Fabrication of patterned recording medium using ion beam proximity lithography

We describe the lithographic structuring of large-area patterned medium samples with sub-50 nm features using ion beam proximity lithography (IBPL). The quality of the patterns formed in IBPL system is primarily limited by the quality of the stencil masks. Hence, the emphasis of this work has been to develop a reliable mask fabrication process that can achieve a size uniformity that is suitable for patterned media. We have developed a mask fabrication approach that incorporates palladium as a hard mask for transferring the lithography pattern through a silicon nitride membrane. A conformal gold coating allows for further reduction of the mask features without a significant increase in the feature size variation. An average standard deviation of 3 nm and 5 nm was measured during various steps of the stencil mask fabrication and after printing using IBPL in PMMA resist, respectively. Patterned medium prototypes with features ranging from 40 nm to 300 nm have been fabricated and magnetic properties measured. A 6-12 fold increase in coercivity was measured for multilayer samples after patterning. Ion irradiation of patterned multilayer samples was also studied as a means to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. Patterned multilayer samples show a decrease in coercivity from 11 kOe for as-patterned to 0.3 kOe for 800 muC/cm² and suggests that ion irradiation can be an integral part of bit patterned medium fabrication for anisotropy control.