Method for etching at least one ion track to a pore in a membrane and electrolyte cell for preparing the membrane

A method of etching at least one ion track in a membrane so as to form a pore, said membrane consisting of a matrix material dielectricum, which can be etched in a preferential manner along an ion track and being disposed in an electrolytic cell with two chambers which are separated from each other by the membrane, said method comprising the steps of:

• supplying to one chamber of said electrolytic cell an etching solution which contains ions which are etch-active with respect to the matrix material of the membrane,
• supplying to the other chamber of said electrolytic cell a stop solution which contains ions of the same polarity as the etch-active ions but which are inactive with respect to the matrix material of the membrane,
• providing a first electrode extending into the etching solution and a second electrode extending into the stop solution and establishing a voltage difference between the first and second electrodes by which an electrical field across the membrane is generated whose strength is time-dependent controllable and whose direction is adjustable,
• monitoring the electric current flowing through the membrane during the etching procedure, while an etching front forming etching pores travels from the side of the etching solution along the ion track through the membrane toward the side of the stop solution,
• when, upon reaching the stop solution side of the membrane, current begins to flow through the etching pores from the etching solution side to the stop solution side, etch-active ions are replaced by etch inactive ions of the same polarity from the stop solution causing the etching procedure to continue at a slowed rate corresponding to the dilution of the etching solution while a thin active membrane layer remains at the bottom of the etching pores which slowly grows sidewardly from the bottom of the etching cavities where the concentration of inactive ions remains lower,
• and, during the last phase of the etching procedure shortly before the penetration of the etching front when the electric current flow through the etching groove rapidly increases and the electric field and the field direction become the determining factor for the progress of etching,
• the etch active ions are retracted from the etching front and replacing them by inactive ions of the same polarity from the stop solution side thereby forming at the bottom of the etching pores thin nano-porous layers which form the active membrane layer, and
• flushing the etching solution from the one chamber containing the etching solution and replacing it by a neutral solution.