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The electron temperature distribution of laser erosion plume after ablation of a tantalum target with excimer laser in vacuum
Optics and Lasers in Engineering 36 (2001) 303–311
The electron temperature distribution of laser erosion plume after ablation of a tantalum target with excimer laser in vacuum O.A. Novodvorskya, C. Wenzelb,*, J.W. Barthab, O.D. Khramovaa, E.O. Filippovaa a
Institute on Laser and Information Technologies, Russian Academy of Science (ILIT RAS), Svyatoozerskaya st. 1, 140700 Shatura, Moscow Region, Russia b Institute of Semiconductor and Microsystems Technology, Dresden University of Technology, D-01062 Dresden, Germany Received 5 January 2001; accepted 10 April 2001
Abstract The erosion plume resulting from an ablation of a tantalum target in vacuum with an excimer laser radiation (308 nm) was studied using the Langmuir probe technique. The spatial and temporal dependencies of the electron probe currents were obtained in real time. The electron temperature of different plume regions was determined from a series of I2U characteristics taken at different distances between the probe and the target. We determined that the plume electron temperature is non-uniform and has a maximum value in front of the plume. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Laser; Ablation; Tantalum; Electron temperature
1. Introduction The process of ablation from a solid target has long been the subject of fundamental research. This is due to both, the scientific complexity of the involved processes and the possibility of a wide practical application [1,2]. One major application of solid target ablation by laser beam is the process of thin film deposition with pulsed laser (PLD). *Corresponding author. Tel.: +0351-463-6413; fax: +0351-463-7172. E-mail address: [email protected] (C. Wenzel). 0143-8166/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 4 3 - 4
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The processes of laser-plasma deposition require information on the characteristics of the erosion plume. The energy spectra of ions are known to play an essential role in the deposition of thin films by physical methods [3], particularly in PLD [4]. The possibility to control the ion energy spectrum, which has a pronounced effect on the deposited film characteristics (type of crystal structure, crystal size, adhesion, etc.) is of great importance [4]. The determination of the plume energy parameters (energy spectrum of ions, electron temperature, density), their dependence on laser pulse energy, spatial evolution and angular distribution are of special importance for the PLD of thin metal films. Time and space resolved measurements made in the spreading direction of plume to substrate yields information on kinetics and velocity of different particles. Probe methods are widely involved in the investigations of laser erosion plume by ablation of metals, semiconductors, ion crystals and ceramics [5,6–9]. It is typical for laser ablation of metals that the target evaporation threshold approaches the threshold of plasma generation [10]. Thus, in pulsed laser deposition the erosion plume of metal is highly ionized [5,11]. In these highly ionized plume plasmas the Langmuir probe records the charged particles, which present a considerable fraction of the plume particles. Due to it’s small size, the probe provides a high spatial resolution. Therefore, the Langmuir probe was used to determine the plume electron temperature [7–9], the velocity distribution [6,9,12] and the energy spectrum of ions [5,13].This paper reports the investigation of the erosion plume by ablation from a tantalum target with an excimer laser radiation (308 nm) using the Langmuir probe with a special possibility to perform a time resolved measurement within the pulse duration. The spatial and temporal distribution of electron and ion probe currents was obtained. The ion velocity, the electron temperature, the ion density distribution in the plume and their dynamics has been measured.
2. Experiment The experiments were run in a vacuum chamber, which was evacuated by a diffusion pump down to pp105 mbar. The laser erosion plasma was generated by an XeCl excimer laser. The beam was focused by a two-lens objective and directed to the target at 501 to the normal. The target has a disc form. The thickness of the 99.9% tantalum foil is either 0.5 or 1 mm. The disc is fixed on an electric drive axis and can be rotated with a frequency of 10 Hz. The focus spot has an ellipsoidal shape with an area of 0.5 mm2 on the target. The target and the vacuum chamber were grounded. The Langmuir probe has a length of 5 mm and is made from a tungsten wire with 0.2 mm diameter placed in a ceramic tube. The probe was positioned parallel to the target surface and could be moved perpendicular in the emission direction between 3 and 160 mm. To study the particle scattering from the target surface, the probe was moved 3 mm steps towards the target and up to 45 mm steps away from the irradiated area. In all measurements, the probe axis was kept parallel to the target surface and the probe was held with its side parallel to the ablation area. The probe potential could be varied within 718 V. The controlled voltage at the
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probe was supplied by a bank of accumulators which was connected to the probe by one pole through a potentiometer, and was grounded by the other pole through a load resistor. The source of controlled voltage was bypassed by a 2.5 mF capacitor in order to stabilize the probe potential when the current is flowing. The value of the load resistance could be varied from 10 to 1000 O discretely. The signal from the load resistor was processed by a high-speed analogue to digital converter (ADC) and stored by a personal computer. The selected time resolution of the ADC was 0.1 ms. The measurement range of the ADC was 75 V. The time-of-flight of charged particles arriving at the probe was counted from the moment of the laser pulse generation recorded by a photodiode and sent to the trigger input of the ADC. The pulse energy was varied between 2 and 25 mJ and the pulse half-height duration was kept 20 ns.
