Rhenium Sputtering Target: Advanced Manufacturing, Compositional Control, And Applications In Semiconductor Deposition

Compositional Design And Alloying Strategies For Rhenium Sputtering Targets

Rhenium is rarely employed as a standalone sputtering target material due to its high melting point (3186 °C), limited availability, and cost. Instead, rhenium is strategically incorporated into multi-component alloy systems to tailor magnetic, electrical, and mechanical properties. The most extensively documented rhenium-containing sputtering target is the CoCrPtBRe alloy, designed for magnetic recording media applications6. In this system, cobalt serves as the ferromagnetic matrix (>50 atom%), chromium (2–18 atom%) segregates to grain boundaries to reduce intergranular exchange coupling, platinum (9–30 atom%) enhances perpendicular magnetic anisotropy, boron (2–14 atom%) refines grain size, and rhenium (2–8 atom%) improves sputtering stability and reduces misfire events during deposition6.

The addition of rhenium to CoCrPtB alloys addresses a critical challenge in high-density magnetic recording: sputtering instability manifested as arcing, particle generation, and compositional non-uniformity. Rhenium's high atomic mass (186.2 g/mol) and refractory nature contribute to more uniform erosion profiles and reduced nodule formation on the target surface6. Comparative studies indicate that CoCrPtBRe targets exhibit significantly lower misfire rates (quantified as arcing events per kilowatt-hour of sputtering power) compared to rhenium-free CoCrPtB targets, with improvements exceeding 40% under identical DC magnetron sputtering conditions6.

Beyond magnetic alloys, rhenium may be considered as a minor alloying addition to ruthenium-based targets for semiconductor applications, although direct patent evidence for Ru-Re binary systems is limited in the retrieved sources. The rationale for such alloying would parallel the use of other platinum-group elements (Pt, Pd, Ir, Os) in ruthenium targets to refine grain structure, suppress oxygen uptake during sintering, and improve film adhesion to silicon substrates111213. Rhenium's position in Group 7 and its similar atomic radius to ruthenium (137 pm vs. 134 pm) suggest potential solid-solution strengthening and grain-boundary pinning effects, though experimental validation is required.

Powder Metallurgy Routes For Rhenium-Containing Sputtering Targets

Raw Material Preparation And Powder Characteristics

The fabrication of rhenium-containing sputtering targets via powder metallurgy begins with high-purity elemental or pre-alloyed powders. For CoCrPtBRe targets, the typical process involves mechanical mixing of elemental powders (Co, Cr, Pt, B, Re) with particle sizes in the range of 1–50 μm6. Rhenium powder is commercially available at purities of 99.9% (3N) to 99.99% (4N), with oxygen content typically below 500 ppm in as-received condition. However, rhenium's affinity for oxygen necessitates stringent handling protocols: powder storage under inert atmosphere (argon or nitrogen), and blending in glove boxes with <1 ppm O₂ and H₂O to prevent surface oxidation1617.

An alternative approach employs gas atomization to produce pre-alloyed powders with homogeneous composition. In this method, a master alloy ingot (e.g., CoCrPtBRe) is induction-melted under vacuum or inert gas, then atomized using high-pressure argon jets to generate spherical powders with controlled size distribution (D₅₀ = 10–100 μm)18. Atomized powders exhibit superior flowability and packing density compared to irregularly shaped mechanically milled powders, leading to more uniform green compacts and reduced porosity in sintered targets18. For rhenium-containing alloys, atomization temperatures must exceed 1600 °C to ensure complete melting and homogenization, requiring specialized crucible materials (e.g., yttria-stabilized zirconia or graphite) to prevent contamination18.

Compaction And Sintering Processes

Following powder preparation, the material undergoes sequential compaction and densification steps. Cold isostatic pressing (CIP) is commonly employed to form green compacts with relative densities of 60–70%2. Typical CIP parameters for rhenium-containing alloys include pressures of 200–400 MPa applied for 5–15 minutes in a flexible rubber mold2. The green compact is then subjected to vacuum sintering or hot isostatic pressing (HIP) to achieve near-theoretical density.

For CoCrPtBRe targets, vacuum sintering is performed at 1100–1300 °C for 2–6 hours under pressures below 10⁻⁴ Pa to minimize oxygen pickup6. The sintering temperature must be carefully optimized: insufficient temperature results in incomplete densification and weak inter-particle bonding, while excessive temperature causes grain coarsening and potential loss of volatile elements (boron sublimation above 1400 °C). Post-sintering relative densities of 97–99% are achievable, with residual porosity concentrated at triple junctions and prior particle boundaries6.

