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This paper analyzes the bias sources of mechanical and electrostatic origin in a tuning fork microelectromechanical systems (MEMS) gyroscope. In a gyroscope which is symmetrical and has no defect, there would be no bias; technological defects should be investigated in order to identify and quantify bias sources, which arise from quadrature and in-phase errors. Technological dispersions within the etching step of the mechanical structure are a major source of errors. Dispersions at the scale of a single MEMS particularly matter by impacting springs and electrodes. Design rules are defined for a z-axis gyroscope in order to curb these errors: In particular, specific drive springs, which are very stiff in the sense direction, and a mechanical decoupling between drive and sense are indicated to decrease errors. Local dispersion of the width of beams has been assessed with a network of electrical test structures and leads to a local standard deviation of 3-6 nm at the scale of a MEMS gyroscope device. The optimized design enables to obtain a total error of less than 300°/s on very compact z-axis gyroscopes with an area of mechanical part of 0.5 mm2.