Emission of ions and electrons correlated with soft and hard x-rays evolution from thermal plasma

A thermal plasma is a high-temperature, ionized gas in which the particles (atoms, molecules, ions, and electrons) are in a state of thermodynamic equilibrium.¹ The plasma torch with an energy ∼10⁻¹⁸ J can produce high density ∼10²³ m⁻³ and high temperature ranging from 29 to 225 eV.^2,3 Thermal plasma is a source of radiations such as ions,^4–6 electrons,^7–9 soft x-rays,^10–13 and hard x-rays.¹⁴ The study of ions and electrons is crucial for several applications and helps to understand the mechanism of their production.^15,16 High-energy ions and electrons are used in the processing of materials, deposition of thin films,⁵ implantation of ions,¹⁷ modification of surface,¹⁸ etc. Metallic ion implantation on polymeric materials can produce a variety of surface structures such as pits, dendrites, tracks, nano-hillocks, and holes.¹⁹ X-rays produced by plasma have applications in various fields of life such as astrophysics,²⁰ bio-medical industry,²¹ nuclear physics, x-ray microscopy, microlithography, micromachining,²² and surface modification.^23,24

Since the past two decades, considerable efforts have been made by the scientific and technological community to detect ions, electrons, and x-rays (soft and hard) because of their numerous scientific and industrial applications.¹⁵ This motivated our experimental study to report simultaneous measurements of ions, electrons, and x-rays using argon as a filling gas.

Various diagnostic techniques are used to characterize plasma species, such as laser-induced breakdown spectroscopy,²⁵ Thomson parabola technique by employing solid-state nuclear track detectors (SSNTDs),^26,27 electric and magnetic probes,²⁸ Faraday cup,²⁹ and Langmuir probe.³⁰ Faraday cup is used to measure energy, flux, and density of ions by the time-of-flight method.^31,32 This is a cost-effective method.²⁶ To characterize electron temperature, electron energy, and electron density, the Langmuir probe is used because of its simplicity and convenience. To observe the characteristics of thermal plasma in ionosphere, Langmuir probes have been placed on satellites.⁸ A single Langmuir probe draws more electronic current and its circuit can be damaged in saturation region because of high electronic current. To overcome this problem, double Langmuir probe is used. The advantage of using double Langmuir probe over the other method is its high time resolution.³³ The electron-ion interaction processes are responsible for the emission of x-rays from plasma.²⁸ Composition and pressure of gas have an influence on SXR emission. Different gases emit soft x-rays of different energies depending on their electronic structure.¹² The electrons are accelerated by the electric field that exists inside the plasma. When these electrons undergo collisions with neighboring particles, they emit recombination and Bremsstrahlung radiations.³⁴ 

There are different detectors to detect x-rays from thermal plasma such as a photomultiplier tube with scintillator, a pinhole camera, CCD detectors, a magnetic spectrometer, and semiconductor detector.³⁵ Semiconductor detectors are the best detectors as they are fast. The advantages of using semiconductor detectors over other detectors are their compact size and low-voltage supply. For time evolution measurement of soft x-rays, filtered BPX65 pin photodiodes were employed as they have fast response, are cost-effective, and have a low-voltage power supply.¹¹ To detect hard x-rays, a photomultiplier tube with NE102A scintillator was utilized.³⁶ 

In the present work, a correlation between plasma parameters like temperature, energy, density of ions, electrons, and x-rays produced by thermal plasma is established by varying operating voltages and keeping flow rate of working gas constant. The plasma was generated using argon gas at atmospheric pressure 760 Torr and at a constant flow rate 5 L/min for 7, 9, and 11 kV.

The purpose of this project is to develop a correlation between time evolution of ions and electrons with x-rays (soft and hard) emitted from argon plasma. Faraday cup was used to monitor the temporal evolution of the ions, ion temperature, ion density, and ion flux from argon plasma. Time-of-flight assembly consisting of two Faraday cups was used to estimate energy of ions. To monitor the temporal evolution of electrons, electron temperature, electron energy, and electron density, double Langmuir probes were employed. Four filtered BPX65 pin photodiodes were deployed for time-resolved detection of soft x-rays (1–10 keV). The glass window of pin photodiodes was replaced by four filters. A photomultiplier tube (98313B) with NE102A scintillator was placed for time-resolved hard x-ray detection.

