Ariel Ruiz

Fabrication of a high anisotropy nanoscale patterned magnetic recording medium for data storage applications

An approach to fabrication of a patterned magnetic recording medium for next generation data storage systems is presented. (Co/Pd)n magnetic multilayers are evaluated as candidates for patterned medium materials for their high and easily controllable magnetic anisotropy. The multilayer films deposited on a Ta seed layer enable high intergranular exchange coupling—an essential feature of a patterned magnetic recording medium. The quality of (Co/Pd)n superlattices was optimized via deposition conditions and monitored using low-angle x-ray diffraction. An estimated in-plane (hard-axis) magnetization saturation field in excess of 40 000 Oe was observed. Vertical (easy-axis) hysteresis loops for as-deposited continuous magnetic multilayers exhibited a low coercivity of 930 Oe, indicating highly uniform (magnetically) films with weak domain wall pinning. Ion-beam proximity lithography was used to pattern magnetic multilayers into 43 nm islands on a 135 nm pitch. Following patterning, easy-axis coercivity increased nearly 15-fold to 12.7 kOe.

Estimation of scattered particle exposure in ion beam aperture array lithography

In ion beam proximity lithography, ions that are incident on the nominally opaque regions of a stencil mask can scatter into the open windows and escape, exposing a wide area of the substrate. Since these ions can lose much of their initial energy in the mask, the scattered particle exposure is concentrated near the resist surface. The resulting loss of contrast can be mitigated to some extent by using aperture array lithography (AAL) where a mask of reduced density minimizes the number of windows from which a scattered ion can escape. Even so, the problem worsens as the pitch of an array, printed by multiple, offset exposures of the AAL mask, shrinks below about 250nm. The only solution is to increase the mask thickness, hence the window aspect ratio, to reduce the escape angles of the scattered particles. In this article, the authors characterize an effective background dose η in the first 75nm of poly(methylmethacrylate) resist for 30keV He+ ion exposures of 0.6μm thick masks with 45, 80, and 110nm cir...

Rapid prototyping of infrared bandpass filters using aperture array lithography

We demonstrate the prototyping of infrared bandpass filters, which consist of cross-shaped openings in a thin gold film, using ion beam aperture array lithography. In the lithography process, a stencil mask containing a periodic array of square apertures is irradiated by a broad beam of helium ions. The ions that pass through the openings expose the resist on a substrate that is placed in close proximity and cross-shaped filter structures are printed by moving the stage underneath the substrate, thus allowing for rapid formation of periodic patterns. We have fabricated filter patterns with peak transmittance ranging from 53% to 67% at wavelengths between 1.2 and 1.3μm that exhibit high reflectance for longer wavelength radiation. The prototyping throughput for masks with 2μm pitch patterns was about 2cm2∕h. The spectral performance of the prototyped filters was measured. Large-area, second-generation masks with 667nm pitch had a lithography throughput of 300cm2∕h and were used to print filter patterns of ...

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.

Fabrication and characterization of annular magnetic nanostructures

Large arrays of permalloy (Ni81Fe19) annular structures were fabricated using a self-aligned patterning process based on ion-beam proximity lithography (IBPL), where a broad beam of energetic He ions is shaped into billions of ion beamlet by a stencil mask to pattern electron beam sensitive resist. IBPL was used to form an array of circular openings in poly(methyl methacrylate) (PMMA) resist, followed by a tone reversal process to form circular pillars in an underlying polymethylglutarimide (PMGI) layer. The PMGI pillars were conformally coated with silicon oxide, which was followed by anisotropic reactive ion etch (RIE) to form silicon oxide rings, which were transferred into the underlying sputter deposited permalloy thin film by ion milling. This fabrication approach was used to make 6 x 6 mm2 arrays of rings on a constant pitch of 670 nm with the outer diameters varied between 350 nm and 450 nm with a fixed inner diameter of 150 nm. Three unique samples that were fabricated and characterized using a vibrating sample magnetometer. The measured M-H loops showed the switching from an onion to a vortex and back to an onion state and are in good agreement with micromagnetic simulations and previously published data.Large arrays of permalloy (Ni81Fe19) annular structures were fabricated using a self-aligned patterning process based on ion-beam proximity lithography (IBPL), where a broad beam of energetic He ions is shaped into billions of ion beamlet by a stencil mask to pattern electron beam sensitive resist. IBPL was used to form an array of circular openings in poly(methyl methacrylate) (PMMA) resist, followed by a tone reversal process to form circular pillars in an underlying polymethylglutarimide (PMGI) layer. The PMGI pillars were conformally coated with silicon oxide, which was followed by anisotropic reactive ion etch (RIE) to form silicon oxide rings, which were transferred into the underlying sputter deposited permalloy thin film by ion milling. This fabrication approach was used to make 6 x 6 mm2 arrays of rings on a constant pitch of 670 nm with the outer diameters varied between 350 nm and 450 nm with a fixed inner diameter of 150 nm. Three unique samples that were fabricated and characterized using a v...

