Sponsor:Packaging Research Center
Personnel: Jae Park and Mark G. Allen
Micromachining techniques are used to realize inductors and transformers integrated with a multichip package, allowing compact integration with chips, sensors, and other components. The processing steps chosen are all low-temperature, which allows the use of low cost substrates such as MCM-L compatible materials. A variety of micromachined inductors and transformers with different geometries and magnetic core materials are designed, fabricated, tested, and compared. Integrated permalloy and orthonol core inductors (15 µm thick) with nominally identical geometries of 4 mm x 1.0 mm x 0.13 mm and 30 turns of multilevel copper coils (40 mm thick) show differences in performance due to differences in core behavior. The permalloy core inductor has a slightly higher inductance, but it has much lower DC saturation current than the orthonol core inductor. The effect of insertion of a core air gap was also studied. Although inductors with no air gap having dimensions of 4 mm x 4 mm x 0.145 mm and 156 turns of multilevel electroplated copper coils (40 mm thick) and electroplated permalloy magnetic core (35mm thick) have slightly higher inductance (about 1.5 mH), air gap inductors have much higher saturation current (I80=250mA). These devices have high current capability (up to 3A steady DC current) and are suitable for low power converter applications.
Micromachined magnetic devices which have low resistance and high values of inductance, Q-factor, coupling factor, and saturation current are useful in many applications such as miniaturized sensors, actuators, filters, and switched power converters integrated with multichip modules or electronic systems. In particular, the use of these devices is necessary in integrated miniaturized DC/DC converters used as power supplies in communications, military/aerospace applications, and computer/peripheral or other portable devices. Requirements on power converters of high efficiency, small size, low weight, and lower cost will require that switching frequencies increase. Miniaturized DC/DC converters using micromachined inductors and transformers have many potential advantages such as high frequency operation, efficiency, quality, low cost, and low power loss. At high switching frequencies, miniaturized surface-mount magnetic components may be able to be replaced by fully integrated magnetic devices.
Desirable characteristics of magnetic cores for integrated inductors and transformers can be summarized as follows: first, high saturation flux in order to obtain high saturation current; second, high permeability to obtain high inductance; third, high resistivity to reduce eddy current loss at high frequencies. In addition, micromachined magnetic devices should be designed to have a completely closed magnetic circuit to minimize leakage flux, since leakage flux does not contribute to the total inductance of the devices and can cause interference with other integrated circuitry on the same substrate. Magnetic properties of electroplated cores for such devices may be very different from magnetic properties of the bulk magnetic materials, thus necessitating an in-situ property assessment. Our approach is to fabricate these required inductive components using low-temperature micromachining techniques in order to enable low-cost, fully integrated versions of these power converter devices.
Personnel: Jae Park, and Mark G. Allen
Although many integrated inductors have been realized using integrated circuit and electronic packaging batch fabrication techniques, their magnetic characteristics are inferior to their discrete counterparts, in part due to the relatively poor magnetic properties of integrated magnetic cores. If the permeability of an integrated magnetic core can be increased, the magnetic characteristics of microinductors based on these cores will improve. To address this issue, batch fabricated, integrated magnetic devices incorporating electroplated magnetically anisotropic cores and electroplated copper coils are investigated. These devices are realized using micromachining and electroplating techniques at low temperature. Three different geometries of inductors, each possessing two different core materials, permalloy (NiFe) and supermalloy (NiFeMo), are presented. The cores have been rendered magnetically anisotropic by application of a magnetic field during electrodeposition, resulting in an easy and hard axis orientation. In addition, some cores consist of a two-layer electrodeposit separated by a polyimide thin film lamination. At low frequencies (less than several hundred kHz), the easy axis devices have higher inductance than the hard axis devices. However, the hard axis devices have better performance at higher frequencies due to a far less steep falloff of material permeability as a function of frequency in the hard axis direction.
