Abstract:
An ion trap device comprises a wafer that supports at least one plate forming an ion trapping region therebetween. The plate has an electrically insulating surface and a multiplicity of electrodes disposed on the insulating surface. The electrodes form at least one ion trap in the trapping region when suitable voltages are applied to the electrodes via conductors coupled to the wafer. The device has a multiplicity of ports for introducing ions into the trapping region and for extracting ions from that region. In embodiments that include a multiplicity of such plates, a first one of the plates is oriented at a non-zero angle to the major surface of the wafer and is rotateably mounted on that surface. In one embodiment, at least two of the plates form an elongated micro-channel having an axis of ion propagation, and the electrodes on at least one of the two plates are segmented along the direction of the axis, thereby forming a multiplicity of ion traps along the axis. A controller applies suitable voltage (e.g., sequentially) to the segmented electrodes, thereby shifting ions from one trap to another. Preferably, the electrodes on the two plates are segmented. Applications to mass spectrometers and shift registers are described.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to ion trap devices and, more particularly, to such devices that are formed by out-of-plane assembly of micro-cavities on a semiconductor or dielectric wafer. 
   2. Discussion of the Related Art 
   Conventional ion traps enable ionized particles to be stored and the stored ionized particles to be separated according to the ratio (M/Q) of their mass (M) to their charge (O). Storing the ionized particles involves applying a time-varying voltage to the ion trap so that particles propagate along stable trajectories therein. Separating the ionized particles typically involves applying an additional time-varying voltage to the trap so that the stored particles are selectively ejected according to their M/Q ratios. The ability to eject particles according to their M/Q ratios enables the use of ion traps as mass spectrometers. 
   Exemplary ion traps are described, for example, by W. Paul et al. in U.S. Pat. No. 2,939,952 issued Jun. 7, 1960. One such ion trap, known as a quadrupole, is described by R. E. March in “Quadrupole Ion Trap Mass Spectrometer,”  Encyclopedia of Analytical Chemistry , R. A. Meyers (Ed.), pp. 11848–11872, John Wiley &amp; Sons, Ltd., Chichester (2000). Both of these documents are incorporated herein by reference. 
     FIG. 1  herein shows one type of quadrupole ion trap  10  that has an axially symmetric cavity  18  akin to that depicted in  FIG. 2  of March. More specifically, the ion trap  10  includes metallic top and bottom end cap electrodes  12 – 13  and a metallic central ring-shaped electrode  14  that is located between the end cap electrodes  12 – 13 . Points on inner surfaces  15 – 17  of the electrodes  12 – 14  have transverse radial coordinates r and axial coordinates z. These coordinates satisfy hyperbolic equations; i.e., r 2 /r 0   2 −z 2 /z 0   2 =+1 for the central ring-shaped electrode  14  and r 2 /r 0   2 −z 2 /z 0   2 =−1 for the end cap electrodes  12 – 13 . Here, 2r 0  and 2z 0  are, respectively, the minimum transverse diameter and the minimum vertical height of the trapping cavity  18  that is formed by the inner surfaces  15 – 17 . Typical trapping cavities  18  have a shape ratio, r 0 /z 0 , that satisfies: (r 0 /z 0 ) 2 ≈2, but the ratio may be smaller to compensate for the finite size of the electrodes  12 – 14 . Typical cavities  18  have a size that is described by a value of r 0  in the approximate range of about 0.707 centimeters (cm) to about 1.0 cm. We refer to cavities of this approximate size as macro-cavities. 
   For the above-described electrode and macro-cavity shapes, electrodes  12 – 14  produce an electric field with a quadrupole distribution inside trapping cavity  18 . One way to produce such an electric field involves grounding the end cap electrodes  12 – 13  and applying a radio frequency (RF) voltage to the central ring-shaped electrode  14 . In an RF electric field having a quadrupole distribution, ionized particles with small Q/M ratios will propagate along stable trajectories. To store particles in the trapping cavity  18 , the cavity  18  is voltage-biased as described above, and ionized particles are introduced into the trapping cavity  18  via ion generator  19 . 1  coupled to entrance port  19 . 2  in top end cap electrode  12 . During the introduction of the ionized particles, the trapping cavity  18  is maintained with a low background pressure; e.g., about 10 −3  Torr of helium (He) gas. Then, collisions between the background He atoms and ionized particles lower the particles&#39; momenta, thereby enabling trapping of such particles in the central region of the trapping cavity  18 . To eject the trapped particles from the cavity  18 , a small RF voltage may be applied to the bottom end cap  13  while ramping the small voltage so that stored particles are ejected through exit orifice  19 . 4  selectively according to their M/Q ratios. The ejected ions are then incident on a utilization device  19 . 3  (e.g., an ion collector), which is coupled to orifice  19 . 4 . 
