ISSN 1027-4510, Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 2016, Vol. 10, No. 5, pp. 1001–1010. © Pleiades Publishing, Ltd., 2016. Original Russian Text © R.H. Khasanshin, L.S. Novikov, S.B. Korovin, 2016, published in Poverkhnost’, 2016, No. 10, pp. 14–24.
Effect of Residual Atmospheric Pressure on the Development of Electrostatic Discharges at the Surface of Protective Glasses of Solar Cells R. H. Khasanshina, *, L. S. Novikovb, and S. B. Korovinc aKompozit
Joint Stock Company, Korolev, Moscow oblast, 141070 Russia Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia cProkhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail:
[email protected] b
Received February 29, 2016
Abstract—Electrostatic discharges obtained upon the irradiation of K-208 glass with 40-keV electrons at a flux density ϕ of 1010 to 2 × 1011 cm–2 s–1 are studied. The residual pressure pv in the vacuum chamber is varied from 5 × 10–5 to 5 × 10–3 Pa. Structural changes in the sample surfaces are studied by atomic-force microscopy. Depending on the pressure level, two types of discharges are observed in experiments at 3 × 1010 ≤ ϕ ≤ 1.2 × 1011 cm–2 s–1: a microprojection at the glass–ionized-residual-atmosphere surface and a discharge which develops along the irradiated surface. It is found that at 5 × 10–5 ≤ pv ≤ 3 × 10–4 Pa and 8 × 1010 ≤ ϕ ≤ 1011 cm–2 s–1, discharges of the first type appear at the beginning of exposure; that is, an increase in microprojections is observed. Further, surface discharges propagate through these microprojections. At 10–3 ≤ pv ≤ 5 × 10–3 Pa and 1010 ≤ ϕ ≤ 5 × 1010 cm–2 s–1, on the contrary, discharges of the second type are realized at the beginning. These discharges result in the appearance of channels with inhomogeneities on the glass, at which subsequently discharges of the first type occur. It is determined by calculations that in the region adjacent to the exposed glass surface, secondary electrons accelerated in a field of charge accumulated in the glass make the main contribution to the ionization of gases. Keywords: electron irradiation, secondary electrons, atomic-force microscopy, discharge channels, microprojections DOI: 10.1134/S102745101605030X
INTRODUCTION The study of processes taking place under the electron irradiation of an insulator and their impact on changes in the structure and properties of the material is of great interest both in terms of the physics of phenomena and from the point of view of solving urgent problems of solid-state radiation physics [1–5], space materials science [6], dosimetry and laser technologies [7, 8], and vacuum pulse power [9]. In particular, the dielectric materials of the outer surfaces of high-orbit satellites accumulate charges under the effect of ionizing radiation, which causes the appearance of surface potentials and strong electric fields resulting in electrostatic discharges with a current amplitude of up to hundreds of amperes and nanosecond leading edges; they interfere with the work of on-board equipment and can cause the destruction of materials [10]. Electrostatic discharges observed upon the irradiation of K-208 and CMG glasses, used as protective
coatings for the solar cells of spacecraft, were previously studied [11–13]. The samples were subject to irradiation with electrons (E0 = 10–40 keV) at ϕ = 1010–1012 cm–2 s–1 and pv = 10–4 Pa. Discharges of two types were observed in the experiments: the discharge of a microprojection at the glass–ionized-surrounding-atmosphere surface led to an increase in the number and size of microprojections, and a discharge of the second type developed at the glass surface to form discharge channels. Both discharges were accompanied by the release of plasma into the environment and changes in the structure of the glass surface. It follows from these experiments that at fixed parameters of E0 and pv, the type of electrostatic discharge is dependent on the value of ϕ. The goal of this work is to show the effects of residual atmospheric pressure in the vacuum chamber on electrostatic discharges under continuous electron irradiation of K-208 glass. The results of experimental studies of electrostatic discharges and their analysis
