Analysis of contamination - a must for ultraclean technology
Contamination analysis has become more and more important in recent years. The demands for purity of liquid chemicals, gases, wafers, and processing are increasing from generation to generation. Instead of requesting the same decrease for all impurities as is frequently done, see Table 1, a more specific procedure is necessary. The transfer of impurities from materials (liquid chemicals, gases, and equipment) onto wafers has to be measured by surface analysis and correlated to the impurities in the materials. This transfer depends on the properties of the wafer surface which have to be optimized. The concentration of impurities at the wafer surface or in a surface layer has to be correlated with devices properties and yield. After further processing then it will be possible to develop specifications which avoid an increase in quality of chemicals, gases, wafers, etc. without differentation thus making ultraclean processing affordable. Of cause in this context, cleaning is of utmost imp ortance, because it can help to clean up impurities after transfer, esp. in cases where the equipment is the cause of a contamination. When investigating the influence of contamination on devices, a clear distribution between the different types of devices is necessary. MOS devices are more sensitive against surface or interface properties whereas bipolar devices are more sensitive against volume properties. All these investigations are only possible by using contamination analysis as a key tool.