Cobalt Nickel Alloy Sputtering Target: Comprehensive Analysis Of Composition, Manufacturing, And Applications In Advanced Thin Film Deposition

Alloy Composition And Metallurgical Design Of Cobalt Nickel Alloy Sputtering Target

Fundamental Compositional Framework And Alloying Strategy

Cobalt nickel alloy sputtering targets are typically formulated within a compositional range where cobalt serves as the primary matrix element (50–95 at.%) and nickel functions as both a structural modifier and magnetic property tuner (5–50 at.%)1. The selection of Co-Ni ratios depends critically on the intended application: high-cobalt compositions (Co > 80 at.%) are preferred for magnetic recording media requiring high coercivity (Hc > 3000 Oe), while balanced Co-Ni ratios (40–60 at.% each) are employed in barrier layer applications where reduced ferromagnetism and improved diffusion resistance are paramount17. Patent literature reveals that cobalt alloy sputtering targets may incorporate additional elements from the group of Ni, Ta, Pt, V, Mo, B, Si, Zn, Ti, Sm, Nb, P, Rh, Pd, Sc, Zr, Fe, Hf, and Re to tailor specific properties1. For cobalt nickel systems specifically, tantalum additions (0.5–10 at.%) are frequently employed to suppress grain growth during deposition and enhance thermal stability of the resulting silicide phases816.

The metallurgical challenge in Co-Ni alloy target design lies in achieving phase homogeneity while maintaining high purity (>99.99%) and controlling oxygen content (<40 ppm)17. Cobalt and nickel form a continuous solid solution across the entire composition range at elevated temperatures, but segregation during solidification can lead to compositional inhomogeneities that manifest as sputtering non-uniformities and particle generation11. Advanced manufacturing routes address this through rapid solidification processing or powder metallurgy approaches that refine microstructure and eliminate macro-segregation19.

Microstructural Characteristics And Phase Distribution

The microstructure of cobalt nickel alloy sputtering targets profoundly influences sputtering performance, particularly PTF and particle generation rates. High-quality targets exhibit a fine-grained structure with average grain sizes below 100 μm, achieved through controlled solidification rates and post-casting thermomechanical processing12. In targets containing boron additions (5–20 at.%), a characteristic columnar microstructure framed by boron intermetallics develops during directional casting, providing mechanical reinforcement while maintaining high PTF (>30% at 3 mm thickness)17. The area ratio of high-purity Ni phases (Ni content >99.5 mass%) should be maintained below 5% to prevent localized magnetic anomalies that degrade PTF uniformity12.

For cobalt-rich compositions, the presence of oxide phases must be strictly controlled. In CoCrPt-based targets containing nickel, the formation of Cr₂O₃ and Co(Cr)-X-O ceramic phases (where X represents metallic elements) should be limited to grain sizes below 3 μm to prevent arc discharge effects and unnecessary particle formation during sputtering3. This is achieved through oxygen-controlled melting environments (vacuum or partial Ar pressure) and the incorporation of oxygen-scavenging elements such as titanium or zirconium in sub-stoichiometric oxide forms (TiOₓ with particle size 0.1–50 μm)18.

Magnetic Property Engineering And PTF Optimization

Pass-through flux (PTF) represents the percentage of magnetic field from the magnetron that penetrates through the target material, directly impacting sputtering efficiency and deposition rate uniformity. For cobalt nickel alloy sputtering targets, achieving high PTF (>75% average with standard deviation <5% perpendicular to the sputtering surface at 3 mm thickness) is essential for commercial viability13. The magnetic moment of Co-Ni alloys can be systematically tuned by adjusting the Ni content: pure cobalt exhibits a saturation magnetization of approximately 1400 emu/cm³, while nickel contributes approximately 485 emu/cm³, allowing linear interpolation of magnetic properties across the composition range17.

Strategic alloying with elements that decrease the Curie temperature of nickel—such as chromium (Cr >18 at.% for low-moment alloys), manganese, or aluminum—enables the production of targets with reduced ferromagnetism at operating temperatures, thereby enhancing PTF1219. For high-moment applications (Cr <18 at.%), the addition of iron (5–40 at.%) in Co-Fe-Ni ternary systems can increase saturation magnetization while maintaining acceptable PTF through microstructural control56. The incorporation of 8–20 at.% of refractory metals (Ta, Zr, Nb, Hf) or aluminum creates a fine dispersion of non-magnetic or weakly magnetic phases that disrupt long-range ferromagnetic ordering, resulting in PTF values ranging from 30% to 70% depending on composition and processing history25.

