Fabricating bi-directional nonvolatile memory cells
View Patent ↗A memory transistor having a pair of separate floating gates overlying end regions of a channel and a control gate that overlies the floating gates and a central region of the channel effectively operates as a pair of floating gate transistors with an intervening select transistor. Each floating gate can be charged to store a distinct binary, analog, or multi-bit value. An erase operation can use a negative voltage on the control and a positive voltage on an underlying well or source/drain region to cause tunneling that discharges one or both floating gates. Applying a limited current to a source/drain region during an erase operation can cause the source/drain region and a floating gate to rise together and avoid band-to-band tunneling and resulting hole injection into the floating gate.
1. A method for manufacturing a memory device, comprising:
forming a first source/drain region, a second source/drain/region, and a channel in a substrate, wherein the channel extends from the first source/drain region to the second source/drain region;
forming a first charge storage region overlying and insulated from a first portion of the channel adjacent the first source/drain region;
forming a second charge storage region overlying and insulated from a second portion of the channel adjacent the second source/drain region, wherein a gap between the second charge storage region and the first charge storage region overlies a central portion of the channel between the first and second portions of the channel; and
forming a control gate overlying and insulated from the first and second charge storage regions, the control gate extending into the gap between the first and second charge storage regions and modulating the central portion of the channel.
2. The method of claim 1 , wherein forming the first and second source/drain regions comprises:
implanting impurities into the substrate using the first and second charge storage regions to define boundaries of implanted areas; and
oxidizing the implanted regions at high temperature to cause the implanted regions to diffuse laterally under the first and second charge storage regions and to form oxide regions over the first and second source/drain regions.
3. The method of claim 1 , further comprising controlling implantation steps that adjust a threshold voltage of the central region relative to threshold voltages of the first and second portions of the channel.
4. The method of claim 1 , wherein forming the first and second source/drain regions precedes forming the first and second charge storage regions.
5. The method of claim 1 , wherein forming the first charge storage region and forming the second charge storage region comprise patterning a conductive layer to form first and second floating gates.