The Silicon Valley chip maker, staggered by a continuing slump in semiconductor sales, also said it will reduce its work week and temporarily shut down some production facilities this spring. The Sunnyvale, Calif., company employs 6,300 in the United States. Layoffs will affect "primarily production workers and certain administrative positions in the company," spokesman Ronald Deutsch said. Administrative workers will begin four-day weekends on alternate weeks starting in mid-March and extending into June, the company said. The layoffs will affect Signetics facilities in Sunnyvale, Sacramento, Orem, Utah, and Albuquerque, N.M.