UV-transparent coating for image sensors

February 8, 2011
CMOS image sensors can now be covered with a transparent protective coating that is permeable to light in the UV and blue spectral range. Credit: Fraunhofer IMS

Image sensors as used in cell phones are partially color-blind. This is because of their coating, which prevents UV light from passing through. CMOS chips have as a result not been suitable for spectroscopy up to now. A new production process makes the coating transparent – and the sensors suitable for special applications.

They have been used as standard in multimedia electronics for a long time, and now they are making rapid inroads in high performance applications: CMOS image sensors are no longer only used in cell phones and digital cameras. The automotive industry, for instance, has discovered the potential of optical semiconductor chips and is increasingly using them in driver assistance systems – from parking aids and road lane detection to blind-spot warning devices. In special applications, however, the sensors that convert light into electrical signals have to cope with diffi cult operating conditions, for example high temperatures and moisture.

For this reason, CMOS devices are covered with a coating. This chemical compound forms hard layers which protect the sensor from mechanical influences and the penetration of moisture and other impurities. The protective coating is applied to the sensor in the final stage of CMOS semiconductor production. The process is called passivation, and is an industry requirement. Unfortunately, up to now this passivation has entailed a problem: the silicon nitride coating limits the range of optical applications because it is impermeable to light in the UV and blue spectral range. CMOS sensors for high-performance applications, used in special cameras are therefore partially color-blind.

Scientists at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg, Germany, have found a solution to this problem: “We’ve developed a new process step,” says Werner Brockherde, head of department at Fraunhofer IMS, “that allows us to produce a protective coating with the same properties but which is permeable to blue and .” The trick is to increase the proportion of nitrogen in the coating. “This reduces the absorption of shortwave light,” explains Brockherde.

In simplified terms, the new coating material will absorbless light of an energy higher than blue light, which means the sensor becomes more sensitive at the blue and UV range. “This makes CMOS image sensors suitable for use in wavelength ranges down to 200 nanometers,” states Brockherde. “With standard passivation the limit was about 450 nanometers.” To change the structure of the silicon nitride for the coating, the Fraunhofer research scientists had to fine-tune the deposition parameters such as pressure and temperature.

With this process development the experts have expanded the range of applications for CMOS image technology. This could revolutionize UV spectroscopic methods, which are used in laboratories around the world, significantly improving their accuracy. Likewise, CMOS image sensors stand to take up a new role in professional microscopy, e.g. in fluorescence microscopes, providing scientists with images of even greater detail.

Explore further: Cypress Completes Acquisition Of IMEC Spinoff FillFactory NV

Related Stories

Cypress Completes Acquisition Of IMEC Spinoff FillFactory NV

August 4, 2004

Belgian CMOS Image Sensor Leader Is Expected to Enable Cypress To Double Its Business in Cell Phones and Expand in Other Markets Cypress Semiconductor Corp. today announced that it has closed its acquisition of FillFactory ...

Kodak, IBM See Eye to Eye on New Image Sensors

September 17, 2004

Eastman Kodak Company and IBM will work together to develop and manufacture image sensors used in such consumer products as digital still cameras and camera phones. The collaboration will mate Kodak's image sensor technology ...

IBM Announces New CMOS Image Sensor Foundry Offering

July 14, 2005

IBM today announced the availability of technology and manufacturing services for complementary metal oxide semiconductor (CMOS) image sensors for use in camera-phones, digital still cameras and other consumer products. The ...

Vigilant windows

March 17, 2009

Is someone sneaking around in front of the window trying to break in? Windows and doors are now being sensitized to suspicious movements: they can detect whether and how quickly something is moving. If it is a person, the ...

Color sensors for better vision

October 5, 2009

CMOS image sensors in special cameras -- as used for driver assistance systems -- mostly only provide monochrome images and have a limited sensitivity to light. Thanks to a new production process these sensors can now distinguish ...

Image sensors for extreme temperatures

September 20, 2010

Image sensors which are used as electronic parking aids in cars or for quality control in production systems have to be able to withstand the often very high temperatures that prevail in these environments. Research scientists ...

Recommended for you

US ends bulk collection of phone data

November 30, 2015

The US government has halted its controversial program to collect vast troves of information from Americans' phone calls, a move prompted by the revelations of former intelligence analyst Edward Snowden.

How can people safely take control from a self-driving car?

November 30, 2015

New cars that can steer and brake themselves risk lulling people in the driver's seat into a false sense of security—and even to sleep. One way to keep people alert may be providing distractions that are now illegal.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.