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are presented, and the mechanisms of discharges occurring at different values of pv are discussed. The focus is on analysis of the development of radiationinduced electrostatic discharges at pv ≥ 10–3 Pa. EXPERIMENTAL Samples of K-208 glass, 40 × 40 × 0.17 mm in size, attached to the cooled grounded metal stage of a UV-1/2 setup (Kompozit, Russia) were irradiated with 40-keV electrons at ϕ = 1010–2 × 1011 cm–2 s–1 and a pressure of pv = 10–5–5 × 10–3 Pa. The distance between electron-beam entrance into the vacuum chamber and the stage was 80 cm. The electron-beam characteristics were recorded by Faraday cups at a distance of 2 cm from the stage with the sample. Before exposure, the samples surfaces were cleaned with distilled water in an ultrasonic bath. Before and after exposure, the surface of the samples were examined using a Solver P47 Multi-Technique SPM atomic force microscope (AFM). EXPERIMENTAL RESULTS The assessment of earlier experiments showed that at pv = 10–4 Pa and fluence values Φ of no greater than 1016 cm–2, discharges of the second type were observed at larger values of ϕ than discharges of the first type for all values of E0 = 10–40 keV. In a new series of experiments, one of the objectives was to determine the pressure dependence of the electron-flux-density threshold ϕcr, at which discharges of the second type begin to form. To solve the problem, the samples were irradiated at different values of ϕ from the above range for every fixed value of pv. As an example, AFM images are shown in Fig. 1, characterizing the structure of the sample surface irradiated at pv = 5 × 10–5 Pa. Based on these results, it was concluded that at a specified pressure and values of ϕ ≤ 1.4 × 1011 cm–2 s–1, the first-type of discharge is the main channel for releasing accumulated charge. The second type of discharges begin to form at a value of ϕ = 1.1 × 1011 cm–2 s–1. The height of microprojections, which are the traces of first-type discharges, reaches 80 nm (Figs. 1a and 1b), and the depth of the channels, which are the traces of second-type discharges, does not exceed 1.5 nm (Figs. 1c and 1d). It should be noted that irradiation under the above conditions leads to the appearance of traces of discharge currents in the glass, which begin near a microprojection and end at the same microprojection (Fig. 1c). It is seen that they are raised above the sample surface by 2 nm on average (Fig. 1d). The characteristic signs of an electrostatic discharge of the first type at pv ≥ 5 × 10–4 Pa, appearing on the surface of irradiated glass, have a flower shape (Fig. 2). In the center of each “flower” (Figs. 2a and 2b),
there is a microprojection with a height from 20 to 70 nm (Fig. 2c) at which converge “petals”, representing traces of discharge processes in the surface glass layer and towering over them by an average of 2 nm (Fig. 2d). If at pv = 5 × 10–5 Pa, the width of petals (Fig. 1d) did not exceed 0.2 μm, then at pv = 5 × 10–4 Pa, this value can reach 1 μm (Fig. 2b), although the values ϕ and Φ are five times smaller. The results of experiments conducted at 5 × 10–4 < pv < 10–3 Pa, 1010 ≤ ϕ ≤ 3.5 × 1010 cm–2 s–1, and 1014 ≤ Φ ≤ 9 × 1014 cm–2 showed that under these conditions, electrostatic discharges of the first type are the main channel for releasing the accumulated charge for most of the irradiated samples. With ϕ and/or Φ increasing in the specified ranges, the sizes of the petals increase. As evidence of the above, a fragment of the irradiated sample surface is given in Fig. 3; it is seen that the height of microprojections reaches 100 nm, the area of individual petals on the glass is approximately 7 μm2, and their height is 3 nm. The character of the traces of electrostatic discharges on the glass surface did not change, as compared with the case shown in Fig. 2. However, at pv = 5 × 10–4 Pa with increasing radiation-flux density to ϕ ~ 4 × 1010 cm–2 s–1, surface discharges were observed in the experiments; the