Patent Publication Number: US-9850123-B2

Title: Stress relief MEMS structure and package

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. application Ser. No. 14/616,017, filed Feb. 6, 2015, entitled “Stress Relief MEMS Structure and Package,” which is incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to MEMS sensors, and, more particularly, to structures for stress relief in MEMS sensors. 
     BACKGROUND 
     Micro-Electrical-Mechanical-Systems (MEMS) such as sensors can be widely used in applications such as automotive, household appliance, building ventilation, and in general industrial applications to sense a physical condition such as pressure, temperature, or acceleration, and to provide an electrical signal representative of the sensed physical condition. 
     Most sensors are sensitive to stress or strain of their physical structure in addition to being sensitive to the measure of interest. Such stresses can be internally generated by virtue of the sensor structure itself, or can be externally generated by the sensor&#39;s enclosure, or packaging. Package stresses, such as thermal stress, can affect the output of a MEMS sensor. One example of package-sensor interaction stresses are stresses caused by the mismatch in the thermal expansion coefficient of components of the package and components of the sensor. Different methods for providing thermal stress relief from packages have been previously disclosed. Some of the conventional methods use flexible support beams to suspend the sensor element. All other conventional methods apply package bonding modification, e.g. by using flexible support diaphragm, flexible support chuck, or putting stress buffer material between the MEMS device and the package. 
     The conventional methods are not suitable for some MEMS pressure sensor packages. For example, the use of flexible support beams requires extra stress relief support structure, which is not hermetic and cannot withstand high applied pressure. Other conventional methods require extra parts or result in a high cost due to complex manufacturing requirements. 
     There is a need for stress relief structures and methods that can be applied to MEMS sensors requiring a hermetic seal and that can be simply manufactured. 
     BRIEF SUMMARY 
     Stress relief structures and methods that can be applied to MEMS sensors, including MEMS sensors that require a hermetic seal, and that can be simply manufactured are disclosed herein below. 
     In one or more embodiments, the system of these teachings includes a sensor having a first surface and a second surface, the second surface being disposed away from the first surface, the second surface also being disposed away from a package surface and located between the first surface and the package surface. The system also includes one or more support members, each support member extending from the second surface to the package surface. The support members are disposed on and operatively connected to only a portion of the second surface. The one or more support members are configured to reduce stress produced by package-sensor interaction. 
     In further embodiments of the system of these teachings, a first support member from the one or more support members surrounds a central region of the second surface, and a second support member from the one or more support members surrounds the first support member. Each support member from the one or more support members comprises a closed structure and/or a hollow cylindrical structure, wherein the closed curves in the cylindrical structure are not circles. Each one of the closed curves comprises a plurality of sections, each section from the plurality of sections being joined to another section from the plurality of sections at one or more points, a slope of each section not being equal to a slope of another section at at least one of the one or more points. 
     In yet further embodiments of the system of these teachings, each support member from the one or more support members is comprised of the same material as a section of the sensor proximate to the second surface and/or the same material as the package surface. The one or more support members comprise one or more first support members extending away from the second surface and one or more second support members extending from the package surface to the second surface, the one or more second support members configured to receive the one or more first support members. Each support member from the one or more first support members extends a distance away from the second surface, where the distance is smaller than a distance between the second surface and the package surface. In one instance, each support member from the one or more support members has a cross-sectional area that varies with distance away from the second surface. In one embodiment, the cross-sectional area decreases with distance away from the second surface. In another embodiment, the cross-sectional area increases with distance away from the second surface. In yet other embodiments, each support member from the one or more support members comprises two subcomponents: a first subcomponent extending from the second surface to a predetermined location between the second surface and the package surface and a second subcomponent extending from the predetermined location to the package surface. In one instance, a cross-sectional area of the first subcomponent is larger than a cross-sectional area of the second subcomponent. In another instance, a cross-sectional area of the first subcomponent is smaller than a cross-sectional area of the second subcomponent. 
