A Novel Selective Freelayer Wet Etching Method for Magnetic Tunnel Junction-Based MRAM

Abstract

This paper describes the development of a novel wet etch process for selective patterning of ferromagnetic freelayers, principally Permalloy, in MRAM magnetic tunnel junction (MTJ) stacks. The method uses a nonadsorbing acid, trifluoromethane sulfonic (TFMSA), as the etchant acid. Freelayer etch selectivity to the alumina tunneling barrier is achieved through the use of long-chain alkane sulfonic acid salts, e.g. sodium 1-tetradecanesulfonate. These inhibitors can increase the alumina barrier etch resistance in the TFMSA solution by up to two orders of magnitude. The importance of the reactive ion etching process used for MTJ cap layer removal prior to the wet etching step is discussed. Open circuit potential (OCP) was shown to be a powerful method for following the progress of freelayer wet etching and for evaluating alumina barrier etch inhibitors.