Control device and method for determining operating point of a reactive hipims process, and method for controlling such a process

Abstract

A control device (100) and method for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process are described. The method comprises a step of monitoring (S101) target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time. The method further comprises a step of, based on the determined discharge current waveform, determining (S102) a value of a control parameter indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse. Moreover, a method for controlling a reactive HiPIMS process, as well as a HiPIMS system (1) comprising the control device (100), are disclosed.

Description

[0001] Control device and method for determining operating point of a reactive HiPIMS process, and method for controlling such a process

[0002] TECHNICAL FIELD

[0003] The present disclosure relates in general to a method for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process. The present disclosure further relates in general to a control device configured to determine a prevailing operating point a reactive HiPIMS process. Moreover, the present disclosure relates in general to a method for controlling a reactive HiPIMS process.

[0004] The present disclosure also relates in general to a computer program as well as a computer-readable medium. Furthermore, the present disclosure relates in general to a high-power impulse magnetron sputtering system.

[0005] BACKGROUND

[0006] Reactive sputtering is a deposition technique that may be used in many industrial processes for the purpose of depositing compound thin films or coatings, which may be designed to optimize various surface properties. Examples of such thin films or coatings include optical coatings, permeation barriers, hard coatings, etc. These compound thin films or coatings are formed of atoms (or clusters thereof) sputtered from a solid target, and a reactive gas. The sputtered matter is deposited on the surface of a substrate where it reacts with the reactive gas, thereby forming the compound thin film or coating. The reactive gas may for example be O2 or N2. The solid target typically consists of a metal, such as Al, Cr, Ti; but could alternatively consist of e.g., an alloy or a non-metal, such as C or B. In the following, reactive sputtering will be described only with reference to a metal target, and hence also sputtered metal atoms, for sake of brevity and clarity. It should however be noted that the same phenomenon and principles also apply in case the target consists of a non-metal or an alloy. Moreover, more than one target may sometimes be utilized during reactive sputtering, in which case the targets e.g., may consist of different metals.

[0007] Reactive sputtering is an attractive process because a wide range of compounds can be achieved from a metal target by addition of an appropriate reactive gas to the noble working gas environment in the deposition chamber. In principle, control of the compound stoichiometry in the deposited thin film or coating may be performed through control of the ratio between flux of sputtered atoms / clusters and reactive gas atoms / molecules during deposition.

[0008] However, deposition of these industrially relevant thin films or coatings by conventional reactive sputtering methods, such as reactive direct current magnetron sputtering (R-DCMS), is in general challenging. This is mainly due to the reactive gas often forming compounds on the solid surface of the target, a phenomenon commonly referred to as target poisoning. This in turn leads to considerably lower deposition rates than obtainable from an elemental target surface. When a compound layer is formed on the sputter target surface, the sputter rate of the target changes, which in turn alters the ratio between flux of sputtered atoms / clusters and reactive gas atoms / molecules to the substrate surface even without a change in external parameters.

[0009] Figure 1 represents a schematic sketch of mass deposition rate as a function of reactive gas flow in a reactive magnetron sputtering process, such as R-DCMS, and where the three main regions of operation of the process are shown. More specifically, the three main regions comprise a metal mode MM region, a transitional mode TM region, and a compound mode CM region. At low reactive gas flows, a metal mode MM is maintained with predominantly metallic film deposited at a high rate. During metal mode MM, substantially all of the supplied reactive gas is incorporated into the deposited metal and the surface of the target is free from compound. Furthermore, in the metal mode MM, the addition of the reactive gas does not significantly affect the sputter process, except that the mass deposition rate may increase with increasing reactive gas flow as the mass of the gas atoms adds to that of the deposited metal. With further increase in the reactive gas flow, the mass deposition rate drops abruptly as a transition from metal mode MM to compound mode CM takes place. This is illustrated in the figure by the jump in mass deposition rate from point A to point B. In the compound mode CM, all the sputtered metal is converted into compound material, and the excess reactive gas reacts with the sputter target surface leading to the above described target poisoning. Films or coatings deposited in the compound mode CM are typically stoichiometric, and further addition of reactive gas will not have any significant effect (except an increase in partial pressure of the reactive gas). The compound mode CM will be maintained even if the gas flow is reduced (below the illustrated point B) until the point C illustrated in the figure is reached. At point C, the reactive gas flow is no longer sufficient to maintain a compound layer on the target surface, and a transition to the metal mode MM occurs, said transition occurring between points C and D. Thus, it can be seen from the figure that the process exhibits hysteresis resulting in a transition mode TM within the area defined by points A-B-C-D. For the same reactive gas flow, the process may be in the metal mode MM (cf. points D to A) or in the compound mode CM (cf. points C to B) depending on the process history. Moreover, a disturbance in the process can cause a transition from one mode to another mode.

[0010] Still, it is desirable to be able to perform reactive sputtering within the transitional mode. This because it enables a high mass deposition rate, which is important from a cost perspective, as well as offers the possibility of tailoring the stoichiometry of the deposited compound, which in turn affects the resulting properties of the thin film or coating. For R-DCMS, the transitional mode is however frequently unstable, and considerable hysteresis in the process parameters (such as target voltage, deposition rate, and reactive gas partial pressure) is very common. The hysteresis can be somewhat counteracted by various feedback systems for controlling the target poisoning, but current solutions are expensive and introduce additional complexity to the overall system. Furthermore, the considerable hysteresis in process parameters means that controlling the process between different operating points may be quite slow.

[0011] One new and promising deposition method, which may also be used for reactive sputtering, is a pulsed plasma sputtering technique called high-power impulse magnetron sputtering (HiPIMS) (sometimes also referred to as high-power pulse magnetron sputtering, HPPMS). HiPIMS utilizes very high power densities of the order of kW / cm2, averaged over the active cathode surface, in short pulses at low duty cycle, usually <10%. A major difference between HiPIMS and conventional sputtering techniques is that HiPIMS has the added advantage of providing substantial positive ion formation of the sputtered material. This in turn results in improved quality of the film / substrate interface and increased possibilities for tailoring coatings, denser coatings, and better deposition on complex geometries (i.e. non-flat substrate surfaces). More importantly, it has been found that HiPIMS has the possibility to considerably reduce hysteresis in the process parameters. Thus, HiPIMS fulfils the preconditions to significantly facilitate obtaining stable operating conditions in the desired transitional mode, and consequently provides possibilities for dramatically increased deposition rates for compound thin films or coatings, in addition to the above mentioned inherent advantages of said process. Therefore, if properly controlled, HiPIMS may have a great impact on the way compound thin films or coatings will be deposited in the future.

