- Email: [email protected]
In-situ analysis of ion-induced physicochemical change of Si surface by secondary electron yield detection
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
In-situ analysis of ion-induced physicochemical change of Si surface by secondary electron yield detection Subhash Ghosh, Prasanta Karmakar
T
⁎
Variable Energy Cyclotron Centre, HBNI, 1/AF, Bidhannagar, Kolkata 700064, India
ARTICLE INFO
ABSTRACT
Keywords: Ion irradiation Silicon Kinetic ion-electron emission Nano-patterning Atomic Force Microscopy
We report the variation of secondary electron yield with ion fluence during 10 keV N+1 and Ar+1 ions bombardment on a Si (1 0 0) surface at normal and oblique incidence. The initial high secondary electron yield from Si surface is observed due to the native oxide layer on Si for both nitrogen and argon ion bombardment. The secondary electron yield is expected to decrease and stabilize for continue ion bombardment, which is observed for the argon ion irradiation. However, a significant variation of the electron yield with continuing nitrogen ion bombardment is observed. Ex situ measurement of ion bombarded surfaces by Atomic Force Microscope shows that the variation of secondary electron yield is correlated with the surface topography development due to ion bombardment. Also, the reaction of the projectile with target atom alters the secondary electron emission in case of N+ ion bombardment. The mean free path length of the secondary electron is estimated using the ParilisKishinevsky theory and the variation is presented. Real-time physicochemical changes of Si surface due to low energy ion bombardment have been probed in-situ by monitoring the secondary electron emission yield measurements.
1. Introduction When an energetic charge particle bombards a solid target, it interacts with the atoms and electrons which leads to the re-distribution, excitation/ionization of the target atoms and emission of electrons. The electrons produced by this process within the target form a cascade by multiple scattering and collision. Those electrons, which escape from the target is known as Secondary Electrons (SE). The secondary electron emission depends not only on the target and projectile combination but also on the energy of the projectile, experimental geometry, elemental combination of target, atomic and electronic structure of the target, the magnitude of the surface electrostatic field, target temperature and degree of magnetization [1,2]. The ion and electron induced electron emission is used in the different field of science. One of the most important applications is the surface imaging by detecting the local angle dependent secondary electron emission in scanning electron and ion microscopy [3]. Secondary electron emission is also a principal phenomenon which is used to detect the ionizing radiation [4]. For the generation and sustaining of glow discharge, SE emission also plays an important role [5]. On the other hand, particle-induced electron emission complicates the measurements of beam currents in irradiation experiments. Low energy ion bombardment cleans the top surface, alters the ⁎
chemical nature by implanting reactive projectiles and also develops nanometer-scale topography on the surface for special cases. Real-time, in-situ monitoring of ion beam induced physicochemical change of the surface is very much important for the surface processing as well as for the understanding of the ion-solid interaction mechanisms. Sometimes, in situ X-ray or visible light scattering techniques are used to monitor the surface pattern formation [6,7]. Some setups also use dual ion and electron beam to modify as well as image the modification [8]. However, these techniques are expensive and complex in nature as well applicable only for very limited cases. A versatile and straightforward in-situ technique for monitoring the surface chemical change and pattern formation during ion bombardment is very much desirable. We demonstrate here that the simple detection of secondary electrons yield during the ion bombardment could detect the chemical change and nanopattern formation on the surface in real time. In the present article, we have studied the change of physical and chemical nature of the Si (1 0 0) surface during the bombardment of 10 keV, N+1 and Ar+1 ions by measuring the real-time secondary electron emission yield. A correlation of electron yield with the erosion of native oxide layer, reactive ion induced compound formation and nanopattern formation is drawn. Detection of pattern formation by monitoring the SE is also verified by ex- situ surface morphology measurement.
Corresponding author. E-mail address: [email protected] (P. Karmakar).
https://doi.org/10.1016/j.nimb.2018.12.055 Received 15 October 2018; Received in revised form 11 December 2018; Accepted 28 December 2018 Available online 05 January 2019 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
Fig. 2. Secondary electron yield vs. ion fluence during 10 keV N+1 ion irradiation on Silicon (1 0 0) surfaces at oblique incidence (θ = 60°).
Fig. 1. Schematic diagram of the Secondary Electron emission measurement system.
fluence during 10 keV N+1 ion bombardment on a silicon (Si) surface at an angle 60° with respect to the surface normal. The variation of yield is presented in three fluence zones. At the beginning of ion bombardment, the secondary electron yield is found about 8. It decreases sharply to 4.2 at ion fluence 1 × 1017 ions/cm2 (zone I). The initial sharp decrease of secondary electron yield is due to the removal of the native oxide layer on Si (1 0 0) surface. It is reported earlier that the electron yield is higher for oxide materials [11,12]. Secondary electron yield is more in silicon oxide than pure Si because of the larger inelastic mean free path of electron in silicon oxide. It is found in zone II that the electron yield again starts to increase at fluence 1 × 1017 ions/cm2 and reaches to 5.0 and again decreases below 4 at the fluence 5 × 1017 ions/cm2. The possible reason for the increase of yield in zone II may be due to nitride formation by implanted nitrogen ions. Recently, Karmakar et al. [13] reported the nitride formation during nitrogen ion implantation on Si at room temperature. For further ion bombardment, when amorphization and damage creation in the Si target exceeds the effect of nitridation, the yield decreases as observed in zone II. Borisov et al. showed the decrease of secondary electron yields due to ion induces damage creation [14]. They investigated the change of secondary electron yield with temperature during 30 keV Ar+ ion bombardment on poly granular graphite and found an increase of yield at high temperature when ion beam induced damages are annealed and secondary electron path length as well as primary ionizing path length are changed due to the change of lattice structures. In zone III, secondary electron yield starts to increase once again at ion fluence 5 × 1017 ions/cm2. At fluence 1 × 1018 the yield reaches to a steady state. This part is also shown separately in Fig. 6. The increase of secondary electron emission in this portion is due to the surface topography development. It is reported earlier that surface ripple pattern is formed in case of nitrogen bombardment in the energy range 10–20 keV [13]. Due to the formation of surface topography the local ion incidence angle changes depending upon the local curvature. The ion beam induced secondary electron emission strongly depends on the local ion incidence angle [15]. To verify the development of surface topography, we performed Atomic Force Microscopy (AFM) measurements of ion-bombarded Si surfaces at various ion fluence. Fig. 3 shows that the AFM images of 10 keV N+ ion (at angle 60°) bombarded Si surfaces for fluence range 2 × 1017–1 × 1018 ions/cm2. No ripple topography is formed below an ion fluence 2 × 1017 ions/cm2. Ripple-like surface morphology starts to develop at an ion fluence 2 × 1017 ions/cm2 and well-defined ripple
