Etching of silicon dioxide

D. J. Monk, D. S. Soane, and R. T. Howe, Hydrofluoric acid etching of silicon dioxide sacrificial... 

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

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. 

Chemical etching is a process for removal of silicon dioxide films through dissolution in solutions. Dissolution of silicon oxides, in the context of this book, is related to the anodic behavior of silicon electrodes. However, the dissolution of anodic oxides is not well studied. In contrast, there is a wealth of information on the dissolution of other types of oxides. Much of this information must also be applicable, at least the qualitative and mechanistic nature, to that of anodic oxides. Also, because oxides of different types are commonly used in device fabrication, compiling the etch rate data of these oxides and those of silicon (presented in Chapter 7) in the same volume would be useful in practice. Additionally, because silica-water interaction, which has been extensively investigated in the geological field, is fundamental to the etching of silicon oxides, some of the results from the investigations on the dissolution of rocks and sands are also included. 

K. Osseo-Asare, Etching kinetics of silicon dioxide in aqueous fluoride solutions A surface com-plexation model, J. Electrochem. Soc. 143, 1339, 1996. 

W. Van Gelder and V. E. Hauser, The etching of silicon nitride in phosphoric acid with silicon dioxide as a mask, J. Electrochem. Soc. 114, 869, 1967. 

R. A. Haken, I. M. Baker, and J. D. E. Beynon, An investigation into the dependence of the chemically-etched edge profiles of silicon dioxide films on etchant concentration and temperature, Thin Solid Films 18, S3, 1973. 

Bertrand and Fleischer (38) have studied the chemical deposition of silicon dioxide on Indium phosphide. They found that on unoxldlzed, etched surfaces, oxide coverage was "always patchy". If the InP had a monolayer of chemisorbed oxygen, an approximately 60A-thlck oxide film could be formed at room temperature through the formation of Sl-O-P bonds at the Interface. 

The membranes of silicon dioxide are etched in vapors of Hydrofluoric (HF 49%) acid. It is a type of dry etching, because contact of the wafer with liquid HF must be avoided. The wafer is placed with the back side down on a pot with the acid. The pot has a diameter smaller than the diameter of the wafer, i.e., if the wafer has a diameter of 4 in., the pot should have a diameter of about 3.8 in. (see Note 9)... 

The rest of the processing will depend on the nature of the device being produced. A simple p-n junction is usually fabricated via photolithography and etching processes. In this method, a layer of silicon dioxide, Si02, is created on the surface of the wafer by heating it in the presence of... 

A fresh surface of thermally grown silicon dioxide, it must be pointed out, is hydrophobic. However, it quickly reacts with water vapor in the atmosphere to form silanol (Si—OH) and gradually becomes hydrophilic. In fact, the chemical vapor deposition of silicon dioxide forms only a silanolated surface. Being fairly hydrophobic, resists do not adhere well to hydrophilic surfaces such as Si02-These surfaces contain hydroxyl groups as illustrated in Reaction [11.1]. The adhesion failure of resist films on such surfaces is often observed in the course of development or wet etching. As a result, a surface treatment to promote adhesion is necessary before the resist film is deposited on such surfaces. ... 

The choice of etchants is dependent on the nature of the substrate to be etched. Silicon dioxide is etched with a mixture of aqueous solution of hydrogen fluoride (HF) and ammonium fluoride (NH4HF). Paris et al. have reported the following functional expression for the etch rate in angstroms per minute of silicon dioxide ... 

Following the STI etch operation, the wafer is again cleaned to remove contamination and surface oxide, after which it undergoes a high-temperature oxidation in a furnace in the presence of oxygen. This results in the growth of a thin layer of silicon dioxide (called the liner oxide) in the exposed walls of the isolation trenches. The liner oxide serves to improve the interface between the silicon and the trench CVD oxide that will be subsequently deposited. The trench is then filled with CVD oxide (see Fig. 16.8). 

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