Patent Publication Number: US-10325707-B2

Title: Integrated field coil for compact atomic devices

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. W15P7T-10-C-A019 awarded by the US Army Communications-Electronics Research, Development and Engineering Center. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Field of the Invention 
     The field of the invention relates to physics packages for atomic devices, and more particularly to field coils for use with atomic clocks. 
     Description of the Related Art 
     Chip-scale atomic devices such as chip-scale atomic clocks (“CSAC”) may involve the interrogation of atomic states in a vapor cell that typically contains alkali species and buffer gasses. The physics packages of these devices are typically constructed using an assembly of multiple component subsystems: an optical source, usually a vertical-cavity surface-emitting laser (“VCSEL”); conditioning optics; a vapor cell; and photodetector. These components must be held in given positions to maintain reproducibility of the optical interrogation. The VCSEL and vapor cell must be thermally biased to stabilize temperature and allow control over a range of external ambient conditions, and the assembly must allow electrical interconnect to the VCSEL, vapor cell, and photodetector modules. Controlled magnetic fields typically must be applied to generate the necessary atomic states in the alkali vapor. These constraints can make assembly difficult, often precluding use of automated assembly technologies and increasing assembly and packaging costs. A need exists to simplify assembly methods and to reduce packaging costs, including methods to assemble the field coil about the vapor cell. 
     SUMMARY 
     A magnetic field coil assembly includes a plurality of stacked dielectric layers, each of the plurality of stacked dielectric layers having a partial-loop conductive trace on a first side of the layer, a via interconnect in communication with the partial-loop conductive trace and extending from the first side of the layer to a side of the layer opposite from the first side, and a vapor cell reception aperture. The magnetic field coil assembly may also include a vapor cell axially extending through the plurality of vapor cell reception apertures sot that the plurality of partial-loop conductive traces is electrically connected serially to form a continuous coil disposed around the vapor cell that would create a magnetic field upon application of a current. 
     In one embodiment, a magnetic field coil assembly may include a dielectric substrate having a vapor chamber center aperture, a plurality of partial-loop conductive traces embedded in the substrate, each of the plurality of planar partial-loop conductive traces extending about a perimeter of the vapor chamber center aperture, a respective plurality of via interconnects disposed orthogonal to the plurality of planar partial-loop conductive traces, the respective plurality of via interconnects electrically connecting the plurality of planar partial-loop conductive traces serially so that the plurality of planar partial-loop conductive traces form a continuous conductive loop about the vapor chamber center aperture, and a vapor chamber disposed in the vapor chamber center aperture so that the plurality of planar partial-loop conductive traces and respective plurality of via interconnects form a magnetic field coil about the vapor chamber. 
     A magnetic field coil assembly may also include a dielectric substrate having a vapor chamber center aperture, a coil embedded in the substrate, the coil extending about a perimeter of the vapor chamber center aperture, and a vapor chamber disposed in the vapor chamber center aperture so that the coil forms a magnetic field coil about the vapor chamber which can generate a magnetic field upon application of a current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is an exploded perspective view of one embodiment of an atomic clock physics package having a vapor cell, photodetector, and VCSEL laser source that may be removably inserted into a sealable slotted container; 
         FIGS. 2 and 3  are exploded perspective views of the atomic clock physics package assembly of  FIG. 1 , with the assembly aligned for slidable coupling within a slotted container and slotted lid having interior electrical contacts; 
         FIGS. 4 and 5  are exploded front perspective views of another embodiment of an atomic clock physics package having a vapor cell, photodetector, and VCSEL laser source that may be removably inserted into a sealable slotted container having interior electrical contacts; 
         FIG. 6  is a perspective view of the fully assembled slotted container assembly; 
         FIGS. 7 and 8  are exploded rear perspective views of the slotted container, base wall and pad substrate first illustrated in the front perspective views of  FIGS. 4 and 5 ; 
         FIG. 9  is a perspective view of the assembled components illustrated in  FIGS. 7 and 8 ; 
         FIGS. 10 and 11  are exploded side perspective views of an atomic clock physics package having a magnetic field coil extending around a vapor cell coupled between first and second vapor cell carriers, with the first and second vapor cell carriers slidably seated into a sealable slotted container; 
         FIG. 12  is a perspective view of the assembled components illustrated in  FIGS. 10 and 11 ; 
         FIGS. 13 and 14  are exploded perspective and assembled perspective views, respectively, of one embodiment of a plurality of partial-loop conductive traces embedded in a multilayered dielectric that collectively form a monolithic coil structure; 
         FIGS. 15 and 16  illustrate front and back suspension structures that may be used to support and thermally isolate the vapor cell from the remainder of the atomic clock physics package structure; and 
         FIGS. 17, 18 and 19  are top plan, side cross-section and perspective views, respectively, of a container lid having surface topology in the form of linear channels for application of a getter coating. 
