WO2017044042A1

WO2017044042A1 - Co-based alloy sputtering target having boride and method for producing the same

Info

Publication number: WO2017044042A1
Application number: PCT/SG2016/050407
Authority: WO
Inventors: Sanghak Lee; Jinling Xu; Arul Kumar KARUPPANNAGOUNDER; Pavan Kumar SURI; James Olszewski; Minisankar APARNADEVI
Original assignee: Heraeus Materials Singapore Pte., Ltd.
Priority date: 2015-09-07
Filing date: 2016-08-23
Publication date: 2017-03-16
Also published as: SG10201507167UA

Abstract

The present invention relates to a sputtering target, comprising a single phase of a cobalt-based alloy, wherein the cobalt-based alloy comprises chromium and optionally platinum as an alloying element, and boride particles, wherein the boride particles have an average particle size of 10 µm or less and are dispersed in the cobalt-based alloy.

Description

Co-Based Alloy Sputtering Target Having Boride and Method For Producing The Same

TECHNICAL FIELD

The present invention relates to a sputtering target comprising a cobalt-based alloy having boride particles for producing magnetic recording media, and a method for producing a cobalt- based alloy sputtering target having boride particles.

BACKGROUND ARTS Hard disk drives used for storage of digital data typically comprise a substrate, such as a glass or aluminum substrate, with various materials deposited layer by layer on the substrate. Each of the materials deposited on the substrate serves a specific purpose so as to assist reading and/or writing data on the media. Each layer is typically deposited via physical vapor deposition techniques, also referred to as sputtering. Usually, the material of the deposited layers is provided from bulk components (i.e. sputtering targets).

Current media architecture is referred to as "perpendicular magnetic recording" (PMR) due to the orientation of the magnetic recording bits in relation to the substrate and is composed of several layers such as Soft Magnetic Under Layer (SUL), Seed Layer (SL), Under layer, multiple recording layers, cap layer, Carbon Overcoat (COC) and a lubrication layer. Each layer has a specific function in enabling the data to be stored and read.

Cap layer is deposited on the top of the perpendicular magnetic recording layer to assist a more uniform inter-granular exchange in the media via cap layer. Useful materials for cap layers are e.g. cobalt-based alloys wherein the cobalt is alloyed with further metallic elements such as, chromium (Cr), boron (B) (e.g. in the form of a boride phase) and additionally platinum (Pt) and ruthenium (Ru). Boron improves the segregation of Cr in the magnetic layer which leads to a sharper transition between the magnetic grains and the non-magnetic Cr-rich grain boundaries. Accordingly, a sputtering target which is used for preparing such a cap layer may comprise these components as well.

US 2013/0341 184 A1 describes a Co-Cr-Pt-B-based alloy sputtering target. The sputtering target comprises mainly Co, Cr, Pt and B-rich layer with high magnetic flux and few microcracks in the B-rich layer. However, the size of B-rich layer is more than 30 μηι and not uniformly distributed in the matrix.

US 2002/0170821 A1 describes an oxide-free Co-Cr-Pt-B sputtering target having multiple phases. The sputtering target does not only comprise dispersed boride particles, but also at least three separate phases of cobalt-based alloys (CoCr, CoPt, CoCrPt), and a separate Pt phase.

In the PMR (perpendicular magnetic recording) architecture, it is desired that each grain of the recording layer is magnetically decoupled from its nearest neighbors. Decoupling is typically achieved by the presence of a non-magnetic oxide layer around each grain. However, when data needs to be stored, multiple grains (typically 8 to 15 grains) form one bit. Given that each grain is magnetically decoupled, a continuous cap layer is helpful in coupling different grains so that the magnetic field can be switched at a given applied magnetic field.

CoCrPtB-based cap layers which are applied onto a magnetic recording layer are described e.g. in US 8,1 10,298 and US 8,614,862.

