Oxidative and carbonaceous patterning of Si surface in an organic media by scanning probe lithography
© Lorenzoni et al.; licensee Springer. 2013
Received: 2 January 2013
Accepted: 26 January 2013
Published: 13 February 2013
A simple top-down fabrication technique that involves scanning probe lithography on Si is presented. The writing procedure consists of a chemically selective patterning in mesitylene. Operating in an organic media is possible to perform local oxidation or solvent decomposition during the same pass by tuning the applied bias. The layer deposited with a positively biased tip with sub-100-nm lateral resolution consists of nanocrystalline graphite, as verified by Raman spectroscopy. The oxide pattern obtained in opposite polarization is later used as a mask for dry etching, showing a remarkable selectivity in SF6 plasma, to produce Si nanofeatured molds.
State-of-the-art technology in patterning semiconductor substrates mainly relies on mask-based techniques such as optical lithography or mask-less techniques like electron beam lithography, which, for their inherent multi-step and large area, parallel processing capabilities are particularly suited for industrial applications such as large numbers of device production in microelectronics and microfabrication in general. Aside some more flexible, fast, and easily modifiable processes, several scanning probe-related lithographies (SPLs) also emerged[1–3] as a research-oriented fast prototyping tool. Nanofabrication by SPL is affordable and very versatile. The advantages of using an atomic force microscope reside in the nanometric accuracy in feature positioning and in the possibility of directly applying multistep processes on pre-patterned substrates with no need for alignment tools and/or photoresist coating.
This makes SPL an ideal tool for flexible and fast prototyping of custom nanodevices. Early studies were mainly focused on oxidation and reduction processes of Si and SiO2 to assess the capability to fabricate semiconductor-insulator nanojunctions, achieving a remarkable ultimate sub-10-nm resolution.
Besides the local oxidation of silicon, the dissociation of organic molecules under an intense electric field (approximately 109 V m−1) localized below a biased AFM tip has been recently used to create nanometer-sized heterojunctions employing common organics[6, 7] organometallics, or fluorinated solvents, obtaining remarkable results in terms of resolution (reaching 2-nm feature size), scalability (employing stamp technology)[5, 6], and writing speed. However, the technique is relatively new, and little effort has been made in extensively exploiting its wide fabrication capabilities.
Water contact angles, height/bias dependence, and correlation coefficient for oxidation on different Si surfaces
Contact angle of water droplet (°)
Slope (nm V−1)
Correlation coefficient (adjusted R2)
Si(OH) native oxide layer
29 ± 0.9
0.40 ± 0.04
81 ± 1.2
0.37 ± 0.01
89 ± 0.8
0.48 ± 0.04
Polished p-type Si(100) wafers (resistivity 1 to 10 Ω cm) were sonicated for 10 min in acetone, ethanol, DI H2O immediately before processing, thus preserving a native SiO2 layer. The exposure of Si surface to a solution of aqueous HF (5 wt.% for 30 s) results in the removal of native oxide and surface H termination (water contact angle ≈ 80°). Silanization of Si(100) wafer has been achieved by exposing the surface, after degreasing, to hexamethyldisilizane (HDMS, ≥99%; Sigma-Aldrich Corporation, St. Louis, MO, USA) vapors for 1 h in moderate vacuum. The obtained wafers showed a water contact angle of approximately 90°. Depositions were performed with an Asylum MFP-3D (Asylum Research, Santa Barbara, CA, USA) operating in contact mode in liquid with integrated software to control lithographic parameters (Microangelo). The liquid environment (1,3,5-trimethylbenzene, ≥99.0%; Sigma-Aldrich) was exposed to typical ambient humidity (35% to 40%).
The probe employed during the fabrication tests was SiN Au-coated Olympus OMLC-RC 800 (k = 0.042 Nm−1, typical tip radius 430 nm), and the maximum bias applicable is ±20 V. It was possible to achieve a writing speed of 10 μm s−1, but the process is better controlled with a speed ranging from 0.2 to 5 μm s−1. Tip's wear does not compromise writing up to 10-mm continuous writing.
Raman spectra have been collected with a micro-Raman spectrometer Horiba T64000 (Edison, NJ, USA). Spectra have been recorded at room temperature, using an incoming laser light line linearly polarized at 514.5 nm from an Argon/Krypton ion laser (Ar/Kr Stabilite 2018-RM, Spectra-Physics, Mountain View, CA, USA), and a power density of about 2 mW μm−2 is used (×100 objective, Olympus SLM plan). The spectrometer resolution was determined by curve fitting the silicon 520 cm−1 band using a linear combination of Gaussian and Lorentian curves achieving full width at half maximum (FWHM) less than 2 cm−1. This silicon band was used for the precise calibration of energy scale.
Kelvin probe force microscopy measures have been performed with Asylum MFP-3D in air at room temperature (RH ≈ 35%) with Pt-coated probe Olympus OMCL-AC240TM. The work function of one reference tip (Φtip = 4.93 ± 0.05 eV) was calibrated by Kelvin probe force microscopy (KPFM) on freshly cleaved highly oriented pyrolytic graphite (HOPG).
