Cobalt Chromium Alloy Sputtering Target: Advanced Manufacturing, Microstructural Engineering, And Applications In Magnetic Recording Media

Compositional Design And Alloying Strategy For Cobalt Chromium Sputtering Targets

The compositional architecture of cobalt chromium alloy sputtering targets is governed by the functional requirements of the deposited thin films, particularly in magnetic recording and electronic applications. The base CoCr system typically contains 33–40 at% Cr, with the balance being Co and unavoidable impurities 3. This chromium concentration range is critical for achieving the desired magnetic anisotropy and corrosion resistance in perpendicular magnetic recording layers. For enhanced coercivity and thermal stability, platinum is incorporated at levels ranging from 8 to 18 at%, forming CoCrPt ternary alloys 4. Platinum additions elevate the magnetocrystalline anisotropy constant (Ku), enabling higher areal density storage by stabilizing smaller magnetic grains against thermal fluctuations.

Beyond the ternary CoCrPt system, advanced targets incorporate boron (B) at concentrations up to 30 at% in the primary alloy phase 1316. Boron serves dual functions: it refines grain size during solidification and promotes magnetic decoupling by segregating to grain boundaries in the deposited film. The Co-Cr-Pt-B alloy phase (A) typically contains B in the range of 0–30 at%, while a secondary alloy phase (B)—comprising Co-B, Co-Cr-B, or Co-Pt-B—contains 0–20 at% B 13. This biphasic microstructure, with phase (A) constituting ≥50 vol% of the target, ensures homogeneous boron distribution without eccentric B aggregate phases, which would otherwise cause sputtering non-uniformities and particle generation 16.

Ceramic oxide additions, particularly SiO₂, TiO₂, and Cr₂O₃, are introduced to further refine the magnetic grain structure in deposited films. However, oxide incorporation presents significant manufacturing challenges due to density mismatches between metal powders (ρ ≈ 8–9 g/cm³) and ceramic powders (ρ ≈ 2–4 g/cm³), leading to compositional inhomogeneity 6. To address this, cobalt oxide (CoO or Co₃O₄) is co-added with non-magnetic oxides; during sintering, cobalt oxide undergoes reduction-oxidation reactions with chromium, forming finely dispersed Cr₂O₃ and Co(Cr)-X-O ceramic phases (where X represents the metallic element of the non-magnetic oxide) 110. Optimal targets exhibit ceramic phase lengths <3 μm, achieved by controlling prealloy powder composition and sintering parameters (temperature, time, atmosphere) 110. This microstructural refinement reduces arcing effects during DC magnetron sputtering and minimizes particle contamination, with high-chromium-containing particles (Cr-rich clusters) limited to maximum diameters ≤40 μm 11.

For specialized applications, additive metals such as tantalum (Ta), zirconium (Zr), niobium (Nb), hafnium (Hf), aluminum (Al), and ruthenium (Ru) are incorporated at 8–20 at% 2414. These refractory elements enhance target mechanical strength, improve sputtering yield, and in ferromagnetic CoCr targets, increase pass-through flux (PTF) from baseline values of 10–20% to 30–70%, thereby improving magnetron sputtering efficiency 514. The selection of additive metals must balance magnetic property modification, phase stability during casting and sintering, and compatibility with downstream film deposition processes.

Manufacturing Methodologies: Melting, Casting, Powder Metallurgy, And Hybrid Approaches

Vacuum Arc Melting (VAM) And Direct Casting Routes

For CoCr and CoCrPt alloy targets without ceramic additions, vacuum arc melting (VAM) offers a direct, cost-effective manufacturing route 2. In this process, elemental cobalt, chromium, and alloying additions (Pt, Ni, Ta, V, Mo, B, Si, Zn, Ti, Sm, Nb, P, Rh, Pd, Sc, Zr, Fe, Hf, Re) are melted in a water-cooled copper crucible under high vacuum (≤10⁻³ Pa) using a consumable electrode arc 2. The VAM process produces ingots with high density (>98% theoretical) and fine, equiaxed grain structures (grain size 50–200 μm) due to rapid solidification rates (10²–10³ K/s) 2. Critically, the fine-grained, homogeneous microstructure enables direct slicing of VAM ingots into sputtering targets without subsequent hot working or powder consolidation, reducing manufacturing cost and lead time 2.

