Thomas F. Marinis

Ultrahigh‐Energy‐Density Microbatteries Enabled by New Electrode Architecture and Micropackaging Design

Progress in microfabrication technology has enabled increasingly compact autonomous microsystems for applications ranging from distributed sensing and communications networks to implantable medical devices. Yet, power sources to enable their widespread adoption have not advanced nearly as rapidly. While batteries remain the most practical form of portable power given their simplicity and reliability, the energy density of commercial technologies decreases rapidly with size, reaching impractically low values well before volumes of interest to microtechnology, for example, <0.1 cm, are reached. This trend corresponds to a rapidly diminishing volume fraction of active materials with decreasing cell size due to internal inactive components and external packaging. Consequently, microsystems can be dwarfed by the battery attached. These wellrecognized limitations have spurred research into alternatives for storing or generating power in small volumes, including miniaturized fuel cells and combustion turbines. Several microbattery technologies have also emerged, with thin-film solid-state batteries being the most developed. While thin-film, solid-state batteries deliver excellent capacity retention over thousands of cycles, they possess very low volumetric energy densities. Electrode thicknesses limited to several micrometers by room-temperature values of the lithium diffusion coefficient result in a 2D geometry dominated by the substrate and other inactive cell components, thus, the performance of thin-film batteries is typically characterized in terms of energy or power per area, rather than per volume or mass. Although areal metrics are appropriate for some uses, many developing applications (e.g., wireless transmission) require small batteries with high volumetric energy and power in nonplanar form factors. Progress in this direction includes reduction of the footprint of thin-film batteries through 3D architectures enabled by advanced microfabrication. However, to our knowledge, fully packaged batteries with demonstrated energy densities exceeding 100WhL 1 in packaged volumes <100mm have not been reported. Microbattery technologies remain undermatched in form factor, performance, and manufacturability to most of the devices they are meant to power. Here, we demonstrate lithium rechargeable microbatteries at the cubic millimeter scale providing energy densities normally achieved only in batteries more than 100 times larger in volume. Two specific materials innovations enable this achievement. The first originates from the surprising discovery that densely sintered electrodes fabricated from brittle intercalation oxides can be repeatedly cycled to full capacity utilization, a result that is wholly unexpected given the well-known cycling-induced fracture (‘‘electrochemical grinding’’) that occurs in intercalation compounds electrodes due to the Vegard’s stresses. This discovery allows us to develop thick, sintered 3D cathodes with areal active materials loadings 4–10 times larger than in conventional lithium ion battery electrodes, yet the electrodes can still be cycled at practical rates. The second innovation is an electroformed packaging approach, which further maximizes active material volume and yields cells with a desirable low-aspect-ratio form factor (e.g., 3mm 3mm 0.7mm) similar to existing miniaturized surface-mount components. Using LiCoO2/Li cell chemistry, we produced microbatteries of 6mm volume with unprecedented energy densities for batteries of this size range. For a charge voltage of 4.6 V, these cells deliver up to 675WhL 1 energy density at C/3 discharge rates (150–200WL 1 power density). At this charge voltage, the cycle life of the oxide is limited, but even comparing to existing primary Li-metal batteries, energy densities are achieved that previously were available only at cell volumes more than 100 times larger (>600mm). Using a lower charge voltage of 4.5 V, more stable cycling of the cells was obtained, while producing energy densities of 400WhL 1 that were previously available in existing rechargeable Li cells only at >1000mm volume. Some measured metrics for our microbatteries, all delivered at C/3 or higher rates, are included in the Supporting Information, Table S1. The theoretical storage energy density of an electrochemical couple is obtained by integrating the thermodynamic voltage capacity curve for capacity-matched electrodes and dividing by the maximum volume that the couple reaches during cycling. Intrinsic values for existing lithium-based systems are universally high (>1000WhL 1 for LiCoO2/graphite, LiMn2O4/graphite, and LiFePO4/graphite, and >2000WhL 1 for LiCoO2/Li and MnO2/Li). [13] It is the amount of inactive components, exceeding two-thirds the total volume and half the total weight even in ‘‘large’’ lithium ion cells, that results in poor historical energy densities. These limitations arise from compromises in electrode

