 Older CMOS technologies and non-MOS technologies commonly use isolation based on LOCOS.STI is created early during the semiconductor device fabrication process, before transistors are formed.The key steps of the STI process involve etching a pattern of trenches in the silicon, depositing one or more dielectric materials (such as silicon dioxide) to fill the trenches, and removing the excess dielectric using a technique such as chemical-mechanical planarization.