Nanoindentation of Silicate Low-K Dielectric Thin Films
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Nanoindentation of Silicate Low-K Dielectric Thin Films Joseph B. Vella1, Alex A. Volinsky1, Indira S. Adhihetty1, N.V. Edwards, and William W. Gerberich2 1
Motorola, Digital DNATM Labs, Process and Materials Characterization Lab, Mesa, AZ. 2 University of Minnesota, Dept. of Chem. Eng. and Mat. Science, Minneapolis, MN.
ABSTRACT The capabilities of nanoindentation to characterize low-k organo silicate glass (OSG) thin films is explored as a relatively rapid and inexpensive metric of mechanical properties, adhesion strength, and fracture toughness. One method of decreasing the static dielectric constant of OSG interlayer dielectrics requires the introduction of porosity in the material which has a dramatic impact on its mechanical and toughness properties. Percolation theory is used to formulate a correlation between porosity and elastic modulus. Using cube corner diamond indentation and scratch testing fracture toughness calculations are also discussed. INTRODUCTION A viable low-K material must be compatible with dual-damascene lithography, as well etching, stripping and cleaning processes--especially CMP and device packaging methods. The reliability of devices containing these multi-layer dual-damascene stacks depends on several factors, including: device mechanical stability; adhesion of the barrier metal to the low-K film; adhesion of etch stop, hard mask and capping layers to the low-K film and barriers; and the ability to polish the Cu and package device without pattern shift. Since the dual-damascene stack is exposed to high shear stresses during the CMP process, any flaws at the interfaces or in the low-K film itself can lead to long-term reliability problems. Given that low-k OSG materials gain their advantageous dielectric properties by the introduction of lattice defects in the form of pores, detailed reliability and compatibility tests are required to integrate new low-K dielectric materials and Cu interconnects. Mechanical properties of thin films often differ from those of the bulk materials. Due to typically high yield strengths thin films can support very high residual stresses. This residual stress can be relieved later during processing or in the actual device operation through thin film fracture, or interfacial delamination. The most important properties of low-k materials necessary to insure device reliability are elastic modulus, hardness, interfacial adhesion, and film fracture toughness. EXPERIMENT Several test structures have been constructed to test OSG low-k dielectric films mechanical properties. OSG films of different thicknesses ranging from 50 nm to 2.5 microns were deposited on oxidized Si wafers (with a 50 nm sputtered TaN glue layer) using a precursor CVD process. Thin film mechanical properties can be measured by tensile testing of freestanding films [1] and by the microbeam cantilever deflection technique [2, 3], but the easiest way is by nanoindentation, where no special sample preparation is required and tests can be performed B12.13.1 Downloaded from https:/www.cambridge.org/c
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