A schematic and a real-time image of the setup is shown in Figs. 1(a) and 1(b), respectively. The experiment was carried out in a circular stainless-steel chamber, which consists of two hollow, moveable cylindrical copper electrodes. These electrodes are connected to a DC power supply ranging from (1–11 kV). Argon was used as a working gas at a flow rate of 5 L/min. Time-of-flight assembly consisting of two Faraday cups was set at a distance of 10 cm from thermal plasma. In the time-of-flight assembly, the distance between two Faraday cups (Faraday cup 1 and Faraday cup 2) was kept 4 cm. The double Langmuir probes were placed inside thermal plasma, parallel to each other to extract information about electron density, electron temperature, and electron energy. The diameter of probe tip was 1 mm. Four BPX65 pin photodiodes were placed into four holes on a circular brass disk placed radially 15 cm apart from thermal plasma. In order to extract time-resolved evolution of soft x-rays, TO-18 glass window of BPX 65 pin photodiodes was replaced with thin sheets of aluminum (24 μm), Mylar (90 μm), copper (80 μm), and silver (10 μm) filters. The scintillator–photomultiplier system was placed 20 cm apart from thermal plasma for hard x-ray detection. All the signals were obtained on two Gw Instek digital oscilloscopes.

After configuring the time-of-flight assembly of two Faraday cups, double Langmuir probe, filtered BPX 65 pin photodiodes, and scintillator–photomultiplier system, thermal plasma was operated using argon gas at constant flow rate of 5 L/min and at operating potential 7, 9, and 11 kV. To detect ions, the Faraday cups (TOF arrangement) were placed 10 cm away from the plasma arc and were negatively biased at −150 V using dc power supply. One Langmuir probe was positively biased, and other probe was negatively biased at 150 V. Filtered BPX65 pin photodiodes were reverse biased at −45 V. The photomultiplier tube was biased with dc power supply at 1400 V. PMT was covered with black paper to avoid noise. After configuring the diagnostic system, argon gas was passed through electrodes to generate plasma. Subsequently, ions, electrons, and soft and hard x-rays were detected during plasma arc. The signals of ions, electrons, and x-rays at 7, 9, and 11 kV were obtained on the oscilloscopes for further analysis.

In order to investigate the impact of ions, electrons, and x-rays (soft and hard) on polished silicon wafer (111), p-type silicon wafer was cut into size 10 × 10 × 10 mm³. Silicon wafers were cleaned for 10 min. in acetone and then air-dried. Argon was used as working gas. To irradiate the silicon wafer with electrons/ions, the sample was positively/negatively biased at 200 V and was placed 5 cm away from the plasma. To expose the silicon wafer with soft x-rays, the sample was covered with Al filter (24 μm). To treat the sample with hard x-rays, it was covered with lead. Samples were exposed for 10 min. at constant flow rate 2 L/min at operating voltage 11 kV. The surface modifications of silicon wafer exposed with ions, electrons, and soft and hard x-rays were observed using an optical microscope.

Two digital storage oscilloscopes were utilized to visualize the signals of ions, electrons, and x-rays (hard and soft). The analysis was made by using Origin software.

Figure 2 represents the temporal profile of Ar plasma ions obtained by time-of-flight assembly consisting of two Faraday cups placed 4 cm apart from each other, plasma operated at (a) 7 kV, (b) 9 kV, and (c) 11 kV. From Fig. 2(a), it is evident that at 7 kV, time delay between two ion signals is 0.1 μs. The calculated energy of argon ion at 7 kV is 33 keV. Figure 2(b) represents that at 9 kV, the time delay between two ion signals is 0.09 μs. The calculated energy of argon ion at 9 kV is 40 keV. Figure 2(c) represents that at 11 kV, time delay between two ion signals is 0.06 μs. The calculated energy of argon ion at 11 kV is 92 keV.

