Advanced Plasma Processing: Etching, Deposition, and Wafer Bonding Techniques for Semiconductor Applications

Plasma processing techniques are one of the cornerstones of modern semiconductor fabrication. Low pressure plasmas in particular can achieve high radical density, high selectivity, and anisotropic etch profiles at low temperatures and mild voltages. This gentle processing environment prevents unwanted diffusion and degradation of materials due to heat and lattice damage from ion bombardment. Plasma treatments have a minimal effect on existing wafer structure, which is a key requirement for large scale integration schemes such as CMOS. In addition, recent progress in plasma-assisted wafer bonding has demonstrated low temperature, low pressure recipes utilizing O2 plasma surface treatment for joining dissimilar semiconductor materials, such as silicon (Si) and indium phosphide (InP) (Fang et al., 2006). In this chapter, we will cover the applications of plasmas to etching and depositing materials, as well as novel processing modalities such as surface treatments in preparation for wafer bonding. All these processes rely on the inductively coupled plasma reactive ion etcher (ICP-RIE) used in the integrated electronics industry, which we will explain in detail. The unique chemical environment of ICP-RIE generated plasmas gives process engineers new capabilities that are not found in other techniques that are compatible with existing architecture requirements. After an overview of the principles of the ICP-RIE, we will describe our work in novel mask materials and processing conditions in plasma etching and deposition. High aspect ratio nanopillars have recently been fabricated using this technique, with features as small as 22 nm etched over 1.25 μm deep. In particular, our use of Al2O3 as a mask material along with cryogenic wafer temperatures has demonstrated to increase the etch selectivity of silicon over mask to more than 5000:1, enabling ultrathin masks for nanoscale pattern transfer (Henry et al., 2009a). After patterning, in situ deposition can encapsulate these structures in preparation for further processing. 5