Patent Application
20240322786
Bulk Acoustic Wave (BAW) resonators are electromechanical devices in which standing acoustic waves are generated by an electrical signal in the bulk of a piezoelectric material. BAW devices use a piezoelectric effect to convert electrical energy into mechanical energy resulting from an applied radio frequency (RF) voltage. BAW devices generally operate at a mechanical resonant frequency that is defined as the frequency for which the half wavelength of sound waves propagating in the device is equal to the total piezoelectric layer thickness for a given velocity of sound in the piezoelectric material. BAW resonators operating in the GHz range (e.g., at about 2 GHz) generally have physical dimensions of tens of microns in diameter with thicknesses of a few microns. The piezoelectric layer of the BAW device is acoustically isolated from the substrate.
Typical packaged electronic BAW devices include a die with one or more electronic components mounted to a die pad within a molded package structure. Bond wires connect die pads of the die with leads that provide external access for soldering to a host printed circuit board (PCB). Glob top materials are used to cover and protect BAW dies or other components of an integrated circuit against contamination and mechanical stress. However, the glob top material may have undesired delamination from the molding material when the materials are curing.
In an example electronic package apparatus, mold compound is removed to create a cavity via laser ablation prior the attachment and integration of a BAW die into the package. The process includes attachment of a bottom die to a leadframe, wire bonding the bottom die to the leadframe, molding the bottom die and leadframe, laser ablation of the molding to create a cavity, attaching a BAW die to the bottom die, and applying a glob top encapsulant over the BAW die to improve the frequency shift margins.
In an arrangement, a semiconductor package comprises a segment of a leadframe having a die attach pad and one or more leads, a semiconductor die electrically connected to the die attach pad; a BAW device attached to (and electrically connected to) the semiconductor die, a mold compound surrounding the first semiconductor die and covering portions of the leadframe and a first portion of a top surface of the semiconductor die, a cavity formed in the mold compound, the cavity exposing the BAW device and a second portion of the top surface of the semiconductor die from the mold compound, and a glob top material disposed within the cavity, the glob top material covering the BAW device and the second portion of the top of the semiconductor die. The semiconductor die is electrically connected to the one or more leads by bond wires that are encapsulated by the mold compound. The semiconductor package may be a quad-flat no-leads (QFN) package.
The cavity has a first height above the semiconductor die, and the glob top material has a second height above the semiconductor die that is less than the first height. The glob top material does not fill the cavity in the mold compound. The mold compound does not cover a top surface of the glob top material.
The BAW device has as top surface and a bottom surface, the bottom surface attached to the top surface of the semiconductor die, the top surface having one or more bond pads, the one or more bond pads electrically connected to contact pads on the semiconductor die by bond wires.
Alternatively, the BAW device is flip-chip mounted on the semiconductor die. The semiconductor package further comprises bond pads on an active surface of the BAW device. The active surface of the BAW device faces the top surface of the semiconductor die. Contact pads are located on the top surface of the semiconductor die, and metal bumps, such as copper, electrically connecting the bond pads to the contact pads.
The glob top is a stress absorbing material that isolates the BAW device from stresses and vibrations in the electronic package. The glob top is a low elastic modulus material that is different from a material used for the mold compound.
In an example method for assembling a stacked-die package, process steps comprise performing a first die attach process that attaches a first side of a first semiconductor die to a die attach pad of a leadframe, performing a first wire bonding process that connects a first bond wire between a first conductive pad of the first semiconductor die to a lead of the leadframe, performing a molding process that encloses a portion of the die attach pad, a portion of the lead, first semiconductor die, and the first bond wire in a mold compound, creating a cavity in the mold compound, the cavity extending from a second side of the first semiconductor die to a top of the mold compound, performing a second die attach process that attaches a second semiconductor die to the second side of the first semiconductor die, and performing a deposition process that deposits a stress absorbing material in the cavity to cover the second semiconductor die. The cavity in the mold compound is created using laser ablation.
The method further comprises applying a laser stop metal to a portion of the second surface of the first semiconductor die before the molding process, and chemically removing an exposed segment of the laser stop metal after the cavity has been created. The cavity in the mold compound is created using laser ablation, and the laser stop metal protects the first semiconductor die from the laser ablation.
