Etching plasmas

Plasma etching is a new technique which allows the measurement of the thickness of the top layer in asymmetric and composite membranes. The uniformity of the structure in the 

Seleciivity and permeation rate as a function of the etching time with PES hollow fibers. Dashed line 1 untreated fibers Curve 2 etched fibers [31]. 

As the name implies, the species that do the etching in this technique are gaseous plasmas generated within an evacuated chamber to which is introduced the etchant precursor gas(es) and which is subjected to high-frequency electric power via external electrodes. These plasmas comprise ionic and neutral fragments. For a precursor etchant gas such as carbon tetrafluoride (CF4), species including C, CF , CF, C, F, CF, CF2, and CF3 have been detected in the generated plasma.  

Mayer and R. A. Baker, Reactive ion beam etching with CF4 characterization of a Kaufman ion source and details of Si02 etching, J. Electrochem. Soc. 129, 585 591 (1982). 

In plasma etchers, specific radicals are selected from the mix of the species generated within the chamber to effect the etching action. For the specific case of species generated from CF4 gas within a plasma chamber, for example, the fluorine radical (F) is selected by means of an appropriately configured perforated aluminum shield or other contraption that blocks the other species from reaching the wafer. In this way, etching of the wafer proceeds only by the reaction of the fluorine radical. Substrates such as silicon, silicon dioxide, and silicon nitride are readily etched by this technique.  

Of course, SEM pictures can also be taken after different periods of plasma etching. This technique was used by van t Hoff [49], Fritzsche et al. [22, 50, 51], 

Khulbe and Matsuura characterized the dense PPO membrane etched by oxygen plasma [54] and studied the structural changes in the depth direction by AFM. Carbon disulfide was used as a solvent when the dense membrane was prepared. It should be recalled that unhke other solvents, carbon disulfide produced a dense PPO membrane, and supernodular aggregates could be observed on its surface. 

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Sugawara M 1998 Plasma Etching Fundamentals and Applications (New York Oxford University Press)... 

Figure C2.13.5. Schematic illustrations of isotropic etching by a neutral gas and anisotropic plasma etching.

Figure C2.18.3. Relationship between ion-assisted etching and directionality in plasma etching, (a) Demonstration of the synergy between ion bombardment and reactive species during ion-assisted etching, (b) Ions incident on an etched feature. This situation prevails in glow discharges when the feature dimensions are much less than the plasma sheath thickness. Reproduced from [35]...

Coburn J W and Winters H F 1979 Ion- and electron-assisted gas-surface chemistry—an important effect in plasma etching J. Appl. Phys. 50 3189-96... 

Chang J P, Arnold J C, Zau G C H, Shin H-S and Sawin H H 1997 Kinetic study of low energy ion-enhanced plasma etching of polysilicon with atomic/molecular chlorine J. Vac. Sc/. Technol. A 15 1853-63... 

Incorporation of cyclic aliphatic (aUcycHc) side groups markedly improves the plasma etch resistance of acryhc polymers, without reduciag optical transparency at 193 nm (91). Figure 32 presents stmctures of some acryhc polymers currendy under study for use ia 193-nm CA resists (92—94). Recendy, polymers with main-chain aUcycHc stmctures have been described that offer similar properties (95,96). 

Plasma etching is widely used in semiconductor device manufacturing to etch patterns in thin layers of polycrystaUine siUcon often used for metal oxide semiconductor (MOS) device gates and interconnects (see Plasma TECHNOLOGY). 

The distribution of impurities over a flat sihcon surface can be measured by autoradiography or by scanning the surface using any of the methods appropriate for trace impurity detection (see Trace and residue analysis). Depth measurements can be made by combining any of the above measurements with the repeated removal of thin layers of sihcon, either by wet etching, plasma etching, or sputtering. Care must be taken, however, to ensure that the material removal method does not contaminate the sihcon surface. 

Surface treatment of the composite can have a significant effect on adhesion. Surface treatment enhances one or more of the mechanisms described previously. Wu et al. [15] studied the effects of surface treatment on adhesive bonding for AS-4/APC-2 laminates. They found that the greatest bond strength was achieved from acid etching and plasma etching the composite surface. Table 1 summarizes the various surface treatments that were evaluated. 

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