Maraging Steel Sputtering Target: Advanced Manufacturing, Microstructural Engineering, And High-Performance Thin Film Deposition Applications

Metallurgical Foundations And Compositional Design Of Maraging Steel Sputtering Targets

Maraging steels derive their name from "martensitic aging" and typically contain 15–25 wt% Ni, 3–5 wt% Mo, 0.2–1.8 wt% Ti, and minor additions of Co (up to 12 wt%) and Al (0.05–0.15 wt%), with carbon content intentionally suppressed below 0.03 wt% to avoid carbide embrittlement. Upon solution treatment at 815–850°C followed by aging at 480–510°C for 3–6 hours, Ni₃(Ti,Mo) and Fe₂Mo intermetallic precipitates nucleate within the low-carbon martensite matrix, elevating tensile strength to 1.8–2.4 GPa while retaining fracture toughness above 50 MPa·m^(1/2). For sputtering target applications, this combination is critical: high yield strength (>1.7 GPa) resists plastic deformation under magnetron bombardment and thermal cycling, while adequate toughness prevents catastrophic fracture during bonding to backing plates or under ion-induced stress gradients 1113.

Compositional tuning for sputtering targets must balance sputter yield, film stoichiometry, and target longevity. Nickel-rich maraging compositions (18–20 wt% Ni) exhibit lower magnetic permeability (relative permeability μᵣ ≈ 1.2–2.5 at room temperature post-aging), facilitating efficient magnetron confinement and uniform erosion profiles 9. Molybdenum additions (3.5–5 wt%) enhance solid-solution strengthening and refine precipitate dispersion, but excessive Mo can elevate sputter threshold energy and reduce deposition rate by 8–12% compared to leaner alloys. Titanium (0.4–1.2 wt%) is essential for Ni₃Ti precipitation kinetics; however, Ti segregation during casting or powder consolidation can create μm-scale Ti-rich clusters that act as particle generation sites during sputtering 119. Cobalt substitution (8–12 wt%) raises the martensite start temperature (Mₛ) and accelerates aging kinetics, enabling shorter heat treatment cycles (down to 2–4 hours at 490°C) without compromising ultimate tensile strength, a cost advantage in high-volume target production 16.

Hydrogen content must be rigorously controlled below 2 ppm (preferably <1 ppm) to prevent hydrogen-induced cracking during thermal cycling between room temperature and sputtering operating temperatures (200–400°C) 3. Vacuum induction melting (VIM) under <10⁻³ Pa base pressure, followed by vacuum arc remelting (VAR) or electroslag remelting (ESR), is standard practice to achieve oxygen levels <50 ppm and nitrogen <30 ppm, thereby minimizing oxide and nitride inclusions that can trigger arcing or particle ejection 1019.

Manufacturing Processes And Microstructural Control For Maraging Steel Targets

Ingot Metallurgy Route: Casting, Forging, And Thermomechanical Processing

The conventional ingot metallurgy route begins with VIM casting of 200–500 kg heats into cylindrical or rectangular ingots (typical dimensions: Ø300–500 mm × 800–1200 mm length). Controlled solidification rates of 5–15 mm/min are employed to suppress dendritic arm spacing below 150 μm and limit macrosegregation of Mo and Ti to <±3% across the ingot cross-section 1516. Post-casting homogenization at 1150–1200°C for 12–24 hours in vacuum or inert atmosphere (Ar, <10 ppm O₂) dissolves residual eutectic phases and equilibrates alloying element distribution, reducing microsegregation to <±1.5%.

Hot forging at 1050–1150°C with cumulative true strain ε ≥ 1.2 (reduction ratio >70%) refines the as-cast grain structure from ASTM 2–4 to ASTM 6–8 (average grain diameter 50–90 μm), enhancing subsequent machinability and reducing anisotropy in mechanical properties 11. Multi-pass forging with intermediate reheating every 20–30% reduction prevents edge cracking and ensures uniform strain distribution; forging pressures of 150–300 MPa are typical for Ø400 mm billets. The forged billet is then solution-treated at 820–840°C for 1 hour per 25 mm thickness, air-cooled to room temperature (cooling rate 15–25°C/min to ensure full martensitic transformation), and aged at 480–500°C for 3–5 hours to achieve target hardness of 50–54 HRC (equivalent to 1.9–2.1 GPa tensile strength).

