the epitaxial layer overlying each subcollector region in the case where a single subcollector is used the trench must cut through the subcollector into the substrate once the epitaxial region 13 is fully grown it is coated with a thick oxide 30 which in turn is coated with a photoresist layer 31 the oxide layer 30 must be at least approximately 20 of the thickness of the epitaxial layer as shown in fig5 the photoresist layer 31 is then exposed and developed using the well known photolitographic techniques to create an opening 32 therein this opening 32 is formed with a gridlike pattern so as to lay out in the photoresist layer 31 the final combination of the isolation grid to be formed in the layer 13 through this formed opening 32 in the photoresist layer a window 33 is opened in the oxide layer 30 by using a known etching technique such as chemical etching once the window 33 is created in the oxide layer 30 the photoresist layer 31 is removed and the unit is placed in a reactive ion etching chamber one ion etching process comprises the introduction of reactive material such as hydrochloric acid at a temperature of approximately 200 u00b0 c in vapor form into the chamber with the device to be etched this vapor is ionized using a plasma so that reactive ions of the reactive material i e the hydrochloric acid are formed these ions are then directed against the surface of the device which is to be etched this technique creates as shown in fig6 a straight vertically walled trench 34 in the epitaxial.