Non-lift-off Block Copolymer Lithography of 25 nm Magnetic Nanodot Arrays

Abstract

Although nanolithographic techniques based on self-assembled block copolymer templates offer tremendous potential for fabrication of large-area nanostructure arrays, significant difficulties arise with both the lift-off and etch processes typically used for pattern transfer. These become progressively more important in the limit of extreme feature sizes. The few techniques that have been developed to avoid these issues are quite complex. Here, we demonstrate successful execution of a nanolithographic process based on solvent annealed, cylinder-forming, easily degradable, polystyrene-b-polylactide block copolymer films that completely avoids lift-off in addition to the most challenging aspects of etching. We report a "Damascene-type" process that overfills the polystyrene template with magnetic metal, employs ion beam milling to planarize the metal surface down to the underlying polystyrene template, then exploits the large etch rate contrast between polystyrene and typical metals to generate pattern reversal of the original template into the magnetic metal. The process is demonstrated via formation of a large-area array of 25 nm diameter ferromagnetic Ni(80)Fe(20) nanodots with hexagonally close-packed order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation. We argue that the approach is generalizable to a wide variety of materials, is scalable to smaller feature sizes, and critically, minimizes etch damage, thus preserving the essential functionality of the patterned material.

Perpendicular magnetic anisotropy in Nd-Co alloy films nanostructured by di-block copolymer templates

“…[9][10][11][12][13] For example, porous alumina templates have been used to fabricate PMA antidot arrays with engineered coercivity at the expense of a very rough topography. [14][15][16] Thin films of self-organized di-block copolymers provide an alternative route to fabricate nanostructured templates for metal films, [9][10][11]13,[17][18][19] that have been used to control different magnetic properties such as bit size in patterned magnetic media, 9 exchange bias in Fe/FeF 2 bilayers 17 or coercivity in permalloy films, 18 and spin valves. 19 Most of the works on local modifications of PMA have been focused on Co(Pt,Pd) and Fe(Pt,Pd) alloys [3][4][5]7,10,12 due to their strong PMA and large Q factors (Q ¼ K N /2pM S 2 with K N the perpendicular anisotropy and M S the saturation magnetization).…”

Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application

“…After successfully reducing pore diameter from 30 nm to 14 nm ( Figure 8 g and h), permalloy was evaporated on the ZnO "nanocrucibles" followed by a "Damascene-type" process reported earlier [ 90 ] to leave well-ordered ZnO nanocrucibles surrounding the permalloy dots (Figure 8i-m). The material showed ferromagnetic multidomain/vortex state at larger diameters but at reduced diameters (14 nm) exhibited superparamagnetism.…”

“…[ 90 ] After overfi lling Ni 80 Fe 20 (permalloy) through molecular beam deposition, planarization using ion beam milling to the PS matrix followed by dry etching enabled the formation of permalloy nanodots as shown in Figure 5 l. The authors also described a climate controlled process enhancing the ordering PS-b -PLA features during self-assembly. A considerable difference in the long range order of the BCP was observed between the two studied annealing environments and thus the resulting evaporated patterned permalloy material.…”

Pattern transfer using block copolymers

“…For example, Olayo-Valles et al [66] [67] demonstrated the fabrication of magnetic dots by overfilling a porous PS template with Ni 80 Fe 20 followed by Ar ion milling and etching (figure 9). A sol-gel replication method was also reported using PS-b-PLA BCPs.…”