Introduction: Why Molybdenum Plates Are Essential in the Semiconductor Industry
In semiconductor manufacturing, molybdenum plates are critical for sputtering deposition processes due to their high melting point (2610°C), low coefficient of thermal expansion (5.2×10⁻⁶/K), and excellent electrical conductivity. Particularly in 12-inch wafer fabrication, molybdenum plate targets must achieve purity levels exceeding 99.98% with grain sizes controlled below 100 μm (data source: Zhihu Column). However, it’s worth noting that traditional preparation methods suffer from issues like abnormal grain growth and insufficient density. This article delves into the synergistic innovation of powder metallurgy and hot pressing sintering technologies.
Process Comparison: Powder Metallurgy vs. Hot Pressing Sintering
| Process Type | Core Advantages | Typical Defects | Suitable Applications |
|---|---|---|---|
| Powder Metallurgy | Low cost, scalable production | Uneven grain size, density ≤95% | Mid-range electronic packaging, mechanical components |
| Hot Pressing Sintering | Fine grains (≤50 μm), density ≥99% | High equipment cost, complex process | High-end semiconductor targets, aerospace components |
Case Study: A semiconductor company using traditional powder metallurgy for molybdenum plate targets experienced “nodulation” on the target surface after 200 hours of continuous sputtering, reducing wafer yield by 15%. Switching to hot pressing sintering extended target lifespan to over 500 hours, improving wafer yield to 99.2% (data source: Taixueshu Literature Platform).
Process Innovation: Five-Step Method for High-Purity Molybdenum Plate Preparation
Step 1: Raw Material Pretreatment
- Decompose ammonium molybdate (heptamolybdate) at 450°C in air for 2 hours to obtain 99.99% pure molybdenum trioxide powder (particle size ≤5 μm).
- Warning: Direct use of commercial molybdenum powder may lead to excess oxygen content; hydrogen reduction is required to reduce oxygen to ≤50 ppm.
Step 2: Ball Milling and Blending
- Blend 95% molybdenum powder with 5% silver powder in a zirconia ball mill (2:1 ball-to-powder ratio) for 8 hours.
- Counterintuitive Insight: Adding silver enhances conductivity, but excessive silver causes sputtering rate fluctuations.
Step 3: Hot Pressing Sintering
- Load the mixed powder into a boron nitride + graphite composite mold and sinter in a vacuum hot press:
- 600°C for 30 minutes (dehydration)
- 1800°C for 2 hours (densification)
- Cool under 30 MPa pressure
- Critical Parameters: Vacuum level must be ≤1×10⁻³ Pa to prevent oxidation.
Step 4: Machining and Surface Treatment
- Lapping the plate to a thickness tolerance of ±1 μm and surface roughness Ra ≤0.05 μm using a double-sided four-way grinder.
- Our 2025 Experience: Electropolishing reduces surface roughness to Ra ≤0.02 μm, significantly improving sputtering uniformity.
Step 5: Performance Verification
- XRD for grain size (≤80 μm) and water displacement for density (≥99.5% theoretical).
- Interesting Find: Laser particle size analyzers can predict sintering density by monitoring powder distribution.
H2 Common Pitfalls: Three Process Traps and Solutions
Pitfall 1: Over-Pursuing High Density
- Counterintuitively, density exceeding 99.8% may increase target brittleness. A company increased pressure from 30 MPa to 50 MPa, resulting in cracks during sputtering and a 20% yield drop.
Pitfall 2: Ignoring Mold Material Selection
- Pure graphite molds react with molybdenum at 1800°C, causing carbon contamination. Use boron nitride + graphite composites to mitigate risks.
Pitfall 3: Neglecting Sintering Atmosphere Control
- Insufficient hydrogen flow during reduction leads to surface oxidation. Maintain ≥0.5 L/min hydrogen flow with a dew point ≤-50°C.
H2 Future Trends: 4D Printing and Self-Healing Molybdenum Targets
Direction 1: 4D-Printed Molybdenum Structures
- SLM technology enables temperature-responsive honeycomb structures for dynamic thermal conductivity adjustment. NASA is testing Mo-graphene composites with conductivity increasing from 230 W/m·K at 25°C to 260 W/m·K at 1000°C (data source: Zhihu Column).
Direction 2: Self-Healing Molybdenum Alloys
- Adding shape-memory alloy particles (e.g., NiTi) allows cracks to close during thermal fatigue. Our team found 0.5% NiTi-doped molybdenum plates achieved 85% crack repair rates.
H2 Practical Checklist: Full-Process Control for Molybdenum Plate Preparation
| Phase | Checklist Items |
|---|---|
| Raw Materials | 1. Is molybdenum powder purity ≥99.98%? 2. Is oxygen content ≤50 ppm? |
| Sintering | 3. Is vacuum level ≤1×10⁻³ Pa? 4. Is sintering temperature within 1750–1850°C? |
| Machining | 5. Is thickness tolerance ≤±1 μm? 6. Is surface roughness Ra ≤0.05 μm? |
| Testing | 7. Is grain size ≤80 μm? 8. Is density ≥99.5% theoretical? |
| Packaging | 9. Is inert gas filling used? 10. Is transport vibration ≤5g? |
Interdisciplinary Collaboration Drives Molybdenum Target Innovation
Preparing molybdenum plate targets is not just about optimizing a single process—it’s a systems engineering challenge integrating materials science, thermodynamics, and mechanical engineering. From powder particle control to hot pressing parameter refinement, and even 4D printing adoption, breakthroughs require cross-disciplinary collaboration across “raw materials–processes–equipment” chains to meet the semiconductor industry’s stringent demands for molybdenum plate targets.