[0012] US 2010 / 0282598 Al discloses an example of a method for controlling a reactive high-power pulsed sputter process. According to said method, a controlled variable is measured, and an adjustable variable is modified based on the measured controlled variable in order to adjust the controlled variable to a predetermined setting value. Furthermore, T. Shimizu et al., "Process stabilization by peak target current regulation in reactive high-power impulse magnetron sputtering of hafnium nitride”, J. Phys. D: Appl. Phys. 49 (2016) 065202, discloses a method for stabilizing a sputtering process in the transition zone during reactive high-power impulse magnetron sputtering (HiPIMS). To stabilize the process conditions at a given set point, a feedback control system was implemented that automatically regulates the pulse frequency, and thereby the average sputtering power, to maintain a constant maximum discharge current. To explore the feasibility of the peak target current regulation technology, the waveforms of the realtime pulse current was monitored to study the process characteristics during reactive HiPIMS of Hf in different Ar / Nj gas mixtures and for different pulse frequencies.

[0013] SUMMARY

[0014] The object of the present invention is to improve accuracy when determining a prevailing operating point of a reactive high-power impulse magnetron sputtering process to thereby e.g., enable improved possibilities for controlling said process to obtain and / or maintain a desired operating point.

[0015] The object is achieved by the subject-matter of the appended independent claim(s).

[0016] The present disclosure provides a method, performed by a control device, for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process. The method comprises a step of monitoring target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time. The method further comprises a step of, based on the determined discharge current waveform, determining a value of a control parameter indicative of an operating point of the process. The control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

[0017] By means of the herein described method, the accuracy when determining a prevailing operating point is improved. This is achieved through, not simply using a global maximum target current (i.e. peak target current), but taking further account to the characteristics of the discharge current waveform. More specifically, the herein described method utilizes a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse. Thereby, a control parameter which monotonically increases or decreases may be achieved, which in turn is important for enabling a prevailing operating point to be accurately determined and allowing appropriate control of the reactive HiPIMS process to obtain and / or maintain a desired operating point.

[0018] The herein described method considerably extends the number of different reactive HiPIMS processes for which a prevailing operating point may be accurately determined, which in turn significantly improves the possibilities for controlling such processes so as to maintain a desired operating point in the transition mode. By stabilizing a reactive HiPIMS process in the transition mode, high deposition rates of compound thin films or coatings having tailored stoichiometry for their intended use may be achieved. Moreover, the herein described method extends the possibilities for scaling up a reactive HiPIMS process to industrial application and thereby significantly improved process economy.

[0019] The above described control parameter may be a function of a plurality of current-to-integral ratios, each of said current-to-integral ratios constituting a ratio of a maximum target current during a predefined time period of the discharge pulse to an integral of the discharge current wave form for the corresponding predefined time period of the discharge pulse. Thereby, the accuracy in determining the prevailing operating point of the reactive HiPIMS process may be further improved, e.g., as this may allow for taking further consideration to the characteristics (such as the shape) of the discharge current waveform. For some reactive HiPIMS processes, this may facilitate achieving a control parameter which is monotonically increasing or decreasing.

[0020] According to a first alternative of the herein described method for determining a prevailing point of a reactive high-power impulse magnetron sputtering (HiPIMS) process, the above described first predetermined time period may correspond to the whole duration of the discharge pulse. The shape and overall evolution of the target current characteristics during the discharge pulse may often typically not be known a priori. Thus, the first predetermined time period may be selected to constitute the whole duration of the discharge pulse since this reduces the risk of failing to include an important fraction of the discharge pulse.

[0021] In case the first predetermined time period corresponds to the whole duration of the discharge pulse, the control parameter may for example constitute a ratio of the global maximum target current of the discharge current waveform to the integral of the discharge current waveform. As already mentioned above, the risk of failing to include an important fraction of the discharge pulse may thereby be reduced. Furthermore, this may facilitate implementation of the herein described method in a control device of a HiPIMS system e.g., since it does not rely on a control parameter being dependent of a plurality of current-to-integral ratios.

[0022] According to a second alternative of the herein described method for determining a prevailing point of a reactive high-power impulse magnetron sputtering (HiPIMS) process, the first predetermined time period may start at a predefined point in time after an expected onset of the target current during the discharge pulse. In other words, the first predetermined time period may be shorter than the duration of the target current during the discharge pulse. An initial part of the discharge pulse during a reactive high-power impulse magnetron sputtering process is typically governed by different physical phenomena compared to the rest of the discharge pulse. It may therefore, at least in some cases, be easier to accurately determine a prevailing operating point when the first predetermined time period is shorter than the whole discharge pulse and is selected such that the initial part of the discharge pulse is not included.

[0023] The method may further comprise determining the prevailing operating point of the process based on an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process. This may for example reduce the risk of an inaccurate result in case of (a) unwanted arc discharges, which may temporarily distort the target current characteristics before being handled by a control device or control system of the HiPIMS system, and / or (b) large variations in target current characteristics between different discharge pulses, for example when operating close to a self-sputter dominated discharge mode.

[0024] The present disclosure also provides a method, performed by a control device, for controlling a reactive high-power impulse magnetron sputtering (HiPIMS) process. The method for controlling the reactive high-power impulse magnetron sputtering process comprises a step of performing the above described method for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering process. The method for controlling the reactive high-power impulse magnetron sputtering process further comprises a step of, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs (optionally at least by a threshold) from a setpoint of the control parameter, adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter. The herein described method for controlling a reactive high-power impulse magnetron sputtering (HiPIMS) process considerably improves the ability to obtain and / or maintain a stable operating point of the process, in particular in the desired transition mode, for essentially the same reasons as already described above with regard to the method for determining a prevailing operating point of a reactive HiPIMS process. More specifically, it enables using a control parameter that monotonically increases or decreases, which in turn reduces the risk of an inappropriate or inaccurate adjustment of the one or more variables when seeking to arrive at the desired operating point, typically in the transision mode, of the process. By ensuring an appropriate and reliable control of the reactive HiPIMS process, high deposition rates as well as high quality of the deposited compound film / coating are inherently enabled. Moreover, the method provides improved possibilities for tailoring stoichiometry of deposited compound films / coatings.

