Nickel Chromium Molybdenum Alloy Sputtering Target: Composition Design, Manufacturing Processes, And Advanced Applications In Semiconductor And Thin-Film Technologies

Alloy Composition Design And Phase Engineering For Nickel Chromium Molybdenum Sputtering Targets

Nickel chromium molybdenum alloy sputtering targets are typically formulated with nickel as the primary matrix element (50–80 at.%), chromium (5–25 at.%), and molybdenum (2–26 at.%), with the balance comprising unavoidable impurities maintained below 100 wtppm to ensure high-purity film deposition 1. The addition of chromium enhances oxidation resistance and mechanical strength, while molybdenum contributes to improved thermal stability and reduced magnetic susceptibility, which is critical for applications requiring non-magnetic or low-moment materials 7. In certain formulations designed for touch-panel conductive substrates, the alloy may contain molybdenum in the range of 2–26 at.% and nickel between 20–80 at.%, with copper constituting the remainder to enable simultaneous etching with copper layers during device fabrication 12.

The microstructural design of these targets prioritizes the suppression of high-purity nickel phases, as excessive segregation of Ni-rich regions (>99.0 mass% Ni) can lead to localized magnetic inhomogeneity and non-uniform sputtering behavior. Advanced nickel alloy targets achieve an area ratio of Ni phase (≥99.0 mass% Ni) below 13%, with average crystal grain diameters controlled to ≤100 μm through optimized thermomechanical processing 1. For applications in semiconductor gate electrodes and barrier layers, alloying elements such as tantalum (0.5–10 at.%) or platinum (22–46 wt.%) are incorporated to stabilize silicide formation (NiSi) and suppress phase transitions to NiSi₂, which would otherwise increase contact resistance and interface roughness 6,8,9,11.

The selection of alloying elements also addresses specific functional requirements:

• Chromium (Cr): Provides solid-solution strengthening and forms protective oxide layers, reducing target oxidation during storage and sputtering. Chromium content above 18 at.% results in low magnetic moment alloys suitable for perpendicular magnetic recording media 16.
• Molybdenum (Mo): Increases recrystallization temperature and thermal stability, enabling targets to withstand high-power sputtering conditions without grain growth or texture evolution. Molybdenum also reduces the Curie temperature of nickel-based alloys, minimizing ferromagnetic behavior 1,7.
• Tantalum (Ta) or Niobium (Nb): Refines grain structure and enhances plastic workability, facilitating target fabrication via rolling or forging. Tantalum additions (1–5 at.%) improve initial magnetic permeability (≥50) and maximum permeability (≥100) on the initial magnetization curve, which correlates with reduced arcing and improved sputtering uniformity 9.

Phase engineering strategies focus on achieving a single face-centered cubic (FCC) structure or a controlled dispersion of secondary phases within the nickel matrix. For example, molybdenum alloy targets designed for electrode and wiring films incorporate a nickel alloy phase dispersed in a molybdenum matrix, with compositions of 10–49 at.% Ni and 1–30 at.% Ti (or Nb), ensuring non-magnetic behavior and high electrical conductivity 4,13. The total alloying element content is typically limited to ≤50 at.% to maintain matrix integrity and avoid brittle intermetallic formation.

Manufacturing Processes And Microstructural Control For Nickel Chromium Molybdenum Alloy Targets

The production of nickel chromium molybdenum alloy sputtering targets employs either powder metallurgy (PM) or vacuum melting routes, each offering distinct advantages in compositional control, microstructural homogeneity, and cost-effectiveness. The choice of manufacturing method depends on target size, alloy complexity, and required purity levels.

Powder Metallurgy Route: Mixing, Consolidation, And Sintering

Powder metallurgy is the predominant method for fabricating multi-component alloy targets, particularly those incorporating refractory elements like molybdenum or tantalum. The process begins with the preparation of high-purity elemental powders (Ni, Cr, Mo) or pre-alloyed powders, which are blended in controlled atmospheres (argon or vacuum) to prevent oxidation. Typical powder particle sizes range from 10 to 150 μm, with finer powders promoting better sintering densification but requiring careful handling to avoid agglomeration 2,4.

The mixed powders undergo cold isostatic pressing (CIP) at pressures of 100–300 MPa to form a green compact with sufficient mechanical strength for handling. This green body is then subjected to vacuum sintering or hot isostatic pressing (HIP) at temperatures between 1100°C and 1400°C, depending on alloy composition. For molybdenum-rich alloys (Mo-Nb or Mo-Ta systems), sintering temperatures may reach 1600°C to achieve >98% theoretical density 2,15. The sintering atmosphere is maintained under high vacuum (<10⁻⁴ Pa) or inert gas to minimize oxygen and nitrogen pickup, which are critical impurities that degrade sputtering performance 9.