microprotrusion sizes did not exceed those of the discharge traces shown in Fig. 2. As the pressure increases to 5 × 10–3 Pa, some discharges of the second type were formed starting at ϕ = 2.3 × 1010 cm–2 s–1 and Φ = 1.5 × 1013 cm–2. With Φ increasing to 2.5 × 1014 cm–2, structures are formed on the glass, which resemble discharge channels (Fig. 4b), but are elevated above the surface (Figs. 4a and 4c). It should be noted that similar formations on the glass irradiated at pv < 10–3 Pa were not observed. A detailed study of these structures showed that they grow over the discharge channels. ANALYSIS OF EXPERIMENTAL RESULTS To date, no conventional quantitative theory exists describing electrostatic discharges under the continuous irradiation of a dielectric material, enabling the unambiguous interpretation of experimental results. This is because in an irradiated dielectric complex interrelated processes occur, for example, the accumulation of charge which induces an high-intensity electric field, electrostatic discharge, secondary electron emission [14], charge relaxation, and the appearance of currents due to radiation-stimulated conductivity. Upon the irradiation of glass, the annealing of structural defects also occurs, the concentration of which is particularly high in the surface layer. In turn, the annealing of defects and dislocations is accompanied by radiation-stimulated strain and radiationenhanced diffusion, which contributes to the acceleration of transport processes that ensure the transfer of
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material to the centers of growth of individual microprojections on the glass surface. These processes lead to changes in the physical properties and structure of the glass. Interacting with an insulator, primary electrons lose most of their energy to the excitation of electrons of the target and become thermalized and captured at traps. This leads to the formation of a region of high charge density in the sample, the distribution maximum of which is shifted with increasing fluence to the irradiated surface due to the deceleration of primary electrons in the field of accumulated charge [12]. The thickness of the flat layer, in which localized electrons are distributed, is determined by their energy, and at E0 = 40 keV, it is approximately 16 μm. However, due to the drift in the field of accumulated charge, localized electrons can migrate both to the irradiated sur-
face and be captured at deeper traps, and toward the substrate, increasing the thickness of this layer and the amount of accumulated charge. Upon reaching the critical value of the electric-field intensity, which for a material with known structure depends mainly on the values of parameters E0, ϕ, and pv, electrostatic discharges start to occur; a decisive role in these discharges is played by gas ionized near the irradiated surface. Upon continuous electron irradiation, the field of the charge accumulated in the insulator affects both the energy of primary electrons Ee(t) reaching the target and the spectrum and the full emission coefficient of an electron emitted from the surface, that is, (1) σ[E e (t )] = δ[E e (t )] + η[E e (t )], where Ee(t) = E0 – eVS(t); VS(t) is the surface potential of the charged dielectric at time t; E0 is the energy of
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primary electrons at VS = 0; δ(t) is the emission coefficient of secondary electrons; and η[Ee(t)] is the numerical reflection coefficient of primary electrons, which is generally dependent on the energy E0 (for example, for layered materials). For the interpretation of experimental data, we consider the ionization of gas near the irradiated surface of the sample. This process can be characterized by the number of ionization events when electrons pass a path length of 1 cm, that is, the parameter that represents the product of the ionization cross section σion(Ee) by the concentration of gas molecules Ng (cm–3). Then the number of ions generated per unit volume in 1 s at the electron current density ϕ can be estimated as
nion = σ ion (E e )N g ϕ.
