A method is provided for forming a short and sharp gate bird's beak in order to increase the erase speed of a split-gate flash memory cell. This is accomplished by implanting nitrogen ions in the first polysilicon layer of the cell and removing them from the area where the floating gate is to be formed. Then, when the polysilicon layer is oxidized to form polyoxide, the floating gate region without the nitrogen ions oxidizes faster than the surrounding area still having the nitrogen ions. Consequently, the bird's beak that is formed at the edges of the polyoxide assumes a sharper shape with smaller size than that is found in prior art. This results in an increase in the erase speed of the memory cell.
A method is disclosed for forming a split-gate flash memory cell where the floating gate of the cell is self-aligned to isolation, to source and to word line. This multi-self-aligned structure, which provides the maximum shrinkage of the cell that is possible, is also disclosed. The multi-self-alignment is accomplished by first defining the floating gate at the same time the trench isolation is formed, and then self-aligning the source to the floating gate by using a nitride layer as a hard mask in place of the traditional polyoxide, and finally forming a polysilicon spacer to align the word line to the floating gate. Furthermore, a thin floating gate is used to form a thin and sharp poly tip through the use of a "smiling effect" to advantage. The tip substantially decreases the coupling ratio of the floating gate to the word line for fast erasing speed, while at the same time increasing the coupling of the source to the floating gate with the attendant increase in the programming speed of the split gate flash memory cell.
A method is disclosed for forming a split gate flash memory cell having a thin floating gate and a sharp poly tip in order to improve the erasing and programming speed of the cell. The method involves the use of an oxide other than the poly oxide that is conventionally employed in forming the floating gate, and also using to advantage a so-called "smiling effect" which is normally taught away. The smiling effect, or an uneven thickening of an oxide layer, comes into play while growing interpoly oxide where concurrently the oxidation of the polysilicon gate advances in such a manner so as to form a sharp and reliable poly tip. The invention is also directed to providing a split gate flash memory cell having a thin floating gate and a poly tip therein.
A method is provided for forming a short and sharp gate bird's beak in order to increase the erase speed of a split-gate flash memory cell. This is accomplished by implanting nitrogen ions in the first polysilicon layer of the cell and removing them from the area where the floating gate is to be formed. Then, when the polysilicon layer is oxidized to form polyoxide, the floating gate region without the nitrogen ions oxidizes faster than the surrounding area still having the nitrogen ions. Consequently, the bird's beak that is formed at the edges of the polyoxide assumes a sharper shape with smaller size than that is found in prior art. This results in an increase in the erase speed of the memory cell.
A method for forming a square oxide structure or a square floating gate without a rounding effect at its corners. A first dielectric layer is formed on a pad layer for a square oxide structure or a polysilicon layer overlying a gate oxide layer for a floating gate, and a second dielectric layer is formed on the first dielectric layer. The second dielectric layer is patterned to form parallel openings in a first direction using a first photosensitive mask. A second photosensitive mask, having a plurality of parallel openings in a second direction perpendicular to the first direction is formed over the second dielectric layer and the first dielectric layer. The first dielectric layer is etched through square openings where the openings in the second photosensitive mask and the openings in the second dielectric layer intersect, thereby forming square openings in the first dielectric layer. The second photosensitive mask and the second dielectric layer are removed. The square oxide structure is completed by etching a trench in the semiconductor structure and forming an STI or LOCOS. The square floating gate is completed by growing polysilicon oxide structures in the square openings in the first dielectric layer and removing the first dielectric layer to form a pattern of openings therebetween, and etching the polysilicon layer through the pattern of openings between the polysilicon oxide structures forming square floating gate polysilicon regions under the polysilicon oxide hard masks.
A method is disclosed for forming a split-gate flash memory cell having a protruding source in place of the conventional flat source. The vertically protruding source structure has a top portion and a bottom portion. The bottom portion is polysilicon while the top portion is poly-oxide. The vertical wall of the protruding structure over the source is used to form vertical floating gate and spacer control gate with an intervening inter-gate oxide. Because the coupling between the source and the floating gate is now provided through the vertical wall, the coupling area is much larger than with conventional flat source. Furthermore, there is no longer the problem of voltage punch-through between the source and the drain. The vertical floating gate is also made thin so that the resulting thin and sharp poly-tip enhances further the erasing and programming speed of the flash memory cell. The vertical orientation of the source structure and the floating gate and the self-alignment of the spacer control gate to the floating gate together makes it possible to reduce the memory cell substantially.
