Abstract:
An apparatus includes a chip-scale atomic clock (CSAC) alkali vapor cell seated on a silicon substrate that is suspended in a package by a metalized Parylene strap having Parylene anchors embedded in a silicon frame, the Parylene strap comprising an extended rigidizing structure, and a plurality of electrical pins extending into an interior of the package, the plurality of electrical pins in electrical communication with the CSAC cell through the metalized Parylene strap, where the CSAC cell is mechanically connected to the package and thermally insulated from the package.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with Government support under Contract No. N6601-02-C-8025 awarded by the U.S. Department of the Navy, Space and Naval Warfare Systems Command (SPAWAR) to Teledyne Scientific &amp; Imaging, LLC (then known as Rockwell Scientific Company, LLC). The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to microstructures, and more particularly to devices for providing structural support and electrical signals to an inner micro-support structure. 
         [0004]    2. Description of the Related Art 
         [0005]    Thermal isolation of micro-scale electrical and optoelectronic components can be important for components that are required to be at a temperature that is de-coupled from their external environment. 
         [0006]    Chip-scale atomic devices such as chip-scale atomic clocks (“CSAC”), for example, may require thermal isolation of particular components from their environment and from the package enclosure in which they sit to reduce thermal losses and hence heating power required to thermally bias the components. Unfortunately, thermal isolation is not the only packaging design consideration. Power and signaling must also be provided to the CSAC components (typically including portions of the “physics package” such as the vapor cell and/or vertical-cavity surface-emitting laser (VCSEL) optical source) to achieve the necessary thermal bias and temperature control, or in the case of the VCSEL to generate the required optical output for generation and interrogation of the atomic states in the vapor cell. These power and signaling requirements necessitate an electrical and physical connection between the physics package components, the enclosure in which they sit and external devices, thus complicating thermal isolation efforts for the physics package. Kapton flex cables may be used for such connections, but their use results in disadvantageous thermal coupling between the physics package components and the enclosure in which they sit. More generally, thermal isolation between adjacent substrates used in other types of systems and other types of physics packages is a problem that is complicated by conflicting requirements of power and signaling communication between them. 
         [0007]    A need continues to exist to provide power and signaling to micro-scale components while minimizing thermal communication with their environment. 
       SUMMARY OF THE INVENTION 
       [0008]    A structure is disclosed that has a microscale rigidized Parylene strap conformally coupled to both a first silicon substrate and to a second silicon substrate such that the first silicon substrate is suspended from the second silicon substrate through the strap. 
         [0009]    A method is disclosed that includes conformally coating Parylene onto a rigidizing structure mold to form a rigidized Parylene layer, etching the rigidized Parylene layer to expose a center portion of a substrate; and etching entirely through an annulus portion of the substrate to free a suspended portion of the rigidized Parylene layer between outer and inner substrate portions so that one of the outer and inner substrate portions are suspended by the other substrate portion by the rigidized Parylene layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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. 
           [0011]      FIG. 1  is a cross-sectional perspective view of one embodiment of a rigidized Parylene strap suspending an inner silicon frame from an outer silicon support; 
           [0012]      FIG. 2  is a cross-sectional view of one embodiment of a CSAC module that uses the rigidized Parylene strap illustrated in  FIG. 1  to suspend a physics package subsystem on an inner silicon frame for thermal isolation and electrical communication; 
           [0013]      FIG. 3  is a perspective view of a TO header style package, inner silicon frame and outer silicon support used in the system of  FIG. 2 ; 
           [0014]      FIG. 4  is a cross-sectional view of another embodiment of a CSAC module that uses the rigidized Parylene strap illustrated in  FIG. 1  to suspend a physics package subsystem on an outer silicon frame for thermal isolation and electrical communication; 
           [0015]      FIG. 5  is a perspective view of the bottom of the rigidized Parylene strap illustrated in  FIG. 1 , showing the honeycomb reinforcement structure. 
