Contact Reactions at Cu / a-Ge Thin Film Couples
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CONTACT REACTIONS AT Cu / a-Ge THIN FILM COUPLES Uwe KOSTER, Klaus P. BLENNEMANN, Axel SCHULTE Dept.Chem.Eng., University of Dortmund, D-44221 Dortmund, F.R. Germany
ABSTRACT The aim of this paper is to investigate phase formation and growth kinetics in thin film Cu/a-Ge difflusion couples (150 nm Cu / 150 nm Ge) by means of cross-sectional transmission electron microscopy. During annealing in the temperature range between 100 and 180 0C a highly supersaturated c-phase was formed first; the growth of this phase exhibits a parabolic dependence, thus indicating diffusion controlled growth; further annealing leads to
a transformation into the orthorhombic s1-phase. The first phase formed during the contact reaction probably depends on the texture and orientation of the copper layer with (111) Cu favouring the formation of the p-phase. When in contact with crystalline Ge the orthorhombic s1-phase is formed directly, probably caused by a lack of driving force for the formation of the c-phase. Crystallization of Cu-contaminated amorphous Ge is characterized by the formation of an extremely fine-grained microstructure; higher Cu contents lead to primary crystallization of the s1-phase (orthorhombic Cu 3Ge) followed by polymorphous crystallization of the
amorphous matrix into crystalline Ge. These results indicate that the early formation of a crystalline interlayer is not due to the reduced crystallization temperature of an amorphous Ge(Cu) film as formed by Cu diffusion into the amorphous Ge.
INTRODUCTION With the further reduction of the lateral dimensions in integrated circuit technology the
pressure to replace aluminium as the element for metallization has increased. Copper has a very low electrical resistivity, but exhibits a high difflusivity in Si or Ge. Monoclinic (or distorted orthorhombic) Cu 3Ge (e -phase), however, has recently be claimed as a very attractive metallization choice for integrated circuits [1] thus leading to a wide interest in understanding
the reaction between Cu and Ge. This phase exhibits not only an extremely small resistivity, but also excellent corrosion behaviour as observed by potentiokinetic methods [2]; the stability against oxidation was first mentioned by Aboelfotoh [1]. Only a few papers describing the reaction between Ge and Cu can be found in literature. Oki et al. [3] observed that a copper layer can reduce the crystallization temperature of an amorphous Ge film from 420TC to about 195TC suggesting some relationship between the eutectic and the crystallization temperature. Herd et al. [4] confirmed the much lower crystallization temperature of germanium deposited onto a thin Cu film (20 nm) and observed that even at room temperature a reaction between Cu and Ge had already taken place, leading to the formation of hexagonal CusGe, even despite an overall composition with about 35 at.% Ge. The CusGe lines in the electron diffraction sharpened as grain growth proceeded; at temperatures above 500*C the authors observed a new fcc structure with a = 0.365 rnm. Hong et al. [5] inv
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