IEDM Image Sensors Content

Solid State Technology published Chipworks analyst Dick James' report from IEDM'08. Here is what Dick says about image sensors:

"Chipworks has been focusing on image sensors for a while now, so Rohm's announcement of a CIGS on CMOS sensor (paper 11.2) caught my attention. CIGS (copper indium gallium selenide) is one of the hot materials in the PV field, so the idea of using it for an image sensor is not that far off the wall -- the big problem has been high dark current.

Rohm gets around this by using a double layer of zinc oxide as the top blanket electrode. One layer is semi-insulating ZnO, and the top sub-layer is Al-doped to give conductivity; this has the dual advantage of isolating the pixels and reducing dark current. The CIGS is co-evaporated on to a molybdenum base layer/back contact (which contacts the top metal layer of the CMOS scanning chip below), with a CdS buffer layer under the ZnO bi-layer.

The pixels are 10μm × 10μm in a 352 × 288 array, so this is clearly a proof of concept. The sensitivity can be extended to sub-lux illumination levels by biasing the photodiode to induce avalanche multiplication. Since the photodiodes are on top of the die, they have an aperture ratio close to 100%, and this coupled with the CIGS spectral response extending into the near-infra-red makes the sensor suitable for automotive and security applications.

In the same session Samsung compared 1.4μm frontside- and backside-illuminated (BSI) sensors (paper 11.4).
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Samsung claims that the technology will extend pixel size down to ~1μm."