Joshua Ballard

Atomic precision lithography on Si

Lithographic precision is as or more important than resolution. For decades, the semiconductor industry has been able to work with ±5% precision. However, for other applications such as micronanoelectromechanical systems, optical elements, and biointerface applications, higher precision is desirable. Lyding et al. [Appl. Phys. Lett. 64, 11 (1999)] have demonstrated that a scanning tunneling microscope can be used to remove hydrogen (H) atoms from a silicon (100) 2 × 1 H-passivated surface through an electron stimulated desorption process. This can be considered e-beam lithography with a thin, self-developing resist. Patterned hydrogen layers do not make a robust etch mask, but the depassivated areas are highly reactive since they are unsatisfied covalent bonds and have been used for selective deposition of metals, oxides, semiconductors, and dopants. The depassivation lithography has shown the ability to remove single H atoms, suggesting the possibility of precise atomic patterning. This patterning proces...

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%.

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 ...

Using patterned H-resist for controlled three-dimensional growth of nanostructures

We present a study addressing the effectiveness of a monolayer of hydrogen as the lithographic resist for controlled three-dimensional (3D) growth of nanostructures on the Si(100) surface. Nanoscale regions on the H-terminated Si(100) were defined by H-desorption lithography via the biased tip of a scanning tunneling microscope (STM) to create well-defined regions of surface “dangling bonds,” and the growth of 3D nanostructures within these regions was achieved using a simultaneous disilane deposition and STM H-desorption technique. We demonstrate that 3D growth is strongly confined within STM depassivated regions while unpatterned H:Si(100) regions are robust against adsorption of the precursor molecules.

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.

Automated 2D micro-assembly using diamagnetically levitated milli-robots

In this article, we demonstrate the application of diamagnetically levitated milli-robots for the 2D micro-assembly of 10-μm polymer microspheres and other silicon microfabricated parts. By using an optical microscope for feedback (imaged at 27 Hz), we are able to demonstrate long-term open-loop stability (up to 78 hr) and sub-micron stability of the levitated micro-robots. Furthermore, due to the low hysteresis and high compliance in the magnetic drive of the milli-robots, we are able to directly use the milli-robots in conjunction with machine vision as a force sensor. Soft polymer-based end effectors are used for the micromanipulation of parts and show modest reliability of pick (>70%) and high reliability of place (>99%) that is insensitive to the pick surface material. Finally, we implement autonomous micro-assembly from randomly deposited microspheres into ordered arrays.

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.

Theoretical and experimental study of tip electronic structures in scanning tunneling microscope

The atomic and electronic structures of pyramidal model STM tips of transition metals (W, Rh, Pd, Ir and Pt) were investigated using density functional theory (DFT) method. The calculated density of states show that d electrons of the apex atoms in the M 4 (M = W, Rh, Pd, Ir, Pt) model tips behave differently near the Fermi level, with the dz 2 state being dominant only for W tip. The electronic structures of pyramid structures of W and Pd single-atom tips with larger sizes are studied and compared. The density of states of Pd apex atom and W apex atom show different occupation of d-bands leading to asymmetric density of states for Pd tip. The asymmetric tunneling currents measured by W and Pt-Ir STM tips are explained by the calculated electronic structures of W and Pd model tips.

Scanning tunneling microscopy using dynamic laser heating

A hydrogen passivated Si(100)-2/spl times/1 surface is studied using ultrafast laser excitation with a novel total internal reflection geometry. The incoherent tip and sample heating dynamics are studied, and it is shown that as long as the STM feedback is faster than the heating and cooling processes, atomic scale imaging can be achieved, even in the presence of significant tip and sample expansion and contraction. This opens the door for studies of surface processes on the ultrafast timescale.