n-well design for trench DRAM arrays

The effects of n-well doping profile on the characteristics of SPT DRAM (substrate plate trench dynamic random access memory) data retention time are described and characterized. A retrograde n-well is shown to be desirable since it offers decreased well resistance without the modification of surface device characteristics. Retention time can be further improved with an n-well doping concentration that decreases monotically with increasing depth rather than a retrograde profile which has a doping peak below the silicon surface. An n-well with such a doping profile has superior median retention time and a reduced number of bits that fail a data retention test. This improvement has been demonstrated by the fabrication of 4-Mb SPT-cell DRAM arrays and test structures.<>