Principle analysis of ultra-low temperature plasma etching technology for plasma surface treatment

Principle analysis of ultra-low temperature plasma etching technology for plasma surface treatment:

Silicon structures with large aspect ratio, such as silicon groove, silicon through-hole, silicon vertebral array and silicon oxide groove, are mainly realized by the following two etching methods of plasma surface processor :1. Bosch etching process; 2. 2. Ultra-low temperature etching process.

Plasma surface treatment machine, Bosch Deep Reactive ion Etching (Bosch Deep Reactive lon Etching, Bosch DRIE) process happened at room temperature, using C4F8 produce protective layer and SF6 gas plasma isotropic Etching, alternative action to generate anisotropic great length-width ratio structure graphics, in the process of the structure of the side walls will be fanned wrinkles. This fan fold is caused by the lateral etching component produced by SF6 plasma etching at room temperature.

In the ultra-low temperature deep reactive ion etching process of plasma surface processor, the protective layer of by-products produced by O2 continuous plasma etching and SF6 plasma etching below -100℃ are used to form smooth structure graphic interval with large depth-width ratio. The main mechanism of low-temperature etching process is to independently control the etching reaction at the bottom and the side wall of the silicon groove, and realize higher silicon etching rate and higher silicon to photoresist etching selection ratio by changing the cathode voltage and reducing the temperature of silicon wafer substrate.

Cold plasma etching process, there exists a byproduct and polymer residues attached on the side wall of the graphics, so as to prevent further etching process, at the same time these by-products deposits on the inner surface of the etching chamber, has affected the further reaction of the surrounding environment, so that the etching rate change as the reaction time, lead to the whole process of etching extremely unstable, even might have an etched end phenomenon. So in the process of etching at room temperature, extra plasma cleaning steps have to be added. It is generally cleaned with O2 plasma to remove the by-products and polymer residues from the etching environment. For the plasma surface processor, the ultra-low temperature plasma etching overcomes this problem from the fundamental principle. In the ultra-low temperature etching process, the silicon wafer or graphical silicon substrate will be cooled to about -100℃, and then SF6/O2 plasma etching will be applied. Some inorganic byproducts containing SiOxFy are adsorbed and form the protective layer of the graphic side wall. When the reaction heats up to room temperature, these byproducts will be adsorbed under the condition of ion bombardment. Therefore, after the plasma surface processor etching, the graphics side wall and the etching cavity side wall will be self-cleaned. In addition, due to the simultaneous etching and side wall adsorption protection steps, the side walls of the feature pattern will become quite smooth. This simultaneous etching and guarding step also speeds up the etching process. Therefore, the process of SF6/O2 continuous plasma etching silicon substrate under ultra-low temperature is called standard ultra-low temperature process.

Precise control of the protective layer containing SiOxFy inorganic by-products forming the graphic side wall will be a key step in the standard ultra-low temperature etching process. Firstly, the content of O2 in SF6/O2 continuous plasma of the plasma surface processor needs to be controlled, so that the protective layer of the by-product can not only protect the graphic side wall, but also enable further plasma etching to take place at the bottom of the trench.

The etching rate comparison shows that the etching rate of photoresist and silicon oxide decreases with the decrease of temperature, especially below -100℃. However, the etching rate of silicon increases to a certain extent when the temperature is lower than -100℃, thus significantly increasing the etching selection ratio of silicon etching to silicon oxide and photoresist. Furthermore, more obvious anisotropic etching characteristics can be realized at temperatures below -100℃.

Therefore, the cathode temperature of plasma etching reaction of plasma surface processor can be lower than -100℃ as the standard of ultra-low temperature etching, and it can also be used as the starting point of subsequent process development and optimization. However, the low temperature process alone does not guarantee good anisotropic etching characteristics, even in the plasma etching process below -120℃. Isotropic phenomena also occur from time to time. So side wall protection plays an important role in reducing lateral etching. True anisotropic etching can only be achieved after the addition of a lateral wall covering to produce gaseous oxygen. In addition, it has been reported that ion bombardment is the decisive factor in many low-temperature plasma etching processes, mainly because reducing the reaction temperature can effectively slow down the surface chemical reaction.

In the low temperature reaction of plasma surface processor, SF6/O2 continuous plasma etching of silicon substrate is called the standard ultra-low temperature process, so as to realize the parallel etching and protection. The macromolecular film of the protective layer mainly contains SiOxFy inorganic compounds. In plasma surface treatment machine plasma etching process, compared with polymer layer containing fluorine, SiOxFy inorganic compound film harder to etching, which needs higher ion bombardment energy in the process of etching to remove silicon groove at the bottom of the protective layer, and avoid obvious lateral etching, more vertical side walls of the structure can be formed. Although higher ion bombardment energy will further increase the etching rate of the photoresist, at very low temperatures (below -100 ° C) the etching rate of the photoresist can be reduced to almost negligible, thus offsetting the effect of increased bombardment energy. An important advantage of low temperature etching is that the structure etched by the PLASMA surface processor has a very low side wall surface roughness.

In addition to improving the etching anisotropy to control the critical size, the research also found that the ultra-low temperature etching process of plasma surface processor can effectively improve the microscopic uniformity and reduce the loading effect. At high temperatures, the etching of large samples of key size (CD) was observed to be slower than the Loading Effect of smaller samples. At this point, the etching rate is limited by the ability of the etched reactants to transport to the surface of the chip. In this state, both the microscopic uniformity and the macroscopic uniformity are poor. With the decrease of temperature, the etching rate is limited by the surface environment due to the formation of residues including semiconductor elements, etching gas and residual background gas on the surface.