Embodiments of the invention relate in general to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of semiconductor integrated circuit devices, which integrate the inductors of the circuits into the package of the devices.
Inductors are essential elements for RF design. Based on planar spiral inductor models first published in 1996 and on the high level of semiconductor technology and device production, planar spiral inductors and planar solenoidal inductors of a wide variety of thin-film single-layered and double-layered designs are available in electronic products with integrated circuits (IC) for RF application. The inductors of these semiconductor products realize the needed inductances by silicon on-chip thin-film spiral and solenoidal designs incorporated into the two-dimensional layout of ICs. Since the inductance of an inductor is proportional to the magnetic permeability of the material inside the inductor, the relatively small inductances sufficient for the RF devices can be generated while accepting the low permeabilities of air and insulators.
On the other hand, when products require higher inductances and have to employ ferromagnetic materials containing iron because of the about 1000 times higher magnetic permeability of iron, the needed inductors are created by piece-part components assembled on the IC surface, thus towering into the third dimension over the two-dimensional IC. As an example, electronic products such as laptop computers, hand-held telephones and notebooks require different electrical supply voltages in order to operate the various component parts (such as integrated circuits, monitors, displays, speakers, clocks, etc.) within their most effective regimes. In addition, these voltages have to be available at reliably constant levels in order to guarantee uniform and trouble-free operation of the component parts. The plurality of voltages and the constant voltage levels are provided and controlled by so-called DC-DC power supply devices.
A typical DC-DC power supply circuit, as it is used in many laptop computers with liquid crystal displays, may have a battery whose voltage is subject to some variation due to usage and ambient temperature. To stabilize the battery voltage output and modulate it for the different voltages required by the various computer components, the circuit includes two lateral field effect transistors (FETs), which are coupled in series with a common terminal; the drain of the first transistor receives the input voltage from a battery, the source of the second transistor is connected to ground potential. The gates of the transistors are operated and coordinated by a driver circuit, which in turn is regulated by a control circuit. The common terminal is connected to an inductor, which stores the energy of the device in the magnetic field inside its solenoid; the magnetic field, in turn, needs high permeability in the inductor to reach high field values. The inductor provides the desired output voltage at the required constant level.
As an electronic device part in a laptop computer, notebook, etc., today's exemplary DC-DC-power supply is built on a rectangular printed circuit board of approximately 16 by 19 mm side length. The plastic packaged ICs, transistors, etc. are physically small (in the millimeter regime) and are soldered on the board, giving the board a slim, essentially two-dimensional appearance (of less than 3 mm height). However, sticking out into the third dimension is the separate piece part of the inductor with an area requirement of 5 by 5 mm and an additional height of 5 mm for the solenoid filled with an iron core (total height about 8 mm).
Applicant recognized that the market trends in electronic products such as laptop computers, notebooks, smartphones and the like demand products, which are thin, light weight, and low cost. As a consequence, he saw that product parts such as DC-DC power supplies with bulky and relatively costly three-dimensional inductors for achieving high magnetic field energy levels need to be modified to achieve slim contours, lower weight, and lower cost.
Applicant found that for creating a volume inside a solenoid sufficient to place an iron core for achieving high magnetic field energy levels, the conventionally employed circular shape of the windings is not essential. Instead, applicant discovered that the cross section of a solenoid winding can be split into a linear portion incorporated into the flat chip surface with the two-dimensional IC, and a three-dimensionally curved portion incorporated into the package encapsulating the chip.
Applicant solved the problem of fabricating a solenoid combining an iron core with the thin contour of a semiconductor device, when he detected that the iron core can be integrated into the standard device backend assembly flow, and that the three-dimensionally curved portions of the solenoid windings can be fabricated by the low cost process of arching wires as in the standard wire bonding technology. The arched wires of the solenoid windings are then integrated into the device package needed anyway to protect the wire bond connections of the chip.