Wafer level vacuum packaging of MEMS sensors

A process has been developed for wafer level vacuum packaging MEMS sensors, which are fabricated from etched, single crystal silicon structures, anodically bonded to metallized glass wafers. Key objectives of the process design were to minimize the number of changes to sensor fabrication, insure a high level of vacuum integrity, and flexible enough to accommodate a wide range of sensor designs. Only a single change to the standard sensor fabrication is required to implement the vacuum sealing process. A seal ring of gold, 250 microns wide by 1 micron thick is applied around the perimeter of the sensor and its electrical contact pads. The key features of this vacuum sealing technology are incorporated in the silicon cap wafer. It is 200 microns thick and contains an array of cavities, 50 microns deep, which align with the MEMS devices on the glass wafer. The opposite side of the wafer is coated with 2000 angstroms of silicon dioxide and is arrayed with aluminum bond pads, which align with those on the sensor wafer. These pads are connected to the sensor by through wafer vias, which are coated with a layer of parylene, one micron thick. The parylene is applied in a vapor deposition process, and then an excimer laser is used to ablate it from the bottom of the vias to allow electrical connections to be made to the aluminum bond pads. The vias are metallized with an adhesion layer of 500 angstroms of titanium and a conduction layer of 2000 angstroms of gold. This metal is photo-patterned, to produce pads that align with those of the sensor, and then all exposed parylene is removed by reactive ion etching. This cap wafer is bonded to the sensor wafer in an ultra-high vacuum system with a base pressure of 10/sup -8/ Torr. The two wafers are held on electrostatic chucks, one of which is hinged, so that in the degas phase, both wafers can be cleaned in-situ with an ion gun. For bonding, the hinge is actuated to position the cap wafer above the sensor wafer. A pair of prisms is positioned between the wafers to allow them to be precisely aligned prior to sealing. The wafers are bonded together by heating them to 300 /spl deg/C and actuating a pair of ball screws, which clamps them together under a load of 500 Newtons. The load and temperature is maintained for one hour to allow the gold of the sensor seal ring to react with the silicon of the cap wafer. The bonded pair is slowly cooled under load to complete the sealing process. The ultimate goal of this sealing approach is to use the control ASIC chip that is paired with the sensor, as the cap structure. This would minimize the length of signal paths between the ASIC and sensor, while realizing a very compact vacuum package.

Isolation of MEMS devices from package stresses by use of compliant metal interposers

Many classes of MEMS devices, such as those with resonant structures, capacitive readouts, and diaphragm elements, are sensitive to stresses that are exerted by their surrounding package structure. Such stresses can arise as a result of changes in temperature, ambient pressure, or relative humidity. We have demonstrated a dramatic reduction in scale factor bias over temperature for a tuning fork gyroscope, by mounting it on an interposer structure within a conventional ceramic chip carrier. Holographic interferometry measurements confirmed that the deformation imposed on a sensor die directly brazed to the package was more than 5 times that of die mounted with an interposer. We have developed several configurations of metal interposer structures for mounting MEMS inertial sensors in standard ceramic chip carrier packages. The interposers are made by first precision chemically etching preforms in metal foil. These preforms are then electroplated with a wire bondable surface finish of gold over nickel. Next, they are excised from the multi-up foil panel and formed to hold the sensor within the package. The interposers are configured with either three or four tabs for holding the MEMS sensors. Gold bumps are applied to these tabs and then the sensors are attached with thermocompression bonding. This assembly is attached to the ceramic package by thermocompression bonding to gold bumps on lands of the wirebond shelf. Wirebonds are made to the I/O pads of the sensor to complete its installation

Vacuum sealed MEMS package with an optical window

A vacuum sealed package with an optical window is a useful diagnostic tool for MEMS devices as well as a critical component of optical devices, such as imaging bolometers, scanning mirrors and variable wavelength filters. In either of these applications, the package must meet a number of stringent requirements. It cannot contaminate devices by either outgassing or shedding particulates. The window must be optically flat to allow devices to be observed or measured with interferometric tools, when the package is used as a diagnostic tool. When it serves as an integral part of an optical MEMS device, the window must also have the requisite transmissibility over the devices operating wavelength range. The vacuum level in many applications can also be quite challenging to achieve. Typically, pressures less than a few millitorr are necessary to prevent gas damping from limiting attainable Q values. Packages utilized for diagnostic purposes are often subjected to harsh environmental testing to evaluate how MEMS devices respond to mechanical shock, vibration, or thermal shock. Consequently, package robustness, particularly the glass to package seal integrity, is an important design element. We have successfully used a sputtered composite structure of gold over platinum over titanium to fabricate a seal ring on the window. The window is attached to a leadless ceramic chip carrier package by soldering with a 50 microns thick eutectic gold-tin preform. The sealing process is to load package assemblies, preforms and windows into a high vacuum system, degas them, raise the temperature of all components to 325degC, bring them into contact, and cool. We have used finite element analysis to optimize the seal geometry as a function of CTE mismatch, solder material, and window material to meet environmental requirements and optical flatness specifications. We have validated these FEM calculations by subjecting sealed packages to mechanical shock and helium leak testing. The optical flatness of windows was evaluated by direct optical interferometry measurements and high resolution measurements on sealed MEMS devices. The gas permeability of sealed packages was evaluated by measuring the Q of resonant devices over a period of several months. This fundamental understanding of window design, validated by experimental testing, extends our MEMS packaging capability to support the needs of both diagnostic investigations and optical device packaging.

