Devarajan Balaraman

Colloidal processing of polymer ceramic nanocomposites for integral capacitors

Polymer ceramic composites form a suitable material system for low temperature fabrication of embedded capacitors appropriate for the MCM-L technology. Improved electrical properties such as permittivity can be achieved by efficient filling of polymers with high dielectric constant ceramic powders such as lead magnesium niobate-lead titanate (PMN-PT) and barium titanate (BT). Photodefinable epoxies as the matrix polymer allow fine feature definition of the capacitor elements by conventional lithography techniques. The optimum weight percent of dispersant is tuned by monitoring the viscosity of the suspension. The dispersion mechanism (steric and electrostatic contribution) in a slightly polar solvent such as propylene glycol methyl ether acetate (PGMEA) is investigated from electrophoretic measurements. A high positive zeta potential is observed in the suspension, which suggests a strong contribution of electrostatic stabilization. By optimizing the particle packing using a bimodal distribution and modified processing methodology, a dielectric constant greater than 135 was achieved in PMN-PT/epoxy system. Suspensions are made with the lowest PGMEA content to ensure the efficiency of the dispersion and efficient particle packing in the dried film. Improved colloidal processing of nanoparticle-filled epoxy is a promising method to obtain ultra-thin capacitor films (<2/spl mu/m) with high capacitance density and improved yield. Capacitance of 35 nF/cm/sup 2/ was achieved with the thinnest films (2.5-3.0 /spl mu/m).

Polymer-ceramic nanocomposite capacitors for system-on-package (SOP) applications

This work focuses on optimizing the dispersion of nanosized ceramic particles for achieving higher dielectric constant, thereby higher capacitance density in polymer/ceramic nanocomposites. It has been observed that high solids loading leads to entrapment of porosity in the microstructure which lowers the effective dielectric constant of the films. The amount of solvent in the suspension and the speed at which spin coating was performed were found to impact the dielectric constant of high filler content nanocomposites. The interplay between the rheological properties of the suspension and processing parameters such as solvent content and coating speeds and its impact on the dielectric properties of the film are discussed. Porosity of thin film composites was measured for the first time to study the impact of these processing parameters. Powders of different particle sizes were mixed to obtain bimodal particle size distribution in order to increase the packing density of the composite. Packing density was improved by modifying the dispersion methodology. A nanocomposite with dielectric constant as high as 135 was obtained for the first time in the low-cost printed wiring board compatible epoxy system. A capacitance densities of /spl sim/35 nF/cm/sup 2/ on a nominal 3.5 micrometer films was achieved on PWB substrates with high yield. The manufacturability of these formulated nanocomposites and their applications as decoupling capacitors have been tested using a large area (300 mm /spl times/ 300 mm) system-on-package (SOP) chip-to-chip communication test vehicle.

Low-cost low actuation voltage copper RF MEMS switches

This paper presents the design, fabrication and testing of capacitive copper RF MEMS switches with various hinge geometries, fabricated on high-resistivity silicon substrates. The switches were fabricated using a simple low-cost four-mask process and 0.6-1.0 /spl mu/m thick membranes were made out of sputtered copper. The capacitive airgap in between the membrane and the signal line is 1.5-2.0 /spl mu/m. The lowest actuation voltage measured on the fabricated switches is 9 V. The measured insertion loss of a fabricated switch and its associated transmission time was 0.9 dB (mainly contributed by the transmission line itself) and the isolation was measured to be 25 dB at 40 GHz.

Chip-to-chip optoelectronics SOP on organic boards or packages

In this paper, we demonstrate compatibility of hybrid, large-scale integration of both active and passive devices and components onto standard printed wiring boards in order to address mixed signal system-on-package (SOP)-based systems and applications. Fabrication, integration and characterization of high density passive components are presented, which includes the first time fabrication on FR-4 boards of a polymer buffer layer with nano scale local smoothness, blazed polymer surface relief gratings recorded by incoherent illumination, arrays of polymer micro lenses, and embedded bare die commercial p-i-n photodetectors. These embedded optical components are the essential building blocks toward a highly integrated SOP technology. The effort in this research demonstrates the potential for merging high-performance optical functions with traditional digital and radio frequency (RF) electronics onto large area and low-cost manufacturing methodologies for multifunction applications.

Magnetic nanocomposites for organic compatible miniaturized antennas and inductors

Current wireless systems are limited by RF technologies in their size, communication range, efficiency and cost. RF circuits are difficult to miniaturize without compromising performance. Antennas and inductors are major impediments for system miniaturization because of the lack of magnetic materials with suitable high frequency properties. Keeping antenna and inductor requirements into consideration, two magnetic nanocomposite systems - silica coated cobalt-BCB and Ni ferrite-epoxy were investigated as candidate materials. Nanocomposite thick film structures (125-225 microns) were screen printed onto organic substrates. Parallel plate capacitors and single coil coplanar inductors were fabricated on these films to characterize the electrical and magnetic properties of these materials at low and high frequencies. Electrical characterization showed that the Co/SiO/sub 2/ nanocomposite sample has a permeability and a matching permittivity of /spl sim/10 at GHz frequency range making it a good antenna candidate. Both polymer matrix composites retain high permeability at 1-2 GHz.

