Patent Publication Number: US-8530249-B2

Title: Middle layer of die structure that comprises a cavity that holds an alkali metal

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     This application is a Divisional of application Ser. No. 11/900,244, filed on Sep. 11, 2007 now U.S. Pat. No. 7,973,611, which is a Divisional of application Ser. No. 10/831,812, filed on Apr. 26, 2004 now U.S. Pat. No. 7,292,111. 
    
    
     BACKGROUND 
     Alkali metals (i.e., cesium) are used by various systems and devices. In order to integrate cesium with elements of a system it may be necessary to encapsulate the cesium in a closed structure. A small system or device may require the closed structure encapsulating cesium to be small. To maintain the integrity of the cesium cell, the inner surfaces of the closed structure are constructed with a material that does not react to cesium or is passive with respect to cesium. 
     In one example, the closed structure encapsulating cesium comprises an ampoule of a borosilicate glass (i.e., Pyrex). Pyrex does not react to cesium. Glass blowing technology is often used to generate the ampoule. A plurality of ampoules may be attached to a manifold and therefore the plurality of ampoules may be filled with cesium simultaneously. To fill the ampoule or plurality of ampoules the ampoule or manifold connecting the plurality of ampoules is infused with cesium. For example, differential heating moves droplets of cesium through a glass tube into an opening in the ampoule. Once the ampoule is filled with cesium, then the opening of the ampoule is pinched or fused to seal the cesium within the ampoule. 
     As one shortcoming, the process of encapsulating cesium within the plurality of ampoules is not automated. Therefore, the process is not well suited for batch fabrication. As another shortcoming, using glass blowing technology to create a small closed structure encapsulating cesium and controlling the dimensions of the small closed structure encapsulating cesium is difficult. The lack of control over the dimensions of the small closed structure encapsulating cesium limits an endurance of the small closed structure encapsulating cesium to effects of shock and vibration. Therefore, the fabrication of the small closed structure encapsulating cesium is dependent on a highly skilled glass blowing technique. As yet another shortcoming, a large closed structure encapsulating cesium requires more power to maintain a temperature the large closed structure encapsulating cesium within a range than the small closed structure encapsulating cesium in environments where the ambient temperature is outside of the range. As yet another shortcoming, the small system or device may not be able to use the large closed structure encapsulating cesium. As yet another shortcoming, the closed structure encapsulating cesium created though glass blowing technology is restricted in functionality to the encapsulation of cesium, and not amenable to function as part of a system or device beyond such functionality. 
     Thus, a need exists for an enhanced closed structure encapsulating an alkali metal. A need also exists for an enhanced process of encapsulating an alkali metal within a closed structure. 
     SUMMARY 
     The invention in one implementation encompasses an apparatus. The apparatus comprises a die structure that comprises a middle layer, a first outside layer, and a second outside layer. The middle layer comprises a cavity that holds an alkali metal, wherein one of the first outside layer and the second outside layer comprises a channel that leads to the cavity. The middle layer, the first outside layer, and the second outside layer comprise dies from one or more wafer substrates. 
     Another implementation of the invention encompasses an apparatus. The apparatus comprises a chamber that accommodates an array of die structures that comprises one or more cavities. The chamber comprises an alkali metal source and an alkali metal source control component. The alkali metal source control component fills a portion of the chamber and the one or more cavities of the array of die structures with a portion of the alkali metal source. 
     Yet another implementation of the invention encompasses an apparatus. The apparatus comprises a first layer of a die structure package that comprises a die structure, a thermal isolator, and an electrical conductor and a second layer of the die structure package that comprises one or more electronic components that provide supplementary functionality to one or more of the die structure, the thermal isolator, and the electrical conductor. The die structure package comprises inorganic materials that serves to promote a reduction of gases released from the die structure package. 
     Still yet another implementation of the invention encompasses a method. A chamber is selected that accommodates an array of die structures that comprises one or more cavities. An inner chamber of the chamber is maintained at a first temperature. An alkali metal source of the chamber is maintained at a second temperature greater than the first temperature. An outer chamber of the chamber is maintained at a third temperature greater than the first temperature and the second temperature. The one or more cavities of the array of die structures is filled with a portion of the alkali metal source. The one or more cavities of the array of die structures is sealed to comprise the portion of the alkali metal source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
         FIG. 1  is a representation of one exemplary implementation of an apparatus that comprises a die structure with a reservoir for an alkali metal. 
