Methods of making a semiconductor memory device
One-transistor (1T) capacitor-less DRAM cells each include a MOS transistor having a bias gate layer that separates a floating body region from a base substrate. The MOS transistor functions as a storage device, eliminating the need of the storage capacitor. Logic “1” is written to and stored in the storage device by causing majority carriers (holes in an NMOS transistor) to accumulate and be held in the floating body region next to the bias gate layer, and is erased by removing the majority carriers from where they are held.
1. A method for making a semiconductor memory device, the method comprising:
providing a semiconductor substrate;
forming shallow-trench-isolations in the semiconductor substrate;
forming floating bodies and a base substrate out of the semiconductor substrate by removing portions of the semiconductor substrate and the shallow-trench-isolations, the floating bodies each being a disconnected portion of the semiconductor substrate held separate from the base substrate by portions of the shallow-trench-isolations which were not removed;
forming a conductive layer between the floating bodies and the base substrate; and
forming MOS transistors each including one of the floating bodies,
wherein the floating body of each of the MOS transistors is adapted to store a data bit during operation of the memory device.
2. The method of claim 1 , further comprising forming a nitride cap on a surface of the semiconductor substrate between the shallow-trench-isolations before forming the floating bodies, the nitride cap further holding the floating bodies separate from the base substrate.
3. The method of claim 1 , wherein the semiconductor substrate comprises a single crystalline silicon.
4. The method of claim 3 , wherein the conductive layer is a polysilicon layer.
5. The method of claim 4 , wherein forming the conductive layer comprises depositing polysilicon into gaps formed between the floating bodies such that the conductive layer is self-aligned to the floating bodies.
6. A method for making a semiconductor memory device, the method comprising:
providing a semiconductor substrate;
forming isolation regions in the semiconductor substrate;
forming floating bodies and a base substrate out of the semiconductor substrate by removing portions of the semiconductor substrate and the isolation regions, the floating bodies each being a disconnected portion of the semiconductor substrate held separate from the base substrate by portions of the isolation regions which were not removed;
forming conductive material between the floating bodies and the base substrate; and
forming field effect transistors each including one of the floating bodies.
7. The method of claim 6 wherein forming the conductive material comprises forming voids in the conductive material.
8. The method of claim 7 comprising forming the voids underneath the floating bodies.
9. The method of claim 8 wherein the respective voids are received underneath only a portion of the floating bodies.
10. The method of claim 7 comprising forming the voids to respectively extend from one isolation region to another isolation region.
11. The method of claim 7 comprising forming the voids underneath the floating bodies, and to respectively extend from one isolation region to another isolation region.
12. The method of claim 6 comprising forming gates of the field effect transistors to be recessed in the semiconductor substrate.
13. The method of claim 6 comprising forming a dummy gate over the respective isolation regions.
14. The method of claim 13 comprising forming gates of the field effect transistors to be recessed in the semiconductor substrate.
15. The method of claim 6 wherein forming the conductive layer comprises depositing and subsequently recessing conductive material.