Surface acoustic wave filer device having a cascade filter structure and a piezoelectric substrate having mirror-polished surfaces

A surface acoustic wave filter device comprising:

• a piezoelectric substrate having first and second surfaces opposite to each other, each of said first and second surfaces being mirror-polished;
• and
• first and second filter stages formed on the first surface of the piezoelectric substrate, respectively, each having a unidirectional input side transducer and a unidirectional output side transducer;
• said input side and output side transducers of the first and second filter stages being arranged so that surface acoustic waves transmitted from the input side transducers propagate toward the respective output side transducers in a predetermined common direction along respective propagation axes which are in parallel with each other;
• said input side and output side transducers of the first and second filter stages each including at least a positive electrode having a plurality of electrode fingers connected by a bus bar, and a negative electrode having a plurality of electrode fingers connected by a bus bar which are arranged between adjacent electrode fingers of the positive electrode, the positive electrode of the output side transducer of the first filter stage and the positive electrode of the input side transducer of the second filter stage being connected with each other;
• said electrode fingers and bus bars of each transducer being formed of an aluminum thin film, said bus bars having a thickness which is greater than the thickness of the electrode fingers.

A surface acoustic wave filter device comprising:

• a piezoelectric substrate having first and second surfaces opposite to each other, each of said first and second surfaces being mirror-polished;
• and
• a package containing the piezoelectric substrate therein and having four corners which are provided with first, second, third and fourth bonding pads, respectively;
• first and second filter stages formed on the first surface of the piezoelectric substrate, respectively, each having a unidirectional input side transducer and a unidirectional output side transducer;
• said input side and output side transducers of the first and second filter stages being arranged so that surface acoustic waves transmitted from the input side transducers propagate toward the respective output side transducers in a predetermined common direction along respective propagation axes which are in parallel with each other;
• said input side and output side transducers of the first and second filter stages each including at least a positive electrode having a plurality of electrode fingers, and a negative electrode having a plurality of electrode fingers which are arranged between adjacent electrode fingers of the positive electrode, the positive electrode of the output side transducer of the first filter stage and the positive electrode of the input side transducer of the second filter stage being connected with each other, said positive and negative electrodes of said input side and output side transducers of the first and second filter stages each including a bus bar extending in said predetermined direction and connecting the respective electrode fingers with each other;
• said bus bars of the negative electrodes of the input side and output side transducers of the first filter stage being connected to the first and second bonding pads of the package, respectively, and said bus bars of the negative electrodes of the input side and output side transducers of the second filter stage being connected to the third and fourth bonding pads of the package, respectively.

A surface acoustic wave filter device comprising:

• a piezoelectric substrate having first and second surfaces opposite to each other, each of said first and second surfaces being mirror-polished;
• and
• first and second filter stages formed on the first surface of the piezoelectric substrate, respectively, each having a unidirectional input side transducer and a unidirectional output side transducer;
• said input side and output side transducers of the first and second filter stages being arranged so that surface acoustic waves transmitted from the input side transducers propagate toward the respective output side transducers in a predetermined common direction along respective propagation axes which are in parallel with each other;
• said input side and output side transducers of the first and second filter stages each including at least a positive electrode having a plurality of electrode fingers connected by a bus bar, and a negative electrode having a plurality of electrode fingers connected by a bus bar which are arranged between adjacent electrode fingers of the positive electrode, the positive electrode of the output side transducer of the first filter stage and the positive electrode of the input side transducer of the second filter stage being connected with each other;
• said electrode fingers and bus bars of each transducer being formed of an aluminum thin film, the bus bar of the positive electrode of the output side transducer of the first filter stage and the bus bar of the positive electrode of the input side transducer of the second filter stage being connected with each other by a conductive pattern formed of an aluminum film on said first surface of the substrate, said bus bars and conductive pattern having a thickness which is greater than that of said electrode fingers.