Chemical etching stage

The blank is then normally electropolished or chemically etched into a sharp point. The two-stage electropolishing process illustrated in Figure 1.3 is commonly employed. 

Fig. 4. Schematic illustration of the final stage of chemical etching in HF solutions where Si-F is converted to Si-H. Note that the HF molecule is inserted into the Si-Si backbond due to polarization of the Si-F bond. Adapted from [37],...

Figure 1.9 SEM images [34] of (a) original porous SiC surface after PECE, (b) early stage formation of columnar pore in cross-section, (c) porous surface structure 20 pm below the original surface after 90 min of RIE (the inset shows the Fourier transform of a larger area of this picture), and (d) the self-ordered columnar porous structure below the cap layer in cross-section. Reproduced from Y. Ke, R.P. Devaty and W.J. Choyke, Self-ordered nanocolumnar pore formation in the photoelectro-chemical etching of 6H SiC, Electrochem. Solid-State Lett., 10(7), K24-K27 (2007). Copyright 2007, with permission from The Electrochemical Society...

The second method employs replication. Direct replica or two-stage replica of the free surface can be used. The free surface may be obtained, e.g., by cryofracture, chemical etching, ion bombardment, etc. Metal shadowing of the replicas with C/Pt coating is frequently used for contrast. 

Starting from a 35-70 mm wide blank tape (a), sprocket holes and window patterns are mechanically fabricated in the film by die punch or laser cut (b). The film is then laminated to 35 p-m thick copper foil (c). Photoresist film is deposited on copper and patterned through a mask (d), before beam leads are formed by chemical etching (e). In the final stage, the connecting pads along the periphery of the integrated circuit are bonded to copper leads (f). 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

Fig. 7.5. Atomic force microscopy (AFM) images in 3D recorded at two scan sizes of the four stages involved in the etching and chemical modification of capillaries for OTCEC.

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