During the fabrication of microwave CVD diamond heat spreaders, the key process parameters that affect the thermal dissipation performance of the diamond thin films include:

1.Microwave Output Power
Mechanism: Increasing microwave output power significantly enhances the growth rate of the diamond thin film, allowing a thicker film to be deposited within the same growth duration.
Impact on Dissipation: The heat dissipation mechanism of diamond thin films is primarily lateral area-plane heat dissipation. When the film is too thin, it cannot achieve effective area-plane dissipation and instead exhibits a point heat dissipation mechanism typical of general materials, which fails to leverage diamond's thermal advantages. As the film thickness increases, the area-plane dissipation mechanism becomes prominent and stabilizes. Therefore, increasing microwave power improves fabrication efficiency to achieve the required thickness faster.

2.Reaction Pressure
Mechanism: Reaction pressure primarily determines film uniformity by affecting the size of the plasma ball. Under a constant microwave output power, lower reaction pressure decreases the gas density in the chamber, increases the electron mean free path, and thus enlarges the plasma ball.
Impact on Dissipation: Excessive pressure shrinks the plasma ball, preventing the intense plasma region from covering the entire substrate surface. This leads to poor film uniformity, where grain sizes at the center and edges differ significantly. Uneven grain sizes severely increase the internal thermal resistance of the film, compromising its thermal conductivity. Thus, lower reaction pressure helps maintain film uniformity and ensures excellent thermal dissipation.

3.Substrate Temperature
Mechanism: Substrate temperature is a critical factor influencing both thin film quality and deposition rate.
Impact on Dissipation: Within the deposition temperature range of 750°C to 950°C, lower substrate temperatures yield smaller grains and a darker overall film, indicating a higher content of non-diamond carbon phases (such as amorphous carbon) that hinder heat conduction. At higher substrate temperatures, the diamond grains are larger with intact crystal structures, and the Raman D/G intensity ratio (the ratio of the diamond D-peak at 1332 cm ⁻¹ to the amorphous carbon G-peak at 1550 cm ⁻¹) increases significantly. This indicates a reduction in non-diamond carbon phases and a substantial improvement in film quality. Higher temperatures also accelerate deposition, thereby reducing the device's surface temperature during operation.

4.Reactant Carbon Source Concentration
Mechanism: The carbon source concentration is controlled by adjusting the flow ratio of methane to hydrogen in the gas mixture.
Impact on Dissipation: Variations in carbon source concentration directly affect thermal performance. Although it was fixed at 2% in this study to isolate other variables, it remains a key parameter for tuning thermal conductivity.

5.Substrate Surface Pretreatment
Mechanism: Before growth, the substrate (such as a single-crystal silicon wafer) is pretreated by abrasive grinding with diamond polishing paste.
Impact on Dissipation: Grinding creates uniform and dense scratch defects on the substrate surface, which highly promote diamond nucleation and high-density growth. This establishes the foundation for achieving continuous, highly thermally conductive diamond thin films.