Abstract:
A method of assembling a remote sensor system to detect a gas or chemical and a remote sensor system are described. The method includes fabricating a sensor, the sensor outputting a sensor signal that changes upon contact of the sensor with the gas or chemical and the sensor having an input port for a clock signal, coupling a capacitor to the sensor, the capacitor output voltage resulting from the sensor signal output by the sensor, and coupling a mixer to the capacitor and a low frequency oscillator, the mixer configured to mix the capacitor output voltage with the low frequency oscillator output to generate an output signal. The method also includes coupling an antenna to the mixer, the antenna configured to transmit the output signal indicating detection of the gas or chemical.

Description:
BACKGROUND 
     The present invention relates to remote sensing, and more specifically, to remote sensing using pulse-width modulation. 
     Remote sensing involves a sensor providing information about conditions at a location without an operator having to be present at that location. Remote sensing facilitates environmental monitoring, for example, without requiring the presence of personnel. Generally, carbon nanotube, graphene, or other two-dimensional materials are used as sensors for environmental monitoring due to their large surface-to-volume ratio and good electronic properties. However, the wireless transmission of the information obtained with the sensors typically suffers from attenuation over longer distances. 
     SUMMARY 
     According to one embodiment of the present invention, a method of assembling a remote sensor system to detect a gas or chemical includes fabricating a sensor, the sensor outputting a sensor signal that changes upon contact of the sensor with the gas or chemical and the sensor having an input port for a clock signal; coupling a capacitor to the sensor, the capacitor output voltage resulting from the sensor signal output by the sensor; coupling a mixer to the capacitor and a low frequency oscillator, the mixer configured to mix the capacitor output voltage with the low frequency oscillator output to generate an output signal; and coupling an antenna to the mixer, the antenna configured to transmit the output signal indicating detection of the gas or chemical. 
     According to another embodiment, a remote sensor system includes a sensor configured to receive an input signal and output a sensor signal that changes upon the sensor contacting the gas or chemical; a capacitor coupled to the sensor, the capacitor configured to output an output voltage resulting from the sensor signal; and a mixer configured to mx the output voltage with a low frequency oscillator output to generate an output signal. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  shows a sensor system according to an embodiment of the invention; 
         FIG. 2  illustrates a shift in the characteristic input signal to drain current Id curve the pFET and the nFET of the sensor system according to embodiments; 
         FIG. 3  details the sensor according to embodiments of the invention; and 
         FIGS. 4-9  are cross-sectional views illustrating some of the processes involved in fabricating the sensor according to an embodiment of the invention, in which: 
         FIG. 4  shows an oxide layer deposited on a complementary metal-oxide-semiconductor (CMOS) circuit; 
         FIG. 5  shows trenches formed in the oxide layer of the structure shown in  FIG. 4 ; 
         FIG. 6  shows the gates formed in the oxide layer; 
         FIG. 7  shows the gate dielectric and channel material deposited over the gates; 
         FIG. 8  shows pFET metal deposited on the gate dielectric; 
         FIG. 9  shows nFET metal deposited on the gate dielectric; and 
         FIG. 10  shows an array of the sensors according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, remote sensors facilitate environmental monitoring and the detection of gas or chemicals (e.g., potentially harmful chemicals) without requiring the presence of any personnel. In existing remote sensor systems, the attenuation of transmitted sensor signals over long distances has affected the utility of remote sensing. For example, when sensor conductance changes due to the concentration of a gas, attenuation of the signal conveying that information affects the accuracy of the information received at a long distance from the sensor. Embodiments of the systems and methods detailed herein relate to a remote sensor system that changes duty cycle (performs pulse width modulation) in response to sensing gas or chemicals such that the sensor output signal may be transmitted long distances without attenuation issues affecting the reception of accurate sensor information. 
       FIG. 1  shows a sensor system  100  according to an embodiment of the invention. The system  100  according to the embodiment detailed herein facilitates wireless transmission of information indicating a concentration of gas or a chemical in the environment in which the system  100  is located, and thus acts as a remote chemical detector. The system  100  includes a sensor  110 . The sensor  110 , detailed with reference to  FIG. 2  below, includes an inverter having a p-channel field effect transistor (pFET)  245  ( FIG. 2 ) and n-channel FET (nFET)  255  ( FIG. 2 ). An output node of the sensor  110  charges and discharges a capacitor  140  based on an input signal  105  and any gas or chemical in the environment, as further discussed below. The input signal  105  may be provided by a known clock circuit that includes a ring oscillator or an LC-tank oscillator, for example. This input signal  105  duty cycle is assumed to be constant (such that any changes in duty cycle of the sensor  110  output may be attributed to gas or chemical exposure.). However, in alternate embodiments, a calibration or regulation of the input signal  105  may be additionally performed by known methods. 
