An x-ray method for direct determination of the strain state and strain relaxation in micron-scale passivated metallizat
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Sean Brennan Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025
John C. Bravman Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 2 February 1993; accepted 24 August 1993)
We describe a method for directly determining the strain state of passivated metal lines. Synchrotron radiation in the grazing incidence geometry is used to directly measure the in-plane interplanar spacing along the length and width of the lines, while the strain normal to the surface of the line is measured using conventional diffraction methods. The entire strain state is thereby defined. Previous work has measured out-of-plane reflections, fit them to a straight line as a trigonometric function of the angle of orientation, and extrapolated to determine the principal strains. The equivalence of the two x-ray methods on the same sample is demonstrated at room temperature before and after thermal cycling. For short time strain relaxation experiments during thermal cycling, measurement of the three principal strains leads to the direct calculation of the stress relaxation. We apply the strain determination technique to Al-0.5% Cu lines passivated with Si 3 N 4 as the lines are thermally cycled from room temperature to 450 °C and back. The strain state, stress state, and strain relaxation of the lines are calculated at several temperatures during thermal cycling.
I. INTRODUCTION The Al used in integrated circuit device fabrication is typically sputter-deposited onto a thermally oxided Si substrate, growing with a well-defined (111) fiber texture.1'2 The Al is lithographically patterned to produce the lines, which are usually passivated with a blanket layer of SiC>2 or Si 3 N 4 at some elevated temperature (e.g., 350-500 °C). Upon cooling, the Al, which has a larger thermal expansion coefficient than the other materials present, and which is rigidly constrained by the passivation layer and the Si substrate, tries to contract more than its surroundings and thus is strained in three directions. This hydrostatic strain can be measured with x-ray diffraction3"6 and used to calculate the hydrostatic stress. Hydrostatic strains in excess of 1.93 X 10~3 have been predicted with finite-element methods and determined with x-ray diffraction.7 This corresponds to a hydrostatic stress on the order of 450 MPa. Simple plastic flow mechanisms such as dislocation glide and climb cannot relieve hydrostatic stress, and, thus, cavitation or
^Currently with Advanced Micro Devices, Integrated Technology Division, P.O. Box 3453, Mailstop 160, Sunnyvale, California 94088-3453. J. Mater. Res., Vol. 9, No. 1, Jan 1994
stress-induced voiding is often observed in these lines as a mechanism for relaxing the large stresses in these lines.8'9 These processes can result in failure of the line and of the chip, and, thus, stress-induced voiding is an important reliability concern in IC's. The relaxation associated with the formation of voids can be measured only if the three components of strai
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