Ionic Liquid Ion Sources (ILIS) for Micro/Nano-Fabrication

Ionic Liquid Ion Source Development 

Ionic Liquid Ion Sources (ILIS) consist of an electrochemically-sharpened tungsten needle, which is covered in an organic ionic liquid. By applying an electric potential between the needle and a downstream metallic extractor, a structure known as a Taylor cone is formed at the tip of the needle. Once we exceed a threshold voltage, ions and droplets are extracted from the cone, and pass through a hole in the extractor. One possible application of ILIS is the field of lithography, specifically for Focused Ion Beam (FIB) applications. The ILIS beam can be focused using special optics and then directed to a substrate for patterning. Traditionally, Liquid Metal Ion Sources (LMIS) have been used in FIB, but the ILIS, which has the same working mechanisms, could bring many advantages to these processes. For instance, the stable operation at low currents helps improve the resolution of the beam. Also, the variety of ionic liquids is immense, which increases the number of possible applications of an ILIS based FIB. [1] Ions present in some of the ionic liquids are reactive, which could eliminate the need of reactive assistance in some FIB etching processes [2].

The use of ionic liquids in focused ion
beams could allow the production of sub-100 nm beams of up to kiloDalton organic ions as well as
reactive species, with the possibility of engineering ionic liquid properties for a specific application.
In addition, using micro-fabricated and nano-structured emitter arrays operating in the PIR can
give access to e
cient and compact positive or negative ion sourcrom spacecraft
thrusters to deep reactive ion etchers (DRIE).
The use of ionic liquids in focused ion
beams could allow the production of sub-100 nm beams of up to kiloDalton organic ions as well as
reactive species, with the possibility of engineering ionic liquid properties for a specific application.
In addition, using micro-fabricated and nano-structured emitter arrays ot
thrusters to deep reactive ion etchers (DRIE).

This work evaluates porous carbon based resorcionol-formaldehyde xerogels as emitter substrates, and the feasibility of laser micro-machining of high aspect ratio emitter features. 

Carbon based ILIS sources and tip details [3]

 

Interaction with materials 

The SPL developed carbon based ILIS has been implemented in an ion gun geometry traditionally used in FIB which was provided by LPN-CNRS. In the irradiation experiments performed, the ion gun with the carbon xerogel ILIS is fired towards a target covered with a copper mask. 

Ion gun setup for irradiation experiments with carbon aerogel ILIS [3]

After irradiation at constant emitter current for several minutes, the shadow mask is removed and the resulting pattern is profiled to determine if the beam exposure resulted in deposition or etching of the substrate.

Sample irradiation results in Si [3]

The beam from the carbon xerogel ILIS has been used to etch silicon, gold and gallium nitride samples, with some deposition observed in gold at the lowest energy tested. The sputtering yields observed in the three materials, with irradiation energies between 2 and 7 keV, are high compared to the sputtering yields obtained with gallium at higher energies.

For further information, please contact David Krejci

References

[1] A. Zorzos and P.C. Lozano, J. Vac. Sci. Technol. B 26, 2097 (2008)
[2] C. Perez-Martinez, S. Guilet, N. Gogneau, P. Gegou, J. Gierak and P.C. Lozano, J. Vac. Sci. Technol. B 28, L25 (2010)

[3] C. Perez Martinez, "Engineering Ionic Liquid Ion Sources for Ion Beam Applications"," PhD thesis, MIT, 2016.