   For quadrupole ion trap  10 , machining techniques are available for fabricating hyperbolic-shaped electrodes  12 – 14  out of base pieces of metal. Unfortunately, such machining techniques are often complex and costly due to the need for the hyperbolic-shaped inner surfaces  15 – 17 . For that reason, other types of ion traps are desirable. 
   A second type of ion trap has a trapping macro-cavity with a right circularly cylindrical shape. This trapping cavity is also formed by inner surfaces of two end cap electrodes and a central ring-shaped electrode located between the end cap electrodes. Here, the end cap electrodes have flat disk-shaped inner surfaces, and the ring-shaped electrode has a circularly cylindrical inner surface. For such a trapping cavity, applying a voltage to the central ring-shaped electrode while grounding the two end cap electrodes will create an electric field that does not have a pure quadrupole distribution. Nevertheless, a suitable choice of the trapping cavity&#39;s height-to-diameter ratio will reduce the magnitude of higher multipole contributions to the created electric field distribution. In particular, if the height-to-diameter ratio is between about 0.83 and 1.00, the octapole contribution to the field distribution is small; e.g., this contribution vanishes if the ratio is about 0.897. For such values of this shape ratio, the effects of higher multipole distribution are often small enough so that the macro-cavity is able to trap and store ionized particles. See, for example, J. M. Ramsey et al., U.S. Pat. No. 6,469,298 issued on Nov. 22, 2002, which is incorporated herein by reference. 
   For this second type of ion trap, standard machining techniques are available to fabricate the electrodes from metal base pieces, because the electrodes have simple surfaces rather than the complex hyperbolic surfaces of the electrodes  12 – 14  of  FIG. 1 . For this reason, fabrication of this second type of ion trap is usually less complex and less expensive than is fabrication of quadrupole ion traps whose electrodes have hyperbolic-shaped inner surfaces. 
   Nevertheless, the metallic components of such ion traps are expensive to manufacture and assemble. Moreover, these metallic components cause equipment in which they are incorporated to be large and bulky. The latter property has limited the widespread application and deployment of these ion traps in equipment such as mass spectrometers and shift registers. 
   Thus, a need remains in the art for a micro-miniature ion trap that can be inexpensively and readily implemented without reliance on the metallic components common to the prior art. In particular, there is a need for such an ion trap that has a micro-cavity that can be readily and inexpensively fabricated and assembled. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with one aspect of our invention, a micro-miniature ion trap device comprises a wafer (or substrate) having a major surface and at least one plate (essentially planar or curved) forming an ion trapping region in proximity thereto. The at least one plate has an electrically insulating surface and a multiplicity of electrodes disposed on its insulating surface. The electrodes form at least one ion trap in the trapping region when suitable voltages are applied to the electrodes via electrical conductors coupled to the wafer. The device has a multiplicity of ports for introducing ions into the trapping region and for extracting ions from that region. A first one of the plates is oriented at a non-zero angle to the major surface of the wafer and is rotateably mounted on that surface. Devices of this type may be useful, for example, as mass spectrometers, atomic clocks, mass filters, or shift registers. 
   By rotateably mounted we mean that the plate can be rotated during assembly of the device, and that it can be fixed in an upright position during operation of the device. 
