Precise etch stop for emitter etching of self-aligned heterojunction bipolar transistors

Dry etching of the AlInAs/GaInAs heterostructure for self-aligned heterojunction bipolar transistors was performed in a Cl,/Ar plasma generated with an electron cyclotron resonance source. Optical emission spectroscopy was used to monitor the Ga emission intensity at 417.2 nm for endpoint detection in order to overcome nonselective etching between AlInAs and GaInAs as well as run-to-run variations in etch rate. The etching of the AlInAs emitter layer was stopped at different overetch times after the increase in the Ga intensity was detected. Ex situ surface analysis was used to characterize the surface after etching. X-ray photoelectron spectroscopy showed that both Al and Ga were present on the surface when overetching 3 s. No Al was detected for the 6 s overetch time, indicating that the AlInAs layer was completely removed. The specific contact resistivity ( p,) at the etched surface was evaluated using transmission line measurements. As the overetch time increased from 3 to 6 s, pC decreased from 7.3 X 10m4 to 4.1 X 10m4 R cm*. This also indicates complete removal of the AlInAs emitter layer. Overetching of the GaInAs base layer should be limited for low base resistance to achieve better device performance. Therefore, reflectometry was used to measure the remaining thickness of GaInAs. For a 6 s overetch, less than 5 nm of the GaInAs base layer was removed and the AlInAs layer was completely etched. The surface morphology was also studied using atomic force microscopy. The root mean square surface roughness of 2.5 nm was obtained for the optimum overetch time. This was significantly lower than the value of 4.4 nm obtained after wet etching.