Package architecture and component design for an implanted neural stimulator with closed loop control

An implanted neural stimulator with closed loop control requires electrodes for stimulation pulses and recording neuron activity. Our system features arrays of 64 electrodes. Each electrode can be addressed through a cross bar switch, to enable it to be used for stimulation or recording. This electrode switch, a bank of low noise amplifiers with an integrated analog to digital converter, power conditioning electronics, and a communications and control gate array are co-located with the electrode array in a 14 millimeter diameter satellite package that is designed to be flush mounted in a skull burr hole. Our system features five satellite packages connected to a central hub processor-controller via ten conductor cables that terminate in a custom designed, miniaturized connector. The connector incorporates features of high reliability, military grade devices and utilizes three distinct seals to isolate the contacts from fluid permeation. The hub system is comprised of a connector header, hermetic electronics package, and rechargeable battery pack, which are mounted on and electrically interconnected by a flexible circuit board. The assembly is over molded with a compliant silicone rubber. The electronics package contains two antennas, a large coil, used for recharging the battery and a high bandwidth antenna that is used to download data and update software. The package is assembled from two machined alumina pieces, a flat base with brazed in, electrical feed through pins and a rectangular cover with rounded corners. Titanium seal rings are brazed onto these two pieces so that they can be sealed by laser welding. A third system antenna is incorporated in the flexible circuit board. It is used to communicate with an externally worn control package, which monitors the health of the system and allows both the user and clinician to control or modify various system function parameters.

Design and characterization of wirebonds for use in high shock environments

MEMS inertial sensors, packaged in hermetic chip carriers, utilize free standing wirebonds to connect to the package I/O pads. Considerable care is taken in design of these wirebonds to balance impedances and minimize cross talk between excitation and readout channels. Many applications of MEMS inertial sensors require that they survive or operate in high acceleration or vibration environments. Any displacement of the wirebonds in these environments could adversely affect sensor bias and scale factor, or in extreme cases, cause sensor failure. Newer generations of high performance, navigation grade, inertial sensors are considerably larger than their predecessors and nearly fill the internal cavity of the chip carrier. Consequently, wirebond geometries can be highly constrained within these packages. Validating the wirebond design in an inertial sensor package, which is subject to a high-G environment, requires extensive testing on appropriate rail gun and shock table equipment. These tests are costly and equipment availability may be limited. It is also difficult to assess the sensitivity to variations in bond geometry or various bond defects using only physical testing. We are developing parameterized finite element models of wire bonds for use as tools to aid in design of sensor packages, and to guide the implementation of quality monitoring test and inspection requirements. Validation of the models is being done by subjecting well characterized wirebond configurations to air gun and drop table shock loads as well as conventional wire pull tests. We are also using a sensitive force gauge to measure the load required to displace a wire normal to the plane of its loop. Analytical expressions have also been developed for simple configurations, which serve as a check on both the finite element model and the experimental measurements.

Gold bump attachment-of MEM sensor die using thermocompression bonding

The conventional method of anaching MEM inertial sensors in ceramic cltip carriers is to braze them to the floor of the package using gold-tin braze of eutectic composition. The attachment leaves the assembly in a state of high residual stress- which raises concerns about its mechanical stability: patlicularly for high performance sensor applications. Themxil compression bonding die to gold bumps willtin chip carrier packages avoids tlie shortcomings of braze attach However. the concentrated loads applied to the sensor die through the gold bumps can resnlt in bond failurc when the assembly is subjected to mechanical or thennal induced loads. Care must be taken in designing gold blimp bonded assemblies for the anticipated operational enviroruncnt. Tlus is particularly true when tlie bumps aTe used to make electrical connections. Titis paper examines the mcchanical performance of gold bump bonded MEM sensor asseniblics that arc subjected to thermal and inertial loads. Meclanical pull tests. liquid to liquid themxil shocks and high-G nil gun tests are used to obtain performance data.