Hot isostatic pressing (HIP) offers superior densification for refractory alloys. In a representative HIP cycle for rhenium-containing targets, the sintered compact is encapsulated in a titanium or molybdenum can, evacuated to <10⁻³ Pa, sealed by electron-beam welding, and subjected to 1300–1400 °C and 150–200 MPa argon pressure for 3–5 hours10. HIP eliminates residual porosity and heals micro-cracks, yielding relative densities exceeding 99.5% and significantly improved mechanical strength (flexural strength >400 MPa for CoCrPtBRe)10. The titanium can is removed by machining or chemical etching post-HIP, leaving a dense, homogeneous target blank ready for final machining10.

Oxygen Control And Impurity Management

Oxygen contamination represents a critical concern in rhenium and rhenium-alloy target fabrication, as oxygen forms stable oxides (Re₂O₇, ReO₃, ReO₂) that decompose during sputtering to generate particles and cause arcing71617. Target specifications typically mandate oxygen content <500 ppm for magnetic recording applications and <200 ppm for semiconductor applications7. Achieving these levels requires multi-stage purification:

• Hydrogen reduction: Rhenium oxide powders are reduced at 800–1000 °C in flowing hydrogen (dew point <-60 °C) to convert ReO₂ and ReO₃ to metallic rhenium, with oxygen removed as water vapor1617.
• Vacuum degassing: Compacted targets are heated to 600–800 °C under high vacuum (10⁻⁵ Pa) for 4–12 hours to desorb surface-adsorbed oxygen and moisture1617.
• Gettering additions: Reactive elements such as titanium, zirconium, or aluminum (0.1–1.0 wt%) are added to scavenge residual oxygen, forming stable oxide inclusions that are less detrimental than dispersed oxygen7. However, excessive gettering additions can alter target stoichiometry and must be balanced against compositional tolerances.

In addition to oxygen, other impurities (carbon, sulfur, alkali metals, radioactive isotopes) must be controlled to parts-per-million levels. For semiconductor-grade targets, boron and phosphorus are particularly critical: specifications often require B <0.1 ppm and P <0.1 ppm to prevent unintentional doping of silicon substrates712. Analytical techniques for impurity verification include glow discharge mass spectrometry (GDMS), inductively coupled plasma mass spectrometry (ICP-MS), and combustion analysis for oxygen, nitrogen, and carbon312.

Melting-Based Fabrication Routes For High-Purity Rhenium Targets

Electron Beam Melting And Vacuum Arc Remelting

An alternative to powder metallurgy is melting-based fabrication, which offers advantages in purity, homogeneity, and elimination of prior particle boundaries. Electron beam melting (EBM) is particularly suited for refractory metals like rhenium due to its high power density (up to 10⁷ W/cm²) and ultra-high vacuum environment (10⁻⁴ to 10⁻⁵ Pa)1213. In a typical EBM process for ruthenium-rhenium alloys (as a proxy for rhenium-containing systems), elemental powders or compacts are loaded into a water-cooled copper crucible, melted by a focused electron beam (accelerating voltage 20–30 kV, beam current 0.5–2.0 A), and solidified into an ingot1213. Multiple remelting cycles (3–5 passes) are performed to ensure compositional homogeneity and reduce volatile impurities (oxygen, nitrogen, hydrogen) through vacuum evaporation1213.

Vacuum arc remelting (VAR) provides an alternative melting route, wherein a consumable electrode (prepared by powder compaction or casting) is melted by a DC arc (1000–3000 A) under vacuum or inert gas, with the molten metal dripping into a water-cooled copper mold to form a solidified ingot4. VAR is effective for removing high-vapor-pressure impurities and refining grain structure through controlled solidification, but requires careful arc stability control to prevent electrode fragmentation and non-uniform melting4.

Forging And Thermomechanical Processing

Cast or melted ingots typically exhibit coarse columnar grains (grain size >5 mm) and crystallographic texture that can lead to anisotropic sputtering behavior and non-uniform film thickness41213. To refine the microstructure, ingots undergo hot forging at temperatures of 1000–1400 °C with total reductions of 50–80%13. Forging breaks up the cast structure, introduces high-angle grain boundaries, and reduces average grain size to 0.5–1.5 mm13. For ruthenium-alloy targets (which serve as a model for rhenium-containing systems), forging is performed in multiple passes with intermediate annealing steps to prevent cracking, and the final forged billet is machined to target dimensions13.

