Abstract:
A sensor device is constructed to maintain a high glass strength to avoid the glass failure at low burst pressure, resulting from the sawing defects located in the critical high stress area of the glass pedestal as one of the materials used for construction of the sensor. This is achieved by forming polished recess structures in the critical high stress areas of the sawing street area. The sensor device is also constructed to have a robust bonding with the die attach material by creating a plurality of micro-posts on the mounting surface of the glass pedestal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/118,618 filed Feb. 20, 2015. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to a sensor device which includes embedded structures in a wafer sawing street and a die mounting surface, to maintain the intrinsic glass strength for a high burst pressure, and to enhance the die adhesion with mounting material for a robust packaging design capable of resisting aggressive thermal shocks, and a hot/humid environment. 
     BACKGROUND OF THE INVENTION 
     Due to the maturity of wafer bonding technology and low MEMS (micro-electrical-mechanical systems) fabrication cost, borosilicate glass is widely used in current MEMS based sensors and actuators. Various types of pressure sensors which use this technology include differential pressure sensors, front side absolute pressure sensors, and backside absolute pressure sensors. 
     In situations where the glass has no major defects, such as those from the starting material from the fabrication process, the borosilicate glass used in the pressure sensors is strong enough to survive a high burst pressure, up to more than 100 bar for a typical structure shape and size. Glass defects may be, however, induced by the device fabrication process. It is the defects induced by the fabrication process that cause the glass to fail at a low pressure, for example, less than 50 bar, compared a structure having a similar size and shape having no defects. One of the major process steps which may induce glass defects is the wafer sawing to singulate the individual devices from the wafer stack, which often creates the mechanical defects around the sidewalls of the glass, such as cracking and chipping. If these defects are located in one of the high-stress areas, such as around the Si and glass interface, and around the die attach interface, the glass is more susceptible to failure when exposed to a high pressure. For example, in a differential or backside absolute pressure sensor, the glass may fail when around 30 bar (or less) of pressure. 
     The defect-free borosilicate glass wafers are normally polished on both surfaces, which may reduce the adhesion with a die mounting material, such as a paste, in a sensor device. The adhesion of the die with the mounting material may be degraded if the sensor is exposed to a hot and humid environment, or experiences thermal shocks, which often results in a device output instability and/or malfunction. 
     Accordingly, there exists a need for an MEMS device which is able to survive when exposed to a high burst pressure, and is robust in a harsh environment, which may include thermal shocks, and high humidity under an elevated temperature. 
     SUMMARY OF THE INVENTION 
     The present invention is a sensor device which is constructed to maintain high glass strength and enhance the device integrity in a harsh environment, which may include thermal shocks, and high humidity under an elevated temperature. 
     It is therefore an object of this invention to eliminate the sawing defects located in the critical high stress areas by forming polished recess structures in the critical high stress area in the sawing street. The recess structures are arranged in such a way that during sawing, the saw blade does not touch the outer walls of the recesses, thus avoiding the creation of any sawing defects in the critical high stress areas. 
     The glass wafers used in the MEMS device according to the present invention are polished to remove the mechanical defects induced in wafer slicing and thinning from the bulk glass material. A highly polished glass surface has weak adhesion to the mounting material. It is therefore another object of present invention to create a micro-roughness structure on the pedestal glass surface facing the die mounting material to increase the mounting area and enhance the interlocking force with the mounting material. 
     In one embodiment, the mounting surface of the MEMS device includes a structure made of an array of micro-posts which protrude from the base (non-micro posts area). There are “gaps” in between the micro-posts which allow gas/vapor to be easily vented out through the gaps of the micro-posts during the die attach process. These micro-posts used in the MEMS device of the present invention have been proved to be efficient in various extended test-to-fail tests, including, but not limited to, thermal shocks, exposure to 85% RH at 85° C., and autoclave. 
     In one embodiment, the present invention is a method of making a pressure sensor, which includes the steps of providing a first wafer having a top surface and a bottom surface, providing a second wafer having a top surface and a bottom surface, and providing a third wafer having a top surface and a bottom surface. The method according to the present invention also includes providing at least one angled recess having at least one angled smooth area formed on the bottom surface as part of the first wafer, providing at least one upper recess having at least one upper smooth area formed on the top surface as part of the second wafer, providing at least one lower recess having at least one lower smooth area formed on the bottom surface as part of the second wafer, and providing at least one outer recess having at least one outer smooth area formed on the bottom surface of the third wafer. The bottom surface of the first wafer is bonded to the top surface of the second wafer at a first bonding interface such that the at least one angled recess is located in proximity to the at least one upper recess. The bottom surface of the third wafer is bonded to the top surface of the first wafer at a second bonding interface such that the at least one outer recess is located in proximity to the second bonding interface. A wafer stack and a saw street area are formed when the first wafer is bonded to the second wafer, and the third wafer is bonded to the first wafer. The wafer stack is partitioned in the saw street area to form at least two pressure sensors. 
