Abstract:
A microfabricated TID comprises a microhotplate and a thermionic source disposed on the microhotplate. The microfabricated TID can provide high sensitivity and selectivity to nitrogen- and phosphorous-containing compounds and other compounds containing electronegative function groups. The microfabricated TID can be microfabricated with semiconductor-based materials. The microfabricated TID can be combined with a microfabricated separation column and used in microanalytical system for the rapid on-site detection of pesticides, chemical warfare agents, explosives, pharmaceuticals, and other organic compounds that contain nitrogen or phosphorus.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to gas chromatographic detectors and, more particularly, to a microfabricated thermionic detector for use with a microanalytical system. 
     BACKGROUND OF THE INVENTION 
     Gas chromatographic (GC) detectors are based on a variety of physical and chemical processes. Some of the most widely used GC detectors function by converting chemical species in the GC effluent to gas-phase ions, which can then be collected and directly transduced to an electrical signal. For example, the flame ionization detector (FID) is the most universally applied GC detector, because of its high sensitivity, wide dynamic range, and ability to detect a variety of organic compounds. However, the FID is a nonspecific detector that responds, in general, to all chemical compounds that contain carbon. 
     Particularly when the compound of interest cannot be easily resolved above background, it may be advantageous to employ a selective detector that responds only to a few limited compounds with little or no response to interfering compounds. A thermionic detector (TID), as known as a nitrogen-phosphorous detector (NPD), is such an element-specific ionization detector. The TID relies on the specific ionization of the sample compound near a hot thermionic surface, resulting from the high selectivity of surface ionization with respect to the ionization potential of the sample compound. Therefore, the TID can provide very specific ionization of sample compounds containing electronegative functional groups, such as nitrogen, phosphorous, halogen atoms, and sulfur. A conventional TID typically has a ratio of 10 4 :1 selectivity over carbon. Because of its sensitivity and selectivity for nitrogen and phosphorus, the TID is especially useful for the analysis of pesticides, chemical warfare agents, explosives, pharmaceuticals, and other organic compounds that contain nitrogen or phosphorus. 
     In  FIG. 1  is shown a conventional TID  10  of the type used with a laboratory-based analytical system. The TID is similar to a FID, except that a rubidium- or cesium-containing bead  11  (e.g., a rubidium silicate bead) is formed on a heater coil  12  (e.g., a platinum wire). The bead  11  is electrically heated by running a current through the coil  12  to provide a thermionic source. The temperature of the thermionic source can affect the sensitivity of the detector, since the source must remain hot enough to produce a reactive chemical environment. Typically, the source is heated to between 600 and 800° C. The bead  11  is situated above a jet  13 , through which passes a sample gas (e.g., containing a nitrogen or phosphorous compound)  15  that can be mixed with hydrogen  16 . Make-up gas (e.g., air or oxygen)  17  can also be introduced to dilute the hydrogen mixture. Typical flow rates for a conventional TID are 2 to 6 mL/min of H 2  and 60 to 200 mL/min of air, dependent on the interior volume of the detector. The negative ions produced by thermochemical reactions that occur when the gases impinge on the low work function surface of the hot bead  11  are collected by a positively biased collector electrode  18  and counted with a sensitive electrometer circuit. A typical circuit can detect 0.4 pg of nitrogen and 0.1 pg for phosphorus. See R. P. W. Scott,  Chromatographic Detectors , Marcel Dekker (1996). 
     Laboratory-based analytical systems can provide precise and accurate results, however, the time between sample collection in the field and the availability of results from the laboratory can often be weeks. Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive on-site detection of particular chemicals, including pollutants, high explosives, and chemical warfare agents. Preferably, these microanalytical systems should provide a high chemical selectivity to discriminate against potential background interferents and the ability to perform the chemical analysis on a short time scale. In addition, low electrical power consumption and reagent usage is needed for prolonged field use. See, e.g., Frye-Mason et al., “Hand-Held Miniature Chemical Analysis System (μChemLab) for Detection of Trace Concentrations of Gas Phase Analytes,”  Micro Total Analysis Systems  2000, 229 (2000). 