3. Results and discussion The experiments delivered time-of-flight curves (TFC) of tantalum ions at different probe-to-target distances, probe potentials and at different energies of laser incident radiation. By changing the sign of the probe potential, we obtained TFC for both, ions and electrons. For characteristic points of the TFC volt–ampere probe curves were generated. Their electron fraction is serving to determine the electron temperature. The potential of ion current saturation can be found from the ion part of the curve. The time-of-flight of the leading ion groups was measured for several probe-totarget distances. The time-of-flight of the signal front edge was found to be proportional to the probe-to-target distance. The ion scatter velocity was determined from the probe curves as the ratio of the probe-to-target distance versus the time difference between the corresponding current and the laser pulse onset. For the leading group of Ta ions the scatter velocity does not depend on probe-totarget distance. We measured a value of n ¼ 1:9 104 m/s. This value of velocity satisfies the dependence nBðMÞ1=2 ; where M is the molecular weight of the element and it agrees with results from [14] for ions of barium, yttrium and copper. The timeof-flight curves of the tantalum ions following the leading group, however, differ considerably at different probe-to-target distances. The evolution of TFC of tantalum ions versus the distance is shown in Fig. 1 for 2 J/cm2 radiation flux. The signal amplitude is significantly reduced as the probe-to-target distance increases, so for convenience the TFC’s in Fig. 1 are not scaled. All the curves were measured for a time-of-flight range between 0 to 100 ms. The time displayed was reduced to 30 ms in order to get a higher resolution in Fig. 1. As can be seen from the curves in Fig. 1, the delay of the signal front edge is proportional to the probe-to-target distance. When the probe is moved to 10 mm away from the target (Fig. 1, curve 1), the time-of-flight signal presents a smooth curve, which has one maximum with a steep front edge and a more flattened back edge tail decreasing to zero within 15 ms. If we assume that this curve corresponds to some smooth distribution of ion velocities, then it should retain its smooth shape even at larger scatter time. However, the time-
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Fig. 1. Dependence of shape of probe time-to-flight curves on probe-to-target separation (1–10 mm, 2– 23 mm, 3–75 mm, 4–113 mm, 5–133 mm). For convenience, the curves are not scaled and the scanning time is shortened.
of-flight signal exhibits a more complicated behavior as the probe-to-target distance increases. With the probe moving away from the target, several maxima can be observed on the TFC, which become even more pronounced as the distance increases. As seen in Fig. 1, TFC has two maxima at the distances of 23 and 75 mm. At 113 mm three maxima can be distinguished, and at 133 mm four maxima are already observed. (In the time of flight range of 30 ms only the first three maxima are visible, the fourth maximum appears at higher delay time.) It turned out that the velocity distribution in the group of particles corresponding to each individual maximum on curves 4 and 5 can be described by a Maxwell distribution or by a shifted Maxwell distribution [13]. These distributions were used to describe the obtained experimental data. After mathematical processing of the experimental data, it turned out that each of the curves 1–5 (Fig. 1) presents a sum of four Maxwell curves with different positions of the maxima. This means, that at a negative probe potential four groups of positively charged particles can be observed in the plume using one set of TFC’s for all of the studied distances [13]. When the probe is located 23 mm from the target, the TFC is described by the curves with maxima at 1.1, 2.1, 4.0 and 16.7 ms, which correspond to particle velocities of about 21, 11, 6 and 1.4 km/s. The curve obtained at 113 mm in the region below 30 ms is well approximated by three curves, which are described by Maxwell distribution. To describe the curve in the region with a scatter time above 30 ms, an additionally shifted Maxwell distribution with relatively low mean velocity has to be used. The maxima of these distributions are at 5.3, 10.2, 17.7 and 49 ms with most probable velocities of about 21, 11, 6 and 2.5 km/s. It can be seen from the velocity values obtained for the distances 23 and 113 mm to the probe that velocities of the first three groups of ions are constant, and the velocity of the fourth group