         FIG. 2  is a sectional representation of the die structure directed along line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a representation of one exemplary implementation of a wafer structure that comprises an array of die structures analogous to the die structure of the apparatus of  FIG. 1 . 
         FIG. 4  is a representation of one exemplary implementation of a chamber structure that serves to fill with cesium the die structure of the apparatus of  FIG. 1 . 
         FIG. 5  a cross-section view of one exemplary implementation of a method of sealing the die structure of the apparatus of  FIG. 1 . 
         FIG. 6  is a representation of one exemplary implementation of a photocell and the die structure of the apparatus of  FIG. 1  fixedly mounted to a first beam structure. 
         FIG. 7  is a representation of another exemplary implementation of a photocell and the die structure of the apparatus of  FIG. 1  fixedly mounted to a first beam structure. 
         FIG. 8  is one representation of one exemplary implementation of a system package that comprises a housing for the die structure of the apparatus of  FIG. 1 . 
         FIG. 9  is another representation of one exemplary implementation of a system package that comprises a housing for the die structure of the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning to  FIG. 1 , an apparatus  100  in one example comprises a die structure  101  that has a reservoir for an alkali metal (i.e., cesium). The apparatus  100  includes a plurality of components that can be combined or divided. The die structure  101  comprises a middle layer  102 , a first outside layer  104 , and a second outside layer  106 . The middle layer  102 , the first outside layer  104 , and the second outside layer  106  comprise dies from a wafer substrate. The middle layer  102 , the first outside layer  104 , and the second outside layer  106  are attached by a method of wafer bonding (i.e., anodic bonding). In one example, one or more outside surfaces of the middle layer  102  are coated with a metal (i.e., tungsten) for anodic bonding with the first outside layer  104  and the second outside layer  106 . Tungsten is inert with respect to cesium. In another example, one or more outside surfaces of the first outside layer  104  and the second outside layer  106  are coated with tungsten for anodic bonding with the middle layer  102 . The first outside layer  104  and the second outside layer  106  may comprise one or more windows to facilitate an entrance and an exit of a laser light. 
     In one example, the die structure  101  comprises a silicon die and two Pyrex dice. For example, the silicon die is formed from a silicon wafer substrate and the two Pyrex dice are formed from one or more Pyrex wafer substrates. In one example, the one or more Pyrex wafer substrates may comprise any borosilicate glass. The middle layer  102  comprises the silicon die. One or more surfaces of the middle layer  102  that may come in contact with cesium are doped with phosphorous and oxidized to protect against a reaction with cesium. For example, the middle layer comprises one or more outer surfaces oxidized by phosphorus doped silicon dioxide. The first outside layer  104  and the second outside layer  106  comprise the two Pyrex dice. Pyrex is inert with respect to cesium and will not react upon contact with cesium, therefore the first outside layer  104  and the second outside layer  106  do not require oxidation to protect against a reaction with cesium. 
     In another example, the die structure  101  comprises three silicon dice. For example, the three silicon dice are formed from one or more silicon wafer substrates. The middle layer  102 , the first outside layer  104 , and the second outside layer  106  comprise the three silicon dice. One or more surfaces of the middle layer  102 , the first outside layer  104 , and the second outside layer  106  that may come in contact with cesium are doped with phosphorous and oxidized to protect against a reaction with cesium. 
     In yet another example, the die structure  101  comprises three Pyrex dice. For example, the three Pyrex dice are formed from one or more Pyrex wafer substrates. The middle layer  102 , the first outside layer  104 , and the second outside layer  106  comprise the three Pyrex dice. 