Novel low temperature hermetic sealing of micropackages

Traditional methods of hermetic packaging are not easily scaled to the task of sealing micropackages, i.e. packages with volumes on the order of one cubic millimeter. Micropackages are fabricated from thin metal shells that are sealed with ceramic or metal covers. They are used to protect MEMS devices, chemical sensors, batteries, and microfluidic components, which all have limited thermal processing tolerance, e.g. less than 150°C. Glass seals are difficult to pattern at this scale and require sealing temperatures in excess of 300°C. Mechanical tolerance issues, fixturing, and electrode design constraints preclude seam sealing. The small size of micropackages makes it difficult to control and remove the heat generated during laser welding. Low temperature soldering is an option for sealing micropackages, but there is a risk of contamination with flux residues and solder wettable metallizations must be provided on both the package and cover. We have successfully sealed ceramic covers to anodized aluminum packages, with internal volumes of one cubic millimeter. The hermetic seal is made by compression of a ring of indium foil between the cover and a flange on the package. The compressive stress on the indium is maintained by an epoxy bond around the perimeter of the package. The seal is made by loading the package into an alignment fixture, placing preforms of indium and epoxy on the package flange, aligning a cover on the package, placing a weight on the cover, and curing the epoxy at 140°C on a hot plate. The process is readily scalable to sealing arrays of packages and covers. It also does not require metalized seal rings on either seal surface as required with soldering. For our application, sealing is conducted within an argon atmosphere, but we believe a dry nitrogen atmosphere would also be adequate.

Electronics Packaging to Isolate MEMS Sensors From Thermal Transients

This paper describes a methodology for passive thermal management of a microelectromechanical system (MEMS) sensor. The MEMS sensor has a temperature rate dependence that must be minimized while the system containing the sensor experiences thermal transients from a cold power-up. A multi-step packaging concept has been developed to manage the thermal transient at the MEMS sensor. The strategy minimizes heat flow to the sensor through use of thin, high thermal resistance kovar leads and an insulating air gap between sensor and electronics. The thermal mass of the sensor assembly is increased with minimal increases in weight and volume by mounting the sensor on a high heat capacity beryllium block. Lastly, a commercial off-the-shelf (COTS) heat sink diverts as much of the heat dissipation into the ambient air as possible. This paper describes the practical implementation of these concepts. This includes the soldering of the leads to the board, the mounting of the component onto the block, and the positioning of the block relative to the frame. This paper also presents a lumped parameter thermal analysis of the packaging. The analysis predicted a maximum dT/dt of 0.4K/minute where dT/dt is the temporal derivative of the MEMS sensor temperature. Test data validating the model is also presented.Copyright

A Metal Interposer for Isolating MEMS Devices from Package Stresses

Many classes of MEMS devices, such as those with resonant structures, capacitive readouts, and diaphragm elements, are sensitive to stresses that are exerted by their surrounding package structure. Such stresses can arise as a result of changes in temperature, ambient pressure, or relative humidity. We have demonstrated a dramatic reduction in scale factor bias over temperature for a tuning fork gyroscope by mounting it on an interposer structure within a conventional chip carrier, Fig. 1. Optimization of a MEMS sensor package for high performance subject to various constraints cannot be accomplished by analysis alone Hanson et al. [1]. There are too many unknown parameters, e.g., material properties, process conditions, and components/package interface conditions, to make this feasible. Extensive performance evaluation of packaged sensors is also prohibitively expensive and time consuming. However, recent advances in optoelectronic laser interferometric microscope (OELIM) methodology Furlong and Pryputniewicz [2] offer a considerable promise for effective optimization of the design of advanced MEMS components and MEMS packages. Using OELIM, sub-micron deformations of MEMS structures are readily measured with nanometer accuracy and very high spatial resolution over a range of environmental and functional conditions. This greatly facilitates characterization of dynamic and thermomechanical behavior of MEMS components, packages for MEMS, and other complex material structures. In this paper, the OELIM methodology, which allows noninvasive, remote, full-field-of-view measurements of deformations in near real-time, is presented and its viability for development of MEMS is discussed. Using OELIM methodology, sub-micron displacements of sensors can be readily observed and recorded over a range of operating conditions, Fig. 2.