     In still further embodiments of the system of these teachings, the system further comprises a soft material disposed between the second surface, the package surface and the one or more support members. The system further comprises a soft material disposed between the second surface, the package surface and the one or more second support members. The system further comprises a soft material disposed between the second surface, the package surface and the one or more first and one or more second support members. The one or more support members comprise at least two support members. A width of one support member is smaller than a width of other support members. A sealing material is applied between the package surface and at least one support member, the sealing material being configured to provide a hermetic seal. At least one support member is bonded to the package surface. 
     In one or more embodiments, the method of these teachings for fabricating a sensor system having reduced stress due to package-sensor interaction includes etching into a surface of a component of a sensor structure, the sensor structure having a first surface, in order to form a second surface of the sensor structure and one or more support structures disposed on the second surface. The second surface is disposed away from the first surface. The second surface is also disposed away from a package surface and located between the first surface and the package surface. The one or more support structures are configured to reduce stress produced by package-sensor interaction. 
     In yet further embodiments of the method of these teachings, the one or more support structures are configured such that, in the sensor system, each support structure from the one or more support structures extends from the second surface to the package surface. The component is a silicon component. A first support member from the one or more support structures surrounds a central region of the second surface and each subsequent support structure surrounds prior support structures. Each support structure from the one or more support structures comprises a closed structure and/or a hollow cylindrical structure, wherein the closed curves in the cylindrical structure are not circles. Each one of the closed curves comprises a plurality of sections, each section from the plurality of sections being joined to another section from the plurality of sections at one or more points, a slope of each section not being equal to a slope of another section at at least one of the one or more points. Each support member from the one or more support structures extends a distance away from the second surface, the distance being smaller than a distance between the second surface and the package surface. 
     In yet further embodiments of the method of these teachings, the method further comprises trimming each support structure such that each support structure from the one or more support structures has a cross-sectional area that varies with distance away from the second surface, the cross-sectional area increasing with distance away from the second surface. In still another embodiment, the method further comprises trimming each support structure such that each support structure from the one or more support structures has a cross-sectional area that varies with distance away from the second surface; the cross-sectional area decreasing with distance away from the second surface. In yet another embodiment, the method further comprises trimming each support structure such that each support structure from the one or more support structures comprises two subcomponents, a first subcomponent extending from the second surface to a predetermined location between the second surface and the package surface and a second subcomponent extending from the predetermined location to the package surface, a cross-sectional area of the first subcomponent being smaller than a cross-sectional area of the second subcomponent. 
     A number of other embodiments of the system and method of these teachings are also disclosed. 
     For a better understanding of the present teachings, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a conventional mounting of a sensor on a package component; 
         FIG. 2  is a cross-sectional view of one embodiment of the system of these teachings; 
         FIGS. 3( a )-3( e )  shows different embodiments of patterns for the stress relief structures of these teachings; 
         FIG. 4  is a cross-sectional view of another embodiment of the stress relief structure of these teachings; 
         FIG. 5  shows a cross-sectional view of yet another embodiment of the stress relief structure of these teachings; 
         FIG. 6  is a cross-sectional view of a further embodiment of the stress relief structure of these teachings; 
         FIGS. 7 a , 7 b    represent cross-sectional views of still another embodiment of the stress relief structure of these teachings; 
         FIG. 8  shows an exemplary embodiment of a sensor structure used in fabricating the system of these teachings; and 
         FIG. 9  shows simulation results for some embodiments of these teachings and for a conventional embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Stress relief structures and methods are disclosed herein below. The stress relief structures and methods of these teachings can be applied to MEMS sensors and result in systems that can be simply manufactured. 
     The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of these teachings, since the scope of these teachings is best defined by the appended claims. Although the teachings have been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims. 
     As used herein, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. 
     “Package-sensor interaction,” as used herein, refers to stresses generated by interaction between the sensor and the packaging. One example, these teachings not being limited to only that example, of package-sensor interaction stresses are stresses caused by the mismatch in the thermal expansion coefficient of components of the package and components of the sensor. 
       FIG. 1  shows a cross-sectional view of a conventional mounting of a sensor on a package component. Referring to  FIG. 1 , a pressure sensor  15  is disposed on the package component  20 . The pressure sensor  15  has a first surface  30  and a second surface  35 . The second surface  35  is mounted directly on a surface  25  of the package component  20 . 
