Abstract:
In this invention the technology is provided for rod shaped conductor member fabrication in situ, in position, in the mass filter spatial configuration by growth through vertically repeated conduit mold formations, filling the conduit increments with to be rod material, and coalescing the growth increments as the rod length is achieved.

Description:
This invention was made with government support under contract #W31P4Q-04-C-R311 awarded by the U.S. Army. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention is in the field of sensing the presence and quantity of chemicals in an ambient using a solid state quadrupole mass spectrometer device; and in particular to the fabrication of the high aspect ratio rod configuration assemblies that are a building block in the fabrication of the solid state quadrupole mass spectrometer device. 
     BACKGROUND AND RELATION TO THE PRIOR ART 
     Quadrupole mass spectrometer apparatus, for use in the sensing of the presence and quantity of chemicals in a gaseous ambient, would include as a main element, a quadrupole mass filter structural assembly capable of atomic selection based on atomic particle mass, together with means for introducing ionized ambient gas into the quadrupole mass filter, means for detecting specific ions in the ionized gas and means for detecting quantity and quality attributes of those selected ions. 
     In operation; to the quadrupole mass spectrometer device, there is supplied to individual diagonally positioned pairs of rod shaped conductor members, combined, direct current (DC) levels and phased radio frequency (RF) signals; such that, for a fixed value of RF and DC voltages, input ion energy, conductor dimensions and frequency: there is produced a hyperbolic field. The hyperbolic field affects the ability of certain ions, having a specific ratio of charge to mass, that in turn is identifiable with certain chemicals, to be diverted for processing in a detection capability. 
     The technology of quadrupole mass spectrometry is being extensively studied in the art. Examples of publications are Peter H. Dawson,  Quadruple Mass Spectroscopy and its Applications , (Elsevier, N.Y. (1976), pp 9-11), and R. A. Syms et al, in IEEE Transactions on Electron Devices 45, 2304(1998). 
     In the translation of the quadrupole mass spectrometry technology into useful devices and apparatus there will be encountered interrelated dimensions within an operating background pressure that in turn is in an assembly of rod shaped members in a spatial volume wherein fields can provide conditions for chemical separation. To the quadrupole mass spectrometer device, there is supplied to individual diagonally positioned pairs of the rod members, combined, direct current (DC) levels and phased radiofrequency (RF) signals; such that, for a fixed value of RF and DC voltages, input ion energy, conductor dimensions and frequency: there is produced a hyperbolic field in the spatial volume. The hyperbolic field affects the ability of certain ions, arriving through the ion path opening, that have a specific ratio of charge to mass, that in turn is identifiable with certain chemicals, to traverse the spatial volume and be processed in a detection capability beyond the ion path exit. 
     The quadrupole mass spectrometer device is sensitive to dimensions and operating pressure. The work of Boomselleck and Ferran, reported in Am. Soc. For Mass. Spec. 12,633 (2001) advances highly useful factors including that maximum operating pressure is inversely proportional to the length dimension of the mass filter, and, that sensitivity is influenced by the “r” and “L” dimensions of the mass filter. The dimensions “r”, “L”, and “D” are labelled in  FIG. 2 . 
     As the art has developed, a promising design for the mass filter has evolved; wherein between upper and lower plane surfaces there is an enclosed spatial volume within which there is located a configuration of four parallel, equidistant as around a bolt circle, rod shaped conductor members, each of which extends between the upper and lower plane surfaces. The promising mass filter design further provides an ion path opening and exit, positioned, centered into and centered exiting from the upper and lower planes within the spatial volume containing the rod shaped conductor member configuration. The rod configuration is a building block in the fabrication of the mass filter. This building block, in turn, permits the fabrication capabilities of the semiconductor industry in producing large area arrays. 
     At this point in the art however there is a significant technical hurdle to be overcome because the length dimension of the mass filter will have to be very long in relation to the horizontal dimensions in other words the mass filter will have to have a large relative length to horizontal aspect ratio. 
     SUMMARY OF THE INVENTION 
     In this invention the technology is provided for rod shaped conductor member fabrication in situ, in position, in the mass filter spatial configuration by growth through vertically repeated conduit mold formations, filling the conduit increments with to be rod material, and coalescing the growth increments as the rod length is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional depiction along a line A-A, of the relative superpositioned relationship of the elements in the mass filter and their positioning in the spatial volume. 
