KR20070121761A - Gated gas sensor - Google Patents

Abstract

An apparatus for detecting an analyte gas is provided. The device may include a signal amplifier that may include a thin film transistor that may include a semiconductor thin film that may include a metal oxide capable of chemically reacting with an analyte gas, such as carbon monoxide. The device can be adjusted to detect the analyte gas by changing the gate voltage of the transistor.

Description

Gate type gas sensor {GATED GAS SENSOR}

The United States of America possesses certain rights in the invention in accordance with Grant No. FA8650-05-M-6562 granted by the US Air Force.

The present invention relates generally to gas sensors. More specifically, the present invention relates to a gas sensor that operates by applying a gate voltage to adjust the detection of a current through a compound layer capable of chemically interacting with an analyte gas. Other methods can also be used to tune the detection of analytes, including optical excitation, chemical dopants, surface chemical layers, and combinations thereof. In addition, applying external forces, such as gate bias, helps to eliminate or extremely reduce the requirement for a heated substrate surface. More specifically, the present invention relates to thin film gated metal oxide detectors adapted for detection of analyte gases such as carbon monoxide.

Heated Metal Oxide Sensors

There is a lot of background in the information regarding metal oxide chemical sensors. These were all located on heated substrates or measured by electrochemical detection. See, for example, "Oxide Materials for Development of Integrated Gas Sensors-A Comprehensive Review" Critical Reviews in Solids State and Materials Sciences, 29: 111-188, 2004 and references therein.

It is aimed at providing sensors for detecting ambient levels of gases, in particular toxic gases. Heated metal oxide sensors are considerably studied in the paper. According to a recent review, Eranna et al. (Page 174, Table 23, indicated herein by reference 1) indicate the range of gases that can be detected and the metal oxides that react for each gas. The inventors have demonstrated a significant improvement in their ability to sense gases using metal oxide surfaces.

Many metal oxides are semiconductors. This means that there is an energy gap between the population of electrons, called the balance band and the conduction band, in which electrons can move through the material. This energy gap is commonly called the band gap and is denoted by E _g . Metal oxide sensors exploit this semiconductor property by promoting electrons from the balance band to the conduction band by heat. This thermal excitation of the electrons can easily change the surface energy of the metal oxide that promotes the chemical reaction.

Typically metal oxides have an amorphous crystal structure. This means that it will be an individual crystal-like grain, but there is no long-range order on the surface. The boundary between each crystal particle creates a grain boundary. The conduction through the metal oxide is limited by the energy barrier created at each grain boundary. In addition to changes in surface energy, heat also changes the energy levels of the barriers created in these individual grain boundaries.

According to one particular example, we will look at the current state of the art and its limitations for detecting carbon monoxide (CO). Carbon monoxide poisoning is an important issue for civil and military sectors. It is estimated that more than 500 people in the United States accidentally die of carbon monoxide ("CO") poisoning each year, more than any other poison. In addition, approximately 10,000 people are treated with symptoms of CO exposure every year. While most of the CO-related events in the home can be identified and treated, this situation is more severe in environments such as cabins where there is a lack of adequately available monitoring devices.

Carbon monoxide has about 210 times greater binding affinity to hemoglobin compared to oxygen. CO is an odorless, tasteless, colorless gas that causes hypoxic hypoxia with reduced oxygen transport capacity in the blood. Carbon monoxide in the blood produces carboxyhemoglobin (COHb) that interferes with the transport of oxygen. At sea level, elevated levels of COHb cause signs that range from headaches to unconsciousness. At 200 ppm, sea level CO causes headaches (equivalent to 15-20% COHb content in the body). At higher altitudes, the effects of CO poisoning and high hypoxia are aggravated, so there is a need for continuous numerical monitoring so that CO in the passenger cabin is below 200 ppm.

Some metal oxide and electrochemical sensors have been operating as CO detection alarms in homes for more than a decade, but they have not been able to measure CO consistently and accurately below the ppm level. Ongoing monitoring of carbon monoxide at the 35 to 200 ppm level presents a challenge to all commercially available CO detector technologies. Continuous monitoring of carbon monoxide is critical in the home, in industry, and in the military sector. Currently, three techniques are used in the manufacture of carbon monoxide alarms. The advantages and disadvantages of each method are outlined below.

Carbon monoxide (CO) detectors based on heated metal oxides:

Semiconductor-based sensors use thin films of tin dioxide heated on ceramic substrates. CO is oxidized on the hot surface. The current increases as the tin dioxide is exposed to carbon monoxide. The microchip control electronics will detect a change in current and will sound an alarm when the level of CO measured by the current exceeds a predetermined threshold. These sensors operate at high temperatures in excess of 400 degrees and consume a lot of power. Such high temperatures cause the sensors to respond to false signals generated by chemically similar analytes. The advantage is that it is inexpensive and easy to produce. Disadvantages are high power consumption, low cycle time, oxygen contamination, reaction to false positive signals, and heating required for system regeneration. The techniques outlined above are still insufficient to provide a complete solution for the continuous detection of carbon monoxide at low ppm levels.

Heated molybdenum oxide (MoO ₃ ) CO sensor.

