To expand the usable spectral range of CCD / CMOS sensors, they can be provided with a specialized coating. When illuminated with UV light, this layer emits visible light, which can be detected by the sensor.
Such a coating lowers the lower end of the spectral sensitivity from 350nm up to 100nm. From 480nm, the coating is almost transparent, so that the sensitivity in the visible and in the near infrared is hardly affected. For a UV coating, especially sensors with elongated pixels are suitable. Some of these sensors can be offered as standard with UV coating (see table). For other sensors this is possible on request.
In a three-year research project, Eureca developed a new way of applying such a coating to image sensors. The development was funded by the »Central Innovation Program of the Mittelstand« (Zentrales Innovationsprogramm des Mittelstandes (ZIM)).
At the heart of the technology is Lumigen, which visibly fluoresces green under UV light and thereby makes the invisible radiation detectable for conventional CCD and CMOS sensors. For high-quality applications, such as precise spectroscopes, a very homogeneous coating of the photosensitive sensors is necessary here, which is why an order by hand does not offer the necessary quality – especially since there is a risk that contacts or surfaces are damaged by the touch.
The method developed by Eureca therefore uses a modified physical vapor deposition (PVD) process, which works without contact and provides reproducible, uniform layer thicknesses. For this purpose, the cover glass of the component to be coated is first removed in a gentle process designed for the respective sensor types, and the sensor itself is cleaned. Afterwards it is placed in a special holder in a vacuum chamber, in which then the Lumigen is deposited homogeneously on the sensor surface during the coating process. Thereafter, the sensor under protective gas is again provided with a cover glass or with a protective film depending on the specification.
By controlling the respective process parameters, it is possible to produce the optimum layer for the respective application. For example, thicker layers lead to a greater scattering of the incident light and can thus be used to reduce interference. The same applies to the coating of microlenses, in which the special surface properties represent a challenge for a good and homogeneous layer. As part of her research, Eureca has also developed methods for layer analysis.
By adjusting the process parameters an optimized layer for the specific application is produced. Thicker layers e.g lead to more dispersion of the incoming light and can thus reduce interferences. This also applies to the coating of microlenses whose surface properties are a challenge for a good and homogeneous layer. For this purpose the company also reasearched methods for layer analysis.
Generally, the treatment extends the usable spectral range of photosensors at the lower end of 450nm to 200nm, under ideal conditions even up to 100nm. On the other hand, above 500nm, the material is almost transparent, so that the sensitivity to light of the visible and near-infrared spectrum is little affected. The lifetime of the coating depends primarily on the level of radiation intensity, the wavelength and the temperatures to which it is exposed, as well as the duration of exposure. Experiences with common spectrometers have already shown that with the short arcs used here, the layer easily lasts several years.
The Lumigen UV sensitization is suitable in principle for different types of sensors, but a main field of application is line sensors, in particular for spectrometers. For optimal optical coupling, sensors with suitable pixel geometries are suitable, for example oblong pixels, as frequently found in spectrometers. However, the coating technology developed by Eureca works independently of the type and geometry of the sensor and can therefore be applied to almost all types available on the market.
Eureca already has a range of different models with coating on offer, but other sensors can be refined on request. A list of sensors currently being coated regularly can be found in the following table:
|Type||Manufacturer||Pixel Number||Pixel Size
[fps or Pixel Frequenzy]
|ICX098BL-UV||Sony||659 x 494||5,6 x 5,6||60 fps|
|ICX414AL-UV||Sony||692 x 504||9,9 x 9,9||60 fps|
|ICX424AL-UV||Sony||692 x 504||7,4 x 7,4||60 fps|
|ILX554B-UV||Sony||2048||14,0 x 56,0||2 MHz|
|ELIS-1024-UV||Dynamax||1024||7,8 x 125,0||30 MHz|
|TCD1205DG-UV||Toshiba||2048||14,0 x 200,0||2 MHz|
|TCD1304DG-UV||Toshiba||3648||8,0 x 200,0||1 MHz|
Example for effective quantum efficiency of a UV coated front side illuminated CCD.
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Last update: 2019-02-04