method and apparatus for optically testing an operating integrated circuit that actively control the operating temperature of the dut . this is chiefly useful with flip - chip packaged ics . the temperature of the dut varies with its operating power consumption , and this fluctuation in temperature adversely affects the results obtained during optical probing or other optical testing . furthermore , the dut may be damaged if its temperature exceeds design limits . the temperature of the dut is controlled by thermally contacting the exposed backside surface of the dut die to a diamond film heat conductor , an associated heat sink structure , and at least one thermoelectric device . the thermoelectric device is controlled by a temperature sensor proximal to the dut . by controlling the amount and direction of the electrical current supplied to the thermoelectric device in response to the sensed temperature , the temperature of the dut is maintained .