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    why metal fill is required in IC layout

    Why metal fill is required to meet minimum or maximum metal density?
    what happen if not fill?

    •   Alt23rd October 2008, 08:04

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    Re: why metal fill is required in IC layout

    As I know, metal filling is related with reliability issues. If the metal density is not constant over the whole chip, the amount of dielectric layer (covered between metal layers) will be different, and it causes the performance variation depends on location.



    •   Alt23rd October 2008, 10:34

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    Re: why metal fill is required in IC layout

    There is a good discussion about this topic here.

    Regards



    •   Alt23rd October 2008, 11:24

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    Re: why metal fill is required in IC layout

    Quote Originally Posted by dipak.rf
    Why metal fill is required to meet minimum or maximum metal density?
    what happen if not fill?
    Metal fills are required to make metal density (more or less) uniform in order to achieve constant metal thickness across chip and flat surface after CMP (chemical mechanical polishing) in copper process. Mechanical properties of dielectric (SiO2 or low K dielectrics) and metal (copper) are different, so that wide metal traces would be polished to a different final thickness as narrow traces, and also dense metal areas would be polished to a different thickness as sparse areas. Uniform metal density helps to avoid such common problems as dishing and erosion.



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    why metal fill is required in IC layout

    Dipak wrote it nice from CMP perspective.

    However, copper-process is around for last 5-6 years or so. Achieving uniform Metal density requirements are there since Aluminum metallization days. CMP was not used those days to achieve planarity. Even conformal dielectrics have been used.

    The core reason - Interconnect parasitics need to be predictable within an acceptable error limit.
    The more the planarity is achieved on the top of an ILD - more closer will be extracted RC prediction, with real measurements. Ancient CVD [Chemical Vapor Deposition] days - closely placed dummy metal islands help maintaining a uniform overall planarity of dielectric layer, which in-turn helps next metallization to be nearly planer - and the next ILD and so on..Density is a point function - it was more important to measure and achieve density within smaller window-size in lower metal-layers, than upper ones.

    The dummy metal-fill patterns are not useless too - they can be utilized to serve as de-caps, serve as shields, thermal-distribution.



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