Structure, Diffusion and Fracture Properties of ULK ILD Films (Han Li)

To meet the interconnect technology requirements in the future generations of high performance semiconductor devices, the dielectric constant of inter-layer-dielectrics needs to be further lowered to reduce the power consumption and line noise. One approach is to introduce pores in organosilicate glass. The first challenge, however, that the ULK dielectric has to face is to survive the mechanically demanding fabrication process, such as chemical mechanical planarization (CMP). It is thus crucial to fully understand how the porosity structure impacts on the strength and fracture resistance of the ULK dielectrics. The matrix and pore structures are engineered to optimize the mechanical properties to meet the process requirements that described in the international technology roadmap for semiconductor 2005.

 

Publication:

H. Li, T. Y. Tsui and J. J. Vlassak, "Water diffusion and fracture behavior in nanoporous low-k dielectric film stacks", J. Appl. Phys. 106, 033503 (2009). (Download)

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Porosity Effects on the Mechanical Properties Nanoporous Low-k Organosilicate Glass Thin Films (Han Li)

The facture and mechanical properties of OSG films with different composition were investigated by means of four-point bend test and bulge test. The stiffness is closely related to the density and the network connectivity of the material. The fracture toughness is found a function of the terminal/networking bonds ratio. It was also observed that the resistance to water-assisted subcritical fracture in ambient strongly dependent on the composition and structure of the material. The fracture behaviors were also studied in various aqueous environments. The degradation of adhesion due to water absorption was first observed.

Publication:

H. Li, Y. Lin, T. Y.Tsui and J. J. Vlassak, "The effect of porogen loading on the stiffness and fracture energy of brittle organosilicates", J. Mater. Res. 24(1), 107-116 (2009). (Download)

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Stiffening Mechanism in Organic-crosslinked Organosilicate Glasses: a Molecular Dynamics Study (Han Li)

It has recently been recognized that organic-crosslinked organosilicates possess significantly improved mechanical strengths in comparison with conventional organosilicates where carbon atoms exist mainly in dangling methyl groups. However, quantitative insights in the reinforcing mechanism at the molecular level remain elusive.

We focus on understanding how the elastic properties of organic-crosslinked organosilicates are correlated with their molecular network structure using molecular dynamics simulations. To elucidate quantitatively the contribution of various structural factors, we choose fully networked amorphous silica as a reference model. By gradually replacing some of the oxygen network atoms with methane units we demonstrate for the first time the possibility of synthesizing organosilicates that are stiffer than silica. The stiffening is achieved via a mechanism in which elastic deformation is mainly associated with the stretching/compressing of Si-O bonds rather than with bending of Si-O-Si as in amorphous silica. At the same time the mass density of the material decreases nearly linearly with carbon concentration, potentially leading to a reduction in the dielectric constant of more than 30%. The role of the dangling groups are also of interest.

Publication:

H. Li, J. M. Knaup, E. Kaxiras and J.  J. Vlassak, "Stiffening of organosilicate glasses by organic cross-linking", 2010. (submitted).

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Environmental Effects on Fracture of Low-k Organosilicate Glass Coatings (Youbo Lin)

The subcritical fracture behavior of OSG films from various barrier layers were systematically studied in ambient and aqueous environments utilizing four-point bend test. XPS was employed to characterize the fractured surfaces. The threshold energy release rate for crack to propagate is a function of the concentration of the reactive species that controls the reaction at the crack tip. In ambient, G_th is proportional to the log of water vapor pressure. In pH solutions, it is linearly dependent on the pH value. In aqueous environment, the transport-controlled region is clearly observed. Analytical models describing the reaction-controlled and transport-controlled fracture behaviors have been reviewed and quantified. It was found that the transport-controlled behavior in pH solutions was due to limited diffusivity of hydroxyl ions to the crack tip. The similarity between subcritical fracture of OSG in aqueous environments and results from a dissolution study suggests that both processes are controlled by the same mechanism.

Publications:

J. J. Vlassak, Y. Lin and T. Y. Tsui, "Fracture of organosilicate glass thin films: environmental effects", Mater. Sci. Eng. A 391, 159-174 (2005). (Download)

Y. Lin, J. J. Vlassak, T. Y. Tsui and A. J. McKerrow, "Subcritical delamination of dielectric and metal films from low-k organosilicate glass (OSG) thin films in buffered pH solutions", Mater. Res. Soc. Symp. Proc. 795, 93-98 (2004). (Download)

Y. Lin, J. J. Vlassak, T. Y. Tsui, A. J. McKerrow, "Environmental effects on subcritical delamination of dielectric and metal films from organosilicate glass (osg) thin films", Mater. Res. Soc. Symp. Proc. 766, E9.4 (2003). (Download)

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Water Diffusion and Degradation of the Fracture Properties of Organosilicate Film Stacks (Youbo Lin)

The degradation of the interfacial fracture toughness of silicon nitride/OSG interfaces as a function of time exposed to water has been examined. Experimental results are in good quantitative agreement with an analytical model that combines water diffusion with subcritical crack growth. Our experiments show that diffusion of water in the OSG film stacks is very fast and the degradation of the interfacial toughness is completely reversible. We also evaluated the effect of interfacial plasma treatments on the degradation process. The plasma treatments result in a significant enhancement of the interfacial toughness in the absence of water, but this enhancement is lost almost completely upon exposure of the film stack to water. If the interfacial toughness exceeds a critical value, crack propagation at the interface is unstable and the crack oscillates between the interface and the bulk of the film. The adhesion degradation data may be used to predict the crack growth in multi-layered structures involving nano-porous OSG layers.

