Performance optimization of microwave-coupled plasma-based ultra-low energy ECR ion source for silicon nano-structuring and application

12 This literature presents a comprehensive optimization of key parameters crucial for generating 13 ion beams in a microwave-coupled plasma -based ultra -low energy Electron Cyclotron 14 Resonance (ECR) ion source, generally used for nano -structuring on solid surfaces. The 15 investigation focuses on developing, accelerating, and extracting Ar + ions from a magnetron 16 (microwave) coupled plasma cup utilizing three-grid ion extraction composed of molybdenum. 17 The study systematically examines the dependence of ion beam current on critical parameters, 18 such as gas pressure, magnetron power, extraction voltage, and ion energies. Additionally, the 19 influence of extraction voltage on beam current is investigated for different ion energies. The 20 variation of beam current as a function of ion energy is explored under constant magnetron 21 current and extraction voltage at various conditions. The Gaussian nature of the beam profile 22 is scrutinized and elucidated within the context of grid extraction -based ion sources. Plasma 23 physics principles are employed to interpret the observed variations in ion current density 24 2 (beam current) with various parameters. The corresponding ion -induced nanopatterning on 25 silicon, using the optimized beam current, is explored in detail. Furthermore, the research 26 delves into the temporal evolution of the surface topography of silicon followed by off-normal 27 incidences (60º and 72.5º) is Ar-ion extracted at 450 eV Ar-ions. The changes in the optical 28 property, resulting from nano-patterned surfaces, investigated using UV-VIS spectroscopy, is 29 correlated the with dimension of nano patterning. This manuscript highlights the potential 30 applications arising from these findings, emphasizing the transformative impact of low energy 31 inert ion induced nano-patterning technologies. 32

Ultra -low energy ECR-based ion source, Optimization of ion current , Surface 33 topography, TEM, UV-VIS spectroscopy 34 *Corresponding author email: [email protected] 35

36 The ion source serves as a fundamental component in numerous scientific and industrial 37 applications, playing a crucial role in generating charged particles (ions) . Following ion 38 production, various systems harness these energetic ions for diverse purposes, spannin g 39

science, high -energy physics, medical applications, and agricultural science [1–5]. 40 Presently, energetic ions find application in various surface treatments such as nano-patterning, 41 sputter etching, controlled defect formation, and more [6,7]. Particularly, ultra-low energy ion 42 beam proves exceptionally valuable for the precise modification of 2D layers [8], ion-induced 43 nano-patterning on semiconductor surfaces[9]. Over the past few decades, ion -induced nano-44 patterning and nanoscale functi onalizations have garnered significant interest, owing to their 45 broad applications in DNA origami [10], tuning wettability [11], electrical and magnetic 46 anisotropy [12,13], isolated dot formation [14], nanoscale plasmonic array[15], field emission 47 [16], etc. Thus, the ion sources generate enormous possibilities for material modifications both 48 3 physically and chemically. Further, diversity exists in energetic ion production mechanisms. 49 The fundamental process of producing ions is the collision of atoms with ions or electrons 50 which may be either elastic or inelastic. In elastic collisions, the internal energy of the colliding 51 particles does not change. Ionization, stripping, electron capture , and excitation of atoms due 52 to collisions are examples of inelastic collisions. The free electrons colliding with atoms also 53 produce ions. Electrons in the gas are heated by the inductively coupled method and then 54 acquire enough energy to generate plasma. Due to several drawbacks in such Townsend 55 discharge, these sources are not used now a days. In recent days, compact broad-beam ion 56 sources are widely used in scientific laboratories to generate ions . Depending upon the 57 mechanism of production of various ions using gaseous plasma, the ion sources can be 58 classified in three ways; direct current (dc) operated ion sources, radio frequency (microwave) 59 ion sources, and microwave ion sources. In the past few decades, DC ion sources were 60 commonly used in the above activities [17–19]. These DC ion sources consist of a hot cathode 61 or filament, which is not especially useful in case s of reactive gas discharge; hence , their 62 lifetime is limited [20,21]. Moreover, the beam current produced by those ion sources is not 63 suitable for modern-day applications. In material science as well as surface science 64 applications, the ion source should be mobile and adaptable to the vacuum system , having a 65 longer lifetime. Further, the ion source should produce a relatively high beam current (i.e., 66 capable of forming a high density of plasma) with lower maintenance. To address this 67 challenge, Electron Cyclotron Resonance (ECR) based ion sources were develope d [22,23]. 