Abstract:
A crystal oscillator is mounted in a flexible harness rather than at discrete points. The crystal oscillator and associated control circuitry may be formed on a common substrate, decreasing component size and minimizing temperature fluctuations by shortening the thermal path between the crystal and the control circuitry.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to sensor packaging, and in particular to packaging for crystal-based devices. 
       BACKGROUND 
       [0002]    As the use of navigation systems increases in both the public and military sectors, there is an incentive to improve the robustness and decrease the size of their individual components. One such component is the crystal oscillator, which supplies a stable clock frequency derived from the mechanical resonance of a piezoelectric crystal. Crystal oscillators can also be found in products such as test equipment, watches, and electronic circuits. Variants of the crystal oscillator engineered to reduce the impact of environmental factors such as temperature and humidity include the temperature-controlled (or -compensated) crystal oscillator (TCXO), the microcomputer-compensated crystal oscillator (MCXO) and the oven-controlled crystal oscillator (OCXO). 
         [0003]    A TCXO typically includes a control chip electrically connected to the piezoelectric crystal oscillator. Traditionally, the control chip and the crystal are packaged in separate carriers which are then bonded together. The crystal is attached to its carrier with epoxy, and electrical connections are made between the two carriers. For example, the two carriers may be positioned one atop the other and soldered together. In some constructions, one end of the crystal is mounted inside its carrier using two small bumps of conductive epoxy. The two bumps provide both the support and electrical contacts for the crystal. 
         [0004]    This scheme exposes the crystal to local stresses at the attachment point that can deleteriously affect its performance and reliability. For example, considerable stress may occur when the package is subjected to an inertial load or a harsh environment. If the elastic limits of the structure (or portions thereof) are exceeded, a permanent change in the TCXO frequency can occur. 
         [0005]    Furthermore, the use of multiple carriers for a single oscillator package constrains its minimum size and can affect the performance of the oscillator as a function of temperature. For example, in the stacked packaging scheme discussed above, the thermal path between the crystal and its control chip is large, since it traverses both carriers. As a result, potentially harmful temperature fluctuations are more likely. 
         [0006]    Finally, discrete packaging of the crystal and control components results in a large overall volume that can limit deployment. 
       SUMMARY 
       [0007]    The foregoing limitations of conventional packaging schemes are herein addressed by mounting the crystal oscillator in a flexible harness rather than at discrete points. Moreover, the crystal oscillator and control circuitry may be formed on a common substrate, decreasing component size and minimizing temperature fluctuations by shortening the thermal path between the crystal and the control circuitry. 
         [0008]    In accordance with the invention, a crystal oscillator is mounted within a harness made of a flexible dielectric material. Advantages of this approach include isolation of shock within the harness rather than allowing it to be transmitted to the crystal; protection against stress; improved thermal isolation of the crystal; and reduction in the signal and thermal path length between the crystal and its control chip. 
         [0009]    The crystal may be attached to the harness using, for example, indium solder or conductive epoxy, which can also establish electrical connections. In some embodiments, the dielectric film is the KAPTON polyimide film supplied by E.I. du Pont de Nemours Co., Wilmington, Del. In other embodiments, the dielectric film may include, or consist essentially of, at least one of Teflon, liquid crystal polymer, polyester, or polyvinyl chloride. The harness may be sputtered with metal and photo-patterned to create electrical connections. 
         [0010]    In an aspect, the invention features a structure including a piezoelectric crystal disposed over at least a first portion of a flexible membrane. The crystal may be in contact with a least two spaced-apart regions of the flexible membrane, and a second portion of the flexible membrane may be disposed over and harness the crystal against the spaced-apart regions. In an embodiment, a conductive adhesive adheres the crystal to at least the first portion of the membrane. The conductive adhesive may be indium solder and/or conductive epoxy. 
