METHOD FOR PRODUCING OXIDE COATINGS

The present invention relates to a High Power Impulse Magnetron Sputtering (HIPIMS) method with a high deposition rate.

Oxide coatings are frequently used for protection of underlying material from oxidation and wear. Such oxide coatings may be deposited by PVD methods such as reactive magnetron sputtering. Using oxygen containing sputtering gas gives rise to problems, because the desired material forms layers with different properties compared to a metallic surface, such as electrically insulating layers, on the target surface. Particularly, when the sputtering yield or electron emissivity of the oxidised target surface is different from that of the metallic surface, hysteresis effects occur that make the process unstable .

Established PVD technology teaches how to reactively deposit oxides by means of dual magnetrons, Dual Magnetron Sputtering (DMS) , and pulsed dc power, in order to avoid forming thick insulating layers on the targets. The problem with low sputtering yield of oxidised targets still remains; in order to obtain the desired compound a certain oxygen partial pressure is required, but at the same time the target surface is covered by oxide as well. The result is the well know hysteresis problem which usually is overcome by a more or less complicated feedback system (pressure, optical or electrical) for controlling the oxygen flow.

High power impulse magnetron sputtering (HIPIMS) has been used for the deposition of metallic coatings or conducting metal nitrides, e.g., CrN by reactive HIPIMS. TiO ₂ has been grown from ceramic TiOi _-8 targets using HIPIMS and high rates were achieved. HIPIMS has been used for reactive sputtering of alumina but with the conventional hysteresis phenomena and oxygen flow feedback control, and with a deposition rate of only 25-30% compared to pulsed DC sputtering. Another issue is that when mixed oxides are deposited the different target metals oxidise, and show hysteresis effects at different reactive gas partial pressures,

making it difficult if not impossible to co-deposit mixed oxide coatings .

It is also known how to operate a magnetron with an aluminium target, and a HIPIMS power supply, in a reactive Ar/oxygen mixture, to deposit an XRD amorphous alumina layer. Albeit the layer contains nanometer size gamma alumina grains, the microstructure is porous, making the deposited layer less suitable in many coating applications.

Deposition using HIPIMS techniques is described in Glocker et al . , "High Power Pulsed Reactive Sputtering of Zirconium Oxide and Tantalum Oxide", 2004 Society of Vacuum Coaters 505/856-7188, ISSN 0737-5921, 47th Annular Technical Conference Proceedings (April 24-29, 2004), Dallas, TX USA, p. 183-186, Konstantinidis et al . , "Titanium oxide thin films deposited by high impulse magnetron sputtering", Thin Solid Films, vol. 515, 23 Nov. 2006, No. 3, p. 1182-1186, Sproul et al . , " The Reactive Sputter Deposition of Aluminium Oxide Coatings Using High Power Pulsed Magnetron Sputtering (HPPMS)", 2004 Society of Vacuum Coaters 505/856-7188, ISSN 0737-5921, 47th Annular Technical Conference Proceedings (April 24-29, 2004), Dallas, TX USA, p. 96-100, and Muenz et al . , DE 10 2005 033 769-Al. However, the above mentioned problems still remain to be solved.

It is an object of the present invention to provide a method in which the deposition rate is higher and in which the need for partial oxygen pressure feedback systems is eliminated. Furthermore, deposition of homogeneous coatings over a large surface is enabled, and with the invented method co-deposition of mixed oxides from targets of different metals or alloys is made possible.

Fig. 1 shows a schematic view of a vacuum deposition system.

Fig. 2 shows a Scanning Electron Microscope micrograph of a coated insert according to one embodiment of the present invention.

Fig. 3 shows an X-ray diffractogram from a coated insert according to one embodiment of the present invention.

According to the present invention there is provided a magnetron sputtering method for producing oxide coatings, especially coatings comprising aluminium oxide, on, e.g., tools for cutting and forming, metallic sheets, and components for the purpose of, e.g., surface engineering, decoration and surface protection. The method applied on the deposition of a coating on a cutting tool insert is preformed according to the following, reference is made to Fig. 1:

A substrate of a hard material, such as cemented carbide, cermet, cubic boron nitride, diamond, ceramic or high speed steel, preferably cemented carbide, is provided, onto which is deposited a well adherent hard and wear resistant coating using magnetron sputtering, for growing, suitably stoichiometric crystalline, oxides, whereby the deposition is based on simultaneously having reactive oxidizing conditions at the substrate, whilst maintaining the target surface in an essentially unoxidized state. This means that the target surface has not reacted with the reactive gas to the extent that it has inflicted hysteretic process behaviour at the selected working point. This is achieved by operating a HIPIMS discharge on one or more target (s) (3), in an argon and oxygen gas mixture, at a peak pulse power higher than 200 Wcrn ^"2 , preferably at least 320 Wcrn ^"2 , and preferably at a total pressure of 15 mTorr or lower, more preferably 10 mTorr or lower, most preferably 6 mTorr or lower, and at a high enough repetition frequency. Upon determining the oxygen stoichiometry of the obtained layer and the oxygen coverage of the sputtering target it is surprisingly found that full stoichiometry of the layer is achieved at conditions under which the target is insignificantly covered by oxygen.