Research into developing such alternative sensors has continued. For example, molybdenum oxide (MoO ₃ ) thin films prepared by sol-gel and RF magnetron sputtering processes have been used previously in the development of CO sensors, which are described in E. Comini, G. Faglia, G. "Carbon Monoxide response of molybdenum oxide thin films deposited by different techniques," in Sberveglieri, C. Cantalini, M. Passacantando, S. Santucci, in Sensors and Actuators B 68, pp. 168-174 (2000), incorporated herein by reference 2. The RF deposited film had a needle-like structure with a longitudinal dimension range of 200-400 nm. The response was measured by applying a constant potential of 1V to the sensing layer and recording the resistance with a picoammeter. This CO sensor acts as a chemical resistor. 8 of Reference 2 shows the dynamic response of the sol-gel sensor and the RF sputtered sensor at 300 degrees to a square concentration pulse of 30 ppm CO. The current change shown in FIG. 8 of Reference 2 is in the pico amp range. This range has the disadvantage that it would be nearly impossible to record this weak output for continuous monitoring without using a very sophisticated device that is not portable. The sensor in Reference 2 was operated at 300 degrees. Heating the sensor substrate consumes a lot of electrical power. This is a disadvantage for portable equipment.

There has also been continued research on how to deposit metal oxides. See, for example, H. Liu, F. Favier, K. Ng, M.P. Zach, R.M. Penner's "Size-selective electrodeposition of meso-scale metal particles: a general method," in Electrochimica Acta 47 pp. The authors of 671-677 (2001) demonstrated that monodisperse nanoparticles of molybdenum dioxide can be grown on conductive surfaces using pulsed voltammetric techniques. 5 of Reference 3 shows an electron scanning micrograph of molybdenum dioxide metallic nanoparticles on a graphite-based flat surface. As shown in this figure, nanoparticles have an apparent size of 100-200 nm, as indicated by the scale line of 1 micron. It is also possible to oxidize existing metal films.

Despite the above techniques, there are strong demands on alternative technologies for heated metal oxide sensors. In particular, there is still a need for gas sensors with low power requirements, extensive environmental operating range, fast response time, high selectivity and high sensitivity.

The inventors have found that any known metal oxide sensor operating by thermal activation can be operated preferably without thermal excitation by applying alternative energy activation methods. Applying an alternative energy method provides a similar change in the conductance mechanism and surface energy state of metal oxides. For example, the application gate bias of a thin film transistor (TFT) architecture reduces the thermal requirements of the sensor described above. FIG. 1 shows how gate bias can manipulate metal oxide surface energy and change the energy barrier through grain boundaries. In addition, the excitation method can be optical, magnetic or a combination thereof.

In some embodiments, the sensing mechanism of the metal oxide sensor mentioned above is independent of the operating temperature. In some such embodiments, the semiconductor channel is electrically manipulated. As an example, the sensor operates at -60F to 140F under the same gate bias.

The TFT architecture may include a semiconductor thin film containing a compound capable of chemically reacting with the analyte gas. It is preferable that the compound is a metal oxide. The chemical reaction is preferably electron transfer. Thus according to an embodiment of the present invention the gas sensor comprises a metal oxide. The metal is preferably a transition metal, more preferably a Group 6B element, even more preferably molybdenum.

The inventors found that making signal amplification of thin film transistor (TFT) architecture overcomes the weak output problem of the sensor detecting the analyte gas.

Also in combination or alternatively, in accordance with an embodiment of the present invention, the gas sensor operates using electron transfer from the analyte gas. This electron transfer can be the result of a catalytic reaction on the sensor surface. Electron transfer may be electron donating. Alternatively, the electron transfer can be electron withdrawing. The gas to be analyzed is preferably carbon monoxide.

Further, in combination or alternatively, according to an embodiment of the present invention, the gas sensor includes a sensor architecture using semiconductor metal oxides. In the sensor architecture, the metal oxide may be in the form of a thin film. For example, the metal oxide may be in the form of nanoparticles or a network of nanoparticles. The sensor architecture preferably includes a thin film transistor architecture.

In addition, or in combination or alternatively, according to an embodiment of the present invention, the gas sensor includes at least one metal oxide nanoparticle produced by a growth method. The thin film can be grown in situ by a first deposition of a metal, followed by oxidation thereof. The metal layer growth method may include solution processing including electrochemical growth, sputtering, metal evaporation, sol gel and nanoparticle solution. The metal layer deposition method may be a combination of the above mentioned techniques. After deposition of the metal layer, the next treatment, oxidation, is necessary to change the metal into the corresponding metal oxide. Such oxidation can be performed thermally, chemically, electrochemically, or a combination thereof. The electrical conductivity through this metal oxide film can be determined by the grain boundary boundary.

Also in accordance with an embodiment of the present invention, the gas sensor includes a microprocessor.

The invention also relates to a gas sensor that operates by applying an external force to adjust the sensor by altering the surface energy of the semiconductor layer and alters the electrical transport through the semiconductor layer. Adjustment of surface energy can be accomplished by sealing the gate. In addition, other methods can be used to tune the detection of the analyte and include optical excitation, chemical dopants, surface chemical layers, magnetic fields, and combinations thereof. In addition, applying external forces, such as gate bias, helps to eliminate or greatly reduce the requirement for a heated substrate surface.