     
    
    
     DETAILED DESCRIPTION 
     A system is disclosed for providing a combined mechanical frame, electrical routing, and embedded magnetic field coil for chip-scale atomic devices to reduce parts count, facilitate assembly and improve consistency in the final assembled component. The system is provided using a plurality of stacked dielectric layers, each of the plurality of stacked dielectric layers having a partial-loop conductive trace on a first side of the layer, a via interconnect in communication with the partial-loop conductive trace and extending from the first side of the layer to a side of the layer opposite from the first side, and a vapor cell reception aperture. A vapor cell may then extend axially through the plurality of vapor cell reception apertures. The plurality of partial-loop conductive traces may be connected electrically in series to form a continuous coil disposed around the vapor cell that would create a magnetic field upon application of a current. 
       FIG. 1  is an exploded perspective view of one embodiment of an atomic clock physics package having a vapor cell, photodetector, and VCSEL laser source that may be removably inserted into a sealable slotted container, subsequent to individual assembly and independent testing, and a lid having interior electrical contacts to exchange signals with the interior vapor cell, photodetector, and VCSEL laser source. A vapor cell substrate  100  having an interrogation chamber  102  (collectively a “vapor cell”  104 ) may be sealed on opposite sides of the interrogation chamber with first and second glass windows ( 106 ,  108 ). The vapor cell  104  and glass windows ( 106 ,  108 ) may be suspended between first and second vapor cell carriers ( 110 ,  112 ) that are each a three-dimensional rigid frame. Each vapor cell carrier ( 110 ,  112 ) may have a respective suspension structure ( 114 ,  116 ) made from a low thermal conductivity material, such as Kapton™ polyimide film or Cirlex™ polymide laminate offered by DuPont USA of Torrence, Calif. (“a suspension structure”). The suspension structure ( 114 ,  116 ) may be attached to the frame and extend into a center region of the frame for suspending and thermally isolating the vapor cell from the remainder of the two vapor cell carriers ( 110 ,  112 ). The VCSEL  118  may be coupled to a VCSEL substrate carrier  120  through a VCSEL substrate  122 . The VCSEL substrate carrier  120  may be formed as a three-dimensional rigid frame. A neutral density (ND) filter  124  may also be coupled to the VCSEL carrier  120  and positioned to receive light emitted from the VCSEL  118 . A wave plate polarizer  126 , preferably consisting of a linear polarizer and a quarter wave plate, may also be coupled to the VCSEL carrier  120  and positioned to receive light (not shown) emitted from the VCSEL  118 . The VCSEL  118 , neutral density filter  124 , wave plate polarizer  126  are aligned so that light emitted from the VCSEL  118  passes through each ( 124 ,  126 ) to interrogate the vapor cell  104 . A photodetector  128  seated on a photodetector carrier  130  is positioned to receive the VCSEL light when emitted from the VCSEL  118  and transmitted through the vapor cell  100 . 
