Fast high voltage for ionization improvement for sputtering process

Abstract

The invention relates to a sputtering process of a target material in a vacuum environment comprising a sputtering target and plasmogenous process gas, wherein a transient electron accelerating pulse is applied to the sputtering target after plasma ignition, whereby applying said electron accelerating pulse involves applying a high negative bias voltage to the sputtering target during a period of time shorter than 1 microsecond, the bias voltage having an absolute value greater than 1 kV. This transitory regime of electron acceleration pulses is referred to as Fast High Voltage for lonization Improvement (FHiVI2). It can be operated alone or in combination with known sputtering processes, e.g. HiPIMS. The inventive process is suitable in sputtering deposition processes but also in other applications, e.g. for space propulsion.

Description

[0001] P-SATTGE-032 / WO 1

[0002] Fast high voltage for ionization improvement for sputtering process

[0003] FIELD OF THE INVENTION

[0004] The present invention generally relates to the sputtering and ionization of elements, preferably metals or their compositions, by physical routes, e.g. physical vapor deposition or ion beam generation, and more specifically by cathodic sputtering.

[0005] BACKGROUND OF THE INVENTION

[0006] Deposition of thin films by physical vapor deposition (PVD) techniques has found widespread use in many industrial sectors. There is an increasing demand for coatings with tailored and enhanced properties such as high hardness, wear and corrosion resistance, low friction, and specific optical or electrical properties as well as decorative colors and often complex combinations of those properties are requested.

[0007] It is known (such as e.g. from WO2019234477 A1 or EP 4 143 360 A2) to implement machines able to carry out the coating of a substrate usually placed in a vacuum chamber, thanks to the deposition, on its surface, of atoms or ions emitted by a solid sacrificial body subjected to the bombardment of high energy particles. In particular, the emission of atoms or ions from the sacrificial body is caused by a plurality of high energy particles, such as for example gas ions in the plasma state, which is caused to hit the sacrificial body, therefore also termed "target". This deposition process is known as cathodic sputtering.

[0008] The target body can be composed of all the elemental materials or their compounds being deposited on the deposition substrate both in their pure and modified state by chemical reactions with the gases that are inside the vacuum chamber. In particular, during the erosion of the target body, the coating material, in the form of atoms, ions, or molecule portions, detaches from the latter and spreads in the surrounding areas, finally adhering on the deposition substrate.

[0009] The erosion of the target body can occur thanks to the use of an ionized gas, in the plasma state, interposed between said target body and said deposition substrate and suitably excited to provide high energy particles intended for hitting the target body in order to cause the afore said erosion thereof.

[0010] Machines and processes for the deposition of material by cathodic sputtering are known and have an electric potential difference between the target body and the deposition substrate. Usually, the target body is the cathode (the more negative pole) whereas the substrate, or another element, is the anode (the more positive pole).

[0011] The moment at which an inert gas, such as for example argon, is filled between said target body and said deposition substrate, in suitable pressure conditions (for example at 1 Pa), the free electrons in the gas are accelerated away from the cathode, the more negative charge, so that they hit the plasmogenous gas atoms and ionize them. In this way, a chain reaction is triggered and a partially ionized gas, or plasma, is achieved. The ionized atoms of the plasmogenous gas are in this case atoms with a positive charge and are thus accelerated by electrostatic attraction towards the cathode, in this case represented by the target body. The ionized atoms collide with the target surface, cause the detachment of coating material (atoms or ions) from the target that, by spreading in a direction opposite to the one taken by the target, is caused to adhere to the deposition substrate. The general physical mechanisms of the cathodic sputtering process which have just been described are valid for electropositive, electronegative, ion-ion plasmas and for all types of plasmogenous gas or gas mixture, whether reactive or not.

[0012] Improved machines and processes are known and further comprise a plurality of magnets (to form a so called "magnetron" thereby depositing material by magnetron sputtering), able to generate a magnetostatic field. Such magnets are usually placed under the target body, so that the resulting magnetic field interacts with the charged particles in the chamber, electrons in particular, interposed between the target body and the deposition substrate, increasing the probability of collisions with subsequent increase of the number of ionized molecules, in order to have higher process yield. It has to be observed that by "yield" of the process the "sputtering yield" is usually meant, i.e. the ratio between the number of atoms eroded from the target and the incident ions on the surface of the target body.

[0013] In a cathodic sputtering process or a magnetron sputtering process, the ions available are mainly the ions of the plasmogenous sputtering gas and ions of the sputtered material are rare. A drawback is thus the low yield of the process, i.e. the low rate between the number of atoms eroded from the target and the ions hitting the surface of the target body, thus causing an increase of time and costs of the process.

[0014] High power impulse magnetron sputtering (HiPIMS) is a plasma-based modern physical vapor deposition (PVD) technique characterized by a very high peak power density (>1 kW / cm2) at the sputtering target and a relatively high plasma density (>1019rrr3) in front of the target with short variable pulse durations (20-200 ps). This technique has been demonstrated to be useful for high-aspect- ratio filling applications and improved target utilization as well as enhancing substrate pre-treatment improving adhesion of hard coatings.

[0015] HiPIMS allows to achieve an increased ionization rate, however with some variability.

[0016] OBJECT OF THE INVENTION

[0017] It is thus an object of the invention to provide an improved sputtering technique, which allows increasing in the ionization rate of the sputtered species.

[0018] SUMMARY OF THE INVENTION

[0019] In order to achieve the above-mentioned object, the present invention proposes a sputtering process of a target material in a vacuum environment comprising a sputtering target and plasmogenous process gas, wherein a transitory (i.e. transient) electron accelerating pulse is applied to the sputtering target after plasma ignition to enhance ionization, whereby applying said electron accelerating pulse involves applying a high negative bias voltage to the sputtering target during a period of up to 1 microsecond, the bias voltage having an absolute value greater than 1 kV.

[0020] As known in the field, the term "plasma" refers to a mixture of electronic charges (electrons) and of ions; it also implies the notion of charge neutrality at the macroscopic scale of the gas in the plasma state.

[0021] In this context, the notion of “plasma ignition” refers to the initiation of the plasma, i.e. the initial process of creating a plasma by applying a voltage bias to the process gas within a plasma chamber. The term “ignited plasma” in turn describes the active state of the plasma during the sputtering process, plasma i.e. a plasma that has been successfully initiated and is sustaining itself within the sputtering chamber.

[0022] In contrast, the term “discharge” implies the presence of electronic charges but does not provide any indication as to the presence of ionic charges. Plasma ignition thus implies that ionization (already) occurred.

[0023] According to the present invention, the electron accelerating pulse is applied after plasma ignition, i.e. on an ignited plasma, and as such is not a discharge establishment pulse nor designed as an ignition pulse. In other words, and as will be apparent to the skilled person, the electron accelerating pulse being applied after plasma ignition, it is a post-ionisation pulse. Indeed, the electron accelerating pulse is an additional pulse applied after an ignition phase leading to plasma ignition, it is thus applied at a time where plasma is formed / ignited.

[0024] It may be noted that in practice, when implementing a sputtering process, a plurality of electron accelerating pulses will be applied, preferably at particular predetermined timings in relation to the operation of a reference plasma process, as will be discussed below.

