An x-ray spectromicroscopic study of electromigration in patterned Al(Cu) lines

Abstract

We studied the surface properties of patterned Al͑Cu͒ lines related to the electromigration phenomena using photoemission spectromicroscopy techniques. We stressed the lines for electromigration in situ in the ultrahigh vacuum microscope chamber and observed the changes on the line surface. Our results show surface precipitation of Cu beneath the Al 2 O 3 layer on the line surface as well as on side walls. Enrichment of grain boundaries in Cu due to electromigration flux was observed in areas downstream of voids with respect to the electron flow. © 1999 American Institute of Physics. ͓S0003-6951͑99͒01801-X͔ Electromigration in Al͑Cu͒ interconnect lines has been studied extensively as an important reliability problem in microelectronic circuits. In particular the role of Cu in improving resistance to electromigration damage has drawn much attention. 1,2 Cu is usually swept away from an area by electromigration before fast Al diffusion leads to appreciable damage in the line. 3 Dominant paths for electromigration flux of atoms are believed to be along grain boundaries and interfaces. Cu has very low solubility in Al at operation temperatures and has been shown to segregate into phase Al 2 Cu precipitates 4 as well as to grain boundaries and interfaces. 3,5 In electron microprobe measurements grain boundaries were shown to become rich in Cu depending on the prior thermal treatment of the film. 2,3 Cu must be effective in reducing the electromigration flux along these dominant paths to produce the observed improvement. Because of its critical role in the electromigration process, it is essential to obtain information on the distribution and chemical state of Cu in the grain boundaries, interfaces, precipitates, and grains during operation and test conditions. Photoemission spectroscopy is a powerful method used in studying physical and chemical properties of solid surfaces. However, the sampling area is ordinarily too large to obtain spatially resolved information on the scale that is necessary for analysis of microscopic changes associated with the electromigration process in interconnect lines. This limitation is overcome in MAXIMUM ͑installed at the Advanced Light Source of Lawrence Berkeley National Laboratory͒, a scanning photoemission spectromicroscope with a spatial resolution better than 0.1 m and an energy resolution of 300 meV. 6 In this study we used a number of contrast mechanisms afforded by this microscope including elemental and chemical sensitivity, and beam induced charging to investigate this problem.For this experiment a 600 nm thick Al 4 wt % Cu alloy was dc magnetron sputter deposited on Si wafers with 200 nm thermally grown SiO 2 . The wafers were not heated during the deposition and the base pressure of the deposition system was better than 1.3ϫ10 Ϫ5 Pa. Single and parallel lines were patterned using conventional photolithography techniques and wet etching. Samples were later annealed at 450°C for 30 min in forming gas and quenched. No passivation was deposited on the patterned wafers. Pattern...

A Selection Rule of Solutes for Void-Resistant Crystalline Metallic Alloys Exposed to Electromigration

“…Careful experiments with Cu-doped Al proved strong void formation by electromigration, as was shown by Luby et al [26] and Vavra et al [35]. A high Cu segregation around voids was also reported in a recent study by Solak et al [25]. This is by no means in contradiction to the common praxis of alloying aluminium with copper in order to extend lifetimes in wide polygranular interconnects, where grain boundaries play a vital role in atomic transport.…”

“…A consequence of such attraction is an increase in population of solute atoms around a [v, s]-pair complex, as well as an attraction of vacancies by the [v, s]-pair complex. Indeed results the attraction between vacancies and solute atoms due to the lattice deformation in an important argument for the fact, that (i) solute or impurity atoms are involved in the coalescence process of electromigration voids, and (ii) that the solute atom concentration is strongly enhanced around voids after electromigration has finished [25,26,28,33,35]. The joint migration of [v, s]-pair complexes has been studied by Monte Carlo simulation in previous work [36], where stability criteria for [v, s]-pairs in dependence on solute atom concentration, migration energy and temperature are formulated.…”

“…Electron microprobe measurement of Al(Cu) interconnects after electromigration stressing have shown that Cu accumulates in grain boundaries [23,24]. In a recent x-ray spectromicroscopic study by Solak et al [25], carried out with Al(4wt%Cu) prior and after electromigration testing, it was found that voids had formed with the interesting property, that inner grain boundaries became enriched in Cu, and this is particularly seen in the downstream direction of the voids. Pronounced accumulation of Copper at the inner walls of voids had been reported by Luby et al [26], too.…”

Measurements of evanescent wave in a sandwich Lamb wave sensor

“…1 The acoustic sensors, with their rapid response, portability, ease of use, and small size, are extremely useful for the analysis of both small and large molecules as well as whole cells interacting with an immobilized binding partner. 2,3 The output signal provides information about the mass loading, intrinsic properties of bound materials, or viscoelastic effects such as structural rearrangements.…”

X-ray microdiffraction study of Cu interconnects

“…[9][10][11][12][13] However, experimental verification is very difficult mainly because of the challenge of stress measurement in the extremely small volumes of lines with micron or submicron spatial resolution. In addition, interconnect lines must be studied in a realistic IC environment, thus limiting the applicability of surface sensitive techniques 14,15 since the complex structure requires a probe capable of penetrating several microns of dielectrics while maintaining a good spatial resolution. X-ray diffraction is a direct technique for strain measurement and has been successfully used for detailed characterization of average strain in interconnect lines.…”