Analysis of local strain in aluminium interconnects by energy filtered CBED

Energy filtered convergent beam electron diffraction (CBED) was used to inv estigate localised strain in aluminium interconnects. The quantitative anal ysis of the experimental patterns is based on a multi-step evaluation proce dure which is the main subject of the present paper. The improvements which were made to the analysis method aim at increasing both the automation and the accuracy. The detection of the higher order Laue zone (HOLZ) line posi tions is performed by means of the Hough transform. The required sub-pixel resolution can be achieved routinely and the achievable accuracy is only li mited by the line width and the amount of noise in the patterns. The determ ination of the strain state is performed via a refinement algorithm which i s based on varying the strain state in the sample coordinate system and sim ulating the patterns for the individual grains until a best fit with the ex periment is obtained. For the simulation we have developed a new correction scheme in which the dynamical effects are treated separately for each indi vidual HOLZ line. The results show that the main source of the observed str ains is the difference in thermal expansion coefficients. The strain is sub stantially reduced underneath a hillock in the interconnect. Asymmetries in the strain distribution around the hillock show that the unidirectional di ffusion during electromigration tests causes peak strains in areas next to the hillock which may be possible failure sites. (C) 2000 Elsevier Science B.V. All rights reserved.