68 ECR ion source is one of the most preferred ion sources for the easy production of ions for 69 different energies and charge states. Since the discharge is maintained in the quartz cup via a 70 strong electric field generated in the cavity , the ECR-based ion sources equipped with 71 microwave cavities neither contain any filament nor any type of electrode . The high plasma 72 density within a quartz cup is confined by solenoid magnets surrounding it, creating a multi -73 4 cusp magnetic field. However, careful attention is required for the microwave coupling to the 74 plasma cup to minimize microwave-reflected power. Mechanical adjustments to the resonator 75 length and waveguide are made to ensure minimal reflection of microwave power. 76 Additionally, maintaining the necessary magnetic field strength is crucial for sustaining the 77 plasma. The ion source's compact design is user-friendly and capable of producing a high beam 78 current density for a single or multi -grid extraction system. As a result, the extracted beam 79 current is influenced by magnetron power, plasma pressure, and extraction voltage. 80 Furthermore, the beam current varies with different ion energies. 81 This article focuses on optimizing the beam current generated by a cost -effective 82 microwave-based Electron Cyclotron Resonance (ECR) ion source and subsequent 83 development of nanoscale patterns on the surface of silicon. The relationship between the beam 84 current and various parameters is extensively examined and elucidated. Experimental 85 parameters, spanning from plasma generation to ion beam extraction, are systematically 86 optimized for the study of low -energy Ar -ion-induced nanostructures on silicon. The 87 dependence of the extracted ion beam on plasma pressure, magnetron power, and extraction 88 grid voltage is documented for different ion energies. Additionally, the manuscript establishes 89 the relationship between ion beam current and the ion energy. Irradiation of p-type single 90 crystal Si (100) surface at off-normal angles (60º and 72.5º) by a 450 eV Ar-ion results in the 91 well-defined formation of nanoscale ripple patterns. The prominence of ripple structures 92 increases with prolonged irradiation time, while bombardment at 72.5º with the same ion beam 93 parameters leads to the coarsening of nanostructures . Cross -sectional transmission electron 94 microscopy (TEM) measurements confirm the formation of nanostructure as observed from 95 atomic force microscopy images (AFM). The thickness of the amorphous thin layer is well 96 agreement with Monte Carlo Simulation (SRIM)[24]. The article further investigates and 97 explains the optical response (by UV-VIS spectrophotometer) of the nano-patterned surfaces 98 5 with the dimension s of nano-patterning (i.e., wavelength and rms roughness) . The potential 99 applications of such na no-patterned silicon surfaces are enlightened. This article underscores 100 the versatility of an optimized broad-beam ultra -low energy ion source, specifically in the 101 context of optimization of inert Ar-ion and subsequent ion-induced silicon nano-patterning. 102 103 Description of the ion source: 104 105 Figure 1: Block diagram consisting of the component of high vacuum plasma ion source 106 equipped with UHV chamber. 107 Figure 1 illustrates the block diagram of the magnetron -coupled ultra -low energy 108 Electron Cyclotron Resonance (ECR) ion source to provide a comprehensive visualization of 109 the entire setup. The schematic representation in Figure. 1 elucidates the process of extracting 110 an ultra-low energy ion beam. An microwave source (magnetron) is connected to the ceramic 111 plasma cup via a waveguide. The gas inlet system facilitates the filling of the plasma cup with 112 gas through a capillary tube. The intense electric field generat ed by the microwave source 113 (magnetron) induces gas breakdown (discharge), leading to the formation of a highly intense 114 plasma. The produced plasma is confined and sustained by a permanent magnet positioned near 115 the ceramic plasma cup (made of Al2O3). For the extraction and focusing of the beam, a gridded 116 electrostatic lens, commonly referred to as an Einzel lens, is employed. The shape and size of 117 the beam are contingent on the extraction voltage applied at the grid and the corresponding ion 118 6 energy. The dir ected beam impacts the silicon target kept in an ultra -high vacuum (UHV) 119 within the target chamber. A faraday cup, connected to a multimeter, measures the beam 120 current, and the corresponding ion fluence is expressed in terms of irradiation time. 121 The sample holder, located in a UHV chamber, is connected to a 5 -axes (x, y, z, θ, φ) 122 manipulator system, offering movement and rotation in all possible directions. The sample is 123 transferred to the ion source using a load -lock system. A later discussion provides a c ross-124 sectional view of the system. 