cvd chemical vapor deposition technique because of the good uniformity and step coverage of tetraethoxysilane teos it can be used for forming the dielectric layer 34 teos can be generated at temperatures of 650 750 u00b0 c and at low pressures by decomposing si c 2 h 5 o 4 with or without oxygen teos could be replaced by borosilicate glass bsg phosphosilicate glass psg or borophosphosilicate glass bpsg wherein bsg is produced by oxidizing b 2 h 6 diborane and sih 4 psg is produced by oxidizing sih 4 and ph 3 phosphine thereafter a silicon nitride layer 36 of 300 2000 u00e5 thickness is deposited over the surface of the dielectric layer 34 by conventional lpcvd low pressure chemical vapor deposition or pecvd plasma enhanced chemical vapor deposition technique as a pad layer which acts as a stop layer for the etching process a mask pattern of a photoresist layer 38 is formed on the silicon nitride layer 36 by using the conventional photographic and photoresist film techniques to form a void region wherein the void region of the photoresist layer 38 is positioned above the source region of the mosfet referring to fig4 the silicon nitride layer 36 and the dielectric layer are etched through the void region of the photoresist layer 38 to expose the surface of the substrate 30 thereby forming a contact window 39 referring to fig5 after stripping the photoresist layer 38 a pair of polysilicon layers 40 and 42 of approximately.