In addition, according to an embodiment of the present invention, the gas sensor includes RF integration for remote sensing.

According to any one of the above-described embodiments of the present invention, the gas sensor can be operated like a dosimeter.

As such, according to an embodiment of the present invention, the gas sensor for detecting the gas to be analyzed may include a first contact, a second contact, a semiconductor layer, an insulating layer, a substrate, and a third contact. Preferably, the third contact functions as a gate contact. The gate contact may be a substrate. It is preferable that a semiconductor layer contains the compound which can chemically react with the gas to be analyzed. The chemical reaction is preferably electron transfer. The insulating layer, the semiconductor layer, the first contact, the second contact, and the third contact are preferably arranged in a predetermined architecture, so that the sensor can be configured with the first and the first, which occur when an electron transfer event occurs between the compound and the analyte. A change in the level of the analyte gas (eg labeled CO) is detected as the change in current between the two contacts. In this embodiment, the CO is oxidized to carbon dioxide (CO ₂ ). When this reaction occurs, electrons are transferred from the CO to the metal oxide surface. This electron transfer can be facilitated when a gator voltage is applied between the first contact and the third contact. The transfer of electrons (or holes) generated through this electron transfer reaction between the analyte and the compound can be manipulated when a gate voltage is applied between the first and third contacts. Manipulation of electron transfer can be accomplished by changing the energy barrier height in the grain boundary layer between nanoparticles. Applying a gate bias can reduce thermal requirements for the sensor and allow operation at room temperature and below without external or internal heated substrates.

In addition, according to an embodiment of the present invention, the gas sensor may include a signal amplifier including a thin film transistor including a semiconductor thin film including an oxide of molybdenum. The signal amplifier may be external. The signal amplifier may be a thin film transistor sensor.

For a more complete understanding of the invention, and its advantages, reference numerals are now made to the following descriptions as shown in the accompanying drawings.

Figure 1 shows the adjustment of grain boundaries with electron transfer by application of surface energy and gate bias for the reaction.

2 illustrates an embodiment of the invention.

3 is a graph illustrating sensitivity based on internal amplification of a gas sensor in accordance with an embodiment of the present invention.

4 shows a gated metal oxide sensor architecture.

5 is a conceptual diagram illustrating additional processing steps for fabricating a gas sensor using a substrate as a third gate contact in accordance with an embodiment of the present invention.

6 is a conceptual diagram illustrating a gas sensor for a remote application in accordance with an embodiment of the present invention.

7 is a response curve of an exemplary gas sensor in accordance with an embodiment of the present invention.

8 is another response curve of an exemplary gas sensor in accordance with an embodiment of the present invention.

Referring now to FIG. 1, FIG. 1 shows the possibility of adjusting the sensor. 1A shows an enlarged surface topography of metal oxide between two contacts. The metal oxide and its contacts are on top of the insulating layer separating the third contacts. Surface topography produces grain boundaries, where the conductivity of electrons or holes is determined by an energy barrier to reach from one grain to an adjacent grain. Referring now to FIG. 1B, we can see the energy barrier associated with the grain boundary indicated in FIG. 1A. In this embodiment, we can adjust the conductivity through the metal oxide layer. This is accomplished by changing the height of the energy barrier at each grain boundary. When a surface reaction occurs, for example when CO oxidizes to CO ₂ , the transferred electrons are captured near the reaction site between these energy barriers. The conventional example has shown that high heat can provide enough energy to transfer electrons beyond the energy barrier. In this embodiment, the inventors reduce the barrier height by applying electrical bias to the contact 3, thus eliminating the requirement for heat. Referring to FIG. 1C, the inventors show how surface reaction energy can be adjusted. In FIG. 1C, the metal oxide is a semiconductor and has a band gap. This band gap is energy and promotes electrons up from the balance band to the conduction band. Conventional examples have demonstrated that heat is required to produce a favorable distribution of electrons in the balance band and the conduction band in order for a surface reaction such as CO to CO ₂ to occur. In this embodiment, the distribution of electrons in the balance band and the conduction band is adjusted by the application of gate bias.

Referring now to FIG. 2, the gas sensor shown in FIG. 2 includes a first contact, a second contact (eg, labeled metal contact pad), a semiconductor (eg, labeled metal oxide nanoparticle layer). Detecting an analyte gas comprising a layer, an insulating layer (e.g. labeled dielectric insulator), a substrate (e.g. labeled Si gate), and a third contact (not shown) in contact with the substrate A gas sensor to illustrate is illustrated. It is preferable that the third contact functions as a gate contact. The semiconductor layer preferably includes a compound capable of chemically reacting with the gas to be analyzed. With continued reference to FIG. 2 and described further below, the insulating layer, semiconductor layer, first contact, second contact, and third contact are preferably arranged in a predetermined architecture, whereby the sensor is configured to include the first contact and the first contact. The change in the analyte gas level (eg labeled CO) is detected in accordance with the conductivity change between the first and second contacts that occurs when a gate voltage is applied between the three contacts.

