In-situ frequency trimming of SAW resonator using conventional crystal resonator fine tuning method with gold thin film addition

SAW devices are routinely frequency trimmed at the wafer level prior to the dicing process so to improve yield at the assembly level. The trimming can be achieved by using wet etching, reactive ion etching, ion beam milling, and other methods. It is also possible to trim the frequency of patterned SAW wafer by depositing dielectric thin films on the wafer but this method is usually done for the purpose of finger shorting protection from metallic particles. Reactive ion etching can be used to trim assembled SAW devices prior to encapsulation. This is seldom employed since it is considered costly, inefficiently, and difficult to perform in-situ. In this paper we propose trimming assembled SAW resonator (SAWR) by using the conventional crystal resonator fine-tuning process with gold thin film addition. Nowadays, the fine-tuning process is a necessary step in crystal resonator production. We show that this is a simple and well-controlled method to trim assembled SAW resonator also. This method provides a wide trimming rage and the gold film is thin enough not to short out the SAW transducers so that the SAW resonator´s integrity can be maintained.