Low energy electron gun for isochromat inverse photoemission

218

Nuclear Instrnments and Methods in Physics Research B53 (1991) 218-222 North~~o~ia~d

providing a low thermal energy spread.

1. htmduction

The study of the interaction of electron beams with solid surfaces has turned out to yield a huge amount of information on these systems. In such studies low energy (below 1 keV) electron guns able to deliver quite high currents to the sample are commonly used. The optimum beam current, however, has to be found as a compromise between the opposite requirements of high signal level versus the need for avoiding sample contamination induced by beam irradiation. Rather high sample currents are in particular needed in inverse photoemission spectroscopy (IPES) [I] since in this case the cross sections of the process are very small. Here, we present the detailed design and the performances of an electron gun particularly suited for IPES application in the ultraviolet (uv) photon energy range. This work essentially fits the specific design objectives appropriate to our IPES apparatus. Our inverse photoe~ssion spectrograph, which is described in detail elsewhere [Z], is based on a uv grating (IO-25 ev) and works in the mul~cha~el isochromat mode, i.e., the electron beam energy is varied while collecting in parallel several spectra at a fixed photon ener= in each channel. The beam spot on the sample constitutes the entrance slit for the grating. The gun must therefore produce a rather well focused electron beam, while the beam energy must be ramped in the 8-50 eV range to obtain wide spectra (> 20 eV) at all the photon energies covered by the grating. Moreover, in order to perform momentum resolved measurements, a narrow beam angle ( < 5 * ) is required [3]. Sample current as constant a~ possibie is also recommendable to simplify the normalization procedures. The typical sample current used in our experiments (about 20 PA, larger currents can damage the surface of many samples) is already so large 0168-583X/91/$03.50

that space charge effects have to be considered at least at the very low energies. The maximum beam size at the sample is determined by the optical system, which has been designed to work with an entrance slit (= beam spot) 2 mm wide. Several electron gun designs for IPES can be found in the literature [4-71. However, none of these designs attains the required features at the same time. In particular the gun thoroughly described by Stoffel and Johnson [7] has been made for purposes very similar to the present ones. The main difference is that such a gun is designed to work at a fixed energy (around 20 e\r) with good focusing properties, or at variable energy (5-30 ev) but not very well focused; in this last case the current at low energies is also quite small, Moreover in our setup the gun dimensions should be as small as possible (actually this is a rather general r~uirement~, while the gun of ref. [7f is quite large. A conveniently small gun is indeed described in ref. [4]; however, it has not been conceived for similar purposes, and consequently some important parameters for k-resolved IPE% seem tu be not well characterized (i.e., operation at low energy, angular divergence, beam spot size). Therefore, we have built a novel gun, very small and particularly suited for is~~ornat IPES performed with a grating. We stress, Ankara that such new design can be very useful in other con~g~ations too, as will be made clear in the follo~ng.

2. Design In order to build an electron gun with very small dimensions, we decided to use aperture lenses. For the same reason, lenses with small diameter are also used. The fraction of the lens diameter filled by the beam

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

F. Ciccacci et al. / Inverse photoemission electron gun

BaO dispenser cathode \

/g

sapphire

El

nacor ceramic

Insulators

krcap

AP4

electrodes

J

Fig. 1. Section view of the electron gun. Ekxtrons are emitted by a BaO cathode indirectly heatad by a filament. The gun is held together by three screws at 120* (only one is shown). The third aperture lens element is wider then usual to lodge the holding screws.

