METHODS AND SYSTEMS FOR GAS SENSING

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent Application 60/983,318, filed on October 29, 2007, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT This invention was made [partially] with U.S. Government support from the U.S. Air Force under contract FA9550-07-1- 0332. The U.S. Government has certain rights in the invention.

BACKGROUND

These teachings relate generally to gas sensing.

Gas sensing is utilized in numerous applications including automotive, medical, environmental monitoring and process control applications.

Many existing NO ₂ gas sensors rely on destructive sensing mechanism, such as the electrochemical oxidation of an electrolyte, etc. Such mechanisms limit the lifetime of a sensor.

There has been substantial research on solid state gas sensors. However, performance problems, related to sensitivity, selectivity and stability, still remain. The

commercialization of thin film carbon technology has been hindered by the lack of ability to form large areas.

Therefore, there is a need for gas sensors with required sensitivity, selectivity and stability.

There is also a need to provide thin film carbon sensors sufficiently large areas.

BRIEF SUMMARY

In one embodiment, the method of these teachings for sensing the presence of a gas includes exposing at least a portion of one or more carbon films to the gas, the gas comprising polar molecules, the one or more carbon films being formed, on a silicon carbide substrate, by solid-state/thermal decomposition of the silicon carbide substrate, and measuring a change in an electrical property of the one or more carbon films.

In another embodiment, the method of these teachings for manufacturing a gas sensor includes forming one or more carbon films on a silicon carbide substrate by solid-state/thermal decomposition of a silicon carbide substrate and interfacing the one or more carbon films to a component for measuring an electrical property of the one or more carbon films.

In one embodiment, the gas sensor of these teachings includes one or more carbon films disposed on a silicon carbide substrate and means for measuring a change in an electrical property of the one or more carbon films .

Other embodiments of the method and system of these teachings are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present teachings are pointed out with particularity in the appended claims. The present teachings are illustrated by way of example in the following drawings in which like references indicate similar elements. The following drawings disclose various embodiments of the present teachings for purposes of illustration only and are not intended to limit the scope of the teachings. For purposes of clarity, not every component may be labeled in every figure. In the figures:

Figure 1 is a schematic flowchart representation of an embodiment of the method of these teachings; Figure 2 is a schematic flowchart representation of another embodiment of the method of these teachings; Figure 3 is a schematic graphical representation of an embodiment of the system of these teachings; Figure 4 is a schematic representation of an exemplary embodiment of a grown carbon film as utilized in these teachings ,- Figure 5a is a schematic graphical representation of the Raman spectrum of an exemplary embodiment of a carbon film of these teachings ;

Figure 5b is a schematic graphical representation of an AFM micrograph of the surface of an exemplary embodiment of a carbon film of these teachings,-

Figure 6 is schematic graphical representation of the temperature variation of the surface resistance (also referred to as the van der Pauw sheet resistance) of a carbon film of these teachings; Figure 7 is schematic graphical representation of the temperature variation of the surface carrier density (also referred to as an Arrhenius plot) of a carbon film of these teachings ; Figure 8 is a schematic graphical representation of the differential resistance response of a carbon film of these teachings to exposure to various polar molecules; Figure 9a is schematic graphical representation of the temporal response of a carbon film of these teachings exposed to NO ₂ ; Figure 9b is a schematic graphical representation of further results of the temporal response of a carbon film of these teachings exposed to NO ₂ ,-

Figure 10 is a schematic graphical representation of the responsivity of a carbon film of these teachings exposed to NO ₂ ; and

Figure 11 represents a schematic graphical representation of the surface potential/work function change as a function of time for a carbon film of these teachings exposed to NO ₂ .

DETAILED DESCRIPTION

The present teachings may be understood by the following detailed description, which should be read in conjunction with the attached drawings. The following detailed description of

certain embodiments is by way of example only and is not meant to limit the scope of the present teachings.

