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Technology basis and perspectives on focused electron beam induced deposition and focused ion beam induced deposition
Nuclear Instruments and Methods in Physics Research B 341 (2014) 37–43
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Technology basis and perspectives on focused electron beam induced deposition and focused ion beam induced deposition Gemma Rius ⇑ Center for Innovative Young Researchers, Nagoya Institute of Technology, Gokiso cho, Showa ku, 466-8555 Nagoya, Japan
a r t i c l e
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Article history: Received 24 August 2013 Received in revised form 6 December 2013 Accepted 20 June 2014 Available online 13 August 2014 Keywords: Nanopatterning Focused electron beam Focused ion beam Charged particle-solid interactions Beam-induced deposition
a b s t r a c t The main characteristics of focused electron beam induced deposition (FEBID) and focused ion beam induced deposition (FIBID) are presented. FEBID and FIBID are two nanopatterning techniques that allow the fabrication of submicron patterns with nanometer resolution on selected locations of any kind of substrate, even on highly structured supports. The process consists of mask less serial deposition and can be applied to a wide variety of materials, depending strictly on the precursor material source used. The basic mechanism of FEBID and FIBID is the adsorption of volatile precursor molecules onto the sample surface and decomposition of the molecules induced by the energetic electron and ion focused beams. The essential similarities of the two techniques are presented and especial emphasis is dedicated to highlighting their main differences, such as aspects related to resolution, deposition rate, deposits purity, substrate integrity, etc. In both cases, the factors interplay and complex mechanisms are still understood in a qualitative basis, so much work can still be done in terms of modeling and simulating the processes involved in FEBID and FIBID. Current work on FEBID and FIBID is presented through examples of achievements, interesting results and novel approaches. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanotechnology is defined by the National Nanotechnology Initiative as the manipulation of matter with at least one dimension sized from 1 to 100 nm or, in an earlier definition, the manipulation of matter on an atomic and molecular scale [1]. Its meaning actually includes the novel (nano)science arising from the restricted dimensionality of nanoscaled particles, nanostructured materials and surfaces, and nanodevices. Nanotechnology spans almost over any science discipline in an ubiquitous manner [2], from physics to biology, from electronics to medicine, in brief, from fundamental science to applications, with potential impact into the most common consumer goods [3]. A remarkable effort of nanotechnology is associated with the development of the production (and characterization) strategies for the functional nanoobjects. Their fabrication, namely nanofabrication, is in some cases seen as a further miniaturization of microfabrication processing [4]. It is seen, then, as a sort of evolutionary route, which, as closely connected to microelectronics planar technology, unquestionably directs toward considering nanopatterning and nanolithography. ⇑ Tel.: +81 052 735 5478. E-mail address: [email protected] http://dx.doi.org/10.1016/j.nimb.2014.06.034 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
In this context, an interest on developing alternative compatible patterning techniques able to precisely define features at the nanoscale is uninterruptedly being sought-after. Beyond more established nanolithographies [5], such as resist-mediated laser or electron beam lithography, focused electron beam induced deposition (FEBID) [6,7] (Fig. 1) and focused ion beam induced deposition (FIBID) [6] have received notable attention due to their potential to directly pattern with high accuracy a variety of materials on almost any substrate, including highly structured surfaces such as sensing probes. Nonetheless, pattern flexibility and alignment capability makes them ideal for interfacing single nanoobjects such as nanoparticles, nanotubes and nanowires [8]. Being referred in the literature with many other names, FEBID and FIBID will be here used as this nomenclature highlights the main focus aspects of present review; the spatial resolution, expressed by the term focused; the use of charged particle beams providing accurate scanning capability, expressed by the terms electron/ion beam; and the concept of the patterning, expressed by the terms induced deposition. In other words and considered in terms of fabrication, FEBID and FIBID do not only represent a simplification of the process sequence, decreasing the number of steps and avoiding eventual resist contamination and so on (Fig. 2), but also provide higher processing flexibility based on the fact that FEBID and FIBID are mask less techniques, the
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Fig. 1. Basic elements and principles of FEBID, (a), and focused electron beam induced etching (FEBIE), (b). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 2. Simplified fabrication of a nanostructure: comparative of the standard sequence by (a) EBL versus (b) FEBID or FIBID patterning. Remarkable reduction of the number of steps is attained.
availability of materials, possibility of mix and match, etc. Although FEBID and FIBID are rooted on a simple principle, and have demonstrated outstanding performance, they involve, actually, very complex concepts which make it difficult to be fully understood, deeply and completely, i.e. quantitatively rather than qualitatively, as a phenomenon. In this brief topical review one would like to highlight the added value and potential of FEBID and FIBID as a methodology for growing a number of material features in ‘‘0’’ to 3D at the nanoscale. The nomenclature choice also aims to reflect a practical point of view on FEBID and FIBID which will be covered here, i.e. seen as a direct patterning technique for efficient prototyping. This short overview article collects valuable references of most significant review literature on FEBID and FIBID, which describe in great detail all the factors, variables and conditions involved, putting the main attention on aspects relevant for processing, being general in terms of the mechanism and modeling matters and shortly mentioning main chemistry aspects. As a result this review would be a comprehensive introduction for those who would like to know about FEBID and FIBID: a nanofeatures patterning strategy based on the use of finely focused charged particle beams. In addition to this, a brief look on a few selected examples is provided. Concerned with the point of view of how FEBID and FIBID can enable novel functional nanomaterials, specific examples of scanning probe microscopy (SPM) probes, ferromagnetic nanowires, transmission electron microscopy (TEM) specimen fabrication and synthesis of patterned graphene layers are illustrated. A few words are given as final remarks on the prospect investigations and future endeavors on FEBID and FIBID.
