After studying the effects of the dark current, dark-current shot noise and the dark fixed-pattern noise on the Photon Transfer Curve (PTC), the influence of the temperature will be studied in this blog.  The dark current is strongly depending on the temperature.  From literature it can be learned that the dark current can double for every 5^oC … 8^oC temperature increase.  Because the dark-current shot noise is related to the dark current (square-root relation), the shot noise component itself is becoming temperature dependent as well.  And the same is true for the dark fixed-pattern noise, at least, if the fixed-pattern noise is generated by non-uniformities in the leakage current.  All three components studied so far depend on the temperature.  Then the interesting question can be raised : “What will be the influence of the temperature on the PTC ?”.

As was the case in the previous blogs, 100+ dark images were generated at various exposure times (between 0 s and 65 s), and in this particular case, also at two different temperatures : 30^oC and 60^oC. The result of this exercise in dark can be seen in the following four figures :

-       Figure 1 contains the average dark signal (left axis) of the generated images and its noise component (right axis) as a function of the integration time (horizontal axis), for images generated at the two temperatures indicated.  To find the noise, all images were averaged on pixel level and the standard deviation on the resulting image was calculated.  This allows obtaining the FPN, because the calculation is done on the averaged pixel signals and consequently, the temporal noise component is excluded (or at least strongly reduced). 

-       Figure 2 shows the dark fixed-pattern noise versus the dark signal, based on the data shown in Figure 1.  Figure 2 is very similar to the Photon Transfer Curve, keeping in mind that this time the FPN is shown.  As can be seen the temperature has no influence on this PTC curve.

-       Figure 3 contains the average dark signal (left axis) of the generated images and its temporal noise component (right axis) as a function of the integration time (horizontal axis), for images generated at the two temperatures indicated.  To find the noise, the standard deviation for the various pixels was calculated, and afterwards these values were averaged.  This allows obtaining the temporal noise, because the calculation is done on the individual pixel values and consequently, the fixed-pattern noise component is excluded. 

-       Figure 4 shows the dark temporal noise versus the dark signal, based on the data shown in Figure 3.  The same conclusion as for Figure 2 is valid : the temperature has no influence on the PTC curve.

Figure 1 : Dark current and its FPN component as a function of the exposure time, for two different temperatures.

As can be seen in Figure 1, the average dark signal is linear with the integration time, indicating that the dark current is responsible for the signal in dark.  The relation between the dark signal and the exposure time (t_exp expressed in ms !) is also indicated in Figure 1. The FPN as a function of exposure time is shown on the right axis.  Because the FPN is composed only out of dark current non-uniformities, the relation between the FPN and the exposure time is linear as well.  The empirical relationship is shown in Figure 1 (t_exp expressed in ms).  From the two formulas shown, it can be deduced that the FPN component is 1/6.6 or 15.2 % of the dark signal.  And this number is valid at any given temperature, because in the sensor under study, FPN is generated only by the dark current.

Figure 2 : Dark FPN versus dark signal for two different temperatures.

The corresponding “PTC” curve is illustrated in Figure 2 : the FPN versus the dark signal is shown.  Notice that the temperature has no effect on the curve.  The slope of the curve should be equal to 1, and its intersection with the horizontal axis is defined by the ratio of the FPN to the average dark current.  Both parameters are not influenced by the temperature.

 Figure 3 : Dark current and its temporal noise component as a function of the exposure time, for two different temperatures.

Figure 4 : “PTC” of the sensor with only dark current shot noise for two different temperatures.

Figures 3 and 4 are showing the dark signal with its temporal noise component, being the dark shot noise.  Clearly visible is the strong influence of the temperature on the dark current, as well as on the dark-current shot noise, both shown in Figure 3.  But similar to Figure 2, the PTC curve shown in Figure 4 is not influenced by the temperature.  This can be easily explained : its slope should be equal to 0.5, and the crossing with the horizontal axis is defining the conversion gain of the system.  The latter is not influenced by the temperature !

From Figure 2 as well as from Figure 4, it can be calculated that the dark current is increased by a factor of 882/66 = 13.36 when switching from 30^oC to 60^oC.  This corresponds to a doubling of the dark current for a temperature increase of 8.2^oC.

