ALUMINUM nitride is of great interest due to its unique

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1 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 11, NOVEMBER HiPIMS Ion Energy Distribution Measurements in Reactive Mode Pierre-Yves Jouan, Laurent Le Brizoual, Mihaï Ganciu, Christophe Cardinaud, Sylvain Tricot, and Mohamed-Abdou Djouadi Abstract In this paper, mass spectrometry was used to measure the ion energy distributions of the main species during the sputtering of an aluminum target in a reactive Ar + N 2 mixture. Both conventional magnetron sputtering (dc) and high-power impulse magnetron sputtering (HiPIMS) were used. It appears that, in the HiPIMS, N + and Al + ions are significantly more energetic (up to 70 ev) than in the dc (< 40 ev). Furthermore, the HiPIMS Al + signal is two orders of magnitude greater than in the dc, and time-resolved measurements indicate that most of the ion flux hits the substrate during the OFF time of the impulse sequence. Index Terms Aluminum nitride, high-power impulse magnetron sputtering (HiPIMS), mass spectrometry. I. INTRODUCTION ALUMINUM nitride is of great interest due to its unique properties. Its direct wide optical band gap of 6.2 ev allows applications in deep ultraviolet optoelectronics [1], microelectronics, and photonics. Its piezoelectric behavior and its high acoustic wave velocity permit excellent acoustic wave applications (SAW and BAW) [2]. Moreover, its high thermal conductivity makes it attractive to be used as a thermal dissipation layer [3]. A wide range of parameters is available in magnetron sputtering to control the growth (gas pressure, nitrogen content, electrical power, target substrate distance, etc.). Those parameters have been largely studied but lead to a wide discrepancy of results concerning the AlN crystalline quality. Other experimental parameters like the magnetron configuration (balanced or unbalanced), the magnetic field distribution, or even the substrate bias have not been so widely investigated. In a previous study, it was shown that the magnetic field strength and distribution are of key importance in order to obtain c-axis-oriented AlN films in a large range of nitrogen partial pressure [4]. Substrate polarization is another experimental parameter that can drastically modify the film growth [5]. Setting the substrate to its self-biased potential can change the energy Manuscript received February 12, 2010; revised May 24, 2010 and August 25, 2010; accepted August 25, Date of current version November 10, This work was supported by the Agence Nationale de la Recherche through its Progremme Blanc under the HIPPOPP Projects P.-Y. Jouan, L. Le Brizoual, C. Cardinaud, S. Tricot, and M. A. Djouadi are with the Institut des Matériaux Jean Rouxel, UMR 6502, CNRS University of Nantes, Nantes, France ( abdou.djouadi@cnrs-imn.fr). M. Ganciu is with the Plasma Department, National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania, and also with the Laboratoire de Physique des Gaz et des Plasmas, Université Paris-Sud (Paris XI), Orsay, France. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TPS of ions bombarding the substrate. In fact, it is well known that AlN films exhibit a columnar-like domain structure with a very small column width when grown under low energetic species flux. When the growth is assisted by ions around 30 ev, the domain boundaries are annihilated, giving rise to wider domains and a film crystallinity improvement [5]. In a previous work [6], we have shown that a slight bias voltage ( 10 to 20) enables to obtain well-crystallized AlN films. These results were confirmed by Sanchez et al. [7] for AlN films deposited by a PECVD method. Nevertheless, due to the low ionization rate of the conventional magnetron systems, only few species (ions) are controlled, and therefore, the detrimental role of energetic neutrals cannot be avoided. High-power impulse magnetron sputtering (HiPIMS) overcomes this problem by ionizing such energetic neutrals and making them more controllable [8], [9]. Therefore, this paper focuses on the energy distribution of the plasma species in both dc and HiPIMS configurations using energyresolved mass spectrometry. Due to the HiPIMS process time dependence, we also used time-resolved measurements to characterize the pulsed plasma. II. EXPERIMENTAL The experiments were performed with an unbalanced magnetron for both the conventional and the HiPIMS discharges. A 50-mm-diameter and 6-mm-thick aluminum target was mounted on this cathode. The deposition of the AlN thin films is done in the nitrided mode of the target. Before any mass spectrometry measurement, the target is first cleaned in a 100% argon atmosphere at 0.27 Pa for 15 min to remove any previous nitride or oxide layer on the Al target. Nitrogen gas is then