HELP :: common-centroid

[hauntedhunter]

I was asked this question in an interview which I couldnt answer properly.Can ayone help please :

Out of the following common-centroid patters which is the best and worst case pattern :

ABBA ABAB AABB
BAAB BABA BBAA

Please help.

 

[mpkp123]

Out of these u can use ABBA or BAAB as the common centroid matching pair.
Because if u see carefully u will find that the chirality is not proper.
And also the edge effects due to eitching are also similar on them in these cases.
Here if u see AABB or BBAA or ABAB or BABA they are not even common centriod.
FOr a pair to be common centroic it should be :
1 Interdigitze.
2. Have there center of gravity point as same.
3.Should be dispersed properly through out the matching array.
4. have similar Etching variations on both sides for both transistors.

Added after 1 minutes:

One more thing their chirality should be the same.

 

[hauntedhunter]

Thanks for your help.Can you explain chirality a lil?And how get that in these cases.

 

[Saravanan.G]

I see three common centroid patterns.
1.all the patters are having common centroid.ie centroid of A and B are same.
2.etching variations will also be same as number of drains facing the field are same for both A and B.in addition dummy poly can be used to minimise the etching variation.
3. there is also an advantage of using
[AABB]
[BBAA]
pattern in term of sharing the drains of same transistor and common nets of A and B with reduced dummmies.

4.only differences are in distributon of fingers .
a good distribution is in pattern
[ABAB]
[BABA]

choosing things which suits better depends on the aspect ratio and total size of transistor.ie
if large size transistor are used we can use two fingers for each A/B in case of [ABAB]
[BABA] pattern
which will give both sharing of drains and good distributin of fingers throughout. than the
AABB
BBAA pattern