Artefact formation in scanning photoelectron emission microscopy

Ultramicroscopy 75 (1998) 35—51

Artefact formation in scanning photoelectron emission microscopy S. Gu¨nther, A. Kolmakov1, J. Kovac2, M. Kiskinova* Sincrotrone Trieste, Science Area Park, Basovizza, 34012 Trieste, Italy Received 23 February 1998; received in revised form 12 June 1998

Abstract The fast developing analytical technique, synchrotron radiation scanning photoemission spectromicroscopy has opened the opportunity for probing surface processes and composition of materials on a submicron spatial scale. Here we describe some artefact and unforeseen phenomena that can occur when a photon flux with high intensity is focused onto a microspot. Using as examples selected data obtained recently with the scanning photoemission microscope built at the ultrabright synchrotron source ELETTRA we illustrate the possible effects of surface morphology and undesired processes, such as photon-assisted carbon deposition, heat dissipation, charging and photon-induced reduction of the sample. All these events can cause severe changes in the chemical maps and photoelectron spectra and provide misleading results. The physical nature of the artefacts are outlined and discussed, as well as the possibilities to reduce their influence or to use them for quantification of some photon-induced phenomena becoming important in spectromicroscopy experiments carried out at the third generation synchrotron sources. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 82.80.P; 79.20.L; 07.85.T; 32.80 Keywords: Spectromicroscopy; Photon-induced phenomena; Scanning photoemission microscopy; Synchrotron radiation

1. Introduction The construction of the third generation low emittance and ultrabright synchrotron radiation

* Corresponding author. 1 Present address: HASYLAB at DESY, Notkestrasse 85, 22603 Hamburg, Germany. 2 Present address: Institute of Surface Engineering and Optoelectronics, Teslova 30, Ljubliana, Slovenia.

sources has become a milestone in X-ray spectromicroscopy. At present the scanning photoelectron emission microscope (SPEM) with zone plate optics built at ELETTRA works routinely with submicron spatial resolution, keeping the same energy resolution and count rate obtained until now only with spatially averaged electron spectroscopy for chemical analysis (ESCA) [1]. Besides that, the acquisition time required for two-dimensional chemical mapping and spectroscopy from selected spots can be reduced to several minutes. The

0304-3991/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 8 ) 0 0 0 4 7 - 3

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present spatial resolution of SPEM ((0.15 lm) is determined by the parameters of the used optical elements and exceeds by almost two orders of magnitude the principal diffraction limit for soft X-rays [2]. Recent advances in nano-fabrication techniques are expected to overcome the technological difficulty to produce high resolution zone plates (0.05 lm) with a large enough diameter and efficiency for being applicable in photoemission, and thus in few years SPEM can approach the mesoscopic world. However, along with the efforts to improve the spatial resolution and the signal level obtained with the photoemission microscope one needs to consider some unforeseen and undesired events that become important with increasing flux density concentrated in a small surface area and limit the applications of the instrument. The SPEM at ELETTRA has already operated for two years and a sufficient data set has been collected to characterise the capabilities of this instrument. The data, obtained studying surface lateral inhomogeneity of a great variety of samples, revealed unforeseen artefacts, specific to this rather new microscopy technique, which deserves special attention and investigation. Some of the observed effects are similar to those reported and extensively studied for scanning Auger microscopy (SAM) and secondary electron microscopy (SEM) [3]. However, the different excitation source and the different nature of the collected electrons have to be considered when comparing the SPEM artefacts with equivalent ones occurring in SAM and SEM. The aim of this paper is to give an overview of the observed artefacts and relate them to their physical origin. In particular the paper will focus on topography effects, deposition of carbon containing contaminants under the focused X-ray beam, heat

dissipation, charging and photon-induced changes in the chemical composition of the irradiated sample.

2. Instrument description The SPEM on ELETTRA is built as a part of an end station at the ESCA microscopy beamline, which is an undulator branch beamline providing high photon flux (typical*1012 photons s~1). The main optical components of the beamline are a prefocusing mirror and a spherical grating monochromator (SGM) with fixed entrance and exit slits, which provides monochromatized X-rays in the range between 200—1200 eV. More details for the beamline components and parameters can be found elsewhere [4]. The microscope is mounted in a separate UHV chamber attached directly to the beamline. Inside the SPEM the X-ray beam is focused to a small submicron-sized spot onto the sample surface. The focusing optical system consists of a Fresnel zone plate lens (ZP) and a pinhole of appropriate size which serves as an order-selecting aperture (OSA) (see Fig. 1). The demagnification of the photon beam is determined by the width of the outer zone, which is 0.1 lm for the zone plates in use at ELETTRA giving a diffraction limited spot diameter of &0.13 lm. The advantage of this focusing optics is that it can work in a wide photon energy range with spatial resolution independent of the energy of the emitted photoelectrons. An important parameter for photoemission spectromicroscopy is the working distance between the sample and the optical assembly in order not to obstruct the path of the photoemitted photoelectrons from the

Fig. 1. Focusing system of the scanning photoelectron emission microscope.

