Wafer carrier arranged to hold a plurality wafers and to inject a fill gas into gaps between the wafers and the wafer carrier for enhanced heat transfer and to promote uniform temperature of the wafers. The apparatus is arranged to vary the composition, flow rate, or both of the fill gas so as to counteract undesired patterns of temperature non-uniformity of the wafers. In various embodiments, the wafer carrier utilizes at least one plenum structure contained within the wafer carrier to source a plurality of weep holes for passing a fill gas into the wafer retention pockets of the wafer carrier. The plenum(s) promote the uniformity of the flow, thus providing efficient heat transfer and enhanced uniformity of wafer temperatures.