The requirements that should be met by a cap layer are uniform thickness, low surface roughness, and specific magnetic properties such as coercivity and magnetic saturation optimized for a given magnetic stack. Magnetic properties of the cap layer are a function of chemical composition. In order to achieve a continuous film with low surface roughness, sputtering of the cap layer is generally done at low partial pressure of the ionization gas, typically a noble gas such as argon. However, at low partial pressure of the ionization gas, the risk of ignition failure increases due to absence of sufficient positive argon ions to initiate and sustain the plasma. Hence, the sputtering target used for providing a cap layer should be able to sputter at pressures that are optimized to result in continuous film with low surface

roughness. The target itself (i.e. its chemical composition and microstructure) can also contribute to ignition failure. Large differences in electrical resistivity in case of composites or a difference in energy threshold of sputtering can result in uneven distribution.

An object of the present invention is to provide a sputtering target which can be used for providing a cap layer in perpendicular magnetic recording media and which keeps the number of ignition failures in a sputtering process very low. A further object of the present invention is to provide an efficient manufacturing process by which a sputtering target complying with the above requirements can be obtained.

SUMMARY OF THE INVENTION

The object is solved by a sputtering target, comprising a single phase of a cobalt-based alloy, wherein the cobalt-based alloy comprises chromium and optionally platinum as an alloying element, and boride particles, wherein the boride particles have an average particle size of 10 μηι or less and are dispersed in the cobalt-based alloy.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are described hereinafter with reference to the following drawings, in which:

Figure 1 shows the microstructure of the sputtering targets in one example of present disclosure.

Figure 2 shows the microstructure of the sputtering targets in one example of present disclosure.

Figure 3 shows the microstructure of the sputtering targets in one example of present disclosure.

Figure 4 shows the microstructure of the sputtering targets in one example of present disclosure.

Figure 5 shows the microstructure of the sputtering target of Example 3.

Figure 6 shows the microstructure of the sputtering target of Example 4.

Figure 7 shows the microstructure of the sputtering target of Example 5.

Figure 8 shows a voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 3.

Figure 9 shows a voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 4.

Figure 10 shows a voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 5.

Figure 1 1 shows the microstructure of the sputtering target of Comparative Example 3.

Figure 12 shows a voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Comparative Example 3. Figure 13 shows enlarged view of one of pulses shown in Figure 12.

Figure 14 shows the microstructure of the sputtering target of Example 6.

Figure 15 shows a voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 6.

DETAILED DESCRIPTION

The sputtering target of the present invention comprises just one phase of a Co-based alloy. There is no second phase like intermetallic phase or separated CoCr and CoPt phase in the matrix. There are not two or more separate Co-based alloy phases. This single phase of a cobalt-based alloy acts as a matrix in which all boride particles of the sputtering target are uniformly dispersed. As the matrix in which the boride particles are dispersed is not formed by two or more phase-separated alloys but rather by a single phase cobalt based alloy, a very homogeneous matrix structure is provided. Due to the small boride average particle size, a very fine dispersion of the boride particles in the cobalt-based alloy matrix is achieved. Furthermore, as there is just a single alloy phase in which all boride particles of the sputtering target are uniformly dispersed, each boride particle is surrounded by the same or at least a very similar matrix composition.

In the present invention, it has surprisingly been realized that a sputtering target having the composition and microstructure as defined above results in a very low number of ignition failures in a DC magnetron sputtering process, even if the sputtering target contains significant amounts of boron and the DC magnetron sputtering process is carried out at a relatively low partial pressure of the ionization gas.

As the target of the present invention can be sputtered at relatively low ionization gas pressure with a low percentage of plasma ignition failures, efficiency of the sputtering process is improved and a cap layer of uniform thickness and low surface roughness can be obtained. As will be discussed below in further detail, a sputtering target having the microstructure outlined above can be obtained if a melt which already includes all the chemical components of the final sputtering target (i.e. a melt comprising cobalt, chromium, boron and optionally platinum), is subjected to a rapid solidification, thereby obtaining an intermediate solid of high structural uniformity which is then subjected to one or more further processing steps so as to obtain the final sputtering target. Preferably, the intermediate solid is not blended anymore with additional components, so as to preserve its uniform microstructure. So, the intermediate solid obtained by rapid solidification preferably has a chemical composition which already

corresponds more or less to the composition of the final target, and the further processing steps are carried out for fine-tuning the microstructure and machining the intermediate solid into the final target.