       FIGS. 2 and 3  are exploded perspective views of the atomic clock physics package assembly of  FIG. 1 , with the assembly aligned for slidable coupling within a slotted container and slotted lid. The VCSEL carrier  120 , vapor cell carriers ( 110 ,  112 ), and photodetector carrier  130  may have respective electrical carrier contacts ( 200 ,  202 ,  204 ,  206 ) for communication of signals and power to their respective coupled components ( 118  (see  FIG. 1 ),  104 ,  128 ). Turning first to the carrier lid  208 , the carrier lid  208  may have a VCSEL carrier lid slot  210 , first and second vapor cell carrier lid slots ( 212 ,  214 ), and a photodetector carrier lid slot  216  for receipt of respective top portions of each of the carriers ( 110 ,  112 ,  120 ,  130 ), as described further below. The VCSEL carrier lid slot  210  may be described as a right rectangular prism shaped slot (alternatively referred to as a channel) having a rectangular aperture in the lid  208 . A plurality of VCSEL carrier lid contacts  217  may be disposed on an interior bottom floor of the VCSEL carrier lid slot  210  and in electrical communication with exterior atomic clock pads (not shown). The VCSEL carrier lid slot  210  is dimensioned to slidably receive and guide a top rectangular portion  218  of the VCSEL carrier  120 . The VCSEL carrier contacts  200  may be in electrical contact with the VCSEL lid slots  212  when the VCSEL carrier  120  is completely and slidably inserted into the VCSEL carrier lid slot  210  to enable communication of VCSEL control signaling and power between the vapor cell and exterior atomic clock physics package pads (not shown). 
     Similarly, the lid  208  may have first and second vapor cell carrier lid slots ( 212 ,  214 ) formed as right rectangular prism slots in interior dimension, and each having a rectangular aperture in the lid  208  to receive the vapor cell carriers ( 110 ,  112 ). Either one or both first and second vapor cell carrier lid slots ( 212 ,  214 ) may have vapor cell lid contacts ( 220   a ,  220   b ) disposed on interior bottom floors of them and in communication with exterior atomic clock pads (not shown). The first and second vapor cell carrier lid slots ( 212 ,  214 ) are dimensioned to slidably receive and top rectangular portions ( 222 ,  224 ) of the first and second vapor cell carriers ( 110 ,  112 ), respectively, thereby slidably holding the first and second vapor cell carriers ( 110 ,  112 ) laterally and horizontally, limiting vertical translation, and guiding the vapor cell contacts ( 202 ,  204 ) into electrical contact with the vapor cell lid contacts ( 220   a ,  220   b ). The vapor cell contacts may enable thermistor, heater, coil, and other communications between the vapor cell and the remainder of the assembly. 
     The photodetector carrier lid slot  216  may also be a right rectangular prism shaped slot having a rectangular aperture in the lid  208 . Photodetector lid contacts  228  may be disposed on an interior bottom floor of the photodetector carrier lid slot  216  and in electrical communication with exterior atomic clock physics package pads (not shown). The photodetector carrier lid slot  216  may be dimensioned to slidably receive and guide a top rectangular portion  226  of the photodetector carrier  130  to establish electrical contact between the photodetector lid contacts  228  and the photodetector carrier contacts  206  when the photodetector carrier  130  is completely and slidably inserted into the photodetector carrier slot  216 . 
     Although the contacts ( 200 ,  202 ,  204 ,  206 ) are illustrated as relatively flat and on respective top ends of the carriers ( 120 ,  110 ,  112 ,  130 ) for electrical connection with lid contacts ( 217 ,  220   a ,  220   b ,  228 ), in an alternative embodiment, the carriers may have carrier contacts ( 200 ,  202 ,  204 ,  206 ) that are configured differently, such as being U-shaped and capping the top ends of the carriers, being spring loaded, or incorporating a plug and socket configuration. In another embodiment, one or more of the carriers ( 120 ,  110 ,  112 ,  130 ) may have a top side that is not at a planar right angle to side portions of the carriers, but rather may form contacts that are angular or nonplanar for receipt into the lid contacts, such as may be the case if the carrier contacts are not embedded in or are not relatively flush on top of the carriers, but rather are formed with flexible metal contacts or contacts which are operable to springily engage lid contacts as the carriers and respective carrier contacts are slidably inserted into the lid slots and abut the respective lid contacts. 
