Pt/PZT/Pt and Pt/Barrier stack etches for MEMS devices in a dual frequency high density plasma reactor

Ion milling has been used in laboratory applications for patterning ferroelectric thin films and noble metal electrodes in Metal/Ferroelectric/Metal stacks. These MFM stacks are used to form several different families of MEMS devices: moving mirrors for optical signal switching applications, for example, utilize the piezoelectric properties of PZT; varactors, or other tunable circuit elements, depend on the dielectric nonlinearity of PZT and BST. The oxidizing environment encountered during the deposition of these ferroelectric films means that some material capable of resisting oxidation (platinum) or capable of forming an electrically conductive oxide (iridium or ruthenium) must be used as the metal electrode in any metal-ferroelectric-metal (MFM) stack. Its' corrosion resistance, electromigration resistance and compatibility with standard IC fabs also make platinum attractive as an interconnect in many other MEMS applications. The physical action of energetic ions (usually argon) can remove surface atoms even when the vapor pressure of the material(s) to be removed is negligibly small. However, when ion milling is used to pattern platinum the removal rate is low (similar to400Angstrom/min), the throughput is low, and the tendency is for the etched material to redeposit along the edge of the etch mask, creating veils, or fences, after the etch mask is removed. These residues, being electrically conductive, can lead to yield-limiting defects in finished devices. In this paper we report on MFM and interconnect stack etch results for MEMS applications from a dual frequency high density plasma etch reactor. Platinum and PZT etch rates greater than 1000Angstrom/min are possible in this reactor at moderate (80degreesC) wafer temperatures using photoresist masks. We can produce good etch profiles with no post-etch residue for MFM stacks like those used for a MEMS-based Atomic Force Microscopy application, for example, which employs a bottom platinum layer 1500Angstrom thick, 2800Angstrom of PZT, and a platinum top electrode of 1500Angstrom. We also present production data from a process for etching a platinum / titanium-tungsten (10%/90%) stack for a micromachined mirror device.