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increases with the distance from the target. An increase in velocity of the fourth ion group is also confirmed by TFC approximation at all the studied probe-to-target distances (10–133 mm). Two-and three-mode distributions of ions by velocities were noted previously by other authors using multi-component targets of ion crystals [15] and HTSC ceramics [16]. In our earlier studies, we have shown that in ablation of Ta the ion probe current saturation is achieved at a probe potential of E10 V [12]. The probe current in the ion current saturation mode is found by Ii ¼ 0:5SenV; where S is the probe area, e the electron charge, n the ion density and V the ion velocity at the boundary of the probe collision-free layer [17]. The ion time-of-flight signal can be transformed to distribution of charge density in the flying plume nðtÞ ¼ IðtÞ=ð0:5SeVÞ ¼ IðtÞt=ð0:5SeLÞ; where t is the arrival time of recorded ions and L the probe-to-target distance. Using this formula, we evaluated the concentration of particles in the plume at different distances from the obtained TFC. It has been found that the maximum concentration of ions in the plume varies from 3.3 1013 cm3 at L ¼ 10 mm to 1.4 1011 cm3 at L ¼ 133 mm. The first group of ions moving with constant velocity are formed at the start of the ablation process in the mode of free collision-less release. We can show that for these ion groups the amplitude of signal reduces with the distance as L2 [12,13], corresponding to a spherical layer scattering. The more slower groups of ions are resulting from the release process characterized by the presence of a Knudsen layer [18]. The time of evaporation can by far exceed the laser pulse duration. Following the hydrodynamic scatter model [19], the density of neutral particles remains maximum at the target surface even though the cloud is scattered to a distance of several centimeters. In the plume, neutral particles have lower propagation velocities than charged particles [14]. There leading part is moving faster and the tail of the fast ion groups can be ionized as a result of resonance recharging. This will lead to an increase in the propagation velocity of the slow ion groups. A long tail in the probe TFC can also be generated during ionization, as a result of the neutral particle flow overlapping the back front of the electron cloud of the plumes charged part [8]. The scattering nature of the plume expansion is also demonstrated by the method of crossing radiation based on changing the flight direction of ions during the crossing of two non-parallel plumes. [20]. The time dependence of the electron probe current has the same complexity as the ion time-of-flight curves. Both are changing considerably with the probe-to-target distance. At long distances there are several maxima which are in their temporal position close to the maxima in the TFC’s of the ion current. A typical plot of an electron current curve with the probe located at the plume axis is shown in Fig. 2. The probe-to-target distance was 23 mm and the laser radiation flux used was 2 J/ cm2. At the start of scaling, a short pulse is seen on the electron current probe curves. This pulse is caused by the photoelectrons, releasing the target surface under laser
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Fig. 2. Curve of electron probe current; the probe is located at 23 mm along the normal to the target.
radiation even before the ablation is starting. A similar pulse of photoelectrons was recorded in ablation of dielectrics [8,15]. The emergence of hot electrons in plasma can be caused by the processes of multiphoton absorption by backward retarding absorption [2] and by build-up of Langmuir vibrations [21]. As a consequence of Coulomb interaction with external layers of the erosion plume, these electrons form a high-energy leading part of ions. Their energy is reaching several hundred eV, which is observed in the experiments from [5,12,13]. Although, due to low depth of external field penetration into the plume plasma, the portion of these ions in the whole mass of plume ions is rather small and makes a few per cent of the overall amount only [12,13]. The electron temperature Te was determined in the different plume parts corresponding to the maxima in time-of-flight curves. The electron temperature was calculated from the plot showing the dependence of the logarithm of the peak amplitude versus the probe potential. The slope of this plot in the region below the electron saturation current is equal to e=ðk Te Þ; where e is the electron charge and k the Boltzmann constant. Te is the electron temperature in eV. This follows directly from the probe equation: Ie pexpðeV=kTeÞ; where V ¼ Vp Vs is probe-to-space voltage, Vp the probe potential, Vs the plasma potential which can be different from zero by the value of ðk Te Þ=e and Ie is the probe current. The probe equation is valid when the positive voltage at the probe is above ðk Te Þ=e: Below this value the ion current is comparable with electron current and the relationship lnIe ðVÞ is shifted from a linear behavior to a non-linear one [17]. The value of Te was determined for the probe-to-target distances of 23, 75 and 133 mm using the three observed maxima. The results are presented in Table 1. The
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Table 1 Values of erosion plume electron temperature (eV) at different probe-to-target separations taken vertically Number of maximum on TFC
Distance from probe to target (mm) 23
1 2 3
75
6.4 5.7 4.8
6.25 5.9 5.0
133 5.8 5.3 4.7