Conclusion : the temperature has a strong influence on the dark-current, on the dark-current shot noise and on the dark current FPN.  On the contrary, if these three parameters are taken into account, the temperature does not influence the PTC curve.  So, it does not matter at which temperature the data is taken to generate a PTC curve.  The various data point on the curve can even be generated at different temperatures.  But important to realize : all images generated to create a single data point on the PTC curve should be taken at the same temperature !

Next time, the model of the sensor will be extended by the incorporation of anti-blooming in the pixels as well as anti-blooming non-uniformities.

Albert 2009-09-24

As most of you probably know, I have the honor of being the Chair of the International Technical Program Committee (ITPC) of the upcoming International Solid-State Circuits Conference (ISSCC). This will take place next February in San Francisco.  Every year, this conference attracts several thousand attendees and the conference can be seen as “The Olympics” of the integrated circuit community.  Being the ITPC chair I had the luxury of choosing the conference theme, and I chose “Sensing the Future”.  Closer to my heart would have been “Imaging the Future”, but of course, ISSCC is about (much) more than image sensors.  Nevertheless, “Sensing the Future” highlights the fact that sensors have become a very important part of the integrated circuit business and will play an increasing role in future electronic applications.

The major part of the conference will be the presentation of 200+ technical papers.  The final paper selection will be done in a few weeks from now, but all the other technical and scientific activities taking place at the ISSCC2010 are already planned. 

It is then interesting to see how, where and when image sensors have found their place in these activities :

-       The conference itself will start on Monday morning with a plenary session.  One of the four plenary speaker will be Tomoyuki Suzuki, vice-president of Sony Corp.  The title of his talk will be: “Challenges of Image Sensor Development”.  I think it has been 10 years since ISSCC had a plenary talk on imagers.  If I remember well, the last one was Bryan Ackland of Bell Labs talking about CMOS imagers.  Suzuki’s abstract indicates that he will give a historical overview of milestones in the development of image sensors reached by his company.  Keep in mind that Sony was the largest image sensor supplier in the CCD era.  But he will also address the new challenges ahead of us, such as back-side illumination, 3D imaging (depth sensing), as well as 3D integration for image sensors. 

-       In total, 6 forums are planned during ISSCC2010.  Over the last 5 years, a forum on image sensors has been organized every year.  Topics like image sensor evaluation, color imaging, noise in image sensors, wide-dynamic range and medical imaging have been covered in the past. 

The day after the technical paper presentations, there will again be a one-day forum on High-Speed Image Sensor Technologies.  The organization of this forum is in the hands of Johannes Solhusvik who was also the technical program chair of the 2009 International Image Sensor Workshop. 

The exact agenda of the forum is not yet fixed, so it is too early to announce the various speakers and their subjects, but I can tell you that 8 or 9 world-experts have been invited to give presentations about the state-of-the-art and recent developments in the field of high-speed image capturing.  Topics dealing with sensor architectures, low noise options, 3D imaging, robotics and high-speed interfacing will also be addressed.  The technical content of a forum is the highest educational level one can attend at ISSCC, so the image sensor forum is absolutely something to look forward to!

Besides the forum on image sensors, an interesting talk (from ST Microelectronics) about TSV technology for camera modules will be part of the forum on 3D integration. 

-       Evening sessions : there will be no specific session devoted to image sensors, but sensors in general can be found in several evening sessions,

-       Short course on CMOS PLL.  Not specifically image sensor related, but PLLs are very important building blocks for all kind of systems-on-chip, so for imaging systems as well.

-       Tutorials : at ISSCC 9 tutorials will be presented and they cover various topics all related to integrated circuits.  One of the tutorials will be given by Kofi Makinwa and he will talk about the “Design of Smart Sensors”.  This is already the third tutorial delivered by Kofi, and as far as I recall this has never happened before at ISSCC.  Kofi is a great speaker and teacher, and that is of course the reason why he has again been invited to talk about sensors in general.

The organizational structure of the educational events of this year’s ISSCC has changed.  We have tried to incorporate more cross-functional topics in the program compared to previous conferences, and this is clearly reflected  in the various activities planned for Sunday, Thursday and the evenings.  In relation to the conference theme “Sensing the Future”, you can be sure that sensors will feature in all these activities, as will image sensors.  I am absolutely looking forward to it all, and I really must congratulate my colleagues for putting together all these great educational events.  The next step will be the compilation of an appealing program for the regular conference as well, and that will be done in a few weeks from now.

Albert 2009-09-13