MODELING AND ALGORITHM DEVELOPMENT FOR AUTOMATED OPTICAL ENDPOINTING OF AN HBT EMITTER ETCH

This paper discusses the development of a high-accuracy endpointing al gorithm for the emitter etch of a heterojunction bipolar transistor (H BT). Fabrication of high-performance HBTs using self-aligned base-emit ter processes requires etching through the emitter layer and stopping with very high accuracy on-the base layer. The lack of selectivity in dry etching coupled with the high etch rates possible in high density plasmas render the use of a standard timed overetch impractical, espec ially as device layers continue to become thinner. The etch process un der study requires the complete removal of an AlInAs emitter while etc hing no more than 5 nm of the underlying GaInAs base layer. Etch produ cts are monitored using optical emission spectroscopy (OES) to determi ne etch endpoint. The process under study relies on the intensity of t he 417.2 nm Ga emission line. The detection of the Ga line indicates t hat the etch has reached the GaInAs layer. However, the presence of a time-varying Ga baseline signal before endpoint and significant noise in the OES signal necessitate more than a simple threshold scheme for critical endpoint detection. The algorithm presented here is based on a generalized likelihood ratio with a signature function. This algorit hm is robust to variance in the optical gains of the measurement equip ment and is applicable to other etch pocesses. Experimental results of automated endpointing using this algorithm are presented in the form of pre- and post-etch ex situ film thickness measurements.