temperature appears to be different at all three maxima lowering to the tail part of the plume. The following formula [22] Te ¼ 2:98104 A1=8 ðZ þ 1Þ5=8 Z 3=4 ðIlÞ1=2 t1=4 K; where A is the atomic weight of an ion, Z the average charge of ions in the cloud, I the laser radiation intensity (W/cm2), lðcmÞ the radiation wavelength, tðcÞ the laser pulse duration, gives an estimation of the electron temperature for tantalum atoms with single ionization (Z ¼ 1) to Te ¼ 4:8 eV, and for Z ¼ 1:5 (tantalum atoms with single and double ionization) to Te ¼ 5:6 eV, respectively. The data are in a good agreement with the experimental results (see Table 1). The amplitudes of the short pulses at the start of the scanning also depend on the probe potential. For them Te is measured to 11.5 eV at a distance of 75 mm and to 7.35 eV at a distance of 133 mm, respectively. The electron TFC’s were also studied when the probe was moved parallel to the target surface at a distance of 3 mm from the surface. In this case three maxima are observed on TFC too. A typical view of the electron current at the probe located near the target surface is given in Fig. 3. It can be seen that the signal shape is considerably different from the probe current curve located at the plume axis. The relationship of the amplitudes of different peaks is also different. The absence of a peak of photoelectrons is remarkable. The onset of the curve shows an individual peak which is weak pronounced or missing when the probe is positioned to the normal axis of the target spot. This peak is caused by the electrons scattered to the outside of the ion cloud, but kept by the Coulomb force. Due to a high concentration of charges in the cloud propagating along the normal, this peak is not detectable in the axis. In the direction parallel to the surface, the concentration of charges in the ion cloud is much lower (diagram of ion scatter Bcos6 y [12]), and the peak can be seen clearly. The dependencies of the signal amplitudes on the probe position were obtained by moving the probe from the normal of the plume to off-axis positions of 15, 30 and 45 mm. Table 2 presents the obtained values of Te : The first and second maximum of the TFC correspond to ion groups propagating at constant velocities. The electron temperature decreases with the distance from the ablation area, as well as in the direction of the plume tail. It should be noted that all three values of electron temperature in different parts of the plume along the surface are lower than those noted along the normal to the surface.
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Fig. 3. Curve of electron probe current; the probe is located at 3 mm from the target surface and moved to 30 mm from the plume axis (probe potential=+8 V, irradiation flux=2 J/cm2)
Table 2 Values of erosion plume electron temperature (eV) at different probe-to-target separations taken horizontally Number of maximum on TFC
Distance from probe to normal (mm) 15
1 2 3
4.2 4.0 3.3
30 3.9 3.65 2.6
45 2.90 1.85 1.80
4. Conclusion The velocity distribution of the ions in the erosion plume during ablation from a tantalum target in vacuum is not smooth in its nature. Individual ion groups can be identified and the velocity distribution of each individual group can be well described by the one-dimensional Maxwell distribution. The leading groups of ions in the plume propagate at constant velocities. Slow groups of ions are accelerated. This can be caused by a variation in the ion concentration as a result of chemi-ionization and recombination processes, as well as a variation of the ion scatter velocity due to second-kind collisions. The accelerated motion of the ions in slow mode shows the dominant role of electron and ion collisions processes with neutral particles in the tail part of the charged area in the plume. In the studied distances from the target the plasma does not yet reach the state of free flight condition. It has been shown that the plume temperature is not uniform. The electron temperature has a maximum
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value in front of the plume, while in the tail the electron temperature is reaching a minimum. Increasing the distance from the target the electron temperature is decreasing in all parts of the plume.
References [1] Hora H. Physics of Laser Driven Plasma. Moscow: Energoatomizdat, 1986. [2] Arutyunyan RV, Baranov VYu, Bolshov LA. Action of laser radiation on materials. Moscow: Nauka, 1989. [3] Roy RA, Catania P, Saenger KL, Cuomo JJ, Lossy RL. Role of energetic atoms and ions in Ta films grown by different physical vapor deposition methods. J Vac Sci Technol B 1993;11(5):1921. [4] Saenger KL. On the origin of apatial nonuniformities in the composition of pulsed-laser-deposited films. J Appl Phys 1991;70:5629. [5] Jordan R, Cole D, Lunney JG, Mackay K, Givord D. Pulsed laser ablation of copper. Appl Surf Sci 1995;86:21. [6] Dyer PE, Greenough RD, Issa A, Key PH. Spectroscopic and ion probe measurements of KrF laser ablated Y–Ba–Cu–O bulk samples. Appl Phys Lett 1988;53(6):534. [7] Von Gutfeld RJ, Dreyfus RW. Electronic probe measurement of pulsed copper ablation at 248 nm. Appl Phys Lett 1989;54(13):1212. [8] Ermer DR, Langford SC, Dickinson JT. Interaction of wide band gap single crystals with 248 nm excimer laser radiation. J Appl Phys 1997;81:1495. [9] Lubben D, Barnett SA, Suzuki K, Gorbatkin S, Greene JE. Laser-induced plasmas for primary ion deposition of epitaxial Ge and Si films. J Vac Sci Technol 1985;B3(4):968. [10] Ready JF. Effects of High Power Laser Radiation. London: Academic, 1971. [11] Witke T, Ziegele H. Plasma state in pulsed arc, laser deposition. Surf and Coat Technol 1997;97:414. [12] Novodvorsky OA, Khramova OD, Shevelev AK, Filippova EO. Probe measurements of XeCl excimer laser ablated Al, Cu, Nb, Ta solid samples. AHPLA ’99; Osaka, Japan, Proc SPIE 3885; 1999: 471-480. [13] Novodvorsky OA, Khramova OD, Filippova EO, Wenzel C, Bartha JW. Energy distribution of ions in plasma formed by laser ablation of metallic Nb and Ta targets. Optics and Lasers in Engineering 1999;32(5):449. [14] Marine W, Gerri M, Scotto dAnielo JM. Analysis of the plasma expansion dynamics by optical timeof-flight measurements. Appl Surf Sci 1992;54:264. [15] Chin JJ, Ermer DR, Langford SC, Dickinson JT. The role of photoelectronic processes in the formation of a fluorescent plume by 248-nm laser irradiation of single cristal NaNO3. Appl Phys 1997;A64:7. [16] Boyarkin OV, Burimov VN, Golubev VS, Zherikhin AN, Popkov VL. Velocity distribution of Ba+ ions in a plasma formed by laser ablation. Izvestiya AN Ser Fizicheskaya 1993;52(12):90. [17] Demidov VI, Kolobkov NV, Kudryavtsev AA. Probe methods of low-temperature plasma investigation. Moscow: Energoatomizdat, 1996. [18] Kelly R, Dreyfus RW. On the effect of knudsen-layer formation on studies of vaporization, sputtering, and desorption. Surf Sci 1988;198:263. [19] Leboenf JN, Chen KR. Modeling of dynamical processes in laser ablation. Appl Surf Sci 1996;96– 98:14–23. [20] Strikovsky MD, Klyuenkov EB, Gaponov SV. Appl Phys Lett 1993;63(8):1146–8. [21] Gorbunov LM, Kirsanov VIJ. Experimental and Theoretical Physics (Rus.) 96; 2(8); 1989: 583. [22] Phipps CR, Turner., Jr TP, Harrison RF. Impulse coupling to targets in vacuum by KrF, HF and CO2 single-pulse lasers. J Appl Phys 1988;64:1083.
The electron temperature distribution of laser erosion plume after ablation of a tantalum target with excimer laser in vacuum O.A. Novodvorskya, C. Wenzelb,*, J.W. Barthab, O.D. Khramovaa, E.O. Filippovaa a
Institute on Laser and Information Technologies, Russian Academy of Science (ILIT RAS), Svyatoozerskaya st. 1, 140700 Shatura, Moscow Region, Russia b Institute of Semiconductor and Microsystems Technology, Dresden University of Technology, D-01062 Dresden, Germany Received 5 January 2001; accepted 10 April 2001
Abstract The erosion plume resulting from an ablation of a tantalum target in vacuum with an excimer laser radiation (308 nm) was studied using the Langmuir probe technique. The spatial and temporal dependencies of the electron probe currents were obtained in real time. The electron temperature of different plume regions was determined from a series of I2U characteristics taken at different distances between the probe and the target. We determined that the plume electron temperature is non-uniform and has a maximum value in front of the plume. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Laser; Ablation; Tantalum; Electron temperature
1. Introduction The process of ablation from a solid target has long been the subject of fundamental research. This is due to both, the scientific complexity of the involved processes and the possibility of a wide practical application [1,2]. One major application of solid target ablation by laser beam is the process of thin film deposition with pulsed laser (PLD). *Corresponding author. Tel.: +0351-463-6413; fax: +0351-463-7172. E-mail address: [email protected] (C. Wenzel). 0143-8166/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 4 3 - 4
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The processes of laser-plasma deposition require information on the characteristics of the erosion plume. The energy spectra of ions are known to play an essential role in the deposition of thin films by physical methods [3], particularly in PLD [4]. The possibility to control the ion energy spectrum, which has a pronounced effect on the deposited film characteristics (type of crystal structure, crystal size, adhesion, etc.) is of great importance [4]. The determination of the plume energy parameters (energy spectrum of ions, electron temperature, density), their dependence on laser pulse energy, spatial evolution and angular distribution are of special importance for the PLD of thin metal films. Time and space resolved measurements made in the spreading direction of plume to substrate yields information on kinetics and velocity of different particles. Probe methods are widely involved in the investigations of laser erosion plume by ablation of metals, semiconductors, ion crystals and ceramics [5,6–9]. It is typical for laser ablation of metals that the target evaporation threshold approaches the threshold of plasma generation [10]. Thus, in pulsed laser deposition the erosion plume of metal is highly ionized [5,11]. In these highly ionized plume plasmas the Langmuir probe records the charged particles, which present a considerable fraction of the plume particles. Due to it’s small size, the probe provides a high spatial resolution. Therefore, the Langmuir probe was used to determine the plume electron temperature [7–9], the velocity distribution [6,9,12] and the energy spectrum of ions [5,13].This paper reports the investigation of the erosion plume by ablation from a tantalum target with an excimer laser radiation (308 nm) using the Langmuir probe with a special possibility to perform a time resolved measurement within the pulse duration. The spatial and temporal distribution of electron and ion probe currents was obtained. The ion velocity, the electron temperature, the ion density distribution in the plume and their dynamics has been measured.