[0025] The above described step of adjusting one or more variable process parameters may comprise adjusting one or more variable process parameters selected from the group consisting of discharge pulse frequency, duration of discharge pulse, duty cycle, discharge voltage, peak power of discharge pulse, average power of discharge pulse, reactive gas flow, and partial pressure of working gas and / or reactive gas.

[0026] The step of adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter may be performed through P regulation, PI regulation, or PID regulation. Suitably, PID regulation may be used.

[0027] Moreover, in accordance with the present disclosure, a control device configured to determine a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process is provided. The control device is configured to monitor target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time. The control device is further configured to, based on the determined discharge current waveform, determine a value of a control parameter indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

[0028] The control device provides the same advantages as described above with regard to the corresponding method for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process. The control device may further be configured to, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs, optionally at least by a threshold, from a setpoint of the control parameter, adjust one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter. This e.g., allows the reactive high-power impulse magnetron sputtering (HiPIMS) process to obtain and / or maintain a stable desired operating point even if operated in a transition mode.

[0029] The present disclosure also relates to a high-power impulse magnetron sputtering system comprising the control device as described herein. The high-power impulse magnetron sputtering system may further comprise a process chamber configured to comprise a target and a substrate, and a power supply arrangement configured to generate plasma discharges between such a target and an anode of the high-power impulse magnetron sputtering system.

[0030] The present disclosure further relates to a computer program comprising instructions which, when executed by the control device as described herein, cause the control device to carry out any one of the methods as described above.

[0031] The present disclosure further relates to a computer-readable medium comprising instructions which, when executed by the control device as described herein, cause the control device to carry out any one of the methods as described above.

[0032] BRIEF DESCRIPTION OF DRAWINGS

[0033] Fig. 1 represents a schematic sketch of mass deposition rate as a function of reactive gas flow in a reactive magnetron sputtering process, and where the three main regions of operation of the process is shown,

[0034] Fig. 2 schematically illustrates one example of a HiPIMS system,

[0035] Fig. 3 illustrates three examples of typical discharge current waveforms that may result depending on the process conditions of a reactive HiPIMS, Fig. 4 illustrates an example of a discharge current waveform with its global maximum target current and the integral of the discharge current waveform for the whole duration of the discharge pulse,

[0036] Fig. 5 illustrates an example of a discharge current waveform, similar to that of Figure 4, as well as a local maximum target current for the first predefined time period when said first predefined time period is shorter than the full duration of the discharge pulse,

[0037] Fig. 6 illustrates an example of a discharge current waveform, similar to that of Figure 4, as well as a plurality of predefined time periods of the discharge pulse including their respective local maximum target currents and integrals of the discharge current waveform,

[0038] Fig. 7 represents a flowchart schematically illustrating one exemplifying embodiment of the herein described method for determining a prevailing operating point of a reactive HiPIMS process,

[0039] Fig. 8 represents a flowchart schematically illustrating one exemplifying embodiment of the herein described method for controlling a reactive HiPIMS process,

[0040] Fig. 9 schematically illustrates an exemplifying embodiment of a device which may comprise, consist of, or be comprised in the herein described control device configured to determine a prevailing operating point of a reactive HiPIMS process,

[0041] Fig. 10 illustrates an exemplifying test result of global maximum target current as a function of reactive gas flow during a reactive HiPIMS process,

[0042] Fig. 11 illustrates the ratio of global maximum target current to the integral of the discharge current waveform as a function of reactive gas flow for the same reactive HiPIMS process setup as in Figure 10,

[0043] Fig. 12 illustrates charge of discharge pulse, i.e. the integral of the discharge pulse, as a function of reactive gas flow for the same reactive HiPIMS process setup as in Figure 10. DETAILED DESCRIPTION

[0044] The invention will be described in more detail below with reference to exemplifying embodiments and the accompanying drawings. The invention is however not limited to the exemplifying embodiments discussed and / or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate the invention or features thereof.

[0045] The term "reactive high-power impulse magnetron sputtering process" is used herein to describe a high-power impulse magnetron sputtering (HiPIMS) process wherein a reactive gas is utilized for the purpose of reacting with sputtered matter so as to be incorporated in a deposited film / coating.

[0046] Moreover, in the present disclosure, the terms "target current" and "discharge current", respectively, are used interchangeably and shall therefore be considered to have the same meaning, unless explicitly disclosed otherwise.

[0047] Figure 2 schematically illustrates one example of a high-power impulse magnetron sputtering (HiPIMS) system 1 in which reactive HiPIMS may be performed. The HiPIMS system 1 comprises a process chamber 2 in the form of a vacuum chamber. The process chamber 2 may for example be essentially cylindrical, but is not limited thereto. During operation of a HIPIMS process, at least one target 3 is arranged in the process chamber 2. The shape of the target 3 may for example be planar rectangular, cylindrical, or hollow cylindrical, but is not limited thereto. During sputtering of the material from the target, the target 3 constitutes the cathode. The target 3 is arranged in the vicinity of a magnetron 4, e.g., by being mounted to the magnetron 4 or held in place relative to the magnetron by a common first holder 5. The magnetron 4 is arranged on the side of the target 3 which is opposite the side of the target 3 that faces the interior of the process chamber 2. The magnetron 4 generates a magnetic field (not shown) at the surface of the target 3 facing the interior of the process chamber 2. This magnetic field serves as a trap for electrons in the plasma during the process.

[0048] The magnetron 4 may be arranged so as to provide the electrical connection of the target 3 to a power supply arrangement 10. Alternatively, the target 3 may be directly connected to the power supply arrangement 10 as shown in the figure. The magnetron 4 may further comprise conduits for enabling cooling of the target (commonly water cooling). The dashed arrows 6 shown in the figure are intended to schematically illustrate a flow of a cooling medium to and from the magnetron 4.

[0049] The HiPIMS system 1 further comprises an anode. The wall(s) 7 of the process chamber 2 may serve as the anode, as shown in the figure. In such a case, the power supply arrangement 10 is electrically connected to the wall(s) of the process chamber 2. Alternatively, the anode may for example be in the form of an anode ring (or any previously known configuration therefore) essentially following the periphery of the target 3, but arranged at a distance from the target 3. Such an anode ring may be supported by the first holder 5.

