Monday, May 27, 2013
Omnivision Angled Implant Takes Advantage of Pixel Layout
Omnivision patent application US20130113969 titled "Method, apparatus and system for providing improved full well capacity in an image sensor pixel" by Sohei Manabe, Keh-Chiang Ku, Vincent Venezia, Hsi-Chih Tai, Duli Mao, and Howard Rhodes proposes an interesting combination of pixel layout and angled implant:
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I see that PD N implant is localized but the pinning P implant is global. This means that the TX is P doped. What is the fundamental advantage of this arrangement??
ReplyDeleteNormally, the pinning layer dosage is not high and can not overcompensate the initial poly N-type implant. So, Tx gate stays N-type.
DeleteOK. Do you mean that there is a N pre-doping on poly gate before its patterning??
DeleteIt depends on the process flow. Some processes have poly implant before patterning, others only dope poly during a diffusion implant stage. In the later case the Tx doping picture becomes quite complex.
DeleteThe gate work function changes with n type or p type and affects the threshold voltage and swing.
DeleteRight, this is one of the reasons why the analysis of that case is quite complex. Another interesting aspect is the transient behavior of the pn junction in poly, as the gate contact only touches the n-type area. In spite of the analysis complexity, some of the commercial sensors work that way.
DeleteYou could use selective silicidation to short the diodes in the poly
DeleteMy feeling is that PD N uses high energy which could be beyond the stopping power of polygate. That is why it's localized.
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