     In the case of a MEMS, ceramic, or other non-metallic sensor, the sensor is generally bonded to a metallic package with solder, brazing, welding, or epoxy type adhesives. Typically, the entire mating surface of the sensor is bonded in order to get the strongest, most hermetic seal, or simply for manufacturing simplicity. 
     In one or more embodiments, the system of these teachings includes a sensor having a first surface and a second surface, the second surface being disposed away from the first surface, the second surface also being disposed away from a package surface and located between the first surface and the package surface, a number of support members, each support member extending from the second surface to the package surface, the support members being disposed on and operatively connected to only a portion of the second surface. The support members are configured to reduce stress produced by package-sensor interaction such as, but not limited to, package-sensor thermal interaction. 
       FIG. 2  shows a cross-sectional view of one embodiment of the system of these teachings. Referring to  FIG. 2 , in the embodiment shown therein, a sensor  15  has a first surface  30  and a second surface  35 . The second surface  35  is disposed away from this surface  25  of the package component  20 . A number of support members  40  extend from the second surface  35  of the sensor  15  to the surface  25  of the package component  20 . The support members  40  are disposed and operatively connected to only a portion of the second surface  35 . As a result of placing the support members between the package component  20  and the sensor  15 , the area of contact between the sensor  15  and the package component  20  is reduced and the connection between the sensor  15  and the package component  20  is more flexible. 
     In the embodiment shown in  FIG. 2 , the material of the support members  40  is the same material as the material of a section of the sensor  15  proximate to the second surface  35 . Proximate, as used herein, refers to next or nearest in space, which is the dictionary definition. It should be noted that other embodiments, including embodiments using other materials, are within the scope of these teachings. 
       FIGS. 3( a )-3( e )  shows different embodiments of patterns for the support members used in these teachings. In the embodiments shown in  FIGS. 3( a )-3( d ) , a first support member  45  surrounds a central region  50  of the second surface ( 35 ,  FIG. 2 ) and each subsequent support member  55  surrounds prior support members. In the embodiments shown in  FIGS. 3( a )-3( d ) , each support member is a closed structure. In the embodiments shown in  FIGS. 3( a )-3( d ) , each support member includes a hollow cylindrical structure  45 ,  55 ,  70 ,  75 ,  60 ,  65 . A cylinder, also referred to as a hollow cylindrical structure, as used here in, is a solid consisting of two parallel planes bounded by identical closed curves that are interconnected at every point by a set of parallel lines, usually perpendicular to the planes. In the embodiments shown in  FIGS. 3( b )-3( c ) , closed curves in the hollow cylindrical structure are not circles. In the embodiments shown in  FIGS. 3( b )-3( c ) , each one of the closed curves has a number of sections  75 , each section  75  being joined to another section  75  at one or more intersection points  70 . A slope of each section  75  is not equal to a slope of another section  75  at one or more of the intersection points  70 . “Slope,” as used herein, refers to the magnitude of the first derivative of the mathematical function defining or approximating the section, where the term “derivative” is used as defined in calculus. The slope of one section, at the intersection point, not being equal to the slope of another section, corresponds to the mathematical condition that the function is continuous but the derivative is not continuous. In the embodiment shown in  FIG. 3( d ) , a width, d 2 , of one support member  60  is smaller than the width, d 1 , of other support members  65 . In the embodiments of  FIGS. 3( a )-3( d ) , where the support members are closed structures and where there are two or more support members, a sealing or bonding material can be applied to one or more support members while one or more other support members are able to move along the surface  25  of the package component  20 . That configuration enables a hermetic seal between the sensor  15  and the package component  20  while retaining flexibility in the structure. In the embodiment shown in  FIG. 3( e ) , the support members  80  surround the central region  50  of the second surface but are not closed structures. 
       FIG. 4  shows a cross-sectional view of another embodiment of the system of these teachings. Referring to  FIG. 4 , in the embodiment shown therein, a soft material  85  is disposed between the second surface  35 , the package surface  25  and the support members  40 . A “soft material,” as used herein, is a material having Young&#39;s modulus that is at least 5 times smaller than a Young&#39;s modulus of the second surface material or a Young&#39;s modulus of the package surface. The soft material can be used to alter the stiffness of the structure. 
