Focused Electron Beam Induced Deposition (FEBID) - A New Tool for 3-D Nanomanufacturing

Abstract

In FEBID a tightly-focused, high-energy electron beam impinges on a substrate and, upon collision interactions, producing lower energy back-scattered (BSE) primary electrons and secondary electrons (SE). Concurrently, the precursor is introduced into the reaction chamber via complete chamber flooding, or filling of a smaller reaction cell within the vacuum chamber or by local gas injection via microneedle. Interactions of adsorbed molecules with BSE and SE of the appropriate energy result in precursor dissociation forming a deposit. FEBID has shown promise for nanoscale deposition of a variety of materials. Potential applications range from 3-D nanofabrication (e.g., STM and AFM tips) to integrated circuit mask repair and nanoscale interconnect fabrication. In an attempt to characterize the FEBID process extensive experimentation has been performed under a variety of conditions and with many different precursors. However, limited advances have been made in providing theoretically sound and experimentally validated understanding of the underlying physics. In this contribution I will discuss fundamental physical and chemical mechanisms underlying FEBID. In general, transport of both the precursor molecules and the primary/secondary electrons facilitate nanostructure deposition. Depending on the operating pressure either continuous or ballistic transport defines delivery of precursor molecules to the substrate surface. At the surface the adsorbed precursor molecules are spatially re-distributed by surface diffusion, and eventually a fraction of the adsorbed molecules becomes converted into a solid deposit. The interactions of primary electrons with the substrate and nanoscaleconfined deposit may induce significant localized heating. Such energy transfer process is complex, involves non-classical heat conduction, and greatly influences the deposition process. The pertinent question is then what controls the FEBID process, i.e., both time dependent growth of the nanostructure and its shape evolution? The answer to this question will be discussed using complimentary theoretical and experimental studies. The interactions of primary electrons with the substrate and nanoscale-confined deposit may induce significant localized heating. Such energy transfer process is complex, involves non-classical heat conduction, and greatly influences the deposition process. Classical heat conduction modeling formalism, based on the Fourier constitutive law, fails to adequately describe heat transport in electron beam heated nanostructures, and I will present a methodology how the time scale analysis of fundamental electron-phonon-exciton interactions can be used to estimate the effect of heating on deposit microstructure. The scaling predictions will be critically compared against experimental data from the literature. At the end, I will also briefly illustrate how understanding of EBID fundamentals is applied to development of two new nanomanufacturing processes relevant to important applications: (1) making a low-temperature Ohmic contact for the next generation of electric interconnects based on ballistically-conducting multiwalled carbon nanotubes (MWCNTs), and (2) resist-free rapid prototyping of high-aspect-ratio three-dimensional semiconductor nanostructures using electron beam assisted deposition of amorphous carbon in combination with metal assisted chemical etching (EBID-MaCE).