Low-density SiOC thin films grown by low temperature plasma-enhanced chemical vapor deposition for high performance acoustic Bragg mirrors

Solidly mounted resonators are an important type of bulk acoustic wave resonators, which are applied in devices such as radio-frequency filters. To decrease acoustic losses into the substrate, dielectric Bragg mirrors are applied between the bottom electrode and the substrate within the layer stack. These mirrors typically consist of alternating high (W) and low acoustic impedance (SiO2) films. To increase the acoustic impedance contrast and thus the performance of the mirror and resonator, one approach is to lower the acoustic impedance of the low acoustic material which depends on the density and elastic modulus. In literature, doping of SiO2 with C has been shown to be an effective approach to reduce the dielectric function with respect to pure SiO2.1
In this study, plasma-enhanced chemical vapor deposition (PECVD) is investigated for growing low acoustic impedance SiOC (carbon doped silicon oxide) thin films on 100 mm diameter Si wafers with native oxide. SiH4, CH4, and N2O are used as the precursors for Si, C, and O, respectively, and Ar is added as neutral species to the processing plasma. The substrate temperature is varied from 50 °C to 350 °C, the total flow rate at a constant pressure (1.2 Torr) from 174 sccm to 698 sccm, and the CH4/N2O flow rate ratio from 0 to 0.2.
Generally, increasing the total flow rate and decreasing the substrate temperature leads to a decrease of the refractive index, which is used as a measure of the density of the film. At 50 °C and a total flow rate of 698 sccm, the refractive index (at 633 nm; spectroscopic ellipsometry) and the mass density (graviometry) can be tuned from 1.37 to 1.42 and 1.6 to 1.9 g/cm3, respectively, by tuning the CH4/N2O ratio, with the lowest values found at CH4/N2O=0.08. The RMS roughness (atomic force microscopy) increases (10-20 nm for around 500 nm thickness) and the films become softer with increasing CH4/N2O ratio (0-0.2), where the elastic modulus drops by 50 % (nanoindentation). Fourier transform infrared spectroscopy furthermore shows changes in the chemical structure with changing ratio of Si, C, and O bonds.
In conclusion, the study shows the potential of using a low temperature PECVD process for tuning the acoustic impedance of SiOC thin films. As the main knob the CH4/N2O flow ratio is investigated which allows to change the chemical and structural properties of the material, supposedly creating a more open structure and weaking the bond strength. In further studies, the films will be tested in a resonator test device and benchmarked against industrial state-of-the-art layer stacks.
1Grill, J. Appl. Phys., 93 (3), 1785, 2003