     The container  230  has an open end  232  and has VCSEL, first and second container vapor cell and photodetector container slots ( 234 ,  236 ,  238 ,  240 ). The slots may have a rectangular cross section to accept sides of the respective rigid-framed carriers ( 120 ,  110 ,  112 ,  130 ) and may extend into side walls of the container to provide proper alignment and fixed spacing for each of the carriers ( 120 ,  110 ,  112 ,  130 ). Each slot may extend from the open end  232  down to a bottom floor (not shown) of the container  230  so that when the carriers ( 120 ,  110 ,  112 ,  130 ) are inserted into the container slots ( 234 ,  236 ,  238 ,  240 ), the top rectangular portions ( 218 ,  222 ,  224 ,  226 ) of the carriers continue to extend beyond the open end  232  (see  FIG. 3 ) for slidable seating in the lid slots ( 210 ,  212 ,  214 ,  216 ). Because the rectangular portions ( 218 ,  222 ,  224 ,  226 ) of the carriers continue to extend beyond the open end  232 , electrical communication may be established between the carrier contacts ( 200 ,  202 ,  204 ,  206 ) and the respective lid contacts ( 217 ,  220   a ,  220   b ,  228 ). A top sealing surface  242  of the container  230  may be configured to sealably couple with a lid sealing surface  242  of the lid  208  using a sealant such as solder so that a vacuum may be maintained within the container after sealing. 
       FIGS. 4 and 5  are exploded front perspective views of another embodiment of an atomic clock physics package having a vapor cell, photodetector, and VCSEL laser source that may be removably inserted into a sealable slotted container, subsequent to individual assembly and independent testing, with the slottable container assembly having interior electrical contacts (rather than electrical contacts on the lid) to exchange signals with the vapor cell, photodetector, and VCSEL laser source. The slotted container  400  may have VCSEL, vapor cell, filter and photodetector carrier container slots ( 402 ,  404 ,  406 ,  408 ) extending from an open side  410  to a back side  412  of the slotted container  400 . A base wall  414  may be sealably coupled to the slotted container  400  at the container&#39;s back side  412 , such as by using a solder preform  416  during assembly. The base wall  414  may have a plurality of VCSEL, vapor cell, and photodetector electrical container contacts ( 418 ,  420   a / 420   b ,  422 ) on or otherwise embedded in the base wall  414 . The container contacts ( 418 ,  420   a / 420   b ,  422 ) are aligned with a respective plurality of carrier contacts (not shown) when the photodetector, vapor cell and VCSEL carriers ( 424 ,  426   428 ) are inserted into their respective carrier slots ( 402 ,  404 ,  408 ). An optic such as a wave plate polarizer (not shown) may be coupled to a filter carrier  430  to be slidably received by the filter carrier slot  406 . Each of the plurality of slotted container contacts is in electrical communication with a respective plurality of pad substrate vias ( 432 ,  434   a ,  434   b ,  436 ) on a pad substrate  438  that are in electrical communication with atomic clock physics package pads (see  FIGS. 7, 8, and 9 ) on an exterior side of the pad substrate  438 . A lid solder preform  440  may be used to create a hermetic seal as between the slotted container  400  and the lid  442 . In one embodiment, the pad substrate  438  and base wall  414  are two-layer co-fired ceramics. 
       FIG. 6  is a perspective view of the fully assembled slotted container assembly. Dashed lines are illustrated on the lid to indicate the locations of interior channels  600  that may be provided for application of getter. 