As already indicated above, the expression "a single phase of a cobalt-based alloy" means that the sputtering target contains just one phase of a cobalt-based alloy. No separation into two or more phases of different Co-based alloys takes place during the manufacturing process of the sputtering target. The term "cobalt-based alloy" means that cobalt is the main component of the alloy, i.e. the amount of cobalt is higher than the amount of any other alloying element which is present in the alloy.

Whether or not the sputtering target contains just one phase of a Co-based alloy can be detected by SEM assisted by EDX. Single phase means that no further Co-based alloy phases with different composition can be detected on elemental analysis, for example, with x1000 magnification taken from a polished surface of the sputtering target which is perpendicular to the sputtering direction. Preferably, the only dispersed phase which is present in the cobalt-based alloy is formed by the boride particles.

Preferably, the sputtering target does not contain oxide particles. Preferably, if platinum is present in the sputtering target, no separate Pt phase or Pt-based alloy phase is formed in the sputtering target.

In the present invention, it is possible that the sputtering target substantially consists of or even consists of the single phase cobalt-based alloy and the boride particles dispersed therein.

The cobalt-based alloy forming the matrix comprises chromium and optionally platinum as an alloying element. In a preferred embodiment, both chromium and platinum are present. The amount of chromium and/or platinum in the sputtering target can vary over a broad range. The sputtering target may comprises chromium in an amount of 2-25 at%, more preferably 10- 20 at%, and platinum in an amount of 0-30 at%, more preferably 5-25 at%. The cobalt of the sputtering target is present in the cobalt-based alloy which forms the single phase matrix as well as in the boride particles which are dispersed in the single phase matrix.

The chromium of the sputtering target is present as an alloying element in the cobalt-based alloy, but may also be present in the boride particles.

If the sputtering target comprises platinum, it is present as an alloying element in the cobalt- based alloy, whereas no platinum or only a very small amount of platinum is present in the boride particles. The amount of boron which is present in the sputtering target can vary over a broad range. The sputtering target may comprise boron in an amount of 19 at% or less, more preferably in an amount of from 2 to 19 at% or from 4 to 14 at%.

The boron is mainly present in the dispersed boride particles. However, the cobalt-based alloy forming the matrix may also contain a low amount of boron, e.g. less than 1000 ppm by weight or less than 500 ppm by weight.

As mentioned above, the sputtering target comprises boride particles which have an average particle size of 10 μηι or less and are dispersed in the cobalt-based alloy.

The average particle size of the boride particles can be determined on a SEM image with x1000 magnification taken from a polished surface of the sputtering target which is perpendicular to the sputtering direction, by using a SEM image analyzer software. The particle size or diameter of a particle is based on the projected area diameter which is the diameter of a sphere having the same projected area as the particle. The projected area diameter d of a particle can be related to the particle projected area A by

d = ((4 X A) / TT)^1/2

or r = (A / TT)^1/2, wherein r is the circle radius (i.e. 0.5 x d)

The particle diameter d is then averaged for all the particles being analysed. Preferably, the boride particles have an average particle size of less than 5 μηι.

Preferably, the boride particles have a maximum diameter which does not exceed 30 μηι. The sputtering properties like number of plasma ignition failures can be further improved if the cross-sectional area of the boride particles in a plane perpendicular to the sputtering direction has a "round" or circle-like shape. During sputtering, the round or circle-shape boride particles will be uniformly deposited to make low surface roughness. The shape of a particle can be expressed e.g. by its aspect ratio. The aspect ratio of a particle is determined by the ratio of major axis to minor axis of an ellipse fitted to the particle. A perfect circle would have an aspect ratio of 1 . The average aspect ratio of the boride particles is determined on a SEM image with x1000 magnification taken from a polished surface of the sputtering target which is perpendicular to the sputtering direction, by using a SEM image analyzer software.

