Patterned media recording has some specific challenges in terms of servo control design which are briefly listed here:  In preliminary research, one of the processes investigated for creating prototypes was ion beam proximity lithography. Ion beam proximity lithography uses stencil masks to produce patterns in ion-sensitive material (resist) which is transferred to magnetic material. The stencil mask contains a thin free standing silicon nitride membrane in which openings are formed. The pattern to be generated is first formed on a substrate that contains a photo-resist using electron beam lithography. Next the substrate is used to transfer the given pattern onto the nitride membrane (stencil mask) using the process of plasma etching. To create sufficient substrates is to maintain size uniformity of the openings which is transferred to the mask during the fabrication process (etching). Many factors contribute to the achievement and maintenance of size uniformity in the mask, such as: pressure, temperature, energy (amount of voltage), and power used when etching. To optimize the process of etching uniform patterns correctly under these parameters, the substrate can be used as a template to fabricate stencil masks of silicone nitride through the process of ion proximity beam lithography. The stencil mask can then be used as a prototype to create pattern media. In 2014, Ricardo Ruiz of Hitachi Global Storage Technologies writes in an upcoming-conference briefing note that "the most promising solution to the lithographic challenge can be found in directed self assembly of block copolymer films which has recently evolved as a viable technique to achieve sub-20nm lithography in time for BPM technology".