In the present study, 25 kinds of specimen with five Al-film thicknesses were prepared to investigate the relation between the internal stress formed during the annealing process and the hillocks. In the preparation of specimens, the governing factors including deposition conditions, annealing temperature, and annealing time, were arranged following the orthogonal table of five-level and six-factorial (L-25(5(6))) design. Stoney's formula is applied to describe the internal stresses before and after annealing (sigma(0) and sigma(f)), respectively. The internal stress arising during the annealing process (sigma(an)) is evaluated using the model developed by Flinn et al. [1]. Then, the response surface methodology (RSM) is used to express the three stress parameters in terms of influential factors. The incipient sigma(an) value for hillocks appearing in the specimens was found to be between -28.7 MPa and -32 MPa in a compressive form. The annealing temperature, time, and Al-film thickness are the three major factors, affecting internal stress sigma(an). An increase in the annealing time reduces the tensile stress or increases the compressive stress, or both. The tensile stress decreases and the compressive stress increases during the annealing process with increasing Al film thickness and annealing temperature. The number of hillocks formed in a unit of area is linearly proportional to both sigma(an) and (sigma(f) - sigma(an)). (C) 2012 Elsevier B.V. All rights reserved.