Abstract:
A compact ionization source includes first and second electrodes, each having a plurality of fingers that are interdigitated with each other. The spacing between the first and second electrode, preferably less than 1 mm, creates a large electric field when a potential is applied across the first and second electrodes. The large electric field creates an ionization volume between the fingers of the first and second electrode and ionizes a portion of the molecules occupying the ionization volume. The interdigitated fingers of the first and second electrodes allow for a narrow gap separating the electrodes while presenting a large flow area for ionizing molecules for downstream analysis.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to devices and methods for generating ions. More specifically, the invention relates to compact devices and methods for generating ions using a corona discharge at or near atmospheric pressure. 
   2. Description of the Related Art 
   Radioactive isotopes such as  241 Am or  63 Ni are commonly used as ionization sources to generate ions in a surrounding gas stream. Radioactive ionization sources have the advantage of simplicity, compactness, durability, and reliability. The regulations associated with these radioactive ionization sources, however, may render the incorporation of radioactive isotopes into a product economically unfeasible. 
   Electric field ionization has the advantage of simple design, relatively simple fabrication, and low power consumption. In electric field ionization, a large electric field between 10 7  to 10 8  V/m is generated between two electrodes. The large electric field accelerates any ions within the field thereby causing the accelerated ions to collide with surrounding gas molecules. The collision of an accelerated ion and a gas molecule creates an ionized molecule. 
   A corona discharge is a type of electric field ionization where a neutral fluid such as, for example, air is ionized near an electrode having a high electric potential gradient. Such a potential gradient is achieved by using a discharge electrode, having a small radius of curvature. The polarity of the discharge electrode determines whether the corona is a positive or negative corona. The corona has a plasma region and a unipolar region. In the plasma region, electrons avalanche to create more electron/ion pairs. In the unipolar region, the slowly moving massive (relative to the electron mass) ions move to the passive electrode, which is usually grounded. If the plasma region grows to encompass the passive electrode, a momentary spark or a continuous arc may occur. The spark or arc may damage the electrodes, produce contaminant ions, and reduce the lifetime of the ionization source. Therefore, there remains a need for devices and methods for compact ionization sources with longer lifetimes. 
   SUMMARY OF THE INVENTION 
   A compact ionization source includes first and second electrodes, each having a plurality of fingers that are interdigitated with each other. The spacing between the first and second electrodes, preferably less than 1 mm, creates a large electric field when a potential is applied across the first and second electrodes. The large electric field creates an ionization volume between the fingers of the first and second electrodes and ionizes a portion of the molecules occupying the ionization volume. The interdigitated fingers of the first and second electrodes allow for a narrow gap separating the electrodes while presenting a large flow area for ionizing molecules for downstream analysis. 
   One embodiment of the present invention is directed to an ionization source comprising: a first electrode having a first plurality of fingers; a second electrode having a second plurality of fingers, the first plurality of fingers being disposed between the second plurality of fingers; and a generator for applying a signal between the first and second electrodes, the signal generating an ionization volume between the first and second electrodes. In some aspects of the present invention, a distance between the first electrode and the second electrode is between 100 μm and 1 μm, preferably 60 μm and 5 μm and most preferably between 40 μm and 10 μm. In some aspects of the present invention, the ionization source further comprises a carbon nanotube layer disposed on a side of the first electrode facing a side of the second electrode. In some aspects of the present invention, the carbon nanotube layer comprises a plurality of carbon nanotubes characterized by a longitudinal axis, the longitudinal axis parallel to a surface normal of the side of the first electrode. In some aspects of the present invention, the ionization source further comprises a diamond-like coating (DLC) layer deposited on the first and second electrodes. In some aspects of the present invention, the DLC layer is comprised of tetrahedral amorphous carbon (ta-C). In some aspects of the present invention, the ta-C is n-doped. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by reference to the preferred and alternative embodiments thereof in conjunction with the drawings in which: 
       FIG. 1  is a side view of an embodiment of the present invention; 
       FIG. 2  is a side view of another embodiment of the present invention; 
       FIG. 3  is a side view of another embodiment of the present invention; 
       FIG. 4  is a side view of another embodiment of the present invention; 
       FIG. 5  is a side view of another embodiment of the present invention. 