Preferably, the boride particles have an average aspect ratio of less than 2.5, more preferably less than 2.0. Preferably, the boride particles have an average perimeter of less than 8μηι, more preferably less than 6 μηι or even less than 5μηι. Perimeter of a particle is the length of its outside boundary and can be determined on SEM images by using a SEM image analyzer software. The perimeter of a particle does not only depend on the particle size but also on its "circularity". The rounder a particle is, the lower is its perimeter at a given particle size.

Preferably, the ratio of the total area (in μηι²) of the boride particles to the total perimeter (in μηι) of the boride particles is within the range of from 0.10 to 0.40, more preferably from 0.16 to 0.35 μηι. Total area of the boride particles is the area which is covered by all the boride particles being analyzed, and total perimeter is the sum of perimeters of all the particles being analyzed, e.g. determined on a SEM image with x1000 magnification taken from a polished surface of the sputtering target which is perpendicular to the sputtering direction. The ratio of total area to total perimeter is a parameter which is commonly used for characterizing particles.

The boride is a (chromium - cobalt) boride. Optionally, the boride may, in addition to chromium, comprise cobalt, but typically in an amount which is lower than the amount of chromium. Optionally, the sputtering target may comprise one or more of the following metal elements in a total amount of not more than 12 at%: Ruthenium (Ru), copper (Cu), silver (Ag), gold (Au), vanadium (V) and tantalum (Ta).

The metal elements Ru, Cu, Ag, Au, V or Ta are mainly present as an alloying element in the Co-based matrix.

Preferably, the sputtering target is made of chromium, boron, optionally platinum, optionally one or more of the following metal elements: Ru, Cu, Ag, Au, V and Ta; the remainder being cobalt (which is the main component) and unavoidable impurities. The preferred amount of each component has been indicated above.

Preferably, if the sputtering target of the present invention is subjected to a DC magnetron sputtering comprising a plurality of pulses and each pulse is composed of a plasma ignition phase with a maximum ignition voltage U, and a plasma sustaining phase with an average sustaining voltage U_{s av}, the ratio U, to U_{s av} being not more than 2.0, less than 7% of the pulses have a plasma ignition phase which is longer than 100 με. Preferably, if the sputtering target is subjected to a DC magnetron sputtering, the frequency of voltage which is higher in the ratio of 1 .6 of the average sustaining voltage U_{s av} of the power, and the maximum ignition voltage U, of time is more than 100 ms is less than 5 % during DC magnetron sputtering. Preferably, if the sputtering target is subjected to a DC magnetron sputtering, the frequency of voltage which is higher in the ratio of 1 .6 of the average sustaining voltage U_{s av} of the power, and the maximum ignition voltage U, of time is more than 100 ms is between about 400 and 900 V DC during DC magnetron sputtering. Preferably, if the sputtering target is subjected to a DC magnetron sputtering, the frequency of voltage which is higher in the ratio of 1 .6 of the base voltage of the power, and the first period of time is less than 50 ms is more than 78 % during DC magnetron sputtering. The present invention also relates to a process for preparing the sputtering target described above, which comprises:

subjecting a first material which comprises cobalt, boron, chromium and optionally platinum to a rapid solidification treatment, thereby obtaining an intermediate material,

subjecting the intermediate material to one or more processing steps for obtaining the sputtering target.

Rapid solidification treatments are generally known to the skilled person and refer to processes wherein a melt is cooled at high cooling rates. A preferred rapid solidification treatment used in the process of the present invention is gas atomization. However, other rapid solidification methods such as vacuum induction melting (by using a device which allows high cooling rates so as to achieve rapid solidification, i.e. a vacuum induction melting device with attached rapid solidification equipment) can be used as well. Appropriate conditions for these methods are known to the skilled person. In a gas atomization process, an inert gas is introduced into a melt and small droplets are generated. By rapidly cooling the droplets, a solid having a very fine and homogeneous microstructure can be obtained. The droplets can be solidified at a cooling rate of e.g. 10²-10⁶ °C or 10³-10⁶ °C per second by controlling gas pressure. However, other cooling rates, in particular higher cooling rates are also possible. The first material to be subjected to the rapid solidification treatment can be a blend of two or more materials having different compositions. The metals can be added to the blend in elementary form or in the form of an alloy or a combination of both. Just as an example, if platinum is added, it can be in elementary form or can be in the form of an alloy together with at least one of the other metals (e.g. a Co-Pt alloy or a Co-Cr-Pt alloy). In total, the blend preferably comprises cobalt, boron, chromium and optionally platinum in amounts which already correspond to the final amounts of each element in the final target.