(filling factor) is then not very small. We use a fill& factor of 0.5: such value cannot be considered so small that lens aberrations are ~~~~~ble~ Therefore a full ray-tracing analysis has been performed to take into account these effects. ~~~~~~~ the gun dimensions also reduces the problems due to the presence of magnetic fields. All the measurements shown here have been performed both with and without p-met& shiefding (the Earth’s magnetic field reduced by a factor of 102): no measurable difference was observed. A drawing of the gun is shown in fig. I: the overall lon~tu~n~ and transverse gun dimensions are 30 and 22 mm, respectively; the sample working distance (measured from to last gun electrode) is I5 mm. A 2 mm plane BaU dispenser cathode [8] is used as the electron emitter in a diode ~nfigura~~o~ in front of an anode containing a small aperture. The cathode operates at a low temperature, about 9OO”C, emitting electrons with a narrow energy dist~bution (halfwidth equal to 2SkT = 0.25 eV [9]). The characteristics of the diode region have already been discussed in the literature [lo]. From such an analysis it turns out that electrons emerge from the anode aperture as though originating from a point source placed behind the aperture (at a distance equal

219

to three times the actual cathode to anode distance), and with an angular spread containing both thermal and geometrical contributions. In our setup the BaO cathode is at zero voltage and the anode is located at a distance d = 3 mm, much larger than the one used in other designs (4,7]. The anode is made out by a Ta foil containing a small aperture (diameter + = 0.5 mm) through which current is mjected into the following part of the gun constituted by the focusing lens elements. Such current depends on the anode potential V, according to a 3/2 power law typical of a diode ~onfiguraiion with current limited by space charge. The anode voltage is maintained constant while changing the beam energy, so that the current injected into the lens region is independent of the beam energy. In this way a relatively flat l,/f,v (current vs beam energy) characteristic is obtained. We find our source configuration very convenient since the large cathode to anode distance considerably reduces the poisoning of the BaU dispensers, then increasing their refiability. Note also that Ta was chosen as anode material since it is one of the few metals which does not cause poisoning of the BaO cathodes. With these precautions, we could operate the gun in ult~~~~ vacuum (UHV) (p < 10e9 Torr} for weeks without any reduction of its performances. Even after venting the vacuum chamber several times with dry nitrogen, followed by pumping down and appropriate cathode regeneration, we did not observe any deterioration of the cathode emission. The anode separates the electron source from the focusing lens region. Kapton washers (not shown in fig, I) insulate the Ta foil form the first electrode of a three apertures lens. The lens elements are made of a nonmagnetic alloy (Arcap AP4), which is known to form conducting surface layers very stable even at high temperatures (> 600 o C). Gold plating of the electrodes is also possible, and indeed necessary in high energy resolution works. However, we found that such procedure was not needed for our applications. The electrical and mechanical separation of the lens elements is achieved by means of three 2-mm sapphire balls located in appropriate holes in each side of the electrodes. Care has been taken to avoid direct view of the ins~ati~8 parts from the electron beam. The emire assembly is held together by three screws, resulting in a compact and rn~ha~~~ly stable structure. The gun is mounted on a CF 35 UHV flange with multipin electrical f~dt~ou~. The three elements aperture lens has been initially designed making use of the lens tables by Harting and Read ill]_ It consists in a zoom lens in which the first element is at V, = 50 V; the last one (Vs) ranges between 8 and 50 V and defines the beam energy. During the scan of V,, the voltage on the central element V, is changed, in order to maintain the beam focus in the same position on the sample, which is also kept at a

220

F. Ciccacci et al. / Inverse photoemission electron gun

voltage V3. The beam is then focused onto the sample in a field free region configuration [12]. This condition is especially necessary in k-resolved IPES, where controlled beam hitting is desired even when the sample/ gun relative geometry is changed. The diameter of the lens apertures is D = 6 mm and the lens produces an image from an object at P = 3.30 in a position Q = 3.80 (distances are measured from the lens center). As mentioned above, to account for aberrations and space charge effects, the lens region has been carefully studied by means of a ray-tracing program: we used the electron optics and gun design program EGUN, developed by Herrmannsfeldt at SLAC [13]. In our analysis we employ as starting conditions a set of electron rays injected into the lens according to what was described above concerning the diode region. In this way, for each beam energy (= eV,>, we are able to find the best value of V, which produces a small spot on the sample with minimum angular divergence. As expected, space charge effects are very important at low energies for currents above few PA. We find, for instance, that at V, = 10 V the best value of I$ corresponds to a lens which, in the absence of space charge effects, would focus the beam not on the sample but closer to the gun end. Electrostatic repulsion, however, reduces the beam convergence so that an essentially parallel beam arrives on the sample. This is achieved at the expense of the beam spot dimension, which turns out to be larger: at low energies (< 13 eV) it exceeds the limits posed by our optical system, when operating the gun at high currents (> 20 PA). In fig. 2 we show the