The second semiconductor die is a bulk acoustic wave (BAW) device.
The method further comprises performing a second die attach process that attaches a first side of BAW device to the second side of the first semiconductor die, and performing a second wire bonding process that connects a second bond wire between a bond pad on a second side of BAW device to a second conductive pad on the first semiconductor die. The stress absorbing material is deposited over the second bond wire.
The method further comprises flip chip bonding the BAW device to the second side of the first semiconductor die so that metal bump bonds electrically couple bond pads on a first side of BAW device to contact pads on the second side of the first semiconductor die. The first side of BAW device faces the second side of the first semiconductor die.
The method further comprises after performing the deposition process, performing a singulation process that separates stacked-die package devices from one another.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, wherein:
The present disclosure is described with reference to the attached figures. The figures are not drawn to scale, and they are provided merely to illustrate the disclosure. Several aspects of the disclosure are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the disclosure. The present disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present disclosure.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts, unless otherwise indicated. The figures are not necessarily drawn to scale. In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. In the following discussion and in the claims, the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are intended to be inclusive in a manner similar to the term “comprising,” and thus should be interpreted to mean “including, but not limited to . . . .” Also, the terms “coupled,” “couple,” and/or or “couples” is/are intended to include indirect or direct electrical or mechanical connection or combinations thereof. For example, if a first device couples to or is electrically coupled with a second device that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and/or connections. Elements that are electrically connected with intervening wires or other conductors are considered to be coupled. Terms such as “top,” “bottom,” “front,” “back,” “over,” “above,” “under,” “below,” and such, may be used in this disclosure. These terms should not be construed as limiting the position or orientation of a structure or element but should be used to provide spatial relationship between structures or elements.
The term “semiconductor die” is used herein. A semiconductor die refers to a thin slice of material, such as a crystalline silicon, that is used to fabricate integrated circuits. A large number of integrated circuits may be created on an active surface of the semiconductor wafer. Discrete semiconductor devices can be integrated circuits with hundreds or thousands of transistors coupled to form a functional circuit, for example a microprocessor or memory device. The semiconductor device may also be referred to herein as a semiconductor wafer or an integrated circuit (IC) die.
The term “semiconductor package” is used herein. A semiconductor package has at least one semiconductor die electrically coupled to terminals and has a package body that protects and covers the semiconductor die. In some arrangements, multiple semiconductor dies can be packaged together. For example, a power metal oxide semiconductor (MOS) field effect transistor (FET) semiconductor device and a second semiconductor device (such as a gate driver die, or a controller die) can be packaged together to from a single packaged electronic device. Additional components such as passive components, such as capacitors, resistors, and inductors or coils, can be included in the packaged electronic device. The semiconductor die is mounted with a package substrate that provides conductive leads. A portion of the conductive leads form the terminals for the packaged device. In wire bonded integrated circuit packages, bond wires couple conductive leads of a package substrate to bond pads on the semiconductor die. The semiconductor die can be mounted to the package substrate with a device side surface facing away from the substrate and a backside surface facing and mounted to a die pad of the package substrate. The semiconductor package can have a package body formed by a thermoset epoxy resin mold compound in a molding process, or by the use of epoxy, plastics, or resins that are liquid at room temperature and are subsequently cured. The package body may provide a hermetic package for the packaged device. The package body may be formed in a mold using an encapsulation process, however, a portion of the leads of the package substrate are not covered during encapsulation, these exposed lead portions form the terminals for the semiconductor package. The semiconductor package may also be referred to as a “integrated circuit package,” a “microelectronic device package,” or a “semiconductor device package.”
The term “bulk acoustic wave (BAW)” is used herein. BAW technology is designed to implement stable, secure, and high-performance communications infrastructure and connectivity. BAW is a micro-resonator technology enabling the integration of high-precision and ultra-low jitter clocks directly into packages that contain other circuits. This allows for cleaner wired and wireless signals to be delivered over networks by a crystal-less package. BAW-based oscillators feature reliable, high-accuracy timing and wake up much faster than quartz crystals. BAW technology can be used to improve network performance and increase immunity to vibration and shock in a wide range of applications. BAW resonator technology enables high-performance, highly accurate resonators. When integrated into a microcontroller package, BAW resonators eliminate the need for external quartz crystals without compromising power, latency or frequency stability. BAW technology is a vital component in advanced filtering solutions for mobile products, as well as the advanced radar, communications systems, and sensor applications.
