How does F-containing gas corrode silicon?

We know that XeF2 and SF6 containing F are both used as silicon corrosion gases, XeF2 is often used as isotropic silicon corrosion gas, and SF6 is often used with CF4 as silicon anisotropic corrosion gas, so can XeF2 and SF6 be replaced with each other?

The answer is no. The fundamental reason is that the reaction mechanism of the two is different. XeF2 is a fluorine-based compound that reacts spontaneously with silicon at room temperature at a rapid rate. In addition to isotropic corrosion of silicon, dry corrosion of Ge and SiGe can also be performed. The equation for the chemical reaction is:

2 XeF₂(g)+Si(s)→SiF₄(g)+2Xe(g)

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This reaction does not require external energy to increase the activation energy of the reaction, so XeF2 corrosive silicon has the characteristics of low cost. At the same time, the by-products of production, SiF4 and Xe, are volatile gases that are easily discharged. SF6 is also a fluorine-based compound, but it is almost impossible to spontaneously react with silicon at room temperature, so SF6 is ionized with the help of external radio frequency to obtain F* free radicals, and 4 F* free radicals combine with Si atoms to form volatile SiF4 by-products that leave the substrate surface and are discharged. The equation for the reaction between SF6 and Si is:

(1)SF→SF_x⁺+F^*+F^-+SF₆^*

(2)F^*+Si→SiF₄↑

This reaction requires the participation of radio frequency, not a pure chemical reaction, but a plasma physicochemical

reaction.

图1 XeF2和SF6分子结构示意图

Due to the different mechanisms by which XeF2 and SF6 etch silicon, there are also different options for choosing a masking layer. The etching selection ratio of XeF2 in pure chemical reactions with Si and SiO2 is greater than 1000:1, while the etching option of SF6 in RIE etching with Si and SiO2 is generally less than 100:1. This means that in the deep silicon process, XeF2 can choose the appropriate thickness of SiO2 as a single masking layer according to the selection ratio, while SF6 generally cannot be used as a single masking layer for DRIE, and needs to form a composite masking layer with photoresist.

Fig.2 Schematic diagram of isotropic and anisotropic corrosion of silicon

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XeF2 etching silicon is a pure chemical reaction, gas molecules can only react to produce corrosion through diffusion and adsorption on the silicon surface, so this reaction has a more obvious size effect, that is, the smaller the etching size, the slower the etching rate. Therefore, when designing the chip graphic structure, it should be noted that the line width and spacing in different areas are not too different.

At the same time, the optimal way for XeF2 etching silicon is to use pulse ventilation to quickly inflate and pump the etching chamber, so that the gas molecules have better penetration and can enter a smaller etching area, and the fast cycle can accelerate the discharge of by-products, thereby inhibiting the size effect.