In recent years, a piezoelectric vibrator, which employs crystal or the like as a time source, a timing source of a control signal or the like, a reference signal source or the like, has been used in a cell phone or a portable information terminal device. Various piezoelectric vibrators have been known as this kind of piezoelectric vibrator, but a surface mounted piezoelectric vibrator has been known as one of the piezoelectric vibrators. As this kind of piezoelectric vibrator, there is generally known a piezoelectric vibrator having a three-layer structure where a piezoelectric substrate including a piezoelectric vibrating reed is bonded to a base substrate and a lid substrate so as to be interposed between the base substrate and the lid substrate in a vertical direction. In this case, the piezoelectric vibrator is received in a cavity (closed chamber) that is formed between the base substrate and the lid substrate. Further, in recent years, there has been developed a piezoelectric vibrator having not a three-layer structure but a two-layer structure.
This type of piezoelectric vibrator has a two-layer structure where a base substrate and a lid substrate are directly bonded to each other and a piezoelectric vibrating reed is received in a cavity formed between both the substrates.
The two-layer structure type piezoelectric vibrators are superior to three-layer structure type piezoelectric vibrators in terms of the reduction in thickness. Accordingly, the two-layer structure type piezoelectric vibrators are preferably used. As one of the piezoelectric vibrators, there is known a piezoelectric vibrator where external electrodes formed on a base substrate are electrically connected to a piezoelectric vibrating reed by using through holes (see Patent Document 1).
As shown in FIGS. 22 and 23, a piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 that are anodically bonded to each other, and a piezoelectric vibrating reed 203 that is sealed in a cavity C formed between both the substrates 201 and 202.
Through holes 204, which pass through the substrate 201, are formed at the base substrate 201, and conductive films 205 are formed so as to extend over the upper and lower surfaces of the base substrate 201 through the inner surfaces of the through holes 204. Portions of the conductive films 205, which are formed on the upper surface of the base substrate 201, function as mounting pads 205a that are electrically connected to the piezoelectric vibrating reed 203. Portions of the conductive films, which are formed on the lower surface of the base substrate 201, function as external electrodes 205b that are electrically connected to the outside. In addition, a part of the mounting pads 205a are used as a bonding film when the base substrate 201 and the lid substrate 202 are anodically bonded to each other. Further, since the mounting pads 205a come into close contact with both the substrates 201 and 202 when the substrates are anodically bonded to each other, the through holes 204 are sealed. Furthermore, a bonding film 206 for anodic bonding is formed between the base substrate 201 and the lid substrate 202 so as to surround the cavity C.
When the base substrate 201 and the lid substrate 202 are anodically bonded to each other in order to manufacture the piezoelectric vibrator 200 having the above-mentioned structure, as shown in FIG. 22, a predetermined voltage is applied between the lid substrate 202 and the conductive films 205 that function as the bonding film 206 and the external electrodes 205b. Accordingly, both the substrates 201 and 202 are solidly bonded to each other by anodic bonding with a part of the conductive films 205 and the bonding film 206 interposed therebetween. As a result, it may be possible to seal the through holes 204 and to seal airtight the piezoelectric vibrating reed 203 in the cavity C. Further, since the piezoelectric vibrating reed 203 can be electrically connected to the outside through the conductive films 205, it may be possible to reliably make the piezoelectric vibrating reed 203 operate in the cavity C.    Patent Citation 1: JP-A-6-343017