After the electrode deposition starts, the atoms on the quartz surface form islands through which no current flows.  As the deposition continues, the islands grow. Eventually, the islands touch, form larger islands, then further coalesce to form a continuous film.  At this point, Rs is large due to the high resistivities of the ultrathin films.  As the film thickness (the “plateback”) increases, the film resistivity decreases.  Eventually, the Q of the resonator rather than the film resistance determines Rs. As the film becomes thick, losses in the film decrease the Q and increase the Rs.
	The “Quartz Resonator Handbook…” referenced below suggests minimum and maximum platebacks for fundamental mode, 3rd overtone and 5th overtone resonators, and for Al, Ag and Au electrodes (p. 115, Table 5).
	The figure above is adapted from the “Quartz Resonator Handbook, Manufacturing Guide for AT Type Units,” R.E. Bennett, Union Thermoelectric Division, p, 113, Figure 51, 1960. A copy of this publication can be found at http://www.ieee-uffc.org/archive (available to IEEE UFFC Society members).
	C. A. Neugebauer, “Condensation, Nucleation, and Growth of Thin Films,“ in Handbook of Thin Film Technology, L. I. Maissel & R. Glang editors, Chapt. 8, McGraw Hill Book Co., 1970.