Patent ID: 8154070

Claim:
A nonvolatile semiconductor memory, comprising: a semiconductor substrate having a step member formed on a main surface thereof, the step member having an upper surface; a first well layer of a first conductivity type, formed on the upper surface of the step member; a control electrode formed over the step member via a gate oxide layer, which is formed on the first well layer; a first diffusion layer and a second diffusion layer, each being of a second conductivity type different from the first conductivity type, the first and second diffusion layers respectively being formed on first and second areas of the substrate main surface, the first and second areas being located at opposite sides of an area where the step member is formed; two second well layers, each being of the first conductivity type, one of the second well layers being formed on the substrate main surface between the first diffusion layer and the first well layer, and the other second well layer being formed on the substrate main surface between the second diffusion layer and the first well layer, wherein the first conductivity type impurities in either of the two second well layers has a density that is lighter than a density of first conductivity type impurities in the first well layer, and a first charge-storage multi-layer and a second charge-storage multi-layer, sandwiching the step member and the control electrode, each of the first charge-storage multi-layer and the second charge-storage multi-layer including a bottom oxide layer, a charge-storage film formed on the bottom oxide layer, a top oxide layer formed on the charge-storage film and a floating electrode formed on the top oxide layer, wherein the step member, the first well layer, the gate oxide layer, and the control electrode are layered in this order, and wherein each of the second well layers is generally L-shaped in cross-section, and has an elongated segment that extends substantially parallel to the main surface of the substrate and another elongated segment that extends substantially perpendicular to the main surface of the substrate.