It's difficult finding things going back that far, but one of the first links I came upon...
http://www.answers.com/topic/ati-technologies-inc
They had fully and partially owned foundries in both Canada and Taiwan (1995 and onwards joint foundry started in Taiwan with a few other partners).
Yes, they actually used to manufacture their own chips in addition to assembling their graphics boards themselves.
Oh, interesting, I hadn't known that ATI had aquired the graphics technology of Tseng Labs, one of my favorite graphics chip makers of the early 90's.
Anyways, I know they still manufactured boards in Canada at least into 2005, but I'm not sure if they continue to do so.
Sapphire was brought on board to boost board production (operating alongside the Canadian board assembly plant) with the 8500 or 9700 series of graphics boards? It may have been earlier than that, but I'm not sure.
ATI started the transition to fabless around 2000 or 2001 I think, but again my memory is a little fuzzy on that one.
Regards,
SB