Microstructure and Electronic Properties of Thin Film Nanoporous Silica as a Function of Processing and Annealing Method
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Microstructure and Electronic Properties of Thin Film Nanoporous Silica as a Function of Processing and Annealing Methods Christine Caragianis-Broadbridge1*, John R. Miecznikowski1, Wenjuan Zhu2, Zhijiong Luo2, Jin-ping Han2 and Ann Hein Lehman1 1 Department of Engineering; Trinity College, Hartford, CT, 06106; 2Department of Electrical Engineering; Yale University, New Haven, CT, 06520; *Visiting Fellow, Yale University. ABSTRACT Alcogels, aerogel precursors, were prepared by hydrolysis and condensation of the metal alkoxide tetraethylorthosilicate and were catalyzed by both acids and bases, according to a standard reaction. Alcogel solution was spin coated onto p-type silicon wafers and fluid extraction was achieved in an uncontrolled (room temperature, atmospheric pressure) environment. Film porosity was retained through surface modification and/or low vapor pressure solvent techniques. The microstructure and electronic properties of the resulting films were evaluated using non-contact atomic force microscopy (nc-AFM), cross sectional scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Metal insulator semiconductor (MIS) devices were prepared and current-voltage and capacitance-voltage measurements were obtained from these devices. Annealing studies reveal a dramatic temperature dependent effect on both the microstructure and electronic properties of the porous silica films. INTRODUCTION Aerogels are highly porous materials with unique optical, thermal, mechanical and electrical properties. Bulk nanoporous silica has been studied and has been shown to exhibit the lowest refractive index (n), thermal conductivity, sound velocity, and dielectric constant (k) of any known material. The potential applications for these materials include sensors, photonic, and electronic devices. Specifically, the low-k properties of thin film silica aerogels (k=1-3) make them highly attractive as a replacement for the conventional insulator (silicon dioxide, k=3.9) which is now utilized for interlevel dielectrics (ILDs) in integrated circuits [1]. The introduction of low-k ILDs into ICs will potentially result in decreased power dissipation, reduced cross-talk and faster achievable switching speeds [2]. The k of silica aerogel is, however, process dependent, while the reliability is similarly determined by such properties as porosity, film uniformity and adhesion. The long-term stability of these films is also an issue, with such environment parameters as annealing response critical to subsequent process integration. Before semiconductor manufacturers can implement thin film nanoporous silica, these issues must be examined in detail. The focus of this research was the fabrication of thin film nanoporous silica while utilizing methods that do not require pore-fluid extraction under supercritical conditions. Standard aerogel preparation techniques require supercritical extraction of pore fluid to prevent pore collapse. Supercritical extraction retains porosity by eliminating the capillary stress exerted by t
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