2. Experimental The experiments were carried out in the 6.4 GHz. Electron Cyclotron Resonance Ion Source (ECRIS) based ion beam system at VECC-RIB, Kolkata. Typical ion current density for 10 keV N+ and Ar+ is 20 µA/cm2. A hemispherical secondary electron yield measurement system shown in Fig. 1 was placed inside an experimental chamber attached to a material science beamline. Continuous pumping brings the pressure of the chamber down to 1 × 10−7 mbar. The pressure at the ion source region was 2 × 10−7 mbar. Mass analyzed nitrogen (N+), and argon (Ar+) ion beams of energy 10 keV were accelerated and used for the experiment. Ion beam bombards the surfaces at normal (0°) as well as at oblique (60°) angle as the well-defined ripple patterns are formed on Si surface by N+ ion beam at 60°. To measure the secondary electron emission yield, we developed a concentric hemispherical analyzer. The silicon target was mounted at the center of the assembly where the ion beam reaches after passing through a series of collimators. The diameters of the first and second collimators are 4 mm, 3 mm, respectively. A hemispherical cup covers the sample to measure all the backward secondary electrons emitted at − /2 to /2 angle. A 6 mm aperture at the center of the cup allows the ion beam to pass and bombard the Si target. Two collimating front plates made of stainless steel are electrically grounded. The hemispherical analyzer is connected to a power supply which can vary from −90 V to +90 V. Secondary electron (SE) yield is defined as the number of secondary electrons emitted from the sample surface per incidence ion. SE yield is calculated by measuring the total current (electron plus ion current) and the actual ion current on the target by applying the appropriate bias to the sample and the cup. The advantage of SE yield measurement by this technique is that it needs a very simple set up, that can measure all the backward secondary electrons. All the experiments were done at room temperature. Due to low power deposition by the ion beam, the temperature rise of the sample is negligible. A Bruker made Atomic Force Microscope (Multimode, Nanoscope V) is used to measure the surface topography of the ion-bombarded Si surfaces. All the measurements were performed in contact mode using silicon nitride prove at room temperature. The data were analyzed by WSxM and Gwyddion™ freeware [9,10]. 3. Results and discussion Fig. 2 shows the variation of electron yield ( ) as a function of 57
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
Fig. 3. AFM images of Silicon surface after 10 keV N+ ion bombardment at oblique incidence (θ = 60°) for fluence (a) 2 × 1017 ions/cm2 (b) 5 × 1017 ions/cm2 (c) 7 × 1017 ions/cm2 (d) 1 × 1018 ions/cm2.
structures are formed at fluence 5 × 1017 ions/cm2. The amplitude of surface patterns (rms roughness) increases with ion fluence. Secondary electrons are generated by the interaction of projectile ions with the target atoms along its path length l . Assume R e (v0 ) is the mean path length of the projectile ion at which it is slowed down to the threshold velocity v0 and loses the power to ionize further target atoms. Thus, R e cos is the depth up to which target atoms could be ionized, where is the ion incidence angle with respect to the surface normal. Now, the electron yield can be written as [14,16,17]:
=
R e (v 0 ) 0
e (v )
e
lcos
dl
=
= 1.16a0 hJ
1
(Z1 + Z2 ) (Z11/2 + Z21/2 )
,
< >
c
(3)
c
In case of 10 keV, N ion implantation on Si (Z1 = 7; Z2 = 14), we obtain the binary collision ionization cross section e (v )= 1.17 × 10−16 cm2 using Eq. (2). Considering the Si number density, =4.9 × 1022 cm−3 and assuming electron emission probability from the surface, = 1, Eq. (3) for < c , can be written as
=
K cos
(4) 6
−1
where K = R = e = 5.73 × 10 cm for the present experimental e condition. Now, the electron yield depends only on local ion impact angle and electron mean free path. Until the surface patterns are formed, the local ion incidence remains at 60°, therefore, electron yield will vary only for the variation of electron mean free path ( ). The variation of electron mean free path strongly depends on the physical and chemical nature of the target atoms. Following the Eq. (4), Fig. 4 shows the change of electron path length ( ) with ion fluence up to 5 × 1017 ions/cm2 due to surface chemical change and ion-induced lattice damage. The observed high (=7.8) (Fig. 2 point A) is due to the presence of native silicon oxide layer on the silicon. Calculated electron mean free path (λ) for silicon oxide layer is found to be 6.68 nm. The measured secondary electron yield ( ) after the removal of the top native oxide layer and subsequent amorphization [13] by 10 keV N+ bombardment is 4.2 (Fig. 2, point B) which corresponds to the secondary electron path length (λ) = 3.6 nm for clean amorphous Si surface. Electron yield after the nitridation (Fig. 2, Point C) is found to be 5, which related to λ = 4.2 nm. The limit
2
v0)10 7]
Re cos
+
(1)
v arctan[0.6(v
×
limit
where ρ is the atomic density of the target, e (v ) is the binary collision ionization cross section, determine by the total energy deposition in the electron shells of both the target and the projectile atoms. It can be expressed as [16,18]. e (v )
limit
(2)
where a0 is the Bohr radius, h is the Plank constant, J is the ionization energy, Z1 and Z2 are the atomic numbers of the projectile and target, is the probability of respectively and v is the projectile velocity. electron emission from the surface, and λ is the secondary electron mean path length, which states that the electrons produced at a depth less than that of λ are only emitted from the surface and detected as secondary electrons. Therefore, Eq. (1) also gives an idea about a particular angle of ion incidence ( c ) , determined by the equation R e cos c where electron emission yield ( ) virtually reaches its limiting value limit = e R e [17].Thus 58
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
cm2 [20], significant damage starts at ∼2 × 1017 ions/cm2 fluence when the effect of crystal damage supersedes the nitridation effect by reducing the path length of electrons produced in the collision. The observed decrease of secondary electron yield at fluence greater than 2 × 1017 ions/cm2 (Fig. 2) is therefore due to the reduction of electron path length as well as alteration of the mean path length (R e ) of the incident projectile ions. Change of electron emission under high fluence ion bombardment on semiconductor are reported earlier [21,22]. We obtain electron path length = 3.41 nm for radiation damaged Si by putting = 3.9 (Fig. 2, point D) and = 60° in Eq. (4). We assume that the beam induced crystal damage is saturated at this ion fluence, therefore no further change of is considered for more ion bombardment. However, change of secondary electron yield with ion fluence beyond 2 × 1017 ions/cm2 is observed (Fig. 2). Further changes of electron yield at higher fluence are due to the change of local ion incidence angle. With the development of surface pattern the incidence angle of the projectile changes locally. The ion incidence angle depends on the slope at the ion impact point. We have calculated the slope variation and thereby change of local ion incidence angle due to the development of such surface topography. On an initially flat surface, the ion incident angle is 60° for all the points on the surface. However, the angle of incidence changes locally if the flatness of the surface is lost. The schematic of the variation of the local ion incidence angle is shown in Fig. 5a. The slope at impact point O is dh/ dx. Now the surface normal makes an angle θe with the global surface normal. Therefore,
Fig. 4. (a) Variation of electron mean free path vs ion fluence due to chemical alteration and radiation damage.
