Selective Material Removal

In-depth selective etching of silicon in alkaline solutions can also utilize the different passivation potentials between p- and -type materials in alkaline solutions such asK0H, EDP, NH40H, hydrazine, andTMAH. In this method, as shown in Fig. 7.62(9), an anodic voltage sufficient to cause passivation of n-Si is applied via an ohmic contact. Due to the potential drop in the reversely biased/in junction, the p-Si is maintained at a potential negative to the passivation potential and is etched. On complete removal of the p-Si, the junction disappears and the etch stops because the n-Si is passivated. A current peak, corresponding to the formation of the 

TABLE 7.8. Examples of Silicon Structures that May Be Fabricated by Etching 

Locally confined etchant can also be used to preferentially etch small holes on silicon. In this technique an active etchant is generated through a reduction reaction at the tip of a fine electrode which is positioned near the silicon surface. In situations where diffusion-controlled process limits the in-depth etch rate, deep etching can obtained using a centrifugal force.  

Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF solutions. The formation of PS was first reported in the late 1950s in studies on electropolishing of silicon. Since then, particularly after 1990 when luminescence of PS was discovered, numerous investigations have been undertaken. These investigations have revealed that PS has extremely rich morphological features with properties that are very different from those of silicon and the formation process of PS is a very complex function of many factors such as HF concentration, type of silicon, current density, and illumination intensity. 

The amount of information on PS in the published literature is enormous and it is not possible to cover all aspects of PS in one chapter. Thus, the focus in this chapter is on the phenomena related to the properties of silicon, such as the formation of PS and the resulting morphology. The phenomena associated with the properties of PS and the applications of PS are only briefly mentioned at the end of the chapter. In particular, there is so much information related to the luminescence of PS that it would require a separate book to organize this body of information. 

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The differential chemical reactivity of the Cu and barrier films when exposed to the slurry chemicals in the polishing environment can lead to the desired selective material removal but can also generate a variety of defects—corrosion pits, fangs due to galvanic corrosion, etc., and the underlying processes can be best investigated using a variety of electrochemical techniques, as described in the chapter authored by Dipankar Roy. 

The existence of isotope shifts and of tunable lasers with narrow Hnewidth leads to the possibHity of separating isotopes with laser radiation (113,114). This can be of importance, because isotopicaHy selected materials are used for many purposes in research, medicine, and industry. In order to separate isotopes, one needs a molecule that contains the desired element and has an isotope shift in its absorption spectmm, plus a laser that can be tuned to the absorption of one of the isotopic constituents. Several means for separating isotopes are avaHable. The selected species may be ionized by absorption of several photons and removed by appHcation of an electric field, or photodissociated and removed by chemical means. 

Selection of the off-take position is important from the standpoint of the amount of material removed. Locating the off-take in the proximity of the material stream or at points of splash will result in greater removal of materials. This positioning may be desirable as a means to control splash effects provided that the off-take velocity is kept low. 

Ion exchange. Ion exchange is used for selective ion removal and finds some application in the recovery of specific materials from wastewater, such as heavy metals. As with adsorption processes, regeneration of the medium is necessary. Resins are regenerated chemically, which produces a concentrated waste stream requiring further treatment or disposal. 

The primary advantage of CD complexation is to stabilize and protect sensitive host molecules, such as flavors, odors, or pharmaceuticals. CDs sharply reduce the volatility, chemical, thermal and photo reactivity of guest moleciiles. More recently, CDs have been used for separation of components in solution. For example, CDs can remove reactive components from fhiit juices to prevent oxidation or eUminate bitterness. Attachment of CDs to chromatographic supports provides chiral separation, selective component removal and modified chemical reactivity. A number of modified and pol3nnerized CD materials have gained acceptance as separation media (9). 

The key to the successful development of this method lies in the development of inexpensive electrodes possessing high electrical conductivity, chemical and physical stability, large selective ion removal capacity, and reasonable electrochemical efficiency. Graphite and carbon-based materials are considered to be the most promising. 

Other precautions must be taken to prevent accidental or occasional increases in concentrations. Materials should be transported in safe containers, spilled material removed rapidly, and floor and wall materials selected to prevent contamination and allow easy cleaning. 

The chemical methods of industrial significance involve the dehydration of aspartic acid to form an acid anhydride, which is then coupled with the phenylalanine or its methyl ester to give the desired product. The two major processes are known as the Z and F processes (Schemes 19 and 20, respectively), named after the protecting group used on the aspartyl moiety.228 231 Both of these processes produce some P-coupled products together with the desired a-aspartame (24), but the selective crystallization removes the undesired isomers. However, because the amino acid raw materials are expensive, they must be recovered from the by-products and waste streams for recycle. 

The CMP output parameters include removal rate, planarization efficiency, surface finish, material removal rate selectivity, wafer-to-wafer uniformity, within-wafer uniformity, dishing and erosion, and defect levels [15]. 

FIGURE 5.25 Material removal selectivity of TEOS, Ti, and TiN to W for polishing on the fixed abrasive pad [80]. 

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