Post-sintering processing includes:

• Hot rolling or forging: Reduces porosity and refines grain structure, with reductions of 30–70% applied at temperatures 50–200°C below the recrystallization temperature to avoid excessive grain growth 1.
• Recrystallization annealing: Performed at 500–950°C to achieve an average grain size of 50–80 μm and to enhance magnetic permeability in nickel-rich targets. Lower annealing temperatures (500–700°C) are preferred for targets requiring fine grains and high initial permeability 6,9.
• Machining and bonding: The sintered billet is machined to final dimensions (typically 3–12 mm thickness for planar targets) and diffusion-bonded to a copper or aluminum backing plate using vacuum brazing or explosive bonding techniques to ensure efficient heat dissipation during sputtering.

Vacuum Melting And Rapid Solidification Processing

For alloys requiring superior chemical homogeneity and minimal segregation, vacuum induction melting (VIM) or vacuum arc remelting (VAR) is employed. The raw materials (pure elements or master alloys) are heated under vacuum or partial argon pressure to a fully liquid state (1500–1700°C), followed by casting into ingots or rapid solidification via melt spinning or gas atomization 16. Rapid solidification processing (RSP) produces fine-grained microstructures (grain size <10 μm) and suppresses the formation of coarse intermetallic phases, which is particularly beneficial for cobalt-chromium-platinum-boron alloys used in magnetic recording applications 16.

The cast or atomized material is then consolidated via hot pressing or spark plasma sintering (SPS) at temperatures 100–300°C below the melting point, achieving near-full density (>99.5%) with minimal grain growth. This route is advantageous for producing large-diameter targets (>300 mm) with uniform composition across the entire sputtering surface, reducing the risk of nodule formation and particle generation during deposition.

Impurity Control And Quality Assurance

Achieving impurity levels below 100 wtppm (excluding gas components) is essential for high-purity film deposition 1,6,9. Oxygen content is typically maintained below 50 wtppm, with nitrogen, hydrogen, and carbon each below 10 wtppm, through the use of high-purity starting materials and controlled atmosphere processing. Advanced targets for semiconductor applications may require oxygen levels below 10 wtppm and total metallic impurities below 10 wtppm, necessitating electrolytic refining of nickel feedstock and electron beam melting of refractory elements 6.

Quality assurance protocols include:

• Glow discharge mass spectrometry (GDMS): Quantifies trace impurities (Fe, Co, Cu, Zn, etc.) at sub-ppm levels.
• X-ray diffraction (XRD): Confirms phase composition and detects secondary phases or amorphous content (which should be <3 vol.% for optimal sputtering performance) 7.
• Electron backscatter diffraction (EBSD): Maps grain size distribution and texture, ensuring normalized grain size uniformity below 20% 10.
• Magnetic permeability testing: Measures initial and maximum permeability to verify low magnetic susceptibility, critical for DC magnetron sputtering applications 9.

Physical And Chemical Properties Of Nickel Chromium Molybdenum Alloy Sputtering Targets

The performance of nickel chromium molybdenum alloy sputtering targets in thin-film deposition processes is governed by a combination of physical, chemical, and magnetic properties that must be optimized for specific applications.

Density And Mechanical Properties

High-density targets (≥98% theoretical density) are essential to minimize void-induced arcing and ensure uniform material removal during sputtering. Typical densities for nickel-based alloy targets range from 8.2 to 8.9 g/cm³, depending on molybdenum content (higher Mo content reduces density due to its lower atomic mass relative to nickel) 4,12. Mechanical properties include:

• Tensile strength: 400–800 MPa, with higher values achieved in fine-grained targets (<50 μm grain size) due to Hall-Petch strengthening.
• Elongation: 10–30%, indicating sufficient ductility for target fabrication and resistance to cracking during thermal cycling.
• Hardness: 150–250 HV, balancing wear resistance during sputtering with machinability during target preparation.

Electrical Resistivity And Thermal Conductivity

Electrical resistivity is a critical parameter for DC sputtering, with typical values ranging from 30 to 80 μΩ·cm for nickel chromium molybdenum alloys. Lower resistivity (30–50 μΩ·cm) is preferred for high-power applications to reduce Joule heating and improve target cooling efficiency 4,13. Thermal conductivity ranges from 15 to 40 W/m·K, with higher values (>30 W/m·K) achieved in copper-containing alloys designed for touch-panel applications 12. Effective thermal management is essential to prevent target overheating, which can lead to grain growth, phase transformation, or target cracking.

