Unable to find a buyer for its gigantic manufacturing plant in Irvine, Motorola Inc. confirmed that it will close the semiconductor factory later this year, eliminating 500 jobs in the process. A Motorola spokesman said there could be relatively few layoffs, though, as most of the workers will be offered jobs at other, out-of-state semiconductor factories operated by the company. The closest are in Phoenix, where Schaumburg, Ill.-based Motorola has a complex of eight factories. The Irvine closure has been anticipated since Motorola announced more than a year ago that it would begin consolidating operations and eliminating stand-alone plants like the 224,000-square-foot facility. Motorola, which bought the building from Western Digital Corp. in 1994 for $111 million, put it up for sale in October, but has been unable to find a serious buyer.