2. FEBID While the invention of scanning electron microscope (SEM) dates back into the late 30s of the last century and electron beam lithography (EBL) is used since the late 60s (Fig. 3), it is not until the middle 80s that the FEBID research starts and, then, gets momentum as seen, for example, in terms of related publications [7]. A simple explanation for this lays in the availability of considerable advances in electron microscopy – electron sources, detectors, digitalization, etc. -, but also in the capacity of beam driving and shutting, which affects dramatically the resolution and capabilities in terms of lithography, enabling complex patterning [9]. Additionally, a massive interest specifically on nanosized matter, as it can be obtained by, for instance, nanopatterning methods, follows the invention of SPM, scanning tunneling microscope (STM) [10] and atomic force microscope (AFM) [11], as well as the discovery of nanomaterials synthesis like fullerenes [12], both paradigmatic examples that lead to the onset of nanotechnology. A typical system for FEBID (or FIBID) essentially consists of an EBL (or FIB) system equipped with an additional gas injection system (GIS) for the volatile precursor supply [ref]. Most relevant parameters during operation include the beam energy and its current density. Beam parameters directly affect the obtained deposition characteristics: rate (height), width (resolution), elemental composition and bonding, conductivity. . . [6,7] As a rule of thumb, the higher the incident energy, the (s)lower the deposition rate (smaller charged particle-precursor collision probability). The thinner the beam, the higher can be attained the resolution. Commonly used acceleration voltages and current densities (beam diameter)
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Fig. 3. EBL systems can consist of a SEM apparatus equipped with an external beam and scan control interface for enabling lithography. Picture courtesy of the Centro Nacional de Microelectronica, Barcelona, Spain.
are a compromise between deposition rate and the resolution, in the order of a few tens of keV, some tens/hundreds of pA, and precisely depending on the application. Similar to the so-called proximity effect in EBL, secondary electrons play a crucial role in FEBID, affecting dramatically the patterning spatial resolution. Scan parameters can also play an important role on the above mentioned structural characteristics of the resulting patterns. Very much related to the system capabilities in terms of lithography, variables such as the scan speed (dwell time and pitch, beam blanking) and strategy (point irradiation, raster scan, vector scan, random irradiation) determine the deposition again in terms of both pattern morphology (therefore, affecting design fidelity, uniformity, etc.) and chemical composition and inner structure aspects (therefore, affecting material functionality). Other system operating characteristics have a more direct effect on the conditions of the particle beam–precursor interaction itself, such as the precursor gas flux and base pressure of the chamber, the position of the nozzle and so forth. After having introduced FEBID (and FIBID) from system engineering and operational point of view, the chemistry and mechanism of the charged particles induced deposition process should not be postponed. A primordial element of the FEBID (and FIBID) is the volatile species used to form the patterned deposits. The concept of the deposition mechanism is that the energetic incident particles collide with the precursor molecules on the surface. The energy transferred from the charged particles to the volatile species causes some molecular decomposition, break of molecule inner bonds, and fixing to the support sample of some of the collisions sub-products. Again, the idea of a very strong, complex and multiplexed interplay between the different variables and factors involved is evidenced. Connected to the unavoidable charged particle interaction, in this case, with the solid substrate, certain effect originated by the beam charges and the electrical conduction of the target surface material is expected. Roughly, substrate factor would be manifested, for example as charging, and it would be affecting remarkably the possibility to define dense patterns as well as limiting the minimum feature size. Moving to the kind of elements that can be deposited, the deposited materials strictly depend on the available volatile precursor molecules. In principle, the precursor sources can be in
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gas, liquid or solid state. In terms of electronic functionality, all kind of materials can be grown: insulators, e.g. SiOx from a TEOS [13]; metallic, e.g. Pt from (CH3)3(CpCH3) [14]; ferromagnetic, e.g. Fe and Co from Fe(CO)9 and Co2(CO)8, respectively [15]. In addition to the precursor chemistry, considering the chemistry of the growth mechanism, the electron, or otherwise ion, beam-induced decomposition of the molecules has a tremendous influence on the resulting purity of the deposits. In consequence, decomposed species determine the deposit functional properties, such as the conduction, but its mechanical characteristics are affected as well. An exhaustive list of deposit-able materials and their available precursors is included, for instance, in [6,7]. As a summary list of the main characteristics of FEBID, the technique allows precise (i) fabrication of nanostructured features in a single step, (ii) over all sorts of substrates, (iii) with an in-plane resolution of 3 nm, and (iv) with a high precision and accuracy in-plane control. However, FEBID strategy limitations include (i) a low deposition rate, and (ii) limited pattern resolution (halo), while composition issues include (iii) limited purity (e.g. low metal elemental content) and, consequently, (iv) high resistivity when metallic deposition is intended. Based on the competence of FEBID (and FIBID) to locally grow features of specific morphology and made of various materials, a wide range of applications are envisioned, such as 3D nanostructures and SPM tips. Straight forwardly, focused charged particle beam induced deposition finds a use on circuit editing and photolithography mask repair. However, it can do a very interesting job when applied to the electrical transport studies of nanowires, nanoparticles and so on. In general, many topics in nanoelectronics can benefit from the use of this technique, such as superconductors [16], magnetic storage and sensing devices [17], nano-antennas [18], to name a few. Some of the applications mentioned here, which are in principle common for FEBID and FIBID, find an convenient feasibility point when combined with the processing capabilities enabled by the use of the FIB, specifically, for example when preparing TEM-transparent samples [19], which will described in the following section on FIBID. To conclude this part, a couple of examples of FEBID applications will be presented next. Although EBL remains as one of the most powerful techniques for nanolithography, it cannot be applied easily to any supports. EBL shows serious limitations for processing highly structured surfaces such as CMOS circuits. Another classic example are SPM tips, where the possibility to strategically deposit small amounts of certain materials (or a combination of several) in the tip apex, for example for C nanotube or Si nanowire growth [20], and functional layers (conductive or magnetic) could be very useful to produce specialized tips for AFM advanced modes such as MFM or SERS-AFM [21]. The possibility to define high aspect ratio deposits shows promise for the fabrication of high resolution tips (Fig. 4). As another application example, an outstanding work using FEBID ferromagnetic deposits is being done by the Spanish group of De Teresa and Ibarra [22–25]. Particularly, they have been studying the magneto transport characteristics of Co and Fe FEBID nanowires which demonstrated high quality. In Ref. [26], giant anomalous Hall could be observed in Fe microwires [26] and, as presented in Ref. [27], domain wall conduit behavior of Co nanowires could be understood (Fig. 5) [27]. The group has also studied interesting properties of superconducting materials such as W [28,29], in this case deposited by FIBID, which technique will be described subsequently.
