  This paper is the first report of the high-temperature characteristics of a
laterally vibrating piezoelectric lithium niobate (LiNbO$_{3}$) MEMS resonator
array up to 500{\deg}C in air. After a high-temperature burn-in treatment,
device quality factor (Q) is enhanced to 508 and the resonance shifts to a
lower frequency and remains stable up to 500{\deg}C. During subsequent in situ
high-temperature testing, the resonant frequencies of two coupled shear
horizontal (SH0) modes in the array are 87.36 MHz and 87.21 MHz at 25{\deg}C
and 84.56 MHz and 84.39 MHz at 500{\deg}C, correspondingly, representing a -3%
shift in frequency over the temperature range. Upon cooling to room
temperature, the resonant frequency returns to 87.36 MHz, demonstrating
recoverability of device performance. The first- and second-order temperature
coefficient of frequency (TCF) are found to be -95.27 ppm/{\deg}C and 57.5
ppb/{\deg}C$^{2}$ for resonant mode A, and -95.43 ppm/{\deg}C and 55.8
ppb/{\deg}C$^{2}$ for resonant mode B, respectively. The temperature-dependent
quality factor (Q) and electromechanical coupling coefficient ($k_{t}^{2}$) are
extracted and reported. Device Q decreases to 334 after high-temperature
exposure, while $k_{t}^{2}$ increases to 12.40%. This work supports the use of
piezoelectric LiNbO$_{3}$ as a material platform for harsh environment
radio-frequency (RF) resonant sensors (e.g. temperature and infrared).
