The Modern Thermocouple 

The modern thermocouple, exemplified by the HP 8481A Power Sensor, has taken advantage of both semiconductor and thin-film technologies. A thin-film resistor, constructed of tantalum nitride and deposited on the surface of a silicon chip, converts the microwave energy to heat. The tantalum-nitride resistive material forms a low-reflection termination for the rf transmission line to frequencies above 18 GHz. Yet the resistor is not at all fragile in contrast to similar terminations constructed of a highly conductive material like most metals. It is largely because of this thin-film resistor that the thermocouple sensor has the lowest reflection coefficient of all the sensing methods. 

The 8481A thermocouple consists of a silicon chip with several other materials deposited on top as shown in Figure 3-3. Each chip actually contains two thermocouples but this discussion will concentrate on only one for a moment. The tantalum-nitride resistor, for converting the rf energy into heat, is deposited on top of the silicon. A silicon-dioxide insulating layer separates the resistor from the silicon. At one end of the resistor, the end near the center of the chip, the insulating layer has a hole through which the resistor is connected to the silicon. The other edge of the resistor and the far edge of the silicon chip have gold beam-lead contacts. The beam leads not only make electrical contact to the external circuits, but also provide mounting surfaces for attaching the chip to a substrate, and serve as thermal paths for conducting heat away from the chip. 
As the resistor converts the rf energy into heat, the center of the chip, which is very thin, gets hotter than the outside edge for two reasons. First. the shape of the resistor causes the current density and the heat generated to be largest at the chip center. Second, the outside edges of the chip are thick and well cooled by conduction through the beam leads. Thus there is a thermal gradient across the chip which gives rise to the thermoelectric emf. The hot junction is the resistor-silicon connection at the center of the chip. The cold junction is formed by the outside edges of the silicon chip and the resistor. The cold junction is separated to form the two contacts to external circuitry. 
The HP 8481A Power Sensor contains two identical thermocouples on one chip, electrically connected as in Figure 3-4. The thermocouples are connected in series