[0050] The HiPIMS system 1 further comprises a power supply arrangement 10 configured to generate plasma discharges between the target 3 and the anode. Thereby, a plasma is generated in front of the surface of the target 3 facing the interior of the process chamber 2. During a deposition process, ions of a process gas (such as argon) are generated in the plasma and drawn out of the plasma and accelerated across a cathode sheath. The target 3 has a lower potential than the region in which the plasma is formed and the target surface therefore attracts ions. The positive ions move towards the target with a high velocity and then impact the target, thereby causing atoms from the target to physically dislodge or sputter from the surface. In contrast to in a conventional DC magnetron sputtering system, the power supply arrangement 10 of the HiPIMS system 1 is configured to operate in a pulsed power mode. Thereby, the power supply arrangement 10 will generate plasma discharges during a number of consecutive pulses, each separated by a duration at which there is no active discharge. During the duration at which there is no active plasma discharge, the plasma will start to decay until the following active discharge pulse. To allow the pulsed power mode, the power supply arrangement 10 may in addition to a DC power supply device comprise a pulsing unit (not shown).

[0051] The power supply arrangement 10 may further comprise a monitoring device 20 configured to record and monitor discharge current and voltage characteristics. Such a monitoring device 20 may for example be used for the purpose of monitoring target current during a discharge pulse to thereby determine a discharge current waveform.

[0052] The HiPIMS system 1 may further comprise a second holder 8 configured to hold one or more substrates 9 onto which for example a thin film or coating is to be deposited. The second holder 8 may be stationary or rotatably arranged during the deposition process. The HiPIMS system 1 may further comprise a bias supply device 11 connected to the second holder 9. The bias supply device 11 may be configured to supply a bias to the one or more substrates 9 during the deposition process. The process chamber 2 comprises a first gas inlet 12. Process gas, for example argon, may be introduced via the first gas inlet 12 by means of a first gas supply device 13 of the HiPIMS system 1. The process chamber 2 may comprise one or more additional gas inlets, if desired. For example, the apparatus may comprise a second gas inlet 14 intended for introduction of a reactive gas. If so, the reactive gas may be introduced via the second gas inlet 14 by means of a second gas supply device 15. It should here be noted that a process gas and a reactive gas may alternatively be introduced into the process chamber 2 via a common gas inlet, such as the first gas inlet 12, if desired. The process chamber 2 further comprises an exhaust 16 configured to allow gas to be led out of the process chamber 2. Said exhaust 16 may typically be connected to a vacuum pump (not shown).

[0053] The HiPIMS system 1 further comprises a control device 100 configured to control one or more of the constituent components of the HiPIMS system 1 and thereby also one or more variable process parameters. For example, the control device may be configured to control the power supply arrangement 10, the first gas supply device 13, the second gas supply device 15, the bias supply device 11, and / or a vacuum pump connected to the exhaust 13. The control device 100 may further be configured to perform the herein described method for determining a prevailing operating point of a reactive HiPIMS process. The control device 100 may, for example, be configured to monitor target current during a discharge pulse using the monitoring device 20. Additionally, the control device 100 may be configured to perform the herein described method for controlling a reactive HiPIMS process. The control device 100 may comprise one or more control units. In case of the control device comprising a plurality of control units, each control unit may be configured to control a certain function / step or a certain function / step may be divided between two or more control units.

[0054] As already mentioned in the background section of the present disclosure, T. Shimizu et al. proposes a method for stabilizing process conditions at a given setpoint (i.e. a desired operating point) within the transition mode. According to said method, waveforms of target current is monitored for the purpose of determining the maximum target current during the discharge pulse and a feedback control system is implemented for maintaining a constant global maximum discharge current (i.e. constant peak target current). The present inventors have however found that, although the proposed method works well for deposition of certain compound thin films or coatings using certain process parameters in a reactive HiPIMS system, it may not be sufficiently accurate, or even possible, for other set-ups. There is therefore still room for further improvements in the control of a reactive HiPIMS process. Depending on process conditions of a reactive HiPIMS process (as well as the selection of material of the target and reactive gas used), the discharge current waveforms may have different shapes. Figure 3 illustrates three examples of typical discharge current waveforms, Wl, W2, W3, that may result depending e.g., on the process conditions of a reactive HiPIMS, each of the discharge current waveforms Wl, W2, W3 defining change in target current over time of a discharge pulse. In the figure, t0represents the point in time at which the discharge pulse is initiated, which in turn corresponds to a point in time at which a discharge voltage is applied. Furthermore, ti represents the point in time at onset of the target current, i.e. the point in time at which a target current starts to increase from zero (as a result of the applied discharge voltage). Moreover, t2represents the point in time at which the target current has dropped to zero substantially at the end of the discharge pulse. The top image illustrates a discharge current waveform Wl having a substantially convex shape until reaching a global maximum target current lpeak substantially at the end of the discharge pulse. In contrast, the middle image illustrates a discharge current waveform W2 having a substantially concave shape until reaching lpeak substantially at the end of the discharge pulse. The lower image illustrates a discharge current waveform W3 having a substantially S-shaped curvature until reaching lpeak, which is reached some time before the end of the discharge pulse. The discharge current waveform W3 also demonstrates, after lpeak, a convexly shaped reduction of target current over time before the sudden drop to zero at t2. It should here be noted that the exemplified discharge current waveforms are merely representative of typical discharge current waveforms of reactive HiPIMS processes and that other shapes of discharge current waveforms are plausible. The reason for a shape / curvature of a discharge current waveform is that different physical phenomena may occur during different parts of the discharge pulse.

[0055] A more detailed description of discharge current waveforms in HiPIMS, and the underlying mechanisms, may be found in D. Lundin, T. Minea, and J. T. Gudmundsson, High Power Impulse Magnetron Sputtering - Fundamentals, Technologies, Challenges and Applications, Elsevier, Amsterdam, 2020, p. 269-272.

