Semiconductor memory device including multi-layer gate structure

A semiconductor memory device comprising:

• a memory cell unit including at least one memory cell transistor, the memory cell transistor including:
• first and second semiconductor layers with a first conductivity which are formed apart from each other in a surface of a third semiconductor layer with a second conductivity opposite to the first conductivity;
• a multi-layer gate electrode which is formed on the third semiconductor layer between the first and second semiconductor layers with a first gate insulating film interposed therebetween, the multi-layer gate electrode including a charge accumulation layer and a control gate;
• and
• a first insulating film formed on the first and second semiconductor layers;
• a select transistor which selects the memory cell unit, the select transistor including:
• fourth and fifth semiconductor layers with the first conductivity which are formed apart from each other in a surface of the third semiconductor layer, the fourth semiconductor layer being connected to the first semiconductor layer;
• a first gate electrode which is formed on the third semiconductor layer between the fourth and fifth semiconductor layers with a second gate insulating film interposed therebetween;
• and
• a second insulating film formed on the fourth semiconductor layer, an interface between the fourth semiconductor layer and second insulating film being on a same plane as a plane of interfaces between the third semiconductor layer and second gate insulating film;
• a first contact plug which is formed on the fifth semiconductor layer, which has an interface with the fifth semiconductor layer, at least a part of the interface being positioned low to have a first stepped portion with respect to the interface between the third semiconductor layer and second gate insulating film, the first contact plug being electrically connected to one of bit and source lines;
• a memory cell array in which a plurality of memory cell units and select transistors are formed;
• a peripheral circuit transistor including sixth and seventh semiconductor layer with the first conductivity formed apart from each other in the surface of the third semiconductor layer, a second gate electrode formed on the third semiconductor layer between the sixth and seventh semiconductor layers with a third gate insulating film interposed therebetween, and a third insulating film formed on the sixth semiconductor layer;
• and
• a second contact plug which is formed on the seventh semiconductor layer, which has an interface with the seventh semiconductor layer, at least a part of the interface being positioned low to have a second stepped portion with a height equal to a height of the first stepped portion with respect to the interface between the third semiconductor layer and third gate insulating film.

A semiconductor memory device comprising:

• a first select transistor formed on a side of an upper surface of a substrate and having a first multi-layer gate;
• a first stepped portion formed by etching the substrate adjacent to the first multilayer gate of the first select transistor such that the first stepped portion forms a cavity in the upper surface of said substrate;
• a first contact plug formed in the first stepped portion;
• a second select transistor formed on a side of the upper surface of the substrate opposite the side of the first select transistor and having a second multi-layer gate;
• a second stepped portion formed by etching the substrate adjacent to the second multi-layer gate of the second select transistor such that the second stepped portion forms a cavity in the upper surface of said substrate;
• a second contact plug formed in the second stepped portion;
• a memory cell transistor formed between the first select transistor and the second select transistor, the memory cell including an third multi-layer gate formed on the upper surface of the substrate and a first impurity diffusion layer formed in the substrate;
• a peripheral circuit region formed on the upper surface of the substrate, including a gate electrode and a second impurity diffusion layer;
• a third stepped portion formed by etching the substrate adjacent to the gate electrode of the peripheral region such that the third stepped portion forms a cavity in the upper surface of said substrate in which at least part of the second impurity diffusion layer is located;
• and
• a third contact plug formed in the third stepped portion.