Thermal and Low-energy Ion- mediated Surface Chemistry of alocarbons on (111)7 7 7 and O 2 Zhenhua e Department of Chemistry 1
Outline Introduction (111) surface Experimental Motivation alocarbons Present work Modification of (111)7x7 and O 2 surfaces by low-energy Ion-mediated surface reactions of three fluorocarbons Formation of novel adstructures and functionalization of (111) surfaces using thermal evolutions of six-member family of chloroethylenes Summary & Outlook 2
Introduction (111) substrate Diamond structure of licon is the predominant material in the semiconductor industry Ideal platform for studying sitespecific surface reactions on (111)7x7 template (111)7x7 surface Reconstruction 1x1 7x7 3
Present work - Procedure Ion-mediated surface reactions Low-energy (50 ev) fluorocarbon ions Chemisorption at RT A family of six chloroethylenes Adsorption Properties (by vibrational EELS) Surface structure (by LEED) Thermal desorption Adsorption Energy & geometry (by DFT) Surface chemistry (by temperature dependent EELS, TDS) Surface condition (111)7x7 Vitreous O 2 Ar sputtered (more defects) 4
Introduction Experimental & theoretical methods elmholtz cage µ metal shield Surface analysis Experimental UV chamber (P<10-10 Torr) Vibrational Electron Energy Loss Spectroscopy (EELS) Thermal Desorption Spectrometry (TDS) Low Energy Electron Diffraction (LEED) Computational Density functional theory (DFT) Gaussian 03 package Geometry and energy of chemisorption 5
Principle: E = hω = E impact E scattered Experimental: EELS Monochromator Analyzer Monoenergetic electron beam Overall energy resolution FWM E 1/2 = Sqrt [( E 1/2M ) 2 ( E 1/2A ) 2 ( E 1/2 non-ideal ) 2 ] Great efforts are needed to get better resolution Tuning the EELS system very time-consuming 6
Experimental: EELS Principle: Energy Loss E = hω = E impact E scattered Dipole Scattering Selection Rules Active Inactive Counts of Scattered Electrons x50 820 1070 1360 120 150 510 1050 135 1510 2900 2980 (111)7x7 (c) 2 C=C 2 1000 L Three frequency ranges (b) 2 C=CF 2 10000 L (a) 2 C=C 2 100 L 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) Scattering Modes: Dipole mode--dominated at specular direction Impact mode--better resolved at off-specular direction 7
Experimental: TDS setup Intensities for different masses amount of desorbed species Desorption temperature bonding energy & adsorption geometry Shape of TDS profiles reaction order Relative Intensity 100 80 60 40 Cracking pattern 2 C=C 2 Wepil NIST Cracking pattern for a specific molecule 20 0 20 30 40 50 60 70 80 90 100 110 m/z 100 L 2 C=C 2 on (111)7x7 2 Temperature (K) 1000 800 600 400 Temperature ramps as a linear function of time Relative Intensity X 0.5 X 0.2 X 0.02 C 2 2 2 C 2 2 2 200 0 0 100 200 300 400 500 600 Time (s) 400 600 800 1000 Temperature (K) 8
Experimental - LEED Structure of the near-surface region Changes of surface structure when exposed to different types/amounts gases (111)7x7 100 L iso-dce/(111)7x7 9
Fluorocarbons: Objective & Motivation To study the interaction of low-energy fluorocarbon ions with (111)7x7 and vitreous O 2 at molecular level (mechanisms and reaction pathways) Scientific reasons alo licon Surface Chemistry Ion Surface Chemistry Practical reasons To understand the fundamental processes in plasma etching and film deposition. New inter-metal dielectric films : alonated O 2 alocarbonated-o 2 10