2. Experiment The experiments were run in a vacuum chamber, which was evacuated by a diffusion pump down to pp105 mbar. The laser erosion plasma was generated by an XeCl excimer laser. The beam was focused by a two-lens objective and directed to the target at 501 to the normal. The target has a disc form. The thickness of the 99.9% tantalum foil is either 0.5 or 1 mm. The disc is fixed on an electric drive axis and can be rotated with a frequency of 10 Hz. The focus spot has an ellipsoidal shape with an area of 0.5 mm2 on the target. The target and the vacuum chamber were grounded. The Langmuir probe has a length of 5 mm and is made from a tungsten wire with 0.2 mm diameter placed in a ceramic tube. The probe was positioned parallel to the target surface and could be moved perpendicular in the emission direction between 3 and 160 mm. To study the particle scattering from the target surface, the probe was moved 3 mm steps towards the target and up to 45 mm steps away from the irradiated area. In all measurements, the probe axis was kept parallel to the target surface and the probe was held with its side parallel to the ablation area. The probe potential could be varied within 718 V. The controlled voltage at the
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probe was supplied by a bank of accumulators which was connected to the probe by one pole through a potentiometer, and was grounded by the other pole through a load resistor. The source of controlled voltage was bypassed by a 2.5 mF capacitor in order to stabilize the probe potential when the current is flowing. The value of the load resistance could be varied from 10 to 1000 O discretely. The signal from the load resistor was processed by a high-speed analogue to digital converter (ADC) and stored by a personal computer. The selected time resolution of the ADC was 0.1 ms. The measurement range of the ADC was 75 V. The time-of-flight of charged particles arriving at the probe was counted from the moment of the laser pulse generation recorded by a photodiode and sent to the trigger input of the ADC. The pulse energy was varied between 2 and 25 mJ and the pulse half-height duration was kept 20 ns.
3. Results and discussion The experiments delivered time-of-flight curves (TFC) of tantalum ions at different probe-to-target distances, probe potentials and at different energies of laser incident radiation. By changing the sign of the probe potential, we obtained TFC for both, ions and electrons. For characteristic points of the TFC volt–ampere probe curves were generated. Their electron fraction is serving to determine the electron temperature. The potential of ion current saturation can be found from the ion part of the curve. The time-of-flight of the leading ion groups was measured for several probe-totarget distances. The time-of-flight of the signal front edge was found to be proportional to the probe-to-target distance. The ion scatter velocity was determined from the probe curves as the ratio of the probe-to-target distance versus the time difference between the corresponding current and the laser pulse onset. For the leading group of Ta ions the scatter velocity does not depend on probe-totarget distance. We measured a value of n ¼ 1:9 104 m/s. This value of velocity satisfies the dependence nBðMÞ1=2 ; where M is the molecular weight of the element and it agrees with results from [14] for ions of barium, yttrium and copper. The timeof-flight curves of the tantalum ions following the leading group, however, differ considerably at different probe-to-target distances. The evolution of TFC of tantalum ions versus the distance is shown in Fig. 1 for 2 J/cm2 radiation flux. The signal amplitude is significantly reduced as the probe-to-target distance increases, so for convenience the TFC’s in Fig. 1 are not scaled. All the curves were measured for a time-of-flight range between 0 to 100 ms. The time displayed was reduced to 30 ms in order to get a higher resolution in Fig. 1. As can be seen from the curves in Fig. 1, the delay of the signal front edge is proportional to the probe-to-target distance. When the probe is moved to 10 mm away from the target (Fig. 1, curve 1), the time-of-flight signal presents a smooth curve, which has one maximum with a steep front edge and a more flattened back edge tail decreasing to zero within 15 ms. If we assume that this curve corresponds to some smooth distribution of ion velocities, then it should retain its smooth shape even at larger scatter time. However, the time-
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Fig. 1. Dependence of shape of probe time-to-flight curves on probe-to-target separation (1–10 mm, 2– 23 mm, 3–75 mm, 4–113 mm, 5–133 mm). For convenience, the curves are not scaled and the scanning time is shortened.
