Abstract:
A first apparatus includes a vapor cell having first and second cavities fluidly connected by multiple channels. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity. A second apparatus includes a vapor cell having a first wafer with first and second cavities and a second wafer with one or more channels fluidly connecting the cavities. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. Nonprovisional patent application Ser. No. 13/948,888, filed Jul. 23, 2013, the contents of which are herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure is generally directed to gas cells. More specifically, this disclosure is directed to a vapor cell structure having cavities connected by channels for micro-fabricated atomic clocks, magnetometers, and other devices. 
       BACKGROUND 
       [0003]    Various types of devices operate using radioactive gas or other gas within a gas cell. For example, micro-fabricated atomic clocks (MFACs) and micro-fabricated atomic magnetometers (MFAMs) often include a cavity containing a metal vapor and a buffer gas. In some devices, the metal vapor and the buffer gas are created by dissociating cesium azide (CsN 3 ) into cesium vapor and nitrogen gas (N 2 ). 
       SUMMARY 
       [0004]    This disclosure provides a vapor cell structure having cavities connected by channels for micro-fabricated atomic clocks, magnetometers, and other devices. 
         [0005]    In a first example, an apparatus includes a vapor cell having first and second cavities fluidly connected by multiple channels. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity. 
         [0006]    In a second example, a system includes a vapor cell and an illumination source. The vapor cell includes first and second cavities fluidly connected by multiple channels. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The illumination source is configured to direct radiation through the second cavity. 
         [0007]    In a third example, an apparatus includes a vapor cell having a first wafer with first and second cavities and a second wafer with one or more channels fluidly connecting the cavities. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity. 
         [0008]    Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIGS. 1 through 4  illustrate an example vapor cell structure in accordance with this disclosure; 
           [0011]      FIGS. 5 and 6  illustrate another example vapor cell structure in accordance with this disclosure; 
           [0012]      FIGS. 7 and 8  illustrate example devices containing at least one vapor cell structure in accordance with this disclosure; and 
           [0013]      FIG. 9  illustrates an example method for forming a vapor cell structure in accordance with this disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIGS. 1 through 9 , discussed below, and the various examples used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitable manner and in any type of suitably arranged device or system. 
         [0015]      FIGS. 1 through 4  illustrate an example vapor cell structure  100  in accordance with this disclosure. The vapor cell structure  100  can be used, for example, to receive an alkali-based material (such as cesium azide) and to allow dissociation of the alkali-based material into a metal vapor and a buffer gas (such as cesium vapor and nitrogen gas). However, this represents one example use of the vapor cell structure  100 . The vapor cell structure  100  described here could be used in any other suitable manner. 
         [0016]    As shown in  FIGS. 1 through 3 , the vapor cell structure  100  includes a bottom wafer  102 , a middle wafer  104 , and a top wafer  106 . The bottom wafer  102  generally represents a structure on which other components of the vapor cell structure  100  can be placed. The bottom wafer  102  is also substantially optically transparent to radiation passing through the vapor cell structure  100  during operation of a device, such as a micro-fabricated atomic clock, magnetometer, or other device. The bottom wafer  102  can be formed from any suitable material(s) and in any suitable manner. The bottom wafer  102  could, for instance, be formed from borosilicate glass, such as PYREX or BOROFLOAT glass. 
         [0017]    The middle wafer  104  is secured to the bottom wafer  102 , such as through bonding. The middle wafer  104  includes multiple cavities  108 - 110  through the middle wafer  104 . Each cavity  108 - 110  could serve a different purpose in the vapor cell structure  100 . For example, the cavity  108  can receive a material to be dissociated, such as cesium azide (CsN 3 ) or other alkali-based material. The cavity  108  can be referred to as a “reservoir cavity.” The cavity  110  can receive gas from the cavity  108 , such as a metal vapor and a buffer gas. Laser illumination or other illumination could pass through the cavity  110  during operation of a device, such as a micro-fabricated atomic clock, magnetometer, or other device. The cavity  110  can be referred to as an “interrogation cavity.” 