125 126 Figure 2: Cross-sectional (schematic) view of microwave coupled ultra-low energy ion beam 127 system. 128 The cross-sectional view of the high vacuum microwave based ECR ion source mentioned 129 above is shown in Figure 2. The entire system consists of a magnetron-coupled ion source, a 130 UHV target chamber with a cylindrical cavity and load lock, a 5-axes manipulator (PREVAC 131 Technologies), and a gas inlet system. This type of magnetron-coupled ion source was first 132 developed by Anton et.al [23]. The ion source is fitted in the cylindrical cavity of the UHV 133 7 target chamber. The inner diameter of the plasma cup is around 52 mm, where the plasma is 134 generated. The cup is surrounded by water-cooled magnets made up of neodymium-iron-boron, 135 which produces a multi-cusp field to confine the plasma. The 2.45 GHz magnetron-based 136 microwave source is attached to the backside of the ion source , as shown in Figure 2. The 137 dimension of the cylindrical resonator (waveguide) is chosen in such a way that it can produce 138 maximum beam current. To generate plasma in the ceramic cup (known as plasma cup), a gas 139 is inserted into it through a capillary tube attached to the gas chamber. A pressure of 10-4 mbar 140 is maintained for sustaining plasma by adjusting a needle valve attached to the gas chamber. 141 The entire length of the ion source is around 130 cm. The extraction of ion beams is 142 accomplished by a three-grid ion optics system, as seen in Figure 2. Wide-ranging extraction 143 voltage is applied to the grid to enable the extraction of an intense beam with different 144 diameters. The circular perfection of the beam shape is evident from observations on the front 145 plate attached to the ultra-high vacuum (UHV) c hamber. In this configuration, the beam 146 current, specific to a given ion energy, can be finely adjusted based on magnetron power, 147 working pressure, and extraction voltage. Consequently, optimizing the dependence of beam 148 current on these parameters is a wor thwhile pursuit, driven by the need to comprehend the 149 underlying scientific principles. Additionally, the ion current (beam current) is influenced by 150 the extracted ion energy and the position of the target. Hence, a comprehensive investigation 151 into the intricate relationship between ion current and the mentioned parameters emerges as a 152 compelling topic in the current scientific context. 153 154 155 156 157 8 Optimization of ion current on various plasma parameters: 158 159 Figure 3: Variation of beam current with (a)–(c) plasma pressure, (d)–(f) microwave 160 (magnetron) power for different ion energy. 161 The variation of beam current with plasma pressure and magnetron ( microwave) power for 162 different ion energies are investigated and presented in Figure 3. Figure 3(a)-(c) demonstrates 163 that the beam current decays almost exponentially with the increase in plasma pressure. The 164 ion current is maximum at plasma pressure 1.5 × 10 -4 mbar irrespective of ion energies. This 165 indicates that the minimum base pressure required for generating plasma is 1.5 × 10-4 mbar. It 166 is also evident that for the same plasma pressure, the beam current is maximum for the highest 167 ion energy. The gas pressure inside the plasma cup is directly proportional to the number of 168 gas molecules present. At low plasma pressure, the mean free path of gas molecule s is larger 169 due to the lesser density of gas molecules, which allows the produced ions to traverse a longer 170 distance without collision. This increases the ionization efficiency , and hence, with fewer no 171 of collisions, the probability for recombination of the ions is very low. Consequently, a large 172 number of ions are extracted, intensifying the beam current. The entire phenomenon can be 173 9 summarised through the equation, λ = (σ.n)-1, where λ is the mean free path of the ions, σ is the 174 cross section for recombination, and n is the density of the ions inside the plasma [25–27]. The 175 mean free path of the ions, determined by the recombination cross section and density of 176 plasma, plays a key role in quantifying the ion current. 177 Further, the conversion of the gas to plasma is governed by a magnetron (microwave) 178 source and therefore, the ion current or plasma density must depend on magnetron power. To 179 understand that, the variation of ion current with microwave power is recorded at different 180 plasma pressures and ion energies, as presented in Figure 3 (d)- (f). In general, the plasma 181 density (n) depends on the microwave frequency (ω) as n = 𝐸𝑅𝐹𝜔2 𝜀 where ERF is the microwave 182 power (energy) and ε is the minimum energy required for ion-electron pair generation [25]. The 183 magnitude of ε is different for different gases. A non -zero magnitude of ε signifies the 184 minimum energy needed to generate plasma, commonly referred to as ionization energy, which 185 is supplied by magnetron power . It