Thermomechanical processing also enables control of crystallographic texture. For example, cross-rolling (alternating rolling directions by 90°) followed by recrystallization annealing can produce a more random texture, reducing the intensity ratio of specific X-ray diffraction peaks (e.g., I(002)/I(101) <1.2 for hexagonal close-packed metals)4. This texture randomization improves sputtering uniformity by minimizing preferential erosion along specific crystallographic planes4.

Microstructural Characterization And Quality Control

Grain Size And Phase Distribution

Grain size is a critical microstructural parameter influencing sputtering performance. Fine-grained targets (grain size <100 μm) exhibit more uniform erosion, reduced particle generation, and improved film thickness uniformity compared to coarse-grained targets (grain size >500 μm)3415. For rhenium-containing CoCrPtBRe targets, optimal grain size is reported as 50–200 μm, achieved through controlled sintering kinetics and boron additions that pin grain boundaries6. Grain size is quantified by optical microscopy on polished and etched cross-sections, using the linear intercept method per ASTM E1123.

Phase distribution is equally important. In CoCrPtBRe alloys, the target microstructure comprises a Co-rich face-centered cubic (fcc) matrix, Cr-rich body-centered cubic (bcc) precipitates at grain boundaries, and intermetallic phases (e.g., CoPt, Co₃B)6. Rhenium partitions preferentially to the fcc matrix due to its similar atomic size to cobalt, enhancing solid-solution strengthening6. Excessive rhenium content (>10 atom%) can promote formation of brittle σ-phase (CoCrRe), which degrades target mechanical integrity and should be avoided6. Phase identification is performed by X-ray diffraction (XRD) with Rietveld refinement, and phase morphology is characterized by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS)59.

Density And Porosity Measurement

Relative density (ρ_rel = ρ_measured / ρ_theoretical × 100%) is a key quality metric, with specifications typically requiring ρ_rel ≥99.0% for magnetic recording targets and ≥99.5% for semiconductor targets1710. Density is measured by the Archimedes method (immersion in water or ethanol) per ASTM B962, with theoretical density calculated from alloy composition and lattice parameters17. Residual porosity is characterized by optical microscopy on polished sections, with pore size distribution and volume fraction quantified by image analysis software10.

For rhenium-containing targets, porosity must be minimized to prevent gas entrapment and particle ejection during sputtering. Pores larger than 10 μm are particularly detrimental, as they can trap argon sputtering gas and subsequently release it as micro-explosions, generating particles that contaminate the deposited film710. HIP processing is highly effective in closing such pores, reducing pore volume fraction from 1–2% (post-sintering) to <0.1% (post-HIP)10.

Oxygen And Impurity Analysis

Oxygen content is verified by inert gas fusion (IGF) analysis per ASTM E1409, wherein a target sample is heated to >2000 °C in a graphite crucible under helium flow, converting oxygen to CO and CO₂ for infrared detection3716. Typical detection limits are 1–5 ppm, with measurement precision ±10% relative standard deviation3. For CoCrPtBRe targets, oxygen specifications are 300–500 ppm, while for semiconductor-grade ruthenium-rhenium alloys (if developed), oxygen must be <200 ppm716.

Trace metal impurities are quantified by GDMS, which provides detection limits of 0.01–1.0 ppb for most elements312. Critical impurities for semiconductor applications include:

• Alkali metals (Na, K): <0.1 ppm each, to prevent mobile ion contamination312.
• Radioactive isotopes (U, Th): <0.01 ppb each, to minimize alpha-particle-induced soft errors in memory devices312.
• Transition metals (Fe, Ni, Cu): <1 ppm each, to avoid deep-level traps in silicon312.

Sputtering Performance And Film Deposition Characteristics

Arcing Suppression And Particle Reduction

Arcing during sputtering is a major yield-limiting factor, caused by localized dielectric breakdown at insulating oxide inclusions or surface contaminants on the target1367. Rhenium additions to CoCrPtB alloys reduce arcing frequency by 30–50% compared to rhenium-free compositions, attributed to rhenium's high electrical