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

Carla Perez-Martinez, Paulo Lozano (Associate Professor)

Timothy Fedkiw (PhD Student) and Jacques Gierak and Stéphane Guilet (LPN/CNRS, France) 

Objective of the Research 

Ionic Liquid Ion Sources (ILIS) are devices that can be used for space propulsion. The source consists 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. The emitted particles provide thrust; this emitters can be microfabricated, and they could be applied to missions in small satellites that require precision.
Another 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].
This work aims to characterize the influence of the emitter-extractor geometry and extraction polarity in the ion beam shape in order to characterize the setup geometry. Additionally, this project also studies the energy characteristics of the different ion species contained in the ILIS beam, by introducing a mass filter in the ion beam path. Also, as part of a collaboration with the French Laboratory for Nanostructures and Photonics, it was found that ILIS are capable of fast etching in silicon, and a summary of the results presented in [2] are summarized below. 

Approach / Tools 

A beam visualization system is used to observe the emitted beam; the ILIS is mounted on a movable stage with submicron precision, which allows modifying the position of the needle with respect to the extractor in situ. The beam is attenuated by a set of grids before hitting a microchannel plate (MCP). The MCP walls emit electrons as they are hit by the ions, and the electrons go on and excite a phosphor screen. This screen glows and the spectra is captured with a camera. The images are then used to determine the ion distribution.

Fig 1. Electrospray Beam Visualization System

The second project is the magnetic filtering of the ILIS beam. For this experiment, the ILIS is placed in front of an optics system that focuses the beam, which then enters a steel structure. The steel structure contains two strong permanent magnets; the steel redirects the magnets field and creates a uniform magnetic field along the beam passage. The beam, which consists of several species of ions, each with different weights and energies, will be split into several beams due to the action of the field. Each of the smaller beams is then captured by a retarding potential analyzer (RPA). The reading of the RPA can be used to determine the energy of each of the species contained in the beam.

Fig 2. Setup for experiments with the magnetic filter, and RPA detail.


Ion Beam Visualization:
The beam of an ILIS using the liquid EMI-BF4 has been produced and visualized with the BVS. It has been found that the source is stable in an alternating polarity regime, and that the beam shape is parabolic, confirming the models in [3] and [4]. Also, unstable behavior has been observed in the formation of several Taylor cones for some emitter positions.

Fig 3. (a) Sample beam profile for ILIS operated at -2.75 kV showing a parabolic profile. (b) Beam profile showing the formation of several Taylor cones for an unstable test.

Development of ILIS for Microfabrication:

During the summer of 2009, in collaboration with the French Laboratory for Nanostructures and Photonics, the potential for ILIS in direct microfabrication was explored. ILIS were found capable of fast etching in monocrystalline silicon. Silicon wafers irradiated with an EMI-BF4 ILIS during half an hour at different currents showed sputtering of materials for ion energies of 15 keV.
An XPS analysis shows that the landing ions create volatile species that accelerate the etching rate, by reacting with the silicon surface. Also, this process leaves a surface roughness almost equal to that of the native surface.

Fig 4. (a) Optical Microscopy Image of pattern imprinted on Si through a TEM copper grid (b) XPS analysis of four regions in the irradiated silicon wafer.


[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] P. Lozano and M. Martinez-Sanchez, J. Colloid Interface Sci. 282, 415 (2005)
[4] P. Lozano and M. Martinez-Sanchez, Efficiency Estimation of EMI-BF4 Ionic Liquid Electrospray Thrusters, in: Proceedings of the 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2005, Tucson, Arizona