Introduction alocarbons Plasma Schematic diagram of Plasma etching system Our simple ion gun igh aspect ratio etch MEMS example alocarbon gas of interest is cracked and ionized by electrons inside the ion gun Postive halocarbon ion beam with high energy (>500 ev) is formed by the lenses Low-energy ions are obtained by the floating voltage on the sample 11
Vibrational frequencies (in cm - 1 ) of reference silicon and related systems Vibrational mode This work Ref. [21] F bend F 2 bend F 3 bend 337 Ref. [22] F 4 bend 380 Ref. [23,24,25,26] C stretch 810-850 750-950 F stretch 850 825 F 2 stretch 340 780-885 300 827, a 870 a, 920, b 965 b 920 F 3 stretch 838, a 1015 b 965 stretch 2080-2180 2088-2120 C stretch 2860 2860-3000 C 2 stretch C 3 stretch 2879-2955 12
Fluorocarbon ions: exposure on 7x7 CF 4 C 2 F 2 C 2 2 F 2 Relative Intensity (arbitrary units) x20 340 δ Fx 885 870 (111)7x7 CF 4 at 50 ev (a) 1 kl (b) 2.5 kl (c) 5 kl (d) 10 kl Relative Intensity (arbitrary units) ν Fx = 780-885 x20 340 850 (111)7x7 C 2 F 2 at 50 ev (a) 1 kl (b) 2.2 kl (c) 5 kl (d) 10 kl ν C 830 2100 (e) 10 kl, 50 ev 300-380 2860 845 ν C = 810-850 ν ν C 2130 (d) 5 kl, 50 ev 3710 120 810 2110 (c) 2 kl, 50 ev 780 (d) (d) 2095 (c) (b) 1 kl, 50 ev (c) 180 2080 (b) 810 (a) 10 kl, 0 ev 160 (b) (a) (a) Relative Intensity (arbitrary units) x50 850 1360 ν (111)7x7 2 C=CF 2 2900 0 500 1000 1500 2000 2500 Energy Loss (cm -1 ) CF 3 (78%) CF 2 (10%) CF (3%) cm -1 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) C 2 F (38%) CF 2 (37%) CF (11%) CF (4%) 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) C 2 2 F 2 (30%) C 2 2 F (20%) CF (15%) C 2 F (12%) C 2 F (11%)
Fluorocarbon ions: (111) vs. O 2 CF 3 incident on 7x7 CF 3 incident on O 2 Relative Intensity (arbitrary units) x5 180 340 810 810 865 885 (111)7x7 (a) 10 kl CF 4 at 50 ev (b) annealed to 700 K (c) annealed to 900 K (d) annealed to 1100 K δ Fx (d) (c) (b) (a) ν C Relative Intensity (arbitrary units) x10 385 δ O- 385 710 180 385 840 950 960 1040 1040 ν -O- Vitreous O 2 (e) sample (c) annealed to 900 K (d) sample (c) annealed to 700 K (c) 10 kl CF 4 at 50 ev (b) 5 kl CF 4 at 50 ev (a) Vitreous O 2 Symmetric 710 Asymmetric 1040 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 Energy Loss (cm -1 ) Energy Loss (cm -1 ) More F x, and fewer C on O 2
Fluorocarbon ions: C 2 F 2 vs. C 2 2 F 2 the effect of C=C bond Relative Intensity (arbitrary units) x10 180 385 850 790 920 385 710 1040 1040 2180 2860 Vitreous O 2 (f) 900 K (e) 830 K (d) 780 K (c) 700 K (b) 10 kl C 2 F 2 at 50 ev (a) Vitreous O 2 ν -O- Relative Intensity (arbitrary units) x20 160 840 385 690 770 1050 2185 Vitreous O 2 2 C=CF 2 at 50 ev 2920 (g) 900 K (f) 810 K (e) 730 K (d) 20 kl (c) 10 kl (b) 2 kl (a) O 2 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) Fewer F x on O 2 produced by ion-irradiation in C 2 2 F 2
