Direct Writing of Metal Nanostructures with Focused Helium Ion Beams

Abstract

A helium ion microscope (HIM) with a focused He⁺-ion beam of variable flux and energy can be used as a tool for local nanoscale surface modification. In this work, we demonstrate a simple but versatile use of the HIM focused He ion beam to fabricate conducting metallic nano- and microstructures on arbitrary substrates of varied types and shapes by directly patterning pre-deposited initially discontinuous and highly insulating (>10 TΩ/sq.) ultrathin metal films. Gold or silver films, measuring 3 nm in thickness, thermally evaporated on solid substrates have a discontinuous nanocluster morphology. Such highly resistive films can be made locally conductive using moderate doses (2 × 10¹⁶–10¹⁷ cm⁻²) of low-energy (30 KeV) ion bombardment. We show that an HIM can be used to directly “draw” Au and Ag conductive lines and other patterns with a variable sheet resistance as low as 10 kΩ/sq. without the use of additional precursors. This relatively straightforward, high-definition technique of direct writing with an ion beam, free from complex in vacuo catalytic or precursor chemistries, opens up new opportunities for directly fabricating elements of conformal metallic nanocircuits (interconnects, resistors, and contacts) on arbitrary organic or inorganic substrates, including those with highly curved surfaces.

1. Introduction

A helium ion microscope (HIM) is frequently employed in nano-fabrication and sample modification applications [1,2]. Previously developed applications utilizing focused ion beams for local material processing include defect engineering [3,4,5,6,7,8], lithography [9], ion implantation [10], direct material removal (milling) [11,12,13], and chemical deposition [14,15,16,17].

Directly “drawing” submicron-scale contacts or other metallic structures with a focused beam of an HIM would significantly enhance opportunities for the fabrication of small electronic devices and the electrical characterization of new functional materials [1,2]. Typically, energetic beam-based methods of metallic nanostructure fabrication rely on relatively complex chemical deposition processes employing toxic precursor compounds, such as W(CO)₆, Fe(CO)₅, or C₉H₁₆Pt, absorbed on a material’s surface and decomposed under a focused particle beam [14,15,18,19,20,21,22]. Such methods require a gas injector system to supply precursor gas primarily to the sample but also to the chamber. Metal deposition in those techniques results from the interaction between the beam, the substrate, and the chemical precursor. The precursor must be injected into the vacuum system near the beam through a delivery needle to achieve a relatively high local pressure, while a sufficiently high vacuum must be maintained elsewhere within the sample chamber [19]. Such a deposition method requires modifications of a commercial instrument.

In this work, we took a different approach of first thermally evaporating an ultrathin, non-conducting metal film of a thickness < 10 nm on a desired substrate and then patterning conducting lines and other nanostructures in such a film using an HIM, thus avoiding the use of specialized materials and complex injection methods. For thermally evaporated metal films, depending on the nature of the substrate, the evaporation conditions, and the nominal thickness of the metal, such films can have a variable sheet resistance $R_{\mathrm{S}}$ covering a vast range, from over 100 TΩ/sq. down to less than 1 Ω/sq. [23]. Electrical transport through such films sensitively depends on the film’s morphology, which can vary from disconnected small islands to a quasi-continuous solid layer [24]. This morphological variability allows for the tuning of the metal evaporation regime to obtain large-area homogeneous coatings of an insulating substrate with a dense layer of poorly connected metal nanoclusters, ensuring a very high $R_{\mathrm{S}}$ value for these samples. Here, we show that such ultrathin nanoclustered metal films can be subsequently used as a canvas for the direct writing of nanoscale conducting electrical circuits. In the proposed method, post-processing can also be applied, for instance, to wash off unused metallic nanoclusters, but such post-processing would be unnecessary in most electrical applications, as the initial (unpatterned) seed layer remains non-conductive.