[0056] For a certain operating point of a reactive HiPIMS process, there will be a characteristic discharge current waveform. In the present disclosure, the characteristic discharge current waveform for the desired operating point (setpoint) of the reactive HiPIMS process is in the following denominated reference discharge current waveform. In case a prevailing operating point of the reactive HiPIMS process is altered, there may not only be a change in the value of global maximum target current, but the shape of discharge current waveform may also be altered and / or a global maximum target current may be reached at a different point in time during a discharge pulse. This is illustrated in the lower image of Figure 3, which illustrates an example of a reference discharge current waveform, Ref, representing a desired operating point of the reactive HiPIMS process. The alteration in the characteristics of the discharge current waveform could for example mean that, even though the process is controlled with the purpose to maintain a constant global maximum target current, the process may not necessarily be stabilized at the desired operating point, at least not for an extended period of time of a reactive HIPMS process.

[0057] In view of the foregoing, it may be understood that using a global maximum target current during a discharge pulse as a reference of a prevailing operating point, and controlling the process to reduce the difference to a setpoint of the global maximum target current may not always result in the ability to maintain a desired stable operating point. In fact, it may even be impossible for certain set-ups (i.e. certain process parameters during a reactive HiPIMS process and / or compound films or coatings to be deposited). The present inventors have however surprisingly found that, through further analysis of the discharge current waveform, the accuracy in determining a prevailing operating point can be significantly improved. Furthermore, the present inventors have found that this also allows for controlling certain reactive HiPIMS processes not previously been able to control using the strategy of seeking to maintain a constant global maximum target current. This in turn also allows for a more accurate control of the reactive HiPIMS process for the purpose of maintaining a desired stable operating point, even in the typically desired transition mode. More specifically, the present inventors have found that consideration of a ratio of maximum target current to integral of the discharge current waveform during one or more predefined time periods of the discharge pulse to be useful. Through consideration of such a ratio, a control parameter which is monotonically increasing, or monotonically decreasing, when passing from a metal mode to a compound mode, or vice versa, of the reactive HiPIMS process may be achieved. This in turn means that a change in value of the control parameter may be predicted, which is important for the ability to accurately identify a prevailing operating point and for allowing a reliable control of the reactive HiPIMS process.

[0058] In view of the foregoing, the present disclosure relates to a method for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process . By means of the herein described method, the prevailing operating point may be more accurately determined which in turn allows for improved possibilities for controlling the HiPIMS process with the purpose of stabilizing the process, typically in the transition mode. It should however be noted that, although the herein described method for determining a prevailing operating point of a reactive HiPIMS process has primarily been developed for the purpose of improving the control of the reactive HiPIMS process, it may also be used for other purposes, for example for detecting possible malfunction in the HiPIMS system or the control thereof during deposition of a compound film or coating.

[0059] The herein described method for determining a prevailing operating point of a reactive HiPIMS process comprises a step of monitoring of target current during a discharge pulse of the process to thereby determine a discharge current waveform. Said discharge current waveform defines change in target current over time during a discharge pulse. The method further comprises a step of, based on the determined discharge current waveform, determining a value of a control parameter that is indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of a (local or global, depending on the circumstances) maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

[0060] More specifically, the above described control parameter is indicative of an operating point of the reactive HiPIMS process by comparison of a current value of the control parameter with a setpoint (or set value) of the control parameter. Such a setpoint of the control parameter may in turn be derived from a reference discharge current waveform, which represents the characteristic discharge current waveform obtained at a desired operating point of the reactive HiPIMS process. The reference discharge current waveform may in turn be determined through performing hysteresis testing of the reactive HiPIMS process to determine a desired operating point, e.g., resulting in an appropriate mass deposition rate for a given composition (including stoichiometry) of the compound to be deposited, for the HiPIMS system used. Such hysteresis testing is as such previously known, and will therefore not be described further herein. When the desired operating point has been found, the resulting discharge current waveform may be recorded and used as reference discharge current waveform.

[0061] According to a first alternative, the above mentioned control parameter constitutes a ratio of the global maximum target current of the discharge pulse (which also constitutes the global maximum target current of the discharge current waveform) to an integral of the discharge current waveform. In such a case, the first predefined time period of the discharge pulse corresponds to the whole duration of the discharge pulse.

[0062] Figure 4 illustrates an example of a discharge current waveform W (similar to the discharge current waveform W3 shown in the lower image of Figure 3), obtained through monitoring of target current during a discharge pulse of a reactive HiPIMS process. Like in Figure 3, to represents the point in time at which the discharge pulse is initiated, ti represents the point in time at the onset of the target current, and t2represents the point in time at which the target current has dropped to zero substantially at the end of the discharge pulse. Moreover, the global maximum target current of the discharge current waveform (and thus also of the discharge pulse) is represented by lpeak. The integral of the discharge current waveform for the whole duration of the discharge pulse represents the charge Qpuise of the discharge pulse, and corresponds to the area under the discharge current waveform, as shown in the figure. Thus, according to the first alternative described above, the control parameter constitutes the ratio of lpeak to Qpuise- In such a case, the first predetermined time period Pl of the discharge pulse corresponds to the whole duration of the discharge pulse, i.e. from to to t2.

[0063] According to a second alternative, the above mentioned control parameter constitutes a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse, wherein the first predefined time period is shorter than the full duration of the discharge pulse. In such a case, depending on whether the first predefined time period encompasses a point in time at which the global maximum target current is reached, the maximum target current of the above mentioned ratio may be a local maximum target current or a global maximum target current of the discharge current waveform. Moreover, the first predetermined time period may start and / or end at any predefined points in time during the discharge pulse, as desired and appropriate for the compound to be deposited during the reactive HiPIMS process and the process parameters used.

[0064] Figure 5 illustrates an example of a discharge current waveform W, similar to that of Figure 4. Moreover, the figure illustrates an example where the first predefined time period Pi of the discharge pulse is shorter than the whole duration of the discharge pulse and starts at a point in time after the onset of target current during the discharge pulse, i.e. after ti. According to the illustrated example, the discharge current waveform W does not reach the global maximum target current lpeak within the first predefined time period Pi. Thus, in case the control parameter constitutes a ratio of maximum target current during the first predefined time period Pi of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse, the control parameter would constitute the ratio of h to Qi. Here, h is the local maximum target current of the discharge current waveform during the first predefined time period Pi.

