**3.1. Platinum–Iridium nanowire electrodeposition**

A series of nanowire-template samples were prepared using different plating solution pH: 1.8, 3.1 and 5.0. Subsequently, nanowires were isolated and examined via SEM to identify what pH produced preferred morphologies. For our application, high-aspect ratio and nonporous wires are desirable. Figure 6 shows a series of representative SEMs of isolated nanowires prepared from solutions of different pH, specifically (from top to bottom): pH = 1.8, 3.1 and 5.0.

Nanowires were first deposited at pH = 1.8, Figure 6a, isolated nanowires showed some mechanical compliance, evidenced by their visible bending in images. However, analysis of image series confirmed that at this pH, high aspect ratio nanowires could not be achieved. It is unclear what the root cause may be. Typically, at such low pH, hydrogen co-deposition played some role in impacting deposition. It is also possible that template dissolution under such high acidity may be taking place.

**2.5. Helium leak testing**

214 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

be correlated to leak rates.

the fixture and the sample to ensure a good seal.

**3.1. Platinum–Iridium nanowire electrodeposition**

to return the leak rate to re-equilibrate.

**3. Results and discussion**

= 1.8, 3.1 and 5.0.

applications.

Helium leak tests are a standard method for assessing electronic package hermeticity. Here we use helium leak tests to provide a preliminary assessment on whether our nanowiretemplate assemblies have hermetic properties worthy of microelectronics packaging

In practice, helium leak tests are performed by subjecting two-dimensional test samples to vacuum on one side, while helium gas is introduced on the opposite side via gas gun. The vacuum side uses a mass spectrometer to measure any helium that has penetrated through the substrate/interconnects as a result of the applied vacuum. Helium concentrations detected can

Helium leak tests were performed using an ASM 182-TD (Alcatel, Inc.) helium leak detector with capability of detecting helium leak rates down to 5 x 10-12 mbar L s-1. Typically, values below 1x10-11 mbar L s-1 are considered hermetic for microelectronics applications [4, 27, 28]. A custom, sample mounting fixture was designed for testing the nanowire embedded membranes. The fixture has an ultra-fine polished surface to ensure a proper seal between the test membrane and the fixture. The fixture was mounted to the inlet aperture of the leak detector using a standard vacuum seal and clamp. Samples are placed over top of a small circular inlet in the center of this fixture and held in place by the applied vacuum, a small amount of vacuum oil is applied to a polymer O-ring which is placed between the surface of

To take measurements samples are mounted and vacuum applied until the flow rate settles below 1 x 10-11 mbar L s-1. A continuous small dose of helium was sprayed from a gas spray gun on the top side of the sample exposed to ambient air. Helium gas was sprayed at the sample surface at a distance of 10cm with a pressure of 20 lbs for 10 seconds. After helium exposure, the highest observed leak rates were measured and recorded. Leak rate measurements were repeated three times for each sample. After each measurement, a time delay was given in order

A series of nanowire-template samples were prepared using different plating solution pH: 1.8, 3.1 and 5.0. Subsequently, nanowires were isolated and examined via SEM to identify what pH produced preferred morphologies. For our application, high-aspect ratio and nonporous wires are desirable. Figure 6 shows a series of representative SEMs of isolated nanowires prepared from solutions of different pH, specifically (from top to bottom): pH

Nanowires were first deposited at pH = 1.8, Figure 6a, isolated nanowires showed some mechanical compliance, evidenced by their visible bending in images. However, analysis of

**Figure 6.** a) SEM micrograph of isolated nanowires with shorter lengths electrodeposited at pH=1.8, (b) SEM micro‐ graph of isolated nanowires with brittle structures electrodeposited at pH=5 and (c) SEM micrograph of isolated dense nanowires with dense structures electrodeposited at pH= 3.1.

At pH = 5.0 Figure 6b, isolated nanowires were fragmented also with small aspect ratio. Porosity within the individual segments could not be confirmed, however, we suspect that either the nanowries themselves are brittle and fragment post-isolation, or that the deposition results in porosity which leads to fragmentation, once the structural support of the template is removed. It is unclear at this time what the mechanism for the resulting microstructure may be.

In contrast to both previous samples, high-aspect ratio nanowires were successfully synthe‐ sized using a plating solution with pH = 3.1, Figure 6c. Image analysis showed that aspect ratios from 20:1 to more than 50:1 were achievable using this approach. More work is needed to better understand what mechanisms are responsible for controlling the transport mecha‐ nisms taking place in the nano-channels. From these data, we determined pH = 3.1 would be capable of producing nanowires with preferred morphological structure.

## **3.2. Controlling Pt: Ir composition with potential**

Co-deposition of binary alloys and multilayer nanowires has been reported elsewhere [29-31]. Here, platinum (Pt) and iridium (Ir) metal atoms are deposited through the AAO channels by electrochemically reducing from platinum ions [PtCl₆]⁴⁻ and iridium ions [IrCl6] 4- in solution. By cycling the potential in a range below the equilibrium potentials for both ion complexes, the kinetics of deposition can be modulated to ensure that both elements are deposited in desirable quantities.

For this study, the potential range that produces a desirable Pt:Ir composition was unknown. We therefore selected five different potential ranges, Table 1, and prepared nanowires using each cycling range. All five potential ranges spanned 250 mV of potential but used different starting potentials (Uo) from Uo = -0.5V to Uo = 0.2V vs. Ag/AgCl.


**Table 1.** Deposition potential ranges and resulting PtIr nanowire fractional composition

Following deposition, nanowires were isolated and composition was tested using energy dispersive spectroscopy (EDS). Results in Table 1 show compositional fractions ranging from 62:38% to 85:15% platinum were attained by varying chemistry. While the exact mechanisms responsible for the differences in concentration are not well understood at this stage, we do know that shifting the potential range used for deposition affects the deposition kinetics inside the nano-channels.

It has been reported that platinum-iridium with 60:40 composition shows preferred properties for neurostimulation applications, e.g. highest electroactivity [32]. We therefore focused on further characterization of nanowires prepared with this deposition range (∆U = 0.00 to -0.15).