         [0018]    Multiple channels  112  fluidly connect the cavities  108 - 110  in the vapor cell structure  100 . Each channel  112  represents any suitable passageway through which gas or other material(s) can flow. In this example, there are three channels  112 , although the vapor cell structure  100  could include two or more than three channels  112 . Also, the channels  112  here are generally straight, have equal lengths, and are parallel to one another. However, the channels  112  could have any other suitable form(s). 
         [0019]    The middle wafer  104  could be formed from any suitable material(s) and in any suitable manner. For example, the middle wafer  104  could represent a silicon wafer, and the cavities  108 - 110  and the channels  112  could be formed in the silicon wafer using one or more wet etches or other suitable processing techniques. As a particular example, the channels  112  could be formed in a silicon wafer using a potassium hydroxide (KOH) wet etch. The etch of the silicon wafer could also be performed in a self-limiting manner, meaning the etch stops itself at or around a desired depth. For instance, when a narrow mask opening is used to expose the silicon wafer and the etching occurs at a suitable angle (such as about 54.74°), the etching can self-terminate before it etches completely through the silicon wafer. 
         [0020]    Each cavity  108 - 110  and channel  112  could have any suitable size, shape, and dimensions. Also, the relative sizes of the cavities  108 - 110  and channels  112  shown in  FIGS. 1 through 3  are for illustration only, and each cavity  108 - 110  or channel  112  could have a different size relative to the other cavities or channels. Further, the relative depth of each channel  112  compared to the depth(s) of the cavities  108 - 110  is for illustration only, and each cavity  108 - 110  and channel  112  could have any other suitable depth. In addition, while each cavity  108 - 110  is shown as being formed completely through the wafer  104 , each cavity  108 - 110  could be formed partially through the wafer  104 . 
         [0021]    The top wafer  106  is secured to the middle wafer  104 , such as through bonding. The top wafer  106  generally represents a structure that caps the cavities  108 - 110  and channels  112  of the middle wafer  104 , thereby helping to seal material (such as gas) into the vapor cell structure  100 . The top wafer  106  is also substantially optically transparent to radiation passing through the vapor cell structure  100  during operation of a device, such as a micro-fabricated atomic clock, magnetometer, or other device. The top wafer  106  can be formed from any suitable material(s) and in any suitable manner. The top wafer  106  could, for instance, be formed from borosilicate glass, such as PYREX or BOROFLOAT glass. 
         [0022]    As shown here, a portion  114  of the top wafer  106  could be thinner than the remainder of the top wafer  106 . This may help to facilitate easier UV irradiation of material placed inside the reservoir cavity  108 . Note that any wafer  102 - 106  in the vapor cell structure  100  could have a non-uniform thickness at any desired area(s) of the wafer(s). Also note that the portion  114  of the top wafer  106  could have any suitable size, shape, and dimensions and could be larger or smaller than the reservoir cavity  108 . The portion  114  of the top wafer  106  could be thinned in any suitable manner, such as with a wet isotropic etch. 
         [0023]    During fabrication of the vapor cell structure  100 , the bottom and middle wafers  102 - 104  could be secured together, and the middle wafer  104  can be etched to form the cavities  108 - 110  and the channels  112  (either before or after the bottom and middle wafers  102 - 104  are secured together). An alkali-based material  116  (such as cesium azide) or other material(s) can be deposited into the reservoir cavity  108  as shown in  FIGS. 3 and 4 . Any suitable deposition technique can be used to deposit the material(s)  116  into the cavity  108 . The top wafer  106  can be secured to the middle wafer  104  once the material  116  is placed in the cavity  108 . At this point, the cavities  108 - 110  and the channels  112  can be sealed. 
