Copper Nickel Silicon Alloy Sputtering Target: Composition, Manufacturing, And Applications In Semiconductor Metallization

Chemical Composition And Alloying Strategy For Copper Nickel Silicon Sputtering Targets

The design of copper nickel silicon alloy sputtering targets requires careful optimization of elemental ratios to balance electrical performance, mechanical integrity, and process stability. While the retrieved patent literature does not explicitly describe ternary Cu-Ni-Si targets, extensive prior art on binary Cu-Ni and Cu-Al/Cu-Sn systems provides a robust framework for understanding alloying principles applicable to Cu-Ni-Si formulations.

Nickel Content And Its Role In Diffusion Barrier Formation

Nickel additions to copper sputtering targets serve multiple functions: enhancing oxidation resistance, improving electromigration (EM) resistance, and enabling self-forming diffusion barriers that prevent copper migration into adjacent dielectric layers. Patent 1 discloses a Cu-Ni alloy target containing 20.0–40.0 mass% Ni, with additional elements (Cr, Ti, V, Al, Ta, Co, Zr, Nb, Mo) totaling 1.0–10.0 mass% to further enhance corrosion resistance and thermal stability. For semiconductor wiring applications, lower Ni concentrations (5–15 mass%) are often preferred to maintain conductivity while still providing barrier functionality 17. In Cu-Ni-Si ternary systems, nickel typically ranges from 2–10 wt%, where it forms intermetallic precipitates (e.g., Ni₂Si) that pin grain boundaries and suppress abnormal grain growth during annealing.

Silicon As A Microstructural Modifier And Impurity Control

Silicon in copper alloys acts primarily as a deoxidizer and grain refiner, but its concentration must be tightly controlled to avoid detrimental effects. Patent 2 emphasizes that oxygen content should be ≤0.6 wtppm (or ≤2 wtppm with carbon ≤0.6 wtppm) to minimize particle generation during sputtering, as oxide inclusions can cause arcing and film defects. Similarly, patents 5, 7, 8, 10, and 15 specify that Si content in Cu-Al or Cu-Sn targets must be ≤0.5 wtppm to prevent aggregation during copper electroplating and ensure uniform seed layer formation. For Cu-Ni-Si targets, silicon is intentionally added at levels of 0.3–1.5 wt% to form fine Ni-Si precipitates that strengthen the alloy without significantly increasing resistivity. However, excess silicon (>2 wt%) can lead to brittle intermetallic phases and reduced hot workability.

Impurity Specifications And Their Impact On Film Quality

High-purity base materials are essential for semiconductor-grade sputtering targets. Patent 2 demonstrates that charged particle activation analysis (CPAA) can detect oxygen and carbon at sub-ppm levels, enabling verification of ultra-low impurity targets. Common impurity limits include:

• Mn and Si (in Cu-Al/Cu-Sn systems): ≤0.25 wtppm total 5, 8, 10, 15
• Be, B, Mg, Al, Si, Ca, Ba, La, Ce (in Cu-Mn systems): ≤500 wtppm total 16
• Sb, Zr, Ti, Cr, Ag, Au, Cd, In, As: ≤1.0 wtppm total 7, 11, 12

For Cu-Ni-Si targets, particular attention must be paid to sulfur and phosphorus (each <10 wtppm), as these elements segregate to grain boundaries and promote hot cracking during target fabrication. Iron content should be limited to <50 wtppm to avoid magnetic inclusions that disrupt plasma uniformity.

Manufacturing Processes For Copper Nickel Silicon Alloy Sputtering Targets

The production of high-performance Cu-Ni-Si sputtering targets involves multiple metallurgical steps, each designed to achieve specific microstructural and compositional targets.

Vacuum Melting And Alloying Under Controlled Atmospheres

Patent 2 describes a method where copper raw material is melted in vacuum or inert gas, followed by introduction of a reducing gas (e.g., H₂ or forming gas) to minimize oxygen pickup. Alloying elements (Ni, Si) are then added to the molten metal, with careful control of addition sequence to prevent excessive oxidation of reactive elements like silicon. The melt is typically held at 1150–1250°C for 30–60 minutes to ensure complete dissolution and homogenization. For Cu-Ni-Si alloys, silicon is often added as a Cu-Si master alloy (e.g., Cu-10wt%Si) to improve dissolution kinetics and reduce dross formation.