observed higher value of λ for silicon oxide and nitride compared to silicon is consistent with earlier observation. Tanuma et al. [19] reported the calculated values of λ for SiO2 and Si3N4 are 5.05 nm and 4.13 nm respectively for 2000 eV electron. Moreover, the observed reincrease of λ after cleaning of the oxide layer confirms the formation of silicon nitride. So far, the variation of electron path length is estimated following the chemical change of Si surface with ion fluence, however, radiationinduced damage of the target atoms strongly influences the value of . Although the surface amorphization starts at ion fluence ∼1015 ions/
tan
e
=
dh dx
i.e e = tan 1 [dh /dx ] If the ion incidence angle is 60 o with respect to the global normal of the surface, the ion impact angle ( ) at point O with respect to the local surface normal would be
Fig. 5. (a) Schematic diagram of the local ion incidence angle on a curved surface. (b) distribution of the local ion incidence angle after the N+1 ion irradiation at various ion fluences. (c) Effective surface area Vs. N+1 ion fluence due to surface topography development. 59
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
Fig. 6. Fluence dependent variation of measured secondary electron yield (solid line). Secondary electron yield calculated from AFM topographic data considering the change of local ion incidence angle distribution (Fig. 5b) and the effective increase of surface area (Fig. 5c).
= 60 o
tan 1 [dh /dx ] ×
180o
The ion local impact angle at every point of the surface could be estimated by taking the derivative of the surface topography. We have taken the dh/ dx where x is along the projection of the ion beam on the surface. We have estimated the change of local ion incidence angle in the X direction (along the projection of ion beam) from the AFM images of Si surface bombarded for different ion fluences. The distribution of local ion incidence angle as a function of lateral length along the projection of ion beam is shown in Fig. 5b for ion fluence 2 × 1017–1 × 1018 ions/ cm2. Local ion incidence angle changes drastically with ion fluence. For unbombarded flat surface or bombarded surface with no ripple patterns the ion incidence angle is 60°, therefore, the distribution of local ion incidence have a narrow peak at 60°. As the surface ripple structures are started to develop at the ion fluence 2 × 1017 ions/cm2 the angle distribution is broadened, but it remains near the 60° similar to that of a flat surface. It is observed from Fig. 5b that at the fluence of 5 × 1017 ions/cm2 when surface ripple structures are fully developed, a new broad distribution peak appears near 80°. With further increase of the ion fluence, the distribution increases to near 80° and decreases at 60°. This indicates that the development of surface topography with increasing ion fluence increases the local ion incidence angle towards the higher value and changes the secondary electron yield. A correlation between secondary electron emission and ion beam induced topography development is drawn here. We assume that the increase of secondary electron yield in zone III of Fig. 2 is mainly due to the ripple pattern formation. SE emission is increased on a patterned surface because of the increase of local ion incidence angle and the effective surface area. Using Eq. (4) we have calculated the secondary electron yield considering the constant electron path length( = 3.41nm) , and the variation of local ion incidence angle from the AFM data (Figs. 3 and 5b). The distribution of local ion incidence angle is shown in Fig. 5b for ion fluence 1 × 1017 ions/cm2 to 1 × 1018 ions/cm2. From the inequality equation R e cos c , we have calculated the critical angle c = 83.72o where the electron yields reaches a saturation value. Considering the impact of local incidence angle from 20° to 83.72°, electron yield ( ) is calculated. Further, the increase of surface area is measured from the AFM images and plotted as a function of ion fluence (Fig. 5c). The calculated SE considering the local incidence angle and surface area increase is plotted (dot) in Fig. 6 where the measured SE during the ion bombardment (line) is also
Fig. 7. Secondary electron yield vs ion fluence during 10 keV (a) N+1 at normal incidence and (b) Ar+1 at oblique (θ = 60°) incidence ion irradiation on Silicon (1 0 0) surface.
shown as a function of ion fluence. The estimation of electron yield from rippled surfaces is consistent with the measured SE. To distinguish the effect of oxide layer, implantation induced chemical change & damage creation and ripple formation on secondary electron emission, we performed two more experiments, one with same energy N+ beam at normal incidence and another with the inert Ar+ beam at an angle 60° with respect to the surface normal. For both the cases surface ripple is not developed on the surface as it is observed for 10 keV N+ bombardments at 60°. In the case of Ar+ ion bombardment, neither chemical change or ripple patterns takes place. Fig. 7(a) shows the variation of secondary electron emission with fluence when 10 keV N+ projectiles hit the Si target at normal incidence angle. It is similar to Fig. 2 but for zone I and II only. Initially, electron yield is very high and decreases with the fluence as native oxide layer present on the sample. At the fluence near about 2 × 1017ions/cm2, electron emission yield is again increased like the region II of Fig. 2 due to the formation of silicon nitride. But, above the fluence of 7 × 1017 ions/cm2 the yield decreases and stabilizes which is dissimilar with the region III of Fig. 2 as the ripple pattern is not formed in this case. It is therefore confirmed that the increase of SE yield in zone III is only observed for the development of surface pattern. This observation also approves our assumption of the saturation of radiation damage and invariance of at higher ion fluence. In the case of 10 keV Ar+1 ion bombardment on Si(1 0 0) surface at oblique incidence, surface patterns are not formed as well no chemical 60
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
