Abstract: A method of sealing an air gap in a layer of a semiconductor structure comprises providing a first layer of the semiconductor structure having at least one air gap for providing isolation between at least two conductive lines formed in the first layer. The at least one air gap extends into the first layer from a first surface of the first layer. The method further comprises forming a barrier layer of a barrier dielectric material over the first surface of the first layer and the at least one air gap. The barrier dielectric material is selected to have a dielectric constant less than 3.5 and to provide a barrier to prevent chemicals entering the at least one air gap.