LIQUID SPUTTER TARGET

Abstract

A sputtering device to sputter a liquid target. The sputtering device including a trough to receive a liquid target material and a device to stir or agitate the liquid target material. The device configured to degas the liquid target material or/and to dissipate solid particles or islands on a surface of the target or/and to move such particles or islands from an active surface region to a passive surface region and/or vice-versa, whereby the passive surface region is at least 50% less exposed to sputtering as the active surface region.

Claims

1) Sputtering device to sputter a liquid target, comprising a trough to receive a liquid target material, wherein the device comprises means being configured to effect at least one of stirring and agitating the liquid target material, degassing the liquid target material, dissipating solid particles or islands on a surface of the target, and moving such particles or islands from an active surface region (S.sub.A) to a passive surface region (S.sub.P) or vice-versa, the passive surface region (S.sub.P) being at least 50% less exposed to sputtering as the active surface region (S.sub.A) wherein the means comprise a planar magnetron system with at least an outer and an inner magnet having reversed polarities and being mounted recessed in the reverse side of the trough, or at or in the bottom of the trough below the liquid level of the target, the planar magnetron system being designed to enable a horizontal and a vertical convection of the liquid target material during sputtering.

2) The device according to claim 1, wherein the target can be electrically connected to one of the cathode of a DC-power supply, the cathode of a pulsed DC-power supply, or to one or several power supplies providing respectively one of a DC/RF-mix, a DC/pulsed DC-mix, an RF/pulsed DC-mix or an DC/RF/pulsed DC-mix.

3) The device according to claim 1, further comprising an anode electrically isolated from the liquid target, the anode being arranged circumferential round the target or/and in a central region of the target.

4) The device according to claim 1, wherein at least a part of the passive surface region (S.sub.P) is projected by a darkroom shield, electrically isolated from the target.

5) The device according to claim 1, wherein a bottom of the trough is formed concave or convex at least in a peripheral region.

6) The device according to claim 1, wherein the means to stir comprise a stirring unit comprising a stirrer positioned in the liquid target material and a drive mounted at or near the reverse side of the trough.

7) (canceled)

8) (canceled)

9) The device according to claim 6, wherein the drive comprises a magnetic unit configured to create a rotating electric field and the stirrer comprises a magnetic bar or is a magnetic stir bar.

10) The device according to claim 1, wherein the means to agitate the liquid target comprise an agitation unit having an inner and an outer agitation-electrode, and a magnet system, whereby at least the inner agitation-electrode is at least in part below the liquid level of the target material, and at least the inner agitation electrode being connected to a DC-current, a pulsed DC-current source or a low frequency AC-current source.

11) The device according to claim 10, wherein the outer agitation-electrode is at least in part below the liquid level of the target material.

12) The device according to claim 10, wherein at least a part of the passive surface region (S.sub.P) is projected by a darkroom shield, electrically isolated from the target and the outer agitation-electrode is the anode.

13) The device according to claim 12, wherein the inner agitation-electrode is at least a part of an inner surface of the trough being in contact with the liquid target.

14) The device according to claim 13, wherein the inner agitation-electrode is the inner surface of the trough being in contact with the liquid target.

15) The device according to claim 1, wherein the liquid target material is at least one of or an alloy of one of the following materials: Ga, GaAs, GaIn, GaAl, InAl, TiGa, Hg, HgAg, HgAu, HgCu, HgIn, HgSn, HgZn.

16) (canceled)

17) (canceled)

18) Vacuum coating system comprising: the sputtering device according to claim 1; a sputter gas-inlet; and a vacuum pump.

19) (canceled)

20) (canceled)

21) The system according to claim 18, wherein the system (10) comprises a measurement device (47) to measure a sputter voltage and an electronic device (48) comprising a comparator (49), one input I.sub.V of the electronic device (48) being connected with the output signal or a transformed output signal S1 of the measurement device (47) and the other input of the comparator being connected with an output from an electronic benchmark database (50) to compare signal S1 and signal S2 from the benchmark database (50) to produce a liquid level signal SLL at an output O of the electronic device (48).

22) The system according to claim 21, wherein the output O is connected to a liquid level indication device (52) and/or to a refill device (53) of the system.

