Sumit K. Soni

Microstructural Origins of Saccharin-Induced Stress Reduction in Electrodeposited Ni

In this work, we provide direct evidence of the fundamental mechanism through which saccharin, a standard industrial Ni electroplating additive, reduces stress in electrodeposited Ni. This was accomplished using real-time in situ stress measurements taken during through-mask electrodeposition of Ni films from a bath where the saccharin concentration was varied. This technique facilitated the direct measure of the effect of saccharin on the stress created at the Ni island boundaries. We demonstrated that increased saccharin concentration in a Ni-sulfamate-based bath resulted in a systematic reduction in the tensile grain-boundary coalescence stress. Based on this and ex situ S concentration measurements of the Ni films, we propose that the reduction in tensile stress was the result of a reduction in the grain-boundary energy due to S incorporation at the island boundaries.

Stress-induced Sn Nanowires from Si–Sn Nanocomposite Coatings

The growth of stress-induced tin (Sn) whiskers has been considered responsible for the failure of many electronic devices and many approaches have been developed to mitigate their growth. In this report, however, we describe a simple approach based on the same mechanism to promote the growth of Sn nanowires. The thermal expansion induced stress was utilized as the driving force to initiate the growth of Sn nanowires from Si–Sn phase-separated nanocomposite coatings. The nanostructure of the Si–Sn matrix was the key to controlling the shape and diameter of Sn nanowires. This approach provides additional flexibility for making desirable metallic nanowires with controlled dimensions.

Residual Stress Mechanisms in Aluminum Oxide Films Grown by MOCVD

Residual stresses in amorphous aluminium oxide films were investigated with in situ wafer curvature measurements. The films were deposited from aluminium tri-isopropoxide, on sapphire substrates. Large tensile stresses of 1-2 GPa occurred during growth. These values are well above the fracture stress in bulk materials, but they are sustainable in thin film form. Subsequent heat treatment of these films produced additional tensile stress, even at low temperatures prior to crystallization. The mechanisms responsible for all of these stress contributions are discussed. The variety of operative mechanisms at low to moderate temperatures in these amorphous films suggests that different processing routes can be used to engineer significant differences in the final stress state of these materials.

Origins of saccharin-induced stress reduction based on observed fracture behavior of electrodeposited Ni films

This research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films where saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate-based electro chemical baths, where it is empirically known that mmol/l concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size up to a few nanometer scales. Some previous observations and several theoretical models suggest that saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which demonstrated island size effects on stress evolution, and enabled a well-defined notch geometry along one of the island boundaries to facilitate fracture experiments. In situ uniaxial tensile testing under in a scanning electron microscope was then used to obtain the fracture strength of such specimens. This technique provided real time recording of microscopic deformation during uniaxial tensile loading. The observed relationships among residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture.