Current Projects

1. 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. Publications: Han Li, Youbo Lin, Ting Y.Tsui and Joost Vlassak, "The effect of porosity on the stiffness and fracture energy of brittle organosilicates", Journal of Materials Research, 24(1) 107-116 (2009). (Download)

2. 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.

3. Nanoindentation analysis of thin films on substrate(Han Li)

A data analysis procedure has been developed to estimate the contact area in an elasto-plastic indentation of a thin film bonded to a substrate. The procedure can be used to derive the elastic modulus and hardness of the film from the indentation load, displacement, and contact stiffness data at indentation depths that are a significant fraction of the film thickness. The analysis is based on Yu’s elastic solution for the contact of a rigid conical punch on a layered half-space and uses an approach similar to the Oliver-Pharr method for bulk materials. The methodology is demonstrated for both compliant films on stiff substrates and the reverse combination and shows improved accuracy over previous methods. Publications: Han Li and Joost J. Vlassak, "Determining the elastic modulus and hardness of an ultra-thin film on a substrate using nanoindentation",Journal of Materials Research, 24(3) (2009). (Download)

4. 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.

Past Projects

1. 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, Gth 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: Youbo Lin, Joost J. Vlassak, Ting Y. Tsui, Andrew J. McKerrow, "Environmental effects on subcritical delamination of dielectric and metal films from organosilicate glass (osg) thin films", MRS Symposium Proceedings 766. E9.4 (2003). (Download) Youbo Lin, Joost J. Vlassak, Ting Y. Tsui and Andrew J. McKerrow, "Subcritical Delamination of Dielectric and Metal Films from Low-k Organosilicate Glass (OSG) Thin Films in Buffered pH Solutions", MRS Symposium Proceedings 795, 93-98 (2004). (Download) Joost J. Vlassak, Youbo Lin and Ting Y. Tsui, "Fracture of Organosilicate Glass Thin Films: Environmental Effects", Materials Science and Engineering A 391, 159-174 (2005). (Download)

2. 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. Publication: Youbo Lin, Ting Y. Tsui, and Joost J. Vlassak, "Water diffusion and fracture in organosilicate glass film stacks", Acta Materialia, in press. (Download) Ting Y. Tsui, Andrew J. McKerrow, and Joost J. Vlassak, "The Effect of Water Diffusion on the Adhesion of Organosilicate Glass Film Stacks", Journal of The Mechanics and Physics of Solids, 54 (5), 887-903 (2006). (Download)

3. 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: Youbo Lin, Ting Y. Tsui, and Joost J. Vlassak, "Octamethylcyclotetrasiloxane-based Low-permittivity Organosilicate Coatings: Composition, Structure and Polarizability", Journal of The Electrochemical Society, 153 (7), F144-F152 (2006). (Download)

4. Constraint effects on cohesive failures in low-k dielectric thin films

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: Ting Y. Tsui, Andrew J. McKerrow and Joost J. Vlassak, "Constraint Effects on Cohesive Failures in Low-k Dielectric Thin Films", Materials Research Society Symposium Proceedings, 863, 3 (2005). (Download) Ting Y. Tsui, Andrew J. McKerrow and Joost J. Vlassak, "Constraint Effects on Thin Film Channel Cracking Behavior", Journal of Materials Research, 20 (9), 2266-2273 (2005). (Download) J. J. Vlassak, "Channel cracking in thin films on compliant substrates", International Journal of Fracture 119 (4), 299-312 (2003). (Download)

5. 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: Zhengwen Li, Roy G. Gordon, Damon B. Farmer, Youbo Lin and Joost Vlassak, "Nucleation and Adhesion of ALD Copper on Cobalt Adhesion Layers and Tungsten Nitride Diffusion Barriers", Electrochemical and Solid-State Letters 8 (7), G182-G185 (2005). (Download)

Updated 2/10/2009