1. A method of forming an integrated circuit device, comprising: forming at least one non-volatile memory cell on a substrate, said at least one non-volatile memory cell comprising a plurality of phase-changeable material regions therein that are electrically coupled in series and collectively configured to support at least 2-bits of data when serially programmed using at least four serial program currents, said forming comprising; forming at least one interlayer insulating layer on the substrate; forming at least one contact hole extending through the at least one interlayer insulating layer; and forming at least two serially coupled phase-changeable material regions in the at least one contact hole.