3. FIBID The system and concept of FIBID are basically the same as the ones used in FEBID. The plain FIB instrument consists of a vacuum
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Fig. 4. Example of FEBID onto an AFM tip apex (right) and AFM tip apex machining for magnetic AFM (see article for details). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 5. (Left) SEM image of a Co nanowire patterned by FEBID. (Right) Magnetization as a function of nanowire width (see article for details). Reprinted from Appl. Phys. Lett. 94, 192509 (2009) with permission Ó 2009 American Institute of Physics [27].
system and chamber, which holds a sample stage and detectors. FIB apparatus key elements are the liquid metal ion source (LIMS) and the ion column, which in essence is analogous to an electron beam column [30]. As in FEBID, for enabling FIBID the system is equipped with a GIS. Due to a number of reasons, traditionally gallium has been the most commonly used LMIS for commercially available FIB instruments. Gallium has a low melting point, vapor pressure and volatility, notable electrical, mechanical and vacuum properties, as well as good emission characteristics, i.e. showing small energy spread and high angular intensity. Yet, recently, the use He ions has received significant attention due to implicit higher resolution and reduction of surface damage [31]. Mostly, the discussion in this article is focused on Gallium-based FIBID [32]. Not only the FIB instrument is similar to a SEM, but in many cases electron and ion columns are made compatible, i.e. integrated in a single tool, the so-called dual beam systems (Fig. 6) [33]. This dual beam option multiplies the analytical capabilities of the tool, enabling for example energy dispersive scattering for in situ chemical analysis. At the same time, it makes safer the ion beam processing since SEM-based navigation can avoid undesired ion implantation and milling in areas sensitive to charging and ion contamination or the FIBID target locations. The use of field emission electron sources, particularly, confers the system not only higher SEM resolution, but also implies that both FEBID and FIBID can be executed in favorable conditions for an optimal resolution, within the same tool.
Fig. 6. Dual beam system equipped for high resolution nanolithography and FEBID and FIBID. Picture courtesy of the Centro Nacional de Microelectronica, Barcelona, Spain.
The different applications of focused charged particle beam induced deposition are, in general, common for FEBID and FIBID. However, as for the preparation of electron transparent specimens for TEM observation combined FIB-based processing is used [34]. The in situ lift out (INLO) method typically consists in, (i) deposition of a protection layer onto a selected area, (ii) strategic removal of material to create a one-point clamped lamella, (iii) attachment for the clamped lamella to a microprobe by local material
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deposition, (iv) release of the lamella, and (v) transfer of the lamella to a TEM grid (optional), which includes additional steps of local material deposition for its sticking and FIB milling for the microprobe-lamella splitting (Fig. 7). Using INLO, TEM specimens may be produced directly from the bulk sample with little or no initial preparation which is obviously advantageous, and the site-specific cross sectional observation of the sample is made possible. As alignment of material deposit(s) and FIB etching is required, combination of the FIB milling with FIBID, rather than FEBID, is often preferred. A similar advantage to the one provided by INLO using combined FIB machining and, specifically, FIBID for TEM specimens is found for the device or circuit modifications [35,36] of prototype chips. For example, it applies to fix design errors, as it involves cutting of vias and fabrication of alternative contacts or dielectric material deposition [37]. Just mentioned for completeness, a FIBID complementary processing by an FIB equipped with a GIS is the locally promoted ion milling, namely focused ion beam induced etching [38]. A summary of the FIBID characteristics include the (i) fabrication of complex structures in a single processing step, (ii) with precise control and alignment capabilities, and (iii) capability to be applied to any material and support, aspects which are analogous to FEBID. Among the differences, FIBID has a higher deposition rate, i.e. better efficiency, reflecting a higher probability of ion–molecule collisions, molecular decomposition, effective momentum transfer, etc. However, this manifests at the same time by a decrease of patterning spatial resolution, which, being of about 3 nm for FEBID, increases to about 10 nm for FIBID. On the other hand, remarkably in the case of the deposition of metallic materials deposition, a higher portion of metal contents is systematically reported [40]. Other aspects related intrinsically to the use of the ions and ionderived phenomena can be considered more critical. As mentioned, energetic ions are eventually implanted, often causing disorder and amorphization of the sample surface, while in other cases even surface sputtering may occur. Seen from a practical point of view of the deposition process, it can be simply said that if the current density of primary beam is too high, ion milling will prevail upon deposition, so no layer or pattern growth will result. Using a FIB instrument the ability to remove, observe and deposit material depends critically on the nature of the ion beam–solid interactions [41]. Among additional limitations of FIBID and FEBID, finite size, charge and the ranges of scattering and diffusion events drives the resolution and patterning distortion. Known as proximity effect
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in EBL [42], Fig. 8 depicts qualitatively and schematically the main concepts involved in the FIBID (and FEBID) at a molecular level and collision cascade region. There is a lot to be done in terms of precisely modeling and simulating FEBID and FIBID processes. Roughly explained for FIBID, an ion beam striking the sample surface, generates many species, including sputtered atoms and molecules, secondary electrons and secondary ions. In the presence of the gas precursor molecules, which happen to be adsorbed onto the sample surface in the vicinity of the GIS nozzle location, precursor molecules only decompose as a consequence of the ion beam delivery. The buildup of material in the region where the ion beam has been scanned occurs where repeated adsorption and decomposition is found. Patterned FIBID can thus take place. Obviously, the electronic nature of the support material is an additional deterministic factor. It is especially important for insulating materials while metals evidence the capability for charge dissipation. Among the charge reduction methods, the idea of charge compensation existing on dual beam systems again appears as a good solution and brings added value to FIBID over FEBID. Additionally, a larger penetration depth of energetic electrons can be an issue on charge sensitive