23) A method for depositing a film on a substrate, whereby at least one first component is sputtered by a sputtering device comprising a liquid metallic target material placed in a target trough and sputtered from the target to the surface of a substrate to be coated, and the target comprises at least an active surface region (S.sub.A) exposed to sputter ions and a passive surface region (S.sub.P) at least 50% less exposed to sputtering as the active surface region (S.sub.A), wherein the method comprises agitation of the liquid target to degas the liquid target material or/and to dissipate solid particles or islands or/and move solid particles or islands from the active surface region (S.sub.A) to the passive surface region (S.sub.P) and/or vice-versa, wherein agitation enforces horizontal and vertical convection in the liquid target during sputtering.

24) The method according to claim 23, wherein the film is a metal, an alloy, or a compound film and at least one first metallic component is sputtered from the target to the surface of the substrate to be coated and in case of a compound film, at least one second component of the compound is introduced as a reactive gas to form the compound in the gas phase or/and on the surface of the substrate and/or the surface of the film.

25) The method according to claim 23, wherein the passive surface region (S.sub.P) is in a peripheral or/and in a central area of the liquid target surface.

26) The method according to claim 23, wherein at least a part of the passive surface region (S.sub.P) is projected by a darkroom shield, electrically isolated from the target to shield the region (S.sub.P) against sputtering or other glow discharge phenomena.

27) The method according to claim 26, wherein the darkroom shield forms a counter electrode of the target.

28) The method according to claim 23, wherein the sputtering device is a magnetron, comprising a magnetron magnet system and a DC-power supply or a pulsed DC-power supply.

29) The method according to claim 23, wherein the agitation of the liquid target is performed by mechanical stirring, by stirring induced by a current flow, by applying ultrasonic waves to the liquid target or by a combination of stirring and applying ultrasonic waves.

30) (canceled)

31) (canceled)

32) The method according to claim 29, wherein the agitation is performed by an agitation unit comprising an inner and an outer agitation-electrode, both being connected to a DC-current source, whereby a DC-current is applied between the agitation-electrodes to generate a circulating movement of the target liquid at least between the agitation-electrodes.

33) The method according to claim 23, wherein the target material is a liquid metal or an alloy of at least two metals having a melting point TM≤300° C. or TM≤70° C. or TM≤40° C.

34) (canceled)

35) (canceled)

36) (canceled)

37) The method according to claim 23, wherein a further target from a different material is used to deposit a compound at the substrate surface.

38) The method according to claim 23, wherein a dependency of one of the electric values sputter voltage, sputter current, or sputter power from the liquid level of the liquid target is used to produce a liquid level signal SLL by measuring the value V1 or a transformed value signal S1, and comparing it with a benchmark value V2 or a respectively transformed value signal S2 corresponding to a defined liquid level.

39) The method according to claim 38, wherein the liquid level signal is used to drive a liquid level indication device and/or to control a target liquid refill device of the system.

40) A method of manufacturing a semiconductor device according to claim 23, wherein the film is deposited on a semiconductive substrate.

41) The system according to claim 18, further comprising: an inlet for reactive gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention shall now be further exemplified with the help of figures. The figures show:

[0040] FIG. 1: A vacuum system comprising a sputter device according to the state of the art;

[0041] FIG. 2: A scheme of an inventive sputter device;

[0042] FIG. 3: An inventive sputter device with magnetic stirrers;

[0043] FIG. 4: An inventive sputter device with Lorentz force stirring;

[0044] FIG. 5: A further inventive sputter device with Lorentz force stirring;

[0045] FIG. 6a-c: Further inventive sputter devices with Lorentz force stirring;

[0046] FIG. 7a-c: Further inventive sputter devices with Lorentz force stirring.

[0047] FIG. 8: A further inventive sputter device with Lorentz force stirring;

[0048] FIG. 9: An inventive device and system comprising a liquid refill system;

[0049] FIG. 10: Voltage/current characteristics;

[0050] FIG. 11: Voltage/liquid level relation.