Sponsor: Packaging Research Center
Personnel: Jae Park and Mark G. Allen
There are a large number of passive components used in consumer electronic products such as VCRs, camcorders, television tuners, and other communication devices. An approach to efficiently realize large interconnected passives networks for these applications is to integrate them directly into a multichip module-laminated (MCM-L) substrate; however, such an approach requires low temperature processes. To realize inductors and transformers compatible with these integrated, low temperature processes, polymer filled magnetically soft ferrites (NiZn and MnZn), in which fine ferrite particles are added to a polyimide matrix to form a composite, are investigated. The electrical resistivity of the fabricated NiZn and MnZn ferrite composites are approximately 1 Mohm-cm and 0.01 Mohm-cm respectively. MnZn ferrite composite has higher saturation flux density (Bs=0.43 T) and initial permeability (µi = 33) than NiZn ferrite composite (Bs = 0.28 T, µi = 25). Using these magnetic composite materials and screen-printing techniques, four different integrated inductor and transformer geometries are fabricated. A two layer vertically stacked spiral microinductor with 50 µm height has a specific inductance of 15 µH/cm2 and quality factor of 15 at 10 MHz, and dc resistance of 2.1ohms. Sandwich type spiral microinductors have specific inductance of 6.5 µH/cm2, quality factor of 17 at 10 MHz, and dc resistance of 1.3 ohms. Sandwich type spiral microtransformers have the best gain characteristics of the four geometries investigated (1.5dB loss for a nominal 1:1 turns ratio device at 25 MHz). The magnetic shielding performance of the ferrite composite material is also investigated. Spiral coils shielded by screen-printed ferrite composite had stray magnetic field emissions reduced by up to 75% when compared to their unshielded counterparts of identical geometry.
Personnel: Jae Park and Mark G. Allen
To meet requirements in mobile communication and microwave integrated circuits, miniaturization of the inductive components that many of these systems require is of key importance. At present, active circuitry is used which simulates inductor performance and which has high Q-factor and inductance; however, such circuitry has higher power consumption and higher potential for noise injection than passive inductive components. An alternate approach is to fabricate integrated inductors, in which lithographic techniques are used to pattern an inductor directly on a substrate or a chip. However, in general, the integrated inductor suffers from low Q-factor and high parasitic effects due to substrate proximity. To expand the applications of integrated inductors, these characteristics must be improved. Packaging-compatible integrated spiral inductors are investigated using polymer/metal multilayer processing techniques and surface micromachining techniques. These inductors have a spiral geometry with an air core and a large air gap (40µm height) between the coils and the substrate (to reduce substrate capacitance), and thick, highly conductive electroplated copper conductor lines (to increase the quality factor). Various inductor geometries are investigated by designing and fabricating several inductors with differing core areas and numbers of turns. The fabricated inductors have a Q-factor of 50~150 at 600~850 MHz and an inductance at these frequencies between 20~70nH. These microinductors have also been integrated with MIM (Metal-Insulator-Metal) capacitors to form integrated passive low-pass filters.
Personnel: Kieun Kim and Mark G. Allen
I am currently working on an integrated, batch fabricated, magnetically-actuated microrelay. These devices, which are smaller than a dime, have already set records for their low contact resistance and ability to switch large current loads. Contact resistance of less than 100 milliohms and switching currents of up to 1.2 amperes have been achieved. Georgia Tech Microrelays have been tested through more than 850,000 operating cycles without failure. The significant issue in using a magnetically-actuated relay is that larger forces can be achieved in the actuation and a greater air gap between contacts is possible as compared to electrostatic relays, thus allowing for a greater off-state resistance. The larger gap holds off a higher voltage, which allows for the switching of higher voltage signals than would be permitted with other types of microrelays. Our microrelays operate at five volts, which would allow them to be driven by digital logic circuits and used as part of equipment for which higher voltages could be undesirable. Due to the batch fabrication techniques, they can be integrated onto circuit boards by using standard microelectronic processing. This also allows for a lower cost per relay and economies of scale. The microrelays range in size from less than three millimeters by four millimeters up to seven millimeters by eight millimeters, and are less than 200 microns in height. The microrelay fabrication is based on standard polyimide mold electroplating techniques and consists of an integrated planar meander coil and one or more pairs of relay contacts positioned above the coil. A movable magnetic plate, made of a magnetic nickel-iron material, is surface micromachined above the contacts. When current is applied to the coil, the magnetic flux generated pulls the nickel-iron plate down until it touches the contacts, closing the circuit. The present design uses a "normally-open," configuration, meaning that a current is required to close the switch. Relays using a "normally closed" configuration, requiring a current to open the switch, and relays that open or close more than one set of contacts at a time are the focus of the current investigation. The relay designed to operate in a normally-closed position works by using a permanent magnet to hold the actuating plate down to keep the contacts closed. When current is sent through the relay's coil, the plate moves up off of the contacts, opening the circuit. This will require the incorporation of permanent magnetic materials in a way compatible with microfabrication. Several permanent magnetic materials, including Neodymium-boron-iron and Samarium cobalt are being investigated.