is evident from the above Figure (3(d)-(e)) that up to the 186 microwave power equal to a critical value, the formation of plasma is forbidden, resulting in a 187 zero-beam current. With the increase of magnetron power beyond the magnitude ε, the beam 188 current increases almost linearly with the input magnetron power, since the plasma density (n) 189 is directly proportional to the microwave power (ERF). The beam current reaches saturation at 190 a specific microwave power level, which varies based on the ion energy. At sufficiently high 191 microwave power, the plasma density (beam current) is high, and the rate of generation and 192 recombination of the ion-electron pair is equal, resulting in a saturation of beam current a s 193 observed in Figure. 3(d)-(f). Further, the cut off power also depends on ion energy. At 194 sufficiently low ion energy, the microwave power required for generating plasma is high. With 195 higher ion energies microwave power required for ion -electron pair generation is also less 196 which is obvious from the above discussion. 197 10 198 Figure 4: Variation of (a)-(c) beam current with ion extraction voltage at different ion 199 energy;(d) beam current with ion energy; (e) beam current with the target position. 200 The extraction of developed Ar-ion beam was governed by a combination of three-grid 201 (concave) ion optics system [28–30]. The beam current and the beam profile depend on the 202 potential applied at the grid and the target position. The change in beam current with the ion 203 extraction voltage recorded for different ion energies is presented in Figure. 4 (a)-(c). Initially, 204 the beam current increases linearly with the applied extraction voltage since more ions are 205 extracted with a higher extraction voltage. Irrespective of ion energy, t he beam current is a 206 maximum extraction voltage of 200 eV. With further increases in extraction voltage, the beam 207 current decreases rapidly. This is governed by two major phenomena, firstly, the space charge 208 effect, i.e., a high extraction voltage , creates an electrostatic field that repels the subsequent 209 ions, causing the beam to spread out ; therefore, the current density reduces. Secondly, during 210 the extraction of the ions, the application of a high extraction voltage leads to the collision of 211 the ions with residual gas molecules, which also causes a significant decrease in beam current 212 11 due to ion-electron recombination. Generally, the extraction voltage is kept fixed to maintain 213 the shape of the beam, essential for uniform irradiation of samples. 214 The dependence of beam current upon ion energy is also investigated in Figure 4(d). It is clear 215 from Figure 4(d) that the beam current increases almost linearly with the increase in ion energy 216 at a fixed extraction voltage and microwave power. When the applied extraction voltage is 200 217 eV and the ion energy is less than 200 eV, then the number of ions that can overcome the barrier 218 of 200 eV is less, causing a low beam current. With the increase in ion energy, the ions achieve 219 sufficient energy to overcome the barrier of 200 eV; hence, the beam current increases. On the 220 other hand, for a particular ion energy, lowering the extraction voltage also results to a lowering 221 of beam current as observed from the above Figure 4 (a)-(c). Therefore, to maintain a proper 222 beam shape and adequate beam current , the extraction voltage and ion energy are to be 223 precisely optimized. Further, the variation of beam current with the target position, known as 224 the beam profile, is also presented in Figure 4(e). The beam profile is Gaussian in nature for 225 concave grid (molybdenum) beam extraction optics. Such a beam profile precisely ensures the 226 target position is for maximum beam current. 227 Nano-structuring on Si surface by 450 eV Ar-ion bombardment: 228 Subsequently, after a detailed optimization of the ultra-low energy Ar-ion beam, the surface 229 topography of Si (investigated by AFM) after the off-normal bombardment of 450 eV Ar- 230 ion at different incidence angles and time is investigated. Figure 5 represents the surface 231 morphology of Si surface after 450 eV Ar-ion bombardment at different incidence angles. 232 The arrow on the right-hand side indicates the direction of the ion beam concerning the 233 surface normal. The irradiation of silicon surface with 450 eV Ar-ion at an angle of 55º 234 leads to no development of surface morphology, and presented in Figure 5(a). However, at 235 an ion incidence angle of 58º, the evolution of surface morphology starts, although no 236 prominent ripple structure is observed (Figure 5(b)). On the other hand, the bombardment 237 12 of 450 eV Ar-ion on the Si surface for an hour at an angle of 60º concerning surface normal 238 leads to the formation of a well-defined nanoscale ripple pattern as observed in Figure 5(c). 