[0025] The invention hence essentially relies on the use of ultra-fast high voltage pulses, leading to high intensity, referred to as electron accelerating pulse, designed to cause a sudden acceleration of the electrons in the gaseous plasma, by contrast to heavier species in the plasma (neutral elements, ions... ) which will not be able to react on the corresponding time. In other words, due to the difference of electric potential and its very fast and short application time, it is possible to accelerate electrons - and electrons only. The other charged species (e.g. ions) will not be accelerated since they will not be sensitive to variations of the electric field that are this fast (of the order of MHz) and short (at the microsecond scale).

[0026] That is to say, applying an electron accelerating pulse after plasma ignition, i.e. on a gas in plasma state and hence comprising both electronic and ionic charges, enables to significantly improve the ionization rate of the sputtered material, hence enhancing ionization.

[0027] The present process may thus be regarded as comprising a phase of plasma ignition and a subsequent phase of (plasma) ionization enhancement, the latter resulting from the application of the electron accelerating pulse on the ignited plasma. Depending on the specific features of the electron accelerating pulse (voltage, duration and / or rise time) and the sputtering target material, improved ionization can be observed concomitantly to the electron accelerating pulse or thereafter. However, even if enhanced ionization appears after the end of the electron accelerating pulse, it is a direct consequence thereof, so that it is considered that the electron accelerating pulse be part of the ionization enhancement phase. In the present text, ionization enhancement phase, phase of ionization enhancement, electron accelerating step and electron accelerating pulse have the same meaning, which implies accelerating electrons - and electrons only - so as to improve the ionization rate of the sputtered material and may thus be used as synonyms.

[0028] Due to its very short application time, the electron accelerating pulse can be referred to as transitory pulse. During a sputtering process, electron accelerating pulses are applied with given periodicity (fixed or varying frequency, depending on the application) and thus forms a transitory regime. In embodiments, the electron accelerating pulses may be applied as a sequence of pulses, referred to as burst pulses or burst mode. Such burst pulses may be repeated during the sputtering process. Implementing the inventive transitory regime of electron acceleration pulses in a plasma chamber may be referred to as Fast High Voltage for Ionization Improvement, in short FHiVI2(or FHiVII). The inventive process is applicable in a vacuum environment, meaning in a vacuum that is maintained in any kind of reactor or chamber, but also in space vacuum.

[0029] The electron accelerating pulse may be generated by a power source configured to supply an output voltage of several kV or greater and with a fast rise time. Indeed, since the electron accelerating pulse has a duration up to 1 ps, the power supply must allow the voltage to rapidly increase to the target pulse value.

[0030] Preferably, the electron accelerating pulse is applied with a voltage rise time of at least 2 kV / ps. In particular, the rise time of the electron accelerating pulse may be higher than 5 kV / ps, preferably higher than 10 kV / ps, 20, 30 or 40 kV / ps. Likewise, the decrease of voltage is preferably controlled with similar rates. Indeed, such voltage rise time supports a sudden acceleration of the electrons in the volume with respect to the heavy(ier) species of the plasma (neutral elements, ions, ... ), and makes it possible to accelerate only electrons.

[0031] In embodiments, the electron accelerating pulse has transient voltages, with a voltage rising period up to the target voltage (i.e. at least 1 kV) followed by a voltage drop (or fall) period.

[0032] As used in the present text, the expression a ‘negative bias voltage’ as applied to the sputtering target means that the sputtering target is put at a lower potential than another reference potential, which may be e.g. the sample holder or the chamber walls. Stated otherwise, the difference of potential between the sputtering target and the reference potential is negative. There is no restriction on the polarity as long as the difference of potential is negative during sputtering phase of the process.

[0033] In embodiments, the electron accelerating pulse is operated with a negative bias having an absolute value higher than 1 ,5 kV, preferably higher than 2 kV and / or lower than 20 kV. That is, the negative voltage bias may be between -1 and -20 KV. It may be desirable to limit the negative bias voltage to a voltage having an absolute value lower than 20 kV in order to limit the production of X- rays and thus prevent an operator from being exposed to such rays. The electron accelerating pulse is thus built as a pulse with high fast voltage, where the high negative bias voltage is applied to reach a target voltage on the target (i.e. difference of potential at the prescribed negative value). The period to reach the target voltage value from the start voltage (cathode voltage value at the moment the electron accelerating pulse is applied) may be referred to as rise period, or establishment time (the time to establish the desired bias voltage on the cathode / target). Conversely, the time period required for the voltage to decrease from the target value to the start voltage value is referred to as fall period. The time period during which the voltage is at the target voltage value is referred to as plateau; it may have a variable length, e.g. ranging from a few ns to a few 100s ns. The electron accelerating pulse thus consists of the rise period, the plateau and the fall period. The duration of the electron accelerating pulse corresponds to sum of these three periods and is 1 ps or less.

[0034] It is not particularly desired to reduce the duration of the electron accelerating pulse. It is rather a matter of ionic plasma oscillations pulsation duration and what matters is that the duration of one electron accelerating pulse does not exceed 1 .0 ps in order to act on electrons and not affect ions. Hence in general, the electron accelerating pulse may have a duration of about 900 ns to 1 000 ns. However, for the sake of exemplification, the electron accelerating pulse preferably has a minimum duration of 400 ns.

[0035] In embodiments, the rise period and fall period may have similar lengths. The rise period, respectively the fall period, may represent up to 45 to 50% of the duration of the electron accelerating pulse, preferably it may represent 25, 20, 15 or 10% of the duration of the electron accelerating pulse. For example, where the rise and fall periods represent each 20%, the plateau (target voltage) may represent 60% of the pulse duration. As understood by the skilled person, the rise period refers to the establishment time of the negative bias voltage (increase of difference of potential). Conversely, the fall time is the time period corresponding to a decrease of difference of potential.

[0036] To recap, one of the merits of the invention is to have identified that such electrons accelerating pulses enable an increase of the number of ions resulting from the species / atoms sputtered from the target material with respect to nonionized atoms sputtered from the target material. Indeed, accelerated electrons on a fast rise time hit species / atoms sputtered from the target material and present in the reactor, thereby ionizing the species / atoms and forming ionized species, or ions. Thenceforth, the energy and directionality of the ions may advantageously be controlled by applying a bias voltage to the substrate, or directly to the sputtering target (e.g. thanks to bi-polar regime).

[0037] Remarkably, the inventive process enables to significantly improve the ionization rate of the sputtered material (i.e. the number of ionized atoms sputtered from the target material with respect to non-ionized, neutral sputtered atoms) without requiring any additional device to be inserted into the reactor. This is of particular relevance as such additional devices (e.g. additional coils or magnets) are generally very disruptive and often cumbersome with respect to the sputtering process, such as but without being limited to a coating / deposition process. Additionally, it is currently believed that the inventive process may, under certain operating conditions, allow controlling the degree of ionization of the sputtered species.

[0038] Another merit of the invention is that it may be implemented with virtually all types of plasma-assisted processes, with numerous possible application fields, as it is independent of the target material to be sputtered as well as of the (inert or reactive) plasmogenous gas. The invention finds particular application in sputtering deposition processes, especially in combination with conventional sputtering deposition processes.