   In accordance with another aspect of invention, at least two of the plates form an elongated micro-channel having an axis of ion propagation, and the electrodes on at least one of the two plates are segmented along the direction of the axis, thereby forming a multiplicity of ion traps along the axis. A controller applies suitable voltage (e.g., sequentially) to the segmented electrodes, thereby shifting ions from one trap to another. Preferably, the electrodes on both of the plates are segmented. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is a schematic, cross sectional view of a prior art ion trap having a macro-cavity; 
       FIG. 2  is a schematic, isometric view of a micro-miniature ion trap device in accordance with an illustrative embodiment of our invention; 
       FIG. 3  is a schematic, isometric view of a wafer-supported vertically oriented plate in accordance with one embodiment of our invention; 
       FIG. 4  is a schematic, isometric view of a wafer-supported obliquely oriented plate in accordance with another embodiment of our invention; 
       FIGS. 5–8  show schematic, cross-sectional views of a wafer at various stages of processing to form a plate that is rotateably mounted on the wafer; 
       FIG. 9  shows a schematic, isometric view of a plate formed by the process described in conjunction with  FIGS. 5–8 ; 
       FIG. 10  is a schematic, isometric view of a shift register in accordance with still another embodiment of our invention; 
       FIG. 11  is a schematic, top view of a shift register in accordance with yet another embodiment of our invention; 
       FIG. 12  is a schematic top view of a shift register in accordance with one more embodiment of our invention; and 
       FIG. 13  is a schematic, isometric view of a curved plate in accordance with another embodiment of our invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Ion Trap Structure and Operation 
   With reference now to the illustrative embodiment of our invention shown in  FIG. 2 , a micro-miniature ion trap  20  comprises at least one plate  22 , which is rotateably or pivotally mounted on a major surface  21 . 1  of a wafer (or substrate)  21  during assembly but fixedly mounted on surface  21 . 1  during operation of the trap. (A pair of plates  22  is shown for purposes of illustration only.) The wafer may be made of semiconductor material, dielectric material, or a combination of both. The ability to pivot or rotate each plate results from processing techniques, which are adapted from the integrated circuit industry and will be described more fully hereinafter. Suffice it to say here that, in one embodiment, such processing results in each plate having a window or aperture  28  formed near the bottom of the electrode so as to define an elongated rail or axle  27 , which extends under a hinge  24 . When the plate  22  is released from its original as-fabricated position  21 . 2  on the surface  21 . 1 , it can be rotated to an upright position as shown and then secured in that position, as described more fully hereinafter. 
   Alternatively, the hinge and axle arrangement of  FIG. 2  may be replaced by micro-fabricated flexible elements (not shown), where one side of such a flexible element is mechanically attached to the plate, and the other side is mechanically attached to the wafer surface. Such flexible elements allow the plate to be rotated to the desired upright position with respect to the substrate surface, without being entirely detached from that surface. 
   When in an upright position, the two plates  22  may be oriented essentially perpendicular to major surface  21 . 1  (as shown). Alternatively, the plates do not have to be oriented perpendicular to major surface  21 . 1 ; that is, for example, one (or more) of the plates  42  ( FIG. 4 ) or  112  ( FIG. 11 ) may be oriented at an acute angle to major surface  21 . 1 . In addition, one (or more) of the plates  114  ( FIG. 11 ) may be essentially parallel to major surface  21 . 1 ; that is, plate  114  remains on the surface of wafer  21  rather than being either released or rotated out of the wafer. In general, the combination of plates may form a three dimensional structure having a polygonic cross-section. Typical shapes include various types of cylinders (e.g., those having circular, oval, rectangular, hexagonal or other cross-sections) and various forms of polyhedrons (e.g., tetrahedrons or pyramids). 
   In addition, the plates may be essentially planar, as shown in  FIG. 2 , or they may be curved, as shown in  FIG. 13 . In the latter case, a curved plate  132  is formed as an essentially planar multi-layered structure with at least two layers  132 . 4  and  132 . 5  having sufficiently different physical properties (e.g., thermal expansion coefficients), so that when the plate is released from the wafer during assembly, the stress inherent between the essentially planar layers  132 . 4 – 132 . 5  causes them curl as shown in  FIG. 13 . Illustratively, the electrodes  132 . 1 ,  132 . 2 , and  132 . 3  are formed on layer  132 . 4  during processing. 
   The plates may be rotated either manually or automatically. In the later case, external energy (e.g., supplied by an electric or magnetic field, or a thermal source) or internal energy (e.g., supplied by an integrated mechanical spring with built-in stress or by chemical changes such as polymer shrinkage) may be used to effect self-assembly. See, for example, the approaches described by the following: V. A. Aksyuk et al., U.S. Pat. No. 5,994,159 issued on Nov. 30, 1999; Y. Yi et al.,  The  10 th    Int. Conf. on Solid - State Sensors and Actuators/Transducers , pp. 1466–1469, Sendai, Japan (June 1999); Y. Yi et al.,  Proceedings of SPIE , Vol. 3511, pp. 125–134 (1998); L. Li et al.,  J. of Microelectromechanical Syst ., Vol. 13, No. 1, pp. 83–90 (February 2004); R. S. Muller et al.,  Proc. of the IEEE , Vol. 86, No. 8, pp. 1705–1720 (August 1998); and M. Gel et al.,  J. Micromech. Microeng ., Vol. 11, pp. 555–560 (2001), all of which are incorporated herein by reference. 
   In order to secure the plates in whatever upright position is desired, a brace or support is provided. Thus,  FIG. 3  depicts an illustrative embodiment of a slotted brace  33  that is pivotally mounted on wafer (or substrate)  31 . When the brace  33  is rotated out of the plane of the wafer, slot  33 . 1  engages an edge  32 . 1  of upright plate  32  and holds it in place. This type of brace is particularly useful when the plate  32  is oriented essentially perpendicular to the major surface  31 . 1 , but can be readily adapted to support plates oriented at other (acute) angles as well. 