           [0016]      FIGS. 6-14  illustrate semiconductor processing steps for an inner annular silicon frame suspended by an outer annular silicon frame by a rigidized Parylene strap; 
           [0017]      FIG. 15  illustrates a plan view of a rectangular inner substrate suspended by an outer rectangular frame by, in one embodiment, a Parylene strap having a box beam configuration; and 
           [0018]      FIG. 16  is a cross section view of the Parylene strap illustrated in  FIG. 15  and along the line  16 - 16 . 
           [0019]      FIGS. 17-24  illustrate processing steps for forming parylene straps which are rigidized with a box-beam structure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    A system for structurally suspending and electrically connecting substrates in a microscale system includes a conformally-coated and rigidized Paraxylyene, referred to herein as a “Parylene,” strap, suspending a frame (which is preferably silicon) from a support (which is preferably silicon). Although the description that follows uses the tradename “Parylene” in place of Paraxylyene, it is understood in this description that references to the term “Parylene” are intended to preferably include at least the Paraxylyene material known in the industry as Parylene-C, and also other Paraxylene formulations which may include Parylene-N, Parylene-D and Parylene-HT. 
         [0021]    In one implementation illustrated in  FIG. 1 , a microscale and rigidized Parylene strap  100  enables one substrate to support and suspend the other substrate for thermal isolation and electrical communication. The rigidized Parylene strap  100  has Parylene  108  conformally coated (or “seated”) on the substrates ( 102 ,  104 ) and is formed with a reinforcement structure extending from one side, such as a honeycomb reinforcement structure  107 . In other embodiments, the rigidized Parylene strap is formed of other reinforcement structures such as in the form of one or more Parylene box-beam structures (see  FIGS. 15 ,  16 ). The rigidized Parylene strap  100  preferably has a plurality of conductive traces  106  (preferably formed of metallic material) deposited on the layer of Parylene  108  to enable electrical communication between the substrates ( 102 ,  104 ), including power signals, while allowing one substrate to be suspended from the other to increase thermal insulation between them. In a preferred embodiment the substrates form a circular inner annulus frame  102  and a circular outer annulus frame  104  with one annulus frame physically suspending the other (See  FIG. 2 ). In other embodiments, the inner and out annulus frames are square annulus frames (e.g.  FIG. 15 ). The substrates are preferably formed of a material such as Silicon (Si) that are etched from a single wafer substrate in which a suspended portion  110  of the Parylene strap is created by etching the wafer to free the strap. Parylene anchor holes are preferably formed by etching in each of the inner annulus frame  102  and outer annulus frame  104  to receive Parylene anchors  112  for increased mechanical adherence to the substrates. In an alternative embodiment the substrate may be formed of Gallium Arsenide (GaAs), borosilicate glass, ceramics or other substrate material. As used in this disclosure, the word “rigidized” is intended to mean a Parylene strap that has a reinforcement structure extending from it on at least one side to change the bending, torsion and vibration characteristics of the otherwise planar Parylene layer, at least across the suspended portion  110 . 
         [0022]      FIG. 2  illustrates one application for the rigidized Parylene strap and substrate assembly illustrated in  FIG. 1  that suspends a portion of the physics package components over a detector in a chip-scale atomic device that is a clock (CSAC) assembly. The physics package components  200  are seated on an inner silicon frame  202  (to define a “substrate frame”) that is suspended from an outer silicon frame  204  preferably through a plurality of rigidized Parylene straps  206 . In an alternative embodiment, the plurality of Parylene straps  206  may consist of one or more rigidized drum straps that extend around a substantial perimeter of the inner and outer silicon frames ( 202 ,  204 ). 
         [0023]    The inner and outer silicon frames ( 202 ,  204 ) are preferably annular, with the Parylene strap  206  metalized with a plurality of conductive traces  106  to provide electrical communication to the physics package components  200 . At least one of the plurality of traces  106  is in communication with an electrical pin  208  of a package base, which may be a package base  210  such as a Transistor Outline Header (“TO Header”) through a lower silicon frame  212  that supports the outer annular silicon frame  204 . Although the electrical signal path between the plurality of traces  108  and electrical pin  208  is illustrated as a combination of surface-level conductive traces  106  and substrate vias  214 , in a preferred embodiment, electrical communication between the lower silicon frame  212  and plurality of conductive traces  106  is by means of trace and trace bonds (not shown). 