This work explores a novel approach of direct HIM fabrication of conducting electrical circuit elements on insulating substrates. Seed films of thicknesses in the range of 3–7 nm were deposited using a simple high-vacuum thermal evaporation method. We show that these highly resistive films can be easily made conductive locally by exposing them to moderate doses of a focused He⁺-ion beam (2 × 10¹⁶–10¹⁷ cm⁻²). Furthermore, we demonstrate the compatibility of this patterning technique with uneven surfaces of high curvature, including the fabrication of contact structures over sharp edges of bulk molecular single crystals, where conventional resist-based lithographic techniques would not be applicable.

2. Methods

Samples were prepared on bare and polymer-coated glass substrates (Gold Seal cover glass, Thermo Fisher Scientific, Waltham, MA, USA). Parylene-N conformal coating was used to coat the glass substrates, leading to improved uniformity of the ultrathin metal seed layer deposited on such a polymer. Parylene-N is a well known non-conjugated polymer forming pinhole-free conformal coatings on a variety of substrates, and it is used as a protective barrier [25] a a robust gate dielectric in emerging field-effect transistor devices [26,27,28,29]. A homebuilt parylene deposition setup was used, as thoroughly described in Ref. [30]. For the preparation of metal structures on a chosen substrate, first, a robust and highly conducting 30–35 nm thick silver layer is deposited through a shadow mask to form contact pads. For relatively small structures, a 25 µm thick silver wire was used as a shadow mask to create a gap between the contact pads. Then, an ultrathin seed metal layer was thermally evaporated in high vacuum (5 × 10⁻⁷–10⁻⁶ mbar) over the structure using gold (99.999% pure) or silver (99.99% pure) pellets (Kurt J. Lesker, Jefferson Hills, PA, USA) placed in a resistively heated tungsten boat. The ultrathin seed metal film was deposited on substrates at room temperature at a rate of 0.3–0.5 Å/s, reaching the final nominal thicknesses in the range of 2.7–7 nm. The nominal film thickness and deposition rate were measured with a quartz crystal microbalance thickness monitor. The resulting samples were carefully patterned with a sharp knife to define resistive channels with a width of approximately 1 mm (Supplementary Materials, Figure S1). Exact channel dimensions were measured using an optical microscope.

Measurements of electrical resistance, $R$, were performed in an inert atmosphere (argon) using a combination of the Keithley 2400 Source Meter and the 6514 Electrometer (Keithley Instruments, Solon, OH, USA). Measurements of $R$ below and above 1 TΩ were performed at a constant bias of 10 and 100 V, respectively. The sheet resistance$R_{\mathrm{S}}$ of the ultrathin gold films used as a seed layer in this study appears to be highly reproducible as long as a freshly grown parylene-N layer is used as a substrate and the correct and reproducible nominal thickness of the metal films is obtained during the evaporation. Even minor variations in this thickness, as small as 10%, can result in more than an order of magnitude difference in $R_{\mathrm{S}}$ for these ultrathin films.

The Rutgers Zeiss Orion Plus helium ion microscope (Carl Zeiss, Oberkochen, BW, Germany) was used for sample imaging and patterning with a focused ion beam at an acceleration voltage of 30 kV. The chamber pressure was 3 × 10⁻⁷ Torr. The ion beam current was in the 3–5 pA range for patterning and ~1.0 pA for imaging. The projected helium ion range (ion penetration depth) in parylene-N films is estimated to be approximately 0.35 µm for a 30 keV beam [31]. Note that the range can be tuned by changing the energy; for example, for 10 keV He ions, the range in the same polymer material would be approximately 0.14 µm.

