Microwave acoustoelectronic sensor on a diamond substrate for the study of molybdenum oxidation

For the first time, a method for studying oxidation processes in metal films has been proposed using a microwave acoustoelectronic sensor based on a diamond substrate. The use of microwave operational frequencies guarantees high sensitivity of a sensor. The piezoelectric transducer, which generates a longitudinal bulk acoustic wave as an operational mode, is well protected from external influences. Using the Mo film as an example, we have demonstrated that the changes in the acoustic properties of a chemical sensor caused by Mo oxidation as a result of annealing in air agree well with the phase transformations of Mo studied by other methods, such as XRD, SEM, AFM. It was found that MoO₂, α-MoO₃, and β-MoO₃ were formed at annealing temperatures above 375 °C. Studying a microwave HBAR-based chemical sensor, we obtained changes in the frequency dependencies of the main acoustic parameters at 6.0, 6.5, 13.7, and 16.4 GHz as a result of the annealing process in air at high temperatures up to 500 °C. It should also be noted that a relatively thick film of Mo (∼1 µm) did not completely transform into oxide phases under the given experimental conditions. The increase in the relative frequency shift, Δf/f, and the Q-factor of overtones at low annealing temperatures up to 300 °C is associated with the compaction of the Mo film, which is a result of a decrease in the average size of the crystallites. In contrast, the decrease in Δf/f and Q values at annealing temperatures above 375-400 °C is associated with a change in the phase composition of the Mo film. Under these conditions, the main factor contributing to the change in the sign and magnitude of Δf/f is a significant increase in the total film thickness. It is well known that the higher the Q-factor, the greater the sensitivity of an acoustoelectronic resonant sensor. In our study, we found that the Q-factor decreases as a result of Mo annealing in the range of 300-500 °C. However, it remains at a value of ∼2000, which is quite sufficient for an accurate measurement of Δf/f values, even at the maximal operational frequency of 16.4 GHz and 500 °C. Because the object under test is placed on a free surface of a diamond substrate, and due to diamond's high chemical tolerance, the sensor studied can be used multiple times to monitor changes in the chemical composition of substances. It will be possible to do this, at least up to a temperature of 500 °C.