of-flight signal exhibits a more complicated behavior as the probe-to-target distance increases. With the probe moving away from the target, several maxima can be observed on the TFC, which become even more pronounced as the distance increases. As seen in Fig. 1, TFC has two maxima at the distances of 23 and 75 mm. At 113 mm three maxima can be distinguished, and at 133 mm four maxima are already observed. (In the time of flight range of 30 ms only the first three maxima are visible, the fourth maximum appears at higher delay time.) It turned out that the velocity distribution in the group of particles corresponding to each individual maximum on curves 4 and 5 can be described by a Maxwell distribution or by a shifted Maxwell distribution [13]. These distributions were used to describe the obtained experimental data. After mathematical processing of the experimental data, it turned out that each of the curves 1–5 (Fig. 1) presents a sum of four Maxwell curves with different positions of the maxima. This means, that at a negative probe potential four groups of positively charged particles can be observed in the plume using one set of TFC’s for all of the studied distances [13]. When the probe is located 23 mm from the target, the TFC is described by the curves with maxima at 1.1, 2.1, 4.0 and 16.7 ms, which correspond to particle velocities of about 21, 11, 6 and 1.4 km/s. The curve obtained at 113 mm in the region below 30 ms is well approximated by three curves, which are described by Maxwell distribution. To describe the curve in the region with a scatter time above 30 ms, an additionally shifted Maxwell distribution with relatively low mean velocity has to be used. The maxima of these distributions are at 5.3, 10.2, 17.7 and 49 ms with most probable velocities of about 21, 11, 6 and 2.5 km/s. It can be seen from the velocity values obtained for the distances 23 and 113 mm to the probe that velocities of the first three groups of ions are constant, and the velocity of the fourth group
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increases with the distance from the target. An increase in velocity of the fourth ion group is also confirmed by TFC approximation at all the studied probe-to-target distances (10–133 mm). Two-and three-mode distributions of ions by velocities were noted previously by other authors using multi-component targets of ion crystals [15] and HTSC ceramics [16]. In our earlier studies, we have shown that in ablation of Ta the ion probe current saturation is achieved at a probe potential of E10 V [12]. The probe current in the ion current saturation mode is found by Ii ¼ 0:5SenV; where S is the probe area, e the electron charge, n the ion density and V the ion velocity at the boundary of the probe collision-free layer [17]. The ion time-of-flight signal can be transformed to distribution of charge density in the flying plume nðtÞ ¼ IðtÞ=ð0:5SeVÞ ¼ IðtÞt=ð0:5SeLÞ; where t is the arrival time of recorded ions and L the probe-to-target distance. Using this formula, we evaluated the concentration of particles in the plume at different distances from the obtained TFC. It has been found that the maximum concentration of ions in the plume varies from 3.3 1013 cm3 at L ¼ 10 mm to 1.4 1011 cm3 at L ¼ 133 mm. The first group of ions moving with constant velocity are formed at the start of the ablation process in the mode of free collision-less release. We can show that for these ion groups the amplitude of signal reduces with the distance as L2 [12,13], corresponding to a spherical layer scattering. The more slower groups of ions are resulting from the release process characterized by the presence of a Knudsen layer [18]. The time of evaporation can by far exceed the laser pulse duration. Following the hydrodynamic scatter model [19], the density of neutral particles remains maximum at the target surface even though the cloud is scattered to a distance of several centimeters. In the plume, neutral particles have lower propagation velocities than charged particles [14]. There leading part is moving faster and the tail of the fast ion groups can be ionized as a result of resonance recharging. This will lead to an increase in the propagation velocity of the slow ion groups. A long tail in the probe TFC can also be generated during ionization, as a result of the neutral particle flow overlapping the back front of the electron cloud of the plumes charged part [8]. The scattering nature of the plume expansion is also demonstrated by the method of crossing radiation based on changing the flight direction of ions during the crossing of two non-parallel plumes. [20]. The time dependence of the electron probe current has the same complexity as the ion time-of-flight curves. Both are changing considerably with the probe-to-target distance. At long distances there are several maxima which are in their temporal position close to the maxima in the TFC’s of the ion current. A typical plot of an electron current curve with the probe located at the plume axis is shown in Fig. 2. The probe-to-target distance was 23 mm and the laser radiation flux used was 2 J/ cm2. At the start of scaling, a short pulse is seen on the electron current probe curves. This pulse is caused by the photoelectrons, releasing the target surface under laser
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Fig. 2. Curve of electron probe current; the probe is located at 23 mm along the normal to the target.