         [0024]    A detector  216  is seated on the package base  210  in a position to receive a laser beam provided by the physics package components  200 . A base  218  of the package base  210  is itself supported by the electrical pins  208  extending through glass welds  220  of the package base  210 , with the physics package components  200 , inner and outer annular substrates ( 202 ,  204 ) and detector  216  components sealed from the environment with a cap  222  that is preferably welded onto the package base  210 . In an alternative embodiment, the chip-scale atomic device  200  is not a CSAC, but any chip-scale device that performs interrogation of atomic states in a vapor cell, such as a chip-scale gyroscope or chip-scale magnetometer. 
         [0025]      FIG. 3  illustrates a perspective embodiment of the inner and outer annular silicon frames and TO header, exposed without the cap and physics package. Electrical pins  208  of the package base  210  are aligned with solder bumps  300  to seat the outer annular frame  204 . The inner annular frame  202  is suspended from the outer annular frame  204  by Parylene straps  206  that also provide electrical communication and thermal isolation between inner and outer annular frames ( 202 ,  204 ). Traces  302  are coupled between the detector  213  and respective electrical pins  304  to provide power and electrical communication between the detector  213  and external electronics (not shown). Although the inner and outer annular frames ( 202 ,  204 ) are illustrated as generally annular, an alternative embodiment they may each be square or conformed to another polygonal shape. Similarly, although four Parylene straps  206  are illustrated to effectuate suspension of the inner annulus frame  202  from the outer annulus frame  204 , in an alternative embodiment the Parylene strap is a Parylene drum extending substantially entirely around and between the frames ( 202 ,  204 ) to provide suspension of the inner annular frame  202 . 
         [0026]      FIG. 4  illustrates an alternative embodiment of physics package components  200  supported by an exterior annular frame  400  that is suspended from an inner annular frame  402  by a plurality of rigidized Parylene straps  404 . In this embodiment, a lower silicon frame  406  supports the inner annular frame  402  and is seated on the electrical pins  208  of a package base  210 . The combination of the outer annular frame  400  suspended by the inner annular frame  402  through the rigidized Parylene straps  404  provide thermal isolation and mechanical support for the physics package components  200  in the center of the assembly over the detector that is positioned in complimentary opposition to the physics package components  200  to receive an uninterrogated laser beam. Although communication between the physics package components  200  and the electrical pins  208  is illustrated by means of substrate vias  214 , in a preferred embodiment, such communication between the electrical pin  208  and inner annular frame  402  is provided by conductive traces and trace bonds, similar to communication between the Parylene strap  404  and physics package components  200 . 
         [0027]      FIG. 5  illustrates a side of the rigidized Parylene strap  100  that has the honeycomb reinforcement structure  107 . In a preferred embodiment, the honeycomb reinforcement structure  107  has walls  110  that extend from the Parylene layer  108  to a height of about 60 um, with the walls of being approximately 17 um wide. Although the honeycomb reinforcement structure  107  is illustrated as hexagonal, the honeycomb reinforcement structure may form a pentagon, heptagon, octagon or other geometric cross section. Other dimensions may be chosen to optimize the mechanical rigidity of the structure. 
