The common questions in analog layout

[dinosaur078]

hi there !

i have some questions for analog layout , could anyone can help me to explain or make it insight

Why need match
Why need same length
Why need unit width to replicate
Why need dummy
Why need shielding
Why care parasitic
Why need seal ring
Why top metal is immune to antenna effect

many thank in advance

 

[dick_freebird]

Why need match

Not all circuits do. Digital doesn't care, single ended
amplifiers don't care. But matching is one of the few
attributes of integrated technology that are superior
to the best-available discrete transistor in the best
(not the one-and-only-at-hand) technology, so these
matching-centric techniques are used to minimize error
and thus valued highly when they -are- employed.

Why need same length

Matching is most accurate when geometries, internal and
close-by, are uniform. Various lithography and proximity-
effect offsets are inherent to fabrication and these are
taken out by using identical-ness.

Why need unit width to replicate

Ditto

Why need dummy

See "proximity effects"

Why need shielding

You'd rather have even more noise?

Why care parasitic

Does your dog like fleas?

Why need seal ring

Some contaminants are mobile in the silicon and/or in
the interfaces between insulating layers. Electric fields
will draw them to wherever they do the most harm,
because the universe hates you. A properly constructed
seal ring prevents, or acceptably slows, such intrusion
and eliminates drifts that make circuits unreliable. It also
prevents passivation chip-off during die singulation that
would make things even more raw.

Why top metal is immune to antenna effect

The antenna effect is about the transistor "victims", not
the metal - the metal geometry, vs how much gate
voltage / current can be withstood, is the key. The
only "advantage" top metal has, is that the circuit
is more fully hooked up and more drains / sources are
protecting gate oxides in shunt. But this is less true
on FDSOI where no global bulk node exists, to shunt to,
and it is not universally true either - only more likely
than on layers below. You still need to verify for peace
of mind and desirable outcome.

 

[dinosaur078]

dear dick_freebird
thank you for your helps so much
could you explain more clearly about "Why need same length"
Matching is most accurate when geometries, internal and close-by, are uniform. Various lithography and proximity-effect offsets are inherent to fabrication and these are taken out by using identical-ness.

i do not understand so much

thank you in advance