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Static aes-the adequate mode of aes for surface reaction and submonolayer adsorption studies
Journal of Electron Spectroscopy and Related Phenomena, 14 (1978) 19-25 @ Elsewer Scientific Pubhshmg Company, Amsterdam - Printed m The Netherlands
STATIC AES-THE ADEQUATE MODE AND SUBMONOLAYER ADSORPTION
A
BENNINGHOVEN,
0
GANSCHOW,
OF AES FOR SURFACE STUDIES
P STEFFENS
REACTION
and L WIEDMANN
Physrkalrsches Instltut, Unrversrtat Munster, Schlossplatz 7, 4400 Munster (W
Germany)
(First recewed 11 October 1977, m final form 12 December 1977)
ABSTRACT
Evidence 1s presented of a need for static AES, 1 e du-ect countmg of the N(E) dlstrlbutlon at low primary electron current densltles and under UHV condltlons, m the mvestlgatlon of metal-adsorbate interactions m the monolayer and submonolayer range After appropriate modifications of the standard equipment, sufficient sensltlvrty can be reached by tlus method without undesired peak dlstortlon by potential modulation and beam-induced surface changes INTRODUCTION
The conventional surface analysis by Auger electron spectroscopyl* ’ using high primary electron beam currents and the potential modulation techmque3 has a number of serxous disadvantages if apphed to the mvestlgatlon of surface reactlons and adsorption of gases m the monolayer and submonolayer range The high electron current densltles usually applied durmg AES analysis are responsible for a number of beam effects such as hydrocarbon crackmg, electroninduced desorptlon or surface diffusion4 For 3-keV electrons, a total electron dose density of about 2 1s the upper hmlt for Auger analysis without lOa A s cm-’ changes of the surface by these effects4 This 1s about lo- 5 times the dose density needed by the usual commercially available AES systems5 for deriving a complete AES spectrum In addition, at these high electron currents contammatlons from the electron gun may influence the surface condltlons conslderably6 The dN/dE spectra obtamed by the potential modulation techmque3 suffer addltlonally from the fact that the slgnal strength depends on the ratio between the energy of the observed Auger peak and the modulation amphtude7-’ Auger features in the energy range below 150 eV, 1 e around the energy of mmlmum escape depth, are especially affected by overmodulatlon, since generally Auger spectra of sohd surfaces are taken with fixed modulation amplitude over an extended energy range This
20 comphcates the interpretation of chemrcal changes durmg surface reactions The Auger peak-shape IS expected to be a function of chemical environment’ O11I, especially m the low-energy range where most of the valence-band and cross transltlons occur So when mvestlgatmg the chemical state of a surface rather than Its elemental composltlon only, one has to look carefully for peak-shape changes Instead of compensatmg such effects by a suitable modulation waveform’ Fmally, m the case of the double-pass cylmdrlcal mirror analyze?, the transmission of the lO-kHz reference to the outer cylinder IS far from satisfactory, the result being a “phase dIstortIon effect”12 which affects the diV/dE spectra drastrtally m the low-energy range Therefore standard AES IS a rather hmlted method for static surface analysrs The features mentloned above therefore require Improvement of the standard techniques towards lower primary electron dose and du-ect detection of the N(E) spectrum EXPERIMENTAL
The standard Physlcal Electronics Inc coaxial electron gun IS dehvered with a gun control wbch IS not deslgned for stable operating condltlons at low beam currents m the range required for static AES This current range can only be achieved by reducing the filament current to a region where no thermal equlhbrmm between the filament and Its surroundmgs ~111occur wlthm a reasonable time Thus the beam current IS very unstable m this range, and quantltatlve measurements are not possible Therefore the gun IS operated at constant filament current throughout, thus estabhshmg constant emlsslon propertles of the filament The beam current IS only controlled by applymg a suitable potential to the Wehnelt electrode Thus the beam can be switched externally without changing the filament parameters, and stable operating condltlons are achieved down to some IO- 1’ A beam current The lower secondary