Since Samsung already has a System-on-Chip made on a 14 nm process (the Exynos 7 Octa 7420, which is found inside the Galaxy S6 and S6 edge), the A9 could also be manufactured on a 14 nm process. This should make it better and more efficient than the A8, which is made on a 20 nm process.
According to Bloomberg, Samsung will start manufacturing A9 chips at the Giheung plant in its home country, South Korea. However, it's not clear when mass production will start. It should happen soon, though, as Apple is likely planning to launch its next generation iPhones, possibly called iPhone 6s and iPhone 6s Plus, sometime in September.
source: Bloomberg via MacRumors