A Fast Micro and Nano-Structuring Method with a High Dimensional Stability Using Ga+ Resistless Lithography (GaRL)

Silicon carbide (SiC) has high mechanical and chemical stability, superior thermal conductivity, excellent breakdown electric field, and wide bandgap, which make them a better choice over silicon in high frequency, high power, and optoelectronic applications.

A highly versatile and rapid micro- and nano-structuring technique that demonstrates superior dimensional stability on flat and pronounced topographies enables rapid prototyping. These capabilities lead to the development of improved sensors, actuators, and micro electro mechanical systems (MEMS).

Fast Micro- and Nano-Structuring Method

These capabilities can be achieved with a process involving GaRL followed by reactive ion etching (RIE) . GaRL involves local Ga implantation through focused ion beam (FIB) with doses much less than FIB direct milling. As a result, strongly retarded etching rates can be achieved for the implanted regions for RIE and wet etching (Figure 1).

Patterning by GaRL and subsequent RIE (schematic)

Figure 1. Patterning by GaRL and subsequent RIE (schematic)

This patterning method holds potential for a myriad of applications, as GaRL is a proven technology in the processing of a variety of materials, including diamond, silicon and silicon dioxide. GaRL can be directly used for the etch mask fabrication if RIE processes are already defined for the materials of interest as only a slight modification is required for the GaRL process for different materials.

Additionally, the control of the ion beam like an electron beam in the case of e-beam lithography, the etch mask itself is maskless and highly versatile. This makes it suitable for prototyping of devices or structures. Furthermore, the highly focused ion beam enables handling dimensions similar to an electron beam, which offers final masking dimensions in the sub-100nm regime.

Experiment Results

Based on this fact, it is demonstrated that the combination of GaRL and succeeding RIE process can be used for the flexible patterning of SiC in the nano-regime. The findings were first reported at the International Conference on Micro and Nano Engineering in London in 2013 (MNE 2013). A FEI Helios Nanolab 600 FIB was employed to perform all FIB experiments using 30keV beam energy.

Implanting doses ranging from roughly1•1015 - 7•1017 cm-2, the dose dependence of the implanted region’s masking behaviour and the superposition of masking and direct milling effects were analysed. Around 50% of the nominal beam diameter was set as the gap between the beam overlap or raster pixels. The nominal beam diameter for the applied current was 17nm. n-doped 4H-SiC samples were used in the experiments.

A PlasmaPro System100 from Oxford Instruments was used for RIE involving SF6 and oxygen as process gases at 4mTorr gas pressure and 50°C chuck temperature. It is necessary to optimize the implanted Ga dose to achieve an efficient etch mask. Figure 2 shows drastically different structures are created after RIE due to different Ga doses. The optimum dose range for etch mask was 2 - 7•1016 cm-2, producing nearly identical structures with smooth surfaces and a height of roughly 135nm.

Figure 2. SEM image (tilted by 52°) of an array of FIB irradiated areas (each of 3x3 µm²) after RIE. Each area was implanted with a different dose as indicated (dose in 1015 cm-2) in the image starting.

FIB milling or FIB material removal occurs along with FIB implantation for higher doses. The masking effect is inadequate for smaller doses, for instance, the masking layer begins ‘peeling off’ after RIE for structures with doses of 8•1015 cm-2 and 1.1•1016 cm-2. Very flexibly micro and nano-structures could be fabricated in 4H-SiC by applying the combination of GaRL and subsequent RIE using such optimized masking doses (Figure 3). The residual Ga-rich masking layer could be detrimental for some applications, and can be removed using a purely physical Ar sputtering process.

SEM image (tilted by 52°) of micro and nano-structures in 4H-SiC fabricated by GaRL and subsequent RIE.

Figure 3. SEM image (tilted by 52°) of micro and nano-structures in 4H-SiC fabricated by GaRL and subsequent RIE.

Conclusion

From the results, it is evident that the combination of GaRL and subsequent RIE can be used for flexible nano-patterning of silicon carbide.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.

For more information on this source, please visit Oxford Instruments Plasma Technology.

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