     As explained in further detail below, gas or chemical exposure of the sensor  110  affects threshold voltages of the pFET  245  and nFET  255 , which, in turn, affects the charging and discharging of the capacitor  140 . This is because charges on absorbed molecules and chemicals can dope the channel materials through electrostatic doping or charge transfer. Because the same species are absorbed by both the pFET  245  and the nFET  255 , the threshold voltage of the pFET  245  changes equally (but with an opposite polarity to) the threshold voltage of the nFET  255 . The opposite polarity results from the characteristic input signal  105  (or input gate bias Vg) to drain current Id curve  201 ,  202  of each of the pFET  245  and the nFET  255 , respectively, as shown in  FIG. 2 . The shift to the dashed curves  201 ′,  202 ′ indicated by the arrows for each of the pFET  245  and nFET  255 , respectively, indicates an exemplary shift based on gas or chemical exposure. In an inverter circuit, such as that of the sensor  110 , only one transistor is “on” or has high drain current Id at a given moment. When the input signal  105  (or Vg) swings from 0 to Vdd, the pFET  245  gets switched off, as indicated by the curve  201 , and the nFET  255  gets switched on, as indicated by the curve  202 . As noted above, when the pFET  245  is on, it charges the capacitor  140 , and when the nFET  255  is on, it discharges the capacitor  140 . Thus, a shift (e.g., from  201  to  201 ′ and from  202  to  202 ′) based on absorbed gas or chemicals affects the length of time that the pFET  245  and, conversely, the nFET  255  stay on, thereby affecting the charging/discharging of the capacitor  140  and, ultimately, the duty cycle of the output signal  160 . 
     According to an embodiment, when the input signal is low, the pFET  245  is conductive and charges the capacitor  140 , and when the input signal is high, the nFET  255  is conductive and discharges the capacitor  140 . Based on the gas or chemical concentration, the threshold voltages of the pFET  245  and nFET  255  may change such that the pFET  245  is on longer and the nFET  255  is (proportionately) on for a shorter length of time. In this exemplary case, the capacitor  140  would be charged longer and the pulse width of the capacitor  140  output voltage  120  would be longer (see e.g., output voltage  120 - 1  in comparison to output voltage  120 - 2 ). Alternately, based on the gas or chemical concentration, the threshold voltages of the pFET  245  and the nFET  255  may change such that the nFET  255  is on longer and the pFET  245  is (proportionately) on for a shorter length of time. In this exemplary case, the capacitor  140  would be discharged longer and the pulse width of the capacitor  140  output voltage  120  would be shorter (see e.g., output voltage  120 - 2  in comparison to output voltage  120 - 1 ). 
     According to the arrangement of the sensor system  100  in the embodiment shown in  FIG. 1 , the effect of chemical exposure of the sensor  110  manifests as a change in duty cycle (or pulse width modulation) of the voltage of the capacitor  140  (output voltage  120 ). This, in turn, facilitates wireless transmission of the sensor  110  information. The capacitor  140  has an initial output voltage  120 - 1 , which may have a duty cycle of 50%, for example. The chemical exposure of the sensor  110  may change the duty cycle to that shown for output voltage  120 - 2 , for example. As noted above, this change from output voltage  120 - 1  to output voltage  120 - 2  indicates a shorter pulse width or less time that the pFET  245  is on relative to the nFET  255  based on the shift in threshold voltages (of equally but with opposite polarities) of the pFET  245  and nFET  255  because of the chemical. A mixer  130  is used to mix the output voltage  120  with a local oscillator  150  to generate the transmitted output signal  160  of the system  100 . The output signal  160  is transmitted via an antenna  165 . Upon receipt at a site remote from a location of the sensor  110 , the duty cycle (pulse width) of the output signal  160  (the duty cycle of the output voltage  120  of the capacitor  140 ) indicates whether gas or chemical was detected and may provide an indication of a characteristic of the gas or chemical at the sensor  110 . The characteristic may be a concentration such that the change in pulse width (duty cycle) of the output signal  160  is proportional to the concentration of the gas or chemical to which the sensor  110  is exposed. In the embodiment shown in  FIG. 1 , the components of the system  100  other than the sensor  110  (e.g., capacitor  140 , low-frequency oscillator  150 ) may be part of a different circuit  101 . According to an exemplary embodiment discussed below, the sensor  110  and the other components of the system  100  may be integrated. 
