Ting Y. Tsui

The Effect of Porogen Loading on the Stiffness and Fracture Energy of Brittle Organosilicates

Integrating porous low-permittivity dielectrics into Cu metallization is one of the strategies to reduce power consumption, signal propagation delays, and crosstalk between interconnects for the next generation of integrated circuits. The porosity and pore structure of these low- k dielectric materials, however, also affect other important material properties in addition to the dielectric constant. In this paper, we investigate the impact of porogen loading on the stiffness and cohesive fracture energy of a series of porous organosilicate glass (OSG) thin films using nanoindentation and the double-cantilever beam (DCB) technique. The OSG films were deposited by plasma-enhanced chemical vapor deposition (PECVD) and had a porosity in the range of 7−45%. We show that the degree of porogen loading during the deposition process changes both the network structure and the porosity of the dielectric, and we resolve the contributions of both effects to the stiffness and fracture energy of the films. The experimental results for stiffness are compared with micromechanical models and finite element calculations. It is demonstrated that the stiffness of the OSG films depends sensitively on their porosity and that considerable improvements in stiffness may be obtained through further optimization of the pore microstructure. The cohesive fracture energy of the films decreases linearly with increasing porosity, consistent with a simple planar through-pore fracture mechanism.

Adhesion Degradation and Water Diffusion in Nanoporous Organosilicate Glass Thin Film Stacks

The diffusion of water in nanoporous organosilicate glass (NPOSG) film stacks causes significant adhesion degradation of the capping layer on top of the NPOSG. We have used this adhesion degradation to estimate the diffusivity of water in an NPOSG film stack. The effective diffusivity is 1.0 × 10- 9 m 2 /s, nearly 2 orders of magnitude larger than in previous generations of dense organosilicate glass film stacks. This result is consistent with the diffusion coefficient measured using secondary-ion mass spectroscopy and a deuterium oxide tracer. An optical microscopy study yields similar results for the diffusion of toluene in NPOSG film stacks, but the optical technique is not suitable for measuring the diffusion coefficient of water.

Subcritical Delamination of Dielectric and Metal Films from Low-k Organosilicate Glass (OSG) Thin Films in Buffered pH Solutions

Understanding subcritical fracture of low-k dielectric materials and barrier thin films in buffered solutions of different pH value is of both technical and scientific importance. Subcritical delamination of dielectric and metal barrier films from low-k organosilicate glass (OSG) films in pH buffer solutions was studied in this work. Crack path and subcritical fracture behavior of OSG depends on the choice of barrier layers. For the OSG/TaN system, fracture takes place in the OSG layer near the interface, while in OSG/SiNx system, delamination occurs at the interface. Delamination behavior of both systems is well described by a hyperbolic sine model that had been developed previously based on a chemical reaction controlled fracture process at the crack tip. The threshold toughness of both systems decreases linearly with increasing pH value. The slopes of the reaction-controlled regime of the crack velocity curves for both systems are independent of pH as predicted by the model. Near transport-controlled regime behavior was observed in OSG/TaN system.

The effects of passivation layer and film thickness on the mechanical behavior of freestanding electroplated Cu thin films with constant microstructure

The goal of this paper is to investigate the effects of film thickness and the presence of a passivation layer on the mechanical behavior of electroplated Cu thin films. In order to study the effect of passivating layers, freestanding Cu membranes were prepared using standard silicon micromachining techniques. Some of these Cu membranes were passivated by sputter depositing thin Ti films with thicknesses ranging from 20 nm to 50 nm on both sides of the membrane. The effect of film thickness was evaluated by preparing freestanding films with varying thickness but constant microstructure. To that effect, coatings of a given thickness were first vacuum annealed at elevated temperature to stabilize the microstructure. The annealed films were subsequently thinned to various thicknesses by means of chemical mechanical planarization (CMP) and freestanding membranes were prepared both with and without Ti passivation. The stress-strain curves of the freestanding Cu films were evaluated using the bulge test technique. The residual stress and elastic modulus of the film are not affected significantly by the passivation layer. The elastic modulus does not change with film thickness if the microstructure keeps constant. The yield stress increases if the film is passivated. For passivated films, yield stress is proportional to the inverse of the film thickness, which is consistent with the formation of a boundary layer of high dislocation density near the interfaces.

