Daryl A. Smith

A nanoscale three-dimensional Monte Carlo simulation of electron-beam-induced deposition with gas dynamics

A computer simulation was developed to simulate electron-beam-induced deposition (EBID). Simulated growth produced high-aspect-ratio, nanoscale pillar structures by simulating a stationary Gaussian electron beam. The simulator stores in memory the spatial and temporal coordinates of deposited atoms in addition to the type of electron, either primary (PE), back-scattered (BSE), or secondary (SE), that induced its deposition. The results provided in this paper apply to tungsten pillar growth by EBID on a tungsten substrate from WF(6) precursor, although the simulation may be applied to any substrate-precursor set. The details of the simulation are described including the Monte Carlo electron-solid interaction simulation used to generate scattered electron trajectories and SE generation, the probability of molecular dissociation of the precursor gas when an electron traverses the surface, and the gas dynamics which control the surface coverage of the WF(6) precursor on the substrate and pillar surface. In this paper, three specific studies are compared: the effects of beam energy, mass transport versus reaction-rate-limited growth, and the effects of surface diffusion on the EBID process.

Fundamental Proximity Effects in Focused electron Beam Induced Deposition

Fundamental proximity effects for electron beam induced deposition processes on nonflat surfaces were studied experimentally and via simulation. Two specific effects were elucidated and exploited to considerably increase the volumetric growth rate of this nanoscale direct write method: (1) increasing the scanning electron pitch to the scale of the lateral electron straggle increased the volumetric growth rate by 250% by enhancing the effective forward scattered, backscattered, and secondary electron coefficients as well as by strong recollection effects of adjacent features; and (2) strategic patterning sequences are introduced to reduce precursor depletion effects which increase volumetric growth rates by more than 90%, demonstrating the strong influence of patterning parameters on the final performance of this powerful direct write technique.

Nanopillar growth by focused helium ion-beam-induced deposition

A 25 keV focused helium ion beam has been used to grow PtC nanopillars on a silicon substrate by beam-induced decomposition of a (CH(3))(3)Pt(C(P)CH(3)) precursor gas. The ion beam diameter was about 1 nm. The observed relatively high growth rates suggest that electronic excitation is the dominant mechanism in helium ion-beam-induced deposition. Pillars grown at low beam currents are narrow and have sharp tips. For a constant dose, the pillar height decreases with increasing current, pointing to depletion of precursor molecules at the beam impact site. Furthermore, the diameter increases rapidly and the total pillar volume decreases slowly with increasing current. Monte Carlo simulations have been performed with realistic values for the fundamental deposition processes. The simulation results are in good agreement with experimental observations. In particular, they reproduce the current dependences of the vertical and lateral growth rates and of the volumetric deposition efficiency. Furthermore, the simulations reveal that the vertical pillar growth is due to type-1 secondary electrons and primary ions, while the lateral outgrowth is due to type-2 secondary electrons and scattered ions.

Understanding the Kinetics and Nanoscale Morphology of Electron‐Beam‐Induced Deposition via a Three‐Dimensional Monte Carlo Simulation: The Effects of the Precursor Molecule and the Deposited Material

The electron-beam-induced deposition of silicon oxide from tetraethyorthosilicate and tungsten from tungsten hexafluoride is simulated via a Monte Carlo simulation. Pseudo one-dimensional nanopillars are grown using comparable electron-beam parameters and a comparison of the vertical and lateral growth rate and the pillar morphology is correlated to the precursor and deposited material parameters. The primary and secondary electrons (type I) are found to dominate the vertical growth rate and the lateral growth rate is dominated by forward and secondary electrons (type II). The resolution and morphology of the nanopillars are affected by the effective electron interaction volume and the resultant surface coverage of the precursor species in the effective electron interaction region. Finally, the simulated results are compared to previously reported experimental results.

Monte Carlo simulation of focused helium ion beam induced deposition

The details of a Monte Carlo helium ion beam induced deposition simulation are introduced and initial results for reaction rate and mass transport limited growth regimes are presented. Reaction rate limited growth leads to fast vertical growth from incident primary ions and minimal lateral broadening, whereas mass transport limited growth has lower vertical growth velocity and exhibits broadening due to scattered ions and secondary electrons. The results are compared to recent experiments and previous electron beam induced deposition simulations.