[0039] It will be appreciated that the inventors have surprisingly found that a successful implementation of the step of accelerating electrons (due to the (at least one) electron accelerating pulse) lies in the precise control of the difference of electric potential applied to the sputtering target. The FHiVI2regime relies on the use of fast, high voltage pulses, hence implementing a high rise time, which allows electrons to acquire a kinetic energy sufficient for leaving the vicinity of the sputtering target and participating efficiently in the ionization of sputtered target material (i.e. in the gas vapor). A fine tuning, i.e. controlled modulation, of the bias voltage and its rise time on the target material allows controlling the ionization rate of the sputtered material (higher bias voltage and / or modulated duration of the step of accelerating electrons leading to higher ionization of sputtered material).

[0040] The FHiVI2regime hence proves very versatile, in that it allows a fine regulation of voltage, duration and application time, allowing, in fine, modulating or more generally controlling the ionization rate of the sputtered material in the gas phase.

[0041] Another merit of the present invention is that the implementation of the FHiVI2regime only requires the introduction, in the power circuit, therefore outside the reactor, of a power source with high frequency and very high voltage switching system with rise times in the order of megahertz. In other words, no structural modifications of a conventional sputtering chamber, a conventional cathode assembly and / or anode are required in order to perform the present inventive process. That is to say, the inventive process may easily be implemented on any kind of conventional sputtering device.

[0042] As indicated above, the FHiVI2regime is compatible with other plasma sputtering processes, namely known conventional sputtering processes. Indeed, the present invention relies on applying at least one electron accelerating pulse on the plasma (i.e. after plasma ignition). Such (conventional) plasma sputtering process may be used for the plasma ignition phase, i.e. to generate the plasma, and hence the vapour of target material, thereby starting the sputtering process. Often, it is this other / conventional plasma sputtering process that is mainly used to control / sustain the sputtering process; it is therefore referred to as “reference sputtering process” in the following.

[0043] In preferred embodiments, the sputtering process comprises operating a reference plasma process by which the sputtering target is submitted to a controlled voltage bias configured to ignite a plasma and sustain the sputtering of target species. The electron accelerating pulse(s) is / are applied at predetermined timings on the ignited plasma.

[0044] The electron accelerating pulses may thus be coupled to this reference sputtering process, being applied simultaneously (meaning applied or over-imposed during the reference sputtering process - i.e. concurrently). This may be the case where the reference process is continuous or pulsed. Where the reference sputtering process is pulsed (reference cathode voltage pulses are applied, whereby a negative voltage bias is applied to the target), electron accelerating pulse(s) may be applied during a negative cathode voltage pulse, directly at the end of such pulse or thereafter. In other words, the reference plasma process drives the cathode voltage according to predetermined patterns able to initiate and sustain the plasma - and therefore able to operate the sputtering process by its own. The electron accelerating pulse(s) are applied on the plasma formed by the reference plasma process, however not during the plasma ignition phase. Often during an industrial process, plasma generation is cyclic / repetitive, in particular in pulsed plasma processes. There, each reference pulse may comprise an ignition phase, and an electron accelerating pulse is applied during the reference pulse or thereafter on the plasma (before extinction / post-discharge), and before the next reference pulse.

[0045] Generally, the reference plasma process may be a process selected from direct current (DC) magnetron sputtering process, pulsed direct current (DC) magnetron sputtering process, radio frequency sputtering process (using RF or microwave sources), high power impulse magnetron sputtering process, or a combination thereof.

[0046] In case of DC regime, the electron accelerating pulses are applied over the reference cathode voltage. In case of pulsed sputtering process, the electron accelerating pulses may be applied during reference cathode voltage pulses, or after (directly or with spacing).

[0047] In embodiments wherein the reference plasma process is a pulsed regime (i.e. application of reference voltage pulses), a reference pulse may be applied from a timing tpp.oN to tpp.oFF and the electron accelerating pulse may applied at a timing between tRp,oN+(tRp,oFF- tpp,0N) / 2 and tRp.oFF.

[0048] In embodiments, the pulsed reference plasma regime may be operated at a frequency F1 , whereby the start of a reference voltage pulse applied to the cathode is separated from the next one by a period T1 =1 / F1 . Preferably, an electron accelerating pulse is applied with respect to each reference voltage pulse at a timing that occurs no later than 50% of the reference voltage pulse period T1 .

[0049] In embodiments, the method comprises the step of generating a static magnetic field in the vicinity of the target during sputtering, in order to increase plasma density and improve target erosion.

[0050] In general, the FHiVI2regime may be operated with a predetermined frequency, e.g. 100 Hz to 250 kHz. That is, two consecutive electron accelerating steps should fall within the prescribed range, also in burst mode. In some applications, in particular with reference processes, suitable operating frequencies may be of 100 Hz or a few hundred Hz. For burst mode, the FHiVI2mode may be operated at 1 kHz and above, e.g. 10 to 100 kHz.

[0051] In embodiments, the reference plasma process is a pulsed regime by which reference voltage pulses are applied with frequency F1 , such as e.g. HiPIMS or RF sputtering, whereby the reference voltage pulses are applied with a reference cathode voltage to the sputtering target for a duration of more than 1 ps. The pulses of the FHiVI2regime may be applied at frequency F2 which is equal to F1 . The same pattern of pulses is thus repeated periodically. For example, one electron accelerating pulse is applied per reference voltage pulse, and this is repeated with same relative timing between the two pulses. This is however only an example and other implementations can be envisaged. Additionally or alternatively, a plurality of electron accelerating pulses may be applied during a pulse of a reference sputtering process, i.e. bursts of electron accelerating pulses combined with a reference sputtering process.

[0052] In embodiments, the reference process is a high power impulse magnetron sputtering process and the reference voltage pulses may have a duration of at least 10 ps, for example between 20 and 100 ps, in particular 30 to 50 ps and / or may have a controlled profile with an initial negative bias peak at no less than - 2 kV, preferably no less than -1.5 kV or -1 kV, to (re)igniteZionize the plasma, followed by a negative bias plateau at no less than -500 V. The electron accelerating pulse is applied after the initial negative bias peak (ignition phase) of the reference voltage pulse, and may be applied during the negative bias plateau and in particular after the reference voltage pulse (i.e. after the plateau).

[0053] In the present text, a negative bias peak of no less than -2 kV means that the absolute value of the negative bias peak is between 0 and 2 kV, i.e. that the initial negative bias peak has an absolute value of no more than 2 kV. Correspondingly, a negative bias plateau at no less than -500 V means that the absolute value of the plateau is between 0 and 500 V, i.e. that the negative bias plateau has an absolute value of no more than 500 V.

[0054] In embodiments, the initial negative bias peak (of a reference voltage pulse) has a duration of 0.5 to 3 ps, preferably of more than 1 .0 ps.

