Abstract: The support is rotated by a central drive shaft extending into a vacuum chamber in sealed relation thereto, with the central drive shaft having support arms secured thereto and each carrying a respective support head, each support head having several substrate holding plates rotatably mounted thereon and coupled together by a respective gear train for turning about their axes of rotation. Each support head is rotatable by a respective support head drive shaft coupled to the central drive shaft through gearing and carrying a driving gear forming part of the gear train. A respective fixed cam plate is secured to each support head and has a stop cooperable with two oppositely acting pawls mounted on the support head. A respective second cam plate is secured to the driving gear of each gear train, and includes cam portions cooperable with the pawls and cam portions cooperable with an abutment on the associated support head.