Embedded structures for high glass strength and robust packaging

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.

Claims

1. An apparatus, comprising: a pressure sensing element; a pedestal having an aperture; a first bonding interface, the pressure sensing element bonded to the pedestal at the first bonding interface; a cavity formed as part of the pressure sensing element; a cap bonded to the pressure sensing element at a second bonding interface; a cavity formed as part of the cap; and a plurality of polished smooth areas, a portion of the plurality of polished smooth areas formed as part of the pressure sensing element adjacent the bonding interface prior to the pressure sensing element and the pedestal being partitioned from a wafer stack, and a portion of the plurality of polished smooth areas formed as part of the pedestal adjacent the bonding interface prior to the pressure sensing element and the pedestal being partitioned from the wafer stack; wherein a portion of the pressure sensing element deflects when pressure is applied to the pressure sensing element in the cavity from the aperture.

2. The apparatus of claim 1, the plurality of polished smooth areas further comprising: at least one angled polished smooth area formed as part of the pressure sensing element adjacent the first bonding interface; at least one upper polished smooth area formed as part of the pedestal adjacent the first bonding interface; and at least one lower polished smooth area formed as part of the pedestal; wherein the at least one angled polished smooth area and the at least one upper polished smooth area reduce the stress concentration in proximity to the first bonding interface, and the at least one lower polished smooth area reduces stress concentration in the pedestal when the pedestal is connected to a housing substrate.

3. The apparatus of claim 2, further comprising: a plurality of rough areas, at least one of the plurality of rough areas formed as part of the pressure sensing element adjacent the at least one angled polished smooth area, and at least one of the plurality of rough areas formed as part of the pedestal adjacent the at least one upper polished smooth area; wherein the plurality of rough areas are formed when the pressure sensing element and the pedestal are partitioned from the wafer stack.

4. The apparatus of claim 3, the plurality of rough areas further comprising: a first rough area formed as part of the pressure sensing element adjacent the at least one angled polished smooth area; a second rough area formed as part of the pedestal adjacent the at least one upper polished smooth area and the at least one lower polished smooth area; wherein the first rough area and the second rough area are formed during a partitioning process, when the pressure sensor is partitioned from the wafer stack.

5. The apparatus of claim 2, the plurality of polished smooth areas further comprising: at least one outer polished smooth area formed as part of the cap in proximity to the second bonding interface; wherein the at least one outer polished smooth area formed as part of the cap reduces thermal stress and bending stress at the second bonding interface.

6. The apparatus of claim 5, further comprising: at least one outer rough area formed as part of the cap in proximity to the at least one outer polished smooth area; wherein the at least one outer rough area is formed during the partitioning process, when the pressure sensor is partitioned from a wafer stack.

7. The apparatus of claim 2, further comprising: a plurality of micro-posts formed as part of the pedestal; and at least one venting area formed as part of the pedestal in proximity to the plurality of micro-posts; wherein air and vapor are vented around the plurality of micro-posts and out of the venting area as the pedestal is bonded to the housing substrate with a mounting material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

(2) 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;

(3) 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;

(4) 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;

(5) FIG. 2B is a magnified view of the circled portion in FIG. 2A;

(6) 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;

(7) 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;

(8) FIG. 3C is a magnified sectional side view of the circled portion in FIG. 3A; and

(9) 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

(10) 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.

(11) 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 10A, 10B, 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 12A which is anodically bonded to a top surface 12B of the glass pedestal 14, forming a bonding interface 12C.

(12) The cavity 22 is etched into the bottom surface 12A 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.

(13) 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.

(14) 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.

(15) As mentioned above, the pressure sensors 10A, 10B are backside absolute pressure sensors 10A, 10B. The sensors 10A, 10B include a cap glass substrate, in the form of a cap 16. In one embodiment, the cap 16 includes sidewalls 16A, 16B, and a cap cavity, shown generally at 24. The cap 16 is anodically bonded to the sensing element 12 as a second bonding interface 12D, 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 10A, 10B 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.

(16) To fabricate the sensors 10A, 10B, 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 12A of the first wafer 38, upper recesses 46 formed on the top surface 12B 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 54A formed as part of the bottom surface 56 of the third wafer 42, as shown in FIGS. 2A and 3A.

(17) The second wafer 40 is bonded to the bottom surface 12A 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 10A, 10B, 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.

(18) 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 10A, 10B are exposed to thermal stress and bending stress. The bonding interface 12C 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 62A formed as part of the pressure sensing element 12, a second rough area 62B formed as part of the pedestal 14, and a third rough area 62C 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 64A formed as part of the pressure sensing element 12 which is part of one of the angled recesses 44, an upper smooth area 64B formed as part of the pedestal 14 adjacent the angled smooth area 64A that was part of one of the upper recesses 46, a lower smooth area 64C formed as part of the pedestal 14 which was part of the lower recess 50, and an outer smooth area 64D formed as part of the cap 16, which was part of one of the outer recesses 54.

(19) 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 10A, 10B from the wafer stack 58, as shown in FIG. 1B, do not touch any polished external side walls (remaining smooth areas 64A, 64B, 64C, 64D) of the recesses 44, 46, 50, 54, so that the areas 64A, 64B, 64C, 64D (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 10A, 10B are robust when exposed to high thermal and bending stresses.

(20) 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 68A, 68B, 68C in FIGS. 1A and 2A. More specifically, there is a first material removal area 68A which is part of the first wafer 38 between each sensing element 12, a second material removal area 68B which is part of the second wafer 40 in the area between each pedestal 14, and a third material removal area 68C which is part of the third wafer 42 in the area between each cap 16. The material removal areas 68A, 68C, 68C 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 10A, 10B is the correct dimensions after the cutting process. The cutting process eliminates each of the material removal areas 68A, 68B, 68C, such that the rough areas 62A, 62B, 62C are left after the cutting process is complete. It is also shown in the Figures that the material removal areas 68A, 68B, 68C are separate from the smooth areas 64A, 64B, 64C, 64D that are part of each of the recesses 44, 46, 50, 54. The smooth areas 64A, 64B, 64C, 64D 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.

(21) 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.

(22) 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 44A, a single wide upper recess 46A, and the single lower recess 50 formed in each saw street 60 separating the individual sensors 10A and 10B from the wafer stack 58.

(23) 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.