Purity is one of the main performance indicators of the target, because the purity of the target has a great influence on the performance of the film. However, in practical applications, the purity requirements of the target are not the same. For example, with the rapid development of the microelectronics industry, the silicon wafer size has increased from 6”, 8” to 12”, and the wiring width has been reduced from 0.5um to 0.25um, 0.18um or even 0.13um, and the previous 99.995% target purity. It can meet the process requirements of 0.35umIC, while the preparation of 0.18um line requires 99.999% or even 99.9999% for target purity.
Impurities in the target solids and oxygen and moisture in the pores are the main sources of contamination of the deposited film. Targets for different applications have different requirements for different impurity contents. For example, pure aluminum and aluminum alloy targets for the semiconductor industry have special requirements for alkali metal content and radioactive element content.
In order to reduce pores in the solids of the target and improve the properties of the sputtered film, the target is usually required to have a higher density. The density of the target not only affects the sputtering rate, but also affects the electrical and optical properties of the film. The higher the target density, the better the performance of the film. In addition, increasing the density and strength of the target allows the target to better withstand the thermal stresses during sputtering. Density is also one of the key performance indicators of the target.
Grain size and grain size distribution
Typically the target is a polycrystalline structure with a grain size on the order of microns to millimeters. For the same target, the sputtering rate of the fine-grained target is faster than that of the coarse-grained target; and the thickness of the target sputter-deposited film with a smaller difference in grain size (distributed uniformly) is more uniform.