Improvement of Mechanical Strength of Porous Low-k Silica Thin Film
Date Issued
2004
Date
2004
Author(s)
Tsai, Cheng-Tsung
DOI
zh-TW
Abstract
As ultra-large-scale circuits (ULSIs) continue to shrink, chip performance becomes increasingly limited by back end of the line due to interconnect delay (RC delay), which cause the increase of power consumption. In order to meet the requirement for ULSIs in submicron region, development of low dielectric constant materials is inevitable. The incorporation of porosity into the films is a feasible method to reduce the dielectric constant. However, the introduction of pores reduces the mechanical strength. Low-k films with insufficient mechanical strength result in failure during CMP process. Improvement the mechanical strength of low-k film is essential.
Several methods were attempted to improve the mechanical strength in this research: aging after spin coating and baking, 70℃ reflux of the precursor of coating solution, addition of TPAOH in the reflux solution, and use PEG1450 as a template. Microscope, dielectric constant, leakage current, mechanical strength, porosity and FTIR were used to analyze the films. It was found from this study that aging the films in the mixture of ammonium hydroxide and ethanol was able to enhance the gelation of the films, and increased the elastic modulus from 5.5GPa to 6.8Gpa. Moreover, the leakage current was reduced. The reflux of TEOS/ethanol solution at 70℃ is another method to improve the mechanical strength. Nevertheless, reflux with template (surfactant Tween80) would produce bigger pores in the film. Reflux without template would reduce the porosity and Si-OH groups in the film; therefore, the film possessed higher mechanical strength and less leakage current than that without reflux. There was no formation of bigger pores during refluxing without template. TPAOH in the reflux solution can improve the condensation of the Si-OH bonds. It was found that the addition of a little TPAOH was able further to increase the mechanical strength of the films. However, when using PEG1450 (a non-surfactant) as the template instead of a surfactant, the dielectric constant of the film was increased to 2.3~2.7, which is not expected in the future application of low k films.
It can be summarized from the experimental results in this study, that the influences of the properties of the porous low-k film are porosity, pore size and the amount of Si-OH bonds in the film. The film with high porosity has low dielectric constant. Appropriate pore size (3~8nm) will help the surface modification and prevent collapsing. The film with less Si-OH bonds has high mechanical strength and low k value. The dielectric constant of the best film prepared in this research was 2.1. The elastic modulus and hardness of the film were 9.9GPa and 1.4Gpa respectively. The leakage current was 1.19E-07 A/cm2.
Subjects
二氧化矽薄膜
低介電係數
機械強度
Silica thin film
low-k film
mechanical strength
Type
thesis