# Thermal Interaction and Cooling of Electronic Device with Chiplet 2.5D Integration
## 4 Near-Chip Cooling  
### Test Vehicle  
Based on the simulation results, a thermal test vehicle for 2.5D integration is designed and fabricated, and a thermal test chip (TTC) is utilized instead of a chiplet in the test vehicle. Depicted in Figure 11a is the schematic of the TTC, where the orange Pt serpentine wire is utilized as heater, with one end connected to the positive terminal and the other end connected to the negative terminal. Temperature measurement of the TTC is determined by measuring the linear relationship between Pt metal resistance and temperature. Pt resistance temperature detectors are manufactured using metal lift-off technology. First, platinum is deposited onto the chip by thin-film deposition. Subsequently, a serpentine pattern is created using photoresist, and then photolithographically exposed, developed, and etched. Finally, the photoresist is removed to complete the fabrication process. The resistance of metals is measured by the four-wire method. The two TTCs are at different dimensions, one with a dimension of 5 mm \(\times\) 5 mm and the other with a dimension of 4 mm \(\times\) 4 mm. To mitigate the thermal interaction between the chiplets, based on the FEM simulation findings, 2 mm is selected as the chiplet spacing in this experiment. As depicted in Figure 11b, the TTCs are bonded to the interposer via microbumps, while the substrate is bonded to the PCB via C4 bumps to enable voltage loading and resistance measurement. The heat sink is made of copper, as depicted in Figure 11c, the microchannel in the heat sink has a width of 0.5 mm and a depth of 1 mm, and the dimensions of the microchannel region are 22 mm \(\times\) 10.8 mm. To prevent the oxidation and corrosion of the copper, a nickel layer is electroplated on the surface of the heat sink. The heat sink, as the metal lid, is contacted with the TTC via the TIM and fixed on the pressboard, as depicted in Figure 11d.  
Figure 11: (**a**) TTC. (**b**) The schematic of chiplet 2.5D integration. (**c**) Heat sink and chiplet 2.5D integration. (**d**) Near-chip cooling test vehicle.  
The entire test closed-loop system comprises several key components: a water tank for storing deionized water as the coolant; valves for controlling the flow of the coolant; a flow meter (OMEGA FLR1004) for measuring coolant flow rate; a data acquisition unit (RIGOL M300) for collecting the Pt resistance values within the chip and the temperature of the coolant at the inlet and outlet of the cooling structure; a DC power supply (Faith FTP020-80-60) for providing heating power to the chip; K-type thermocouples (5 TC-TT-K-40-36) positioned at the inlet and outlet of the cooling structure to measure water temperature; and a plunger pump (PB-6L5M) for supplying coolant.  
To accurately characterize the temperature of the chiplet, the temperature-resistance curves of the temperature-measuring resistors in Figure 11a need to be corrected. The 2.5D integrated structure in Figure 11c is placed in a thermostat (temperature measurement accuracy 1.1 K) and the resistance values of chiplet1 and chiplet2 are measured at various temperatures to obtain the temperature-resistance curves, as depicted in Figure 12. The curves are fitted using a linear equation and the relationship between temperature and thermal resistance is expressed as:  
\[T_{1} =0.2975R_{1}-349.8 \tag{14}\] \[T_{2} =0.2530R_{2}-260.6 \tag{15}\]