The term “glob top” is used herein. In packaged integrated circuits, glob top refers to a stress absorbing material that encapsulates all or a portion of a semiconductor die, such as a BAW die. The glob top may encapsulate related wire bonds that electrically connects the semiconductor die to other components. Performance of some devices can be improved by isolating a die within a package from mechanical stress, shock, and/or vibration incident on the outer surfaces of the package. The glob top or stress absorbing material structurally isolates a die from external mechanical stress. The glob top material may be dispensed as a liquid or gel and is subsequently cured using UV, thermal, or time cure processes. The cure process selected depends on the type of glob top material selected. Glob top encapsulants are typically one- or two-part epoxies or UV curable compounds that are specially formulated to flow easily in response to stress applied during application but have rapidly increasing viscosity after placement. This change in viscosity allows the material to flow smoothly into narrow spaces without damaging semiconductor dies or bond wires while preventing the material from flowing beyond the target area. Once cured, glob tops protect die and wire bonds from contaminants and moisture while providing sufficient mechanical support to prevent damage during handling and assembly.
The example electronic device 100 of
The BAW resonator die 101 has a top surface 110 with one or more one or more bond pads 111 and a bottom surface 112 that is mounted on the top side 106 of second semiconductor die 102. The bond pads 111 on BAW resonator die 101 are connected to conductive bond pads 107a on the second semiconductor die 102 using bond wires 113. BAW resonator die 101 may provide a reference oscillator or clock signal to second semiconductor die 102. Although not shown, the BAW resonator die 101 can have more than one bond pad 111, such as to add a ground connection or other connections to second semiconductor die 102.
A mold compound 114 encapsulates some or all of the leadframe 104, second semiconductor die 102, and bond wires 109. Mold compound 114 may be a thermoset epoxy resin mold compound that is applied during a molding process or some other epoxy, plastic, or resin. Die attach pad 103 and leads 108 are exposed through the sidewalls of mold compound 114 for attachment to other devices, such as a printed circuit board (PCB). In other arrangements, leads 108 may have an exposed portion that extends beyond the sidewalls of mold compound 114.
A cavity 115 is formed within mold compound 114 during the manufacturing process. A stress absorbing structure 116 within cavity 115 encapsulates the BAW resonator die 101. The stress absorbing structure 116 functions as a glob top and forms a bubble-like structure over BAW resonator die 101. The stress absorbing structure 116 extends across a portion of the top surface 106 of second semiconductor die 106 and encapsulates all exposed surfaces of BAW resonator die 101, bond pad(s) 111, and bond wire(s) 113.
In one arrangement, the stress absorbing structure 116 is a flexible material that isolates all or a portion of the BAW resonator die 101 from external mechanical stress (e.g., shock and/or vibration) incident on the packaged electronic device 100. One suitable example material for the stress absorbing structure 116 is an epoxy material. In one example, the stress absorbing structure 116 includes a thermally cured epoxy material. In one example, the stress absorbing structure 116 is a low viscosity epoxy encapsulant, such as an epoxy laminate. In other arrangements, stress absorbing structure 116 is a low elastic modulus material, such as silicone rubber. The stress absorbing structure 116 helps isolate stress from the BAW resonator die 101. For example, stress from external forces can transfer stress to the BAW resonator die 101. The stress absorbing structure 116 helps to prevent coupling stresses from the second semiconductor die 102 or the mold compound 114 into the BAW resonator die 101.
In prior semiconductor packages having a BAW resonator die, a glob top of stress absorbing material covered the BAW resonator die, and the package mold compound 114 typically covered and encapsulated the entire glob top. In such designs, package stress (such as stress due to change in temperature) causes a frequency shift in the BAW device oscillator or microresonator, which is a critical item that needs to be controlled within certain limits. The mold compound encapsulating the package and situated above the BAW resonator die directly influences the package stress. The configuration disclosed herein, with a mold-free cavity design, avoids the package stress issues found in prior designs.
