REVIEW PHOTONICS WEST REPORT


One step closer to commercial graphene-CMOS sensors


Greg Blackman takes a look at the first image sensors based on graphene-CMOS integration that were on display at the show


B


roadband imaging based on graphene-CMOS integration has been demonstrated at this year’s Photonics


West. Frank Koppens from the Institute of Photonic Sciences (ICFO) in Barcelona, Spain presented several applications of graphene- CMOS integration, including the first monolithic integration of an electronic CMOS integrated circuit with graphene, operating as a broadband photodetector. Meanwhile, Finnish company Emberion was exhibiting a new VIS-SWIR VGA (640 x 512 pixels) image sensor comprising a graphene photodetector array monolithically built onto a tailor-made CMOS chip. Graphene is a one-atom thick layer of carbon. It has impressive material properties, one of which is its broadband absorption – detectors have been built for visible, infrared and even terahertz radiation. The EU is half way through its €1 billion Graphene Flagship programme, where the goal is to take the material out of the laboratory and make commercial products out of it. Koppens commented during his


presentation at Photonics West that a key advantage of graphene is the material can be combined with platforms like silicon CMOS. ‘You can take advantage of all the power


that already exists in silicon CMOS technology and leverage the billions that have been invested in silicon CMOS and expand the functionality,’ he said. Because graphene is a 2D material it can be


transferred on top of silicon rather than having to grow the material. The layers are bound via van der Waals forces, which are still extremely strong, Koppens noted, because the material is only one atom thick. Koppens’ team took a CMOS integrated


circuit and replaced all the photosensors with graphene, layering the material on top of the silicon CMOS circuitry. The graphene was connected to the circuitry with vertical vias. The ICFO chip consists of 388 x 288 pixels. Photodetectors have already been made


with graphene that can sense across a broadband spectral range, but these are still based on one single pixel. The graphene in the ICFO chip was sensitised with colloidal quantum dots to make it sensitive from 300nm to 2,000nm, covering UV, visible and infrared light. ‘At this point there is no technology that can do that on the market,’ Koppens said. Infrared imagers can be costly pieces of equipment; the sensors are not based on silicon and can require cooling. A broadband


“You can take advantage of all the power that already exists in silicon CMOS technology and leverage the billions that have been invested in silicon CMOS and expand the functionality”


Leti demonstrates curved optical sensors


The French research institute, Leti, has developed a prototype technology able to curve optical sensors and micro-displays. Curving components like CMOS or CCD imagers can improve their performance, enhance the field of view, and compensate for optical aberrations.


Leti demonstrated its prototype


called Pixcurve at SPIE Photonics West in San Francisco from 27 January to 1 February. ‘Curved sensor technology is a


disruptive approach for imaging applications such as photography, videography, computer vision,


surveillance and many other applications,’ said Bertrand Chambion, one of the co-authors of a paper presented at Photonics West. ‘In recent years, we have seen very strong interest in curved electronics, particularly for opto-electronics systems whose performance improves, while size, complexity and cost are reduced.’ The demonstration at the


show was based on a 1/1.8-inch format, 1.3-million pixel CMOS image sensor. The standard sensor structure consists of a 7.74 x 8.12mm silicon die glued on a ceramic package. Electrical


connections are wire bonded from the die to the package surface and, then, to the interconnection board. A glass cover is placed on top for mechanical protection. Leti’s technology uses a grinding


process to get the sensor below 100µm thick, which makes it mechanically flexible. It is then glued onto a curved substrate, which determines its final shape. A wire bonding process


developed for electrical connections is used to prevent damage on the thinned dies. The radius of curvature is R=65mm. Leti’s process is compatible with


any sensor size and with large- scale manufacturing processes. Potential uses for the technology include curving sensors for mobile phones, cameras, telescopes, medical imaging tools and industrial control equipment. Other uses include infrared sensors for astronomy, defence, drones, as well as micro-displays for automotive applications, and augmented and virtual reality. Leti also presented a paper at Photonics West on its new lens- free microscope, which provides point-of-care diagnosis for spinal meningitis. The microscope


Emberion’s image sensor comprising a graphene photodetector array built onto a tailor-made CMOS chip


imager operating into the infrared and made with existing silicon CMOS techniques could open up a range of uses from driverless cars to wearable electronics. ‘It’s [graphene-CMOS integration] not


better than silicon,’ Koppens said. ‘It’s almost impossible to compete with silicon CMOS [for visible light detection]. But compared to InGaAs, it’s comparable or better, but you also get the visible wavelength range for free.’ Sensitivity at each of the pixels on the ICFO chip is high. The team has demonstrated imaging through fog with the sensor, which is important for automotive and self-driving cars. The sensor has also been shown to be sensitive to starlight, meaning it can be used for night vision, which requires extremely high sensitivity between 1,500nm and 2,000nm. Emberion’s VIS-SWIR VGA image sensor


14 Electro Optics March 2018


@electrooptics | www.electrooptics.com


Emberion


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