Growth and characterization of EBID-fabricated suspended nanostructures

While the standard beam-induced deposition approach is to grow deposits on a substrate, deposition of self-supporting suspended structures can be obtained by slowly moving the beam laterally from an elevated edge, in order to grow an overhanging deposit behind the beam path. An example of nanofabrication by lateral-EBID is given in fig. 1, where a suspended nanowire has been deposited across two vertical pillars. The structure is obtained (fig. 1c) by moving the electron beam from the top of left pillar toward the right one with a scan speed of 33 nm/s, at steps of 5 nm, under a Pt-metallorganic gas flow. As indicated by the lateral dimensions in fig. 1a and fig. 1b, an advantage of this deposition method is the high lateral resolution because of the lack of secondary emissions from the substrate, that enlarge the deposit footprint well above the beam spotsize.The growth mechanism at the basis of lateral deposition, as proposed by Liu and co-workers, is depicted in fig. 2, where a gaussian electron beam, positioned over a thin edge and then moved to the right, is considered. On the first spot, the beam generates a gaussian deposit above (A) and below (B) the edge (because of the deep electron penetration), having a volume and gaussian width which are proportional to the beam dwell time and also depend on the beam shape. When the beam is shifted laterally it falls on a point of the deposit that may be above (O1a), at the same level of (O2), or below (O3b) the edge, depending on the amplitude of the shift, and thus give rise to upward (A1B1), parallel (A2B2) or downward (A3B3) growth, respectively. If the lateral shift is much larger than the deposit width, lateral growth doesn’t take place. The process is ruled by the interplay of several parameters, such as the deposition rate, the beam dwell time, the lateral shift amplitude and the beam shape. In most studies, these are often resumed into the beam shift speed which is also a reasonable parameter for a given beam shape and gas flux. A major capability offered by lateral deposition is the three-dimensional (3D) nanofabrication. Matsui and co-workers demonstrated the potentiality of the technique with several examples of both functional (nanocoils, electrical circuit elements, nanogrippers) and artwork shapes (nanoglass) produced with IBID of carbon precursor. In the case of EBID, Ooi and co-workers presented the fabrication of tools and probes based on suspended nanowires for the manipulation and observation, with SNOM microscopy, of DNA fibers. The structures were realized by lateral-EBID of carbon-contamination gas. Another noteworthy example are the 10 nm-size nanotweezers, with a gap of 25 nm, fabricated at the ends of conventional Si microtweezers by lateral-EBID of C contamination by Boggild et al. Several works were devoted to the study of the growth mechanism and material properties. EBID of 3D freestanding nanostructures from a Cu precursor was explored by Utke et al.. Suspended horizontal nanorods were used as a support for the growth of vertical pillars, and it was found that the reduced thermal conductivity with respect to a bulk substrate, resulted in a thermal decomposition of the precursor with higher crystallinity of the Cu deposit. Fujita et al. fabricated 5 nm-width suspended nanowires by lateral-EBID of C contamination with a SEM. They compared the suspended growth with the one of vertical pillars and concluded that the higher resolution in the former was due to the reduced secondary electrons generation within the structure. Liu and co-workers studied the lateral deposition of W precursors with TEM high energy electrons (200keV), deriving the growth model presented in fig. 2. Lateral EBID has been also investigated on bulk substrates. By varying the lateral scan speed, the inclination of pillars deposited from Au and Mo precursors [ ] and the periodicity of arch-like structures grown from Cu precursor [ ] have been studied. The geometry of suspended depositions, grown from a gold precursor in an environmental SEM, has also been explored [ ].In the following paragraphs we will focus on lateral EBID of suspended nanostructures from Pt-metallorganic and TEOS precursors, performed with an SEM. We will cover the growth mode as a function of e-beam parameters, the characterization (structure and composition) and sculpting by TEM, and, limited to the Pt structures, the thermal processing, the electrical characterization, and the structural modifications under high electrical current densities.