       FIG. 3  details the sensor  110  according to embodiments of the invention. Some of the processes involved in fabricating the sensor  110  are detailed below.  FIG. 2  details the components of the sensor  110  and specifically shows that the channel material  230  is exposed to the environment. As a result, the sensor  110  detects chemicals in the environment. The sensor  110  includes an oxide layer  210  with a gate  220  formed therein. Although the orientation shown in  FIG. 3  does not show an electrical connection between the gate  220  of the pFET  245  and the gate  220  of the nFET  255 , the gates  220  are electrically connected and receive the same input signal  105 . A gate dielectric  225  is formed over the gate  220  and oxide layer  210 . The pFET  245  and nFET  255  are defined by deposition of pFET metal  240  and nFET metal  250 . The drain terminals of the pFET  245  and nFET  255  are electrically connected. The channel material  230  formed above the gate  220  (on the gate dielectric  225 ) may be a carbon nanotube (CNT) or graphene ribbon. Graphene is essentially a very thin (two-dimensional) layer of pure carbon. Absorption of gas or chemicals in the environment onto the graphene results in doping of the pFET  245  and nFET  255 . This, in turn, affects the threshold voltage (the minimum gate-to-source voltage differential needed to create a conducting path between the source and drain terminals) of the pFET  245  and nFET  255 . Detecting the shift in threshold voltages equates to detecting the concentration of chemical to which the sensor  110  is exposed. According to the embodiment detailed with reference to  FIG. 1 , the shift in the threshold voltages is not only detected but also indicated remotely because the system  100  facilitates manifesting the shift in threshold voltages in a capacitor  140  and transmitting the capacitor  140  output voltage  120  wirelessly. 
     The gas or chemical exposure of the channel material  230  (e.g., graphene) changes the conductance of the sensor  110  by changing the threshold voltages of the pFET  245  and the nFET  255 . This change in conductance is converted to pulse width modulation of the output voltage  120  of the capacitor  140  of the system  100 , as discussed above. Initially, with no chemical exposure, the pFET  245  may charge the capacitor  140  and the nFET  255  may discharge the capacitor  140  equally in the ideal case. In alternate embodiments, because the duty cycle of the output voltage  140  (the output signal  160 ) may not necessarily be 50% with no gas or chemical exposure, a calibration may be performed. That is, the duty cycle of the output signal  160  without any gas or chemical exposure of the sensor  110  may be used as a baseline (in a calibration process) to determine the change in duty cycle following gas or chemical exposure rather than assuming a duty cycle of 50% as a default. Alternatively, a reference sensor may be encapsulated such that its electrical properties do not change based on chemical exposure. A comparison of the transmitted output signal  160  to the signal output by the reference sensor may be used in lieu of a calibration to address any uncertainty in the clock input signal  105 . 
       FIGS. 4-9  are cross-sectional views illustrating some of the processes involved in fabricating the sensor  110  according to an embodiment of the invention.  FIG. 4  shows an oxide layer  210  deposited on a complementary metal-oxide-semiconductor (CMOS) circuit  410 . In alternate embodiments, the oxide layer  210  may be deposited on a substrate. When the sensor  110  is formed on a CMOS circuit  410  that includes the capacitor  140  and transmission components, a separate circuit including those components need not be coupled to the sensor  110 . Lithography and a reactive ion etch (RIE) process are used to form trenches  510  in the oxide layer  210 , as shown in  FIG. 5 . As shown in  FIG. 6 , a gate metal deposition is performed followed by chemical mechanical planarization to form the embedded gate  220 .  FIG. 7  shows the result of completing two processes. A gate dielectric  225  is deposited by atomic layer deposition (ALD) or chemical vapor deposition (CVD). The gate dielectric  225  may be comprised of hafnium oxide (HfO 2 ), silicon dioxide (SiO 2 ), or aluminum oxide (Al 2 O 3 ), for example. This deposition is followed by deposition and patterning of the channel material  230 . As noted above, the channel material  230  may be comprised of carbon nanotube (CNT), graphene, or another two-dimensional semiconductor. In  FIG. 8 , the result of depositing and patterning a pFET metal  240  is shown. The pFET metal  240  may be a high workfunction metal such as palladium (Pd), nickel (Ni), or chromium (Cr), for example.  FIG. 8  shows the result of depositing and patterning an nFET metal  250 . The nFET metal  250  may be a low workfunction metal such as scandium (Sc) or erbium (Er), for example. At this stage, the sensor  110  may be connected to the CMOS circuit  410  to form the sensor system  100 . In alternate embodiments, the sensor  110 , formed on a substrate, may be connected to the other components of the system  100 . 
       FIG. 10  shows an array of the sensors  110  according to embodiments discussed herein. Two or more sensors  110  may be arranged in an array on the CMOS circuit  410 , for example, with each sensor  110  of the array designed to be sensitive to a different variety of chemicals. A selector  1010  may determine which of the sensors  110  transmits the output signal  160 . Power consumption of the array of sensors  110  may be controlled by using a timing circuit  1020  that activates and de-activates each sensor  110 , clock input (that provides the input signal  105 ), and transmission circuit as needed. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.