[0055] Preferably, when combined to a reference plasma process, the electron accelerating pulses are designed / optimized such that the floating potential in the vicinity of the cathode becomes positive. In other words, the electron accelerating pulses have a predetermined timing (duration) and amplitude configured to achieve a positive floating potential in the vicinity of the cathode. Positive floating potential is due to ionised species and thus reflects the increase of ionisation rate achieved by the application of an electron accelerating pulse after plasma ignition. Visualising the positive floating potential in the vicinity of the cathode is made easier by applying a polarity inversion step. The determination of the floating potential is known to the skilled person, and he is able by experimentation to determine when the vicinity of the cathode becomes positive by varying voltage and timing, for given operating conditions.

[0056] By “in the vicinity of the cathode” it is typically meant a space, or volume, that is situated close to the cathode, before the free surface of the target (i.e. from which target material is sputtered), e.g. covering a distance of 20 to 40 mm.

[0057] In embodiments, the reference pulse may be applied from a timing tpp.oN to tpp.oFF and the electron accelerating pulses is applied at a timing defined with respect to a predetermined pulverised species and such that said pulverized species have moved at a distance Ai from the sputtering target of the reference pulse, the application timing teap.oN of the electron accelerating pulse being calculated by resolving: wheretis the mean ionisation length, m is the mass of the sputtered species, Ei its initial kinetic energy and y is a transport coefficient > 1 .

[0058] The distance Ai covers the average distance that the sputtered species were able to travel but also considers all the species that are at this distance, therefore even the neutral species that backscatter by elastic collisions or that come from previous sputtering phases of the target and that are there but in a so-called thermalized state (energy close to 0 eV).

[0059] The above equation allows optimizing the timing of the electron accelerating pulse, having in mind that the plasma does not develop throughout the volume of the reactor but tends to exist around the cathode, whereby a certain time is needed to the pulverized species to pass into the densest plasma to be ionized and to obtain a maximum electron density in volume. In other words, the electron accelerating pulses are preferably applied not too close after the end of the reference pulse to allow displacement to a region where they will be influenced by the electron accelerating pulse(s).

[0060] The transport coefficient y reflects the possible deviations of the trajectory of the atoms during their transport and their slowing down (reduction of Ei) towards the place where they will be most likely to be ionized. These changes in direction and this slowing down are mainly due to the presence of gas atoms on the trajectory of the species to be ionized. In particular, y=1 when no deviation nor slowing down occurs, i.e. corresponding to a straight and uniform trajectory, and y>1 when modification of the trajectory and slowing down occur.

[0061] It should however also be appreciated that, alternatively, the FHiVI2regime can be implemented as a stand-alone regime, where bursts of electron accelerating pulses are applied to the target and will generate the plasma and subsequently provide the electron acceleration effect described above. This regime is referred to as ‘autonomous burst mode’. Here, there is no conventional reference plasma process that is applied in parallel. The voltage regulation is carried out by applying bursts of electron accelerating pulses. In such case, the first electron accelerating pulse of the burst sequence acts as ignition pulse (the ignition phase) and the subsequent accelerating pulses provide the enhanced ionization effect in the sense of the invention.

[0062] In burst mode, the electron accelerating pulses may be operated in sequences wherein the pulses are spaced by a predetermined time interval. In autonomous burst mode the electron accelerating pulses are not coupled with a reference plasma process, as the sequence of electron accelerating pulses is configured to generate a plasma, sustain the sputtering of target species and accelerate electrons. At least one electron accelerating pulse is performed after plasma ignition, as a post-ionisation pulse.

[0063] As explained, the FHiVI2regime implies the implementation of electron accelerating steps, either as repeated, individual electron accelerating steps, or as bursts pulses (burst pulses may be combined with a reference sputtering process or as autonomous mode).

[0064] The electron accelerating pulses may be repeated (combined with a reference sputtering process or as autonomous mode) with regular or variable frequency. There is in general no specific limit on the number of repetitions and the repetition frequency, the electron accelerating pulses being however separated by more than 1 ps and preferably more than 2 or 3 ps.

[0065] Preferably, the repetition rate of electron accelerating pulses is no more than 250 kHz, in order to avoid potential boundary effects. This ensures a spacing (the period) of at least 4 ps between two consecutive electron accelerating pulses. Spacing between two consecutive electron accelerating pulses may also be referred to as delay between consecutive pulses.

[0066] This is particularly desirable when operating burst pulses in combination with a pulsed mode reference puttering process.

[0067] One burst of pulses itself may comprise a few electron accelerating pulses, up to 100 pulses. For example, a set of 4, 5, 6, 7 or 8 pulses, or 10, 20, 30 or 50 pulses, or higher. In embodiments, a polarity inversion step (bipolar step) may be applied before and / or after the electron accelerating pulse, and possibly directly after.

[0068] In embodiments, the vacuum chamber may comprise more than one sputtering target, and the electron accelerating pulses may be applied to one or more of the sputtering targets, as desirable.

[0069] The sputtering target may comprise one or more materials, e.g. any metal or compound, in particular metals, metallic alloys, metal oxides ceramics, polymers or combination thereof.

[0070] In embodiments, the target material is selected from the post-transition and transition metals groups notably comprising tungsten, aluminium, copper, nickel, titanium, platinum, palladium, silver and gold, their respective alloys, their respective hydrides, their respective oxides, their respective nitrides and the mixtures thereof. Carbon and some alkaline earth metals and metalloids as magnesium, calcium, boron and silicon are also considered as target material.

[0071] Any appropriate plasmogenous gas may be employed, e.g. selected from reactive and inert gases. The plasmogenous process gas may be pure or mixed.

[0072] In embodiments, the plasmogenous process gas is selected from the group comprising helium, neon, argon, krypton, xenon, hydrogen, nitrogen, oxygen, and mixtures thereof.

[0073] In embodiments, the present process, respectively the reference supporting processes, are operated to generate a cold plasma.

[0074] Once again, the present invention relies on applying an electron accelerating pulse on the activated plasma, after plasma ignition. The inventive process is thus particularly adapted to be implement as sputtering process, wherein a negative bias voltage, referred to as reference cathode voltage, is applied to the sputtering target for a duration of more than 1 ps; and at least one electron accelerating pulse is applied during the reference process simultaneously. Here the FHiVI2regime is combined with a reference process. In case of DC regime, the electron accelerating pulses are applied over the reference cathode voltage. In case of pulsed sputtering process, the electron accelerating pulses may be applied during a reference cathode voltage pulse, or after (directly or with spacing).

[0075] During the reference plasma process the reference cathode voltage applied to the sputtering target may have an absolute value of no more than 2 kV or less than 1.5 kV or less than 1 KV. In case the reference cathode voltage is pulsed, the pulses have a duration that of at least 5, 10, 15 or 20 ps.

[0076] According to another aspect, the invention relates to a sputtering deposition process as claimed in claim 21 .

[0077] According to still another aspect, the invention relates to method of generating a propulsion plasma according to claim 22. The FHiVI2regime is indeed also appropriate to improve ionization rate of a sputtering process implemented for spatial propulsion.

[0078] According to yet another aspect, the invention relates a method of generating an ion beam as claimed in claim 23.

[0079] According to yet another aspect, the invention relates a plasma sputtering device as claimed in claim 24. The plasma sputtering device comprises a controller configured for implementing a sputtering process according to the first aspect of the invention. In particular, the controller is configured to cause the power source to deliver an electron accelerating pulse as a post-ionization pulse, i.e. after initiation of plasma generation.