       FIG. 5  shows a cross-sectional view of yet another embodiment of the system of these teachings. Referring to  FIG. 5 , in the embodiment shown therein, the support members include a first number of support members  90  extending away from the second surface  35  and a second number of support members  95  extending from the package surface  25  to the second surface  35 , the second number of support members  95  are configured to receive the first number of support members  90 . In the embodiment shown in  FIG. 5 , each support member from the first number of support members  90  extends a distance away from the second surface  35 , the distance being smaller than a distance between the second surface  35  and the package surface  25 . Also in the embodiment shown in  FIG. 5 , a soft material  85  is disposed between the second surface  35 , the package surface  25  and the first and second number of support members  90 ,  95 . It should be noted that the soft material can be applied onto the first number of support members  90  before the system is assembled or onto the second number of support members  95  before the system is assembled. (The soft material can be applied by any conventional techniques suitable for soft materials. Conventional deposition techniques may be suitable for some materials. For other soft materials, layer by layer deposition techniques, spray techniques, pulsed laser evaporation, matrix assisted pulsed laser operation, and direct topical application can be used to apply the material.) Embodiments in which the soft material is disposed on the second surface  35 , the package surface  25  and the second number of support members  95  and embodiments in which the soft material is disposed on the second surface  35 , the package surface  25  and the first number of support members  90  are also within the scope of these teachings. The embodiment shown in  FIG. 5  enables a configuration in which to control stiffness of the structure by placing the soft material between the first and second number of support members  90 ,  95 . It should be noted that embodiments in which a soft material is not used are also within the scope of these teachings. 
     Embodiments of support members such as those shown in  FIGS. 3( a )-3( d ) , in which each support member includes a hollow cylindrical structure, can be used in embodiments of the system in which a sealing material is applied between the package surface and at least one support member. The sealing material provides a hermetic seal between the package surface and at least one support member. Sealing material is applied using conventional techniques such as, but not limited to, topical application of an adhesive or a sealing compound. Embodiments of support members such as those shown in  FIGS. 3( a )-3( d )  can also be used in embodiments of the system in which at least one support member is bonded, using conventional bonding techniques, to the second surface ( 25 ,  FIG. 2 ) in order to provide a hermetic seal. In some embodiments, the support member surrounding and closest to the central region of the second surface is sealed or bonded to the second surface. It should be noted that other embodiments, in which other support members are sealed or bonded to the second surface are also within the scope of these teachings. 
       FIG. 6  is a cross-sectional view of a further embodiment of the system of these teachings. Referring to  FIG. 6 , in the embodiment shown therein, the support members  105  are comprised of the same material as a material of the package surface  25 . It should be noted that embodiments in which other materials are used for the support members  105  are also within the scope of these teachings. 
       FIGS. 7 a , 7 b    represent cross-sectional views of still another embodiment of the stress relief package of these teachings. Referring to  FIG. 7 a   , in the embodiment shown therein, each support member  115  has a cross-sectional area that varies with distance away from the second surface  35 . Cross-sectional area of a support member or of component of a support member, as used herein, refers to a cross-sectional area substantially perpendicular to a centerline of each support member, the centerline extending from the second surface  35  towards the package surface  25 . In the embodiment shown in  FIG. 7 a   , the cross-sectional area of each support member  115  increases with distance, the distance being measured from the second surface towards the package surface (hereinafter referred to as distance away from the second surface) but it should be noted that embodiments in which the cross-sectional area of each support member decreases with distance away from the second surface  35  are also within the scope of these teachings. 
     Referring to  FIG. 7 b   , in the embodiment shown therein, each support member has two subcomponents; a first subcomponent  125  extending from the second surface  35  to a predetermined location between the second surface  35  and the package surface  25  and a second subcomponent  135  extending from the predetermined location to the package surface. In the embodiment shown in  FIG. 7 b   , a cross-sectional area of the first subcomponent  125  being smaller than a cross-sectional area of the second subcomponent  135 . Embodiments in which the cross-sectional area of the first subcomponent is larger than the cross-sectional area of the second subcomponent are also within the scope of these teachings. Embodiments such as those shown in  FIGS. 7 a , 7 b    enable affecting the propagation of stresses from the package component  20  ( FIG. 2 ) to the sensor  15  ( FIG. 2 ). In the instance where the stresses are thermal stresses, embodiments such as those shown in  FIGS. 7 a , 7 b    enable affecting the thermal resistance and thermal conductance. 