     The method of making a pressure sensor according to the present invention also includes providing a portion of the at least one angled smooth area to be located outside the saw street area, providing a portion of the at least one upper smooth area to be located outside the saw street area, providing a portion of the at least one lower smooth area located outside the saw street area, and providing a portion of the at least one outer smooth area located outside the saw street area. The wafer stack is partitioned such that each of the portion of the at least one angled smooth area, the portion of the at least one upper smooth area, the portion of the at least one lower smooth area, and the portion of the at least one outer smooth area remain intact on the pressure sensor after the wafer stack is partitioned. 
     The method of making a pressure sensor according to the present invention also includes the steps of providing a first material removal area formed as part of the first wafer in proximity to the angled recess and located in the saw street area, providing a second material removal area formed as part of the second wafer in proximity to the upper recess and located in the saw street area, and providing a third material removal area formed as part of the third wafer in proximity to the outer recess and located in the saw street area. Each of the first material removal area, the second material removal area, and the third material removal area is eliminated as the wafer stack is partitioned 
     The method of making a pressure sensor according to the present invention also includes providing at least one pressure sensing element formed as part of the first wafer, providing at least one pedestal formed as part of the second wafer, and providing at least one cap formed as part of the third wafer. The wafer stack is partitioned such that one of the at least two pressure sensors have the at least one pressure sensing element, the at least one pedestal, and the at least one cap. 
     The method of making a pressure sensor according to the present invention also includes the steps of providing a plurality of micro-posts formed on the bottom surface of the glass pedestal, and providing at least one venting area in proximity to at least one of the plurality of micro-posts. The pedestal is bonded to a housing substrate, venting air and vapor away from the at least one venting area as the pressure sensor is attached to a housing substrate. 
     The method of making a pressure sensor according to the present invention also includes the steps of providing a cavity formed as part of the at least one pressure sensing element, providing a cap cavity formed as part of the cap, and providing an aperture formed as part of the pedestal, which is in fluid communication with the cavity. The method also includes detecting the pressure applied to the pressure sensing element in the cavity by measuring the pressure sensor output change due to the deflection of the pressure sensing element. 
     The method of making a pressure sensor according to the present invention also includes the steps of forming at least a partial vacuum in the cap cavity by bonding the bottom surface of the third wafer to top surface of the first wafer. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  is a sectional side view of a first embodiment of a wafer stack used to make a plurality of pressure sensor structures, according to embodiments of the present invention; 
         FIG. 1B  is a sectional side view of a first embodiment of two pressure sensors after a wafer stack is partitioned, according to embodiments of the present invention; 
         FIG. 2A  is a second sectional side view of a second embodiment of a wafer stack used to make a plurality of pressure sensor structures, according to embodiments of the present invention; 
         FIG. 2B  is a magnified view of the circled portion in  FIG. 2A ; 
         FIG. 3A  is a sectional side view of a third embodiment of a wafer stack used to make a plurality of pressure sensor structures, according to embodiments of the present invention; 
         FIG. 3B  is a sectional side view of a third embodiment of two pressure sensors after a wafer stack is partitioned, according to embodiments of the present invention; 
         FIG. 3C  is a magnified sectional side view of the circled portion in  FIG. 3A ; and 
         FIG. 4  is an enlarged view of the circled portion shown in  FIG. 1B  of micro-posts formed on part of a pedestal, according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     A first embodiment of a MEMS sensor structure according to the present invention is shown in  FIGS. 1A-2B , generally at  10 . Typically, the MEMS fabrication process generates thousands of the same devices from one wafer stack. For simplicity,  FIGS. 1A-2A  only show a portion of a wafer stack used to make two sensor structures  10 . In this embodiment, the pressure sensor structure  10  includes two backside absolute pressure sensors, shown generally at  10 A,  10 B, each of which includes a silicon structure in the form of a pressure sensing element  12 , and pedestal glass structure in the form of a glass pedestal  14 . Formed as part of the glass pedestal  14  is an aperture  20 , and the aperture  20  is in fluid communication with a cavity, shown generally at  22 . The pressure sensing element  12  includes a bottom surface  12 A which is anodically bonded to a top surface  12 B of the glass pedestal  14 , forming a bonding interface  12 C. 
