By characterization of Fe/Si multilayers with a range of layer thickness we show that the diffusion of Si into Fe makes the Si layers 11.4±1.2 Å thinner than expected. We can use this information to improve growth of multilayers with high precision of layer thickness. Furthermore, the interdiffusion of Si into Fe creates a magnetically dead layer at the interface of about 5 Å. We have used reactive sputtering of Si with N 2 as a method to reduce interdiffusion. With the ability to control thickness more accurately and to reduce interdiffusion we have deposited polarizing Fe/SiN x supermirrors with neutron spin-up reflectivity in excess of 85% at λ/ θ=0.3°/ λ (i.e., “ m=3”), allowing us to obtain neutron polarization above 95% in reflection geometry and above 90% in transmission geometry, respectively.