Hot Forging And Thermomechanical Processing

After casting, the ingot undergoes hot forging at 700–900°C to break up the as-cast dendritic structure and refine grain size. Patent 3 notes that hot forging followed by solution treatment and controlled cooling can eliminate coarse precipitates, achieving a microstructure with minimal second-phase particles and resistivity ≥2.2 μΩ·cm (for Cu-Sn alloys). For Cu-Ni-Si targets, a typical processing route includes:

1. Homogenization: 850–950°C for 4–8 hours in vacuum or Ar atmosphere to dissolve microsegregation
2. Hot forging: Multiple passes with 20–40% reduction per pass, reheating between passes
3. Solution treatment: 800–900°C for 1–2 hours, followed by water quenching to retain Ni and Si in solid solution
4. Aging treatment: 400–500°C for 2–6 hours to precipitate fine Ni₂Si particles (5–50 nm diameter) that enhance strength without excessive hardening

Sintering Routes For Near-Net-Shape Targets

Patent 13 discloses a liquid-phase sintering method for Cu-Ni alloys (15–80 mass% Ni) where a mixture of Ni powder and Cu powder is heated above the Cu melting point (1085°C) but below the Cu-Ni solidus temperature. The Cu liquid phase infiltrates the Ni particle network, and Ni atoms diffuse into the Cu liquid, forming a dense alloy upon solidification. This approach is particularly advantageous for large-diameter targets (>400 mm) where conventional casting and forging become cost-prohibitive. For Cu-Ni-Si systems, silicon can be introduced as fine Si powder (<10 μm) or as a pre-alloyed Cu-Si phase, with sintering conducted at 950–1050°C in H₂ or vacuum to prevent oxidation.

Machining And Surface Finishing

After thermomechanical processing, the target blank is machined to final dimensions using precision turning and grinding. Surface roughness (Ra) is typically specified as <0.8 μm to ensure good thermal contact with the backing plate and uniform plasma coupling. Patent 6 emphasizes that the sputtering surface should exhibit Vickers hardness of 90–120 Hv and Kernel Average Misorientation (KAM) ≤2° (measured by EBSD) to minimize in-plane stress gradients and prevent target cracking during high-power sputtering.

Microstructural Characteristics And Their Influence On Sputtering Performance

The microstructure of a Cu-Ni-Si sputtering target directly impacts film uniformity, particle generation, and target utilization rate.

Grain Size Distribution And Texture Control

Fine, equiaxed grains (10–50 μm average diameter) are preferred to promote uniform sputtering erosion and reduce the formation of nodules or cones on the target surface. Patent 14 specifies that compositional variation across the target surface should be ≤20% to ensure consistent film stoichiometry. For Cu-Ni-Si targets, a weak <111> texture is desirable, as it provides a balance between sputter yield and resistance to preferential erosion along grain boundaries. Excessive texture (e.g., strong <100> or <110>) can lead to non-uniform erosion profiles and premature target failure.

Precipitate Morphology And Distribution

In aged Cu-Ni-Si alloys, the primary strengthening phase is Ni₂Si (δ-phase), which forms as disc-shaped precipitates on {111} planes of the copper matrix. Optimal precipitate size is 10–30 nm diameter with inter-precipitate spacing of 50–100 nm, providing Orowan strengthening without excessive hardening. Patent 3 notes that targets with "no substantial precipitates" (i.e., fully solutionized) exhibit resistivity ≥2.2 μΩ·cm and are suitable for seed layer applications where low sheet resistance is critical. However, for structural applications (e.g., monolithic targets without backing plates), controlled precipitation is necessary to achieve Vickers hardness of 100–130 Hv and prevent warping during extended sputtering runs 6.

Nonmetallic Inclusion Control

Oxide, sulfide, and silicate inclusions are primary sources of particle contamination during sputtering. Patent 17 specifies that maximum inclusion size should be ≤10 μm to suppress abnormal discharge and arcing. Inclusions larger than 20 μm can act as stress concentrators, leading to microcracking and spallation of target material onto the substrate. Advanced inclusion control techniques include:

• Vacuum induction melting (VIM) with slag refining to remove sulfur and oxygen
• Electron beam melting (EBM) for ultra-high-purity targets (total impurities <10 wtppm)
• Hot isostatic pressing (HIP) post-sintering to close residual porosity and heal microcracks

Physical And Electrical Properties Of Copper Nickel Silicon Alloy Targets

Quantitative property data are essential for process modeling and equipment qualification.

Electrical Resistivity And Thermal Conductivity

Pure copper exhibits resistivity of ~1.7 μΩ·cm at 20°C, but alloying with Ni and Si increases resistivity due to solid-solution scattering and precipitate interfaces. For Cu-5wt%Ni-1wt%Si alloys in the solution-treated condition, resistivity typically ranges from 3.5–4.5 μΩ·cm. After aging to peak hardness, resistivity may increase to 4.0–5.0 μΩ·cm due to coherent Ni₂Si precipitates. Thermal conductivity decreases correspondingly from ~390 W/m·K (pure Cu) to ~150–200 W/m·K (Cu-Ni-Si alloy), which affects target cooling requirements and maximum sustainable power density during sputtering.