introduced (N 2 / (Ar + N 2 )=25% to 35%), and the pressure is adjusted down to 0.27 Pa. The target is nitrided for 15 min. The total gas mass flow rate is 40 sccm. The power applied to the target in the dc reactive magnetron sputtering is 150 W. In the HiPIMS, high voltage pulses are applied to the cathode by a custom-made power supply [10]. The applied voltage is 1 kv, the pulse width is 28 μs, and the frequency (repetition rate) is 1.6 khz. The ion energy distributions (IED) were measured with an energy-resolved mass spectrometer from Hiden (EQP 1000). The spectrometer is differentially pumped by a turbomolecular pump allowing a lower pressure limit of Pa. The mass spectrometer aperture is located in front of the target at a distance of mm. Fig. 1 represents a schematic of the /$ IEEE

2 3090 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 11, NOVEMBER 2010 Fig. 2. IEDs of a dc magnetron sputtering discharge at a distance of 30 mm for the grounded configuration. Fig. 1. Schematic of Hiden (EQP 1000) spectrometer. Distance between the target and the spectrometer head varies from 30 to 100 mm. New electrode with 380-μm slit was added [12]. used mass spectrometer. The principle of mass spectrometry for ions is simple; as species entering the first electrode of the spectrometer are already ionized, no further ionization step is needed. Therefore, these species are directly selected versus their mass/charge ratio to obtain the IEDs. A detailed description of the technique measurement for the HiPIMS discharges is given in [11]. The spectrometer is referenced to the ground (0 V). Nevertheless, in order to optimize the species collection, the extractor potential was varied. The variation had been taken into account in order to obtain the correct value of the IED for the ground reference. To retrieve the actual energy of species arriving at the floating substrate, a special setup inspired by Vlcek et al. [12] was used. The substrate is replaced by a conducting plate of 30 mm in diameter located in front of the spectrometer aperture and isolated from the ground potential. A 380-μm slit is located at the middle of this plate to allow plasma species to get through and reach the spectrometer aperture. When the plasma is ON, the plate reaches the floating potential (V f ). This potential is measured through a high-impedance oscilloscope. For the time-resolved measurements, the triggering signal was taken from the pulse generator of the power supply. The transit times in the apparatus were calculated for each ion and are expressed as follows [12]: t transit = S ext. 2M i e(e Vext )+ e V axis + S en. M i 2e Vaxis + S mass. M i + S det. 2M i 2e Vtr e Vdyn. In this formula, S ext, S en, S mass, and S det are the lengths of the extractor, the energy sector, the mass quadrupole, and the detector, respectively. V ext and V axis are the potentials applied to the extractor (0 V) and along the drift tube ( 40 V), respectively. V tr is the transit energy of ions in the drift tube (3 V) while V dyn is the voltage of the first dynode of the detector ( 4000 V). These values lead to a transit time of 89 μs forar +,74μs for N + 2,72μs foral+, and 52 μs forn +. The difference in the transit time for an ion at 1 and 100 ev ranges from 2.5 to 3.6 μs for the investigated ions. Therefore, the gate width for the acquisition was set to 4 μs to ensure that all the ions have been collected by the detector while keeping an adequate time resolution. III. RESULTS The IED of a dc magnetron sputtering discharge was performed in the following conditions: an applied power of 150 W and a pressure of 0.27 Pa in a mixture of 75% Ar + 25% N 2 gas. The spectrometer head was positioned in front of the target at a distance of 30 mm and was grounded. Fig. 2 presents the results for the 25% N % Ar discharge in the dc mode. Al +,N +,Ar +, and N + 2 species are observed, but no higher ionization states of aluminum or nitrogen were detected. The curves for the different species have a maximum around 5 ev, and the energy tail extends toward 20 ev for Al + and N + species, whereas it is below 15 ev for argon and nitrogen ions. Two ion populations are visible on the IED of Al + and N +. The low-energy peak corresponds to low-energy ions coming from the gas plasma (Ar and N 2 ). The second peak and the broad energy tail result from energetic sputtered neutrals ionized by the electron impact during their transport between the target and the mass spectrometer head. This second peak has already been observed during the sputtering of an Al target in an Ar + O 2 gas [13]. Fig. 2 clearly shows the two measured peaks on the 14-amu curve. In fact, the first peak comes from dissociated N 2 gas species (its energy is a representative to the plasma potential), and the second peak comes from ionized sputtered N atoms.

