1. Field of the Invention
The present invention relates generally to a surface acoustic wave apparatus using a surface acoustic wave (SAW) element and an elastic wave apparatus using a film bulk acoustic resonator (FBAR), and, more particularly, to a surface acoustic wave apparatus and an elastic wave apparatus incorporated into a mobile communication device such as a cellular phone and used for a filter and an antenna sharing unit or duplexer utilizing surface acoustic waves (SAW) or a film bulk acoustic resonator (FBAR).
2. Description of the Related Art
Recently, corresponding to the demand for miniaturization of mobile devices, a surface acoustic wave apparatus is incorporated into a cellular phone and the like to act as a filter and an antenna duplexer, because of a miniaturized property thereof.
By the way, generally in cellular phones, static electricity charged in human bodies or objects is applied to internal components through antennas and others. As such an internal component, the surface acoustic wave apparatus has a surface acoustic wave element made up of interdigital (IDT) electrode fingers built up with the thin-film conductor consisting primary of aluminum on a piezoelectric crystal substrate.
Intervals between the interdigital (IDT) electrode fingers are mainly determined by the center frequency of the filter and, in the case of 1 to 2 GHz band filters, an interval between the adjacent electrode fingers are 0.1 to 0.5 μm. Therefore, an electric discharge is generated between the electrode fingers when static electricity greater than 50 to 100 V is applied to electrodes. At this point, a discharge current with a high current density instantaneously flows through the electrode fingers at the discharging portion, melts and cuts the electrode fingers and deteriorates or destroys properties of the filter.
Especially, a surface acoustic wave apparatus used as an antenna-duplexer is located at the position electrically connected immediately to an antenna and tends to be affected by such electrostatic discharges.
However, in the surface acoustic wave filter of the antenna duplexer itself, measures have not been taken for the electrostatic discharges until now, and the surface acoustic wave filter has been protected from the electrostatic discharges by inserting a discrete component between the antenna and the surface acoustic wave filter to pass electric charges due to static electricity to ground.
For example, in IEEE Ultrasonics Symp. pp. 9–12, (1998), Hukushima et al., especially in FIG. 5 and page 11, proposed inserting a n type, LC high-pass matching circuit between the antenna and the surface acoustic wave filter and passing static charges from a hot terminal to ground through inductances connected in parallel to protect the surface acoustic wave filter.
Also, a varistor has been used as a discrete component for taking measures to the electrostatic discharges. The varistor has a nonlinear resistance property exhibiting high resistance at low applied voltage and drastically reducing the resistance when the applied voltage becomes high, and thereby, when high voltages are applied, components can be protected from the electrostatic discharges by passing static charges to ground.
However, since the conventional varistor configured as a discrete component has a large capacity (capacitance) not negligible in comparison with 1 to 2 GHz in order to increase a value of the passing current and therefore has a disadvantage that the property of the surface acoustic wave filter is changed by the varistor connecting in parallel with the surface acoustic wave filter. The varistors are used for protecting components from the electrostatic discharges when relatively low frequencies of 100 MHz or lower are handled.
Further, as prior art for taking measures to the electrostatic discharges in the surface acoustic wave apparatus, propositions have been made for the technology using a thin-film resistive element (see, e.g., Japanese Patent Application Laid-Open Publication No. 1996-167826), the technology using a impedance matching coil (see, e.g., Japanese Patent Application Laid-Open Publication No. 1999-274886) or the technology using a capacitive element (see, e.g., Japanese Patent Application Laid-Open Publication No. 2001-168672).
Although the inventions described in Japanese Patent Application Laid-Open Publication Nos. 1996-167826 and 1999-274886 are configured such that electrostatic currents are passed to ground through a resistor or a coil, respectively, since the resistor or the coil is inserted in parallel with a surface acoustic wave element, the property of the surface acoustic wave filter is inevitably affected as is the above case for using the large-capacity varistor.
Also, the invention described in Japanese Patent Application Laid-Open Publication No.2001-168672 is configured such that leak discharges are easily generated by providing a sharp protrusion on a capacitive electrode. In such a configuration, once leak discharge occurs at the capacitive electrode, the protruding leading edge is melted and cut, and subsequent electrostatic discharges can not be induced.
Since mobile devices such as cellular phones are carried by a hand, it is difficult to block external static electricity. Moreover, even if a method is used for protecting the surface acoustic wave filter and the antenna as a circuit from static electricity with prior art described above, in the process when cellular phones are assembled by cellular phone manufacturers, the surface acoustic wave filters itself before mounted into the cellular phones does not have resistance to the electrostatic discharges.