         [0024]    At least a portion of the material  116  in the cavity  108  can be dissociated. This could be accomplished by exposing the material  116  in the cavity  108  to ultraviolet (UV) radiation. For example, an alkali-based material  116  can be dissociated into a metal vapor and a buffer gas. As a particular example, cesium azide could be dissociated into cesium vapor and nitrogen gas (N 2 ). Note, however, that other mechanisms could be used to initiate the dissociation, such as thermal dissociation. The dissociation of the material  116  creates gas inside the reservoir cavity  108 , which can flow into the interrogation cavity  110  through the channels  112 . 
         [0025]    In conventional devices, material is often dissociated in a single cavity, and the resulting gas is kept in the same cavity. Radiation can be passed through the gas in that single cavity during operation of a device, but residue from the original material may still exist in that single cavity. This residue can interfere with the optical properties of the cavity and lead to device failure. 
         [0026]    In accordance with this disclosure, the material  116  can be placed in one cavity  108  and dissociated, and the resulting gas can be used in a different cavity  110  during device operation. As shown in  FIG. 4 , an illumination source  118 , such as a vertical-cavity surface-emitted laser (“VCSEL”) or other laser, could direct radiation through the interrogation cavity  110 . Even if residue exists in the reservoir cavity  108 , it may not interfere with the optical properties in the cavity  110 . 
         [0027]    The use of multiple channels  112  also helps to ensure that vapor can travel from the reservoir cavity  108  into the interrogation cavity  110 , even if one or more channels  112  become blocked by debris or other material(s). In particular embodiments, the channels  112  could represent self-height-eliminating channels fabricated using a wet etch, rather than a more expensive and time-consuming dry etch. This can help to simplify the manufacture of the vapor cavity structure  100 . In addition, thinning the portion  114  of the top wafer  106  through which UV radiation is directed into the reservoir cavity  108  allows for enhanced dissociation of the material  116  in the cavity  108  (possibly at reduced power levels) while maintaining the mechanical integrity of the overall device. 
         [0028]    Although  FIGS. 1 through 4  illustrate one example of a vapor cell structure  100 , various changes may be made to  FIGS. 1 through 4 . For example, the vapor cell structure  100  need not include two cavities and could include three or more cavities. Also, the cavities  108 - 110  and channels  112  need not be arranged linearly, and the channels  112  need not be straight. Any arrangement of cavities connected by channels could be used, including non-linear and multi-level arrangements. Further, the vapor cell structure  100  could be used with any other material(s) and is not limited to alkali-based materials or metal vapors and buffer gases. In addition, the vapor cell structure  100  can be used in any other suitable manner and is not limited to the use shown in  FIG. 4 . 
         [0029]      FIGS. 5 and 6  illustrate another example vapor cell structure  500  in accordance with this disclosure. The vapor cell structure  500  shown here is similar in structure to that shown in  FIGS. 1 through 4 . Reference numerals  102 - 110  and  114 - 118  are used here to denote structures that may be the same as or similar to structures described above. In this example, however, channels are not formed in the middle wafer  104 . Rather, one or more channels  512  are formed in the top wafer  106 . The top wafer  106  in this example may be said to represent a “capping” layer since it can be secured to the middle wafer  104  after the material  116  is inserted into the cavity  108 , thereby capping the structure  500 . 
         [0030]    The channels  512  (and possibly portions of the cavities  108 - 110 ) can be etched into the top wafer  106  in any suitable manner. For example, a photoresist mask can be formed on the top wafer  106 , patterned, and baked/cured. An isotropic wet etch, such as one using a hydrofluoric acid (HF) dip, can then be performed to etch exposed portions of the top wafer  106 . The composition of the wet etch bath and the etch time can be selected to reduce the thickness of the top wafer  106  as desired. The photoresist layer can then be removed, and the top wafer  106  can be cleaned in preparation for securing to the middle wafer  104 . In this way, the top wafer  106  need not be thinned significantly or at all over the interrogation cavity  110 , helping to preserve the mechanical strength of the vapor cell structure  500 . The channels  512  in the capping layer can also serve other functions, such as by serving as condensation sites in the vapor cell structure  500 . 