3. Results

Figure 1a describes the proposed mechanism of ultrathin metal film patterning with a focused HIM ion beam, in which ion bombardment sputters atoms from metal clusters, leading to a better inter-cluster linkage and increasing the film’s conductivity. Depending on the substrate material, its temperature, and the seed metal layer thickness, the film’s “as deposited” morphology can vary. For example, the 7 nm (nominal thickness) silver film on a glass, shown in Figure 1b, has tightly packed metal particles separated by relatively narrow gaps. Upon He⁺-ion beam exposure, the seed film undergoes a clear morphological change (Figure 1c,d). Figure 1c shows the result of subjecting the seed film to selective HIM exposure (“writing”) with a 10¹⁶ cm⁻² dose of a 30 KeV He⁺ beam. The most evident result is the increase in the local brightness of the HIM image’s patterned regions, consistent with a significant increase in the film conductivity, as described below. The dominant aspects of interaction between the focused ion beam and the nanoclustered metal film, underlying the patterning mechanism, are unknown; however, two main processes might be responsible for the increased electrical conductivity of the patterned regions. High-energy ion bombardment of metal nanoparticles may result in sputtering and the redeposition of metal atoms [32,33], leading to a smoother surface [11] and better particle interconnectivity, thus overcoming the percolation threshold for charge transport. Other possible processes include chemical modifications of the substrate’s surface (dangling bond formation), decreasing the tunneling barrier between the particles [24], or the possible chemical bonding of metal atoms to the modified substrate [32,33]. As shown in Figure 1d, tens of nm-wide homogeneous lines can be drawn at an increased beam dose.

To understand the effect of HIM patterning on the electrical conductivity of the resultant films and evaluate the patterning efficiency, we designed device layouts suitable for measurements of the resistivity of the as-evaporated and HIM-patterned regions. Figure 2a,b schematically show the two types of device structures (on glass and polymer substrates, respectively) used to measure the films’ conductivity (see also Supplementary Figure S1). The thick continuous silver contact pads, deposited through a 25 μm thick wire mask, define a short channel in these devices (corresponding to the gap marked “25 μm” in Figure 2b,d). An ultrathin silver or gold seed layer is deposited onto the entire device to cover the 25 µm long gap between the thick contacts. The sample is then manually patterned under a zoom microscope using the tip of a sharp knife to isolate selected contact pairs. Figure 2b shows an HIM image of one of these devices on glass, with a conducting silver stripe drawn by the focused ion beam. The width of the patterned stripe connecting the contacts is ~1 µm.

The resistivity of unpatterned (as-evaporated) ultrathin silver films on glass is usually rather unpredictable. This effect might be associated with the inhomogeneous and hard-to-control distribution of static charges on glass substrates [23]. Although static charges can be eliminated by annealing these substrates at about 300 °C in vacuum right before metal evaporation [23], this approach is not compatible with thermally unstable substrates such as glass coated with organics or functionalized with molecular (mono) layers, or stacks of materials with different coefficients of thermal expansion.

While the HIM patterning on glass shown in Figure 2b already serves as a proof-of-principle demonstration, we aalso studied alternative types of dielectric substrates, based on a non-conjugated highly resistive parylene-N, the use of which leads to much more reproducible patterning results.

The use of a highly robust conformal parylene coating is a well known methodology in applications requiring protective or electrically insulating barriers [26,27,28,29,30]. In our study, glass substrates were pre-coated with parylene-N, followed by the deposition of contacts and the seed layer (Figure 2c). In this case, gold was used for the seed layer to improve the long-term stability of the ultrathin films in air. Figure 2d shows an HIM image of the corresponding device after HIM patterning. In this example, five parallel µm-wide stripes were drawn with the ion beam in the channel to improve the accuracy of subsequent resistivity measurements.

Table 1. Sheet resistance and nominal resistivity values of ultrathin unpatterned (as-evaporated) and HIM-patterned gold films on an insulating polymer (parylene-N) substrate, along with a substrate-only control measurements.

	${\mathit{R}}_{\mathbf{S}}$ (Ω/sq.)	$\mathit{\rho }$ (Ω·cm)	d (nm)	Dose (cm−2)
As-evaporated (unpatterned) ultrathin seed gold film on parylene-N	4 × 1013	1.3 × 107	3.3	0
The same film after the HIM patterning	7 × 106	2.3	3.3	2 × 1016
As-grown (unpatterned) parylene-N alone (no gold)	>4 × 1015		1000	0
The same (gold-free) parylene-N after the HIM patterning	2 × 1013		1000	2 × 1016

for the gold film, a nominal thickness, $d$, as measured using a quartz crystal microbalance thickness monitor during the thermal evaporation, is indicated. For parylene-N, $d$ was determined by optical ellipsometry. The accuracy of the thickness measurements is about 10%. the nominal bulk resistivity of the gold film calculated as $\rho \equiv R_{\mathrm{S}}·d$. For bare parylene-N, the sheet resistance of both unpatterned and patterned films is likely dominated by very small near-surface/surface conductivity, making the estimates of bulk resistivity, in this case, inaccurate.