Figure 1 shows a flowchart representation of an embodiment of the method of these teachings for sensing the presence of a gas comprised of polar molecules. Referring to Figure 1, the method shown therein includes exposing at least a portion of one or more carbon films to the gas {step 20, Fig. 1) , the gas comprising polar molecules, the one or more carbon films being formed, on a silicon carbide substrate, by solid-state/thermal decomposition of the silicon carbide substrate, and measuring a change in an electrical property of the one or more carbon films (step 30, Fig. 1) . In one embodiment, the carbon film is a nanocrystalline graphite film. Other embodiments can be obtained by varying the ambient (including vacuum) in which the solid-state/thermal decomposition occurs. The thickness of the carbon film is a function of the annealing temperature (the temperature at which the solid- state/thermal decomposition occurs) . The electrical property can be, but is not limited to, resistance, differential resistance (allowing sensing of the concentration of the gas) or surface potential/work function.

Figure 2 depicts a schematic flowchart representation of an embodiment of the method of these teachings for manufacturing a gas sensor of these teachings. Referring to Figure 2, the method includes forming one or more carbon films on a silicon carbide substrate by solid- state/thermal decomposition of a silicon carbide substrate (step 40, Fig. 2; annealing the silicon carbide substrate at temperatures greater than 1000°

C) and interfacing the one or more carbon films to a component for measuring an electrical property of the one or more carbon films (step 50, Fig. 2) . Carbon films of large area can be formed by the method of these teachings. The method of these teachings can be carried out on silicon carbide grown on a variety of substrates such as, but not limited to, silicon or Sapphire or others .

The method shown in Figure 2 can include the step of patterning the one or more carbon films. The method can also include the step of patterning electrical contacts for the one or more carbon films. In one instance, the patterning can be achieved utilizing conventional lithography and conventional material removal {or deposition) techniques. The step of forming the one or more carbon films can include the step of obtaining one or more carbon films of predetermining thickness, the thickness being a function of the annealing temperature and the duration of the annealing. In one embodiment, the one or more carbon films comprise one or more nanocrystalline graphite films. In other embodiments, other allotropes of carbon are formed, the other allotropes being obtained by performing the annealing under different ambient (such as, but not limited to, vacuum, halocarbons or HCl gas) . The component for measuring the electrical property of a gas (such as, in one instance, one or more resistors and circuits to measure the resistance or differential resistance of the one or more carbon films) can be fabricated utilizing conventional semiconductor processing or can be conventional discrete components (an exemplary embodiment, not a limitation of these teachings, of components used to measure differential resistance is shown International Patent Publication Number WO

97/01753, which is incorporated by reference herein is entirety) . The electrical property can be, but is not limited to, resistance, differential resistance {allowing sensing of the concentration of the gas) or surface potential/work function.

Figure 3 shows an embodiment of the gas sensor of these teachings. Referring to Figure 3, one or more carbon films 110 are disposed on a silicon carbide substrate 120. The silicon carbide substrate 120 can, in one embodiment, be disposed on silicon (Si), sapphire or any other substrate. The one or more carbon films on SiC are formed by annealing a SiC substrate at high temperatures greater than 1000 ⁰ C (shown schematically in Figure 4) . At these high temperatures, the Si sublimes, leaving behind carbon to rearrange into an appropriate structure depending on the conditions. The higher the temperature and the longer the anneal , the thicker the active layer. The thickness can be controlled to cater to the relevant application. This anneal can be carried out in a vacuum, or in an ambient, including halocarbons and HCl gas in order to control the allotrope of carbon that is formed. In one instance, the one or more carbon films 110 comprise one or more nanocrystalline graphite films.

Referring again to Figure 3, the one or more carbon films are patterned. Electrical contacts 125, providing an electrical connection to the one or more carbon films 110, are deposited and patterned. In one instance, the patterning can be achieved utilizing conventional lithography and conventional material removal (or deposition) techniques. Components for measuring a change in an electrical property 130 of the one or more carbon

films 110 are interfaced to the one or more carbon films 110. The change in the electrical properties of the one more carbon films 110 results from gas molecules adsorbed on the surface of the one or more carbon films 110. After measurement, the surface of the one or more carbon films 110 may be reset by, for example, but not a limitation of these teachings, desorbing the gas molecules by application of ultraviolet light or by heat treatment (in one instance, at approximately 150 ⁰ C) . The sensing of the gas only requires the expenditure of a modest amount of power (in one instance, less than lOμwatts) .

