25-08-2021 | | By Sam Brown
Recently, French startup Silina demonstrated its ability to curve 275 CMOS sensors in a single manufacturing step while Sony revealed its latest curved image sensor. What advantages does a curved sensor have over a flat sensor, what exactly has been demonstrated, and how will the benefits of curved sensors help everyday items?
Image sensors used in the vast majority of cameras are based on semiconductor technology thanks to the ability of semiconductor devices to detect photons (i.e. light). To keep sensor costs low and manufacture as simple as possible, they utilise the same construction methods as typical semiconductor devices, meaning that they are flat in design. However, flat image sensors are far from optimum design, and their flatness introduces many optical defects and artefacts that can ruin images.
The first challenge presented by flat sensors is that focus is reduced towards the edges of the sensor. This is because lenses used to focus light are curved, and their focal point changes with angle.
The second challenge arises from the first challenge with the changing focal point. Projecting a curved image onto a flag sensor makes the image less bright towards the ends of the sensor. This creates images that appear to be darker around the edges and thus reduces the overall image quality. The third challenge arises from both the first and second challenges. These optical defects can be fixed using multiple lens stages, introducing added weight and increased cost.
If a curved sensor could be developed, then the cost of camera systems would significantly reduce while the quality of images would increase. However, creating a curved image sensor is not easy as semiconductor manufacturing techniques use additive and subtractive processes on a flat substrate. Creating curved surfaces is more complex, considering that semiconductor manufacturing processes also rely on centrifugal methods to deposition some layers.
However, researchers from a startup company Silina have recently demonstrated their ability to curve 275 standard CMOS sensors in less than an hour. Silina themselves do not manufacture CMOS or CCD sensors, but they can bend pre-existing products already on the market to custom specifications. Their process also allows controlling the curvature of the sensors to create various shapes, including spherical, aspherical, freeform, and custom shapes. However, what is not clear is precisely how the company have achieved the ability to curve the devices; it is most likely that heat is used to make the silicon-based sensors pliable, but other methods could be used.
Sony has also been working on curved sensors for several years but has recently filed for a new patent that describes an entire camera module with a simplified lens and sensor. While such a design may not be practical for many years, it does demonstrate how imaging technology could continue to improve.
While curved sensors improve image quality, it is the use of simpler lenses that will make them desirable to manufactures. In particular, smartphones have the most to gain from curved sensors as they would produce smaller, more compact cameras utilising lighter lenses. This weight reduction will also benefit drones and other remotely piloted devices by reducing fuel usage.
Using a simpler lens also reduces the need for complex image preprocessing, which potentially allows for higher-speed operation. Furthermore, having all points of an image correctly focused could also lead to improving auto-focus to better track objects as they move.