Graph in Fig. 3 represents the change in energy, temperature, and flux of argon ions with the changing operating voltage. The graph shows that flux and energy of argon ions escalate by increasing thermal plasma operating voltage. The increase in the operating voltage of thermal plasma enhances the rate of ionization leading to an increase in collision rate. The velocity of argon ions escalates, resulting in an increase in the kinetic energy of the ions; hence, the ion temperature is enhanced. The separation of ions and electrons becomes stronger. This establishes a strong electric field, which leads to the enhancement of ion flux. There are Coulomb's repulsive forces present between the ions causing them to scatter. A detailed interpretation shows that ions in thermal plasma undergo acceleration depending upon their m/z ratio and strength of self-generated electric field. The electrons due to lesser size move faster than ions, yet they are unable to leave the plasma because of the large space charge they accumulate. The space charge field produced by the Coulomb attraction of ions by electrons tends to accelerate ions in accordance with their charge Ze.³⁷ The ions get acceleration in the presence of a strong self-generated electric field and cross the double layer. Ions proceed in the forward direction in a conical path and are detected by the Faraday cups placed on the plasma axis.^38–40 However, less ions reach the second cup because of being at longer distance due to which intensity of argon ions at FC1 is higher than at FC2. Optimal results in the time evolution of ions are obtained when the thermal plasma is operated at 11 kV. Figure 4 represents a schematic illustrating space charge field produced due to the attraction of ions by electrons.

Plasma parameters, i.e., electron temperature, electron energy, electron density, and plasma frequency, have been obtained by employing double Langmuir probe. For this purpose, thermal plasma is operated at constant operating voltage of 11 kV, as we have obtained best results in the case of ions at this operating potential. Figure 5 represents the temporal evolution profile of electrons and ions obtained employing double Langmuir probe at biasing voltages: (a) 50 V, (b) 100 V, (c) 150 V, (d) 200 V, (e) 250 V, and (f) 300 V. Graph shows that intensity of electrons is higher than ions. In Fig. 5(a) at the beginning of electron signal, i.e., at t = 0, the peak appears because electrons are accelerated by positive biasing and develop a space charge around the probe tip forming a so-called Debye sphere and same is true in the case of ions. However, the peak intensity is less than that of the first peak because most of the electrons have taken U-turn after the setting up of the self-generated electric field and ions have escaped in the forward direction out of the plasma forming a beam, which is detected by the Faraday cup (Fig. 2). It is clear from the signal in Fig. 5(a) that at t = 100 ns, intensity of electrons signal is higher than ions because of the obvious reason for difference in the masses. Second peak also shows that electron signal persisted for 30 ns, whereas the ionic signal for 20 ns. Ions because of being massive surround the tip faster. Following this, plasma breaks down, and negative signal appears because of the noise. After 250 ns, plasma still persists, which causes the third peak that shows that electrons last for 120 ns and ions last for 200 ns. The electrons are more intense than those of ions. However, at this instance, ions have broader peak than electrons due to increase in the rate of ionization. Particles present in electric and magnetic field experiences Lorentz force. They are considered as simple harmonic oscillator with the cyclotron frequency. The smaller the particle mass, the smaller will be its gyro-radius, and higher the magnetic field. Electrons gain more energy which intensifies the collision between electrons and neutral particles, so ionization rate enhances.⁴¹ The rest of the signals Figs. 5(b)–5(f) can be explained in the same manner; nevertheless, the time evolution or intensities of the electrons and ions might vary from signal to signal.

When Langmuir probe tips are inserted into the thermal plasma, several physical processes take place. When one of the probe tip is biased negatively relative to other, an electric field is created around the plasma region. Electric field exerts a force on charged particle, causing them to move. Electrons being negatively charged and much lighter than ions experience a force in the direction opposite to the electric field. Therefore, they are attracted toward positively biased probe. Ions, on the other hand, are positively charged particles. They experience a force in the same direction as that of electric field and are attracted toward the negatively biased probe. The attraction of charged particles toward the probes results in current flow between the probes. The thermal motion of particles in a plasma depends on their temperature. Electrons being much lighter have high thermal velocities than ions at same temperature. The higher thermal velocity further contributes to increase in intensity in the Langmuir probe signal.

