Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Atomic scale observation of interface defect formation processes between Si and SiO2 Satoshi Yamasaki a,b, Wataru Futako a, Norikazu Mizuochi a,b a Diamond Research Center, AIST b Tsukuba University scenario ・ Back Ground ・ Ultra-High Vacuum Electron Spin Resonance (UHV-ESR) ・ db termination process of Si (111) surface ・ Interface defect creation mechanism during Si oxidation processes Oxidation of Si (111) Oxidation of Si (100) ・ Conclusion

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Electron spin resonance (ESR) Direct observation of unpaired electron spin Dangling bond MOS device Control of interface defect ・ creation mechanism ・ structure ・ minituarization gate insulator interface new materials ・ defect density ・ wave function identification defect structure In-situ ESR / under UHV circumstance ・ in-situ observation of oxidation processes of crystalline Si Si db Background

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Si oxidation process Oxygen atoms insert Si-Si bond different from thin film deposition O 2 molecule goes through a-SiO 2 network Desociation of O 2 to O atoms Insert Si-Si bond Crystalline Si amorphous SiO 2 Si(100)

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 P b center adatom db Adatom SiO 2 Si PbPb O PbPb SiO 2 P b center is... ･ The typical Si/SiO 2 interface defect ･ Electrically active interface defects. E.H. Poindexter, et al. J. Appl. Phys., 56 (1984) 2844 ･ Typical density is 4 ~ 5 x10 12 cm -2. ( H anneal.) A. Steasmans, Phys. Rev. B 48 (1993) 2418 ･ Simple idea The density is determined by the accumulated interface strain. What determines interface defect ?

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 [ 111 ] Si(111) PbPb Interface Si dangling bond With 3 Si-Si backbonds *Brower, APL 43, 1111 (1983) **Stesmans, APL, 48, 973 (1996) SiO 2 /Si(111) interface *Poindexter et al., JAP, 52, 879 (1981). Stesmans, JAP, 83, 2449 (1998) **Stesmans et al., PRB58, (1998) 〈 211 〉 〈 111 〉 [100] SiO 2 /Si(100) interface P b1 *, ** P b0 * Si(100) SiO 2 Three kinds of P b centers

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Clean Si surfaces How the spin nature changes during O termination of Si(111) ? Starting surface of Si oxidation Clean Si surfaces are needed for precise measurements Si(111) 7x7 surface reconstruction Si(100) 2x1 surface reconstruction

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Ultra-High Vacuum ESR System RHEED ESR quartz tube Magnet Sample with holder Single crystal silicon 45 x 3 x 0.3 mm R > 1000  cm both side mirrored Base Pressure: ~ Torr Transfer rod Microwave 9.6GHz e-gun Screen H2H2 Gas Supply (needle valve) DC Heater & Filament + - DC V DC Purifier O2O2 AES

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Oxygen termination process of Si (111) surface How spin nature changes ?

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Structural model for Si(111)-(7x7) corner adatom (6) center adatom (6) rest atom (6) corner hole atom (1) unfaulted halffaulted half K.D. Brommer et al. Surf. Sci. 314 (1994) 57 corner hole unfaulted half RHEED Pattern dangling bond (19) surface electronic band

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Experimental Procedure Ultra-high vacuum Base press. ~ Torr flashing, 1300 ℃ anneal （ clean surface ） O 2 exposure at room temperature Dose ~ Langmuir [Pressure, Torr] x [Exposure Time, 1 sec] evacuation ~ Torr ESR measurements （ room temperature ） Si oxidation and ESR measurement

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Surface Cleaning Process P b center (No signal) ADDB P b center Termination ADDB Adatom O Si PbPb SiO 2 Si(111) 7x7 :No signal broadening Si (RHEED)

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Many db overlapping of wave function 2 dim. metallic state (not localized) no signal (broadening?) Start to be localized Many db - big signal wide spectrum broadening effect dipole-dipole interaction almost O coverage small No. of db sharp spectrum O termination spin center Model for spin center termination

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Termination of ad-atom dbs at Room Temp. P b center (No signal) ad-atom db P b center Termination Si ad-atom db Si Small amount of O 2 ESR signal appears (RT) O 2 ExposureDecrease of signal intensity & linewidth

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Many db overlapping of wave function 2 dim. metallic state (not localized) no signal (broadening?) Start to be localized Many db - big signal wide spectrum broadening effect dipole-dipole interaction almost O coverage small No. of db sharp spectrum O termination spin center Model for spin center termination

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Termination of ad-atom dbs (room temp.) FWHM (mT) Spin density (10 12 cm -2 ) Oxygen dose (L) ad-atom dangling bond ~ Torr O 2 R.T. No signal T. Umeda et al. Appl. Surf. Sci (2000) 299 Si ad-atom db Si Ad-atom db density and line width decreases with increase of O coverage

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Many db overlapping of wave function 2 dim. metallic state (not localized) no signal (broadening?) Start to be localized Many db - big signal wide spectrum broadening effect dipole-dipole interaction almost O coverage small No. of db sharp spectrum O termination spin center Model for spin center termination

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Anisotropy of g value B//[112] (90 deg) B B//[111] (0 deg) B Sample Electronic structure of ad-atom db is similar to P b center does not matter whether SiO 2 on it or not

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 O In-situ ESR spectra during oxidation P b center (No signal) ADDB P b center Termination ADDB Si PbPb Adatom Si ADDB Si SiO 2 PbPb

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Oxygen termination process of Si (111) Dynamic change of surface dbs of Si (111) was observed using ultra-high vacuum ESR system. Wavefunction of clean Si(111) surface dbs (7 x 7 structure ) does not localized. Electronic structure of ad-atom db is similar to P b center.

