VHF Monolithic Crystal Filters Fabricated by Chemical Milling

Abstract

The fundamental frequency limitations imposed by conventional quartz wafer machining can be overcome by chemical milling. Using this process, two-pole, fundamental mode AT-cut monolithic filters have been fabricated at frequencies from 70 to 250 MHz. Four and six-pole tandem monolithic filters have also been constructed. Chemical milling allows the realization in an inductorless tructure of filter bandwidths previously unattainable at VHF. Capabilities and limitations are discussed. Introduction The introduction of dual-mode resonators in the '60's greatly advanced the VHF crystal filter art by simplifications resulting from the reduced number of components as compared with discreteresonator filters. Most importantly, it afforded improvements by eliminating the need for balanced transformers, which increase in difficulty with increasing frequency. Further restrictions remain, however, associated with the maximum fundamental frequency obtainable using conventional wafer lapping and polishing techniques. Since, at a given frequency and bandwidth, filter impedance is proportional to the third power of the overtone, increasing the realizable fundamental frequency greatly extends the capabilities of crystal filters. Vig, et a1 [ l , 21 developed the use of chemical etching for polishing quartz, and suggested that etching might be used to fabricate resonators having the ring-supported wafer structure proposed by Guttwein, Ballato, and Lukaszek [3] and others. The development of such chemically etched, or milled, VHF and UHF ring-supported resonators has been carried out in our organization [ 4 ] . The same process techniques have also been applied to the fabrication of monolithic two-pole filters at fundamental frequencies up to 250 MHz. Four and six-pole tandem monolithic filters have also been produced. This paper describes some of the filters which have been developed and discusses the capabilities and limitations of this approach. Two-Pole Fabrication Figure 1 shows the ringsupported wafer structure of a chemically milled two-pole monolithic filter. Aluminum electrodes were deposited using photofabricated aperture masks and electron-beam-gun evaporation. The electrode configuration shown is similar to that used by us for conventional VHF two-poles. An alternative arrangement, discussed below, was also employed. The two-pole devices were packaged in standard 3-lead holders dimensionally equivalent to type HC-l8/U except for a reduced height of 0.450 inch (11.4 mm). FIG. 1. RING-SUPPORTED MONOLITHIC FILTER WAFER As frequency and bandwidth increase, required electrode dimensions decrease. The use of conventional aperture masks to define electrode patterns is limited by aperture dimensional tolerances and by front-to--back mask registration errors. To alleviate the former to some degree and to essentially eliminate the effect of mis-registration, the novel electrode configuration shown in figure 2 was devised and used to fabricate the 252 MHz two-pole devices shown in the next section, A s can be seen, the lectrode dimensions for each resonator are independent of lateral mis-registration. The effects of electrode connecting tabs in these configurations can be approximated analytically and have been taken account of in design. Qualitative effects of tabs have been discussed previously [ 5 ] . L fiRa . . . . . . . 5 MILS (130p) FIG. 2. NOVEL ELECTRODE CONFIGURATION USED FOR 252 MHz TWO-POLE FILTER Results Table 1 summarizes the primary characteristics of seven representative filters. All seven use fundamental mode, AT-cut two-poles. Figures 3-12 show attenuation and delay responses. The responses are characterized by low insertion loss, reasonable unwanted mode response, and high ultimate attenuation. Natural impedances run from 470 to 1400 ohms.