[0051] FIG. 2 shows a schematic drawing of an inventive sputter device comprising a mechanical stirrer 25, driven by a drive 28 via an axis 26. The axis 26 is guided by a liquid tight, and in the case the trough bottom forms a part of the bottom of the vacuum chamber, also a vacuum tight feedthrough 27 in the bottom of the target trough 2. Otherwise as obvious to the man of art a respective vacuum tight feedthrough has to be provided in the bottom of the vacuum chamber of the system. By setting the stirrer to work, the liquid material of the target is stirred and the static surface 4 of the liquid turns to a dynamic surface 4′ symbolized by its waveform. Actual dynamic surfaces 4′ however will rather be formed vortex-like at least with embodiments as drafted in FIGS. 2 and 3. The bottom of the trough 24 can be formed concave as shown in the figure or convex as mentioned above at least in a border area of the trough-bottom and symbolized by dotted lines 24′ or be completely concave, as symbolized by dashed lines 24″ or completely convex to compensate for the high surface tension of liquid metals which might otherwise cause non-wettable surface areas in and near the circumferential inner edge of the trough and thereby forming gas pockets under the liquid surface, which might effect gas break through during the sputter process. For better heat dissipation a liquid circuit 36 can be implemented directly into the trough-bottom and be combined, e.g. with a closed cycle cooling system. Alternatively a combined heating and cooling system can be used to temper the target material between and/or before sputter processes take place, which has the benefit that target materials having a higher melting point, e.g. above room temperature, can be liquefied before sputtering takes place and thereby avoid local and critical phase shift during the sputter process, e.g. on the surface of the target. Before or in the beginning of the sputter process, e.g. in dependency of the sputter power or a temperature output of a temperature measurement device, not shown, which can be placed in a side wall of the trough, the heating and cooling system can be switched from heating to cooling. The anode 7 may project above a border area, e.g. a circumferential border area of the liquid target 3, and thereby form a passive surface region S.sub.P. Swivel-mounted shutter 45 facilitates to protect the substrate surface against unintentional coating, e.g. during initial sputtering to clean or degas the liquid target. Such shutter can also be realized as iris-wheel, or slide shutter.

[0052] As mentioned features like shutter(s), concave or convex trough bottom 24′, 24″, liquid circuit 36, provision of a heating and cooling system and the like as mentioned above can be useful for any embodiment of the present invention even when no explicit reference is made to it.

[0053] The same refers to certain additional features which can be applied to the vacuum coating system 1 to make deposition processes comprising an inventive sputter device even more efficient. Such features, which have been superimposed to the vacuum system in FIG. 1 for ease of use, are a separate reactive gas-inlet 22 in the cover 11 of the vacuum chamber, means 17, 18 for applying a bias to the substrate holder and means to cool and/or heat 16 the substrate holder, respectively the substrate. It should be mentioned that such features are not known to be disclosed by state of the art documents together in combination with liquid sputtering.

[0054] By use of the separate reactive gas-inlet 22 near to the substrate 14 surface, partial pressure of the reactive gas can be reduced, the gas-inlet 21 can be used as inert gas-inlet only and be placed in the height of the target surface 4, 4′ allowing a better flushing or protection of the surface 4, 4′ from reactive gas. Both measurements can further reduce target poisoning and formation of solid particles or islands on active surfaces S.sub.A of the target surface 4, 4′. Additionally, gas showers may be used along the circumference of the substrate holder 15 for the reactive gas inlet 22 and/or along the circumference of the target for the sputter-gas inlet 21.

[0055] The means for applying a bias to the substrate holder may comprise a bias-power supply 17, which again may be a DC (−) but usually at least for non-conductive substrate materials will be a high-frequency (HF), for instance a radio-frequency (RF) power supply, connected via a power line 18 and a feedthrough to the substrate holder 15. The holder 15 on the other hand may comprise a holder-circuit for cooling or for cooling and heating with respective cooling or cooling and heating means. By use of an electrostatic chuck (ESC) clamping and thereby shadowing of certain areas of the substrates 14 surface can be avoided.

[0056] It has to be mentioned that essential features of the state of the art sputter device 1′, as magnet system 5, power supply 23, power line 8 and the like as shown in and described with FIG. 1 has to be assumed to be present also with FIG. 2 to 5, also if not explicitly shown or mentioned to maintain the clarity of the drawings.

[0057] FIG. 3 demonstrates a further type of mechanical stirring, showing two magnetic bars 29 positioned in the liquid target 3 which are driven by magnetic stirrers 30 on axis 31. Such stirrers 30 can be replaced in a known way by stirrers using a rotating electrical field instead of further rotating magnets. Such magnetic stirring means 30, 31 can be placed, e.g. within the race track of a magnetron target. Further supporting or stand alone agitation of the liquid target material can be provided by ultrasonic sources 32 mounted at the sidewalls and/or at the bottom of the trough 2 (see FIG. 4).

