Introduction

Abstract Up until the late 1960s, chemical etching, also called wet solvent etching, was the key technology to integrated circuit manufacturing. Indeed, from the standpoint of manufacturing, wet etching provides low cost and often infinite selectivity. But, inherent limitations in wet etching preclude the use of this technique for micron and submicron pattern sizes. The most serious limitation is that of isotropic etching, which results in undercutting of the mask material and hence limits the minimum size of the pattern. To obtain high-density packing of microelectronic circuits, anisotropic etching is essential. By the early 1970s, CF /02 dry etching was widely adopted for patterning. It was widely recognized that dry etching offered the possibility of a vertical etch rate that greatly exceeded the horizontal etch rate (namely anisotropic etching). As a result, these dry etching techniques, which can generate anisotropic etch profiles, came into prominence. Dry etching soon began to appear in IC fabrication. However, these dry etching technologies, like wet processes, had their resolution limited by undercutting, and other, even more anisotropic etching techniques appeared. Among these were reactive ion etching (RIE). The RIE technique which has been widely adopted for IC fabrication until now provides moderate selectivity, etch rate and high anisotropy. In the mid-to-late 1970s, research began on a dry etch technique which would combine the positive features of RIE with directional bombardment of the surface by ions accelerated through electrostatic sheaths. Figure 1.1 shows how integrated circuits have evolved and how the number of individual components per circuit has increased with time. It is seen in the figure that the component density of IC circuits has doubled every year and this yearly doubling rate has been maintained from the first IC in 1960 until recently. At the same time, the design feature size has shrunk from 6 μm to 0.8 μm in the last 15 years, and the smallest dimensions are expected to decrease to 0.1 μmin the near future.