N+1 and Ar+1 ion bombardment at an oblique angle (60 °) has been studied. The experiment shows that the secondary electron yield varies with the variation of ion fluence. At the very beginning of the irradiation, electron yield is very high and sharply decreases for both N+ and Ar+ bombardment due to the removal of native silicon oxide layer. With continue bombardment, the yield again increases due to the projectile (nitrogen) ion-induced chemical change to silicon. When defect creation supersedes the chemical effect the yield decreases again. The chemical change is not observed for Ar+ ion bombardment where the decrease is found only due to top oxide layer cleaning and ioninduced damage creation. At higher fluence, if surface patterns are developed, the variation of electron yield is observed because of the change of local ion incidence angle and increase of effective surface area. Thus we found, ion beam induced electron emission from the elemental surface depends both on chemical and physical condition of the bombarding surface. Ion beam induced surface cleaning, projectile induced chemical change, and nanometer scale topography development result in a variation of the secondary electron emission. The variation of SE yields due to the change of secondary electron mean free path and local ion incidence angle has been correlated by the Parilis-Kishinevsky theory [16]. Without the essence of any additional physical or chemical characterization tools, real-time physicochemical change of the target surface during ion bombardment could be monitored. Acknowledgments We acknowledge all the members of RIBF group, VECC, Kolkata for kind support during the experiment. The authors are especially thankful to Mr. Dipak Bhowmik and Mr. Doddi Lavanya Kumar for their help during the operation of ion beam system and data collection as well as AFM measurements. We are grateful to the Accelerator Technology development mechanical Section of VECC for fabricating the hemispherical electron yield measurement system. References [1] D. Hasselkamp, Kinetic electron emission from solid surfaces under Ion bombardment, in: G. Hohler (Ed.), Particle Induced Electron Emission II, Springer-Verlag, Berlin Heidelberg, New York, 1992, pp. 1–95. [2] M. Rosler, W. Brauer, J. Devooght, J.C. Dehaes, A. Dubus, M. Cailler, J.P. Ganachaud, Particle Induced Electron Emission I, Springer – Verlag, Berlin Heidelberg GmbH, 1991. [3] J. Goldstein, H. Yakowitz, Practical scanning electron microscopy: electron and ion microprobe analysis, in: H.Y. Joseph Goldstein (Ed.), Practical Scanning Electron Microscopy: Electron and Ion Microprobe Analysis, Plenum Press, the University of Michigan, 1975, p. 582. [4] D.A. Bromley, Detectors in nuclear science, Nucl. Instrum. Meth. 162 (1979). [5] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley & Sons, Hoboken, NJ, 2005. [6] E. Chason, M.J. Aziz, Spontaneous formation of patterns on sputtered surfaces, Scr. Mater. 49 (2003) 953. [7] C.S. Madi, E. Anzenberg, K.F. Ludwig, M.J. Aziz1, Mass Redistribution Causes the Structural Richness of Ion-Irradiated Surfaces, Phys. Rev. Lett. 106 (2011) 066101. [8] R.J. Young, M.V. Moore, Dual-beam (FIB-SEM) systems, in: L.A. Giannuzzi, F.A. Stevie (Eds.), Introduction to Focused Ion Beams Instrumentation, Theory, Techniques and Practice, Springer, Boston, 2005, pp. 247–268. [9] I. Horcas, R. Fernandez, J.M. Comez-Rodriguez, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705. [10] D. Nečas, P. Klapetek, Gwyddion: an open-source software for SPM data analysis, Cent. Eur. J. Phys. 10 (2012) 181. [11] J. Maul, K. Wittmaack, Secondary ion emission from silicon and silicon oxide, Surf. Sci. 47 (1975) 358–369. [12] A. Marcak, C. Corbella, T.D.L. Arcos, A.V. Keudell, Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor, Rev. Sci. Instrum. 86 (2015) 106102. [13] P. Karmakar, B. Satpati, The influence of projectile ion induced chemistry on surface pattern formation, J. Appl. Phys. 120 (2016). [14] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Ion-induced electron emission – monitoring the structure transitions in graphite, Nucl. Instrum. Meth. Phys. Res. Sect. B 230 (2005) 443–448. [15] B. Sevensson, G. Holmen, Electron emission from ion-bombarded aluminum, J. Appl. Phys. 52 (1981).
Fig. 8. AFM images of silicon surface after 10 keV Ar+ ion bombardment at oblique incidence (θ = 60°) for fluence (a) 1 × 1017 ions/cm2 (b) 2 × 1017 ions/cm2 (c) 5 × 1017 ions/cm2 (d) 7 × 1017 ions/cm2. (e) Distribution of local ion incidence angle on silicon surface before and after Ar+1 ion irradiation at various ion fluence.
alteration is expected due to chemical inertness of Ar. Fig. 7(b) shows the variation of electron yield with ion fluence. It shows a similar behavior as observed in Fig. 2 for zone I only. The cleaning of the native oxide layer by Ar+ is similar as N+ at the beginning of ion bombardment, but at higher fluence, no change of yield is observed because of the inertness of Ar with silicon a well it forms no surface patterns. Fig. 8(a–d) shows the ex-situ AFM images after the irradiation with Ar+ projectile at difference fluences. It shows no prominent pattern on the Si surface. Fig. 8e shows the distribution of the local ion incidence angles after Ar+ ion irradiation at various ion fluences. As no pattern is formed, it remains the same as the virgin Si surface. Therefore, the presence of native oxide layer, ion induced chemical alteration and lattice damage, as well as surface pattern formation affect the yield of the emitted secondary electron. Thus, detection of secondary electrons during the ion bombardment can be used as an in situ probe for monitoring the real-time physicochemical change of Si surface. 4. Conclusion The ion-induced electron emission from the Si (1 0 0) during 10 keV 61
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar [16] E.S. Parilis, L.M. Kishinevsky, N.Y. Turaev, B.E. Baklitzky, F.F. Umarov, V.K. Verleger, S.L. Nizhnaya, I.S. Bitensky, Atomic Collisions on Solid Surfaces, Elsevier, Amsterdam, 1993. [17] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Angular and temperature dependences of ion induced electron emission of polycrystalline graphite, Vacuum 73 (2004) 65–72. [18] N.N. Andrianov, V.S. Avilkina, A.M. Borisov, E.S. Mashkova, E.S. Parilis, The study of graphite disordering using the temperature dependence of ion-induced electron emission, Vacuum 86 (2012) 1630. [19] S. Tanuma, Calculations of electron inelastic mean free paths III. Data for 15
inorganic compounds over the 50-2000 eV range, Surf. Interface Anal. 17 (1991) 927–939. [20] A.I. Titov, C.E. Christodoulides, G. Carter, M.J. Nobes, The depth distribution of disorder produced by room temperature 40 keV N+ ion irradiation of silicon, Radiat. Eff. 41 (1979) 107–111. [21] I.N. Evdokimov, E.S. Mashkova, V.A. Molchanov, A new method of observing defects annealing in crystals, Phys. Lett. 25A (1967) 619–620. [22] B.A. Brusilovsky, Kinetic ion induced electron emission from the surface of random solids, Appl. Phys. A 50 (1990) 111–129.
