Electrochemically Assisted Maskless Selective Removal of Metal Layers for Three-Dimensional Micromachined SOI RF MEMS Transmission Lines and Devices

Abstract

Abstract-This paper presents a novel electrochemically assisted wet-etching method for maskless selective removal of metal layers. This method has been developed as the key process step for enabling the fabrication of low-loss 3-D micromachined silicon-oninsulator-based radio-frequency microelectromechanical systems transmission line components, consisting of a silicon core in the device layer covered by a gold metallization layer. For this application, the full-wafer sputtered metallization layer must be locally removed on the handle layer to guarantee for a well-defined and low-loss coplanar-waveguide propagation mode in the slots of the transmission line. It is not possible to use conventional photolithography or shadow masking. Gold areas to be etched are biased by a 1.2-V potential difference to a saturated calomel reference electrode in a NaCl(aq) solution. The measured etch rate of the proposed local electrochemically biased etching process is 520 nm/min, and no detectable etching was observed on unbiased areas even after a 1-h etch. The suitability of different adhesion layers has been investigated, and Ti-based adhesion layers were found to result in the highest yield. The new etching method has been successfully applied for the fabrication of transmission lines with integrated microswitches, lowering the insertion loss of the waveguide at 10 GHz from 1.3 to 0.3 dB/mm. The issue of unwanted thin metallic connections caused by secondary deposition during sputtering is discussed but found not to significantly affect the process yield. Finally, local removal of gold on isolated features even within the device layer is presented for locally removing the metallization on stoppers of laterally moving electrostatic actuators, to drastically reduce the mechanical wear on stopper tips.[ 2010-0326]Index Terms-Coplanar waveguide (CPW), radio-frequency (RF) microelectromechanical systems (MEMS), transmission lines.

Gold etching for microfabrication

“…A related technique for patterning thin metal films is electrochemical etching or 'deplating' [18,59,60]. In this approach, an external current or potential is used to drive the anodic dissolution process rather than relying on the action of oxidising agents.…”

“…Electrochemical etching of Au seed layers in thiourea solutions has also been investigated, but due to local variations in the etch rate, it was difficult to develop a stable and repeatable process [18]. More recently, selective removal of gold thin films in chloride media using electrochemical etching has also been used to successfully fabricate RF MEMS devices [60].…”

Microwave MEMS devices designed for process robustness and operational reliability

“…the states are maintained without applying any external actuation energy, resulting in true static zero-power-consumption. External voltage only needs to Transmission losses are lower in the 3D micromachined SOI RF MEMS transmission line due to much lower electrical field penetration in the substrate and a larger conductor volume for the signal current without skin-depth limitations [67]. Fig.…”

High-Directivity MEMS-Tunable Directional Couplers for 10–18-GHz Broadband Applications

“…The top metallization is achieved by highly directional e-beam evaporation of a 1 µm thick gold layer using 50 nm of titanium as adhesion layer, followed by a short wet etch-back in order to remove potential deposition traces on the sidewalls. Finally, the metal coating on the substrate and in the unwanted areas is removed by electrochemically assisted selective etching of gold in an electrically biased potassium iodide and sodium sulfite solution [27]. All the wet steps are followed by critical point drying step.…”