Different types of ASICs?

[Michael]

Hello there!

Talking about ASICs, what's (in detail if possible ) the
difference between "gate arrays" versus "full custom"?

I mean price, number of gates on chip, speed.

Thanks
--
Mike

 

[David Jones]

In article <biup3v$d8isk$5@ID-50260.news.uni-berlin.de>,
Michael <NOSPAM@invalid.com> wrote:

Hello there!

Talking about ASICs, what's (in detail if possible ) the
difference between "gate arrays" versus "full custom"?

I mean price, number of gates on chip, speed.

"Full-custom" is just that - you control the patterning of all layers of
the chip.

In a "gate array", the base layers are set down in a pattern defined
by the vendor. Typically, the N/P-diffusions, polysilicon and sometimes
first layer metal are set down. Your design fixes the metal layers
on top of the base.

Price: gate-array is usually a bit cheaper, since fewer custom masks
need to be cut. A full set of masks on a 0.25u can run up to $500,000.
With a gate array, you don't need the base layer masks.

Gate-array delivery may also be faster, as the fab can prefabricate
the wafers up to Metal-1, then set aside. You wait only for the
patterning of the top layer metals.

As for gate capacity, full-custom will have a slight advantage, as
each gate is only as big as it needs to be. With a gate array,
your gates are "quantized" to fit the pre-patterned base layers.

Speed: again full-custom has the advantage as circuits can be
individually optimized, whereas in the gate array, device sizes
are quantized.

Also look out for custom blocks such as analog cores and memories.
With full-custom you can do anything, but with gate-array you must
choose a vendor that offers those primitives in the base array.

The general trade-off is flexibility vs. cost and delivery time.