[0065] Moreover, Qi represents the integral (and thus charge) of the discharge current waveform for said first predefined time period Pi. According to the illustrated example, the first predefined time period Pi starts after ti ,as already mentioned above. In a typical HiPIMS discharge, a delay is always noticed between the onset of the target voltage and the onset of the discharge current. This delay depends on the working gas pressure, gas composition, target material, and applied discharge voltage (as further described in D. Lundin, T. Minea, and J. T. Gudmundsson, High Power Impulse Magnetron Sputtering - Fundamentals, Technologies, Challenges and Applications, Elsevier, Amsterdam, 2020, p. 63-64) . It is often hard to predict the exact length of this delay, making an analysis of the prevailing process conditions solely based on the first portion (typically the first 10-20 microseconds) of the discharge pulse rather difficult. Therefore, it may be advantageous to select the first predefined period of time Pi of the discharge pulse to start after a certain amount of time has passed from an expected onset of the target current (except when the first predefined period of time corresponds to the whole duration of the discharge pulse).

[0066] Although Figure 5 illustrates an example where the discharge current waveform W does not reach the global maximum target current lpeak within the first predefined time period Pi, it should be noted that the first predefined time period Pl may alternatively encompass the global maximum target current lpeak- In such a case, the control parameter may constitute the ratio of lpeak to Qi since k would correspond to lpeak.

[0067] According to a third alternative, the above mentioned control parameter is dependent at least of the ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse. In such a case, the control parameter may further be dependent of a ratio of maximum target current during a second predefined time period of the discharge pulse to an integral of the discharge current waveform for said second predefined time period of the discharge pulse.

[0068] In fact, the control parameter indicative of an operating point of the reactive HiPIMS process may be a function of a plurality of current-to-integral ratios, wherein each of said current-to-integral ratios constitutes a ratio of a maximum target current during a predefined time period of the discharge pulse to an integral of the discharge current wave form for the corresponding predefined time period of the discharge pulse. By way of example, such a function may be described in accordance with Equation 1 below: where

[0069] S represents the control parameter indicative of an operating point; ••• , Ineach represents a maximum target current during a predefined time period of the discharge pulse, said predefined time period being numbered 1, 2, . . n (n being an integer);

[0070] Qi> Q > ■■■> Qn each represents an integral of the discharge current waveform for the corresponding predetermined time periods, numbered 1, 2, . . n , of the discharge pulse; kltk2, ... kneach represents a coefficient; and a, b, ... , x each represents an exponent applied to a respective ratio.

[0071] It should here be noted that, in case the control parameter is dependent on more than one ratio of maximum target current during a predefined time period of the discharge pulse to an integral of the discharge current waveform for the corresponding predefined time period of the discharge pulse (i.e. dependent of a two or more current-to-integral ratios pertaining to different predefined time periods of the discharge pulse), the predefined time periods may, or may not, have the same duration. Moreover, the different predefined time periods may be separated in time from each other. It is also possible that at least two of the predefined time periods are partially overlapping in time, which e.g., may occur in case the first predefined time period corresponds to the whole duration of the discharge pulse whereas a second predefined time period is shorter than the whole duration of the discharge pulse.

[0072] Figure 6 illustrates an example of a discharge current waveform W, similar to that of Figure 4. Moreover, the figure illustrates a plurality of predefined time periods of the discharge current waveform W. More specifically, the figure illustrates a first predefined time period Pi, a second predefined time period P2, and a third predefined time period P3. According to the example shown, each of Pi, P2, and P3 is shorter than the full duration of the discharge pulse and starts after the target current has been initiated during the discharge pulse. However, it may also be possible that one of Pi, P2, and P3 corresponds to the full duration of the discharge pulse. Moreover, Pi, P2, and P3 are here shown to be of different durations. It is however also possible that at least two of Pi, P2, and P3have substantially the same duration, albeit starting at different points in time. The figure further illustrates the maximum target current , l2, and l3, respectively, for each of the predefined time periods Pi, P2, and P3. It can be noted that k here corresponds to the global maximum target current Ipeak as the discharge current waveform reaches lpeak within the first predefined time period Pi. In contrast, each of l2and h constitutes a local maximum target current for their respective predefined time periods P2and P3. The figure also illustrates the integrals of the discharge current waveform for the respective predefined time periods Pi, P2, and P3 of the discharge pulse, i.e. Qi, Q2and Q3.

[0073] As previously mentioned, the control parameter indicative of an operating point of the reactive HiPIMS process may be a function of a plurality of current-to-integral ratios, wherein each of said current-to-integral ratios constitutes a ratio of a maximum target current during a predefined time period of the discharge pulse to an integral of the discharge current wave form for the corresponding predefined time period of the discharge pulse. Thus, if considering the example shown in Figure 6, the control parameter may be a function of at least two of the ratios I1 / O1, 12 / Q.2, and I3 / Q3.

[0074] It should here be noted that although it is possible to determine a prevailing operating point of the reactive HiPIMS process through consideration of a single value of the control parameter indicative of an operating point (in other words, a value of the control parameter derived from a single discharge pulse), it may be suitable to consider an average of values of the control parameter derived from a number of consecutive discharge pulses of the reactive HiPIMS process. In other words, the method may comprise a step of determining the prevailing operating point of the process based on an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process. By way of example, an averaged value of the control parameter over 5, 10, 15 or 20 consecutive discharge pulses may suitably be used. Suitably, the number of consecutive discharge pulses considered could be from 5 to 10.

[0075] Figure 7 represents a flowchart schematically illustrating one exemplifying embodiment of the herein described method for determining a prevailing operating point of a reactive HiPIMS process. The method comprises a step S101 of monitoring of target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time. The method further comprises a step S102 of, based on the determined discharge current waveform, determining a value of a control parameter indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse. The method according to the exemplifying embodiment may further comprise a step S103 of, through comparison of the determined value of the control parameter with a setpoint of the control parameter, determining the prevailing operating point of the reactive HiPIMS process. Said setpoint of the control parameter may for example be derived through analysis of a reference discharge current waveform, said reference discharge current waveform corresponding to a discharge current waveform obtained when the reactive HiPIMS process is at a desired operating point thereof.

[0076] The present disclosure further relates to a method for controlling a reactive high-power impulse magnetron sputtering (HiPIMS) process to achieve and / or maintain a desired operating point. Said method is performed by a control device configured therefore. The method for controlling a reactive HiPIMS process comprises performing the steps of the above described method for determining a prevailing operating point of a reactive HiPIMS process. The method for controlling a reactive HiPIMS process further comprises a step of, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs, optionally at least by a threshold, from a setpoint of the control parameter, adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter. These steps may for example be repeated continuously or performed at regular time intervals, as desired.

