A substrate carrier is described that uses a proportional thermal fluid delivery system In one example the apparatus includes a heat exchanger to provide a thermal fluid to a fluid channel of a substrate carrier and to receive the thermal fluid from the fluid channel, the thermal fluid in the fluid channel to control the temperature of the carrier during substrate processing. A proportional valve controls the rate of flow of thermal fluid from the heat exchanger to the fluid channel, A temperature controller receives a measured temperature from a thermal sensor of the carrier and controls the proportional valve in response to the measured temperature to adjust the rate of flow.