Gel-free pressure sensor package

Abstract

A gel-free sensor package is disclosed. In one embodiment, the package includes a microelectromechanical system (MEMS) die having a first substrate that in turn includes a first surface on which a MEMS device is formed. The package also includes a polymer ring having an inner wall extending between first and second surfaces that are opposite on a back side. The first surface of the polymer ring is bonded to the first surface of the first substrate to define a first cavity in which the MEMS device is housed. A body of a molding compound having a second cavity that is concentric with the first cavity enables fluid communication between the MEMS device and an external environment of the package.

Description

Technical Field

[0001] This invention relates to the packaging of microelectromechanical systems (MEMS). Specifically, this invention relates to the packaging of gel-free pressure sensors. Background Technology

[0002] Microelectromechanical systems (MEMS) are semiconductor devices with embedded mechanical components. MEMS devices include, for example, pressure sensors, accelerometers, gyroscopes, microphones, digital mirror displays, microjets, etc. MEMS devices including pressure sensor devices can be used in a wide range of applications. This disclosure will be described with reference to a MEMS pressure sensor (hereinafter referred to as the pressure sensor), and it should be understood that this disclosure is not limited thereto.

[0003] The packaging of pressure sensors presents significant challenges, at least in part, due to their interaction with the external environment, the fragility of many types of pressure sensors, and severe cost constraints. In fact, many applications of pressure sensor devices require small size and low-cost packaging to meet high cost targets. Summary of the Invention

[0004] In one embodiment, a package is disclosed comprising a microelectromechanical system (MEMS) die, the MEMS die including a first substrate having a first surface on which a MEMS device is formed. A polymer ring is bonded to the first surface of the first substrate to create a first cavity therein accommodating the MEMS device. The package further includes a molding compound body including a second cavity concentric with the first cavity to allow fluid communication between the MEMS device and an external environment of the package. The polymer ring may include an inner wall extending between back-facing first and second surfaces, wherein the first surface of the polymer ring is bonded to the first surface of the first substrate. The molding compound body may include a first surface flush with the second surface of the polymer ring. The first surface of the polymer ring may be bonded to the first surface of the first substrate.

[0005] A seal can be formed between the first surface of the polymer ring and the first substrate, the seal inhibiting the flow of molding compound into the first cavity during the encapsulation process. The MEMS die may include a second substrate, the second substrate including a first recess. The first substrate may additionally include a cantilever platform structure having a platform and an arm extending from the platform, wherein the platform and the arm are suspended over the first recess. The MEMS device may be formed on the platform, and the polymer ring may be bonded to the arm but not to the platform. The MEMS die may include conductive traces formed on the platform and the arm, wherein the polymer ring contacts a portion of the conductive traces formed on the arm but not a portion of the conductive traces formed on the platform. The MEMS die may be mounted on a control die including bonding pads, wherein the MEMS die includes bonding pads formed on the first surface, wherein the encapsulation further includes a bonding wire connecting one of the control die bonding pads to one of the first surface bonding pads, and wherein the molding compound body encapsulates the bonding wire. The polymer ring may include fingers extending laterally between a pair of first surface bonding pads. MEMS devices can be pressure sensors.

[0006] A method for forming a packaged device is also disclosed, comprising applying a polymer film to a wafer comprising microelectromechanical systems (MEMS) dies, each of which includes a first surface forming a MEMS device. Subsequently, a portion of the polymer film is removed to create a ring, wherein the polymer ring creates a first cavity therein for receiving the MEMS device. The method may further include heating the wafer prior to removing the portion of the polymer ring to bond the polymer film to the first surface. The method may further include dicing the wafer to separate the MEMS dies, adhering one of the separated MEMS dies to an integrated circuit (IC) die, after adhering one of the separated MEMS dies to the IC die, adhering the IC die to a lead frame, connecting bonding pads on the MEMS die to bonding pads on the IC die using bonding wires, contacting the polymer ring with a mold to create a seal between the mold and the polymer ring, and encapsulating a portion of the MEMS die and the bonding wires with a molding compound while the polymer ring is in contact with the mold. The polymer ring in the method may include an inner wall extending between a first and a second surface facing each other, wherein the first surface of the polymer ring is bonded to the first surface. The method may further include curing the molding compound to produce a molding compound body, wherein the molding compound body includes a second cavity concentric with the first cavity to allow fluid communication between the MEMS device and the external environment of the packaging device.