(2)
The full ionization cross section is calculated according to [15] by summing the partial cross sections taking into account all electronic shells, as
σ
ion
(E e ) =
∑m σ n
ion n (E e ),
(3)
n
where mn is the population of the nth shell, and Ee is the energy of an ionizing electron,
4 π a0 R (E e + U n + E n )U n (4) 2 ⎡⎛ 1 U n ⎞ E e Un Un Ee ⎤ × ⎢⎜ − ln +1− − ln , 2⎟ E e U n + E e U n ⎦⎥ ⎣⎝ 2 2E e ⎠ U n where En is the average energy of an electron in a molecule at the nth shell (in eV), ΔUn is the electron binding energy at the nth shell (in eV), a0 is the Bohr radius (5.292 × 10–9 cm), and R is the Rydberg constant σ ion n (E e ) =
2
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Fig. 3. AFM images of the irradiated sample (ϕ = 3.4 × 1010 cm–2 s–1, Φ = 8.5 × 1014 cm–2): (a) and (b) 3D and 2D images of frames 10 × 10 μm, and (c) and (d) cross sections of the 2D frame along lines 1–2 and 3–4.
(13.61 eV). The parameters used in Eq. (4) for the main atmospheric gases are shown in the table [16]. It should be noted that the area adjacent to the exposed glass surface is under special conditions. First, it has a high density of gases due to the contribution of molecules removed from the surface during electron-stimulated desorption and oxygen atoms released upon the breakage of bonds in SiO2 molecules and diffusion to the glass surface. Second, the ionization of gases in this field occurs under the action of both primary electrons and electrons emitted from the surface and accelerated by the field of charge accumulated in the glass. Third, this field attracts ions formed in neighboring areas. It follows from the calculation results of σion(Ee) according to Eq. (3), which is consistent with experimental data [15–17], that the ionization cross section of gases first increases to a maximum and then
decreases with energy increasing from the threshold value. The results of calculations of the cross sections of the main atmospheric gases as functions of the energy of ionizing electrons are presented in Fig. 5. For example, in the case of a nitrogen molecule, σion(Ee) is the ionization cross section of N2 with a yield of only Ν 2+ ions (Fig. 5a), and σ ion total (E e ) is the ionization cross section with a yield of Ν 2+, N+, and Ν 2++ (Fig. 5b). Calculations showed that for a nitrogen –16 cm–2 molecule, the maximum of σ ion total (E e ) = 2.5 × 10 is achieved at Ee = 98 eV, and at an energy level of 0.05, 0.3, and 40.0 keV, this value is 1.91, 1.82, and 0.03 × 10–16 cm–2, respectively. With the pressure varying in the range of 10–5 to 5 × 10–3 Pa, the specific rate of ion generation in the residual atmosphere of the vacuum chamber by primary
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electrons under these experimental conditions ranges from 1.25 × 10 4 to 6.25 × 106 ions/(cm3 s). However, in the surface region of the irradiated glass, the concentration of ions may exceed these values. We consider this issue in more detail. It is known that the flow of electrons emitted from a solid surface upon irradiation consists of primary electrons, which are elastically or nonelastically reflected from the target, and true secondary electrons, that is, electrons of the target, excited with radiation, which have overcome the potential barrier, and left its surface. The initial energy of the major part of secondary electrons leaving the surface of an uncharged target is independent of E0 for a wide range of primary electron energies and is a few electron volts [14]. However, in the case of the continuous irradiation of a dielectric material, secondary electrons are accelerated in the field of the accumulated charge. For example, in a
field with the strength 10 kV/cm, secondary electrons acquire an energy of 100 eV even at a distance of 10 μm from the surface, which corresponds to the maximum ionization cross section of the main atmospheric gases. The results of estimations suggest that under the experimental conditions, the contribution of secondary electrons to the ionization of gas near the irradiated surface is greater than the corresponding contribution of primary electrons. The concentration of ions in this area is approximately 36 times larger than that in the vacuum chamber at a distance of 0.2 m from the sample stage, and it is sufficient to maintain gas discharge forms. In the time intervals between discharges, the density of positive ions near the irradiated surface increases with increasing field strength, especially in the vicinity of the microprojection tips, where the field strength has local maxima. Indeed, if h and r are the microprojection height and the radius of its top,
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σion, 10–16 сm–2
–16 σion сm–2 total, 10
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Fig. 5. Dependence of the ionization cross sections of the electron energy with the yield of (a) only one ionized molecule M 2+ and (b) all major types of ions.
