Introduction

Before the transistor action was first confirmed by W. Shockley, J. Bardeen and W. H. Brattain at Bell Laboratories in 1948, photolithography had been well developed and widely used to fabricate printing plates. Therefore, it seems natural that this technology was readily applied to the fabrication of planar transistors which were designed about ten years after the invention of transistors. An example of photolithographic process applied to the fabrication of semiconductor devices is shown in Fig. 1.1, where the process to etch pattemwise the oxide layer formed on a silicon wafer is illustrated.