James H. G. Owen

Multimode hydrogen depassivation lithography: A method for optimizing atomically precise write times

A method to enhance the speed of scanning tunneling microscope based hydrogen depassivation lithography is presented. In order to maximize patterning speed while maintaining the capability to retain atomic precision with respect to line edges and feature positions, a multimode technique is used where the modes are characterized either by large or small spot sizes. For areas where atomically precise lithography is required, a tip sample bias of 4–4.5 V is used. In other areas, such as in the center of a large solid pattern, large (∼7 nm) linewidth field emission lithography with a tip sample bias of 8 V is used. A method to generate an optimized set of writing vectors for each mode is described and applied to a fundamental square pattern on the Si(100) surface with an experimental 78% write time reduction. An analysis of the optimal vectors indicates that patterning times may be reduced by up to 95%.

Selectivity of metal oxide atomic layer deposition on hydrogen terminated and oxidized Si(001)-(2×1) surface

In this work, the authors used density-functional theory methods and x-ray photoelectron spectroscopy to study the chemical composition and growth rate of HfO2, Al2O3, and TiO2 thin films grown by in-situ atomic layer deposition on both oxidized and hydrogen-terminated Si(001) surfaces. The growth rate of all films is found to be lower on hydrogen-terminated Si with respect to the oxidized Si surface. However, the degree of selectivity is found to be dependent of the deposition material. TiO2 is found to be highly selective with depositions on the hydrogen terminated silicon having growth rates up to 180 times lower than those on oxidized Si, while similar depositions of HfO2 and Al2O3 resulted in growth rates more than half that on oxidized silicon. By means of density-functional theory methods, the authors elucidate the origin of the different growth rates obtained for the three different precursors, from both energetic and kinetic points of view.

Patterned atomic layer epitaxy of Si/Si(001):H

We aim to develop techniques for the building of atomically precise structures. On the H-terminated Si(001) surface, H atoms can be selectively removed using an STM tip with appropriate lithography conditions, creating arbitrary patterns of reactive dangling bonds with atomic precision. The exposed patterns are used as templates for the growth of Si and Ge by gas-source epitaxy, using disilane and digermane as the precursor gases. The quality of the epitaxy, in terms of island size and defect density of the second and subsequent monolayer (ML), is dependent upon the electron exposure. Good-quality growth of the second and following MLs requires a multiple of the exposure required for good-quality growth of the first ML. This is interpreted in terms of remanent hydrogen in island sites in the first ML.

Pattern transfer of hydrogen depassivation lithography patterns into silicon with atomically traceable placement and size control

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the masks being used in order to maintain exquisite control over both feature size and feature density. Here, the authors demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top–down atomic control over nanofabrication. First, hydrogen depassivation lithography is performed on hydrogen terminated Si(100) using a scanning tunneling microscope, which spatially defined chemically reactive regions. Next, atomic layer deposition of titanium dioxide produces an etch-resistant hard mask pattern on these regions. Reactive ion etching then transfers the mask pattern onto Si with pattern height of 17 nm, critical dimension of approximately 6 nm, and full-pitch down to 13 nm. The effects of linewidth, template atomic defect density, and line-edge roughness are examined in ...

Spurious dangling bond formation during atomically precise hydrogen depassivation lithography on Si(100): The role of liberated hydrogen

The production of spurious dangling bonds during the hydrogen depassivation lithography process on Si(100)-H is studied. It is shown that the number of spurious dangling bonds produced depends on the size of the primary pattern on the surface, not on the electron dose, indicating that the spurious dangling bonds are formed via an interaction of the liberated hydrogen with the surface. It is also shown that repassivation may occur if hydrogen depassivation lithography is performed near an already patterned area. Finally, it is argued that the product of the interaction is a single dangling bond next to a monohydride silicon on a silicon dimer, with a reaction probability much in excess of that previously observed.

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

A self-tuning controller for high-performance scanning tunneling microscopy

The loop gain in feedback control system of a Scanning Tunneling Microscope (STM) is proportional to a quantum mechanical property of the STM tip and sample, known as the Local Barrier Height (LBH). Variations in LBH can negatively affect the stability of feedback loop and increase the risk of tip-sample crash. In this paper, we propose a method for online estimation of the LBH and accordingly tuning the gains of a proportional-integral (PI) controller. Experimental results confirm enhanced stability of the STM with the tuning algorithm in effect.

Atomically Precise Manufacturing: The Opportunity, Challenges, and Impact

Fifty years ago, Richard Feynman famously stated that “I am not afraid to consider the final question as to whether, ultimately—in the great future—we can arrange the atoms the way we want” (Feynman, “There’s Plenty of Room at the Bottom”, speech on December 29th 1959 at the annual meeting of the American Physical Society at the California Institute of Technology). Twenty years ago, Don Eigler of IBM, did arrange atoms the way he wanted (Eigler and Schweizer, Nature 344:524, 1990). We contend that in the very near future, that arranging atoms the way we want will become a manufacturing technology. This technology will start small, very small, in making practical and profitable products, and from there scale-up to a wide range of products and applications with very large economic and societal impacts. We will explain some of the details of the path that we are on to achieve Atomically Precise Manufacturing (APM), some of the challenges we must overcome to succeed, and the surprising number of applications that we have identified that are waiting for us to exploit.