Dramatic Reduction of Gate Leakage Current of Ultrathin Oxides - PowerPoint PPT Presentation

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Dramatic Reduction of Gate Leakage Current of Ultrathin Oxides through Oxide Structure Modification Zhi Chen Zhi Chen, Jun , Jun Guo Guo, and , and Chandan Chandan B. B. Samantaray Samantaray Department of Electrical and Computer Engineering Department of Electrical and Computer Engineering University of Kentucky University of Kentucky Lexington, KY 40506 Lexington, KY 40506 University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Outline Outline Theory of Hydrogen/Deuterium Isotope Effect Theory of Hydrogen/Deuterium Isotope Effect • • Experimental Evidence for Origin of Isotope Effect Experimental Evidence for Origin of Isotope Effect • • Discovery of Phonon Energy Discovery of Phonon Energy- -Coupling Enhancement Coupling Enhancement • • Dramatic Improvement of Quality of Gate Oxides Dramatic Improvement of Quality of Gate Oxides • • University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Hydrogen Passivated MOSFET Hydrogen Passivated MOSFET Polysilicon Metal Source Metal Drain Deposited Gate Contact Contact Insulator SiO 2 SiO 2 n + n - n - n + Hot Electrons near Drain p-Si substrate p-Si substrate O O O O H O Si O Si O Si O Si O H SiO 2 O O O O H H H Si Si Si Si Si Si Si Si Si Si e - e - Si Si Si Si Si Si Si Si Si e - e - e - Hot electr ons desorb hydr ogen, cr eating interface states which degrade device performance. University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Van de Walle & Jackson Theory ( Van de Walle et al., Appl. Phys. Lett. vol. 69, 2441 (1996) ) Two competing processes: Two competing processes: Hot electron excitation causes Si Si- -H/D bond breaking. H/D bond breaking. • Hot electron excitation causes • De- -excitation is due to energy coupling from excitation is due to energy coupling from Si Si- -D to phonon. D to phonon. • De • Reason: ν ν ∝ ∝ 1/m based on 1/m based on IR spectroscopy theory IR spectroscopy theory Reason: ν ~ 650 Si- -H H vibrational vibrational frequency frequency ν ~ 650 cm -1 . Si 1 ( ≈ 460 cm frequency ≈ Si- -D D vibrational vibrational frequency 460 cm - (Si-Si TO phonon mode) Si -1 De De- -excitation is more efficient for excitation is more efficient for Si Si- -D bonds than for D bonds than for Si Si- -H bonds H bonds ---This is why This is why Si Si- -D bonds are stronger than D bonds are stronger than Si Si- -H ones. H ones. --- University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Schematic of Hydrogen/Deuterium Effect: Energy coupling from Si-D bending mode to Si-Si TO phonon mode No coupling from Si-H bending mode to Si-Si TO phonon mode Si Si O O Si Si O O Energy Coupling Energy Coupling No Coupling No Coupling O O O O SiO 2 SiO D D H H 2 Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Direct Measurement of the Vibrational Frequency of Si-H/D Bonds No experimental data available for Si No experimental data available for Si- -D D vibrational vibrational frequency in the frequency in the SiO 2 /Si interface SiO 2 /Si interface Only in the deuterated amorphous Si ( α -Si), the vibrational frequency (510 cm -1 ) was measured*. However, the chemical environment of the amorphous Si is very different from that of crystal Si. * J.-H. Wei, M.-S. Sun, and S.-C. Lee, Appl. Phys. Lett. 71, 1498 (1997). University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Origin of the Isotope Effect: No Energy Coupling from Si-H to Si-Si TO phonon No difference between the H-annealed sample and the as-oxidized one, except for the Si-H bending vibration. ( Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003) ) Si-Si TO X4 point 0.010 Si/oxide, hydrogen annealed Absorbance Si/oxide, unannealed Si-O TO 0.008 rocking 0.006 Si-Si Si-O LO 0.004 LO Si-H Si-O TO 0.002 Bending bending 0.000 875 825 775 725 675 625 575 525 475 425 -1 ) Wavenumber (cm University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Origin