   Alternatively, as shown in  FIG. 4 , when plate  42  is oriented at an acute angle to the major surface  41 . 1  of wafer (or substrate)  41 , a support  43  having a shelf  43 . 1  may be utilized. That is, the height and slant of the shelf  43 . 1  may be adapted to support the plate at the desired acute angle θ to the major surface  41 . 1 . 
   Once the plates are properly positioned they define an ion trapping micro-cavity between them. As shown in  FIG. 2 , ions  29 . 1  are injected into the trapping region from an ion generator  29 . In order to trap these ions each plate is provided with an array of electrodes  22 . 1 – 22 . 3 , which are disposed on an insulating surface  22 . 5  of each plate  22 . More specifically, the array includes upper and lower electrodes  22 . 1  and  22 . 2 , respectively. These two electrodes are typically connected to a source of (DC) reference potential, typically ground. A third (middle) electrode  22 . 3  is disposed between the upper and lower electrodes. A time varying (e.g., RF) voltage is applied to the third electrode. The combination of these voltages forms a parabolic trapping potential well in the micro-cavity between the two plates  22 , as is well known in the art. (In the case where only a single plate is used, all of the electrodes would, of course, be located on that plate, and the trapping potential well would be formed in near proximity to the plate.) 
   To this end the separation of the plates  22  from one another and the height of the trap (i.e., the distance from the top of upper electrode  22 . 1  to the bottom of lower electrode  22 . 2 ) should be approximately equal. Illustratively, the dimensions of the electrodes range from about 3 to 200 μm. However, the shape of the electrodes need not be rectangular; in general, the shape should preferably optimize the quadrupole potential field for trapping an ion. On the other hand, the dimensions of the plates are preferably at least two to three times that of the electrodes. 
   Once trapped, an ion is released as in the prior art; that is, by applying an additional small, ramped AC voltage to the RF electrode  22 . 3 . 
   In general, the requisite voltages are applied to the DC electrodes  22 . 1 – 22 . 2  via bonding pad  25 . 2  and conductor  25 , and to the RF electrode  22 . 3  via bonding pad  26 . 2  and conductor  26 . Alternatively, the bonding pads may be replaced by integrated electronic circuits generating the requisite electrical signals. The conductors  25 – 26 , which may be made of metal or polysilicon, each include a flexible segment  25 . 1 – 16 . 1 , which enable the plates  22  to be rotated without breaking the electrical connection between the bond pads  25 . 2 – 26 . 2  and the electrodes  22 . 1 – 22 . 3 , respectively. Illustratively, the flexible segments  25 . 1 – 26 . 1  are depicted as being serpentine sections of suspended wire located within window  28  of plate  22 . The segments are relatively short, typically 1 to 5 μm long, to reduce fringing electrical fields, which can perturb the trapping potential. 
   For convenience we have depicted the conductors and electrodes as being located on the same surface and hence of the same plane of a plate, but they could be located on different planes. For example, the electrodes could be located on the front surface of the plate, with the conductors being located on the back surface. The latter design would improve shielding; i.e., reduce fringing electric fields. 
   In an alternative embodiment, the flexible segments  25 . 1 – 26 . 1  are replaced by micro-fabricated metal (e.g. solder) joints (not shown). Such joints would be first melted to allow the plates  22  to be rotated into the desired upright position. After the plates are rotated, the joints would be allowed to cool down and solidify, providing the required electrical connection between conductors  25 ,  26  and electrodes  22 . 1 – 22 . 2 ,  22 . 3 , respectively. They also may serve an additional function of fixing the plate  22  in its desired upright position. 
   Ion Trap Fabrication 
   With reference now to  FIGS. 5–9 , we briefly describe how to fabricate a rotateable plate  82  ( FIG. 8 ) using well-known silicon integrated circuit processing techniques as they are commonly applied to micro-electro-mechanical systems (MEMS) technology. See, for example, H. Zhang, “MEMS Devices and Design,” Course No. 04813190, Lecture 2, pp. 39–43 (Spring 2004), which is incorporated herein by reference and can be found at internet website http://ime.pku.edu.cn/mems/courses/device&amp;design/Lecture — 13_Device Design.pdf. 
   Beginning with  FIG. 5 , a first sacrificial layer  52  of a silicon oxide is deposited on a single crystal silicon wafer  51 . Then a first polysilicon (poly) layer is deposited and patterned to form the patterned poly layer  53 , which will ultimately be released to form plate  82 . 
