Si has been etched with an electron cyclotron resonance source in a Cl-2 plasma to fabricate microsensors with high-aspect-ratio structures. Resonating elements in Si with an aspect ratio of 20 were generated. For these high-aspect-ratio sensing elements, the etch rate and profile strongly depend on feature size and etch depth. When the spacing between elements was increased from 2 to 50 mu m, the Si etch depth varied from 21.5 to 24.6 mu m for a 156 min etch. Typical etch conditions were 100 W microwave power, 100 W rf power, at 3 mTorr and 8 cm source distance. The Si etch rate decreased linearly from 156 to 119 nm/min as the trench aspect ratio varied from 2 to 34. The etch rate also decreased from 151 to 124 nm/min in 2-mu m-wide trenches and from 163 to 141 nm/min in 10-mu m-wide trenches as the pressure was increased from 3 to 30 mTorr. Etch profile was vertical at 3 mTorr and it became undercut at pressures greater than or equal to 10 mTorr. Larger undercut widths were found for trenches with narrower spacings. As microwave power was varied from 50 to 250 W, the etch rate increased from 88 to 206 nm/min for 2-mu m-wide openings and from 96 to 251 nm/min for 25-mu m-wide features. The amount of undercut increased with microwave power but decreased with trench opening width. The etch profile was also significantly influenced by wafer temperature. For a 2-mu m-wide opening with etch depth of 15 mu m, the Si etch profile was vertical for temperature less than or equal to 40 degrees C and had 1 mu m undercut width at 350 degrees C. Etch rate increased with temperature and trench width. Wafer clamping and He backside cooling were necessary to provide efficient temperature control. To achieve uniform etch depth and vertical etch profile for various feature sizes, low ion flux, low pressure, and controlled wafer temperature are required.