Identification of lattice defects in Cu thin films by positron annihilation spectroscopy

Abstract

Positron lifetime measurements have been performed for Cu thin films in order to identify lattice defects. Theoretical positron lifetimes of vacancy clusters of Cu are also calculated for identification of the experimental values. Cu thin films were prepared by electro chemical deposition (ECD) and physical vapour deposition (PVD). All of the Cu thin films have longer positron lifetimes than bulk Cu does. This indicates that lattice defects are introduced during deposition. However composition analysis of positron lifetimes shows that whereas vacancy clusters exist in the ECD-Cu thin film, the positron lifetime of defect component in the PVD-Cu thin films is shorter than that of mono-vacancy. On the other hand, the positron lifetime of mono-vacancy was detected in PVD-Cu-0.5 at%Sb thin film prepared by PVD. In order to elucidate the effect of Sb, first-principles calculations have been performed for Cu including Sb and monovacancy. The result suggests that mono-vacancies are introduced because of the binding energy between a Sb atom and mono-vacancy. 1 Introduction The dual-damascene fabrication process is presently recognized as a standard Cu interconnection technique. In this technique, metallic barrier layers and Cu seed layers are deposited in successive order on the inside walls of via holes or trenches using a sputtering method (physical vapour deposition; PVD). The Cu interconnections are then embedded into via holes or trenches using an electrochemical deposition (ECD).As the diameter of the via holes becomes smaller and smaller, the relative embedding performance tends to decline. The high pressure anneal process is attracting attention because of its excellent capabilities in the filling performance of high aspect-ratio via holes and improved adhesion of wiring Cu interconnections. The PVD method is attractive from the viewpoint of simplifying the entire process and alloying of Cu-film to further enhance the electron migration resistance for example. However the PVDCu films requires higher temperatures than the ECD films to fill via holes completely by the high pressure annealing process [1,2]. On the other hand, it was found that the addition of Sb to the PVD-Cu lowers the required temperature. In the present study, positron lifetime measurement and its theoretical calculation have been performed for the ECD and PVD-Cu thin films in order to identity lattice defects, which can affects the filling behaviour.

Frozen-in vacancies in PVD-Cu films with improved high-pressure reflowability studied using a slow positron beam

“…The positron lifetime for this sample was 111 ps. This value agreed with the reported lifetime of positrons that are delocalized and annihilated in interstitial sites [3]. This indicates that the S value of the annealed bulk-Cu sample, observed at E = 25 -30 keV, was the characteristic value of S for a Cu perfect crystal.…”

“…This means that the S value in this energy range corresponds to the annihilation of positrons in the 1 µm pure-Cu film. A previous study on conventional positron annihilation lifetime spectroscopy indicated that almost all positrons which were implanted into a PVD-pure-Cu film were trapped at grain boundaries and annihilated there because of submicron grain sizes [3]. This was the reason for the higher value of S for a pure-Cu film compared to the value of a Cu perfect crystal.…”

“…The Cu-Sb film thus contains a large amount of vacancies compared to a pure-Cu film. It is well known that the binding energy between a Sb atom and a Cu mono-vacancy has a relatively high value (0.32 eV experimental value [9], 0.39 eV calculated value [3]). Therefore, we can interpret this drastic change in the S value for the Cu-Sb dilute alloy film in the following way: vacancies were introduced during deposition by the high binding energy between a Sb and Cu mono-vacancy.…”

“…The value of S for the annihilation of positrons trapped by vacancies is larger than that for positrons annihilated from the interstitial sites. Recent positron annihilation spectroscopy studies have suggested that ECD-Cu films have a larger amount of vacancy clusters than PVD-Cu films [2][3][4]. In this paper, we investigate lattice defects in a PVD-CuSb dilute alloy film compared to a PVD-pure-Cu film by measuring the Doppler broadening of the annihilation radiation using a slow positron beam.…”

A Study of Difference in Reflow Characteristics Between Electroplated and Sputtered Cu in a Dual-Damascene Fabrication Process for Silicon Semiconductor Devices

“…From these considerations, it is concluded that the defect component of s d1 is due to dislocations or grain boundaries. 38 From these results, we can say that the reflow behavior of Cu films is accelerated by the presence of vacancies or dislocations, which are present in large numbers compared with equilibrium point defects in as-deposited Cu films. The Cu films are embedded in the vias and trenches by the abrupt migration of vacancies or dislocations during isochronal annealing above 250°C.…”