3 JOUAN et al.: HiPIMS ION ENERGY DISTRIBUTION MEASUREMENTS IN REACTIVE MODE 3091 Fig. 3. Al + IEDs for different values of the mass spectrometer extractor potential from 0 to 30 V (substrate distance is 10 cm). Fig. 4. Al + IEDs for grounded and floating substrates at 30 mm for a dc argon and nitrogen magnetron sputtering discharge (0.27 Pa). However, no extraction potential is applied to attract ions inside the spectrometer (grounded configuration). When the extractor is referenced to a negative potential, more ions are attracted inside the spectrometer. Low-energy ions which are not as directional as high-energetic ions are thus more sensitive to this negative potential. In order to check this, a dc magnetron argon discharge was analyzed, and the Al + energy distribution was recorded for different reference potential values. It appears that the low-energy part of the curve is enhanced, but the energetic tail remains unchanged (see Fig. 3). The low-energy species are effectively collected, and therefore, one may have the best representation of the true ion energy for a potential value of 30 V. Nevertheless, it is worth to notice that, as only argon is used, the Al + distribution is wider than that obtained with a mixture of Ar/N 2 (see Fig. 2). As V f ranges from 20 to 30 V in the dc magnetron sputtering, the reference potential of the spectrometer was set to 30 V to prevent low-energy ions from slowing down between the plate and the entrance aperture. The added plate with the slit was therefore grounded or let at a floating potential, and Fig. 4 shows the IED for Al + ions for the grounded and floating configurations. The two experiment setups have a different ion transmission inside the spectrometer, so the curves are normalized to their area to enable a direct comparison. The IED is broader, and the energetic tail slightly extends at higher energies in the floating case. In fact, when the substrate becomes floating from grounded, the difference between the plasma potential and the substrate potential increases from 2 to 30 V which leads to a 1.5 times thicker sheath [14]. Consequently, the ions are accelerated, gain more energy, and have more collisions in the sheath which leads to the IED broadening. These results concerning the IED may explain previous works where we have shown that a floating potential or a slight bias voltage ( 10 to 20) improves significantly the crystalline quality of AlN films [7], [8]. However, the low ionization degree of a conventional magnetron sputtering plasma implies that most of the incoming species at the substrate surface are neutrals. Among these species, energetic backscattered neutrals coming from the target could play a detrimental role during the growth and could not be controlled. Increasing the ionization degree of the plasma could thus help to get rid of these energetic neutrals, which can be done by replacing them with ions. The HiPIMS produces such a high amount of ions in the plasma due to an ionization rate of 10% to 70% for power densities around 1 3 kw/cm 2 [11]. Moreover, as the plasma production is significantly affected by a reactive gas and the changes of the plasma conditions induced by the nitrogen had to be taken into account, the plasma potential was measured at different nitrogen contents. It appears that no significant change was observed on the plasma potential when nitrogen is introduced in both the dc and HiPIMS processes. For the HiPIMS, the mean plasma potential value is between 3 and 4 ev for 35% nitrogen content but does not vary significantly when the percentage of nitrogen is changed. In our experimental conditions (pulsewidth = 28 μs, repetition rate = 1.6 khz, and applied voltage = 1 kv), the typical temporal current and voltage curve of the cathode is shown in Fig. 5(a). The discharge voltage does not drop from 0to 1000 V at t = 0 because there is a preionization voltage ( 300 V) in between the pulses [10]. At t = 7 μs, the discharge current rises, and the floating potential V f decreases down to 120 V. V f increases at 5 μs, exhibits a shoulder at t = 12 μs ( 70 V), then quickly increases again around 22 V until the end of the pulse, and finally reaches a value close to 0 V shortly after the pulse end. To investigate the origin of the V f curve shape, the floating potential for a HiPIMS discharge with an aluminum target under 100% argon gas and with a nitrided Al target under Ar + N 2 mixed gas (30% N 2 ) is displayed in Fig. 5(b). The first peak minimum in the absolute value is associated with the primary electrons generated at the beginning of the discharge. Gas ions are then attracted by this negative potential, and V f increases. However, as shown in Fig. 5(b), the shoulder that follows this first peak is enhanced in the reactive Ar + N 2 case. At t =12μs [see Fig. 5(a)], the cathode current is rising, and secondary electrons are emitted by the target and are responsible for the observed shoulders. The secondary electron emission yield is higher for AlN (0.3 electron/ion) than for a pure Al target (0.08 electron/ion) [15] and may lead to a more intense peak. One may suppose that the highly negative floating