devices, such as CMOS [43]. A major drawback of FIBID is that, different from electrons, atoms from the ion beam can compromise the purity of the deposits in terms of elemental contents, especially when FIBID deposits are aimed to be used as structural materials of devices, instead of INLO previously presented. Actually, the use of complex precursor molecules and non-ideal or incomplete decomposition can lead to undesired impurity levels and relevantly affect conductivity. In some cases, mild thermal annealing is used to improve these two properties in both FIBID and FEBID deposits [44]. Related to the utility of post deposition processing and as a final example, some of our work done on carbon patterns grown by FIBID is briefly introduced. In this case, the investigation aimed to radically transform ultrathin carbon layers of FIBID (FIBID-C) as route to obtain patterned graphene-like materials deposited on insulators (SiO2). Previous works on FIBID-C [45–47], the amorphous nature of as-deposited FIBID-C, H and Ga impurities and the mechanical characteristics (predominance of sp3 bonds) were determined. In that case, even when high temperature treatment was applied the change was essentially on the deposits chemistry, reduction of H and Ga contents, but structural change, variation of sp2/sp3, was not dramatically observed. In contrast, we achieved [48] and further analyzed [49,50] the graphitization of the FIBID-C by using metal-induced crystallization, particularly, post FIBID treatment by extrinsic Ni foils at relatively high temperatures (Fig. 9). Further details of this work, such as using other catalytic metals, are also included in [51]. The electrical properties of FIBID-C as a function of thermal treatment conditions are currently being studied. The study is the preliminary stage demonstrator of a convenient and simple integrative strategy towards graphene electronic devices in a planar technology fashion. Far from being an optimized process for, particularly, growing patterned graphene, it may be suggestive of impelling system developments and further studies on this, carbon, and other crystalline materials.
4. Summary and outlook
Fig. 7. Fabrication of an electron transparent specimen for TEM observation by INLO. Reprinted from ‘FIB Lift-Out specimen Preparation Techniques’ with permission Ó 2005 Springer [39].
In summary, the main characteristics of FEBID and FIBID have been presented. FEBID and FIBID allow the fabrication of submicron patterns with nanometer resolution on selected locations of any kind of substrate, including highly structured or non-flat supports. The process consists of mask less serial deposition of a wide variety of materials, which strictly depends on the precursor material source used. Deposits are characterized by a high smoothness of the layer, while complex and 3D patterning has been widely
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Fig. 8. Schematic depiction of atomic level mechanism for FEBID and FIBID (see article for details). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 9. High resolution cross section TEM micrograph of a completely graphitized ultrathin FIBID-C layer, resulting from the metal-assisted crystallization by thermal treatment with the Ni foil-based methodology.
demonstrated in the literature. FEBID and FIBID proceed by adsorption of the volatile precursor molecules onto the sample surface, decomposition of the molecules induced by the energetic electron and ion focused beams and fixing to the sample support. The essential similarities of the two techniques have been covered. On the one hand, the basic instrumental systems are equivalent, having as the key parts an electron or ion column and a GIS, made operative by a control and beam driving interface. On the other hand, the deposition relies on a fundamental mechanism based on the electron and ion momentum transfer to the precursor molecules alike. Especial emphasis has been put on highlighting the differences between FEBID and FIBID. FEBID has higher spatial resolution and a low deposition rate. Deposited material is free of particle beam contamination, but shows low purity, particularly, for metal deposition. Trapped electrons in the interfaces can be an issue on sensitive substrates or devices. FIBID has better process efficiency, but lower spatial resolution. Major concerns are due to the ions themselves. Finite size of energetic ions can cause substrate surface damage, such implantation, amorphization and sputtering, especially at the early stage of the deposition or if conditions are not correctly chosen. Atoms from the ion beam
can also remain as a mass fraction of the deposits, compromising their purity. In both cases, the exact mechanisms and their quantitative understanding, such as the cross section of the electron and ion induced dissociation of the volatile molecules, are still unknown. Much work can still be done in terms of modeling and simulating the processes involved in FEBID and FIBID. The complex interplay of the number of variables involved in the mechanism, but also of the many scan parameters and extra factors, requires profuse and accurate trial and optimization in order to establish the exact process conditions adapted for each specific application. The evaluation of the performance of helium ion microscope for FIBID could also be considered, as its sub-nanometer beam size provides significant higher resolution capability and chemical inertness implies safer processing [52]. As additional remarks on the future research work related to FEBID and FIBID, the time will show us if the potential of charged particle induced deposition is finally exploited, e.g. for the direct patterning of nanostructures at wafer scale. For this, a possibility would be to combine the vast knowledge and instrumental available for more conventional electron [53] and ion beam [54] and plasma processing [55] with strategies for flexible and feature definition by parallel or serial programmable and integrative routines. If made compatible with existing processing sequences, therefore, this technology would enable batch nanopatterning of complex structures and selected materials, made by tuning the materials used and combined recipes with other pre and post deposition processes. In any case, the current works based on focused beams continuously provide examples of successful accomplishments, interesting results and novel approaches, which will continue to serve as demonstrators of the capability of the strategy and to provide deeper understanding, or perhaps even to inspire and justify tool and technological developments of wider and higher impact.
Acknowledgements The opportunity to write this article is indebted to the organizers of Symposium W: ion beam applications: new and innovative approaches of the E-MRS Spring Meeting 2013. The author is also expressly grateful to her collaborators at the Centro Nacional de Microelectronica – CSIC (Barcelona, Spain), the Toyota Technological Institute (Nagoya, Japan), the Nagoya University (Nagoya, Japan), the Politecnico di Torino (Torino, Italy) and the University of Houston (Houston, USA).
