Referring to FIGS. 5 and 6, it is understood that the disc-like rotor assembly 10 comprises the disc-like bottom plate 11 made of rigid material such as stainless steel and ceramics, two upper plates 12 and 13, and the chip 20 composed of the base plate 20a and the planar plate 20b clamped between the bottom plate 11 and the upper plates 12 and 13.