Some specific applications, such as optical devices, require non-conventional layouts. In this context, the known OPC solutions developed during decades and optimized for CMOS planar applications are facing significant challenges. Standard design files format as well as OPC algorithms are indeed suitable for 0-45-90° edges (also called Manhattan layouts) and other angle edges can lead to bad OPC results, huge run time, large file size, and even run crashes. While innovative developments are on going from OPC suppliers’ side, we have to use smartly the conventional OPC platforms to achieve accurate, fast and cost-effective solutions. Taking the example of optical diffusers application, we will discuss the implementation of such an OPC flow, including rule-based correction, SRAF insertion, model-based correction, and mask sign-off strategy.
Despite of the fact that thousands of CD-SEM (critical dimension scanning electron microscope) images are acquired in a daily basis in a fab, limited metrology is performed. Usually these images will not serve other purposes after they are collected and measured, but as they are stored, post-process analysis can be applied. Initially, most of these images are used to perform CD metrology, even though many other types of metrics could be extracted from the same images, especially when using contour metrology. In this paper two use cases will be explored, where contour-based image processing is performed on typical inline metrology targets. In both cases, initial intended metric was CD but thanks to contour based image computing, complementary information can be extracted. In the first use case, CD and overlay metrics can be extracted, while in the second CD, etch slanting and asymmetry analysis is performed across the wafer. Contour-based metrology offers new capabilities to dissociate several layers (e.g. via and line) or elements (e.g. top and bottom) in the image so that interlayer and intralayer metrics, other than width dimensions, can be computed. Besides, a solution not integrated in the tool provides excellent versatility to re-process images, thus allowing the obtention of new metrics, which can be very helpful also for retro-analysis.
Microlens arrays are used on CMOS image sensors to focus incident light onto the appropriate photodiode and thus
improve the device quantum efficiency. As the pixel size shrinks, the fill factor of the sensor (i.e. ratio of the
photosensitive area to the total pixel area) decreases and one way to compensate this loss of sensibility is to improve the
microlens photon collection efficiency. This can be achieved by developing zero-gap microlens processes. One elegant
solution to pattern zero-gap microlenses is to use a grayscale reticle with varying optical densities which locally
modulate the UV light intensity, allowing the creation of continuous relief structure in the resist layer after development.
Contrary to conventional lithography for which high resist contrast is appreciated to achieve straight resist pattern
profiles, grayscale lithography requires smooth resist contrast curve. In this study we demonstrate the efficiency of
grayscale lithography to generate sub-2μm diameter microlens with a positive-tone photoresist. We also show that this
technique is resist and process (film thickness, development normality and exposure conditions) dependent. Under the
best conditions, spherical zero-gap microlenses as well as aspherical and off-axis microlenses, which are impossible to
obtain with the conventional reflow method, were obtained with satisfying process latitude.
Microlenses arrays are commonly used in CMOS images sensors to focus the incident light onto the photosensitive area of the pixel. These microlenses are fabricated using a thermal reflow method. Currently, due to the fast evolution of CMOS Imager technology, the understanding of the mechanisms involved in microlens formation becomes essential to better control what occurs during the process. We have seen in a previous study that the complexity of the reflow method comes from the competition between two phenomena occurring during the melt bake step: on one hand the surface tension tends to push the resist patterns into a spherical shape, on the other hand the resist crosslinking reaction drastically increases the resist viscosity hindering the microlens formation. In this paper the influence of resist crosslinking, resist volume and resist/substrate interface on the final shape of the microlens has been investigated. It appears that the contact angle between microlens and substrate varies depending on substrate wettability but is the same whatever the resist volume for a given substrate/resist combination. The microlens shape depends also significantly on bake temperature and crosslinking kinetics. In fact the right tuning of process conditions seems to be the key parameter in the control of the final microlens shape because it enables to adjust the kinetics of each mechanism and thus favour the microlens formation with regards to resist crosslinking.
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