     Although conventional TIDs are now widely used in laboratory-based analytical systems, there remains a need for a microfabricated TID that can be used with microanalytical systems. Typically, the microfabricated TID can be combined with a microfabricated separation column and used in gas chromatography analysis to detect the nitrogen and/or phosphorous content in analytes eluted from the column. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a microfabricated TID, comprising a microhotplate comprising a resistive heating element; a lid disposed on a side of the microhotplate to thereby provide a detection chamber; a low work function material disposed on a surface of the microhotplate exposed to the detection chamber to provide a thermionic source; at least one gas inlet attached to the detection chamber for introduction of a sample gas thereinto; an exhaust gas outlet attached to the detection chamber for removal of decomposition products therefrom; and an ion collection electrode disposed in the detection chamber proximate the thermionic source, wherein the electrode collects charge generated by the sample gas reacting with the thermionic source when the microhotplate is heated by the resistive heating element and a voltage is applied between the thermionic source and the electrode. The low work function material can comprise an alkali- or alkaline-earth-containing material, TiN, or LaB 6 . 
     Such a microfabricated TID can provide high sensitivity and selectivity to nitrogen- and phosphorous-containing compounds and other compounds containing electronegative function groups and can minimize the usage of reagents, including hydrogen. Furthermore, the microfabricated TID does not depend on the adsorption of vapor to produce a detector signal. Therefore, compared to polymer-coated microbalances that are commonly used in microanalytical systems, the response of the microfabricated TID is instantaneous. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form part of the specification, illustrate the present invention and, together with the description, describe the invention. In the drawings, like elements are referred to by like numbers. 
         FIG. 1  shows a schematic illustration of a conventional thermionic detector. 
         FIG. 2  shows a schematic illustration of a microfabricated thermionic detector. 
         FIG. 3  shows the response of the microfabricated thermionic detector to dimethyl methyl phosphonate (DMMP) and hexane. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Miniaturization of GC detectors for portable applications has been hampered by the large heat losses experienced by microsystems, due to the large surface-to-volume ratio at the microscale. However, a microcombustor, including an on-chip flame ionization detector (microFID), has recently been disclosed in U.S. Pat. No. 6,786,716 to Gardner et al., which is incorporated herein by reference. That microcombustor uses a catalyst-coated microhotplate to overcome heat losses and provide sustained combustion on the microscale. The microhotplate comprises a thin-film heater/thermal sensor patterned on a thin insulating support membrane that is suspended from its edges over a substrate frame. This microhotplate has very low heat capacity and thermal conductivity and is an ideal platform for heating thermionic source materials placed on the surface of the support membrane. 
     Alternative microhotplates, comprising thermally isolated low mass supports with integral heaters, can be microfabricated with semiconductor industry technologies and are suitable platforms for the microfabricated TID of the present invention. For example, a microbridge can be formed by patterning a thin filament of the resistive heater material on a silicon wafer and underetching the filament to leave a microbridge. The thermionic source material can be placed on the thermally isolated microbridge and the analytes can be flowed over the microbridge. See R. P Manginell et al., “In-Situ Monitoring of Micro-Chemical Vapor Deposition (μ-CVD): Experimental Results and SPICE Modeling,”  Tech. Digest  1998  Sol. State Sensor and Actuator Workshop,  371 (1998) and R. P Manginell et al., “Selective, pulsed CVD of platinum on microfilament gas sensors,”  Tech. Digest  1996  Sol. State Sensor and Actuator Workshop,  23 (1996), both of which are incorporated by reference. Another suitable microhotplate is the heated pivot plate resonator. The pivot plate resonator is basically a small plate, or paddle, that pivots about two torsional pivot arms and thereby enables the measurement of adsorbed mass by detecting the oscillatory motion of the paddle. Thin-film heaters can be included in the fabrication of the paddle to provide a low heat capacity, thermally stable platform on which the thermionic source material can be placed. See U.S. patent application Ser. No. 10/903,329 to Manginell et al., which is incorporated herein by reference. 
