Resist mask

Step 9. Si02 is blanket deposited over the substrate. The resist (mask 3) that has openings over the Si02 is deposited and patterned. The exposed Si02 is etched down to the source, drain, and gate layers, creating contact windows for metallisation. 

Fig. 6.6 PS layer thickness inhomogeneities as a result of different kinds of masking layers and doping densities, (a) While underetching is minimal for a silicon nitride mask, (b) a resist mask shows severe under-etching.

The selective Cu deposition process was suggested by Ting and Paunovic (13) as an alternative means of fabricating multilevel Cu interconnections (Fig. 19.4). The first step in this through-mask deposition process (14) is the deposition of a Cu seed layer on a Si wafer, and then a resist mask is deposited and patterned to expose the underlying seed layers in vias and trenches. In the next step, Cu is deposited to fill the pattern. After the Cu deposition mask is removed, the surrounding seed layer is etched and dielectric is deposited. Electroless Cu deposition has been suggested for the blanket and selective deposition processes (15). 

It was found that 5-nm-thick resist-mask polysilane films worked well in a direct lithography process on silicon substrates, resulting into a line width of 40 nm prepared by scanning probe microscope lithography, using a carbon nanotube tip.57 Thin PMPS films of 6—8 nm, with a molecular weight of 30,000 were prepared by spin casting and cured at 150°C to obtain a smooth surface. It has been interpreted that moisture was essential for the oxidation of the polysilane. The proposed mechanism involved dissociation of Si-Si bonds in polysilane by the electron injection from the carbon nanotube tip catalyzed by moisture. 

Then, the patterned resist is developed by a dry process, followed by etching of the substrate through the resist mask window. 

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. 

A resist mask is realized onto a silver film through standard lithographic techniques, such as optical or E-beam lithography (Fig. 14.8a) ... 

L. T. Romankiw, A Review of Plating Through Polymeric Resist Masks, Extended Abstracts of the Electrochemical Society, 79-2, No. Abstract No. 462, 1165-1166, The Electrochemical Society Inc., Pennington, NJ (1979). 

Sometimes, the last reaction is used to obtain a negative-tone resist mask by means of the DNQ-novo-lac positive photoresist.Imidazole, triethanol amine, and other bases can be used as tone-modifiers. 

Strictly speaking, the dry-process compatibility for a resist is very process dependent, and must be measured for the specific process involved. A general idea of compatibility, however, can be obtained by doing a CF4/O2 plasma etch test vs Si02 and/or PMMA references, and this test has been adopted as a quick screen test for dry-process compatibility for new resists. The relative etch ratios vs these references usually, but not always, remain constant when the process requiring resist masking is changed (e.g., PE to RIE), thus, what is measured is resist compositionally-dependent. 

Pittman, M. Ueda, C. Chen, C. Cook, J. Helbert, and J. Kwiatkowski, Synthesis, radiation degradation, and electron beam resist behavior of fluorine containing vinyl polymers, J. Electro chem. Soc. 128, 1759 (1981) M. Kakuchi, S. Sugawara, K. Murase, and K. Matsuyama, Polymeric resist mask composition, U.S. Patent No. 4,125,672 (1978). 

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