Fluorocarbon ions: TDS CF 4 and C 2 F 2 C 2 2 F 2 Relative Intensity 11 10 9 8 7 6 5 4 3 2 1 0 2 1 0 2 1 (c) mass 85 (b) mass 66 (a) mass 47 0 300 400 500 600 700 800 900 1000 Temperature (K) 10 kl CF 4 at 50 ev (111)7x7 Vitreous O 2 10 kl C 2 F 2 at 50 ev (111)7x7 Vitreous O 2 Relative Intensity 6 5 4 3 2 1 0 2 1 0 (b) mass 85 (a) mass 66 300 400 500 600 700 800 900 1000 1100 Temperature (K) 10 kl C 2 2 F 2 at 50 ev (111)7x7 Vitreous O 2 10 kl C 2 2 F 2 (111)7x7 Vitreous O 2 More desorption of F 2 and F 4 from O 2 ion-irradiated in CF 4 or C 2 F 2 Fewer desorption of F 2 and F 4 from O 2 ion-irradiated in C 2 2 F 2 than from si(111)7x7
Fluorocarbon ions (50 ev): Summary C x F y or C 2 x F y Bombardment F 4 desorption F 2 and/or 2 Carbon state transition plus... Reaction layer = F x and carbonaceous layer 350-700 K C alloy and/or ydrocarbon fragments 700-1000 K licon carbide Carbon diffusion into bulk CF x layer is only formed at sufficiently high dose of CF 3 ion incidence (>3x10 16 cm -2,. Toyoda,. Morishima, R. Fukute, Y. ori, I. Murakami, and. Sugai, J. Appl. Phys. 95 (2004) 5172) In our case, the estimated dose of CF 3 ion incident on the sample surface is ~10 16 cm -2 for 10,000 L exposure of CF 3 ions Ion current ~ 200nAcm -2 at 1x10-6 Torr ion dose ~ 1x10 16 cm -2 for 10,000 L of CF 3 ions 17
Fluorocarbon ions (50 ev): Summary The resulting surface products on (111)7x7 and vitreous O 2 obtained by the irradiation of low-energy fluorocarbon ions from three fluorocarbons with different F-to-C ratio have been examined systematically On (111)7x7, the ion-irradiation in the three fluorocarbons produced similar reaction layer: fluorosilyl (F x ) and carbonaceous cluster (C and/or hydrocarbon fragments) On vitreous O 2, more fluorosilyl (F x ) formed than on 7x7 surface when the surfaces exposed to the same dose of ion-irradiation generated in CF 4 or C 2 F 2 But, the same dose of ion-irradiation generated in C 2 2 F 2 produced a smaller amount of fluorosilyl (F x ) on (111)7x7 than on vitreous O 2 The presence or absence of a C=C bond in the sputtering gas and the F-to-C ratio of a sputtering gas are found to play important roles 18
Introduction Chloroethylenes Common industrial chemicals for processing and treatment of silicon wafers An ideal platform for studying geometric isometric effects and the effects of halogen substitution on organosilicon chemistry Competition among several plausible reaction mechanisms: cycloaddition to diradical on surface, and formation of surface vinyl via cleavage of C bonds as well as dative bonding resulting from the inductive effect of atoms Adsorbates of interest TCE PCE 912 cluster diradical Å 4.6 2.4 4. 3 2.9 Å Å MCE content isometric iso- trans- cis-dce 3.3 Å 19 Å
(111)7x7 20
Chloroethylene chemisorption: content effect Relative Intensity (arbitrary units) x50 820 1070 1360 120 510 820 (111)7x7 C sp3 (c) 2C=C2 Cycloaddition 2900 νc2 (b) 2C=C 150 510 2980 1050 ν1510 C=C 1000 2000 135 0 C sp2 (a) 2C=C2 3000 4000 5000 Energy Loss (cm-1) ν = 510 cm-1 Dechlorination Primary channel for 2 and more substitution 21
Chloroethylene - isometric effect Relative Intensity Normalized to the Elastic Peak (%) Different adstructures Chlorovinyl and vinylidene Low saturation exposure for molecules with =C 2 group Steric effect between cis-dce and trans-dce 1.0 0.8 0.6 0.4 0.2 C stretch 2980 cm -1 trans-dce cis-dce iso-dce TCE Relative Intensity (arbitrary units) x50 160 A 120 135 130 510 1150 740 780 1050 1360 820 1150 1510 2980 (111)7x7 (d) trans - C 2 2 2 (c) cis-c 2 2 2 (b) iso-c 2 2 2 (a) C 2 3 0.0 0 200 400 600 800 1000 Exposure (L) the relative intensity of EELS feature Saturation exposure 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) ν = 510 ν C=C =1510 ν C =2980 22