     The cavity  22  is etched into the bottom surface  12 A of the pressure sensing element  12 , and includes four inner surfaces, where only a first inner surface  26  and a second inner surface  28  are depicted in  FIGS. 1A and 1B , because  FIGS. 1A and 1B  are cross-sectional views. Each of the four inner surfaces terminates into a backside surface  30 , which is part of a diaphragm  32 . In one embodiment, the cavity  22  is formed using an anisotropic chemical etching by potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), etc, or a dry etch by deep reactive ion etch (DRIE), but it is within the scope of the invention that other processes may be used. 
     The pressure sensing element  12  is made from a single crystalline silicon, and includes the diaphragm  32  having a top surface  34 , and the cavity  22  having surfaces  26 ,  28 , and  30 . The top surface  30  of the cavity  22  is also the backside surface of the diaphragm  32 . The pressure sensing element  12  also includes a bridge circuitry  36  on the top surface  34  of the diaphragm  32 . In one embodiment, the bridge circuitry  36  contains at least four separate piezoresistors connected by P+ doped and/or metal interconnects. The piezoresistors may be placed in one of several configurations. The piezoresistors may be located close to one side of the edge of the diaphragm  32 , close to four sides of the edge of the diaphragm  32 , or distributed in one direction across the diaphragm  32 . For drawing simplicity,  FIGS. 1A and 1B  do not include these details, such as interconnects, or the location of each piezoresistor. Instead, reference numeral  36  in  FIGS. 1A and 1B  is used to represent a generic bridge circuitry, which may be in any configuration and location as generally known in the field. 
     The diaphragm  32  is relatively thin, and the thickness of the diaphragm  32  depends upon the diaphragm size and the pressure sensing range. The diaphragm  32  deflects in response to pressure applied to the backside surface  30  through the aperture  20  of the substrate  14  and the cavity  22 , as shown in  FIGS. 1A and 1B . The deflections of the diaphragm  32  resulting from the applied pressure causes an imbalance in the bridge circuitry  36  such that the output of the bridge circuitry  36  correlates to the pressure signal. 
     As mentioned above, the pressure sensors  10 A,  10 B are backside absolute pressure sensors  10 A,  10 B. The sensors  10 A,  10 B include a cap glass substrate, in the form of a cap  16 . In one embodiment, the cap  16  includes sidewalls  16 A,  16 B, and a cap cavity, shown generally at  24 . The cap  16  is anodically bonded to the sensing element  12  as a second bonding interface  12 D, such that the cap cavity  24  is on top of diaphragm  32 , and the pedestal  14  is anodically bonded to the pressure sensing element  12  in such a way that the cap cavity  24  is hermetically sealed as at least a partial vacuum. This allows the pressure sensors  10 A,  10 B to measure absolute pressure. The length and width of the cap cavity  24  are bigger than but close to the length and width of the diaphragm  32 . 
     To fabricate the sensors  10 A,  10 B, there is a first wafer  38  which is used to create the pressure sensing elements  12 , a second wafer  40  used to create the pedestals  14 , and a third wafer  42  used to create each cap  16 . Also formed as part of the sensor structure  10  are multiple embedded structures, which in this embodiment are recesses. More specifically, there are angled recesses  44  formed on the bottom surface  12 A of the first wafer  38 , upper recesses  46  formed on the top surface  12 B of the second wafer  40 , and one lower recess  50  formed from the bottom surface  52  of the glass pedestal  14 . There are also outer recesses  54  formed on the bottom surface  56  of the third wafer  42  used to form each cap  16 . In an alternate embodiment, instead of the two outer recesses  54  being formed as part of the third wafer  42 , as shown in  FIG. 1A , there may be a single outer recess  54 A formed as part of the bottom surface  56  of the third wafer  42 , as shown in  FIGS. 2A and 3A . 
     The second wafer  40  is bonded to the bottom surface  12 A of the first wafer  38 , and the third wafer  42  is bonded to the top surface  34  of the first wafer  38  to form a wafer stack, shown generally at  58  in  FIG. 1A . All recesses are at least partially located in an area of what is referred to as a “saw street,” shown in the area indicated at  60 , where the saw street  60  is the area which is cut to separate the wafer stack  58  in the embodiment shown in  FIG. 1A , into the individual sensors  10 A,  10 B, shown in  FIG. 1B . In the embodiment shown in  FIGS. 1A and 1B , the area of the saw street  60  is cut between the two angled recesses  44 , between the two outer recesses  54 , and through the third wafer  42  between the two upper recesses  46  and in the area of the wide recess  50 . 