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Technology basis and perspectives on focused electron beam induced deposition and focused ion beam induced deposition Gemma Rius ⇑ Center for Innovative Young Researchers, Nagoya Institute of Technology, Gokiso cho, Showa ku, 466-8555 Nagoya, Japan
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Article history: Received 24 August 2013 Received in revised form 6 December 2013 Accepted 20 June 2014 Available online 13 August 2014 Keywords: Nanopatterning Focused electron beam Focused ion beam Charged particle-solid interactions Beam-induced deposition
a b s t r a c t The main characteristics of focused electron beam induced deposition (FEBID) and focused ion beam induced deposition (FIBID) are presented. FEBID and FIBID are two nanopatterning techniques that allow the fabrication of submicron patterns with nanometer resolution on selected locations of any kind of substrate, even on highly structured supports. The process consists of mask less serial deposition and can be applied to a wide variety of materials, depending strictly on the precursor material source used. The basic mechanism of FEBID and FIBID is the adsorption of volatile precursor molecules onto the sample surface and decomposition of the molecules induced by the energetic electron and ion focused beams. The essential similarities of the two techniques are presented and especial emphasis is dedicated to highlighting their main differences, such as aspects related to resolution, deposition rate, deposits purity, substrate integrity, etc. In both cases, the factors interplay and complex mechanisms are still understood in a qualitative basis, so much work can still be done in terms of modeling and simulating the processes involved in FEBID and FIBID. Current work on FEBID and FIBID is presented through examples of achievements, interesting results and novel approaches. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanotechnology is defined by the National Nanotechnology Initiative as the manipulation of matter with at least one dimension sized from 1 to 100 nm or, in an earlier definition, the manipulation of matter on an atomic and molecular scale [1]. Its meaning actually includes the novel (nano)science arising from the restricted dimensionality of nanoscaled particles, nanostructured materials and surfaces, and nanodevices. Nanotechnology spans almost over any science discipline in an ubiquitous manner [2], from physics to biology, from electronics to medicine, in brief, from fundamental science to applications, with potential impact into the most common consumer goods [3]. A remarkable effort of nanotechnology is associated with the development of the production (and characterization) strategies for the functional nanoobjects. Their fabrication, namely nanofabrication, is in some cases seen as a further miniaturization of microfabrication processing [4]. It is seen, then, as a sort of evolutionary route, which, as closely connected to microelectronics planar technology, unquestionably directs toward considering nanopatterning and nanolithography. ⇑ Tel.: +81 052 735 5478. E-mail address: [email protected] http://dx.doi.org/10.1016/j.nimb.2014.06.034 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
In this context, an interest on developing alternative compatible patterning techniques able to precisely define features at the nanoscale is uninterruptedly being sought-after. Beyond more established nanolithographies [5], such as resist-mediated laser or electron beam lithography, focused electron beam induced deposition (FEBID) [6,7] (Fig. 1) and focused ion beam induced deposition (FIBID) [6] have received notable attention due to their potential to directly pattern with high accuracy a variety of materials on almost any substrate, including highly structured surfaces such as sensing probes. Nonetheless, pattern flexibility and alignment capability makes them ideal for interfacing single nanoobjects such as nanoparticles, nanotubes and nanowires [8]. Being referred in the literature with many other names, FEBID and FIBID will be here used as this nomenclature highlights the main focus aspects of present review; the spatial resolution, expressed by the term focused; the use of charged particle beams providing accurate scanning capability, expressed by the terms electron/ion beam; and the concept of the patterning, expressed by the terms induced deposition. In other words and considered in terms of fabrication, FEBID and FIBID do not only represent a simplification of the process sequence, decreasing the number of steps and avoiding eventual resist contamination and so on (Fig. 2), but also provide higher processing flexibility based on the fact that FEBID and FIBID are mask less techniques, the
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Fig. 1. Basic elements and principles of FEBID, (a), and focused electron beam induced etching (FEBIE), (b). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 2. Simplified fabrication of a nanostructure: comparative of the standard sequence by (a) EBL versus (b) FEBID or FIBID patterning. Remarkable reduction of the number of steps is attained.
availability of materials, possibility of mix and match, etc. Although FEBID and FIBID are rooted on a simple principle, and have demonstrated outstanding performance, they involve, actually, very complex concepts which make it difficult to be fully understood, deeply and completely, i.e. quantitatively rather than qualitatively, as a phenomenon. In this brief topical review one would like to highlight the added value and potential of FEBID and FIBID as a methodology for growing a number of material features in ‘‘0’’ to 3D at the nanoscale. The nomenclature choice also aims to reflect a practical point of view on FEBID and FIBID which will be covered here, i.e. seen as a direct patterning technique for efficient prototyping. This short overview article collects valuable references of most significant review literature on FEBID and FIBID, which describe in great detail all the factors, variables and conditions involved, putting the main attention on aspects relevant for processing, being general in terms of the mechanism and modeling matters and shortly mentioning main chemistry aspects. As a result this review would be a comprehensive introduction for those who would like to know about FEBID and FIBID: a nanofeatures patterning strategy based on the use of finely focused charged particle beams. In addition to this, a brief look on a few selected examples is provided. Concerned with the point of view of how FEBID and FIBID can enable novel functional nanomaterials, specific examples of scanning probe microscopy (SPM) probes, ferromagnetic nanowires, transmission electron microscopy (TEM) specimen fabrication and synthesis of patterned graphene layers are illustrated. A few words are given as final remarks on the prospect investigations and future endeavors on FEBID and FIBID.
