Diamond has excellent physical, chemical, and electrical properties, such as a wide bandgap, high thermal conductivity, high hardness, high transmittance, and high stability. Therefore, diamond has a wide range of applications in electronic devices, optical components, biomedicine, and microelectromechanical systems. In high-power electronic devices, the high thermal conductivity of diamond can effectively dissipate heat, and bonding with gallium nitride substrates can improve the heat dissipation performance and reliability of gallium nitride power devices. Diamond itself also has a wide bandgap and high electron and hole mobility, making it highly promising for development in the field of power semiconductors. In the field of optics, diamond's high transmittance and high refractive index can be used to manufacture high-efficiency optical components such as waveguides, improving the efficiency and durability of optical systems. In addition, the high hardness and high temperature resistance of diamond make it advantageous in microelectromechanical systems (MEMS). MEMS technology is widely used in sensors and various micro mechanical devices, and currently the working temperature of manufactured sensors has exceeded 600 ℃. Due to the superior material properties of diamond, diamond based microelectromechanical systems can improve measurement accuracy and extend service life.

Plasma etching

These characteristics of diamond provide broad prospects for its various applications, but correspondingly, its processing difficulty is also more challenging compared to other materials. At present, the main etching methods for diamond include laser etching, wet etching, metal etching, and plasma etching. Among various etching methods, plasma etching has become the main means of diamond etching due to its high precision, low damage, and controllable process. It has been widely studied and applied in the fields of diamond device manufacturing and microfabrication. Plasma etching includes reactive ion etching, inductively coupled plasma etching, and electron cyclotron resonance etching. It is a technique that uses active particles in plasma to undergo physical and chemical reactions with material surfaces to achieve etching. Plasma is usually generated by gas excitation through radio frequency (RF) or microwave (MW), containing various active particles such as electrons, ions, and free radicals. Plasma etching involves both physical and chemical etching processes. Specifically, plasma bombards the surface of materials, sputtering material atoms from the surface. Free radicals in the plasma react chemically with the material surface to generate volatile gases, thereby achieving material removal.

Gas selection for diamond plasma etching

In the study of plasma etching of diamond, commonly used etching gases include oxygen (O2), hydrogen (H2), chlorine (Cl2), carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), argon (Ar), and their mixed gases. The mechanism of plasma etching diamond varies among different gases. The main reaction of oxygen etching diamond is the reaction between O and the surface C of diamond to generate CO or CO2 for etching diamond. Hydrogen gas reacts with H and C to generate CxHy for etching diamond, which includes various hydrocarbons. Due to the different etching rates between different hydrocarbons, anisotropic etching occurs in hydrogen plasma etching. Other etching gases such as (F, Cl2) mainly react with C to generate corresponding compounds (CF4, CCl4), which desorb from the surface to etch diamond. However, the mechanism of argon etching is different. Argon is an inert gas that does not undergo chemical reactions with carbon, and its etching effect is mainly achieved through physical sputtering. In addition to etching gas, changes in etching power and pressure can also lead to different etching effects. High power can generate high-energy ions and electrons, thereby increasing the etching rate, but it can also lead to a decrease in etching selectivity. Higher pressure usually increases the collision frequency of plasma and reduces their energy. Therefore, low pressure usually provides a higher etching rate.

In terms of mixed gas etching, hydrogen oxygen mixed gas etching combines the advantages of oxygen plasma and hydrogen plasma, which can significantly reduce surface roughness while achieving etching. It is widely used in diamond pretreatment due to its ability to remove damage caused by polishing. Other mixed gases besides hydrogen and oxygen, such as carbon tetrafluoride and oxygen (CF4/O2), argon and oxygen (Ar/O2), and argon and chlorine (Ar/Cl2), are commonly used for etching to optimize the etching rate and surface morphology. In CF4/O2 mixed gas etching, the etching rate is significantly improved under the same conditions compared to O2, and the addition of fluoride ions can effectively reduce micro mask effects and decrease roughness. The mixed gases of Ar/O2 and Ar/Cl2 combine the physical etching effect of Ar, and through the combination of physical and chemical etching, preferentially react with the carbon sputtered by Ar, reducing the preferential etching at the defect site, lowering the roughness, and increasing the etching rate.