Saturday, February 28, 2015
Omnivision Proposes DTI with Charge Transfer Assist
Omnivision patent application US20150048427 "Image sensor pixel cell with switched deep trench isolation structure" by Sing-chung Hu, Rongsheng Yang, Gang Chen, Howard E. Rhodes, Sohei Manabe, and Hsin-chih Tai proposes a BSI pixel with DTI, filled by a lightly doped poly. A negative pulse applied to the DTI poly allows increasing PD doping and full well while maintaining a low image lag:
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Why pulse the DTI ? We can negatively bias the DTI continuously which has been experienced by IMEC and ST.
ReplyDelete-yang ni
Keeping DTI negative all the time helps to reduce dark current. Pulsing it negative during the charge transfer helps to reduce image lag. As long as the lag is low, the PD doping can be increased and, thus, PD full well capacity can be made higher.
DeleteThis is the difference between OVT and ST/IMEC approaches.
For me, you can use higher doping level when the DTI is constantly biased too. This is equivalent to dope the DTI sidewall. The image lag will also be reduced by the constant negative bias as well.
Delete-yang ni
Yes, the constant negative bias reduces the lag, but it also reduces the full well. The pulsed DTI bias reduces the PD full well only at the moment of charge transfer, when the PD charge should be emptied out anyway.
DeleteThis is not after all a new idea! There is a patent from Sharp or some other Japanese image sensor company from the early 2000's that does the same thing -- use a negative pulse -- essentially applied across the sidewall portion of the photo-diode junction to assist readout, i.e., remove lag. They may not have had a poly filled STI. But the crux of this method is the negative pulse! It appears that patents may be recycled as fresh ideas/patents when the memory of them has faded and with only very small changes to make them look different. e.g., a negative pulse applied from the left vs right, etc.
ReplyDeletewhat is to be blamed for this is not just the patent system, but also the incentive systems for patent generation that exist in corporations today!