technological interest in miniaturized, multi-parameter (e.g., gas, humidity, chemical, temperature, biological, and pressure) sensor devices has grown in recent years. The need for such devices spans a wide range of industries and applications, such as the medical instrumentation, food and agriculture, paper, automotive, electric appliance, petrochemical, and semiconductor industries, as well as the military, in, for example, gas, humidity, chemical, temperature, biological, and pressure sensing applications. The wide range of environments to which these devices are exposed may limit the candidate materials that can be used to build the devices. A number of gas, humidity, chemical, temperature, biological, and pressure sensor devices have been developed and built for specific applications. However, these devices do not demonstrate a suitable combination of robustness, sensitivity, selectivity, stability, size, simplicity, reproducibility, reliability, response time, resistance to contaminants, and longevity. Thus, what are still needed, in general, are multi-gas, vapor, and biological sensor devices, among other sensor devices, that exploit the unique properties of certain thin films and nano and pico sized structures, including their adjustable pore size, customized functionality, high surface area, high adsorption/desorption rate under optimized conditions, high chemical stability, and mass and stress changes associated with the physisorption of gas and vapor molecules.
Power consumption, response time, mechanical strength, and crosstalk between unit sensor devices are major areas of concern with respect to stress, strain, mass change, and thermally-sensitive Microelectromechanical Systems (MEMS), such as gas, humidity, chemical, temperature, and pressure sensor devices, as well as calorimeter and microheater resonant devices, in general. For example, lower power consumption may be desired for portable and wireless devices. Response time and sensitivity may be critical in many sensing applications, such as in sensing warfare agents, measuring low dew points, detecting trace gases, etc., but may be difficult to optimize with conventional multi-gas and vapor sensor devices without making sacrifices with respect to other performance parameters. Power consumption and crosstalk between unit sensor devices may be affected by the rigidity of the resonating structure. Typically, resonant devices are actuated electrostatically, which requires high voltages or very narrow gaps on order of a few hundred nanometers between the driving electrodes in order to generate high enough forces that would deflect a spring element (e.g., a membrane, a cantilever, or a diaphragm). While high voltages suggest high power consumption, controlling very narrow gaps in a repeatable and reliable manner requires complex fabrication processes and very tight fabrication tolerances that may ultimately drive the sensor cost too high. Thus, resonant MEMS that are built with highly compliant materials with low power consumption actuators and very sensitive and low response time read out mechanisms may provide low power consumption, sensitive, and fast response time read out mechanisms.
Two additional areas of concern are raised with respect to miniaturized vapor (e.g., humidity) sensor devices, among other sensor devices. First, the sensing films associated with such vapor sensor devices may become significantly swollen while at relatively high humidity due to their high affinity for water vapor. The swelling of these sensing films generates lateral stresses that impinge upon the thin membranes, potentially breaking them. Second, sensing films having larger surface areas are desired in order to reduce the thickness of the sensing films at a given mass. Reducing the thickness of the sensing films and incorporating nanostructures (e.g., nano-spheres, nano-rods, nano-fibers, etc.) into the sensing materials decreases the diffusion time constant of the water adsorption/desorption, reducing the response time of the vapor sensor devices. Thus, what are needed are micro-machined resonant gas and vapor sensor devices, among other sensor devices, that utilize, for example, high-aspect ratio silicon microstructures etched adjacent to the thin membranes. These silicon microstructures may serve as stress relievers at varying vapor (e.g., humidity) levels and provide large surface areas for the sensing films, increasing the sensitivity of the vapor sensor devices.
According to some embodiments, a resonant MEMS sensor includes a sensing material on a spring element. The sensor may also include a detector adapted to determine a resonant frequency associated with the spring element, wherein the resonant frequency changes upon the exposure of the sensing material to an analyte.
A resonant sensor may be used to determine if an analyte is present and/or to quantify an amount of “analyte.” As used herein, the term “analyte” may refer to any substance to be detected and/or quantified, including a gas, a vapor, and/or a bioanalyte. For example,
A current IBIAS flows through a conductor, such as a wire 230, located on, within, or in close proximity to the spring element 210. According to other embodiments, the current IBIAS instead flows through the spring element 210 itself. A constant magnetic flux density or field BBIAS is also present (e.g., stemming from a magnet or integrated coil not illustrated in
When the current IBIAS is an Alternating Current (AC), the spring element 210 will vibrate between being deflected upwards and downwards. Moreover, the amplitude of deflection that is experienced by the spring element 210 at a constant bias current amplitude and for a given surrounding atmosphere will vary depending on the frequency of the current IBIAS. For example,
According to this embodiment, a sensing material 450 is formed or deposited on the spring element 410 such that a resonant frequency associated with the spring element 210 will change when the sensing material 410 is exposed to an analyte. The sensing material 450 may provide a high selectivity which will have a sensitivity to a particular gas, vapor, or bioanalyte depending on its composition, physical, and chemical properties. That is, the presence of a first analyte or class of analytes (e.g., water vapor or alcohol) will change the resonant frequency while the presence of other analytes (e.g., N2 and CO) will not. The sensing material 450 may be, for example, a thin film of zeolite, a polyelectrolyte, a carbon nanotube, mesoporous silicon/oxide, or other material applied to a surface of the spring element 410 opposite from the surface upon which the conductor 430 is mounted. According to another embodiment, the sensing material 450 may instead be formed on the same surface of the spring element 410 as the conductor 430.
