High frequency fundamental resonators and filters fabricated by batch process using chemical etching

The small sized 1st intermediate frequency (IF) filters at center frequency range of 70 MHz to 150 MHz and passband widths of ±5 to ±100 kHz with sharp selectivity are required in mobile communication systems such as mobile and portable cellular phone. Our solution to employ fundamental mode monolithic crystal filter (MCF) assembled in surface mountable package. We describe here in detail, the design approach including batch process etching technology. Through photolithography, 56 patterns are chemically etched on one wafer (25 mm×20 mm). Then, a similar etching process automatically adjusts the wafer thickness in accordance with frequency. For the MCF, the frequency of split electrodes and the degree of coupling between them are automatically adjusted by an accurately positioned mask evaporation process controlled by a computer. Further, we describe suppression of the spurious response, technology for realizing wide bandwidth and high stopband attenuation characteristics. By our above developed technology, we achieved 90 MHz miniaturized IF filter with 1/15 volume reduction of conventional 3rd overtone mode MCF, still possessing the same characteristic of conventional one. Also, we achieved 130 MHz of middle band with suppression of spurious response in wide stopband frequency range, and 71 MHz linear phase wide ±88 kHz band with the group delay distortion 0.9 μs over f₀±80 kHz