FIG. 26 shows a relationship between the roughness of the surface (particularly the top surface) of the variable resistance material 32 of the fourth embodiment and the characteristics of a memory cell MC. A forming voltage and a threshold voltage are shown as typical characteristics. As shown in FIG. 26, the forming voltage and the threshold voltage depend on the roughness of the surface of the variable resistance material 32. Where the surface roughness is small, the forming voltage and the threshold voltage are low. The reason is that where the surface roughness is high, the non-uniformity of the electric field generated in the variable resistance material 32 is high.

<4.4. Advantages>

According to the fourth embodiment, the surface of the variable resistance material 32 is etched by low-angle IBE. Therefore, the surface roughness of the variable resistance material 32 is low. In the variable resistance material 32 having a low surface roughness, the non-uniformity of the electric field is suppressed. Therefore, a memory device 1 including a memory cell MC having high performance can be realized by the low forming voltage and the low threshold voltage.

<5. Modification>

In the above, the embodiments were described referring to an example in which each memory cell MC has a selector in the lower portion and a variable resistance element VR in the upper portion. However, each memory cell MC may have a selector in the upper portion and a variable resistance element VR in the lower portion.