Many layers of dielectric films are used to make an integrated circuit. IC manufacturing requires dielectric films to have certain properties in order for the circuit to function as designed. For example, there is a general need for materials with low dielectric constants (low-k). Using low-k materials as the intermetal dielectric (i.e., the layer of insulator separating consecutive levels of the conductive metal interconnects) reduces the delay in signal propagation due to capacitive effects, otherwise know as the RC delay. A dielectric material of low dielectric constant will have low capacitance, and hence the RC delay of an IC constructed with such a material will be lower as well.
As another example, there is a general need for materials with specific tensile or compressive stresses. Increasing shallow trench isolation (STI) film tensile stress increases transistor drain current and device performance because the electron and hole mobilities are higher. Other applications require dielectric films to have compressive stress. These and other properties may be met on the film as deposited, or after treatment.
One such treatment may be a thermal process in which the substrate is heated to a temperature for a time. A thermal treatment may remove unwanted particles from the film, or change its stresses. These thermal processes, however, have certain difficulties. In particular, substrate temperatures generally need to be high (i.e., greater than about 400° Celsius) with exposure times typically on the order of hours. As is well known in the field, these conditions can damage copper containing devices, especially in the application wherein a low-k dielectric is being cured. Also, the use of nickel silicide precludes inducing film stress by using temperatures above 400° C. while some SiN films have a cure temperature up to 480° C.
To overcome these disadvantages of thermal processing, another technique has been developed, which involves exposing the film to UV radiation. Irradiation of the low-k or spacer nitride films permits modulation of desired film properties such as dielectric constant or film stress at lower temperatures. However, the use of UV radiation in such processes may result in deleterious side-effects unless special care is taken to deliver the precise wavelengths and to maintain the wafer temperature at an optimal level.
What is needed therefore are improved methods for treating dielectric films with UV radiation to obtain desired film properties.