[0077] The one or more variable process parameters may for example be selected from the group consisting of discharge pulse frequency, duration of discharge pulse, duty cycle, discharge voltage, peak power of discharge pulse, average power of discharge pulse, reactive gas flow, and partial pressure of working gas and / or reactive gas. In other words, the above mentioned step of adjusting one or more variable process parameters may comprise adjusting one or more variable process parameters selected from the above mentioned group. Suitably, the step of adjusting one or more variable process parameters may comprise adjusting one or more variable process parameters selected from the group consisting of discharge pulse frequency, discharge voltage, and reactive gas flow.

[0078] The above mentioned step of adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter may be implemented using a proportional- integral-derivative (PID) regulation, proportional-integral (PI) regulation, or a proportional (P) regulation. Suitably, PID regulation is used.

[0079] Figure 8 represents a flowchart schematically illustrating one exemplifying embodiment of the herein described method for controlling a reactive HiPIMS process. The method comprises a step S101 of monitoring of target current during a discharge pulse of the reactive HiPIMS process to thereby determine a discharge current waveform that defines change in target current over time. The method further comprises a step S102 of, based on the determined discharge current waveform, determining a value of a control parameter indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse. The method further comprises a step S104 of determining whether the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs (optionally at least by a threshold) from a setpoint of the control parameter. If not, the method may revert to start as shown in the figure. The method further comprises, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs (optionally at least by the threshold) from the setpoint of the control parameter, a step S105 of adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter. After step S105, the method may revert to start as shown in the figure.

[0080] The performance of the herein described method for determining a prevailing operating point of a reactive HiPIMS process as well as the performance of the herein described method for controlling a reactive HiPIMS process may each be governed by programmed instructions. These programmed instructions may take the form of a computer program which, when executed by a computer, cause the computer to effect desired forms of control action. Such a computer may for example be comprised in the control device as described herein. A computer is in the present disclosure considered to mean any hardware or hardware / firmware device implemented using processing circuity such as, but not limited to, a processor, Central Processing Unit (CPU), a controller, an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, an applicationspecific integrated circuit, or any other device capable of electronically performing operations in a defined manner.

[0081] The above described programmed instructions, which may take the form of a computer program, may be stored on a computer-readable medium. Hence, the present disclosure also relates to a computer-readable medium storing instructions, which when executed by a computer (such as a computer of the herein described control device), cause the computer to carry out the herein described method for determining a prevailing operating point of a reactive HiPIMS process and optionally also the herein described method for controlling a reactive HiPIMS process. The computer-readable medium may be a non-transitory computer-readable medium, such as a tangible electronic, magnetic, optical, infrared, electromagnetic, and / or semiconductor system, apparatus, and / or device. The present disclosure further relates to a control device configured to determine a prevailing operating point of a reactive HiPIMS process, and preferably also control said process. The control device may be configured to perform any one of the steps of the method for determining a prevailing operating point of a reactive HiPIMS process as described above as well as any one of the steps of the method for controlling a reactive HiPIMS process as described above.

[0082] More specifically, in accordance with the present disclosure, a control device configured to determine a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process is provided. The control device is configured to monitor target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time. The control device is further configured to, based on the determined discharge current waveform, determine a value of a control parameter indicative of an operating point of the process. Said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

[0083] The control device may further suitably be configured to, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs, optionally at least by a threshold, from a setpoint of the control parameter, adjust one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter.

[0084] In case the herein described control device configured to determine a prevailing operating point of a reactive HiPIMS process is not also configured to control said reactive HiPIMS process, it may be configured to communicate with another control device which in turn is configured to perform control of the reactive HiPIMS process in accordance with the above described method therefore.

[0085] The herein described control device may comprise one or more control units. In case of the control device comprising a plurality of control units, each control unit may be configured to control a certain function / step or a certain function / step may be divided between more than one control units. Figure 9 schematically illustrates an exemplifying embodiment of a device 500. The control device 100 described above may for example comprise the device 500, consist of the device 500, or be comprised in the device 500.

[0086] The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read / write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I / O means, an A / D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

[0087] There is provided a computer program P that comprises instructions for determining a prevailing operating point of a reactive HiPIMS process. The computer program comprises instructions for monitoring of target current during a discharge pulse of the reactive HiPIMS process to thereby determine a discharge current waveform that defines change in target current over time. The computer program further comprises instructions for, based on the determined discharge current waveform, determining a value of a control parameter indicative of an operating point of the reactive HiPIMS process, wherein said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

[0088] The computer program may further comprise instructions for controlling the reactive HIPIMS process. More specifically, the computer program may comprise instructions for, when the value of the control parameters differs, optionally at least by a threshold, from a setpoint of the control parameter, adjusting one or more variable process parameters so as to reduce the difference between the value and the setpoint of the control parameter.

[0089] The program P may be stored in an executable form or in a compressed form in a memory 560 and / or in a read / write memory 550.

[0090] The data processing unit 510 may perform one or more functions, i.e. the data processing unit 510 may effect a certain part of the program P stored in the memory 560 or a certain part of the program P stored in the read / write memory 550. The data processing device 510 can communicate with a data port 599 via a data bus 515. The nonvolatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit 510 via a data bus 511. The read / write memory 550 is adapted to communicate with the data processing unit 510 via a data bus 514. The communication between the constituent components may be implemented by a communication link. A communication link may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link.

[0091] When data are received on the data port 599, they may be stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 is prepared to effect code execution as described above.

[0092] Parts of the methods herein described may be affected by the device 500 by means of the data processing unit 510 which runs the program stored in the memory 560 or the read / write memory 550. When the device 500 runs the program, methods herein described are executed.

[0093] Experimental results

[0094] To illustrate the above discussed problems, and the solution thereto provided by the herein described methods, the following exemplifying test results may be considered.

