Non-volatile nanocrystal memory and method therefor
A nanocrystal non-volatile memory (NVM) has a dielectric between the control gate and the nanocrystals that has a nitrogen content sufficient to reduce the locations in the dielectric where electrons can be trapped. This is achieved by grading the nitrogen concentration. The concentration of nitrogen is highest near the nanocrystals where the concentration of electron/hole traps tend to be the highest and is reduced toward the control gate where the concentration of electron/hole traps is lower. This has been found to have the beneficial effect of reducing the number of locations where charge can be trapped.
1. A method for forming a semiconductor device comprising:
providing a semiconductor substrate;
forming a first insulating layer over a surface of the semiconductor substrate;
forming a layer of nanocrystals over a surface of the first insulating layer; and
forming a second insulating layer having a graded nitrogen content over the layer of nanocrystals.
2. The method of claim 1 , further comprising forming a third insulating layer over the second insulating layer, wherein the third insulating layer has a relatively lower nitrogen content than the second insulating layer.
3. The method of claim 2 , wherein forming the third insulating layer comprises forming the third insulating layer to have a nitrogen content that less than the lowest nitrogen content of the graded nitrogen content of the second insulating layer.
4. The method of claim 2 , further comprising annealing the semiconductor device in an ambient comprising oxygen at a temperature of 700 to 1100 degrees Celsius.
5. The method of claim 2 , wherein forming the third insulating layer further comprises uses plasma nitridation to modulate the nitrogen content of the third insulating layer.
6. The method of claim 5 , wherein using plasma nitridation comprises reducing a plasma energy of the plasma nitridation source over time to form the graded nitrogen content.
7. The method of claim 5 , wherein using plasma nitridation comprises reducing a flow of the nitrogen source over time to form the graded nitrogen content.
8. The method of claim 1 , wherein forming the second insulating layer comprising forming the second insulating layer using silicon-oxynitride.
9. The method of claim 1 , wherein forming the second insulating layer comprises forming the second insulating layer having a peak nitrogen content of about two atomic percent to about ten atomic percent.
10. The method of claim 1 , wherein forming the second insulating layer further comprises using plasma nitridation to modulate the nitrogen content of the second insulating layer.
11. The method of claim 10 , wherein using plasma nitridation comprises reducing a plasma energy of the plasma nitridation source over time to form the graded nitrogen content.
12. The method of claim 10 , wherein using plasma nitridation comprises reducing a flow of the nitrogen source over time to form the graded nitrogen content.
13. The method of claim 1 , wherein forming the second insulating layer further comprises using chemical vapor deposition by flowing gases comprising silicon, an oxidation source, and a nitrogen source.
14. The method of claim 13 , wherein forming the second insulating layer further comprises reducing a flow of the nitrogen source over time to form the graded nitrogen content.
15. The method of claim 1 , wherein forming the first insulating layer comprises forming the first insulating layer having a nitrogen content that decreases as distance from the semiconductor substrate increases.
16. The method of claim 1 , wherein forming the second insulating layer comprises having relatively more nitrogen at an interface with the layer of nanocrystals and relatively less nitrogen as distance from the layer of nanocrystals increases.
17. The method of claim 1 , wherein the nanocrystals of the layer of nanocrystals comprises at least one of silicon, germanium, or a metal.
18. The method of claim 1 , further comprising coating the nanocrystals with a material comprising nitrogen.
19. The method of claim 1 , wherein forming the first insulating layer comprises forming the first insulating layer from an oxide comprising at least one of silicon, hafnium, lanthanum, and aluminum.
20. The method of claim 1 , wherein forming the second insulating layer further comprises using plasma oxidation to improve the stoichiometry of the second insulating layer.