Publications:

Y. Lin, T. Y. Tsui and J. J. Vlassak, "Water diffusion and fracture in organosilicate glass film stacks", Acta Mater. 55, 2455-2464 (2007). (Download)

T. Y. Tsui, A. J. McKerrow and J. J. Vlassak, "The effect of water diffusion on the adhesion of organosilicate glass film stacks", J. Mech. Phys. Solids 54(5), 887-903 (2006). (Download)

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Composition, Structure and Polarizability of Low-k Organosilicate Glass Thin Films (Youbo Lin)

Inter-layer-dielectric (ILD) in integrated circuits is conventionally silica, which has been replaced by organosilicate glass with low dielectric constant. The composition, structure and dielectric properties of OSG deposited by PECVD were systematically studied. The films incorporate more C and H, and less O, when the siloxane precursor/oxygen ratio increases. The reduction in the dielectric constant is due to both a decrease in dipole density and molecular polarizability. The lower dielectric constant of OSG as compared to silica is attributed mainly to the much lower density of OSG. A detailed FTIR analysis reveals that the film becomes less cross-linked when the carbon content increases because the siloxane network is more and more interrupted by -CH₃ and -H groups. We proposed a quantitative model for the OSG structure based on the functional groups identified using FTIR. The model makes it possible to determine the electronic polarizabilities of the individual bond configurations in OSG. The inverse IR absorption cross-sections for various bonds were derived from the model for future use.

Publication:

Y. Lin, T. Y. Tsui and J. J. Vlassak, "Octamethylcyclotetrasiloxane-based low-permittivity organosilicate coatings: composition, structure and polarizability", J. Electrochem. Soc. 153(7), F144-F152 (2006). (Download)

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Constraint Effects on Cohesive Failures in Low-K Dielectric Thin Films (Youbo Lin)

One of the most common forms of cohesive failure observed in brittle thin films subjected to a tensile residual stress is channel cracking, a fracture mode in which through-film cracks propagate in the film. The crack growth rate depends on intrinsic film properties, residual stress, the presence of reactive species in the environment, and the precise film stack. The effect of various buffer layers sandwiched between a brittle carbon-doped-silicate (CDS) film and a silicon substrate on channel cracking of the CDS film has been investigated. Channel cracking is enhanced if the buffer layer is more compliant than the silicon substrate. Crack velocity increases with increasing buffer layer thickness and decreasing buffer layer stiffness. This is caused by a reduction of the constraint imposed by the substrate on the film and a commensurate increase in energy release rate. The degree of constraint is characterized experimentally as a function of buffer layer thickness and stiffness, and compared to the results of a simple shear lag model.

Publications:

T. Y. Tsui, A. J. McKerrow and J. J. Vlassak, "Constraint effects on cohesive failures in low-k dielectric thin films", Mater. Res. Soc. Symp. Proc. 863, 3 (2005). (Download)

T. Y. Tsui, A. J. McKerrow and J. J. Vlassak, "Constraint effects on thin film channel cracking behavior", J. Mater. Res. 20(9), 2266-2273 (2005). (Download)

J. J. Vlassak, "Channel cracking in thin films on substrates of finite thickness", Int. J. Fract. 119(4), 299-312 (2003). (Download)

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Adhesion Study of ALD Dielectric and Metallic Thin Films (Youbo Lin)

Atomic layer deposition (ALD) can be used to make conformal and thin coatings. In semiconductor fabrication process, ALD is an ideal technique to deposit thin metallic and barrier layers in trench and via with high aspect ratio. The adhesion of these thin coatings remains a major concern during the fabrication process. Good adhesion has been found for ALD SiO₂/WN/Co/Cu structures. Other structures, such as TaN/Ru/Cu and WN/Ru/Cu were also investigated to determine the minimum thickness of the barrier films that are mechanically and electrically reliable. ALD is also a potential technique for pore sealing in porous low-k dielectrics. The sealing layer has been found to adhere well to the low-k dielectric.

Publication:

Z. Li, R. G. Gordon, D. B. Farmer, Y. Lin and J. Vlassak, "Nucleation and adhesion of ALD copper on cobalt adhesion layers and tungsten nitride diffusion barriers", Electrochem. Solid-State Lett. 8(7), G182-G185 (2005). (Download)

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Flexed Overlayer (FOL) Technique for Measuring Adhesion in Small Samples (Han Li)

Flexed overlayer (FOL) technique is developed for measuring adhesion in small samples. When an FOL sample is loaded, a crack nucleates at a notch in the top of the sample and penetrates into the ceramic/metallization interface, growing along that interface. Growth of the interfacial crack decreases the sample stiffness (applied load divided by load point displacement), making it possible to extract the crack length from the change in sample stiffness and to calculate the critical energy release rate for delamination. This critical energy release rate provides an estimation of the adhesion energy of the target interface. The central section of the fixture where the sample is glued is designed to be much thinner than the rest of the fixture in order to maximize the change in sample stiffness as the crack propagates.