Method for making bipolar transistor having an enhanced trench isolation

A method for making a bipolar transistor comprising the steps of:

• injecting an impurity into a silicon substrate to form a conductive buried collector;
• forming a collector layer over the silicon substrate;
• sequentially forming a first insulating layer, a first polysilicon layer, a silicon oxide layer, a first nitride layer and a second polysilicon layer on the collector layer;
• defining active and inactive regions using a patterned photoresist layer formed on the second polysilicon layer;
• removing layers formed on the first polysilicon layer of the inactive region;
• forming a side wall nitride layer at both sides of the layers formed on the active region;
• depositing a second insulating layer thereon;
• removing the side wall nitride layer and performing an etching to form a trench;
• filling an isolation insulating layer into the trench;
• polishing the isolation insulating layer up to a surface of the silicon oxide layer;
• sequentially forming a second insulating layer on the isolating insulating layer and the silicon oxide layer;
• removing the first polysilicon layer and the first insulating layer formed on an inactive region other than an active region defined by the trench;
• thermal-oxidizing the collector layer formed on the inactive region to form a thermal oxide layer;
• removing the second insulating layer and sequentially forming a third polysilicon, a third insulating layer and a nitride layer;
• etching back layers formed on a portion of the first insulating layer to form an opening in the active region;
• forming a first side wall on both edges of the opening and removing the first insulating layer;
• forming an intrinsic base at a region where the first insulating layer is removed to electrically connect the intrinsic base with an extrinsic base in self-alignment;
• forming a second side wall on both sides of the first side wall;
• and
• forming an emitter layer on the intrinsic base.