We have used electron beam lithography to make very small (<30 nm linewidth) mechanical structures in single-crystal silicon. These structures can be driven capacitively by applying a voltage between the suspended portion and the underlying substrate. Optical interference techniques are used to detect and measure the motion of the structures with resonant frequencies above 40 MHz. We employed a design consisting of a square mesh with a 315 nm period, which results in a low mass (similar to 1 x 10(-13) g) and large relative surface area (10(-6) cm(2)). Also, by making suboptical-wavelength features, the optical properties can be altered, leading to an improved measurement sensitivity. We measured the oscillations at small amplitudes where the detected change in the optical reflection is proportional to the drive amplitude.