Low - k BEOL Mechanical Modeling

Low dielectric constant (low-k) is achieved often at the cost of degraded mechanical properties, making it difficult to integrate the dielectric in the back end of line (BEOL) and to package low-k chips. Development of low-k technology becomes costly and time-consuming. Therefore, more frequently than before, people resort to modeling to understand mechanical issues and avoid failures. In this paper we present three multilevel patterned film models to examine channel cracking in low-k BEOL. The effects of copper features, caps and multilevel interconnects are investigated and their implications to BEOL fabrication are discussed. INTRODUCTION Low-k materials have been aggressively pursued for the insulator to further reduce RC delay and cross talk in BEOL after copper replaced aluminum as the conductor. However it is very challenging to integrate low-k materials because their mechanical properties are severely compromised. Compared to oxide they generally have lower modulus and hardness, higher coefficient of thermal expansion (CTE), lower cohesive strength, and weaker adhesion. The properties usually get worse when pores are introduced to lower the k further. As a consequence, various mechanical failures, such as film cracking and delamination, may occur during BEOL integration, low-k chip packaging and reliability testing. The driving force to advance a crack is the energy release rate (ERR). If it is above the cohesive strength of the ILD, the crack will grow. The cohesive strength of low-k materials, which depends on the chemistry and the environment, can be measured by carefully controlled experiments. The reader is referred to a recent review [1] and the references therein for the cohesive strength. Instead, this paper focuses on the driving force only. The energy release rate can be calculated analytically for simple geometries, but numerical methods such as finite element have to be used for complicated structures. Generally the driving force calculated depends on the crack length. In the case of channel cracking the energy release rate increases with the crack length and levels off eventually. If the plateau value (steady-state energy release rate) is less than the cohesive strength, a defect of any size introduced during BEOL processing will not propagate. Therefore, in the worst case scenario, one only needs to consider a semi-infinite crack to calculate the energy release rate [2]. For given mechanical properties of a low-k material and copper/low-k integration process, we present channel cracking models to explore the dependence of the energy release rate on the interconnect structure and geometry.