An In Situ Transmission Electron Microscopy Study During NH 3 Ambient Annealing of Cu-Cr Thin Films
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Z. ATZMON*, R. SHARMA*, J.W. MAYER*, AND S.Q. HONG** * Center for Solid State Science, Arizona State University, Tempe, AZ 85287 **Dept. Mater. Sci. Eng., Cornell University, Ithaca, NY 14853
ABSTRACT Nitridation of Cu-Cr alloy films under an NH 3 ambient was studied using in situ transmission electron microscopy. Cu-Cr thin films (40-100 nm) were deposited on a single crystal NaCl substrate by electron beam coevaporation, and were heat treated up to 750 0C at 2.5-3.0 Torr NH 3 . The films were also vacuum (10-6 Torr) annealed under the same conditions for comparison. Initial observation of Cu and Cr crystallization occurred at 470'C for both environmental conditions. The nitridation process of Cr to form CrN was observed initially at 580'C and was followed by evolution of faceted Cu grain growth in the CrN matrix.
INTRODUCTION Copper is being extensively studied as a potential new interconnect material for microelectronics devices due to its physical properties such as low electrical resistivity, high melting temperature, and high electromigration resistance. However, the application of Cu in the technology of thin films is restricted by many problems which are encountered during the fabrication process. Some of the major difficulties include: poor adhesive behavior to dielectric films such as Si0 2 and Si 3N4 [1,2], strong diffusion tendency into adjacent materials such as Si and Si0 2 [3], and propensity for reaction with silicides. During this reaction Cu usually diffuses through the silicide film to the silicide/Si interface to form Cu 3Si. In NiSi/Si structure, no sign of dissociation of NiSi was observed [2]. In some systems, such as Pd2Si/Si [2], CoSi 2/Si, CrSi 2/Si, and TiSi 2/Si [4], Cu has been observed to diffuse into the silicide layer and silicide/Si interface to react with both the Si and the metal in the silicide to form Cu 3 Si and intermetallic phases. It is essential that a diffusion barrier for Cu-based metallization be used. An effective diffusion barrier should be a thin, conducting, thermally stable layer in between Cu and underlying material. From the above requirements, several barrier layers have been tested, most of them based on a refractory metal nitride such as TiN [5], TaN and Ta2N [6], and NbN [2]. In the present research the Cu-Cr system is used with Cu films containing up to 50% Cr. Nitridation of Cr under an NH 3 ambient is an attractive process to form a CrN layer, which is a strong candidate as a diffusion barrier 245 Mat. Res. Soc. Symp. Proc. Vol. 317. @1994 Materials Research Society
for Cu-based metallization. This process is based on the thermodynamic concept that Cr 3N is unstable, since the heat of formation for this phase is 4.5 kcal/g while for CrN it is -14.0 kcal/g [7]. The equilibrium phase diagram for the Cu-Cr system shows that the system is immiscible, namely, there is no intermediate phase formation during the reaction. This fact makes control of the nitridation process easier since the products of reaction favor the formation of chromium nitride and copper
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