The exemplary gas sensor disclosed herein is a miniaturized, low power, fast responding CO sensor based on a metal oxide nanoparticle network applied to a thin film transistor (“TFT”) architecture. Any metal oxide is an n-type semiconductor, which shows an increase in conductivity due to electron transfer caused by oxidation or reduction of the analyte gas. This change in conductivity is proportional to the concentration of the analyte gas. The nanoparticle network approach offers significant improvements in sensitivity and selectivity that surpass the most successful commercial technologies for CO detectors based on metal oxide films for the following reasons: The nanostructured interaction of metal oxide nanoparticle networks provides higher sensitivity compared to commercial detectors. It is possible to operate in the lower ppm range (eg 0-200 ppm), which cannot be achieved in thick tin oxide thin films based on commercial detectors. Moreover, metal oxide nanoparticle networks can be refreshed by ambient oxygen. In addition, the TFT design allows for increased sensitivity due to built-in gain. The gain comes from the nonlinear current versus voltage curve that is characteristic of semiconductors. In addition, the salient features of the proposed device that provide improvements over existing technologies include the following advantages: inherent gain through TFT architecture; On-chip design and integration; Quick response and continuous monitoring; Leaflash generation through chemistry and gate voltage; Quantitative response; And time-integrated response to cumulative impressions. TFT architectures can be created using CMOS processes for manufacturing in very parallel and low cost.

The ability to apply a gate bias can adjust the sensitivity of the device. Previous work by Fan et al. [Fan, Z. et al. "ZnO nanowires field-effect transistor and oxygen sensing property" Appl. Phys. Lett. 2004 , (85) 24, 5923-5925, indicated herein by reference 4, present an oxygen sensor made of zinc oxide (ZnO) nanowire field effect transistors. This study showed that the sensitivity of the device to oxygen varies as a function of gate bias. However, this device operates with a basic difference from the present invention. Oxygen in this device is physically adsorbed to the ZnO surface. Oxygen adsorption atoms (adsorbed atoms) are negative electrical and move electron density from the semiconductor, altering the distribution of current carriers. This change in current concentration is the same as that which occurs when the gate bias is applied to any semiconductor, and by definition is a change in current due to the field effect. This conventional device operates as a ChemFET. Such devices will not operate at ambient environmental conditions due to surface saturation at normal oxygen concentrations. This is not a sensor but only a physical change in transistor response due to a change in the environment.

The present invention works through chemical reactions on metal oxide surfaces. This chemical reaction can add (via oxidation) or remove (via reduction) electrons from the semiconductor layer of the sensor. Changes in the number of electrons will change the current through the device. Even if the material is a poor semiconductor, the removal of electrons will always cause a change in current, but oxygen adsorption that changes the electron density or distribution will not.

Prior to that, Dalin [Dalin, J. "Fabrication and characterization of a novel MOSFET gas sensor" Final Thesis at Linkopings Institute of Technology, Fraunhofer Institute for Physical Measurement Techniques, Frieburg, Germany, 6-5-2002, LiTH-ISY-EX -3184, herein indicated 5, shows that the gate bias can regulate the current through the tin oxide (SnO ₂ ) gas sensor operated at 200 ° C or 280 ° C. This is a heated sensor. The level of current through the sensor has changed, but the sensitivity has not. For example, FIG. 6.5 of Reference 5 shows the actual current values for different CO concentrations at the variable gate bias. This figure shows the significant change in sensor response current at each gate bias. In FIG. 6.6 of Reference 5, the response of the heated SnO ₂ sensor to CO is plotted as an initial current above the actual current. In this figure, measurements can be made, but as a function of the gate biaser, there is a small change in sensitivity. If we examine the change by the reaction, we can see that the baseline of the reaction changes almost the same as the exposed reaction. This indicates that the gate bias changes the current level through the sensor but does not increase the sensitivity. The sensor in Reference 5 did not operate at low temperatures.

The gated metal oxide sensor of the present invention does not require heat for operation. This gated metal oxide sensor operates from -60 to 100 degrees Celsius or more. This suggests an increased response at low temperatures.

Technical Approach-Chemistry:

Continuous oxide chemistry is the driving force of the present invention for sensing electron transfer from the surface reaction of the analyte gas. Gases to be analyzed include, but are not limited to, carbon monoxide (CO) and other electron donating and / or electron accepting species. The success of the sensor requires a maximized semiconductor reaction in the metal oxide. While certain metal oxide thin film / nanoparticle systems are deemed appropriate in the present invention, more specific transitional metal oxides such as molybdenum oxide (MoO ₃ ) are used because of their inherent properties for CO. Metal oxides exist in many forms. For example, molybdenum oxide may be MoO, MoO ₂ , MoO ₃ depending on the oxidation state of the metal. Although the present invention cites MoO ₃ as a specific example, it may be applied to other molybdenum oxides of the present invention. MoO ₃ is an n-type semiconductor, which will oxidize Co through electron transfer, which will cause a measurable change in resistance. Molybdenum trioxide includes molybdenum in its hexavalent state. Hexavalent molybdenum has no electrons in its 4d orbital. Thus, oxidation of carbon monoxide means electron transfer from CO to Mo ⁺⁶ . Following its initial electron transfer step, several reaction pathways are possible for the next oxidation of carbon monoxide to carbon dioxide. Most commonly, this pathway means a fast free radical chain stage, which is transferred to a fast and sensitive sensor.