239 The growth of the ripple becomes more prominent with the increase in bombardment time. 240 The amplitude of the ripple grows larger with longer bombardments of Ar-ions. 241 242 Figure 5: AFM image (2D and 3D) of the evolution of surface morphology by 450 eV Ar-ion 243 bombarded at different incidence angles and time. The arrow indicates the ion beam direction. 244 To visualize the growth of the ripple, the 3D AFM images are presented along with the 245 2D images. It is clear from the height scale associated with the images, that the ripple height 246 increases with bombardment time. The first Fourier transform (FFT) image of nano-patterned 247 surface is inset at the right lowest corner of each image. In the present case, the fluence is 248 replaced by irradiation time. The quality and the growth of the nano -structures are 249 quantitatively discussed in Figure 6, where a detailed variation of ripple wavelength, rms 250 roughness, and the power spectral density is discussed. 251 13 Figure 5(g) represents the cross-sectional transmission electron microscopy (TEM) image of 252 450 eV Ar-ion bombarded Si surface at an angle of 60º with a time of 3 hours. The presence of 253 Ar-ion-induced surface corrugation in terms of ripple-like nanostructure is evidenced in Figure. 254 5(g). Although the amplitude of the ripples is not sufficiently large, the observed ripple 255 wavelength of around 31 nm from the TEM image, is consistent with that of AFM data 256 (presented in Figure 6(e)). However, in addition to the ripple-like nanostructure, an ultra-thin 257 amorphous layer formation occurs due to Ar -ion bombardment. The thickness of the 258 amorphous layer is around 1.5 nm, which is consistent with the penetr ation depth of the Ar-259 ions (1. 2 nm), estimated by Monte Carlo Simulation (Fig 5(f)) [24]. Therefore, the 260 topographical image is consistent with the cross-sectional image, indicating a clear signature 261 of ripple-like nano-structure formation. 262 263 14 Figure 6: Variation of (a) – (c) surface height modulation of nano -patterned silicon surface; 264 (d) ripple wavelength and (e) rms roughness with irradiation time. Variation of power spectral 265 density of nanopatterned silicon surface on (g) parallel and (h) perpendicular direction. 266 Figure 6 (a)-(c) indicate the variation of the surface profile of the nano-patterned AFM 267 images shown in Figure 5. The height profile is the direct evidence of variation of ripple 268 amplitude with irradiation time. The increase in ripple height with irradiation time is shown in 269 Figure 6 (a)-(c). Further, the fluctuation in ripple height or amplitude generally termed rms 270 roughness, is also investigated in Figure 6(d). It is clear from Figure 6(d), that rms roughness 271 increases linearly with the irradiation time (fluence). Further, the ordering of the nanostructure 272 with the bombardment time , examined in terms of ripple wavelength, is presented in Figure 273 6(e). From Figure 6(e) initially, t he ripple wavelength increases as the bombardment time 274 increases from one hour to two hours. With a further increment in irradiation time, the change 275 of ripple wavelength is negligible, i.e., a saturation of ripple wavelength is observed. The 276 degree of similarity between two spatial morphologies is generally quantified by auto -277 correlation length or self-correlation length, as presented in Figure 6(f). It is clear from Figure 278 6(f), that the auto-correlation length decreases with bombardment time. This indicates a more 279 ordered ripple st ructure is found to develop with higher irradiation time. To understand the 280 growth of the ripple structure, the power spectral density factor along the parallel and 281 perpendicular direction of the developed ripple is presented in Figure 6 (g)-(h). The prominent 282 peak present in Figure 6(g) indicates, the development of ripple structure along the x direction 283 (parallel) with a particular wavevector ( kx). Besides, the absence of a ripple wavevector in 284 perpendicular mode is evidenced in Figure 6(h). Therefore, 450 eV Ar-ion bombardment on 285 Si, leads to well-defined parallel mode ripple formation at off-normal incidence. 286 15 287 Figures 7: AFM images (2D and 3D) of the evolution of nano -structure on Si surface with 288 time under 450 eV Ar-ion bombardment at an angle of 72.5º. 289 Figure 7 illustrates the surface topography after 450 eV Ar-ion bombarded the silicon surface 290 at an angle of 72.5º with different bombardment time. Corresponding 3D AFM images are also 291 presented along with 2D surface topography. Generally, the transformation of well -defined 292 nano ripples into nano-facets occurs at this near-grazing angular region[31]. In the present case, 293 although no prominent nano-facet formation is observed, a clear signature of transformation of 294 ripple structure towards nano-facets is seen. At sufficiently large bombardment time (10 hours), 295 nano-facets like structures with larger dimensions are developed, although the facets are not 296 well organized. It is also evident that the rms roughness is also increasing with bombardment 