Figure 6 represents the I–V curve obtained from double Langmuir probe characterized by three regions: the electron saturation region (I), transition region (II), and the ion saturation region (III). When the probe potential is positive relative to thermal plasma, electrons are attracted to Langmuir probe tip, while ions are repelled resulting in a disappearance of small ion current. A flat portion, called electron saturation region, is achieved at 200 V. When the probe potential is negative relative to plasma, electrons begin to repel and ions are accelerated leading to a decrease in electron current. At −200 V, probe repels all electrons, which is known as floating potential. A flat portion, called ion saturation region, is achieved at −200 V. Furthermore, decreasing the potential results in ion saturation region.²⁷ 

Table I represents the electron temperature, electron energy, electron density, plasma frequency, and Debye's length of thermal plasma torch at different biasing voltages. Plasma parameters are derived using current–voltage characteristic curve and by applying formulas of standard probe theory.²⁷ 

Figure 7 shows the variation in electron density, electron energy, electron temperature, and plasma frequency with the change in biasing voltage of Langmuir probe. Figure 7(a) shows that biasing potential of Langmuir probe creates an electric field around the probe. This electric field attracts electrons from surrounding plasma toward the probe surface. As biasing potential increases, the electric field becomes stronger, resulting in a higher attraction force on electrons. Consequently, more electrons are collected by the probe, leading to an increase in electron density.⁴²  Figure 7(b) shows that biasing potential also affects the energy of the electrons in the plasma. As electrons are attracted toward the positively biased probe, they gain energy from the electric field. With a higher biasing potential, the electric field is stronger, resulting in a greater energy transfer to the electrons. Consequently, the average energy of the collected electrons, as measured by the Langmuir probe, increases with the biasing potential. Increasing the biasing potential of the Langmuir probe can lead to an increase in the electron temperature. Figure 7(c) shows that stronger electric field accelerates electrons, causing them to collide more frequently with other particles in the plasma. These collisions result in energy exchange and thermalization, leading to an increase in the electron temperature. As far as electron frequency is concerned Fig. 7(d), when biasing potential is increased, electrons move faster than ions establishing a self-generated electric field. When electrons interact with electric field, they move in circular or helical path and do not transfer their energy to the surrounding electrons. As they move, they curve around and reverse their direction, forming a U-shaped trajectory. However, due to Coulomb's attraction of positive biasing, they are attracted toward positive probe tip. Plasma frequency arises due to the balance between the electric force that causes electrons to move apart and the Coulomb force that pulls them back together. So, plasma frequency increases with the increase in biasing voltage.

Figure 8 shows soft x-ray signals, detected by BPX 65 pin photodiode, thermal plasma operated at (a) 7 kV, (b) 9 kV, and (c) 11 kV. Signal I is received through Al filter of 24 μm thickness. Signal II is received through Mylar filter of 90 μm thickness. Signal III is received through Cu filter of 80 μm thickness. Signal IV is received through silver filter of 10 μm thickness.

The soft x-ray signals in Fig. 8(a) at operating voltage of 7 kV show that the duration of the whole signal is 2700 ns (2.7 μs), which consists of three strong pulses appearing at 6–6.7, 6.7–7.4, and 7.4–8.7 μs. It is well-known fact that electrons are responsible for the production of soft x-rays in the plasma under various interaction between ions and electrons.^36,43–46 However, the soft x-ray signals last much longer (2700 ns in our case) than those of electrons' and ions' signals (300 ns in our case). This is because of the obvious reason that electrons and ions are immediately detectable as soon as the plasma is formed. However, soft x-rays were produced as a consequence of the interaction between ions and electrons, which off course take a longer time. The normal timeframes for these interactions in a hot plasma are typically in the range of nanoseconds, although they can vary greatly depending on the particular plasma circumstances. There are small spikes in the signals during first 6 μs, the x-ray signal consisting of discontinuity and edges. This is due to recombination process as the ions and electrons do not have sufficient energy at initial stages. So, ions and electrons recombine and give recombination radiation. This transition is called free-bound transition.³⁴ 