The present invention includes, but is not limited to, molybdenum oxide nanoparticles for CO detection. Molybdenum oxide exhibits any inherent properties suitable for the present invention. Other metal oxides can be used in the solid state sensor design, but none have the same properties as molybdenum oxide. Two other metals in Group 6B are chromium (Cr) and tungsten (W). They have similar chemical properties to molybdenum oxides. CrO ₃ at the top of the cycle will be more reactive. This reactivity is expressed in cost. More reactive species will produce a more stable product that reverses the reaction, ie increases the difficulty of relashing the sensor. Increased responsiveness will also reduce the selectivity of the sensor. Conversely, WO ₃ at the bottom of the row is less reactive and will reduce sensitivity, but more easily reverses. Molybdenum oxide has the highest reactivity while having the easiest reversibility. Metal oxides other than Group 6B did not exhibit the required sensitivity or selectivity for CO. This includes tin dioxide.

Chemical obstructions such as moisture vapor for the proposed sensor system are not a problem. Automotive, or other contaminants possible in an industrial or home environment, such as carbon dioxide, nitrogen dioxide, or saturated hydrocarbons, are not expected to interfere with this sensor system. They will not fix MoO ₃ through electron transfer and therefore will not generate a signal. It is possible to design metal oxide materials that will be specific and selective to a particular analyte. It is also possible to design metal oxide materials to exclude the sensitivity of a particular analyte. Additionally, gate bias applied to metal oxide materials can enhance selectivity for chemically similar analytes.

The use of spherical nanoparticle membranes has several advantages over other types of nanoparticles. Spherical nanoparticles increase the probability of active surface atoms (diameter in the range of 5-300 nm). The atoms in the middle of the particle, called "bulks," do not contribute electrically to any reaction or binding event. When a reaction or binding event occurs, these surface atoms further contribute to the overall electrical structure of the nanoparticles. This increased contribution translates into a signal that is increased slightly. Semi-spherical nanoparticles have a similar surface to bulk ratio. This will include nano- "bumps" on the substrate or small grains of material on the surface. In addition, very thin films of metal oxides will exhibit increased sensitivity due to the high surface in bulk ratio. This will occur when the thickness is close to the divide-length for the semiconductor material, but overall conductivity must also be considered.

TFT sensor design:

Due to the semiconductor nature of MoO ₃ , the present invention uses a thin film transistor (TFT) architecture to maximize the signal output of the CO sensor. Molybdenum oxide is an n-type semiconductor material. This means that the conductivity through the material can be controlled by a third terminal, commonly called a gate. Since this is an n-type semiconductor, the resistance will decrease as it is moved to a positive gate voltage. Electron transfer by oxidation from CO to CO ₂ will increase the number of electrons in the MoO ₃ film, thus increasing the number of carriers and the current through the device. This is equally effective as applying a positive gate voltage.

Referring back to FIG. 2, FIG. 2 shows a metal oxide semiconductor thin film transistor for CO detection. Electron transfer occurs when CO is oxidized at the surface of the metal oxide. This surface reaction changes the carrier concentration of the n-type semiconductor and is measured by the current change.

The semiconductor nature of MoO ₃ contributes to the increased sensitivity of the sensor. TFT architecture has an inherent gain, which means that very small changes in gate voltage (e.g., electron transfer caused by oxidation of CO) produce changes in current. The electron transfer event from the oxidation of a gas such as CO to the metal oxide nanoparticle framework is shown in FIG. 1. As shown in FIG. 1, this electron transfer event causes a change in current, which can reach orders of magnitude depending on the slope of the current versus gate voltage curve. This curve is based on n-type semiconductor material. The present invention can be adjusted for gases that can be reduced by making semiconductor channel p-types. In this type of device, the surface will provide electrons that allow the oxidized chemical species to be reduced. The change from n-type to p-type will require a change in the chemical properties of the metal oxide by changing the doping of the base metal, the alloy of other metals or a small amount of additional material.

Referring now to FIG. 3, FIG. 3 shows the current versus gate voltage for an n-type semiconductor, which is in terms of the current ΔI as a result of changing the small gate voltage ΔV due to electron transfer from CO. High intrinsic change.

As can be seen in FIG. 3, the movement from point A to point B requires only a small change in gate voltage. This small change in voltage causes a large change in current (approximately three orders of magnitude), thus increasing the inherent ΔV / ΔI. This signal amplification allows for an unparalleled level of sensitivity or otherwise cannot be achieved in chemistoristor conversion. In addition, adjustment of the gate voltage causes the thin wire to operate in the I-V curve, here the most sensitive region of the slope, so the sensitivity of the device is the steepest.