In the process flow of the invention, each solenoid winding is constructed of a layer portion and a wire portion. The layer portion is realized as an elongated trace of stripe-shaped metal thin film; in preferred embodiments, the stripe lays flat on the insulation over the chip surface. The layer may be created by depositing metal using techniques such as plating, silk screening, sputtering, evaporation, and chemical vapor deposition, followed by a stripe-patterning step as needed. Each stripe has a first end and a second end; consecutive stripes are preferably arrayed parallel to each other, spaced apart by a respective insulating gap. After covering the center portions of the stripes with an insulating film, an iron sheet is deposited by standard semiconductor fabrication steps on the insulating film extending across the parallel stripes and gaps. The wire portion is realized as a wire or ribbon arch spanning from one stripe to the next.
The wires are welded to the stripes and connect the stripes so that the first wire, welded to the first end of the first stripe, spans an arch over the stripe center portion and the adjoining gap to the second end of the adjacent second stripe; the second wire, attached to the first end of the second stripe, spans an arch over the stripe center portion and the adjoining gap to the second end of the adjacent third stripe; and so on to the last stripe. Consecutive wires are preferably arrayed parallel to each other. Thus, a solenoid with orderly windings for a constant clock direction emerges. The welding can be achieved by low-cost techniques such as ball bonding, stitch bonding, and pressure bonding. The solenoid is connected to external parts by spanning a wire from the second end of the first stripe to a package contact pad, and another wire from the first end of the last stripe to another package contact pad, creating an inductor. For protection, all wires may be embedded in the packaging compound encapsulating the chip surface employed for package robustness.
Some embodiments may have two elongated solenoids arranged in parallel and serially connected so that the electrical current continues in the same clock direction in both solenoids. With this arrangement, the iron core of the first solenoid may continue, after a U-turn, as the core of the second solenoid, enhancing the inductor performance. In other embodiments, the parallel solenoid arrangement has an iron core closed as a ring by adding another U-turn to the iron portions through the individual solenoids, resulting in a specially powerful inductor and energy storage device.
It is a technical advantage of the invention that the fabrication of the inductors need only low cost common package manufacturing processes and equipment. It is another technical advantage that the inductors of the invention can be integrated into the packages as part of the slim device dimensions.
As an example, a packaged 48-pin TSSOP embodiment has its inductor with 15 windings of 150 μm arch height and an iron plate of 2.75 mm by 0.5 mm by 0.05 mm integrated into its slim package of outline 12.5 mm by 6.1 mm by 1.2 mm. The height of only 1.2 mm represents a reduction of more than 80% compared to the conventional height of about 8 mm.
The exemplary embodiment of the invention illustrated in
Laying flat on the dielectric layer 102, and adhering to it, is a plurality of deposited metal stripes 120. The stripes are arrayed parallel to each other. Preferably, the stripes have the same length 123 and the same width 121. Each stripe has a first end 124 and a second end 125; preferably, the first ends 124 of the stripe plurality are linearly arrayed, and the second ends 125 are linearly arrayed. The metal of the stripes is exposed at each first and second end. The center portion of the stripes, however, is covered by an insulating film 126 laying over the stripes, which is patterned to leave the first (124) and second (125) ends of the strips un-covered. Film 126 may be made of polyimide, silicon dioxide, or any other suitable insulator. Adjacent stripes are spaced from each other by gaps 122, which are controlled by dielectric material (dielectric layer 102 and insulating film 126). As a consequence, adjacent stripes are electrically isolated from each other. Gaps 122 have preferably the same width for all gaps. The stripes are made of a first metal selected for high electrical conductivity. Preferably, the first metal is copper and the stripe a layer of copper in the thickness range from about 10 to 75 μm, covered by a bondable metal film (such as gold or silver); alternatively, the first metal may be silver or a metal alloy.
The metal of the stripes 120 is deposited on the dielectric layer 102 as a film or layer. The deposition methods include plating, sputtering, evaporating, silk screening, and chemical vapor deposition. If the layer is deposited as a large-area layer, follow-up photoresist and etching steps are required for patterning the layer into the plurality of stripes 120.
