Effect of conductor surface roughness and geometry on microwave loss
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Low Temperature Co-fired Ceramic (LTCC) technology is being used for various high frequency applications such as T/R modules for radar, bluetooth communication modules, LAN switches, and automotive radar. One of the key properties of the electronic packages for these applications is microwave loss. LTCC metallizations are typical thick film conductors. Conventional thick film technology has relatively low resolution, poor edge definition, and rough conductor surface. There is a concern about the magnitude of microwave losses resulting from surface roughness and line geometry of the conductor. Photoimageable technology offers fine line (such as 25 micron lines and spaces), better edge definition, and smoother surface. However, the trade-off is that photoimageable process involves several additional manufacturing steps. An understanding of possibilities and limitations of photoimageable technique and improved conventional screen printing methods is helpful for appropriate selection of microwave circuits manufacturing methods. This paper describes effect of surface roughness of conductor on microwave losses at frequencies < 25 GHz. The insertion loss of a conventional thick film silver conductor was compared to that of a photoimageable silver conductor using the ring resonator configuration and 96% alumina substrate. It is shown that, in the frequency range of 1 to 25 GHz, the insertion loss of photoimageable silver is slightly better than that of conventional screen-printed silver conductor using the ring resonator setup. The implication of this finding for LTCC is discussed. It is proposed that conventional screen-printing may be adequate for some microwave circuits that can tolerate >30 micron lines and spaces and operates below 25 GHz.
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