an apparatus for fabricating a semiconductor device having cooling jackets for preventing a gas from being exuded in a reaction chamber , thereby minimizing the generation of contaminating particles . the apparatus includes a reaction chamber having four cooling jackets respectively mounted on a first side wall adjacent to a wafer transfer chamber , a second side wall opposite to the first side wall , an upper wall and a bottom wall . a gate valve is disposed between the reaction chamber and the wafer transfer chamber and has a fifth cooling jacket . while fabricating a polysilicon film using the above apparatus , a pressure of a cassette chamber is controlled to be less than about 0 . 05 mtorr . alternatively , a pressure of a cooling chamber and the wafer transfer are both controlled to be less than about 1 . 0 μtorr .