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

sample to the analyser. This imposes a requirement for using large diameter zone plates which still offer moderate resolution. The specimen is mounted on a positioning and scanning system. This system consists of a combination of UHV mechanical stepper motors for xyz coarse movements and piezoelectric positioners for xy fine scanning. The coarse stage allows 25 mm travel in x, y and z direction with the smallest step size of 1 lm. The fine stage piezo motors provide a scanning frame of 70 lm2 with a step size from 5 nm to 1 lm. For collecting the photoelectrons emitted from the microprobe the SPEM uses a 30° acceptance angle, 100 mm mean radius hemispherical analyser with 5 different sized entrance slits and 16 channel multidetector (VSW-SCA), mounted at 70° with respect to the incident X-ray beam. This measurement set-up enhances the surface sensitivity of the instrument. The energy resolution and transmission can be optimised according to the experiment requirements by changing the size of the SGM slits, the analyser slits and pass energy. The typical energy resolution used in the experiments described here is 0.4 eV. The microscope has also a highly sensitive photodiode mounted behind the sample to detect the transmitted X-rays (when the sample is transparent). This facility is commonly used for spatial resolution tests using transparent patterned samples. Scanning photoelectron spectromicroscopy experiments are based on two complementary operation modes. (i) Imaging, where by scanning the sample with respect to the focused beam chemical maps can be generated by simultaneously monitoring the photoelectron (PE) yield at a selected photoelectron kinetic energy (usually corresponding to a core level line of an element). These images provide information about the spatial chemical variations at the probed surface. (ii) Spectroscopy, where conventional energy distribution curve (EDC) spectra are taken on a microspot selected from the images. This mode uses the whole analytical power of ESCA in order to obtain information on the chemical state and local electronic structure of the probed area. By processing the microspot spectra more precise

37

evaluation of the local concentration and chemical state of each element present in the irradiated area can be made. The SPEM chamber is UHV connected to a sample preparation chamber, where basic surface science tools (Low Energy Electron Diffraction, Auger Electron Spectroscopy and Mass Spectroscopy) and facilities for sample cleaning and in situ sample preparation are available. The samples, mounted on a special sample carrier, can be introduced through a fast-entry lock and then be transported to the SPEM or the preparation chamber.

3. Photon-induced contamination of the probed spot: carbon deposition In the first measurements performed with SPEM we had to deal with local deposition of a carbon layer when the focused X-ray beam irradiated the same spot for a longer time or when many images were taken on the same surface area. Fig. 2 shows an image of a bimetal Au—Ag/Si(1 1 1) interface obtained with the analyser tuned to the C 1s PE

Fig. 2. 25.6]25.6 lm2 C 1s image of a sample showing that the smaller, previously scanned (imaged) area appears brighter due to the X-ray beam-induced local enhancement of the carbon coverage.

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S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

peak. The bright rectangular feature is the area of the surface from where several images were collected before that. The contrast clearly shows an enhanced carbon concentration in this already irradiated area. Carbon deposition on the probed area is a well known phenomenon in SEM, where the illuminated areas appear darker in the secondary electron image. Since these microscopes normally work in the pressure range of 10~6—10~8 mbar the formation of carbon layers on the illuminated parts of the sample is explained by electron beaminduced cracking of the C containing gases (CO, hydrocarbons, CO ) which are contained in the 2 residual gas and thus enhancing their effective adsorption sticking coefficient. As in SEM we relate the effect displayed in Fig. 2 to photon assisted processes that enhance the sticking of carbon containing molecules present in the residual gas. It should be noted that the SPEM at ELETTRA works at a base pressure in the low 10~10 mbar range, where the partial pressure of the C containing gases P(C) is )5]10~11 mbar. Therefore the observed effect of carbon deposition induced by the photon beam should be quantified and related to the impingement rate of molecules from the residual gas. We can estimate the deposition rate by positioning the X-ray beam on a non-illuminated spot of the sample (which is carbon free) and performing local spectroscopy versus illumination time. Subsequently recorded Ag 3d and C 1s photoelectron spectra (with peak positions at kinetic energies of 125 and 210 eV in this experiment) showed that after 30 min illumination time the intensity of the former dropped by 30%, whereas that of the C 1s intensity had increased. For one monolayer (ML) of C deposited on the sample we estimated a damping factor ranging between 0.5 and 0.7 considering the universal electron attenuation length curves for the given electron kinetic energy and the analyser grazing geometry [5]. This gives a deposition rate of (0.5—1) ML in 30 min or a flux of (1—3)]10~2 ML/min. If we consider the residual gas as the only contamination source with a partial pressure P(C))5]10~11 mbar and the packing density of Si(1 1 1) sample (1 ML equals 7.8]10~14 atoms] cm~2) the impingement rate of carbon containing molecules on the sample of )1]10~3 ML/min is

to be expected. The fact that the measured deposition rate on the illuminated spot exceeds the expected value by a factor of 10—30 implies that a “local carbon source” close to the surface should exist. Analogously to a gas doser such a local source can provide an enhancement of the impingement rate on the sample without increasing the background pressure [6]. The most likely “local carbon source” are the optical elements (OSA and ZP) placed rather close to the sample. (For the used zone plates the focal distance at a photon energy of 500 eV ranges between 5 and 8 mm, which means that the OSA—sample distance is less than 3 mm.) The intensive unfocused photon beam illuminating the optics can induce desorption of carbon containing gases or fragments increasing locally their partial pressure. We found out that the carbon deposition onto the irradiated parts of the sample is most severe after mounting a new optical system and decreases with operation time of the microscope because the optical elements are degassed with time.

