Influence of underlying interlevel dielectric films on extrusion formation in aluminum interconnects

Abstract

Knowledge of the mechanical properties of interlevel dielectric films and their impact on submicron interconnect reliability is becoming more and more important as critical dimensions in ultralarge scale integrated circuits are scaled down. For example, lateral aluminum (Al) extrusions into spaces between metal lines, which become more of a concern as the pitches shrink, appear to depend partially on properties of SiO2 underlayers. In this article nanoindentation, wafer curvature, and infrared absorbance techniques have been used to study the mechanical properties of several common interlevel dielectric SiO2 films such as undoped silica glass using a silane (SiH4) precursor, undoped silica glass using a tetraethylorthosilicate precursor, phosphosilicate glass deposited by plasma-enhanced chemical vapor deposition and borophosphosilicate glass (BPSG) deposited by subatmosphere chemical vapor deposition. The elastic modulus E and hardness H of the as-deposited and densified SiO2 layers are measured by nanoindentation. The coefficients of thermal expansion (CTE) of the densified layers are estimated by temperature-dependent wafer curvature measurements. Fourier transform infrared spectroscopy is used to obtain the chemical structures of all SiO2 layers. Among the four common interlevel layers, BPSG exhibits the smallest modulus/hardness and a relatively small amount of moisture loss during anneal. The BPSG shows the highest CTE, which generates the smallest thermal stress due to a closer match in the CTE between Al and SiO2. BPSG again has the lowest as-deposited compressive stress and the lowest local Si–O–Si strain before annealing. The center frequency of the Si–O bond stretching vibration exhibits a linear dependence on total film stress. The shifts of Si–O peaks for all the SiO2 layers also correlate well with the stress hysteresis obtained from wafer curvature measurements. Stress interactions between the various SiO2 underlayers and the Al metal film are also investigated. The impact of dielectric elastic properties on interconnect reliability during thermal cycles is proposed.