[0058] FIG. 4 and FIG. 5 show a different principle to stir or agitate liquid targets, making use of a Lorentz force phenomenon as explained in the summary of the invention. Thereby FIG. 4 shows a simple set up with a central two-dimensional, e.g. circular or linear electrode 33 or 3-dimensional, e.g. rod or wall like electrode 33′ (dashed lines) depending on the target geometry which can be round, square rectangular or otherwise elongated. The electrode is fed by a separate electrode current-supply 34 and is feed through electrically isolated from the trough. The current can be a DC or a pulsed DC current. With this arrangement the anode can have the function of the outer agitation-electrode, with the agitation current flowing from the inner agitation electrode via liquid target and the conductive plasma to the anode. The DC-potential of the electrode can be set between the anode potential, which is usual ground, and the cathode potential of the sputter target, which can be between at least −100 and about −4 kV. Therefor the DC-potential of the agitation electrode can be set from −10 to about −3.9 kV for usual sputter processes. Additional to the magnetron magnets 5, 5′, 5″ further magnets 44 can be provided, e.g. as side magnets at the circumference of the trough, to enforce vertical magnetic field components within the liquid target material. Such magnets can be solid permanent magnets or an electric magnet with a magnetic force that can be controlled by the applied current. Such magnets might be arranged as a ring (or any given array of magnets.

[0059] A Further embodiment of the invention is shown in FIG. 5 with an inner, here horizontally linear agitation-electrode 33″, and an outer agitation-electrode 35, here in form of a closed loop, as shown in the superposed vertical section in dashed lines. Inner electrode 33″ and outer electrode 35 are connected to different poles of the electrode current-supply 34 in analogy to the connection described with FIG. 4. Such agitation-electrodes can be positioned in parallel to the targets race track to provide a circular or rotational movement along the race track as symbolized by dashed arrows 40. It should be mentioned that rotational direction can be reversed, if polarities of the outer and inner magnetron magnets are reversed. Agitation electrodes 33, 33′, 33″, and 35 can be designed in one piece or as split electrodes comprising several electrode sections or sub-electrodes. In another embodiment such electrodes can be designed swivel- or movable-mounted and submersible into the target liquid (not shown). Such embodiments as well as mechanical stirring can be used for instance when agitation should be performed before the sputter process starts or agitation induced by the sputter electrodes does not suffice.

[0060] FIGS. 6 to 7 refer to operative inventive sputter devices with Lorentz force stirring, which use the magnetron magnet-field 6 and DC or pulsed DC target currents 42 to stir the liquid target 3. Therefor no separate agitation electrode(s) 33, 35 are needed. Nevertheless, it may be useful to ad one or further separate agitation electrodes, e.g. similar to the examples as shown above. As with FIGS. 3 to 5, only features essential for the understanding of the shown embodiment or present process are shown, whereas for other features, like power lines and supplies, substrate holder, pumping system or the like, which are necessary to run a sputter operation it is referred to FIG. 1 and respective description.

[0061] With FIGS. 6a to 7c a simplified explanation of current induced vertical convection is symbolized by circular arrows 39. It should be mentioned that such vertical convection needs a certain minimum depth of the liquid in the trough, which should be at least 6 mm, however 8 mm or 10 mm or deeper will work better. FIG. 6a shows an inventive sputter device 1 in operation having a trough 2 dimensioned to allow vertical 39, as well as horizontal convection which overlaps with the circular movement 40 in the racetrack as shown in FIG. 5. Both types of convection only symbolize the vertical and the horizontal component of the overall-convection and therefor both have their origin in current(s) and magnetic field(s) applied to the liquid target 2. It should be mentioned that some of the following explanatory information is not based on proven theory but only deducted as a first guess from experimental results and common physical knowledge and might need some further improvement. As symbolized by arrow 41 a plasma current of a pulsed or static DC-driven sputter magnetron discharge enters the target about normal to the surface below the magnetic tunnel of the magnetic field 6. The arrows 41 symbolize the place of the highest current density which corresponds about to the middle of the so called race-track formed on the target surface. Under the presumption that both the liquid target 3 and the trough 2 provide in each case an evenly distributed conductivity, e.g. of at least 0.1 MS/m for metallic or carbon materials like graphite, a current flow as symbolized by arrows 42 will result, see FIG. 7a. The internal current flow in the liquid target 3 together with the magnetic field 41 from the magnet system 5 of the sputter device 1, will produce the current induced vertical convection 39. It should be mentioned that the magnetic field 41 provided by magnet system 5 is, of course also present within the liquid target 3 but is not shown to avoid overloading the drawings.

