In the production of semiconductor devices, silicon wafers must be strictly cleaned. Trace contamination can also cause device failure. The purpose of cleaning is to remove surface contamination impurities, including organic and inorganic substances. Some of these impurities exist on the surface of the silicon wafer in the form of atoms or ions, and some in the form of thin films or particles. Organic contamination includes photoresist, organic solvent residues, synthetic waxes, and grease or fibers from human contact with devices, tools, and utensils. Inorganic pollution includes heavy metal gold, copper, iron, chromium, etc., which seriously affects the minority carrier lifetime and surface conductance; alkali metals such as sodium cause serious leakage; particle pollution includes silicon slag, dust, bacteria, microorganisms, organic colloidal fibers, etc. , will lead to various defects. There are two ways to remove pollution: physical cleaning and chemical cleaning.
Classification
physical cleaning
There are three methods of physical cleaning. ①Brushing or scrubbing: It can remove particle contamination and most films that stick to the film. ②High pressure cleaning: The surface of the film is sprayed with liquid, and the pressure of the nozzle is as high as several hundred atmospheres. High-pressure cleaning relies on jetting, and the film is not prone to scratches and damage. However, high-pressure spraying will generate static electricity, which can be avoided by adjusting the distance and angle of the nozzle to the film or adding antistatic agent. ③Ultrasonic cleaning: Ultrasonic sound energy is introduced into the solution, and the contamination on the film is washed away by cavitation. However, it is more difficult to remove particles smaller than 1 micron from patterned wafers. Increase the frequency to the ultra-high frequency band, and the cleaning effect is better.
chemical cleaning
Chemical cleaning is to remove invisible pollution of atoms and ions. There are many methods, such as solvent extraction, pickling (sulfuric acid, nitric acid, aqua regia, various mixed acids, etc.) and plasma method. Among them, the hydrogen peroxide system cleaning method has good effect and little environmental pollution. The general method is to first clean the silicon wafer with an acid solution with a composition ratio of H2SO4:H2O2=5:1 or 4:1. The strong oxidizing property of the cleaning solution decomposes and removes the organic matter; after rinsing with ultrapure water, use an alkali with a composition ratio of H2O:H2O2:NH4OH=5:2:1 or 5:1:1 or 7:2:1 Due to the oxidation of H2O2 and the complexation of NH4OH, many metal ions form stable soluble complexes and dissolve in water; then use the composition ratio of H2O:H2O2:HCL=7:2:1 or 5 :2:1 acidic cleaning solution, due to the oxidation of H2O2, the dissolution of hydrochloric acid, and the complexation of chloride ions, many metals generate complex ions that dissolve in water, so as to achieve the purpose of cleaning.
Radiotracer atomic analysis and mass spectrometry analysis showed that the hydrogen peroxide system had the best cleaning effect, and all the chemical reagents H2O2, NH4OH, and HCl could be completely volatilized. When cleaning silicon wafers with H2SO4 and H2O2, sulfur atoms of about 2×1010 atoms per square centimeter will be left on the surface of silicon wafers, which can be completely removed by the latter acid cleaning solution. Using H2O2 system to clean silicon wafers has no residue and is less harmful, which is also beneficial to workers' health and environmental protection. After each step of cleaning solution in wafer cleaning, rinse thoroughly with ultrapure water.