[0095] Figure 10 illustrates an exemplifying test result of global maximum target current lpeak as a function of reactive gas flow during a reactive HiPIMS process. This exemplifying test result stems from a reactive HiPIMS process in which a circular titanium target having a size of about 700 cm2was used, and nitrogen was used as the reactive gas. The discharge voltage was kept constant at 700 V for each discharge pulse. The pulse frequency was 800 Hz, and the duration of each discharge pulse was 30 ps. The flow of process gas (argon) was kept constant at 100 seem, whereas the reactive gas flow was varied between about 2 seem to about 6 seem as shown in the figure.

[0096] The arrows in Figure 10 are used for the purpose of illustrating which part of the lpeak curve that belongs to an increasing and a decreasing reactive gas flow, respectively. It can be seen from the figure that the global maximum target current lpeak initially increases with increasing reactive gas flow when moving from the metal mode MM due to increasing reactive gas flow. However, the global maximum target current lpeak does not increase monotonically when moving from the metal mode MM, via the transition mode TM, to the compound mode CM. Furthermore, the global maximum target current lpeak does not decrease monotonically when moving from the compound mode CM to the metal mode MM. This leads to considerable difficulties in determining a prevailing operating point of the reactive HiPIMS process, and more importantly, in the control of the reactive HiPIMS process to arrive at a desired operating point. More specifically, it may not be possible to determine whether the reactive gas flow needs to be increased or decreased in order to reduce a possible difference between a global maximum target current, determined from a discharge current waveform, and a setpoint of the global maximum target current.

[0097] However, Figure 11 illustrates the ratio of global maximum target current lpeak to the integral of the discharge current waveform Qpuise as a function of reactive gas flow for the same reactive HiPIMS process setup as in Figure 10. Here it can be seen that the ratio monotonically increases when moving from the metal mode MM, via the transition mode TM, to the compound mode CM. Moreover, the ratio monotonically decreases when moving from the compound mode CM to the metal mode MM, passing through the transition mode TM. This demonstrates that the ratio lPeak / QpUise is a control parameter suitable for determining a prevailing operating point of the reactive HiPIMS process as well as the control of said process.

[0098] For sake of comparison, Figure 12 illustrates the charge Qpuise of the discharge pulse, i.e. the integral of the discharge current waveform, as a function of reactive gas flow for the same reactive HiPIMS process setup as in Figures 10 and 11. It can here be seen that, like lpeak as shown in Figure 10, Qpuise does not constitute a control parameter which is monotonically increasing or monotonically decreasing when moving from the metal mode MM to the compound mode CM, and vice versa. Therefore, Qpuise alone also does not constitute a control parameter which may be used for accurately determining a prevailing operating point and controlling the reactive HiPIMS process to obtain and / or maintain a desired operating point.

[0099] It should here be noted that although Figures 10-12 relate to deposition of a Ti-N compound film using certain process parameters, many other reactive HiPIMS set-ups also demonstrate that the global maximum target current lpeak is not a control parameter which monotonically increases or monotonically decreases when moving from the metal mode to the compound mode, or vice versa. However, as already mentioned above, the present inventors have found that, by instead selecting the control parameter to be, or be dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse it is possible to obtain a control parameter which is monotonically increasing or monotonically decreasing when moving from the metal mode to the compound mode, or vice versa, passing through the transition mode.

Claims

CLAIMS1. A method, performed by a control device (100), for determining a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process, the method comprising the following steps: monitoring (S101) target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time, based on the determined discharge current waveform, determining (S102) a value of a control parameter indicative of an operating point of the process, wherein said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

2. The method according to claim 1, wherein said control parameter is a function of a plurality of current-to-integral ratios, each of said current-to-integral ratios constituting a ratio of a maximum target current during a predefined time period of the discharge pulse to an integral of the discharge current wave form for the corresponding predefined time period of the discharge pulse.

3. The method according to any one of claims 1 or 2, wherein said first predefined time period corresponds to the whole duration of the discharge pulse.

4. The method according to claim 3, wherein the control parameter constitutes a ratio of a global maximum target current of the discharge current waveform to an integral of the discharge current waveform.

5. The method according to any one of claims 1 or 2, wherein said first predefined time period starts at a predefined point in time after an expected onset of the target current during the discharge pulse.

6. The method according to any one of the preceding claims, further comprising determining the prevailing operating point of the process based on an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process.

7. A method, performed by a control device (100), for controlling a reactive high-power impulse magnetron sputtering (HiPIMS) process, the method comprising the following steps: performing the method for determining a prevailing operating point of a reactive high- power impulse magnetron sputtering process according to any one of the preceding claims, when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs, optionally at least by a threshold, from a setpoint of the control parameter, adjusting (S105) one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter.

8. The method according to claim 7, wherein the step of adjusting one or more variable process parameters comprises adjusting one or more variable process parameters selected from the group consisting of discharge pulse frequency, duration of discharge pulse, duty cycle, discharge voltage, peak power of discharge pulse, average power of discharge pulse, reactive gas flow, and partial pressure of working gas and / or reactive gas.

9. The method according to any one of claims 7 or 8, wherein the step of adjusting one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter is performed through P regulation, PI regulation, or PID regulation.

10. A control device (100) configured to determine a prevailing operating point of a reactive high-power impulse magnetron sputtering (HiPIMS) process, wherein said control device (100) is configured to: monitor target current during a discharge pulse of the process to thereby determine a discharge current waveform that defines change in target current over time, and based on the determined discharge current waveform, determine a value of a control parameter indicative of an operating point of the process, wherein said control parameter constitutes, or is dependent of, a ratio of maximum target current during a first predefined time period of the discharge pulse to an integral of the discharge current waveform for said first predefined time period of the discharge pulse.

11. The control device (100) according to claim 10, wherein the control device further is configured to:when the value of the control parameter, or an average of values of the control parameter derived from a plurality of consecutive discharge pulses of the process, differs, optionally at least by a threshold, from a setpoint of the control parameter, adjust one or more variable process parameters so as to reduce the difference to the setpoint of the control parameter.

12. A high-power impulse magnetron sputtering (HiPIMS) system (1) comprising the control device (100) according to any one of claims 10 or 11.

13. A computer program comprising instructions which, when executed by a control device (100) according to any one of claims 10 or 11, cause the control device (100) to carry out the method according to any one of claims 1 to 9.

14. A computer-readable medium comprising instructions which, when executed by a control device (100) according to any one of claims 10 or 11, cause the control device (100) to carry out the method according to any one of claims 1 to 9.

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