翻译(非平衡磁控溅射阴极的优化研究)

Studies on the optimisation of unbalanced magnetron sputtering cathodes AbstractThe optimisation is reported on the design of unbalanced magnetron (UBM) sputtering cathodes. For the study, a planar circular cathode backed by a double-coil electromagnet (compatible for a 100mm diameter target) was developed. The variation of the structure and strength of the magnetic field in front of the target was investigated for different current combinations in the electromagnetic coils, and its effect on the sputtering process was analysed. The observations on the magnetic field geometry revealed some interesting features, such as the balancing point of the fields along the axis (null-point), and the zero axial region over the target surface (Bz=0 ring). The positions of both could be controlled by adjusting the ratio of the electric current in the coils. The magnetic field null-point could be used as a reference for the region of homogeneous film growth. The Bz=0 ring was the location where the glow discharge concentrated (or where the maximum target erosion occurred). The diameter of the ring determined the area covered by the discharge and thus the sputtering e¦ciency. The optimum substrate position can be Þxed according to the position of the null-point and optimisation of sputtering can be achieved by adjusting the diameter of the Bz=0 ring. The results of this study should be helpful in the designing of an ideal UBM using permanent magnets as well as electromagnets.1. IntroductionThe unbalanced magnetron (UBM) has been the focus of interest for the PVDcommunity for over a decade [1]. While the conventional magnetron sputtering has already made its impact on the thin-film coating industry, the” unbalanced” version proves more advantageous because it combines high deposition rates and low-energy ion bombardment at the substrate. The ion bombardment is achieved by modifying the conventional magnetron (CM) with an additional magnetic field, which deviates the trajectories of secondary electrons from the glow discharge region concentrated near the target, towards the substrate. This would create an ionised region near the substrate and hence causes ion bombardment on the growing film, thereby modifying the properties of the coating [1Ð4].The first ever detailed study of the UBM, is due to Window and Savvides [5]. They analysed dfferent modes of unbalancing the magnetron using permanent magnets and electromagnets, and proved that a magnetic field which converges towards the substrate (Type II, as they designated) gives the desired unbalancing effect. This field distribution was found to maintain considerable ion bombardment over the substrate at low energies, with a high ratio of ion-to-deposited atom.Subsequently, a host of cathode designs appeared in different sizes and shapes, most of them custom-built for laboratory research [1Ð4]. Twin- and multi-cathode configurations were developed in due course, to coat industrial components and a few of such coating systems have appeared in the commercial market also. An excellent review in this regard can be found in Ref. [1].Despite these developments, the understanding about the effects of unbalancing the magnetron on the properties of the Þlms grown, is far from complete. The optimisation studies and the characterisation of the UBM cathodes are rarely reported [6, 7]. Some factors yet remain uninvestigated, like the change in the structure and strength of the magnetic field during unbalancing, and the positioning of the substrates in the unbalancedÞeld, which are crucial in practical considerations. This paper presents preliminary attempts done to have a more complete understanding of the UBM process, namely the effciency optimisation of the cathode.For the study, we designed and fabricated a planar circular magnetron, driven by a double-coil electromagnet. The electromagnet version of the UBM has an advantage that it facilitates the manipulation of the field distribution through the adjustment of currents in the respective coils [4]. A systematic study on the design of the UBM, the principal features of the unbalanced magnetic field and their impact on the optimisation of the sputtering process are discussed in this paper.2. Design of the magnetronThe electromagnet-driven sputtering cathode poses several practical constraints in designing [1, 4]. Electromagnets are normally bulky and heavy, and the coils give rise to outgassing problems inside the vacuum chamber. Therefore, mounting of the magnet outside the chamber is advisable. At the same time, the poles should be as close as possible to the target, to utilise the magnetic field effciently to the maximum extent. The magnet should be electrically isolated from the cathode, to ensure operational safety.Moreover, water flow is to be incorporated for the cooling of the cathode. The present magnetron cathode has been developed considering all these constraints.A schematic of the cathode assembly is shown in Fig. The cathode is made in the form of a flange (200 mm dia,6 mm thick copper disc) which incorporates a target holder (120 mm dia, 6mm thick copper disc). Both are brazed concentrically after machining water channel on the joining faces. The water feeders (6 mm dia copper tubes) are brazed onto the backside of the flange.The electromagnet is made with planar circular pole faces, so that a cylindrically symmetric magnetic field is created in front of the target. It hasa central pole (20 mm dia cylinder) and an annular outer pole (100 mm ID,120 mm OD), both 130 mm long, connected by a backplate. The yoke is fabricated out of a single soft-iron piece, avoiding joints to minimise field losses. Insulated copper wire (18 SWG) is