It is furthermore surprisingly found that the hysteresis effects are reduced or eliminated, therefore not reducing the deposition rates, as is the case of pulsed DC sputtering. Consequently, the need for partial reactive gas pressure feedback systems is eliminated. In the absence of hysteresis at the working point, the process can be operated in a stable mode with well defined characteristic parameters for each selected value of oxygen flow.

In HIPIMS the high peak pulse power renders the ion density of sputtered atoms very high, therefore sputtering is performed by target metal ions, which leads to a lower deposition rate in HIPIMS. Surprisingly the rate achieved by applying the invented method is at least 3 times higher compared to prior art.

Cutting tools of suitable types, provided as substrates, may be coated, in a vacuum chamber, with a metal oxide coating according to the following embodiment of the invention:

After subjecting the substrates (10) to standard cleaning procedures they are mounted on a substrate holder (4) operable to rotate during deposition, wherein the deposition area, i.e. the sum of the exposed surfaces of the substrate holder (4) and the substrates (10) mounted thereon, is preferably at least 10 times the sum of the area(s) of the one or more target (s), preferably 10-25 times the sum of the area(s) of the one or more target (s), preferably with a minimum distance of 6 cm, more preferably between 8 and 20 cm from at least one magnetron sputter source with a target (3) . The substrate holder (4) is either electrically floating, grounded or connected to an optional bias power supply (8), with dc or ac or pulsed dc operation mode, according to what is common practice for someone skilled in the art. The material of the target (3) is selected according to the metal composition of the desired coating, preferably an AlMe alloy, most preferably Al. The type of magnetron source onto which the target (3) is mounted may be any standard commercial or special design, with balanced or unbalanced or adjustable magnet systems, and its shape may, for instance, be circular or rectangular.

Deposition is carried out in the vacuum chamber (1), which is first evacuated by a high vacuum pump (2) to below 5xlO ^~4 Torr, preferably below 5xlO ^~6 Torr. Before the deposition the substrate (10) is preferably heated to a temperature of between 300-900 ⁰ C, preferably 450-700 ⁰ C, most preferably 570-630 ⁰ C. Ar is used as sputtering gas, which is let into the chamber (1) through a first valve or mass flow controller (5) . A constant Ar flow is used, rendering a chamber pressure of 15 mTorr or lower, preferably 1-10 mTorr, most preferably 3-8 mTorr. The deposition is initiated preferably in pure Ar sputtering gas, but may also

start in conjunction with, or even after introducing oxygen in the system through a second valve or a mass flow controller (6) . The oxygen gas is preferably introduced at a constant flow, which is maintained during the deposition. In case of initiation in pure Ar sputtering gas, the oxygen gas (O ₂ ) is preferably introduced at a constant flow of 1-8 seem, preferably 2-5 seem, preferably after a delay of less than 10 minutes, more preferably 1-3 minutes. The oxygen gas (O ₂ ) flow is chosen so that stoichiometric oxides are obtained, which flow is dependant on the deposition system characteristics, such as, e.g., the target size, and pumping speed. The appropriate oxygen flow value is empirically determined by the skilled artisan, by adjusting the flow value according to measurements of the coating oxygen stoichiometry . However, for normal size production systems, oxygen gas (O ₂ ) flow of 200-1000 seem, may be used.

The deposition is started by applying negative voltage pulses, generated by a power supply (7), of 200-2000 V, preferably 500-1000 V, most preferably 650-750 V, to the target relative to the anode (the vacuum chamber walls) . The pulse length is preferably 2 to 200 μs, more preferably 10 to 100 μs, more preferably less than 100 μs, more preferably less than 75 μs, most preferably 20 to 40 μs and the repetition frequency 100 Hz to 10 kHz, more preferably 300 Hz to 3 kHz, most preferably 500 Hz to 1.5 kHz. The power supply (7) comprises a pulsing unit fed with a preferably constant DC voltage source or most preferably an integrated pulsed dc power supply, suitable to the purpose. The pulses, preferably having a constant voltage, ignite a magnetron sputtering glow discharge (9) with a current rising to a maximum peak during each pulse, at which the peak pulse power is reached. After reaching maximum the current is constant or drops to a minimum 50% of the peak value. Preferably the current maximum is, however, reached at the end of the voltage pulse. In between two consecutive pulses there is no electric power input to the deposition system, therefore the plasma decays by ion/electron recombination. The time between pulses is selected by the operator skilled in the art, so that the glow discharge is preferably never extinguished. Further-

more, the peak current pulse maximum, length, and repetition frequency is selected high enough, i.e. according the limits given above, so that hysteresis effects with respect to oxygen flow in characteristic parameters, e.g., deposition rate, target voltage, optical emission and coating as well as target oxide coverage, are reduced or eliminated.

