Nanoindentation Analysis of Viscoelastic Thin Films: Strain Rate and Adhesion Effects
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Nanoindentation Analysis of Viscoelastic Thin Films: Strain Rate and Adhesion Effects Manuel Luis B. Palacio*, Tracey Scherban**, Brad Sun***, Jessica Xu**, William W. Gerberich* *Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN **Intel Corporation, Hillsboro, OR ***Intel Corporation, Santa Clara, CA
ABSTRACT The mechanical properties of two viscoelastic thin films were measured, a polyimide and a low-k polymer dielectric. The static modulus and hardness were obtained from nanoindentation experiments using an elastic-plastic unloading model (J. Mater. Res., 13, 421 (1998)). Nanoindentation creep tests were also performed, where a value for the modulus was extracted by fitting the data to an equation based on the threeelement standard linear solid model. Aside from being comparable to values reported in the literature, the moduli from creep experiments are at most 1.3 times lower than the static moduli. This decrement can be attributed to the differences in strain rate for the two methods and the effect of adhesive forces.
INTRODUCTION Due to the need for films with low dielectric constant (“low-k”) to meet electrical performance targets in the integrated circuit, polymer thin films have recently been considered as replacements for SiO2 and fluorinated silica. However, these films have much lower mechanical strength, typically having modulus values less than 10 GPa compared to 50 GPa and higher for silica and fluorinated silica. Since polymers are viscoelastic, it is important from a reliability standpoint to evaluate this behavior. Nanoindentation is a suitable technique due to its high load and depth resolution. Polyimide has been used for many technology generations for “passivation” or protection of the silicon die before it is assembled into a package. It is interesting to note that polyimide was one of the first polymers to be evaluated as an interlayer dielectric (ILD), although it did not gain widespread use since its dielectric constant is close to that of fluorinated silica [1].
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EXPERIMENTAL Materials Two polymer-coated Si wafers were processed at Intel Corporation. One was coated with 3.8 µm of polyimide (to be referred to as “PI”) while the other one had 1.9 µm of a low dielectric constant polymer (to be referred to as “low-k”). Indentation Standard indentation and indentation creep experiments were performed using the Hysitron Triboscope (Hysitron Inc., Minneapolis, Minnesota) in conjunction with an atomic force microscope. For both experiments, two tips were used: one is a 1µm diamond conical tip with a 90o apical angle and the other one is a 1µm tungsten conical tip, where the included angle is 85o. The tungsten tip was fabricated using the method of Grunlan, et al. [2]. For standard indents, a trapezoidal load-time profile with 50 µN/s loading/unloading rate was employed. The maximum load varied from 25 µN to 2000 µN. The holding time is half the loading time. The creep indents were performed by keeping the ratio of holding time to pre-hol
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