4. Topographic artefacts It is desirable that the PE images reflect only the distribution of the chemical species present on the surface under investigation but in the set-up used in the scanning photoelectron microscopes this is possible only with perfectly polished samples. This is because with an HS analyser mounted in grazing geometry the probed depth is reduced and the local curvature of the surface results in pronounced topographic artefacts. Fig. 3 shows a Mo grid imaged using transmission and photoemission detection modes. The image 3a displays the intensity of the transmitted photons measured by a photodiode and transformed into an electrical current, corresponding to the displayed linear grey scale. The images 3b,c show the intensity of the Mo 3d and the C 1s core level photoelectrons, respectively. As can be expected in the transmission image the holes of the grid appear bright, whereas in the photoelectron images they are dark. A distinctive feature in the photoelectron images is the enhanced brightness on the left side of each hole, which is a simple

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

39

Fig. 3. 300]300 lm2 images of a Mo test-grid obtained by (a) measuring the transmitted light, (b) the emitted Mo 3d photoelectrons and (c) the C 1s photoelectrons. The topographic artefacts due to the local curvature of the surface are clearly seen in (b) and (c). Dividing of (b) to (c) yields an image (d) where the topographic effects are almost removed.

geometric effect considering that the analyser is positioned on the right side of the displayed images. For our geometry the angle between the surface of the grid and the analyser is 20°, whereas the angle between the local surface plane of the left edge of the holes and the analyser is close to 90°. Thus, the photoelectron signal collected from the edge is stronger because of the enhanced probing depth. The displayed “ratio” image (Mo 3d/C 1s) in Fig. 3(d) serves as an example how to tackle this geometric problem as will be discussed further below. Another type of a topographic artefact due to the grazing collection angle is the shadowing of the photoelectrons which can become rather severe

when features with a certain height are present on the surface. Fig. 4 shows a big MoO crystal 3 on a flat Al O support which hosts several 2 3 smaller MoO particles on top. All particles of 3 this agglomerate are imaged with a shadow on their left side because they prevent the photoelectrons emitted from the left vicinity of each feature to reach the analyser in line-of-sight. Again, as in Fig. 3b,c the local surface plane of the right edge of each crystal is nearly perpendicular to the analyser axis and therefore appears brighter. In brief, due to the grazing acceptance angle of the SPEM all imaged high objects appear as being illuminated from the right side (where the electron analyser is placed).

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S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

Fig. 4. 60]60 lm2 Mo 3d image of a MoO crystal deposited 3 on a flat Al O support. The shadow is due to the fact that the 2 3 high crystal prevents the photoelectrons emitted from the left vicinity of the crystal to reach the analyser which is mounted in grazing geometry on the right side of the displayed image.

The above described topography-related enhancement or reduction of the photoelectron intensity obscures the real lateral chemical contrast and requires image processing to correct for this artefact. Assuming that the geometric artefacts are generated in the same way for emitted electrons independently of their kinetic energy, the correction for the topography-related artefacts can be achieved by numerically dividing images that were subsequently recorded with the analyser tuned to different kinetic energies. This correction procedure works satisfactorily when the effective mean free path (EMFP) of the photoelectrons under investigation does not differ too much and the angular distribution of the emitted photoelectrons is comparable within the acceptance angle of the analyser. Note that our SPEM accepts a wide range of emitted photoelectrons (70°$15°) and thus averages over a wide angular range. The result of handling the topographic artefacts of Fig. 3b and Fig. 3c by image processing is displayed in Fig. 3d where the Mo 3d image, obtained by collecting photoelectrons with kinetic energy of

262 eV, was divided by the C 1s image obtained by collecting photoelectrons with kinetic energy of 210 eV. It can be seen that the enhanced brightness on the left edge of each hole is greatly reduced in the “ratio” image Fig. 3d. Inside the holes of the grid the photoelectron signal approaches zero counts. Therefore these parts appear in Fig. 3d with a high amount of scatter because here one divides zero counts of the one image by zero counts of the second. This difficulty occurs as well when dealing with shadowing of photoelectrons as in Fig. 4. If the count rate of a shadow region drops to values close to zero the division by a second image produces a high amount of scatter in these regions. Here the background of secondary electrons also determines whether the count rate in the shaded regions drops completely to zero and thus can influence the appearance of the shadows imaged in SPEM. In brief, we can summarise that while the correction of shadows cannot be solved generally, the topographic artefacts due to the local curvature or the surface can be strongly reduced by producing “ratio images”. A problem with this procedure arises when the specimen contains more than two chemical species (A,B,C,2) which are distributed unevenly on the surface. In this case a ratio image (e.g. the A/B-image) does not necessarily reveal the true distribution of each element on the surface, because the influence of the remaining species (e.g. the C-component) has to be considered as well. Here, as was already described in detail in Ref. [7], images reflecting the distribution of a single component can be obtained by dividing its image (e.g. the A-image) by a weighted sum of all images of the components present on the surface under investigation (the weighted A#B#C#2 image). The disadvantage of this technique is that the “weight” factors, that should account for the sensitivity of the microscope towards each of the contained species, are not known a priori. The value of these sensitivity factors can be evaluated taking into account the cross-sections for photoionization of the selected core levels, the kinetic energy of the emitted photoelectrons and the analyser geometry [9]. However, the best values for the relative sensitivity factors can be obtained experimentally, i.e. by calibrating the SPEM measuring a known

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

two component system. For example, in the case of the investigation of a bimetal Ag#Au/Si(1 1 1) interface we used the well known (J3]J3)Au/Si(1 1 1) and (J3]J3)-Ag/Si(1 1 1) surfaces for calibrating the sensitivity of the SPEM with respect to a monolayer of Au on top of a Si(1 1 1) crystal or a monolayer of Ag, respectively [8].