However, VAM-cast targets may exhibit residual segregation of high-melting-point elements (Cr, Pt) and formation of coarse intermetallic phases (e.g., CoCr σ-phase, Co₃Pt ordered phase) if cooling rates are insufficient. To mitigate segregation, VAM ingots are subjected to homogenization annealing at 1150–1300°C for 4–24 hours, followed by controlled cooling 3. For CoCr-based targets, post-casting thermomechanical processing—comprising hot rolling at 1150–1300°C with total reduction ratios of 50–90%, followed by heat treatment at 500–1200°C—refines the microstructure and reduces residual stress 3. This processing sequence transforms the as-cast dendritic structure into a recrystallized, equiaxed grain structure with improved sputtering uniformity.

Powder Metallurgy (PM) Routes For Ceramic-Containing Targets

Targets incorporating ceramic oxides (SiO₂, TiO₂, Cr₂O₃) or borides cannot be produced via conventional casting due to immiscibility and density segregation. Instead, powder metallurgy (PM) routes are employed, involving powder synthesis, mixing, compaction, and sintering 61017. The critical challenge in PM processing is achieving homogeneous distribution of metal and ceramic phases.

Powder Synthesis: CoCr or CoCrPt prealloy powders are produced by gas atomization of VAM-melted ingots, yielding spherical particles with diameters of 10–100 μm and oxygen contents <500 ppm 610. Gas atomization under inert atmosphere (Ar or N₂) prevents oxidation and ensures powder flowability. For boron-containing targets, Co-Cr prealloy powder is first synthesized, then mechanically alloyed with elemental boron powder to form Co-Cr-B composite powder with controlled boride particle size (<5 μm) and distribution 17.

Wet Mixing And Slurry Processing: To overcome density-driven segregation, wet mixing techniques are employed 610. Ceramic powders (e.g., SiO₂, particle size 0.1–1 μm) are first dispersed in a solvent (ethanol, isopropanol) with dispersants (polyethylene glycol, polyvinyl alcohol) to form a stable slurry. Platinum powder (particle size 1–5 μm) is then added and ultrasonically mixed, allowing Pt particles to adsorb onto ceramic surfaces via electrostatic or steric stabilization 6. This Pt-ceramic slurry is subsequently mixed with CoCr prealloy powder, forming a CoCrPt-ceramic composite slurry with uniform phase distribution 6. The slurry is spray-dried or freeze-dried to produce free-flowing granules suitable for compaction.

Compaction And Sintering: The composite powder is uniaxially pressed at 100–300 MPa to form green compacts with relative densities of 60–75% 10. Green compacts are then sintered in vacuum (≤10⁻³ Pa) or inert atmosphere at temperatures of 900–1200°C for 2–8 hours 10. During sintering, solid-state diffusion densifies the compact to >95% theoretical density, while carefully controlled oxygen partial pressure (pO₂ ≈ 10⁻¹⁵–10⁻¹⁸ atm at sintering temperature) governs oxide phase formation. Cobalt oxide (CoO) added to the powder mixture undergoes reduction-oxidation with chromium, forming finely dispersed Cr₂O₃ and Co(Cr)-X-O phases with characteristic lengths <3 μm 110. Post-sintering, targets may undergo hot isostatic pressing (HIP) at 1000–1150°C and 100–200 MPa to eliminate residual porosity and improve mechanical integrity.

Hybrid Approaches: Rapid Solidification And Mechanical Alloying

For targets requiring ultrafine microstructures, rapid solidification methods such as melt spinning or gas atomization followed by mechanical alloying are employed 11. Melt spinning produces ribbons with cooling rates of 10⁵–10⁶ K/s, yielding amorphous or nanocrystalline structures (grain size <50 nm) 11. These ribbons are pulverized and consolidated via hot pressing or spark plasma sintering (SPS) at 800–1000°C under 50–100 MPa for 5–30 minutes 11. SPS enables rapid densification while preserving nanocrystalline grain sizes, resulting in targets with superior sputtering uniformity and reduced particle generation.

Mechanical alloying (MA) of elemental or prealloy powders in high-energy ball mills (attritor, planetary mill) for 10–50 hours produces composite powders with nanoscale phase dispersion and high defect densities 11. MA powders are consolidated via hot pressing or HIP, yielding targets with homogeneous ceramic dispersion and refined metal grain structures. However, MA introduces contamination from milling media (Fe, Cr, W) and requires careful control of milling atmosphere (Ar, N₂) and process control agents (stearic acid, ethanol) to prevent excessive oxidation or nitridation.

Microstructural Characterization And Control Of Phase Morphology

The microstructure of cobalt chromium alloy sputtering targets critically determines sputtering performance, film uniformity, and defect generation. Key microstructural features include grain size and morphology, phase distribution, ceramic particle size and spacing, and residual stress.

