Nonvolatile semiconductor storage device
According to one embodiment, a control gate is formed on the semiconductor substrate and includes a cylindrical through hole. A block insulating film, a charge storage film, a tunnel insulating film, and a semiconductor layer are formed on a side surface of the control gate inside the through hole. The tunnel insulating film comprises a first insulating film having SiO 2 as a base material and containing an element that lowers a band gap of the base material by being added. A density and a density gradient of the element monotonously increase from the semiconductor layer toward the charge storage film.
1. A nonvolatile semiconductor storage device, comprising:
a semiconductor substrate;
a control gate disposed above the semiconductor substrate and having a through hole extending from a top surface to a bottom surface thereof;
a block insulating film disposed on a side surface of the control gate inside the through hole;
a charge storage film disposed on the side surface of the block insulating film inside the through hole;
a tunnel insulating film disposed on the side surface of the charge storage film inside the through hole; and
a semiconductor layer disposed on the side surface of the tunnel insulating film inside the through hole,
wherein the tunnel insulating film includes insulating films including a first insulating film, a second insulating film, . . . , an i-th insulating film (i is an integer and i≧3) laminated from a side of the charge storage film, having SiO 2 as a base material, and containing an element that lowers a band gap of the base material by being added and
if densities of the element of the first insulating film, the second insulating film, . . . , the i-th insulating film are n 1 , n 2 , . . . , n i and thicknesses of those are r 1 , r 2 , . . . , r i , respectively, relationships n 1 >n 2 . . . >n i and (n 1 −n 2 )/(r 1 +r 2 )>(n 2 −n 3 )/(r 2 +r 3 ) . . . >(n i-1 −n i )/(r i-1 +r i ) are satisfied.
2. The device of claim 1 , wherein the element is N.
3. The device of claim 1 , wherein the element is one of Al and Hf.
4. The device of claim 1 , wherein the i-th insulating film in contact with the semiconductor layer comprises SiO 2 .
5. The device of claim 1 , wherein the charge storage film comprises a material having a band gap smaller than the band gap of the tunnel insulating film.
6. The device of claim 2 , wherein each of the first insulating film, the second insulating film, . . . , the i-th insulating film of the tunnel insulating film has a composition ratio of (SiO 2 ) x (Si 3 N 4 ) 1-x (0.75≦x≦1).
7. The device of claim 3 , wherein each of the first insulating film, the second insulating film, . . . , the i-th insulating film of the tunnel insulating film has a composition ratio of (SiO 2 ) x (Al 2 O 3 ) 1-x (0≦x≦1) or (SiO 2 ) x (HfO 2 ) 1-x (0≦x≦1).
8. The device of claim 1 , wherein each of the first insulating film, the second insulating film, . . . , the i-th insulating film has a constant composition ratio.
9. The device of claim 1 , wherein the through hole is a cylindrical hole.