Role of On-Chip Optical Communication in Overcoming CMOS Scaling Challenges

Complementary metal-oxide-semiconductor (CMOS) technology has been the foundation of the semiconductor industry for the past fifty years, especially in the microprocessor domain. This technology is a predominant one used in the fabrication of integrated circuits (ICs), including microprocessors, microcontrollers, memory chips, and various digital logic circuits.

What is a complementary metal oxide semiconductor technology?

Complementary metal-oxide-semiconductor technology creates logic functions using complementary and symmetrical pairs of p-type and n-type MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).

CMOS technology utilizes NMOS (n-channel MOSFET) and PMOS (p-channel MOSFET) transistors to achieve low power consumption and high efficiency, as shown in Fig. 1. The term "complementary" refers to how these transistors work together to switch logic levels efficiently. In a CMOS circuit, when one type of transistor is on, the other is off, which minimizes power consumption since static power is only dissipated during the switching process.

Fig. 1: Illustrates the CMOS structure. Source: Rakesh Kumar, Ph.D.

What is CMOS scaling?

CMOS scaling is a critical aspect of semiconductor technology that involves reducing the physical dimensions of CMOS devices to improve performance, increase density, and reduce power consumption. This process has been a driving force behind the advancements in microelectronics, enabling the production of faster, smaller, and more efficient integrated circuits.

Traditional CMOS scaling methods have been approaching the physical limits of accessible materials since 2005. Over time, new technologies have emerged, including the gate-all-around field-effect transistor (GAAFET) design and the fin field-effect transistor (FinFET).

Challenges Faced by CMOS Scaling Technology

CMOS has been a key driver of advancements in microelectronics. However, this scaling presents several challenges that must be addressed to continue improving performance and efficiency. When MOSFET dimensions get closer to the 3-nanometer range, the device has several issues.

These challenges include:

● Power density and heat dissipation

● Increased complexity and cost

● Structural limitations

● Short-channel effects

● Leakage current

● Cross-talk, interferences, closeness, and density restrictions

● Overheated failure mechanisms

● Quality and reliability rule constraints

The material's intrinsic potentials, like the thermal voltage, cause the heat dissipation to grow exponentially concerning the MOSFET transistors' on-chip density. The device eventually becomes unfeasible or impractical due to cooling requirements.

Interconnect delay is significantly influenced by second-order phenomena such as electron migration, quantum tunneling, and parasitic capacitances, which slow down on-chip component communication and reduce processing power.

Addressing the Challenges Faced by CMOS Scaling Technology

Worldwide groups and research teams have identified on-chip optical communication and interconnects as a potential solution for the CMOS scaling issue. On-chip optical communication and interconnects represent a significant advancement in the design of integrated circuits, particularly for applications requiring high data throughput and low power consumption.

Since optical communication would greatly reduce the need for the metal interconnects currently used as communication lines between on-chip components, it would significantly address CMOS technology's challenges.

What is on-chip optical communication?

This technology uses the properties of light to transmit data across chips, offering several advantages over traditional electrical interconnects. By doing this, the second-order phenomenon issue that was previously described would be resolved, and additional transistor integration would be possible on the chip surface.

Reduced Complexity and Size

By substituting large, intricate electronic metal interconnects with optical connections, space may be freed up, and communication can be shrunk. Optical interconnects can solve many problems currently present due to their metal lines, size, and complexity.

Improved Space and Structure

One of the most important issues in microelectronics is efficient scalability. Metal electronic interconnects, like copper wires, require space to accommodate their dimensions, plus insulation for the horizontal and vertical spaces between them.

On the other hand, waveguides that are compact in size can be used to construct on-chip optical interconnects. Similar to regular lines, these waveguides are often several hundred nanometers wide and need space between them; yet, this still uses space more efficiently than metal lines.

Stacking waveguides is more compact than stacking metal lines. The waveguides, typically constructed of silicon oxide or polymers, can be built directly on the chip substrate, guiding the optical signals and enabling compact data routing within the chip.

Improved Power Density and Heat Dissipation

Splitting lines are necessary for metal electronic interconnects to lower currents and stop overheating failure mechanisms, including electromigration and heating. Furthermore, quality and reliability regulations require additional line enlargement even after splitting. Importantly, this complexity rises with the number of data routes and linkages. However, many data streams may be easily delivered across a single waveguide using optical communication, eliminating the requirement for separate wiring layers and streamlining the routing procedure.

Reduced Proximity and Interference

Another issue that optical communication addresses is the close proximity and occasionally high density of metal interconnects, which can result in undesirable interference and crosstalk from electromagnetic coupling. This tendency is lessened because the light signals in optical communication interact differently from one another due to differences in wavelengths, polarization modes, etc. Higher data transfer density within a specific chip region is thus made possible by the closer packing of optical interconnects.

Compact Size

The chip's performance can be simplified and improved by having optical and electronic components on the same wafer. In addition to electronic components, devices such as lasers, modulators, and photodetectors can be mounted directly on a silicon substrate. Space can be saved since separate, external optical devices are not needed when these optical components are integrated with other chip parts.

In conclusion, on-chip optical communication and interconnects offer a promising solution for overcoming the limitations of traditional electrical interconnects in high-performance computing systems.

Summarizing the Key Points

● CMOS technology has been essential in the semiconductor industry for over 50 years, particularly in the microprocessor domain.

● Scaling CMOS devices presents challenges such as power density, heat dissipation, interconnect delays, etc.

● On-chip optical communication offers a promising solution to address these challenges by reducing reliance on metal interconnects.

● Optical interconnects can enhance data transfer density and efficiency, allowing for the integration of compact electronic and optical components on the same chip.

● The integration of optical communication technologies can improve performance and lower power consumption in future microelectronics.

Reference

Avi Karsenty, “Overcoming Silicon Limitations in Nanophotonic Devices by Geometrical Innovation: Review,” IEEE Photonics Journal 15, no. 4 (August 1, 2023): 1–19, https://doi.org/10.1109/jphot.2023.3295077.

Sanjay Vidhyadharan, “VLSI Design CMOS Scaling Technology Scaling, Constant Field Scaling, Constant Voltage Scaling.,” January 23, 2022,https://www.youtube.com/watch?v=2j3A3HdAcRc.