[0062] It has to be further noted that with low induced currents or at least at the beginning of a sputter process, mainly such vertical convection 39, resulting in an essentially linear horizontal movement on the liquid surface could be achieved. Such vertical movement however will continue also with higher currents and will superimpose the horizontal movement to provide an effective agitation of the liquid, as far as the liquid is provided with a sufficient depth in the trough. Thereby liquid at the surface and therewith possible contaminations or surface islands, e.g. from compounds like GaN formed at the surface during a reactive sputter process, are moved from both sides essentially perpendicularly towards the racetrack. Such agitation allows to clean the target under a closed shutter 45 in a first sputter step with low sputter power and resulting low sputter currents in the liquid target, and thereby to start the deposition process only after cleaning has been finished by opening the shutter 45, see. FIG. 2. Furthermore when a reactive sputter process should be applied, which is the usual mode when sputtering liquid targets to coat a substrate, coating parameters like sputter-power and partial pressure of a reactive gas, e.g. nitrogen, oxygen or carbon containing gas like methane can be modified in such a way that the target surface or at least the race track is free from any disturbing solid compounds of a critical size floating on the liquid target. Such particles might cause process irregularities leading to annoying particles on a substrate surface to be coated.

[0063] By applying a higher sputter power, e.g. together with a higher flow of reactive gas which can be performed just before opening the shutter for deposition. With higher sputter power and resulting currents, the horizontal component of the Lorentz-power becomes strong enough to enforce also a rotational movement of the target liquid at least within the region of the racetrack. Such superposed horizontal and vertical movements in the target liquid can be used to furnish immediately subcritical small solid compound particles into the race track, which is the most active surface region S.sub.A, where they are sputtered away without forming harmful particles on a substrate surface. Thereby forming of particles of overcritical size or solid islands can be avoided effectively at least within active surface regions.

[0064] In FIGS. 6b and 6c a further embodiment of an inventive sputter device 1 is shown having it's magnet system 5′ mounted in respective recesses in the upper side of the trough bottom 24. Such an embodiment allows better access to the magnet system for servicing. Therewith another magnet system can be implemented for other sputter processes more easily, e.g. for sputtering another type of liquid target material, without the need to dismount the whole trough from the coating system 10. Furthermore, such embodiments allow higher liquid levels due to the higher position of the magnets which allows to reduce service time for material supply. As an alternative to extend such service intervals, a supply of liquid target material can be foreseen allowing to feed fresh target material e.g. by feeding a wire of the respective material (Ga, In, or respectively adapted alloys) into the liquid target, or via a supply pipe from a material supply positioned inside or outside the vacuum coating system (not shown), whereby the feed inlet can be under the surface of the target level. With modifications as shown in FIG. 6a to 6c, FIG. 7a and FIG. 8 below, essentially the whole of the troughs inner surface which is electrically connected to the cathode of the sputter supply, is also in electrical contact with the liquid target, and therewith is used as an inner agitation-electrode. That applies also to magnets 5′ at the bottom of the trough as they are electrically conductive and therefor act too as electrode surface. Flow installations 46 as shown in FIG. 6c can be provided to help develop faster or vary a flow pattern from the agitation electrodes. Such installations can be put on as shown or formed in the inner trough bottom or sidewalls. These installations 46 can be too conductive like the trough surface or may be made of or coated with isolating material which gives a further parameter to influence the flow pattern, as will be explained in the following with FIGS. 7b and 7c. Side magnets 44 as shown in FIG. 6c provide a further parameter to influence magnetic field and flow pattern of the liquid movement.