2. FEBID While the invention of scanning electron microscope (SEM) dates back into the late 30s of the last century and electron beam lithography (EBL) is used since the late 60s (Fig. 3), it is not until the middle 80s that the FEBID research starts and, then, gets momentum as seen, for example, in terms of related publications [7]. A simple explanation for this lays in the availability of considerable advances in electron microscopy – electron sources, detectors, digitalization, etc. -, but also in the capacity of beam driving and shutting, which affects dramatically the resolution and capabilities in terms of lithography, enabling complex patterning [9]. Additionally, a massive interest specifically on nanosized matter, as it can be obtained by, for instance, nanopatterning methods, follows the invention of SPM, scanning tunneling microscope (STM) [10] and atomic force microscope (AFM) [11], as well as the discovery of nanomaterials synthesis like fullerenes [12], both paradigmatic examples that lead to the onset of nanotechnology. A typical system for FEBID (or FIBID) essentially consists of an EBL (or FIB) system equipped with an additional gas injection system (GIS) for the volatile precursor supply [ref]. Most relevant parameters during operation include the beam energy and its current density. Beam parameters directly affect the obtained deposition characteristics: rate (height), width (resolution), elemental composition and bonding, conductivity. . . [6,7] As a rule of thumb, the higher the incident energy, the (s)lower the deposition rate (smaller charged particle-precursor collision probability). The thinner the beam, the higher can be attained the resolution. Commonly used acceleration voltages and current densities (beam diameter)
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Fig. 3. EBL systems can consist of a SEM apparatus equipped with an external beam and scan control interface for enabling lithography. Picture courtesy of the Centro Nacional de Microelectronica, Barcelona, Spain.
are a compromise between deposition rate and the resolution, in the order of a few tens of keV, some tens/hundreds of pA, and precisely depending on the application. Similar to the so-called proximity effect in EBL, secondary electrons play a crucial role in FEBID, affecting dramatically the patterning spatial resolution. Scan parameters can also play an important role on the above mentioned structural characteristics of the resulting patterns. Very much related to the system capabilities in terms of lithography, variables such as the scan speed (dwell time and pitch, beam blanking) and strategy (point irradiation, raster scan, vector scan, random irradiation) determine the deposition again in terms of both pattern morphology (therefore, affecting design fidelity, uniformity, etc.) and chemical composition and inner structure aspects (therefore, affecting material functionality). Other system operating characteristics have a more direct effect on the conditions of the particle beam–precursor interaction itself, such as the precursor gas flux and base pressure of the chamber, the position of the nozzle and so forth. After having introduced FEBID (and FIBID) from system engineering and operational point of view, the chemistry and mechanism of the charged particles induced deposition process should not be postponed. A primordial element of the FEBID (and FIBID) is the volatile species used to form the patterned deposits. The concept of the deposition mechanism is that the energetic incident particles collide with the precursor molecules on the surface. The energy transferred from the charged particles to the volatile species causes some molecular decomposition, break of molecule inner bonds, and fixing to the support sample of some of the collisions sub-products. Again, the idea of a very strong, complex and multiplexed interplay between the different variables and factors involved is evidenced. Connected to the unavoidable charged particle interaction, in this case, with the solid substrate, certain effect originated by the beam charges and the electrical conduction of the target surface material is expected. Roughly, substrate factor would be manifested, for example as charging, and it would be affecting remarkably the possibility to define dense patterns as well as limiting the minimum feature size. Moving to the kind of elements that can be deposited, the deposited materials strictly depend on the available volatile precursor molecules. In principle, the precursor sources can be in
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gas, liquid or solid state. In terms of electronic functionality, all kind of materials can be grown: insulators, e.g. SiOx from a TEOS [13]; metallic, e.g. Pt from (CH3)3(CpCH3) [14]; ferromagnetic, e.g. Fe and Co from Fe(CO)9 and Co2(CO)8, respectively [15]. In addition to the precursor chemistry, considering the chemistry of the growth mechanism, the electron, or otherwise ion, beam-induced decomposition of the molecules has a tremendous influence on the resulting purity of the deposits. In consequence, decomposed species determine the deposit functional properties, such as the conduction, but its mechanical characteristics are affected as well. An exhaustive list of deposit-able materials and their available precursors is included, for instance, in [6,7]. As a summary list of the main characteristics of FEBID, the technique allows precise (i) fabrication of nanostructured features in a single step, (ii) over all sorts of substrates, (iii) with an in-plane resolution of 3 nm, and (iv) with a high precision and accuracy in-plane control. However, FEBID strategy limitations include (i) a low deposition rate, and (ii) limited pattern resolution (halo), while composition issues include (iii) limited purity (e.g. low metal elemental content) and, consequently, (iv) high resistivity when metallic deposition is intended. Based on the competence of FEBID (and FIBID) to locally grow features of specific morphology and made of various materials, a wide range of applications are envisioned, such as 3D nanostructures and SPM tips. Straight forwardly, focused charged particle beam induced deposition finds a use on circuit editing and photolithography mask repair. However, it can do a very interesting job when applied to the electrical transport studies of nanowires, nanoparticles and so on. In general, many topics in nanoelectronics can benefit from the use of this technique, such as superconductors [16], magnetic storage and sensing devices [17], nano-antennas [18], to name a few. Some of the applications mentioned here, which are in principle common for FEBID and FIBID, find an convenient feasibility point when combined with the processing capabilities enabled by the use of the FIB, specifically, for example when preparing TEM-transparent samples [19], which will described in the following section on FIBID. To conclude this part, a couple of examples of FEBID applications will be presented next. Although EBL remains as one of the most powerful techniques for nanolithography, it cannot be applied easily to any supports. EBL shows serious limitations for processing highly structured surfaces such as CMOS circuits. Another classic example are SPM tips, where the possibility to strategically deposit small amounts of certain materials (or a combination of several) in the tip apex, for example for C nanotube or Si nanowire growth [20], and functional layers (conductive or magnetic) could be very useful to produce specialized tips for AFM advanced modes such as MFM or SERS-AFM [21]. The possibility to define high aspect ratio deposits shows promise for the fabrication of high resolution tips (Fig. 4). As another application example, an outstanding work using FEBID ferromagnetic deposits is being done by the Spanish group of De Teresa and Ibarra [22–25]. Particularly, they have been studying the magneto transport characteristics of Co and Fe FEBID nanowires which demonstrated high quality. In Ref. [26], giant anomalous Hall could be observed in Fe microwires [26] and, as presented in Ref. [27], domain wall conduit behavior of Co nanowires could be understood (Fig. 5) [27]. The group has also studied interesting properties of superconducting materials such as W [28,29], in this case deposited by FIBID, which technique will be described subsequently.