62
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
In-situ analysis of ion-induced physicochemical change of Si surface by secondary electron yield detection Subhash Ghosh, Prasanta Karmakar
T
⁎
Variable Energy Cyclotron Centre, HBNI, 1/AF, Bidhannagar, Kolkata 700064, India
ARTICLE INFO
ABSTRACT
Keywords: Ion irradiation Silicon Kinetic ion-electron emission Nano-patterning Atomic Force Microscopy
We report the variation of secondary electron yield with ion fluence during 10 keV N+1 and Ar+1 ions bombardment on a Si (1 0 0) surface at normal and oblique incidence. The initial high secondary electron yield from Si surface is observed due to the native oxide layer on Si for both nitrogen and argon ion bombardment. The secondary electron yield is expected to decrease and stabilize for continue ion bombardment, which is observed for the argon ion irradiation. However, a significant variation of the electron yield with continuing nitrogen ion bombardment is observed. Ex situ measurement of ion bombarded surfaces by Atomic Force Microscope shows that the variation of secondary electron yield is correlated with the surface topography development due to ion bombardment. Also, the reaction of the projectile with target atom alters the secondary electron emission in case of N+ ion bombardment. The mean free path length of the secondary electron is estimated using the ParilisKishinevsky theory and the variation is presented. Real-time physicochemical changes of Si surface due to low energy ion bombardment have been probed in-situ by monitoring the secondary electron emission yield measurements.
1. Introduction When an energetic charge particle bombards a solid target, it interacts with the atoms and electrons which leads to the re-distribution, excitation/ionization of the target atoms and emission of electrons. The electrons produced by this process within the target form a cascade by multiple scattering and collision. Those electrons, which escape from the target is known as Secondary Electrons (SE). The secondary electron emission depends not only on the target and projectile combination but also on the energy of the projectile, experimental geometry, elemental combination of target, atomic and electronic structure of the target, the magnitude of the surface electrostatic field, target temperature and degree of magnetization [1,2]. The ion and electron induced electron emission is used in the different field of science. One of the most important applications is the surface imaging by detecting the local angle dependent secondary electron emission in scanning electron and ion microscopy [3]. Secondary electron emission is also a principal phenomenon which is used to detect the ionizing radiation [4]. For the generation and sustaining of glow discharge, SE emission also plays an important role [5]. On the other hand, particle-induced electron emission complicates the measurements of beam currents in irradiation experiments. Low energy ion bombardment cleans the top surface, alters the ⁎
chemical nature by implanting reactive projectiles and also develops nanometer-scale topography on the surface for special cases. Real-time, in-situ monitoring of ion beam induced physicochemical change of the surface is very much important for the surface processing as well as for the understanding of the ion-solid interaction mechanisms. Sometimes, in situ X-ray or visible light scattering techniques are used to monitor the surface pattern formation [6,7]. Some setups also use dual ion and electron beam to modify as well as image the modification [8]. However, these techniques are expensive and complex in nature as well applicable only for very limited cases. A versatile and straightforward in-situ technique for monitoring the surface chemical change and pattern formation during ion bombardment is very much desirable. We demonstrate here that the simple detection of secondary electrons yield during the ion bombardment could detect the chemical change and nanopattern formation on the surface in real time. In the present article, we have studied the change of physical and chemical nature of the Si (1 0 0) surface during the bombardment of 10 keV, N+1 and Ar+1 ions by measuring the real-time secondary electron emission yield. A correlation of electron yield with the erosion of native oxide layer, reactive ion induced compound formation and nanopattern formation is drawn. Detection of pattern formation by monitoring the SE is also verified by ex- situ surface morphology measurement.
Corresponding author. E-mail address: [email protected] (P. Karmakar).
https://doi.org/10.1016/j.nimb.2018.12.055 Received 15 October 2018; Received in revised form 11 December 2018; Accepted 28 December 2018 Available online 05 January 2019 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 441 (2019) 56–62
S. Ghosh, P. Karmakar
Fig. 2. Secondary electron yield vs. ion fluence during 10 keV N+1 ion irradiation on Silicon (1 0 0) surfaces at oblique incidence (θ = 60°).
Fig. 1. Schematic diagram of the Secondary Electron emission measurement system.
fluence during 10 keV N+1 ion bombardment on a silicon (Si) surface at an angle 60° with respect to the surface normal. The variation of yield is presented in three fluence zones. At the beginning of ion bombardment, the secondary electron yield is found about 8. It decreases sharply to 4.2 at ion fluence 1 × 1017 ions/cm2 (zone I). The initial sharp decrease of secondary electron yield is due to the removal of the native oxide layer on Si (1 0 0) surface. It is reported earlier that the electron yield is higher for oxide materials [11,12]. Secondary electron yield is more in silicon oxide than pure Si because of the larger inelastic mean free path of electron in silicon oxide. It is found in zone II that the electron yield again starts to increase at fluence 1 × 1017 ions/cm2 and reaches to 5.0 and again decreases below 4 at the fluence 5 × 1017 ions/cm2. The possible reason for the increase of yield in zone II may be due to nitride formation by implanted nitrogen ions. Recently, Karmakar et al. [13] reported the nitride formation during nitrogen ion implantation on Si at room temperature. For further ion bombardment, when amorphization and damage creation in the Si target exceeds the effect of nitridation, the yield decreases as observed in zone II. Borisov et al. showed the decrease of secondary electron yields due to ion induces damage creation [14]. They investigated the change of secondary electron yield with temperature during 30 keV Ar+ ion bombardment on poly granular graphite and found an increase of yield at high temperature when ion beam induced damages are annealed and secondary electron path length as well as primary ionizing path length are changed due to the change of lattice structures. In zone III, secondary electron yield starts to increase once again at ion fluence 5 × 1017 ions/cm2. At fluence 1 × 1018 the yield reaches to a steady state. This part is also shown separately in Fig. 6. The increase of secondary electron emission in this portion is due to the surface topography development. It is reported earlier that surface ripple pattern is formed in case of nitrogen bombardment in the energy range 10–20 keV [13]. Due to the formation of surface topography the local ion incidence angle changes depending upon the local curvature. The ion beam induced secondary electron emission strongly depends on the local ion incidence angle [15]. To verify the development of surface topography, we performed Atomic Force Microscopy (AFM) measurements of ion-bombarded Si surfaces at various ion fluence. Fig. 3 shows that the AFM images of 10 keV N+ ion (at angle 60°) bombarded Si surfaces for fluence range 2 × 1017–1 × 1018 ions/cm2. No ripple topography is formed below an ion fluence 2 × 1017 ions/cm2. Ripple-like surface morphology starts to develop at an ion fluence 2 × 1017 ions/cm2 and well-defined ripple