Grain Structure And Texture

CoCr-based targets exhibit either equiaxed or elongated grain structures depending on processing history. VAM-cast targets typically have equiaxed grains with diameters of 50–200 μm, while hot-rolled targets develop elongated grains with aspect ratios of 2:1 to 5:1 3. Grain size influences sputtering erosion uniformity: finer grains (<100 μm) yield more uniform erosion profiles and reduced surface roughness evolution during sputtering. Crystallographic texture, quantified by X-ray diffraction (XRD) pole figures, affects sputtering yield anisotropy. Targets with random texture (texture coefficient <1.5) exhibit isotropic sputtering, while targets with strong <111> or <100> fiber textures show directional sputtering yield variations of 10–30% 3.

For Co-Cr-Pt-B alloy targets, the microstructure comprises an island-shaped rolled structure formed from a Co-rich primary phase surrounded by a Cr-rich or B-rich secondary phase 789. The average size of these island structures should be ≤200 μm to ensure uniform sputtering and minimize particle generation 78. This morphology is achieved by controlled solidification (cooling rate 10–100 K/s) followed by hot rolling at 900–1100°C with reduction ratios of 60–80% and final annealing at 600–900°C for 1–4 hours 7. The island structure reduces segregation-induced residual stress and provides a uniform distribution of magnetic and non-magnetic phases in the deposited film.

Ceramic Phase Distribution And Size Control

In CoCrPt-oxide composite targets, the size, morphology, and spatial distribution of ceramic phases (Cr₂O₃, SiO₂, TiO₂, Co(Cr)-X-O) govern arcing behavior and particle generation during sputtering. Optimal targets exhibit ceramic phase lengths <3 μm and number densities of 10⁵–10⁶ particles/mm² 110. Ceramic phases with lengths >5 μm act as preferential sites for arc initiation due to localized electric field enhancement and differential sputtering rates relative to the metal matrix 10. Targets with >500 Cr₂O₃ clusters/mm² having maximum lengths >5 μm exhibit unacceptable arcing frequencies (>10 arcs/hour) and particle generation rates (>0.1 particles/cm²/hour) 10.

Ceramic phase refinement is achieved by controlling prealloy powder composition and sintering parameters. Specifically, using CoCr prealloy powder with Cr content of 35–38 at% (rather than elemental Cr powder) reduces the driving force for Cr₂O₃ coarsening during sintering 110. Sintering at lower temperatures (900–1000°C) and shorter times (2–4 hours) limits Cr₂O₃ grain growth via Ostwald ripening, while maintaining sufficient densification through liquid-phase sintering assisted by low-melting eutectics (Co-Cr eutectic at 1350°C, Co-B eutectic at 1100°C) 10. Post-sintering rapid cooling (>50 K/min) suppresses Cr₂O₃ precipitation during cooling, preserving the fine ceramic dispersion achieved at sintering temperature.

High-Chromium-Containing Particles And Compositional Homogeneity

High-chromium-containing particles (Cr content >70 at%, size 10–100 μm) represent a critical defect class in CoCrPt-based targets 11. These particles form via microsegregation during solidification or via Cr₂O₃ reduction and Cr agglomeration during sintering. Targets with high-Cr particles exceeding 40 μm in maximum diameter exhibit localized sputtering rate variations (±15–30% relative to matrix) and serve as nodule initiation sites 11. Nodules—hemispherical protrusions of re-deposited material—grow during sputtering and eventually detach, generating macroscopic particles (>1 μm) that contaminate the deposited film and cause device yield loss.

Reduction of high-Cr particle size and population density is achieved through rapid solidification processing (melt spinning, gas atomization) followed by powder consolidation 11. Rapid solidification suppresses microsegregation by reducing diffusion distances (proportional to √(D·t), where D is diffusivity and t is solidification time), yielding compositional homogeneity at the 1–10 μm scale 11. Alternatively, mechanical alloying of elemental powders produces composite powders with nanoscale Cr dispersion, which upon consolidation yield targets with Cr-rich particles <10 μm 11. Post-consolidation heat treatment at 800–1000°C for 1–4 hours homogenizes residual compositional gradients via solid-state diffusion without inducing excessive grain growth.

Magnetic Properties And Pass-Through Flux (PTF) Optimization

For ferromagnetic CoCr-based targets used in DC magnetron sputtering, magnetic permeability (μr) and pass-through flux (PTF) are critical performance parameters. PTF, defined as the ratio of magnetic flux passing through the target to the flux generated by the magnetron, quantifies the efficiency of magnetic field coupling to the plasma 514. Conventional CoCr and CoCrPt targets exhibit low PTF (10–20%) due to high magnetic permeability (μr > 100) and saturation magnetization (Ms ≈ 1.0–1.4 T), which shunt magnetic flux within the target rather than allowing it to penetrate to the plasma region 5[14