Semiconductor structure and method for forming the same

1. A semiconductor structure, comprising:

a substrate;

a fin structure disposed on the substrate, wherein:

the fin structure includes a channel region, a source region, and a drain region,

the channel region is located between the source region and the drain region,

the channel region includes a first nanowire and a second nanowire above the first nanowire,

the first nanowire contains first threshold-voltage adjustment ions,

the second nanowire contains second threshold-voltage adjustment ions,

a first opening is formed between the first nanowire and the substrate, and between the source region and the drain region, and

a second opening is formed between the first nanowire and the second nanowire, and between the source region and the drain region;

a first gate structure, formed in the first opening and across and surrounding the first nanowire, and including a first gate electrode layer and a first work function layer surrounding the first gate electrode layer; and

a second gate structure, formed in the second opening and across and surrounding the second nanowire, and including a second electrode layer between the first nanowire and the second nanowire, and a second work function layer, the first work function layer covering a bottom surface and a part of sidewall surfaces of the second electrode layer, and the second work function layer covering a top surface and another part of the sidewall surfaces of the second electrode layer, wherein:

the first threshold-voltage adjustment ions are different from the second threshold-voltage adjustment ions in type, concentration, or a combination thereof; and

a thickness of the first work function layer is higher than a thickness of the second work function layer, or a type of the first work function layer is opposite to a type of the second work function layer.

2. The semiconductor structure according to claim 1 , wherein:

a type of the first threshold-voltage adjustment ions is opposite to a type of the second threshold-voltage adjustment ions;

when the semiconductor structure is N-type, the first threshold-voltage adjustment ions are N-type, and the second threshold-voltage adjustment ions are P-type; and

when the semiconductor structure is P-type, the first threshold-voltage adjustment ions are P-type, and the second threshold-voltage adjustment ions are N-type.

3. The semiconductor structure according to claim 2 , wherein:

a concentration of the first threshold-voltage adjustment ions is in a range of approximately 5.0E17 to 7.0E19 atoms per cubic centimeter (atom/cm 3 ); and

a concentration of the second threshold-voltage adjustment ions is in a range of 0 to approximately 4.0E19 atom/cm 3 .

4. The semiconductor structure according to claim 1 , wherein:

a type of the first threshold-voltage adjustment ions is same as a type of the second threshold-voltage adjustment ions;

when the semiconductor structure is N-type, the first threshold-voltage adjustment ions and the second threshold-voltage adjustment ions are N-type;

when the semiconductor structure is P-type, the first threshold-voltage adjustment ions and the second threshold-voltage adjustment ions are P-type; and

a concentration of the first threshold-voltage adjustment ions is higher than a concentration of the second threshold-voltage adjustment ions.

5. The semiconductor structure according to claim 4 , wherein:

the concentration of the first threshold-voltage adjustment ions is in a range of approximately 1.5E18 to 1.0E20 atom/cm 3 ; and

the concentration of the second threshold-voltage adjustment ions is in a range of 0 to approximately 8.0E19 atom/cm 3 .

6. The semiconductor structure according to claim 1 , wherein:

the type of the first work function layer is same as the type of the second work function layer;

when the semiconductor structure is N-type, the first work function layer and the second work function layer are made of an N-type work-function material;

when the semiconductor structure is P-type, the first work function layer and the second work function layer are made of a P-type work-function material; and

the thickness of the first work function layer is higher than the thickness of the second work function layer, wherein:

the P-type work-function material includes tantalum nitride (TaN x ), aluminum nitride (AlN x ), or a combination thereof, and

the N-type work-function material includes titanium aluminum alloy.

7. The semiconductor structure according to claim 6 , wherein:

the thickness of the first work function layer is in a range of approximately 25 Å to 120 Å; and

the thickness of the second work function layer is in a range of 0 Å to approximately 100 Å.

8. The semiconductor structure according to claim 1 , wherein:

the type of the first work function layer is opposite to the type of the second work function layer;

when the semiconductor structure is N-type, the first work function layer is made of an N-type work-function material, and the second work function layer is made of a P-type work-function material; and

when the semiconductor structure is P-type, the first work function layer is made of the P-type work-function material, and the second work function layer is made of the N-type work-function material, wherein:

the P-type work-function material includes tantalum nitride (TaN x ), aluminum nitride (AlN x ), or a combination thereof, and

the N-type work-function material includes titanium aluminum alloy.

9. The semiconductor structure according to claim 8 , wherein:

a thickness of the first work function layer is in a range of approximately 30 Å to 130 Å; and

a thickness of the second work function layer is in a range of 0 Å to approximately 20 Å.

10. A method for forming a semiconductor structure, comprising:

forming a channel-region structure on a substrate, wherein:

the channel-region structure includes a first sacrificial layer disposed on the substrate, an initial first nanowire disposed on the first sacrificial layer, a second sacrificial layer disposed on the initial first nanowire, and an initial second nanowire disposed on the second sacrificial layer;

forming a first isolation layer on the substrate, the first isolation layer covering a portion of a sidewall surface of the first sacrificial layer;

forming a dummy gate structure on the substrate across the channel-region structure;

forming a source region and a drain region respectively on two sides of the channel-region structure;

forming a second isolation layer on the substrate, the second isolation layer covering a sidewall surface of the dummy gate structure and exposing a top surface of the dummy gate structure;

removing the dummy gate structure to form a top opening above the initial second nanowire, removing the first sacrificial layer and the second sacrificial layer to form a first opening between the initial first nanowire and the substrate and between the source region and the drain region and form a second opening between the initial second nanowire and the initial first nanowire and between the source region and the drain region; and

doping first threshold-voltage adjustment ions into the initial first nanowire to form a first nanowire, and doping second threshold-voltage adjustment ions into the initial second nanowire to form a second nanowire;

forming a first gate structure in the first opening and across and surrounding the first nanowire, the first gate structure including a first gate electrode layer and a first work function layer surrounding the first gate electrode layer; and

forming a second gate structure in the second opening and across and surrounding the second nanowire, the second gate structure including a second electrode layer between the first nanowire and the second nanowire, and a second work function layer, the first work function layer covering a bottom surface and a part of sidewall surfaces of the second electrode layer, and the second work function layer covering a top surface and another part of the sidewall surfaces of the second electrode layer, wherein:

the first threshold-voltage adjustment ions are different from the second threshold-voltage adjustment ions in type, concentration, or a combination thereof; and

a thickness of the first work function layer is higher than a thickness of the second work function layer, or a type of the first work function layer is opposite to a type of the second work function layer.