Dual-level stacked flash memory cell with a MOSFET storage transistor
View Patent ↗The present invention is a dual-level flash memory cell design that stores 3 or more bits of information per transistor. The dual-level memory cell stores two lower bits in a first level and stores an upper bit in a second level. The lower bits are programmed, erased and read by alternate modes of operation wherein active regions operate as source and drain, and then drain and source. The upper bit is programmed and erased independent of the lower bits. However, reading of the upper bit depends upon read values of the lower bits. Additional levels are employed to store more than 3 bits of information.
1. A dual level flash memory cell comprising;
a p-type substrate;
a first active region and a second active region formed in the substrate;
a first charge trapping insulator dielectric formed on the first active region, the second active region, and the substrate, wherein the first charge trapping insulator dielectric is operable to store electrons for first and second bits of information;
a first poly layer formed on the first charge trapping insulator dielectric;
a second charge trapping insulator dielectric formed on the first poly layer, wherein the second charge trapping insulator dielectric is operable to store electrons for a third bit of information; and
a second poly layer formed on the second charge trapping insulator dielectric.
2. The memory cell of claim 1 , wherein the first charge trapping insulator dielectric is comprised of a bottom dielectric, a narrow bandgap material formed on the bottom dielectric, and a top dielectric formed on the narrow bandgap material.
3. The memory cell of claim 2 , wherein the bottom dielectric is comprised of silicon dioxide, the top dielectric is comprised of silicon dioxide, and the narrow bandgap material is comprised of nitride.
4. The memory cell of claim 2 , wherein the bottom dielectric is comprised of silicon dioxide, the top dielectric is comprised of silicon dioxide, and the narrow bandgap material is comprised of silicon-germanium.
5. The memory cell of claim 2 , wherein the bottom dielectric is relatively thin and the top dielectric is relatively thick.
6. The memory cell of claim 2 , wherein the bottom dielectric is relatively thick and the top dielectric is relatively thin.
7. The memory cell of claim 1 , the first charge trapping insulator dielectric is formed within a trench in the substrate between the first active region and the second active region so as to comprise a vertical structure of the first charge trapping insulator dielectric to facilitate scaling, wherein the trench, the first active region and the second active region all intersect a horizontal plane.
8. The memory cell of claim 7 , wherein the vertical structure is V-shaped.
9. The memory cell of claim 7 , wherein the vertical structure is U-shaped.
10. The memory cell of claim 1 , further comprising:
a third charge trapping insulator dielectric formed on the second poly layer, wherein the third charge trapping insulator dielectric is operable to store electrons for a fourth bit of information; and
a third poly layer formed on the third charge trapping insulator dielectric.
11. The memory cell of claim 10 , further comprising one or more additional pairs of charge trapping insulator dielectrics/poly layers, wherein each pair comprises:
an additional charge trapping insulator dielectric formed on the memory cell; and
an additional poly layer formed on the additional charge trapping insulator dielectric.
12. The memory cell of claim 1 , wherein the third bit of information is operable to be read by applying a read voltage to the second poly layer, wherein the read voltage is a function of values of the first and the second bits of information.
13. The memory cell of claim 1 , wherein the first poly layer and the second poly layer are comprised of metal.
14. The memory cell of claim 1 , the first charge trapping insulator dielectric extending in a continuous manner over
at least some of the first active region,
exposed portions of the substrate between the first active region and the second active region and
at least some of the second active region.