Polycrystalline silicon thin film transistor device and method of fabricating the same

1. A polycrystalline silicon thin film transistor device, disposed on a substrate, the polycrystalline silicon thin film transistor device comprising:

a buffer layer, disposed on the substrate, wherein the buffer layer has dopants;

a polycrystalline silicon layer, disposed on the buffer layer, wherein the polycrystalline silicon layer comprises a channel, a source doped region, and a drain doped region, and the source doped region and the drain doped region are respectively located on two sides of the channel;

a gate insulating layer, disposed on the polycrystalline silicon layer; and

a gate electrode, disposed on the gate insulating layer, and corresponding to the channel of the polycrystalline silicon layer,

wherein the polycrystalline silicon layer is formed by performing a thermal process to convert an amorphous silicon layer into the polycrystalline silicon layer by means of polycrystalization, and simultaneously to out-diffuse a portion of the dopants in the buffer layer into the polycrystalline silicon layer for adjusting a threshold voltage.

2. The polycrystalline silicon thin film transistor device according to claim 1 , wherein the dopants in the buffer layer comprises P-type dopants or N-type dopants.

3. The polycrystalline silicon thin film transistor device according to claim 1 , wherein the buffer layer is a single-layer-structure buffer layer, and the dopants are located in the single-layer-structure buffer layer.

4. The polycrystalline silicon thin film transistor device according to claim 3 , wherein the single-layer-structure buffer layer is a porous buffer layer.

5. The polycrystalline silicon thin film transistor device according to claim 1 , wherein the buffer layer is a multi-layered-stack-structure buffer layer, and comprises at least one bottom buffer layer located on the substrate and a top buffer layer located on the at least one bottom buffer layer, and the dopants are located in the top buffer layer of the multi-layered-stack-structure buffer layer.

6. The polycrystalline silicon thin film transistor device according to claim 5 , wherein the top buffer layer is a porous buffer layer.

7. The polycrystalline silicon thin film transistor device according to claim 1 , further comprising:

an interlevel dielectric layer located on the gate electrode; and

a source electrode and a drain electrode located on the interlevel dielectric layer, wherein the source electrode and the source doped region are electrically connected, and the drain electrode and the drain doped region are electrically connected.

8. A method of fabricating the polycrystalline silicon thin film transistor device as claimed in claim 1 , the method comprising:

providing the substrate;

forming the buffer layer having the dopants on the substrate;

forming the amorphous silicon layer on the buffer layer having the dopants;

performing the thermal process, to convert the amorphous silicon layer into the polycrystalline silicon layer by means of polycrystalization, and simultaneously to out-diffuse a portion of the dopants in the buffer layer into the polycrystalline silicon layer for adjusting the threshold voltage;

patterning the polycrystalline silicon layer to form an active layer;

forming the gate insulating layer on the active layer;

forming the gate electrode on the gate insulating layer; and

forming the source doped region and the drain doped region in the active layer.

9. The method according to claim 8 , wherein the dopants in the buffer layer comprise P-type dopants or N-type dopants.

10. The method according to claim 8 , wherein the buffer layer is a single-layer-structure buffer layer.

11. The method according to claim 10 , wherein before performing the thermal process, the dopants are located in the single-layer-structure buffer layer.

12. The method according to claim 8 , wherein the buffer layer is a multi-layered-stack-structure buffer layer, and comprises at least one bottom buffer layer located on the substrate and a top buffer layer located on the at least one bottom buffer layer.

13. The method according to claim 12 , wherein before performing the thermal process, the dopants are located in the top buffer layer of the multi-layered-stack-structure buffer layer.

14. The method according to claim 8 , wherein the step of forming the buffer layer having dopants on the substrate comprises:

performing a deposition process to deposit the buffer layer on the substrate, and simultaneously introducing a gas that contains the dopants during the deposition process, to form the dopants in the buffer layer.

15. The method according to claim 8 , wherein the step of forming the buffer layer having dopants on the substrate comprises:

performing a deposition process to deposit the buffer layer on the substrate; and

performing an ion implantation process to form the dopants in the buffer layer.

16. The method according to claim 8 , wherein the thermal process comprises an excimer laser annealing (ELA) process.

17. The method according to claim 8 , wherein the thermal process comprises a solid phase crystallization (SPC) process.

18. The method transistor device according to claim 8 , further comprising:

before performing the thermal process, first performing a dehydrogenation process on the amorphous silicon layer, and simultaneously further diffusing a portion of the dopants in the buffer layer into the amorphous silicon layer.

19. The method according to claim 8 , further comprising:

forming an interlevel dielectric layer on the gate electrode; and

forming a source electrode and a drain electrode on the interlevel dielectric layer, wherein the source electrode and the source doped region are electrically connected, and the drain electrode and the drain doped region are electrically connected.