[0007] In another embodiment, a device is disclosed comprising a microelectromechanical system (MEMS) die including a first substrate and a polymer ring. The first substrate includes a first surface on which a pressure sensor device is formed. The polymer ring is engaged with the first surface of the first substrate to create a first cavity in which the pressure sensor device is received. The device may further include a molding compound body including a second cavity concentric with the first cavity to allow fluid communication between the pressure sensor device and an external environment of the package. The polymer ring may include an inner wall extending between back-facing first and second surfaces, wherein the first surface of the polymer ring is engaged with the first surface of the first substrate. The molding compound body includes a first surface flush with or substantially flush with the second surface of the polymer ring. The MEMS die may include a second substrate including a first recess. The first substrate further includes a cantilever platform structure having a platform and an arm extending from the platform, wherein the platform and the arm are suspended over the first recess, wherein the pressure sensor device is formed on the platform, and wherein the polymer ring is engaged with the arm but not the platform. Attached Figure Description

[0008] Figure 1 The top view and side section view of the substrate are shown.

[0009] Figure 2 This illustrates the later stages of the manufacturing process. Figure 1 Top view and side section view of the structure.

[0010] Figure 3 This illustrates the later stages of the manufacturing process. Figure 2 Top view and side section view of the structure.

[0011] Figure 4 This illustrates the later stages of the manufacturing process. Figure 3 Top view and side section view of the structure.

[0012] Figure 5 This illustrates the later stages of the manufacturing process. Figure 4 Top and side views of the structure.

[0013] Figure 6 It shows Figure 4 The top and side views of the structure with alternative thick-film polymer rings are shown.

[0014] Figure 7 The wafer was cut as shown Figure 5 Top and side views of the device.

[0015] Figure 8 This illustrates the process of creating one type of encapsulation for a molded body. Figure 7 The device shown.

[0016] Figure 9 These are top and cross-sectional side views of the stress-isolated pressure sensor die after the encapsulation process of creating the molded body. Detailed Implementation

[0017] Typical pressure sensor packaging involves placing the pressure sensor within a cavity-type package made of a non-conductive material. Cavity packaging reduces or eliminates stress on the pressure sensor, which can alter its performance in unpredictable ways. However, creating the cavity for the pressure sensor presents challenges.

[0018] Pressure sensors have many different configurations and can be packaged in different ways using different manufacturing processes. This disclosure describes the manufacture and packaging of a reference stress-isolated pressure sensor die (hereinafter referred to as stress-isolated Pcell), but it should be understood that this disclosure is not limited thereto.

[0019] Figure 1The diagram shows a top view and a side cross-sectional view of a first substrate 102 contained within a wafer (not shown) during the initial stage of the manufacturing and packaging process of a stress-isolated Pcell die. The first substrate 102 is etched using known techniques to form corresponding recesses 104. Specifically, the depth to which the recesses 104 are etched from surface 106 into the first substrate 102 is less than the thickness of the first substrate 102.

[0020] Figure 2 This illustrates the later stages of the manufacturing process. Figure 1 The diagram shows a top view and a side cross-sectional view of the structure. A wafer (not shown) containing a second substrate 204 is attached to surface 106 of a first substrate 102 such that a recess 104 is inserted between the second substrate 204 and the first substrate 102. In an embodiment, the second substrate 204 may be included in a crystal wafer attached to the wafer containing the first substrate 102 using a silicon-on-insulator (SOI) process, fused bonding, or another similar process. After the second substrate 204 is attached to the first substrate 102, a buried cavity or recess 206 is formed.