297 time. 298 Surface nano-structuring by energetic ion bombardment is a consequence of ion-beam-299 induced off-normal (60º and 72.5º ) sputtering of surface atoms and their consecutive 300 redistribution [9,32,33]. During ion bombardment, the unequal radius of curvature of the surface 301 16 ensures unequal deposition of energy at different points on the surface, which results in unequal 302 sputtering at those points. This generates surface instability, and consequently , the surface 303 atoms are redistributed to stabilize the surface. These two effects jointly trigger nano-pattern 304 formation on the surface. A first theoretical model was proposed by Bradley and Harper [34], 305 based on curvature-dependent sputtering of surface and near-surface atoms. Later, Carter and 306 Vishnyakov introduced the concept of redistribution of surface atoms [35]. In present days, 307 several experiments have been carried out to understand other factors that contribute to nano-308 pattern formation, such as preferential and differential sputtering, the role of surface and beam 309 impurities, the effect of chemical compound formation, and compound ion irradiation. In cases 310 of ultra-low energy ion bombardment, the rate of sputtering is lower compared to medium-311 energy ion bombardment; therefore, in this case, mass redistribution of the surface atoms plays 312 a key role. Being inert, the reaction between Ar -ion and Si atoms is forbidden, ensuring the 313 absence of the chemical aspect of pattern formation. However, the native silicon oxide layer 314 partially sputters out with the bombardment of Ar -ion. In the present case , the sputtering of 315 elemental Si atoms takes place along with the Si atoms in oxide form. This is also a key factor 316 in generating surface instability. The surface morphology largely varies due to different 317 amounts of near-surface mass transport by the surface-confined ion -enhanced viscous 318 flow[36]. Here in the present case, up to an ion incidence angle of 58º, the surface remains 319 unstable under 450 eV Ar-ion bombardment. Due to such sputtering , a well -defined ripple 320 formation is found after an hour of 450 eV Ar-ion bombardment . With the increase in 321 bombardment time, Si atoms in both elemental and oxide form sputter more, and a well-ordered 322 ripple is obtained. However, due to the presence of the ripple, the surface becomes anisotropic. 323 The consequence of such an anisotropic nature of the surface is investigated and discussed in 324 the upcoming section. 325 326 17 Application of nano-patterned Si surface: 327 328 Figure 8: (a) UV-VIS-IR spectra of pristine and nano-patterned surfaces, (b) variation of rms 329 roughness and reflectivity with ripple wavelength. 330 The optical response of pristine and Ar-ion-induced nano-patterned silicon surfaces are 331 investigated through UV-VIS reflectivity & presented in Figure 8. Figure 8(a) depicts the 332 change in reflectivity of the silicon surface due to the presence of nano -pattern. With the 333 increase of bombardment time , reflectivity decreases drastically. Further the change in 334 reflectivity in UV region with respect to rms roughness and ri pple wavelength is also 335 investigated in Fig 8(b). It is clear from the Fig 8(b), that the reflectivity decreases with the 336 increase in ripple wavelength. Further, variation of the rms roughness of the Ar -ion irradiated 337 nano-patterned Si surfaces is nominal. In general, the presence of nano-patterns on the surface 338 enhances the reflection of UV-VIS light, due to light trapping by multiple reflections [37–39]. 339 The presence of a well-defined ripple pattern, formed due to Ar-ion bombardment (for three 340 hours), leads to the development of a well-defined nanopattern on the silicon surface and hence 341 the reflectivity is minimal compared to the other two surfaces. The change in reflectivity 342 depends on the change in the electronic structure as well as surface topography of the material. 343 Such a change in electronic structure can contain several factors, like the change in chemical 344 18 nature or impurity incorporation on the surface, and amorphization of the surface. Ar-being 345 inert causes no chemic al modification of the silicon surface, along with the absence of the 346 trapped Ar-ion on silicon surface (concluded from TEM image in Fig 5(g)), particularly in this 347 lower energy regime. Therefore, i n the present case, the amorphization due to ion beam 348 sputtering, the amorphization of the silicon surface, and the nanostructure formation on the 349 surface, together change the electronic density of the material, causing a lowering in 350 reflectivity. The tailoring of the reflectivity by developing nano-structure is widely applicable 351 for anti-reflective (ARC) coating and photovoltaic device applications [40,41]. 