[0080] Benefits of the inventive process may be summarized as follows:

[0081] - Any kind of plasma generating method may be used to generate a plasma;

[0082] - Any kind of material may be sputtered (no limitations regarding the nature of the target material);

[0083] - Any kind of plasmogenous gas may be used to generate a plasma;

[0084] - Easy implementation on already existing cathode sputtering devices;

[0085] - Improvement of the density (and possibly the degree) of ionization of species sputtered from the target material, without having to add coils around / in the reactor (in case of plasma generation by radio frequency sputtering) or additional magnet (in case of plasma generation by magnetron sputtering); - Regulation of the density of ionization of the sputtered material, e.g. via the difference of electric potential applied to the sputtering target during the step of accelerating electrons, its rise time, duration of its application and / or the number of repetitions of the step of accelerating electrons, e.g. via a burst mode;

[0086] - Applicable to any technical field associated with the use of a discharge regime in manufacturing processes: etching of cathode material, semiconductor doping, plasma assisted deposition / coating, plasma propulsion;

[0087] - Energy efficient process, faster deposition of target material, faster coating of substrates.

[0088] BRIEF DESCRIBTION OF THE DRAWINGS

[0089] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0090] Fig. 1 is a principle drawing illustrating the principle of the process according to the invention;

[0091] Fig. 2 are graphs representing the voltage and current variations at the sputtering target as functions of time for a first embodiment of a process according to the present invention implementing a conventional reference plasma process where a) represents whole sequence and b) is an enlarged view of the electron accelerating pulse;

[0092] Fig. 3 are graphs representing a) the voltage applied to the sputtering target being at the cathode, b) the current at the cathode and c) the resulting floating potential of the target as a function of time for the first embodiment of a process according to the present invention and for a comparative process;

[0093] Fig. 4 is a mass spectrum plotting the intensity of tungsten ions (W+) as a function of time for the first embodiment of the inventive process and for a conventional process;

[0094] Fig. 5 a) is a graph representing the intensity of the emission line of ionized tungsten W II as a function of time for the first embodiment of the inventive process and for a conventional process; and b) is a graph representing the respective intensities of the emission line of neutral tungsten W I and of the emission line of ionized tungsten W II as a function of time for the first embodiment of the inventive process;

[0095] Fig. 6 is a graph representing the ratio of the respective intensities of the emission line of neutral tungsten W I and of the emission line of ionized tungsten W II as a function of the bias voltage applied during the electron accelerating pulse;

[0096] Fig. 7 are graphs representing the voltage variations at the sputtering target as functions of time for other embodiments of the process according to the present invention;

[0097] Fig. 8 are graphs representing voltage and current variations at the sputtering target vs. time for a second embodiment of a process according to the present invention (whole sequence);

[0098] Fig.9 are graphs representing a) the voltage pulses applied to the sputtering target being at the cathode and b) the current at the cathode as a function of time for the second embodiment of a process according to the present invention;

[0099] Fig. 10 is a graph representing the voltage at the sputtering target as a function of time with indication of various observation times for the floating potential measured in the plasma near the cathode; and

[0100] Fig. 1 1a-e are two-dimensional representations of the evolution of the floating potential in front of the cathode as a function of the distance from the target (y- axis) and bias voltage of the electron accelerating pulse at the times of Fig. 9.

[0101] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0102] The inventive process will now be described with reference to Fig. 1 and Fig. 2, in an exemplary context of a HiPIMS deposition process as reference process.

[0103] A. The sputtering device

[0104] It may be noted that the inventive process may be implemented in a conventional sputtering device or system, such as a conventional cathodic sputtering device, which will therefore only be briefly described herein. In particular, it is not required to modify the (internal) structure of the process chamber (reactor). It may be noted that the present device, and exemplary process described herein, could be used in an industrial setting for surface treatment of materials, to deposit thin material layers on a sample / substrate.

[0105] As illustrated in Fig.1 , a sputtering device 10 comprises a target assembly 12 and a substrate assembly 16 arranged inside a process chamber 24 (or reactor) that defines an enclosed volume 26.

[0106] The sputtering process is typically conducted at pressures lower than atmospheric (e.g. below 10 Pa), i.e. in a vacuum environment. The process chamber 24 may thus be referred to as vacuum chamber.

[0107] In a sputtering deposition process the target assembly is often referred to as cathode, while the substrate assembly is the anode. The reactor walls may also be considered as anode if connected to the substrate holder.

[0108] The target assembly 12 typically comprises a sputtering target 14, which constitutes a solid material source. It Is received on a target holder 15, which forms the cathode electrode. The sputtering target 14 may consist of a single material or of a plurality of materials. The sputtering cathode may comprise any metal or compound, in particular metals, metallic alloys, metal oxides ceramics, polymers or combination thereof.

[0109] At the substrate assembly 16, a substrate 18, or deposition substrate, is arranged on a substrate holder 20 in front of the target assembly 12 and is spaced with respect to the same. The substrate holder 20 forms the anode electrode. Fig. 1 shows only one substrate 18, but there could be more on the holder 20. The substrate 18 can be any kind of element, part or components to be coated with target material ejected from the source (target 14).

[0110] A switch 21 allows connecting the substrate 18 to ground, leaving the substrate disconnected at ‘floating potential’, or applying a bias voltage when connected to an auxiliary voltage supply 23. The skilled person may thus select between these options depending on the operating mode. Reference sign 22 designates a power supply that is configurable to apply the energy needed to generate and sustain the plasma necessary for the sputtering process.

[0111] The power supply 22 may comprise direct current (DC) - pulsed (pulsed DC, HiPIMS, or not - and / or frequency power sources (radiofrequency or microwave frequency), depending on the application. The power supply 22 may further include controls to adjust the power levels, voltage, and current, as well as monitoring instruments to observe the process parameters and make adjustments, as needed.

[0112] In particular, the power supply 22 is configurable to operate according to the desired modes or regimes as described below, notably in the inventive FHiVI2regime.

[0113] In general, the power supply 22 is connected at one end to the target assembly 12 and at the other end to the substrate assembly 16 (as shown here) or to the walls of the reactor 24, in order to apply predetermined bias voltage levels to the cathode. In the context of the invention, the power supply is dimensioned to operate at several kV. The potential of the target assembly 12 is noted Vt and the reference potential is noted Vr.

[0114] A source of plasmogenous gas (not shown) allows introducing plasmogenous gas into the volume 26 between the cathode assembly 12 and the substrate 18.

[0115] During the sputtering process, the plasmogenous gas will be ionized through application of high voltage. The ionized gas, placed at suitable pressure conditions (for example 1 Pa), forms the plasma that provides ion species that will hit and erode the sputtering target, thereby ejecting / dislodging atoms, or more generally ‘sputtered species’ from the target.

[0116] The sputtered species (often atoms and / or neutrally charged small clusters) may then travel across the vacuum chamber and deposit onto the substrate placed opposite the target. This deposition forms a thin film on the substrate surface.

