Wrought Copper Nickel Grade Sputtering Target: Composition, Manufacturing, And Performance Optimization For Semiconductor Applications

Chemical Composition And Alloying Strategy Of Wrought Copper Nickel Sputtering Targets

Wrought copper nickel grade sputtering targets are formulated to balance electrical conductivity, mechanical integrity, and film deposition uniformity. The fundamental composition consists of copper (Cu) as the matrix element and nickel (Ni) as the primary alloying addition, with nickel concentrations typically spanning 15–80 mass% depending on the target application 3. For interconnect seed layer applications, alloy compositions containing 20.0–40.0 mass% nickel have been demonstrated to provide optimal oxidation and corrosion resistance while maintaining acceptable resistivity for subsequent electroplating processes 1. In certain formulations, minor additions of elements such as chromium (Cr), titanium (Ti), vanadium (V), aluminum (Al), tantalum (Ta), cobalt (Co), zirconium (Zr), niobium (Nb), or molybdenum (Mo) are incorporated at total levels of 1.0–10.0 mass% to further refine grain structure, suppress abnormal grain growth, and enhance thermal stability during sputtering 1.

The selection of nickel content is governed by the trade-off between electrical resistivity and chemical stability. Pure copper exhibits resistivity near 1.7 μΩ·cm at room temperature, whereas copper-nickel alloys with 20–40 mass% Ni typically exhibit resistivities in the range of 10–30 μΩ·cm 1. This increase in resistivity is acceptable for seed layer applications where the primary function is to provide a continuous, adherent base for electroplated copper, rather than to serve as the final low-resistance interconnect. The enhanced oxidation resistance of copper-nickel alloys is attributed to the formation of a protective nickel-rich oxide layer on the surface, which inhibits further oxidation and prevents the formation of volatile copper oxides during storage and handling 1.

Impurity control is critical for high-performance sputtering targets. Unavoidable impurities—particularly sulfur (S), lead (Pb), oxygen (O), and carbon (C)—must be minimized to prevent particle generation, arcing, and non-uniform film deposition. For high-purity copper-nickel targets, sulfur content is typically restricted to ≤10 ppm and lead to ≤2 ppm to suppress the formation of low-melting-point phases and reduce the risk of nodule formation during sputtering 18. Oxygen content is controlled through vacuum melting and inert-atmosphere processing to avoid the formation of oxide inclusions that can act as particle sources 13.

Wrought Processing Route And Microstructural Evolution

The term "wrought" refers to the thermomechanical processing sequence employed to convert cast ingots into dense, fine-grained sputtering targets with controlled crystallographic texture and minimal defects. The wrought processing route for copper-nickel sputtering targets typically comprises the following stages:

Vacuum Melting And Casting

High-purity copper and nickel raw materials are melted under vacuum or inert atmosphere (argon or nitrogen) to minimize oxidation and gas pickup 3. The molten alloy is cast into ingots using controlled cooling rates to avoid macrosegregation and coarse dendritic structures. For copper-nickel alloys with nickel contents of 15–80 mass%, the solidification behavior is governed by the Cu-Ni phase diagram, which exhibits complete solid solubility across the composition range. However, localized microsegregation of nickel can occur during solidification, necessitating subsequent homogenization treatments 3.

Hot Forging And Homogenization

The as-cast ingot is subjected to hot forging at temperatures typically ranging from 700°C to 950°C to break up the cast structure, close internal porosity, and promote recrystallization 16. Hot forging is performed in multiple passes with intermediate reheating to achieve cumulative reductions of 50–80% in cross-sectional area. This process refines the grain structure and homogenizes the nickel distribution, reducing the amplitude of compositional gradients inherited from casting 3. Following hot forging, the material is subjected to a homogenization heat treatment at 400–700°C for 1–3 hours to further equilibrate the microstructure and relieve residual stresses 16.

Cold Rolling And Recrystallization

After homogenization, the forged billet undergoes cold rolling at room temperature with reductions of 40–75% to further refine the grain size and introduce controlled dislocation density 16. Cold rolling imparts significant stored energy in the form of dislocations and subgrain boundaries, which serve as driving forces for subsequent recrystallization. The cold-rolled material is then subjected to a recrystallization heat treatment at 450–700°C for 1–3 hours to nucleate and grow new, strain-free grains 16. The recrystallization temperature and time are carefully controlled to achieve an average grain size of 10–100 μm, which is optimal for uniform sputtering and minimal particle generation 3. Grain sizes below 10 μm can lead to excessive grain boundary area and increased risk of abnormal grain growth during sputtering, while grain sizes above 100 μm can result in non-uniform erosion profiles and reduced target utilization 17.

