With the continuous advancement of modern technology and increasingly fierce market competition, the life cycle of products is becoming shorter and shorter, so manufacturers must take some measures to speed up the product development cycle. In addition, the application of vision systems in various industries is becoming more and more extensive, and multiple factors such as its performance and cost need to be considered at the same time.
In this case, manufacturers can focus resources on the added value of the vision system, such as improving performance, adding functionality, improving reliability, and so on. Through this method, the market competitiveness of the vision system can be improved, and a larger market share and profit can be obtained.
In September 2022, Teledyne e2v launched Optimom 2M, the first in a series of MIPI CSI-2 modules, to address this challenge. The module combines the latest innovations in imaging and optics into a one-stop imaging solution by mounting a proprietary image sensor onto a panel with fixed lens or multi-focal lens technology. But what exactly are these innovations, and how do they work? What benefits do they bring to vision-based systems?
Image Sensor Innovation
The Optimom 2M module uses Topaz 2M, a 2-megapixel global shutter CMOS image sensor that combines multiple innovations, from pixel structure to packaging to chip design itself.
In a world where pure product performance is the sole driver of product development, vision system manufacturers choose the largest possible pixel size to maximize the sensitivity and full well capacity of the device. However, in the real world, price, form factor, and power consumption are all important considerations, so vision system manufacturers must balance their maximum performance within size and cost constraints by finding the image sensor with the best optoelectronic performance. The desire to improve the optical performance of the system while also being able to accommodate a certain optical format.
Depending on the target optical surface, the maximum acceptable pixel size can become a technical challenge. Furthermore, moving from one optical format to a smaller one (eg, from 1.1 inches to 1 inch) often implies a substantial reduction in pixel size, as highlighted in Figure 2.
Figure 1｜Topaz 2M sensor and finger size comparison
The Topaz 2M has an extremely small global shutter pixel, which allows it to match compact and cost-effective 1/3" lenses, while still maximizing sensitivity and signal-to-noise ratio. This pixel, developed by TowerJazz using its 65nm technology, enables it to perform global shutter operation in a small size of 2.5μm by utilizing the concept of a shared pixel structure. In the Topaz 2M sensor, the 8T shared pixel structure is adopted, and the two pixels on the diagonal share 8 transistors, so the advanced functions of the 6T pixel structure are combined, such as intra-pixel correction (also known as CDS or correlated double sampling), And the high sensitivity of the 4T structure, because the surface of each pixel only occupies 4 transistors.
Well-thought-out optical stack
Based on this structure, Topaz 2M sensor and Optimom 2M module have higher sensitivity thanks to the pioneering optical stack structure on top of the pixel. The pixel pitch is optimized with a gap-free top lens to avoid light loss and unwanted reflections, but the real innovation lies in this "dual light pipe" architecture, which passes micro-fibres (differential reflections) created in the sensor's optical stack. high-efficiency materials) direct the light onto a photodiode.
Figure 3 | Cross-sectional view of the optical structure of an integrated pixel. The image shown in Figure 3 shows a cross-sectional view of the optical stack embedded in the product.
Chip packaging
In addition to optimizing pixel size and optical structure, image sensors are now benefiting from advances in packaging technology that reduce sensor cost, weight and form factor. In recent years, wafer-level packaging technology has flourished in the market, especially in consumer applications such as mobile, automotive or wearable devices.
While traditional ceramic (CLGA) packages have been used in the industry for many years, recent technological advances in shrinking pixel size have opened the door to wafer-level packaging, even for high-end image sensors in industrial inspection, logistics, or robotics. Traditional ceramic (CLGA) packaging requires the die to be individually packaged into a ceramic structure with spaced pads on the back for connection to the sensor board, while wafer-level packaging is mass-produced.
In the case of fan-out wafer-level packaging, the silicon wafer is diced into individual sensor dies, which are all embedded in a reshaped glass-substrate wafer, which is then diced into individually packaged sensors. The optimization of process and package size goes a step further for another type of wafer-level packaging, chip-scale packaging, in which the silicon wafer is encapsulated directly into the material, eliminating the intermediate step of molding a glass substrate around it. These process technologies make image sensors smaller and more compact. For these two types of wafer-level packaging, the backside connection of the image sensor to the circuit board is ensured by balls that provide higher density connections, which is the solution for producing miniature and lightweight imaging for embedded systems such as drones or autonomous guided vehicles. An excellent solution for the program.
The latest combination of these pixel, sensor structure, and packaging innovations has allowed a new generation of image sensor form factors to be reduced by a factor of four in just five years, see Figure 4 for a timeline and example comparison. The package use sapphire window to meet their hermetic and transmittance requirements.
Figure 4 | Image sensor form factor evolution since 2016 as packaging and pixel technology improve
In addition to packaging technology, the design of the sensor die to be packaged also has an impact on the final system size. One of the key tricks available to image sensor manufacturers is to minimize the final system housing size by matching the center of the package to the exact same location as the center of the optics. Figure 5 illustrates the effect of optical center and package center mismatch, which is still seen in some image sensors today.
Figure 5 | Illustration of the space loss when the optical center of the die is not centered in the package
A new technology
While pixel shrinking has a positive impact on image sensor cost and size, it has a rather negative impact on optical system versatility, especially depth of field.
Depth of field can be defined as the difference between the closest and farthest distances at which an object can be captured with a sufficient level of sharpness, which decreases as pixels shrink and there is less tolerance for out-of-focus images. For applications that require capturing objects at different working distances, such as package tracking in logistics centers, system builders often look for smaller aperture optics (typically F/7.0 or F/8.0) to allow Maintain sufficient depth of field.
Unfortunately, stopping down the aperture comes at the expense of sensitivity, as less light passes through the lens and is captured by the image sensor. Therefore, the challenge of focusing technology now is to achieve a greater depth of field while maintaining the high sensitivity of the vision system. This is exactly what the multifocal lens technology developed in the Optimom 2M optical module solves, which combines a large aperture of F/4.0 with a wide working distance from 10cm to infinity.
Figure 6 | Left: Rear view of the Optimom 2M module with FFC/FPC connectors for controlling sensor and lens focus. Right: Front view of Optimom 2M module with embedded multifocal lens.
This proprietary lens stacking technology achieves these properties by adjusting the facet curvature of the lens to adjust the focal point. The control of the lens shape is ensured by means of I2C protocol signals, which are managed directly through the standard FFC/FPC connector on the back of the module board. The connector handles MIPI CSI-2 data output, clock management, and image sensor and multifocal lens control via I2C. This concept enables multi-focus to offer several advantages over other focus adjustment technologies, such as a fast response time of less than 1ms and resistance to electromagnetic effects.
Summarize
Optimom 2M optical modules achieve advanced optoelectronic performance and high versatility by utilizing multiple innovative technologies. The embedded image sensor combines innovations in pixel structure, optical stacking, and die packaging to achieve a small and lightweight design that can match inexpensive S-type lenses while maintaining high sensitivity levels. The optional integrated multifocal lens relies on a new focus adjustment technology to simultaneously achieve a wide working distance, high sensitivity and fast response time.
This article was written by Marie Charlotte Leclerc, Product Manager, Teledyne e2v (Grenoble, France). For more information, please visit https://www.teledyne-e2v.com/.
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