     Turning to  FIG. 2  (a cross section  2 - 2  of  FIG. 1 ), the middle layer  102  comprises a cavity  108  that serves as at least a portion of the reservoir for the alkali metal. The first outside layer  104  comprises a channel  110  that leads into the cavity  108  from outside the die structure  101 . In one example, the channel  110  comprises a minimal size that allows cesium to access the cavity  108 . In one example, one or more surfaces of the cavity  108  and the channel  110  comprise a material that does not react to contact with cesium. In another example, the one or more surfaces of the cavity  108  and the channel  110  comprise an outer layer (i.e., a coating) that does not react to contact with cesium. In yet another example, all surfaces of the cavity  108  and the channel  110  that may come in contact with cesium comprise a material or the outer layer that does not react to contact with cesium. 
     In one example, the die structure  101  comprises a cube with sides equal to two millimeters, and the cavity  108  comprises a cube shaped void within the die structure  101  with sides equal to one millimeter. The die structure  101  with sides equal to two millimeters is useful to applications that require the die structure  101  to be small. The cavity  108  with sides equal to one millimeter is advantageous to applications that require maintenance of a temperature of the cesium in the cavity  108  to be within a range that is above the ambient temperature. The small size of the cavity  108  promotes a reduction of the amount of power used to heat the cesium in the cavity  108 . 
     Turning to  FIG. 3 , a wafer structure  130  illustrates an array of die structures analogous to the die structure  101 . The die structure  101  comprises one of plurality of die structures generated on the wafer structure  130  by micro-electromechanical system (“MEMS”) batch fabrication technology. The wafer structure  130  may comprise a single wafer or a plurality of wafers bonded together. The wafer structure  130  serves to illustrate the batch fabrication capability of micro-electromechanical systems technology that creates the wafer structure  130 . In one example, the wafer structure  130  comprises the single wafer. The single wafer corresponds to one layer of the middle layer  102 , the first outside layer  104 , and the second outside layer  106  shown in  FIGS. 1 and 2 . In another example, the wafer structure  130  comprises three wafers bonded together. The three wafers bonded together correspond to the middle layer  102 , the first outside layer  104 , and the second outside layer  106  shown in  FIGS. 1 and 2 . 
     The wafer structure  130  yields one or more die structures analogous to the die structure  101 . How many of the one or more die structures the wafer structure  130  yields is dependent on a size of the die structure  101  and a size of the wafer structure  130 . In one example, the wafer structure  130  yields one hundred die structures analogous to the die structure  101 . In another example, the wafer structure  130  yields one thousand die structures analogous to the die structure  101 . The batch fabrication capability of micro-electromechanical systems technology allows for generation of multiple reservoirs for cesium (i.e., the die structure  101 ) on the wafer structure  130 . Micro-electromechanical systems technology is able to create structures on the wafer structure  130  made of silicon, glass, or other material with feature sizes in the micrometer range. Micro-electromechanical systems technology is able to create the multiple reservoirs for cesium that are substantially smaller than reservoirs for cesium made by previous methods. Micro-electromechanical systems technology allows more controllability than glass blowing to enable creation of the die structure  101  to sustain effects of shock and vibration. 
     Turning to  FIG. 4 , a chamber structure  136  that serves to fill with cesium the die structure of the apparatus  100 . The chamber structure  136  fills with cesium and seals the array of die structures analogous to the die structure  101 . In one example, the chamber structure  136  fills and seals the wafer structure  130  with cesium. The chamber structure  136  comprises an inner chamber  140 , an outer chamber  141 , a platform  142 , a sealing mechanism  143 , a cesium source  144 , a cesium source valve  145 , a gas source  146 , a gas source valve  147 , a pump  148 , and a pump valve  149 . 
     The outer chamber  141  encapsulates the inner chamber  140 . The wafer structure  130  rests on the platform  142  within the inner chamber  140 . In one example, the sealing mechanism  143  comprises a plug installation component. The sealing mechanism  143  works with the platform  142  to seal the cesium in the wafer structure  130 . In one example, cesium source  144  comprises an alkali metal source and the cesium source valve  145  comprises an alkali metal source control component. The cesium source  144  attaches to the inner chamber  140  to form a channel between the inner chamber  140  and the cesium source  144 . The channel between the inner chamber  140  and the cesium source  144  is controlled by the cesium source valve  145 . The cesium source valve  145  controls opening and closing of the channel between the inner chamber  140  and the cesium source  144 . 