Final Cold Forming And Surface Finishing

Following recrystallization, the target material undergoes a final cold forming step at a cold working rate of ≤50% to achieve the desired target dimensions and surface finish 16. This step also introduces a controlled level of residual stress and dislocation density, which can enhance mechanical strength and reduce warping during bonding to the backing plate. The sputtering surface is then machined and polished to a surface roughness (Ra) of <0.5 μm to ensure uniform plasma coupling and minimize arcing 13.

Microstructural Characterization And Quality Metrics

The microstructural quality of wrought copper-nickel sputtering targets is assessed using a combination of optical microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) techniques. Key quality metrics include:

• Average Grain Size: Targets with average grain sizes of 10–100 μm exhibit optimal sputtering performance, with finer grains (10–50 μm) preferred for high-power-density applications to minimize localized heating and thermal stress 17.
• Grain Size Uniformity: The standard deviation (1-sigma) of grain size should be <15% throughout the target to ensure uniform erosion and consistent film thickness 8. Non-uniform grain size distributions can lead to differential sputtering rates and the formation of surface roughness features that degrade film quality.
• Kernel Average Misorientation (KAM): KAM values ≤2° indicate low residual strain and a well-recrystallized microstructure, which correlates with reduced particle generation and stable plasma conditions during sputtering 16.
• Crystallographic Texture: For copper-nickel targets, a random or weakly textured microstructure is generally preferred to avoid anisotropic sputtering behavior. However, certain applications may benefit from controlled textures (e.g., <111> or <200> fiber textures) to optimize film stress and adhesion 11.
• Phase Purity: EBSD and XRD analyses should confirm the absence of secondary phases (e.g., intermetallic compounds or oxide inclusions) that can act as particle sources or cause arcing 3. For copper-nickel alloys, the area ratio of high-purity Ni phases (Ni content ≥99.5 mass%) should be ≤5% to prevent localized compositional inhomogeneities 10.

Mechanical And Physical Properties Of Wrought Copper Nickel Targets

Wrought copper-nickel sputtering targets exhibit a combination of mechanical strength, ductility, and thermal stability that is critical for reliable sputtering performance. Key properties include:

• Yield Strength: Wrought copper-nickel targets typically exhibit yield strengths of 150–300 MPa, depending on nickel content and cold working history 8. Higher yield strengths reduce the risk of target deformation during handling and bonding, but excessive strength can compromise ductility and increase the risk of cracking during thermal cycling.
• Vickers Hardness: The sputtering surface hardness of wrought copper-nickel targets ranges from 90 to 120 Hv, with higher hardness values correlating with finer grain sizes and higher dislocation densities 16. Hardness values in this range provide a balance between wear resistance and ductility, minimizing the risk of brittle fracture during sputtering.
• Electrical Resistivity: As noted earlier, copper-nickel alloys with 20–40 mass% Ni exhibit resistivities of 10–30 μΩ·cm, which is acceptable for seed layer applications 1. Resistivity increases with nickel content due to increased electron scattering from solute atoms and grain boundaries.
• Thermal Conductivity: Copper-nickel alloys exhibit thermal conductivities of 20–50 W/(m·K), significantly lower than pure copper (≈400 W/(m·K)) but sufficient for heat dissipation during sputtering 1. Lower thermal conductivity can lead to localized heating and thermal stress, necessitating careful control of sputtering power density and cooling conditions.
• Coefficient Of Thermal Expansion (CTE): Copper-nickel alloys exhibit CTEs of 14–17 × 10⁻⁶ K⁻¹, which must be matched to the backing plate material (typically copper or molybdenum) to minimize thermal stress and prevent delamination during sputtering 16.

Sputtering Performance And Film Deposition Characteristics

The sputtering performance of wrought copper-nickel targets is evaluated based on film deposition rate, film uniformity, particle generation, and plasma stability. Key performance metrics include:

• Deposition Rate: Copper-nickel targets exhibit deposition rates of 50–200 nm/min at typical sputtering powers (2–5 W/cm²) and argon pressures (2–10 mTorr), depending on target composition and magnetron configuration 4. Higher nickel contents generally result in lower deposition rates due to the higher sputtering threshold energy of nickel compared to copper.
• Film Uniformity: Wrought targets with uniform grain structures and controlled textures produce films with thickness non-uniformities of <3% (1-sigma) across 300 mm wafers 7. Non-uniform grain size distributions or surface roughness can lead to localized variations in sputtering yield and film thickness.
• Particle Generation: High-quality wrought copper-nickel targets exhibit particle counts of <0.1 particles/cm² (≥0.2 μm) during steady-state sputtering, as measured by in-situ laser scattering or post-deposition wafer inspection 13. Particle generation is minimized by controlling impurity levels (S, Pb, O), eliminating oxide inclusions, and ensuring uniform grain structures.
• Plasma Stability: Stable plasma conditions are characterized by low voltage fluctuations (<5% RMS) and the absence of arcing events during sputtering 18. Plasma stability is enhanced by fine grain structures, low surface roughness, and the absence of secondary phases or inclusions that can cause localized field enhancements.

