Hitesh Arora

Bit-Patterned Magnetic Recording: Theory, Media Fabrication, and Recording Performance

Bit-patterned media (BPM) for magnetic recording provides a route to thermally stable data recording at >1 Tb/in2 and circumvents many of the challenges associated with extending conventional granular media technology. Instead of recording a bit on an ensemble of random grains, BPM comprises a well-ordered array of lithographically patterned isolated magnetic islands, each of which stores 1 bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this paper, we describe a BPM fabrication method that combines rotary-stage e-beam lithography, directed self-assembly of block copolymers, self-aligned double patterning, nanoimprint lithography, and ion milling to generate BPM based on CoCrPt alloy materials at densities up to 1.6 Td/in2. This combination of novel fabrication technologies achieves feature sizes of <;10 nm, which is significantly smaller than what conventional nanofabrication methods used in semiconductor manufacturing can achieve. In contrast to earlier work that used hexagonal arrays of round islands, our latest approach creates BPM with rectangular bit cells, which are advantageous for the integration of BPM with existing hard disk drive technology. The advantages of rectangular bits are analyzed from a theoretical and modeling point of view, and system integration requirements, such as provision of servo patterns, implementation of write synchronization, and providing for a stable head-disk interface, are addressed in the context of experimental results. Optimization of magnetic alloy materials for thermal stability, writeability, and tight switching field distribution is discussed, and a new method for growing BPM islands from a specially patterned underlayer-referred to as templated growth-is presented. New recording results at 1.6 Td/in2 (roughly equivalent to 1.3 Tb/in2) demonstrate a raw error rate <;10-2, which is consistent with the recording system requirements of modern hard drives. Extendibility of BPM to higher densities and its eventual combination with energy-assisted recording are explored.

A silica sol–gel design strategy for nanostructured metallic materials

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Block Copolymer Self-Assembly–Directed Single-Crystal Homo- and Heteroepitaxial Nanostructures

Polymer Templating for Metals Polymer templating has been used to fabricate a wide range of ordered materials, both due to the ability to pattern the polymers easily over a large area and their facile removal. However, the process is somewhat limited to the incorporation of materials that will flow easily into the templated areas. Arora et al. (p. 214) show that current techniques can be extended to the patterning of metals, through guided epitaxial growth. An excimer laser was used to control the flow of material into patterned templates formed from block copolymers. Patterns created on surfaces by phase-separating polymers direct the growth of crystalline inorganic nanostructures. Epitaxy is a widely used method to grow high-quality crystals. One of the key challenges in the field of inorganic solids is the development of epitaxial single-crystal nanostructures. We describe their formation from block copolymer self-assembly–directed nanoporous templates on single-crystal Si backfilled with Si or NiSi through a laser-induced transient melt process. Depending on thickness, template removal leaves either an array of nanopillars or porous nanostructures behind. For stoichiometric NiSi deposition, the template pores provide confinement, enabling heteroepitaxial growth. Irradiation through a mask provides access to hierarchically structured materials. These results on etchable and non-etchable materials suggest a general strategy for growing epitaxial single-crystal nanostructured thin films for fundamental studies and a wide variety of applications, including energy conversion and storage.

Block Copolymer Directed Nanoporous Metal Thin Films

Porous metal thin films have high potential for use in applications such as catalysis, electrical contacts, plasmonics, as well as energy storage and conversion. Structuring metal thin films on the nanoscale to generate high surface areas poses an interesting challenge as metals have high surface energy. In this communication, we demonstrate direct access to nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles, and intermetallics.

Colloidal Self-Assembly-Directed Laser-Induced Non-Close-Packed Crystalline Silicon Nanostructures

This report describes an ultrafast, large-area, and highly flexible method to construct complex two- and three-dimensional silicon nanostructures with deterministic non-close-packed symmetry. Pulsed excimer laser irradiation is used to induce a transient melt transformation of amorphous silicon filled in a colloidal self-assembly-directed inverse opal template, resulting in a nanostructured crystalline phase. The pattern transfer yields are high, and long-range order is maintained. This technique represents a potential route to obtain silicon nanostructures of various symmetries and associated unique properties for advanced applications such as energy storage and generation.

Template-Assisted Direct Growth of 1 Td/in2 Bit Patterned Media

We present a method for growing bit patterned magnetic recording media using directed growth of sputtered granular perpendicular magnetic recording media. The grain nucleation is templated using an epitaxial seed layer, which contains Pt pillars separated by amorphous metal oxide. The scheme enables the creation of both templated data and servo regions suitable for high density hard disk drive operation. We illustrate the importance of using a process that is both topographically and chemically driven to achieve high quality media.

“Nothing” can be better: Study of porosity in the charge trap layer of Flash memory

Discrete charge storage devices based on traps and nanocrystals (NCs) are shown to alleviate the problems of stress induced leakage currents (SILC) that ultimately limits the tunnel oxide scaling. Also, electric field enhancement in the tunnel oxide due to metal NCs1 boosts carrier injection efficiency from the channel thereby lowering program/erase voltages. This study investigates another possibility of generating such field asymmetry in the gate stack through the use of nano-porous dielectrics. The inherent difference in programming and retention mechanisms (F-N against direct tunneling) may be further widened through engineered pores. We investigate the effect of nano pores (NPs) in the charge storage layer through simulation and then present experimental results of porous TiO2.