Removal of Glass Covers from Image Sensors

Often the standard cover glass of CCD/CMOS image sensors cause disturbing reflections or interferences. The remedy here is the removal of the cover glass.

The removal of the cover glass also offers interesting options in further steps to refine a sensor, so that relatively inexpensive sensor types can also be used for high-end applications. Examples of this are the placement of custom filters or the application of a UV-sensitive coating on the sensor surface.

Over the last few years, Eureca has developed several methods to remove the glass covers of CCD/CMOS sensors. Thus, a suitable method is available for many types. The different procedures have different failure rates due to damage to the sensors. In the best case, this rate is only a few percent.

The development of suitable methods for the cover glass removal of image sensors was also part of a research program at Eureca, which was funded by the »Central Innovation Program of the Mittelstand« (Zentrales Innovationsprogramm des Mittelstandes (ZIM)).

We process both sensors from our own product range as well as sensors, which are provided by the customer. Customer positions are first subjected to extensive quality control prior to processing in order to examine them for suitability for subsequent processing, for example to detect changes in the manufacturer's production in good time. Only after successful testing and release by the customer is further processing carried out.

After removing the glass cover, all sensors are optically checked for possible damage and, if necessary, cleaned. An additional electro-optical test with the inclusion of test images is possible, provided that we have a suitable camera board for the respective sensor. For sensors that are processed in larger quantities, this test is always performed.

As a protection during transport to the customer a special adhesive film is applied, which can be removed without residue before use.

Years of experience with large quantities delivered have shown that the sensors without cover glass have the same lifespan as the original sensors. However, without cover glass, the sensors are understandably quite sensitive and must be protected from moisture, dirt and mechanical contact during operation.

Erfolgreich verarbeitet werden folgende Typen:

• all sensors of the ILX series from Sony (e. g.ILX551B)
• all sensors of the ICX series from Sony (e. g. ICX285AL)
• all sensors of the TCD series from Toshiba (e. g. TCD1304DG)
• sensors of the IMX series from Sony
  • Pregius and STARVIS generations, including the latest Pregius S generation (e. g. IMX174, IMX250, IMX304, IMX421, IMX540)
  • latest InGaAs generation (e. g. IMX990)
• OnSemi: CMV4000, Python1300/5000, AR0521
• e2V: EV76C570, EV76C560ABT
• SiOnyx: XQE-0920, XQE-1310
• GPixel: Gsense2020, GMAX3265

The various methods used involve different levels of risk, for example contamination of the sensor surface. Thanks to Eureca's many years of experience and continuous further development of the methods, effective techniques are available with which the contamination of the sensors after processing can be significantly reduced. While cleanroom quality cannot be achieved in most cases, the result is usually sufficient for most of the applications and products targeted.

A little statistics for classification

The following graph shows the results for 81 sensors. The data was determined by taking a large number of test images using homogeneous illumination. The average of these images was used to calculate the mean gray value and the standard deviation. The relative standard deviation is obtained by normalizing the standard deviations to the mean grey value. The averaged image data mainly consists of the fixed pattern noise (FPN) of the sensor. A comparison of these results before and after cover glass removal provides an estimate of the increase in FPN due to processing. We found that the average increase in the mean relative standard deviation after processing is about 2.06 × 10⁻³.

Further information on this topic can be found in our PDF document Glass cover removal from image sensors.

After the actual cover glass removal, the sensor can be further processed. Examples of such refinements are:

filter

The cover glass is replaced directly by one or a combination of multiple filters. This generally improves the signal quality considerably, as undesired spectral ranges can be masked out in a targeted manner. The direct placement on the chip gives a compact and above all robust unit without additional holders for external filters in the beam path. For applications where the sensors or cameras are cooled, this is done under an argon atmosphere to avoid condensation of trapped moisture.

fiber optics

To improve the optical signal, a fiber optic can be put onto the CCD sensor. In addition to optics that depict true to scale, reductions can also be realized. Compared to conventional lenses, fiber optics significantly reduce light losses.

coatings

By applying special materials to the chip, the sensitivity can be extended to spectral ranges outside the usual range. These include, for example, fluorescent substances for the detection of UV or X-rays. Eureca offers a special lumigen-based coating, which is applied in an in-house coating facility.