Christian Schwalb

Focused electron beam induced deposition: A perspective

Summary Background: Focused electron beam induced deposition (FEBID) is a direct-writing technique with nanometer resolution, which has received strongly increasing attention within the last decade. In FEBID a precursor previously adsorbed on a substrate surface is dissociated in the focus of an electron beam. After 20 years of continuous development FEBID has reached a stage at which this technique is now particularly attractive for several areas in both, basic and applied research. The present topical review addresses selected examples that highlight this development in the areas of charge-transport regimes in nanogranular metals close to an insulator-to-metal transition, the use of these materials for strain- and magnetic-field sensing, and the prospect of extending FEBID to multicomponent systems, such as binary alloys and intermetallic compounds with cooperative ground states. Results: After a brief introduction to the technique, recent work concerning FEBID of Pt–Si alloys and (hard-magnetic) Co–Pt intermetallic compounds on the nanometer scale is reviewed. The growth process in the presence of two precursors, whose flux is independently controlled, is analyzed within a continuum model of FEBID that employs rate equations. Predictions are made for the tunability of the composition of the Co–Pt system by simply changing the dwell time of the electron beam during the writing process. The charge-transport regimes of nanogranular metals are reviewed next with a focus on recent theoretical advancements in the field. As a case study the transport properties of Pt–C nanogranular FEBID structures are discussed. It is shown that by means of a post-growth electron-irradiation treatment the electronic intergrain-coupling strength can be continuously tuned over a wide range. This provides unique access to the transport properties of this material close to the insulator-to-metal transition. In the last part of the review, recent developments in mechanical strain-sensing and the detection of small, inhomogeneous magnetic fields by employing nanogranular FEBID structures are highlighted. Conclusion: FEBID has now reached a state of maturity that allows a shift of the focus towards the development of new application fields, be it in basic research or applied. This is shown for selected examples in the present review. At the same time, when seen from a broader perspective, FEBID still has to live up to the original idea of providing a tool for electron-controlled chemistry on the nanometer scale. This has to be understood in the sense that, by providing a suitable environment during the FEBID process, the outcome of the electron-induced reactions can be steered in a controlled way towards yielding the desired composition of the products. The development of a FEBID-specialized surface chemistry is mostly still in its infancy. Next to application development, it is this aspect that will likely be a guiding light for the future development of the field of focused electron beam induced deposition.

A Tunable Strain Sensor Using Nanogranular Metals

This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals.

Tuning the electrical conductivity of Pt-containing granular metals by postgrowth electron irradiation

We fabricated Pt-containing granular metals by focused electron beam–induced deposition from the (CH3)3CH3C5H4Pt precursor gas. The granular metals are made of platinum nanocrystallites embedded in a carbonaceous matrix. We exposed the as-grown nanocomposites to low-energy electron beam irradiation and measured the electrical conductivity as a function of irradiation dose. Postgrowth electron beam irradiation transforms the matrix microstructure and thus the strength of the tunneling coupling between Pt nanocrystallites. For as-grown samples (weak tunnel coupling regime) we find that the temperature dependence of the electrical conductivity follows the stretched exponential behavior characteristic of the correlated variable-range hopping transport regime. For briefly irradiated samples (strong tunnel coupling regime) the electrical conductivity is tuned across the metal-insulator transition. For long-time irradiated samples the electrical conductivity behaves like that of a metal. In order to further anal...

Universal Conductance Correction in a Tunable Strongly Coupled Nanogranular Metal

We present temperature-dependent conductivity data obtained on a sample set of nanogranular Pt-C with finely tuned intergrain tunnel coupling strength g. For samples in the strong-coupling regime g > g(C), characterized by a finite conductivity for T→0, we find a logarithmic behavior at elevated temperatures and a crossover to a √T behavior at low temperatures over a wide range of coupling strengths g(C) ≈ 0.25 < g ≤ 3. The experimental observation for g > 1 is in very good agreement with recent theoretical findings on ordered granular metals in three spatial dimensions. The results indicate a validity of the predicted universal conductivity behavior that goes beyond the immediate range of the approach used in the theoretical derivation.

Binary Pt–Si Nanostructures Prepared by Focused Electron-Beam-Induced Deposition

Binary systems of Pt-Si are prepared by electron-beam-induced deposition using the two precursors, trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPt(Me)(3)) and neopentasilane (Si(SiH(3))(4)), simultaneously. By varying the relative flux of the two precursors during deposition, we are able to study composites containing platinum and silicon in different ratios by means of energy-dispersive X-ray spectroscopy, atomic force microscopy, electrical transport measurements, and transmission electron microscopy. The results show strong evidence for the formation of a binary, metastable Pt(2)Si(3) phase, leading to a maximum in the conductivity for a Si/Pt ratio of 3:2.

Superconductivity and metallic behavior in PbxCyOδ structures prepared by focused electron beam induced deposition

Focused electron beam induced deposition as a direct-write approach possesses great potential to meet the demands for superconducting nanostructure fabrication especially regarding its 3D patterning capabilities combined with the high resolution in the nanometer regime. So far, however, it was not possible to fabricate superconducting structures with this technique. In this work, we present a lead-based superconductor prepared by focused electron beam induced deposition by dissociation of the precursor tetraethyllead. The as-grown structures exhibit metallic behavior and a minimum resistivity in the normal state of ρ = 16 μΩcm at T = 9 K followed by a superconducting transition at Tc = 7.2 K.

In situ growth optimization in focused electron-beam induced deposition

Summary We present the application of an evolutionary genetic algorithm for the in situ optimization of nanostructures that are prepared by focused electron-beam-induced deposition (FEBID). It allows us to tune the properties of the deposits towards the highest conductivity by using the time gradient of the measured in situ rate of change of conductance as the fitness parameter for the algorithm. The effectiveness of the procedure is presented for the precursor W(CO)6 as well as for post-treatment of Pt–C deposits, which were obtained by the dissociation of MeCpPt(Me)3. For W(CO)6-based structures an increase of conductivity by one order of magnitude can be achieved, whereas the effect for MeCpPt(Me)3 is largely suppressed. The presented technique can be applied to all beam-induced deposition processes and has great potential for a further optimization or tuning of parameters for nanostructures that are prepared by FEBID or related techniques.

Superconductivity in the system MoxCyGazOδ prepared by focused ion beam induced deposition

We have prepared the new amorphous superconductor

Analysis of local deformation effects in resistive strain sensing of a submicron-thickness AFM cantilever

\rm Mo_xC_yGa_zO_{\delta}

Improved Understanding of Material Behavior using Correlative In-situ Techniques

with a maximum critical temperature