Morphology of Damage in Al Films Tested Under Electromigration Conditions Using the Drift Velocity Method

  • PDF / 1,906,950 Bytes
  • 6 Pages / 414.72 x 648 pts Page_size
  • 42 Downloads / 206 Views

DOWNLOAD

REPORT


ABSTRACT Pure Al films deposited by the partially ionized beam technique at bias lavels of 0, 3 and 6 kV onto TiN sublayers were patterned for measurement for direct drift velocities. Then films were annealed in a vacuum 10-4 Pa for I hour. Electromigration tests were performed in an air atmosphere at a temperature of 280'C. The current density during tests was 6.105 A/cm 2 . Examinations in the SEM showed that the morphology of the damage on cathode and anode ends of the Al stripes depends on the bias during deposition. In the case of 0 kV bias, voids nucleate on the back and side edges of the cathode end and propagate toward the anode end. Hillock and whisker formation was observed on the anode end. The height and diameter of the whiskers were about 50 and 1-2 ptm, respectively. In the case of 3 and 6 kV bias the drifting edge was smooth. Only hillocks (no whiskers) grew on the anode edge. The morphological differences of these films are discussed on the basis of self-ion bombardment effects. INTRODUCTION Recently we have shown [1,2] that the structure and properties of pure aluminum films can be varied in a wide range using bombardment by self-ions. It has also been demonstrated that the time to failure in an EM (electromigration) test of the annealed pure Al films deposited at the accelerating voltage 5 kV is about an order of magnitude greater than that of the films deposited at 3 kV and without an accelerating voltage [2]. An increased resistance to EM in Al films deposited by a partially ionized beam (PIB) is usually considered to be due to a strong (11l) texture in the films [3,4]. The deposition technique which we used [5] differs from the PIB deposition technique proposed by Mei and Lu [6] and provides a high ionization efficiency (about 10%). It has been shown that the increase in the ion-to-atom ratio and ion energy result in the depression of preferred orientation [7]. We also observed no well-defined texture in our films. Therefore, we attributed the higher EM resistance of these films to a greater number of equilibrium special GB (grain boundary) in the annealed Al films prepared under the higher bias [2]. In this work we studied the EM behavior of pure Al films on TiN sublayer using direct drift velocity measurements. The Al films were prepared by the PIB deposition described in [5]. EXPERIMENTAL PROCEDURE The test structure which was used to measure the aluminum drift velocity is similar to that described in [8]. The structure was fabricated as follows. 501 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society

Silicon wafers were oxidized to form a SiO 2 layer about 100 nm thick. Then TiN was rf-sputtered to form a film 250 nm thick on the oxide. After that, high purity aluminum was deposited using PIB technique [5] at a rate of about 2 nm/s onto unheated substrate. The ion-to-atom ratio was about 6%. The ions were accelerated by potentials of 0, 3 and 6 kV applied to the accelerating electrode located near the substrate. The thickness of the Al film was 300 nm. The aluminum was ph