Figure 9 illustrates the schematic of Bremsstrahlung x-ray production. Due to lower mobility of ions compared to electrons, ions remain within the plasma plume, generating a substantial self-generated electric field. This field causes electrons to decelerate and eventually reverse direction toward the ions. During this retrograde motion, electrons release energy in the form of x-rays, resulting in a pronounced second peak. This phenomenon, known as free-free transition or Bremsstrahlung, contributes to the significant energy release observed. Additionally, line radiations (bound-bound transitions), generated by energetic electrons, contribute to a third peak in the emission spectrum.^13,34

The temporal response of the II and III signals in Fig. 8(a) is almost same with the only difference that the intensities of the signals from Ag filter are less than those for the rest of the signals. Since soft x-rays have less energy than hard x-rays, materials can more readily absorb them. Silver, being a dense metal, can absorb a significant number of soft x-rays, which reduces the number of photons that can reach the detector. This absorption leads to a decrease in the signal intensity and affects the resolution.

The signals in Figs. 8(b) and 8(c) depict the same temporal behavior. The intensities of soft x-ray signals are increased by increasing operating plasma potential because of the increased electron and ions densities. Soft x-ray emission in both the cases lasts for 3 μs (3000 ns) both for 9 and 11 kV. It is essential to note that the plasma properties, such as density and temperature, have a substantial impact on the interaction periods for these processes. Higher density and temperature of plasma accelerate collision rates, promoting x-ray production. Optimal results in the time evolution of soft x-ray are attained when the thermal plasma has been operated at 11 kV.

Figure 10 shows the temporal evolution of hard x-rays from thermal plasma at operating plasma voltage (a) 7 kV, (b) 9 kV, and (c)11 kV. It appears from the graphs that the hard x-ray signals last for 1 μs (1000 ns) regardless of the operational voltage. Figure 10(a) shows that at a lower operating voltage (7 kV), at t = 0, there are very small number of hard x-rays that have been emitted from the plasma. The emission starts at 0.3 μs (300 ns) and increases a bit afterward. However, the intensity of the hard x-rays at 7 kV is much smaller than those at 9 and 11 kV [(b) and (c), respectively]. When the plasma operating potential is increased to 9 and 11 kV, in both the cases, the emission starts immediately after the formation of the plasma, i.e., at t = 0. The reason is that at higher operating voltages, the input energy is more than sufficient for the processing gas atoms to be ionized immediately. In the case of argon gas, ionization energy is 16 eV. In a plasma, the production of x-rays generally depends on the energy of the charged particles (electrons and ions) and the atomic structure of the gas. When the operating voltage is increased, it typically leads to an increase in the energy of the charged particles in the plasma. The electrons in the plasma gain more energy. This higher electron energy results in more energetic collisions with argon atoms, leading to higher-energy x-ray production through collisional excitation and ionization processes.⁴⁷ Higher operating voltage leads to higher ionization rates in the plasma. Highly ionized argon ions can undergo recombination with electrons, releasing energy in the form of HXRs during the de-excitation process.⁴⁸ 

The emission is much stronger and intense in the case of plasma operated at 11 kV than at 9 kV. This is because of the setting up of a stronger self-generated electric field at higher operating potentials because of the larger input energies. Due to this, electrons are accelerated bearing higher energies. The production of hard x-rays involves collision of high-energy electrons with target atom or ions, causing them to undergo transitions. At higher energies, Auger electrons can also be produced during the de-excitation of highly ionized ions. Auger electrons have energies in the hard x-ray range, and the higher operating voltage can enhance their production, contributing to the stronger x-ray emission.¹⁵ The conditions within the plasma can vary with time and location, leading to fluctuations in properties like electron density, temperature, and magnetic field strength. These variations can have a more significant impact on hard x-ray emission due to the higher energies involved. Consequently, these plasma instabilities contribute to the oscillation in the observed hard x-ray signals.^49,50 Optimal results in the time evolution of hard x-ray are attained when the thermal plasma has been operated at 11 kV.

In order to compare the utilization of these radiations emitted from the plasmas for material processing, the ions, electrons, and soft and hard x-rays were made to fall on the Si target. Figure 11 shows optical images of silicon wafer (a) untreated and (b) treated with argon ions. Optical image in Fig. 11(b) clearly shows the existence of latent damaged trails formed by ions, which is a typical signature mark when a material is irradiated with ions.

