Mechanical Stability of Porous Low-k Dielectrics

Abstract

This paper reviews the mechanical and fracture properties of porous ultralow-k dielectrics with the focus on chip package interaction related issues. It is shown that the mechanical and fracture properties of porous ultralow-k dielectric films are closely linked with porosity, pore morphology, network structure and deposition technology, while their fracture properties are also sensitive to reactive species in the environment. The survivability of low-k dielectrics upon integration, package assembly and subsequent reliability tests is therefore a combination of their mechanical stability, fracture properties, the specific mechanical or thermo-mechanical load and environmental effects. For the past three decades, the semiconductor industry has been improving the performance of microelectronic devices and the functionality of advanced integrated circuits through the continuous scaling of integrated circuits and the maximization of transistor density.1 Consequently, this encourages the introduction of new materials, processes, chip designs, and packaging strategies into micro-/nano-electronic products. Among these material innovations are insulating materials with a dielectric constant (k) less than that of SiO 2 , so called low-k dielectrics. During the last 15 years, various materials and methods have been developed for fabricating low-k dielectric films, among which the plasma enhanced chemical vapor deposition (PECVD) technology of porous organosilicate glasses (OSG) is the most popular due to its better compatibility with the technological needs.2-5 The reduction in k for low-k dielectrics is being pursued through the introduction of controlled levels of porosity. However, finding a good low-k material has proven to be much more challenging than first expected due to a number of integration and reliability issues.1 Besides the need of being compatible with the different lithography, etching, stripping and cleaning processes that are used in state-of-the-art integration schemes, they must also have sufficient mechanical strength to withstand the high shear stresses as well as harsh chemical environments that are involved during the chemical mechanical polishing process without cohesive or adhesive failure occurring.6-8 On top of that, since low-k dielectrics exhibit intrinsic tensile stresses and have increased coefficients of thermal expansion compared to SiO 2 , thin film cracking and adhesion are serious thermal-mechanical reliability issues for low-k dielectric materials (Figure 1). 9,10 Thermo-mechanical deformation of the package during package assembly and subsequent reliability tests can induce large local stresses that can initiate and propagate cohesive and/or adhesive cracks in different back-end of line (BEOL) layers.11 Therefore, a careful characterization of the mechanical integrity of potential new low-k candidates is required to integrate these new materials and Cu interconnects and to assure reliability during chip packaging and under field conditions. In the microelectronics industry, the elas...

Extraction of elastic modulus of porous ultra-thin low-k films by two-dimensional finite-element simulations of nanoindentation

“…From these equations, equivalent cone angles to represent Berkovich (a ¼ 65. 3 ) and cube corner (a ¼ 35.3 ) indenters in 2D are found to be 70. 3 and 42.3 , respectively.…”

“…3 ) and cube corner (a ¼ 35.3 ) indenters in 2D are found to be 70. 3 and 42.3 , respectively. This is done by calculating A c from Eq.…”

“…2 A considerable amount of these issues are associated with the inferior mechanical properties of the ultra-low-k materials. 3 On top of that, since these materials exhibit intrinsic tensile stresses and have higher coefficients of thermal expansion compared with SiO 2 , thin film cracking and adhesion are serious thermo-mechanical reliability issues. 4 Therefore, it is important to assess the mechanical properties of these films in a reliable and reproducible way.…”

“…To date, nanoindentation is the most commonly used technique in the semiconductor industry to assess the mechanical properties of thin low-k films such as elastic modulus and hardness. 3 Although the measurement of elastic properties with nanoindentation seems almost a routine job, the analysis is rather complex due to the porous nature of the films and the reduced film thicknesses (<100 nm) that are used in advanced technology nodes. Recently, Vanstreels et al demonstrated a clear correlation between the internal pore structure of ultra low-k films and their mechanical response upon indentation, thereby improving the fundamental understanding on how the measured elastic modulus value is influenced by porosity, the pore structure, the matrix stiffness, and material/probe interactions.…”

In-situ surface and interface study of atomic oxygen modified carbon containing porous low-κ dielectric films for barrier layer applications

“…7 Organosilicates (OSGs) are a promising class of organic-inorganic hybrid dielectric material for current and next-generation micro and nanoelectronic devices. 8 The introduction of hydrocarbon organic groups to the silicon oxygen network lowers the dielectric constant by decreasing the total density and polarizability of the material. Further reductions in the j value can be achieved by introducing porosity into these materials.…”