Evaluation parameters and application of dry etching
Evaluation parameters of RIE etching
In RIE etching process, there are several key parameters used to evaluate the etching effect and process quality. These parameters include etching rate, selection ratio, critical size (CD), size offset, and etching pattern Angle.
Etch Rate (ER)
Etching rate refers to the amount of material removed per unit time. In order to improve process efficiency, it is usually desired that the etching rate be as high as possible. However, too high an etching rate may affect the uniformity and accuracy of the etching, so it is necessary to find a balance between rate and accuracy.
Selectivity ratio
Selection ratio refers to the etching rate ratio between the etched target material and the mask or substrate. A higher selection ratio means less damage to the mask or substrate when removing the target material. Especially in the processing of grid structure, the control of selective ratio is particularly important because the oxide film is very thin.
Critical Dimension, CD
Key size refers to the size of the image after etching. The control of CD directly affects the electrical performance of the circuit, such as the threshold voltage of the transistor. In the manufacturing process, the change of CD must be strictly controlled to improve the performance and yield of the product.
The size offset is the difference between the size of the etched image and the size of the original mask. With precise etching control, the size offset can be controlled to a minimum to ensure the accuracy of the graphics.
The ideal etching pattern should have vertical sidewalls close to 90°. Excessive angles (chamfering) can lead to charge accumulation in subsequent processes, which can affect device performance, while reverse angles (inverse chamfering) can create shadowing areas, which can adversely affect subsequent ion implantation.
RIE technology can be used to etch a variety of semiconductor materials, including metals, insulators, and silicon-based materials. The etching process requirements of different materials are significantly different, so the selection of RIE etching gas and process parameters need to be optimized for specific materials.
Metal etching
In semiconductor manufacturing, metal etching is mainly used for the processing of interconnects. Common etched metal materials include aluminum, copper and tungsten. Taking aluminum as an example, chlorine gas (Cl₂) is commonly used to etch aluminum, and its reaction product is the volatile AlCl₃, which can be quickly removed. To improve the selection ratio, auxiliary gases such as SiCl₄ can be used to protect the mask while ensuring the etching rate.
Common insulation materials etched include silicon dioxide (SIO2) and silicon nitride (Si₃N₄). Silica is usually etched in fluoro based gases such as CF₄ and CHF ₄. The fluorine radical reacts with the silica to form the volatile SiF₄. Silicon nitride etching can be performed by mixing gases (CF₄ with O₂, etc.) to improve the selection ratio and etching rate.
The etching of silicon materials is mainly used to form the gate structure of the trench isolation or transistor. Monocralline silicon etching usually uses bromine based gases such as HBr. During etching, the bromine radical reacts with the silicon to form the volatile SiBr₄, ensuring the anisotropy and high selectivity of the etching. Polysilicon etching is widely used in gate etching, chlorine gas (Cl₂) and HBr gas mixture can increase etching speed and ensure good sidewall protection.