Alternatively to a blend, the first material can be a single piece of material, e.g. an alloy already containing cobalt, boron, chromium and optionally platinum. Just as an example, said first material can be obtained from a blend of different materials by melting and casting.

Preferably, the intermediate solid is not blended anymore with additional components, so as to preserve its uniform microstructure. So, the intermediate solid obtained by rapid solidification preferably has a chemical composition which already corresponds more or less to the composition of the final target, and the further processing steps are carried out for fine-tuning the microstructure and machining the intermediate solid into the final target.

Prior to the rapid solidification treatment, a deoxidation step can be carried out, e.g. with reduction atmosphere using inert gas.

If the rapid solidification treatment is a gas atomization, the intermediate material is a powder which is subjected to a sintering step. Prior to sintering, the powder might be subjected to a compaction step (e.g. uniaxial pressing or cold isostatic pressing).

Appropriate sintering methods are commonly known to the skilled person. Preferably, the powder is sintered by spark plasma sintering or hot pressing such as vacuum hot pressing. However, other commonly known sintering methods can be used as well. It is also possible to carry out two sintering steps, e.g. spark plasma sintering followed by hot isostatic pressing.

The appropriate sintering temperature may depend on the composition and can be e.g. between 700-1500°C. The heating rate applied during the sintering step can be e.g. within a range of from 5-500°C per minute. Preferably, a pressure of up to 200 MPa is applied. Preferably, the sintering step is continued until a sputtering target having a relative density of at least 95%, more preferably at least 97% is obtained.

Finally, the sputtering target can be machined to its final dimensions. The present invention is illustrated in further detail by the following Examples.

EXAMPLES

If not stated otherwise, the microstructure of the sputtering targets was analyzed as follows:

A surface of the sputtering target which is perpendicular to the sputtering direction was polished by conventional metallurgical analysis method. A SEM image with x1000 magnification was taken from the polished surface of the sputtering target. An image analyzer software (Image J 2.0) was used for determining average particle size of the boride particles, average aspect ratio of the boride particles, average perimeter of the boride particles, and area fraction covered by the boride particles. For elemental analysis of the detected phases, the SEM device (JEOL JSM-6610LV) was equipped with EDX (Oxford INCA x-act).

Example 1 : Sputtering Target of Co -18.02 at.% Cr - 9.89 at.% B composition obtained by rapid solidification and sintering

The composition of a target produced in Example 1 is Co-18.0Cr-9.9B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr and Co-B alloy as melt stock was combined in the weight ratio of 80.25 wt.% of Co, 17.7 wt.% of Cr and 2.05 wt.% of B. The material was then melted in a vacuum induction melter between 1200 to 1300°C and solidified to give chemically

homogeneous material. The solidified mass was atomized at a temperature of 1250 to 1400°C to give powder of near spherical shape. The powder was charged into a graphite mold and sintered by SPS (spark plasma sintering) at 1 100°C under a pressure of 35 MPa for 10 minutes. The sintered body was machined to obtain a sputtering target.

Figure 1 shows the microstructure of the sputtering targets. The boride particles have an average particle size of 4.46 micrometers. Furthermore, the boride particles had an average aspect ratio of 1.88 and an average perimeter of 5.20 μηι. The ratio total area/total perimeter was 0.33 μηι. The oxygen content of the sputtering target was 161 ppm.

