A method of plasma etching one or more features in a silicon substrate, the method comprising the steps of: performing a main etch using a cyclical etch process in which a deposition step and a main etch step are alternately repeated, wherein the main etch is performed to etch a major portion of the one or more features in the silicon substrate; and performing an over etch to complete the plasma etching of the features; in which: the over etch comprises at least one deposition step, one or more over etch steps of a first kind, one or more over etch steps of a second kind and one or more over etch steps of a third kind, each of the over etch steps of the first, second and third kind (54, 64, 74) comprising etching by ion bombardment of the silicon substrate; wherein ion bombardment during the one or more over etch steps of the second kind has an inward inclination radially towards the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind, and ion bombardment during the one or more over etch steps of the third kind has an outward inclination radially away from the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind in which the over etch steps of the second and third kinds (64, 74) are performed to correct asymmetries introduced by the one or more over etch steps of the first kind; and wherein the inclination of the ion bombardment during the over etch is controlled using an electrical bias power which is applied to the substrate to produce an electrical bias, in which the electrical bias power is an RF power. performing a main etch using a cyclical etch process in which a deposition step and a main etch step are alternately repeated, wherein the main etch is performed to etch a major portion of the one or more features in the silicon substrate; and performing an over etch to complete the plasma etching of the features; in which: the over etch comprises at least one deposition step, one or more over etch steps of a first kind, one or more over etch steps of a second kind and one or more over etch steps of a third kind, each of the over etch steps of the first, second and third kind (54, 64, 74) comprising etching by ion bombardment of the silicon substrate; wherein ion bombardment during the one or more over etch steps of the second kind has an inward inclination radially towards the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind, and ion bombardment during the one or more over etch steps of the third kind has an outward inclination radially away from the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind in which the over etch steps of the second and third kinds (64, 74) are performed to correct asymmetries introduced by the one or more over etch steps of the first kind; and wherein the inclination of the ion bombardment during the over etch is controlled using an electrical bias power which is applied to the substrate to produce an electrical bias, in which the electrical bias power is an RF power. the over etch comprises at least one deposition step, one or more over etch steps of a first kind, one or more over etch steps of a second kind and one or more over etch steps of a third kind, each of the over etch steps of the first, second and third kind (54, 64, 74) comprising etching by ion bombardment of the silicon substrate; wherein ion bombardment during the one or more over etch steps of the second kind has an inward inclination radially towards the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind, and ion bombardment during the one or more over etch steps of the third kind has an outward inclination radially away from the centre of the silicon substrate with respect to ion bombardment during the one or more over etch steps of the first kind in which the over etch steps of the second and third kinds (64, 74) are performed to correct asymmetries introduced by the one or more over etch steps of the first kind; and wherein the inclination of the ion bombardment during the over etch is controlled using an electrical bias power which is applied to the substrate to produce an electrical bias, in which the electrical bias power is an RF power.