The sensing material 450 may act as a chemical transducer. For example, the sensing material 450 may adsorb an analyte to be sensed and convert the adsorbed analyte into a mass, heat, stress, and/or strain change. In any of these cases, the frequency at which the spring element 410 will resonate will be altered (e.g., because the combined mass or stress of the spring element 410, conductor 430, and thin film sensing material 450 has been changed). The sensing material 450 may be, for example, an organic material (e.g., a polymer or copolymer), an inorganic material (e.g., a zeolite, a carbon nanotube, or ceramic material), or an organic/inorganic composite or nanocomposite. Moreover, the sensing material 450 may be nanostructured (e.g., exhibiting nanomorphologies by contrast to bulk crystalline or amorphous morphologies).
Assume, for example, that the dashed curve in
The materials used as the sensing material 450 will determine the analyte or analytes that can be detected. For example, when the analyte to be detected is CO an appropriate sensing material 450 might be a layer that includes ZSM-5, MFI, a polymer, a nonocomposite, and/or other materials. As another example, when the analyte is CO2 the sensing material 450 might be a layer that is comprised of ZS500A, Zeochem Z10-02, SAP-34, carbon nanotubes, AFR, and/or other materials. When the analyte is O2 the sensing material 450 could be a layer of A-type zeolites, zeolite SX6, and/or zeolite rho. As yet another example, when the analyte is ammonia the sensing material 450 may include zeolite 4A, zeolite 5A, zeolite 13X, FAU, and/or polyelectrolytes.
A sensing material 450 comprised of zeolites SX6, CaX, LTA, and/or zincophosphate might be used, for example, to detect N2. Moisture or humidity (e.g., H2O) might be detected using polyelectrolytes (e.g., polystyrene sulfonic acid) and/or A-zeolite. As another example, when the analyte to be detected is CH4 the sensing material 450 might be a layer of LTA and/or zincophosphate. Moreover, NA-Y could be used to detect NOx.
When the sensing film is a zeolite, the selectivity can be achieved based on molecular size exclusion, molecular geometry exclusion, and/or electrostatic interactions between the analyte and the sensing material (e.g., polarity). The pore size in zeolitic structures may be controlled by the AL/Si atomic ratio and the synthesis parameters (e.g., temperature and pressure).
At Step 706, a conducting layer is deposited and patterned on the first insulating layer. For example, the cross section view of
At Step 708, an area of the second insulating layer is etched away. At Step 710, silicon associated with the exposed area of the second insulating layer is etched away to form a cavity that extends through the wafer thickness to the first insulating layer. As a result, the portion of the first layer suspended over the cavity acts as a flexible membrane (and is supported by the wafer that surrounds the cavity). Consider, for example,
A sensing layer is then formed on first insulating layer proximate to the cavity at Step 712. For example,
According to some embodiments, a sensing layer is formed proximate to the cavity and then a sensing material is added to the sensing layer. Consider, for example, a MEMS sensor that may be used to detect and/or quantify the presence of CO2. In this case, the sensing material might be a single carbon nanotube or a plurality of carbon nanotubes. Note that the carbon nanotubes might be single wall (SWNT) or multi-wall (MWNT) carbon nanotubes. Moreover, the carbon nanotubes might be added to the sensing layer via solution deposition of dispersed nanotubes in an appropriate solvent.
MWNTs and SWNTs can be dispersed in a number of non-aqueous solvents, including 1,2-dichlorobenzene (12-DCB), chloroform (CHCl3) or dimethylformamide (DMF), to allow for their effective deposition onto the sensing layer. Mechanical stirring or more effective low-intensity sonication (water bath) might be used to aid in the dispersion of the nanotubes in the chosen solvent. The resulting MWNT or SWNT solutions can be drop-cast or spin-cast onto the sensing layer to deposit the respective nanotubes therein. The relatively high volatility of CHCl3 and DMF enables convenient nanotube deposition.