     The gas source  146  attaches to the inner chamber  140  to form a channel between the inner chamber  140  and the gas source  146 . The channel between the inner chamber  140  and the gas source  146  is controlled by the gas source valve  147 . In one example, the gas source valve  147  comprises a gas source control component. The gas source valve  147  controls opening and closing of the channel between the inner chamber  140  and the gas source  146 . 
     The pump  148  attaches to the inner chamber  140  to form a channel between the inner chamber  140  and the pump  148 . The channel between the inner chamber  140  and the pump  148  is controlled by the pump valve  149 . In one example, the pump valve  149  comprises a pump control component. The pump valve  149  controls opening and closing of the channel between the inner chamber  140  and the pump  148 . 
     A description of an exemplary operation of the apparatus  100  is now presented, for explanatory purposes. Prior to filling the wafer structure  130  with cesium, the temperature in the inner chamber  140  is elevated and the pump  148  evacuates the inner chamber  140  to remove any impurities from the array of die structures analogous to the die structure  101  in the wafer structure  130 . The inner chamber  140  isothermally maintains a temperature that corresponds to a desired vapor pressure. In one example, the desired vapor pressure comprises the partial pressure of cesium. Thus, the amount of cesium in the die structure  101  may be precisely determined. Control of a temperature of the inner chamber  140  and control of a temperature of the cesium source  144  serves to allow control of an equilibrium partial pressure of the inner chamber  140  and control of the amount of cesium in the die structure  101 . The cesium source  144  maintains a temperature greater than the temperature of the inner chamber  140  by around one degree Celsius during filling and sealing of the wafer structure  130 . The temperature gradient between the inner chamber  140  and the cesium source  144  facilitates a transport of cesium from the cesium source  144  to the inner chamber  140  when the cesium source valve  145  is open. 
     The gas source  146  comprises gas that is inert with respect to cesium. The gas enters the inner chamber  140  when the gas source valve  147  is open. The gas enters the cesium source  144  when the gas source valve  147  and the cesium source valve  145  are open. The gas entering the cesium source  144  facilitates a transport of cesium from the cesium source  144  to the inner chamber  140  when the cesium source valve  145  is open. 
     The outer chamber  141  maintains a temperature greater than the temperature of the inner chamber  140  by around ten degrees Celsius during filling and sealing of the wafer structure  130 . The temperature gradient exists between the inner chamber  140  and the outer chamber  141  so that cesium will not deposit on surfaces of the chamber structure  136  that are adjacent to the outer chamber  148 . 
     At a first time, the inner chamber  140  comprises a vapor mixture of cesium and inert gas. The inner chamber  140  comprises an equilibrium vapor pressure. The cesium of the vapor mixture fills the wafer structure  130 . At a second time, the sealing mechanism  143  traverses the array of die structures analogous to the die structure  101  sealing each die structure of the array of die structures analogous to the die structure  101  to generate an array of die structures analogous to the die structure  101  containing cesium. A computer automates the platform  142  and the sealing mechanism  143  so that the sealing mechanism  143  has knowledge of the position of each die structure in the array of die structures analogous to the die structure  101 . 
     At a third time, the cesium source valve  145  and the gas source valve  147  are closed, the pump valve  149  is opened, and the temperature in the inner chamber  140  is elevated. The pump  148  removes any excess cesium from the inner chamber  140 . A cutter component separates the array of die structures analogous to the die structure  101  containing cesium which generates a plurality of individual cesium-filled die structures analogous to the die structure  101 . Thus, the batch fabrication of the plurality of individual cesium-filled die structures  150  analogous to the die structure  101  on the wafer structure  130  comprises an automated process. An atomic clock comprises one exemplary employer of the individual cesium-filled die structure  150 . 
