The present invention relates to a container for shielding an induction coil. It further relates to a physical vapor deposition system comprising such a container.

The induction heating of substances and more particular the heating of metals, such as the heating and melting of one or more metals or metal alloys in physical vapour deposition (PVD) processes, may give rise to undesirable sparking and arcing between equipment components with different potentials due to contamination of the induction coil by the (metal) vapour. This will result in short-circuit effects, such as sparking and arcing, and an instable deposition process. Without cleaning between each session these issues will become more severe.

The occurrence of sparking and arcing is dependent on the voltages and currents used in the induction heating, the geometry of the process parts, the process gas pressure and composition, and the partial gas pressures and compositions related to volatile system components. With sparking a disruptive discharge of electricity is meant that takes place between two places having a large potential difference. The spark is preceded by ionization of the path. With arcing a luminous electrical gas discharge is meant with high current density and low potential gradient. The ionization necessary to maintain the large current is provided mainly by the evaporation of some of the material of the equipment components between which the arcing takes place. With PVD both sparking and arcing may damage the equipment and will result in process stops and are thus disadvantageous for productivity.

Another factor that may give rise to power loss and damaging the induction coil is the undesired generation of plasma's. Plasma formation may occur between points where a potential difference is present, therewith creating an electrical field. The higher the field strength, the conductivity of the gas and the sensitivity of the gas to ionisation, the easier it is to form a plasma. The conductivity of the gas and its sensitivity to ionisation depend on gas composition, pressure of the gas and the distance between which the potential difference is present. The plasma may wear down the coil, increasing the risk of sparking, arcing and coating contamination.

One approach to minimise the risk for sparking of the induction coil is to position the crucible and induction coil outside the vacuum chamber and thereby separating it from vapours and process gasses. In this situation sparking will only occur at extremely high potential difference levels of many kilovolts, see also F. Paschen, 'Ueber die zum Funkenubergang in Luft, Wasserstoff und Kohlensaure bei verschiedenen Drucken erforderliche Potentialdifferenz, Annalen der Physik 273 (5): 69-75'. However, placing the induction coil with the crucible outside the vacuum chamber complicates the process considerably by introducing the requirement of vacuum seals that can cope with high temperature differences of the crucible compartment and the rest of the vacuum chamber.

Another approach, applying a protective coating directly onto the induction coil to prevent short-circuit effects is described in WO2014131519A1. However, once contaminated, replacing this type of shielding is a time consuming, difficult and expensive process.

It is therefore an object of the invention to provide a shielding of the induction coil that can easily be replaced, exchanged and cleaned. It is another object of the invention to limit the exposure of the induction coil from a process gas.

It is another object of the invention that the induction coil can be placed under medium vacuum conditions.

One or more of these objectives are reached with a container according to claim 1 - 10 and a physical vapor deposition system according to claim 11.

In the first aspect, there is provided a container for shielding an induction coil (10) from contaminants, comprising an inner cylinder (2) and an outer cylinder (3) around the inner cylinder, a top closure ring (1) on top of the cylinders (2, 3) and a bottom plate (5) at the bottom of the cylinders (2, 3), thereby defining an intermediate chamber (20) between the inner cylinder and outer cylinder for receiving an induction coil, and an inner chamber (30) within the inner cylinder and wherein the outer cylinder (3) is made of a single piece.

Advantageously, by locating the induction coil in the intermediate chamber between the inner and outer cylinder the induction coil is mechanically shielded from the contaminants in the inner chamber, as well as from the contaminants and process gasses outside the container. Hence, protecting the induction coil against contamination or the direct flow/influence of a process gas flow. If this shielding becomes contaminated by the metal vapour it does not affect the induction field created by the coil, in contrast to a coating/contamination on the coil itself, thereby preventing sparking and arching. It is an easy constructible solution and can be adjusted according to the application, such as the size of the induction coil and the temperature requirements. The container itself can be placed inside a vacuum chamber of the PVD process and does not result in any possible safety issues. Furthermore it was found by the inventors that the container effectively shields the coil from metal vapour contamination and the formation of an undesired plasma. By making the outer cylinder in a single piece, the vapour/gas leakage is minimized.

