about the measurement of the surface potential of ... - Mohamed Belhaj

Jul 6, 2001 - Benavides and M José Yacaman, Mexico, Vol 1, p. 467. [3] D. C Joy and C. S. Joy , Micron. 27 (1996) 247. [4] H. Gong, C. Le Gressus, K.H. Oh, ...
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Vide : Scien. Tech. Appl. (Numéro spécial : 4th Internat. Conf. on Electric Charges in Non-Conductive Materials. Tours (F), 1-6 July 2001

ABOUT THE MEASUREMENT OF THE SURFACE POTENTIAL OF ELECTRON-IRRADIATED INSULATORS IN SEM BY USING THE BREMSSTRAHLUNG HIGH-ENERGY-CUTOFF. M. Belhaj a - O. Jbara a - S. Fakhfakha a,b - M.N. Filippov c - E.I. Rau c - M.V. Andrianov c a) DTI UMR 6107 CNRS Faculté des Sciences, BP 1039, F-51687 REIMS CEDEX 2 France b) Laboratoire des dielectriques Faculté des sciences, Sfax, Tunisia. c) Department of Physics, Moscow State University, 119899 Moscow We have performed the measurement of the surface potential on single crystal of Al2O3 under electron irradiation in scanning electron microscope (SEM), using both the bremsstrahlung high-energy-cutoff and electron spectroscopy. A disagreement between these two methods was observed. The causes of this disagreement are analyzed. In particular we show that the determined values of the surface potential from the bremsstrahlung high-energy-cutoff could be largely underestimated. Introduction To determine the charging magnitude in SEM, without the use of electrode contact, a lot of SEM charge measurement methods have been developed. Among these methods, the measurement of the surface potential from the shift of the high energy cut-off of the x-ray continuous radiation (bremsstrahlung) emitted from the sample is largely used [1-4]. We have performed the measurement of the surface potential on a Al2O3 single-crystal for a variety of primary beam energy E0 using the high energy cut-off of the X-ray bremsstrahlung. Simultaneously we have measured Vs by monitoring the shift occurring on energy spectra of the secondary (SEs) and backscattered electrons (BSEs) emitted from the charged sample surface [5,6]. A big disagreement between the two methods is observed, particularly when the primary beam energy is high. The goal of this work is to discuss the origin of the difference between the measured value of the surface potential by these two methods. Throughout some experiments, we show that the position of the experimental high energy cut-off of the x-ray bremsstrahlung is not directly connected to the builtup surface potential. Thus, the measured Vs from the x-ray spectroscopy method can be seriously underestimated. Charging Charging effects in electron irradiated insulators result from a competition between the secondary electron emission which contributes to a positive charging and the trapping electrons in the sample. During the irradiation the progressive trapping of incident and secondary electrons in the interaction volume leads to a progressive increase of a negative space charge associated to a decrease of the effective secondary electron escape depth and of secondary electron yield. The final state equilibrium may corresponds to a positive charge or to a negative one. The total electron emission yield approach is often used to predict the sign of the charge. Following this approach, this sign is positive when primary beam energy is situated between the two critical energies E1 and E2: energies, where the total electron emission yield σ = δ + η (δ: secondary electron yield; η: backscattering coefficient), is equal to the unity, while it is negative outside this interval. A detailed and critical analysis of this approach is given in [7]. Methods The experiments were carried out in SEM Philips 505 (vacuum: around 10-6 Torr). equipped with a PGT SiLi energy dispersive spectrometer (EDS) and a high compact electron toroidal

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Vide : Scien. Tech. Appl. (Numéro spécial : 4th Internat. Conf. on Electric Charges in Non-Conductive Materials. Tours (F), 1-6 July 2001

