1. A method for forming a semiconductor device comprising: providing a substrate prepared with a second gate structure; forming an inter-gate dielectric on the substrate and over the second gate; forming a first gate, wherein the first gate is adjacent to and separated from the second gate by the inter-gate dielectric; patterning the substrate to form a split gate structure with the first and second adjacent gates, wherein the inter-gate dielectric layer below the first gate and between the first and second gates forms a charge storage layer, the first and second gates include first and second adjacent sidewalls which are adjacent to the first and second gates and first and second non-adjacent sidewalls; forming a first diffusion region adjacent to the first non-adjacent sidewall of the first gate and a second diffusion region adjacent to the second non-adjacent sidewall of the second gate; and providing the split gate structure with an e-field equalizer which at least extends from the first non-adjacent sidewall to below the first gate, wherein the e-field equalizer improves uniformity of e-field across the first gate during operation, thereby improving charge trapping capabilities of the charge storage layer.
2. The method of claim 1 wherein the first gate comprises a control gate and the second gate comprises a select gate.
3. The method of claim 2 wherein forming the first diffusion region includes forming a first extension region adjacent to the edge of the first gate and forming the second diffusion region includes forming a second extension region adjacent to the edge of the second gate.
4. The method of claim 1 wherein the first diffusion region serves as a source terminal and the second diffusion region serves as a drain terminal.
5. The method of claim 1 wherein the inter-gate dielectric layer comprises nano-crystals having charge storage capacity.
6. The method of claim 1 wherein the e-field equalizer comprises a heavily doped e-region of a first polarity type adjacent to the first diffusion region and contacts at least a portion of the charge storage layer.
7. The method of claim 6 wherein forming the e-field equalizer comprises implanting first polarity type dopants.
8. The method of claim 7 wherein forming the e-field equalizer comprises implanting the first polarity type dopants at an angle.
9. The method of claim 8 wherein the angled implant is performed with a mask layer.
10. The method of claim 9 wherein the mask layer comprises a window, wherein the window is adjacent to the first terminal side of the gate structure.
11. The method of claim 9 wherein the mask layer has a thickness between about 1000 to 3000 Å.
12. The method of claim 8 wherein the angle of the implant is between about 20 to 45°.
13. The method of claim 10 wherein the window has a width between about 0.1 to 1 μm.
14. The method of claim 5 wherein the inter-gate dielectric layer comprises a bottom oxide layer.
15. The method of claim 14 wherein the bottom oxide layer is formed using thermal oxidation.
16. The method of claim 14 wherein the bottom oxide layer comprises a thickness gradient.
17. The method of claim 14 further comprises implanting different concentration of dopant species into the substrate prior to forming the bottom oxide layer, wherein the dopant concentration gradient controls the thickness gradient of the bottom oxide layer.
18. The method of claim 17 wherein the dopant species implanted retards oxidation.
19. A method of forming a device comprising: providing a substrate with a first feature having first and second sidewalls wherein the first feature includes a charge storage layer below the first feature and on the substrate; forming a first diffusion region in the substrate adjacent to the first sidewall of the first feature; and providing the first feature with an e-field equalizer which at least extends from the first sidewall to below the first feature, wherein the e-field equalizer improves uniformity of e-field across the distance between the first and second sidewalls of the first feature during operation, thereby improving charge trapping capabilities of the charge storage layer.
20. The method of claim 1 wherein the e-field equalizer comprises an inter-gate dielectric layer having a bottom oxide layer with a thickness gradient.
21. The method of claim 19 further comprises a second feature adjacent to the second sidewall of the first feature.
22. The method of claim 21 wherein the charge storage layer comprises nano-crystals having charge storage capacity.
23. The method of claim 19 wherein the e-field equalizer comprises a heavily doped e-region of a first polarity type adjacent to the first diffusion region and contacts at least a portion of the charge storage layer.