In an embodiment of the invention the inner chamber (30) is covered with a top inner ring. The top inner ring can serve as a connection with the depositing unit, e.g. a vapour deposition box. The top inner ring is preferably easy to process, stable at high temperature and non-conductive. Preferably, the top inner ring is made of a composite material, containing AI ₂ O ₃ , S1O ₂ and/or Fe ₂ C> ₃ .

In an embodiment of the invention the inner chamber (30) is adapted for receiving a crucible. The inner chamber provides a shielding of the induction coil from the metal vapour surrounding the coil and possible process gasses being used. By allowing the crucible containing the melt to be received in the inner chamber, the melt can easily be interchanged and/or replenished for example with a liquid feeding unit.

In an embodiment of the invention all components are substantially non-magnetic, displaying no or a small response to the applied electromagnetic field generated by the induction coil. By ensuring that all components of the container are substantially non magnetic, the components will not be impacted by the electromagnetic field around the coil. Furthermore, non-magnetic components are preferred as they will not influence the induction field of the coil. As such interference from the components of the container with the induction field and electromagnetic field is minimised. In an embodiment of the invention all components are non-conductive and preferably have a dielectric figure of at most 10 (DIN 53483). By ensuring that all components of the container are non-conductive, the components itself will not be heated. As such the induction is efficiently used to only heat the contents in the crucible..

In an embodiment of the invention the inner cylinder, top closure ring, bottom plate and/or top inner ring are temperature stable up to at least 600 °C. Depending on the application, most components of the container should be able to withstand high temperatures, for example those involved in the PVD process. Preferably, the top closure ring is made from a construction material on mica basis, such as Eritherm 600 M. This material has been selected for high temperature stability (up to 600 °C), non-conductivity, its process-ability and dimensional stability, such as a high compressive strength of at least 100 N/mm ² .

In an embodiment of the invention, the top inner ring is made from a material which is temperature stable up to 1000 °C, preferably up to 1200 °C, to withstand the elevated temperature of the vapor deposition box. Preferably the top inner ring is made from a Al ₂ 0 ₃ -Si0 ₂ -Fe ₂ C> ₃ based material, such as Imidesign® F -1220 °C. ln an embodiment of the invention, the inner cylinder is temperature stable up to 800 °C. As the inner cylinder will be close to the melt in the crucible, the inner cylinder has to withstand the highest temperature. The inner cylinder is therefore preferably made from a construction material on mica basis, such as Eritherm 800 M. This material has been selected for its very high temperature stability (up to 800C), non-conductivity, its process-ability and dimensional stability, such as a high compressive strength of at least 100 N/mm ² .

The outer cylinder is made of a single piece. By making the outer cylinder in a single piece, the vapour/gas leakage is minimized. Preferably, the outer cylinder is made from borosilicate glass. Borosilicate glass is easy to process, stable at high temperature (up to 170°C), non-conductive and dimensionally stable. Since it is positioned further away from the melt in the crucible, a lower temperature impact is expected with respect to the inner cylinder. Furthermore the borosilicate glass can be constructed in single piece and doesn’t require a weld, thereby minimizing the vapour/gas leakage. In an embodiment the bottom plate is from RVS, preferably RVS 304. This material has been selected for its very high temperature stability (up to 800 °C), non-conductivity, its process-ability and dimensional stability. The bottom plate may be made of two pieces, enabling a correct placement of the other components of the container.

In an embodiment of the invention the bottom plate comprises an opening within the inner chamber for connecting to a pressure means such as a vacuum pump.

In an embodiment of the invention the bottom plate comprises one or more openings within the intermediate chamber for connecting the induction coil to a power unit. Furthermore, The bottom place contains openings to enable to pump the air out of the shielding and/or to balance the vacuum of the PVD installation with the pressure inside the shielding.

In an embodiment of the invention, the pressure in the intermediate chamber is higher than in the inner chamber. A small pressure difference between the chambers may further reduce potential contamination in the intermediate chamber.

In the second aspect, there is provided a physical vapor deposition system comprising a vapor distribution box, a crucible, an induction coil and a container according to the embodiments described above in a vacuum chamber, wherein the crucible is placed within the inner cylinder in the inner chamber, wherein the induction coil is placed around the inner cylinder in the intermediate chamber to heat the crucible through the inner cylinder such that the inner cylinder is shielding the induction coil from hazardous contaminants present in the inner chamber and such that the outer cylinder is shielding the induction coil from hazardous contaminants and process gasses present in the vacuum chamber, and wherein the outer cylinder is made of a single piece. Such a physical vapor deposition system provides an adjustable, reversible system to protect the induction coil from the hazardous contaminants of the crucible as well as from the contaminants and process gasses in the vacuum chamber, thereby making the system more robust and easier to clean.