     In  FIG. 2  is shown an exemplary microfabricated TID  20  of the present invention, wherein the microhotplate comprises a heated membrane  22  that is suspended from a substrate  23 . The TID  20  can be microfabricated from semiconductor-based materials used in the microelectromechanical systems (MEMS) and integrated circuit (IC) industries. The microfabricated TID  20  comprises a low work function thermionic material deposited on the heated membrane  22  of the microhotplate. The low work function material thereby provides a heated thermionic source  21 . A gas tight lid  24  attaches to the detection chamber side of the substrate  23  to seal the detection chamber  30 . The detection chamber  30  can preferably have a diameter of a few millimeters and a height of about 0.15-1 mm. The lid  24  can have a gas inlet  32  for introduction of the sample and carrier gases into the detection chamber  30  and an exhaust gas outlet  36  for removal of the gaseous decomposition products  37 . The gas inlet  32  can further comprise a pre-mixer section  35  for pre-mixing the sample and carrier gases  33  and  34 . For example, the sample gas  33  can comprise a nitrogen- and/or phosphorous-containing compound. The carrier gas  34  can comprise hydrogen and/or make-up gas. Alternatively, the gases can be fed into the detection chamber  30  through separate gas inlets (not shown). Capillary tubes can be used as the gas inlet  32  and exhaust gas outlet  36 . A resistive heating element  26  can be disposed on the detection chamber side (as shown) or on the opposite side of the membrane  22 . The resistive heating element  26  can be resistively heated by electricity supplied by a power supply and control circuit  27 . Electrical contact to the resistive heating element  26  can be established with perimeter bond pads (not shown). The perimeter bond pads and thin membrane  22  thermally and physically isolate the resistive heating element  26  and the detection chamber  30  from the electrical power source  27  and the substrate  23 . The circuit  27  enables active temperature control of the thermionic source  21  by varying the power into the resistive heating element  26  to maintain a set resistance. The set point resistance, and therefore temperature, is user controlled. A feedback mechanism maintains constant heating element resistance, and hence constant thermionic source temperature. The microfabricated TID  20  has a high thermal sensitivity of typically better than 0.4 mW/° C. The microfabricated TID  20  can attain a temperature of 200° C. in less than 20 msec. 
     Thermal energy is provided by the heat from the electrically heated resistive heating element  26 , not by combustion. Unlike the FID, there is no self-sustaining flame and the chemistry occurs only near the surface of the hot, thermionic source  21 . As a result, nitrogen and phosphorous are ionized with high specificity by a surface-dominated process. The sensitivity of the microfabricated TID  20  depends on both the efficiency of the ion production as well as the efficiency of the ion collection. Therefore, the thermionic source  21  is preferably positioned so that the sample compounds impinge on the surface of the source and the resulting ionization is measured by an ion collector electrode  28  that is proximate the source  21 . The efficiency of ion collection, in turn, depends on the magnitude and direction of the electric field produced within the detection chamber  30 . Therefore, the placement of the electrode  28  will affect the response of the microfabricated TID  20 . Therefore, the pattern of the electric field within the detector volume should be designed to maximize ion collection. Further, the collector electrode  28  should be well insulated electrically from neighboring structures to minimize leakage currents. 
     The ion collection electrode  28  can be positively biased relative to the thermionic source  21  to collect negatively charged ions emitted from the thermionic source when the source is heated by the microhotplate. The applied potential should be sufficiently large to accelerate the negative ions to the ion collection electrode, yet not so large as to result in arcing or ion multiplication. The electric field can be less than about 200 V/cm. Since the electrode spacing can be less than 0.1 cm, applied voltages of less than 20 V can be used. A sensitive electrometer  29  can be used to measure the ionization current generated by the thermionic source  21  and collected by the collection electrode  28 . 
     Microfabrication of the Thermionic Detector 
     The microfabricated TID  20  can be formed by a fabrication method similar to that for the microcombustor. Unlike the microcombustor, that comprises a catalytic material for sustaining combustion, the microfabricated TID  20  of the present invention comprises a low work function material for the thermionic generation of negative ions. However, with the exception of the replacement of the combustion-sustaining catalytic material of the microcombustor with the low work function thermionic material, the processing steps of material deposition, photolithography, masking, etching, mask stripping and cleaning required to form the microfabricated TID  20  are similar to those disclosed by Gardner et al. and are generally well-known in the semiconductor IC industry. 