2. Experimental The experiments were carried out in the 6.4 GHz. Electron Cyclotron Resonance Ion Source (ECRIS) based ion beam system at VECC-RIB, Kolkata. Typical ion current density for 10 keV N+ and Ar+ is 20 µA/cm2. A hemispherical secondary electron yield measurement system shown in Fig. 1 was placed inside an experimental chamber attached to a material science beamline. Continuous pumping brings the pressure of the chamber down to 1 × 10−7 mbar. The pressure at the ion source region was 2 × 10−7 mbar. Mass analyzed nitrogen (N+), and argon (Ar+) ion beams of energy 10 keV were accelerated and used for the experiment. Ion beam bombards the surfaces at normal (0°) as well as at oblique (60°) angle as the well-defined ripple patterns are formed on Si surface by N+ ion beam at 60°. To measure the secondary electron emission yield, we developed a concentric hemispherical analyzer. The silicon target was mounted at the center of the assembly where the ion beam reaches after passing through a series of collimators. The diameters of the first and second collimators are 4 mm, 3 mm, respectively. A hemispherical cup covers the sample to measure all the backward secondary electrons emitted at − /2 to /2 angle. A 6 mm aperture at the center of the cup allows the ion beam to pass and bombard the Si target. Two collimating front plates made of stainless steel are electrically grounded. The hemispherical analyzer is connected to a power supply which can vary from −90 V to +90 V. Secondary electron (SE) yield is defined as the number of secondary electrons emitted from the sample surface per incidence ion. SE yield is calculated by measuring the total current (electron plus ion current) and the actual ion current on the target by applying the appropriate bias to the sample and the cup. The advantage of SE yield measurement by this technique is that it needs a very simple set up, that can measure all the backward secondary electrons. All the experiments were done at room temperature. Due to low power deposition by the ion beam, the temperature rise of the sample is negligible. A Bruker made Atomic Force Microscope (Multimode, Nanoscope V) is used to measure the surface topography of the ion-bombarded Si surfaces. All the measurements were performed in contact mode using silicon nitride prove at room temperature. The data were analyzed by WSxM and Gwyddion™ freeware [9,10]. 3. Results and discussion Fig. 2 shows the variation of electron yield ( ) as a function of 57
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Fig. 3. AFM images of Silicon surface after 10 keV N+ ion bombardment at oblique incidence (θ = 60°) for fluence (a) 2 × 1017 ions/cm2 (b) 5 × 1017 ions/cm2 (c) 7 × 1017 ions/cm2 (d) 1 × 1018 ions/cm2.
structures are formed at fluence 5 × 1017 ions/cm2. The amplitude of surface patterns (rms roughness) increases with ion fluence. Secondary electrons are generated by the interaction of projectile ions with the target atoms along its path length l . Assume R e (v0 ) is the mean path length of the projectile ion at which it is slowed down to the threshold velocity v0 and loses the power to ionize further target atoms. Thus, R e cos is the depth up to which target atoms could be ionized, where is the ion incidence angle with respect to the surface normal. Now, the electron yield can be written as [14,16,17]:
=
R e (v 0 ) 0
e (v )
e
lcos
dl
=
= 1.16a0 hJ
1
(Z1 + Z2 ) (Z11/2 + Z21/2 )
,
< >
c
(3)
c
In case of 10 keV, N ion implantation on Si (Z1 = 7; Z2 = 14), we obtain the binary collision ionization cross section e (v )= 1.17 × 10−16 cm2 using Eq. (2). Considering the Si number density, =4.9 × 1022 cm−3 and assuming electron emission probability from the surface, = 1, Eq. (3) for < c , can be written as
=
K cos
(4) 6
−1
where K = R = e = 5.73 × 10 cm for the present experimental e condition. Now, the electron yield depends only on local ion impact angle and electron mean free path. Until the surface patterns are formed, the local ion incidence remains at 60°, therefore, electron yield will vary only for the variation of electron mean free path ( ). The variation of electron mean free path strongly depends on the physical and chemical nature of the target atoms. Following the Eq. (4), Fig. 4 shows the change of electron path length ( ) with ion fluence up to 5 × 1017 ions/cm2 due to surface chemical change and ion-induced lattice damage. The observed high (=7.8) (Fig. 2 point A) is due to the presence of native silicon oxide layer on the silicon. Calculated electron mean free path (λ) for silicon oxide layer is found to be 6.68 nm. The measured secondary electron yield ( ) after the removal of the top native oxide layer and subsequent amorphization [13] by 10 keV N+ bombardment is 4.2 (Fig. 2, point B) which corresponds to the secondary electron path length (λ) = 3.6 nm for clean amorphous Si surface. Electron yield after the nitridation (Fig. 2, Point C) is found to be 5, which related to λ = 4.2 nm. The limit
2
v0)10 7]
Re cos
+
(1)
v arctan[0.6(v
×
limit
where ρ is the atomic density of the target, e (v ) is the binary collision ionization cross section, determine by the total energy deposition in the electron shells of both the target and the projectile atoms. It can be expressed as [16,18]. e (v )
limit
(2)
where a0 is the Bohr radius, h is the Plank constant, J is the ionization energy, Z1 and Z2 are the atomic numbers of the projectile and target, is the probability of respectively and v is the projectile velocity. electron emission from the surface, and λ is the secondary electron mean path length, which states that the electrons produced at a depth less than that of λ are only emitted from the surface and detected as secondary electrons. Therefore, Eq. (1) also gives an idea about a particular angle of ion incidence ( c ) , determined by the equation R e cos c where electron emission yield ( ) virtually reaches its limiting value limit = e R e [17].Thus 58
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cm2 [20], significant damage starts at ∼2 × 1017 ions/cm2 fluence when the effect of crystal damage supersedes the nitridation effect by reducing the path length of electrons produced in the collision. The observed decrease of secondary electron yield at fluence greater than 2 × 1017 ions/cm2 (Fig. 2) is therefore due to the reduction of electron path length as well as alteration of the mean path length (R e ) of the incident projectile ions. Change of electron emission under high fluence ion bombardment on semiconductor are reported earlier [21,22]. We obtain electron path length = 3.41 nm for radiation damaged Si by putting = 3.9 (Fig. 2, point D) and = 60° in Eq. (4). We assume that the beam induced crystal damage is saturated at this ion fluence, therefore no further change of is considered for more ion bombardment. However, change of secondary electron yield with ion fluence beyond 2 × 1017 ions/cm2 is observed (Fig. 2). Further changes of electron yield at higher fluence are due to the change of local ion incidence angle. With the development of surface pattern the incidence angle of the projectile changes locally. The ion incidence angle depends on the slope at the ion impact point. We have calculated the slope variation and thereby change of local ion incidence angle due to the development of such surface topography. On an initially flat surface, the ion incident angle is 60° for all the points on the surface. However, the angle of incidence changes locally if the flatness of the surface is lost. The schematic of the variation of the local ion incidence angle is shown in Fig. 5a. The slope at impact point O is dh/ dx. Now the surface normal makes an angle θe with the global surface normal. Therefore,
Fig. 4. (a) Variation of electron mean free path vs ion fluence due to chemical alteration and radiation damage.
