In many applications of quartz crystal oscillators, the oscillator noise is an important parameter. For example, excessive oscillator noise will limit the performance of radar, navigation and communications systems. In a properly designed high precision oscillator, the quartz crystal resonator is often the major noise determining component. Reducing the noise of the resonator has been an important goal of resonator and oscillator designers.
The gold electrodes that are used on most high precision resonators can be a significant noise source, especially when the electrodes are deposited through masks which are not in intimate contact with the surfaces of the quartz plate. Specifically, when gold has been deposited onto a quartz resonator substrate through a shadow mask, thinning of the gold has been observed around the edges of the pattern. Because the mask has not been in intimate contact with the quartz, that is, has not been in touch with the major surfaces of the quartz plate, gold metal forms shadows around mask edges.
For example, when a 100 nanometer thick circular gold electrode is deposited onto a quartz plate through a 5 millimeter diameter opening in a mask that is not in direct contact with the surfaces of the quartz plate, it has been found that the electrode thickness does not change abruptly from 100 nm inside the 5 mm diameter circle to zero outside the 5 mm circle. The transition is gradual; that is, a narrow rim is formed outside the 5 mm circle. Across the width of this rim, the thickness decreases gradually from 100 nm to zero. The ultrathin gold film in such a rim can be very noisy. Furthermore, when this thinning gold is less than 40 nm thick, it adheres rigidly to the quartz substrate, whereas the rest of the gold electrode, that is thicker, adheres very weakly. The rim, therefore, can be a source of noise and other instabilities in a quartz resonator. A similar rim is formed when the electrode is of a material other than gold. Ultrathin films, in general, tend to be noisy and are, therefore, detrimental to the performance of high stability resonators.
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