Purity is one of the main performance indexes of target, because the purity of target has a great influence on the performance of the film. However, in practical applications, the purity requirements of target materials are not the same. For example, with the rapid development of the microelectronics industry, the size of silicon wafer has been developed from 6 ", 8 "to 12", and the wiring width has been reduced from 0.5um to 0.25um,0.18um and even 0.13um. In the past, the target purity of 99.995% can meet the technological requirements of 0.35umic, while the target purity of 0.18um line is 99.999% or even 99.9999%.
Impurities in target solids and oxygen and water vapor in pores are the main sources of pollution. Different target materials have different requirements for different impurity content. For example, pure aluminum and aluminum alloy targets used in the semiconductor industry have special requirements for alkali metal content and radioactive element content.
In order to reduce the porosity in the solid of target and improve the performance of sputtering films, high density of target is usually required. The density of the target affects not only the sputtering rate but also the electrical and optical properties of the film. The higher the density of the target, the better the performance of the film. In addition, increasing the density and strength of the target makes it better withstand the thermal stress during sputtering. Density is also one of the key performance indexes of target material.
Grain size and grain size distribution
The target material is usually polycrystalline, with grain sizes ranging from microns to millimeters. For the same target, the sputtering rate of the target with small grains is faster than that of the target with large grains. The thin films deposited by target sputtering with smaller grain size difference (uniform distribution) have more uniform thickness distribution.
