1. Technical Background and Evolutionary Drivers
In semiconductor manufacturing, interconnect technology is a key factor determining chip performance. Early aluminum interconnects could not meet the requirements for processes below 0.18 microns due to high resistivity (2.82 μΩ·cm) and electromigration issues. Copper, with its lower resistivity (1.68 μΩ·cm) and significantly stronger resistance to electromigration (over 10 times that of aluminum), emerged as a replacement material. However, the etching challenges of copper (inability to generate volatile by-products) led to the birth of the Damascene process—bypassing the difficulties of directly etching copper by “first etching the dielectric layer, then filling with metal.”
2. Core Process of the Damascene Technology
The core steps of the copper Damascene process include:
1. Dielectric Layer Patterning
– An insulating layer is deposited on Low-k dielectric materials (such as SiOCH), and through photolithography and dry etching, vias (vertical connections) and trenches (horizontal wiring) are formed.
– The Dual Damascene technology allows for simultaneous etching of vias and trenches, reducing process steps and costs.
2. Barrier Layer and Seed Layer Deposition
– A Ta/TaN barrier layer (approximately 2-5 nm thick) is deposited in the trenches using Physical Vapor Deposition (PVD) or Atomic Layer Deposition (ALD) to prevent copper diffusion into the silicon substrate.
– A thin copper seed layer (about 50 nm) is deposited as a conductive substrate for electroplating copper.
3. Electroplating Copper Filling and Planarization
– Electrochemical deposition (ECD) fills copper into the trenches, ensuring void-free filling by optimizing the electroplating solution formulation and parameters (such as current density and temperature), especially in structures with a depth-to-width ratio exceeding 10:1.
– Chemical Mechanical Polishing (CMP) removes excess copper and the barrier layer from the surface, achieving global planarization with surface roughness controlled within 0.5 nm.
3. Technical Advantages and Performance Breakthroughs
1. Enhanced Electrical Performance
– The low resistivity of copper reduces signal transmission delay by 20%-30%, making it particularly suitable for high-frequency scenarios in 5G and AI chips.
– The enhanced resistance to electromigration extends the chip lifespan to over 10 times that of aluminum interconnects.
2. Process Compatibility and Integration Density
– Supports multi-layer interconnects (8-12 layers), with line widths reduced to below 4 nanometers, promoting the development of 3D packaging (such as hybrid bonding technology).
– The use of Low-k dielectrics (dielectric constant ≤ 2.5) reduces parasitic capacitance and minimizes signal crosstalk.
3. Cost and Yield Balance
– The Dual Damascene process reduces the number of photolithography steps, and integrated etching equipment (such as the dynamic plate gap adjustment technology developed by Zhongwei Company) improves yield for nodes below 28 nanometers.
4. Key Challenges and Innovative Solutions
1. Resistance Surge at Nanoscale
– When wire dimensions shrink below 3 nanometers, surface scattering effects cause a significant increase in copper resistivity. Solutions include:
– Introducing ruthenium (Ru) or cobalt (Co) as ultra-thin barrier layers (≤1 nm) to reduce cross-sectional area ratio.
– Developing tungsten-copper composite via structures (such as the IMEC solution), which reduce resistance by 50% in 10 nanometer vias.
2. Electromigration and Reliability Issues
– Depletion of copper atoms at the cathode leads to void formation, necessitating optimization of the trench depth-to-diameter ratio (such as HD/D1) and seed layer deposition direction to reduce defects.
– Employing Self-Aligned Via technology to minimize the impact of process offsets on reliability.
3. CMP Process Limits
– The demand for atomic-level planarization has led to the development of new polishing slurries (such as cerium oxide-based slurries) and adaptive pressure control technologies.
5. Future Trends and Domestic Progress
1. Directions for Process Innovation
– Semi-Damascene process: Combining self-aligned patterning technology at the 1.5 nanometer node to support 14-22 nanometer metal spacing, enhancing post-integration density.
– Air gap interconnects: Forming air layers through selective etching in tight spacing to further reduce dielectric constant.
2. Packaging and Heterogeneous Integration
– Hybrid Bonding: Utilizing the ultra-flat copper surface after CMP for direct chip bonding, increasing interconnect density to levels of 10^6/mm².
– Through-Silicon Via (TSV) technology: Supporting HBM memory stacking through copper electroplating filling in three-dimensional structures.
3. Breakthroughs in Domestic Equipment and Materials
– Zhongwei Company’s integrated Damascene etching equipment has been adapted for logic chips below 28 nanometers, and domestic CMP equipment (such as Huajin Semiconductor patents) has made progress in polymer dielectric processing.
6. Conclusion
The copper Damascene process, through the synergistic innovation of materials and processes, has broken through the physical limits of aluminum interconnects, becoming a core technology in the semiconductor miniaturization process. As processes advance towards 1.5 nanometers, the complexity of processes and the demand for new materials will continue to escalate, while breakthroughs in domestic equipment and materials will provide critical support for the global supply chain. In the future, this technology will not only drive performance leaps in logic and memory chips but also empower advanced packaging and heterogeneous computing, ushering in a new era of semiconductor integration.