     Turning to  FIG. 5 , a cross-section view of the individual cesium-filled die structure  150  illustrates one embodiment of a method of sealing a reservoir  152  containing cesium of the individual cesium-filled die structure  150 . The method of sealing the reservoir  152  employs a ring  154  and a plug  156 . In one example, the ring  154  and the plug  156  comprise a metal ring and a metal plug. For example, the ring  154  and the plug  156  comprise a metal that does not react with cesium (i.e., copper). An anodic bond attaches the ring  154  to a surface of the first outside layer  104  in a closed loop around the channel  110 . A compression bond attaches the plug  156  to the ring  154  thus sealing an opening of the reservoir  152  containing cesium. The ring  154  and the plug  156  may comprise a platinum coating to prevent oxidation. The platinum coating maintains the sealed integrity of the reservoir  152  containing cesium. 
     Another embodiment of the method of sealing the reservoir  152  containing cesium of the individual cesium-filled die structure  150  is to compression bond a Pyrex or tungsten cover to an opening of the channel  110 . The sealing mechanism  143  may apply the Pyrex or tungsten cover to the opening of the channel  110 . Tungsten is inert with respect to cesium and also bonds well with borosilicate glass (i.e., Pyrex). Yet another embodiment of the method of sealing the reservoir  152  containing cesium of the individual cesium-filled die structure  150  is to anodically bond a metal disk to the opening of the channel  110 . 
     Turning to  FIGS. 6-7 , the individual cesium-filled die structure  150  and a photocell  166  are shown fixedly mounted in a first orientation to a first beam structure  168  in  FIG. 6 . The individual cesium-filled die structure  150  and the photocell  166  are shown fixedly mounted in a second orientation to a second beam structure  170  in  FIG. 7 . The first and second beam structures  168  and  170  comprise thermal isolators for the individual cesium-filled die structure  150 . The first and second beam structures  168  and  170  comprise long beams with small cross-sectional areas. The small cross-sectional areas serve to reduce a conductive loss of heat from the reservoir  152  containing cesium. The first and second beam structures  168  and  170  also comprise a high aspect ratio. The high aspect ratio serves to increase a rigidity of the first and second beam structures  168  and  170 . In one example, the first and second beam structures  168  and  170  comprise dimensions of one hundred micrometers by five hundred micrometers by seven millimeters. In one example, the first and second beam structures  168  and  170  comprise ceramic wafers that are shaped by a laser cutting tool. In another example, the first and second beam structures  168  and  170  comprise glass wafers. One of the first and second beam structures  168  and  170  may replace one of the first outside layer  104  and the second outside layer  106  in the individual cesium-filled die structure  150 . In one example, the second beam structure  170  replaces the second outside layer  106  in the individual cesium-filled die structure  150 . The middle layer  102  and the first outside layer  104  bond to the second beam structure  170  to form the individual cesium-filled die structure  150 . 
     Referring to  FIG. 6 , the second outside layer  106  and the photocell  166  comprise one or more metal bonding pads  174 . The one or more metal bonding pads  174  facilitate an connection between the second outside layer  106  and the photocell  166 . The one or more metal bonding pads  174  may comprise gold for compression bonding at a temperature of approximately two hundred degrees Celsius. The second outside layer  106  comprises a recess  178 . The recess  178  provides a location to accommodate a vertical cavity surface emitting laser  180  (“VCSEL”). The vertical cavity surface emitting laser  180  may comprise an attached heater. In one example, the vertical cavity surface emitting laser  180  and the recess  178  extend two hundred micrometers into the second outside layer  106 . One advantage of a silicon version of the second outside layer  106  is that silicon provides an attenuation for the vertical cavity surface emitting laser  180 . 
     The first outside layer  104  comprises a mirror  182  on a boundary between the first outside layer  104  and the reservoir  152  containing cesium. The mirror  182  comprises a dielectric material that is inert with respect to cesium. The first outside layer  104  comprises a heater  184  on an outer surface opposite the mirror  182 . 
     Conducting wires  185  connect the photocell  166 , the vertical cavity surface emitting laser  180 , and the heater  184  to electrical contacts  186  on the first beam structure  168 . A wire bonder connects the conducting wires  185  to the electrical contacts  186 . For the configuration shown in  FIG. 6 , the wire bonder bonds wires on surfaces which lie in perpendicular planes to the beam structure  168 . For the configuration shown in  FIG. 7 , the wire bonder bonds wires on surfaces which lie in parallel planes to the beam structure  170 . The beam structures  168  and  170  comprise conducting traces  188 . The conducting traces  188  may function both as electrical connections and mounting pads. 
