Introduction
The metal–oxide–semiconductor field-effect transistor (MOSFET) has been a pivotal element in the evolution of complementary metal–oxide–semiconductor (CMOS) technology, driving the advancement of integrated-circuit products. Over the past six decades, the physical gate length of MOSFETs has been scaled to sub-20 nanometres, propelled by the demand for higher speed, energy efficiency, and integration density. However, the downscaling of transistors while keeping power consumption low has become increasingly challenging, even for state-of-the-art fin field-effect transistors. In this article, we will delve into the challenges faced by modern CMOS scaling, providing a comprehensive assessment of the existing and future CMOS technologies.
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Challenges of Modern CMOS Scaling:
As semiconductor technology advances, modern CMOS scaling faces intricate challenges. Shrinking transistors to boost performance encounters barriers like power consumption, heat dissipation, and quantum effects. Balancing these factors becomes pivotal for sustaining the pace of innovation in electronic devices.
Short-Channel Effects (SCEs)
Short-channel effects (SCEs) refer to the adverse effects that occur in metal–oxide–semiconductor field-effect transistors (MOSFETs) as the device size enters the 1-μm regime. These effects are primarily caused by the reduction in the physical gate length and the resulting increase in the electric field at the drain end of the channel.
As the gate length decreases, the control of the channel by the gate weakens, leading to various adverse effects such as increased leakage current,reduced carrier mobility, anddecreased threshold voltage control.
Historically, researchers focused on reducing the physical gate oxide thickness and engineering the doping profile of the source, drain, and channel to mitigate these effects. However, the semiconductor field has introduced novel materials and device architectures, such as strained channels, high-dielectric-constant (k) metal gates (HKMG), silicon-on-insulator (SOI), and fin field-effect transistors (FinFETs), to actively suppress short-channel effects (SCEs) and counter other adverse impacts.
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Limitations of Traditional Scaling Efforts
The continuous scaling of CMOS technology has encountered obstacles, primarily due to the limitations of traditional scaling efforts. The latest International Roadmap for Devices and Systems (IRDS) indicates that scaling at sub-5-nm technology nodes will stall at physical gate lengths of 14 nm and 12 nm for low-power (LP) and high-performance (HP) applications, respectively, presenting a significant challenge for further scaling.
The technology node hoax
The naming of technology nodes, like 65 nanometres or 28 nanometres, came from the minimum gate length of the transistor. For example, if the distance is 65 nanometres, it’s called a 65-nanometer transistor.
For a long time, the gate length of the transistor matched the technology name, as was the case for 28 nanometres and some 22 millimetres processes. However, recent technology nodes, like 10 nanometers or 5 nanometers, do not correspond to any gate anymore. Transistors no longer look the same, as the industry has shifted from planar structures to finFET-like structures.
For example, in 10 nanometer technology, a half-pitch distance (minimum distance between similar features) is 32 nanometers, gate length is 19 nanometers, and fin width is 7.2 nanometers. The term “10 nanometers” is more of a marketing number that benchmarks new technology against conventional planar transistors. The number, like 5 nanometers, is used if it delivers the expected performance of a 5nm gate length device, regardless of the actual dimensions.
Device Electrostatics and Variability
Improving device electrostatics and addressing variability issues in ultra scaled MOSFETs have become critical challenges in modern CMOS scaling.
Device electrostatics here refers to the quantification and improvement of the scalability of FETs.
The natural length λ quantifies the scalability of FETs by capturing the steepness of the potential variation from the source or drain to the channel.
Efficient modulation of the channel potential is crucial for controlling the mobile charge-carrier population, and addressing these electrostatic challenges is essential for scalability.
The above flow chart illustrates FET scaling scenarios hierarchically using a tap. The tap, operating similarly to FETs, controls the flow of water or charge carriers from source to drain through a channel using a knob or gate. Different forms of taps represent various scaling scenarios. The natural length is denoted by λ.
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Challenges in Emerging Transistor Technologies
While emerging transistor technologies such as nanowire (NW) and nanosheet (NS) FETs show promise in improving device current and electrostatics, they also present challenges in increasing device integration density beyond a certain point. Additionally, practical approaches to increase device density involve three-dimensional integration, which stacks either devices or dies in the vertical space. However, this method poses fabrication challenges.
These challenges include thermal budget and heat dissipation issues, as well as the need for high layer density and high local connectivity-enabled designs. The complexity of these fabrication processes also comes with associated costs.
Conclusion:
As we have explored above,challenges faced by modern CMOS scaling are multifaceted, encompassing short-channel effects, limitations of traditional scaling efforts, device electrostatics, and the challenges of emerging transistor technologies. Additionally,as we navigate the road to sub-10-nanometre gate lengths, it is essential to address these challenges. Stay tuned for our next article, where we will explore potential solutions to overcome these challenges and delve into the future of transistor technologies.