electron yield at low beam currents has to be compensated by enhanced detectlon sensltlvlty We have accomphshed this by using a new preamphfier capable of detectmg multlpher pulses up to a pulse-pair resolution of 8 ns together with a dlgltal ratemeter, both developed by one of us (P S ) The preamphfier drives both the standard lock-m amplifier and the ratemeter The latter instrument allows the followmg modes of operation (1) direct pulse-counting, (11) differentiation of mcommg pulse rate with respect to time, thus deriving dN/dE during an energy sweep, and (m) lock-m technique by provldmg a smusoldal reference output and phase-sensltrve detection of mcommg pulses Finally, the sensltlvlty at low electron energies was enhanced by increasing the acceleration voltage between the inner cyhnder of the CMA and the first dynode of the multlpher from 45 V, as scheduled by the manufacturer, to 200 V, so that the secondary electrons now arrive at the dynode with energy E = Ekln + 200 eV, which IS necessary
d,
a
Figure 1 AES spectrum of silver, dN/dE by standard lock-m techmque Primary electron energy 3 keV, beam current density 0 6 A cm-z, sweep rate and total electron density not leportedl”
loo
200
300
400 electron
eV
energy
Figure 2 AES spectrum of sliver, dN/dE by standard lock-m technique 3 keV, beam current density 0 2 A cm-2, sweep rate 6 7 eV s-l, total electron
Primary electron energy dose density 12 A s cm-2
22 for ensuring a nearly constant detection range
probablhty
throughout
the whole energy
RESULTS
The characterlstlc features of our Improved experlmental condltlons will be shown by comparison of AES spectra from a clean silver surface Figure 1 shows a standard AES spectrum by Palmberg et al 13 taken by standard lock-m techmque at 50 ,xA beam current The Improvement m the low-energy range by usmg a higher accelerating voltage as mentioned above IS shown m Fig 2 Figure 3 shows the N(E) spectrum of silver taken m the pulse counting mode with a beam current density of lo- ’ A cm- ’ (5 lo-’ A onto a beam spot of about 0 8 mm diameter), correspondmg to a total dose density of about 2 lo- 4 A s cm- * This IS considered as the upper limit of statrc analysls4 Dose densltles lower by one or two orders of magnitude are still possible, but accompamed by some loss m the signal-to-noise ratlo The signal-to-background ratio of about umty m the N(E) spectrum has to be compared with the general belief’ 4 that electron-excited AES from sohd surfaces has to be done m the lock-m mode smce the bad slgnal-to-background ratio does not allow direct detectlon For comparison, the dN/dE spectrum taken by dn-ect dlfferentlatlon of the rncommg pulses under Identical condltlons IS shown In Fig 4 The relevant structure of the silver spectrum IS revealed clearly, although the signal-to-noise ratlo has deteriorated as compared to the high-current lock-m spectrum of Fig 2 On the other hand, the fine structure of the secondary electron spectrum m the low-energy range IS resolved considerably better than by standard lock-m technique We emphasrze that
kcps 300 G z zoc
100
0
0
2o0300
0
lm
ev
&xl electron
energy
Figure 3 AES spectrumof silver, N(E) by direct countmg Primary electron energy 5 keV, beam current density 10-G A cm-z,
sweep rate 2 eV s-l,
total electron dose density 2
10-a A s cm-2
23
I
I
xx,
0
2cQ
300
400
ev
ekctrm energy Figure 4 AES spectrum of sliver, dN/dE by direct dlfferenttatlonof mcommg pulses Experimental parametersas m Fig 3
o0
'
5
x) L oxygen dose
Figure 5 0 KLL signal from molybdenum as a function of oxygen exposure Beam current density 0 2 A cm-2 (0 0 0) and 4 10e8 A cm-2 (I I I)
m the 0-50-eV range the spectrum of Fig 4 IS the derlvatlve of the N(E) &strlbutlon, whereas Frg 2 or even Fig 1 are obviously not This IS mamly due to the overmodulatlon and phase dlstortlon errors mentioned above The need for static analysis IS demonstrated by some results obtained during the mvestigatlon of molybdenum-oxygen mteractlon l5 In order to get reproducible AES results dunng oxygen exposure of MO, electron currents as low as 2 lo- lo A (correspondmg to 4 IO- a A cm- 2, had to be apphed Figures 5 and 6 show the course of the 0 KLL and MO NVV signals during oxygen exposure for static condltlons and for high current density lock-m measurements The differences due to beam-
10 oxygen
L dose
Figure 6 MO IV23VV slgnal as a function of oxygen exposure ExperImentalparametersas m Fag 5 asslsted changes of the Mc+O mteractlon are obvious The changes m the curve shapes of Figs 5 and 6 were not investigated systematically as a function of the electron beam current density Curves taken with a beam current density of 1 IO- 6 A cmm2 rather resembled the “high current” shape, whereas m the case of tltamum and cobalt 15, this current density was low enough for static condltlons Therefore It IS assumed at present that the upper current density llmlt for static AES must be determined for each system CONCLUSION It has been shown that static AES 1s an adequate mode of operation for mvestlgatlons of surface reactlons m the monolayer range The primary electron beam current can be chosen low enough to prevent undesired beam effects wlthout substantial loss In sensltlvlty, If dn-ect pulse counting of the secondary electrons 1s used Furthermore, the secondary electron dlstrlbutlon 1s represented better by the orlgmal N(E) spectrum and Its direct derlvatlve than by the dN/dE spectra obtamed by potential modulation REFERENCES 1 2 3 4 5 6 7 8 9
L A Harris, J Appl Phys , 39 (1968) 1419 K Mullcr, Sprmger Tracts Mod Phys , 77 (1975) 97 L B Leder and J A Simpson, Rev Scr In&rum, 29 (1958) 571 M Gettmgs and J P Coad, AERE-Report No 8188 (1976), and hterature cited therem P W Palmberg, J Vat Scz Techno!, 12 (1975) I79 L Wledmann, 0 Ganschow and A Bennmghoven, J Electron Spectrosc Relat Phenom , 13 (1978) 243 J T Grant, T W Haas and J E Houston, Surf SCI ,42 (1974) 1 R W Sprmger and D J Packer, Rev SCI Instrum , 48 (1977) 74 J T Grant, R G Wolfe, M P Hooker, R W Springer and T W Haas, J Vat Scr Technoi , 14 (1977) 232
25 10 11 12 13 14 15
P H P P
J Bassett and T E Gallon, J Electron Spectrosc Relat Phenom , 2 (1973) 101 E Bishop, J C Rlvlere and J P Coad, Surf Scl , 24 (1971) 1 W PaImberg, Physzcal ECectronrcs Inc Techmcal Note, 1976 W Palmberg, G E fiach, R E Weber and N C MacDonald, ffandbook ofAuger Electron Spectroscopy, Physical Electronics Inc , Edma, 1972 See, for example, A Joshl, L E Davrs and P W Palmberg, m A W Czandema (Ed ), Methods of Surface Analysrs, Elsevler, Amsterdam, 1975, p 159 L Wledmann, 0 Ganschow and A Bennmghoven, 24th NatzonaI Vacuum Symposium, Boston (1977). to be published m J Vat Scr Techno! (1978)
STATIC AES-THE ADEQUATE MODE AND SUBMONOLAYER ADSORPTION
A
BENNINGHOVEN,
0
GANSCHOW,
OF AES FOR SURFACE STUDIES
P STEFFENS
REACTION
and L WIEDMANN
Physrkalrsches Instltut, Unrversrtat Munster, Schlossplatz 7, 4400 Munster (W
Germany)
(First recewed 11 October 1977, m final form 12 December 1977)
ABSTRACT
Evidence 1s presented of a need for static AES, 1 e du-ect countmg of the N(E) dlstrlbutlon at low primary electron current densltles and under UHV condltlons, m the mvestlgatlon of metal-adsorbate interactions m the monolayer and submonolayer range After appropriate modifications of the standard equipment, sufficient sensltlvrty can be reached by tlus method without undesired peak dlstortlon by potential modulation and beam-induced surface changes INTRODUCTION
The conventional surface analysis by Auger electron spectroscopyl* ’ using high primary electron beam currents and the potential modulation techmque3 has a number of serxous disadvantages if apphed to the mvestlgatlon of surface reactlons and adsorption of gases m the monolayer and submonolayer range The high electron current densltles usually applied durmg AES analysis are responsible for a number of beam effects such as hydrocarbon crackmg, electroninduced desorptlon or surface diffusion4 For 3-keV electrons, a total electron dose density of about 2 1s the upper hmlt for Auger analysis without lOa A s cm-’ changes of the surface by these effects4 This 1s about lo- 5 times the dose density needed by the usual commercially available AES systems5 for deriving a complete AES spectrum In addition, at these high electron currents contammatlons from the electron gun may influence the surface condltlons conslderably6 The dN/dE spectra obtamed by the potential modulation techmque3 suffer addltlonally from the fact that the slgnal strength depends on the ratio between the energy of the observed Auger peak and the modulation amphtude7-’ Auger features in the energy range below 150 eV, 1 e around the energy of mmlmum escape depth, are especially affected by overmodulatlon, since generally Auger spectra of sohd surfaces are taken with fixed modulation amplitude over an extended energy range This
20 comphcates the interpretation of chemrcal changes durmg surface reactions The Auger peak-shape IS expected to be a function of chemical environment’ O11I, especially m the low-energy range where most of the valence-band and cross transltlons occur So when mvestlgatmg the chemical state of a surface rather than Its elemental composltlon only, one has to look carefully for peak-shape changes Instead of compensatmg such effects by a suitable modulation waveform’ Fmally, m the case of the double-pass cylmdrlcal mirror analyze?, the transmission of the lO-kHz reference to the outer cylinder IS far from satisfactory, the result being a “phase dIstortIon effect”12 which affects the diV/dE spectra drastrtally m the low-energy range Therefore standard AES IS a rather hmlted method for static surface analysrs The features mentloned above therefore require Improvement of the standard techniques towards lower primary electron dose and du-ect detection of the N(E) spectrum EXPERIMENTAL
The standard Physlcal Electronics Inc coaxial electron gun IS dehvered with a gun control wbch IS not deslgned for stable operating condltlons at low beam currents m the range required for static AES This current range can only be achieved by reducing the filament current to a region where no thermal equlhbrmm between the filament and Its surroundmgs ~111occur wlthm a reasonable time Thus the beam current IS very unstable m this range, and quantltatlve measurements are not possible Therefore the gun IS operated at constant filament current throughout, thus estabhshmg constant emlsslon propertles of the filament The beam current IS only controlled by applymg a suitable potential to the Wehnelt electrode Thus the beam can be switched externally without changing the filament parameters, and stable operating condltlons are achieved down to some IO- 1’ A beam current The lower secondary electron yield at low beam currents has to be compensated by enhanced detectlon sensltlvlty We have accomphshed this by using a new preamphfier capable of detectmg multlpher pulses up to a pulse-pair resolution of 8 ns together with a dlgltal ratemeter, both developed by one of us (P S ) The preamphfier drives both the standard lock-m amplifier and the ratemeter The latter instrument allows the followmg modes of operation (1) direct pulse-counting, (11) differentiation of mcommg pulse rate with respect to time, thus deriving dN/dE during an energy sweep, and (m) lock-m technique by provldmg a smusoldal reference output and phase-sensltrve detection of mcommg pulses Finally, the sensltlvlty at low electron energies was enhanced by increasing the acceleration voltage between the inner cyhnder of the CMA and the first dynode of the multlpher from 45 V, as scheduled by the manufacturer, to 200 V, so that the secondary electrons now arrive at the dynode with energy E = Ekln + 200 eV, which IS necessary
d,
a
Figure 1 AES spectrum of silver, dN/dE by standard lock-m techmque Primary electron energy 3 keV, beam current density 0 6 A cm-z, sweep rate and total electron density not leportedl”
loo
200
300
400 electron
eV
energy
Figure 2 AES spectrum of sliver, dN/dE by standard lock-m technique 3 keV, beam current density 0 2 A cm-2, sweep rate 6 7 eV s-l, total electron
Primary electron energy dose density 12 A s cm-2
22 for ensuring a nearly constant detection range
probablhty
throughout
the whole energy
RESULTS
The characterlstlc features of our Improved experlmental condltlons will be shown by comparison of AES spectra from a clean silver surface Figure 1 shows a standard AES spectrum by Palmberg et al 13 taken by standard lock-m techmque at 50 ,xA beam current The Improvement m the low-energy range by usmg a higher accelerating voltage as mentioned above IS shown m Fig 2 Figure 3 shows the N(E) spectrum of silver taken m the pulse counting mode with a beam current density of lo- ’ A cm- ’ (5 lo-’ A onto a beam spot of about 0 8 mm diameter), correspondmg to a total dose density of about 2 lo- 4 A s cm- * This IS considered as the upper limit of statrc analysls4 Dose densltles lower by one or two orders of magnitude are still possible, but accompamed by some loss m the signal-to-noise ratlo The signal-to-background ratio of about umty m the N(E) spectrum has to be compared with the general belief’ 4 that electron-excited AES from sohd surfaces has to be done m the lock-m mode smce the bad slgnal-to-background ratio does not allow direct detectlon For comparison, the dN/dE spectrum taken by dn-ect dlfferentlatlon of the rncommg pulses under Identical condltlons IS shown In Fig 4 The relevant structure of the silver spectrum IS revealed clearly, although the signal-to-noise ratlo has deteriorated as compared to the high-current lock-m spectrum of Fig 2 On the other hand, the fine structure of the secondary electron spectrum m the low-energy range IS resolved considerably better than by standard lock-m technique We emphasrze that
kcps 300 G z zoc
100
0
0
2o0300
0
lm
ev
&xl electron
energy
Figure 3 AES spectrumof silver, N(E) by direct countmg Primary electron energy 5 keV, beam current density 10-G A cm-z,
sweep rate 2 eV s-l,
total electron dose density 2
10-a A s cm-2
23
I
I
xx,
0
2cQ
300
400
ev
ekctrm energy Figure 4 AES spectrum of sliver, dN/dE by direct dlfferenttatlonof mcommg pulses Experimental parametersas m Fig 3
o0
'
5
x) L oxygen dose
Figure 5 0 KLL signal from molybdenum as a function of oxygen exposure Beam current density 0 2 A cm-2 (0 0 0) and 4 10e8 A cm-2 (I I I)
m the 0-50-eV range the spectrum of Fig 4 IS the derlvatlve of the N(E) &strlbutlon, whereas Frg 2 or even Fig 1 are obviously not This IS mamly due to the overmodulatlon and phase dlstortlon errors mentioned above The need for static analysis IS demonstrated by some results obtained during the mvestigatlon of molybdenum-oxygen mteractlon l5 In order to get reproducible AES results dunng oxygen exposure of MO, electron currents as low as 2 lo- lo A (correspondmg to 4 IO- a A cm- 2, had to be apphed Figures 5 and 6 show the course of the 0 KLL and MO NVV signals during oxygen exposure for static condltlons and for high current density lock-m measurements The differences due to beam-
10 oxygen
L dose
Figure 6 MO IV23VV slgnal as a function of oxygen exposure ExperImentalparametersas m Fag 5 asslsted changes of the Mc+O mteractlon are obvious The changes m the curve shapes of Figs 5 and 6 were not investigated systematically as a function of the electron beam current density Curves taken with a beam current density of 1 IO- 6 A cmm2 rather resembled the “high current” shape, whereas m the case of tltamum and cobalt 15, this current density was low enough for static condltlons Therefore It IS assumed at present that the upper current density llmlt for static AES must be determined for each system CONCLUSION It has been shown that static AES 1s an adequate mode of operation for mvestlgatlons of surface reactlons m the monolayer range The primary electron beam current can be chosen low enough to prevent undesired beam effects wlthout substantial loss In sensltlvlty, If dn-ect pulse counting of the secondary electrons 1s used Furthermore, the secondary electron dlstrlbutlon 1s represented better by the orlgmal N(E) spectrum and Its direct derlvatlve than by the dN/dE spectra obtamed by potential modulation REFERENCES 1 2 3 4 5 6 7 8 9
L A Harris, J Appl Phys , 39 (1968) 1419 K Mullcr, Sprmger Tracts Mod Phys , 77 (1975) 97 L B Leder and J A Simpson, Rev Scr In&rum, 29 (1958) 571 M Gettmgs and J P Coad, AERE-Report No 8188 (1976), and hterature cited therem P W Palmberg, J Vat Scz Techno!, 12 (1975) I79 L Wledmann, 0 Ganschow and A Bennmghoven, J Electron Spectrosc Relat Phenom , 13 (1978) 243 J T Grant, T W Haas and J E Houston, Surf SCI ,42 (1974) 1 R W Sprmger and D J Packer, Rev SCI Instrum , 48 (1977) 74 J T Grant, R G Wolfe, M P Hooker, R W Springer and T W Haas, J Vat Scr Technoi , 14 (1977) 232
25 10 11 12 13 14 15
P H P P
J Bassett and T E Gallon, J Electron Spectrosc Relat Phenom , 2 (1973) 101 E Bishop, J C Rlvlere and J P Coad, Surf Scl , 24 (1971) 1 W PaImberg, Physzcal ECectronrcs Inc Techmcal Note, 1976 W Palmberg, G E fiach, R E Weber and N C MacDonald, ffandbook ofAuger Electron Spectroscopy, Physical Electronics Inc , Edma, 1972 See, for example, A Joshl, L E Davrs and P W Palmberg, m A W Czandema (Ed ), Methods of Surface Analysrs, Elsevler, Amsterdam, 1975, p 159 L Wledmann, 0 Ganschow and A Bennmghoven, 24th NatzonaI Vacuum Symposium, Boston (1977). to be published m J Vat Scr Techno! (1978)