Abstract:
An ion thrusting system is disclosed comprising an ionization membrane having at least one area through which a gas is passed, and which ionizes the gas molecules passing therethrough to form ions and electrons, and an accelerator element which accelerates the ions to form thrust. In some variations, a potential is applied to the ionization membrane may be reversed to thrust ions in an opposite direction. The ionization membrane may also include an opening with electrodes that are located closer than a mean free path of the gas being ionized. Methods of manufacture and use are also provided.

Description:
This application is a continuation of U.S. patent application Ser. No. 10/452,343 filed Jun. 2, 2003 now U.S. Pat No. 6,828,552, which is a divisional of U.S. patent application Ser. No. 10/180,813 entitled “Field Ionizing Elements and Applications Thereof” filed Jun. 25, 2002, now U.S. Pat. No. 6,642,526 which claims benefit of U.S. Provisional Application No.: 60/301,092, filed Jun. 25, 2001, U.S. Prov. App. No.: 60/336,841 filed on Oct. 31, 2001, and U.S. Provisional Application No. 60/347,685 filed on Jan. 8, 2002, all of which are hereby fully incorporated by reference. 

   This invention was made in part with Government support under contract NASA-1407 awarded by NASA. The Government has certain rights in this invention. 

   BACKGROUND 
   Many different applications are possible for ionization systems. For example, it is, desirable to form a pumpless, low mass sampling system for a mass spectrometer. 
   Conventional mass spectrometers often use “hard” techniques of producing ion fragments, in which certain parts of the molecule are forcibly removed, to form the fragmented ion. For example, the fragments may be produced by ultraviolet, radioactive, and/or thermal electron ionization techniques. Some of these techniques, and specifically the thermal technique, may require a vacuum to enhance the life of the filament source. 
   Different systems which use ionization are known. A quadrupole and magnetic sector/time of flight system ionizes a sample to determine its content. These devices, have limitations in both operation and size. Many devices of this type may operate over only a relatively small mass sampling range. These devices may also suffer from efficiency issues, that is the ions might not be efficiently formed. 
   Many of these systems also require a very high vacuum to avoid ion collisions during passage through the instrument. For example, the systems may require a vacuum of the level of such as 10 −6  Torr. A vacuum pump must be provided to maintain this vacuum. The vacuum pump consumes power, may be heavy, and also requires a relatively leak free environment. This clashes with the usual desire to miniaturize the size of such a device. 
   Other applications could be desirable for ionization, if an ionization system were sufficiently small. However, the existing ionization systems have problems and difficulties in fabrication which has prevented them from being used in certain applications. 
   SUMMARY 
   The present application describes a special ionization membrane, along with applications of this special ionization membrane that are facilitated by the membrane. 
   A first application uses the ionization membrane as part of a mass spectrometer. 
   Another application uses the ionization membrane for other applications. According to an aspect of this invention, the electrodes are formed closer than the mean free path of a specified gas, for example the gas being considered. This may ionize gas molecules in free space. Different applications of this soft ionization technique are described including using this system in a mass spectrometer system, such as a rotating field mass spectrometer. This may also be used in a time of flight system. 
   In an embodiment, a pumpless mass spectrometer is described which does not include a pump for either forming the vacuum or for driving the ions. 
   Another embodiment describes using this system for an electrochemical system. Another application describes using this system in propulsion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described in detail with reference to the accompanying drawings, wherein 
       FIG. 1  shows Paschen curves for various gases; 
       FIGS. 2   a – 2   c  show details of the special ionization membrane of the present system, with  FIG. 2   b  showing a cross-section along the line  2   b — 2   b  in  FIG. 2   c  and  FIG. 2   a  showing a close-up detail of one of the holes in  FIG. 2   b;    
       FIG. 3  shows an ion mobility spectrometer; 
       FIG. 4  shows a solid-state ionization membrane being used in an electrochemical device; 
       FIG. 5  shows the ionization membrane being used as a propulsion system; and 
       FIG. 6  shows this propulsion system in its housing with top and bottom accelerator grids. 
   

   DETAILED DESCRIPTION 
   Gas may be ionized in a high electric field. Avalanche arcing may be produced by the gas ionization. It has been found by the present inventor, however, that when the “mean free path” between molecules is greater than electrode separation, only ionization occurs. 
     FIG. 1  shows the Paschen curves for various gases. This represents the breakdown voltage of the gas at various characteristic points. On the left side and under each Paschen curves ionization of the gas occurs using the special membrane described herein. This technique is “soft” in the sense that it ionizes without fragmenting the molecular structure of the gas being ionized. That means that large organic compounds can be analyzed without breaking them into smaller atomic fragments. 