         [0011]    Embodiments of the invention may include the following features. At least one metal film may be disposed over and in contact with the flexible membrane, and may be in contact with the crystal. The metal film may include at least one of aluminum or gold. The flexible membrane may be attached to a first surface of a substrate, and may be attached by at least one of solder bumps or gold rivets. The membrane may be disposed in a cavity in the substrate, and a thin foil may be disposed over the cavity and seal it. Control circuitry may be disposed on a second surface of the substrate, and at least one conductive via may connect the control circuitry to the crystal. The first surface of the substrate may be substantially parallel to and opposite the second surface, and at least a portion of the conductive via may be disposed within the substrate. The control circuitry may be configured to provide a temperature-dependent correction voltage. The flexible membrane may include a dielectric material such as a polyimide, Teflon, liquid crystal polymer, polyester, or polyvinyl chloride. 
         [0012]    In another aspect, the invention features a method of making a crystal oscillator including providing a substrate with a first surface, disposing a piezoelectric crystal over at least a portion of a flexible membrane, disposing the membrane and crystal over the first substrate surface, and disposing control circuitry over a second surface of the substrate. An electrical connection may be formed between the crystal and the control circuitry. The first surface of the substrate may be substantially parallel to and opposite the second surface, and forming the electrical connection may including forming at least one conductive via through the substrate. In an embodiment, the method includes forming a cavity in the substrate, and the first surface may be at least partially within the cavity. The crystal may be placed in contact with at least two spaced-apart regions of the flexible membrane. A second portion of the flexible membrane maybe be disposed over the crystal so as to harness the crystal against the spaced-apart regions of the flexible membrane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0014]      FIG. 1  is a plan view of a flexible membrane for supporting a crystal in accordance with the invention; 
           [0015]      FIG. 2  shows the flexible membrane illustrated in  FIG. 1  with a crystal mounted thereon; 
           [0016]      FIG. 3  is a cross-sectional view of a substrate, mounted in a flexible membrane as shown in  FIG. 2 , attached to a substrate; and 
           [0017]      FIG. 4  is a cross-sectional view of a crystal oscillator-based device incorporating the elements shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 1 , a flexible membrane may be prepared for utilization as a harness for a piezoelectric crystal. Membrane  100  includes, or consists essentially of, a flexible material, which may be a dielectric material (i.e., an electrically insulating material). For example, membrane  100  may be a polyimide film such as the KAPTON polyimide film supplied by E.I. du Pont de Nemours Co., Wilmington, Del. In other embodiments, membrane  100  includes, or consists essentially of, at least one of Teflon, liquid crystal polymer, polyester, or polyvinyl chloride. Membrane  100  may be approximately rectangular, and may have areal dimensions of approximately 6 millimeters (mm) by approximately 3 mm In other embodiments, membrane  100  may take any shape suitable to the application, e.g., a square, quadrilateral, other polygon, or even a circle. The thickness of membrane  100  may be approximately 12 to 25 micrometers. Generally, the shape, areal size, and thickness of membrane  100  will be large enough to accommodate a piezoelectric crystal (as described below). Preferably, membrane  100  is sized to be as small as possible while still accommodating a given piezoelectric crystal. 
         [0019]    Membrane  100  may include one or more slits representatively illustrated at  110   1 ,  110   2  as well as two or more contacts  120   1 ,  120   2 . Slits  110  may be cuts made through substantially the entire thickness of membrane  100 , and may roughly divide membrane  100  into two or more portions. A first portion of membrane  100 , representatively illustrated at  125   1 ,  125   2 , may be defined as the peripheral region(s) between slits  110  and the outer boundaries of membrane  100 . A second portion  130  of membrane  100  may be defined as the region between the two outermost slits  110 . Contacts  120  may include, or consist essentially of an electrically conductive material, preferably a metal such as aluminum or gold. Contacts  120  may cross at least one of slits  110  in order to facilitate electrical contact with a subsequently mounted piezoelectric crystal. 
         [0020]    Referring to  FIG. 2 , crystal  200  may be configured so that it will be in contact with first portion  125  at a plurality of points, or even in contact with membrane  100  across the entire area of overlap of crystal  200  and first portion  125 . In an embodiment, crystal  200  is wider than membrane region  130 , such that when the crystal is slipped through slits  110 , portions of the crystal  200  extend to each side of membrane region  130  to overlap the membrane regions  125   1 ,  125   2 . As a result, membrane region  130  secures the crystal  200  against regions  125  in the manner of a harness. Crystal  200  may include, or consist essentially of, a piezoelectric material such as man-made or natural quartz. The size and dimensions of membrane  100  are selected such that membrane  100  has a larger cross-sectional area than crystal  200 . For example, crystal  200  may be approximately rectangle-shaped, and may have areal dimensions of approximately 3 mm by approximately 1.5 mm. The thickness of crystal  200  may be approximately 0.08 mm. 
