Masking materials

As with any other fabrication process, masks are needed to define the features to be etched. It is common that the etch used for the semiconductor also etches the masking material. For this reason many different masks are used in etching, including photoresist, dielectric films, and metals. Masking can be a complex issue, especially when very deep etches (>5 fim) are performed with high aspect ratios (148). 

Many different materials can be used to spatially mask an implant on the semiconductor surface. Such masks include photoresist, dielectrics, and metals. In order to be an effective implant mask the material should be thick enough to prevent the implant from penetrating the mask and entering the sample. A minimum thickness for stopping 99.99% of the ions in the masking material is + 3.72Ai p (168). 

Combustible masking materials such as organic char may be partially or completely removed by periodic elevations of the catalyst bed temperature. Noncombustible masking materials may be removed by air lancing or aqueous washing generally with a leaching solution (20,21). 

By the end of the war, poison gases filled one in four artillery shells used by both sides. In military terms, however, poison gas failed. Since masks provided quite effective protection, poison gas was never a decisive weapon on the Western Front the fatality rate for firearms was ten times higher. Poison gas was not used in the next world war. In fact, if World War I had continued, chemical warfare would have backfired on the Germans. Prevailing winds blow eastward, and Germany had run out of mask material and had no fabric to reclothe soldiers blistered by corrosive gases. 

Combining flow manifolds, 73 272 Combipress, molecular formula and structure, 5 161t Combustible masking materials, 10 91 Combustion, See also Fire entries in diesel engines, 72 420-421 energy loss from, 70 138 of ethers, 70 579-580 explosives and propellants during, 70 719... 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

Surface layers of silicon oxide are important in semiconductor device fabrication as interlayer dielectrics for capacitors, isolation of conducting layers, or as masking materials. However, anodic oxides, due to their relatively poor electrical properties, breakdown voltage, and leakage current, have not yet found much use in device technology, and cannot compete with thermal oxides obtained at high temperatures of 700 to 900 °C. 

Recently a detailed study has been made of the methods required to obtain spectra of micro and ultra-micro amounts of sample 117). A focussing micro-illuminator is used for the detection of 1-10 micrograms of samples. Often, one can obtain the spectrum of impurities like fish-eyes in plastics by merely punching a hole the size of the imperfection in a masking material like cardboard and run the spectrum until the signal-to-noise level has reached a sufficient value to make an identification of the impurity. One can carry out a similar measurement in regions where the impurity appears to be absent and subtract the two spectra to enhance the spectra of the impurity relative to the more dominant polymer. 

Figure 11. Schematic diagram comparing the control of etch profiles by the use of erodible and nonerodible mask materials. (Reproduced with permission from reference 199. Copyright 1980 John Wiley.)...

Efforts continue in the Far East, particularly in Japan, by Horie et al., on photosensitive polyimides containing epoxide groups [44,45]. These studies focus on the chemical amplification of photo crosslinks in the resulting materials for use in making mask materials in silicon chip fabrication. 

An understanding of gas-phase and surface chemistry is particularly important to the next generation of MOVPE processes involving selective epitaxy [18] and atomic layer epitaxy (ALE) [19]. In the first process, the compound semiconductor is deposited selectively on substrate areas opened in a suitable masking material (e.g., SiOz). This is achieved by operating under conditions where nucleation occurs only on the substrates. Slight variations in processing environment and the presence of impurities can cause nucleation on the mask and result in loss of selectivity. 

Uses Chief combustible ingredient of black powder. Component of gas-mask material. For the latter purpose an activated product is being used. 

Since the displacement reaction initiates at the surface of the metal, it is also possible to use a resist mask to restrict the replacement of the metal in desired confined regions. In general, the SGDR process can be carried out using either aqueous or organic solutions, therefore no specific limitations on the choice of both substrate and mask materials are imposed by the chemical environment of the reaction. 

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