Some analysts claim that Moore’s Law is coming to a screeching halt with the arrival of 3nm. Most of the time this conversation centers around problems with quantum effects in scaled-down devices; however, another (and arguably more significant) problem is the interconnect, which is increasingly becoming a system bottleneck.
A less-discussed scaling trend is that of the interconnect. Image used courtesy of Stanford University
The combined need for device improvement beyond Moore’s Law, along with the diminishing functionality of traditional interconnect schemes, are paving way for the field of silicon photonics. While this field isn’t necessarily new, it is gaining traction. In December 2020, Intel discussed at its annual “Lab Days,” that the company is actively researching silicon photonics “to overcome the limits of electrical I/O by advancing the integration of optical and silicon technologies for future data centers and networks connected by light.”
In this article, we’ll discuss challenges with conventional ICs, the need for silicon photonics, and highlight recent research in which scientists used silicon photonics to create an improved ultrasound sensor.
The Interconnect Bottleneck
There are two overall trends happening in the IC world currently: devices are getting smaller and chip areas are getting bigger (due to increased integration). The result of these trends is that the interconnect, not the device, is actually becoming one of the biggest bottlenecks in IC design.
Interconnects on an IC are directly affected by their geometry and position near one another. Image used courtesy of Stanford University
Interconnects, in general, are being forced into smaller geometries with decreased width and thickness while global interconnects, like the clock line, are also becoming longer to accommodate larger chip sizes.
Additionally, increased integration is creating more metal layers that are closer to one another. All of this results in hugely increased interconnect impedances, where parasitic resistances, capacitances, and inductances are becoming more prominent than that of the transistors themselves.
The interconnect is becoming an IC design bottleneck. Image used courtesy of Grzela et al.
As such, the interconnect is one of the largest contributors to both system delay and power consumption in the form of data movement energy.
Silicon Photonics: A Possible Solution
The solution to these problems, as many believe, is silicon photonics.
Silicon photonics use light for communication. Image used courtesy of Luxtera and IEEE Spectrum
Silicon photonics is a form of IC where photonic integrated circuits are built on silicon. In these circuits, data is transmitted and moved around the chip via light through waveguides as opposed to electrical signals through copper interconnects. The result is a technique that side-steps the increasing concern of the interconnect bottleneck, creating faster ICs without having to scale down the transistor.
The technology has been mostly confined to research to this point, but there is a strong push to eventually bring it to the industry for speed and power-hungry use cases like data centers.
Ultrasound Sensor Based on Silicon Photonics
Proving the value of silicon photonics, a group of researchers from the Interuniversity Microelectronics Center (IMEC) in Europe has developed a silicon photonics-based ultrasound sensor.
Conventional ultrasound sensors use arrays of piezoelectric devices that rely on mechanical resonance at specific ultrasound frequencies. Unfortunately, these techniques are limited by many factors. For instance, the smaller the piezo, the less sensitive it is, hindering its ability to be built into large arrays.
Ultrasound sensor layout. Image used courtesy of Westerveld et al.
In their article in Nature, the researchers propose a new method that uses a “split-rib” silicon photonic waveguide. The system consists of a main part placed on a movable membrane along with a fixed rib on a substrate. The two are placed in a 15nm-wide gap containing a strong electric field.
When an ultrasound wave distorts the membrane, the electric field will greatly vary the waveguide’s refractive index, which will change the resonance properties of the fixed rib. The researchers were able to use tunable lasers to read out this wavelength, producing an accurate signal.
The Power of Light
The ultrasound sensor proposed in this research is said to have a sensitivity 100 times greater than conventional sensors, showing the power that silicon photonics can offer. As Moore’s Law comes to an end and interconnects become increasingly difficult to work with, a switch to silicon photonics may be just the solution to keep the field advancing.