5. Interaction zone In SPEM experiments, where the diameter of the focused photon beam on the sample is less than a micron, a very small area interacts with the irradiating photon beam. This area, displayed in Fig. 5a and b, will be called here “the interaction zone”. Inside this zone the introduced power of the photon beam has to be dissipated into the bulk of the sample and the induced local charge due to emission of photoelectrons has to be compensated. If the thermal or the electrical conductivity of the sample is not sufficient the illuminated zone will heat-up and/or charge-up. Since the typical cross-

41

sections for photoionization are of the order of 0.1 Mb for soft X-rays [9] and reflection can be neglected, the beam penetrates approximately 0.1—1 lm below the surface. Due to the energy dependence of the effective mean free path (EMFP) of electrons in a solid the emission of the core level photoelectrons or auger electrons is exclusively from the top few layers, while the slow secondary electrons travel longer distances without significant energy losses and can reach the surface from depths up to 500 A_ [1]. Since the total electron yield for X-ray excitation consists mainly of secondary electrons [10], the emission of secondary electrons also controls the generated total photoelectron yield in SPEM measurements. Therefore, if charging of the illuminated area occurs it will be related to a depth of )500 A_ . Photons ionizing the atoms from deeper layers cause only the production of electron—hole pairs that will not contribute to a homogeneous surface charge. For the following evaluations and discussion we will simplify the problem. In order to estimate the temperature and the potential of the interaction

Fig. 5. Drawing of the interaction zone of the focused X-ray beam and the sample for the two geometries (a) and (b) treated analytically for estimation of the heat and the electrical current flow from the illuminated spot to the bulk of the sample or the sample holder.

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zone it will be considered as a homogeneously heated or charged disk (or a half sphere) with diameter equal to the size of the focused beam. The thickness of the “heated” disk is equal to the photon penetration depth (0.1—1 lm) and of the “charged” disk to the escape depth of the secondary electrons (500 A_ ). In order to estimate the photoelectron yield one has to consider that each photoionization process leads to the ejection of a primary photoelectron as well as instantaneously emitted auger electrons that occur while the core level hole is filled (if fluorescence decay is neglected). While travelling through the solid these electrons undergo inelastic kinetic energy losses mainly due to electron collisions that result in the emission of secondary electrons and/or secondary auger electrons [11]. The last step to be taken into account is the escape of the electrons from the solid by overcoming the energy barrier into vacuum (for metals the so-called work function). Henke et al. treated the problem of X-ray excited electron emission both theoretically and experimentally [12]. It was shown that for a photon energy of about 1000 eV the total electron yield stays in the order of 0.1 electron per incident photon for metals whereas the yield of insulators may exceed this number by a factor up to 30. This increase is in accordance with the longer EMFP for slow ((10 eV) electrons in insulators due to the low probability for electron—electron interactions so that secondary electrons created in deeper layers still can reach the surface and escape from the solid. In the present calculations of the photocurrent induced in the illuminated area we assume that the photoelectron yield per incident photon is 0.1 and 1 for metals and insulators, respectively.

These values are within the numbers obtained in the calibration measurements at BESSY and HASYLAB [13]. It should also be noted that pulsed character of the synchrotron light which in the case of the ELETTRA synchrotron source (0.1 ns emission with 0.5 GHz repetition rate) gives a factor of 20 times higher short term photon flux. For reasons of simplicity we will use the average value of flux in the focused beam measured in SPEM: 109!1010 photons s~1 [4]. Thus, the average photocurrent induced by irradiation ranges between &10—100 pA for metals (0.1 electrons/photons ]109—1010 photons s~1) and 100— 1000 pA for insulators. This value is in agreement with the photocurrent of 50 pA we measured on a Mo test sample with the SPEM. When working with the usual for SPEM photon energy of 400—900 eV the heat dissipated in the interaction zone is &0.1—1 lW (using an average photon energy of 500 eV and the photon flux in the spot). Cooling processes due to the emission of photoelectrons can be neglected as being a (5%effect because of the low photoelectron yield (0.1—1 electron per photon) and the low average energy of the emitted secondary electrons. The energy loss via radiation cannot exceed the black body radiation loss of 40]10~3 W/cm2(0.1 lm)2"4] 10~12 W at 300 K and can be neglected up to temperatures as high as 3000 K (which means always). In the next two paragraphs we will use the estimated photocurrent and the heating power to describe the temperature and the charging of the illuminated zone for metallic, semiconducting and insulating samples. As examples we use the physical properties of Ag, Si and sapphire which are taken from Ref. [14] and are listed in Table 1.

Table 1 Typical material parameters for metals, semiconductors and insulators (taken from Ref. [14]) Metal (Ag)

Semiconductor (Si)

Insulator (sapphire)

Density o (in 103 kg/m3) Thermal capacity c (in Ws/kg K) Thermal conductivity j (in W/mK) Electrical conductivity p, (in 1/) cm)

10.5 240 430 108

3.99 753 42 (10~14

Dielectric constant e Electrical strength

R

2.34 703 80—150 Undoped: 5]10~4 Doped e.g.: 5]105 11

7.5—10.5 17]106 V/m

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

5.1. Heat dissipation In the case of microscopy the total power introduced into the sample by irradiation is low ((1 lW) but its local density is extremely high (104 W/cm2). Therefore it has to be estimated if local heating of the surface occurs. First, we have to show that local equilibrium is reached fast for typical solids and that we can estimate the temperature gradients in the steady state approximation. The typical time, q, in which the heat gradient of lateral dimension d is reduced to 1/e of its initial value is q"d2oc/j, where o is the density, c is the specific heat capacity and j is the heat conductivity of the sample. Using the typical material parameters for metals (e.g. Ag), semiconductors (e.g. Si) and insulators (e.g. sapphire) given in Table 1 [14] and an initial gradient of &1 lm yields q"10~7—10~8 s. Therefore we can assume that the temperature approaches its equilibrium value fairly fast and the solution of a stationary heat equation will provide a sufficient estimate of the temperature. The two limiting cases that can be considered as extreme cases are displayed in Fig. 5a and b. The geometry used for the first approximation which underestimates the heat flow is shown in Fig. 5a. It treats the heat flow confined within a depth equal to the photon penetration depth of z"0.1—1.0 lm. The boundary conditions are chosen symmetrically assuming that the interaction zone is uniformly heated disk of a radius r "0.1 lm, whereas the 1 sample is kept at room temperature (by the sample holder) at a concentric ring of radius r "2 mm, 2 thus ¹(r )"300 K. For the cylindrical symmetry 2 shown in Fig. 5a, the temperature gradient ¹(r) can be solved analytically from the stationary heat equation:

