Report : Solid-State Image Sensors at IEDM, Dec. 5th, 2011

 

Paper 1 : Extremely-low noise CMOS image sensor with high saturation capacity, by K. Itonaga (Sony).

It was reported that for S/N in dark it is primarily needed to lower the noise floor of the sensor.  This can be done by optimizing the technology to lower the amount of defects and/or by increasing the area of the source-follower transistor.  On the other hand, to increase the S/N at bright conditions, it is needed to increase the saturation level, for instance by choosing a larger PD area.  In this paper both items are improved and optimized.  The level of defects is reduced by no longer making use of STI in the image part.  The complete pixel array is made on the same gate dielectric and isolations are introduced by means of p+ implants.  As a result the 1/f noise is going down as well as the dark current.  The technology was demonstrated by means of a 16 Mpixel sensor, 1.12 um pixel, 15fps.  The pixels are configured in as 2 x 4 shared pixels, without a select transistor resulting in 10 transistor per cell of 8 pixels (1.25 T/pixel).  Noticable, the length of the source follower gate is 1.14 um, being the optimum between thermal noise and 1/f noise.  Nice paper, but unfortunately no performance numbers were given during the presentation.

Paper 2 : High performance 300 mm backside illumination technology for continuous pixel shrinkage, by D. Yaung (TSMC).

The author reported about several issues encountered in the development of the BSI process, and told us that they were solved by further optimization of the tools, by reducing particles, by reducing defects, etc.  Results were shown to illustrate that the BSI process is under control, for instance the 1/f noise of the source-follower in the BSI process is better than the 1/f noise of the source-follower in the FSI process.  QE values for a 0.9 um pixel were shown : 50 % in blue, 47 % in green en 45 % in red.  The pixels were realized in a 65 nm process with a remaining thickness of the silicon equal to 2 um … 4 um.  In the case of the 0.9 um pixel, the optical cross-talk is about 4 times as large as in the 1.1 um version.  As can be expected, the QE and the optical cross-talk, both become worse if the pixels shrink.  Simulated data for pixels down to 0.6 um were shown. 

Paper 3 : A 1.4 um front-side illuminate image sensor with novel light-guiding structure consisting of stacked lightpipes, by H. Watanabe (Panasonic).

The major FSI limitations are the reduced aperture size of the pixels (coming close to the wavelength of red light) and the propagation distance (several um).  The newly developed technology is characterized by :

          45 nm Cu metallization technique, this allows a very low optical stack,

          Stacked lightpipe consisting of a high-refractive index material (Si3N4) and separation wall (SiO2) between the colour filters.  So in principle, the colour filters already act as a lightpipe.

Some data : QE in green 74 % in comparison with 69 % for the BSI and 43 % for the FSI without stacked lightpipe.  Angular response is 80 % at an angle of 20o.  Further demonstration is done by a 14 Mpixel device with 1.4 um pixel size and a dark current of 9.9 e/s @ 60 oC.

Nice paper with good results and a good presentation.

Paper 4 : Investigation of dark current random telegraph signal in pinned photodiode CMOS image sensors, by V. Goiffon (ISAE).

CMOS image sensors demonstrate blinking pixels due to current fluctuations in the source-follower (SF-RTS) and dark current fluctuations in the photodiode (DC-RTS).  The paper concentrated on the latter effect.  The frequency of the DC-RTS blinking pixels seems to be much lower than the frequency of the source-follower RTS pixels.  The authors built their own analysis tool based on edge detection in the temporal domain.

The following experimental observations were made :

          DC-RTS depends proportionally on the integration time,

          There seems to be no correlation between the RTS pixels and the dark current in the bright pixels,

          There seems to be no correlation between the RTS pixels and the average dark current,

          There seems to be no correlation between the switching time of the RTS pixels and their amplitude,

          Activation energy seems to be 0.6 eV.

          When the TG is put into accumulation, all RTS pixels are gone !

In conclusion : the DC-RTS pixels are metastable defects characterized as a SRH generation center, that are located at the STI edge or at the depletion edge of the transfer transistor !

Very well structured paper, with a lot of experimental data.

 

Paper 5 : A CMOS compatible Ge-on-Si APD operation in proportional and Geiger modes at infrared wavelengths, by. A. Sammak ((TU Delft).

The author described their technology flow to make ultra-shallow p+ junctions.  This is based on the deposition of pure B on Si or pure Ga on Si.  For the Ge-on-Si devices, they propose to use the combination of pure Ga on top of pure B on top of Ge.  Devices were made and first results were shown also for the near-IR spectrum.

 

 

Paper 6 : Enhanced angle sensitive pixels for light field imaging, by S. Sivaramakrishnan (Cornell Universy).

Angle sensitive pixels based on dedicated metal grids were already demonstrated at previous conferences.  One of the drawbacks of these devices is the presence of the double metal grid.  This reduces the quantum efficiency to 12 %.  In this paper two new techniques were introduced to overcome the issues of quantum efficiency :

          The bottom metal grid was replaced by means of 2 interleaved finger-structured diodes,

          The top metal grid was replaced by a phase-grating in SiO2.  The latter is processed later in-house with simple processing steps, but the width of the phase-grating is defined by a metal grid defined by the CMOS processing fab.  After the phase-grating is defined, the metal mask is removed.  Very cleaver solution to obtain very fine linewidths defined by the fine-detail CMOS process, but ultimately generated by classical etching tools.

At the end of the road, an angle sensitive device is realized without any metal grid above the pixels.  The QE obtained was reported to be around 45 %.

 

Paper 7 : A 192×108 pixel ToF-3D image sensor with single-tap concentric-gate demodulation pixels in 0.13 um technology, by T.-Y. Lee (Samsung).

A device is developed with 192×108 pixels, 12 bit ADC built in a 0.13 um.  The light source for the ToF measurement is an LED with 850 nm wavelength.   The pixel pitch is 28 um, and is using a single tap solution for the detection of the incoming signal.  Although this single tap gives pretty good results, it should be noticed that 75 % of the incoming information is not used.  The measurement error is smaller than 1 % for distances up to 7 m.

The detection contrast and the quantum efficiency are essential parameters in ToF sensors.  The QE can be further optimized by going to BSI.  Other options for further improvements are the optimization of the light source,  optimization of the optics and implementing a correction of the ambient light.

Albert

05-12-2011.

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