Chloroethylene: thermal desorption Relative Intensity (arbitrary units) 1000 L cis-c 2 2 2 on (111)7x7 x 0.2 x 0.05 2 x 0.02 2 (f) mass 98 (e) mass 96 (d) mass 63 (c) mass 36 (b) mass 26 (a) mass 2 400 600 800 1000 1200 Temperature (K) 2 2 300 K 300 K 300 K (VI) (IV) (V) 350 K 2 >450 K >450 K 2 (VI) 2 II >580 K I 2 2 III hydrocarbon fragments or C Three states of acetylene desorption Low temperature desorption Resulting from β- elimination of 2-chlorovinyl Desorption from di-σ bonded vinylene 23
Novel adstructure - vinylidene Relative Intensity (arbitrary units) x25 135 510 820 1050 880 1360 2080 ν C=C 1510 ν C2 2980 (111)7x7 2 C=C 2 (i) 950 K (h) 890 K (g) 840 K (f) 770 K (e) 720 K (d) 670 K (c) 580 K (b) 450 K (a) 100 L First observation of vinylidene on semiconductor surface produced by RT adsorption of iso-dichloroethylene on (111)7x7 The presence of the ν C=C EELS feature Supported by DFT calculations D 2 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) ν = 510 cm -1 ΔE=- 510.2 kj mol -1 δ ρ C2 24
Trichloroethylene thermal evolution First observation of acetylide on semiconductor surface produced by RT adsorption of isodichloroethylene on (111)7x7 (d) The presence of the ν C EELS feature at 3300 cm -1 2 300 K (VI) (VII) >450 K 2 2 >450 K 2 3 2 (IX) (X) (VIII) >580 K 3 hydrocarbon fragments and/or C mono-σ bonded chlorovinyl (ċ=c) di-σ bonded chlorovinylene (ċ=ċ) Relative Intensity (arbitrary units) x25 120 510 1150 820 2080 2900 ν C=C 1510 ν C 2 2980 (111)7x7 2 C=C (i ) 970 K (h) 920 K (g) 850 K 3300 (f ) 700 K (e) 580 K (d) 450 K (a) 100 L 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) (c) 400 K (b) 350 K Mono-σ bonded -C C 25
Cycloaddition, dechlorination and dative adsorption Summary of geometries found in the roomtemperature adsorption of Chloroethylenes 2 300 K (I) Dechlorination occurs for all the chloroethylenes [22] cycloaddition found in monochloroethylene on (111)7x7 Dative adsorption involving the chlorine () for all chloroethylenes on (111) (II) 300 K 2 2 2 300 K 26
Summary First systematic study of thermal evolution of a family of chloroethylenes on (111)7x7 surface Dechlorination occurs for all the chloroethylenes; [22] cycloaddition found monochloroethylene/(111); Dative adsorption involving the for all chloroethylenes/(111) First observation of novel adstructures formed by chemisorption of chloroethylenes: in particular, di-σ bonded vinylidene on semiconductor surface First comprehensive study of the interactions of low fluorocarbon ions with (111) and O 2 surfaces using more-surface sensitive technique: vibrational EELS The presence or absence of a C=C bond in the sputtering gas and the F-to-C ratio of a sputtering gas are found to play important roles on the resulting surface products obtained by the irradiation of low-energy fluorocarbon ions 27