An AC source and the conductor 822 (e.g., doped silicon, platinum, or other conducting material) provides current IBIAS and a magnet 870 (e.g., a permanent magnet, a solenoid, or an integrated coil) creates a constant magnetic field BBIAS such that current IBIAS will cause the membrane 860 to vibrate. The resonant frequency of the membrane 860 will depend in part on the analytes found in gas mixture that enters the cavity 850 and is physically adsorbed by the sensing material 840. In this way, a detector adapted to determine the resonant frequency can be used to measure and quantify the presence of an analyte, and some examples of detectors are described with respect to
Such an approach may provide a sensitive, selective, fast responding, robust, and accurate analyte detector. Moreover, the design can be used to detect different analytes and/or different amounts of analytes (e.g., by changing the materials used in the sensing material 840 and/or the geometry of the membrane 860 and/or the material used to fabricate the membrane).
Note that after a target analyte has been adsorbed by the sensing material 840, the analyte would need to be removed before the sensor would be able take another measurement. To accelerate the removal of the analyte from the sensing material 840, micro-heaters may be used to temporarily increase the temperature of the sensing material 840 (causing the desorption of the analyte from the sensing material 840). Other methods can be used to accelerate the desorption of the analyte from the sensing material 840, including exposure to light at specific wavelengths. According to some embodiments, the conductor 822 carrying current IBIAS may be used as a micro-heater. According to other embodiments, one or more separate micro-heaters may be used instead of (or in addition to) the conductor 822.
Also note that the resonant frequency of the membrane 860 could change because of factors other than the adsorption process. For example, a substantial change in pressure and/or temperature might change the resonant frequency of the membrane 860—which could affect the measurement and quantification of the target analyte.
In this way, the reference portion of the sensor 1200 may be used to determine if a change in resonant frequency is due to a factor other than the presence of the analyte. For example, a change in temperature might change the resonant frequency of the membranes over both the first cavity 850 and the reference cavity 852 an equal amount (and therefore the sensor 1200 would not generate an output indicating that the analyte is present). A change in the resonant frequency of the membrane over the first cavity 850, however, without a corresponding change in the membrane over the reference cavity 852 would generate such an output.
To determine the resonant frequency of a spring element, a detector may sample amplitudes of deflection over a range of frequencies driven from the current conductor layer. The frequency associated with the greatest deflection can then be identified as the resonant frequency.
In particular
As described with respect to
The following illustrates various additional embodiments of the present invention. These do not constitute a definition of all possible embodiments, and those skilled in the art will understand that the present invention is applicable to many other embodiments. Further, although the following embodiments are briefly described for clarity, those skilled in the art will understand how to make any changes, if necessary, to the above-described apparatus and methods to accommodate these and other embodiments and applications.
For example, a MEMS resonant sensor could be associated with a system 2000 such as the one illustrated in
Information from the MEMS resonant sensor 1910 is provided to a sensor dependent device 1920 (e.g., via an electrical signal). Such a system 1900 might be associated with, for example, a consumer device (e.g., an alarm to be used in a home), an industrial process control device, or a Heating, Ventilation, Air Conditioning (HVAC) device. Some medical examples include a breath analyzer, a blood alcohol measuring device, and a blood glucose monitor device. Similarly, the system 2000 might be an air quality device, an emissions management device, a leak detector, a poison detector, a flammable material detector, a chemical weapon detector, a toxic material detector, an explosive material detector, a hydrogen economy detector, a pharmaceutical process control device, and/or a bioanalyte sensor. Note that the system 1900 might be associated with any combination of these examples.
Moreover, although particular layouts have been described herein, embodiments may be associated with other layouts. For example, according to some embodiments, only one end of a spring element might be anchored and/or the spring element might be a beam. As another example, a spring element could be supported above a wafer (instead of being suspended over a well). Similarly, a MEMS resonant sensor could use any combination of amplitude detection techniques described herein. Some embodiments might simply keep the frequency of current IBIAS constant and measure the amplitude of deflection (e.g., when any amount of an analyte to be detected will reduce the amount of deflection). As still another example, the velocity at which the spring element moves might be measured.
According to some embodiments, a sensor will always scan through a pre-determined set of frequencies (e.g., from an fMIN to an fMAX). According to another embodiment, the sensor may sample a mid-range frequency and then select the next frequency to be sampled (e.g., higher or lower than the last sampled frequency by smaller and smaller increments). For example, the sensor might initially sample the amplitude of deflection when current IBIAS has the following frequencies: fMIN, f1, f2, and fMAX. If f2 had the greatest maximum amplitude of deflection, then the next two frequencies to be sampled might be (f1+f2)/2 and (f2+fMAX)/2.
Some embodiments have described a magnetic source that provides a magnetic field B. Note that such a magnet force may be provided and/or located in any number of different ways. For example,
In addition, in some embodiments a resonant frequency measuring device might be used to measure the resonant frequency shift directly. For example, second resonator might be used to measure such a shift.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
