Dielectric breakdown of alkali-free boroaluminosilicate glass thin films

Open Access
Author:
Serra, Jessica Lynn
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 28, 2010
Committee Members:
  • Carlo G Pantano, Thesis Advisor
  • Michael T Lanagan, Thesis Advisor
Keywords:
  • dielectric breakdown
  • glass thin films
Abstract:
Some glasses have exhibited dielectric breakdown strengths two orders of magnitude higher than polycrystalline ceramic materials. Their high dielectric breakdown strength makes glass an ideal candidate for use in high energy density capacitors for pulsed power applications. It has been observed in recent studies that a commercial alkali-free boroaluminosilicate glass, Schott AF45TM, exhibits a dielectric breakdown strength of 12-13 MV/cm. In order to successfully measure the dielectric breakdown strength of this glass, samples 5-30 microns thick were used during testing. It was desirable to continue testing glass below 5 microns thick in the pursuit of measuring the intrinsic dielectric breakdown strength. In this study, thin film deposition was used to produce glass less than 5 microns thick. Because of a built-in thickness gradient in the films across the wafer surface, each sample provided a range of thicknesses to measure. AF45 was used as the target material. RF magnetron sputtering was chosen over other deposition techniques, such as evaporation or chemical vapor deposition, because of its ability to sputter insulators, the ability to use a piece of commercial glass as the target material, the increased film density and good compositional control. Several characterization techniques were used to understand the properties of the films. The films were examined with field emission scanning electron microscopy and observed to be microstructure free. The composition, measured by x-ray photoelectron spectroscopy, differed from that of the bulk glass. The films also appear to have a significant amount of water included in the structure and a lower density based on spectra obtained from Fourier transfer infrared spectroscopy measurments. The dielectric breakdown strength of the films was measured to be 9.5-10 MV/cm, about 20% lower then the dielectric breakdown strength of the thinnest sample of bulk glass. The decreased breakdown strength measured for the thin films is attributed to several factors. The presence of water and the low density of the films can allow for charge to pass through the glass films more easily than in the bulk. Microstructural defects such as voids and impurities within the deposited film, electrodes and especially at the film/electrode interface, are obvious candidates based upon previous work with bulk dielectrics. The compositional non-uniformity observed in the films could further enhance structural defect formation, beyond those intrinsic to the deposition process itself.