Processing and Dielectric Properties of Nanocomposite Thin Film “Supercapacitors” for High-Frequency Embedded Decoupling

The embedded decoupling capacitor problem has been pursued by several groups and industry around the world over the past decade. Currently, popular ceramic-polymer composites can only provide limited capacitance, typically within 10 nF/cm2 . With the reliability and processing constraints imposed, the capacitance density would be much lower. Newer capacitor concepts such as supercapacitors can overcome the limitations of existing polymer based capacitors and are now being considered. These concepts rely on nanostructured electrodes for high surface area per unit volume resulting in ultrahigh capacitance densities and unconventional polarization mechanisms such as electrical double layer and interfacial polarization. Supercapacitive structures lead to ultrahigh capacitance densities of the order of hundreds of microfarads. However, manufacturers report that the properties are unstable at high frequencies, typically even at tens of megahertz. To adapt these structures for mid-to-high-frequency decoupling, it is hence essential to systematically characterize the high-frequency dielectric properties of the thin nanocomposite films and nanostructured electrodes. This paper reports complete electrical characterization of a part of such a system, carbon black-epoxy nanocomposites. The high-frequency properties of the cured films were evaluated with a multiline calibration technique by measuring -parameters of transmission lines fabricated on the top of the dielectrics. Though the nanostructured carbon black epoxy composites showed high dielectric constant of 1000 at low frequencies, the high frequency (0.5-4.5GHz) dielectric constant was found to be only up to 10 times that of the base polymer matrix. The measured dielectric constant at gigahertz frequencies increased from 15-30 when the filler content was increased from 3.8% to 6.5%, with excessive leakage currents. Based on these measurements, conduction and polarization relaxation mechanisms will be assessed and the suitability of the thin film supercapacitors for high-frequency decoupling applications will be discussed.

Embedded decoupling capacitor performance in high speed circuits

Embedded decoupling is normally considered a better solution than surface mount decoupling for suppressing the switching noise of a high speed digital board/package because of its shorter leads that result in smaller parasitic inductance. This leads to lower impedance over a higher frequency band. It is presumably better in reliability and lowers the cost as well. Designers tend to use large value capacitors for efficient decoupling. Usually, to increase capacitance of an embedded capacitor, one can use a material with higher dielectric constant, design larger electrodes, and reduce the thickness of the dielectric. However, these strategies may sometimes lead to lower performance at high frequency band. This paper will discuss the pros and cons of different embedded capacitor approaches through simulation. As an application example, a typical power/ground network with an embedded capacitor will be compared with that of surface mount discrete capacitor.

Development of high-k embedded capacitors on printed wiring board using sol-gel and foil-transfer processes

Sol-gel ceramic films were fabricated for organic system-on-package compatible integral capacitor applications. The films were synthesized on Ti and Ni foils which were then transferred onto organic boards using a lamination step. SrTiO/sub 3/ and BaTiO/sub 3/ films were synthesized with capacitance as high as 700 nF/cm/sup 2/ and loss as low as 0.005. It should be noted that the high permeability of Ni (approximately 100 in bulk form) and lower conductivity compared to copper decreases the skin depth and increases the resistivity of copper. This can have a deleterious effect on Q. More studies are underway to investigate this effect.

Integrating High-k Ceramic Thin Film Capacitors into Organic Substrates Via Low-Cost Solution Processing

Current organic package-compatible embedded decoupling capacitors are based on thick film (8-16 m) polymer-ceramic composites with dielectric constant (k) of 20-30 and do not have sufficient capacitance density to meet the impedance requirements for emerging high-speed circuits and high power density microprocessors. High-k/high capacitance density ceramics films that can meet the performance targets are generally deposited by high-temperature processing or costly vacuum technology (radio frequency sputtering, PECVD) which are expensive and also incompatible with organic packages. The objective of this project is to develop ultra thin films (100-300nm) with high dielectric constant using organic compatible processes to meet future decoupling applications. In the current study, direct deposition of crystalline ceramic films on organic boards at temperatures less than 100C was demonstrated with the hydrothermal method. Post-hydrothermal treatments were shown to minimize the defects in the as-synthesized hydrothermal barium titanate films and improve the breakdown voltage (BDV) and leakage characteristics. Thin films with high capacitance densities and breakdown voltages of 10V were demonstrated. As an alternate technique, sol-gel technology was also demonstrated to integrate ceramic thin films in organic packages. A major barrier to synthesis of sol-gel films on copper foils is the process incompatibility of the sol-gel barium titanate with the copper electrodes. To enable process compatibility, process variables like sol pyrolysis temperature and time, and sintering conditions/atmosphere were optimized. Capacitance densities above 1.1F/cm was demonstrated on commercial copper foils with a BDV above 10 V. The two technologies reported in this study can potentially meet midfrequency decoupling requirements of digital systems.

Simultaneous switching noise suppression using hydrothermal barium titanate thin film capacitors

This paper reports the integration of hydrothermal barium titanate thin film embedded capacitors in organic printed wiring boards. These capacitors have 300 nm thick dielectrics with k>350, can attain capacitances of 1 /spl mu/F/cm/sup 2/ and are ideal for decoupling applications. In order to evaluate these films for simultaneous switching noise suppression, a clock distribution network was designed using a clock driver with one input and four differential outputs. The design consists of a clock driver and four pairs of impedance controlled transmission lines with embedded decoupling capacitors. In order to evaluate the effects of capacitance value and the type of capacitor /sub i/screte vs. embedded, coupons with different embedded capacitance values and discrete capacitors were fabricated on the same board. The fabricated structures were simulated using the transmission matrix method (TMM) in the frequency domain and a macromodeling method in the time domain. This paper demonstrates for the first time that the simple low-cost, aqueous based low-temperature film growth technique can provide the best solution for embedded decoupling capacitor problems in organic packages.