   Details of the membrane are shown in  FIGS. 2A–2C , with  FIGS. 2A &amp; 2B  showing cross sections of the membrane of  FIG. 2C . The miniature ionization device  99  is formed by micromachining an array of small holes  100  through a relatively thin membrane  105 . The membrane  105  may be, for example, of sub micron thickness. The material  106  of the substrate itself may be silicon or any other easy-to-machine material. Metal electrodes  120 , 122  are located on respective sides of the membrane  100 . The metal can be any material such as chrome or titanium or gold. 
   In formation of the membrane  99 , a plurality of holes such as  130  are formed from the bottom  132 . The holes may generally taper as shown towards the top portion  133  of the hole. The top portion  133  of the hole  130  may have a dimension  137  which may be, for example, 2 to 3 microns. Openings may be formed in the top metal coating  120 , and in the bottom metal coating  122 . For example, the hole may be formed by focused ion-beam milling (maskless process). 
   The substrate material  106  also includes a dielectric layer  134  which can be for example, silicon nitride, alumina, or any other similar material that has a similar dielectric breakdown. The thickness  136  of the dielectric layer sets the distance between the metal electrodes  120  and  122 . The dielectric thickness can be to 200–300 nm The dielectric can in fact be thinner than 200 nm, in fact can be any thickness, with thicknesses of 50 nm being possible. 
   In a preferred system, the distance between the electrodes  120 ,  122  is less than 1 micron. When this small separation is maintained, electric field strengths on the range of mega volts per meter are produced for each volt of potential difference between the electrodes  120 ,  122 . 
   The inventor has noted that the membranes could not be formed simply from the thin, sub micron elements. Membranes that are formed in this way could be too fragile to sustain a pressure difference across the membrane, or to survive a minor mechanical shock. In this embodiment, the thicker supporting substrate part  105  is used, and is back-etched through to the membrane. By forming the substrate in this way, that is with a relatively thick substrate portions such as  105 / 106 , separated by back etched holes such as  100 , the structure of the device can be maintained while keeping a relatively small distance between the electrodes. 
   An embodiment is described herein which uses the field ionizer array, which may be a micromachined field ionizer membrane, with a lateral accelerator, which is coupled to a mass spectrometer. An array of cathodes may be deployed to detect the position of impinging of the particles. 
   The cathode electrodes may be derived from an active pixel sensor array of the type described in U.S. Pat. No. 5,471,215, and as conventional may include various types of on-chip matrix processing. This system may use an electrode sensor of 1024 by 1024 pixels, with sub pixel centroiding and radial integration. The active pixel sensor itself may have a sensitivity on the order of 10 −17  amps. By adding pixel current processing, another two orders of magnitude of sensitivity may be obtained. 
   Forming the mass spectrometer in this way enables the device to be formed smaller, lighter, and with less cost than other devices of this type. This enables a whole range of applications; such as in situ biomedical sampling. One application is use of the miniature mass spectrometer is for a breathalyzer. Since there are no electron beam filaments and the like, any of the system components can operate at relatively higher pressures, for example 5 to 7 Torr pressures or higher. With a Faraday cup electrometer ion detector, sub femtoamp levels of sensitivity may be obtained. This system could be used as a portable device for finding various characteristics in exhaled breath. For example, detection of carbon monoxide in exhaled breath may be used as a screening diagnostic for diabetes. 
   Another application of this system is for use in a miniature ion mobility spectrometer as shown in  FIG. 3 . Conventional ion mobility spectrometers use a shutter gate. This provides short pulses of ions. The shortened pulses of ions are often limited to about 1 percent of the total number of ions that are available for detection. However, resolution of such a device is related to the width of the ion pulse. The width of the ion pulse cannot be increased without correspondingly decreasing the resolution. 
   In the improved system of  FIG. 3 , total and continuous ionization of sample gas and continuous introduction of all ions into the chamber is enabled. Sample gases are introduced as  600  into the ionization membrane  605  of the type described above. In general, the ionization membrane  605  could include either a single pore device or could have multiple pores within the device. 
   Ions  610  from the membrane exit the membrane as an ion stream. Electrons in contrast move back behind (that is, to the other side of) the membrane, and may further contribute to the ionization of the incoming gases. The atoms or molecules are carried through the body of the spectrometer by a gas feed system  625 . The gas feed system includes either an upstream carrier gas supply and Venturi sampler, or a downstream peristaltic pump. 
   The ions are drawn towards the filter electrode  615  which receive alternating and/or swept DC electric fields, for the transverse dispersal of the ions. A repetitive ramping of the DC fields sweeps through the spectrum of ion species. 