TFT architecture can be fabricated by methods including the method of electrochemically depositing MoO 3 nanoparticles on a conductive substrate as described below. In addition, the metal oxide may be deposited by a solution method. Metal oxides may also be grown by oxidizing thin metal films or metal nanoparticle films.

We consider nanoparticles growing in two parallel approaches using, for example, Gamry Potentiostat with a PC interface. In Approach I, the process involves the direct deposition of nanoparticles on a conductive substrate, which then converts the surface portion of the conductive substrate into an insulating layer. In Approach I, a two-step process involves indirect deposition of nanoparts on a conductive substrate and removal of nanoparticles from the conductive substrate, followed by deposition of nanoparticles on the insulating substrate. Direct and indirect deposition are described by way of example, such as electrochemical growth, but alternative deposition methods known in the art, such as sputtering, thermal evaporation, electron beam evaporation, etc., may be considered. It can be understood that there is. Also depending on the deposition method, the initial deposition can occur on any suitable surface, selecting between conductors, insulators and semiconductors.

Manufacturing steps:

Approach I: Electrochemical Growth of Metal Oxide Nanostructures:

Referring now to FIG. 3, FIG. 3 illustrates an electrochemical fabrication step to create a MoO ₃ TFT-based CO sensor. Step A will electrochemically deposit a nanoparticle film on the SI substrate. Step B will thermally grow the gate oxide.

Still referring to FIG. 3, the inventors will begin with a conductive silicon substrate and grow the nanoparticles directly until the conductive film is achieved and reaches step B. The silicon substrate will eventually function as a global back gate. Electrochemical fabrication of metal oxide nanostructures will be carried out by a two step process, which involves a nucleation step (high negative voltage for a short time of less than 10 seconds), and water / organic A long growth stage (up to 10 minutes) with low negative voltage in the metal oxide solution. Electrochemical preparation can be performed in a constant voltage mode (chronoamperometry) or in a constant current mode (chronopotentiometry).

Thermal oxidation will follow after electrochemical growth to grow silicon oxide under the molybdenum oxide layer. (Step B) This 1000 ° C. thermal step will provide two functions. First, it will provide high quality thermal silicon oxide gate insulators and avoid the alignment problem of photolithography. Secondly, the thermal layer will anneal the semiconductor molybdenum oxide film to increase conductivity. Higher conductivity will reduce the need for complex electronics to reduce noise when measuring low current signals. As an alternative to direct electroplating of metal oxides, a second similar approach is possible.

2. Approach II: Indirect Electrochemical Growth of Metal Oxide Nanospheres:

The present invention will involve a procedure wherein molybdenum oxide will nucleate and grow on a conductive substrate (eg, a newly cleaved graphite surface), followed by removal of nanoparticles from the conductive substrate and collection in solution Will follow. This will allow us to solve the deposition of nanoparticles onto any insulating substrate. We will create conductive nanoparticle films using drop casting or Langmuir-Blogett solution based technology. These two techniques are known for depositing continuous nanoparticle films. These techniques are shown in FIG. Once the molybdenum oxide film is formed, we will move it to the front end device processing.

Referring again to FIG. 4, FIG. 4 shows indirect electrochemical fabrication steps for creating a MoO 3 TFT based CO sensor. Step A will electrochemically deposit the nanoparticle film on a conductive (eg graphite) substrate, followed by a harvest step where the nanoparts are dispersed in a liquid suspension. And the nanoparts can be deposited by a language such as a Langblad blowjet film or dropcast on an insulating substrate.

3. Approach III: Oxidation after metal deposition.

The present invention may involve a procedure wherein molybdenum is deposited onto the substrate. This metal layer may be deposited by liquid suspension of metal nanoparticles, thermal evaporation, sputtering, electron beam evaporation or other techniques known in the art. This metal layer will be deposited on the insulating substrate, followed by oxidation to the metal oxide. The metal layer can be oxidized by heating in an oxygen containing environment. Controlling the oxidation temperature and time will control the conductivity of the metal layer. The metal may be oxidized at a temperature of 100 ° C to 1400 ° C. The metal can also be oxidized chemically or electrochemically. Once this metal oxide layer is formed, we can move on to final processing.

4, final photolithography manufacturing steps and device assembly:

Referring now to FIG. 5, FIG. 5 illustrates a photolithography fabrication step for a MoO ₃ CO sensor.

Upon completing Approaches I, II, or III, several additional steps will be necessary to complete the device. Referring again to FIG. 5, the first metal contact electrode that generates the source and drain will be double-plated onto the MoO ₃ film using standard photolithography and lift-off. (Step C) The contacts will be used to measure the resistance or current through the active region of the device. Next, the active region of the device will be patterned using photolithography as shown in step D. The photoresist will create a protective film for subsequent reactive ion etching. This etching step will remove molybdenum oxide in unwanted areas on the chip, prevent crosstalk between adjacent sensors, and determine the dimensions of the active termination device. The final step will create a hole in the insulated silicon oxide, and then create a gate contact to the underlying substrate by metallization (as shown in step E). After completion of wafer fabrication, they will be broken into individual sensor elements. The sensors will then be mounted to the multi-pin header using an appropriate epoxy. And the three contacts (source, drain and gate) will be wired to the header pin using a packing method known to those skilled in the art.

