etching is used to shrink the profile of the photoresist 34 as depicted in fig3 b the edges of the photoresist are now inside the etched b c mesa 22 the plasma etching process is optimally performed in an oxygen plasma with a pressure of 0 7 torr and a flow rate of 20 sccm and rf power of 500 w the plasma etching causes an undercut 37 shown in fig3 b the undercut 37 is essential for lifting off the evaporated oxide in the next step without this undercut the oxide will either not lift off at all or it will lift off partially with uneven edges and with a low yield next the layer of sio x 30 is evaporated and those silicon oxide portions on the photoresist layer 34 are lifted off this process uses the same photoresist 34 that was used during etching of the base collector mesa to protect the base emitter mesa this process results in a self aligned deposition which minimizes the dimensions and therefore minimizes the c bc and would result in good processing yield the sio x 30 that fills the etched mesa is shown in fig3 c the deposition of the sio x layer 30 is conventional and employs evaporation techniques well known in this art it can be seen that the sio x 30 overlaps the base 12 to ensure that in the next process step during deposition of the base metal 16 the base and collector terminals of the device are not shorted conventional processing is resumed to pattern photoresist and evaporate and lift off the base metal 16 as shown in fig3 d it can be seen that at the edge of the b c mesa the base metal 16 overlaps the sio x layer 30 which.