W. F. van Dorp

A critical literature review of focused electron beam induced deposition

An extensive review is given of the results from literature on electron beam induced deposition. Electron beam induced deposition is a complex process, where many and often mutually dependent factors are involved. The process has been studied by many over many years in many different experimental setups, so it is not surprising that there is a great variety of experimental results. To come to a better understanding of the process, it is important to see to which extent the experimental results are consistent with each other and with the existing model. All results from literature were categorized by sorting the data according to the specific parameter that was varied (current density, acceleration voltage, scan patterns, etc.). Each of these parameters can have an effect on the final deposit properties, such as the physical dimensions, the composition, the morphology, or the conductivity. For each parameter-property combination, the available data are discussed and (as far as possible) interpreted. By combining models for electron scattering in a solid, two different growth regimes, and electron beam induced heating, the majority of the experimental results were explained qualitatively. This indicates that the physical processes are well understood, although quantitatively speaking the models can still be improved. The review makes clear that several major issues remain. One issue encountered when interpreting results from literature is the lack of data. Often, important parameters (such as the local precursor pressure) are not reported, which can complicate interpretation of the results. Another issue is the fact that the cross section for electron induced dissociation is unknown. In a number of cases, a correlation between the vertical growth rate and the secondary electron yield was found, which suggests that the secondary electrons dominate the dissociation rather than the primary electrons. Conclusive evidence for this hypothesis has not been found. Finally, there is a limited understanding of the mechanism of electron induced precursor dissociation. In many cases, the deposit composition is not directly dependent on the stoichiometric composition of the precursor and the electron induced decomposition paths can be very different from those expected from calculations or thermal decomposition. The dissociation mechanism is one of the key factors determining the purity of the deposits and a better understanding of this process will help develop electron beam induced deposition into a viable nanofabrication technique.

Ultrahigh resolution focused electron beam induced processing: the effect of substrate thickness

It is often suggested that the growth in focused electron beam induced processing (FEBIP) is caused not only by primary electrons, but also (and even predominantly) by secondary electrons (SEs). If that is true, the growth rate for FEBIP can be changed by modifying the SE yield. Results from our Monte Carlo simulations show that the SE yield changes strongly with substrate thickness for thicknesses below the SE escape depth. However, our experimental results show that the growth rate is independent of the substrate thickness. Deposits with an average size of about 3 nm were written on 1 and 9 nm thick carbon substrates. The apparent contradiction between simulation and experiment is explained by simulating the SE emission from a carbon substrate with platinum deposits on the surface. It appears that the SE emission is dominated by the deposits rather than the carbon substrate, even for deposits as small as 0.32 nm(3).

Nanometer-scale lithography on microscopically clean graphene

Focused-electron-beam-induced deposition, or FEBID, enables the fabrication of patterns with sub-10 nm resolution. The initial stages of metal deposition by FEBID are still not fundamentally well understood. For these investigations, graphene, a one-atom-thick sheet of carbon atoms in a hexagonal lattice, is ideal as the substrate for FEBID writing. In this paper, we have used exfoliated few-layer graphene as a support to study the early growth phase of focused-electron-beam-induced deposition and to write patterns with dimensions between 0.6 and 5 nm. The results obtained here are compared to the deposition behavior on amorphous materials. Prior to the deposition experiment, the few-layer graphene was cleaned. Typically, it is observed in electron microscope images that areas of microscopically clean graphene are surrounded by areas with amorphous material. We present a method to remove the amorphous material in order to obtain large areas of microscopically clean graphene flakes. After cleaning, W(CO)(6) was used as the precursor to study the early growth phase of FEBID deposits. It was observed that preferential adsorption of the precursor molecules on step edges and adsorbates plays a key role in the deposition on cleaned few-layer graphene.

Solutions to a proximity effect in high resolution electron beam induced deposition

A new type of proximity effect in electron beam induced deposition (EBID) is presented. Proximity effects are well known in resist based electron beam lithography (EBL), where they present themselves as an extra and unintentional exposure of the resist layer surrounding the irradiated areas. Several types of proximity effects have already been reported for EBID, which are of a different nature. The authors report a proximity effect where the amount of deposited mass increases with each new line that is deposited. This effect occurs when parallel lines are deposited at a spacing close to the width of the lines. The increase in deposition rate was found to be dependent on the angle between the irradiated target and the incident electron beam. Results from a simulation based on this model qualititatively show the same trend. A successful strategy for reducing the effect is presented.

Statistical variation analysis of sub-5-nm-sized electron-beam-induced deposits

We report on the statistical analysis of the variations in the size and position of sub-5nm tungsten-containing dots in regular arrays deposited by electron-beam-induced deposition. Full widths at half maximum of the dots are 4.2 and 2.0nm in average. It can be observed in the recorded annular dark-field images that there is a variation in intensity for these dots. We have analyzed these variations and it is found that the relative standard deviation for the mass per dot is 0.092 for the 4.2nm dots and 0.26 for the 2.0nm dots. Comparing this to a relative standard deviation in the estimated number of precursor molecules that are pinned down per dot of 0.041 for the 4.2nm dots and 0.11 for the 2.0nm dots, it appears that the dot-to-dot variation in mass for both dot sizes compares reasonably well with the values expected from Poisson statistics on the number of molecules per dot. It can be concluded that at these dimensions, the statistics on the number of pinned precursor molecules dominates the control o...