[0021] Figure 3 This illustrates the later stages of the manufacturing process. Figure 2 The structure is shown in top and side cross-sectional views. A MEMS pressure sensor device 302 (hereinafter referred to as the pressure sensor device), conductive traces 304, and bonding pads 306 are formed on the surface 208 of a second substrate 204. In some embodiments, the MEMS pressure sensor device 302 can be fabricated on the second substrate 204 using, for example, surface micromachining processes to form a capacitive pressure sensor. In one embodiment, the device 302 can be produced by forming a polysilicon layer (not shown) on the substrate 204. A sacrificial layer (not shown) is then formed on top of the polysilicon layer. Another polysilicon layer (not shown) is formed on top of the sacrificial layer. The sacrificial layer is then removed, resulting in parallel plates of polysilicon, the top of which can bend with changes in the pressure of the fluid exposed therein. Changes in the distance between the parallel plates cause changes in the capacitance between them, which can be measured and converted into pressure data. The conductive traces 304 and bonding pads 306 can be fabricated in conjunction with the fabrication of the MEMS pressure sensor device 302. Metal deposition, patterning, and etching can be performed to form the bonding pads 306. Conductive traces 304 interconnect the MEMS pressure sensor device 302 with bonding pads 306 to form a suitable electrical connection. Other manufacturing activities may be performed conventionally, but for clarity of description, they are not discussed or shown herein.

[0022] Figure 4 This illustrates the later stages of the manufacturing process. Figure 3The structure is shown in top and side cross-sectional views. A portion of the second substrate 204 is etched or otherwise removed to form the cantilever platform structure 402. For example, a mask (not shown) may be used to cover or otherwise protect areas of the second substrate 204 that are not etched. Thus, the mask provides a pattern for forming the opening 404 through the second substrate layer 204 and for generating the cantilever platform structure 402 consisting of the platform 406 and the arm 408. The removal process can be performed using any of a number of known etching processes to produce the cantilever platform structure 402. Figure 4 The etching process results in the formation of an opening 404, and thus a cantilever platform structure 402. Note that conductive traces 304 are appropriately arranged such that they are located on the arm 408 to interconnect the pressure sensor device 302 with the bonding pad 306.

[0023] Pressure sensor devices (e.g., pressure sensor device 302) are typically housed in cavities to protect them from encapsulation stresses. A crystalline silicon cap (not shown) above the cavity housing the pressure sensor device can also be used to protect it from environmental stresses and molding encapsulation materials. The cap should include orifices located away from the pressure sensor device so that the pressure sensor device can withstand external gas pressures. However, adding a silicon cap is expensive. The silicon cap will need to be bonded to the structure supporting the pressure sensor device via a bonding process that may require a bonding material made of glass frit or metal that contacts the structure and the silicon cap to ensure bonding occurs between them prior to the encapsulation process. Some pressure sensors require silicone gel to protect components (e.g., bonding wires) from exposure to moisture and other environmental factors. The disadvantage of using gel is that it is relatively expensive and requires large quantities. Another expensive step in manufacturing encapsulated pressure sensors is the use of a membrane in the membrane-assisted molding of the encapsulation. Once used, the membrane cannot be reused. This disclosure overcomes the need for a cap covering the cavity, gel, and membrane-assisted molding.