[0117] The FHiVI2regime is a transitory regime using electron accelerating pulses which are designed as high voltage pulses -high negative bias greater than -1 kV (i.e. greater than 1 kV in absolute value) of very short duration of less than (or up to) 1 microsecond. Such duration is considered appropriate for most cases, and can be shortened depending on the application. It is generally desirable that the duration of the electron accelerating pulse is shorter than ionic plasma oscillations for the respective species.

[0118] According to the equation of Langmuir waves, ionic plasma oscillations pulsation duration are defined as : [s] ; with e the charge, m,- the mass of the considered ions, n the plasma density and wpLthe ionic plasma oscillation. For each electron accelerating pulses a high negative voltage bias is applied to the sputter target 14. That is the voltage is controlled by the power supply 22 to reach a predetermined voltage (i.e. at least -1 KV) within the duration of the pulse. As used herein, the “high negative voltage bias” means that the difference of potential between the sputtering target, at potential Vt, and a reference at potential Vr, e.g. chamber walls and / or the substrate, is negative: Vt-Vr <0.

[0119] This can be obtained when where both Vt and Vr are negative or positive, or where Vt is negative and Vr positive.

[0120] In practice, the negative voltage bias applied to the cathode may be of at least -1 kV, meaning -1.5 kV, -2 kV, or -5 kV, etc. For example, the negative voltage bias may be between -1 kV and -20 kV.

[0121] In the following, for ease of speech, the terms “rise time” or “increase” may be used to describe the voltage control by the power supply to operate the electron accelerating step, because the potential difference increases.

[0122] B. of a first

[0123] Figs. 1 and 2 illustrate an embodiment of the inventive process, wherein the electron acceleration pulses are applied during a conventional HiPIMS deposition process (the reference sputtering process).

[0124] As will be known to those skilled in the art, HiPIMS is an advanced magnetron sputtering process. Therefore, magnetic element(s) (e.g. magnets or electromagnets, not shown) are arranged at the target assembly, spaced apart from the sputtering target material. The magnetic elements are configured to generate static magnetic fields that increase plasma density, maintain a stable and uniform plasma discharge, leading to improved target erosion and ultimately to consistent coating quality across the substrate at higher deposition rate (increased deposition / process yield).

[0125] The method starts with operating a reference sputtering process, here a HiPIMS mode (or regime), which will generate a plasma and start the sputtering and deposition process.

[0126] As conventionally known in the field, the HiPIMS regime involves applying high- energy pulses of power to the target material. During such pulses, a high voltage bias (the reference cathode voltage) is applied with very short variable pulse durations (e.g. 50 to 200 ps).

[0127] During this step, and represented in Fig. 1 a, the plasma is generated after an electronic avalanche, leading to the ionization of the plasmogenous gas in the reactor, and the parallel production of electrons 32 in the gas. The plasmogenous gas ions 28 accelerated in the direction of the target 14 induce the sputtering of material of the target material, i.e. the sputtered species 30. The volume is thus enriched in species from the target and in electrons.

[0128] Turning to Fig.2 a), a HiPIMS pulse is indicated P1 , which here is a voltage signal with a peak-type transient phase of several microseconds and then a plateau at about -0.4 kV. As is known in the art, these first microseconds of the HiPIMS pulse allow igniting the plasma, hence corresponding to a plasma ignition phase. Next, the FHiVI2regime is coupled to the HiPIMS mode, by applying an electron accelerating pulse, noted P2, directly at the end of the HiPIMS pulse P1. In this embodiment, the electron accelerating pulse has a duration of less than 1 ps (about 950 ns) and a negative bias voltage of -2 kV is applied to the target.

[0129] The electron accelerating pulse is a transitory pulse, in that it has a very short duration according to the equation of Langmuir waves, in particular compared to the HiPIMS pulses. It can be observed that the electron accelerating pulse here consists mainly of voltage transients. More specifically, the electron accelerating pulse structure consists of: a rising period: the first portion (or rising edge) of the curve where the voltage increases negatively from the starting point Vs (voltage at the start of the electron accelerating pulse) down to the target value, i.e. here -2 kV; a very short plateau where the voltage remains at the target, here less than about 100 ns; and a fall period: the cathode voltage decreases from the plateau towards zero.

[0130] By definition, the duration of the pulse is measured from the moment the pulse is applied (with start voltage Vs being the cathode voltage at the moment the accelerating pulse is applied) until the voltage decreases from the plateau and reaches again the same value Vs.

[0131] It may be noted that whereas in this embodiment the FHiVI2pulse P2 is applied directed after the HiPIMS pulse P1 , other configurations are possible. For example, pulse P2 may be performed (at least partially) simultaneously to step P1 or after a predetermined time after pulse P1 . The voltage and duration of pulse P2 may be modulated depending on its time of application.

[0132] In the shown example, the difference of electric potential applied during the electron accelerating pulse is, compared to the HiPIMS pulse, quite high (fivefold the plateau of the HiPIMS pulse) and very rapid (the -2 kV target is reached in approximately 350 ns, i.e. with a rise speed dV / dt of about 5.7 kV / ps). These temporal characteristics (e.g. rise time, fall time) of the difference of electric potential applied to the cathode / sputtering target provoke a sudden accelerations of the electrons in the volume with respect to the heavy species of the plasma (neutral elements, ions, ... ). In other words, due to the bias voltage of the electrons accelerating step and its very fast application time, i.e. rise time, it is possible to accelerate electrons 32 - and electrons only. The other charged species (e.g. ions) will not be accelerated since they will not be sensitive to variations of the electric field that are that fast (of the order of MHz) and of short duration (at the microsecond scale). The acceleration of the electrons is represented in Figi b). The electrons 32 which have acquired a kinetic energy corresponding approximately to the difference of electric potential applied during the step of accelerating electrons then leave the vicinity of the cathode / sputtering target and are able to participate in the ionization of the material vaporized from the target material 14, i.e. the atoms 30 detached from the target material 14 by the hitting ions 28 from the plasma along their path toward to the substrate 18, thereby forming ions 34 (see Fig. 1 c).

[0133] In this embodiment, the sputtering process comprises an additional step of applying a positive voltage bias to the cathode (or positive impulse), thereby performing a polarization inversion step. This is referred to as bipolar step. The bipolar step, P3 in Fig.2 b, starts directly at the end of step P2. The amplitude of the positive impulse, its duration and the moment of its application can be independently controlled. This results in the acceleration of the positively charged species (ions) towards the substrate.

[0134] As can be observed, electron accelerating pulses have specific voltage and current characteristics. For the voltage, a single, simple pulse shape is observed (in particular, no combination of several waveforms is observed). As for the current characteristics, there is a very rapid increase in (negative) current over times of the order of a hundred nanoseconds and a plateau that is established very quickly. This plateau is characteristic of the charged state of the plasma.

[0135] The electron accelerating pulse according to the present invention thus provides for both a strong displacement current (often intense but local displacement of charge densities which generates the propagation of an electromagnetic wave in the medium to be considered, and results in capacitive effects) and a strong conduction current (movement of charges by circulation thereof in the electric circuit, resulting in resistive effects) as opposed to a discharge establishment pulse providing only a displacement current. C. Further results

[0136] The graphs of Fig.2 have been obtained with a sputtering device as illustrated in Fig.1 . The target material was tungsten and the target was spaced by 17.5 cm from the substrate. The working pressure in the chamber was 1 Pa. Pure argon was used as plasmogenous gas.