[0065] FIGS. 7b and 7c shows sputter devices with a different guidance of the current within the liquid target material 3 as compared to FIG. 7a. With FIGS. 7b and 7c only a part of the inner surface of the trough which is in contact with the liquid target is used as the inner agitation-electrode. Therefor electrically isolating material 43′, 43″ which is inert against the liquid target material, e.g. Teflon, can be placed as a cover or coating on certain surface areas in the trough. Thereby current flow is varied in the target material as symbolized by arrows 42′ and 42″ which also provides different flow patterns which are liquid movements 39′, 39″ in the liquid and on the surface of the target 3. This allows to adapt and optimize agitation and flow design easily within the target according to the actual process needs. An alternative but less flexible and more expensive embodiment would be to take an electrically isolating trough body and provide respective contacts or contact areas on its inner surface. Such use of electrically isolating material 43′, 43″ can be also combined additionally with other embodiments, e.g. as shown in FIGS. 1, 4, 5, 6b, 6c and 8.

[0066] Sputtering tests to sputter Gallium under inert and reactive sputter conditions have been performed in a water cooled round stainless steel (AISI 1.4301) trough with an inner diameter of 200 mm and 15 mm depth. Within 10 mm of the inner circumference the bottom has been given a concave boarder area up to a height of 5 mm from the bottom to compensate for the high surface tension of Gallium on stainless steel and thereby flatten the Ga-surface within this area. 1.5 to 3 kg Ga have been filled into the trough to a height of about 8 to 14 mm. The magnet system 5″ as used was an unbalanced magnetron system mounted in respective recesses in the lower (reverse) side of the trough bottom 24, with magnetic axes M.sub.A of inner and outer magnets vertically but reversed oriented, as shown in FIG. 8. Thereby magnets, respectively the poles of the magnets could be placed near to the liquid surface region, that is in a distance of 17 to 35 mm to the maximum, and a high magnetic flux and parallelism of the magnetic field to the surface of the unstirred target could be provided. Magnetic flux density was measured within the racetrack region at a radius of 75 mm from the trough center about 5 mm above the Ga-surface, whereby the horizontal component Bx was about 0.038 T and the vertical component was about 0.013 T, giving an overall flux |B| of about 0.05 T.

[0067] Therefore at this place about 75% of the field are contributed by the horizontal, essentially surface parallel component B.sub.x and only about 25% by the respective vertical component B.sub.y. It could be shown that a magnetic field having a horizontal component B.sub.x of at least 60% or even better equal or higher 70% within the race track, respectively within an area of the race track projection within and above for at least 5 to 10 mm distance from the stationary liquid surface could help to agitate the liquid more effectively. Similar strong and surface parallel magnetic fields can be provided e.g. with a configuration as shown in FIGS. 6b and c, with magnets near to the liquid surface due to their position within the trough. It should be mentioned that side magnets 44 may help but are not a condition to provide such field characteristic.

[0068] A further variation which can be used to improve the parallelism of the magnetic field within the liquid gallium target and at least directly above its surface can be to provide a central yoke 5′″ made of magnetizable material having a Curie temperature >500° C. in the middle of the target as shown with dashed lines in FIG. 8. Such a yoke can cooperate with magnets 5″ to design the magnetic field lines in a more surface parallel way.

[0069] It should be mentioned that with magnetic systems 5, 5′, 5″ (5′″), comprising inner or central magnets and outer magnets with respect to the center of the target, usually a highly unbalanced magnetic field will result due to the higher number or, if a ring-magnet is used, the bigger size of the peripheral magnet(s), especially with reference to a circular target design. Nevertheless within the liquid target and near its surface a nearly symmetric parallel magnetic field can be applied by respective design of the magnetic system as shown.

[0070] The process has been divided into two process steps, with the first step behind a closed shutter at low sputter power under inert gas to condition the liquid target by an essentially linear horizontal movement as described above, followed by a second step with higher sputter power under a mixture of inert and reactive gas, whereby the shutter has been opened after a few seconds to deposit a compound layer on a polished wafer substrate. The following parameters have been applied to produce a flawless compound (GaN) layer on the wafer surface.