lists the average electrical resistance and other parameters of unpatterned (as-evaporated) and patterned ultrathin gold films on parylene-N. It also shows how the typical damage resulting from HIM patterning affects the underlying dielectric (parylene-N) at the same dose level. Patterning with a dose of 2 × 10¹⁶ cm⁻² results in a conductive metal film with minor (typically negligible) damage to the underlying dielectric substrate, as seen from the sheet resistance R_S of the corresponding layers (Table 1). The nominal bulk resistivity ρ of about 2.3 Ω·cm achieved for the patterned ultrathin gold film is still many orders of magnitude higher than that of bulk solid gold but sufficiently low for most nanoscale device applications operating at sub-µA current levels. The ratio of resistivities of unpatterned and patterned ultrathin gold films is about 10⁷, sufficiently high for most device applications.

The exact mechanism underlying the increased conductivity of ultrathin metal films as a result of ion bombardment is still unknown. Control measurements show that writing with the HIM’s ion beam on a clean, as-grown parylene-N surface without any gold on it does lower the sheet resistance of these polymer films from over 4000 to about 20 TΩ/sq. (Table 1), but they remain many orders of magnitude more resistive than the patterned gold seed layer on the same type of parylene-N substrates. Another control test, involving pre-treating a bare parylene-N surface with the HIM’s ion beam first and then evaporating the ultrathin gold seed layer on it, showed that this does not alter the highly insulating nature of the subsequently deposited seed layer. The latter test shows that treating a bare parylene-N surface with a He⁺-ion beam does not generate a sufficient number of active sites (dangling bonds) to appreciably change the surface energy, or those sites become passivated, for instance, by oxidation, when the sample is exposed to air (see, e.g., [34]) right before the gold deposition step. This test also suggests that the direct interaction of the energetic helium ions with the gold clusters themselves is likely essential for the formation of a more continuous, conducting metallic layer in the HIM patterning. We thus arrive to the two most feasible tentative explanations of the patterning mechanism: (a) kinetic sputtering of metal atoms from the nanoclusters under ion bombardment, leading to a lateral redistribution of the metal and improved interconnectivity between neighboring metal clusters, and (b) in vacuo chemical activation of the substrate’s surface, for instance, via a dangling bond formation, leading to much stronger bonding/interaction between the substrate and the metal, thus triggering wetting, observed here as a morphological change and improvement in electrical conductivity in the patterned regions. We also note that under the conditions of our experiments, the common ion beam-induced buildup of hydrocarbon contaminants on patterned regions (see, e.g., Refs. [15,35]) might directly or indirectly affect the film’s conductivity. Such a buildup represents another form of surface modification that could partially be responsible for the enhanced conductivity observed here. Supplementary Figure S6 shows the effect of such contamination on the secondary electron imaging contrast of HIM-patterned thick continuous metal films: here, a reduced secondary electron emission from the patterned regions leads to an inverted contrast (the patterned regions appear darker than the unpatterned ones) in comparison with the HIM patterning of ultrathin sub-conductive metal films. Each of these processes or their combination might be relevant.

The local heating of samples due to the energy absorbed from the ion beam can facilitate these processes. However, we do not believe that gold nanoparticle melting occurs under our experimental conditions using relatively mild ion beam doses, as melting in high vacuum would likely lead to accelerated metal evaporation and rapid material loss, contradicting the formation of continuous metallic films. Additionally, we note that the interaction of ultrathin metal films with a single-ion beam investigated here is distinctly different from the effect of metal sputtering and the resultant metal surface smoothing previously observed with gas cluster ion beams [36]. Finally, while gas cluster ions have significantly lower penetration depth, single-ion beams allow for better lateral focusing, important for high-resolution direct writing applications. Future imaging studies using high-resolution atomic force microscopy (AFM) and transmission electron microscopy (TEM) can shed more light on the mechanism of this type of patterning. In addition, X-ray photoelectron spectroscopy (XPS) can reveal details of surface composition.