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 SiO 2 formation process of Si (111) surface To get several SiO 2 layers, high temperature oxidation is needed. How interface P b center of Si(111) created ?

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Experimental Procedure Ultra-high vacuum Base press. ~ Torr flashing, 1300 ℃ anneal （ clean surface ） O 2 exposure 780 ℃ Dose ~ Langmure, L [Pressure, Torr] x [Exposure Time, sec] evacuation ~ Torr ESR measurements （ room temperature ） Si oxidation and ESR measurement

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Formation of P b center (780  C) RCA cleaning (No signal) no change! SiO 2 formation Si PbPb PbPb

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 SiO 2 Thickness 8A 4A 14A SiO 2 (experiment): 100A Si clean surface (experiment) simulation SiO 2 thickness was estimated from comparison with simulated AES spectra. For AES simulation, experimental data of Si, bulk-SiO 2 was used. Escape depth of electron was assumed ~6A.

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Formation of P b center (780  C) RCA cleaning (No signal) no change! SiO 2 formation Si PbPb PbPb

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Simulation of the ESR signal O Si PbPb SiO 2 O Si ？ 3200L [111] (P b ;Lorentzian) unknown (Gaussian) Experimental Simulation

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Anisotropy of g value O Si PbPb SiO 2 [111] [112] [110] isotropic amorphous or random At the present stage, the origin is not known.

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Defect density of ultra-thin SiO 2 780C Constant defect density ! (d>2.5A) almost the same density as thick SiO 2 case RCA cleaning Si PbPb PbPb

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Defect density of ultra-thin SiO 2 Constant defect density ! (d>2.5A) immediately reach a usual defect density for thick SiO 2. macroscopic lattice mismatch between Si/SiO 2 must be a gradual change against thickness. This is not the case! Interface defect is the results of microscopic chemical reaction of Si oxidation !

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 SiO 2 formation process of Si (111) surface Interface defect creation process during Si (111) oxidation was observed using UHV ESR system at 780 ˚C. Even for monolayer oxidation, interface defect density reached close to the value of thick SiO 2 case (~ 5x10 12 cm -2 ). These results suggest that interface defect of P b centers are created by atomic level chemical reactions, Not macroscopic stress between Si and SiO 2.

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 SiO 2 formation process of Si (100) surface Si (100) is a standard surface in the field of Si technology. Si (100) / SiO 2 has two kind of P b centers; P b0 and P b1. What is the difference of creation process of P b0 and P b1 P b0 SiO 2 Si(100) [111] SiO 2 Si(100) [211] P b1

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Experimental Procedure Ultra-high vacuum Base press. ~ Torr flashing, 1300 ℃ anneal （ clean surface ） O 2 exposure 600 ℃ or 800 ℃ Dose ~ Langmuir, L [Pressure, Torr] x [Exposure Time, 1 sec] evacuation ~ Torr ESR measurements （ room temperature ） Si oxidation and ESR measurement

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 In-situ ESR spectra during oxidation Wet SiO 2 2x1 clean surface 15s 5400s 200s P b0 P b1 SiO 2 Si(100) [211] P b0 P b1 SiO 2 Si(100) [111] B//[100]

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Simulation of the ESR signal (deconvolution) 1800s P b0 Experimental Simulation P b1 SiO 2 Si(100) [111] SiO 2 Si(100) [211]

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Interface defect at Si(100)/ SiO 2 [P b1 ] begins to increase after [P b0 ] saturation Density ~ 10 4 sec is close to thick SiO 2 case Si Total P b0 P b1 clean surface ~6 Å SiO 2

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Discussion A. Stirling et al. PRB 85(2000)2773 1st-Principles calculation For Si oxidation process Interface always moves to next new deeper layer. ・ interface Si layer close to Si side makes P b0 ・ 2nd step Si oxidation makes P b1 It is known that island growth occurs. Interface defect is not located at the step edge line. Interface defect is determined by microscopic chemical reaction not macroscopic stress. the same as the Si (111) case,

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Tentative model for defect generation P b1 P b0

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 SiO 2 formation process of Si (100) surface Interface defect creation process during Si (100) oxidation has been observed using UHV-ESR system. P b0 center is formed at 1st Si layer oxidation P b1 center is formed at 2nd Si layer oxidation Density of P b0 and P b1 centers are determined at the initial process. (independent of thickness) Interface defect is determined by microscopic chemical reaction. not macroscopic stress.

Nano and Giga Challenges in Microelectronics Symposium and Summer School, Cracow, September 13-17, 2004 Conclusion Dynamic change of surface and interface db of Si was successfully observed using ultra-high vacuum ESR system. Oxygen termination process of Si (111) Wavefunction of Si(111) surface dbs does not localized. Electronic structure of ad-atom db is similar to P b center. Interface defect creation process during Si (100) oxidation P b0 center is formed 1st Si layer oxidation P b1 center is formed 2nd Si layer oxidation Experimental results suggest that interface defect of P b, P b0, P b1 centers are created by atomic level chemical reactions, Not macroscopic stress between Si and SiO 2. Comparison with theoretical work is important.