First Step—Linear Surface Movement for Target Conditioning:

[0071] Flow Ar: 20 sccm

[0072] Flow N2: 0 sccm

[0073] Total pressure: 0.29 Pa

[0074] Sputter power (DC): 50 W

[0075] Sputter current: 0.19 A

[0076] Sputter voltage: 268 V

Second Step—Rotational Surface Movement for Coating:

[0077] Flow Ar: 20 sccm

[0078] Flow N2: 10 sccm

[0079] Total pressure: 0.29 Pa

[0080] Sputter power (DC): 600 W

[0081] Sputter current: 1.71 A

[0082] Sputter voltage: 350 V

[0083] Alternatively the following steps have been applied:

[0084] First step—conditioning of the target herewith is performed with high sputter power which provides high steering and heat, both resulting in a very effective degassing of the liquid Gallium, e.g. compared with linear surface movement. A dummy wafer has been used with this conditioning step:

[0085] Flow Ar: 30 sccm

[0086] Flow N2: 60 sccm

[0087] Total pressure: 0.33 Pa

[0088] Sputter power (DC): 1700 W

[0089] Sputter current: 3.7 A

[0090] Sputter voltage: 460 V

[0091] Second Step—Rotational Surface Movement for Coating:

[0092] Flow Ar: 14 sccm

[0093] Flow N2: 72 sccm

[0094] Total pressure: 0.36 Pa

[0095] Sputter power (DC): 500 W

[0096] Sputter current: 1.33 A

[0097] Sputter voltage: 376 V

[0098] In the following standard ranges which are applicable and ranges which gave very good results for GaN-coatings are given for every parameter of the coating process:

[0099] Flow Ar: 5-70 sccm, 8-14 sccm

[0100] Flow N2: 0-100 sccm, 5-80 sccm

[0101] Total pressure: 0.1-0.8 Pa, 0.2-0.6 Pa

[0102] Sputter power (DC): 10-600 W, 20-500 W

[0103] Sputter current: 0.05-10 A, 0.08-8.38 A

[0104] Sputter voltage: 200V-800V, 261V-538V

[0105] It should be mentioned that critical minimal dimensions to agitate a liquid target trough 2 by means of Lorentz-force in a good and reproducible way, seem to be, e.g. an inner diameter of a round or an inner smaller side of a rectangular trough of at least 55 mm or 60 mm and a depth allowing at least a liquid level of 4 mm or 6 mm with projecting edges of at least 8 or 10 mm to avoid spill out of target liquid during agitation or filling. Whereas for the critical maximum dimension a size of 2,000 mm still seems to be manageable with a reasonable effort. For a higher material reservoir liquid levels up to 20 or 30 mm seem to be manageable with respective magnet systems, e.g. on or within the upside of the trough bottom to ensure a manageable distance from the liquid surface.

[0106] Due to the symmetric set-up and respective fluid movement, round targets have been proved to be a good solution for inner trough diameters from 100 and 1000 mm, especially from 200 to 800 mm.

[0107] FIG. 10 shows two typical current-voltage characteristics for a liquid level target as described with FIG. 8. No optional central yoke 5′″ has been used. The lower curve has been taken with a low liquid Ga-level in the trough 2, which refers to about 8 mm level above the trough bottom. The upper curve has been taken after refilling the trough with a high liquid Ga-level, which refers to about 13 mm above the trough bottom. Surprisingly it could be shown that the offset of the sputter voltage between the two characteristic curves always had a characteristic constant offset of about 68.70 V and sputtering with a higher liquid level results in a higher sputter voltage and power as far as the sputter current is kept constant. Alternatively when sputter power should be kept constant, e.g. when power control is used to control the target power supply 23, the target current will be lower with a freshly refilled high level target than with a target having a low liquid level.

[0108] Without being able to give proof of it by now, this effect is supposed to be due to a more pronounced “dynamo-effect” of the higher rotating target mass which induces a higher electrical resistance to the sputter current flow. Such findings could be used by the inventors to deduce a method and device to continuously monitor the liquid level in the target trough and indicate the need to refill to the operator and/or initiate automatic refill of a low liquid level target between deposition processes.

[0109] In FIG. 11 and the following example of a look up table a simple linear scheme has been extrapolated from the low (“before fill”) and high (“after fill”) target voltage values at one ampere from FIG. 10.

[0110] Lookup Table for constant target current of 1 amper:

TABLE-US-00001 Ga (kg) fill level (mm) U (V) 1.0 5.5 335 1.5 8.0 370 2.0 10.5 405 2.5 13.0 440 3.0 15.5 475

[0111] Such schemes made for several different target currents and therewith different target power levels could be used in electronic lookup tables serving as a database to indicate the level of the liquid gallium level and output a control signal SLL to control a liquid level refill device. Filling was initiated with about 1.5 kg of gallium left in the target trough on which one kg liquid gallium has been added to a total of 2.5 kg target liquid.

