The evolution of mobile image sensors is ultimately tied to advances in pixel technology. The growing demand for high-quality images in smaller, thinner devices has made "fine pixel" technology a core mission for the mobile image sensor industry.

In line with this trend, Samsung System LSI continues to advance technology with its experience in small pixel image sensors. The recently released mobile image sensor ISOCELL JNP is the industry's first sensor to use nanoprism technology, pushing the physical limits of pixels.

Let's explore how Nanopris, the first technology to apply Meta-Photonics to image sensors, was born and how it is implemented in ISOCELL JNP. 

Smaller pixels, more light

The sensitivity of image sensors is a key factor in achieving clear and vivid images. Pixel technology has been evolving to capture as much light as possible. For example, there have been various technologies such as front-side illumination (FSI) to back-side illumination (BSI), as well as deep trench isolation (DTI).

In particular, in order to achieve high-resolution images without increasing the size of smartphone camera modules, technology has been moving towards smaller and smaller pixels. However, this gradually reduces the sensitivity of the unit pixel and causes image quality degradation due to crosstalk between pixels. As a result, the problem of drastic image quality degradation in low-light environments is unavoidable.

To solve this problem, Samsung introduced the front deep trench isolation (FDTI) structure to create a physical barrier between pixels, and developed ISOCELL 2.0 to even isolate the color filter on top of the pixel. In addition, Samsung also considered innovating the optical structure of the pixel itself, even using peripheral light that the existing structure cannot receive. Nanoprism was born with this consideration. 

Nanoprism: Refracting light to collect more light 

Nanoprism is a new technology first proposed in 2017, based on the meta-photonics light source technology accumulated by Samsung Advanced Institute of Technology (SAIT) for many years. Unlike the super-lens research that was active in the field of meta-photonics research at the time and aimed to minimize light dispersion, nanoprism uses the reverse idea of maximizing dispersion to separate colors. Nanoprism is a prism structure based on a metasurface that can perform color separation.

So, what changes does it have compared to existing pixel structures? In existing microlens-based optical systems, the microlenses and pixel color filters are matched in a 1:1 ratio, so the pixels can only receive light of the color corresponding to the color filter of each pixel. In other words, there is a physical limitation that only light of the same size as the defined pixel can be received. 

However, nanoprisms set up an optimized light path by placing a nanoscale structure in the place of a microlens, so that light can be directed to each color-matched pixel. In short, the amount of light received by each pixel increases because the light lost due to color mismatch can be sent to adjacent pixels through refraction and scattering of light. Nanoprisms enable pixels to receive more light than existing microlens structures, and have the potential to improve the problem of reduced sensitivity caused by smaller pixels.

Nanoprisms in Image Sensors

Commercializing Meta-Photonics technology for image sensors is a challenging task. Ensuring customer reliability and technical readiness is critical. In order for the product to function properly, not only does the structure of the nanoprism need to be realized, but dozens of indicators must also be met.

Samsung's relevant teams worked closely together to repeat the design-process-measurement cycle, considering and reflecting various situations from the initial design stage and establishing a reliable verification procedure to do their best to ensure performance.

As the name of Nanoprism suggests, it is extremely difficult from process development to mass production because precise and complex nanometer (nm) structures must be realized in pixels. In order to put this new technology into practice, special technologies and methods were introduced, including CMP (chemical mechanical polishing) and low-temperature processes for nanoprism realization, and TDMS (thermal desorption mass spectrometry) for image sensor production.

ISOCELL JNP for brighter, clearer images

ISOCELL JNP with nanoprism has been mass-produced this year and is used in recent smartphones to further enhance the user experience. By receiving more light without loss, bright and clear photos can be taken even in difficult lighting conditions. In fact, ISOCELL JNP with nanoprism is 25% more sensitive than the previous generation ISOCELL JN5 with the same specifications.

Of course, increasing the size of the image sensor can improve the overall performance of the camera, but in the field of mobile devices, there is a limit to increasing the size of the image sensor indefinitely due to design limitations such as the "camera bump". Samsung System LSI is trying to break through this limitation with nanoprism technology. This technology improves the sensitivity and color reproduction of each pixel even when the pixels are getting smaller and smaller, and is applied to ISOCELL JNP.

The demand for high-resolution images in the mobile market will continue to grow. Therefore, the trend of pixel miniaturization will continue. Even if the pixels become smaller, pixel technology needs to be developed to ensure high sensitivity, quantum efficiency and noise reduction. Nanoprism technology is one of the technologies that improves sensitivity, and Samsung aims to innovate further in the direction of exceeding existing physical limitations.

Building on this collaboration, continued cross-functional and cross-team efforts are aimed at exploring new directions for next-generation image sensor technology.

Source: Content compiled from Samsung

Reference link

https://semiconductor.samsung.com/news-events/tech-blog/nanoprism-optical-innovation-in-the-era-of-pixel-miniaturization/

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