Microprocessor Development and Device Integration:

Microprocessors developed as part of the present invention will have the ability to adjust gate voltage, measure current through our CO sensor, calculate CO concentration, drive a digital display, and output to an alarm device. The device box will include both a piezo-based acoustic alarm and an LED based visual alarm. The microprocessor will be required to operate one or more input channels. A non-operating control sensor will be integrated into the device to counteract aging and temperature flow. The control channel will be measured with the active sensor during each sampling period. Data samples are averaged to filter noise and converted to CO concentration levels. The on-board LCD display will be updated every 20 seconds or less.

Referring now to FIG. 6, FIG. 6 illustrates radio frequency (RF) integration for remote applications.

The same architecture will be fully compatible with low-power RF links for remote readout. Referring again to FIG. 6, FIG. 6 shows a possible arrangement. Software performance will handle normal “in operation”, battery status reports, and readings of measurement changes. An approximate duty cycle of 0.1% activity is assumed to report the CO change occurring at about 10 second intervals. This maintains about 2mW of static power consumption and occurs at 0.1% intervals. This low average power consumption allows for long battery life.

The invention will be more readily and fully understood by the following examples. This example shows a gas sensor according to an embodiment of the present invention.

example

The sensor was prepared by growing molybdenum trioxide, such as an exemplary sensing compound, in a thin film arranged as part of a thin film transistor architecture. Growth is followed by electron beam evaporation of molybdenum followed by thermal oxidation of molybdenum. The membrane structure is characterized using an electron scanning microscope (SEM). The membrane had a nanoparticle structure. The deposited metal film had a thickness of less than 20 nm.

7 shows the sensor's response to a continuous flow of carbon monoxide at 50 ppm in a sealed chamber. The sensor was operated at room temperature. The response was measured as a normalized ratio of resistance (R) in the presence of carbon monoxide to resistance (R ₀ ) in the absence of carbon monoxide as a function of time. The response was measured when the gate voltages were at two different values, + 5V and -5V. The lines marked ON and OFF indicate the amount of CO turned on and off. The upper curve (-5Vg) shows the response to CO. The curve below (+5 Vg) shows no response to CO. This result demonstrates that the response of the sensor can be adjusted by changing the gate voltage.

8 shows the response of the same sensor to a continuous flow of carbon monoxide at increasing levels of concentrations of 2 ppm, 5 ppm, 10 ppm, and 20 ppm. The sensor was operated at room temperature. The response was measured as a normalized ratio of resistance (R) in the presence of carbon monoxide to resistance (R ₀ ) in the absence of carbon monoxide as a function of time. The results demonstrate the sensor's sensitive response to low levels of gas. The response to CO is linear.

This detailed result also demonstrates that the requirement for heating the sensor to operate at room temperature (22 degrees C) is eliminated by using a gate bias.

The inventors have found that they can operate at lower temperatures (eg -60 degrees F) than at room temperature. Thus, the sensors can operate at ambient temperatures facing up to 40,000 feet above ground, and are therefore adapted for use in aircraft or other high altitude applications. Thus, the method of operating the sensor may include adjusting the gate voltage in accordance with the temperature.

The inventors also found that the gate bias can be adjusted for various analyte gases at a wide range of concentrations.

Thus, a method of operating a sensor in accordance with an embodiment of the present invention may include adjusting any one or combination of gate bias and sensing compounds to select an analyte.

Although the invention and its advantages have been described in detail, it should be understood that various alterations and modifications can be made here without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (64)

