Closed porosity aluminosilicate for electronic packaging applications
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V. M. Hietala and C. J. Brinker Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 29 June 1992; accepted 30 December 1992)
The electrical properties of sol-gel prepared aluminosilicate films were investigated for suitability in electronic applications. The aluminosilicate films exhibited apparent closed porosity and a dielectric constant as low as = 5 with processing temperatures from 373 K to 873 K. The porosity was inaccessible to nitrogen at 77 K, helium at 293 K, and water vapor at 293 K. Both bulk and thin-film samples were analyzed for hydroxyl and carbon contents to elucidate the relative dependence of the measured electrical properties on processing conditions. Experiments indicate it is possible to vary the porosity in bulk material in ways that should improve electrical properties.
I. INTRODUCTION Aluminosilicate xerogels prepared by sol-gel methods typically have nitrogen surface areas >100 m 2 /g (as do most sol-gel derived materials). A bulk aluminosilicate of composition 1:1 molar ratio of Al;Si exhibited an anomalously low surface area of < 1 m 2 /g when prepared by a variety of sol-gel routes.1"3 The surface area and skeletal density of the 1:1 material are compared in Fig. 1 with other similarly prepared aluminosilicate compositions. The dramatic decrease in surface area and density has been attributed to closed porosity, and from helium displacement measurements of the skeletal densities, closed porosities of 15% to 20% are calculated for the 1:1 composition. Films prepared from this material exhibited BET surface area of ~ 1 cm 2 /cm 2 (accessible surface area per area of film), as measured using a surface acoustic wave (SAW) technique.4'5 The unique property of a closed cell ceramic material, which can be obtained at low processing temperatures (373 K to 873 K), suggests uses in electronics packaging applications, as a result of its low gas/vapor permeability and conceivably low dielectric constant. Electrical and physical properties were measured as a function of heat treatment atmosphere, time, and temperature, and the effect on these properties due to the carbon and hydroxyl content in the films was determined.
ratios 1 EtOH: 1 HC1: 0.8 TEOS) was added to the flask, and then the aluminum tri-sec butoxide was mixed with this solution. This was allowed to react (with slight agitation) for several minutes, then diluted with ethanol to form a solution of 2 wt. % solids, and refluxed at 353 K overnight. A gel was formed by adding water (molar ratios 100 H 2 O: Si) to the sol, with the gel time tg ~ 4 h at 298 K. To form a bulk powder, the gel was allowed to dry at room temperature for several days. The film sample was prepared by spin coating a 4 in. silicon wafer at 2000 rpm with the same sol at t/t%t\ =0.5. A relatively uniform coating was indicated by the wafer color, which turned a dark blue after heating. The wafer was then sectioned into pieces for each of the different heating procedures described below.
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II. EXPERIMENTAL The bulk and film materials were prepar
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