A support (1) is attached fixedly to the center of a glass substrate (5). The support (1) is surrounded at a predetermined distance by a weight (2) in the form of a square frame. The weight (2) is supported movably on the support (1) by thin beams (11-18), which extend along the sides of the support (1). The support (1), the weight (2), and the beams (11-18) are formed of a silicon substrate.
