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Through its dependence on low symmetry crystal phases, ferroelectricity isinherently a property tied to the lower temperature ranges of the phase diagramfor a given material. This paper presents conclusive evidence that in the caseof ferroelectric AlScN, low temperature has to be seen as a purely relativeterm, since its ferroelectric-to-paraelectric transition temperature isconfirmed to surpass 1100{\deg}C and thus the transition temperature ofvirtually any other thin film ferroelectric. We arrived at this conclusionthrough investigating the structural stability of 0.4 - 2 ${\mu}$m thickAl$_{0.73}$Sc$_{0.27}$N films grown on Mo bottom electrodes via in situhigh-temperature X-ray diffraction and permittivity measurements. Our studiesreveal the wurtzite-type structure of Al$_{0.73}$Sc$_{0.27}$N is conservedduring the entire 1100{\deg}C annealing cycle, apparent through a constant cover a lattice parameter ratio. In situ permittivity measurements performed upto 1000{\deg}C strongly support this conclusion and include what could be theonset of a diverging permittivity only at the very upper end of the measurementinterval. Our in situ measurements are well-supported by ex situ (scanning)transmission electron microscopy and polarization and capacity hysteresismeasurements. These results confirm the structural stability on thesub-${\mu}$m scale next to the stability of the inscribed polarization duringthe complete 1100{\deg}C annealing treatment. Thus, AlScN is the first readilyavailable thin film ferroelectric with a temperature stability that surpassesvirtually all thermal budgets occurring in microtechnology, be it duringfabrication or the lifetime of a device - even in harshest environments.

On the exceptional temperature stability of ferroelectric AlScN thin films