Figure 12 shows the simulations of argon ion penetration depth of damage in silicon wafer due to implantation of ions using SRIM software. The calculation of SRIM is executed for 10 000 incident argon ions. Energy of argon ion is selected 92 keV calculated by Faraday cup.

Figure 13 shows the TRIM results: (a) total displacements by argon ion, (b) collision events of argon ion with silicon wafer, and c) ion range into silicon wafer. To predict the depth distribution of defects in silicon wafer, TRIM is used.³⁵ The ion vacancies, damage, and ion range in silicon wafer are calculated by TRIM.⁵¹ The simulations show damage distribution perpendicular to sample surface.⁵²  Figures 13(a) and 13(b) intimate that 1368 silicon atoms are displaced per argon ion. 1260 vacancies are created per argon ion implantation. Figure 13(c) shows that range of argon ion into silicon sample is 759 Å.

It appears from Fig. 14(a), the electrons irradiation has damaged the Si surface more than that of ions. The reason might be the higher density of electrons.⁵³ The density of argon ions is 1.43 × 10²⁴ m⁻³, while density of electrons is 87.9 × 10²³ m⁻³. Soft x-rays have a lesser effect as compared to the Si exposed with hard x-rays Fig. 14(c). The reason is the higher energy of the hard x-rays. In the case of hard x-rays irradiated Si, at few places ablation has been observed. The ablation has formed erupted volcano-like structures (encircled).

Overall, ions, electrons, and x-rays are interconnected through their ability to ionize, excite, and interact with matter. Soft x-rays are produced as a consequence of interaction between electrons and ions. In plasma, electrons and ions are detected earlier than soft x-rays. So, the soft x-ray signals last much longer (3000 ns) than those of electrons' and ions' signals (300 ns). Electrons have greater impact on silicon wafer than ions. Irradiating silicon wafers with electrons is utilized in semiconductor fabrication processes to introduce controlled defects, tailor material properties, and improve device performance through techniques like dopant activation and radiation hardening.

Radiations (ion, electron, soft, and hard x-rays) from thermal plasma have been investigated with a view to understand their correlation. The energy, flux, and temperature of argon ions are enhanced with the increase in the operating potential. The 11 kV operating potential is found to be the best for the ion emission in our case. The value of electron temperature, electron density, and electron energy of thermal plasma evaluated by Langmuir probe is 29–225 eV, 10²³ m⁻³, and 10⁻¹⁸ J, respectively, when plasma produced at 11 kV. SXR and HXR evolution increased as the thermal plasma voltage increased, with the lowest emission at 7 kV and the highest at 11 kV. More gas is ionized at higher voltages, and more ions and electrons with greater energies form and accelerate toward the cathode and anode, respectively. As a result, x-ray emission increased. Soft x-ray signals last much longer (3000 ns) than those of electrons' and ions' signals (300 ns). 11 kV is found to be the best voltage for ion, electron, and soft and hard x-ray emissions.

Soft x-ray signals obtained by BPX65 pin diode with Al filter of 24 μm thickness showed best resolution.

Treating silicon wafer with electrons damaged the sample more than ions. Ablation occurred when Si wafer was treated with hard x-rays. The ablation has formed erupted volcano-like structures.

This research is crucial for advancing understanding of plasma physics, which underpins various technological fields. Potential applications include improved diagnostic and imaging techniques in medical and industrial settings, advancements in energy production technologies such as nuclear fusion, and enhanced security screening methods utilizing x-ray technology.

The authors have no conflicts to disclose.

Aneesa Naveed Ahmad: Conceptualization (equal); Methodology (equal). Muhammad Shahid Rafique: Conceptualization (equal); Project administration (equal); Supervision (equal); Writing – original draft (equal). Muhammad Arslan: Formal analysis (equal). Tehreem Arshad: Project administration (equal). Ayesha Armani: Methodology (equal). Muhammad Mudassar: Conceptualization (equal). Fakhar Siddiq: Resources (supporting). Fazila Javed: Investigation (equal); Project administration (equal). Imran Shahadat: Investigation (equal). Abdul Muneeb: Data curation (equal). Hafsa Mahmood: Data curation (equal). Mubashra Amir: Formal analysis (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.