Si dry etching was conducted with a Sentech ICP-RIE SI 500 plasma etcher (Sentech Instruments GmbH, Berlin, Germany). Working parameters for SF6 were as follows: gas flow 30 sccm, 1 Pa, RF/ICP power 600, and RF plate power 18 W. For pseudo Bosch (SiF6 + C4F8), gas SiF6 flow 30 sccm, C4F8 flow 32 sccm, 1 Pa, RF/ICP power 600, and RF plate power 18 W. Each sample has been finally cleaned by oxygen plasma. Fabricated masters have been imaged in tapping mode with standard Si cantilevers (Nanosensors PPP-NCH, Nanoworld AG, Neuchâtel, Switzerland; nominal resonant frequency ca. 330 kHz, force constant ≈ 42 Nm−1, polygon-based pyramidal tip with half cone angles of 20° to 30° with a tip apex radius below 10 nm). To minimize tip's convolution artifacts, some samples have been imaged using high aspect ratio tips (Nanosensors AR5-NCHR; nominal resonant frequency ca. 330 kHz, force constant ≈ 42 Nm−1) with half cone angle smaller than 2.8°. Energy diffraction spectroscopy (EDS) elemental analysis was performed by a X-Max large area analytical EDS silicon drift detector (Oxford Instruments, Oxford, UK) with (Mn Kα typically 125 eV) mounted on a JEOL 7500 FA SEM (Akishima, Tokyo, Japan).
Results and discussion
To clarify the solvent decomposition mechanism under a positively biased tip, further investigation is needed although the mechanism proposed by Vasko et al., in our case involving electron tunneling from the substrate to the tip and formation of reaction intermediates, could provide a valid explanation.
Writing is successfully performed in both polarization on p-doped Si(100) wafers having three different surface terminations: H:Si(100), CH3:Si(100), and Si(100) with native oxide layer of 1.7 to 2 nm, as measured by ellipsometer (data not shown). The formation and the geometry of the water meniscus is ruled by a number of factors including capillary forces, electric field gradients, ambient humidity, as well as the wetting behavior of the substrate. Oxide growth is confined by the water meniscus and thus sensitive to surface preparation that affects the capillary condensation at the water/silicon interface. As the surface becomes more hydrophilic, line width raises above 100 nm (Figure 4c,d,e) but is not inhibited. As water contact angle increases, the meniscus is likely to condense with different geometries resulting in narrower features (approximately 40 nm). Line height and width written by solvent decomposition (Figure 4f) still depend on the bias applied, but the non-linear behavior indicates a different undergoing mechanism with respect to local oxidation.
At a first glance (Figure 5), the sample shows the three intense Raman features also present in graphite and the two overlapping bands around 1,600 cm−1 present in nanographite at approximately 1,595 cm−1 (G band), approximately 1,349 cm−1 (D band) and approximately 2,698 cm−1 (G′ band). According to the three-stage model of classification of disorder introduced by Ferrari and Robertson, the Raman spectrum is considered to depend on the degree of amorphization, the disorder, clustering of sp2 phase, presence of sp2 rings or chains, and ratio between sp2 and sp3 bonds.
C constant depends on the wavelength; at 514.5 nm, its value is equal to 44 Å. Therefore from Equation 1, it is possible to estimate a grain size La = 36 ± 2 Å (Figure 5e). Our results are also consistent with a high content of sp2 hybridized carbon, as already reported by Suez et al. for features deposited from a liquid aliphatic precursor (hexadecane). A more detailed evaluation of the band around 1,600 cm−1 (Figure 5f), by a multipeak fit, reveals that the three components could represent the sample spectra. The two components (G and D′) are present in the nanocrystalline graphite, and a third component around 1,570 (lowered G peak) is due to mainly sp2 amorphous carbon.
The difference in work function measured allows to clearly resolve patterned graphitic bodies and partially confirms the prevalent graphitic composition of the features although it was not possible to get a quantitative explanation of the local work functions measured.
We illustrated a simple and inexpensive nanofabrication method that can produce oxide or pure graphitic nanofeatures by means of SPL, employing almost any commercial AFM, avoiding subtractive fabrication methods including electron beam lithography and focused ion beam. Secondly, choosing a proper organic precursor, we show that the technique is accessible to most AFM users with no need of dedicated setups in ambient environment. The reaction leading to carbon deposition is likely to happen in both polarities, but when the tip is biased negatively, the competing oxidation masks solvent decomposition. The method, combined with dry etching allows the fast prototyping of Si masters ideal for replica molding/nanoimprinting. As a possible prototype, we realized several Si masters with satisfactory aspect ratio and we showed how to hybridize microlithography with SPL, connecting Al micropatterns with nanopatterns.
This work was entirely supported by the Italian Institute of Technology (IIT). We specially appreciate the support coming from the facilities of the Nanostructures Department.
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