We study wet chemical etching of the low-temperature grown GaAs (LT-GaAs) using the resistive mask produced with electron-beam lithography. Efficient etching of the mesa-structure (i. e. mesa) is developed allowing to control exact parameters of the mesa in the given areas of LT-GaAs. The technology includes an essential retreatment of the LT-GaAs surface before applying the resist to improve adhesion, as well as treatment in oxygen plasma after its development. Applying the proposed technology, we fabricated large-area photoconductive THz emitter with sophisticated topology of the mesa-structure. Keywords: optoelectronic source, terahertz radiation, wet chemical etching, mesa structure, semiconductors, electron-beam lithography.