Processes & Materials for CMOS Scaling & New Devices

Processes & Materials for CMOS Scaling & New Devices

Pushing the Limits of Moore’s Law and Beyond

For over five decades, CMOS (Complementary Metal-Oxide-Semiconductor) technology has driven the semiconductor revolution, doubling transistor density approximately every two years in accordance with Moore’s Law. However, as scaling approaches atomic dimensions, traditional silicon-based processes face physical and material limitations that challenge further miniaturization.

This article explores the advanced processes, new materials, and device innovations that are redefining CMOS scaling — including FinFETs, Gate-All-Around (GAA) FETs, nanosheet/nanoribbon transistors, high-k/metal gate stacks, EUV lithography, strain engineering, and 2D materials. It also examines post-CMOS device concepts such as tunnel FETs, negative-capacitance FETs, and spintronic/quantum devices that aim to sustain the pace of innovation in the era “Beyond Moore.”

1. Introduction: The Evolution and Challenge of Scaling

Since the 1970s, CMOS scaling has relied on Dennard’s scaling principles — reducing transistor dimensions while maintaining constant electric fields to improve speed and density without increasing power.
However, as feature sizes fall below 5 nm, these assumptions no longer hold.
Key challenges include:

• Short-channel effects (SCEs) and leakage current.

• Power density and heat dissipation.

• Variability due to process fluctuations.

• Quantum tunneling and mobility degradation.

To sustain progress, the industry has shifted toward new materials, novel device architectures, and advanced process technologies — combining both “More Moore” (continued scaling) and “More than Moore” (new functionality and integration).

2. Evolution of CMOS Device Architectures

2.1 Planar MOSFET (Pre-2010 Era)

• Traditional bulk CMOS with 2D gate control.

• Scaling limited below ~30 nm due to poor electrostatics.

• Used SiO₂/Poly-Si gate stacks.

2.2 FinFET Technology (22 nm–7 nm)

• Tri-gate structure wraps the gate around the channel on three sides.

• Improved control over short-channel effects and reduced leakage.

• Uses high-k dielectrics and metal gates for better performance.

• Introduced in Intel’s 22 nm (2011) and widely adopted across foundries.

2.3 Gate-All-Around (GAA) and Nanosheet FETs (5 nm and Beyond)

• Gate surrounds the channel on all sides, offering full electrostatic control.

• Stacked nanosheet/nanoribbon channels enable drive current tuning by width adjustment.

• Key Benefit: Balances performance and leakage better than FinFETs.

• Implemented in 3 nm and upcoming 2 nm technology nodes.

2.4 Forksheet and CFET (Complementary FET)

• Forksheet FETs: N-type and P-type transistors placed closer by dielectric isolation.

• CFETs: Stacks NMOS and PMOS vertically to reduce footprint.

• These architectures represent 3D transistor integration for density scaling beyond 2D lithography limits.

3. Key Materials for Continued CMOS Scaling

3.1 High-k/Metal Gate (HKMG) Stacks

• Replaced traditional SiO₂/Poly-Si gates.

• Common materials: HfO₂ (high-k dielectric), TiN/TaN (metal gates).

• Reduce gate leakage and enable thinner equivalent oxide thickness (EOT).

• Now standard since the 45 nm node.

3.2 Strain Engineering

• Mechanical strain enhances carrier mobility.

• Tensile strain for NMOS (boosts electron mobility).

• Compressive strain for PMOS (boosts hole mobility).

• Implemented using SiGe channels, stress liners, and strained epitaxy.

3.3 Channel Material Engineering

3.4 Low-k and Ultra-Low-k Dielectrics

• Used in interconnects to reduce RC delay and power consumption.

• Air-gap dielectrics and porous SiCOH films are being adopted.

3.5 Metal Contacts and Silicides

• Replacing high-resistance tungsten (W) with cobalt (Co) and ruthenium (Ru) for reduced contact resistance.

• Self-aligned silicide (NiSi, CoSi₂) ensures low contact resistance at nanoscale.

