Environmental Effects on Subcritical Delamination of Dielectric and Metal Films from Organosilicate Glass (OSG) Thin Fil
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Environmental Effects on Subcritical Delamination of Dielectric and Metal Films from Organosilicate Glass (OSG) Thin Films Y. Lin, J.J. Vlassak, T.Y. Tsui1, and A.J. McKerrow1 DEAS, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA 1 Silicon Technology Development, Texas Instruments Inc., Dallas, TX 75243, USA ABSTRACT Subcritical delamination of dielectric and metal films from organosilicate glass (OSG) thin films was studied in controlled ambient with different levels of relative humidity and in aqueous environments of varying pH. The material systems studied include OSG/SiO2, OSG/TaN and OSG/SiNx. For both sets of experiments, subcritical crack growth in OSG is found to be described by a model originally developed for soda-lime silicate glass. The threshold energy release rate for water molecule-assisted cracking varies linearly with the natural logarithm of water partial pressure. In aqueous environments, the threshold value decreases linearly with increasing pH in accordance with a simple model. The slope of crack growth rate curve also decreases with increasing pH. INTRODUCTION Organosilicate glasses (OSG) are leading candidates among new low-k dielectric materials that are being assessed for use as interlayer dielectric (ILD) in high-performance interconnects. OSG is essentially silicon dioxide in which a fraction of the Si-O bonds have been replaced with bonds to organic groups, typically methyl groups (e.g., -CH3). As a result, OSG has a network structure similar to that of fused silica, but less dense because of the presence of the -CH3 groups. The dielectric constant is reduced from 4 to approximately 2.8 – 3.1. A consequence of this change in bonding and density is that the mechanical properties (hardness, toughness, modulus, etc.) of OSG films are inferior to those of silica. During the many wet-processing steps involved in semiconductor device fabrication, OSG films are subject to mechanical loads in aggressive chemical environments. Typical examples include loads that arise during chemical-mechanical polishing (CMP) or dicing, as well as loads due to residual stresses in the film stack. Under such conditions there is a concern that OSG films may be vulnerable to stress-corrosion, leading to delamination of the film stack. To better understand this issue, we present an investigation of subcritical delamination of various dielectric and metal barrier films from OSG thin films in controlled ambient with different levels of relative humidity and in aqueous environments of varying pH. EXPERIMENTAL DETAILS Subcritical crack growth was studied by means of the four-point bending technique. To this purpose, 500 nm OSG films were deposited onto 200 mm silicon wafers using PECVD. The OSG films were capped with three different barrier layers: 80 nm of SiNx, 30 nm of TaN, and 250 nm of PECVD SiO2 (TEOS precursor). The TaN wafers were subsequently sputter coated with 150 nm of Cu; the SiNx and SiO2 wafers were coated with a 70 nm adhesion layer of Ti, followed by 300 nm of Cu. Four-point-bend s
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