Patent Application
20110042767
The present invention relates to filters in an image sensor, such as a chip scale package image sensor.
Chip scale package (CSP) image sensors are commonly used in mobile phone cameras and many other devices. These types of image sensors are relatively expensive to manufacture, however, in the long-term, increased yield may mitigate some of the expenses. A CSP for a typical image sensor may include a silicon chip, on which an image sensor is constructed, and a cover, such as glass, which may reduce ingress and contamination of the sensor surface during the construction process. A typical image sensor may also include bonding to bond the cover to the silicon and electrical connections between the active surface (i.e. the surface on which the sensor is built) and the rear thereby allowing signals to pass to external electronics where appropriate.
A CSP is typically manufactured using a wafer related process to bond a wafer of silicon to the cover. This combination of wafer and cover is then processed to provide connectivity before the wafer is cut into chip scale packages. This may often require a process for “thinning” the silicon wafer for back to front connectivity. This can be seen in
There are a number of advantages and disadvantages associated with current CSP image sensor packaging. Some of these disadvantages become accentuated when the CSP concept uses an IR to UV filter, which may be made by vacuum deposition of dielectric layers on a separate piece of glass.
Referring to
Attempts to overcome the problems have relied on the use of glass that is expansion matched to the silicon and carried out as follows. The silicon and an uncoated glass wafer are typically prepared by bonding, revealing contacts, etc. The wafer is then cut to produce single CSPs that are cleaned and placed in a coating jig. The packages are then coated with a filter on the upper surface of the glass to complete the finished CSP. A disadvantage of this process is the production of particles during the cutting and handling of the CSP prior to application of the filter coating, which introduces contamination to the final CSP. This produces increased yield loss on a potentially expensive component during a low-cost part of the procedure.
Another attempt has been to coat the glass on both sides to try and keep the glass form flat. The process then proceeds to bond the silicon and coated glass wafer together and cut them into the individual modules. This process is typically highly accurate to ensure that any defect size is less than a pixel diagonal of the sensor on the lower surface. In addition there is potential for variations in the amount of coating which can continue to generate warping of the glass wafer and therefore uneven thinning of the silicon wafer.
In view of the foregoing background, it is therefore an object of the present invention to overcome at least some of the problems associated with the prior art. A further object of the present invention is to make a CSP comprising a filter using deposition of dielectric layers as part of the CSP while in a wafer form.
The object is also accomplished by a device as set out in the accompanying claims. According to one embodiment, there is provided a method of forming an image sensor having a sensor, a cover, and a filter. The method may include applying a filter layer to a cover layer by masking the cover layer with a predetermined pattern, and applying the filter layer by a deposition process. The method may further include bonding the cover layer to a sensor layer including a plurality of sensors. The predetermined pattern may result in a filter layer which is aligned with each sensor and includes gaps in the filter layer around each sensor.
The present embodiment may provide a number of advantages, in particular, the problems of differential thermal expansion between a coating and the wafer scale package are significantly reduced thereby reducing yield loss. As stress considerations are less of an issue, the filter may be designed with other criteria, for example, achieving appropriate color accuracy for a camera in the final application. As stress is less of an issue, the glass layer may have a reduced thickness which may improve the space available for other camera elements. This may result in an improved camera tracked length. In some situations two different glass layers may be used: one for the cover and one to support the filter. The present embodiments may provide a means or an approach by which one layer of glass is used.
The present embodiments may also have a number of advantages in terms of handling the CSP and reducing contamination. As the filter layer may not be impacted by the cutting places, there is less debris and contamination. In addition, as the filter is applied by a deposition process, a clean room environment is typically maintained and less contamination is likely. The present embodiments may result in a higher yield without additional costs or processing.
Reference will now be made, by way of example, to the accompanying drawings, in which:
The present embodiments include a method and chip that reduce the effect of differential thermal expansion of the silicon and glass layers. This reduces production loss due to cutting the combined layer into individual chips, and enables the use of a viable coating process.
Referring to
The glass wafer may then be bonded with the silicon wafer, and any subsequent backgrind process may result in significantly less thickness variations that would be detrimental to the TSV process. The final silicon glass combination or laminate is shown as 306.
The shadow mask (not shown) may be a contact mask or near contact mask having a reduced height to allow the first filter characteristics to remain within the requirements for a clear aperture at the upper surface of the CSP for the final resulting camera. The mask is in the form of a grid that coincides with the “step repeat” of the sensors on the silicon wafer. This may ensure that each filter is correctly positioned relative to the final individual CSPs.
As described, the filter is applied before bonding the silicon and glass layers. However, the filter could be applied after the two layers have been bonded together.
The filter is formed of an appropriate material depending on the frequency for transmission and suppression of radiation. For a UV-IR filter, a typical material may include multiple thin film layers of silicon oxide with silicon nitride. Similarly different materials for the glass wafer and silicon wafer may be appropriate in different circumstances. For example, borosilicate or alumino-silicate float drawn glasses.
As a consequence of the grid-like filter layer, there are less stresses between the filter coating and the glass wafer. Thus bowing and warping of the overall package which could ultimately result in stress failure of the module may be reduced or avoided. As a first approximation, the linear stress in the module may be reduced by a ratio of the dimension of the coated area to that of the step repeat distance in that direction.
A significant advantage is that, as stress is reduced, the ability to tune the filter to give optical and reliability properties may come before the reduction of stress. This provides more flexibility in the design process, and may enable an improved optical standard to be achieved.
An advantage over implementing the filter in a two sided coating is that near field defect criteria, where defects are compared to pixel size, are reduced and may be substantially removed. Instead, with the present embodiments, the size of the allowable defects is based upon the exit pupil of the objective lens, and not the effects of the coating.
It will be appreciated that the grid may take many different forms depending on the underlying arrangement of sensors or other devices which require this type of filter. The grid may be regular or irregular and the relative sizes of this filter and the gap between may also be variable. It should be noted that reference to light or radiation is intended to encompass all frequencies of radiation in which a digital image sensor may operate.
The application of a filter in accordance with the present embodiments may be applied to any combination of material, not just glass and silicon as described herein. The application of the filter may apply to any situation where warping due to thermal expansion differences occurs.
The digital image sensor in accordance with the present embodiments is suitable for use in any device which makes use of a digital image sensor. For example, the digital image sensor may be used in a camera, in camera modules, or in a mobile telephone or any other computer related equipment. It will be appreciated that the embodiments may be varied in many different ways and still remain within the intended scope of and spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0914350.4 | Aug 2009 | GB | national |
