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To examine and evaluate for providing the guidelines of insulation design in Micro-Electro-Mechanical Systems (MEMS) devices, the samples prepared by etching insulation gaps measured from 5 to 40 μm were investigated in this paper. The pre-breakdown current-voltage curves and relationships between breakdown voltages and gap widths were obtained on test samples fabricated with aluminum film electrodes in standard MEMS processes. Combined with observation of electrode surface by scanning electron microscopy (SEM), breakdown mechanism of MEMS planar electrode structures was discussed. Results show that the pre-breakdown current lies between 1 nA and 100 nA, and Fowler-Nordheim plot verifies that field emission don't dominate the breakdown process, which don't agree with many studies. Instead Townsend avalanche mechanism could still be used to explain the gap breakdown in the range of 5 μm to 40 μm, but the minimum breakdown voltages need to be modified for micro scale electrodes. SEM analysis shows that the breakdown damage is crater-like and anode surface suffers more damage. Paschen's curves for describing the electric breakdown phenomena of gaseous dielectrics needs to be re-examined for designs and operations of MEMS to avoid or to encourage breakdown.