  We demonstrate a "direct writing" method for the fabrication of planar
Josephson junctions from high quality superconducting niobium titanium nitride
(NbTiN) thin films using focused He-ion beam irradiation. Compared to the
materials previously used in such processing, YBCO and MgB$_2$, NbTiN has much
better mechanical and electrical properties, as well as good corrosion
resistance. We show that we can control the suppression of superconductivity in
NbTiN as a function of the helium ion beam fluence, and that this controllable
critical temperature suppression combined with the high spatial resolution and
position control of the He-ion beam in a helium ion microscope enables us to
successfully fabricate Josephson junctions with highly tunable weak links.
Because of the continuous nature of the disorder-induced metal-insulator
transition, this method allows the creation of barriers with wide range of
resistivities ranging from the metallic to the insulating state, with the
critical current and the junction resistance varying over two orders of
magnitude. Electrical transport measurements show that junctions follow closely
the ideal resistively and capacitively shunted junction behavior, have high
characteristic voltages ($0.2-1.4$ mV) and show Shapiro steps up to very high
orders. This suggests that these type of junctions are suitable for a wide
range of applications in superconducting electronics and quantum information
technology, with the bonus that a whole device can be fabricated from just a
single thin film, with the excellent electrical and microwave characteristics
offered by NbTiN.
