William J. Borland

Extrinsic scaling effects on the dielectric response of ferroelectric thin films

Scaling effects in polycrystalline ferroelectric thin films were investigated by preparing barium titanate in a manner that maintained constant composition and film thickness while allowing systematically increased grain size and crystalline coherence. The average grain dimensions ranged from 60to110nm, and temperature dependence of permittivity analysis revealed diffuse phase transitions in all cases. Maximum permittivity values ranged from 380 to 2040 for the smallest to largest sizes, respectively. Dielectric hysteresis is evident at room temperature for all materials, indicating stability of the ferroelectric phase. Comparison of permittivity values at high electric fields indicates that the intrinsic dielectric response is identical and microstructural artifacts likely have a minimal influence on film properties across the sample series. Permittivity values, however, are substantially smaller than those reported for bulk material with similar grain dimensions. X-ray line broadening measurements were ...

Permittivity scaling in Ba1−xSrxTiO3 thin films and ceramics

A dramatic enhancement in the electromechanical response of barium titanate thin films is demonstrated by understanding and optimizing the relationship between organic removal, crystallization, and microstructure, which therefore results in pore elimination, larger grain sizes, and superior densification. The combination enables one to produce bulk-like dielectric properties in a thin film with a room temperature permittivity value above 3000. This advancement in complex oxide thin film processing science creates a new perspective from which to compare, parameterize, and better understand a collection of literature data concerning the manner in which the dielectric response of BaTiO3 depends upon physical dimensions. We are consequently able to apply a single physical model to bulk ceramic and thin film systems, and so demonstrate that the existence of parasitic interfacial layers are not needed to explain dielectric scaling. This work is instrumental in illustrating that extrinsic contributions to scalin...

Ceramic Resistors and Capacitors Embedded in Organic Printed Wiring Boards

Combining thick-film and printed wiring board processes allows thick-film ceramic resistors and capacitors to be embedded in printed wiring boards (PWB). The resistor materials are based on lanthanum boride and cover the range of 10 ohm/square to 10 Kohm/square resistivities. The capacitor material is based on doped barium titanate. Both systems are designed to be “thick-film” printed on copper foil in the locations desired in the circuit and the foil is then fired in nitrogen at 900°C to form the ceramic component on the copper foil. The foil is then laminated, component face down, to FR4 using standard prepreg. The inner layer is then etched to reveal the components in a FR4 matrix. The resistors can be trimmed to tight tolerance at this stage and the components tested. The inner layer can then be laminated into a multilayer PWB. The process is described and the influence of board design, PWB processing and materials are presented and discussed. Examples of circuits using embedded thick-film passives are shown and results of reliability studies are presented.Copyright

Formation of high quality screen-printed contacts to homogeneous high sheet resistance emitters (HHSE)

In this paper we report on the formation of high quality contacts to HHSE with a sheet resistance variation of ≥30 Ω/sq using a commercial front-side Ag paste, PV16A from DuPont. We fabricated and characterized solar cells with emitter sheet resistances of 65, 75, 85, 95 and 105 Ω/sq. We found that emitter sheet resistances in the range of 65–95 Ω/sq can be contacted with low average series resistance of 0.63–0.77 Ω-cm² and high fill factor (FF) of 77.4–78.8%. The 75 Ω/sq emitter gave the best average efficiency of 18.3%, followed by 18.2% for the 65 Ω/sq, 18.0% for the 85 Ω/sq, and finally 17.7% for the 95 Ω/sq. The 105 Ω/sq emitter gave a low FF due to high series resistance, but the shunt resistance and ideality factor were excellent, which suggests the paste was not encroaching the shallow emitter junction.

Cu-Compatible Ultra-High Permittivity Dielectrics for Embedded Passive Components

Barium titanate thin films have been prepared by chemical solution deposition on 18 μm thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO 3 grains with diameters between 0.1 and 0.2 μm, and an equiaxed morphology. The average film thickness is 0.6 μm and high resolution cross sectional microscopy shows no indication of interfacial phases. The BaTiO 3 films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities greater than 3000 are observed, with loss tangents under 2.5%. The BaTiO 3 phase exhibits pronounced ferroelectric switching and coercive field values near 20 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100 °C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature – as compared to typical film processing – provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO 3 films on copper foil represents an important step towards high capacitance density embedded passive components.

