Plasma etching discussion

In what follows, the fundamentals of plasma engineering are discussed with emphasis on plasma etching. Discussion pertains to the kind of plasmas used in electronic materials processing. Similarities and differences with electrochemical reactor engineering are pointed out along the way, and are summarized in Section 8. 

This case study is based on real industrial data collected from a plasma etching plant, as presented and discussed in Reece et al. (1989). The task of the unit is to remove the top layer from wafers, while preserving the bottom one. Four different objectives and performance variables are considered ... 

Dry etching techniques permit etch processes to be carried out in various modes. These can be described as purely chemical, purely physical, and a mixture of chemical and physical. With plasma etching and RIE, we have concentrated on chemical and ion assisted processes. In this section, etching methods that depend either solely or primarily on physical processes (momentum transfer) will be discussed briefly. 

The authors wish to thank E. A. Chandross for many helpful discussions, M. Y. Heilman for the determination of polymer molecular weights, J. Frackoviak for the electron beam exposures and SEM micrographs, and A. C. Adams for the plasma etch data. 

A brief description of the key plasma diagnostic techniques, which have been especially useful in delineating the gas phase processes in fluorocarbon plasmas, will be given followed by an extensive discussion of plasma etching and polymerization mechanisms. 

Some of the modern surface analysis methods have been used to study the surface chemistry of both plasma etched surfaces and plasma polymerized thin films. Much of this work has involved exposure of these surfaces to air as the samples are transferred from the plasma system to the surface analysis system. However, in a few cases surface analysis has been performed in the plasma system after the plasma gas has been pumped away. This work will be discussed in more detail later in this chapter. 

We wish to acknowledge helpful discussions with W. E. Quinn on the mechanism of oxygen plasma etching. 

Plasmas are also used for the low temperature deposition of thin solid films, for example amorphous hydrogenated silicon, diamond, and a host of other materials. Since the fundamentals of plasma physics and chemistry are the same for both plasma etching and plasma assisted chemical vapor deposition (PECVD), the latter will only be discussed briefly in Section 6.6. A review of PECVD can be foxmd in [14]. Sputtering is discussed by Chapman [15], and plasma polymerization is covered by Yasuda [16]. 

Process and system non-uniformity (8-3) can affect selectivity requirements. Similarly, the non-uniformity significantly affects anisotropy requirements as well. In general, there are essential trade-offs among anisotropy, selectivity, and uniformity of plasma etching processes, which are discussed, in particular, by Flamm and Herb (1989) and Liebermann and Lichtenberg (1994). 

Plasma can also be effectively applied for etching layers of silicon doped with refractory or rare metals, semiconductors, or via interconnects made with rare or refractory metals. Some of those etching reactions are illustrated in Table 8-4. These processes normally use fluorine-containing gases as a feedstock and have hexafluorides as volatile etching products. However, chlorine is preferred in some specifle cases. Only the most common plasma etching systems are discussed here much more detail on the subject can be found, particularly in the reviews of Flamm (1989) and Orlikovsky (2000). 

The fourth chapter examines the nodular structure of the membrane surface observed under AFM. It has been known for a long time that macromolecules form nodules at the membrane surface, and the size and the shape of the nodules strongly govern the membrane performance. In conjunction with an advanced technique such as plasma etching, AFM can reveal the nodular structure at the membrane surface more clearly than any other technique. In this chapter, the relationship between the nodular structures and the membrane preparation conditions is discussed for fiat sheet membranes in the first part and hollow fibers in the second part. This chapter also deals with the roughness at the membrane surface. 

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