Comparative Example 1 : Sputtering Target of Co - 17.47 at.% Cr - 9.59 at.%B composition obtained by commercial melting and casting

The composition of a target produced in comparative example 1 is Co-17.5Cr-9.6B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr and Co-B alloy as melt stock was combined in the weight ratio of 80.25 wt.% of Co, 17.7 wt.% of Cr and 2.05 wt.% of B. The material was then melted in a vacuum induction melter between 1200 to 1300°C and solidified in the graphite mold. The solidified ingot was rolled and machined to obtain a sputtering target.

Figure 2 shows the microstructure of the sputtering targets. The boride particles exist between matrix with inter-dendritic structure. Example 2: Sputtering Target of Co - 14.13 at% Cr - 14.01 at% Pt - 9.82 at% B composition obtained by rapid solidification and sintering

The composition of a target produced in Example 2 is Co-14.1 Cr-14.0Pt-9.8B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr, Co-Cr-Pt and Co-B alloy as melt stock was combined in the weight ratio of 50.6 wt.% of Co, 10.08 wt.% of Cr, 37.82 wt.%Pt and 1 .5 wt.% of B. The material was then melted in a vacuum induction melter between 1350 to 1450°C and solidified to give chemically homogeneous material. The solidified mass was atomized at a temperature of 1350 to 1500°C to give powder of near spherical shape. The powder was charged into a graphite mold and sintered by SPS at 1050°C under a pressure of 35 MPa for 10 minutes. The sintered body was machined to obtain a sputtering target.

Figure 3 shows the microstructure of the sputtering target. The boride particles had an average particle size of 2.77 micrometers.

Furthermore, the boride particles had an average aspect ratio of 1 .52 and an average perimeter of 2.56 μηι. The ratio total area/total perimeter was 0.24 μηι. Comparative Example 2: Sputtering Target of Co - 14.15 at% Cr -14.00 at% Pt - 9.82 at% B composition obtained by commercial melting and casting

The composition of a target produced in Comparative Example 2 is Co-14.2Cr-14.0Pt-9.8B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr,

Co-Cr-Pt and Co-B alloy as melt stock was combined in the weight ratio of 50.6 wt.% of Co, 10.08 wt.% of Cr, 37.82 wt.%Pt and 1 .5 wt.% of B. The material was then melted in a vacuum induction melter and solidified in the graphite mold. The solidified ingot was rolled and machined to obtain a sputtering target.

Figure 4 shows the microstructure of the sputtering targets. The microstructure is typical dendritic structure. The connected boride particles shown in the figure have an average particle size of more than 30 micrometers.

Examples 3-5: Sputtering Target of Co -15.23 at% Cr - 14.02 at% Pt - 8.55 at% B composition obtained by rapid solidification and using different sintering conditions The composition of a target produced in Examples 3-5 are Co-15.2Cr-14.0Pt-8.6B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr, Co-Cr-Pt and Co-B alloy as melt stock was combined in the weight ratio of 50.3 wt.% of Co, 10.7 wt.% of Cr, 37.6 wt.%Pt and 1 .4 wt.% of B. The material was then melted in a vacuum induction melter between 1350 to 1450°C and solidified to give chemically homogeneous material. The solidified mass was atomized at a temperature of 1350 to 1500°C to give powder of near spherical shape. For example 3, the powder was charged into a graphite mold and sintered by SPS at 1 100°C under a pressure of 35 MPa for 10 minutes. For example 4, the powder was charged into a graphite mold and sintered by SPS at 1 100°C under a pressure of 35 MPa for 10 minutes, followed by hot isostactic pressing (HIP) at 1000°C. For example 5, the powder was charged into a graphite mold and sintered by SPS at 1050°C under a pressure of 35 MPa for 10 minutes. The sintered bodies of Examples 3 to 5 were machined to obtain a sputtering target.

The boride particles of Example 3 had an average particle size of 2.84 μηι, an average aspect ratio of 1 .67, and an average perimeter of 2.85 μηι. The ratio total area/total perimeter was 0.23 μηι. The sputtering target had an oxygen content of 291 ppm. The microstructure of the sputtering target of Example 3 is shown in Figure 5.