Pulsed Helium Ion Beam Induced Deposition: A Means to High Growth Rates

The sub-nanometer beam of a helium ion microscope was used to study and optimize helium-ion beam induced deposition of PtC nanopillars with the (CH3)3Pt(CPCH3) precursor. The beam current, beam dwell time, precursor refresh time, and beam focus have been independently varied. Continuous beam exposure resulted in narrow but short pillars, while pulsed exposure resulted in thinner and higher ones. Furthermore, at short dwell times the deposition efficiency was very high, especially for a defocused beam. Efficiencies were measured up to 20 times the value for continuous exposure conditions. The interpretation of the experimental data was aided by a Monte Carlo simulation of the deposition. The results indicate that two regimes are operational in ion beam induced deposition (IBID). In the first one, the adsorbed precursor molecules originally present in the beam interaction region decompose. After the original precursor layer is consumed, further depletion is averted and growth continues by the supply of molecules via adsorption and surface diffusion. Depletion around the beam impact site can be distinguished from depletion on the flanges of the growing pillars. The Monte Carlo simulations for low precursor surface coverage reproduce measured growth rates, but predict considerably narrower pillars, especially at short dwell times. Both the experiments and the simulations show that the pillar width rapidly increases with increasing beam diameter. Optimal writing strategy, good beam focusing, and rapid beam positioning are needed for efficient and precise fabrication of extended and complex nanostructures by He-IBID.

Electron Beam Induced Deposition and Etching: Fundamentals, Challenges and Nanotechnology–based Applications

Precise spatial patterning of advanced materials with minimum error is critical for fabrication at reduced length scales. Electron beam induced processing (EBIP) has emerged as a method to define with high spatial precision nanoscale features and elements. However, control of the composition coordinate was proven an elusive challenge when using the EBIP approach for direct–write deposition (electron–beam–induced deposition, EBID). The compositional characteristics of EBID deposits will be discussed, the fundamental reasons for the occurrence of the impurity problem and current approaches to best maximize deposit purity will be discussed. Both EBID as well as the electron–beam–induced etching process (EBIE) require a fundamental understanding of the governing parameter space to obtain the desired, nanoscale end–product. The tremendous and complex EBID/EBIE parameter space includes the local precursor gas flux, primary electron beam energy, electron beam current, surface diffusion rates of adsorbed precursor species, thermal effects on desorption, and the cascade of electron species produced by elastic and inelastic scattering processes. In the case of EBIE, strongly surface bound etch by– products can significantly rate–limit EBIE efficiency. Characterization of this process for the etching of silicon by electron dissociated XeF2 will be discussed. In this presentation, a variety of experimental studies will be presented to demonstrate the various electron-gas, gas-solid, and electron-solid interactions that are relevant to the electron beam induced processing technique. Reaction rate–limited and mass transport–limited EBID growth modes will be discussed in terms of their respective effects on deposition rate and deposition efficiency in atoms deposited per electron. A well developed computer simulation based on Monte-Carlo calculation sequences will also be presented and compared to various experimental observations. Lastly, several nanoscale device applications will be demonstrated including EBID lithography, scanning probe tip editing, and other nanotechnology–based applications. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609099176 318

Understanding the growth mechanisms of electron beam induced deposition via a Monte — Carlo based, 3D growth simulation

Electron beam induced deposition (EBID) and etching (EBIE) is rapidly becoming the method of choice for nanoscale selective processing because is it a softer less damaging process relative to focused ion beam processing. Deposition with tungsten-hexafluoride (WF6) and tetra-ethyl-ortho-silicate (TEOS) sources have been shown to efficiently deposit tungsten and SiOx, respectively; however the distinct differences in material properties affect the final deposit morphology. Initial results from experiments show the distinct shapes formed from the two dissociation reactions have been reproduced using a Monte-Carlo based 3D algorithm which was designed specifically to predict such behavior. The effective Bethe stopping range determines the resultant nanopillar morphology under similar WF6 and TEOS EBID conditions. Simulations and experimental results show that the morphology is cylindrical when the fiber height is greater than the effective range and is conical when less than the effective range.