Dissipative Kerr solitons in ultra-high- Q resonators are extremely sensitive to the thermal behavior of the resonators. Especially for resonators with hydrophilic surfaces, moisture continuously adsorbs on their surfaces and causes additional absorption loss that results in an excessive thermal shift of resonance frequency. This change makes soliton mode locking more challenging or even impossible. Here, we report hydrophobic monolayer passivation using hexamethyldisilazane on ultra-high- Q silica wedge resonators. It was experimentally confirmed that the Q-factor and dispersion were maintained after passivation, and excess thermal shift by moisture was inhibited for more than three days in the atmosphere. Soliton mode locking was successfully performed with the resonator one month after passivation. (C) 2021 Optical Society of America.