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1. A method of forming a hole or cavity or channel in a region of an electrically insulating substrate, comprising the steps:
a) providing an electrically insulating substrate, b) applying, by means of a voltage supply, a voltage across a region of said electrically insulating substrate, said region having a size less than that of the entire substrate, said voltage being sufficient to give rise to a significant increase in electrical current through said region and to a dielectric breakdown (DEB) through said region, c) applying heat in a directed and locally restricted manner to said region only by using a laser or other focused light source or a gas flame, or by applying an AC voltage to said region so as to increase the temperature of said region to define the location where dielectric breakdown is to occur, said heat originating either from said laser or other focussed light source or said gas flame or from components of said voltage applied in step b), said heat being applied so as to reduce the amplitude of voltage required in step b) to give rise to said current increase through said region, wherein step b) is performed and ended using an electronic feedback mechanism operating according to user-predefined parameters, said electronic feedback mechanism controlling the properties of said applied voltage and/or of said electrical current, wherein said electronic feedback mechanism comprises an analysis circuit which is a current analysis circuit or a voltage and current analysis circuit, alone or as part of a user-programmed device, said analysis circuit controlling voltage supply output parameters in relation to a trans-substrate voltage and current flow according to user-predefined procedures, wherein step b) occurs by the placement of electrodes at or near said region by placing one electrode on one side of that substrate and by placing another electrode on another side of said substrate, and by application of said voltage across said electrodes.
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