Researchers Design Innovative Color Sensors that are Less Expensive to Manufacture

As a part of the FOWINA project, novel color sensors with a special microlens arrangement have been created by researchers from the Fraunhofer Institute for Integrated Circuits IIS in Erlangen and the Fraunhofer Institute for Silicate Research ISC in Würzburg.

FOWINA – Accurate micro color sensors for chip-level integration. (© K. Selsam, Fraunhofer ISC)

The sensors can be realized directly on the chip and integrate numerous functions in very minimal space. Their very slim design makes the sensors appropriate for a broad range of applications, such as in color-modifiable LED lamps or mobile devices.

Color sensors are used in LEDs, displays, and other tech devices to produce true colors. Their fabrication requires the use of special nanoplasmonic structures. These structures filter the incident light, permitting only precisely demarcated regions of the color spectrum to reach the detector surface. The ability to manipulate the angle of incidence is pivotal to the correct functioning of the color filters. Conventional sensors comprise of macroscopic elements to enhance the filter’s accuracy and avoid untrue colors by masking out light at unwanted angles, but these added elements greatly increase the component’s build size.

Ultraslim sensors for cameras and smartphones

To handle this disadvantage, the two Fraunhofer Institutes involved in the FOWINA project, IIS and ISC are developing an all-in-one solution that incorporates multiple functions in a minimum of space. Color-filter structures, evaluation circuits for signal processing, angular filters to control the incident light, and photodiodes to turn light energy into electrical energy are all included in the color sensor chip. This very compact design makes it possible to construct novel, ultraslim color sensors that can be added into smartphones, cameras, and a number of other products. FOWINA is an in-house project sponsored by Fraunhofer as part of its internal program of SME-oriented research. The German acronym stands for “controlling the angular spectrum of nanostructured color sensors using micro-optical beam-shaping elements.”

In addition to their high scale of integration, which enables a maximum of functions to be incorporated into a small surface, the novel sensors are easier and thus less expensive to fabricate than their precursors. Fraunhofer IIS is responsible for designing the sensor IC including the nanoplasmonic color filters. The latter can be made cost efficiently along with the photodiodes and evaluation circuits using one and the same CMOS process, i.e. a single technology.

Fraunhofer ISC is in charge of fabricating the arrays of microstructures that work as the angular filter elements in the sensors. “We use the advanced technique of two-photon polymerization, which enables the creation of almost any type of microstructure or structured surface,” says Dr. Sönke Steenhusen, a research scientist at Fraunhofer ISC. To accelerate the manufacturing process, Fraunhofer ISC uses nanoimprint technology—an extremely precise and field-established lithographic method—to copy the microstructures. This technique also allows various structures to be integrated on the same substrate.

Restricting the angle of incident light

During the course of the FOWINA project, Fraunhofer ISC has realized the best-possible color-filter performance by limiting the angle of incident light to a tolerance range of +/-10 degrees using micro-optical structures. This enables the color of LEDs, for instance, to be actively modified. Another advantage is the extremely high surface accuracy of the microlenses, which focus the light on the color filters in a targeted manner. The material used by Fraunhofer ISC to fabricate the arrays is a special inorganic-organic hybrid polymer, which displays high thermal, chemical, and mechanical stability and can be easily altered to the needs of specific applications by adjusting its molecular structure.

At present, the two collaborating Fraunhofer Institutes are enhancing the design and manufacturing processes for the color sensors, with the aim to scaling up to industrial applications and, at a later stage, mass production of the sensors.

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