         FIG. 2  is a cross sectional depiction of the mass filter building block, where a protective membrane has been removed for visibility; and wherein there is shown, positioned along a cross section line A-A, two of the four equidistant, as in a bolt circle type, rod element configuration made up of serially, incrementally fabricated in accordance with this invention, rod elements. 
         FIG. 3  is a depiction of alignment guiding as incremental fabrication proceeds. 
         FIG. 4  is a flow chart of the main process steps in the incremental rod fabrication. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     This invention is directed to the in situ fabrication of rod shaped elements that are to be an integral part of the hyperbolic field apparatus of the mass filter and which will be located within the mass filter. The mass filter and it&#39;s operation is described in connection with  FIGS. 1 and 2  wherein each component has the same reference numeral. 
     Referring to  FIGS. 1 and 2  together, in the quadrupole mass filter  1 , at site locations between the parallel surfaces  2  and  3  of a planar type member  4  such as a wafer, of a generally non conductive material of for example Silicon (si); the quadrupole mass filter device  1  is positioned, in which there is a configuration  5  of four, parallel rod shaped conductor members, ( 6 , 7 , 8  &amp;  9  in  FIGS. 2 and 8  &amp;  9  in  FIG. 1 ), positioned orthogonal with respect to the parallel surfaces  2  &amp;  3  in  FIGS. 1 and 2  and equidistantly positioned from each other as in a bolt circle. A membrane on surface  2  has been removed for better viewing in  FIG. 2 . 
     The four, rod shaped conductor members,  6 , 7 , 8  &amp;  9 , in each mass filter device, surround a spatial volume  10  in the material of the wafer. The wafer material, within what is to be the spatial volume  10  will be later removed by etching. An ion path passes through the top one of the parallel wafer surfaces  2 , 3 , extends through the enclosed spatial volume  10  and continues out the bottom one,  3 , of the parallel wafer surfaces,  2  &amp;  3 . 
     There is delivered to pairs of diagonally positioned individual rod members ( 6 , 9  &amp;  7 , 8 ); with dimensional relations “r”, “D” and “L” combined, direct current (DC) levels and phased radio frequency (RF) signals; whereby, a hyperbolic cylindrical field is produced in the spatial volume  10 . The connections are shown in  FIG. 2  where they are more visible but the actual connections are made below surface  3  in  FIG. 1 . 
     The ionized ambient gas; in which the ions are at fixed energy that must be controlled by applied potentials, is focused into the ion path at the opening  11  in the top one of the parallel wafer surfaces through the hyperbolic field in the spatial volume and out the ion path exit  12  in the bottom one of the parallel surfaces. The ions in the ionized ambient gas that have a specific ratio of charge to ion mass are selected in passing through the field in the spatial volume and exit in the ion path into the subsequent stage. The actual dimensions involved are quite small and the vertical distance between the surfaces is much greater than the horizontal distances between rods. Those dimensions in turn affect many parameters of the mass filter. Assuming, as an example, a mass filter that is built in silicon and occupies about a cubic centimeter in volume. The length of the rods  6 - 9  will affect the traverse rate of the ions and hence the quantity of ions that are selected in the filter. The work of Ferran and Boomselleck, titled “ Effects of quadrupole analysers for RGA ”, published in JVST A—Vacuum, Surfaces, and Films, page 1258 (2001) provides a good perspective in selection. The length of the rods  6 - 9  labelled distance “L” in  FIG. 2 , would be about 500 micrometers. The radius of an individual rod, labelled dimension “r” in  FIG. 2 , would be about 4.56 micrometers and the diameter of a circle that tangentially touches all four of the rods  6 - 9  in the example configuration, labelled dimension “D” in  FIG. 2  would be about 8 micrometers. 
     The work of Peterman et al, titled “ Building thick photoresist structures from the bottom up ” published by J. Microtech. Microeng. 13, 380(2003), demonstrates forming high aspect ratio structures from single layers and the ability to add an additional layer. 
     This invention provides the ability to form structures from many layers sufficient to meet the complex future dimensional requirements. 
     Referring to  FIG. 3  a depiction is provided of the incremental fabrication operation of the invention, in which, using reference numerals as used in  FIGS. 1 and 2  on a substrate equipped with registration capability, serial depositions of which four can be accomplished and are shown, which in turn permit substantial extension in the vertical direction. In  FIG. 3 , the registration capability is illustrated as a guidance symbol  12  on a substrate  11  that is is positioned with reference to the plane and location on that plane of the bottom of each of the four rod members  6 - 9 . The registration capability can provide orientation for runout as the vertical dimension gets longer and arrays extend over greater area in the horizontal direction. The guidance symbol can also be placed on subsequent layers. 
