At the LSBI we aim to fabricate so-called multimodal devices, that is, neural implant capable of stimulating the nervous system through multiple modes, simultaneously. In particular, we are interested in designing an implant that would combine electrical and optical capabilities. Doing so requires the use of transparent conductors in order to maximize the amount of light transmitted to the tissue.

Indium Tin Oxide

Indium-tin-oxide (ITO) has been the center of growing attention because of its exciting and attractive properties. Its relatively low electrical resistivity (\(2-4\cdot10^{-4}\ \left[\Omega\cdot cm\right]\)), combined with high optical transmittance (>85%) make it a material of choice in both research institutes and industry \cite{Kim_1999}.
For example, ITO films have long found applications in solar cells. As early as 1978, Cheek et al. showed the fabrication and characterization of ITO/polycrystalline silicon solar cells using ion-beam sputtering techniques \cite{Cheek_1978}. A few years later, Saim and Campbell presented a fabrication technique based on conventional thick film printing methods. The ITO film was used here not only as a conducting surface layer, but also as an anti-reflective coating \cite{Saim_1987}. More recently, ITO has also been used as a semiconductor material in solar cells \cite{Yu_2016}.
Galstian et al. developed a electrically tunable liquid crystal autofocus lens. To achieve electrical contact while keeping optical transmittance at its best, the lens needs to be coated with transparent electrodes. Indium-tin-oxide is thus the ideal material for this application \cite{Galstian_2017}.
Since the 1990s, ITO thin films were also found in liquid-crystal displays \cite{Ishibashi_1990,Sawada_2001}. More recently, Yan et al. demonstrated the use of refractive index matching of ITO films with respect to their glass substrate, yielding to higher  optical transmittance \cite{Yan_2009}.
In a completely different field of application, Briggs patented the use of ITO for aircraft windshield deicing. The invention consists of the sandwiching of a transparent resistive interlayer between a pair of transparent sheets. Applying an AC signal to that interlayer will generate heat, thus helping with the deicing. Typically, that resistive layer can be made of ITO \cite{briggs2011aircraft}. 

Methods

Deposition

The ITO films were formed using a commercial sputtering system (Alliance Concept AC450). Sputtering is a deposition technique in which a target material is bombarded with the highly energetic ions from a plasma, leading to the ejection of atoms toward a sample. On the AC450, the quality of the resulting film can be controlled via six parameters (Table \ref{tab:parameters}):
  1. The electrical field applied between the target material and the chuck holding the sample. The strength and nature of the field are controlled by the power values of its DC and RF components.
  2. The base pressure, i.e. the quality of the vacuum inside the chamber at the start of the sputtering phase.
  3. The temperature of the chuck holding the sample.
  4. The distance between the target and the chuck holding the sample during the deposition.
  5. The argon flow fed inside the chamber and used to create and sustain the plasma.
  6. The oxygen flow fed inside the chamber. Not only are the oxygen atoms used to generate the plasma, they also contribute to the creation of defect inside the ITO lattice - this is the main mechanism through which ITO is made conductive.
For this project, we decided to limit our investigation to five parameters by keeping the distance between the target and the chuck constant across all experiments. We also chose to keep the RF component of the electric field null and to only vary the value of the DC power. This last decision was made based on the fact that DC recipes naturally lead to faster deposition rates and thus faster turnaround, a critical consideration during the prototyping phase of a device.
In order to ensure that the quantity of deposited material would be sufficient for the characterization process, we set a target thickness of 100nm. We thus started our investigation by assessing the deposition rate for each set of parameters during a series of calibration runs. To do so, we ran each recipe for 10 minutes, before measuring the resulting film thickness using a mechanical profilometer (Bruker DektakXT). Based on these measurements, we were able to estimate each deposition rate, which we then used to compute an appropriate deposition time for the characterization samples.