[0024] Figure 5 It shows Figure 4The structure shown produces a top and side view of a stress-isolated Pcell die 500 in a later stage of the manufacturing process. As shown, a thick-film, flexible, conductive or non-conductive polymer ring 502 is formed on a first substrate 204. It should be noted that in the example embodiment, the ring 502 is square and its thickness is approximately 200 micrometers. In other embodiments, the thick-film, flexible polymer ring may be in the form of a circular device. However, for illustrative purposes only, a square ring 502 will be assumed. The polymer ring 502 surrounds the pressure sensor device 302 and the platform 406. The polymer ring 502 has a first flat surface 503 and a back-facing flat surface (not shown) in contact with the surface 208 of the substrate 204. The polymer ring 502 has parallel sidewalls 507 perpendicular to or substantially perpendicular to the surface 208 of the second substrate 204. The inner sidewalls 507B create cavities 505 for the pressure sensor device 302. In one embodiment, a thick film or flexible film of a polymer (polytetrafluoroethylene, perfluoroalkoxy, etc.) can be applied to the substrate containing... Figure 4 The ring 502 is formed by fabricating a wafer with a structure that creates a bond between the thick-film polymer and the second substrate 204. In one embodiment, the bond can be formed by thermal bonding. Once the polymer film is bonded to the surface 208 of the substrate 204, the ring 502 can be formed by imaging holes in the polymer film using known photolithography techniques and then chemically or dry etching to remove a portion of the thick film. Alternatively, a sheet of the thick-film polymer ring 502 can be attached to a wafer containing a substrate using standard manufacturing techniques. Figure 4 A sheet of thick-film polymer rings is formed before the wafer with the structure. Assuming the pre-formed sheet of thick-film ring 502 is precisely aligned with the wafer, a bond is created between the polymer and the second substrate 204. In one embodiment, the bond can be created by heating the ring 502 and the wafer. The polymer ring will remain flexible after heating and sufficiently flexible at the mold temperature (approximately 175°C) to maintain a seal with the mold during subsequent encapsulation processes, which will be described more fully below. Figure 6 It shows Figure 4The diagram shows a top and side view of a structure having an alternative thick-film, flexible, conductive, or non-conductive polymer ring 602 bonded thereto. This alternative thick-film polymer ring 602 includes fingers 604 extending between bonding pads 306. In one embodiment, the fingers 604 may provide insulation between bonding lines that will subsequently be added to connect the bonding pads 306 to bonding pads of underlying control circuitry (e.g., a control die). In one embodiment, the ring 602 and fingers 604 are formed of a conductive polymer (e.g., a polymer filled with conductive particles (e.g., Ag, Cu, Ni)) that provides EMC isolation. In another embodiment, the ring 602 is a non-conductive polymer, while the fingers 604 are a conductive polymer. In yet another embodiment, the ring 602 and fingers 604 are non-conductive polymers, but the fingers 604 may subsequently be metallized to become conductive using any of many known techniques (e.g., sputtering, plating, inkjet printing, etc.). If conductive, one or more of the ring 602 and / or the finger 604 can be electrically coupled to a grounding pad, which can be formed on surface 208 and below one of the ring 602 or the finger 604.

[0025] Finally, polymer rings 502 or 602 are used during encapsulation to create an encapsulation cavity for the pressure sensor device 302. Importantly, the use of flexible polymer rings 502 or 602 eliminates the need for expensive films during encapsulation molding, and the polymer rings 502 or 602 create a seal between the polymer ring and the mold and between the polymer ring and the second substrate 204 to prevent molding compounds from flowing into the polymer cavity 505.

[0026] Include Figure 5 The wafer of stress-isolated Pcell 500 in section 6 can be diced to produce individual or grouped stress-isolated Pcell dies. The wafer can be inverted during dicing to prevent residue from entering cavity 505 during the dicing process. Figure 7 The diagram shows a top view and a side view of structure 700, which includes components after wafer dicing and after mounting on an integrated circuit control die 702, which in turn is mounted on a lead frame 704. Figure 5 The stress-isolated P-cell die structure. The control die 702 may be in the form of, for example, an application-specific integrated circuit or another type of signal processor, and includes bonding pads 706 connected to bonding pads 306 of the stress-isolated P-cell die 500 via bonding lines 710. Although not shown in the figure, additional bonding lines may provide data communication between the control die 702 and the lead frame 704.

[0027] It can at least partially surround Figure 7The structure 700 shown forms a molded body, thus creating a molded body cavity concentric with the polymer cavity 505. The molded body encapsulates bonding pads 306 and 706 and bonding lines 710. The molded body adequately protects the component including the bonding lines 710, eliminating the need for gel filling that may be used in prior art packages. Figure 8 This illustrates the process of creating one type of encapsulation for a molded body. Figure 7 The structure 700 is shown. Specifically, structure 700 is placed on the surface 804 of a machine including a mold 802 that contacts the upper surface 503 of a flexible polymer ring 502, while an encapsulating molding compound (e.g., molten epoxy molding compound) flows into a mold cavity 806. The mold 802 contacts the upper surface 503 of the polymer ring to create a seal that prevents the encapsulating molding compound from flowing into the cavity 505 created by the polymer ring 502. Subsequently, the molding compound is cured to create a hardened body surrounding structure 700.

[0028] Figure 9 These are top and cross-sectional side views of structure 700 after the encapsulation process of creating the molded body 902. The first surface 904 of the molded body 902 is flush or substantially flush with the surface 503 of the polymer ring 502. As can be seen in this figure, a cavity 505 is retained during the encapsulation process by means of the polymer ring 502. In this configuration, the pressure sensor device 302 is in fluid communication with its environment via the cavity 505. The resulting package including the molded body 902 provides a means to connect the MEMS pressure sensor device 302 and the packaged control die 702 to a device in the external environment, such as a printed circuit board, via pins (not shown) of the lead frame 704.

