UC Berkeley

Low pressure plasma processing is used extensively in the semiconductor industry for modern technology. As a result, the understanding of plasma/material interactions is very important to the improvement and design of materials and treatment methods. This dissertation focuses on the understanding of interactions between individual plasma species and materials that are used in the fabrication of the interconnect in semiconductor devices, namely porous low-k dielectrics.

Proper insulation of the metal lines in the interconnect relies on maintaining the integrity of the insulator's dielectric properties. As the features of the interconnect are shifting to smaller dimensions, techniques used to implement lower dielectric permittivity have also lowered the material's resistance to plasma-induced damage. As a result, plasma processing of these materials have become a major challenge for future advances in interconnect technology. This study examines specific species generated from typical plasma discharges and their effect on porous low-k materials.

The first part of this dissertation studies the mechanism for photon-induced effects and demonstrates the influence of emission wavelength on carbon removal from carbon-doped oxide films. VUV photons emitted from the plasma were observed to break Si-C bonds. However, oxidizing gas species must be present in the background to cause carbon removal. Depending on gas chemistry, VUV photons of different wavelengths are emitted, which affects the depth of the damage penetration into the film. Shorter wavelength emissions are absorbed by the SiO2-like damage layer that is produced after carbon extraction, thereby preventing further removal of carbon.

The dissertation continues by examining the effects of photons and radicals individually by isolating the species to obtain separate exposures. By doing so, radicals and photons generated from O2 plasma were observed to remove carbon in different ways, creating different carbon profiles as a function of depth. 130 nm wavelength photons are fairly transparent through the material, leading to a gradual removal of carbon throughout the modification depth. In contrast, oxygen radicals are diffusion-limited, leading to removal of carbon occurring as a front. Modeling of damage effects by these species was performed in each case, and good predictions of their behavior were obtained. However, direct plasma exposures were observed to behave differently, exhibiting much less carbon removal than predicted by the model. Further experimentation found evidence that synergy between photon and radical species led to an effective decrease in the diffusivity of the modified material, reducing subsequent plasma damage.

Through fundamental study of plasma/material interactions, the role of photons and radicals in plasma-induced damage has been determined. As a result, treatment methods to reduce plasma damage based on inhibiting these species can be designed. These include plasma filtration techniques, densification by ion bombardment, and the use of plasmas with reducing chemistries. While the scope of this research has focused on interactions with dielectric thin films, these approaches to plasma interactions are relevant to other processes that rely on plasma processing.