Unpatterned (as-evaporated) ultrathin seed layers of gold and silver usually exhibit a steady, long-term (over hundreds of hours) drift of resistance likely associated with the coalescence of metal nanoparticles (Ostwald ripening) and their interaction with the atmosphere [37]. As shown in Supplementary Figure S2, not only does the HIM patterning drastically reduce the $R_{\mathrm{S}}$ of such films, it also noticeably slows down this drift. The stabilization of $R_{\mathrm{S}}$ is consistent with the metal forming a better bond with the substrate, thus reducing the long-term ripening of the patterned gold film.

To understand how the resistance of patterned metal films depends on the level of ion beam exposure, we measured the $R_{\mathrm{S}}$ of patterned gold films prepared at various doses (Figure 3). Supplementary Figure S3 shows the corresponding calculated nominal bulk resistivity $\rho$ of the patterned films, determined as $\rho =R_{\mathrm{S}}·d$, where $d$ is the nominal thickness of the evaporated film, as measured with a quartz crystal microbalance thickness monitor during film deposition. The noticeable spread of points in Figure 3 and Figure S3 is primarily due to inaccuracies in the film thickness measurements of about 10%. As expected for ultrathin metal films in the vicinity of their percolation threshold, their resistance is very sensitive to the exposure dose. Increasing the dose tenfold results in a nearly five-order-of-magnitude decrease in the gold film’s $R_{\mathrm{S}}$. Some damage to the underlying polymer substrate (parylene-N) also occurs with ion irradiation, leading to a decrease in the resistance of the bare substrate (Table 1), which might be due to the polymer carbonization under ion bombardment. Nevertheless, even after exposure to a substantial HIM dose of 10¹⁷ cm⁻², the sheet resistance of the bare (no metal) parylene-N substrate remains above ~4 × 10¹¹ Ω/sq., still many orders of magnitude greater than the $R_{\mathrm{S}}$ of the gold film patterned at the same dose (Figure 3). This suggests that the method is practically feasible for most electronic device applications, although substrate materials more tolerant to ion beam exposure might be required to widen the range of possible applications of this technique [34].

Finally, we assessed the feasibility of HIM patterning on uneven surfaces. Typical fabrication methods for electrical circuits, such as lithography, are based on spin coating a liquid solution of a resist material on flat substrates, which is unsuitable for strongly warped, non-flat surfaces. Our new method allows for the fabrication of conductive circuits on sharply bent dielectric surfaces. To demonstrate this, we used a sharp edge of an organic molecular single crystal, rubrene, coated with a 1 µm thick parylene-N layer. Figure 4a shows such a sharp crystallographic edge formed between (001) and (100) facets of one of those rubrene crystals. The crystal was positioned and fixed on a substrate holder with a thermal wax such that its edge faced upwards. Thick continuous silver contact pads were thermally evaporated on both facets through a shadow mask made out of a 25 µm thick silver wire, creating a short channel running approximately along the crystal edge (Figure 4b,c). An ultrathin insulating gold seed layer was then evaporated over the gap between the thick electrodes. Figure 4d shows the HIM-patterned “T”-shaped contacts drawn in the seed layer right next to the crystal edge. Such precisely positioned contacts can be used, for instance, to apply strong local electric fields over desired warped structural features of studied materials.

4. Conclusions

In summary, we have developed a new, simple, and robust method for the fabrication of nanoscale electrically conductive circuits on arbitrary dielectric substrates by using a combination of a conventional thermal evaporation and a focused He-ion beam of a basic helium ion microscope. The method does not rely on any toxic or complex catalysis or precursor chemistries and allows for nanoscale resolution. The resistance of circuit elements directly “drawn” with the HIM can be easily tuned in a wide range (from insulating to highly conductive) by adjusting the ion beam dose. Complex circuitry can be drawn conformally over non-flat surfaces, including sharp bends and crystal edges. This method not only unlocks new or previously challenging industrial and research applications of the HIM but also provides a new “tool” for the creation of nanoscale devices requiring contacts in extreme geometries.