As shown in
As an example in
For a very elongated solenoid made of a number of windings, the energy of the magnetic field inside the solenoid is proportional to the inductance of the solenoid and the square of the current through the windings. The inductance, in turn, is proportional to the square of the number of windings, to the cross section of the solenoid, and to the permeability inside the solenoid, and inverse proportional to the length of the solenoid. The dominant factors determining a high energy of the magnetic field inside a solenoid is the high permeability achievable by a ferromagnetic material core, the number of windings, and the amount of current through the windings. While the quoted dependencies are qualitatively valid for short linear solenoids, they are quantitatively valid for circular solenoids; consequently, circular solenoids with iron cores (see
Ferromagnetic material may also be called ferroelectric material.
Ferroelectricity has also been shown to exist in soft biological tissues such as fat. For example, collagen's building block—the amino acid glycine—is ferroelectric when its molecules are arranged in a crystalline lattice. Proteins such as microtubules have been reported to be ferroelectric. Therefore, the term ferromagnetic material in this disclosure may also include biological material which exhibit ferroelectric properties.
As
Exemplary device 100 exhibits a package 160, preferably formed by a polymeric encapsulation compound, such as an epoxy-based molding compound, embedding all bonding wires. Device package 160 has the slim contours of a standard semiconductor device, such as the length, width, and height of a Quad Flat No-Lead (QFN) or Small Outline No-Lead (SON) device. Since the embedding in the device encapsulation also includes the solenoid wires 140, the solenoid of the invention is fully integrated into the device package and its slim contours; since it includes a ferromagnetic core, it achieves high magnetic field energies. The solenoid thus no longer needs the bulky and costly three-dimensional shape of the inductors in customary DC-DC power supplies.
As an example, a packaged 48-pin TSSOP embodiment has its inductor with 15 windings of 150 μm arch height and an iron plate of 2.75 mm by 0.5 mm and 50 μm height integrated into its slim package of outline 12.5 mm by 6.1 mm by 1.2 mm. The height of only 1.2 mm represents a reduction of more than 80% compared to the conventional height of about 8 mm (of which less than 3 mm are for the body and about 5 mm for the discrete solenoid component).
The top view of the exemplary device 100 in
While the insulating film covering the center portions of the stripes is not shown in
The stripes serve as the two-dimensional portions of the forth-coming solenoid windings. Since the magnetic performance of the solenoid is proportional to the square of the windings number, it is advantageous to pack as many stripes as possible into the given length of the dielectric-covered chip surface, whereby the aspect ratio between stripe height and gap width needs to be taken into account. Consequently, the stripes are preferably positioned in an orderly sequence, oriented in parallel and with equal gaps between adjacent stripes, and have the same length with their endpoints arrayed linearly. Since the magnetic performance of the solenoid is proportional the square of the electric current through the stripe, it is advantageous to minimize the resistance per square of the stripe and to make the electrical resistance of the stripe equal to the resistance of the equally long wire-to-be-employed.
In the next process step, also indicated in
In the next process step, shown in
The other half of the solenoid is formed by the flat stripes, resulting in a solenoid for an electric current in continuous clock direction. The cross section of the solenoid windings is split into the stripe-portion on the two-dimensional chip surface, and the three-dimensionally curved portion of the wire spans, which preferably is incorporated into the package encapsulating the chip. Inside the solenoid is the iron core 130.
As an alternative process step, the sequence of wire arches is pre-fabricated as a half coil and placed on the stripe ends in one piece. Inside the half coil is the iron core of the solenoid.
In the next process step, the solenoid is connected to an external power supply. The result is shown in
After the step of connecting the chip terminals 103 to the contact pads 111 of the substrate (in
The embodiment of
The high inductances of the embodiments of
While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the invention applies to devices with any type of substrate including to leadframe-based devices. Other substrates include multi-layer insulating polymeric or ceramic substrates, or other boards used in semiconductor devices. As another example, the invention not only applies to QFN/SON type devices, but to any type of semiconductor devices.
As another example, the method can be extended to incorporate any volume into the package of semiconductor devices when the volume can be broken up into a substantially two-dimensional portion, which can be integrated into the chip-based elements, and a substantially three-dimensional portion, which can be integrated into the package-based elements. The invention is thus applicable to certain MEMS-type devices.
As another example, while the method of easily integrating an iron core into the standard semiconductor assembly flow is preferred for magnetic-based devices, it can applied to other devices in need of other cores or crystals.
It is therefore intended that the appended claims encompass any such modifications or embodiments.