Atomic layer etching (ALE) is a technique for removing thin layers of material using sequential reaction steps that are self-limiting. ALE has been studied in the laboratory for more than 25 years. Today, it is being driven by the semiconductor industry as an alternative to continuous etching and is viewed as an essential counterpart to atomic layer deposition. As we enter the era of atomic-scale dimensions, there is need to unify the ALE field through increased effectiveness of collaboration between academia and industry, and to help enable the transition from lab to fab. With this in mind, this article provides defining criteria for ALE, along with clarification of some of the terminology and assumptions of this field. To increase understanding of the process, the mechanistic understanding is described for the silicon ALE case study, including the advantages of plasma-assisted processing. A historical overview spanning more than 25 years is provided for silicon, as well as ALE studies on oxides, III–V compounds, and other materials. Together, these processes encompass a variety of implementations, all following the same ALE principles. While the focus is on directional etching, isotropic ALE is also included. As part of this review, the authors also address the role of power pulsing as a predecessor to ALE and examine the outlook of ALE in the manufacturing of advanced semiconductor devices.

As we enter the era of ultra-large-scale integrated circuit manufacture, plasma etching grows more important for fabricating structures with unprecedented dimensions. For feature sizes below 1 μm and aspect ratios (depth/width) much larger than one, etching rates have been observed to depend on aspect ratio and pattern density. Such dependencies tend to increase the cost of manufacturing because even small changes in device design rules, cell design, or wafer layout can result in time-consuming, new plasma process development. In addition, microscopically nonuniform etching affects the trade-off between chips lost from failure to clear and chips lost by damage from overetching. Although aspect ratio and pattern dependent etching have been observed for a large variety of material systems and processing conditions, the fundamental causes underlying these effects are poorly understood. Partly, this results from use of confusing and conflicting nomenclature and a lack of careful, quantitative comparisons between experiment and theory. In this article we review recent literature on microscopic uniformity in plasma etching and carefully define terminology to distinguish between aspect ratio dependent etching (ARDE) and the pattern dependent effect known as microloading. For ARDE, we use dimensional analysis to narrow the range of proposed mechanisms to four which involve ion transport, neutral transport, and surface charging. For microloading, we show that it is formally equivalent to the usual loading effect, where the reactant concentration is depeleted as a result of an excessive substrate load.

No abstract

A "reference cell" for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

No abstract