SWIR (short-wavelength infrared) image sensor is very important in the quality inspection of various industries. The SWIR spectrum is a series of infrared spectra outside the visible light band, which can be used for product inspection to detect tiny defects or pollutants that are invisible to the human eye. The SWIR image sensor recently developed by Sony has achieved a smaller size and more pixels by deploying the industry’s smallest 5 µm pixels. This image sensor is unparalleled because its dynamic range also covers the visible light spectrum. Behind this unique image sensor is an unprecedented challenge story in the development process. The story of how to overcome these challenges reveals the expertise that only Sony semiconductor Solutions Group (hereinafter referred to as the “Group”) possesses.

Sony first pointed out that SWIR (Short Wave Infrared) is a special range of infrared. Thermal imaging cameras can be used to detect certain ranges of infrared light to measure, for example, the temperature of the human body. Others have different attributes. SWIR is the wavelength range between 0.9 and 2.5 µm.

It is different from visible light in terms of reflection and absorption, and SWIR image sensors use these characteristics to visualize things that the human eye cannot perceive. At the same time, some materials allow short-wave infrared light to penetrate, which makes it possible to inspect what is behind these materials.

This kind of light scatters less than visible light, and this feature is very useful for visualizing objects in a foggy or foggy environment.

Thanks to its original technology, the SWIR image sensor developed by Sony can not only visualize up to 1.7 µm, but also capture images in the visible spectrum.

  Sony talks about the smallest SWIR image sensor ever

From the application point of view, Sony pointed out that SWIR light has the characteristics of penetrating silicon and visualizing fog. Using these characteristics, image sensors can be applied to various industries, including semiconductor and food production, mainly for inspection purposes. Taking the semiconductor industry as an example, semiconductor wafers are usually made of silicon that absorbs visible light. Using a traditional digital camera under visible light conditions, it looks like an ordinary metal plate. At the same time, short-wave infrared penetrates silicon. Therefore, when visualized in an image captured by a camera equipped with a SWIR image sensor, the chip looks like a transparent glass plate, and cracks in the wafer or contaminants in the semiconductor can be detected.

In addition, water is transparent under natural visible light, but it absorbs specific wavelengths in the short-wave infrared band, so it will appear colorful in the captured image. This feature can be used to detect moisture, so the sensor can be used for food detection, such as detecting bruises and scratches on fruits that are difficult to detect under visible light.

When asked about the difficulty of designing such sensors, Sony responded: SWIR image sensors have been used in many industries, but they have major technical problems. One of them is the pixel size. The pixels in traditional sensors are large, and the size makes it difficult to place multiple pixels in one sensor. It poses a challenge to improve the resolution. In terms of image quality, it is also difficult to obtain clear images, so camera manufacturers have to arrange heavy post-processing. In addition, since the sensor is an analog type, the camera must convert the signal into digital data, which puts a lot of pressure on the camera’s processing circuit and requires design knowledge to implement it. They further pointed out that traditional SWIR image sensors use bump connections, which include metal balls to connect the indium phosphide layer and the silicon layer in the pixel. In order to use bump connection to reduce the pixel size, it is necessary to accurately align the metal balls with the micron-level bump pitch, which is difficult to achieve by itself. This technical difficulty hinders the miniaturization of pixels, leading to an increase in the price of image sensors.

It is because of the consideration of the use of bump connections that hinder the miniaturization of SWIR image sensors. Sony uses Cu-Cu connection to solve this problem, which is a stacking technology that Sony has developed for image sensors over the years. According to Sony, when stacking the back-illuminated CMOS image sensor part (top chip) and the logic circuit (bottom chip), it is a technology that provides electrical continuity through the connected Cu (copper) pads. Compared with through-silicon via (TSV) wiring that penetrates through electrodes around the pixel area, this method provides more design freedom, improves productivity, allows more compact size, and improves performance. This technology will make it possible to align pixels with fine pitches.

At the same time, conventional image sensors use silicon as a photoelectric conversion layer, but this material does not absorb light in the SWIR range. Therefore, Sony uses indium gallium arsenide (InGaAs) as a photodiode material that can absorb the SWIR spectrum and convert light energy into electrical signals. This material has never been used in Sony’s image sensors before, but another division of Sony has compound semiconductor technology that produces InGaAs.

In addition, the group has circuit technology that converts light energy into digital signals. In fact, Sony has mastered the necessary technologies to meet the challenges, including the compound semiconductor technology required for SWIR image sensors. Sony believes that by combining these technologies, it is possible to realize a SWIR image sensor at an unprecedented miniature scale and increase the number of pixels.

  Sony talks about the smallest SWIR image sensor ever

Sony said that the company already believes that the group’s technical expertise can be successful, so they are focusing on deploying the group’s technology to make the final image sensor useful to customers. The first thing Sony did was to use a column-parallel A/D conversion circuit, a technology developed for CMOS image sensor technology, to digitize SWIR image sensor output. This eliminates the need for camera manufacturers to provide A/D conversion components for post-processing. It also helps solve the image quality issues that Sony’s competitors must address. Therefore, Sony believes that it will help significantly reduce the image editing steps for camera manufacturers.

Sony continues to point out that the SWIR image sensor has a photodiode made of InGaAs. Indium phosphide (InP), which is essential for the manufacture of InGaAs, forms a layer that blocks visible light. However, the Cu-Cu connection that Sony is considering uses structurally allows the thickness of the InP layer to be reduced. This prompted us to develop a unique SWIR image sensor that can also capture images under visible light.

Sony further said that this is the first SWIR image sensor developed using InGaAs in the history of the group. Therefore, the company thoroughly reviewed the challenges to be solved and there were more than 300 in the early stages of development. With further development, Sony found more problems that needed to be dealt with. It turns out that there are so many challenges waiting. There are also some unique phenomena of InGaAs, which are unknown to silicon, and we have to discuss how to deal with these phenomena in the final product.

Sony finally said that the increased number of pixels through pixel miniaturization has brought huge benefits. The image sensor can identify the smallest damage that could not be detected before, thereby improving the quality of detection. Digitization will make camera design easier, which in turn will help save costs, etc., and make image sensors more affordable for customers. Since Sony believes that it can bring better quality inspection to the world, Sony believes that this SWIR image sensor will play an important role in it.

“Our SWIR image sensor has the group’s widely acclaimed features, such as ease of use, high performance, etc.. I believe it will cause a sensation in the market. I think these features will also help pave the way for new applications.” Sony Finally.

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