Phonon lithography enabling fabrication of superior high-resolution electrical contacts on 1D and 2D materials

Session

PSA.1 - 1D & 2D Materials

Authors

Dr Felix Holzner (1)

Affiliations

1. Heidelberg Instruments Nano

Keywords

nanolithography 

non-invasive

phonon

thermal scanning probe

Abstract text

We present a novel nanolithography method based on local heat transfer that enables easy and accurate fabrication of high-resolution electrical contacts on top of 1D and 2D materials. The quality of the electrical contacts is superior compared to contacts made by conventional methods due to the non-invasive nature of the nanolithography process.

High-resolution electrical contacts on 1D and 2D materials are usually done by electron beam lithography (EBL). Accurate overlay of the electrodes with respect to individual flakes and nanowires can be tedious by EBL as it usually requires marker structures close to the flakes or nanowires and additional imaging steps to precisely determine the positions relative to the markers. A fundamental issue using EBL on 1D and 2D materials is the fact that the high energy electrons can generate various defects in the materials like vacancies, trapped charge or creation of covalent bonds with the organic resist molecules. Such defects can strongly deteriorate the properties of the 1D or 2D materials leading e.g. to poor quality non-ohmic metal contacts with high Schottky barriers and large contact resistances and hence reduced device performance [1].

Thermal Scanning Probe Lithography (tSPL) is using phonons to locally modify materials. The technology has recently entered the market as a versatile nanolithography method and a first true alternative to EBL for a wide variety of applications including direct writing of high-resolution electrical contacts [2]. Core of the technology is a heatable probe tip that is used for both patterning and simultaneous inspection of complex nanostructures. The heated tip can pattern very high-resolution (< 10 nm half-pitch) nanostructures by locally evaporating resist materials. The structures are inspected by the cold tip in parallel with the patterning process, enabling stitching and markerless overlay with sub-5 nm accuracy [3]. The technique is compatible with all the common pattern transfer processes [4,5].

Here, we show that tSPL can be used for shaping 2D materials with very high precision (Figure 1a-b) [6] and for forming high-quality metal contact electrodes on them (Figures 1 c–d) [7]. The fabricated devices exhibit vanishing Schottky barrier heights (around 0 meV, Figure 1d), record-high on/off ratios of 1010, no hysteresis, and subthreshold swings as low as 64 mV per decade. We also report of InAs nanowire transistors with high resolution metal top gates fabricated by t-SPL [8]. The transistors show excellent switching behavior and a subthreshold slope close to 60 mV per decade at room temperature. The improved performance is also attributed to the absence of electrons that could be trapped in the thin gate oxide during usual electron beam lithography.

In summary, we present how nanolithography using phonons is used to fabricate high quality electrical devices out of 1D and 2D materials.

Figure 1. (a) AFM image of 18-nm half-pitch 1L MoS₂ nanoribbon array patterned along the zigzag direction. (b) A close-up of the region marked with a white dashed box in (a). Figures (a) and (b) from Ref [6]. (c) Optical image of a 1L MoS₂ FET with a h-BN gate dielectric where the source, drain and top-gate electrodes have been patterned with a NanoFrazor. (d) Gate voltage dependence of Schottky barrier height of a 1L MoS₂ FET with Al/Au contacts (V_ds = 2 V). The deviation from the linear response at low V_bg (dashed red line) defines the flat band voltage and the SBH. Inset, corresponding temperature-dependent transfer curves (V_ds = 2 V). Figures (c) and (d) from Ref. [7].

References

[1]    Allain et al., Nature Materials 14, 1195–1205 (2015).

[2]    Howell et al., Nature Microsystem & Nanoeng, accepted

[3]    Rawlings et al., ACS Nano 9, 6188 (2015).

[4]    Wolf et al., J. Vac. Sci. Technol. B 33, 02B102 (2015).

[5]    Kulmala et al., Proc. SPIE 1058412 (2018).

[6]    Ryu and Knoll, Electrical Atomic Force Microscopy for Nanoelectronics, 143-172 (Ed. U. Celano), Springer Nature Switzerland AG (2019).

[7]    Zheng et al., Nature Electronics 2 17-25 (2019).

[8]    Wolf et al., Proc. SMTA PanPac (2019).