[0029] This disclosure is intended to explain how to design and use various embodiments according to the invention, rather than to limit the true, established, and fair scope and spirit of the invention. The foregoing description is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Furthermore, the wording or terminology used herein is for descriptive purposes and not for limitation. Modifications or variations may be made in light of the foregoing teachings. The embodiments were chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable those skilled in the art to use the invention in various embodiments and with various modifications suitable for the particular intended use. All such modifications and variations, and all their equivalents, are within the scope of the invention as defined by the appended claims when interpreted with the fair, lawful, and justly conferred breadth of the rights, and may be amended during the pending period of this patent application.

Claims

1. A package, characterized in that, include: Microelectromechanical systems (MEMS) chips, including: A first substrate, the first substrate including a first surface on which a MEMS device is formed; and The second substrate includes a first groove formed on a first side of the second substrate and not extending through the second substrate, wherein a second surface of the first substrate is attached to the first side of the second substrate such that the first groove is located below the MEMS die. A polymer ring is bonded to the first surface of the first substrate to create a first cavity in which the MEMS device is accommodated. A molded compound body includes a second cavity concentric with the first cavity to enable fluid communication between the MEMS device and the external environment of the package.

2. The packaging according to claim 1, characterized in that, The polymer ring includes an inner wall extending between a first surface and a second surface that are opposite each other, wherein the first surface of the polymer ring is bonded to the first surface of the first substrate.

3. The packaging according to claim 1, characterized in that, A seal is formed between the first surface of the polymer ring and the first substrate, the seal inhibiting the flow of molding compound into the first cavity during the encapsulation process.

4. The packaging according to claim 1, characterized in that: The first substrate further includes a cantilever platform structure having a platform and an arm extending from the platform, wherein the platform and the arm are suspended above the first groove. The MEMS device is formed on the platform; The polymer ring is attached to the arm, but not the platform.

5. The packaging according to claim 4, characterized in that: The MEMS die includes conductive traces formed on the platform and the arm; The polymer ring contacts a portion of the conductive trace formed on the arm, but does not contact a portion of the conductive trace formed on the platform.

6. The packaging according to claim 4, characterized in that: The MEMS die is mounted on a control die including bonding pads; The MEMS die includes bonding pads formed on the first surface; The package further includes bonding lines that connect one of the control die bonding pads to one of the bonding pads formed on the first surface; The molding compound body encapsulates the bonding line.

7. A method for forming a packaging device, characterized in that, The method includes: A polymer film is applied to a wafer containing a microelectromechanical system (MEMS) die, each of which includes: A first substrate, including a first surface on which a MEMS device is formed; and A second substrate, the second substrate including a first groove, the first groove being formed on a first side of the second substrate and not extending through the second substrate, wherein a second surface of the first substrate is attached to the first side of the second substrate such that the first groove is located below the MEMS die; and A portion of the polymer film is removed to create a polymer ring, wherein the polymer ring creates a first cavity therein to house the MEMS device.

8. The method according to claim 7, characterized in that, Further includes: The wafer is cut to separate the MEMS die; One of the separate MEMS dies is adhered to the integrated circuit IC die; After attaching one of the separated MEMS dies to the IC die, the IC die is attached to the lead frame; Use bonding wires to connect the bonding pads on the MEMS die to the bonding pads on the IC die; The polymer ring is brought into contact with the mold to create a seal between the mold and the polymer ring; as well as When the polymer ring contacts the mold, a portion of the MEMS die and the bonding wires are encapsulated with a molding compound.

9. A device, characterized in that, include: Microelectromechanical systems (MEMS) chips, including: A first substrate, the first substrate including a first surface on which a pressure sensor device is formed; and A second substrate, the second substrate including a first groove formed on a first side of the second substrate and not extending through the second substrate, wherein a second surface of the first substrate is attached to the first side of the second substrate such that the first groove is located below the pressure sensor device; and A polymer ring is bonded to the first surface of the first substrate to create a first cavity in which the pressure sensor device is accommodated.

10. The device according to claim 9, characterized in that, It further includes a molded compound body, the molded compound body including a second cavity concentric with the first cavity to enable fluid communication between the pressure sensor device and the packaged external environment.

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