Cold rolling or precision machining reduces the solution-treated and aged billet to final target thickness (typically 6–12 mm for planar targets, 8–15 mm for rotary targets). Intermediate stress-relief annealing at 650–680°C for 1–2 hours after every 15–20% cold reduction prevents residual stress accumulation (target residual stress <100 MPa) and maintains dimensional tolerance within ±0.05 mm over Ø300 mm diameter 9. Final machining employs carbide or CBN tooling at cutting speeds of 40–80 m/min with flood coolant to achieve surface roughness Ra <0.4 μm on the sputtering face, minimizing arcing initiation sites.

Powder Metallurgy Route: Atomization, Consolidation, And Densification

For complex target geometries or alloy compositions prone to segregation (e.g., high-Ti maraging grades), powder metallurgy (PM) offers superior microstructural homogeneity. Gas atomization (typically Ar at 3–5 MPa) of VIM-melted maraging steel produces spherical powders with D₅₀ = 20–60 μm and oxygen pickup limited to 200–400 ppm 715. Sieving to −45 μm + 15 μm fraction ensures optimal packing density (tap density 4.2–4.6 g/cm³ for 18Ni maraging powder) prior to consolidation.

Hot isostatic pressing (HIP) at 1150–1200°C and 100–150 MPa for 2–4 hours achieves >99.5% theoretical density (ρ = 8.05–8.15 g/cm³ for 18Ni-300 grade) with equiaxed grain structure (ASTM 7–9, 30–60 μm) and minimal residual porosity (<0.3 vol%, pore size <10 μm) 716. Lower HIP pressures (80–100 MPa) can be employed if powder is pre-compacted by cold isostatic pressing (CIP) at 200–300 MPa, reducing HIP cycle cost by 20–30%. Post-HIP solution treatment and aging follow the same thermal schedules as wrought material, yielding comparable mechanical properties (tensile strength 1.85–2.05 GPa, elongation 8–12%).

Spark plasma sintering (SPS) at 950–1050°C under 50–80 MPa uniaxial pressure for 5–10 minutes offers rapid densification with finer grain size (ASTM 9–11, 15–40 μm) and reduced energy consumption (cycle time <30 minutes vs. 4–6 hours for HIP), but requires careful control of heating rate (50–100°C/min) to prevent thermal runaway and non-uniform densification across large-diameter targets (>Ø200 mm). SPS-processed maraging targets exhibit 5–8% higher hardness (52–56 HRC) due to refined precipitate spacing (mean inter-precipitate distance λ = 8–15 nm vs. 15–25 nm in HIP material), potentially extending target life by 10–15% in high-power-density sputtering (>10 W/cm²) 314.

Bonding To Backing Plates And Structural Integrity

Maraging steel targets are typically bonded to oxygen-free copper (OFC) or aluminum alloy backing plates (6061-T6, 2024-T3) via diffusion bonding, brazing, or elastomer bonding to facilitate heat dissipation and mechanical support 2511. Diffusion bonding at 500–600°C under 10–30 MPa for 1–3 hours in vacuum (<10⁻⁴ Pa) creates a metallurgical interface with shear strength >150 MPa, but requires surface preparation (grinding to Ra <0.2 μm, degreasing, and Ar plasma cleaning) to eliminate oxide films that inhibit atomic interdiffusion 18. Interlayer materials such as Ti foil (25–50 μm thickness) or electroplated Ni (5–10 μm) can enhance bond strength by 20–40% through formation of intermetallic phases (TiNi, Ni₃Al) that accommodate thermal expansion mismatch (CTE of maraging steel ≈ 10.5 × 10⁻⁶ K⁻¹ vs. 16.5 × 10⁻⁶ K⁻¹ for Cu, 23 × 10⁻⁶ K⁻¹ for Al) 518.

Brazing with Ag-Cu-Ti or Au-Ni filler metals at 780–850°C for 10–30 minutes yields joint strengths of 180–250 MPa but introduces residual stress (50–120 MPa tensile at the target/braze interface) due to CTE mismatch and solidification shrinkage. Post-braze stress relief at 480°C for 2 hours reduces residual stress by 40–60% without degrading target mechanical properties. Elastomer bonding (e.g., silicone or epoxy adhesives cured at 120–180°C) offers lower thermal conductivity (0.2–0.8 W/m·K vs. >300 W/m·K for diffusion bonds) but accommodates CTE mismatch through viscoelastic deformation, reducing interfacial stress to <30 MPa and enabling target reuse after sputtering 26.