In one instance, not a limitation of these teachings, the electrical property is resistance and the component 130 includes a resistor and electronic sub- components to measure a change in the resistance of the one or more carbon films when exposed to a gas, where the gas comprises polar molecules. (For example, the sub- components can be similar to those shown in International Patent Publication Number WO 97/01753, which is incorporated by reference herein is entirety.) In another instance, a differential resistance is measured. (In one instance, not a limitation of these teachings, the component measures a change in differential resistance of one carbon film and the component includes a memory subcomponent in order to measure the differential resistance. A component for measuring differential resistance can sense concentration of a gas.) In another instance, the one or more carbon films 110 comprise two carbon films and the differential resistance is measured between the two carbon films (see for example, International Patent Publication Number WO 97/01753, which is incorporated by reference herein in its entirety) . In another

instance, the change in surface potential/work function of the carbon film, when exposed to a gas comprising polar molecules, is measured. The surface potential/work function can be, in one instance, measured utilizing scanning Kelvin probe microscopy (SKPM) or, in another embodiment, a dedicated component, such as, but not limited to, the component disclosed in Bergstrom, P. L.; Merchant, R.; Wise, K.D.; Schwank, J. W., Dielectric Membrane Technology For Conductivity And Work-function Gas Sensors, The 8th International Conference on Solid-State Sensors and Actuators, 1995 and Eurosensors IX., Transducers '95, Volume 2, Date: 25-29 Jun 1995, Pages: 993 - 996, which is incorporated by reference herein in its entirety.

In order to better illustrate the present teachings, an exemplary embodiment is disclosed herein below. It should be noted that these teachings are not limited only to the exemplary embodiment .

Nominally on-axis Si-face semi- insulating 6H SiC substrates were used in the exemplary embodiment. The silicon carbide substrates were annealed at 2000 ⁰ C for 10 minutes at pressures <2xlO ^"7 Torr for 10 minutes to form the graphite films. No special surface treatment was employed. The resulting films were characterized by Raman spectroscopy (excited by a 514.5 nm Ar laser) , atomic force microscopy (AFM) and van der Pauw geometry Hall measurements at a magnetic field of 2kG. Electrical contacts were formed by evaporating Ti/Au at the corners of the 0.5cmx0.5cm samples. Hall measurements were also performed with the samples immersed in acetone and water.

The sensitivity to low concentrations of NO ₂ gas in nitrogen was evaluated by monitoring the 2-terminal resistance of a sample at a bias voltage of 5OmV. In all measurements, currents <0.18mA were drawn. Before each measurement, the surface of the material was reset with UV light to desorb any molecules on the surface, until the measured resistance was constant in time i.e. to stabilize and clean the surface. All NO ₂ measurements were performed at room temperature.

Figure 5a) shows the Raman spectrum of the films produced in the exemplary embodiment. The two signature peaks for carbon are seen at -1350cm ^"1 , corresponding to amorphous carbon (termed the D band) and at ~1580 cm ^"1 , corresponding to single crystal graphite (termed the G-band) . The ratio of the two peaks gives an indication of the grain size in the resulting film. Analysis reveals this film to be nanocrystalline graphite (NCG) . The AFM micrograph of the surface in Figure 5b) is consistent with this interpretation, showing small grains ~20nm in size. The grain size may be increased by pre- treating the surface of the SiC to start with an atomically flat surface. Nevertheless, the films were substantially smooth, with an RMS roughness of ~3nm, although aggressive handling resulted in scratching of the surface, consistent with the "softness" ascribed to graphite. The areas between scratches maintained their integrity as measured by AFM.