352 The formation of nano -structure on the silicon surface by inert ion bombardment is a 353 consequence of ion-induced instability on the surface by the interplay between sputtering and 354 mass redistribution of surface atoms [42,43]. During ion bombardment, the sputtering of the 355 native silicon oxide layer along with the bulk silicon takes place. The rate of sputtering of 356 silicon oxide and the eleme ntal silicon is different which initiates the instability at the initial 357 time of bombardment. Further, the instability is enhanced by differences in the sputtering yield 358 of silicon in native oxide and elemental form. These two effects are jointly responsib le for 359 developing the nano-pattern on the surface. With the increasing bombardment time, the rate of 360 sputtering of Si in elemental and compound form increases, and a well -defined periodic 361 structure is observed. Further, during sputter erosion, the native oxide layer is mostly removed, 362 forming a silicon ripple structure . Further, the exposure of the nano -patterned silicon surface 363 to air during optical measurement ensures the formation of non -uniform silicon oxide on the 364 nano-patterned silicon surface. Due to such preferential spatial formation of silicon oxide, the 365 change in reflectivity is triggered. Therefore, nano -patterned silicon surfaces can be an 366 alternative for memory devices. 367 368 369 19

370 In this comprehensive study, the intricacies of an ultra-low energy magnetron-based Electron 371 Cyclotron Resonance (ECR) ion source are studied systematically by exploring optimal 372 parameters to achieve stable and intense beam currents. The cost -effectiveness and versatility 373 of this ion source make it particularly noteworthy, offering a practical solution for generating 374 reasonable beam currents. Notably, the ion source opera tes within an ultra -high vacuum 375 environment, rendering it valuable for both implantation and deposition processes. Our 376 meticulous investigation into the ECR-based ultra-low energy ion source lays the groundwork 377 for ion beam-induced nano-structuring and layer-wise material modification, affording precise 378 control over ion penetration depth and fluence. The manuscript emphasizes an intriguing 379 alternative perspective by highlighting the in-depth optimization of low energy ion source and 380 inert ion -induced nano -patterning as a viable approach for ARC coating . Additionally, the 381 manuscript underscores the potential of nano -patterned silicon surfaces as an alternative 382

for tailoring reflectivity, particularly for Anti-Reflective Coating (ARC) applications. 383 This study not only advances our understanding of ECR -based ion sources but also opens 384 avenues for innovative applications in nanotechnology and materials science. 385

386 The authors thank the Department of Atomic Energy, Govt. of India for financial support. 387

388 (1) Bhowmick, S.; Mukherjee, J.; Satpati, B.; Karmakar, P. Appl Surf Sci 2022, 578. 389 doi:10.1016/j.apsusc.2021.152079 390 (2) Barofsky, D. F. Mass Spectrometric Analyses in Agriculture and Natural Product 391 Research; 1999; Vol. 29 392 20 (3) Gambino, N.; Myalsky, S.; Adler, L.; De Franco, A.; Ecker, F.; Guidoboni, G.; Kurfürst, 393 C.; Penescu, L.; Pivi, M.; Schmitzer, C.; Strasik, I.; Wastl, A. Impact of Ion Source 394 Stability for a Medical Accelerator. In Journal of Instrumentation; Institute of Physics 395 Publishing, 2019; Vol. 14. doi:10.1088/1748-0221/14/05/C05017 396 (4) Gammino, S. Ion Sources for Medical Applications. In CERN Yellow Reports: School 397 Proceedings; CERN, 2017; Vol. 1, pp 59–70. doi:10.23730/CYRSP-2017-001.59 398 (5) Satoh, K.; Oono, Y. Studies on Application of Ion Beam Breeding to Industrial 399 Microorganisms at Tiara. Quantum Beam Science . MDPI June 1, 2019. 400 doi:10.3390/qubs3020011 401 (6) Mukherjee, J.; Bhowmik, D.; Mukherjee, M.; Satpati, B.; Karmakar, P. J Appl Phys 402 2020, 127. doi:10.1063/1.5144960 403 (7) Mukherjee, J.; Bhowmik, D.; Bhattacharyya, G.; Satpati, B.; Karmakar, P. Journal of 404 Physics Condensed Matter 2022, 34. doi:10.1088/1361-648X/ac4937 405 (8) De Lima, L. H.; Cun, H. Y.; Hemmi, A.; Kälin, T.; Greber , T. Note: An Ion Source for 406 Alkali Metal Implantation beneath Graphene and Hexagonal Boron Nitride Monolayers 407 on Transition Metals. In Review of Scientific Instruments ; 2013; Vol. 84. 408 doi:10.1063/1.4848936 409 (9) Norris, S. A.; Aziz, M. J. Ion -Induced Nanopatterning of Silicon: Toward a Predictive 410 Model. Applied Physics Reviews . American Institute of Physics Inc. March 1, 2019. 411 doi:10.1063/1.5043438 412 (10) Teshome, B.; Facsko, S.; Keller, A. Nanoscale 2014, 6, 1790 –1796. 413 doi:10.1039/c3nr04627c 414 21 (11) Bhowmik, D.; Karmakar, P. Surf Coat Technol 2020, 385. 415 doi:10.1016/j.surfcoat.2020.125369 416 (12) Mollick, S. A.; Singh, R.; Kumar, M.; Bhattacharyya, S.; Som, T. Nanotechnology 2018, 417 29. doi:10.1088/1361-6528/aaaa74 418 (13) Parida, B. K.; Kundu, A.; Hazra, K. S.; Sarkar, S. Appl Phys A Mater Sci Process 2021, 419 127. doi:10.1007/s00339-021-05117-0 420 (14) Bhowmick, S.; Mukherjee, J.; Satpati, B.; Karmakar, P. Appl Surf Sci 2022, 578. 