Semiconductor structure and method of fabrication thereof with mixed metal types
A semiconductor structure includes a first PMOS transistor element having a gate region with a first gate metal associated with a PMOS work function and a first NMOS transistor element having a gate region with a second metal associated with a NMOS work function. The first PMOS transistor element and the first NMOS transistor element form a first CMOS device. The semiconductor structure also includes a second PMOS transistor that is formed in part by concurrent deposition with the first NMOS transistor element of the second metal associated with a NMOS work function to form a second CMOS device with different operating characteristics than the first CMOS device.
1. A method for forming a semiconductor structure, comprising;
forming first and second PMOS transistor elements and first and second NMOS transistor elements, wherein the first PMOS transistor element and the first NMOS transistor element establish a first CMOS transistor pair, and the second PMOS transistor element and the second NMOS transistor element establish a second CMOS transistor pair, by:
forming a first threshold voltage setting region extending under a gate of the first and second PMOS transistor elements, the first threshold voltage region defining a depletion zone when a voltage to the transistor elements is applied;
forming a second threshold voltage setting region extending under a gate of the first and second NMOS transistor elements, the second threshold voltage region defining a depletion zone when the voltage to the transistor elements is applied;
forming a substantially undoped channel region, disposed above the threshold voltage setting regions and below the gates of each of the first and second PMOS and NMOS transistor elements;
concurrently depositing a first gate metal only on the gate of the first PMOS transistor element of the first CMOS transistor pair and the gate of the second NMOS transistor element of the second CMOS transistor pair; and
concurrently depositing a second gate metal only on the gate of the first NMOS transistor element of the first CMOS transistor pair and the gate of the second PMOS transistor element of the second CMOS transistor pair.
2. The method of claim 1 , wherein the second gate metal is a modification of the first gate metal.
3. The method of claim 1 , wherein at least one of the first gate metal and the second gate metal has a work function between 100 millivolts from band edge to midgap.
4. The method of claim 1 , wherein at least one of the first gate metal and the second gate metal has a work function selected to be between midgap and 300 millivolts from midgap.
5. The method of claim 1 , further comprising:
forming the gates of the first and second PMOS transistor elements and the first and second NMOS transistor elements after forming respective source and drain regions.
6. The method of claim 1 , further comprising:
forming the gates of the first and second PMOS transistor elements and the first and second NMOS transistor elements prior to forming respective source and drain regions.
7. The method, of claim 1 , further comprising:
forming a body tap region for at least one of the first and second PMOS transistor elements and first and second NMOS transistor elements to selectively apply a bias thereto.
8. The method of claim 1 , wherein the undoped channel is formed by epitaxial growth of silicon.
9. The method of claim 1 , wherein the first NMOS transistor element is formed to operate with a predetermined first leakage, the second NMOS transistor element being formed to operate with a second leakage at least ten times lower than the predetermined first leakage.
10. The method of claim 1 , wherein the first NMOS transistor element is formed to operate with a predetermined first leakage, the second NMOS transistor element being formed to operate with a second leakage at least fifty times lower than the predetermined first leakage.