     All recesses  44 ,  46 ,  50 ,  54  are fabricated prior to the wafer bonding step forming the wafer stack  58 . This fabrication method provides the capability of chemically etching and polishing the surfaces of all recesses  44 ,  46 ,  50 ,  54  to eliminate or minimize the mechanical defects created during fabrication of the recesses. During operation, the sensors  10 A,  10 B are exposed to thermal stress and bending stress. The bonding interface  12 C between the sensing element  12  and the pedestal  14 , and the bonding interface between the pedestal  14  and a housing substrate, are each exposed to a high stress. There are several areas which may have a rough surface after being cut through with the saw blade, and several areas which are smooth and polished, that were part of the various recesses  44 ,  46 ,  50 ,  54  prior to the wafer stack  58  undergoing the cutting process. More specifically, there is a first rough area  62 A formed as part of the pressure sensing element  12 , a second rough area  62 B formed as part of the pedestal  14 , and a third rough area  62 C formed as part of the cap  16 . There are also several smooth areas which remain after the cutting process, which were part of the recesses  44 ,  46 ,  50 ,  54  prior to the cutting process. More specifically, there is an angled smooth area  64 A formed as part of the pressure sensing element  12  which is part of one of the angled recesses  44 , an upper smooth area  64 B formed as part of the pedestal  14  adjacent the angled smooth area  64 A that was part of one of the upper recesses  46 , a lower smooth area  64 C formed as part of the pedestal  14  which was part of the lower recess  50 , and an outer smooth area  64 D formed as part of the cap  16 , which was part of one of the outer recesses  54 . 
     The widths and locations of the recesses  44 ,  46 ,  50 ,  54  in the Figures are arranged in a such a way that the sawing blades used to singulate individual sensors  10 A,  10 B from the wafer stack  58 , as shown in  FIG. 1B , do not touch any polished external side walls (remaining smooth areas  64 A,  64 B,  64 C,  64 D) of the recesses  44 ,  46 ,  50 ,  54 , so that the areas  64 A,  64 B,  64 C,  64 D (which are exposed to high stress) remain intact after the cutting process, and are smooth with minor defects or are defect free. This structure and fabrication method shown in  FIGS. 1A to 1B  helps maintain strength such that the sensors  10 A,  10 B are robust when exposed to high thermal and bending stresses. 
     In addition to the recesses  44 ,  46 ,  50 ,  54 , there are also areas of material which are removed during the cutting process. These areas are shown at  68 A,  68 B,  68 C in  FIGS. 1A and 2A . More specifically, there is a first material removal area  68 A which is part of the first wafer  38  between each sensing element  12 , a second material removal area  68 B which is part of the second wafer  40  in the area between each pedestal  14 , and a third material removal area  68 C which is part of the third wafer  42  in the area between each cap  16 . The material removal areas  68 A,  68 C,  68 C are excess material that is part of each respective wafer in the wafer stack  58 , but are intended to be removed during the cutting process such that each sensor  10 A,  10 B is the correct dimensions after the cutting process. The cutting process eliminates each of the material removal areas  68 A,  68 B,  68 C, such that the rough areas  62 A,  62 B,  62 C are left after the cutting process is complete. It is also shown in the Figures that the material removal areas  68 A,  68 B,  68 C are separate from the smooth areas  64 A,  64 B,  64 C,  64 D that are part of each of the recesses  44 ,  46 ,  50 ,  54 . The smooth areas  64 A,  64 B,  64 C,  64 D have undergone the etching and polishing process, as mentioned above, and are not damaged during the cutting process, and are therefore able to maintain bond integrity, and are more resistant to bending and thermal stresses. 
     Referring again to the Figures generally, each embodiment includes a plurality of micro-posts  66  formed as part of the bottom of the glass pedestal  14 , which are shown in  FIG. 4  in an enlarged bottom view. The micro-posts  66  protrude a few of microns (in length) from the bottom surface  52  of the pedestal  14 , and arranged in such a way that the vapor and air is vented out from the bottom surface of the glass pedestal  14  during the bonding to the housing substrate. More specifically, the bottom surfaces  70  of the micro-posts  66  are connected to the housing substrate by a mounting material. There is venting area, shown generally at  72  in between each of the posts  66 , which allows for the venting of air and vapor during the die mounting process around the posts  66 , and out of the venting area  72 . Examples of the flow path for the air or vapor are shown by the arrows  74 . This ensures no voids in the mounting material are created during the bonding of the pedestal  14  to the housing substrate. 
     Another embodiment of the present invention is shown in  FIGS. 3A-4B , with like numbers referring to like elements. In this embodiment, instead of having multiple angled recesses  44  formed between the pressure sensing elements  12  and pedestal  14 , there is only a single angled recess  44 A, a single wide upper recess  46 A, and the single lower recess  50  formed in each saw street  60  separating the individual sensors  10 A and  10 B from the wafer stack  58 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.