         [0021]    An adhesive material may be used in order to maintain contact between crystal  200  and membrane  100  (i.e., to prevent crystal  200  from slipping out of the harness provided by membrane region  130  during operation). The adhesive material may be placed at one or more of the points of contact between crystal  200  and membrane  100 . When placed on or within membrane  100 , crystal  200  may overlap at least one of the contacts  120 . In order to facilitate electrical contact between crystal  200  and contacts  120 , the adhesive material may be conductive, e.g., may be indium (In) solder or conductive epoxy. Electrical contact between crystal  200  and membrane  100  may exert less deleterious stress upon crystal  200  than connections between a crystal and a rigid package due to the flexibility of membrane  100 . 
         [0022]      FIG. 3  shows how membrane  100  (with crystal  200  therein) may be attached to a substrate  300 . The material and structure of substrate  300  is dictated by the desired application. For example, substrate  300  may have a thickness of approximately 0.5 mm, and may include, or consist essentially of, a rigid material such as alumina, silicon, quartz, or liquid crystal polymer. Membrane  100  may be attached to a surface  305  of substrate  300  at the ends of membrane  100 , and the points of connection may substantially overlap with the locations of contacts  120 . Membrane  100  may be attached to substrate  300  using, e.g., gold rivets or solder including or consisting essentially of In or an indium-tin alloy, and may be substantially parallel to surface  305 . In an embodiment, surface  305  follows a stepped cavity  310  milled into substrate  300 , and membrane  100  is attached to opposing steps  315   1 ,  315   2  of cavity  310  so as to remain suspended above the recess  320 . Cavity  310  may have a total depth d of, for example, approximately 0.125 mm. Cavity  310  may be at least substantially sealed by a foil cover  320 , which is bonded to substrate  300  by, e.g., an adhesive material or solder. Foil  320  may be relatively thin, i.e., it may have a thickness of approximately 0.05 mm. Placing membrane  100  within sealed cavity  310  may insulate membrane  100  and crystal  200  from the ambient environment during subsequent operation. 
         [0023]    With continued reference to  FIG. 3 , and also to  FIG. 4 , a block of control circuitry  405  may be formed on a second surface  325  of substrate  300 . Second surface  325  may be substantially parallel to and opposite first surface  305 . Control circuitry  405  may include a plurality of passive and active electronic devices, such as transistors, varactors, and/or thermistors, and may be fabricated in whole or in part directly on substrate  300  or bonded to a face thereof as a complete, pre-fabricated chip. Control circuitry  405  may include, for example, a conventional compensation module that provides a temperature-dependent correction voltage. Control circuitry  405  may be enclosed within the thickness of a dielectric film  410  disposed over second surface  325 , thus protecting control circuitry  405  from environmental factors during operation. Electrical contact between membrane  100  and control circuitry  405  is established by the formation of at least one conductive via  420  (there may be a plurality of vias  420 , two of which are representatively illustrated). Vias  420  are formed by etching through at least substrate  300  (and, if appropriate, through dielectric film  410  as well) and refilling the resulting void with a metal such as gold or copper. Vias  420  may directly connect membrane  100  to control circuitry  405 , or may instead connect, via a metal film  430 , to one or more vias  440  formed from control circuitry  405  through dielectric film  410 . The resulting device  400 , formed by the interconnection of membrane  100  (with crystal  200 ) and control circuitry  405 , may be subsequently electrically connected to other devices or circuits by one or more conductive input/output connections  450 . 
         [0024]    Device  400 , with crystal  200  harnessed within membrane  100  and electrically connected to control circuitry  405 , may be utilized in a wide range of applications. Device  400  may, for example, be configured to generate a high-frequency (e.g., approximately 20 GHz) clock signal, and may be utilized as a TCXO, MCXO, or OCXO in accordance with designs well known in the art. Device  400  may be utilized in test equipment or global positioning system (GPS) applications or other navigational systems. 
         [0025]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein.