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Vacuum ultraviolet resonance line radiation source from rare gas atoms and ions for UHV photoelectron spectroscopy
Spectroscopy and Related Phenomena, 14 (1978) 143-153 @ Elsevler Sclentdk Pubhshmg Company, Amsterdam - Prmted m The Netherlands
Journal of Electron
VACUUM ULTRAVIOLET RARE GAS ATOMS AND SCOPY
G M LANCASTER*,
RESONANCE LINE RADIATION SOURCE FROM IONS FOR UHV PHOTOELECTRON SPECTRO-
J A TAYLOR**,
Departments of Chemrstry and Physzcs,
A IGNATIEV
Unrversrty
and J W RABALAIS
of Houston,
Houston, Texas 77004 ( U S A )
(First received 18 January 1978, In final form 15 February 1978)
ABSTRACT
A source and dfferentlal pumpmg system for producing high mtenslty resonance lme radiation from rare gas atoms and ions for ultrahgh vacuum (UHV) photoelectron spectroscopy has been developed Photoelectron count rates from a gold sample, as measured with a double-pass cyhndrlcal mirror analyzer at pass energy 15 eV and 0 10 eV resolution, are - 300,000 c s- ’ for the He(I) (2 1 22 eV) lme and - 30000 c s- 1 for the He(IL) (40 8 1 eV) line The source design 1s based on the prmclple of the electrostatic charged particle oscillator and 1s capable of sustalmng discharges over the pressure range 1 to - lo- 6 torr The &scharge segment consists of a cylmdncal cold cathode surrounding two tungsten rod anodes which are held at high posltlve potential Three stages of tifferentlal pumping are employed m order that the vacuum m the mam spectrometer chamber can be maintained at 2 x lo- ’ * torr during operation The calculated hehum flow reachmg the mam chamber under these comhtlons 1s < 10 1 s- ’ Details of the construction and operatmg characterlstrcs of the source are presented INTRODUCTION
The incident ra&atlon used m photoelectron spectroscopy must be monochromatic so that monoenergetlc electrons can be eJected from a given level Synchrotron radiation IS ultimately the most desirable for PES because it offers a broad contmuous range of photon energies However, smce a synchrotron IS not conveniently available for all experiments, there 1s still a need for mtense monochromatic
* R A Welch Foundation predoctoral fellow * Present address Physlcal Ekctromcs Industries, Inc ,6509 Flymg Cloud Dr , Eden Pranxe, Mmnesota 55344, U S A l
144 radiation sources The “rale ultlme”, 1 e the resonance lme produced by the electromc transltlon from the first excited state to the ground state of the atom or Ion, of the rare gases has been used most effectively m PES I- 6 In particular the He(I) and He(I1) lines at 584 33 A (21 22 eV) and 303 78 ii (40 81 eV), respectively, have been used most extensively Due to its high energy, the He(I1) hne 1s the most favorable for studying valence bands of sohds because (1) matrix element modulation effects are much less important than at lower photon energies, (11) the photoelectrons have sufficiently high kmetlc energies so that they are well removed from the melastlcally scattered electrons at low energy, and (in) semi-core levels can often be studled because of the extended range The construction of a source of stable mtense He(I1) radiation has proved to be dlfficult6 Burger and Maler’ have constructed a UV source m whch the He(I1) mtenslty 1s enhanced with respect to the He(I) intensity This source design IS based on the prmclple of the electrostatic charged particle oscillator developed by McIlralth and co-workers7 - 1’ The device consists of a cylmdrlcal cathode surroundmg two anode wires which are symmetrically disposed about the cylinder axls An electron startmg from rest wlthm a specified region follows a long oscillatory path which always passes between the two wn-es The discharge is, therefore, concentrated about an axial plane normal to the plane of the anode wires The long electron path length (up to -5 km) ’ - 1’ allows the discharge to be maintained down to - lo- 6 torr without apphcatlon of a magnetic field The efficiency of such a cold cathode discharge has resulted m its apphcatlon as an lomzatlon gauge’ ‘9 1 ‘, an ion sourcegl “9 13 and a molecular beam detector’ 4 as well as a UV source4 The UV source of Burger and Maler4 was used for gaseous PES stu&es and employed wn-e anodes without any source coohng capablhtles Hence, the power dlsslpatlon was llmlted by the wire anodes, and discharge currents of only - 25 mA could be used provldmg photoelectron counting rates m the range of -500 c s- 1 It was found that below - lo- 3 torr the He(I1) Aux was comparable to the He(I) flux The low total intensity of this source hindered Its acceptance as a useful He(I1) source for PES apphcatlons Thl% paper presents a detailed description of the construction and performance charactenstlcs of a UHV compatible source of the UV resonance lme radratlon from rare gas atoms and ions along with the details of an efficient dlfferentlal pumping system The source 1s based on the prmclple of the charged particle oscillator and uses tungsten rod anodes and water-coolmg of the source body m order to facllltate power dlsslpatlon The maxlmum He(I) and He(I1) mtenslty, measured as the photoelectron count rate from a gold film, occurs at a discharge pressure of - 8 x lo- 2 torr It 1s shown that, at Gus pressure, the source 1s operating m a transltlon mode between the true oscillatory mode and general glow discharge mode By means of the dlfferentlal pumpmg system the pressure of the mam chamber can be mamtamed at 2 x IO- lo torr durmg operation of the source.
145 GENERAL
DESCRIPTION
OF THE APPARATUS
The apparatus consists of a UV source, a dBerentla1 pumpmg and hght colhmatmg system and a UHV chamber for electron spectroscopy studies Smce the UHV chamber with Its double-pass cylmdncal mirror analyzer’ 5 and associated equipment for XPS, Auger spectroscopy and SIMS has been fully described elsewhere’ 6, only the UV source and Its pumping system will be described here The UV source, the dlfferentlal pumpmg system and the hght colhmatmg system are constructed from 304 stamless steel, machmable ceramic1 7 and Pyrex glass All permanent Jomts are made by Hellarc welding and all demountable Jomts are sealed by Conflat flanges18 with copper gaskets Vlton O-rings are present only m the faces of the gate valves The system IS completely bakeable to 250°C by means of a heating mantel constructed from heating tapes and ceramic fiber blanket’ ’ The source 1s 22 86 cm long, the differential pumping system 1s 13 34 cm long and the mam chamber flange on which the differential pumping system mounts IS 22 86 cm from the face of the sample Thus, the sample IS 36 20 cm from the discharge region The incident angle between the photon beam and surface 1s IO” and the angle between the CMA axis and surface IS 60” The resolution of the system 1s -0 10 eV measured as the full-width at halfheight of the Ar 2p3,2 line at 1 x lop4 torr Ar and a CMA pass energy of 15 eV It should be noted that this resolution 1s hmlted by the CMA analyzer and not by the UV source UItrawolet sowce The UV source, as illustrated rn Fig 1, consists of a grounded cyhndrlcal stainless steel cathode which IS 22 86 cm long with an outer diameter of 3 81 cm and mner diameter of 3 48 cm Two 0 48 cm tungsten rods separated by 0 37 cm and symmetrically displaced about the cylinder axis serve as the anodes The anode rods are isolated from the cathode by machinable ceramic disks The holes m the ceramic are sufficiently large to allow for thermal expansion of the rods caused by electron end plates 0
w stainless steel qstainless end pla tes and
connectors
0
0
Ia mochlnable II
bolts and
0
0
ceramic set screws
Figure 1 Scale drawing of the cold cathode discharge W bolts_onto the-r&t end of the source
,1
source The dlfferentlal pumping system
146 bombardment Larger holes were also drilled m the ceramics to provide adequate conductance between the source and differential pumpmg system The anode potenteal, O-5 klir, 1s supplied through a high voltage feed-through at the rear of the source Grounded stamless steel end-plates cover the inner side of the ceramic disks m order to reflect electrons traveling m the axial dtrectlon back mto the source volume Photons produced along the axls of the source can emerge through a 0 25 cm exit aperture between the anodes at the front end of the lamp An aperture with a radius that IS less than the inner diameter of the cathode dlvlded by SIX allows sampling of the radlatlon without allowmg electrons to escape along the axial plane” The lamp IS cooled by clrculatmg chdled water through 0 64 cm tubing that enclrcles the cathode The discharge gas used m these experiments was 99 9995 ‘A pure helium which flows through an activated-charcoal trap mamtamed at 77 K and a GranvdlePhrIhps variable leak valve before entering the rear of the source Power to the anodes IS supphed by a O-5 kV, O-500 mA Savant power supply Smce the resistance of the source vanes from mfimty before mltlatlon of a discharge to several kQ during discharge, it IS necessary to provide a ballast reslstor m series with the source to prevent current surges during mltlatlon of a discharge This ballast was supplied by a variable resistor set at 16 kQ At a He-pressure of 0 08 torr, the resistance of the source IS -9 kD
D@erentlal
pumpmg
system
The pumpmg system, illustrated m Fig 2, consists of three stages of differential pumping The first stage 1s across a 1 5 mm 1 d and 3 0 cm long Pyrex caprllary and IS pumped by means of a chilled-water trapped 325 1 s- ‘, 5 08 cm 011dlffuslon pump*’ This stage can be eliminated by opening a 1 91 cm control valve m a bypass to the first diffusion pump The second stage IS across a slmllar capillary and IS pumped by means of a hqmd nitrogen trapped 325 1 s- I, 5 08 cm oil diffusion pump The third
UHV
chamber
Figure 2 Scale drawmg of the dlfferentlal pumpmg system The UV source bolts onto the left end of the pumping system The mam spectrometer chamber IS at the r&t sxde of Frg 2
147 stage IS across a 2 5 mm 1 d and 22 86 cm long Pyrex capillary and IS pumped through the mam chamber by means of a 450 1 s- ’ turbomolecular pump2’ and a 500 1 s- ’ 10n pump’ * A gate valve mounted on 6 99 m o d flanges 1s used to Isolate the source and dlfferentlal pumpmg system from the mam chamber A double-sided 6 99 cm o d flange IS used to connect the gate valve to the flange of the mam chamber The kmfe edge and tapped holes for a 3 38 cm mlm-flange are machmed mto the inner face of the double-sided flange Thus supports a mml-flange with a glass-kovar seal for the third stage of dfferentlal pumping and hght colhmatlon to the sample The pressure at various stages of the dlfferentlal pumpmg IS morutored by a lomzatlon gauges, and finally a nude thermocouple gauge, two Bayard-Alpert iomzatlon gauge ’ * m the mam chamber The thermocouple gauge was calibrated against a McLeod gauge for hehum gas and IS located m the discharge regon (Fig 2) before the first stage of dlfferentlal pumping This allows measurement of the pressure directly m the discharge region over the range l-lo-’ torr In order to measure pressures below 1O- 3 torr, the bypass valve across the first stage of dlfferentlal pumping IS opened and the discharge pressure IS momtored mth the first lomzatlon gauge This ehmmates the first stage of dlfferentlal pumpmg, however, it IS not needed under these condltlons since the discharge pressure IS less than lo- 3 torr Openmg the bypass valve causes a drastic change of the pumping speed m the discharge regon which affects the photon mtensrty Plots of photoelectron mtenslty vs pressure show a dlscontmulty at lo- 3 torr due to this altered pumping speed Typical operating pressures of the vanous stages are listed m Table 1 The fiow of helium gas Q (torr 1 s- ’ ) mto the mam chamber across the final 9 m capillary IS given byz3 Q=CP, where
C (1 s- ‘) IS the conductance
TA3LE
1
TYPICAL OPE DIFFERENTIA SOURCE
of the capillary
and Pa (torr)
ATING PRESSURES (TORR) OF THE VARIOUS STAGES PUMPING FOR DIFFERENT DISCHARGE PRESSURES
IS the pressure
OF IN THE w
Dzscharge region
1st ronrzatlon gauge
2nd lomzatton gauge
Maw2 chamber
1 1 1 1 1
6 x 10-a 4 x 10-d 1 6 x 1O-4 1 x 10-S 1 x 10-a
2 8 6 4 2
4 3 2 2 2
x x x x
10-l 10-s 10-s B lo-a&
x x x x x
10-B lo-’ lo-’ 10-V lo-’
a Measured with the bypass valve to the first dlffuslon pump open
x x x x x
10-10 10-10 10-10 10-10 10-10
148 differential flowz3
across the capillary
Under
our condltlons
of predommantly
molecular
where I and L are the radms and length of the capdlary m cm, T 1s the absolute temperature and M 1s the molecular weight of the gas With the pressure condltlons hsted m Table 1, the amount of helium reaching the mam chamber 1s - 9 torr 1 s- ’ Residual gas analysis In the mam chamber using a General Electric monopole mass spectrometer showed no hydrocarbon contammatlon and only a very small mcrease m the helium signal under operating condltlons Charactematlon of the source Since the photoelebLron count rate 1s proportional to the flux of lomzmg photons, the relative mtensltles (fluxes) of He(I) and He(I1) radlatlon have been monitored by measuring the photoelectron spectrum of an evaporated gold film The evaporated film was sputter cleaned with 1 keV Arf Ions and the X-ray photoelectron spectrum showed no detectable contammatlon Typical He(I) and He(l[I) spectra of the film as collected m a multichannel analyzer are shown m Fig 3 Intensity vs discharge pressure plots were obtained by momtormg the He(I) gold band at a
IONIZATION
ENERGY
(eV)
Figure 3 TypIcal He(I) and He(I1) photoelectron gold film obtamed with the UV source
spectra of the valence
bands of an evaporated
149
--
5-
25 ii
2 ---N
0 U
-
0 >
/
z--
/
1
He
/
__--
/)@
20
I mA I 8
I
10-z
10-l
PRESSURE
-0
10-3
(torr)
Flgure 4 He(I) photoelectron count rate and voltage across the source as a function of discharge pressure at a constant current of 20 mA
20mA I I
10-l P R ESSURE
I
10-2 (torr)
Figure 5 HellI) photoelectron count rate and voltage across the source as a function of discharge pressure at a constant current of 20 mA
km&c energy of 10 5 eV and the He(H) gold band at a kmetlc energy of 30 5 eV correspondmg to a bmdmg energy of 6 4 eV The photoelectron mtenslty vs discharge pressure at constant discharge current (20 mA) 1s shown m Figs 4 and 5 for He(I) and He(II), respectively Figs 4 and 5 also show the change m voltage as a function of pressure In order to mamtam a constant current as pressure 1s reduced, the voltage across the source must be
He I 3000
:
v
\_____
0
1
10-l PRESSURE
10-2
(tom)
Figure 6 He(I) photoelectron count rate and dmharge at a constant voltage of 3000 V
current
as a function of discharge pressure
He II 3000 \--
PRESSURE
--+
v
o--Cy -I-
E z w
(torr)
Figure 7 He(II) photoelectron count rate and dwzharge current as a function of discharge pressure at a constant voltage of 3000 V
mcreased to compensate for the mcreasmg source resistance The He(I) and Se(I1) mtensltles are maxImum at pressures of 9 x lo- ’ and 8 5 x lo-’ torr, respectively The sharp inflection pomts at these pressures suggest a change m the fundamental operating mode of the source The photoelectron mtenslty vs discharge pressure at constant voltage (3000 V) 1s shown m Figs 6 and 7 for He(I) and He(II), respectively Figs 6 and 7 also
151 show the change m discharge current as a function of pressure At constant voltage the discharge current decreases as the helmm pressure decreases (source resistance increases) From Fig 6 It 1s evident that the He(I) mtenslty and discharge current follow each other very closely as pressure IS changed In contrast the He(II) mtenslty peaks at -0 16 torr and then follows the current closely at lower pressures The source 1s capable of sustammg a discharge at a pressure of 5 x TO- 5 torr, although the mtenslty of both He(I) and He(I1) at pressures below lo- 3 torr 1s less than the mtenslty at higher pressure Higher discharge currents and, hence, higher mtensltles at these low pressures could be achieved by using voltages above 5 kV DISCUSSION
It 1s Interesting to note that the change m the fundamental mode of operation of the source at 0 090 and 0 085 torr (Figs 4 and 5) 1s dependent on the electron mean free path The mean free path for electrons moving through a gasz4 IS ,$I = 4,/2L with L =
1/(J2rU2dZ)
(3)
where n 1s the number of gas molecules per unit volume and d IS the effective molecular diameter of the gas From the above equations the electron mean free path through helium IS equal to the cathode radms (1 74 cm) at a pressure of 0 083 torr The mflectlon pomts m Figs 4 and 5 correspond to this pressure At pressures below this pomt, the electron mean free path between lomung colhsrons with He-atoms 1s greater than the radms of the cathode The electrons must perform oscillatory traJectones m order to aclueve this path length At pressures above this pomt where the electron mean free path IS less than the radius of the cathode, the source operates m a conventional glow discharge mode without srgmficant contnbutlon to the lonizatlon process from 0scilIatmg electrons Rushton et al I3 have shown that there are three basic modes of operation of such a source At pressures less than ~5 x lo- 3 torr, L,, 1s many times greater than the cathode radius and the source operates as a true charged particle oscillator At pressures (-0 085 torr m our case) where &I IS equal to or greater than the cathode radms, the system 1s m a general glow discharge mode and there IS no slgmficant electron osclllatlon At intermediate pressures the system 1s m a transition mode where L,, 1s several Inches and both osclllatmg and &scharge modes occur The inflection points m Figs 4 and 5 illustrate quite dramatically the begmnmg of the osclllatmg mode of operation The ratlo of He(II)/He(I) mtenslty increases as the pressure 1s lowered below lo-’ torr However, the low current results m a low photon flux For example, at a He-pressure of 2 x lo- 4 torr with a &scharge current are produced for the of 20 mA at 4600 V, only 1500 c s - ’ of He(H) photoelectrons gold band indicated above A He(I1) PES mtenslty of -30000 c s- ’ and a He(I) PES mtenslty of - 300,000 c s- 1 can be obtamed on the gold sampIe at a He-pressure
152 of - 8 x 10- ’ torr with a discharge current of 120 mA at 3000 V and voltage across the source of 1080 V Thus, operation m the transltlon region between the true oscillatory mode and the general glow discharge mode provides the highest absolute intensity of both He(I) and He(I1) photoelectrons The use of water-cooled tubes rather than rods for the anodes would provide better power dlsslpatlon and operation at hgher currents Also, our source 1s situated 36 20 cm from the sample because of the extensive differential pumping system and large size of our spectrometer chamber This length could be greatly reduced m order to mmlmlze photon losses and increase the flux at the sample If one IS wrllmg to sacrifice mtenslty for an improved He(II)/ He(I) ratlo, then It 1s feasible to operate at lower He-pressure CONCLUSIONS
A UHV compatible source for resonance hne radiation from rare gas atoms and Ions based on the prmclple of the charged particle oscillator along with its differential pumpmg system has been described in detail The advantages of this source for PES apphcatlons are as follows (1) It 1s constructed from stainless steel, tungsten, machinable ceramic and Pyrex caplllanes, making It very rugged and durable The Pyrex caplllarles could be replaced by metal or ceramic caplllanes, since they serve only for dlfferentlal pumping, m order to eliminate all fragile components (n) The low discharge pressure condltlons coupled with the dlfferentlal pumping system make the design partrcularly appropriate for UHV stu&es of surfaces (111) The discharge 1s very stable and the intensity 1s sufficient to allow dn-ect recording of the spectrum or accumulation m a multlchannel analyzer (IV) The photoelectron count rates produced from a gold film are -300,000 c s-l for He(I) and -30000 c s-l for He(I1) at - 8 x lo- 2 torr with a discharge current of 120 mA at 3000 V (v) The source 1s simple to construct, all external parts are at ground potential and there 1s no danger of coolmg water leakage mto the vacuum system (VI) Relative mtensltles of He(I1) radiation which are comparable to those of He(I) radlatlon can be obtamed at pressures below - 10m3 torr, however the absolute rntenslty under these condltlons 1s lower than the mtenslty at lugher pressures ACKNOWLEDGMENT
Acknowledgment IS made to the donors of the Petroleum Research Fund, admmlstered by the American Chemical Society, and to the U S Army Office of Research for support of this research
153 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
J W Rabalals, Prwmples uf UItravloIet Photoelectron Spectroscopy, Wdey-Intersclence, New York, NY, 1977 D W Turner, A D Baker, C Baker and C R Brundle, Molecular Photoelectron Spectroscopy, Wiley, New York, NY, 1970 L Asbrmk, 0 Edqmst, E Lmdholm and L E Sehn, Chem Phys Letf , 5 (1970) 192 F Burger and J P Marer, J Phys E, 8 (1975) 420, J Electron Specfrosc ReIat Phenom , 5 (1974) 783 R T Poole, J Llesegang, R C G Leckey and J G Jenkm, J Electron Spectrosc Refat Phenom , 5 (1974) 773 N Ueno, A Ikegaml, Y Hayasl and S Klyono, Jpn J Appl Phys , 16 (1977) 1655 A H McIlralth, Nature, 212 (1966) 1422 A H McIlralth, J Vat Scr Technol, 9 (1972) 209 R K Fitch, T MuIvey, W J Thatcher and A H McIh-alth, J Phys D, 3 (1970) 1399 R K Fitch, T Mulvey, W J Thatcher and A H McTlraIth, J Phys E, 4 (1971) 553 R K Fitch and G J Rushton, Vacuum, 20 (1970) 445 R K Fitch and G J Rushton, J Vat Scz Technol , 9 (1972) 379 G J Rushton, K R O’Shea and R K Fetch, _J Phys D, 6 (1973) 1167 G N Peggs and A H McIlraith, J P&U E, 6 (1973) 1137 Physical Electronics Industries, Inc , Eden Prairie, Minnesota J A Taylor, G M Lancaster, A Ignatiev and J W Rabalals, J Chem Phys , 68 (1978) 1776 Corning Glass Works, Cormng, New York Varlan Vacuum Divlslon, Palo Alto, Cahforma Thermal Corporation, Huntsville, Alabama A H McIlralth, private commumcatlon Cooke Vacuum Products, Inc , Norwalk, Conn Leybold-Heraeus Vacuum Products, Inc , Monroevllle, PennsylvanIa S Dushman, Sczentrfic Foundations of Vacuum Technzque, 2nd edn , Wdey, New York, NY, 1962, p 80 J Roboz, Zntroductron to Mass Spectrometry, Wiley-Interscience, New York, NY, 1968
Journal of Electron
VACUUM ULTRAVIOLET RARE GAS ATOMS AND SCOPY
G M LANCASTER*,
RESONANCE LINE RADIATION SOURCE FROM IONS FOR UHV PHOTOELECTRON SPECTRO-
J A TAYLOR**,
Departments of Chemrstry and Physzcs,
A IGNATIEV
Unrversrty
and J W RABALAIS
of Houston,
Houston, Texas 77004 ( U S A )
(First received 18 January 1978, In final form 15 February 1978)
ABSTRACT
A source and dfferentlal pumpmg system for producing high mtenslty resonance lme radiation from rare gas atoms and ions for ultrahgh vacuum (UHV) photoelectron spectroscopy has been developed Photoelectron count rates from a gold sample, as measured with a double-pass cyhndrlcal mirror analyzer at pass energy 15 eV and 0 10 eV resolution, are - 300,000 c s- ’ for the He(I) (2 1 22 eV) lme and - 30000 c s- 1 for the He(IL) (40 8 1 eV) line The source design 1s based on the prmclple of the electrostatic charged particle oscillator and 1s capable of sustalmng discharges over the pressure range 1 to - lo- 6 torr The &scharge segment consists of a cylmdncal cold cathode surrounding two tungsten rod anodes which are held at high posltlve potential Three stages of tifferentlal pumping are employed m order that the vacuum m the mam spectrometer chamber can be maintained at 2 x lo- ’ * torr during operation The calculated hehum flow reachmg the mam chamber under these comhtlons 1s < 10 1 s- ’ Details of the construction and operatmg characterlstrcs of the source are presented INTRODUCTION
The incident ra&atlon used m photoelectron spectroscopy must be monochromatic so that monoenergetlc electrons can be eJected from a given level Synchrotron radiation IS ultimately the most desirable for PES because it offers a broad contmuous range of photon energies However, smce a synchrotron IS not conveniently available for all experiments, there 1s still a need for mtense monochromatic
* R A Welch Foundation predoctoral fellow * Present address Physlcal Ekctromcs Industries, Inc ,6509 Flymg Cloud Dr , Eden Pranxe, Mmnesota 55344, U S A l
144 radiation sources The “rale ultlme”, 1 e the resonance lme produced by the electromc transltlon from the first excited state to the ground state of the atom or Ion, of the rare gases has been used most effectively m PES I- 6 In particular the He(I) and He(I1) lines at 584 33 A (21 22 eV) and 303 78 ii (40 81 eV), respectively, have been used most extensively Due to its high energy, the He(I1) hne 1s the most favorable for studying valence bands of sohds because (1) matrix element modulation effects are much less important than at lower photon energies, (11) the photoelectrons have sufficiently high kmetlc energies so that they are well removed from the melastlcally scattered electrons at low energy, and (in) semi-core levels can often be studled because of the extended range The construction of a source of stable mtense He(I1) radiation has proved to be dlfficult6 Burger and Maler’ have constructed a UV source m whch the He(I1) mtenslty 1s enhanced with respect to the He(I) intensity This source design IS based on the prmclple of the electrostatic charged particle oscillator developed by McIlralth and co-workers7 - 1’ The device consists of a cylmdrlcal cathode surroundmg two anode wires which are symmetrically disposed about the cylinder axls An electron startmg from rest wlthm a specified region follows a long oscillatory path which always passes between the two wn-es The discharge is, therefore, concentrated about an axial plane normal to the plane of the anode wires The long electron path length (up to -5 km) ’ - 1’ allows the discharge to be maintained down to - lo- 6 torr without apphcatlon of a magnetic field The efficiency of such a cold cathode discharge has resulted m its apphcatlon as an lomzatlon gauge’ ‘9 1 ‘, an ion sourcegl “9 13 and a molecular beam detector’ 4 as well as a UV source4 The UV source of Burger and Maler4 was used for gaseous PES stu&es and employed wn-e anodes without any source coohng capablhtles Hence, the power dlsslpatlon was llmlted by the wire anodes, and discharge currents of only - 25 mA could be used provldmg photoelectron counting rates m the range of -500 c s- 1 It was found that below - lo- 3 torr the He(I1) Aux was comparable to the He(I) flux The low total intensity of this source hindered Its acceptance as a useful He(I1) source for PES apphcatlons Thl% paper presents a detailed description of the construction and performance charactenstlcs of a UHV compatible source of the UV resonance lme radratlon from rare gas atoms and ions along with the details of an efficient dlfferentlal pumping system The source 1s based on the prmclple of the charged particle oscillator and uses tungsten rod anodes and water-coolmg of the source body m order to facllltate power dlsslpatlon The maxlmum He(I) and He(I1) mtenslty, measured as the photoelectron count rate from a gold film, occurs at a discharge pressure of - 8 x lo- 2 torr It 1s shown that, at Gus pressure, the source 1s operating m a transltlon mode between the true oscillatory mode and general glow discharge mode By means of the dlfferentlal pumpmg system the pressure of the mam chamber can be mamtamed at 2 x IO- lo torr durmg operation of the source.
145 GENERAL
DESCRIPTION
OF THE APPARATUS
The apparatus consists of a UV source, a dBerentla1 pumpmg and hght colhmatmg system and a UHV chamber for electron spectroscopy studies Smce the UHV chamber with Its double-pass cylmdncal mirror analyzer’ 5 and associated equipment for XPS, Auger spectroscopy and SIMS has been fully described elsewhere’ 6, only the UV source and Its pumping system will be described here The UV source, the dlfferentlal pumpmg system and the hght colhmatmg system are constructed from 304 stamless steel, machmable ceramic1 7 and Pyrex glass All permanent Jomts are made by Hellarc welding and all demountable Jomts are sealed by Conflat flanges18 with copper gaskets Vlton O-rings are present only m the faces of the gate valves The system IS completely bakeable to 250°C by means of a heating mantel constructed from heating tapes and ceramic fiber blanket’ ’ The source 1s 22 86 cm long, the differential pumping system 1s 13 34 cm long and the mam chamber flange on which the differential pumping system mounts IS 22 86 cm from the face of the sample Thus, the sample IS 36 20 cm from the discharge region The incident angle between the photon beam and surface 1s IO” and the angle between the CMA axis and surface IS 60” The resolution of the system 1s -0 10 eV measured as the full-width at halfheight of the Ar 2p3,2 line at 1 x lop4 torr Ar and a CMA pass energy of 15 eV It should be noted that this resolution 1s hmlted by the CMA analyzer and not by the UV source UItrawolet sowce The UV source, as illustrated rn Fig 1, consists of a grounded cyhndrlcal stainless steel cathode which IS 22 86 cm long with an outer diameter of 3 81 cm and mner diameter of 3 48 cm Two 0 48 cm tungsten rods separated by 0 37 cm and symmetrically displaced about the cylinder axis serve as the anodes The anode rods are isolated from the cathode by machinable ceramic disks The holes m the ceramic are sufficiently large to allow for thermal expansion of the rods caused by electron end plates 0
w stainless steel qstainless end pla tes and
connectors
0
0
Ia mochlnable II
bolts and
0
0
ceramic set screws
Figure 1 Scale drawing of the cold cathode discharge W bolts_onto the-r&t end of the source
,1
source The dlfferentlal pumping system
146 bombardment Larger holes were also drilled m the ceramics to provide adequate conductance between the source and differential pumpmg system The anode potenteal, O-5 klir, 1s supplied through a high voltage feed-through at the rear of the source Grounded stamless steel end-plates cover the inner side of the ceramic disks m order to reflect electrons traveling m the axial dtrectlon back mto the source volume Photons produced along the axls of the source can emerge through a 0 25 cm exit aperture between the anodes at the front end of the lamp An aperture with a radius that IS less than the inner diameter of the cathode dlvlded by SIX allows sampling of the radlatlon without allowmg electrons to escape along the axial plane” The lamp IS cooled by clrculatmg chdled water through 0 64 cm tubing that enclrcles the cathode The discharge gas used m these experiments was 99 9995 ‘A pure helium which flows through an activated-charcoal trap mamtamed at 77 K and a GranvdlePhrIhps variable leak valve before entering the rear of the source Power to the anodes IS supphed by a O-5 kV, O-500 mA Savant power supply Smce the resistance of the source vanes from mfimty before mltlatlon of a discharge to several kQ during discharge, it IS necessary to provide a ballast reslstor m series with the source to prevent current surges during mltlatlon of a discharge This ballast was supplied by a variable resistor set at 16 kQ At a He-pressure of 0 08 torr, the resistance of the source IS -9 kD
D@erentlal
pumpmg
system
The pumpmg system, illustrated m Fig 2, consists of three stages of differential pumping The first stage 1s across a 1 5 mm 1 d and 3 0 cm long Pyrex caprllary and IS pumped by means of a chilled-water trapped 325 1 s- ‘, 5 08 cm 011dlffuslon pump*’ This stage can be eliminated by opening a 1 91 cm control valve m a bypass to the first diffusion pump The second stage IS across a slmllar capillary and IS pumped by means of a hqmd nitrogen trapped 325 1 s- I, 5 08 cm oil diffusion pump The third
UHV
chamber
Figure 2 Scale drawmg of the dlfferentlal pumpmg system The UV source bolts onto the left end of the pumping system The mam spectrometer chamber IS at the r&t sxde of Frg 2
147 stage IS across a 2 5 mm 1 d and 22 86 cm long Pyrex capillary and IS pumped through the mam chamber by means of a 450 1 s- ’ turbomolecular pump2’ and a 500 1 s- ’ 10n pump’ * A gate valve mounted on 6 99 m o d flanges 1s used to Isolate the source and dlfferentlal pumpmg system from the mam chamber A double-sided 6 99 cm o d flange IS used to connect the gate valve to the flange of the mam chamber The kmfe edge and tapped holes for a 3 38 cm mlm-flange are machmed mto the inner face of the double-sided flange Thus supports a mml-flange with a glass-kovar seal for the third stage of dfferentlal pumping and hght colhmatlon to the sample The pressure at various stages of the dlfferentlal pumpmg IS morutored by a lomzatlon gauges, and finally a nude thermocouple gauge, two Bayard-Alpert iomzatlon gauge ’ * m the mam chamber The thermocouple gauge was calibrated against a McLeod gauge for hehum gas and IS located m the discharge regon (Fig 2) before the first stage of dlfferentlal pumping This allows measurement of the pressure directly m the discharge region over the range l-lo-’ torr In order to measure pressures below 1O- 3 torr, the bypass valve across the first stage of dlfferentlal pumping IS opened and the discharge pressure IS momtored mth the first lomzatlon gauge This ehmmates the first stage of dlfferentlal pumpmg, however, it IS not needed under these condltlons since the discharge pressure IS less than lo- 3 torr Openmg the bypass valve causes a drastic change of the pumping speed m the discharge regon which affects the photon mtensrty Plots of photoelectron mtenslty vs pressure show a dlscontmulty at lo- 3 torr due to this altered pumping speed Typical operating pressures of the vanous stages are listed m Table 1 The fiow of helium gas Q (torr 1 s- ’ ) mto the mam chamber across the final 9 m capillary IS given byz3 Q=CP, where
