Nanowires with high aspect ratio can become unstable due to Rayleigh-Plateau instability. The instability sets in below a certain minimum diameter when the force due to surface tension exceeds the limit that can lead to plastic flow as determined by the yield stress of the material of the wire. This minimum diameter is given dm ≈ 2σS/σY , where σS is the surface tension and σY is the Yield force. For Ag and Cu we estimate that dm ≈ 15nm. The Rayleigh instability (a classical mechanism) is severely modified by electronic shell effect contributions. It has been predicted recently that quantum-size effects arising from the electron confinement within the cross section of the wire can become an important factor as the wire is scaled down to atomic dimensions, in fact the Rayleigh instability could be completely suppressed for certain values of kF r0. Even for the stable wires, there are pockets of temperature where the wires are unstable. Low-frequency resistance fluctuation (noise) measurement is a very sensitive probe of such instabilities, which often may not be seen through other measurements. We have studied the low-frequency resistance fluctuations in the temperature range 77K to 400K in Ag and Cu nanowires of average diameter ≈ 15nm to 200nm. We identify a threshold temperature T* for the nanowires, below which the power spectral density SV(f) ~1/f. As the temperature is raised beyond T* there is onset of a new contribution to the power spectra. We link this observation to onset of Rayleigh instability expected in such long nanowires. T* ~ 220K for the 15nm Ag wire and ~ 260K for the 15nm Cu wire. We compare the results with a simple estimation of the fluctuation based on Rayleigh instability and find good agreement.
We have studied the conductance fluctuation in metal film which is under electromigration stressing. The apparatus used by us allows measurement of noise with an ac 5-probe technique with a superimposed dc stressing current (typically 2MA/cm2). This allows measurement of noise in the film at different stages of the electromigration process till the film is damaged completely. We study both the spectral power SV(f) and also the probability density function (PDF) from the time series. The electromigration stressing was done to elevated temperature on Al and Cu metal lines grown by RF magnetron sputtering. Principal motivation of the investigation is to study low frequency defect relaxations in the metal film due to electromigration that give rise to conductance fluctuations with a spectral power SV(f)∝ 1/fα. SV(f) (both magnitude as well as the spectral power quantified through α) shows changes continuously and some times non-monotonically during the electromigration process and it is large just before the damage of the film. It was also observed that the PDF width increases significantly during the course of the em stressing and it changes from a Gaussian to a non-Gaussian PDF.