spectrometer (ETS) specially adapted for applications in SEM [8]. The samples are locally irradiated in SEM fast scanning mode (50 frames per second). The raster was maintained at approximately 50X50 µm². When the accumulated charge reaches its equilibrium state (i.e. stabilization of the SEM image distortion or/and the sample holder current [9]), the X-ray and the electron spectra are monitored. The two methods used to measure the surface potential are described here briefly X-ray spectroscopy: The high energy cut-off in the X-ray spectra corresponds to the energy of the incident electrons striking the sample surface. It is generally admitted that, if the surface potential is VS, the experimental cut-off occurs at the energy Ecut-off = E0+eVS (Duane-Hunt limit). Ecut-off, being the intersection of the high energy part of the X-ray spectrum with the energy axe’s. Thus, the surface potential can be determined from the spectrum as VS = - (E0 - Ecut-off) / e . Electron spectroscopy: The electron spectra obtained using the (ETS), allows a direct measurement of the surface potential [6,7]. In fact, when the sample is negatively charged. The (SEs) and (BSEs) emitted from the sample surface are accelerated between the negatively charged surface and the grounded entrance of the (ETS). As a consequence the spectrum is simultaneously compressed and shifted toward the high energy region. The energy position of the maximum of the (SEs) distribution corresponds to the value of the surface potential multiplied by the electron charge, -eVS. Results and discussions The results of VSx (energy shift of the high energy cut-off of the X-ray bremsstrahlung) and VS (energy shift of the SEs peak) measurements versus E0 (primary beam energy) on single crystal of Al2O3 using a primary beam current of 0.5 nA are shown on (fig.1). If at low primary beam energies VSx is consistent with VSe, a net difference appears when, E0 is greater than 8 keV. Higher is the primary beam energy and higher is the difference between measured surface potential by the two methods. We should note that the measurement of VSe are possible only if the energy shift of the SEs distribution exceeds 2 keV (i.e the negative surface potential > 2kV) because, the ETS can detect only electrons with energies ranging from 2 to 40 keV [8]. For this reason the measurements of VSe are carried out for primary electron energies higher than 5 keV. e

Fig.1. The determined surface potential by the two methods as function of the primary beam energy (VSx: deduced from x-ray spectroscopy) and (VSe: deduced from electron spectroscopy).

Fig.2. The measured spectrum of the electrons emitted from non grounded Cu-coated/ Al2O3. In the inset, the x-ray spectra emitted from the Cucoated/ Al2O3 (grounded (2) and not grounded (1))

For given experimental parameters, generation emission and detection processes of X-ray photons and electrons depend on the sample matrix as well as on the magnitude of the accumulated charge. In order to separate these two effects a special sample has been prepared. Two copper disks (2 mm in diameter and about 30 nm in thickness) were evaporated through a mask on an Al2O3 surface. The sample was set on the grounded sample holder, made of chrome-nickel steel. One of the Cu-

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Vide : Scien. Tech. Appl. (Numéro spécial : 4th Internat. Conf. on Electric Charges in Non-Conductive Materials. Tours (F), 1-6 July 2001

coated areas was connected to the ground in order to maintain its surface potential to zero. The other Cu-coated area was kept floating. In this way only the charging effects differentiate the two Cu-coated Al2O3 areas under electron irradiation. Both grounded and no grounded Cu-coated areas were alternatively submitted to electron irradiation, with 16 keV primary beam energy and 10 nA primary beam current. The shift of the (SE) distribution (fig. 2) on the electron spectrum emitted from the non-grounded Cu-coated Al2O3 indicates that VSe = - 11.5 kV. The X-ray spectra from the two Cu-coated Al2O3 (grounded and not grounded) are shown (inset of fig.2). In the X-ray spectrum 1 (non-grounded Cu-coated Al2O3), no Cu line was observed . This means that the primary electron beam striking the charged Cu-coated Al2O3 has an energy below the CuK level (8.994 keV). As a consequence, the surface potential must be at least 16 - 8.994 ≈ 7 kV. However, VSx deduced from the energy position of the X-ray bremsstrahlung cut-off is 16 – 13.7 = 2.3 kV. It can be also seen, that CrKα and NiKα X-ray lines are clearly seen in spectrum 1, even through the primary beam irradiated area is free from the corresponding elements. In the case of grounded Cu-area, the X-ray spectrum 2 is free from these lines. Knowing that the position of the collimated X-ray detector does not allows to detect X-ray emitted by the pole pieces, these lines are certainly, issued from the sample holder made from chrome-nickel steel. These last results involves, another electronic source strongly delocalized and related to the surface potential build-up. In fact the high energy part of bremsstrahlung in spectrum 1 (in box on the fig.2) is certainly related to this source and not directly to the primary electron beam. The origin of this delocalized electrons source and therefore the qualitative explanation for the observed disagreement between these two methods may be deduced from the following arguments:

Fig. 3. formation of the x-ray Bremsstrahlung in Fig. 4. Illustration of the effect of the pole pieces SEM materials. In addition to the acceleration of the (SEs) and (BSEs) in the electric field generated by the accumulated charge on the surface, there angular distributions are strongly distorted. The flux of the emitted electrons tends to be collimated on the pole pieces (specially the secondary electrons (SEs), due to there low energy emission < 50 eV). This collimating process is the origin of the so-called "pseudo-mirror effect" in SEM that was described elsewhere [10]. At the equilibrium charging state σ = 1, the total number of emitted electrons from the sample surface per second is equal to the primary beam current. Thus when the charged sample was irradiated by primary electrons I0, the pole pieces are irradiated by a current I’, comparable in magnitude with I0. Unlike the case of conductive sample, I’ is formed by (SEs) and (BSEs) which have a relatively high energy, ranging from -eVSe. to E0. (i.e. from –11.5 keV to 16 keV in the present experiment). The interactions of these electrons with the pole pieces give rise to low energy secondary electrons (SEs’) and backscattered electrons (BSEs’) with energies in the interval between 50 eV and E0. Hence, the pole pieces in turn acts as an additional source of electrons irradiating back a non charged areas of the sample as well as the sample holder and other elements of the specimen chamber (inset of fig.3). In the energy range of (SEs) and (BSEs), starting here from 11.5 keV, the backscattering coefficient of the pole pieces is not very dependant on

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the incident primary beam energy, but strongly dependant on the mean atomic number of the pole pieces materials. This coefficient can vary approximately from 0.1 for carbon to 0.35 for copper, so the number of (BSEs’) per second can reach a non negligible values as 10% to 35% of I0. The interaction of these electrons (BSEs’) with a non-charged region of the specimen, the sample holder or some other elements of the specimen chamber give rise to an additional x-ray emission. In fact, the whole experimentally observed X-ray bremsstrahlung spectrum is the sum of the two components, the bremsstrahlung photons emitted from the primary beam irradiated region at the electron excitation source of energy E0 + eVS and those generated by the (BSEs’) (fig.3). As a result, the position of experimentally observed high-energy cut-off limit of the bremsstrahlung is not in accordance with the effective landing energy, E0 + eVS of the primary electrons. To highlight the role of the pole pieces materials in the process described above, we have carried out an additional experiment. Under the pole pieces, we have placed (i) brass disk and then (ii) carbon coated disk (fig. 4). In the neighbour of the sample, at approximately 1 mm of distance a piece of pure titanium was placed. In both cases (i) and (ii) the non-grounded Cu-coated Al2O3 was irradiated with 16 kV primary beam accelerating voltage and 10 nA primary beam current. we note that in the second configuration (carbon coated disk) the intensity of Ti Kα is lower than in the first configuration (brass disk), because the backscattering coefficient of carbon is also lower in comparison with that of brass. Conclusion When an insulator charges under electron irradiation in SEM an additional electronic source irradiating back the sample as well as the sample holder appears. This source is caused by the SEs and BSEs emitted from the sample, which are first accelerated by the external electric field and next backscattered by the pole pieces. As a result, the experimentally observed x-ray bremsstrahlung spectrum is the sum of two processes different in origin : - The excitation of bremsstrahlung due to the primary electron beam. - The excitation of bremsstrahlung by an additional source formed by the backscattered electrons (BSEs') emitted from the pole pieces. As a consequence the experimental measured high-energy cut-off limit of the x-ray bremsstrahlung for charged samples is not directly related to the surface potential. References [1] G. F. Bastin and H. J. M. Heijligers, in Electron Probe Quantification, Edited by K. F. J. Heinrich and D. E. Newbury ( Plenum, New York, 1991), p. 193. [2] J.J. Hwu and D. C. Joy, Proceeding of Electron Microscopy 1998, Edited by H. A Calderon Benavides and M José Yacaman, Mexico, Vol 1, p. 467. [3] D. C Joy and C. S. Joy , Micron. 27 (1996) 247. [4] H. Gong, C. Le Gressus, K.H. Oh, X. Z. Ong and B. T. G. Tan, J. Appl. Phys. 74 (1993) 1944 . [5] M. Andrianov, A. Gostev, E. Rau, J. Cazaux, O. Jbara and M. Belhaj, Surf. Invest. 2 (2000) 9. [6] O. Jbara, M. Belhaj, S. Odof , K. Msellak, E.I. Rau and M.V. Andrianov, Rev. Sci. Instrum. 72 (2001). [7] J. Cazaux, J. Appl. Phys. 85 (1999) 1137. [8] E.I. Rau and V.N.E. Robinson, Scanning 18 (1996) 556. [9] J. Bigarré, S. Fayeule, O. Paulhe and D. Tréheux, IEEE Anual repport Conference on Electrical Insulation and dielectric Phenomena, 1997, p. 101 [10] M. Belhaj, O. Jbara, S. Odof , K. Msellak, E.I. Rau and M.V. Andrianov, Scanning 22 (2000) 352.

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