Sensor for detecting the presence of the gas to be analyzed, The sensor functions by adjusting the surface energy of the semiconductor layer and the conductivity of the semiconductor layer, Board; Insulating layer; A semiconductor layer separated from the substrate by the insulating layer; A first contact in contact with the semiconductor layer; A second contact in contact with the semiconductor layer; And A third contact in contact with said substrate, The semiconductor layer includes a compound capable of chemically reacting with the analyte gas; The third contact is a conductive layer on the substrate separated from the semiconductor layer by an insulator; And The insulating layer, the semiconductor layer, the first contact, the second contact, and the third contact are arranged in a predetermined architecture such that the sensor occurs when a voltage is applied between the first contact and the third contact. The analyte gas presence detection sensor which detects a change in the level of the analyte gas in accordance with a change in current between the first contact and the second contact. The method of claim 1, The compound comprises a metal oxide, analyte gas presence detection sensor. The method of claim 2, Said metal comprises a transition metal. The method of claim 3, The metal comprises a mixture of metals. The method of claim 3, The mixtures are secondary, tertiary, quadranary mixtures and additional metals serve as dopants for changing orbital energy levels. . The method of claim 3, The transition metal comprises group 6B elements. The method of claim 6, And the group 6B element includes molybdenum oxide. The method of claim 1, Wherein said chemical reaction comprises electron transfer. The method of claim 8, The electron transfer comprises electron donation to the compound by the analyte gas. The method of claim 8, Wherein the electron transfer comprises an electron withdrawal to the compound by the analyte gas. The method of claim 1, The analyte gas includes carbon monoxide, the analyte gas presence detection sensor. The method of claim 1, The semiconductor layer comprises a thin film, analyte gas presence detection sensor. The method of claim 12, The thin film comprises a plurality of nanoparticles containing the compound, the gas to be detected sensor. The method of claim 13, The nanoparticles are spherical, analyte gas presence detection sensor. The method of claim 13, The nanoparticles are not spherical, analyte gas presence detection sensor. The method of claim 2, The continuous metal oxide has a rough surface, wherein topography forms grain boundaries. The method of claim 1, And a gate bias affects conductivity through the semiconductor layer. The method of claim 16, Gate bias affects energy barriers between grain boundaries. The method of claim 13, And the nanoparticles have a homogeneous size. The method of claim 1, The architecture comprises a thin film transistor architecture. The method of claim 1, Wherein said sensor is made by a method comprising growing nanoparticles comprising said compound. The method of claim 12, Wherein the growth of the semiconductor layer comprises electrodeposition. The method of claim 12, The growth of the semiconductor layer comprises a sputtering (sputtering), analyte gas presence detection sensor. The method of claim 12, Wherein the growth of the semiconductor layer occurs directly on the insulating layer or a conductive precursor of the insulating layer. The method of claim 12, Wherein the growth of the semiconductor layer occurs indirectly away from a surface of the insulating substrate or from a precursor of the insulating substrate. The method of claim 25, The method further comprises applying the nanoparticles to the surface. The method of claim 1, Wherein said sensor is made by a method comprising the deposition of a metal thin film followed by its transformation into a metal oxide. The method of claim 27, Said transformation to metal oxide is produced by thermal annealing. The method of claim 27, Wherein said transformation into a metal oxide is produced by a chemical reaction. The method of claim 27, Said transformation to metal oxide is produced by an electrochemical reaction. The method of claim 21, The method further comprises thermal annealing the nanoparticles. The method of claim 1, The gas sensor further comprises a microprocessor adapted to regulate the voltage. The method of claim 1, The gas sensor further comprises a radio frequency link adapted to remotely readout a change in current. The method of claim 1, The gas sensor is adapted to function as a dosimeter. The method of claim 2, Sensor event detection sensor, wherein the detection event occurs without any requirement for heat. The method of claim 2, Wherein the sensor operates independently of temperature and is a function of gate voltage. The method of claim 1, The sensor may be operated at varying conductivity levels. The method of claim 37, Wherein the electrical current level is less than 1 nanoampere. In the method for detecting the presence of the gas to be analyzed using a semiconductor layer, Wherein the semiconductor layer is altered in a manner selected from the group consisting of (a) adjustment of surface energy of the semiconductor layer, (b) adjustment of conductivity of the semiconductor layer, and (c) combinations thereof. . The method of claim 39, The analyte gas is carbon monoxide (CO), analyte gas presence detection method. The method of claim 39, And said semiconductor layer has surface topography. The method of claim 41, wherein Wherein said surface photography produces grain boundaries. The method of claim 42, wherein And an energy barrier is present in the grain boundary. The method of claim 43, And the semiconductor layer is part of a sensor. The method of claim 39, And the semiconductor layer is adjusted by application of a bias through a third contact. The method of claim 43, Wherein said energy barrier is adjusted by radiation. 47. The method of claim 46 wherein And the radiation is light. The method of claim 43, The energy barrier is controlled by a magnetic field. The method of claim 43, And the energy barrier is adjusted by varying the semiconductor material contained in the semiconductor layer. The method of claim 44, Wherein the semiconductor layer is a different metal oxide. The method of claim 44, And the semiconductor layer is a mixture of metal oxides. The method of claim 39, The surface energy is adjusted by promoting electrons from one energy band to another. The method of claim 52, wherein The energy band is a molecular orbital, analyte gas presence detection method. The method of claim 39, The surface energy of the semiconductor layer is adjusted by application of a gate bias through the third contact. The method of claim 54, Said sensitivity is in accordance with an adjustment of surface energy. The method of claim 39, And said surface energy of said semiconductor layer is adjusted by exposure to radiation. The method of claim 39, And said surface energy of said semiconductor layer is adjusted by a magnetic field. The method of claim 43, The energy barrier is adjusted by varying the semiconductor material. The method of claim 58, Wherein the semiconductor material is a different metal oxide. The method of claim 58, Wherein the semiconductor material is a mixture of metal oxides. The method of claim 39, Wherein the semiconductor layer is part of a sensor that is sensitive to multiple analytes by adjusting gate bias. 62. The method of claim 61, The sensor is selective for each analyte by application of a specific gate bias. As a gas sensor, The gas sensor includes a signal amplifier, the signal amplifier comprises a thin film transistor, the thin film transistor comprises a semiconductor thin film made of molybdenum oxide, and the gas sensor comprises (a) adjusting the surface energy of the semiconductor thin film, and (b) controlling the conductivity of the semiconductor thin film, and (c) changing the semiconductor thin film in a manner selected from the group consisting of a combination thereof. In the method for detecting the gas to be analyzed, The method employs the use of the sensor of claim 1.

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