Feasibility studies on the planar millimeter-wave cavity structures have been made. The structures could be used for linear accelerators, free electron lasers, mm-wave amplifiers. or mm-wave undulators. The cavity structures are intended to be manufactured by using DXL (deep x-ray lithography) microfabrication technology. The frequency of operation can be about ZOGHz to 300GHz. For most applications, a complete structure consists of two identical planar half structures put together face-to-face. Construction and properties of the constant gradient structures that have been investigated so far will be discussed. These cavity structures have been designed for 120GHz 2d3-mode operation.Fl E E I v ED FEE? 0 8 19% 0 s T J