SF6, or sulfur hexafluoride, is a gas commonly used in dry etching, mainly used for silicon etching in the semiconductor production process. The SF6 gas has an octahedral structure, consisting of a central sulfur atom surrounded by six fluorine atoms. Its non-polar properties make it an insulating gas in high-voltage electrical equipment. The physical properties of SF6 include colorless, odorless, non-flammable, non-toxic, insulating, heavier than air, cooling capacity, high dielectric strength, thermal stability, and poor solubility in water but soluble in non-polar organic solvents.
In terms of chemical properties, SF6 hardly reacts with other substances at room temperature, but will decompose under strong ultraviolet light.
SF6 is generally produced by industry and its content in nature is very small. The reaction equation for SF6 production is:
2 CoF₃ + SF₄ + [Br₂] → SF₆ + 2 CoF₂ + [Br₂]
When SF₄, CoF₃ and Br₂ are mixed together and heated at 100°C, a reaction occurs between them. In this reaction, a part of SF₄ and CoF₃ reacts to produce SF₆ and CoF₂. Bromine is not consumed in the reaction, it only acts as a catalyst.
Is SF6 dangerous?
Although SF6 is non-toxic in its pure state, it will displace oxygen from the air, and a volume concentration greater than 19% in the air will cause suffocation. Therefore, it is very important to detect SF6 leaks in a timely manner and take appropriate preventive measures in the semiconductor manufacturing process.
SF6 use in semiconductor Industry
SF6 is widely used in semiconductor manufacturing. In the silicon etching process, SF6 is used as the main etching gas, and it works together with the generated volatile gases SF4 and C4F8 to achieve deep silicon etching. In addition, SF6 is often used for dry etching of Mo and W metals, reacting with these metals to generate volatile hexafluorides MoF₆ and WF₆. Although SF6 is not the preferred gas for aluminum etching, it can be used as an auxiliary gas to enhance the etching rate of aluminum when mixed with gases such as Cl₂.
Silicon Etching
In the silicon etching step, only the silicon at the bottom where the passivation film has been removed is etched. SF6 gas is introduced, and SF6 is dissociated in the plasma to generate a variety of decomposition products, including highly active fluorine atoms (F).
SF6-->SF4+F2-->SF2+2F2-->F+...
The generated fluorine atoms react with the silicon surface to generate silicon tetrafluoride (SiF4), a volatile compound that is easily discharged from the chamber.
Si+4F-->SiF4
Bottom passivation layer etching
At this stage, a passivation layer is formed on both the sidewall and the bottom. However, we only want to keep the passivation layer on the sidewall to protect the sidewall from being etched, but we need to remove the passivation layer on the bottom in order to etch downwards. Therefore, SF6 gas will be introduced at this time to attack the passivation layer on the bottom. After the passivation layer on the bottom disappears, SF6 continues to etch the silicon, and the cycle repeats itself, like a never-ending loop.
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