observed higher value of λ for silicon oxide and nitride compared to silicon is consistent with earlier observation. Tanuma et al. [19] reported the calculated values of λ for SiO2 and Si3N4 are 5.05 nm and 4.13 nm respectively for 2000 eV electron. Moreover, the observed reincrease of λ after cleaning of the oxide layer confirms the formation of silicon nitride. So far, the variation of electron path length is estimated following the chemical change of Si surface with ion fluence, however, radiationinduced damage of the target atoms strongly influences the value of . Although the surface amorphization starts at ion fluence ∼1015 ions/
tan
e
=
dh dx
i.e e = tan 1 [dh /dx ] If the ion incidence angle is 60 o with respect to the global normal of the surface, the ion impact angle ( ) at point O with respect to the local surface normal would be
Fig. 5. (a) Schematic diagram of the local ion incidence angle on a curved surface. (b) distribution of the local ion incidence angle after the N+1 ion irradiation at various ion fluences. (c) Effective surface area Vs. N+1 ion fluence due to surface topography development. 59
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Fig. 6. Fluence dependent variation of measured secondary electron yield (solid line). Secondary electron yield calculated from AFM topographic data considering the change of local ion incidence angle distribution (Fig. 5b) and the effective increase of surface area (Fig. 5c).
= 60 o
tan 1 [dh /dx ] ×
180o
The ion local impact angle at every point of the surface could be estimated by taking the derivative of the surface topography. We have taken the dh/ dx where x is along the projection of the ion beam on the surface. We have estimated the change of local ion incidence angle in the X direction (along the projection of ion beam) from the AFM images of Si surface bombarded for different ion fluences. The distribution of local ion incidence angle as a function of lateral length along the projection of ion beam is shown in Fig. 5b for ion fluence 2 × 1017–1 × 1018 ions/ cm2. Local ion incidence angle changes drastically with ion fluence. For unbombarded flat surface or bombarded surface with no ripple patterns the ion incidence angle is 60°, therefore, the distribution of local ion incidence have a narrow peak at 60°. As the surface ripple structures are started to develop at the ion fluence 2 × 1017 ions/cm2 the angle distribution is broadened, but it remains near the 60° similar to that of a flat surface. It is observed from Fig. 5b that at the fluence of 5 × 1017 ions/cm2 when surface ripple structures are fully developed, a new broad distribution peak appears near 80°. With further increase of the ion fluence, the distribution increases to near 80° and decreases at 60°. This indicates that the development of surface topography with increasing ion fluence increases the local ion incidence angle towards the higher value and changes the secondary electron yield. A correlation between secondary electron emission and ion beam induced topography development is drawn here. We assume that the increase of secondary electron yield in zone III of Fig. 2 is mainly due to the ripple pattern formation. SE emission is increased on a patterned surface because of the increase of local ion incidence angle and the effective surface area. Using Eq. (4) we have calculated the secondary electron yield considering the constant electron path length( = 3.41nm) , and the variation of local ion incidence angle from the AFM data (Figs. 3 and 5b). The distribution of local ion incidence angle is shown in Fig. 5b for ion fluence 1 × 1017 ions/cm2 to 1 × 1018 ions/cm2. From the inequality equation R e cos c , we have calculated the critical angle c = 83.72o where the electron yields reaches a saturation value. Considering the impact of local incidence angle from 20° to 83.72°, electron yield ( ) is calculated. Further, the increase of surface area is measured from the AFM images and plotted as a function of ion fluence (Fig. 5c). The calculated SE considering the local incidence angle and surface area increase is plotted (dot) in Fig. 6 where the measured SE during the ion bombardment (line) is also
Fig. 7. Secondary electron yield vs ion fluence during 10 keV (a) N+1 at normal incidence and (b) Ar+1 at oblique (θ = 60°) incidence ion irradiation on Silicon (1 0 0) surface.
shown as a function of ion fluence. The estimation of electron yield from rippled surfaces is consistent with the measured SE. To distinguish the effect of oxide layer, implantation induced chemical change & damage creation and ripple formation on secondary electron emission, we performed two more experiments, one with same energy N+ beam at normal incidence and another with the inert Ar+ beam at an angle 60° with respect to the surface normal. For both the cases surface ripple is not developed on the surface as it is observed for 10 keV N+ bombardments at 60°. In the case of Ar+ ion bombardment, neither chemical change or ripple patterns takes place. Fig. 7(a) shows the variation of secondary electron emission with fluence when 10 keV N+ projectiles hit the Si target at normal incidence angle. It is similar to Fig. 2 but for zone I and II only. Initially, electron yield is very high and decreases with the fluence as native oxide layer present on the sample. At the fluence near about 2 × 1017ions/cm2, electron emission yield is again increased like the region II of Fig. 2 due to the formation of silicon nitride. But, above the fluence of 7 × 1017 ions/cm2 the yield decreases and stabilizes which is dissimilar with the region III of Fig. 2 as the ripple pattern is not formed in this case. It is therefore confirmed that the increase of SE yield in zone III is only observed for the development of surface pattern. This observation also approves our assumption of the saturation of radiation damage and invariance of at higher ion fluence. In the case of 10 keV Ar+1 ion bombardment on Si(1 0 0) surface at oblique incidence, surface patterns are not formed as well no chemical 60
