This article presents a study concerning a cryo-chuck device used in an inductively coupled plasma reactor for deep anisotropic etching of narrow trenches (2 mum wide) in silicon wafers of 5 in, in diameter. Thermal mechanisms in a mechanically clamped cryo-chuck system have been studied. First, wafer deformations have been measured in the chuck device. Deformations occur because of the helium backside pressure allowing thermal transfer to cool the wafer. In a second step, these deformations have been used in calculations considering the heat transfers in the substrate. In a last step, in situ temperature measurements have been made on wafers during a process using a LUXTRON (R) probe. This study shows the influence of the backside helium gap variation and the clamping ring temperature on the uniformity of substrate cooling.