Silicon dioxide selective etching

A number of other types of processes that can be considered a form of self-assembly at surfaces are just beginning to appear. The selective oxidation of silicon, followed by etching of the silicon dioxide, as a route to silicon nanowires is an example172 the galvanic deposition of platinum on selenium nanostructures, followed by removal of the selenium, to make nanowalls with complex shapes is a second173,174. 

The major problem, then and now, in attempting to deposit blanket tungsten is the adhesion to silicon dioxide. Unless some steps are taken beforehand, the H2 reduction of WF6 on Si02 will not produce an adherent film. The solution to this problem was to undertake the selective process initially.18 It is then pointed out that WF6 will decompose on Si, but it will slowly etch Si02. Once the Si02 has been etched by WF6, then the W deposited from the H2 reduction process adheres very well. 

Transparent, conducting, tin oxide coatings are used in applications where light must pass through the substrate in order to strike the active element such as a photoconductive or photoelectric material. Chemically deposited films of silicon dioxide serve as masks on semiconductor materials for selective doping in the preparation of integrated circuits that can later be removed by chemical etching. 

Ion track formation in dielectric materials is becoming more and more interesting, since there are promising applications in sub micro and nanotechnology [1-3], Some papers have already reported experiments concerning the formation of nanometer-sized pores in silicon dioxide by means of MeV ion beams, and subsequent selective etching of ion tracks by liquid or vapor hydrofluoric acid [4-6],... 

Ephrath, L.M. Selective etching of silicon dioxide using reactive ion etching with CF4/H2. J. Elec-trochem. Soc. 1979, 126, 1419-1421. 

The processing sequence for silicon dioxide (SiOi) depends on its specific use. For example, silica for use as inter-metallization insulation the order is deposition, densi-fication by annealing, and etching to the correct configuration. CVD processes for Si02 films can be characterized by chemical reaction type, the growth pressure, or deposition temperature. The choice of route is often dictated by requirements of the thermal stability of the substrate or the conformality. Table 5-4 summarizes selected... 

In plasma etchers, specific radicals are selected from the mix of the species generated within the chamber to effect the etching action. For the specific case of species generated from CF4 gas within a plasma chamber, for example, the fluorine radical (F) is selected by means of an appropriately configured perforated aluminum shield or other contraption that blocks the other species from reaching the wafer. In this way, etching of the wafer proceeds only by the reaction of the fluorine radical. Substrates such as silicon, silicon dioxide, and silicon nitride are readily etched by this technique. ... 

The Flamm formulas lead to the expression for the silicon-to-silicon dioxide etching selectivity ... 

The etching process results in removal of the silicon dioxide in the selected areas defined, for example, by the clear or opaque areas in the mask outlining the circuit pattern, so that the silicon is exposed. 

Figure 9. Formation of micropillars from a solid precursor (schematic). (1) A175 nm thick thermal silicon dioxide film is grown on a silicon wafer. (2) A photoresist is applied by spin coating. (3) The oxide film Is selectively exposed, and then (4) selectively plasma etched with CHFs. (5) The photoresist is stripped, and (6) the silicon water is plasma etched with CI2/BCI3. (7) The residual oxide film is removed with a HF etch, and the silicon miaopillars as-formed are ready for use. Courtesy of Dr. A. Perez, Cornell University, Ithaca, NY.

Fig. 13.1 a Chrome on quartz hard photomask, b Schematic illustration of photoresist (purple layer) exposure to UV light through a quartz-chrome hard mask. Exposure of positive resist (fe ) and negative resist (right) followed by resist removal and subsequent silicon dioxide (white layer) etching to selectively expose the silicon substrate (blue layer)... 

---