Harvest Imaging Blog

“A 1/2.65inch 44Mpixel CMOS image sensor with 0.7 um pixels fabricated in advanced full-depth deep-trench isolation technology” by Samsung.  Basically the title says it all.

Remarkable technology that is used to create the deep-trenches in a pixel of 0.7 um.  On the backside of envelope I calculated the trench should be around 85 nm in width, with an aspect ratio of 69.  Next an isolation is provided on the sides of the trenches and the poly-Si filling takes place.  For the latter it is even more challenging : the aspect ratio is increased to 110.  It is really incredible that these things are possible in CMOS technology.

Some numbers :

• Pixel size : 0.7 um,
• Full well : 6000 electrons,
• Temporal noise : 1.4 electron,
• Dark current at 60 deg.C : 1.3 electron.

It is not just the trenches that are further optimized in the technology (compared to the 0.8 um pixel pitch that is most probably used in the 108 Mpixel device), also the boxing of the CFA is changed : a low refractive index grid is used instead of tungsten.  This results in an increase of about 15 % In QE(green).

The sensor presented is making use of the quad Bayer structure.  This is a very attractive architecture for binning options.  With binning the SNR can be increase considerably for applications in harsh light conditions (at the expense of resolution of course).  The results are shown in the figure below.

 

Artilux presented “An up-to-1400 nm 500MHz Demodulated time-of-flight image sensor on a Ge-on-Si platform”.  It is of course well known that the silicon response is limited up to a wavelength of 1100 nm.  If one is interested in detection of longer wavelengths, another material than silicon is needed, for instance germanium (Ge).

The reason to go for longer wavelengths, such as in time-of-flight applications, can be found in :

• issues with eye-safety between 800 nm and 1100 nm,
• background sunlight : the sun does not emit at 1300 nm.

If one compares the absorption spectrum of germanium with the one of silicon, indeed germanium seems to be a viable candidate to replace silicon for longer wavelengths.  The paper presented puts a very thin Ge toplayer on a silicon readout circuit.  The idea is not really new, this was already done earlier by Noble Peak (USA).  A cross section of the Si readout structure provided with a thin Ge layer (using a Si carrier ?) is shown below.

The authors claim to reach a QE equal to 50 % at 1500 nm and further optimization is still possible in optimizing the microlenses for this wavelength.  Nevertheless, there still some work needed to lower the dark current.   In the ToF application a modulation depth of 90 % can be obtained at 500 MHz modulation frequency, resulting in a measurement error less than 0.5 % for a distance of 1 m.

Albert, 20 February 2020.

 

 

 

 

This entry was posted on Thursday, February 20th, 2020 at 6:31 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.