C (1 s- ‘) IS the conductance
TA3LE
1
TYPICAL OPE DIFFERENTIA SOURCE
of the capillary
and Pa (torr)
ATING PRESSURES (TORR) OF THE VARIOUS STAGES PUMPING FOR DIFFERENT DISCHARGE PRESSURES
IS the pressure
OF IN THE w
Dzscharge region
1st ronrzatlon gauge
2nd lomzatton gauge
Maw2 chamber
1 1 1 1 1
6 x 10-a 4 x 10-d 1 6 x 1O-4 1 x 10-S 1 x 10-a
2 8 6 4 2
4 3 2 2 2
x x x x
10-l 10-s 10-s B lo-a&
x x x x x
10-B lo-’ lo-’ 10-V lo-’
a Measured with the bypass valve to the first dlffuslon pump open
x x x x x
10-10 10-10 10-10 10-10 10-10
148 differential flowz3
across the capillary
Under
our condltlons
of predommantly
molecular
where I and L are the radms and length of the capdlary m cm, T 1s the absolute temperature and M 1s the molecular weight of the gas With the pressure condltlons hsted m Table 1, the amount of helium reaching the mam chamber 1s - 9 torr 1 s- ’ Residual gas analysis In the mam chamber using a General Electric monopole mass spectrometer showed no hydrocarbon contammatlon and only a very small mcrease m the helium signal under operating condltlons Charactematlon of the source Since the photoelebLron count rate 1s proportional to the flux of lomzmg photons, the relative mtensltles (fluxes) of He(I) and He(I1) radlatlon have been monitored by measuring the photoelectron spectrum of an evaporated gold film The evaporated film was sputter cleaned with 1 keV Arf Ions and the X-ray photoelectron spectrum showed no detectable contammatlon Typical He(I) and He(l[I) spectra of the film as collected m a multichannel analyzer are shown m Fig 3 Intensity vs discharge pressure plots were obtained by momtormg the He(I) gold band at a
IONIZATION
ENERGY
(eV)
Figure 3 TypIcal He(I) and He(I1) photoelectron gold film obtamed with the UV source
spectra of the valence
bands of an evaporated
149
--
5-
25 ii
2 ---N
0 U
-
0 >
/
z--
/
1
He
/
__--
/)@
20
I mA I 8
I
10-z
10-l
PRESSURE
-0
10-3
(torr)
Flgure 4 He(I) photoelectron count rate and voltage across the source as a function of discharge pressure at a constant current of 20 mA
20mA I I
10-l P R ESSURE
I
10-2 (torr)
Figure 5 HellI) photoelectron count rate and voltage across the source as a function of discharge pressure at a constant current of 20 mA
km&c energy of 10 5 eV and the He(H) gold band at a kmetlc energy of 30 5 eV correspondmg to a bmdmg energy of 6 4 eV The photoelectron mtenslty vs discharge pressure at constant discharge current (20 mA) 1s shown m Figs 4 and 5 for He(I) and He(II), respectively Figs 4 and 5 also show the change m voltage as a function of pressure In order to mamtam a constant current as pressure 1s reduced, the voltage across the source must be
He I 3000
:
v
\_____
0
1
10-l PRESSURE
10-2
(tom)
Figure 6 He(I) photoelectron count rate and dmharge at a constant voltage of 3000 V
current
as a function of discharge pressure
He II 3000 \--
PRESSURE
--+
v
o--Cy -I-
E z w
(torr)
Figure 7 He(II) photoelectron count rate and dwzharge current as a function of discharge pressure at a constant voltage of 3000 V
mcreased to compensate for the mcreasmg source resistance The He(I) and Se(I1) mtensltles are maxImum at pressures of 9 x lo- ’ and 8 5 x lo-’ torr, respectively The sharp inflection pomts at these pressures suggest a change m the fundamental operating mode of the source The photoelectron mtenslty vs discharge pressure at constant voltage (3000 V) 1s shown m Figs 6 and 7 for He(I) and He(II), respectively Figs 6 and 7 also
151 show the change m discharge current as a function of pressure At constant voltage the discharge current decreases as the helmm pressure decreases (source resistance increases) From Fig 6 It 1s evident that the He(I) mtenslty and discharge current follow each other very closely as pressure IS changed In contrast the He(II) mtenslty peaks at -0 16 torr and then follows the current closely at lower pressures The source 1s capable of sustammg a discharge at a pressure of 5 x TO- 5 torr, although the mtenslty of both He(I) and He(I1) at pressures below lo- 3 torr 1s less than the mtenslty at higher pressure Higher discharge currents and, hence, higher mtensltles at these low pressures could be achieved by using voltages above 5 kV DISCUSSION
It 1s Interesting to note that the change m the fundamental mode of operation of the source at 0 090 and 0 085 torr (Figs 4 and 5) 1s dependent on the electron mean free path The mean free path for electrons moving through a gasz4 IS ,$I = 4,/2L with L =
1/(J2rU2dZ)
(3)
