Modification of Stress in CVD Tungsten Silicide (Polycide) Film by SiO 2 Capping
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MODIFICATION OF STRESS IN CVD TUNGSTEN SILICIDE (POLYCIDE) FILM BY SIO2 CAPPING Joshua Pelleg and E. Elish Department of Materials Engineering, Ben Gurion University of the Negev Beer Sheva, 84105, Israel ABSTRACT Stresses in chemical vapor deposited polycide tungsten silicide (poly-Si/WSi2) were evaluated at each stage of fabrication. The individual layers of the Si/SiO2/Poly-Si/WSi2/Poly-Si multilayer structure were deposited sequentially on separate wafers and subjected to x-ray diffraction analysis in the as deposited and annealed conditions to determine the changes in strain occurring in WSi2. Samples cut from wafers containing all the layers were capped with a 25nm thermal oxide and the strain in the WSi2 film was also analyzed by XRD. The change in strain of the WSi2 layer, following each step of the fabrication process, was evaluated by the lattice parameter variation of the c axis. The layers of the multilayered film affect the stress in the WSi2. A poly-Si layer on top of WSi2 reduces its stress, since it introduces a compressive component, which further decreases upon annealing. It also maintains a Si supply at the polySi/SiO2 interface, thus, eliminating Si outdiffusion during heat treatment in an oxygen containing ambient. Capping the system by a thin oxide layer modifies the stress pattern of the WSi2, which becomes compressive. INTRODUCTION Refractory metal silicides, such as tungsten silicide (WSi2) have become widely used in the past years, as low resistance gate electrodes and interconnect material in very large scale integration (VLSI), due to their well-established thermal stability and fabrication compatibility with the polycide technique [1-5]. A typical CVD WSix multi layer structure, used in the industry is Si/SiO2/Poly-Si/ WSix [3, 6]. Peeling is a serious problem in chemical vapor deposited (CVD) tungsten silicide [5-7] and is one of the limitations affecting more efficient advancement of this silicide in the industry. It is the consequence of high stresses acting in the film. The presence of very large stresses in thin films is a well known phenomenon and as a consequence stress related failure, such as peeling, plastic deformation, cracking or even fracture may occur. Of particular interest is the strain distribution in multi layer, thin film structure [8]. WSi2 produced by the silane process suffers from poor step coverage and adhesion and tend to peel off, especially over extreme topography [2]. Nevertheless, because of its many advantages, CVD WSix has been extensively used as polycide (WSix/poly-Si) gate and interconnections for VLSI [4]. Understanding the reasons and origin of the stresses present in WSix films is essential; controlling them is crucial and reducing their magnitude, is of great importance in order to obtain stable thin film layers with good adhesion to the substrate. Stresses present in the WSi2 layer have been investigated before by one of the techniques based on curvature measurement [2, 9 –13]. Various X-ray diffraction (XRD) techniques were also used, for measurin
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