The deposition is continued for a suitable period of time, after which the inserts are left to cool below 200 ⁰ C, and promptly removed from the vacuum system. In one embodiment of the method the area of the target (3) is 1000-2000 cm ² , and the substrate holder (4) area is 10000- 20000 cm ² . For the alternative embodiment the maximum sputter current is 980-2800 A, preferably 1120-1680 A, and a maximum instantaneous power of 200-2000 kW, preferably 320-4000 kW. In another embodiment of the method there is provided more than one magnetron sputtering source with targets, and these targets may have metal compositions being the same or differing from each other.

In one embodiment the one or more targets are of the same metal composition being an AlMe alloy, where Me is one or more of the metals of the group Mg, Zn, Mn, Fe, Co, Ni, Zr, Cd, Cu, Cr and Sn and the gas mixture in the vacuum chamber is a mixture of Ar and oxygen gas (O ₂ ) , whereby the deposited coating comprises a two phase oxide, or a mixed oxide of the type (AIi- _x Me _x ) ₂ O ₃ or spinel (Me) _x Al ₂ O ₃₊ X (0<x≤l). Preferably the one or more targets of the same metal composition are of Al, whereby the deposited coating comprises a crystalline phase of alumina, more specifically gamma or alpha alumina, deposited to a thickness of preferably 0.1-30 μm, more preferably 1-10 μm, most preferably 2-5 μm. Most preferably the deposited coating comprises single phase alpha-alumina.

In one further embodiment of the method two or more targets, having metal compositions differing from each other, and a gas mixture of Ar and oxygen gas (O ₂ ) is used, whereby a multilayer or mixed oxide of the type (Ali_ _x Me _x ) ₂ O ₃ or spinel (Me) _x Al ₂ 0 _3+x (0<x≤l), or a two phase oxide is deposited.

In between and on top of the metal oxide coating, other layers of wear resistant oxides, carbides or nitrides may be deposited, by methods as known in the art.

According to the present invention there is also provided oxide coatings made according to the method, especially coatings comprising aluminium oxide, suitable for, e.g., tools for cutting and forming, typically having a substrate of a hard material such as cemented carbide, cermet, cubic boron nitride, diamond, ceramic or high speed steel, preferably cemented carbide, or suitable for metallic sheets, and components for the purpose of, e.g., surface engineering, decoration and surface protection .

Example 1.

A cutting tool insert of commercial grade HlOF of composition WC10%Co and insert style SNUN1204 (12x12x4 mm size), was coated in an ultra high vacuum deposition system, see Fig. 1, according to the following. The insert (10) was cleaned using standard procedures and mounted on a substrate holder (4), operable to rotate during deposition, having an area of 200 cm ² , at a position about 10 cm above a magnetron sputter target (3) . The substrate holder (4) was electrically floating. The target (3) was an Al disc with a diameter of 50 mm, and a thickness of 3 mm. Deposition was carried out in a vacuum chamber (1) which was first evacuated to 5xlO ^~7 Torr. Before the deposition started the insert (10) was heated to an approximate surface temperature of 600 ⁰ C. Ar was used as sputtering gas, which was let into the chamber (1) through a first mass flow controller (5) . A constant flow of 100 seem argon was used which resulted in a total pressure in the chamber (1) of 6 mTorr.

The deposition was started by applying negative voltage pulses of 700 V to the target (3) relative to the anode (the chamber walls) . The pulse length was 35 μs and the repetition frequency 1 kHz. For this purpose a power supply (7) was used, comprising a pulsing unit (SPIK, Melee Gmbh) fed with a constant DC voltage source (Pinnacle, Advanced Energy) . The resultant pulses with a constant voltage ignited a magnetron sputtering

glow discharge (9) with a current rising to a maximum during the pulse. The deposition was initiated in pure Ar sputtering gas, introduced through a first mass flow controller (5) . After about 2 minutes after the start of the deposition, oxygen gas (O ₂ ) was introduced through a second mass flow controller (6) at a constant flow rate of 3.4 seem. Since the deposition was carried out in a system without hysteresis the process could be operated in a stable mode with a well defined maximum sputtering current in the pulses for the selected value of oxygen flow. The voltage and oxygen flow selected for this particular deposition run imparted a time averaged power of 110 W. For these conditions the maximum sputter current was determined to 9.1 A, i.e. a maximum instantaneous power of 6.4 kW. The deposition was continued for 3 h, after which the insert was left to cool below 200 ⁰ C, before being removed from the vacuum system.

Analysis of the insert, showed in Fig. 2, revealed that the coating (a) on the substrate (b) consisted of a well adherent aluminium oxide layer, transparent in the visible optical range and with a thickness of 2.0 μm, corresponding to a deposition rate of 0.67 μm/h, see also Table 1. The coating was found to comprise one stoichiometric crystalline phase, alpha-Al ₂ O ₃ , identified by at least 5 peaks as is shown in the X-ray diffractogram in Fig. 3. All other peaks than the designated as alpha originate in the substrate.

Table 1.

From Table 1 it can be seen that the deposition rate when using the method according the present invention is at least 3 times higher compared to prior art.