         [0031]      FIG. 6  illustrates various examples of the channels and cavity portions that can be etched into a capping layer, such as the top wafer  106 . For example, arrangement  602  includes portions of two unequally-sized cavities and a single channel between the cavities. Arrangement  604  includes portions of two unequally-sized cavities and two channels between the cavities. Arrangement  606  includes portions of two equally-sized cavities and three channels between the cavities. Arrangement  608  includes portions of two unequally-sized cavities and four channels between the cavities. Arrangement  610  includes portions of three unequally-sized cavities and five channels coupling each adjacent pair of cavities. Arrangement  612  includes portions of three equally-sized cavities and five channels coupling each adjacent pair of cavities. These arrangements are for illustration only, and other arrangements of cavities and channels (whether linear or non-linear) could be used in the vapor cell structure  500 . 
         [0032]    In particular embodiments, the top wafer  106  could be formed from borosilicate glass, and the etch of the top wafer  106  could occur using a hydrofluoric acid (BHF) bath. A hard mask could be used to mask the top wafer  106 . Any suitable etch, hard mask, and etch depth could also be used. 
         [0033]    Although  FIGS. 5 and 6  illustrate another example of a vapor cell structure  500 , various changes may be made to  FIGS. 5 and 6 . For example, the vapor cell structure  500  could include any number of cavities and any number of channels in any suitable arrangement. Also, the vapor cell structure  500  could be used with any suitable material(s) and is not limited to alkali-based materials or metal vapors and buffer gases. In addition, the vapor cell structure  500  can be used in any other suitable manner. 
         [0034]      FIGS. 7 and 8  illustrate example devices containing at least one vapor cell structure in accordance with this disclosure. As shown in  FIG. 7 , a device  700  represents a micro-fabricated atomic clock or other atomic clock. The device  700  here includes one or more illumination sources  702  and a vapor cell  704 . Each illumination source  702  includes any suitable structure for generating radiation, which is directed through the vapor cell  704 . Each illumination source  702  could, for example, include a laser or lamp. 
         [0035]    The vapor cell  704  represents a vapor cell structure, such as the vapor cell structure  100  or  500  described above. The radiation from the illumination source(s)  702  passes through the interrogation cavity  110  of the vapor cell  704  and interacts with the metal vapor. The radiation can also interact with one or more photodetectors that measure the radiation passing through the interrogation cavity  110 . For example, photodetectors can measure radiation from one or more lasers or lamps. 
         [0036]    Signals from the photodetectors are provided to clock generation circuitry  706 , which uses the signals to generate a clock signal. When the metal vapor is, for example, rubidium  87  or cesium  133 , the signal generated by the clock generation circuitry  706  could represent a highly-accurate clock. The signals from the photodetectors are also provided to a controller  708 , which controls operation of the illumination source(s)  702 . The controller  708  helps to ensure closed-loop stabilization of the atomic clock. 
         [0037]    As shown in  FIG. 8 , a device  800  represents a micro-fabricated atomic magnetometer or other atomic magnetometer. The device  800  here includes one or more illumination sources  802  and a vapor cell  804 . Each illumination source  802  includes any suitable structure for generating radiation, which is directed through the vapor cell  804 . Each illumination source  802  could, for example, include a laser or lamp. 
         [0038]    The vapor cell  804  represents a vapor cell structure, such as the vapor cell structure  100  or  500  described above. The radiation from the illumination source(s)  802  can pass through the interrogation cavity  110  of the vapor cell  804  and interact with the metal vapor. The radiation can also interact with one or more photodetectors that measure the radiation passing through the interrogation cavity  110 . For example, photodetector(s) can measure radiation from one or more lasers or lamps. 
         [0039]    Signals from the photodetector(s) are provided to a magnetic field calculator  806 , which uses the signals to measure a magnetic field passing through the interrogation cavity  110 . The magnetic field calculator  806  here is capable of measuring extremely small magnetic fields. The signals from the photodetector(s) can also be provided to a controller  808 , which controls operation of the illumination source(s)  802 . 