     There are four layers of light responsive materials labelled layers A-D. Each layer is the outermost layer for a vertical section. 
       FIG. 4  is a step by step flow chart of the main process steps in the incremental rod fabrication. 
     In connection with  FIGS. 1-4  a procedure is described based on recent developments in the thick type of light responsive, commonly called photoresist, technology. The technology and the structure produced presents a number of fabrication challenges, including; the structure and the parts thereof are tall and must be parallel to one another to a high degree of precision; the opposing pairs of the rod elements must be able to sustain a high voltage difference without electrical leakage or breakdown; access and exiting for ions must be centered in the rod configuration and the configuration structure must be housed and supported. 
     The procedure is based on thick photoresist technology. Some examples described in the art are NANO S U 8-2000, NANO PMMA, of Micro Tm Chem Newton, Mass. and BPR 100 Photoresist. Electronic Materials, Marlborough, Mass. 
     A pedestal as indicated at level A of  FIG. 3  is fabricated in the SU-8, rods are formed in a cavity formed from the wafer then covered top and bottom with a membrane of SU-8. The bottom of the wafer serves as a pedestal for rods and electrical contacts to opposing pairs of rods. Referring to  FIG. 4  a flow chart as provided of the main process steps in the incremental rod fabrication. The process begins at Step  1  by the oxiding of the thin 150 micrometer base plate  11  substrate and using lift off to pattern the oxide using Mask  1 . The mask set is shown in  FIG. 4 . The patterned wafer is then etched using deep reactive ion etching to produce through wafer holes in the pattern of the rod and access hole layout. The wafer is then given a high temperature thermal oxide to produce an insulating barrier. In  FIG. 4 , Step  5   a  carrier wafer is prepared with the pattern, Mask  2 , for electroplating the rods but not the access holes. The through etched wafer is aligned and attached to the patterned carrier wafer and the through holes in the wafer filled with Au metal. In Step  7  the electroplating carrier wafer is removed and a low temperature oxide is deposited over the entire structure. The oxide is then patterened in Step  8  with mask  3  to produce openings for making electrical connections for oneset of rods. Metal  1 (Cr/Au) is deposited and patterened with Mask  4  by lift off to produce the connecting lines for the first set of rods. Another low temperature oxide, Step  13 , is deposited over the entire structure again to provide electrical isolation between the two sets of connection lines. This oxide is patterned with Mask  5  and openings etched. Metal layer  2 (Cr/Au) is deposited and patterned with mask  6  and liftoff, Step  18 . Other metals may be used for metals  1  &amp;  2 . Prepare a new carrier wafer using Mask  2 , Step  20 , for electroplating the rods, electrical contact is made the metal interconnect lines just completed. The wafer must be flipped over to mate to the carrier wafer. The oxide on the new top side must now be patterned with Mask  8  and etched (Step  22 ) to open up the holes for electroplating the rods. 
     The cavity wafer that encloses the quadrupole devices is prepared, (Step  21 ) The cavity openings are laser machined through the 500 micrometer thick wafer. The cavity openings become accessible at Mask  7 . Spin coat the wafer with 100 micrometer thick layer of BPR resist (Step  23   a ) using Mask  8  to pattern the openings for electroplating the rods. (Step  23   b ), electroplate the next 100 micrometer length of rod through the thickness of the resist. Repeat Steps  23   a  and  23   b  four more times to produce the 500 micrometer tall rods (Steps  23   c  through  23   j ). Dissolve the BPR resist. Spin a 20 micrometer thick layer of SU-8 on the laser machined cavity wafer (Step  24 ) and attach to the wafer with the electroplated rods. The cavity is then filled with PMMA and planarized in (Step  25 ). A thick layer of SU-8 is spun onto the planarized surface and patterned with (Mask  9 ) to produce the access holes. Finally a metal film is deposited over the SU-8 surface to make a ground plane for the device. The carrier wafer is then removed and the PMMA dissolved out of the structure using acetone. The mask set can make an array of 8×8 of the basic device layout. 
     What has been described is the technology of the building of rods for quadrupole mass filter devices where the length is extended by incremental layers.