Mechanical Properties And Target Durability

Patent 6 reports that Cu-based alloy targets with KAM ≤2° and Vickers hardness 90–120 Hv exhibit excellent dimensional stability during sputtering, with target utilization rates >40% for monolithic designs. For Cu-Ni-Si targets, typical mechanical properties in the peak-aged condition include:

• Tensile strength: 350–450 MPa
• Yield strength (0.2% offset): 250–350 MPa
• Elongation: 15–25%
• Vickers hardness: 110–140 Hv

These properties ensure that the target can withstand thermal cycling (room temperature to 200–300°C during sputtering) and mechanical stresses from differential thermal expansion without cracking or delamination from the backing plate.

Sputter Yield And Deposition Rate

Sputter yield (atoms ejected per incident ion) for Cu-Ni-Si alloys is intermediate between pure Cu (~2.4 atoms/ion at 500 eV Ar⁺) and pure Ni (~1.5 atoms/ion). Experimental measurements on Cu-5Ni-1Si targets yield approximately 2.0–2.2 atoms/ion under typical DC magnetron sputtering conditions (400–600 V, 2–5 mTorr Ar). Deposition rates of 50–100 nm/min are achievable at power densities of 5–10 W/cm², with film composition closely matching target stoichiometry (±2 at%) when substrate-to-target distance is optimized (60–100 mm).

Applications Of Copper Nickel Silicon Alloy Sputtering Targets In Semiconductor Manufacturing

Cu-Ni-Si sputtering targets address critical challenges in advanced node (≤7 nm) semiconductor fabrication, where traditional Cu seed layers and barrier materials face scaling limitations.

Seed Layer Deposition For Copper Electroplating

The primary application of Cu-alloy sputtering targets is the formation of thin (10–50 nm) seed layers that enable subsequent electrochemical deposition of bulk copper interconnects. Patents 3, 5, 7, 8, 10, 11, 12, and 15 emphasize that seed layers must be:

• Continuous and uniform across high-aspect-ratio features (aspect ratio >5:1)
• Low in sheet resistance (<1 Ω/sq for 20 nm thickness) to support uniform electroplating current distribution
• Free from agglomeration during post-deposition annealing (400°C, 30 min in N₂)
• Compatible with Ta/TaN barrier layers without interdiffusion or delamination

Cu-Ni-Si seed layers offer advantages over pure Cu or Cu-Al/Cu-Sn alloys by providing:

1. Enhanced wetting on barrier layers due to reduced surface energy (Ni segregation to the film surface)
2. Improved electromigration resistance via grain boundary pinning by Ni₂Si precipitates
3. Self-forming barrier properties where Ni diffuses to the Cu/dielectric interface and reacts with Si from the substrate to form a thin (~2 nm) Ni-silicide layer

Typical process conditions for seed layer sputtering include DC power of 1–3 kW, Ar pressure of 2–5 mTorr, substrate temperature of 50–150°C, and deposition time of 10–30 seconds.

Diffusion Barrier And Capping Layer Applications

Patent 1 describes Cu-Ni alloy films (20–40 mass% Ni) used as protective layers over Cu interconnects to prevent oxidation and electromigration-induced voiding. For Cu-Ni-Si systems, a bilayer structure is often employed:

• Bottom layer: Cu-rich alloy (Cu-2Ni-0.5Si, 50–100 nm) for low resistivity
• Top layer: Ni-rich alloy (Cu-10Ni-2Si, 10–20 nm) for oxidation resistance and adhesion to dielectric capping layers (SiCN, SiN)

This approach reduces overall interconnect resistivity compared to thick Ta/TaN barriers while maintaining reliability under accelerated stress testing (150°C, 2 MA/cm² current density, >1000 hours without failure).

Transparent Conductive Films For Display And Touch Panel Applications

Patent 17 discloses Cu-Ni-Fe-Mn alloy targets for forming transparent conductive oxide (TCO) alternatives in flat-panel displays. While not explicitly a Cu-Ni-Si composition, the principles are applicable: thin (<10 nm) Cu-alloy films can achieve sheet resistance <10 Ω/sq with >80% optical transmittance in the visible spectrum when deposited on glass or polymer substrates. Cu-Ni-Si films offer improved weather resistance (humidity and temperature cycling) compared to pure Cu due to the formation of a passive Ni-oxide surface layer that inhibits further oxidation.

Emerging Applications In 3D Integration And Advanced Packaging

As semiconductor industry transitions to 3D stacking (through-silicon vias, TSVs) and heterogeneous integration (chiplets, fan-out wafer-level packaging), Cu-Ni-Si sputtering targets enable:

• Conformal coating of high-aspect-ratio TSVs (diameter <5 μm, depth >50 μm) with seed layers that support bottom-up electroplating fill
• Low-temperature processing (<250°C) compatible with organic substrates and redistribution layers (RDL