Applications Of Wrought Copper Nickel Sputtering Targets In Semiconductor Manufacturing

Seed Layer Deposition For Copper Electroplating

One of the primary applications of wrought copper-nickel sputtering targets is the deposition of seed layers for copper electroplating in advanced semiconductor interconnects 4. The seed layer serves as a continuous, conductive base that enables uniform electroplating of copper into high-aspect-ratio trenches and vias (aspect ratios >10:1). Copper-nickel seed layers offer several advantages over pure copper:

• Enhanced Adhesion: The nickel component promotes adhesion to barrier layers (e.g., TaN, TiN) through the formation of interfacial nickel-metal bonds 7.
• Improved Step Coverage: Copper-nickel alloys exhibit higher adatom mobility during sputtering, resulting in better step coverage on sidewalls and bottom surfaces of high-aspect-ratio features 4.
• Oxidation Resistance: The nickel-rich surface oxide layer prevents oxidation of the seed layer during air exposure between sputtering and electroplating steps, ensuring consistent electroplating nucleation 1.

Typical seed layer thicknesses range from 20 to 100 nm, with resistivities of 10–30 μΩ·cm acceptable for subsequent electroplating 7. The seed layer is deposited at substrate temperatures of 100–300°C to promote adhesion and minimize residual stress.

Barrier Layer And Capping Layer Applications

Copper-nickel alloys are also employed as barrier layers to prevent copper diffusion into adjacent dielectric materials and as capping layers to protect copper interconnects from oxidation and electromigration 1. For barrier layer applications, nickel-rich compositions (40–80 mass% Ni) are preferred to maximize diffusion resistance. Barrier layer thicknesses of 5–20 nm are typical, with deposition performed at low substrate temperatures (<200°C) to minimize interdiffusion with the underlying dielectric 5.

Capping layers are deposited on top of copper interconnects to improve electromigration resistance and reduce oxidation during subsequent processing steps. Copper-nickel capping layers with 20–40 mass% Ni exhibit electromigration activation energies of 1.0–1.2 eV, significantly higher than pure copper (0.7–0.9 eV), resulting in improved reliability at elevated operating temperatures 1.

Thin-Film Resistor And Sensor Applications

Copper-nickel alloys are widely used in thin-film resistor applications due to their stable resistivity and low temperature coefficient of resistance (TCR). Alloys with 40–50 mass% Ni (e.g., Constantan) exhibit TCR values of <50 ppm/K and resistivities of 40–50 μΩ·cm, making them suitable for precision resistor networks in analog and mixed-signal integrated circuits 1. Wrought copper-nickel sputtering targets enable the deposition of uniform resistor films with thickness tolerances of <2% and resistivity variations of <5% across 300 mm wafers.

Copper-nickel thin films are also employed in thermocouple and temperature sensor applications, where the Seebeck coefficient of the Cu-Ni alloy is exploited to generate a voltage proportional to temperature difference. Alloys with 10–20 mass% Ni exhibit Seebeck coefficients of 20–40 μV/K, suitable for on-chip temperature monitoring in power management and thermal management applications 1.

Process Optimization And Troubleshooting For Wrought Copper Nickel Target Sputtering

Power Density And Thermal Management

Sputtering power density is a critical parameter that influences deposition rate, film stress, and target lifetime. For wrought copper-nickel targets, optimal power densities range from 2 to 5 W/cm², depending on target composition and cooling efficiency 4. Higher power densities (>5 W/cm²) can lead to excessive target heating, thermal stress, and the formation of surface roughness features that degrade film quality. Effective thermal management is achieved through the use of high-conductivity backing plates (e.g., oxygen-free copper) and efficient water cooling systems that maintain target temperatures below 150°C during sputtering 6.

Argon Pressure And Gas Purity

Argon pressure and purity are key factors that affect plasma density, ion energy, and film microstructure. For copper-nickel sputtering, argon pressures of 2–10 mTorr are typical, with lower pressures (2–5 mTorr) favoring higher ion energies and denser films, and higher pressures (5–10 mTorr) promoting lower ion energies and more conformal step coverage 4. Argon purity should be ≥99.999% (5N) to minimize contamination from oxygen