       FIGS. 7 and 8  are exploded rear perspective views of the slotted container, base wall and pad substrate first illustrated in the front perspective views of  FIGS. 4 and 5 . Photodetector electrical container contacts ( 418 ,  420   a / 420   b ,  422 ) (see  FIG. 4 ) of the base wall  414  are in electrical communication with rear electrical traces  702  through internal vias (not shown). The pad substrate  438  may have internal conductive vias (not shown) to provide electrical communication between the rear electrical traces  702  and atomic clock physics package pads  704 . The base wall  414  may be coupled to the pad substrate  438  to establish a base wall assembly  706 . The base wall assembly  706  may be coupled to the slotted container  400  using the solder preform  416 . In  FIG. 9 , the assembly is illustrated fully assembled. 
       FIGS. 10 and 11  are exploded side perspective views and  FIG. 12  an assembled view of an atomic clock physics package having a magnetic field coil extending around a vapor cell coupled between first and second vapor cell carriers, with the first and second vapor cell carriers slidably seated into a sealable slotted container subsequent to individual assembly and independent testing. The magnetic field coil  1000  may be disposed circumferentially about a vapor cell  1002  to provide a known magnetic field for generating appropriate atomic states in alkali vapor of the vapor cell  1002 . The vapor cell  1002  and glass windows ( 1004 ,  1006 ) may be suspended between first and second vapor cell carriers ( 1008 ,  1010 ). Each vapor cell carrier ( 1008 ,  1010 ) may have a respective low thermal dissipation suspension structure ( 1012 ,  1014 ) attached to the frame and extending into a center region of the frame for suspending and thermally isolating the vapor cell from the remainder of the two vapor cell carriers ( 1008 ,  1010 ). The first and second vapor cell carriers ( 1008 ,  1010 ) may be slidably seated into first and second vapor cell carrier slots ( 1016 ,  1018 ). 
     A VCSEL  1020  may be coupled to a substrate spacer  1022 , with a filter package  1024 , preferably a wave plate polarizer and ND filter, also coupled to the substrate spacer  1022  and disposed in front of the VCSEL  1020 . The substrate spacer  1022  is coupled to the VCSEL substrate carrier  1026  and the VCSEL substrate carrier  1026  slidably seated into a VCSEL substrate carrier slot  1027 . 
     A photodetector  1028  may be seated on a photodetector carrier  1030  that is slidably seated in a photodetector carrier slot  1032 . The VCSEL  1020 , vapor cell  1002  and photodetector  1020  are positioned so that light emitted from the VCSEL  1020  is directed through the vapor cell  1002  to impinge on the photodetector  1020 . A container lid  1034  may have a container facing VCSEL carrier lid slot, first and second vapor cell carrier lid slot, and photodetector carrier lid slot (each not shown) for slideably receiving the respective VCSEL substrate carrier  1026 , first and second vapor cell carriers ( 1008 ,  1010 ) and photodetector carrier  1030 . The container lid  1034  may also sealably couple to an open end  1036  of the container  1038 . 
       FIGS. 13 and 14  are exploded perspective and assembled perspective views, respectively, of one embodiment of a plurality of partial-loop conductive traces embedded in a multilayered dielectric that collectively form a monolithic coil structure. The coil structure may be used to both suspend a vapor cell through low thermal dissipation suspensions while enabling a magnetic field to be applied to the vapor cell. The plurality of dielectric layers  1300 , such as layers of ceramic  1302 , are positioned in a stacked arrangement, with each ceramic layer  1302  in the shape of a square or rectangular toroid that establishes a center aperture  1304 . Each ceramic layer  1302  may have a partial loop conductive trace  1306  extending about its top face  1308 , with one end of the conductive trace connected to a via interconnect (not shown) for electrical connection to a pad or other terminal on a bottom face of the ceramic layer for electrical connection to a bottom facing adjacent ceramic layer (electrical connections indicated by dashed lines). The other end of the metal trace on the top face may be connected to a top facing ceramic layer. At either end of the stacked layers of ceramic are vapor cell electrical pads  1310 . An alternate embodiment uses metallized edge traces on the edges of the individual ceramic layers to electrically interconnect the partial loop conductive traces on adjacent layers. In such an embodiment, the edge trace on a particular layer would be in electrical communication with the partial-loop conductive trace on such layer. 
