See also TSMC OIP: What To Do With 20,000 Wafers Per Day although I screwed up the math and it is really over 50,000
I think that there are two reasons for doing this. The first is that TSMC is pumping out nodes very fast. Risk production for 10FF is Q4 of 2015 (which starts inext week) and so there are only 6 quarters between 10FF and 7FF if all the schedules hold. I think that makes it hard for Xilinx to get two whole families designed with some of the design work going on in parallel. It costs about $1B to create a whole family of FPGAs in a node. On the business side of things, 10nm would be a short-lived node. The leading edge customers would move to 7nm as soon as it was available so the amount of production business to generate the revenue to pay for it all and make a profit might well be too limited.
I contacted Xilinx to try and they pretty much confirmed my guess:
The simple reason is that our development timelines & product cadence lined up better with 7nm introduction. TSMC has a very competitive process technology and world class foundry services and their timeline for their 7nm introduction lines up well with our needs and plans.
There have been rumors that Intel might skip 10nm too, although the recent rumors are that they will tape out a new 10nm core M processor early next year. I don't know lf anything much that Intel has said about 7nm, either from a technology or a timing point of view.
See also Intel to Skip 10nm to Stay Ahead of TSMC and Samsung?
See also Intel 10nm delay confirmed by Tick Tock arrhythmia leak-"The Missing Tick"
That brings up the second big reason. All processes with the same number are not the same. TSMC's 16FF process has the same metal stack (BEOL) as their 20nm process. It is their first FinFET process and so presumably they didn't want to change too many things at once. Interestingly, Intel made the same decision the other way around at 22nm, where they had their first FinFET process (they call it TriGate) but kept the metal pitch at 80nm so it could still be single patterned. The two derivative TSMC 16nm processes, 16FF+ and 16FFC, have the same design rules and so the same 20nm metal. This limits the amount of scaling from 20nm to 16nm. There is a big difference in speed and power but not so much in density.
See also Scotten Jones's tables in Who Will Lead at 10nm?
At 10nm Intel has a gate pitch of 55nm and a metal 1 pitch of 38nm (multiplied together gives 2101nm2 although I get 2090nm2). TSMC at 10nm has a gate pitch of 70nm and a metal 1 pitch of 46nm, for an area of 3220nm2. But perhaps more tellingly, Intel's 14nm has a gate pitch of 70nm (same as TSMC's 10nm) and a metal 1 pitch of 52nm, only a little looser than TSMC's 10nm pitch of 46nm. So another reason Xilinx might skip 10nm is that it would not look good against Altera's products in 14nm.
TSMC say that 10nm is about 50% smaller than their 16nm processes. TSMC said that 7FF will be 45% of the area of 10FF. Without any information to go on, it is still clear that Intel's 7nm will be higher density than TSMC's. The TSMC 7nm process will probably close to the Intel 10nm process. This is not necessarily a criticism of anyone. Intel is totally focused on bringing out server microprocessors and can read the riot act to all their designers as to how restrictive their methodology has to be and the designers have to suck it up. TSMC has to accept a much wider range of designs from a broad group of customers that they do not control in the same way.
|Intel: you will do designs this way|
Intel designers: but...
Intel: you will
Intel designers: OK
|TSMC: you will do designs this way|
Apple engineers: no we won't
One wrinkle in all of this is also the Intel acquisition of Altera, Xilinx's primary competitor. They seem to have been struggling to tape-out their designs in Intel's 14nm process. If Intel is serious about using FPGAs in the datacenter, especially if they want to put the arrays on the same substrate as the processor, then they will need to get Altera's fabric into 10nm and then 7nm hot on the heels of the server processors themselves. Xilinx's worst nightmare would be if they produced a family of arrays in TSMC 10nm (only slightly better than Intel 14nm) and Altera got a family out in Intel's 7nm which is a generation ahead.
So, Xilinx skipping 10nm and Altera being acquired by Intel with an opaque roadmap makes for an interesting spectator sport.