may be formed by depositing an n type impurity doped polysilicon on substrate 100 fig2 a and 20b show the step of forming a trench isolation layer 116 a first oxide film 106 a polysilicon film 108 a second oxide film 110 and a nitride film 112 are sequentially formed on the resultant structure shown in fig1 the films 106 108 110 and 112 together serve as a mask layer for forming a trench isolation region thereafter the mask layer is etched by a lithographic process to form a window then the substrate 100 is deeply etched to below the drain region 102 using the mask layer as an etch mask to thereby form a first trench then for strengthening the electrical insulation between the devices p type impurity ions 113 are implanted through the window in the mask layer to thereby form a p impurity layer 114 beneath the first trench thereafter an insulating material e g an oxide is deposited on the entire surface of the resultant structure and is etched back to fill the interior of the first trench with the insulating material thereby forming trench isolation region 116 the insulating material is preferably etched back to first oxide film 106 fig2 a to 21c show the steps of forming a buried bit line 122 and fig2 b and 21c are cross sectional views taken along lines aa 39 and bb 39 in fig2 a respectively a predetermined portion of an active region between adjacent trench isolation regions 116 is etched through a suitable window.