3. FIBID The system and concept of FIBID are basically the same as the ones used in FEBID. The plain FIB instrument consists of a vacuum
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Fig. 4. Example of FEBID onto an AFM tip apex (right) and AFM tip apex machining for magnetic AFM (see article for details). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 5. (Left) SEM image of a Co nanowire patterned by FEBID. (Right) Magnetization as a function of nanowire width (see article for details). Reprinted from Appl. Phys. Lett. 94, 192509 (2009) with permission Ó 2009 American Institute of Physics [27].
system and chamber, which holds a sample stage and detectors. FIB apparatus key elements are the liquid metal ion source (LIMS) and the ion column, which in essence is analogous to an electron beam column [30]. As in FEBID, for enabling FIBID the system is equipped with a GIS. Due to a number of reasons, traditionally gallium has been the most commonly used LMIS for commercially available FIB instruments. Gallium has a low melting point, vapor pressure and volatility, notable electrical, mechanical and vacuum properties, as well as good emission characteristics, i.e. showing small energy spread and high angular intensity. Yet, recently, the use He ions has received significant attention due to implicit higher resolution and reduction of surface damage [31]. Mostly, the discussion in this article is focused on Gallium-based FIBID [32]. Not only the FIB instrument is similar to a SEM, but in many cases electron and ion columns are made compatible, i.e. integrated in a single tool, the so-called dual beam systems (Fig. 6) [33]. This dual beam option multiplies the analytical capabilities of the tool, enabling for example energy dispersive scattering for in situ chemical analysis. At the same time, it makes safer the ion beam processing since SEM-based navigation can avoid undesired ion implantation and milling in areas sensitive to charging and ion contamination or the FIBID target locations. The use of field emission electron sources, particularly, confers the system not only higher SEM resolution, but also implies that both FEBID and FIBID can be executed in favorable conditions for an optimal resolution, within the same tool.
Fig. 6. Dual beam system equipped for high resolution nanolithography and FEBID and FIBID. Picture courtesy of the Centro Nacional de Microelectronica, Barcelona, Spain.
The different applications of focused charged particle beam induced deposition are, in general, common for FEBID and FIBID. However, as for the preparation of electron transparent specimens for TEM observation combined FIB-based processing is used [34]. The in situ lift out (INLO) method typically consists in, (i) deposition of a protection layer onto a selected area, (ii) strategic removal of material to create a one-point clamped lamella, (iii) attachment for the clamped lamella to a microprobe by local material
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deposition, (iv) release of the lamella, and (v) transfer of the lamella to a TEM grid (optional), which includes additional steps of local material deposition for its sticking and FIB milling for the microprobe-lamella splitting (Fig. 7). Using INLO, TEM specimens may be produced directly from the bulk sample with little or no initial preparation which is obviously advantageous, and the site-specific cross sectional observation of the sample is made possible. As alignment of material deposit(s) and FIB etching is required, combination of the FIB milling with FIBID, rather than FEBID, is often preferred. A similar advantage to the one provided by INLO using combined FIB machining and, specifically, FIBID for TEM specimens is found for the device or circuit modifications [35,36] of prototype chips. For example, it applies to fix design errors, as it involves cutting of vias and fabrication of alternative contacts or dielectric material deposition [37]. Just mentioned for completeness, a FIBID complementary processing by an FIB equipped with a GIS is the locally promoted ion milling, namely focused ion beam induced etching [38]. A summary of the FIBID characteristics include the (i) fabrication of complex structures in a single processing step, (ii) with precise control and alignment capabilities, and (iii) capability to be applied to any material and support, aspects which are analogous to FEBID. Among the differences, FIBID has a higher deposition rate, i.e. better efficiency, reflecting a higher probability of ion–molecule collisions, molecular decomposition, effective momentum transfer, etc. However, this manifests at the same time by a decrease of patterning spatial resolution, which, being of about 3 nm for FEBID, increases to about 10 nm for FIBID. On the other hand, remarkably in the case of the deposition of metallic materials deposition, a higher portion of metal contents is systematically reported [40]. Other aspects related intrinsically to the use of the ions and ionderived phenomena can be considered more critical. As mentioned, energetic ions are eventually implanted, often causing disorder and amorphization of the sample surface, while in other cases even surface sputtering may occur. Seen from a practical point of view of the deposition process, it can be simply said that if the current density of primary beam is too high, ion milling will prevail upon deposition, so no layer or pattern growth will result. Using a FIB instrument the ability to remove, observe and deposit material depends critically on the nature of the ion beam–solid interactions [41]. Among additional limitations of FIBID and FEBID, finite size, charge and the ranges of scattering and diffusion events drives the resolution and patterning distortion. Known as proximity effect
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in EBL [42], Fig. 8 depicts qualitatively and schematically the main concepts involved in the FIBID (and FEBID) at a molecular level and collision cascade region. There is a lot to be done in terms of precisely modeling and simulating FEBID and FIBID processes. Roughly explained for FIBID, an ion beam striking the sample surface, generates many species, including sputtered atoms and molecules, secondary electrons and secondary ions. In the presence of the gas precursor molecules, which happen to be adsorbed onto the sample surface in the vicinity of the GIS nozzle location, precursor molecules only decompose as a consequence of the ion beam delivery. The buildup of material in the region where the ion beam has been scanned occurs where repeated adsorption and decomposition is found. Patterned FIBID can thus take place. Obviously, the electronic nature of the support material is an additional deterministic factor. It is especially important for insulating materials while metals evidence the capability for charge dissipation. Among the charge reduction methods, the idea of charge compensation existing on dual beam systems again appears as a good solution and brings added value to FIBID over FEBID. Additionally, a larger penetration depth of energetic electrons can be an issue on charge sensitive devices, such as CMOS [43]. A major drawback of FIBID is that, different from electrons, atoms from