Dual conducting floating spacer metal oxide semiconductor field effect transistor (DCFS MOSFET) and method to fabricate the same
Dual Conducting Floating Spacer Metal Oxide Semiconductor Field Effect Transistors (DCFS MOSFETs) and methods for fabricate them using a process that is compatible with forming conventional MOSFETs are disclosed. A DCFS MOSFET can provide multi-bit storage in a single Non-Volatile Memory (NVM) memory cell. Like a typical MOSFET, a DCFS MOSFET includes a control gate electrode on top of a gate dielectric-silicon substrate, thereby forming a main channel of the device. Two electrically isolated conductor spacers are provided on both sides of the control gate and partially overlap two source/drain diffusion areas, which are doped to an opposite type to the conductivity type of the substrate semiconductor. The DCFS MOSFET becomes conducting when a voltage that exceeds a threshold is applied at the control gate and is coupled through the corresponding conducting floating spacer to generate an electrical field strong enough to invert the carriers near the source junction. By storing charge in the two independent conducting floating spacers, DCFS MOSFET can have two independent sets of threshold voltages associated with the source junctions.
1. A dual conductive floating spacer memory cell, comprising:
a semiconductor substrate having impurities of a first conductivity type, the semiconductor substrate having a first surface at which is formed a first source/drain region, a second source/drain region and a channel region between the first source/drain region and the second source/drain region, each of the first and second source/drain region having impurities of a second conductivity type;
a control gate formed above the channel region, electrically isolated therefrom by a gate dielectric layer;
a first conductive floating spacer and a second conductive floating spacer each having a first side and a second side, each conductive floating spacer being formed adjacent the control gate on the first side and each being electrically isolated by a dielectric layer from the control gate, such that the control gate and the dielectric layer form a substantially vertical sidewall on the second side of each of the first and second conductive floating spacers, the first and second conductive floating spacers being formed above the channel region and overlapping the first and second source/drain regions, respectively, each separated therefrom by a tunnel oxide region; and
dielectric spacers formed along the substantially vertical sidewalls.
2. A dual conductive floating spacer memory cell as in claim 1 , further comprising trench isolation regions in the semiconductor substrate.
3. A dual conductive floating spacer memory cell as in claim 1 , wherein the first conductivity type is P-type and the second conductivity type is N-type.
4. A dual conductive floating spacer memory cell as in claim 1 , wherein the first conductivity type is N-type and the second conductivity type is P-type.
5. A dual conductive floating spacer memory cell as in claim 1 , wherein the dual conductive floating spacer memory cell is formed in a NAND-type non-volatile memory array.
6. A dual conductive floating spacer memory cell as in claim 1 , wherein the dual conductive floating spacer memory cell is formed in a NOR-type non-volatile memory array.