CIS: The force behind the sensor proliferation

Translated from EEtimes

Our focus on technology is often on the main drivers of electronics: CMOS logic and memory. While these may account for the majority of devices built using Lam’s research tools, creating systems that are useful to humans requires many other specialized technologies. Many of these technologies impact our interactions with electronics. For example:

Sensors – CMOS Image Sensors (CIS) and Micro-Electro-Mechanical Systems (MEMS)

Radio frequency circuits for sending and receiving wireless signals

Power electronics, made of MOSFETs, insulated gate bipolar transistors (IGBTs), and bipolar CMOS-DMOS (BCD) technology

Optics - Displays and Photonics

These technologies impact a wide range of systems in the commercial, industrial, and automotive markets. Of recent significant interest are Internet of Things (IoT) devices and cellular technology (particularly 5G), both of which make extensive use of specialized technologies. These markets will account for 30% of global IC demand by 2023. (IC Insights, McClean and OSD Reports, 2019)

Camera in the spotlight

Among sensors, CIS sensors have become particularly important recently. In the consumer market, more and more cameras are being stuffed into smartphones. They have become the main gimmick in mobile phone marketing. In 2017, Apple spent about 10% of the information release time of the iPhone X promoting the camera; two years later, it spent 49% of the time promoting the iPhone 11's camera instead of other features.

Future car designs will use multiple cameras of different types in addition to radar and/or lidar to aid advanced ADAS and autonomous driving. These cameras will effectively surround the car, eliminate blind spots, and provide better situational awareness for both the driver and the autonomous system.

The CIS market is expected to grow significantly over the next few years, with a unit compound annual growth rate (CAGR) of 6.6% through 2024. The fastest-growing segments of the CIS market will see significantly higher growth rates, with consumer growing at a 24.6% CAGR and automotive growing at a 14.1% CAGR.

CIS categories include cameras that record visible light and cameras that work in infrared (IR) or near infrared (NIR) environments. Visible light cameras provide a video stream for identifying objects and the overall environment around them. In the infrared realm, it is becoming increasingly useful in facial recognition using structured light, which does not require illuminating the face with visible light.

Build the best CIS chip

Early CIS chips received illumination from the top or front of them. This is the most obvious way to get photons through the thin silicon layer and into the sensing device. But the downside is that metallization and other chip features obscure some of the light—only light that passes through these obstacles is captured.

Newer CIS devices emit light on the backside and have no such obstacle. Of course, this means that the backside of the wafer must be thinned to capture as many photons as possible without having them scattered and absorbed by the thicker wafer. Because the backside of the CIS wafer is illuminated, the front side can be combined with other wafers without affecting the perception of light. So new integration is possible by combining image sensors, memory and other logic functions in a single package.

However, doing so requires advanced technologies to achieve the most efficient light capture. Two typical examples: deep trench isolation (DTI) and through silicon via (TSV).

DTI allows pixel circuits to be more effectively isolated from each other. When photons enter a pixel, they are scattered and move around - potentially drifting from the pixel they entered into to a neighboring pixel. When high resolution is required, this creates a blurring effect as pixels bleed into one another. DTI effectively builds a wall between pixels, keeping the number of photons contained and producing a sharper image.

Die stacking relies on TSVs. The front side of a die has all the metal interconnects and any traditional substrate, so when you connect the front sides of two dies together, those signals can be combined. But the back side of the die doesn't have those signals. So when you stack the back side of a die onto one front side or another, there has to be some way to get the signals from the front side of the die to the back side of the die so they can connect. TSVs are deep metal "pipes" drilled through the silicon that do this.

DTI and TSV are important technologies that require careful and precise processes to be effective. These are technologies that Lam is relatively good at implementing, and as the CIS market grows, Lam expects the demand for these technologies to grow significantly. While CIS and other specialized technologies may not receive the same sustained attention as mainstream technologies, they are just as important to the effectiveness of the systems we will see and interact with in the coming years.