AB

Q r ¹(r )" ln 2 #¹(r ). 1 2 2pzj r 1

(1)

Typical values of solids (see Table 1) yield maximum heating of the illuminated disk of the order of (10~3—10~1) K. The second extreme case which overestimates the heat flow is modelled by using the spherical geometry shown in Fig. 5b where the heat can be dissipated into the bulk of the sample. Here we

43

assume an uniformly heated half sphere with a radius equal to the light penetration depth (0.1—1 lm) and a half sphere with a radius of 2 mm which is fixed to room temperature. The analytical solution of this model (not shown here) yields a maximum temperature increase of the irradiated zone approximately 10 times lower than the above calculated value for cylindrical geometry. The main result of these estimations is that the heat conductivity of all solids (metals, semiconductors and insulators) is sufficiently high to dissipate the power introduced by the incident Xray beam without a significant temperature increase as long as they are thermally well connected to a sample holder. This finding is important to rule out temperature effects when on some solid samples local chemical changes are observed in the area irradiated by the focused X-ray beam. It should be noted that this situation can completely change if particles or fibres without good thermal contact to the sample support have to be investigated. For example, it is easy to calculate that a thermally insulated micron-sized particle can heat up to its sublimation temperature almost instantaneously after being hit by the focused X-ray beam. 5.2. Local charging The phenomenon local charging is not new [1,5]. For example, in order to obtain information about the charging geometry Cazaux treated the case of an uniformly charged disk in detail [15]. Lately, he used this results in his studies on the radiation damage of biological samples probed by X-ray microscopy [16]. Considering the great differences in the electrical conductivity for different materials (see Table 1) it is obvious that the charge neutralization of the irradiated zone differs substantially for metals, semiconductors and insulators. In order to calculate the surface potential of the irradiated surface we will perform estimations similar to those described in the previous section using equations nearly identical to Eq. (1). For example in the case of dynamical equilibrium one has to exchange only heat flow with electrical current. First, the time necessary to achieve equilibrium has to be estimated. Assuming ohmic conduction of

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Table 2 Decay time q for charging of metal, semiconductor or insulator samples (e.g. Si and sapphire)

q

Metal

Si doped

Si undoped

Sapphire

“0” s, instantaneous

10~16 s

10~7 s

'104 s&3 h

the sample it can be shown that generally the decay time q necessary to neutralise a localised charged area to 1/e of the initial charging potential is ee q" 0, p

(2)

where e and e are the dielectric constants of the 0 material and of the vacuum field, respectively, and p is the electrical conductivity of the material3. Typical values taken from Table 1 yield for metal, doped Si, undoped Si and sapphire the time scales listed in Table 2. As can be expected no charging occurs in metals, whereas insulators can remain charged on a time scale of hours. Thus, they will charge up linearly with time when illuminated for periods shorter than q. On the other hand a semiconductor like Si (doped and undoped) reaches equilibrium fairly fast so that the solution of a stationary potential equation will provide a good estimation. The illuminated area is assumed to be homogeneously charged and the sample connection to ground is at a distance of approximately 2 mm away from the interaction zone. The two geometries illustrated in Fig. 5 can be treated similarly to the estimations of the heat dissipation. In the case of Fig. 5a the electrical current is restricted to the “near-surface region” and cylindrical symmetry is assumed. This estimate corresponds to the solution of a charged cylinder of a sufficiently large length in Ref. [15]. In the second geometry displayed in Fig. 5b electrical conduction into the bulk is allowed and the problem is treated with spherical symmetric boundaries. Equivalent to the “heat dissipation” phenomenon the geometry (a) will underestimate and (b) will overestimate the real sample conductivity.

3 This formula is a result of Gaussian law and the first Maxwell equation.

In the following solutions, we will consider explicitly the case (a) only. All values given for charging will be related to these solutions. The analytical expressions for case (b) are very similar and yield an estimation of the potential approximately an order of magnitude lower. For both geometries the first Maxwell potential equation can be solved analytically, assuming the validity of Ohms law: j"pE, as follows:

P

P

1 Q E dA" o d»" ee ee 0 0 and

P

P

I" j dA"p E dA,

(3a)

(3b)

By solving Eq. (3a) the capacity, C, of the charged region for a given geometry is obtained whereas the second Eq. (3b) yields an analytical expression for the resistivity, R, between the charged part of the surface and the connection to ground potential in the steady state regime. For the case displayed in Fig. 5a the illuminated spot is assumed as a homogeneously charged disk of radius r "0.1 lm and thickness z"500 A_ (es1 cape depth of secondary electrons), the electrical conductivity p is restricted within a depth of 500 A_ and the connection to ground º"0 V is provided at a cylinder with r "2 mm (e and e are the 2 0 dielectric constants of the sample and the vacuum field, respectively). The solutions of (3a) and (3b) are 2pee z 0 , C" ln(r /r ) 2 1 1 R" ln(r /r ). 2 1 2ppz