wound on respective poles, to make inner and outer coils. The inner coil has 1880 turns and the outer coil, 1000. As the coils can carry the operating currents (up to 3 A) without overheating, no special cooling is provided for the magnet.The electromagnet is placed concentrically behind the water-cooled cathode. The magnet is isolated from the cathode using PTFE insulation. The assembly is mounted in sputter-down configuration on the top plate of the chamber, with the necessary vacuum seals and electrical isolation.3. CharacterisationA knowledge of the magnitude and direction of the magnetic field near the cathode is essential while optimising the parameters for sputtering. However, this is not easy as the field varies continuously from point to point.The reported data on the magnetic field geometry of the magnetrons are based on computer simulation curve fi tting techniques [6, 8Ð10]. In the present case we made use of the compass needle, the classical tool for plotting the magnetic field, and plotted the actual field. The field line plots corresponding to the inner and outer coils are shown in Fig. 2a and b, respectively. Any variation in the coil currents results in a change in the field strength however, without altering the general nature of the field distribution. The plot for the combination field of both the coils is shown in Fig. 3. When both the coils are powered with opposite polarity, most of the field lines emanating from the central pole converge to the outer pole.A tunnel of field lines is formed in between the circular poles. As the outer pole becomes stronger, additional field lines from the periphery bend over the tunnelled region and run downwards parallel to the axis forming a typical Type II unbalanced field [5]. This field structure is observed to be highly sensitive to the ratio of currents in the coils which is in conformity with the earlier observations [6].The field strength measurements are done with the help of a Gaussmeter using transverse-type Hall probe. This seems more straightforward than the computational methods [8, 9], and of great help in the real-time optimization of the magnetron. As the field is cylindrically symmetrical, the radial and axial components (Br and Bz, respectively) would suffice to give necessary information about it. The vector sum of these components at any point gives the total B at that point.The field values have been measured in front of the target. When the inner coil alone is operating, the axial field maximum (Bz on the magnet axis) at the target surface is 350Ð680G, and the radial field maximum (Br along the target surface) is 150Ð270G for 1Ð3A currents respectively (Fig. 4a and b). The outer coil, due to its larger diameter, contributes only a fraction of these values (20Ð80G in the centralregion, for 1Ð3A currents).This additional field, however, is sufficient to create the unbalance.The field strength distribution in front of the magnet (total B) is determined graphically, after measuring the Br and the Bz values at each discrete point and taking their vector sum. The total field at 1.5A current in each coil, is represented as an isogram in Fig. 5.It is known that the discharge characteristics are controlled by the Br component in front of the target surface.The glow discharge will concentrate in the region where the Br components orient parallel to the target, in other words, where the Bz components become zero [8]. It is the locus of Bz =0 points which determine the erosion track. Therefore, an attempt has been made to study the variation in the Bz components along the target surface and the effect of outer coil current on it. The plot for fixed inner coil current of 1.5A at different outer coil currents is shown in Fig. 6. It can be observed that the Bz=0 ring (the radius of which is indicated by the cross-over point on the x-axis) shrinks as the outer field becomes stronger.4. Discussionhe studies on the field distribution gives an insight into the creation of a Type II unbalanced field in front of the target. It is to be noted that a null-point forms on the magnet axis where the field lines meet and balance (Fig.3). The distance of this point from the magnet plane changes according to the ratio of currents in the coils. The null-point appears to be an interesting feature, which was not analysed in the earlier UBM studies. The Þeld structure above and below this point is totally di¤erent, and hence the same will be the case with the ion (or electron) bombardment during sputtering.A comparison of Figs. 3 and 5 will give a clear idea about the magnitude and direction of the unbalanced field. The value of totalB decreases along the axis and becomes zero at the null-point. The zero-B region at the null-point spreads across (at about 5 cm dia) as a cup shaped surface, parallel to the target plane. The field does increase thereafter, but the increment is not substantial. The total B remains in between 10 and 20G (which is the field reported for most of the UBM cathodes [11]) to a distance of several centimeters beyond null-point. The field in the region below the null-point is almost constant in magnitude and direction throughout a large volume, as evident from Fig. 5. This ensures a uniform ion bombardment and a homogeneous film growth, even on a considerably large,three-dimensional substrate.These studies indicate the methodology of optimising the UBM sputtering, for an ideal combination of high deposition rate and ion bombardment on the substrate.4.1. Optimisation of the substrate positionThe characteristics of the unbalanced field (Figs. 3 and 5) imply that the substrate positioning is very crucial in UBM sputtering. To avail the true advantage of the