Manufacturing Processes For Cobalt Nickel Alloy Sputtering Target Production

Vacuum Arc Melting And Casting Routes

Vacuum arc melting (VAM) represents a primary manufacturing route for cobalt nickel alloy sputtering targets, offering advantages in achieving high density and fine grain microstructure directly from the cast ingot1. In this process, raw materials comprising pure cobalt, nickel, and alloying elements are melted in a water-cooled copper crucible under high vacuum (typically <10⁻⁴ Torr) using a consumable or non-consumable electrode arc. The rapid solidification inherent to VAM (cooling rates of 10²–10⁴ K/s) suppresses segregation and refines grain structure, often eliminating the need for extensive post-casting deformation processing1. Targets produced via VAM can be fabricated by directly slicing the ingot into discs of appropriate thickness (typically 3–6 mm for planar magnetron sputtering), followed by surface grinding to achieve the required flatness (<0.1 mm total indicator runout) and surface finish (Ra <0.8 μm)1.

For compositions containing reactive elements such as boron, titanium, or aluminum, induction melting under controlled atmosphere (vacuum or partial pressure of argon) is preferred to minimize oxidation and volatilization losses1719. The molten alloy is poured into preheated graphite or ceramic molds to control solidification rate and minimize thermal gradients that can induce cracking in brittle intermetallic phases. Directional solidification techniques, where the mold is withdrawn from the heating zone at controlled rates (1–10 mm/min), produce columnar grain structures aligned perpendicular to the sputtering surface, which can enhance PTF and reduce in-plane anisotropy17.

Powder Metallurgy And Consolidation Techniques

Powder metallurgy (PM) routes offer superior compositional control and microstructural homogeneity for complex cobalt nickel alloy systems, particularly those containing immiscible elements or requiring ultra-fine grain sizes19. The PM process begins with the preparation of alloy powders through gas atomization, where a molten stream of the target composition is disintegrated by high-velocity inert gas jets (typically argon or nitrogen at pressures of 3–7 MPa), producing spherical particles with diameters ranging from 10 to 150 μm19. Alternatively, mechanical alloying of elemental or master alloy powders in high-energy ball mills can produce nanocrystalline or amorphous precursors that, upon consolidation, yield extremely fine-grained targets with enhanced sputtering uniformity19.

Powder consolidation is achieved through hot pressing, hot isostatic pressing (HIP), or spark plasma sintering (SPS). Hot pressing involves uniaxial compression of the powder charge in a graphite die at temperatures of 0.7–0.9 Tm (where Tm is the melting temperature) and pressures of 20–50 MPa, typically in vacuum or inert atmosphere18. For cobalt-based alloys containing sub-stoichiometric titanium oxide (TiOₓ) particles, hot pressing at 900–1100°C for 1–3 hours achieves densities exceeding 95% of theoretical density while maintaining homogeneous distribution of the oxide phase18. HIP processing, conducted at similar temperatures but under isostatic gas pressure (100–200 MPa), eliminates residual porosity and produces targets with densities >99.5% theoretical, critical for minimizing particle generation during sputtering19.

Post-Consolidation Processing And Quality Control

Regardless of the primary manufacturing route, post-consolidation processing is essential to optimize target performance. Stress-relief annealing at temperatures of 400–600°C for 2–8 hours in vacuum (<10⁻⁵ Torr) or high-purity argon atmosphere removes residual stresses induced by solidification or powder consolidation, reducing the propensity for cracking during bonding or sputtering17. For targets requiring enhanced PTF, a second annealing step at higher temperatures (700–900°C) may be employed to promote grain growth to the optimal size range (50–100 μm) and homogenize the distribution of secondary phases13.

Bonding of the sputtering target to a backing plate (typically oxygen-free high-conductivity copper or aluminum alloys) is accomplished through diffusion bonding, brazing, or elastomer bonding, depending on the target composition and application requirements414. Diffusion bonding, conducted at 0.6–0.8 Tm under vacuum and applied pressure (5–20 MPa), creates a metallurgical bond with minimal interface resistance, essential for efficient heat dissipation during high-power sputtering4. Quality control protocols include ultrasonic inspection to detect internal defects, PTF mapping across the target surface using Hall effect sensors, chemical analysis via inductively coupled plasma mass spectrometry (ICP-MS) to verify composition and impurity levels, and microstructural characterization through optical and electron microscopy13.

Physical And Chemical Properties Of Cobalt Nickel Alloy Sputtering Target

Density, Melting Point, And Thermal Properties

The density of cobalt nickel alloy sputtering targets varies linearly with composition according to the rule of mixtures, ranging from approximately 8.90 g/cm³ for pure nickel to 8.86 g/cm³ for pure cobalt, with typical Co-Ni alloys exhibiting densities of 8.88–8.92 g/cm³ depending on the Co:Ni ratio and the presence of lighter alloying elements such as aluminum or boron17. High-density targets (>99% theoretical) are essential to minimize void-related particle generation and ensure consistent sputtering rates across the target surface18.