0

z (mm)

ULr-L-l

5,

0

5

IO

v3= 25

I5

20

3c

7

1:

‘5

a

50 2G

-

IO

IO

20

30 v,

40

50

tjQ

(Volt)

Fig. 3. Typical I/V characteristic for two different extraction conditions. Notice how the fixed extraction voltage permits to work with a quite constant current above 5 V.

results of the ray-tracing analysis for different beam energies at a current of 18 PA.

3. Performance The gun performance has been tested in an UHV chamber equipped with a Faraday cup mounted on an xyz manipulator. In order to perform measurements of the beam profile, we use a Faraday cup with a small (0.5 mm) entrance aperture. The total beam current is instead measured by a larger collector. As expected, the beam current increases with the anode voltage V,: in our gun V, ranges between 50-100 V. The maximum value of V, is determined by the need of operating the BaO dispenser far from saturation, i.e., under the condition of current limited by space charge. Typical curves for beam current versus energy are shown in fig. 3 for two different extraction conditions. It is seen that in both cases a quite smooth and monotonic I/V characteristic is obtained in our working region (8-50 V), with variations below 30%. In these and all of the following measurements, V, is fixed to yield the desired current

and then kept constant.

The first lens element

is

at V, = 50 V, the voltages of the third ( V3) and of the

1J--Ln-V3=

5

SAMFLE

(mm)

z

0

25

20 2 2

55

33

SAMPLF

z (mm) Fig. 2. Result of the ray-tracing analysis of the three aperture lens region. The beam current is 18 pA, while its energy is 10 eV (top), 25 eV (middle), and 50 eV (bottom). The value of V, is set to obtain the best focusing conditions on the sample. Note that the origin is taken at the Ta foil anode. The rays emerge from the anode as originated from a point source

located 9 mm behind the origin (see text).

second ( Vz) one are instead changed to sweep the beam energy and to maintain the correct focusing conditions, respectively. Operating the gun then requires two constant (V, and VI) and two variable ( Vz and V,) voltages. In particular, V, has been first set according to the ray-tracing results, and then experimentally adjusted to the value corresponding to optimum focusing conditions, as seen by the beam profile measurements (see below). In fig. 4 we present such values as a function of V,. The experimentally found values are very close to the ones expected on the basis of the ray-tracing analysis and, for the high energy side, also close to the values

F. Ciccacci et al. / Inverse photoemission electron gun

221

0.5 0.0 -g

0.5

.g

0.0

z

2.0

2

1.0 0.0

Fig. 4. Values of the focusing voltage Vz as a function of the beam energy eP’,. The solid line represents a best quadratic fit to the data points: Vz= -0.059 Vl -I10.1V,i 54.8.

obt~nab~e from the lens tables. The full line in fig. 4 represents the best quadratic fit to the data points. Such behavior can be easily reproduced by suitable electronics ~mponents having the voltage V, as input. In this way, in addition to the required constant voltages V, and V,, the gun would be driven by only one programmable power supply, providing V, and in turn Y2. The beam current profiles measured with the gun focused at 15 mm from the last electrode are shown in fig. 5. The lower panel contains the results for three different beam energies, for a total average current of

-2

-1

0

1

2

Fig. 5. a) Lower panel: beam profiles for different beam energies (15, 30, and 50 ev) taken with a total current of 25 PA. x is the lateral displacement from the gun axis. The curves have been normalized to the same height. b) Upper panel: beam profile at 10 eV for two different total currents: 15 and 25 PA. The curves are normahzed to the same area.