Effects of silicon carbide composition on dielectric barrier Voltage Ramp and TDDB reliability performance

Silicon carbide films containing either nitrogen or oxygen were integrated within a dual-level metal copper interconnect and characterized using Voltage Ramp and TDDB testing. Oxygen containing silicon carbide films were characterized by poor dielectric breakdown properties, but their performance improved with a short soak in ambient at elevated temperatures. This data suggests that oxygen containing silicon carbide films have poor moisture barrier properties. Similar evaluation of nitrogen containing silicon carbide films revealed materials properties that were more similar to those of silicon nitride. TDDB comparison of all three dielectric films is consistent with conclusions from the Voltage Ramp study.

Effects of elastic modulus on the fracture behavior of low-dielectric constant films

A model that predicts channel-crack propagation behavior in silica-based low-/spl kappa/ dielectrics (low-/spl kappa/) was developed. A solid-mechanics theory that governs fracture behavior was used to obtain low-/spl kappa/ material constants. These fracture parameters were used to predict crack behaviors in five low-/spl kappa/ films with distinct elastic moduli. The model developed demonstrates that crack propagation rate is extremely sensitive to modulus, especially when the material is compliant.

Environmental Effects on Crack Characteristics for OSG Materials

To improve capacitance delay performance of the advanced back-end-of-line (BEOL) structures, low dielectric constant organosilicate glass (OSG) has emerged as the predominant choice for intermetal insulator. The material has a characteristic tensile residual stress and low fracture toughness. A potential failure mechanism for this class of low-k dielectric films is catastrophic fracture due to channel cracking. During fabrication, channel cracks can also form in a timedependent manner due to exposure to a particular environmental condition, commonly known as stress-corrosion cracking. Within this work, the environmental impacts of pressure, ambient, temperature, solution pH, and solvents upon the channel cracking of OSG thin films are characterized. Storage under high vacuum conditions and exposure to flowing dry nitrogen gas can significantly lower crack propagation rates. Cracking rates experience little fluctuation as a function of solution pH; however, exposure to aqueous solutions can increase the growth rate by three orders of magnitude.

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. In this paper, we investigate 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. The results show that 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 that was proposed previously.

Effects of dielectric liners on TDDB lifetime of a Cu/ low-k interconnect

Thin films of silicon nitride (SiN) or silicon carbonitride (SiCN) were deposited as liners at metal-1 in a dual level metal Cu/organosilicate glass interconnect. Breakdown field and time dependent dielectric breakdown lifetime testing of comb/serpent test structures with SiN or SiCN liners showed improvements in performance, relative to a baseline no liner split. Two dimensional electric field simulations demonstrated that the dielectric liner significantly reduced the electric field at the Cu/OSG/etch stop interface.

Fracture Property Improvements of a Nanoporous Thin Film via Post Deposition UV Curing

As silicon-based microelectronic devices continue to aggressively scale down in size, traditional BEOL dielectric materials have become obsolete due to their relatively high dielectric constant. Organosilicate glass (OSG) materials have emerged as the predominant choice for intermetal dielectrics in advancing technology nodes. A potential failure mechanism for this class of low-k dielectric films during the manufacturing process is catastrophic fracture due to channel cracking. To improve the mechanical strength and stability of these silicon-based materials, the use of post-deposition curing processes is under evaluation. In this work, the effects of UV curing on the properties of OSG films were characterized. After UV curing, film hardness and elastic modulus are improved, with no change in the residual film stress. The average film density increases linearly as a function of UV exposure time. Channel crack propagation velocities decrease with UV exposure. The improvements in the mechanical properties of OSG films are believed to correlate with the increasing Si-O-Si bond population. Comparisons between post-deposition UV and Electron Beam curing processes are provided.