[0112] A logic scheme to operate a liquid level refill device 53 and monitor the liquid level in the target trough 2 is shown in FIG. 9. A measurement device 47, here to monitor the target to ground voltage is electrically connected between the cathodic trough and ground potential and forwards a voltage signal S1, which usually will be a digitally transformed voltage signal, to one input I.sub.V of an electronic device 48 which is connected to one input of a comparator 49. The electronic device 48 can be a separate controller or a computer to operate the refill device 53 or a control circuit within the system control of the coating system. A second signal S2 is applied to the other input of the comparator 49. The second signal S2 being generated by an electronic lookup table 50 selected in accordance to a respective target current level or range measured within the circuitry of the target power supply 23 and given as current signal SC to an input I.sub.C/P of the electronic device 48 to select the respective lookup table 50 for the present current flow. Alternatively, lookup tables can be also foreseen for constant power levels or ranges of the supply 23. In this case a power signal SP, which again will usually be a digitally transformed signal, can be used to select the respective lookup table. Lookup table 50 comprises several values S2.1, S2.2 . . . S2.n according to the required accuracy of the liquid level detection. Such values can be given sequential to one comparator with S1 unchanged or in parallel to a respective series of comparators as shown. With each signal S2.n the respective liquid level value SLL.n is given to a logic element 51 which forwards a respective value SLL to indication device 52 and refill device 53 only when the output signal of the comparator 49 refers to S1=S2. Thereby display of the liquid level with level meter 52 and operation of the refill system 53 can be controlled. The latter comprising a refill channel 54 to fill the target liquid below the liquid surface 4, e.g. sidewise through the trough or in a periphery, e.g. inactive surface region. It should be mentioned that instead of lookup tables also an algorithm or a combination of lookup table(s) and an algorithm could be used to calculate respective SLL signals from basically measured or transformed voltage S1 and current SC or power signals SP.

[0113] As mentioned in the common part of the description above any of the electrical values like sputter voltage, sputter current or sputter power can be used to produce or deduce signal S1, one of the remaining values is kept constant and the last value is varied to produce the respective lookup table or find the algorithm which fits to the respective target geometry and plasma regime(s) to produce comparative benchmark signal S2 for different liquid levels.

[0114] Despite of the fact that in the forgoing description the invention has been demonstrated by means of various embodiments, any feature as disclosed with one embodiment is assumed to be disclosed with any other embodiment, as long such combination is not obviously conflicting or inconsistent with other features of the respective embodiment for the man of art.

REFERENCE NUMBERS

[0115] 1, 1′sputter device [0116] 2 target trough [0117] 3 liquid target [0118] 4 static surface [0119] 4′ dynamic surface [0120] 5, 5′ magnet system [0121] 6 magnetic field [0122] 7 anode [0123] 8 power line target supply [0124] 9 cooling device [0125] 9′ clamp [0126] 10 vacuum coating system [0127] 11 cover [0128] 12 sidewall [0129] 13 substrate port [0130] 14 substrate, wafer [0131] 15 substrate holder [0132] 16 means to cool and/or heat [0133] 17 bias power supply [0134] 18 power line bias supply [0135] 19 vacuum pump [0136] 20 pump-port [0137] 20′ pump valve [0138] 22 gas inlet [0139] 23 sputter power supply [0140] 24 bottom of the trough [0141] 24′ bottom of the trough partially concave [0142] 24″ bottom of the trough completely concave [0143] 25 mechanical stirrer [0144] 26 axis [0145] 27 feedthrough [0146] 28 drive [0147] 29 magnetic bar [0148] 30 stirrer [0149] 31 axis of the stirrer [0150] 32 ultrasonic source [0151] 33 central agitation electrode [0152] 34 separate electrode current-supply [0153] 35 outer agitation electrode [0154] 36 liquid circuit [0155] 37 shield [0156] 38 vacuum chamber [0157] 39 vertical current induced liquid movement [0158] 40 horizontal current induced liquid movement [0159] 41 current flow in sputter plasma [0160] 42, 42′, 42″ current flow in liquid target [0161] 43′, 43″ isolating cover or coating [0162] 44 side magnets [0163] 45 shutter [0164] 46 flow installation [0165] 47 measurement device [0166] 48 electronic device [0167] 49 comparator [0168] 50 lookup table [0169] 51 logic element [0170] 52 liquid level indication device [0171] 53 liquid level refill device