Structural reinforcement strategies include peripheral flange forging 11 and central boss protrusions 13 to counteract target warpage under thermal gradients (ΔT = 100–200°C between sputtering surface and backing plate during high-power operation). Finite element analysis (FEA) indicates that a 5 mm thick, 15 mm wide forged flange increases target stiffness by 35–50%, reducing maximum deflection from 0.8 mm to 0.3 mm under 5 kW magnetron power, thereby maintaining target-to-substrate distance within ±0.5 mm tolerance critical for uniform film thickness 1113.

Sputtering Performance Characteristics And Process Optimization

Sputter Yield, Deposition Rate, And Film Composition

Maraging steel exhibits sputter yields of 0.8–1.4 atoms/ion under Ar⁺ bombardment at 400–600 eV, intermediate between pure Fe (1.2–1.6 atoms/ion) and refractory metals like Mo (0.5–0.9 atoms/ion) 1415. Deposition rates of 15–35 nm/min are achievable at 2–5 kW DC magnetron power (power density 3–8 W/cm²) and 0.3–0.8 Pa Ar pressure, with rate scaling linearly with power up to 8 W/cm² before onset of target overheating (surface temperature >450°C) and accelerated erosion 14. Reactive sputtering in Ar/N₂ or Ar/O₂ atmospheres (N₂ or O₂ partial pressure 0.05–0.2 Pa) enables deposition of maraging steel nitride or oxide films with tunable stoichiometry (Fe₃Ni-N, Fe-Ni-O phases) for magnetic or corrosion-resistant coatings, though deposition rate decreases by 30–50% due to target surface poisoning and reduced sputter yield 1719.

Film composition typically mirrors target composition within ±2 at% for major elements (Fe, Ni, Mo) under optimized conditions (target-to-substrate distance 60–100 mm, substrate bias 0 to −50 V), but Ti and Al can exhibit preferential sputtering (film Ti content 10–20% lower than target) due to their lower surface binding energies (4.85 eV for Ti vs. 5.42 eV for Ni). Pulsed DC or RF sputtering at 50–250 kHz with duty cycle 60–80% mitigates preferential sputtering and reduces arcing frequency by 40–70% compared to continuous DC, particularly beneficial for targets with residual oxide inclusions 11019.

Magnetic Properties And Magnetron Efficiency

Post-aging maraging steels exhibit saturation magnetization Mₛ = 1.2–1.6 T and coercivity Hc = 80–200 A/m, classifying them as soft ferromagnetic materials with relative permeability μᵣ = 50–150 at low field (<1 kA/m) 917. For magnetron sputtering, lower μᵣ is advantageous: targets with μᵣ < 10 (achievable via over-aging at 550–600°C for 10–20 hours to coarsen precipitates and reduce magnetic domain wall pinning) enable stronger magnetic field penetration (surface field 200–400 G vs. 100–200 G for μᵣ = 50–100), resulting in 15–25% higher plasma density and more uniform erosion track width (30–50 mm vs. 20–35 mm for standard aging) 49.

Magnetic flux leakage measurements across the target surface should exhibit variation <12% (standard deviation of normal field component <25 G over Ø300 mm) to ensure uniform sputter erosion and prevent localized hotspots 9. Targets with excessive magnetic inhomogeneity (flux variation >20%) develop asymmetric erosion grooves (depth variation >2 mm over 50 mm track width), reducing material utilization from 30–35% to 20–25% and increasing particle generation risk as the erosion groove approaches the target/backing plate interface 613.

Particle Generation, Arcing, And Target Life

Particle contamination in sputtered films originates from three primary mechanisms: (1) ejection of μm-scale inclusions (oxides, nitrides, undissolved precipitates) from the target surface, (2) flaking of re-deposited material from chamber walls or target periphery, and (3) micro-arcing at surface defects or grain boundaries with high electrical resistivity contrast 11019. For maraging steel targets, oxide inclusions (primarily Al₂O₃, TiO₂, and complex (Fe,Ni)(Mo,Ti)O₄ spinels) with diameter >5 μm are the dominant particle source; maintaining oxide number density <5 particles/mm² (diameter ≥10 μm) on the sputtering surface reduces particle-induced film defects by 60–80% 19.

Arcing frequency correlates strongly with target surface roughness and residual stress: targets with Ra >0.6 μm or compressive residual stress >200 MPa exhibit 3–5× higher arcing rates (>10 events/kWh) compared to optimized targets (Ra <0.4 μm, residual stress