Trench capacitors based dynamic random access memory (DRAM) cells is a useful approach in downsizing the memory chip area. This approach has the ability to preserve cell capacitance for storing charges (information bits) within a smaller memory cell area.
The following thesis focuses on the technology and modelling of the trench capacitors in DRAM cells. Major issues of the technology of trench capacitors are documented in Chapter 2, which include the fabrication and formation process, doping techniques, trench etching criteria, dielectric quality and breakdown voltage. In addition, various structures of the DRAM employing the trench capacitor technology are also reviewed. In Chapter 3, a description of DRAM cell operation and the theoretical point of view of modelling the trench capacitors will be talked upon.
In the next section, an approximation method to calculate the charge stored in the trench capacitors is presented. This method combines the numerical accuracy available from a two-dimensional semiconductor device simulator, called MEDICI, with the theoretical computations associated with closed form equations. Four types of trench capacitor cells (inversion store, diffusion store, substrate plate and stacked trench cells) are modelled and results obtained showed that under certain bias and substrate doping conditions, the approximation method produces results that are not demonstrated by the closed form equations. Therefore, the approximation method is useful for quick and accurate calculations of charge stored in the trench cells. Finally, a review of the overall reliability and technology of trench capacitors is presented in conclusion.