film ring a very uniform temperature distribution over substrate surface 25 e g a 6 u2033 silicon wafer can be achieved with a uniformity of better than a 1 k temperature difference the material of heating device 21 may be silicon which has a good thermal conductivity and is stable with respect to gaseous hydrogen fluoride as an alternative a teflon plate may also be used either carrying the heating elements or provided with a duct system through which hot water flows at a freely selectable temperature regulated by a second thermostat fig2 shows another possible embodiment of cover 20 of the device for carrying out the process according to the present invention silicon substrate 25 is not clamped overhead against a top heating plate 20 but instead is placed on a heated bottom substrate plate 27 which is connected to the top plate substrate plate 27 contains one of the heating devices 21 described previously cover 20 is heated by heating elements 21 to prevent condensation of hf gas the temperature of heating plate 27 carrying silicon substrate 25 is monitored and has a homogeneous distribution over the substrate surface heating plate 27 and cover plate 20 are connected by at least two hollow teflon webs 26 for example in the case of electric heating elements 21 the.