Thomas N. Marieb

Observations of electromigration induced void nucleation and growth in polycrystalline and near‐bamboo passivated Al lines

Electromigration voiding in passivated pure Al lines was observed in situ using high voltage scanning electron microscopsy. Two different types of lines were investigated; one deposited under high purity conditions with a near‐bamboo microstructure, and one deposited under conventional conditions with a polycrystalline microstructure with stress voids present. The samples were observed while being tested under accelerated electromigration conditions. Samples were then thinned and analyzed with transmission electron microscopy techniques to investigate the relationship between line microstructure and void nucleation. Electromigration voids and stress voids were seen to nucleate at very specific, unique, microstructural sites that require the intersection of a grain boundary with the line sidewall. Void movement after initiation is also dictated by microstructure, with voids only growing into and causing failure in grains oriented with a {111} plane near perpendicular to the line. The conditions for void nu...

Observation of voids induced by mechanical stress and electromigration in passivated Al lines deposited at different purity levels

Two pure Al passivated line samples were prepared under different metal deposition conditions, and were electromigration tested in a high voltage scanning electron microscope (HVSEM). The sample prepared under ultrahigh purity conditions showed a high resistance to void initiation and a very large grain size, while the more conventionally deposited metal displayed comparatively poor electromigration resistance. The sample prepared at lower purity conditions showed initial stress voids, but these were not the site for electromigration damage.

Void phenomena in passivated metal lines: Recent observations and interpretation

Since the pioneering TEM work of Blech and Meieran thirty years ago, it has been evident that direct in-situ observations of electromigration in progress can provide information essential to the understanding of the phenomenon. Unfortunately, the practical difficulties of carrying out such observations have been so great that almost all experimental work on electromigration in passivated metal has been limited to lifetime measurements and post-mortem examination of failed specimens. Without an adequate qualitative picture, theoretical efforts at detailed modeling of the phenomenon have had limited success. In particular, the extremely large variance observed in life tests of virtually identical lines has not been satisfactorily explained. To make possible the necessary in-situ observations, Intel and Stanford have, for some years, been engaged in the development of specialized SEM technology leading to a High Voltage Scanning Electron Microscope (HVSEM) with complete computer control. A special computer g...

Measurement and Modeling of Intrinsic Stresses in CVD W Lines

We have studied stress states in chemical vapor deposited (CVD) tungsten (W) for both blanket films and lines, to understand better the mechanical implications of intrinsic stress for interconnection structures. Since W has a low mobility at its deposition temperature, a very large intrinsic stress develops during deposition. Intrinsic strains in blanket W films were measured with an X-ray technique. The measured strains correspond to a biaxial tensile stress of the order of 1 GPa. This result was used to provide an initial strain input in a finite element calculation to obtain intrinsic stress states in W lines. SEM observation of cross sections of the metal lines enabled us to determine the growth pattern of the W, and infer the boundary conditions during growth. Finite Element Method (FEM) calculations of the room temperature stress in the lines, including both intrinsic and thermal components, are in good agreement with X-ray determinations.

Direct observation of the growth and movement of electromigration voids under passivation

In situ high voltage scanning electron microscope (HVSEM) examinations of accelerated electromigration tests were performed. The samples tested were Al‐1% Si lines 3 μm wide, 1 μm high, and 300 μm long. Voiding processes were observed through passivation and the early stages of voiding were documented. Voids nucleated at the sidewall of the line at the metal‐passivation interface. The voids then grew in a wedge‐like manner. The later stages of movement and coalescence of voids, leading to eventual failure, were also imaged. A simple model of the early stages of voiding is proposed.

The relationship between resistance changes and void volume changes in passivated aluminum interconnects

We performed electromigration tests on 11 passivated Al lines at 212/spl deg/C and 3 MA/cm/sup 2/ inside a high voltage scanning electron microscope (HVSEM). Our automated experimental apparatus saved an image of the entire test structure every six minutes while it continuously measured and recorded the resistance of each line. We determined the void volume as a function of time using digital image processing. While it is straightforward to determine resistance changes knowing the changes in the void geometry, it remains to be seen whether one can perform the reverse calculation. From our data, the amount of information that can be extracted solely from the resistance versus time curves appears limited. The nucleation time of the first void can usually be found, but it is not possible to detect the nucleation of subsequent voids. The resistance change depends much more critically on void shape than void size, making an estimation of the void volume from the resistance impossible.