UHF AT-cut crystal resonators operating in the fundamental mode

The UHF AT-cut crystal resonators operating in the fundamental mode were manufactured using both wet chemical etching technology with high etching rate and reactive ion etching technology with low etching rate for high-precision thickness adjustment in combination. In order to decrease the thermal stress at the interface between the crystal and a electrode, which is caused by a difference of thermal expansion coefficients, a gold electrode was selected as this material has the approximate thermal expansion coefficient as the crystal. The electrode thickness was also examined to have the most suitable plate back ratio. The ratio of the film thickness of the reverse side electrode to the thickness of the crystal substrate (9%), together with the film thickness of the main electrode (60 nm), creates a resonance at center frequency of 622 MHz with a favorable frequency temperature characteristic of frequency shifts within 110 ppm at the temperature range from -35°C to +85°C. Furthermore, the application of this resonator to the configuration of VCXO for the Colpitts oscillation circuit produced a frequency control characteristic of ±120 ppm/+2.5 V±2 V and a phase noise characteristic of -100 dBc/Hz at an offset frequency of 1 kHz