Narrow and low-lossYBa2Cu3O7−&dgr;(YBCO) transmission lines for use in multichip modules have been attempted to be developed. 18-cm-long YBCO coplanar transmission lines with widths down to 5 &mgr;m were prepared by a composite patterning process combining Ar-ion milling and wet-etching. The transmission losses of the packaged 5-, 10-, and 25-&mgr;m-wide lines, respectively, were −1.57, −0.85, and −0.55 dB at 20 GHz and 55 K. These values provide similar surface resistances of 0.59–0.80 m&OHgr; at 20 GHz and 55 K. This indicates successful fabrication of a 5-&mgr;m-wide YBCO coplanar line without notable extrinsic loss increase resulting from process damage. The attenuation constants of these lines are approximately two orders of magnitude lower than for 10-&mgr;m-wide Cu microstrip lines. These results show that the YBCO coplanar lines with widths down to 5 &mgr;m have great potential for use in multi-chip modules. ©1997 American Institute of Physics.