Focused electron beam induced processing and the effect of substrate thickness revisited

The current understanding in the study of focused electron beam induced processing (FEBIP) is that the growth of a deposit is mainly the result of secondary electrons (SEs). This suggests that the growth rate for FEBIP is affected by the SE emission from the support. Our experiments, with membranes thinner than the SE escape depth, confirm this hypothesis. We used membranes of 1.4 and 4.3 nm amorphous carbon as supports. At the very early stage, the growth is support-dominated and the growth rate on a 4.3 nm thick membrane is three times higher than on a 1.4 nm thick membrane. This is consistent with Monte Carlo simulations for SE emission. The results suggest that SEs are dominant in the dissociation of W(CO)6 on thin membranes. The best agreement between simulations and experiment is obtained for SEs with energies between 3 and 6 eV.With this work we revisit earlier experiments, working at a precursor pressure 20 times lower than previously. Then, despite using membranes thinner than the SE escape depth, we did not see an effect on the experimental growth rate. We explain our current results by the fact that very early in the process, the growth becomes dominated by the growing deposit itself.

Selective functionalization of patterned glass surfaces

Tailored writing and specific positioning of molecules on nanostructures is a key step for creating functional materials and nano-optical devices, or interfaces for synthetic machines in various applications. We present a novel approach for the selective functionalization of patterned glass surfaces with functional probes of any nature. The presented strategy is optimized for imaging fluorophore labeled nanostructures for (single-molecule) fluorescence microscopy. The first step in the protocol is coating a glass surface, here a microscope cover slide, with a 60 nm thick diamond-like carbon film. Subsequently, the pattern is defined by either writing silicon oxide on the coating with a focused electron beam, or by etching the coating with a focused ion beam to expose the glass surface. Finally, the pattern is silanized and functionalized. We demonstrate the selective binding of organic fluorophores and imaging with high contrast, especially in total-internal-reflection mode. The presented approach is flexible and combines bottom-up assembly with high-resolution lithography on glass cover slides to precisely position and image functional molecules of any type.

The nucleation stage in electron beam induced deposition

With electron beam induced deposition structures as small as 1 nm can be deposited. The nucleation stage of the growth of such deposits is studied in dependence of the substrate material and substrate temperature. On amorphous carbon foils larger deposit sizes are obtained at higher substrate temperatures during growth. On SiN membranes unusually high growth rates are observed which seem to relate to charging effects.

In situ monitoring and control of material growth for high resolution electron beam induced deposition

During electron beam induced deposition on electron transparent membranes, the transmitted annular dark field (ADF) signal can be monitored. A method was developed to use the ADF signal to obtain insight into the growth process and to control the mass of individual nanometer-sized deposits. Arbitrary two-dimensional patterns can be defined. The smallest sampling time of the ADF signal monitoring is presently about 40ms. For arrays of dots that were deposited, the growth of each individual dot was monitored. It is observed that the growth is different for each dot, although the average deposit growth rate is linear with the dwell time. Apart from monitoring the ADF signal during the growth, the amount of deposited mass can be controlled for individual deposits by terminating the growth process when the ADF signal exceeds a threshold value. The dynamic ADF feedback control was applied to reduce variations in deposit mass. This attempt did not succeed, but the method was successfully applied to prevent the o...

Synthesis of Nanostructures using Electron Beam Induced Deposition

Nanostructures can be fabricated in-situ in the electron microscope by Electron Beam Induced Deposition (EBID. A precursor gas is admitted in the vacuum of the microscope, and adsorbed precursor molecules are dissociated by both the primary and the secondary electrons, leaving nonvolatile parts behind on the sample (Figure 1). Different precursor gases allow to deposit Pt, W, Au, etc. The deposits usually contain a large percentage of carbon. The old-fashioned carbon contamination deposits in electron microscopes is also a form of EBID. To obtain the smallest possible deposits, we studied the electron-sample interaction and carefully tuned the process parameters. We performed EBID experiments in the environmental Scanning Transmission Electron Microscope (STEM) of ASU at a beam energy of 200 keV, a 0.3 nm probe and W(CO)6 as a precursor gas. Typical precursor gas pressures during the deposition are 10 -3 Torr, and to reduce contamination from the microscope the substrates are held at an elevated temperature of 100-150 ˚C. We deposited arrays of nanometer sized dots on thin membranes of SiN, amorphous carbon or graphite[1]. The imaging and monitoring of the growth were performed with the annular dark field (ADF) signal. The smallest deposits we have made so far have an average volume of 0.4 nm 3 .