Magnetic Annular Nanostructure Fabrication Using Ion Beam Proximity Lithography

We describe the fabrication of large-area magnetic ring structures using ion beam proximity lithography (IBPL) to pattern an array of circular openings and then use a conforming oxide coating to define the ring structure through the sidewall coating. Arrays of Permalloy rings with sub 500 nm outer diameter and 150 nm inner diameter on a 650 nm pitch over a 5.5 mm times 6 mm area were fabricated to study transitions between the micromagnetic configurations within these structures. The results suggest that the field required for onion-to-vortex transition and field required for vortex-to-onion transition to be 0Oe and 400Oe, respectively.

Fabrication of High Anisotropy Nanoscale Patterned Magnetic Recording Medium for Data Storage Applications

Conventional magnetic recording systems based on continuous medium recording are rapidly approaching their superparamagnetic limit. A shift to patterned media, where the data are stored in arrays of discrete nanomagnets, will help extend the areal bit densities due to a significant increase in the thermal activation volume. One of the key challenges is the development of a cost-effective strategy for media manufacturing. In this work, we present ion beam proximity lithography (IBPL) as a low cost tool for media patterning.(Co/Pd)n magnetic mutlilayers were used as a patterned medium material. Such magnetic multilayers exhibit very large and easily tunable vertical magnetic anisotropy, which makes them suitable for ultra-high density magnetic recording applications. The magnitude of the anisotropy can be varied by controlling the quality of the interfaces and/or by changing the thicknesses of the individual layers in the Co/Pd bi-layer stack. Also, an appropriate choice of a buffer/seed layer can help promote enhanced intergranular exchange coupling, an essential attribute of patterned medium materials. Magnetic films were deposited by magnetron sputtering in 2.5mTorr Ar pressure at room temperature on silicon wafers coated with a 0.5mum thermal oxide. A 5nm Ta seed was used to promote exchange-coupled films. The deposition conditions and the thicknesses of individual Co (5.2 Aring) and Pd (6.6 Aring) layers were optimized to achieve the largest vertical anisotropy, smallest coercivity (to minimized domain wall pinning), and the remnant squareness of one. X-ray diffraction was used as a benchmarking tool to precisely gauge the period of the (Co/Pd)n superlattices and the thicknesses of individual Co and Pd layers. Optimized films had a surface roughness of less than lnm. Medium patterning was accomplished using IBPL, a high-throughput direct write lithography where a large array of ion beamlets shaped by a stencil mask is used to write an arbitrary device pattern. In IBPL system used in this work, helium ions are extracted from a duo-plasmatron ion source and are then accelerated through a constant gradient tube towards a mask (silicon nitride stencil membrane). A 30 keV He+ ion-beam with an ion current density of 140nA/cm2 was used. HSQ, a high resolution negative tone resist, was used for patterning. The sample was developed in 0.24N TMAH and the pattern was transferred into the multilayers using HSQ as the hard mask. Reactive ion etching (RIE) with CHF3 was used to remove HSQ. SEM micrograph of a patterned medium prototype with 43nm features on a 135nm pitch and the vertical M-H loops for the continuous and patterned medium are shown. A 15x coercivity increase as a result of patterning can be observed.