(4a) (4b)

The equilibrium potential of the charged spot is achievable by calculating º"R]I with

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

I"50—500 pA (typical measured photocurrent) and R obtained from Eq. (4b). For the characteristic decay time in which the equilibrium is achieved a value q"R]C"ee /p is 0 obtained in agreement with the general result of Eq. (2). The initial potential rises for a period t@q, *º/*t, can be evaluated neglecting the ohmic connection to ground, R, and considering the photocurrent, I, and the capacity, C, of the charged illuminated spot (Eq. (4a)). This yields an initial slope *º/*t"I/C. For all solutions given above we can include the case of pure surface conductivity. We have to assume that while the thickness of the charged disk stays constant at z"500 A_ the current flow takes place on the surface (zP0) and the term p]z has to be replaced by the surface conductivity ph. Typical values for ph for highly insulating materials, as epoxy or vinyl plastic, have been measured in the range of 10~15 1/) [17]. By comparing p]z (p] z"10~14 1/)cm]500 10~10 m"5]10~22 1/)) with ph"10~15 1/), it is obvious that the surface conductivity becomes an important carrier transport mode in the case of a highly insulating material like sapphire. If we consider this type of conductivity we can estimate the potential of the illuminated spot when being discharged by ohmic conduction only after it reaches the steady-state conditions as shown in Table 3. The values in Table 3 already show that the charging induced by an intense focused X-ray beam becomes a severe problem. This is a consequence of the extremely small capacity of the illuminated area which is connected to ground through a high resistivity. Already for undoped Si measurable charging of the surface might be expected. On the other hand, considering the values of Table 3, the sap-

45

phire surface should charge up so severely that all photoelectrons emitted from the irradiated sample would be recaptured due to the positive surface potential and no electron could leave the sample. In this case, a surface potential would be established that equals the maximum kinetic energy of the emitted photoelectrons which cannot exceed the photon energy of the irradiating X-ray beam, in our case ht"500 eV. As will be shown later we measured surface potentials up to 250 V for sapphire samples in our SPEM experiments, i.e. already half the primary photon energy. The fact that the extreme case of a charged surface that does not allow any electron to escape is not reached in reality is due to four physical reasons: First, as the surface charges up only slightly, the slow secondary electrons are attracted by the surface potential and cannot leave the sample. Thus, already after charging up to approx. #10 V the effective photocurrent drops due to the strongly reduced secondary electron emission. This decrease is very pronounced in the case of insulators and may range between a factor of 10 and 100 [12]. This reduction of the effective photocurrent leads to surface voltage values already close to those measured in reality. Second, the Eqs. (4a) and (4b) assume an ohmic conductivity of the sample which is violated for a heavily irradiated surface (e.g. photo-induced conductivity occurs). For the geometry of SPEM this effect can be neglected. As long as only a single spot of the sample is irradiated a potential drop takes place on the non-irradiated part of the surface. But during imaging the sample is scanned under the X-ray beam and a much larger area is subsequently illuminated. If the irradiation generates surface defects (see the last Section), which increase the surface conductivity, the charge can be

Table 3 Surface potential of different samples illuminated in the SPEM assuming a photocurrent of 50 pA (500 pA for insulators) and ohmic discharging only. Note that the expected surface potential exceeds the energy (h t"500 eV) of the irradiating soft X-rays

Charging

(Metal Ag)

Si doped

Si undoped

Sapphire

10~11 V

3]10~9 V

3V

1012 » (bulk conductivity)

H

8]105 » (surface conductivity)

'ht

46

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

distributed over a much larger area and a significant potential drop can be expected. The third process that reduces the surface potential is the electrical break through caused by the high electrical fields at the boarder of the charged spot with the non-irradiated dielectric, which again is very sensitive to the local geometry of the surface. As a fourth effect specific for the SPEM set-up using a zone plate optical system we have to consider that a small quantity of the photoelectrons emitted from the order selecting aperture (OSA) and the zone plate can “see” the positive surface potential of the charged sample because they are not well shielded. These electrons will be attracted and discharge the surface. All the above-mentioned events that can reduce the charging are active only if the surface has been irradiated long enough. Therefore we can estimate the initial linear voltage raise for an insulator that has just been exposed to irradiation. For Si and sapphire we can estimate an initial potential increase of *º/*t"I/C'18]106 V/s with C calculated from Eq. (4a) and a photocurrent of I'50 pA. This steep slope confirms that if the ohmic conductivity is insufficient to neutralise the irradiated sample within a decay time of q(10~7 s the surface will charge up significantly. An insulator like sapphire will nearly instantaneously charge up when being irradiated in the SPEM. Already after t(10~7 s it reaches a surface potential of &2 V. After this initial rise the above-mentioned processes become active and slow down the charging. Therefore, if time-dependent charging occurs on insulators probed by the SPEM it is always caused by a non stable steady-state surface potential due to changes in the surface conductivity rather than by a slow initial charging slope. Differential charging in photoemission experiments is an interesting phenomenon that accounts for peak broadening in conventional XPS and has been extensively discussed in Ref. [18]. In previous measurements differential charging was observed with electron imaging X-ray microscopes where the sample is homogeneously illuminated by a standard X-ray source and the lateral resolution is achieved by electron optics of the analyser which magnifies the image of the irradiated surface [19,20]. In SPEM the charging geometry is signifi-