radiation even before the ablation is starting. A similar pulse of photoelectrons was recorded in ablation of dielectrics [8,15]. The emergence of hot electrons in plasma can be caused by the processes of multiphoton absorption by backward retarding absorption [2] and by build-up of Langmuir vibrations [21]. As a consequence of Coulomb interaction with external layers of the erosion plume, these electrons form a high-energy leading part of ions. Their energy is reaching several hundred eV, which is observed in the experiments from [5,12,13]. Although, due to low depth of external field penetration into the plume plasma, the portion of these ions in the whole mass of plume ions is rather small and makes a few per cent of the overall amount only [12,13]. The electron temperature Te was determined in the different plume parts corresponding to the maxima in time-of-flight curves. The electron temperature was calculated from the plot showing the dependence of the logarithm of the peak amplitude versus the probe potential. The slope of this plot in the region below the electron saturation current is equal to e=ðk Te Þ; where e is the electron charge and k the Boltzmann constant. Te is the electron temperature in eV. This follows directly from the probe equation: Ie pexpðeV=kTeÞ; where V ¼ Vp Vs is probe-to-space voltage, Vp the probe potential, Vs the plasma potential which can be different from zero by the value of ðk Te Þ=e and Ie is the probe current. The probe equation is valid when the positive voltage at the probe is above ðk Te Þ=e: Below this value the ion current is comparable with electron current and the relationship lnIe ðVÞ is shifted from a linear behavior to a non-linear one [17]. The value of Te was determined for the probe-to-target distances of 23, 75 and 133 mm using the three observed maxima. The results are presented in Table 1. The
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Table 1 Values of erosion plume electron temperature (eV) at different probe-to-target separations taken vertically Number of maximum on TFC
Distance from probe to target (mm) 23
1 2 3
75
6.4 5.7 4.8
6.25 5.9 5.0
133 5.8 5.3 4.7
temperature appears to be different at all three maxima lowering to the tail part of the plume. The following formula [22] Te ¼ 2:98104 A1=8 ðZ þ 1Þ5=8 Z 3=4 ðIlÞ1=2 t1=4 K; where A is the atomic weight of an ion, Z the average charge of ions in the cloud, I the laser radiation intensity (W/cm2), lðcmÞ the radiation wavelength, tðcÞ the laser pulse duration, gives an estimation of the electron temperature for tantalum atoms with single ionization (Z ¼ 1) to Te ¼ 4:8 eV, and for Z ¼ 1:5 (tantalum atoms with single and double ionization) to Te ¼ 5:6 eV, respectively. The data are in a good agreement with the experimental results (see Table 1). The amplitudes of the short pulses at the start of the scanning also depend on the probe potential. For them Te is measured to 11.5 eV at a distance of 75 mm and to 7.35 eV at a distance of 133 mm, respectively. The electron TFC’s were also studied when the probe was moved parallel to the target surface at a distance of 3 mm from the surface. In this case three maxima are observed on TFC too. A typical view of the electron current at the probe located near the target surface is given in Fig. 3. It can be seen that the signal shape is considerably different from the probe current curve located at the plume axis. The relationship of the amplitudes of different peaks is also different. The absence of a peak of photoelectrons is remarkable. The onset of the curve shows an individual peak which is weak pronounced or missing when the probe is positioned to the normal axis of the target spot. This peak is caused by the electrons scattered to the outside of the ion cloud, but kept by the Coulomb force. Due to a high concentration of charges in the cloud propagating along the normal, this peak is not detectable in the axis. In the direction parallel to the surface, the concentration of charges in the ion cloud is much lower (diagram of ion scatter Bcos6 y [12]), and the peak can be seen clearly. The dependencies of the signal amplitudes on the probe position were obtained by moving the probe from the normal of the plume to off-axis positions of 15, 30 and 45 mm. Table 2 presents the obtained values of Te : The first and second maximum of the TFC correspond to ion groups propagating at constant velocities. The electron temperature decreases with the distance from the ablation area, as well as in the direction of the plume tail. It should be noted that all three values of electron temperature in different parts of the plume along the surface are lower than those noted along the normal to the surface.
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Fig. 3. Curve of electron probe current; the probe is located at 3 mm from the target surface and moved to 30 mm from the plume axis (probe potential=+8 V, irradiation flux=2 J/cm2)
Table 2 Values of erosion plume electron temperature (eV) at different probe-to-target separations taken horizontally Number of maximum on TFC
Distance from probe to normal (mm) 15
1 2 3
4.2 4.0 3.3
30 3.9 3.65 2.6
45 2.90 1.85 1.80
4. Conclusion The velocity distribution of the ions in the erosion plume during ablation from a tantalum target in vacuum is not smooth in its nature. Individual ion groups can be identified and the velocity distribution of each individual group can be well described by the one-dimensional Maxwell distribution. The leading groups of ions in the plume propagate at constant velocities. Slow groups of ions are accelerated. This can be caused by a variation in the ion concentration as a result of chemi-ionization and recombination processes, as well as a variation of the ion scatter velocity due to second-kind collisions. The accelerated motion of the ions in slow mode shows the dominant role of electron and ion collisions processes with neutral particles in the tail part of the charged area in the plume. In the studied distances from the target the plasma does not yet reach the state of free flight condition. It has been shown that the plume temperature is not uniform. The electron temperature has a maximum
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value in front of the plume, while in the tail the electron temperature is reaching a minimum. Increasing the distance from the target the electron temperature is decreasing in all parts of the plume.
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