Investigation of diffusion and segregation in thin films

Motivation

  • Copper alloys are considered as potential next generation interconnects in microelectronics. Alloying should improve electromigration behaviour without significant increase in specific resistance. Therefore the alloying element should develop its effect in the suspected main path of electromigration, i.e. the grain boundaries and interfaces.Preparation of Cu alloys and understanding the effect of dopants require to investigate the interdiffusion of the elements and segregation of dopants towards the diffusion path, i.e. interface Cu/barrier and grain boundaries, respectively.

  • Electron spectroscopies (AES, XPS) are suited to study surface segregation because of their intrinsic surface sensitivity and the fact that the measurements are carried out under ultra high vacuum conditions were the reaction of the segregating species with the ambient can be minimized.

Ag on Cu films

  • We have studied the dissolution and segregation of thin layers of Ag deposited on sputter cleaned Cu thin films by Auger electron spectrometry (AES). Deposition and annealing were carried out in the preparation chamber of the Auger microprobe PHI660.
  • Silver layers of 0.6 nm thickness were deposited onto sputter cleaned Cu films. With annealing in ultra high vacuum the steady-state Ag coverage decreases to about 1 monolayer (Fig. a: anneal_1), i.e. silver is partly dissolved in the Cu film. The Ag surface layer can be removed by sputtering and renewed several times by annealing (anneal_2, _3) due to segregation of Ag dissolved in Cu.
  • The dissolution of 0.6 nm Ag (= 0.2 at-% in the Cu film) increases the specific resistivity in comparison with annealed Cu film without Ag addition (Fig. b).
  • In contrast, the evolution of the microstructure (grain size, lattice parameters, strain) is dominated by annealing, no significant differences by addition of Ag were found.


CuAg Surface composition
CuAg Resistivity

Surface composition measured by AES after different treatments. The coverage (number of adsorbed atoms per Cu atom) is given in monolayers (ML).

Specific resistivity of sputtered Cu layers after different treatments.

 



Segregation of impurities

  • Impurities – either present in the samples or adsorbed from the ambient – can influence the diffusion into the sample, across the surface and the segregation from the sample interior to the surface.

 

  • The figures below show the varying surface coverage (i.e. the number of adsorbed atoms per Cu atom) measured by AES during annealing of different Cu samples under similar heating conditions. The samples are distinguished by their different purity (depending on the origin) and thickness. Purity, microstructure, and the total amount of impurities (layer thickness) determine the kinetic curves.


Cu Segregation ECD
Cu Segregation PVD

Sample: layer of electrodeposited Cu

(d = 2 µm)

Sample: layer of magnetron sputter deposited Cu

 (d = 200 nm)

Cu Segregation foil

Sample: polycrystalline Cu foil

(d = 250 µm)