The Fire will play for the MLS Cup on Nov. 23 at the Home Depot Center against the winner of tonight's Kansas City Wizard-San Jose Earthquake game.
Justin Mapp and Damani Ralph set up the winner 10 minutes into overtime. Mapp carried the ball up the right side and around defender Marshall Leonard before centering to Ralph, who redirected it to Armas. He scored from six yards.
Chicago is 17-9-2 in its playoff history and 13-1-1 overall at home in the postseason.