Magnetic Properties And Sputtering Behavior

Magnetic susceptibility directly impacts sputtering uniformity in magnetron systems. Non-magnetic or low-moment targets (initial magnetic permeability <50) are required for applications such as perpendicular magnetic recording media, where ferromagnetic contamination would degrade signal-to-noise ratios 10,16. Nickel alloy targets with chromium content >18 at.% or molybdenum additions exhibit reduced Curie temperatures (<100°C) and low saturation magnetization (<0.1 T), enabling stable plasma confinement and uniform erosion profiles 1,7.

For targets with higher nickel content (>70 at.%), heat treatment at recrystallization temperatures (500–950°C) is employed to increase initial magnetic permeability to ≥50 and maximum permeability to ≥100, which correlates with reduced arcing frequency and improved target utilization 9. The relationship between grain size, magnetic permeability, and sputtering performance is well-established: finer grains (<80 μm) with equiaxed morphology provide more uniform magnetic domain structures, reducing localized heating and nodule formation.

Chemical Stability And Oxidation Resistance

Chromium additions (5–25 at.%) form a passive Cr₂O₃ layer on the target surface, protecting against atmospheric oxidation during storage and pre-sputtering conditioning. This oxide layer is readily sputtered away during the initial cleaning phase, exposing the underlying alloy for film deposition. Molybdenum contributes to high-temperature oxidation resistance, with alloys containing >10 at.% Mo exhibiting weight gain rates <0.5 mg/cm² after 100 hours at 600°C in air 7. For applications requiring long-term stability in humid environments, nickel chromium molybdenum targets demonstrate superior moisture resistance compared to pure nickel or nickel-copper alloys, with corrosion rates <1 μm/year in 85% relative humidity at 85°C 4,13.

Applications Of Nickel Chromium Molybdenum Alloy Sputtering Targets In Semiconductor And Electronic Devices

Barrier Layers And Diffusion Control In Solder Bump Interconnects

Nickel chromium molybdenum alloy sputtering targets are extensively used to deposit barrier layers in flip-chip and wafer-level packaging technologies, where they prevent tin (Sn) diffusion from lead-free solder bumps into underlying copper or aluminum metallization. The barrier layer must exhibit excellent wettability with Sn-based solders while inhibiting intermetallic compound (IMC) formation that would increase contact resistance or cause mechanical failure 3,5. Nickel alloy targets containing 1–30 at.% Cu and 2–25 at.% of V, Cr, Al, Si, Ti, or Mo deposit films with contact angles <30° on Sn-Ag-Cu solder and suppress Sn diffusion to depths <50 nm after reflow at 260°C for 60 seconds 3,5.

The mechanism of diffusion inhibition involves the formation of a stable Ni₃Sn₄ intermetallic layer at the solder interface, which acts as a kinetic barrier to further Sn penetration. Chromium and molybdenum additions refine the Ni₃Sn₄ grain structure and increase its activation energy for grain boundary diffusion, extending the effective barrier lifetime from <500 thermal cycles (pure Ni) to >2000 cycles (Ni-Cr-Mo alloy) in accelerated aging tests (125°C, 100% relative humidity) 3. This performance is critical for automotive and aerospace electronics, where solder joints must withstand extreme thermal cycling (-40°C to 150°C) and mechanical vibration.

Gate Electrode Materials And Silicide Formation In Advanced CMOS Devices

In sub-22 nm CMOS technology nodes, nickel silicide (NiSi) contacts are preferred over cobalt silicide (CoSi₂) due to their lower formation temperature (400–500°C vs. 600–700°C) and reduced silicon consumption. However, NiSi films are prone to agglomeration and phase transformation to high-resistivity NiSi₂ at temperatures above 600°C, limiting their thermal budget compatibility 6,8,11. Nickel alloy sputtering targets containing 0.5–10 at.% tantalum or 22–46 wt.% platinum stabilize the NiSi phase by increasing the activation energy for NiSi → NiSi₂ transformation from 2.1 eV (pure Ni) to >3.0 eV (Ni-Ta or Ni-Pt alloys) 6,8,9,11.

The deposition of Ni-Ta or Ni-Pt films (10–30 nm thickness) on silicon substrates, followed by rapid thermal annealing (RTA) at 400–500°C for 30–60 seconds, produces NiSi contacts with sheet resistances of 8–12 Ω/sq and thermal stability up to 700°C for 30 minutes without phase transformation or agglomeration 6,11. Tantalum additions (1–5 at.%) also reduce particle generation during sputtering from >0.5 particles/cm² (pure Ni target) to <0.05 particles/cm² (Ni-Ta target), improving yield in 300 mm wafer fabrication 9. Platinum-containing targets (22–46 wt.% Pt) further enhance wettability on silicon and suppress interface rough