     The fabrication of the TID  20  comprises the steps of forming the suspended membrane  22  on the substrate  23 , forming the resistive heating element  26  on the suspended membrane  22  to provide the microhotplate, and depositing the low work function thermionic material on the suspended membrane  22  to provide the thermionic source  21 . The substrate  23  used to form the microfabricated TID  20  generally comprises a semiconductor (e.g., silicon or gallium arsenide) or a dielectric (e.g., a glass, quartz, fused silica, a plastic, or a ceramic), with a thickness generally about 400-500 μm. The suspended membrane  22  is typically formed as a rectangle or polygon with lateral dimensions from about one to a few millimeters on a side (e.g., a square of 1-3 mm on a side), or alternatively as a circle or ellipse with a size from one to a few millimeters. The suspended membrane  22  is supported at its edges by attachment to the substrate  23 . The membrane  22  can be sufficiently thick (generally about 0.4-1 μm total thickness) for robustness as required for handling and to support the resistive heating element  26  and the thermionic source  21 . Additionally, the membrane  22  can be sufficiently robust to withstand any stress induced by a mismatch in thermal expansion coefficients of the membrane  22  and the supporting substrate  23  upon heating to a thermionic temperature of over several hundred ° C. Low-pressure chemically vapor deposited silicon nitride is a preferred membrane material due to its low stress, low thermal conductivity, low heat capacity, and compatibility with IC processing steps. The low thermal conductivity minimizes heat loss to the substrate  23  and the low heat capacity enables heating of the detection chamber  30  to elevated temperatures. Other materials such as polycrystalline silicon, silicon oxynitride, and silicon carbide can also be used to form the membrane  22 . 
     A silicon nitride suspended membrane  22  can be fabricated on a silicon substrate  23  by through-wafer etching of a silicon wafer. Either Bosch etching or KOH etching can be used to release the membrane  22 , with no discernable operational differences between the completed devices made by either method. In the case of Bosch etching, an etch stop layer, for example 0.5 μm thermally-grown oxide, can be used to prevent undesired etching of the low-stress silicon nitride membrane layer. Any residual oxide remaining after the Bosch etch can be stripped in buffered HF. For KOH etching, no additional etch stop layer is required. 
     Prior to silicon etching by either method, the thin-film resistive heating element  26  can be patterned on the membrane layer on the opposite side of the silicon wafer from the etch window. The resistive heating element  26  generally can comprise one or more circuitous metal traces formed from one or more layers of deposited metals including platinum, molybdenum, titanium, chromium, palladium, gold, and tungsten that can be patterned on the upper (i.e., detection chamber) side of the membrane  22 . Alternatively, the resistive heating element  26  can be patterned on the underside of the membrane  22 . To form a platinum resistive heating element  26 , a 10-nm-thick adhesion layer of titanium or tantalum, or oxides of these materials, can be deposited on the silicon nitride membrane layer  22  through a patterned photoresist mask having a circuitous opening therethrough, followed by deposition of a 170-nm-thick layer of platinum. The resistive heating element  26  generally covers about 50% of the area of the suspended membrane  22  that forms the detection chamber  30 . The resistive heating element  26  can double as a temperature sensor by monitoring the resistance change of the wire caused by thermal fluctuations. 
     The electronic work function of the thermionic source  21  is determined by the composition of the thermionic surface. The composition preferably provides a low work function surface to facilitate the emission of charged particles from the heated thermionic source. The low work function material can be an alkali- or alkaline-earth-containing glass or ceramic material. The alkali metal is preferably Cs or Rb. The alkaline-earth metal is preferably Sr. Alternatively, other low work function materials can also be used, such as TiN or LaB 6 . Preferably, the thermionic material is thermally robust and will not corrode the underlying membrane  22  and resistive heating element  26 . The thermionic source  21  is disposed on the surface of the heated membrane  22  that is exposed to the detection chamber  30 . The thermionic source  21  should preferably be thick enough to provide sufficient thermionic activity, but thin enough to allow for adequate heat transfer between the microhotplate surface and the thermionic material surface in contact with the gases to be analyzed. 
     Reliable deposition of thermionic materials is highly desirable in order to achieve consistent microfabricated TID performance. Typically, the thermionic material can be spray coated on the surface of the microhotplate. Slurry deposition and chemical vapor deposition also can be used to deposit thermionic materials in the past. A micropen deposition technique can also be used. The micropen dispenses a controllable volume of paste per time, which enables control of thickness by varying print volume, paste concentration, and write speed. 
     Another simple technique to deposit the low work function material is via sol-gel processes. For example, alkali metal hydroxide catalyzed sol gels comprising silicate ions and alkali metal ions, including cesium and rubidium, can be used. For example, a sol gel precursor solution, comprising tetraethylorthosilicate, cesium hydroxide, ethanol, and excess water can be sprayed, drop coated, or spin coated onto the surface of the microhotplate. Suitable ratios of alkali metal to silicon oxide are about 1 to 30% by weight. 
