An improved catheter is provided that is particularly useful for simultaneously mapping electrical activity at multiple locations within the heart. The etching gas includes approximately equal amounts of a hydrogen-free fluorocarbon, most preferably C4F6, and oxygen and a much larger amount of argon diluent gas. The lid carries a locking mechanism which when in the close position urges the central part of the lid toward the bottom section. For example, the via can extend from a topside of the device to a backside so that the electrically conductive material connects a topside contact to a backside contact.