the ion beam can compromise the purity of the deposits in terms of elemental contents, especially when FIBID deposits are aimed to be used as structural materials of devices, instead of INLO previously presented. Actually, the use of complex precursor molecules and non-ideal or incomplete decomposition can lead to undesired impurity levels and relevantly affect conductivity. In some cases, mild thermal annealing is used to improve these two properties in both FIBID and FEBID deposits [44]. Related to the utility of post deposition processing and as a final example, some of our work done on carbon patterns grown by FIBID is briefly introduced. In this case, the investigation aimed to radically transform ultrathin carbon layers of FIBID (FIBID-C) as route to obtain patterned graphene-like materials deposited on insulators (SiO2). Previous works on FIBID-C [45–47], the amorphous nature of as-deposited FIBID-C, H and Ga impurities and the mechanical characteristics (predominance of sp3 bonds) were determined. In that case, even when high temperature treatment was applied the change was essentially on the deposits chemistry, reduction of H and Ga contents, but structural change, variation of sp2/sp3, was not dramatically observed. In contrast, we achieved [48] and further analyzed [49,50] the graphitization of the FIBID-C by using metal-induced crystallization, particularly, post FIBID treatment by extrinsic Ni foils at relatively high temperatures (Fig. 9). Further details of this work, such as using other catalytic metals, are also included in [51]. The electrical properties of FIBID-C as a function of thermal treatment conditions are currently being studied. The study is the preliminary stage demonstrator of a convenient and simple integrative strategy towards graphene electronic devices in a planar technology fashion. Far from being an optimized process for, particularly, growing patterned graphene, it may be suggestive of impelling system developments and further studies on this, carbon, and other crystalline materials.
4. Summary and outlook
Fig. 7. Fabrication of an electron transparent specimen for TEM observation by INLO. Reprinted from ‘FIB Lift-Out specimen Preparation Techniques’ with permission Ó 2005 Springer [39].
In summary, the main characteristics of FEBID and FIBID have been presented. FEBID and FIBID allow the fabrication of submicron patterns with nanometer resolution on selected locations of any kind of substrate, including highly structured or non-flat supports. The process consists of mask less serial deposition of a wide variety of materials, which strictly depends on the precursor material source used. Deposits are characterized by a high smoothness of the layer, while complex and 3D patterning has been widely
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Fig. 8. Schematic depiction of atomic level mechanism for FEBID and FIBID (see article for details). Reprinted from J. Vac. Sci. Technol. B 26, 1197 (2008) with permission Ó 2008 American Vacuum Society [6].
Fig. 9. High resolution cross section TEM micrograph of a completely graphitized ultrathin FIBID-C layer, resulting from the metal-assisted crystallization by thermal treatment with the Ni foil-based methodology.
demonstrated in the literature. FEBID and FIBID proceed by adsorption of the volatile precursor molecules onto the sample surface, decomposition of the molecules induced by the energetic electron and ion focused beams and fixing to the sample support. The essential similarities of the two techniques have been covered. On the one hand, the basic instrumental systems are equivalent, having as the key parts an electron or ion column and a GIS, made operative by a control and beam driving interface. On the other hand, the deposition relies on a fundamental mechanism based on the electron and ion momentum transfer to the precursor molecules alike. Especial emphasis has been put on highlighting the differences between FEBID and FIBID. FEBID has higher spatial resolution and a low deposition rate. Deposited material is free of particle beam contamination, but shows low purity, particularly, for metal deposition. Trapped electrons in the interfaces can be an issue on sensitive substrates or devices. FIBID has better process efficiency, but lower spatial resolution. Major concerns are due to the ions themselves. Finite size of energetic ions can cause substrate surface damage, such implantation, amorphization and sputtering, especially at the early stage of the deposition or if conditions are not correctly chosen. Atoms from the ion beam
can also remain as a mass fraction of the deposits, compromising their purity. In both cases, the exact mechanisms and their quantitative understanding, such as the cross section of the electron and ion induced dissociation of the volatile molecules, are still unknown. Much work can still be done in terms of modeling and simulating the processes involved in FEBID and FIBID. The complex interplay of the number of variables involved in the mechanism, but also of the many scan parameters and extra factors, requires profuse and accurate trial and optimization in order to establish the exact process conditions adapted for each specific application. The evaluation of the performance of helium ion microscope for FIBID could also be considered, as its sub-nanometer beam size provides significant higher resolution capability and chemical inertness implies safer processing [52]. As additional remarks on the future research work related to FEBID and FIBID, the time will show us if the potential of charged particle induced deposition is finally exploited, e.g. for the direct patterning of nanostructures at wafer scale. For this, a possibility would be to combine the vast knowledge and instrumental available for more conventional electron [53] and ion beam [54] and plasma processing [55] with strategies for flexible and feature definition by parallel or serial programmable and integrative routines. If made compatible with existing processing sequences, therefore, this technology would enable batch nanopatterning of complex structures and selected materials, made by tuning the materials used and combined recipes with other pre and post deposition processes. In any case, the current works based on focused beams continuously provide examples of successful accomplishments, interesting results and novel approaches, which will continue to serve as demonstrators of the capability of the strategy and to provide deeper understanding, or perhaps even to inspire and justify tool and technological developments of wider and higher impact.
Acknowledgements The opportunity to write this article is indebted to the organizers of Symposium W: ion beam applications: new and innovative approaches of the E-MRS Spring Meeting 2013. The author is also expressly grateful to her collaborators at the Centro Nacional de Microelectronica – CSIC (Barcelona, Spain), the Toyota Technological Institute (Nagoya, Japan), the Nagoya University (Nagoya, Japan), the Politecnico di Torino (Torino, Italy) and the University of Houston (Houston, USA).
G. Rius / Nuclear Instruments and Methods in Physics Research B 341 (2014) 37–43
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