Figure 6 shows the van der Pauw sheet resistance (the sheet or surface resistance measured by the van der Pauw method as described in van der Pauw, L.J. (1958) . ¹¹ A method of measuring specific resistivity and Hall effect of discs of arbitrary shape" Philips Research Reports 13: 1-9) of a 0.5cmxO.5cm

sample as a function of temperature. At lower temperatures, the resistance increases, in contrast with metallic conduction expected for single crystal graphite, which displays the opposite tendency. Hall measurements were also performed, although no definite carrier type could be identified, indicating mixed carrier types. In addition, adsorption of water vapor on the surface may obscure identification of carrier type. Nevertheless, the magnitude of the Hall voltage gives an estimate of the carrier concentration, presented as an Arrhenius plot in Figure 7. Although the Hall voltage signs did not indicate any single carrier type, the results were consistent from measurement to measurement. The carrier concentration has a weak temperature dependence, indicating that there is not a significant bandgap. The lack of bandgap is an indicator of the graphitic (rather than amorphous carbon) nature of this film.

Figure 8 shows the van der Pauw sheet resistance of the NCG immersed in two polar liquids. The 2 terminal resistance is also shown for comparison (includes contact resistance) . The differential change in the 2 terminal resistances is comparable, indicating that the measured sensitivity arises from the NCG, and not due to the sensitivity of the metal/graphite interface. It also indicates that the contact resistance is not significant. It should be noted that the Hall voltages of acetone and water are of opposite polarity, indicating a change in carrier type in the NCG, although neither solvent gave a clear carrier type.

Figure 9a shows the differential resistance change of a NCG film exposed to NO ₂ and also shows the effect of OV reset.

Figure 9b shows the differential resistance change of a NCG film exposed to various concentrations of NO ₂ diluted in nitrogen as a function of time. At a bias of 5OmV, currents<0.18mA were used (corresponding to power <9μW) . The longer the sample was exposed to NO ₂ , the larger the change in resistance at any given concentration. This indicates the cumulative nature of the sensing mechanism. The sensitivity- arises from several NO ₂ molecules adsorbing on the surface of the NCG. The longer the exposure time, the greater the number of molecules adsorbed on the surface, leading to a greater change in resistance. The fact that the surface can be reset with UV light supports this interpretation, as the high energy UV photons provide sufficient energy to drive the desorption of the NO ₂ molecules.

Due to the cumulative nature of the sensitivity, it may be desirable for an NCG NO ₂ sensor to be operated in differential resistance mode (i.e. dR/dt) to accurately sense the gas concentration. In differential mode, selectivity between water vapor and NO ₂ can be obtained as well, as the change in resistance is of opposite sign for the two molecules (see Figure 8, Figure 9a or 9b) . In Figure 10, the slope of the differential resistance change (shown in Figures 9a, 9b)) is shown as a function of NO ₂ concentration. It can be seen that the responsivity i.e. dR/dt, is a strong function of NO ₂ concentration and can be used as the metric for quantifying NO ₂ concentration.

It should be noted that though the response to adsorbant NO ₂ on the surface is a strong function of NO ₂ concentration, the recovery time constant of the resistance after NO ₂ turn-off is

not {Figures 9a, 9b) ) . This behavior indicates that the desorption of NO ₂ is not dependent on the adsorbate history of the film, but only on the interaction between the NO ₂ and the NCG. These observations also indicate that the surface of the NCG is substantially well behaved. Such stability is important for the lifetime of a NCG sensor.

Figure 11 shows the change in surface potential/work function in response to exposure of the nanocrystalline graphite film to 600ppb NO ₂ gas, as measured by SKPM. Referring to Figure

11, the surface work function exhibits a similar recovery time constant as the resistance in Figure 9.

It should be noted that the present teachings are not limited to the exemplary embodiment, which is presented to better illustrate the teachings. Various other embodiments, as described hereinabove, are within the scope of the present teachings .

Although the teachings have been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims .

What is claimed is: