26th EuMC * 9-12 September 1996 l Hotel Hilton Atrium * Prague * Czech Republic Low loss multilayer coplanar waveguide transmission lines on silicon substrate for MMICs D Budimir, I D Robertson, A H Khalid*, and AA Rezazadeh* Communications Research Group, *Physical Electronics Research Group, Department of Electronic and Electrical Engineerinlg, King's College, University of London, Strand, London WC2R 2LS, United Kingdom. Tel:+44 171 8732889. Fax:+44 171 8732446 email:budimir@orion.eee.kcl.ac.uk Abstract Multilayer V-shaped coplanar waveguide transmission lines on a Silicon Substrate fabricated by using MMIC technology are proposed and the performance is investigated experimentally and with electromagetic simulations. These structures can eliminate the problem of current crowding at the edges of the signal and ground conductors and reduce insertion loss. Introduction Multilayer MMIC techniques are now well established [1,2] and thin-film microstrip has been used extensively in a number of circuits. For reduced loss, V-shaped valley microstrip has been proposed as an improved transmission line structure for multilayer MMICs [3]. Ultra low impedance multilayer transmission lines have been shown to have important applications in matching networks [4,5]. In conventional MMICs these lines cannot be readily used. In CPW circuits a practical impedance limit is imposed by fabrication of the very narrow slot, and the current crowding at the edges of the conductors results in high losses. To reduce this problem this paper presents a new method of realising low loss CPW transmission lines on multilayer MMICs. In these multilayer CPW lines, the conductor gap width limitation and high current crowding are overcome by employing a V-shaped signal conductor, with ground planes which overlap on different metal levels. The cross section of the multilayer V-shaped CPW transmission line structures are shown in figure 1. The multilayer V-shaped CPW lines are fabricated on the upper face of a Si integrated chip. The full-wave analysis program, em was employed for electromagnetic simulation. Configuration The structures that have been considered are shown in cross section in Figure 1. These stuctures can eliminate the current concentration at the edges of the conductors and reduce insertion loss. Since the distance between the ground plane and the centre conductor can be individually controlled on 3 metal levels, the concentration of the current is dispersed along several conductor edges, i.e., the centre and both edges of the valley CPW conductor. Therefore, the degree of current spreading in the V-shaped CPW conductor becomes higher than in the conventional CPW conductor for equal conductor widths and the same characteristic impedances. Fabrication and Performance An experimental multilayer fabrication process for the realisation of the novel 3 dimensional passive structures [5] has been developed by the Physical Electronics Research Group, at King's College. With reference to Figure 1, the top section of the structure (M3) is made from 3 jm thick aluminium metalisation layer. The middle and bottom sections are made from 1 jim thick aluminium metalisation layers, M2 and Ml, respectively. Layers of polimide are employed for separation of the two metal layers M3/M2, M2/M1 and MI/Silicon substrate. The relative dielectric constant and the thickness of the polyimide are 3.4 and 2 jim, respectively. The relative dielectric constant and the thickness of the Silicon substrate are 11.9, 2000 Q/cm and 340 jim, respectively. The measured transmission loss of the fabricated multilayer V-shaped CPW lines are shown in Figure 2. The measured was made using a cascade Microtech wafer probe station and a HP8510B vector analyzer. A comparison of the measured and simulated S-parameters for V-shaped multilayer CPW line is plotted in Figure 3. As can be seen, a very good agreement is achieved, despite the highly three dimensional nature of the structure. The charactenstic impedance of these particular CPW multilayer transmission lines were 22 Q, 24 Q, and 25 Q . The experimental process is curently being refined for reduced passive component losses. Conclusion The coplanar waveguide structure has been investigated for multilayer MMICs in order to eliminate the current concentration at the edges of the conductors. These lines are very effective in reducing insertion loss. Experimental investigation and simulation of the performance of these lines have been presented. This kind of transmission line is expected to find application particularly in MMIC power amplifiers. Acknowledgement This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom. 697