     In such a manner, the partial-loop conductive traces  1306  are connected serially to create an electrically continuous coil disposed about the center apertures  1304  of the multilayered dielectric to enable a magnetic field to be generated about the aperture  1304  upon application of an electrical signal to the coil. The plurality of via interconnects in the individual layers  1302  may be distributed substantially equally angularly about a perimeter of the plurality of stacked dielectric layers  1302 . The assembly may also include a rigid bottom support layer  1312  disposed on a side of the plurality stacked dielectric layers opposite from the rigid top support layer  1314 . 
     Although the illustrated embodiment has a single loop trace, alternately called a partial-loop conductive trace, on each ceramic layer, in other embodiments each ceramic layer may have two or more loops, depending on size constraints. In such an embodiment, there may be two turns per layer and a total of 16 layers. Each of the partial-loop conductive traces  1306  may have a trace width of 250 microns, a trace thickness of 5 microns, and the two loops be spaced 250 microns apart from one another. 
       FIGS. 15 and 16  illustrate front and back suspension structures that may be used to support and thermally isolate the vapor cell from the remainder of the atomic clock structure. The front and back suspension structures may be front and back low dissipation suspension structures ( 1500 ,  1502 ) made of low thermal conductivity materials, such as Kapton or Cirlex, coupled to first and second sides ( 1504 ,  1506 ) of the plurality of stacked dielectric layers  1508 . A vapor cell  1510  may be coupled to the front and back suspension structures ( 1500 ,  1502 ). 
       FIGS. 17, 18 and 19  are top plan, side cross-section and perspective views, respectively, of a container lid having getter channels for application of a getter coating to absorb undesirable vacuum gases, such as O 2 , H 2 O, CO, CO 2  and N 2 , to better maintain the vacuum environment within the hermetically sealed container during use. The lid  1700  may have a metal or ceramic undulating inner surface consisting of a plurality of channels, grooves or troughs (“channels  1702 ”) extending from a first end  1704  to a second end  1706  and extending from an inner surface  1800  of the lid down and into the lid&#39;s material. The channels  1702  may extend longitudinally and be spaced apart in a parallel arrangement. In alternative embodiments, the channels extend laterally (i.e., extending between the longer sides of the rectangular lid) or may form a pattern of relatively smooth dimples, rather than channels, that collectively increase the surface area of the inner surface from what would otherwise exist without such dimpling. The channels or dimples, otherwise referred to as an “undulating surface,” may be formed by machining, embossing, etching, direct casting, by additive manufacturing or by other methods. By way of example, and not limitation, the getter applied to the channels may be any one of the thin film getters, hydrogen getters, evaporable getters or non-evaporable getters (NEG) offered by SAES Getters S.p.A of Italy. 
     The channels may have a generally circular or oval cross-section along their length. In one implementation of a container lid having a length of 14.5 mm, a width of 11.0 mm, five channels may be provided having a channel length (C L ) of 10 mm, a width (C W ) of 1.4 mm and a radial depth (Rd) of approximately 0.5 mm. In such a case, the surface area presented by the undulating surface may have 30% greater surface area than what would otherwise exist without such channels. In other embodiments, the undulating surface may extend up and away from the inner surface  1800  to form longitudinal crowns (now shown), rather than channels extending down into the surface material. Similarly, a dimpled undulating surface may be replaced with a surface having mounds, bumps or other additive material that collectively increase the surface area presented on the inner surface  1800  from what would otherwise exist with a planar surface. 
     The channels may not extend to the outer perimeter of the container lid, but rather the lid may have a flat and metalized bonding surface  1708  extending about the perimeter to enable coupling and vapor sealing of the lid with a container  1038  (see  FIGS. 10-12 ). 
     While various implementations of the embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. While reference is made to an atomic clock physics package, this can equally apply to other compact atomic devices, such as magnetometers or gyroscopes.