Impact of Carbon on short channel behavior in deep submicronic N MOSFET ’s

1. Impact of Carbon on short channel behavior in deep submicronic N MOSFET ’s K. Romanjek1, G. Ghibaudo1, T. Ernst2 1) IMEP, ENSERG, BP 257, 23 rue des Martyrs, 38016 Grenoble, cedex 16, France 2) CEA/LETI, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France

2. Outline • Introduction • Device fabrication and parameter extraction technique • N MOSFET’s structure • Y function method • Impact of Carbon on electrical parameters • Threshold voltage • Low Field mobility • Transconductance parameter • Boron pockets influence • Comparison between transistors with and without pockets • Neutralization of Boron pockets using body bias • Conclusion 1

3. Substitutional Carbon atoms limit Boron diffusion Retrograde doping profile Introduction Why Carbon incorporation in deep submicronic N MOSFET’s ? Better control of short channel effects TEM picture by CEA-Leti 2

4. Device fabrication and parameter extraction technique N MOSFET’s structure • Transistors studied : • NMOSFET’s • tcap = 2nm after oxidation • tSi:C = 3, 6 or 10nm • x = 0.6 or 1% • toxide = 2nm • W = 10µm • L = 10µm to 50nm • fabricated by CEA-Leti 3

5. low Vd Vg >> Vt Device fabrication and parameter extraction technique Y function method 4

6. 40 C 0.6 % 10 nm 30 C 1 % 6 nm Si:C C 1 % 10 nm 20 10 Threshold voltage variation (%) 0 Epitaxy Si Implanted channel -10 -20 Si -30 0,01 0,1 1 Channel length (µm) Impact of Carbon on electrical parameters Threshold voltage (1/2) • Better control of the • roll off : • Epitaxial channel as compared to implanted one • Si:C transistors compared to Si transistors 5

7. 40 C 0.6 % 10 nm C 1 % 6 nm 30 C 1 % 10 nm C 1 % 10 nm 20 C 1 % 3 nm Threshold voltage variation (%) 40nm Si PSD /C 1 % 10 nm 10 0 -10 -20 0,01 0,1 1 Channel length (µm) Impact of Carbon on electrical parameters Threshold voltage (2/2) Si:C parameters : • 3 nm Si:C layer thickness is not sufficient to limit Boron diffusion • 0.6 % of Carbon is sufficient to control the rool-off • 40nm PSD layer isn’t necessary to have a better control of the roll off 6

8. 400 350 300 250 Low field mobility (/V /s cm2) 200 150 C 1 % 10 nm 100 Si:C Epitaxy Si C 1 % 6 nm Si 50 Implanted channel C 0.6 % 10 nm 0 0 0,2 0,4 0,6 0,8 1 1,2 Channel length (µm) Impact of Carbon on electrical parameters Low Field mobility • Even for long transistors we don’t have a clear gain due to carbon incorporation • For Si:C transistor a strong decrease is noticed • For ultrashort channel length there is a convergence of all curves. 7

9. 25 Implanted channel Si Epitaxy Si 20 15 Gm/Gm(Lmax) 10 1/L C 1 % 10 nm 5 C 1 % 6 nm Si:C C 0.6 % 10 nm 0 0,01 0,1 1 Channel length (µm) Impact of Carbon on electrical parameters Transconductance parameter The transconductance parameter is studied to account for the possible variation of difference between electrical and technological channel length • Even for Si transistors we have a slight degradation may due to Boron Pockets • For Si:C transistor a strong degradation is noticed 8

10. Boron pockets influence Comparison between transistors with and without pockets (1/2) • Transistors without pockets have a stronger roll off than transistors with pockets : Boron pockets contribute to the control of short channel effects. • For Si:C transistors, a good control of the roll off still persists even for transistors without pockets : Carbon incorporation also controls the short channel effects. 9

11. Boron pockets influence Comparison between transistors with and without pockets (2/2) • Transistors with pockets have a higher degradation than transistors without pockets : Boron pockets contribute to degradation of the carrier transport. • For Si:C transistors, a strong degradation still persists even for transistors without pockets : Carbon incorporation also degrades the carrier transport . 10

12. Impact of Carbon on electrical parameters Neutralization of Boron pockets using body bias (1/2) • For Si transistors : roll on due to implant pockets is totally neutralized by body bias. • For Si:C transistors : a good control of the roll off still persists even when Boron pockets are neutralized. 11

13. Impact of Carbon on electrical parameters Neutralization of Boron pockets using body bias (2/2) • For Si transistors : degradation due to implant pockets totally neutralized by body bias. • For Si:C transistors : a degradation still persists even when Boron pockets are neutralized. 12

14. Conclusion Short channel effects : • As expected, a good control of the roll off is noticed for Si:C transistors. • By a method based on a body bias, we have separated the contribution due to Boron pockets and have shown thata Si:C layer contributes mainly to the control of the roll off. • The most critical parameter is the Si:C layer thickness, it has to be large enough to be a good barrier for Boron diffusion in order to obtain a retrograde doping profile. Carrier transport : • Unfortunately, we have observed a degradation of carrier transport in Si:C transistors may due to a high percentage of Interstitial Carbon atoms which create defects in the oxide and/or in the channel. • We have shown that this degradation is partially due to Boron pockets, but the major part is due to Carbon incorporation. 13