     The ion collection electrode  28  can be embedded in the detection chamber side in the lid  24 , as shown. The lid  24  is preferably fabricated of a high-temperature dielectric material, such as Pyrex or ceramic. Alternatively, an electrode  28 ′ can be patterned on the same substrate as the heater. Alternatively, the collection electrode  28  can be microfabricated in a micromachined channel placed over the thermionic source  21  or in the exhaust gas outlet  37 . The measured ionization current can be quite small. Therefore, the electrode  28  should be sufficiently large to collect substantially all of the ionization current generated by the thermionic source  21 . The collection electrode  28  can be of a stable, conducting material, such as a metal or a doped semiconductor. For example, the electrode  28  can be a small planar nickel electrode, of approximately 2 mm diameter. 
     Operation of the Microfabricated Thermionic Detector 
     In operation, the sample and carrier gases are flowed over the hot thermionic source. The sample compounds interact directly with the thermionic surface, which is heated by the resistive heating element. The electronegative products of the decomposition are selectively ionized by surface ionization on the thermionic source. The ionized decomposition products are then collected by the positively biased ion collection electrode, which can be kept at a potential of less than one hundred volts relative to the thermionic source. 
     The operating parameters are the electronic work function of the thermionic source material, the temperature of the thermionic source, and the chemical composition of the gas mixture surrounding the source. The thermionic surface temperature must be hot enough to form the electronegative decomposition products that are selectively ionized at the thermionic surface. Further, the specific response to the different compounds is highest when the thermionic surface temperature is relatively low. As the surface temperature is increased, the microfabricated TID responds to a wider variety of compounds. The operating temperature of the thermionic source is typically in excess 400° C., depending on the work function of the particular thermionic material used. The temperature of the thermionic source is affected by the current to the resistive heating element and heat losses to the surrounding microfabricated TID structure and to the gas flowing over the thermionic source. In particular, heat loss to the gas is related to the thermal conductivity of the gas mixture and the gas flow rate over the thermionic source. Typical gas flow in the microfabricated TID is as low as 2 milliliters per minute of air. The microfabricated TID can operate with or without hydrogen and, because of the small size of the device, makeup gas is not normally required. 
     The detection will also be very sensitive to the detailed electronegative character of the sample compound&#39;s molecular structure. Therefore, the compound-to-compound response of the microfabricated TID can vary widely. Further, the composition of the gas environment can be varied to suppress the response to certain compounds while enhancing the response to others. For example, the sensitivity and selectivity of the microfabricated TID for nitrogen and phosphorous can be strongly dependent on hydrogen flow rate. Further, when the thermionic source is operated in an oxygen-containing gas environment, rather than hydrogen, the response to halogenated compounds will be enhanced relative to nitrogen-containing compounds. The presence of the electronegative oxygen molecule changes the transfer of negative charge in a manner that enhances the specific ionization of halogenated compounds. 
     In  FIG. 3  is shown the response of a microfabricated TID to a phosphorous-containing compound, dimethyl methyl phosphonate (DMMP), and a normal alkane, hexane. The thermionic material comprised a cesium hydroxide catalyzed sol gel. The composition of the sol gel was 2.31 grams CsOH monohydrate dissolved in 5 mL of 18 mΩ de-ionized water with the addition of 1 mL tetraethylorthosilicate. The resulting biphasic mixture was stirred at 1200 rpm on a 50° C. hotplate until a single phase was obtained. The sol gel mixture was spray coated on the membrane of the microfabricated TID. The membrane was heated to 600° C. using 370 mW of power. Two separate injections were made into the microfabricated TID. The first was a headspace injection of DMMP what has a vapor pressure of approximately 1 mmHg at 25° C. The second injection was hexane that has a vapor pressure of 151 mmHg at 25° C. The collected ionization current was measured with a commercial bias circuit and electrometer. As expected with the selective detector, there is a large difference in the response of the DMMP and the hexane. Selectivity was approximately 90,000:1 moles of phosphorous to mole of carbon. The microfabricated TID was also sensitive to nitrogen and did not require the use of hydrogen to detect nitro-functionalized compounds. 
     The present invention has been described as microfabricated thermionic detector. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.