**2.5. Helium leak testing**

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 applications.

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 be correlated to leak rates.

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 the fixture and the sample to ensure a good seal.

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 to return the leak rate to re-equilibrate.
