Chemical etchant

Most molded plastics have a very smooth, hydrophobic surface that must be modified. Chemical etchants are used to oxidize and roughen the surface. The resultant hydrophilic surface promotes good metal-to-plastic adhesion. The etchant is usually a solution of chromic acid and sulfuric acid pure chromic acid can also be used. 

For the more complex alloys anodic polarisation offers a vital extra variable in the use of selective etchants and has been widely exploited for alloys for which simple chemical etchants have proved inadequate. Besides many types of alloys steelthis technique has been applied to copper and the Cu-Be-Ni-Zr alloys " , uranium-base alloys " , Ni-Al alloys , tin-bearing invar and many others. 

Poly(methyl methacrylate) (PMMA) is a classical one-component, positive resist system. PMMA is a single, homogeneous material that combines the properties of excellent film-forming characteristics, resistance to chemical etchants and intrinsic radiation sensitivity. 

In the ion induced damage mechanism energetic ions break crystal bonds on the film surface thereby making the film more accessible and more reactive to the active chemical etchant. However, the side walls remain relatively unperturbed, and etching proceeds at the nominal chemical etch rate. Consequently, material removal proceeds far more rapidly in the ion flux direction, resulting in anisotropy. In actuality, the surfaces exposed to the plasma are likely to be composed of a chemisorbed coating of etchant... 

Some nonconductors, such as the polymers polycarbonates and polystyrenes, must be subjected to a surface treatment prior to activation to ensure good adhesion of palladium nuclei. Surface treatment can include the use of chemical etchants for plastics or reactive gas plasma treatments (66). 

Etchant Resistance. Etchant resistance refers to the ability of the resist to withstand the etching environment during the pattern transfer process. The most common method of pattern transfer is wet chemical etching which places emphasis on the adhesion and chemical stability of the resist. Etchant solutions may be either acidic or basic, depending on the type of substrate to be etched. For example, buffered hydrofluoric acid is used to etch Si02. However, lateral penetration of the chemical etchant is significant for thick substrate films and... 

The preparation or etching of compound semiconductors is more complex due to the potential of altering the surface stoichiometry. Shiota et al. (36) used AES to show that the final As/Ga at the surface of GaAs was very dependent on the chemical activity of wet chemical etchants. Bertrand was able to follow the changes in the chemical bonding of Ga and As on p-type GaAs etched in HC1 or Br in methanol and relate this to Schottky barrier heights of similarly prepared surfaces with Pb contacts (37). 

In the case of a photoresist, the ultimate definable feature size together with the ability of the material to withstand either chemical etchants or plasma environments determines the domain of utility. The feature size is in turn determined by the wavelength required for exposure, the sensitivity and contrast of the resist, and the dimensional stability of the material during exposure, development, and subsequent processing. Adhesion of the resist to the substrate is critical both for patterning and use, and adhesion can be affected by surface preparations, and by residual stresses developed during deposition and cure. While photo-imagable polyimides have been introduced, their principal intended application is as a component of the finished part, either as passivant or interlevel dielectric (see below). 

J. J. Gannon and C. J. Nuese, A chemical etchant for the selective removal of GaAs through Si02 masks, J. Electrochem. Soc. 121, 1215, 1974. 

M. T. Kelly, J. K. M. Chun, and A. B. Bocarsly, High efficiency chemical etchant for the formation of luminescent porous silicon, Appl. Phys. Lett. 64(13), 1693, 1994. 

Many chemical etchants are mixtures of acids with a solvent such as water. Acids oxidize atoms of a specimen surface and change them to cations. Electrons released from atoms of specimen surfaces are combined with hydrogen to form hydrogen gas. For more noble materials, etchants must contain oxidizers (such as nitric acid, chromic acid, iron chloride and peroxides). Oxidizers release oxygen, which accepts electrons from atoms of the specimen surface. Table 3.1 lists some commonly used etchants, their compositions and applications. 

In the process of fabrication these materials have been exposed to a wide range of materials or chemicals, ranging from plasmas to aggressive chemical etchants. Environmental effects may also be produced as a result of exposure to ambients such as vacuum and high temperature, as well as air laden with moisture and a variety of contaminants. 

The purpose of the resist is to protect the desired areas from the attack of the chemical etchants while the exposed portions of the substrate are being dissolved, and the Importance of the adhesion of the resist material to the substrate Is quite obvious. Lack or Inadequency of adhesion culminates In undercutting which limits the pattern resolution and consequently the device yield. Ideally, one would like to have patterns with vertical walls and the phenomenon of undercutting results In slanted patterns. 

Several chemical etchants can etch silicon isotropically or anisotropically, be dopant-dependent or not, and have a wide range of selectivity to silicon, which determines the appropriate masking material. Brief but clear descriptions of the silicon chemical etchants and their properties can be found in [2-4, 7]. 

Alkanethiolate coating of the gold particles protects them against action of chemical etchants. Protected colloids dissolve much more slowly in aqua regia than their unprotected counterparts. The level of protection observed from monolayers on the flat gold surface, however, is never achieved, probably because of the high curvature of the surface of gold... 

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