respectively, the field strength at the microprojection top can be estimated using the equation Em(t) ≈ ES(t)h/r [18], where ES(t) is the field strength at the surface of the sample at the time t. The movement of ions in the direction of the microprojection leads to an increase in the space charge density and a local increase in the electric-field strength, while bombardment of the tops of the microprojection by ions accelerated by means of the field warms it and stimulates the emergence of discharge. Multiple repetitions of discharges at microprojections, observed in experiments, leading to their growth through the accumulation of cooled glass, are caused by the fact that the microprojections and some areas in the glass surrounding them, in which previous discharges evolved, have a higher conductivity compared to glass outside these regions. The latter is because the violation of stoichiometry and the destruction of the material in these areas lead to the formation of systems of localized states in these regions, responsible for the appearance of the electronic component of the current [19]. At pv ≥ 5 × 10–4 Pa, traces of the first-type electrostatic discharge generally have the shape of a flower consisting of petals, rising above the surface of the glass (Fig. 2) and converging at the base of the microprojection. Perhaps the emergence of the petals is due to intense gas formation, destruction, and the local expansion of glass, caused by the passage of high currents over its surface layer and exchange processes between the glass surface and gas ionized in discharges. For example, plasma formed at the surface discharges supplies charges to compensate for unscreened charges and prevents an increase in the electric field [18]. The experimental results showed that with increasing pressure in the vacuum chamber, discharges of the
second type occur at a lower density electron flux. It is seen from Fig. 6 that with pv increasing from 10–5 to 5 × 10–3 Pa, the value of ϕcr decreased from 1.7 × 1011 to 1.4 × 1010 cm–2 s–1. One of the important features of the results of irradiation at pv ≥ 10–3 Pa is the appearance of structures on the irradiated glass (Fig. 4) that have not been observed at lower pressures. Their formation can be explained by the existence of a mechanism that decreases the amount of charge accumulated in the insulator. At an early stage of irradiation, channels of a depth from 1.5 to 5 nm are formed in the glass surface layer as a result of second-type discharges. Upon further irradiation, while the tangential component of the field has not yet reached the value necessary for the development of surface discharge, first-type discharges occur at irregularities in the channel strucϕcr, 1010 сm–2 s–1 18 15 12 9 6 3 0 0.01
0.1
3 1 pv, 10 Pа
Fig. 6. Dependence of the critical electron flux density on pressure.
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Fig. 7. AFM image of the irradiated sample (ϕ = 1.0 × 1011 cm–2 s–1, Φ = 1.1 × 1015 cm–2): (a) frame 10 × 10 μm, and (b) and (c) cross sections of the frame along lines 1–2 and 3–4.
tures. As a result, microprojections are built-up along the channels.
microprojections formed at an earlier stage of irradiation (Fig. 7a).
The proposed hypothesis was partially confirmed after the irradiation of samples at the same values of ϕ and pv but at lower values of the fluence: Φ1 = 1.4 × 1013 cm–2 and Φ2 = 6.9 × 1013 cm–2. AFM analysis revealed that there are discharge channels on the glass surfaces irradiated with fluences Φ1 and Φ2; however, in the latter case, individual groups of microprojections are observed along the channels. We note that consistent implementation of the two types of discharges also took place at pv = 10–4 Pa [12]. However, under these conditions, the order of the discharges has been reversed. At the initial stage of irradiation, firsttype discharges took place with the formation and growth of microprojections, at which discharges of the second type evolved in the subsequent period. At pv = 10–4 Pa, discharges of the second type appear when ϕ ≥ 7 × 1010 and in the range of ϕ values from 1010 to 8 × 1011 cm–2 s–1, they pass predominantly through
Thus, in the case of pv = 10–4 Pa with increasing electron f lux density to 1011 cm–2 s–1, discharges of the second type begin to dominate, and at ϕ ≥ 1.4 × 1011 cm–2 s–1 become dominant (Fig. 8), and discharges of the first type are not observed. In this case, discharge channels with a depth of up to 3 nm are formed on the glass surface. CONCLUSIONS Based on the analysis of experimental results obtained under the continuous irradiation of K-208 glass samples in a vacuum chamber with 40-keV electrons and calculations, it can be argued that the mechanisms of the observed electrostatic discharges are largely determined by the pressure of the residual atmosphere. In particular, we show that an increase in pressure in the vacuum chamber from 10–5 to 5 × 10–3 Pa
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Fig. 8. AFM image of the irradiated sample (ϕ = 1.4 × 1011 cm–2 s–1, Φ = 1.1 × 1015 cm–2): (a) and (b) 3D and 2D images, and (c) cross section of the frame 10 × 10 μm along line 1–2.
