)Motorola, Digital DNA Labs, Process and Materials Characterization Lab, Mesa, AZ 85202, USA
)In order to meet the next generation device requirements the semiconductor industry is gradually moving from well-established Al/SiO2 technology to new Cu/low-k interconnect structures. Obvious benefits of the transition include increased device operating frequency, lower power consumption, and reduced interline cross talk. This transition is however challenging, since a number of integration and reliability issues are introduced with this new materials system. Nanoindentation is a versatile technique for measuring films mechanical properties (elastic modulus, hardness, interfacial adhesion, and film fracture toughness). It is an alternative to tensile testing of freestanding films [1] and to the microbeam cantilever deflection technique [2-4], but is much easier, since no special sample preparation is required and tests can be performed quickly and inexpensively. Nanoindentation works using the same principals as conventional hardness tests, but is performed on a much smaller scale using high-resolution load and displacement sensing equipment. During the measurement a sharp diamond indenter is forced into a material surface while continuously recording both the force and the indentation depth. Mechanical properties are measured by either analyzing the continuous load-displacement profile or by measuring the material response to a frequency modulated force oscillation that is imposed on the indentation tip during indentation. Both elastic modulus and hardness can be readily extracted directly from the nanoindentation curve [4-7]. Since the depth resolution is on the order of nanometers, it is possible to indent even very thin (<100 nm) films. Indentation has been also used to measure thin film adhesion [8-13], where the mechanical energy release rate, or practical work of adhesion is calculated based on the size of delamination that can be generated by high load (200-800mN) indentation. Indentation techniques have also been to measure fracture toughness. When a sharp tip such as Vickers, Berkovich or a cube corner diamond is indented into bulk brittle materials, radial cracking can occur after a critical load has been reached. This method allows one to calculate fracture toughness based on the maximum indentation load and the crack length [14-16].
All measurements can be performed at elevated temperatures, in various ambient environments. This talk is an overview of mechanical properties (measured by nanoindentation) and their effect on reliability of various materials systems, from low-K dielectrics to copper interconnects.
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