The boride particles of Example 4 had an average particle size of 3.19 μηι, an average aspect ratio of 1 .67, and an average perimeter of 3.21 μηι. The ratio total area/total perimeter was 0.25 μηι. The sputtering target had an oxygen content of 291 ppm. The microstructure of the sputtering target of Example 4 is shown in Figure 6.

The boride particles of Example 5 had an average particle size of 2.16 μηι, an average aspect ratio of 1 .75, and an average perimeter of 2.29 μηι. The ratio total area/total perimeter was 0.16 μηι. The sputtering target had an oxygen content of 291 ppm. The microstructure of the sputtering target of Example 5 is shown in Figure 7.

Each of the sputtering targets prepared in Examples 3-5 was subjected to a DC magnetron sputtering under the following conditions:

Base Pressure: 7.3 x10^"3 Torr

Argon flow rate:

Step 1 : 0.5s/30 seem (standard cubic cm/minute) (1 seem = 2.7x10¹⁹ atoms/cm³) Step 2: 1 s/20 seem

Ar Pressure: 7.3 mTorr

Power: 0.71 kW

Bias: OV

A voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 3 is presented in Figure 8.

A voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 4 is presented in Figure 9.

A voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 5 is presented in Figure 10. Comparative Example 3: Sputtering Target of Co-15.40 at% Cr-14.08 at% Pt-8.56 at% B composition obtained by commercial melting and casting

The composition of a target produced in Comparative Example 3 is Co-15.4Cr-14.1 Pt-8.6B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr, Co-Cr-Pt and Co-B alloy as melt stock was combined in the weight ratio of 50.3 wt.% of Co, 10.7 wt.% of Cr, 37.6 wt.%Pt and 1 .4 wt.% of B. The material was then melted in a vacuum induction melter and solidified in the graphite mold. The solidified ingot was rolled and machined to obtain a sputtering target. The microstructure of the sputtering target of Comparative Example 3 is shown in Figure 1 1 .

The sputtering target prepared in Comparative Examples 3 was subjected to a DC magnetron sputtering under the same conditions as used for the targets of Examples 3-5. A voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Comparative Example 3 is presented in Figure 12. In Figure 13, one of those pulses is shown which had several misfire events in the plasma ignition phase, which is why it took about 230 ms to ignite the plasma. For the sputtering process, it would be preferred that the plasma ignites at a voltage which is close to the voltage applied for sustaining the plasma. If the ignition voltage exceeds the plasma sustaining voltage, a voltage spike is present in the plasma ignition phase (Ui). However, if such a voltage spike is present, its time span should be as short as possible, e.g. less than 100 με or even shorter. The time span of such a spike corresponds to the ignition phase time span. The more time is needed for igniting the plasma, the less time is available during the pre-fixed pulse duration for sputtering the material, moreover, the yield of thin film is lower than normal process result in low productivity. For each of the sputtering tests of Examples 3-5 and Comparative Example 3, the percentage of pulses having a plasma ignition phase which exceeds 100 ms was determined. The results are shown in the following Table 1 :

Table 1

As demonstrated by the data of Table 1 , the sputtering targets of the present invention show a significantly reduced ignition failure. If compared to Comparative Example 3, the sputtering targets of the present invention significantly reduced the percentage of pulses having a plasma ignition phase of more than 100 ms, thereby resulting in a pronounced improvement of sputtering efficiency.

Furthermore, the sputtering targets of the present invention show reduced arcing and provide a sputtered film of very uniform compositional distribution.

Example 6: Sputtering Target of Co-6.94 at% Cr-17.91 at% Pt-1 .95 at% Ru-10.52 at% B composition obtained by rapid solidification and sintering

The composition of a target produced in Example 6 is Co-6.94Cr-17.91 Pt-1 .95Ru-10.52B and the target was produced and evaluated as follows. Melt stock consisting of Co, Cr, and including a combination of material such as Co-Cr, Co-Cr-Pt-Ru and Co-B alloy as melt stock was combined in the weight ratio of 47.5 wt.% of Co, 4.6 wt.% of Cr, 44.2 wt.%Pt, 2.5wt.%Ru and 1 .2 wt.% of B. The material was then melted in a vacuum induction melter between 1350 to 1450°C and solidified to give chemically homogeneous material. The solidified mass was atomized at a temperature of 1350 to 1500°C to give powders of near spherical shape. The powders was charged into a graphite mold and sintered by SPS at 1050°C under a pressure of 35 MPa for 10 minutes. The sintered body was machined to obtain a sputtering target.