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Multiple devices may be manufactured at the same time on a leadframe sheet. After such processes are completed, the semiconductor wafer, leadframe, and mold compound are severed (“singulated” or “diced”) with a cutting tool, such as a saw or laser, into separate semiconductor packages. Each semiconductor package including a singulated leadframe, at least one IC die, electrical connections between the die and leadframe (e.g., a roughened conductive bump), and the mold compound which covers at least part of these structures.
In one arrangement, an electronic package, such as a semiconductor device or semiconductor package, comprises a segment of a leadframe having a die attach pad and one or more leads and a semiconductor die electrically connected to the die attach pad. A BAW device is attached to, and electrically connected to, the semiconductor die. A mold compound surrounds the first semiconductor die and covers portions of the leadframe and a first portion of a top surface of the semiconductor die. The mold compound has a cavity area wherein the mold compound has been removed from the cavity area or was never formed in the cavity area. The mold compound does not cover the BAW device or a second portion of the top surface of the semiconductor die within the cavity area. A glob top material is disposed within the cavity area (i.e., the cavity area is at least partially filled with the glob top material). The glob top material covers the BAW device and the second portion of the top of the semiconductor die (i.e., the glob top material covers areas that are not covered by the mold compound). The semiconductor die is electrically connected to the one or more leads by bond wires that are encapsulated by the mold compound. The glob top is a stress absorbing material that isolates the BAW device from stresses and vibrations in the electronic package. The glob top is a low elastic modulus material that is different from a material used for the mold compound.
The cavity area has a first height above the semiconductor die. The glob top material has a second height above the semiconductor die that is less than the first height (i.e., the glob top material does not protrude above the top surface of the mold compound). The glob top material does not fill the cavity area in the mold compound. The mold compound does not cover a top surface of the glob top material.
The BAW device has as top surface and a bottom surface. In one configuration, the bottom surface is attached to the top surface of the semiconductor die. The top surface has one or more bond pads. The one or more bond pads are electrically connected to contact pads on the semiconductor die by bond wires.
In another configuration, the BAW device is flip-chip mounted on the semiconductor die. The electronic package further comprises bond pads on an active surface of the BAW device. The active surface of the BAW device faces the top surface of the semiconductor die. Contact pads are on the top surface of the semiconductor die. Bumps electrically connect the bond pads to the contact pads.
A example method for assembling a stacked-die package comprises performing a first die attach process that attaches a first side of a first semiconductor die to a die attach pad of a leadframe, performing a first wire bonding process that connects a first bond wire between a first conductive pad of the first semiconductor die to a lead of the leadframe, and performing a molding process that encloses a portion of the die attach pad, a portion of the lead, first semiconductor die, and the first bond wire in a mold compound. The method further comprises creating a cavity in the mold compound, the cavity extending from a second side of the first semiconductor die to a top of the mold compound, performing a second die attach process that attaches a second semiconductor die to the second side of the first semiconductor die; and performing a deposition process that deposits a stress absorbing material in the cavity to cover the second semiconductor die. The second semiconductor die is a BAW device. After performing the deposition process, a singulation process is performed to separate stacked-die package devices from one another.
The cavity in the mold compound may be created using laser ablation. The method further comprises applying a laser stop metal to a portion of the second surface of the first semiconductor die before the molding process and chemically removing an exposed segment of the laser stop metal after the cavity has been created. The laser stop metal protects the first semiconductor die from the laser ablation.
In one arrangement, the method further comprises performing a second die attach process that attaches a first side of BAW device to the second side of the first semiconductor die and performing a second wire bonding process that connects a second bond wire between a bond pad on a second side of BAW device to a second conductive pad on the first semiconductor die. The stress absorbing material is deposited over the second bond wire.
In another arrangement, the method further comprises flip chip bonding the BAW device to the second side of the first semiconductor die so that metal bump bonds electrically couple bond pads on a first side of BAW device to contact pads on the second side of the first semiconductor die, wherein the first side of BAW device faces the second side of the first semiconductor die.
While various examples of the present disclosure have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed examples can be made in accordance with the disclosure herein without departing from the spirit or scope of the disclosure. Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. Thus, the breadth and scope of the present invention should not be limited by any of the examples described above. Rather, the scope of the disclosure should be defined in accordance with the following claims and their equivalents.