unbalancing (i.e. to have su¦cient ion bombardment), the substrate should be kept below the null-point. The immediate vicinity of the null-point is not advisable because of low values of Þe ld strength (Fig. 5). Keeping substrate far below, however, will result in the reduction of deposition rate. The optimum position will be in between 1 and 2 cm below the null-point.4.2. Optimisation of the sputteringSputtering experiments done (with OFHC copper target in argon) at various combinations of the magnetic fields, showed that the sputtering parameters are strong functions of the field structure in front of the target. The ratio of the inner and the outer coil currents affects the field line shape of the tunnelled region, which in turn, decides the area of the target covered by the glow discharge. Most effcient sputtering (for a given magnetic field strength and pressure) occurs when the glow discharge covers the maximum target area. This condition is satis fied only when the Bz=0 ring takes a position where the circular area inner to it equals the annular area outside. In the case of a 100mm target, this position corresponds to the Bz=0 ring diameter of 70mm. Circular planar UBMs of any size can be optimised in a similar way, by manipulating the currents in the magnetic coils.The optimisation is tested in actual sputtering conditions by observing the discharge current, which is an indication of efficiency of ionisation at any given voltage. Fig. 7 shows the variation of the discharge current as a function of outer coil current, for different inner coil currents at a fixed discharge voltage of 500V. For a given inner field, the discharge current goes to a maximum at a particular value of outer field. The peak values of discharge current depict the case of optimum sputtering with highest sputtering rate, obtainable at the magnetic field and pressure values. The field ratios at each peak correspond to a Bz=0 ring of diameter 70mm, confirming the geometrical condition for glow discharge coverage.4.3. The erosion zoneObservations on the erosion zone at the optimised conditions is done by sputtering an OFHC target in argon at 0.1Pa. The erosion zone practically covered the whole target area. The region immediate to the Bz=0 ring eroded more, leaving a broad X-shaped groove. After eroding the target to a few millimetres depth, the target utilisation is calculated. The volumetric utilisation is found to be 42%. This is obviously a better value when compared with the utilisation in the range 25-30% reported for planar circular magnetron cathodes [1, 12].5. ConclusionA systematic analysis of the magnetic field in front of a planar circular magnetron cathode is carried out in order to have an optimised "unbalanced" sputtering. An unbalanced magnetron cathode based on double-coil electromagnet has been designed and fabricated for the purpose. The variations in the structure and strength of the field are recorded at different combinations of currents in the coils of the magnet, and their effect on the sputtering is observed. The investigation indicated the methodology of unbalancing and optimising the sputtering cathode design.A typical Type II unbalanced field can be obtained by giving additional field at the periphery of the cathode. A “null-point” occurs on the axis where the fields due to the coils balance. This point can be taken as a reference to decide the substrate position, so that the real unbalance effect can be availed.The glow discharge will be concentrated on the target surface along the circle where the Br components fall parallel (or where the Bz components go to zero). The optimum sputtering occurs when this ring occupies such a position that the glow discharge covers the maximum area on the target. This condition can be achieved by carefully matching the ratio of the magnetic fields and identified practically by observing the Bz=0 position. The optimized Bz=0 ring position will give the largest discharge current during sputtering, for given values of cathode voltage, magnetic field strength and pressure. This optimised condition also gives the highest sputtering rate and the broadest erosion zone. This study shows the possibility of optimising a circular planar UBM cathode during the design stage itself by observing the Bz=0 ring diameter, without going for actual sputtering. The procedure of optimisation (i.e.finding the null-point and identifying the Bz=0 ring) is rather simple that it can be done using a Hall probe. The same methodology is as well applicable to UBM based on permanent magnet. The field structure manipulation in this case can be done through the adjustment of the magnet position or by employing appropriately designed pole pieces.AcknowledgementsThe financial aid for the work under the Indo-US(ONR Grant No. N00014-95-/-1282) fund is gratefully acknowledged. The authors thank the other colleagues in the laboratory for their helpful discussions.References[1] Rohde SL. In: Francombe MH, Vossen JL, editors. Physics ofthin Þlms New York: Academic Press. 1994;18.[2] Window B. Surf Coat Technol 1995;71:93.[3] Mohan Rao G, Mohan S. Ionics 1995;12:53.[4] Sproul WD. Surf Coat Technol 1991;49:284.[5] Window B, Savvides N. J Vac Sci Technol 1986;A4:196.[6] Kadlec S, Musil J. J Vac Sci Technol 1995;A13:389.[7] Muralidhar GK, Musil J, Kadlec S. Rev Sci Instr 1995;66:4961.[8] Perlov CM, Brauer JR. IEEE Trans Magn 1986;MAG-22:831.[9] Wong MS, Sproul WD, Rohde SL. Surf Coat Technol 1991;49:121.[10] Murphy MJ, Cameron DC, Karim MZ, Hashmi MSJ. Surf CoatTechnol 1993;57:1.[11] Musil J, Kadlec S, Munz WD. J Vac Sci Technol 1991;A9:1171.[12] Schiller S, Heisig U, Goedicke K. Thin Solid Films 1977;40:327.非平衡磁控溅射阴极的优化研究摘要报道了非平衡磁控溅射阴极的运转设计。