   Next, as shown in  FIG. 6 , a second sacrificial layer  62  of a silicon oxide is deposited on the patterned poly layer  53  and the exposed portions of first sacrificial layer  52 . The two sacrificial layers  52  and  62  are patterned to open windows  74 , as shown in  FIG. 7 . Then, a second poly layer  73  is deposited over the wafer and into the windows  74 . Poly layer  73  is patterned to form hinge  84  ( FIG. 8 ). Finally, both sacrificial layers  52  and  62  are etched away in order to release the plate  82 , as shown in  FIG. 8 . An isometric view of the plate  82 , after having been released from wafer  51  and rotated, is shown in  FIG. 9 . Also shown are the first poly layer  53 , which forms the plate itself, and the second the second poly layer  73 , which forms the hinge. 
   Note, for simplicity we have omitted from the foregoing description the fact that, before etching away the two sacrificial layers, metallization layers and insulating dielectric layers would have to be deposited and patterned in order to form electrodes  22  and conductors  25 – 26 . 
   It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments that can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, although the micro-miniature ion traps of  FIGS. 1–9  can be readily used in mass spectrometer applications, they can also be modified to construct shift register devices, as described below. 
   Shift Register Devices 
   With reference now to  FIG. 11 , we illustrate an embodiment of an ion-trap-based shift register device  110  in which at least two plates  112 – 114  are positioned to form an ion propagation micro-channel therebetween. Illustratively, plate  112  is oriented at an angle θ (0°&lt;θ≦90°) to the top major surface of wafer  121 , and plate  114  lies within the top major surface. The electrodes  116  on at least one of the plates  112  are segmented to form a multiplicity of ion traps along the channel axis. On the other plate  114  the electrodes  118  are illustratively not segmented. 
   When suitable AC voltages are applied (e.g., sequentially) to the segmented middle electrodes  116 . 3 , a multiplicity of ion traps is created in tandem in the channel. When ions  119 . 1  from ion generator  119  are injected into the channel, they are shifted from one ion trap to another until they exit the shift register device and are incident on a utilization device (not shown). 
   Preferably, however, the electrodes on both plates are segmented, as shown in an alternative embodiment of  FIG. 10 . Here the shift register device  100  is shown in top view to depict a pair of plates  102 – 104 , which are oriented essentially parallel to one another and perpendicular to the major surface of the supporting wafer (not shown). The plates define therebetween an ion propagation micro-channel, which guides ions injected from ion generator  109  to utilization device  108 . On each of the plates the DC and AC electrodes  106  previously described are segmented. A controller  107  applies suitable voltages to the electrodes to create a multiplicity of ion traps along the axis of propagation. The AC voltages are applied (e.g., sequentially) to the segmented middle electrodes in order to move the ions along the micro-channel in shift register fashion. 
     FIG. 10  also depicts a second set of plates  102   a – 104   a , which are oriented illustratively at right angles to plates  102 – 104  to demonstrate that the propagation path can be made to turn corners. To this end, the corner section  103  appears to have extra electrodes  103 . 1 – 103 . 1   a  on the outer plates  104 – 104   a , respectively, that have no counterparts on the inner plates  102 – 102   a . However, this problem can be addressed in several ways. First, the spacing and size of the AC and DC electrodes  106 . 1  on the inner plate  102  near the corner section  103  can be reduced so that a sufficient number of electrodes can be located near the corner, thereby preserving a 1:1 correspondence between the segmented electrodes on the outer and inner plates. Alternatively, the illustrative sequential pulsing protocol of the AC electrodes can be paused as an ion enters corner section  103 . More specifically, the innermost AC electrodes  106 . 2 – 106 . 2   a  on the inner plates  102 – 102   a , respectively, may be pulsed repeatedly while sequentially pulsing the AC electrodes  103 . 1 – 103 . 1   a  on the outer plates  104 – 104   a , respectively, of the corner section  103  until the ion propagates around the corner section  103  and the enters the micro-channel between plates  102   a  and  104   a , whereupon the normal sequential pulsing of the AC electrodes on plates  102   a – 104   a  would resume. 
   An extension of the principle that ion propagation path can be made to turn corners is depicted in  FIG. 12 , a Y-branch device, which incorporates electrode configurations akin to those described with reference to  FIG. 10 . Ions from source  129  are made to propagate along a main channel  122  to a region where the main channel splits or branches into N channels  124 . 1  to  124 .N. Then, control signals from a controller (not shown) cause the ions to propagate along one or more of the branching channels  124 . 1  to  124 .N.