In some embodiments, the first conductive shield 150 may conform in shape with and/or lie in close contact to an inner wall of the first blind hole 141. In particular, the first conductive shield 150 may cover a major part of or the whole inner wall surface of the first blind hole. The first conductive shield 150 may lie close to the inner wall of the first blind hole 141, such that no spaces or gaps are provided between the inner wall of the first blind hole 141 and the first conductive shield 150. Spaces between the first conductive shield and the inner wall of the first blind hole may potentially generate new triple points, i.e. new junctions of metal, dielectric and vacuum, and hence new positions of potential field enhancements, such that such spaces are beneficially reduced or avoided.

In some embodiments, which can be combined with other embodiments described herein, the first conductive shield 150 may cover 80% or more, particularly 90% or more of the inner wall surface of the first blind hole 141. Specifically, essentially the entire inner wall surface of the first blind hole 141 may be covered by the first conductive shield 150. This reduces the risk of generating new triple points at positions inside the first blind hole.