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Will High Power Impulse Magnetron Sputtering (HIPIMS) Be the Right Technique for Nb/Cu Coated 1.5 GHz Superconducting Cavities? Meng Yin1,2,3, O. Azzolini2, G. Keppel2, C. Pira2, A. Rossi2, V. Palmieri2. 1 Università di Padova, Padova, Italy. 2 INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy. 3 China Institute of Atomic Energy, Beijing, China. The main advantage is the control over energy and direction of the deposition material. In some sense, HIPIMS can be seen as a method that combines the advantages of conventional magnetron sputtering and arc evaporation. It produces highly ionized droplet free plasma. The plasma conditions in HIPIMS can be used for a resputtering of the growing film through the bombardment of the ions of the target material itself. The process result is dense, droplet free films. From the point of view of accelerator techniques, coatings produced with HIPIMS are highly interesting as it has been proven that the film properties are enhanced. The main problems with the current version of HIPIMS are reduction of deposition rate and the transition to an arc discharge. The deposition rate for the HIPIMS discharge is expected to be in the range 30-80 % compared to a conventional magnetron discharge, with the same average power. The other problem with HIPIMS is the arcing tendency on the cathode surface, especially for targets with low melting point (luckily this is not the case of Nb). INTRODUCTION In order to save building costs of superconducting resonant cavities for particle accelerators, the development of the sputtering technique for the deposition of niobium (Nb) superconducting films onto copper cavities has started in the 1980’s at CERN [1]. However high energy gradients remain unachievable when using thin films. The main limit of the Nb/Cu cavity is the Q-slope, that makes the Q factor decreasing at high accelerating fields. Exploring new sputtering configurations is one of the principal way to find a solution to the Q-slope problem. High Power Impulse Magnetron Sputtering (HIPIMS) is an evolution of the magnetron technique which relies on 100 μs high voltage pulses of the order of 1 kV compared to the 300 V of the standard DC magnetron process [2]. In this work, an R&D effort has been undertaken to study the HIPIMS, to improve it and understand the correlation between the sputtering parameters and the film morphology, the superconducting properties and the RF film quality. HIPIMS The common way of generating plasma for material processing by magnetron sputtering is to use two metal electrodes: a cathode and an anode (usually the grounded chamber walls) enclosed in an evacuated vacuum chamber. In order to generate the plasma a source of power is needed, such as a direct current (DC) power supply. Sputtering film technique has the following drawbacks: 1. the working gas is trapped in the film; it may cause intrinsic defects inside of the grain. The impurities of the working gas are not good for the thin film; 2. the deposition energy is low, which does not help to avoid columnar grains. So, we investigate HIPIMS to avoid these drawbacks. HIPIMS is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. HIPIMS utilizes extremely high power densities of the order of kWcm−2 in short pulses (impulses) of tens of microseconds at low duty cycle (on/off time ratio) of <10 %. A distinguishing feature of HIPIMS is its high degree of ionization of the sputtered metal and high rate of molecular gas dissociation [3]. In HIPIMS there are three principal parameters that drive the process: Voltage (V), Frequency (F) and Pulse Time (PT), as it can be seen in figure 1. LNL Annual Report Fig. 1. HIPIMS cycle. EXPERIMENTAL SYSTEM For our experiments we used a deposition system where the target is a niobium post magnetron with an external source of magnetic field, instead of a cylindrical magnetron like in the standard CERN configuration [4]. To make a deposition in this system requires two power supplies, one for the cathode and one to feed the coil. The compact vacuum system is capable of moving inside and outside a big magnetic coil. The cathode is composed by a Niobium post magnetron (magnetron diameter 43 mm, 103 Applied, General and Interdisciplinary Physics increases with Frequency. No correlation with PT. SUPERCONDUCTIVE PROPERTIES – RRR. The superconducting properties of the samples are not good, yet. Both the Tc and RRR are lower than needed; also our standard (coated in DC mode) presents RRR lower than 10, and Tc less than Tc of bulk Nb. This means that the problem is caused by the sputtering system and not by the technique. Despite this, we have found some interesting correlation between the superconductive films RRR and HIPIMS process V and F. RRR reaches a maximum for 800 V, then decreases at very low values for 1000 V. RRR decreases linearly with Frequency. This behavior is inverse with respect to grain dimension. We have not found a clear relationship between pulse time and RRR. wings diameter 65 mm) fixed in the centre of the cavity through a water cooled steel tube. For the deposition we used quartz samples fixed in a sample holder with the same shape of the cavity. In this first experiment we have deposited Nb thin film only in the centre of the cavity (the iris). The ultimate pressure is in the order of 10-9 mbar. Sputtering pressure is 1⋅10-2 mbar in Ar. The sketch and the photo of the used set-up are shown in figure 2. CONCLUSIONS a) At the end, we will try to answer to the difficult question presented in the title of this report. We did not arrive to high values of RRR for our samples: maximum values were around 9 (normally, cavities have RRR higher than 20). But also in DC mode we obtained the same results in the same system, so this implies that it is not a problem of technique, but there is something wrong in our deposition system. The electron bombardment, the uncooled cathode and the high temperature reached by the cavity during the process are the principal candidates to explain the film pollution. On the other hand, we found that HIMPIMS parameters (in particular voltage and frequency) influence the microstructure and the RRR of the coated films. This means that if we can find the right deposition parameters we would obtain a high quality superconductive film. At the moment the goal is to identify and solve the problem of pollution of the film, in order to obtain high RRR values. Only after having obtained high purity films, HIPIMS can be studied deeply and used for deposition of the entire surface of the cavity. After that, the second stage of optimization of the deposition will be possible in order to deposit a copper cavity and measure the features. b) Fig. 2. 3D image (a) and photo (b) of the deposition system. EXPERIMENTAL SYSTEM The Nb films was characterized by X-ray diffraction (XRD), Profilometer, measure of Residual Resistivity Ratio (RRR) and Critical Temperature (Tc). We correlate deposition rate, film microstructure and superconductive properties, to the three principal parameters of HIPIMS process: Voltage (V), Frequency (F) and Pulse Time (PT). DEPOSITION RATE. The results of our experiments is that the deposition rate increases with the increasing of values of each of these three parameters: voltage, frequency and pulse time increase. V, F and PT. In particular the relationship with F and PT are linear, whereas increase in deposition rate with V goes exponentially. RETICULAR PARAMETER. For voltage lower than 800 V, the reticular parameters (a) is lower than bulk Nb reticular parameter. When voltage is 1000 V, the reticular parameters (a) is higher than bulk Nb reticular parameter. This means that for high sputtering potential Nb films pass from a compressive stress to a tensile stress. GRAIN DIMENSION. A maximum in grain dimension when Voltage is 800 V. Grain dimension also exponentially LNL Annual Report [1] [2] [3] [4] 104 C. Benvenuti et al., Physica, C 316 (1999) 153-188. V. Kouznetsov, Swedish Patent 525231, (2005). V. Kouznetsov, Patent WO 2005/005684 A1, (2005). C. Benvenuti et al., Physica, C 351 (2001) 421. Applied, General and Interdisciplinary Physics