In order to determine the dielectric constant of a layer deposited on a semiconductor wafer (2), the density of the layer is obtained. To obtain that density, the wafer (2) without the layer is weighed in a weighing chamber (4) in which a weighing pan (7) supports the wafer on a weighing balance. The weight of the wafer is determined taking into account the buoyancy exerted by the air on the wafer (2). Then the layer is deposited on the wafer (2) and the weighing operation repeated. Alternatively a reference wafer may be used. If the material of the layer is known, the weight of the layer can be used to derive its density using a thickness measurement. Alternatively, if the density is known, the thickness can be obtained.