       FIG. 6  is a top view of the embodiment shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a side view of an embodiment of the present invention. In  FIG. 1 , a first electrode  110  and a second electrode  115  are disposed on a substrate  120  and separated by a gap  130 . A DC or RF signal  140  is applied between the first and second electrodes. A DC, pulsed DC, or radio frequency signal may be applied between the first and second electrodes using commonly known methods for generating the applied signal. The electric field generated by signal  140  creates an ionized volume  135  in the gap  130  between the first and second electrodes. 
   The configuration shown in  FIG. 1  may be fabricated using well-known microelectronic processing methods. The electrodes may be Pt, Au, Cr, Cu, Ni, or other suitable electrode materials that may be sputtered, chemical vapor deposited or electroplated onto the substrate. The substrate is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts. 
     FIG. 2  is a side view of another embodiment of the present invention. In  FIG. 2 , a first electrode  210  is deposited on a substrate  220 . An insulator  250  is disposed on a portion of the first electrode  210  and a second electrode  215  is disposed on the insulator  250 . A voltage potential, not shown, is applied between the first and second electrode and creates an ionized volume  235  between the first and second electrodes. The embodiment shown in  FIG. 2  may be fabricated using any of the microelectronic processing methods known in the microelectronic processing arts. The electrodes may be Pt, Au, Cr, Cu, Ni, or other suitable electrode materials that may be sputtered, chemical vapor deposited or electroplated onto the substrate. The insulator is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts. Similarly, the substrate is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts. 
     FIG. 3  is a side view of another embodiment of the present invention. In  FIG. 3 , ionizer  300  includes a first electrode  310  and a second electrode  315 . Each electrode  310 ,  315  is preferably comb shaped, when seen from above, with the fingers of one electrode interdigitated with the fingers of the other electrode such that each finger of the first electrode is between fingers of the second electrode. The first and second electrodes are spaced apart such that the gaps between neighboring fingers define channels having a volume  335  where molecules may be ionized. The distance between neighboring fingers is preferably between 1-100 μm, more preferably between 5-60 μm, and most preferably between 10-40 μm. 
     FIG. 6  is a top view of the embodiment shown in  FIG. 3 . In  FIG. 6 , structures identical to structures in  FIG. 3  are referenced with the corresponding reference number in  FIG. 3 .  FIG. 6  shows the comb shaped first and second electrodes with interdigitated fingers. In  FIG. 6 , each electrode is shown with five fingers for purposes of clarity but it should be understood that electrodes with more than one finger are within the scope of the present invention.  FIG. 6  also illustrates that the gap between the first and second electrodes forms a continuous serpentine channel with a small channel width. The length of the channel may be controlled by the number of fingers in the first and second electrode. Increasing the length of the channel by increasing the number of fingers in the first and second electrodes increases the flow area through the ionizer. Thus, the interdigitated electrodes creates a volume with a large flow area while maintaining a narrow gap. 
   Each electrode  310 ,  315  includes a metal layer  320  deposited on substrate  325 . The metal layers  320  may be Pt, Au, Cr, Cu, Ni, or other suitable electrode materials that may be sputtered, chemical vapor deposited or electroplated onto the substrate. The substrate is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts such as, for example, glass, alumina, and quartz. An optional second metal layer  322  may be deposited on the face of the substrate opposite the first metal layer  320 . In a preferred embodiment, the second metal layer  322  is held at or near the same voltage potential as the first metal layer  320 . 
