Split gate NAND flash memory structure and array, method of programming, erasing and reading thereof, and method of manufacturing
A split gate NAND flash memory structure is formed on a semiconductor substrate of a first conductivity type. The NAND structure comprises a first region of a second conductivity type in the substrate with a second region of the second conductivity type in the substrate, spaced apart from the first region. A continuous first channel region is defined between the first region and the second region. A plurality of floating gates are spaced apart from one another with each positioned over a separate portion of the channel region. A plurality of control gates are provided with each associated with and adjacent to a floating gate. Each control gate has two portions: a first portion over a portion of the channel region and a second portion over the associated floating gate and capacitively coupled thereto.
1. A method of erasing a plurality of flash memory cells in a flash memory structure formed in a semiconductor substrate of a first conductivity type wherein said structure has a first region of a second conductivity type in said substrate; a second region of the second conductivity type in said substrate, spaced apart from said first region, thereby defining a continuous first channel region therebetween; a plurality of floating gates, spaced apart from one another, each positioned over a separate portion of the channel region, wherein each floating gate defines a flash memory cell; a plurality of control gates, each associated with and adjacent to a floating gate, each control gate having two portions: a first portion over a portion of the channel region and a second portion over the associated floating gate and capacitively coupled thereto; wherein said method comprising:
erasing a first plurality of floating gates in a first pass by:
applying a first positive voltage to a plurality of first control gates, wherein each of said first control gates is not immediately adjacent to one another;
applying a second voltage, lower than said first positive voltage to a plurality of second control gates, wherein each second control gate is immediately adjacent to one of said first control gates to which said first positive voltage is applied, thereby erasing the first plurality of floating gates' with each of the first plurality of floating gates associated with each of said second control gates to tunnel electrons from each floating gate to said each first control gate adjacent to said each associated second control gate; thereafter
erasing a third plurality of floating gates in a second pass by:
applying a third positive voltage to a plurality of third control gates, wherein each of said third control gates is not immediately adjacent to one another, with a third control gate between a pair of the first control gates;
applying a fourth voltage, lower than said third positive voltage to a plurality of fourth control gates, wherein each fourth control gate is immediately adjacent to one of said third control gates to which said third positive voltage is applied, thereby erasing the third plurality of floating gates' with each of the third plurality of floating gates associated with each of said fourth control gates to tunnel electrons from each floating gate to said each third control gate adjacent to said each associated fourth control gate.
2. The method of claim 1 wherein said first control gates are alternate control gates; and wherein said second control gates are alternate control gates, and wherein said third control gates during said second pass are said second control gates of said first pass and said fourth control gates during said second pass are said first control gates of said first pass.
3. The method of claim 2 wherein said first voltage is substantially the same as said third voltage.
4. The method of claim 3 wherein said second voltage is substantially the same as said fourth voltage.
5. The method of claim 4 wherein said second voltage is at ground.
6. The method of claim 4 wherein said second voltage is a negative voltage.
7. The method of claim 2 wherein said first and second regions are at ground.
8. The method of claim 1 wherein said first control gates are not alternate control gates and wherein said second control gates are not alternate control gates.
9. The method of claim 8 further comprising:
applying a fifth voltage to all control gates other than said first control gates and second control gates, while erasing the second floating gates;
wherein said fifth voltage is substantially at ground.
10. The method of claim 9 further comprising:
applying a sixth voltage to all control gates other than said third control gates and fourth control gates, while erasing the fourth floating gates;
wherein said sixth voltage is substantially at ground.