cantly different, because the highly focused X-ray beam illuminates only submicron portions of the surface and the images are obtained by scanning of the sample with respect to the beam. This opens an opportunity of investigating by SPEM the differential charging phenomenon in steady-state conditions with spatial resolution not achievable before. If time-dependent charging occurs on a time scale comparable to the acquisition time of an image it is almost impossible to set appropriate imaging parameters as will be illustrated with the help of Fig. 6. Here an area of a sapphire sample, covered by a thin laterally heterogeneous MoO x phase, was repetitively imaged. The electron analyser was tuned to collect the Mo 3d photoelectrons, taking into account the energy shift induced by the charging. In this mode the areas emitting photoelectrons with a kinetic energy within the selected energy window of the analyser appear bright so that equipotentials of a charged surface can be imaged. Since the charging conditions are not stable the equilibrium surface potential changes with time. Therefore the “clouds” of equally charged areas move laterally in time as can be seen from the sequence of the images in Fig. 6. The fairly stable feature that occurs in the upper right corner of each image also is due to charging but its nature is not well understood yet. The example shown in Fig. 6 illustrates that imaging insulating samples with poorly defined topographic features yields very ambiguous data since topographic and charging effects cannot be easily separated. Thus, in order to study the differential charging phenomenon, conditions have to be established where the surface repeatably charges up to a stable potential. In this case differential charging artefacts can be “imaged” by SPEM, as illustrated in Fig. 7. The test sample was prepared by deposition of a thick Ag film onto a sapphire single crystal through a stripe-like mask at room temperature. The produced Ag patch was connected to ground potential (which also provided coupling to the spectrometer) and represented the non-charged part of the imaged surface. The Ag coverage gradually decreases moving away from the border of patch, which represents the place where the mask cuts the Ag flux during deposition. In this “shadow” region of the mask Ag clusters form on

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

47

Fig. 6. A sapphire sample which contains a thin MoO phase is subsequently imaged with the analyser tuned to the Mo 3d core level x photoelectrons accounting for their deceleration due to charging. Since the charging of the sample changes on a time scale comparable to the image acquisition time the ‘‘moving bright parts’’ correspond to Mo 3d photoelectron emitting areas which are charged within the range of the selected energy window of the analyser. Image size: 51.2]51.2 lm2.

Fig. 7. (a) 64]64 lm2 image of a grounded Ag patch deposited on an insulating sapphire surface by vapor deposition through a rectangular mask. The image was obtained by tuning the analyser to the kinetic energy of the Ag 3d photoelectrons emitted from the Ag patch. (b) Photoelectron spectra corresponding to the three distinctive areas of the surface, indicated by 1, 2 and 3 in the image.

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S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

the sapphire surface. They are too small to be resolved by SPEM, but their presence is proved by the Ag 3d spectra measured on the areas away from the patch. These Ag clusters are laterally disconnected from each other because the Ag coverage drops below the percolation regime. Therefore, these clusters are disconnected from the ground potential of the thick Ag patch and thus electrically float on the charging insulating sapphire surface. Fig. 7a shows the map of the border region obtained with the analyser tuned to the Ag 3d photoelectrons kinetic energy corresponding to the grounded Ag patch. A notable feature is the narrow stripe which appears on the right part of the imaged area parallel to the Ag border. Analysis of the photoelectron spectra taken from different selected areas of the surface (Fig. 7b) helped us to understand the origin of this artefact feature. Since the surface charges up nearly linearly with increasing distance from the Ag patch the C 1s photoelectrons, which for the uncharged surface have kinetic energy of 207 eV, are slowed down until they enter the

selected energy window corresponding to the lower kinetic energy Ag 3d photoelectrons. This results in the bright stripe in Fig. 7a which corresponds actually to a surface potential equal to the difference between the Ag 3d and C 1s photoelectron kinetic energy. Fig. 8 schematically summarises this situation and displays the measured surface potential as a function of the distance to the grounded Ag patch. It should be noted that the averaged potential gradient of 5]106 V/m in the vicinity of the Ag patch reaches an electrical field already close to break through conditions for sapphire (17]106 V/m see Table 1), especially if one takes into account that locally the electrical field can exceed the determined average value since the potential drop can only take place between the separated Ag clusters. The examples above show that sapphire crystals can reach very high charging potentials (up to 250 V) when being irradiated in the SPEM, which makes the microscopy experiments practically impossible. One way to avoid this is to use thin oxide films grown on top of a metallic foil. For example,

Fig. 8. Drawing of the topography of the sample, described in Fig. 7. Beside the grounded Ag patch small isolated Ag clusters, not spatially resolvable by SPEM, are pesent on the surface. They electrically float on the charging support. The determined surface potential is displayed. Note that the potential gradient is very high (see text).

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

we found out that a 200 A_ thick c-Al O oxide 2 3 layer on top of a Al foil can be used as a support sample for model catalytic studies with SPEM. In this case the thickness of the insulator is much less than the irradiated spot size and the geometry becomes comparable to the set-up of standard XPS measurements if we neglect the surface conductivity. The main advantage of this set-up is that now the insulating film is thinner than the escape depth of secondary electrons. Thus, a significant part of the emitted photoelectrons does not contribute to charging because it can be neutralised by the conductive support foil. This accounts for a smaller effective photocurrent. The second important effect reducing the charging is the fact that in this geometry the neutralising current flows inside the irradiated part of the sample (as in a standard XPS configuration). Thus, photoinduced carriers can contribute to the electrical conductivity before recombining. This will lower the effective resistivity of the irradiated area of the oxide layer. As a third reason why charging of thin oxide films remains low is that, although the surface may not be charged more than a few eV, the electrical field between the surface and the conducting support will be extremely high if the thickness of the oxide layer is less than 100 nm. Due to this high field even tunnelling of electrons from the support towards the charged surface can become possible [21]. If these kinds of samples are used as a support for particles thicker than one micron the beam focused onto the particles does not penetrate the thin oxide film of the support any more. Thus, the abovementioned effects that reduce the surface charging cannot take place. In this case differential charging of the particles with respect to the support film occurs. In a system where micron sized MoO 3 crystals were placed on a support consisting of a 200 A_ thick alumina film on an Al foil we observed that the crystals were charged differentially up to #10 V with respect to the support.

