Effect of Conductor Surface Roughness and Geometry on Microwave Loss

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. INTRODUCTION Recent advancements in Low Temperature Co-fired Ceramic (LTCC) technology have made it increasingly a material of choice for high frequency packages and interconnects. The benefits of using LTCC technology include proven reliability, size reduction over standard hybrid circuits, superior electrical performance, ability to embed passive components, and better thermal management. Recent development of silver conductor system makes it possible to further advance the technology for low cost and high performance applications [1]. One of the key properties of interest to the packaging designers is the total material loss. LTCC metallizations are typical thick film conductors. Microwave loss typically comes from the following origins: ceramic, metal conductor, metal-ceramic interface, radiation, and surface roughness. There is a concern about the contribution to the microwave loss from the surface roughness. It has been well known that microwave propagates along the conductor surface (skin effect) [2]. For example, the skin depth of silver conductor is about 0.64 micron at 10 GHz. Therefore, surface roughness and conductor trace geometry (edges) may have significant effect on the microwave loss at high frequencies. This paper reports the attenuation of ring resonators built using a conventional silver conductor and a photoimageable silver conductor.