Semiconductor device and method of fabricating the same

A semiconductor device having first and second transistor regions and a field region, the device comprising:

• a semiconductor substrate having a first type conductivity;
• a first trench in the substrate at the field region separating the first and second transistor regions from each other;
• a second trench in the substrate over the first trench;
• a first field oxide layer in the first trench;
• a second field oxide layer in the second trench over the first field oxide layer;
• third and fourth trenches at the first and second transistor regions of the second field oxide layer;
• first and second gate oxide layers between the second and third trenches and the second and fourth trenches, wherein the first and second gate oxide layers have thicknesses different from each other;
• first and second gates in the third and fourth trenches at the first and second transistor regions, respectively;
• first and second impurity regions of a second conductivity type outside the second trench on the substrate and on the bottom surface of the second trench;
• and
• third and fourth impurity regions of a second type conductivity on the bottom surface of the second trench and outside the second trench.

A semiconductor device having first and second transistor regions and a field region, the device comprising:

• a semiconductor substrate having a first type conductivity;
• a first trench in the substrate at the field region separating the first and second transistor regions from each other;
• a first field oxide layer in the first trench;
• a well region having a second type conductivity at the first transistor region;
• a second trench over the first trench in the substrate, the second trench having a width greater than the first trench;
• a second field oxide layer in the second trench over the first field oxide layer;
• third and fourth trenches in the second field oxide layer;
• first and second gate oxide layers between sides of the second trench and the third and fourth trenches, wherein the first and second gate oxide layers have thicknesses different from each other;
• first and second gates in the third and fourth trenches, respectively;
• first and second impurity regions of a second type conductivity outside the second trench on the substrate and on the bottom surface of the second trench;
• and
• third and fourth impurity regions of a second type conductivity on the bottom surface of the second trench and outside the second trench.

A method of fabricating a semiconductor device having first and second transistor regions and a field region, the method comprising the steps of:

• forming a first trench in the field area of a semiconductor substrate, thereby defining the first and second transistor regions;
• forming a second trench over the first trench in the semiconductor substrate;
• forming a first field oxide layer in the first trench;
• forming first, second, third and fourth impurity regions outside the second trench and on the bottom surface of the second trench except where the first field oxide layer is formed;
• forming a second field oxide layer in the second trench over the second and third impurity regions including the first field oxide layer;
• forming third and fourth trenches in the second field oxide layer, so as to define first and second gate oxide layers;
• and
• forming first and second gates in the third and fourth trenches.