49

trast in the images does not correspond to the real lateral distribution of the chemical constituents of the surface. In this last chapter we will describe one more event which results in unexpected core level energy shifts of a chemical origin. This is a phenomenon related to the interaction of the photons with the matter where the energy transfer and the following excitation and de-excitation events can result in breaking of chemical bonds and ejection of atoms or molecular fragments from the surface [22]. This means that if the cross-section of these photon-induced processes is very high the chemical composition of the irradiated surface can change significantly. The SPEM experiments performed on ELETTRA are actually among the first microscopy measurements where along with appreciating the advantages of using an ultrabright light source we faced the undesired photon-induced effects due to the very intense photon flux of '109 photons s~1 focused onto a small area of )0.03 lm2. Note that

5.3. Photo-induced reduction In the previous section we discussed the severe differential charging artefacts which produce misleading contrast features due to slowing down the emitted core level photoelectrons. Thus, the con-

Fig. 9. Evolution of the Ti 2p spectra as a function of illumination time which shows the reduction of the TiO surface by the 2 focused X-ray beam.

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S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

even in the best conventional XPS microprobe machines the photon flux of the same magnitude irradiates an area of more than 2500 lm2. Similar effects as the ones described here were reported very recently for synchrotron radiation spectromicroscopy of Ba—K bismuthates and porous Si performed with a spot of order of 1 lm2 [23,24] Fig. 9 shows the Ti 2p spectrum taken with SPEM from a TiO surface after 30 min irradiation 2 with the focused photon beam. It is clear by the shifted components of the Ti 2p spectra towards metallic Ti 2p that the illuminated area is highly reduced. This loss of oxygen is nearly not visible if non focused X-rays illuminate the surface. As has already been shown in Section 5.1 the local heating of the illuminated spot is negligible so that it is only a photon-induced process that leads to reduction of TiO . The loss of oxygen is due to the ejection of 2 O` ions, a process already observed for several maximal valence oxides, including TiO and ex2 plained by Knotek and Feibelman as due to an

inter-atomic Auger decay [22,25,26]. Thus, with the electron analyser tuned to the metallic Ti 2p peak, one can study the photon-induced reduction of the sample with time following the increase of the peak intensity. Fig. 10 shows a similar case for a sample where a thin MoO phase covers a TiO substrate. Here x 2 the small rectangular area had been scanned in front of the beam before the displayed bigger image was taken collecting the photoelectrons with a kinetic energy corresponding to the (metallic) Mo 3d peak. Similar to the case of TiO , the MoO phase 2 x of the already scanned area has undergone photon-induced reduction and it appears bright in Fig. 10a because of the presence of reduced Mo. Note that this contrast does not reveal lateral inhomogeneity of the molybdena phase but only proves the change of its chemical state due to the interaction with the X-ray beam. This can be confirmed by measuring the Mo 3d spectra of the non-illuminated part of the surface as a function of

Fig. 10. (a) 64]64 lm2 Mo 3d image containing a smaller area that has been previously scanned illustrates the photon induced reduction of a thin MoO phase on a TiO support surface. By tuning the analyser energy window to the expected kinetic energy of the x 2 Mo 3d photoelectrons emitted from a reduced surface already irradiated parts of the surface appear bright. (b) Evolution of the Mo 3d region with irradiation time showing the shift towards higher kinetic energy due to photon-induced reduction of the Mo oxide.

S. Gu¨ nther et al. / Ultramicroscopy 75 (1998) 35—51

time. These results are displayed in Fig. 10b. Clearly the Mo 3d core level peak shifts towards lower binding energy (as expected for reduced Mo) with increasing exposure time to the X-rays. It should be noted that also charging may contribute to a certain extent to this shift, although we confirmed that on the surface investigated, this effect accounts for shifts less than 1 eV.

6. Conclusions Using selected results obtained with the recently developed scanning photoemission microscopy technique we demonstrate different artefacts which may complicate the interpretation of the data and in some particular cases even make it impossible to perform chemical (core level) mapping. Despite the same physical origin of most of the artefacts observed in SPEM the difference in the type of the excitation source and the nature of the collected photoelectrons affect the artefact appearance in comparison with images obtained by other scanning microscopy techniques, like SEM and SAM. Since in SPEM core level photoelectrons are collected to obtain images a new type of artefact was observed due to occurrence of photon-induced processes, like charging and photon-induced desorption, that can affect the energy position of the selected core level line. We make an attempt to quantify these effects that are specific only for SPEM. We show that by choosing suitable measurement conditions, phenomena like charging and photon-induced reduction of a laterally inhomogeneous specimen can be imaged, which appears as a great advantage of the core level scanning microscopy.

Acknowledgements We gratefully acknowledge the fruitful discussions with M. Marsi, L. Gregoratti and L. Casalis and the excellent technical support of D. Lonza and G. Sandrin. We also would like to thank M. Gentili and Enzo DiFabrizio from IESS (CNRRome) for providing the zone plate optics. The

51

work was financed by an EC grant under contract ERBCHGECT920013 and by Sincrotrone Trieste SCpA.

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