Fig. 6. Full width at half m~mum of the electron beam spot as a function of the distance from the last electrode, L The gun

was set to get optimum focusing conditions at z = 15 mm, and to deliver an average current of 15 pA. 25 PA. For the sake of clarity, the curves have been nor~liz~ to the same height. It it seen that the beam sizes compatible with our needs (FWHM, full width at half maximum smaller than 2 mm) are obtained. The beam width decreases at high energies, where space charge effects are expected to be less relevant. Such effects are instead well seen in the top panel, referring to the results for a beam energy of 10 eV. It is evident, here, that when the total current exceeds some 20 PA, the beam size broaden too much to allow a correct operation of our IPES spectrograph. Therefore, a safe operation at all the energies is obtained running the gun at lower total current. The collimation of the beam was checked by measuring the variation of the beam profile along the axis with the gun always focused at the same position. The results are collected in fig. 6, where the beam FWHMs measured at different distances from the gun are presented. From such curves, we can estimate the full angular convergence (or divergence) of the beam. We get a value of about To at 10 e?J, which reduces below the detectable limit (3”) at slightly larger energies. These values are well within the requirements for &resolved spectroscopies [3]. In conclusion, we have produced an electron gun that meets all the design objectives and in turn the requirements as stated in the intr~uction. The gun is actually well suited for isochromat IPES in the uv range.

Acknowledgements

The authors thank L. Tecchio, G. Lampel, A. Zecca, and E. Kisker for fruitful discussions. This work has

F. Ciccacci et al. / Inverse photoemission

222 been

supported

ione

(Italy),

(Italy),

and

by the Minister0 the the

Consigho Istituto

della

Nazionale Nazionale

Pubblica

Istruz-

delle

Ricerche

di Fisica

Nucleare

(Italy).

References [l] For a recent review on Inverse Photoemission Spectroscopy see for instance: N.V. Smith, Rep. Prog. Phys. 51 (1988) 1227. [2] M. Sancrotti, L. Braicovich, C. Chemelli, F. Ciccacci, E. Puppin, G. Trezzi and E. Vescovo, to be published Rev. Sci. Instrum. (1991). [3] The necessary angular resolution can be estimated from the free electron energy E = h2k2/2m. Then k = [0.54 E”2] A-‘, with E in eV, and kparall = k sin(a/2), where a is the full divergence angle. A kparall < k~BrillouinZoneJ is required for good momentum resolution. [4] P.W. Erdman and E.C. Zipf, Rev. Sci. Instrum. 53 (1982) 225. [5] Th. Fauster, F.J. Himpsel, J.J. Donelon and A. Marx, Rev. Sci. Instrum. 54 (1983) 68.

electron gun

[61K. Desinger,

V. Dose, M. Goebl and H. Scheidt, Solid State Commun. 49 (1983) 479. [71 N.G. Stoffel and P.D. Johnson, Nucl. Instr. and Meth. A234 (1985) 230. PI CT10 dispenser cathode, Thomson Tubes Electroniques, Av. De Rocheplaine, 38120 St. Egreve, France. [91 W. Franzen and J.H. Porter, Adv. Electron. Electron Phys. 39 (1975) 73. [lOI See for example ref. [7], and references therein. [111 E. Harting and F.H. Read, Electronic Lenses (Elsevier, Amsterdam, 1976). [W We note that field free region conditions are not rigorously achieved in our setup, since the vacuum chamber is of course grounded. However in our system the chamber walls are very far away with respect to the sample/gun distance, so that such conditions are well approximated. In different geometries, it can be convenient to keep the sample and last gun element also grounded, as obtainable by polarizing the BaO cathode and coherently shifting all the other voltages. SLAC Rep. 331 UC-28 (A) (1988). [I31 W.B. Herrmannsfeldt,