The boride particles of Example 6 had an average particle size of 4.05 μηι, an average aspect ratio of 1 .81 , and an average perimeter of 4.7 μηι. The ratio total area/total perimeter was 0.25 μηι. The sputtering target had an oxygen content of 1 14 ppm. The microstructure of the sputtering target of Example 6 is shown in Figure 14.

The sputtering target prepared in Example 6 was subjected to a DC magnetron sputtering under the same conditions as used for the targets of Examples 3-5 and Comparative Example 3.

A voltage vs. time graph showing some pulses of the DC magnetron sputtering process of the sputtering target of Example 6 is presented in Figure 15. The percentage of pulses having a plasma ignition phase which exceeded 100 ms was determined. The results are shown in the following Table 2.

Table 2

As demonstrated by the voltage-time-graph of Figure 15 and the data of Table 2, the sputtering target of inventive Example 6 shows a significantly reduced ignition failure. None of the pulses had a plasma ignition phase of more than 100 ms, thereby significantly improving efficiency of the sputtering process.

Claims

A sputtering target, comprising a single phase of a cobalt-based alloy, wherein the cobalt-based alloy comprises chromium and optionally platinum as an alloying element, and boride particles, wherein the boride particles have an average particle size of 10 μηι or less and are dispersed in the cobalt-based alloy.

The sputtering target according to claim 1 , which comprises chromium in an amount of 4-25 at%, and platinum in an amount of 0-30 at%.

The sputtering target according to one of the preceding claims, comprising boron in an amount of 19 at% or less

The sputtering target according to one of the preceding claims, wherein the boride particles have an average aspect ratio of less than 2.5

The sputtering target according to one of the preceding claims, wherein the boride particles have an average perimeter of less than 8 μηι.

The sputtering target according to one of the preceding claims, wherein the ratio of the total area (in μηι²) of the boride particles to the total perimeter (in μηι) of the boride particles is within the range of from 0.10 to 0.40.

The sputtering target according to one of the preceding claims, further comprising at least one of Ru, Cu, Ag, Au, V or Ta in a total amount of 12 at% or less.

The sputtering target according to one of the preceding claims, wherein, if the sputtering target is subjected to a DC magnetron sputtering comprising a plurality of pulses and each pulse is composed of a plasma ignition phase with a maximum ignition voltage U, and a plasma sustaining phase with an average sustaining voltage U_s,av, the ratio U, to U_s,av being not more than 2.0, less than 7% of the pulses, more preferably less than 5% of the pulses, even more preferably less than 2% of the pulses have a plasma ignition phase which is longer than 100 με. The sputtering target according to one of the preceding claims, wherein, if the sputtering target is subjected to a DC magnetron sputtering, the frequency of voltage which is higher in the ratio of 1 .6 of the base voltage of the power, and the first period of time is more than 100 ms is less than 5 % during DC magnetron sputtering.

A process for preparing the sputtering target according to one of the claims 1 to 9, which comprises:

subjecting a first material which comprises cobalt, chromium, boron, and optionally platinum to a rapid solidification treatment, thereby obtaining an intermediate material, subjecting the intermediate material to one or more processing steps for obtaining the sputtering target.

The process according to claim 10, wherein the rapid solidification treatment is a gas atomization.

The process according to claim 10 or 1 1 , wherein the first material which is subjected to a rapid solidification treatment has a composition which corresponds to the composition of the sputtering target.

The process according to one of the claims 10 to 12, wherein the intermediate material is subjected to at least one sintering step.

The process according to claim 13, wherein the sintering step is a spark plasma sintering, a hot isostatic pressing, or a combination thereof.