   In a preferred embodiment, electrodes  310 ,  315  are fabricated using deep reactive ion etching (DRIE) methods in the MEMS/semiconductor processing arts. In accordance with such methods, a metal layer  320  is first deposited on a first major surface of a continuous substrate  325 . Optionally, a second metal layer  322  is then deposited on a second major surface of the substrate using photolithographic techniques. The metal layer(s) are then etched to separate electrodes  310 ,  315  and the substrate is etched through to define the gaps between the electrode fingers. 
   A voltage source  340  applies a voltage potential across the first and second electrodes, which creates an electric field in the volume  335  between the electrode fingers. The voltage is selected such that the electric field generated in volume  335  is sufficient to create an ionization region within volume  335  and ionize a portion of the molecules in the volume. The voltage source  340  may apply a DC voltage to create a corona discharge in volume  335  or may apply an RF voltage to generate a plasma in the volume. 
   Deflector electrode  360  may be disposed above and/or below the ionizer to drive ions from the volume  335  to another location for analysis. The “pass-through” design of ionizer  300  enables a gas to enter plenum volume  370 , ionize a portion of the gas in ionizer  300 , and have the ions removed to a second plenum volume  372  for downstream analysis. The “pass-through” design of ionizer  300  alternatively allows ions generated in ionizer  300  to be transported from the ionizer to the second plenum volume  372  by establishing a flow from the first plenum volume  370  to the second plenum volume  372 . 
     FIG. 4  is a side cross-sectional view of another embodiment of the present invention. In  FIG. 4 , structures similar to those shown in  FIG. 3  are referenced with a corresponding reference number incremented by  100 .  FIG. 4  shows ionizer  401  attached to holding substrate  430 . Ionizer  401  includes a first electrode  410  and a second electrode  415 . Each electrode  410 ,  415  is preferably comb shaped, when seen from above, with the fingers of one electrode interdigitated with the fingers of the other electrode such that each finger of the first electrode is between fingers of the second electrode. The first and second electrodes are spaced apart such that the gaps between neighboring fingers define channels having a volume  435  where molecules may be ionized. The distance between neighboring fingers is preferably between 1-100 μm, more preferably between 5-60 μm, and most preferably between 10-40 μm. 
   Each electrode  410 ,  415  includes a metal layer  420  deposited on substrate  425 . The metal layers  420  may be Pt, Au, Cr, Cu, Ni, or other suitable electrode materials that may be sputtered, chemical vapor deposited or electroplated onto the substrate. The substrate is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts such as, for example, glass, alumina, and quartz. An optional second metal layer  422  may be deposited on the face of the substrate opposite the first metal layer  420 . In a preferred embodiment, the second metal layer  422  is held at or near the same voltage potential as the first metal layer  420 . In a preferred embodiment, electrodes  410 ,  415  are fabricated as described in conjunction with  FIG. 3  using deep reactive ion etching (DRIE) methods in the MEMS/semiconductor processing arts. 
   A carbon nanotube layer  428  is disposed on the sides of the first electrode  410  facing the second electrode. In a preferred embodiment, the carbon nanotubes in layer  428  are oriented such that the axis of the carbon nanotube is generally parallel to the surface normal of the electrode side surface. The carbon nanotube layer may be fabricated in situ by biasing the electrodes and using plasma enhanced CVD methods such as those described in, for example, Chhowalla et al., “ Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition ,” J. Appl. Phys., vol. 90, no. 10 (November 2001), which is incorporated herein by reference. 
   It is believed, without being limited to a particular theory, that the small radius of curvature at the ends of the carbon nanotubes creates a large electric field concentration such that ignition of a corona occurs at a lower applied potential across the first and second electrodes. 
   A voltage source (not shown) similar to voltage source  340  of  FIG. 3  applies a voltage potential across the first and second electrodes, which creates an electric field in the volume  435  between the electrode fingers. The voltage is selected such that the electric field generated in volume  435  is sufficient to create an ionization region within volume  435  and ionize a portion of the molecules in the volume. The voltage source may apply a DC voltage to create a corona discharge in volume  435  or may apply an RF voltage to generate a plasma in the volume. 