[0137] HiPIMS + bipolar pulse parameters: HiPIMS pulse of -400 V for 22 ps followed by a positive impulse of 150 V for about 10 ps. Pulse frequency 200 Hz.

[0138] FHiVI2pulse parameters: -2 kV pulse with a duration of 900 ns. Pulse frequency 200 Hz. The FHiVI2pulse is applied at the end of the HiPIMS pulse.

[0139] Contrary to the representation of Fig.1 , the substrate is not grounded but left at floating potential.

[0140] The implementation of the electron accelerating pulse, resp. of the FHiVI2regime, in combination with the reference sputtering process (HiPIMS) induces notable modifications in the plasma present in the reactor, in particular on the directly measurable electrical characteristics thereof.

[0141] Fig.3 is illustrate plots of a) voltage, b) current and c) floating potential over time for the sputtering process with Fig.2 (i.e. with FHiVI2) and b) for the reference process (HiPIMS+bipolar) alone.

[0142] The application of the electron acceleration pulse causes a sharp decrease of voltage, because of the additional difference of electric potential being applied to the sputtering target. The current measured at the cathode increases to reach a maximum value (in absolute value) of approximately 12 A in 0.8 ps whereas in the absence of an electron acceleration step, the current does not exceed, in absolute value, 5 A (Fig. 3b). This increase in current reflects an increase in the concentration of charged species, i.e. ions and electrons, at the cathode.

[0143] The potential at the substrate holder is left floating (i.e. not grounded); its evolution of thus dependent on the plasma itself (Fig.3c). When entering the bipolar step P3, the floating potential increases more rapidly for the process using FHiVI2than according to the reference method comprising only the HiPIMS discharge. This faster increase in the floating potential indicates that the ions flow gained in energy and / or that a higher quantity of positive charges (ions) in the reactor is also generated, i.e. the charges being generated during the electron acceleration step.

[0144] The faster variation of the floating potential observed with the FHiVI2regime highlights a modification of the plasma composition in the reactor in terms of charges and therefore of the concentration of ions over time, due to the application of an additional difference of electric potential (negative bias voltage of the electron accelerating pulse).

[0145] Time- and energy-resolved mass spectrometry measurements of the plasma in the reactor (Fig. 4) as well as optical emission spectroscopy measurements (Fig. 5a) also show an increase in signal intensity for the tungsten ions W+, and therefore an increase in the concentration of tungsten ions W+, when the sputtering process comprises an electron acceleration pulse as compared to a conventional HiPIMS process (i.e. without FHiVI2).

[0146] The optical emission spectroscopy measurements further indicate that the plasma obtained using the process according to the invention comprises approximately 1.4 times more tungsten ions W+(W II) than neutral tungsten W (W I) (Fig. 5b), in this specific case.

[0147] Further results were obtained by varying the absolute value of the bias voltage Vt applied to the sputtering target during the electron accelerating pulse (Fig. 6). The ratio of the respective intensities of the emission line of neutral tungsten W I and of the emission line of ionized tungsten W II increases with an increase (in absolute value) of the cathode bias voltage Vt.

[0148] Figs. 8 to 10 illustrate another embodiment of the inventive process, wherein the electron acceleration pulses are applied during a conventional HiPIMS deposition process (the reference plasma process).

[0149] As for the first embodiment, the method starts with operating a reference sputtering process, here a HiPIMS mode (or regime), which will generate a plasma and start the sputtering and deposition process. On Fig.8, a HiPIMS pulse is indicated P’1 , which here is a voltage signal with a peak negative transient phase of several microseconds and then a plateau at about -0.7 kV.

[0150] Next, the FHiVI2regime is coupled to the HiPIMS mode, by applying an electron accelerating pulse, noted P’2, at the end of the HiPIMS pulse P’1 , after expiry of a time period of about 28 ps. With the HiPIMS pulse starting at 0 ps, the electron accelerating pulse P’2 starts at about 49 ps. The electron accelerating pulse P’2 has a duration of less than 1 ps (about 950 ns) and, in the embodiment of Fig. 8 and 9, a negative bias voltage of -3 kV is applied to the target.

[0151] The timing of application of the electron accelerating pulse P’2 may vary after the end of the HiPIMS pulse P’1 , as shown on Fig. 9. While delaying application of the electron accelerating pulse P’2 after the end of the reference plasma pulse P’1 does not affect the voltage (as expected, as the voltage is applied by the power source - Fig. 9a) the resistivity of the environment, in particular of the plasma, varies thereby negatively affecting the current (Fig. 9b).

[0152] As for the first embodiment, the electron accelerating pulse is a transitory pulse, in that it has a very short duration, in particular compared to the HiPIMS pulses. It can be observed that the electron accelerating pulse here consists mainly of voltage transients.

[0153] By definition, the duration of the pulse is measured from the moment the pulse is applied (with start voltage Vs being the cathode voltage at the moment the accelerating pulse is applied) until the voltage decreases from the plateau and reaches a smaller absolute negative voltage on the same order of Vs, preferably of Vs and more preferably of 0 V.

[0154] In this embodiment, the sputtering process comprises an additional step of applying a positive voltage bias to the cathode (or positive impulse), thereby performing a polarization inversion step. This is referred to as bipolar step. The bipolar step, P’3 in Fig.8, starts directly at the end of step P’2. The amplitude of the positive impulse, its duration and the moment of its application can be independently controlled. This results in the acceleration of the positively charged species (ions) towards the substrate. A spatial and temporal measurement of the charge fluxes (flux of positive charges and flux of negative charges) near the cathode is carried out by measuring the floating potential in front of the cathode at different times (to to tE - see Fig. 10), while maintaining constant the moment of application of the electron accelerating pulse (i.e. start of step P’2 at 49 ps) and varying the absolute value of the negative bias voltage of the electron accelerating pulse from 0 to 3 kV.

[0155] Fig. 11 a-e report the measurement of the floating potential at different timings (to to t ), thereby illustrating - at different times - the consequence of the different regimes in the volume. A positive floating potential reflects a predominance of ions flow (flux of positive charges) while a negative floating potential reflects a predominance of electrons flow (flux of negative charges).

[0156] Time to corresponds to the time preceding the application of the FHiVI2regime (see Fig. 10). The presence of a vertical color gradient only in the direction away from the target (Fig. 11 a) indicates an invariant situation regardless of the voltage conditions applied during the FHiVI2regime (i.e. not influenced by the negative bias voltage of the electron accelerating pulse). This invariance is entirely expected since at this measurement time, the FHiVI2regime has not yet been applied. This mapping also makes it possible to highlight a distance for which the electron flow remains predominant (20 mm from the surface of the target).

[0157] The measurement time ts corresponds to the end time of the FHiVI2pulse (see Fig. 10), i.e. one microsecond after its start and precedes the application of the bipolar step P’3. From this measurement time, the mapping (Fig. 11 b) shows a strong difference depending on the negative bias voltage of the electron accelerating pulse. For a negative bias voltage from 0 to -1 kV the volume in front of the cathode exhibits a rather ionic flux dominance and for a negative bias voltage from -1 to -3 kV the volume then exhibits a rather electronic flux dominance. In other words, this mapping (Fig. 11 b) highlights the consequence of the sudden acceleration of electrons for negative bias voltages greater than - 1 kV.

