Method of manufacturing semiconductor device

A method of manufacturing a semiconductor device comprising the steps of:

• (a) providing a semiconductor substrate having a first semiconductor region of first conductivity type in a surface;
• (b) forming a first gate electrode over the semiconductor substrate through a first insulating film;
• (c) forming a second gate electrode adjacent to a first sidewall of the first gate electrode through a second insulating film containing a charge accumulation area, over the semiconductor substrate through the second insulating film;
• (d) forming a second semiconductor region of the first conductivity type in an upper surface of the semiconductor substrate in a first region adjacent to a second sidewall opposite to the first sidewall of the first gate electrode;
• (e) forming a first diffusion layer in the upper surface of the semiconductor substrate in the first region by implanting impurities of second conductivity type different from the first conductivity type;
• (f) forming a second diffusion layer having a larger depth than the first diffusion layer, in the upper surface of the semiconductor substrate in a second region adjacent to a third sidewall of the second gate electrode opposite to the first gate electrode by implanting impurities of the second conductivity type;
• (g) heating the semiconductor substrate after whichever step is last taken among the step (d), the step (e), and the step (f);
• (c1) after the step (c), forming a third semiconductor region in the upper surface of the semiconductor substrate in the first region by implanting impurities of the second conductivity type before whichever step is taken earlier, either the step (e) or the step (f);
• and
• (c2) after the step (c), forming a fourth semiconductor region in the upper surface of the semiconductor substrate in the second region by implanting impurities of the second conductivity type before whichever step is taken earlier, either the step (e) or the step (f),
• wherein a drain region including the first diffusion layer, a source region including the second diffusion layer, the first gate electrode and the second gate electrode constitute a memory cell of a nonvolatile memory,
• wherein in the step (g), a first diffusion region of the second conductivity type covering the first diffusion layer and a second diffusion region of the second conductivity type covering the second diffusion layer are formed in the semiconductor substrate by diffusing impurities of the second conductivity type in each of the first diffusion layer and the second diffusion layer by heating,
• wherein an impurity concentration of the second conductivity type of the third semiconductor region is lower than the impurity concentration of the second conductivity type of the first diffusion layer, and
• wherein the impurity concentration of the second conductivity type of the fourth semiconductor region is lower than the impurity concentration of the second conductivity type of the second diffusion layer.