decreases the minimum density electron flux ϕcr, required for the development of discharges of the second type, from 1.7 × 1011 to 2.3 × 1010 cm–2 s–1. At pv ≥ 5 × 10–5 Pa, the traces of first-type discharges on the glass surface generally have the form of a flower. There is a microprojection at the center,
where the discharge occurs and at which petals, that is, structures towering above the sample surface, converge. They are formed because of the removal of gas, degradation, and local expansion, accompanying the passage of large currents over the surface layer of the glass and exchange processes between the sample sur-
Parameters used to calculate the ionization cross sections of the main atmospheric gases Nitrogen Shell no. 1 2 3 4 5 6
Oxygen
Carbon dioxide
∆Un, eV
En, eV
mn
∆Un, eV
En, eV
mn
∆Un, eV
En, eV
mn
15.58 17.07 21.00 41.72 – –
54.91 44.34 63.18 71.13 – –
2 4 2 2 – –
12.07 19.64 19.79 29.82 46.19 –
84.88 59.89 71.84 90.92 79.73 –
2 4 2 2 2 –
13.77 19.70 20.27 21.62 40.60 46.19
64.43 49.97 71.56 74.66 78.38 79.73
4 4 2 2 2 2
En is the average energy of the electron in a molecule at the nth shell; ΔUn is the binding energy of electron at the nth shell; mn is the population of the nth shell. JOURNAL OF SURFACE INVESTIGATION: X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES
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face and gas ionized in discharges. With pv increasing from 5 × 10–5 to 5 × 10–4, the width of petals increases from 0.2 to 2 μm. At pv ≥ 10–3 Pa, the successive realization of the two types of discharges takes place, leading to the formation of structures that are not observed at lower pressures. In the first stage of irradiation, discharges of the second type occur, and channels with irregularities are formed, at which discharges of the first type are subsequently realized, accompanied by the appearance of clusters of microprotrusions along the channels. In the area adjacent to the irradiated surface, the main contribution to the ionization of gases is made by secondary electrons accelerated in the field of charge accumulated in the glass. REFERENCES 1. S. M. Brekhovskikh, Yu. N. Viktorova, and L. M. Landa, Radiation Effects in Glasses (Energoizdat, Moscow, 1982) [in Russian]. 2. A. W. Zanderna, T. E. Madey, and C. J. Powell, Beam Effects, Surface Topography and Depth Profiling in Surface Analysis (Kluwer, New York, 2004). 3. J. F. Denatale and D. G. Howitt, Nucl. Instrum. Methods Phys. Res., Sect. B 1, 489 (1984). 4. S. G. Boev and V. Ya. Ushakov, Radiation Accumulation of Charge in Solid Dielectrics and Methods of Diagnosis (Energoatomizdat, Moscow, 1991) [in Russian]. 5. T. Gavenda, O. Gedeon, and K. Jurek, Nucl. Instrum. Methods Phys. Res., Sect. B 322, 7 (2014).
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Translated by O. Zhukova
JOURNAL OF SURFACE INVESTIGATION: X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES
Vol. 10
No. 5
2016