[0158] From tc onwards, the dominance of the ion flux begins to appear for applied negative bias voltages (UFHIVF) inferior or equal to -2 kV (Fig. 11 c). As the measurement time increases, the dominance of the ion flux extends more and more towards low negative bias voltages in absolute value (Fig. 11 d). Finally, the measurement time tE shows a threshold condition at -1.5 kV beyond which the state of charges is clearly different near the cathode (Fig. 11 e). Thus, for UFHIVI2voltages lower than this condition, the observed dominance of electrons close to the target supports that the FHiVI2conditions are not sufficient to effectively modify the volume, by ionizing the sputtered neutral atoms. On the contrary, beyond this threshold condition, a dominance of ions is observed, the result of an effective action of the FHiVI2regime which cannot be obtained by any other discharge regime. This result allows to show the change of behavior induced by the FHiVI2regime.

[0159] E. Autonomous burst mode

[0160] As mentioned above, the inventive FHiVI2regime can be implemented as a stand-alone regime (i.e. without coupling to a reference sputtering process), in so-called “autonomous burst mode”, where bursts of electron accelerating pulses are applied to the target and will generate the plasma and subsequently provide the electron acceleration effect described above. In general, series of pulses may each comprise 3 to 10, such as 2 to 8, e.g. 6, pulses with a frequency of 20 kHz and series may be repeated at a frequency of about e.g. 200 Hz.

[0161] An example of the FHiVI2regime operated in autonomous burst mode is shown in Fig. 7a: each burst comprises a set of 6 consecutive electron accelerating pulses with a frequency of 20 kHz (i.e. period of 50 ps) and the burst frequency is 200 kHz (i.e. the spacing between two consecutive bursts is 50 ps).

[0162] Another example is shown in Fig. 7b, where each electron accelerating pulse within the bursts is directly followed by a polarity inversion step. Alternatively, the polarity inversion steps (if any) may be applied solely for the last burst of the series.

Claims

Claims1. A sputtering process of a target material in a vacuum environment comprising a sputtering target and plasmogenous process gas, characterized in that a transitory, electron accelerating pulse is applied to the sputtering target after plasma ignition to enhance ionization, whereby applying said electron accelerating pulse involves applying a high negative bias voltage to the sputtering target during a period of time shorter than 1 microsecond, the bias voltage having an absolute value greater than 1 kV.

2. The sputtering process according to claim 1 , wherein the negative bias voltage of the electron accelerating pulse has an absolute value higher than 1 .5 kV, preferably higher than 2 kV and / or lower than 20 kV.

3. The sputtering process according to claim 1 or 2, comprising: operating a reference plasma process by which the sputtering target is submitted to a controlled voltage bias configured to ignite a plasma and sustain the sputtering of target species; and the electron accelerating pulse(s) is / are applied at predetermined timings on the ignited plasma.

4. The sputtering process according to claim 3, wherein the reference plasma process is selected from a direct current (DC) magnetron sputtering process, pulsed direct current (DC) magnetron sputtering process, radio frequency sputtering process (using RF or microwave sources), high power impulse magnetron sputtering process, or a combination thereof.

5. The process according to claim 4, wherein the reference plasma process is a pulsed regime by which reference voltage pulses are applied with frequency F1 , whereby the reference voltage pulses are applied with a reference cathode voltage to the sputtering target for a duration of more than 1 ps.

6. The process according to claim 5, wherein the reference voltage pulses have a duration of at least 10 ps, for example between 20 and 100 ps, in particular 30 to 50 ps.

7. The process according to claim 5 or 6, wherein the reference voltage pulses have a controlled profile with an initial negative bias peak at an absolute value of no more than 2 kV, preferably no more than 1.5 kV or 1 kV, to (re)ignite / ionize the plasma, followed by a negative bias plateau at an absolute value of no more than 500 V.

8. The process according to claim 7, wherein the initial negative bias peak of the reference voltage pulse has a duration of 0.5 to 3 ps, preferably of more than 1.0 ps.

9. The process according to any one of claims 5 to 8, wherein the reference voltage pulse is applied from a timing tRp.oN to tRp.oFF and the electron accelerating pulse is applied at a timing between tRp,oN+(tRp,oFF- tRp,0N) / 2 and tRP.OFF.

10. The process according to any one of claims 5 to 8, wherein the electron accelerating pulses are applied at a predetermined timing after a reference voltage pulse and before the next one.11 . The process according to claim 10, wherein the electron accelerating pulse is applied at a timing that occurs no later than 50% of the period of the reference voltage pulse.

12. The process according to any one of the preceding claims, wherein the electron accelerating pulses have a predetermined timing and high negative bias voltage configured to achieve a positive floating potential in the vicinity of the cathode.

13. The process according to any one of claims 3 to 12, wherein the reference voltage pulse is applied from a timing tpp.oN to tRp.oFF and the electron accelerating pulse is applied at a timing defined with respect to a predetermined pulverised species and such that said pulverized species have moved at a distance from the sputtering target, the application timing teap.oN of the electron accelerating pulse being calculated by resolving:whereis the mean ionisation length, m is the mass of the sputtered species, Etits initial kinetic energy and y is a transport coefficient > 1 .

14. The sputtering process according to any of the preceding claims, wherein a voltage rise time of the electron accelerating pulse is higher than 2 kV / ps in absolute value.

15. The sputtering process according to claim 14, wherein the voltage rise time is higher than 5 kV / ps, preferably higher than 10 kV / ps.

16. The sputtering process according to any one of the preceding claims, wherein an establishment time of the negative bias voltage of the electron accelerating pulse represents up to 45 to 50% of the duration of the electron accelerating pulse, preferably 25, 20, 15, or 10%.

17. The sputtering process according to any one of the preceding claims, comprising performing a plurality of electron accelerating pulses at a predetermined frequency, which is no more than 250 kHz, in particular being applied as sequences of pulses.

18. The sputtering process according to any of claims 1 or 2, wherein sequences of electron accelerating pulses are performed, the sequences being configured to generate a plasma, sustain the sputtering of target species and accelerate electrons.

19. The sputtering process according to any one of the preceding claims, wherein a polarity inversion step is applied before and / or after the electron accelerating pulse.

20. The sputtering process according to any one of the preceding claims, wherein the plasmogenous process gas is selected from the group comprising helium, neon, argon, krypton, xenon, hydrogen, nitrogen, oxygen, and the mixture thereof.21 .A sputtering deposition process for depositing a layer of a target material onto a substrate in a reactor comprising a sputtering target and plasmogenous process gas, by performing the sputtering process according to any one of the preceding claims.

22. A method of generating a propulsion plasma using the sputtering process according to any one of claims 1 to 20.

23. A method of generating an ion beam using the sputtering process according to any one of claims 1 to 20.

24. A plasma sputtering device comprising a vacuum chamber comprising a sputtering target and connected to a source of plasmogenous gas, and a power supply, characterized by a control unit configured operate the power supply in order to implement the method according to any one of claims 1 to 20.

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