where n 1s the number of gas molecules per unit volume and d IS the effective molecular diameter of the gas From the above equations the electron mean free path through helium IS equal to the cathode radms (1 74 cm) at a pressure of 0 083 torr The mflectlon pomts m Figs 4 and 5 correspond to this pressure At pressures below this pomt, the electron mean free path between lomung colhsrons with He-atoms 1s greater than the radms of the cathode The electrons must perform oscillatory traJectones m order to aclueve this path length At pressures above this pomt where the electron mean free path IS less than the radius of the cathode, the source operates m a conventional glow discharge mode without srgmficant contnbutlon to the lonizatlon process from 0scilIatmg electrons Rushton et al I3 have shown that there are three basic modes of operation of such a source At pressures less than ~5 x lo- 3 torr, L,, 1s many times greater than the cathode radius and the source operates as a true charged particle oscillator At pressures (-0 085 torr m our case) where &I IS equal to or greater than the cathode radms, the system 1s m a general glow discharge mode and there IS no slgmficant electron osclllatlon At intermediate pressures the system 1s m a transition mode where L,, 1s several Inches and both osclllatmg and &scharge modes occur The inflection points m Figs 4 and 5 illustrate quite dramatically the begmnmg of the osclllatmg mode of operation The ratlo of He(II)/He(I) mtenslty increases as the pressure 1s lowered below lo-’ torr However, the low current results m a low photon flux For example, at a He-pressure of 2 x lo- 4 torr with a &scharge current are produced for the of 20 mA at 4600 V, only 1500 c s - ’ of He(H) photoelectrons gold band indicated above A He(I1) PES mtenslty of -30000 c s- ’ and a He(I) PES mtenslty of - 300,000 c s- 1 can be obtamed on the gold sampIe at a He-pressure
152 of - 8 x 10- ’ torr with a discharge current of 120 mA at 3000 V and voltage across the source of 1080 V Thus, operation m the transltlon region between the true oscillatory mode and the general glow discharge mode provides the highest absolute intensity of both He(I) and He(I1) photoelectrons The use of water-cooled tubes rather than rods for the anodes would provide better power dlsslpatlon and operation at hgher currents Also, our source 1s situated 36 20 cm from the sample because of the extensive differential pumping system and large size of our spectrometer chamber This length could be greatly reduced m order to mmlmlze photon losses and increase the flux at the sample If one IS wrllmg to sacrifice mtenslty for an improved He(II)/ He(I) ratlo, then It 1s feasible to operate at lower He-pressure CONCLUSIONS
A UHV compatible source for resonance hne radiation from rare gas atoms and Ions based on the prmclple of the charged particle oscillator along with its differential pumpmg system has been described in detail The advantages of this source for PES apphcatlons are as follows (1) It 1s constructed from stainless steel, tungsten, machinable ceramic and Pyrex caplllanes, making It very rugged and durable The Pyrex caplllarles could be replaced by metal or ceramic caplllanes, since they serve only for dlfferentlal pumping, m order to eliminate all fragile components (n) The low discharge pressure condltlons coupled with the dlfferentlal pumping system make the design partrcularly appropriate for UHV stu&es of surfaces (111) The discharge 1s very stable and the intensity 1s sufficient to allow dn-ect recording of the spectrum or accumulation m a multlchannel analyzer (IV) The photoelectron count rates produced from a gold film are -300,000 c s-l for He(I) and -30000 c s-l for He(I1) at - 8 x lo- 2 torr with a discharge current of 120 mA at 3000 V (v) The source 1s simple to construct, all external parts are at ground potential and there 1s no danger of coolmg water leakage mto the vacuum system (VI) Relative mtensltles of He(I1) radiation which are comparable to those of He(I) radlatlon can be obtamed at pressures below - 10m3 torr, however the absolute rntenslty under these condltlons 1s lower than the mtenslty at lugher pressures ACKNOWLEDGMENT
Acknowledgment IS made to the donors of the Petroleum Research Fund, admmlstered by the American Chemical Society, and to the U S Army Office of Research for support of this research
153 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
J W Rabalals, Prwmples uf UItravloIet Photoelectron Spectroscopy, Wdey-Intersclence, New York, NY, 1977 D W Turner, A D Baker, C Baker and C R Brundle, Molecular Photoelectron Spectroscopy, Wiley, New York, NY, 1970 L Asbrmk, 0 Edqmst, E Lmdholm and L E Sehn, Chem Phys Letf , 5 (1970) 192 F Burger and J P Marer, J Phys E, 8 (1975) 420, J Electron Specfrosc ReIat Phenom , 5 (1974) 783 R T Poole, J Llesegang, R C G Leckey and J G Jenkm, J Electron Spectrosc Refat Phenom , 5 (1974) 773 N Ueno, A Ikegaml, Y Hayasl and S Klyono, Jpn J Appl Phys , 16 (1977) 1655 A H McIlralth, Nature, 212 (1966) 1422 A H McIlralth, J Vat Scr Technol, 9 (1972) 209 R K Fitch, T MuIvey, W J Thatcher and A H McIh-alth, J Phys D, 3 (1970) 1399 R K Fitch, T Mulvey, W J Thatcher and A H McTlraIth, J Phys E, 4 (1971) 553 R K Fitch and G J Rushton, Vacuum, 20 (1970) 445 R K Fitch and G J Rushton, J Vat Scz Technol , 9 (1972) 379 G J Rushton, K R O’Shea and R K Fetch, _J Phys D, 6 (1973) 1167 G N Peggs and A H McIlraith, J P&U E, 6 (1973) 1137 Physical Electronics Industries, Inc , Eden Prairie, Minnesota J A Taylor, G M Lancaster, A Ignatiev and J W Rabalals, J Chem Phys , 68 (1978) 1776 Corning Glass Works, Cormng, New York Varlan Vacuum Divlslon, Palo Alto, Cahforma Thermal Corporation, Huntsville, Alabama A H McIlralth, private commumcatlon Cooke Vacuum Products, Inc , Norwalk, Conn Leybold-Heraeus Vacuum Products, Inc , Monroevllle, PennsylvanIa S Dushman, Sczentrfic Foundations of Vacuum Technzque, 2nd edn , Wdey, New York, NY, 1962, p 80 J Roboz, Zntroductron to Mass Spectrometry, Wiley-Interscience, New York, NY, 1968