Paul Ruchhoeft

Patterning curved surfaces: Template generation by ion beam proximity lithography and relief transfer by step and flash imprint lithography

Submicron patterning of 1 in. diameter curved surfaces with a 46 mm radius of curvature has been demonstrated with step and flash imprint lithography (SFIL) using templates patterned by ion beam proximity printing (IBP). Concave and convex spherical quartz templates were coated with 700-nm-thick poly(methylmethacrylate) (PMMA) and patterned by step-and-repeat IBP. The developed resist features were etched into the quartz template and the remaining PMMA stripped. During SFIL, a low viscosity, photopolymerizable formulation containing organosilicon precursors was introduced into the gap between the etched template and a substrate coated with an organic transfer layer and exposed to ultraviolet illumination. The smallest features on the templates were faithfully replicated in the silylated layer.

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.

Ion beam aperture-array lithography

We explore a new technique, ion beam aperture-array lithography (AAL), for fabricating low cost, large area nanostructure arrays. In this approach, a broad beam of light ions illuminates a stencil mask containing a periodic array of circular apertures. The array of transmitted beamlets is moved over the substrate, thereby printing an array with the same period but arbitrary unit cell. AAL has several advantages over ion beam proximity lithography: (1) the mask pattern density is lower, (2) the unit cell patterns are defined by software, and (3) dose and shape correction can be applied to compensate for system blur. Dose optimization methods are discussed and experimental results with 150 nm apertures presented.

Ion projection lithography: International development program

Ion projection lithography (IPL) has demonstrated not only the resolution required for next-generation lithography (50 nm resolution at >4:1 aspect ratio) [Bruenger et al., Microelectron Eng. 46, 477 (1999)] but also cost advantages with respect to other competing technologies [Gross et al., J. Vac. Sci. Technol. B 16, 3150 (1998)]. This article reports on the progress of a worldwide development program, with the target to manufacture a process development tool and create the necessary mask infrastructure to demonstrate that IPL is a viable industrial lithography technology for the future. An overview of papers, reporting on the progress in critical areas, is given and new, experimentally validated, simulations of complementary mask stitching are shown for the first time. Longitudinal and lateral offsets of up to 32 nm for 100 nm critical dimensions are possible with linewidth variations less than 11 nm. Our concept for beta tools, based on a powerful new stitcher strategy, is described. This will lead to...

An ADI-FDTD method for periodic structures

In this communication, the alternating-direction implicit finite-difference time-domain (ADI-FDTD) method is extended to analyze periodic structures. In the implicit updates of the ADI-FDTD method, the periodic boundary condition leads to a cyclic matrix. Instead of inverting the cyclic matrix directly, the problem is converted into two auxiliary linear systems that can be solved using the tridiagonal matrix solver. Consequently, only 7n arithmetic operations are required for each implicit update and the efficiency of the ADI-FDTD method is retained. Numerical examples further demonstrate the effectiveness of this periodic ADI-FDTD method.

A proximity ion beam lithography process for high density nanostructures

Image contrast in proximity ion beam lithography is limited by scattered ions which enter the opaque regions of the mask and exit through the sidewalls of the mask windows. The scattering angles are widely distributed resulting in a ‘‘proximity effect’’ whose range is on the order of the mask‐to‐wafer gap. This problem becomes more severe with increasing pattern density and sets the resolution limit for high density patterns such as interdigital transducers. The only way to counteract this effect is to limit the ion range to a fraction of the mask thickness so that the scattered ions can be recaptured by adjacent sidewalls. This article explores the dependence of image contrast on resolution, pattern density, and beam energy in proximity ion beam lithography. Patterns with feature sizes in the range from 20 to 50 nm and 0.4 μm pitch have been printed with a linewidth change of only 3 nm for a 10% change in dose.

Extraction of a nearly monoenergetic ion beam using a pulsed plasma

A nearly monoenergetic ion beam was extracted from a capacitively coupled pulsed Ar plasma. The electron temperature decayed rapidly in the afterglow, resulting in uniform plasma potential, and minimal energy spread for ions extracted in the afterglow. Ion energy was controlled by a dc bias on a ring electrode surrounding the plasma. Langmuir probe measurements indicated that this bias simply raised the plasma potential without heating the electrons in the afterglow. A rejection grid downstream of the plasma allowed ions to pass only during a selected time window in the afterglow. The energy spread was 3.4 eV full width at half maximum for a peak ion beam energy of 102.0 eV. This energy spread is about an order of magnitude narrower than the beam extracted from the continuous plasma.

Nanofabrication using nanotranslated stencil masks and lift off

We propose and demonstrate a technique for forming nanometer-scale metal features based on evaporation onto a substrate through a stencil mask. In this work, the stencil mask is laterally translated by a piezoflexure stage, between evaporations of different metals. The metals are chosen based on their etch chemistry to allow one material to be lifted off with respect to another. In this way, sidewall features are formed with dimensions and spacings controlled by moving the translational stage, which has 1 nm resolution.

Recording Physics, Design Considerations, and Fabrication of Nanoscale Bit-Patterned Media

Recording physics, design considerations, and fabrication of bit-patterned magnetic medium for next generation data storage systems is presented. (Co/Pd)N magnetic multilayers are evaluated as candidates for bit-patterned medium recording layer materials for their high and easily tunable magnetic anisotropy. The optimized patterned multilayers used in this study had coercivities in excess of 12-14 kOe. Bit patterning was accomplished using ion-beam proximity printing, 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. It is found that the nature of magnetization reversal strongly depends on bit edge imperfections and is likely to contribute to switching field distribution.

Detection and Monitoring of Microparticles Under Skin by Optical Coherence Tomography as an Approach to Continuous Glucose Sensing Using Implanted Retroreflectors

We demonstrate the feasibility of using optical coherence tomography (OCT) to image and detect 2.8 μm diameter microparticles (stationary and moving) on a highly-reflective gold surface both in clear media and under skin in vitro. The OCT intensity signal can clearly report the microparticle count, and the OCT response to the number of microparticles shows a good linearity. The detect ability of the intensity change (2.9%±0.5%) caused by an individual microparticle shows the high sensitivity of monitoring multiple particles using OCT. An optical sensing method based on this feasibility study is described for continuously measuring blood sugar levels in the subcutaneous tissue, and a molecular recognition unit is designed using competitive binding to modulate the number of bound microparticles as a function of glucose concentration. With further development, an ultra-small, implantable sensor might provide high specificity and sensitivity for long-term continuous monitoring of blood glucose concentration.