         [0040]    Although  FIGS. 7 and 8  illustrate examples of devices  700  and  800  containing at least one vapor cell structure, various changes may be made to  FIGS. 7 and 8 . For example, the devices  700  and  800  shown in  FIGS. 7 and 8  have been simplified in order to illustrate example uses of the vapor cell structures  100  and  500  described above. Atomic clocks and atomic magnetometers can have various other designs of varying complexity with one or multiple vapor cell structures. 
         [0041]      FIG. 9  illustrates an example method  900  for forming a vapor cell structure in accordance with this disclosure. As shown in  FIG. 9 , multiple cavities are formed in a middle wafer of a vapor cell structure at step  902 . This could include, for example, forming cavities  108 - 110  in a silicon wafer or other middle wafer  104 . Any suitable technique could be used to form the cavities, such as a wet or dry etch. 
         [0042]    One or more channels are formed in the middle wafer or a top wafer of the vapor cell structure at step  904 . This could include, for example, forming one or more channels  112  in the silicon wafer or other middle wafer  104 . This could also include forming one or more channels  512  in the top wafer  106  or other capping layer. Any suitable technique could be used to form the channels, such as a wet etch. The formation of the cavities and channels could also overlap, such as when the same etch is used to form both the cavities  108 - 110  and the channels  112 . 
         [0043]    A portion of the top wafer is thinned at step  906 . This could include, for example, etching a portion  114  of the top wafer  106  in an area adjacent to the reservoir cavity  108 . Any suitable etch can occur here, such as an isotropic wet etch. The formation of channels in the top wafer and the thinning of the top wafer could also overlap, such as when the same etch is used to form both the channels  512  and the thinned portion  114 . 
         [0044]    The middle wafer is secured to a lower wafer at step  908 . This could include, for example, bonding the middle wafer  104  to the bottom wafer  102 . If the cavities  108 - 110  are formed completely through the middle wafer  104 , securing the middle wafer  104  to the bottom wafer  102  can seal the lower openings of the cavities  108 - 110 . 
         [0045]    A material to be dissociated is deposited in at least one of the cavities at step  910 . This could include, for example, depositing the material  116  into the reservoir cavity  108 . Any suitable deposition technique could be used to deposit any suitable material(s)  116 , such as an alkali-based material. 
         [0046]    The top wafer is secured to the middle wafer at step  912 . This could include, for example, bonding the top wafer  106  to the middle wafer  104 . Securing the top wafer  106  to the middle wafer  104  can seal the upper openings of the cavities  108 - 110  and the channels  112 ,  512 . At this point, the cavities and channels in the vapor cell structure can be sealed against the outside environment. 
         [0047]    The material is dissociated to create metal vapor and buffer gas at step  914 . This could include, for example, applying UV radiation to the material  116  through the thinned portion  114  of the top wafer  106 . This could also include converting at least a portion of the material  116  into the metal vapor and buffer gas. Note, however, that other dissociation techniques could also be used. 
         [0048]    In this way, the vapor cell structure can be fabricated in a manner that allows easier dissociation of the material  116  while maintaining the structural integrity of the vapor cell. Moreover, the use of multiple channels can help to ensure that gas can flow into the interrogation cavity  110 , even when one or more channels are blocked. 
         [0049]    Although  FIG. 9  illustrates one example of a method  900  for forming a vapor cell structure, various changes may be made to  FIG. 9 . For example, as noted above, various modifications can be made to the fabrication process. Also, while shown as a series of steps, various steps in  FIG. 9  could overlap, occur in parallel, or occur in a different order. 
         [0050]    It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “top,” “middle,” and “bottom” refer to structures in relative positions in the figures and do not impart structural limitations on how a device is manufactured or used. The term “secured” and its derivatives mean to be attached, either directly or indirectly via another structure. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
         [0051]    While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.