4 3092 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 11, NOVEMBER 2010 Fig. 6. Time-integrated IEDs of a HiPIMS discharge at a distance of 30 mm for a floating substrate (pressure of 0.4 Pa, a pulsewidth of 28 μs, a repetition rate of 1.6 khz, and applied voltage = 1 kv). Fig. 5. (a) Waveforms of the cathode voltage and current in HiPIMS for a pressure of 0.4 Pa. Floating potential of a substrate at 30 mm is also shown. (b) Floating potential of the substrate under 100% Ar and under 70% Ar + 30% N 2. potential is sustained for a longer time due to the secondary electron emission of the Al nitrided target even if it begins to be more insulating at the surface. It is obvious that these measurements which used a simple plate of 30 mm in diameter are not exactly the measure of the plasma floating potential, but they are a representative of the substrate time-dependent potential. In the HiPIMS discharge conditions (pulsewidth = 28 μs, repetition rate = 1.6 khz, and applied voltage = 1 kv), for a floating substrate located at 30 mm from the target, ionized Al +,N +,N + 2, and Ar+ were detected (see Fig. 6). Higher ionization state Al ++ and Ar ++ (not shown) were also observed with a maximum signal of 10 4 counts/s. According to the time-integrated IED displayed in Fig. 6, the overall signal of each ion investigated is higher than that observed in the dc mode, particularly at a high energy. Moreover, the target species (Al + and N + ) have a larger energy tail of up to 70 ev for aluminum ions and up to 50 ev for N + ions. The ratio of the Al + /Ar + signal reaches 0.71, accounting for the high ionization rate in the HiPIMS, whereas it is lower in the dc mode. As in the dc mode, the energy spectra are composed of a low-energy peak corresponding to thermalized ions in the plasma and of a higher energy tail. The highly negative potential applied to the cathode ( 1 kv) is the primary source of energy for the plasma species. The high-energy tail originates from sputtered neutrals which are ionized on their way to the substrate. These postionized neutrals are sputtered by Ar + ions Fig. 7. Relative composition of the detected species (Al, Ar, N, and N 2 ions) in the HiPIMS plasma versus the delay time (zero is set at the beginning of the pulse). but also by the reflection of ionized target species attracted by the highly negative target potential. Concerning the measurements at the floating potential, for the HiPIMS experiments, the floating substrate, which is referenced to the ground, varies during the pulse. At the beginning (7 10 μs), its value is relatively high ( 120 V); after that, the value remains constant around 20 V, and then, it reaches zero. Nevertheless, it is worth to notice that ions are detected after at least 20 μs, and therefore, the floating potential seen by ions is around 20 V or even less. Moreover, the integrated measurements are done during a larger time than the pulse duration (during the whole period and even the OFF time). In this case, for both the integrated and time-resolved measurements, the floating potential is mainly around zero. The time-resolved measurements were performed to investigate the plasma composition during the impulse sequence. Fig. 7 show the ion signal (energy-integrated) versus time for Al +,N +,N + 2, and Ar+. As it is shown, the HiPIMS discharge is dominated by Al + ions. This is the opposite in the dc where Ar + is the dominant ion. However, in the HiPIMS, a high power density at the target causes the gas to undergo very strong rarefaction and could explain the lower detected signal of Ar +.

5 JOUAN et al.: HiPIMS ION ENERGY DISTRIBUTION MEASUREMENTS IN REACTIVE MODE 3093 target is nitrided due to the high secondary electron emission yield of AlN. Moreover, for the HiPIMS, the mass spectrometry measurements show that the main ionic species detected is Al + whose intensity is 150 times higher than that in the dc mode. The time-resolved studies of the HiPIMS plasma show that most of the ionic flux reaches the substrate during the pulse OFF time. In the case of the AlN HIPIMS sputtering plasma, the large fraction of energetic ions coming from the target have been used for momentum transfer, and better crystallized AlN films were obtained. Fig. 8. XRD rocking curve for AlN films deposited by dc and HiPIMS magnetron sputtering. For each ion, the signal increases quickly, shortly after the end of the pulse (28 μs), and is the maximum for a delay of 50 μs. Vasina et al. [16] and Konstantinidis et al. [17] which had also conducted HiPIMS experiments with short pulse duration show clearly that the collected ion current at the substrate is delayed from the beginning of the pulse. This delay can be explained by geometrical consideration, magnetic field configuration, etc. In our experiments, one has to take into account that the target mass-spectrometer first electrode distance is 35 mm, which may explain the delay, and the observed maximum could correspond to the end of the pulse. Therefore, the diffusion rate of the plasma can be estimated to 0.6 cm μs 1, which is in accordance with previously published data [18] where the diffusion is dominated by elastic scattering. One may conclude that, for the short pulse duration, most of the ion flux reaches the substrate at the end of the pulse and during the OFF time of the impulse sequence. The IED measurements in the HiPIMS configuration show that ions could be very energetic. However, despite these large measured energies, AlN films have better crystallinity in the HiPIMS than in the dc mode. The rocking curve measurements (Fig. 8) of the (0002) diffraction peak indicate an FWHM of 1.6 in the dc reactive magnetron sputtering (150 W under 2 mtorr and 65% Ar + 35% N 2 gas mixture) and an FWHM of 1.1 in the HiPIMS (220 W under 3 mtorr and 65% Ar + 35% N 2 gas mixture). Even if the ion species in the HiPIMS discharge are more energetic than in the dc, the momentum transfer seems to be lower than that of the energetic backscattered neutrals. The experimental parameters of the HiPIMS have to be investigated now in order to limit the energy tail distribution and, therefore, the momentum transfer to further optimize the AlN thin film crystalline quality. IV. CONCLUSION The HiPIMS plasma IED has been measured and has shown to be broad and to have a significantly more energetic tail compared to a conventional dc discharge. 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