   An important feature of this device is the high field strengths which can be obtained. At moderate field strengths, for example &lt;100,000 volts per meter, the mobility of ions at atmospheric and moderate pressures is constant. However, at higher field strengths, such as 2 million volts per meter or greater, the mobility of the ions is nonlinear. The mobility changes differentially for high and low mobility ions. This change may be, for example, by 20 percent. Therefore, by applying a waveform that is formed of a short high-voltage and a long low or negative voltage to the filter electrodes, the ion species is disbursed between the filter electrodes. This waveform may be selected to provide a zero time averaged field. In operation, the ions are transported laterally by a carrier gas stream. A low strength DC field may be supplied in opposition to the other field. This fields applied to the filter electrode may straighten the trajectory of specific ion species, allowing their passage through the filter. The other ion species collide with the electrodes. Sweeping of the DC field may facilitate detection of the complete ion spectrum. 
   Detector electrodes  620  are located downstream of the filter electrodes  615 . The selected ions have straightened trajectories, and these detector electrodes  620  deflect the straightened-trajectory ions into detection electrodes, where they are detected. The detected current provides a direct measure of the number of ions. The number of ions is effectively proportional to the vapor concentration. 
   It should be understood that this gas feed system could be either upstream or downstream in this way. 
   Another embodiment uses this ionization technique to form a free space ion thruster. 
   Yet another embodiment describes use of an ionizer of this type in a fuel cell. Previous fuel cell proton exchange membranes have used platinum or other electrooxidation catalysts to facilitate proton transfer. In this system, the oxidation gas or gases  700  is passed through the pores of a membrane  705  under an extreme electric field as shown in  FIG. 4 . The oxidation gas or gases  700  are completely ionized on passage through the membrane. The gas  708  once ionized, now has a positively charged aspect. The gas  708  drifts to the membrane  710  where the electrooxidized state of the gas enhances its transfer through the cathode. The transfer of atomic species through the membrane in this way reduces the partial pressure between the ionizer  705  and the membrane  710 , this causing further inflow through the ionizer pores of the oxidation gas  702 . The ionizer potential may alternatively be maintained positive with respect to the cathode membrane in order to accelerate the ions to an increased velocity before imprinting on the cathode membrane which forms the accelerator grid. 
   Another embodiment, shown in  FIG. 5 , uses this ionization membrane as part of a miniature ion thruster. This may form a thrust system using propellant gas. Propellant gas  800  is ionized by passing it through the pores of a membrane  805  of the type described above, under a high electric field. This forms positively charged ions  809  from the gas. The ions  809  enter another field  808  between the membrane and a porous accelerator grid  810 . This other field  808  accelerates the ions to an increased velocity, and expels them from the thruster as  820 . 
   The electrons are caused to move back behind the membrane where a small electric field and magnetic field may linearly and rotationally accelerate the electron beam around to eject the electrons from the thruster with the same vector but reduced velocity as the ion beam. Since the ion and electron currents are substantially identical, this system becomes effectively charge neutral. 
   This system may use a small tube  820  of 1.5 cm long; 2 mm in diameter, of dielectric materials such as quartz. The tube  820  may be eutectically bonded to the top of the membrane  805 . The micromachined conductive grid is similarly affixed to the top of the tube. The bottom of the membrane may also be eutecticly bonded to a thruster housing  825 . That housing may contain another accelerating grid  830  and magnets. 
   An exterior view of the structure is shown in  FIG. 6 , which shows the tube for any particular accelerator grid potential, the thrust of the engine is determined by the gas flow through the membrane pores. This system may use a plurality of miniature ionization tubes such as the one described above, that are disbursed across the surface of the structure. These tubes may be deployed individually or collectively by connecting them into a circuit. The ions from each of these tubes are accelerated under the influence of a localized electric field that is along the vector representing the least distance to the peripheral grid. The aggregate thrust is the geometrically integrated mass-momentum of all connected free space ion thrusters. 
   In this embodiment, a bipolar ion thruster may allow reversing the electrode potentials on the ionization membrane, causing the electrons to pass through the membrane, while ions move behind the membrane. The high velocity ions are expelled from the front of the thruster, and electrons are expelled from the rear of the thruster. This engine can therefore be reversed in this way. 
   When used in a vacuum, a low-pressure gas may need to be introduced into the membrane aperture that has a velocity sufficient to carry the gas into the ionization field. Gas expands in a vacuum and has its molecules accelerated to supersonic speed while cooling, and directed through the membrane. Once ionized, the accelerating ions will create a partial vacuum behind them, which partial vacuum encourages further gas flow through the membrane. Gas that remains behind the membrane is ionized, and its negative field directs those ions through the membrane. 
   This system may have many different applications including biomedical applications such as a breath analyzer, as well as applications in other systems. It may have applications environment monitoring, personal monitoring, reviewing of water quality, automobile MAP control, detection of explosives, chemical and biological agent detection, and in an artificial nose type product.