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N+1 and Ar+1 ion bombardment at an oblique angle (60 °) has been studied. The experiment shows that the secondary electron yield varies with the variation of ion fluence. At the very beginning of the irradiation, electron yield is very high and sharply decreases for both N+ and Ar+ bombardment due to the removal of native silicon oxide layer. With continue bombardment, the yield again increases due to the projectile (nitrogen) ion-induced chemical change to silicon. When defect creation supersedes the chemical effect the yield decreases again. The chemical change is not observed for Ar+ ion bombardment where the decrease is found only due to top oxide layer cleaning and ioninduced damage creation. At higher fluence, if surface patterns are developed, the variation of electron yield is observed because of the change of local ion incidence angle and increase of effective surface area. Thus we found, ion beam induced electron emission from the elemental surface depends both on chemical and physical condition of the bombarding surface. Ion beam induced surface cleaning, projectile induced chemical change, and nanometer scale topography development result in a variation of the secondary electron emission. The variation of SE yields due to the change of secondary electron mean free path and local ion incidence angle has been correlated by the Parilis-Kishinevsky theory [16]. Without the essence of any additional physical or chemical characterization tools, real-time physicochemical change of the target surface during ion bombardment could be monitored. Acknowledgments We acknowledge all the members of RIBF group, VECC, Kolkata for kind support during the experiment. The authors are especially thankful to Mr. Dipak Bhowmik and Mr. Doddi Lavanya Kumar for their help during the operation of ion beam system and data collection as well as AFM measurements. We are grateful to the Accelerator Technology development mechanical Section of VECC for fabricating the hemispherical electron yield measurement system. References [1] D. Hasselkamp, Kinetic electron emission from solid surfaces under Ion bombardment, in: G. Hohler (Ed.), Particle Induced Electron Emission II, Springer-Verlag, Berlin Heidelberg, New York, 1992, pp. 1–95. [2] M. Rosler, W. Brauer, J. Devooght, J.C. Dehaes, A. Dubus, M. Cailler, J.P. Ganachaud, Particle Induced Electron Emission I, Springer – Verlag, Berlin Heidelberg GmbH, 1991. [3] J. Goldstein, H. Yakowitz, Practical scanning electron microscopy: electron and ion microprobe analysis, in: H.Y. Joseph Goldstein (Ed.), Practical Scanning Electron Microscopy: Electron and Ion Microprobe Analysis, Plenum Press, the University of Michigan, 1975, p. 582. [4] D.A. Bromley, Detectors in nuclear science, Nucl. Instrum. Meth. 162 (1979). [5] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley & Sons, Hoboken, NJ, 2005. [6] E. Chason, M.J. Aziz, Spontaneous formation of patterns on sputtered surfaces, Scr. Mater. 49 (2003) 953. [7] C.S. Madi, E. Anzenberg, K.F. Ludwig, M.J. Aziz1, Mass Redistribution Causes the Structural Richness of Ion-Irradiated Surfaces, Phys. Rev. Lett. 106 (2011) 066101. [8] R.J. Young, M.V. Moore, Dual-beam (FIB-SEM) systems, in: L.A. Giannuzzi, F.A. Stevie (Eds.), Introduction to Focused Ion Beams Instrumentation, Theory, Techniques and Practice, Springer, Boston, 2005, pp. 247–268. [9] I. Horcas, R. Fernandez, J.M. Comez-Rodriguez, WSXM: a software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705. [10] D. Nečas, P. Klapetek, Gwyddion: an open-source software for SPM data analysis, Cent. Eur. J. Phys. 10 (2012) 181. [11] J. Maul, K. Wittmaack, Secondary ion emission from silicon and silicon oxide, Surf. Sci. 47 (1975) 358–369. [12] A. Marcak, C. Corbella, T.D.L. Arcos, A.V. Keudell, Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor, Rev. Sci. Instrum. 86 (2015) 106102. [13] P. Karmakar, B. Satpati, The influence of projectile ion induced chemistry on surface pattern formation, J. Appl. Phys. 120 (2016). [14] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Ion-induced electron emission – monitoring the structure transitions in graphite, Nucl. Instrum. Meth. Phys. Res. Sect. B 230 (2005) 443–448. [15] B. Sevensson, G. Holmen, Electron emission from ion-bombarded aluminum, J. Appl. Phys. 52 (1981).
Fig. 8. AFM images of silicon surface after 10 keV Ar+ ion bombardment at oblique incidence (θ = 60°) for fluence (a) 1 × 1017 ions/cm2 (b) 2 × 1017 ions/cm2 (c) 5 × 1017 ions/cm2 (d) 7 × 1017 ions/cm2. (e) Distribution of local ion incidence angle on silicon surface before and after Ar+1 ion irradiation at various ion fluence.
alteration is expected due to chemical inertness of Ar. Fig. 7(b) shows the variation of electron yield with ion fluence. It shows a similar behavior as observed in Fig. 2 for zone I only. The cleaning of the native oxide layer by Ar+ is similar as N+ at the beginning of ion bombardment, but at higher fluence, no change of yield is observed because of the inertness of Ar with silicon a well it forms no surface patterns. Fig. 8(a–d) shows the ex-situ AFM images after the irradiation with Ar+ projectile at difference fluences. It shows no prominent pattern on the Si surface. Fig. 8e shows the distribution of the local ion incidence angles after Ar+ ion irradiation at various ion fluences. As no pattern is formed, it remains the same as the virgin Si surface. Therefore, the presence of native oxide layer, ion induced chemical alteration and lattice damage, as well as surface pattern formation affect the yield of the emitted secondary electron. Thus, detection of secondary electrons during the ion bombardment can be used as an in situ probe for monitoring the real-time physicochemical change of Si surface. 4. Conclusion The ion-induced electron emission from the Si (1 0 0) during 10 keV 61
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S. Ghosh, P. Karmakar [16] E.S. Parilis, L.M. Kishinevsky, N.Y. Turaev, B.E. Baklitzky, F.F. Umarov, V.K. Verleger, S.L. Nizhnaya, I.S. Bitensky, Atomic Collisions on Solid Surfaces, Elsevier, Amsterdam, 1993. [17] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Angular and temperature dependences of ion induced electron emission of polycrystalline graphite, Vacuum 73 (2004) 65–72. [18] N.N. Andrianov, V.S. Avilkina, A.M. Borisov, E.S. Mashkova, E.S. Parilis, The study of graphite disordering using the temperature dependence of ion-induced electron emission, Vacuum 86 (2012) 1630. [19] S. Tanuma, Calculations of electron inelastic mean free paths III. Data for 15
inorganic compounds over the 50-2000 eV range, Surf. Interface Anal. 17 (1991) 927–939. [20] A.I. Titov, C.E. Christodoulides, G. Carter, M.J. Nobes, The depth distribution of disorder produced by room temperature 40 keV N+ ion irradiation of silicon, Radiat. Eff. 41 (1979) 107–111. [21] I.N. Evdokimov, E.S. Mashkova, V.A. Molchanov, A new method of observing defects annealing in crystals, Phys. Lett. 25A (1967) 619–620. [22] B.A. Brusilovsky, Kinetic ion induced electron emission from the surface of random solids, Appl. Phys. A 50 (1990) 111–129.
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