Poly effects and dummy poly need in iC fabrication

Hi...

i need to know some basics regarding poly in IC fabrication.

first of all.. why poly is extended outwards from the diffusion in MOSFETs...

The need of dummy poly..( i know dummy poly is used while etching..) but it is said that.. dummy poly is very much needed for minimum length devices than long channel devices..

What makes the importance of dummy poly in long and short channel devices..?

How poly orientation affect the yield ? which is better horizontal or vertical?

thanks

Hi...

i need to know some basics regarding poly in IC fabrication.

first of all.. why poly is extended outwards from the diffusion in MOSFETs...

ans: Bcoz outwards extention of poly s treated as width over hang for poly. so dat whnen in etching process if at all the width of etched out more, to protect the active area we have extentionof width outwards in both the sides. and one more thing if the width s etched more then der might be changes of tunneling effect(direct path betw source and drain).

The need of dummy poly..( i know dummy poly is used while etching..) but it is said that.. dummy poly is very much needed for minimum length devices than long channel devices..

ans: bcoz in short chanells we c more well proximity effects so to protect our actual devices we have two to three rows and coloumn of dummies. well proximity effect means, when we fabiricate nwell the ions will bobard to the well edge and divert from the position where it has to sit. so we find difference in concentration compared to middle of well and near the edges. so we can expect different behavior btw middle device and edge device(vt varies ) so to avoid or have same behavior we have two to three rows and colomns of dummyies in short chanels.

What makes the importance of dummy poly in long and short channel devices..?

ans : short channel i hav explained above. for long channel etching the main consern.

How poly orientation affect the yield ? which is better horizontal or vertical?

ans: both are prefered but through out the chip we should have either horizontal or vertical.

thanks for your reply..

and can you please explain the tunneling effect... and.. change in dummy poly widths of NMos and PMos makes any trouble..

because we are facing such situation in UMC pdk.. where the dummy poly length is different for NMOs and Pmos for a certain Width.. if we increase the widths of NMOS and PMOS the dummy poly length also same..

so what is the need for such change in dummy poly length..?

thanks..