     Referring to  FIGS. 7 a  and 7 b   , in the embodiment shown there in, the support members comprise the same material as the material of the sensor. It should be noted that embodiments in which the support members comprise different materials from the material of the sensor also within the scope of these teachings. 
     In one or more embodiments, the method of these teachings for fabricating a sensor system having reduced stress due to package-sensor interaction includes etching into a surface of a component of a sensor structure, the sensor structure also having a first surface, in order to form a second surface of the sensor structure and one or more support structures disposed on the second surface. The second surface is disposed away from the first surface. The second surface is also disposed away from a package surface and located between the first surface and the package surface. The support members are configured to reduce stress produced by package-sensor interaction. In this embodiment, the surface of the component that provides the second surface of the sensor is etched to produce multiple continuous changes and support members. In several embodiments, the support structures described hereinabove corresponding to  FIGS. 2, 3 ( a )- 3 ( e ), and  5  are produced by etching. 
     In order to elucidate the above embodiment of the method of these teachings, an exemplary embodiment is presented herein below.  FIG. 8  shows an exemplary sensor structure  17 , which, after processing, provides the sensor  15  and support structures  40  of  FIG. 2 . Referring to  FIG. 8 , in the embodiment shown there in, the sensor structure  17  has a first surface  25 , an inner surface  36  and a section  37  disposed away from the inner surface  36 . Etching into the surface  39  of the section  37 , the second surface  35  and the support structures  40  ( FIG. 2 ) are formed. 
     In one instance, the section  37 , that provides the second surface of the sensor structure, is comprised of silicon. 
     In one instance, the method of these teachings also includes trimming each support member such that each support member has a cross-sectional area that varies with distance away from the second surface. In one embodiment, the cross-sectional area increases with distance away from the second surface. In another embodiment, the cross-sectional area decreases with distance away from the second surface. 
     In another instance, the method of these teachings also includes trimming each support member such that each support member comprises two subcomponents. The two subcomponents include a first subcomponent extending from the second surface to a predetermined location between the second surface and the package surface and a second subcomponent extending from the predetermined location to the package surface. In one embodiment, a cross-sectional area of the first subcomponent is smaller than a cross-sectional area of the second subcomponent. Embodiments in which the cross-sectional area of the first subcomponent is larger than the cross-sectional area of the second subcomponent are also within the scope of these teachings. 
     There are a number of advantages of the present teachings over conventional systems and methods. One of those advantages is that, in the systems of the present teachings, the bonding or contact area with the package surface is reduced and the stresses are localized to isolated positions. The configuration of the present teachings suppresses stress during the packaging process. The embodiments having multiple closed support structures enable improving the package hermeticity by enabling substantially sealing soldering or sealing the space between one or more of the support structures and the package surface (in one instance, removing air bubbles from the contact area). Substantially sealing the space can be accomplished by applying sealing material or soldering material to the support structure faces. 
       FIG. 9  shows simulation results for some embodiments of these teachings and for a conventional embodiment. Referring to  FIG. 9 , in the results shown there in, “no-ring up” indicates conventional flat to flat surface bonding. “1-500 μm up” indicates a single silicon ring shaped support member, as in  FIG. 2 , the single silicon ring shaped support member having a 500 μm width (width being d 1  or d 2  in  FIG. 3( d ) ). Referring again to  FIG. 9 , “5-100 μm up” indicates five 100 μm wide silicon ring shaped supports as in  FIG. 2 . Referring again to  FIG. 9 , “1-100 μm” indicates a single 100 μm wide silicon ring shaped support member, as in  FIG. 2 . As can be seen from  FIG. 9 , reduced contact area or with multiple subcomponents, as obtained in the present teachings, reduces the performance drift (e.g. capacitance drift of a capacitive pressure sensor) from temperature changes. 
     For the purposes of describing and defining the present teachings, it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     Although the invention has been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.