[CCoE Notice] TcSUH's Bi-Weekly Seminar: Jack Wolfe "Image Noise in Helium Lithography"

[Lewis, Lindsay R]

Prof. John C. "Jack" Wolfe

Hugh Roy and Lillie Cranz Cullen Distinguished University Professor,

Dept. of Electrical and Computer Engineering; Professor of Physics

_______________________________________________________________________

FRIDAY, JUNE 24, 2011

University of Houston Science Center, Room 102

12:00 Noon - 1:00 p.m.



"Image Noise in Helium Lithography"



Synopsis: In the 1990's "nanotechnofabrication" meant building structures with spatial dimensions below 100 nm

(30-50 nm was readily attainable in this timeframe). Today, however, sub-10 nm structures are required for many

applications, including the fabrication of integrated circuits, magnetic storage media, graphene nanoribbons, and

pinning center arrays. This talk describes image noise, one of the principal challenges to emerge as lithographic

technology advances from ~100 nm to the sub-10 nm domain.



Abstract: A distinctive feature of resist exposure by energetic helium ions or neutral atoms is that critical exposure

densities are very low, about 100 times smaller than for electrons. Thus, particle distributions are sparse, leading to

significant statistical fluctuations in the deposited energy density even in Polymethylmethacrylate (PMMA), a

relatively insensitive, non-chemically amplified resist. The impact is first seen as roughness in the region of partial

exposure on a feature's edge where the bunching of just a few particles may cause the energy density to shift

above or below the resist development threshold. As feature size is reduced, however, fluctuations in the total

number of particles (shot noise) become larger as a fraction of average dose, potentially causing over and under

exposure of the entire feature. This talk presents an integrated study of image noise in helium lithography that

compares shape variations in neutral particle mask images with the predictions of a Monte-Carlo model. The model

accounts for the following: 1) Poisson statistics of the particle emission process, 2) the variable spatial distribution of

the particles within the aerial image, 3) the effect of scattering on the particle distribution at various depths in the

resist, and 4) smoothing of the deposited energy distribution by exposure and development processes. Proximity

lithography experiments were carried out using 10 keV neutral helium atoms in 20 nm thick PMMA resist under

conditions of 12.7 nm [FWHM] penumbral image blur. The energy smoothing function is assumed, based on previous

experiments, to be Gaussian and its standard deviation s treated as a free parameter. Model predictions of the

power spectral density of line edge roughness agree with experiment for σ =5.0 ± 0.5 nm. The model predicts that

using a resist with a critical dose 20 times higher than PMMA and reducing penumbra to 0.5 nm [FWHM], for

example by reducing the proximity gap, would reduce shape fluctuations to less than 0.5 nm [FWHM] for dense 10

nm dot arrays.



Biographical Sketch: Jack Wolfe earned his B.S. and Ph.D. degrees in Physics at the University of Rochester. He joined the ECE department at UH in 1976. His work over the past 3 decades has focused on development of nanofabrication technology for applications in data storage, integrated circuit manufacturing, and energy. He served as Program Chair of the 1999 International Symposium on Electron, Ion, and Photon Beam Technology and Nanofabrication, the preeminent conference on lithography and nanofabrication. He has served as Department Chair of ECE, Interim Dean of the College of Engineering, and Interim Department Chair of ECE. He holds the position of Hugh Roy and Lillie Cranz Cullen Distinguished University Professor. He holds a joint appointment in the Physics Department where he plans to become involved in curriculum development.



Persons with disabilities who require special accommodations in

attending this lecture should call (713) 743-8210 as soon as possible.

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