Synthesis and properties of barium titanate stannate thin films by chemical solution deposition

Barium titanate stannate (BaTi1−xSnxO3, 0 ≤ x ≤ 0.25) thin films were deposited directly on copper foil substrates via a chelate chemical solution process. The films were subsequently crystallized in a reducing atmosphere such that substrate oxidation was avoided and that the 2-valent state of tin could be stabilized. Despite the stabilization of the low-melting temperature SnO oxidation state at high temperatures, the final grain size was smaller with increased tin incorporation similar to other B-site substituted BaTiO3 films. Temperature and field-dependent dielectric measurements revealed a reduction in dielectric constant and dielectric tuning with increasing tin concentration. The reduction in permittivity with reduced grain size is consistent with the well-known trends for ceramic barium titanate and in combination with a defect-dipole model involving Sn acceptors, can be used to explain the experimental trends. Phase transition frequency dependence was studied and for compositions containing up to 25 mole percent tin. No phase transition dispersion was observed and thus no strong evidence of relaxor-like character. The phase transition became increasingly diffuse with deviation from Curie–Weiss behavior, but the observed transition temperatures agreed well with bulk reference data.

Decoupling of High Performance Semiconductors Using Embedded Capacitor Technology

The integration of embedded DuPont ceramic capacitors and HiK polyimide based planar capacitor materials in IC packages has been investigated by a joint program between DuPont and Georgia Institute of Technology packaging research center (PRC). Test vehicles with different types of structures were designed, fabricated and tested for individual device characterization. The test vehicles included embedded ceramic-fired-on-foil capacitors with microvia interconnects and two sequential industry standard build-up layers on each side of a BT core. Feature sizes were 12 micron lines and spaces and 50 micron diameter microvias. Other test vehicles used a core layer without build-up layers, planar capacitor layers and arrays of discrete capacitors with different size, capacitance, and interconnection designs. Each capacitor variation was electrically characterized. The electrical performance data from the test vehicles was used to perform simulations to determine the designs offering the most effective power delivery and noise decoupling for a package having 2007 ITRS die and substrate features. The embedded ceramic capacitors show a significant improvement in power system noise decoupling and charge supply to the IC in the mid-frequency range due to the low inductance design.

Materials Interactions in the Firing of Copper Thick Film Multilayer Ceramics

In recent years, copper thick film materials have gained rapid acceptance in ceramic multilayer interconnect boards because of their ability to meet advanced packaging requirements at reasonable cost. With high volume production, however, problems such as opens, shorts, blisters and porosity have been experienced. Many of these failures may be attributed to undesirable materials interactions caused by reducing conditions which can be caused by incomplete burnout of thick film organics during firing. This paper considers the interactions in current copper thick film material systems. Primary emphasis will be placed on chemical interactions that occur in both ideal and non-ideal conditions during firing. Failure modes will be related to chemistry and processing. New systems designed to overcome processing sensitivity will be discussed.

Low-temperature nitride transformation reactions

This study illustrates a novel method of transforming between two refractory nitrides at temperatures well below their respective melting points. Silicon nitride (Si3N4) is an excellent thermal and electronic insulator, with applications in the microelectronic, automotive and technical ceramic industries. Thermodynamically, there is a significant decrease in the Gibbs Free Energy inherent in the transformation between Si3N4 and a number of refractory metal nitrides; however, these transformation reactions are limited in the pure state by a kinetic barrier at any temperature appreciably lower than the melting point of Si3N4 (∼2173 K). Results of this study illustrate the successful conversion of powdered amorphous Si3N4 to TiN. The transformation is made possible by a liquid phase present in a number of Ti-based alloys at temperatures in the vicinity of 800°C. Since both nitrides (SiNx and TiN) are refractory, the presence of the liquid phase provides a high-diffusivity pathway, thus overcoming the kinetic barrier associated with the otherwise thermodynamically favorable reaction.

Contacts to silicon using a silver paste containing a phosphorus source

Performance enhancing phosphorus additives were found to be beneficial in front side conductive silver pastes. The phosphorus source at low levels increased the cell efficiencies from 0.1% to 0.3% on poly-crystalline and mono-crystalline Si cells. Better particle dispersion is seen when the phosphorus source is present. Additionally, specific contact resistivity measurements were made to quantify the contact resistivity.