         [0028]      FIGS. 6 through 14  illustrate the fabrication steps for the inner and outer annular silicon frames ( 402 ,  400 ) and Parylene strap  404  combination first illustrated in  FIG. 4 . A film of silicon dioxide is deposited on a substrate, preferably a silicon substrate  602 , and then the silicon dioxide film is patterned into islands  600  (alternatively referred to as “dielectric pads”). A plurality of blind anchor holes  702  are etched into the substrate to facilitate later anchoring of a Parylene layer to the substrate  602 . An extended rigidizing structure mold  604 , preferably in a honeycomb pattern, is etched into the substrate to receive conformally coated Parylene which will ultimately form a rigidized honeycomb structure (See  FIG. 1 , reference numeral  107 ). In  FIG. 8 , a layer of Parylene  800  is deposited over the oxide pads  600  and into the rigidizing structure mold  604  and anchor holes  702  (forming respective Parylene tabs) to form a conformally seated Parylene layer having a rigidizing structure  802  that is as-yet embedded in the silicon substrate  602 . In  FIG. 9 , the Parylene layer at a substrate center portion  900  is removed and the silicon dioxide pads  600  partially exposed to define the parylene straps ( 902 ,  904 ) In  FIG. 10 , the first and second Parylene straps ( 902 ,  904 ) are coated with patterned metal to form a plurality of traces  1000  and an optional second layer of Parylene (not shown) may be deposited to protect the traces. In  FIG. 11 , substrate  602  is attached face-down to a handle wafer  1102  using photoresist layer  1100  as adhesive. In  FIG. 12 , metal substrate contacts  1200  are deposited on the back side of substrate  602 . A second photo resist layer  1202  is formed on the back side of the substrate  602  to enable etching, in  FIG. 13 , of the substrate  602  and formation of the inner annular silicon frame  1300  and outer annular silicon frame  1302 . In FIG.  14 , the handle wafer  1102  and photo resist  1100  are removed to expose the now-defined rigidized Parylene strap  1400 . 
         [0029]      FIG. 15  illustrates one embodiment of an inner substrate  1500  (preferably a silicon substrate) that is suspended by an outer substrate  1502  (also preferably a silicon substrate) through Parylene straps  1504  that has a box-beam strap portion. The rigidized Parylene straps  1504  have Parylene anchors  1506  at their proximal and distal ends embedded in each of the inner and outer substrates ( 1500 ,  1502 ). At least one of the rigidized Parylene straps  1504  has a metalized trace  1508  deposited on the straps and extending between the inner and outer substrates ( 1500 ,  1502 ) to provide electrical communication between them. The Parylene straps  1504  preferably include a box beam strap portion  1510  formed during the strap&#39;s fabrication process to provide increased resistance to torsion and bending moments. In an alternative embodiment, the box beam strap portion  1510  is instead a honeycomb reinforcement structure (not shown). Also, the metalized trace  1508  may be deposited on the box beam structure  1510  or may consist of a plurality of metalized traces. 
         [0030]      FIG. 16  illustrates a cross section of a Parylene strap about the line  16 - 16  in  FIG. 15 . A box beam type structure  1600  formed of Parylene is established on the Parylene layer  1602 . The dimensions of the box beam structure would be chosen to control the stiffness of the strap in bending and torsion. A metallic trace  1604  is seated on the Parylene layer  1602 . 
         [0031]      FIGS. 17-24  illustrate one embodiment of fabrication steps for the rigidized Parylene straps illustrated in  FIG. 16 .  FIG. 17  illustrates the silicon substrate  1700  before processing.  FIG. 18  shows formation of anchor recesses  1800  in the silicon wafer  1700 .  FIG. 19  illustrates a base Parylene layer  1902  deposited on the surface of the wafer  1700  and into and substantially filling the anchor recesses  1800  to form Parylene tabs  1903 . Metallic traces  1904  are patterned on the base Parylene layer  1902 . In  FIG. 21 , a rigidizing structure mold, preferably in the form of a thick resist layer  2000 , is coated on a portion of the base Parylene layer  1902 , with the resist having dimensions that will form the cavity of the box beam type rigidizing structure. A second layer of Parylene  2100  is confomally deposited on the thick resist  2000 . In  FIG. 22 , release holes  2200  are etched to enable removal of the thick resist layer  2000 . In  FIG. 23 , the substrate is immersed in solvent which dissolves the thick photoresist structure  2000  through the release holes  2200  forming the box beam cavity  2300 . In  FIG. 24 , the silicon wafer  1700  is etched to create inner and outer substrates and to suspend a portion of the now-rigidized Parylene strap  2400 . Other sacrificial materials besides photoresist may also be used. 
         [0032]    While various implementations of the application 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.