Outlook Further studies of surface chemistry mediated by the low-energy fluorocarbons ions using more chemical-sensitive techniques (e.g. AES, electronic EELS, XPS) Extend the current study of chloroethylenes on (111)7x7 to bromoethylenes and iodoethylenes on (111)7x7 to explore the sitespecific surface reactions of (111)7x7 surface Study post adsorption of lager hydrocarbons with unsaturated bonds (e.g. styrene) on silicon surface with the vinyl and vinylidene to explore the further functionalization of silicon surface for molecular electronics 28
Thank you Dr. K.T. Leung Dr. Dan Thomas, Dr. Pu Chen and Dr. Qing-Bin Lu Dr. Qiang Li and Xiaojin Zhou Xiang Yang and Sergey Mitlin Dr. Michael Thiam, Dr. Nina einig Dr. ui Yu Entire staff of the Science Shops We, the willing Led by the unknowing Are doing the impossible For the ungrateful We have done so much for so long With so little We are now qualified To do anything With nothing 29
Chemisorption theory: searching an elephant E. Shustorovich, Accounts of Chemical Research, 21 (1988)183. 30
(111)7x7: DAS model 31
Trichloroethylene thermal evolution First observation of acetylide on semiconductor surface produced by RT adsorption of isodichloroethylene on (111)7x7 (c) (d) The presence of the ν C EELS feature at 3300 cm -1 2 300 K 300 K (V) (VI) (VII) 350 K >450 K 2 2 >450 K 2 3 2 (IX) (X) (VIII) >580 K 3 hydrocarbon fragments and/or C Relative Intensity (arbitrary units) x25 120 510 1150 820 1510 2080 2900 2980 (111)7x7 2 C=C (i ) 970 K (h) 920 K (g) 850 K 3300 (f ) 700 K (e) 580 K (d) 450 K (a) 100 L 0 1000 2000 3000 4000 5000 Energy Loss (cm -1 ) -C C (c) 400 K (b) 350 K 32
(111)7x7 33
In our case, the estimated dose of CF 3 ion incident on the sample surface is ~10 16 cm -2 for 10,000 L exposure of CF 3 ions Ion current ~ 200nAcm -2 at 1x10-6 Torr ion flux ~ (2x10-7 Cs -1 cm -2 )/(1.6x10-19 C) =1.25x10 12 cm -2 s -1 ion dose ~ 1.25x10 12 cm -2 for 1L of exposure of CF 3 ions ion dose ~ 1x10 16 cm -2 for 10,000 L of CF 3 ions 34
Fluorocarbon ions (100 ev): A related work At the small dose of CF 3 ions (<0.2x10 16 cm -2 ), formation of fluorosilyl (F x ) are preferred. With increasing of dose, carbon atoms are accumulated on the surface. Such carbon layer inhibits the chemical reaction of F atoms with the, and enhances CF bond formation, together with CF 2 desorption from the surface. At the sufficiently high dose (>3x10 16 cm -2 ), a balance between the C atom supplied by CF 3 incidence and the C atom lost by CF x radical desorption is established, while F atoms on the surface approach a steady state determined by CF 3 ion incidence and the F x radical desorption. CF x layer is only formed at sufficiently high dose of CF 3 ion incidence In our case, the estimated dose of CF 3 ion incident on the sample surface is ~10 16 cm -2. Toyoda,. Morishima, R. Fukute, Y. ori, I. Murakami, and. Sugai, J. Appl. Phys. 95 (2004) 5172. 35
Surface analysis techniques
Low Energy Electron Diffraction Auger Electron Spectroscopy Ion Gun Electron gun In our case, the estimated dose of CF 3 ion incident on the sample surface is ~10 16 cm -2 for 10,000 L exposure of CF 3 ions Ion current ~ 200nAcm -2 at 1x10-6 Torr ion flux ~ (2x10-7 Cs -1 cm -2 )/(1.6x10-19 C) =1.25x10 12 cm -2 s -1 ion dose ~ 1.25x10 12 cm -2 for 1L of exposure of CF 3 ions Thermal Desorption Spectrometry ion dose ~ 1x10 16 cm -2 for 10,000 L of CF 3 ions Electron Energy Loss Spectroscopy 37