   Deflector electrode  460  may be disposed above and/or below the ionizer to drive ions from the volume  435  to another location for analysis. The “pass-through” design of ionizer  401  enables a gas to enter plenum volume  470 , ionize a portion of the gas in ionizer  401 , and have the ions removed to a second plenum volume  472  for downstream analysis. The “pass-through” design of ionizer  401  alternatively allows ions generated in ionizer  401  to be transported from the ionizer to the second plenum volume  472  by establishing a flow from the first plenum volume  470  to the second plenum volume  472 . 
     FIG. 5  is a side cross-sectional view of another embodiment of the present invention. In  FIG. 5 , structures similar to those shown in  FIG. 3  are referenced with a corresponding reference number incremented by  200 . Ionizer  502  includes a first electrode  510  and a second electrode  515 . Each electrode  510 ,  515  is preferably comb shaped, when seen from above, with the fingers of one electrode interdigitated with the fingers of the other electrode such that each finger of the first electrode is between fingers of the second electrode. The first and second electrodes are spaced apart such that the gaps between neighboring fingers define channels having a volume  535  where molecules may be ionized. The distance between neighboring fingers is preferably between 1-100 μm, more preferably between 5-60 μm, and most preferably between 10-40 μm. 
   Each electrode  510 ,  515  includes a metal layer  520  deposited on substrate  525 . The metal layers  520  may be Pt, Au, Cr, Cu, Ni, or other suitable electrode materials that may be sputtered, chemical vapor deposited or electroplated onto the substrate. The substrate is preferably silicon but may also be selected from insulator materials known in the microelectronic process arts such as, for example, glass, alumina, and quartz. An optional second metal layer  522  may be deposited on the face of the substrate opposite the first metal layer  520 . In a preferred embodiment, the second metal layer  522  is held at or near the same voltage potential as the first metal layer  520 . In a preferred embodiment, electrodes  510 ,  515  are fabricated as described in conjunction with  FIG. 3  using DRIE methods in the MEMS/semiconductor processing arts. 
   A diamond-like coating (DLC) layer  529  covers the first and second electrodes  510 ,  515 . In a preferred embodiment, the DLC layer is formed using filtered cathodic vacuum arc (FCVA) as described in Satyanarayana et al., “ Field emission from tetrahedral amorphous carbon ,” Appl. Phys. Lett., vol 71, no. 10, (September 1997), which is incorporated herein by reference. 
   It is believed that, without being limited to a particular theory, the n-doped tetrahedral amorphous carbon (ta-C) in the DLC layer results in field emission of electrons at field strengths of about 10 V/μm. The chemical inertness and high hardness of the DLC layer is believed to contribute to improving the electrode lifetime. 
   A voltage source (not shown) similar to voltage source  340  of  FIG. 3  applies a voltage potential across the first and second electrodes, which creates an electric field in the volume  535  between the electrode fingers. The voltage is selected such that the electric field generated in volume  535  is sufficient to create an ionization region within volume  535  and ionize a portion of the molecules in the volume. The voltage source may apply a DC voltage to create a corona discharge in volume  535  or may apply an RF voltage to generate a plasma in the volume. 
   Deflector electrode  560  may be disposed above and/or below the ionizer to drive ions from the volume  535  to another location for analysis. The “pass-through” design of ionizer  502  enables a gas to enter plenum volume  570 , ionize a portion of the gas in ionizer  502 , and have the ions removed to a second plenum volume  572  for downstream analysis. The “pass-through” design of ionizer  502  alternatively allows ions generated in ionizer  502  to be transported from the ionizer to the second plenum volume  572  by establishing a flow from the first plenum volume  570  to the second plenum volume  572 . 
   Having thus described at least illustrative embodiments of the invention, various modifications, and improvements will readily occur to those skilled in the art and are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.