421 doi:10.1016/j.apsusc.2021.152079 422 (15) Kasani, S.; Curtin, K.; Wu, N. A Review of 2D and 3D Plasmonic Nanostructure Array 423 Patterns: Fabrication, Light Management and Sensing Applications. Nanophotonics. De 424 Gruyter 2019. doi:10.1515/nanoph-2019-0158 425 (16) Saini, M.; Singh, R.; Sooraj, K. P.; Basu, T.; Roy, A.; Satpati , B.; Srivastava, S. K.; 426 Ranjan, M.; Som, T. J Mater Chem C Mater 2020, 8, 16880 –16895. 427 doi:10.1039/d0tc03862h 428 (17) Abdel Rahman, M. M.; Abdel Salam, F. W.; Soliman, B. A. Improved Treatment of 429 Home-Made Glow Discharge Ion Source. In Journal of Physics: C onference Series ; 430 Institute of Physics, 2022; Vol. 2304. doi:10.1088/1742-6596/2304/1/012011 431 (18) Publications, T. J. G.; Gaus, A. D.; Htwe, W. T.; Brand, J. A.; Gay, T. J.; Schulz, M.; 432 Gay, T. J. DigitalCommons@University of Nebraska -Lincoln 433 DigitalCommons@University of Nebraska -Lincoln Energy Spread and Ion Current 434 Measurements of Several Ion Energy Spread and Ion Current Measurements of Several 435 Ion Sources Sources Energy Spread and Ion Current Measurements of Several Ion 436 Sources; 1994 437 22 (19) Schmor, P. W. A REVIEW OF POLARIZED ION SOURCES; 1996 438 (20) Angra, S. K.; Kumar, P.; Dongaonkar, R. R.; Bajpai, R. P. Unstable Plasma 439 Characteristics in Mirror Field Electron Cyclotron Resonance Microwave Ion Source ; 440 2000; Vol. 54 441 (21) Asmussen, J. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 442 1989, 7, 883–893. doi:10.1116/1.575815 443 (22) Schmidt, A. A.; Offermann, J.; Anton, R. Thin Solid Films 1996, 281–282, 105–107. 444 doi:10.1016/0040-6090(96)08586-0 445 (23) Anton, R.; Wiegner, T.; Naumann, W.; Liebmann, M.; Klein, C.; Bradley, C. Review of 446 Scientific Instruments 2000, 71, 1177–1180. doi:10.1063/1.1150420 447 (24) Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. Nucl Instrum Methods Phys Res B 2010, 448 268, 1818–1823. doi:10.1016/j.nimb.2010.02.091 449 (25) Ohtsu, Y. Physics of High-Density Radio Frequency Capacitively Coupled Plasma with 450 Various Electrodes and Its Applications. In Plasma Science and Technology - Basic 451 Fundamentals and Modern Applications ; IntechOpen, 2019. 452 doi:10.5772/intechopen.78387 453 (26) Chu, P. K.; Qin, S.; Chan, C.; Cheung, N. W.; Ko, P. K. IEEE Transactions on Plasma 454 Science 1998, 26, 79–84. doi:10.1109/27.659535 455 (27) Ghosh, S. N.; Dhungel, S. K.; Yoo, J.; Gowtham, M.; Yi, J.; Bora, D. Study of High -456 Density Helicon-Plasma Generation and Measurement of the Plasma Parameters by 457 Using a Frequency-Compensated Langmuir Probe; 2006; Vol. 48 458 (28) Yang, Y. R.; Fu, S. H.; Ding, Z. F. AIP Adv 2022, 12. doi:10.1063/5.0082813 459 23 (29) Fournier, P.; Lisi, N.; Meyer, C.; Scrivens Cern, R.; Divisi on, P. S.; Ostroumov, P. 460 Experimental Characterisation of Gridded Electrostatic Lens (GEL) Low Energy Beam 461 Transport (LEBT) for the Laser Ion Source (LIS) and Effect of a Wire Grid on the 462 Extraction Electrode; 1999 463 (30) Dudin, S. V.; Rafalskyi, D. V. European Physical Journal D 2011, 65, 475 –479. 464 doi:10.1140/epjd/e2011-20402-y 465 (31) Deka, A.; Barman, P.; Mukhopadhyay, M. K.; Bhattacharyya, S. R. Surfaces and 466 Interfaces 2021, 25. doi:10.1016/j.surfin.2021.101242 467 (32) Jain, I. P.; Agarwal, G. Ion Beam Induced Surface and Interface Engineering. Surface 468 Science Reports. Elsevier B.V. 2011, pp 77–172. doi:10.1016/j.surfrep.2010.11.001 469 (33) Datta, D. P.; Garg, S. K.; Basu, T.; Satpati, B.; Hofsäss, H.; Kanjilal, D.; Som, T . Appl 470 Surf Sci 2016, 360, 131–142. doi:10.1016/j.apsusc.2015.10.133 471 (34) Bradley, R. M.; Harper, J. M. E. Journal of Vacuum Science & Technology A: Vacuum, 472 Surfaces, and Films 1988, 6, 2390–2395. doi:10.1116/1.575561 473 (35) Carter, G.; Vishnyakov, V. Roughening and Ripple Instabilities on Ion -Bombarded Si; 474 1996 475 (36) Vorathamrong, S.; Panyakeow, S.; Ratanathammaphan, S.; Praserthdam, P. AIP Adv 476 2019, 9. doi:10.1063/1.5084344 477 (37) Mennucci, C.; Muhammad, M. H.; Hameed, M. F. O.; Mohamed, S. A.; Abdelkh alik, 478 M. S.; Obayya, S. S. A.; Buatier de Mongeot, F. Appl Surf Sci 2018, 446, 74 –82. 479 doi:10.1016/j.apsusc.2018.02.186 480 (38) Garnett, E.; Yang, P. Nano Lett 2010, 10, 1082–1087. doi:10.1021/nl100161z 481 24 (39) Amalathas, A. P.; Alkaisi, M. M. Nanostructures for Light Trapping in Thin Film Solar 482 Cells. Micromachines. MDPI AG September 1, 2019. doi:10.3390/mi10090619 483 (40) Mousavi, B. K.; Mousavu, A. K.; Busani, T.; Zadeh, M. H.; Brueck, S. R. J. Journal of 484 Applied Mathematics and Physics 2019, 07, 3083 –3100. 485 doi:10.4236/jamp.2019.712217 486 (41) Zang, K.; Jiang, X.; Huo, Y.; Ding, X.; Morea, M.; Chen, X.; Lu, C. Y.; Ma, J.; Zhou, 487 M.; Xia, Z.; Yu, Z.; Kamins, T. I.; Zhang, Q.; Harris, J. S. Nat Commun 2017, 8. 488 doi:10.1038/s41467-017-00733-y 489 (42) Madi, C. S.; Anze nberg, E.; Ludwig, K. F.; Aziz, M. J. Phys Rev Lett 2011, 106. 490 doi:10.1103/PhysRevLett.106.066101 491 (43) Chan, W. L.; Chason, E. J Appl Phys 2007, 101. doi:10.1063/1.2749198 492 493 494