Posted: July 24, 2017
Contributing Authors: Ray Fontaine
The third topic addressed in the IISW 2017 paper was an overview of the trends we’re seeing in pixel isolation structures. These structures are critical to the performance of 1.0 µm to 1.4 µm generation pixels in smartphone cameras. Going forward, we expect to see these structures deployed in other imaging applications, and for sub-micron pixels.
The full version of the TechInsights paper will be posted to the IISS website in August. As always, a thank-you is due to the IISW organizing committee for the opportunity to present and to our talented lab staff and analysts who generate the underlying data. Stay tuned – there is never a dull moment in the image sensors sector!
Pixel Isolation Structures
A wide range of structures have been fabricated on back-illuminated substrates to provide improved electrical and optical isolation. DTI process modules have been implemented both early in the process flow (prior to active device formation) and much later (after CIS back-of-die thinning). Most B-DTI structures in use to date have been filled with dielectrics, including a variety of high-k films.
The first DTI structures for back-illuminated CIS were found in 2010 in the HTC One UltraPixel camera chip fabricated by STMicroelectronics . These early front-fabricated DTI (F-DTI) structures were built using silicon-on-insulator starting substrates. Current back-illuminated CIS chips are typically fabricated on back-thinned epi/bulk substrates.
Sony’s first observed DTI effort was a B-DTI structure found in the IMX147 in 2013 . The B-DTI penetrated about 0.3 µm deep into a 2.8 µm thick substrate and was filled with a first interfacial oxide followed by HfO, TaO, oxide, a Ti-based liner and W metal. This chip is the only example of a metal filled trench from Sony. By 2015 Sony switched to a deeper, dielectric filled B-DTI structure comprising a thin interfacial oxide, AlO, TaO, and an undoped oxide core.
Fig. 3 shows a recent, dielectric-filled B-DTI structure from the 1.4 µm pixel generation IMX260 featuring a 2.9 µm thick substrate. The B-DTI extends to a nominal depth of 1.9 µm from the back surface, although it extends to a depth of 2.4 µm deep at B-DTI intersections.
B-DTI and F-DTI structures have been integrated to Samsung’s ISOCELL technology platform. Analyses to date indicate Samsung’s preference for full-depth F-DTI for 1.0 µm pixel generation ISOCELL and partial-depth B-DTI for 1.12 µm and 1.4 µm pixel generation ISOCELL structures.
The first observed B-DTI structures from Samsung, implemented in a 1.12 µm pixel generation chip in 2015, featured B-DTI trenches extending 1.3 µm deep into a 2.6 µm deep substrate.
OmniVision introduced partial B-DTI structures through its PureCel Plus-S technology platform, analyzed in 2016 . The TSMC-fabricated structures for a 1.0 µm pixel generation CIS, extended 0.45 µm deep into the back surface of a 2.5 µm thick substrate. The B-DTI trench fill comprises an interfacial oxide, first deposited HfO, TaO, oxide, Ti-based liner, and a W core. To date, this is the only observed DTI structure on an OmniVision chip and the first metal filled B-DTI trench since Sony’s effort in 2013.
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