Melting points of Co-Ni alloys exhibit a solidus-liquidus range rather than a sharp melting temperature, with the solidus temperature decreasing from 1495°C for pure cobalt to approximately 1455°C for pure nickel, and reaching a minimum of approximately 1430°C near the equiatomic composition1. This relatively narrow melting range facilitates casting and reduces the risk of hot tearing during solidification. Thermal conductivity of Co-Ni alloys at room temperature ranges from 60 to 90 W/(m·K), intermediate between pure cobalt (100 W/(m·K)) and pure nickel (91 W/(m·K)), with the exact value depending on composition, grain size, and the presence of secondary phases17. Adequate thermal conductivity is critical for dissipating the heat generated during sputtering (power densities of 5–50 W/cm²), preventing target overheating that can lead to grain growth, phase transformations, or catastrophic failure.

The coefficient of thermal expansion (CTE) for Co-Ni alloys ranges from 12 to 14 × 10⁻⁶ K⁻¹ over the temperature range of 20–500°C, closely matching that of common backing plate materials such as copper (CTE ≈ 17 × 10⁻⁶ K⁻¹) and aluminum (CTE ≈ 23 × 10⁻⁶ K⁻¹)17. Minimizing CTE mismatch between target and backing plate is essential to prevent debonding or cracking during thermal cycling inherent to sputtering operations.

Mechanical Properties And Sputtering Yield

The mechanical properties of cobalt nickel alloy sputtering targets are dominated by the face-centered cubic (fcc) crystal structure common to both cobalt (above 422°C) and nickel. Room-temperature tensile strength ranges from 400 to 800 MPa depending on composition and processing history, with yield strengths of 200–500 MPa and elongations of 10–40%17. Targets containing refractory metal additions (Ta, Nb, Hf) or boron intermetallics exhibit higher strength (tensile strength >1000 MPa) but reduced ductility (elongation <10%), necessitating careful control of bonding parameters to avoid cracking25.

Hardness values for Co-Ni alloy targets typically range from 150 to 300 HV (Vickers hardness), with higher values associated with fine-grained microstructures and the presence of hard secondary phases11. While increased hardness generally correlates with improved wear resistance during sputtering, excessively hard targets (>350 HV) may exhibit reduced sputtering yield and increased particle generation due to brittle fracture mechanisms15.

Sputtering yield, defined as the number of target atoms ejected per incident ion, is a critical performance parameter that depends on target composition, ion energy, ion mass, and angle of incidence. For cobalt nickel alloy targets sputtered with argon ions at typical magnetron energies (300–800 eV), sputtering yields range from 1.5 to 3.0 atoms/ion, with cobalt-rich compositions exhibiting slightly higher yields than nickel-rich compositions due to differences in surface binding energy (4.39 eV for Co vs. 4.44 eV for Ni)17. The addition of lighter elements such as boron or aluminum can reduce the average sputtering yield, while heavier elements such as tantalum or platinum increase it, allowing compositional tuning to match specific deposition rate requirements38.

Chemical Stability And Oxidation Resistance

Cobalt nickel alloys exhibit moderate oxidation resistance at elevated temperatures, forming protective oxide scales that limit further oxidation. In air at temperatures below 400°C, both cobalt and nickel form thin, adherent oxide layers (CoO and NiO, respectively) with parabolic growth kinetics, providing adequate protection for short-term exposures17. However, at temperatures above 600°C, the formation of volatile cobalt oxides (Co₃O₄) and the development of non-protective, porous oxide scales can lead to accelerated oxidation, particularly in cobalt-rich compositions1.

For sputtering target applications, oxidation is primarily a concern during storage, handling, and the initial stages of sputtering before the target surface is cleaned by ion bombardment. To minimize surface oxidation, targets are typically stored in vacuum-sealed packaging with desiccants and may be subjected to in-situ sputter cleaning (pre-sputtering) for 5–30 minutes prior to deposition13. The oxygen content of the bulk target material should be maintained below 40 ppm to prevent the formation of oxide inclusions that can act as particle generation sites17.

Corrosion resistance of Co-Ni alloys in aqueous environments is generally good, with both elements forming passive oxide films in neutral and alkaline solutions. However, in acidic environments (pH <4), particularly those containing chloride ions, localized corrosion (pitting) can occur, especially at grain boundaries or secondary phase interfaces17. For targets used in reactive sputtering processes involving oxygen or nitrogen plasmas, the formation of surface oxides or nitrides is intentional and controlled through process parameters, but the bulk target composition must be designed to minimize subsurface oxidation that can alter sputtering characteristics over the target lifetime3.

Applications Of Cobalt Nickel Alloy Sputtering Target In Advanced Technologies

Magnetic Recording Media And Data Storage Devices

Cobalt nickel alloy sputtering targets play a pivotal role in the fabrication of magnetic recording media for