Anisotropic Silicon Etching

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

An interesting demonstration of profile control via alteration of the specific chemistry is that of silicon etching in C1F3 mixtures (86). Because a pure chemical (isotropic) etchant (F atoms) is combined with an ion-bombardment-controlled (anisotropic) etchant (Cl atoms), a continuous spectrum of profiles with varying anisotropies is generated by changing the gas composition. 

Figure 2.9 shows the shapes obtained when silicon is etched anisotropically. The four (111) levels cut the surface of a disk of (100) material along the (11 0) directions within it. The etchant, in turn, attacks each layer of the (100) levels and removes... 

Mechanism of Anisotropic Etching. The rich details as to anisotropic etch rates, nature of reactions, and surface topography indicate that a complex mechanism is involved in silicon etching in alkaline solutions. A coherent mechanistic model has to address three basic aspects (1) the physical cause of the difference in the removal rates of atoms from the surface of different crystal orientations, (2) the kinetic processes that actualize such physical cause, and (3) the surface condition that determines the global removal rate of the surface atoms. 

H. G. Linde and L. W. Austin, Catalytic control of anisotropic silicon etching, Sensors Actuators A 49, 181, 1995. 

Anisotropic etching, that is, different dissolution rates on different crystal planes, is a characteristic feature of silicon etching in alkaline solutions. Strictly speaking, the etch rate of silicon always depends, to a various extent, on crystal orientation in all etching solutions, acidic or alkaline. However, the etch rate difference among different planes is small in acidic HF solutions compared to those in alkaline solutions. Figure 26 shows the etch rate ratios of(100)/(lll) and (110)/(111) planes in various solutions. 

Fig. 3. Panorama of plasma etching using silicon etching with chlorine as an example. This figure also shows the disparate length scales involved from the reactor, to the sheath, to the microfeature, to the atomic scale. Cl radicals and CIJ ions are generated in the plasma by electron impact of gas molecules (a). Ions accelerate in the sheath and bombard the wafer along the vertical direction (b), thereby inducing anisotropic etching of microscopic features to yield SiCU, a volatile product (c). Ion bombardment creates a modified layer at the surface where Cl is mixed within the Si lattice (d).

Etching of silicon by fluorine atoms is the most practically important and the best characterized surface etch process (Flamm, 1989, 1990 Winters Cobum, 1992 Lieberman Lichtenberg, 1994). We consider first the pure chemical mechanism of the process, leading to isotropic silicon etching then we discuss ion energy-enhanced anisotropic etching. 

Fig. 12 SEM images of the composed etching profiles (a), (b) patterns realized by anisotropic silicon etching, deposition of passivation polymer layer, and isotropie...

Anisotropic silicon etching Deep reactive-ion silicon etching Electrochemical etching ... 

Silicon has a diamond cubic crystal structure. The Miller indices of the main crystallographic planes of silicon are (100), (110), and (111), respectively. In the wet bulk micromachining, there are two silicon etching methods isotropic (direction-independent) and anisotropic... 

In contrast to isotropic etching, anisotropic etching is a fabrication technique that removes material in specific directions allowing for the production of geometric characteristics such as sharp corners, flat surfaces, and deep cavities. Relative to microfabrication, selectivity can be defined as the ratio of the etch rate of the target material to the etch rate of other materials. The definition can also be applied more specifically to single crystalline materials such as silicon where it would be considered the ratio of the etch rate in the target direction to the etch rate in other directions. 

Fabrication of 3D Microfiuidic Structures, Rgure 12 SEM images of the composed etching profiles (a), (b) patterns realized by anisotropic silicon etching, deposition of passivation polymer layer and isotropic silicon etching (c), (d) patterns etched like (a) and (b), passivation with polymer and additional S13N4 layer (5). (With permission from Elsevier)... 

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