NOVELTY - A substrate support assembly (238) comprises thermally conductive body (224), substrate support surface (234), heater(s) (232), and at least two cooling channels. The substrate support surface is on the surface of the thermally conductive body and supports the large area substrate (240) on it. The heaters are embedded within the thermally conductive body. The cooling channels are embedded within the thermally conductive body to be coplanar with heaters.    USE - A substrate support assembly useful for supporting a large area substrates, e.g. for fabricating solar cell, solar panel, flat panel display (FPD), flexible display, organic light emitting diode (OLED) displays, flexible organic light emitting diode (FOLED) display, polymer light emitting diode (PLED) display, liquid crystal displays (LCD), organic thin film transistor, active matrix, passive matrix, top emission device, and/or bottom emission device (claimed) inside a process chamber. Can also be used for regulating temperature of substrate in process chamber and for processing solar cells.    ADVANTAGE - The assembly provides improved substrate support that improves temperature uniformity across the surface of substrate inside a process chamber. The assembly provides improve film properties.    DETAILED DESCRIPTION - Each cooling channel comprises at least two branched cooling passages. The cooling fluid is cooling gas, cooling liquid, water, clean dry air, compressed air, and/or cooling oil. The cooling fluid is flowed inside the cooling channels at 10-25degreesC. The cooling channels are formed by forging, welding, friction stir welding, explosive bounding, e-beam welding, and/or abrasion. The thermally conductive body comprises an aluminum material. The substrate support surface can support a large area substrate having a dimension of greater than or equal to370 mm x 470 mm. The fluid inside the channels is flowed at 100-200degreesC. The temperature of the large area substrate is maintained constantly by a combination of turning on/off the two power sources. INDEPENDENT CLAIMS are included for:    (1) large area substrate processing apparatus; and    (2) maintaining the temperature of a large area substrate inside a process chamber (202).    DESCRIPTION OF DRAWING(S) - The figure shows a cross sectional view of the process chamber with substrate support assembly.    Process chamber (202)    Thermally conductive body (224)    Heaters (232)    Substrate support surface (234)    Substrate support assembly (238)    Substrate (240)