A method of tuning the thermal conductivity of an electrostatic chuck (ESC) support assembly comprises measuring the temperature at a plurality of sites on a support assembly surface in which each site is associated with a given cell, determining from the measurements any fractional reduction in area suggested for each cell, and removing material from the support assembly surface within each cell in accordance with the suggested fractional reduction in order to decrease thermal conductivity in that cell. The material removal can result in an improvement to the equilibrium temperature uniformity of the electrostatic chuck support assembly at the chuck surface of an electrostatic chuck bonded to the support assembly surface, or can result in an equilibrium temperature profile of the ESC support assembly which approaches or achieves a target equilibrium temperature profile. Thermal conductivity tuning can thus take place by a method comprising defining a cell structure, determining the target areal density of each cell and removing a fractional area of material to achieve the target areal density for that cell. Material removal can be effected by drilling, routing, laser machining or grit blast machining on an X-Y table.