4. Process Innovations Enabling Advanced Nodes

4.1 Extreme Ultraviolet (EUV) Lithography

• Wavelength: 13.5 nm (vs 193 nm DUV).

• Enables single-exposure patterning at sub-10 nm pitches.

• Reduces multiple patterning steps, improving yield and cost.

• Challenges: Source power, resist sensitivity, and mask defects.

4.2 Atomic Layer Deposition (ALD)

• Offers angstrom-level precision for thin films (gate oxides, spacers).

• Ensures conformal coating in 3D structures (FinFET, GAA).

4.3 Selective and Self-Aligned Processes

• Selective epitaxy: Grows materials only where desired (e.g., SiGe channels).

• Self-aligned double/quad patterning (SADP/SAQP): Achieves fine line spacing with high precision.

4.4 3D Integration and Backside Processing

• Backside Power Delivery Network (BSPDN): Moves power routing beneath transistors to reduce droop and congestion.

• Wafer bonding and TSV (Through-Silicon Via): Enable 3D stacking for logic and memory integration.

4.5 Advanced Metallization

• Cu → Co → Ru → Mo transition for lower resistivity and electromigration resistance.

• Semi-damascene and dual-damascene processes used for fine-pitch interconnects.

5. Device Innovations for “Beyond CMOS” Scaling

As CMOS scaling approaches physical limits, researchers are exploring new physics-based device concepts:

These emerging devices aim to complement or replace CMOS in specialized applications — particularly for ultra-low-power and neuromorphic systems.

6. Interconnect and Packaging Synergy

As transistor scaling slows, interconnect resistance and parasitics dominate delay and power.
To address this:

• 3D stacking and chiplets reduce global interconnect length.

• Hybrid bonding enables dense die-to-die interconnects (<10 µm pitch).

• Co-design of devices, interconnects, and packaging (DTCO/PTCO) ensures optimized system-level performance.

Advanced materials like graphene, carbon nanotubes (CNTs), and optical interconnects are also being explored for future nodes.

7. Reliability and Variability Considerations

7.1 Variability Sources

• Random dopant fluctuations (RDF).

• Line edge roughness (LER).

• Work function variation (WFV).

• Channel strain non-uniformity.

7.2 Reliability Mechanisms

8. Sustainability and Manufacturing Efficiency

As fabrication complexity grows, sustainability becomes a critical concern:

• Green manufacturing: Reducing chemical and water usage in fabs.

• Energy-efficient etching and deposition.

• Recycling and circular material flow for rare metals.

• Carbon footprint reduction in EUV and vacuum processes.

Future fabs (e.g., TSMC’s 2 nm and Intel’s 18A) integrate AI-based process control for yield optimization and waste minimization.

9. The Path Forward: “More Moore” and “Beyond Moore”

The roadmap for semiconductor innovation combines:

• Continued CMOS scaling with GAA, CFET, and nanosheet devices.

• New materials and 2D semiconductors for enhanced mobility and control.

• 3D monolithic integration and advanced packaging for system-level scaling.

• Co-optimization of design, process, and architecture (DTCO/PTCO/STCO).

• Hybrid CMOS–quantum systems for the ultimate computing frontier.

Together, these approaches ensure that Moore’s Law evolves — not ends — through innovation in materials, processes, and integration.

The journey of CMOS scaling has transformed silicon into the most engineered material in history.

Yet, as transistors approach atomic scales, new materials, device architectures, and fabrication processes are redefining the boundaries of possibility.

From FinFETs and nanosheets to 2D materials and quantum devices, the next era of semiconductor progress will be driven not by shrinking dimensions alone, but by inventing new materials and leveraging new physics.

The fusion of process innovation, material science, and device engineering will power the next generation of electronics — from AI processors to quantum systems — ensuring that CMOS technology remains the foundation of the digital world for decades to come.

VLSI Expert India: Dr. Pallavi Agrawal, Ph.D., M.Tech, B.Tech (MANIT Bhopal) – Electronics and Telecommunications Engineering