     Turning to  FIGS. 8 and 9 , a die structure package  190  comprises a housing for the individual cesium-filled die structure  150 . The die structure package  190  comprises inorganic materials. Inorganic materials are free from outgassing. Inorganic materials do not release gas due to a pressure decrease or temperature increase. The die structure package  190  comprises a base  192  and a cover  194 . In one example, the die structure package  190  comprises a ceramic die structure package.  FIG. 8  illustrates a top view of the base  192 .  FIG. 9  illustrates a cross-section view of the die structure package  190 . In one example, the individual cesium-filled die structure  150  and the beam structure  168  are fixedly mounted to the base  192 . In another example, individual cesium-filled die structure  150  and the beam structure  170  are fixedly mounted to the base  192 . The die structure package  190  comprises a first layer and a second layer. The first layer comprises cesium-filled die structure  150 , the beam structure  168 , and an electrical conductor. The second layer of the die structure package  190  comprises supplemental electronics  196  that provide supplementary functionality to the cesium-filled die structure  150 , the beam structure  168 , and the electrical conductor. The cover  194  comprises a recess to accommodate a getter  198  mounted to the cover  194 . 
     Referring to FIGS.  6  and  8 - 9 , a vacuum evacuates a space  199  within the die structure package  190  between the base  192  and the cover  194 . The base  192  and the cover  194  are tightly bonded together defining a boundary of the vacuum which surrounds the individual cesium-filled die structure  150 . Materials of the die structure package  190  are inorganic to insure vacuum integrity. The getter  198  absorbs matter that may be present in the space  199  after the base  192  and cover  194  are tightly bonded together. The beam structure  168  suspends and thermally isolates the individual cesium-filled die structure  150  within the space  199 . The beam structure  168  electrically connects the individual cesium-filled die structure  150  to the electronics  196 . In one example, the first beam structure  168  comprises an outer layer of a low emissivity metal (i.e., titanium, aluminum, or gold) to minimize a loss of thermal energy due to radiation. Lithography removes a portion of the metal layer to define electrically isolated portions, to create the electrical contacts  186 , and to create the conducting traces  188 . The electrical contacts  186  and conducting traces  188  are capable of carrying current, voltage, and power signals. Additionally, the conducting traces  188  may function as mounting pads for bonding the beam structure  168  to the base  192 . Thus, the die structure package  190  in conjunction with the beam structure  168  thermally isolates, electrically connects, and suspends the individual cesium-filled die structure  150 . 
     The individual cesium-filled die structure  150  is thermally isolated by the vacuum enclosed by the die structure package  190 , the beams of the beam structure  168  comprise a metal coating, and the individual cesium-filled die structure  150  is small. Therefore, the heater  184  requires small amounts of power to maintain the individual cesium-filled die structure  150  within a temperature range of fifty to eighty degrees Celsius in an environment where the ambient temperature is cooler than fifty degrees Celsius. 
     The individual cesium-filled die structure  150  comprises one or more components that serve to add functionality of a die structure application to the individual cesium-filled die structure  150 . The one or more components are coupled with the die structure. One example of the die structure application comprises the atomic clock. The atomic clock comprises one exemplary application that utilizes the individual cesium-filled die structure  150 . The individual cesium-filled die structure  150  mounts to the beam structure  168  and the die structure package  190  covers the individual cesium-filled die structure  150 . The atomic clock comprises a small cesium-based atomic clock. A geometry of the individual cesium-filled die structure  150  and the beam structure  168  may be tailored to the atomic clock to endure shock and vibration effects. The atomic clock benefits from an ability to create devices and structures on the individual cesium-filled die structure  150 . The features of the atomic clock are easily integrated into the individual cesium-filled die structure  150 . The atomic clock benefits from micro-electromechanical systems technology to produce a plurality of atomic clocks though batch fabrication. 
     The steps or operations described herein are just exemplary. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified. 
     Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.