of the Isotope Effect: Energy Coupling from Si-D to Si-Si TO phonon & Si-O TO rocking mode (Chen et al. Appl. Phys. Lett. ) The absorbance of the Si-Si TO phonon mode and the Si-O TO rocking mode are all enhanced significantly (>25%) after deuterium anneal. ( Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003) ) S i-S i T O X 4 p o in t 0 .0 1 2 S i/o x id e , u n a n n e a le d 0 .0 1 0 S i/o x id e , d e u te riu m S i-O T O S i-D Absorbance 0 .0 0 8 ro c k in g B e n d in g 0 .0 0 6 S i-S i L O 0 .0 0 4 S i-O L O S i-O T O 0 .0 0 2 b e n d in g 0 .0 0 0 8 6 5 8 1 5 7 6 5 7 1 5 6 6 5 6 1 5 5 6 5 5 1 5 4 6 5 4 1 5 -1 ) W a v e n u m b e r (c m University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD New finding: Energy is coupled from Si-D bending mode to Si-Si TO phonon mode and also to Si-O TO rocking mode (Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003)) Energy Coupling Energy Coupling Si Si O O Si Si O O No Coupling No Coupling O O O O SiO 2 SiO D D H H 2 Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Energy Coupling Energy Coupling University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Challenge: How to further enhance the energy coupling? Hypothesis: Hypothesis: Shift the Shift the Si Si- -D D vibrational vibrational mode toward mode toward Si Si- -Si Si TO phonon mode. TO phonon mode. Method 1: Mechanical stress just a little shift (~6-8 cm -1 ) Method 2: Electrical stress Method 3: Thermal stress How? University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Surprising Discovery: Phonon Energy-Coupling Enhancement The absorbance of the Si-Si TO phonon mode,the Si-O TO rocking mode, and Si- Si LO mode are all enhanced significantly (>50%) after rapid thermal processing (RTP). There is further enhancement after deuterium annealing. T ox =23 nm. 0.014 Si-Si TO X4 point 0.012 Si-O TO 3 rocking 0.010 Si-D Bending Absorbance 2 Si-Si 0.008 1. Si/Oxide, Unannealed LO 2. Si/Oxide, RTP annealed 0.006 1 3. Si/Oxide, RTP plus D2 0.004 annealed 0.002 0.000 640 615 590 565 540 515 490 465 440 415 Wavenumbers(cm -1 ) University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD The Enhancement is not due to the Surface Plasmon. It is well-known that the surface plasmon on the nanoscale metallic islands also produces strong surface-enhanced IR spectra. In order to avoid the metallic island- like surface, we used n - wafer (n=2×10 14 cm -3 and ρ =20.8 Ω -cm) for experiments. Si-O TO 0.014 rocking 0.012 Si-Si TO X4 point 0.010 Absorbance 1. Si/Oxide, Unannealed 2 Si-Si 0.008 2. Si/Oxide, RTP annealed LO 0.006 1 Si-O 0.004 LO 0.002 0.000 635 615 595 575 555 535 515 495 475 455 435 415 Wavenumbers(cm-1) University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Dependence of Enhancement on the Oxide Thickness For thick oxide (T ox =80 nm), there is almost no enhancement except for the Si-Si LO mode after rapid thermal processing (RTP)---- implying stress-related phenomena. This also suggests that there should be no effect for the polysilicon/oxide stack. 0.035 Si-Si TO X4 point 0.030 0.025 Si/Oxide, Unannealed Absorbance Si/Oxide, RTP annealed Si-O TO 0.020 rocking 0.015 Si-Si LO 0.010 0.005 0.000 650 630 610 590 570 550 530 510 490 470 450 430 410 Wavenumbers(cm -1 ) University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering

2005 ISDRS, Bethesda, MD 2005 ISDRS, Bethesda, MD Dependence of Enhancement on the Cooling Time The enhancement is strongly dependent on the cooling time ----implying the stress-related phenomena. Si-Si TO 0.018 X4 point 0.016 Si-O TO 4 1. Si/Oxide, Unannealed rocking 0.014 2. Si/Oxide, RTP annealed, Absorbance 0.012 cooling 10 minutes 0.010 3 Si-Si 3. Si/Oxide, RTP annealed, LO Si-O cooling 5 minutes 0.008 LO 2 4. Si/Oxide, RTP annealed, 0.006 cooling 20 seconds 1 0.004 0.002 0.000 650 630 610 590 570 550 530 510 490 470 450 430 Wavenumbers(cm-1) University of Kentucky University of Kentucky Dept. of Electrical and Computer Engineering