It has been confirmed by x-ray photoelectron spectroscopy that the Ti nitride can be athermally formed without substrate disruption by simultaneous NH3 exposure and Ti evaporation to a clean Si(100)-2X1 held at 40 K under ultrahigh vacuum. One of the critical points in the present nitride formation at 40 K is solving the kinetic constraint problem by simultaneous adsorption of consisting elements. Compared to TiN formed by other techniques, this athermally formed Ti nitride does not contain oxygen contamination but has TiN precursors, TiNxHy. Postheat treatment partially converts TiNxHy to Ti nitride and induces the excessive metallic Ti to be mixed with the previously formed Ti nitride. When the Ti nitride is exposed to the air, Ti in the Ti nitride is preferentially oxidized and forms the Ti oxide on the top side. By mild-annealing it up to 550 degrees C under ultrahigh vacuum, the oxygen is transferred from Ti oxide to the Si substrate and the original Ti nitride is gradually recovered. Even for annealing up to the temperature high enough to desorb the oxidized Si, the Ti nitride hardly desorbs.