The invention is further explained by the non-limiting example shown in FIG. 1 - 3.

FIG. 1. Shows an exploded view of a container of the invention.

FIG. 2. Shows a cross section of the container of the invention in combination with a crucible and induction coil.

FIG. 3. Shows an embodiment of the physical vapor deposition system.

FIG. 1 shows an exploded view of a container according to the invention. The top closure ring (1) of the container is made from Eritherm 600 M, a non-conductive material with a high temperature stability (up to 600 °C). The top closure ring is made in two pieces (1, T) for ease of assembly and disassembly of the container. Since the top closure ring is positioned further away from the melt in the crucible, a lower temperature impact is expected with respect to the inner cylinder. The inner cylinder (2) is made from Eritherm 800 M, a non-conductive material with a very high temperature stability (up to 800 °C). Since it is positioned close to the melt in the crucible, it should be able to withstand high temperatures. The outer cylinder (3) is made from Borosilicate glass, a non-conductive material with a high temperature stability, which can be made from a single piece. As the outer cylinder (3) is made from a single piece, the vapour and gas leakage can be minimized.

The top inner ring (4) is from Al ₂ 0 ₃ -Si0 ₂ -Fe ₂ 0 ₃ compound, having high temperature stability and non-conductivity. Through the top inner ring, the vapour distribution box (60) can be connected to the crucible in the container. The bottom plate (5) is made from RVS 304, which has been selected for its high temperature stability, its process-ability and mechanical properties. The bottom plate supports the other parts of the coil shielding structure and is made from two pieces (5, 5’) enabling a correct placement of the other parts of the coil shielding. The bottom plate also contains openings to enable to pump the air out of the shielding and as an access point for cables, for example to connect the induction coil to a power unit.

FIG. 2 shows a cross-section of the container within a physical vapour deposition set up, wherein the container shields the induction coil (10). During the process a metal is being molten in a crucible (40) by induction heating. The induction heating is being performed by a water cooled induction coil around the crucible. The crucible is placed in the inner chamber within the inner cylinder (2) and the induction coil is placed in the intermediate chamber, between the inner cylinder (2) and the outer cylinder (3). As the induction coil is located in the intermediate chamber, it is shielded from the metal vapour and process gasses present in the inner chamber and/or the vacuum chamber. The melt in the crucible is typically heated up to 500 °C - 800 °C during the PVD process.

The assembled components mechanically shield the induction coil from surrounding vapours and gasses, and at the same time does not implode upon creating a vacuum in the coating deposition chamber, due to their dimensional stability and gas openings in the bottom plate. The vacuum is created through openings in the shielding not accessible by the metal vapour, for example in the bottom plate of the coil shielding structure. Several solutions for the pressure regulation inside the shielding might be used. For example, instead of using an opening in the shielding one can think of an active pressure-regulation system that tunes the pressure inside the shielding in agreement with the main vacuum chamber and thereby making it completely vapour and gas tight.

FIG. 3. Shows the container in the vacuum chamber. As can been seen in FIG. 3, the container is located within the vacuum chamber (100), at a pressure p. Furthermore a plasma sputtering unit (70) is present in the same vacuum chamber. It is clear that the container also shields the induction coil from potential contamination generated in the vacuum chamber.

Two PVD campaigns were compared wherein a steel strip was cleaned by a plasma sputtering unit and subsequently coated with zinc, without (A) and with (B) the container. In campaign B, the container was placed in the vacuum chamber and a crucible containing zinc was placed in the inner chamber and the induction coil was placed in the intermediate chamber of the container. The findings are summarised in the table below: As can be derived from the table, using the container according to the invention, the Zn contamination, cleaning frequency and short circuit stops were significantly improved, thereby increasing the duration of a campaign without cleaning by at least 5 times. In addition, the frequency of short circuit stops were significantly reduced. Furthermore, the type of cleaning for campaign B was much easier as the Zn could be easily wiped of the cylinders, whereas campaign A required a cleaning of the coil itself. Hence, the use of a container according to the invention in a PVD system improves the stability and run time of the coating campaigns and simplifies the cleaning of the system in between campaigns.