A reaction mechanism is proposed for the dissolution process of silicon dioxide networks in aqueous HF-based solutions. Etch experiments with thermally grown silicon dioxide were used to create a model for the etch process. Literature data on the etching of other vitreous silicon dioxide materials were used to refine the model. A new method, using a quartz microbalance, is used to monitor the etch rate in situ and to establish the reactive species. The first reaction step determines the rate of the etch process. It is the substitution of a surface SiOH group, which is bonded to three bridging oxygen atoms, by an SiF group. Due to an acid/base equilibrium reaction of the silanol groups on the surface with its protonated and deprotonated forms, the substitution reaction rate is pH dependent. At low pH (<1.5) water is eliminated from the protonated silanol group and an HF 2ion or an H 2 F 2 molecule supplies an Fthat binds to the positively charged silicon atom. At higher pH values (>1.5), the elimination of an OHgroup from the SiO 2 surface becomes the major reaction route. Once the OHgroup is eliminated, an HF 2molecule supplies an Fion. The pK a value of the deprotonation reaction increases due to the buildup of surface charge at pH > 4. Consequently, the SiOH surface concentration and the etch rate are higher than expected from a simple acid/base equilibrium reaction. All subsequent reaction steps to remove the Si-F unit from the SiO 2 matrix are fast reaction steps (18-20 times faster) involving HF 2addition reactions on F x Si-O bonds. Using this reaction model, published etch rate data of multicomponent glasses can be understood. Metal ions in glass break up the SiO 2 network and create Si atoms bonded to less than four bridging oxygen atoms. The nonbridging oxygen atoms are terminated by a metal ion, and the silicon bonded to these oxygen atoms etches as fast as the Si-F units in vitreous silicon dioxide. Therefore, the etch rates of multicomponent glasses are higher than that of vitreous silicon dioxide.
Neutral (arenethiolato)copper(I) complexes [CuSC6H3(CH(R')NMe2)-2-R"-3]3 (R' = H, R" = H, Cl; R' = Me, R" = H) with an intramolecularly coordinating ligand have been prepared and characterized. Crystals of [CuSC6H4(R-CH(Me)NMe2)-2]3-THF are hexagonal, space group P6}, with a -13.743 (1) A, 6 = 11.248 (1) A, V = 1839.8 (3) A3, Z = 2, and final R = 0.054 for 1448 reflections with I > 2.5
A reaction mechanism for the etching of silicon nitride layers in aqueous hydrofluoric acid solutions is proposed. The surface of Si 3 N 4 consists of SiNH 2 groups that are etched from the solid matrix via three possible routes. Depending on the pH, these SiNH 2 groups are protonated (pK a ϭ 1.4) to SiNH 3 ϩ . At ϽpH 4, the rate-limiting step consists of an elimination of NH 3 and a subsequent addition of F Ϫ or HF to the vacant surface site to form Si-F. At ϾpH 3, the elimination of NH 2 Ϫ is assisted by HF 2 Ϫ , followed by a transfer of one of the fluorides of HF 2 Ϫ to the vacant site. All subsequent reaction steps to remove the SiF unit are nucleophilic substitution reactions with low activation energies. The etch rates and mechanism of different types of silicon nitride films are compared with that of SiO 2 etching. Therefore, etch selectivity between these two materials can be explained. The theory is also applicable for silicon hydrogen passivation.
The impact of organic contamination on the quality of 5-nm-thick gate oxide structures, both before and after gate oxidation, is studied. Sources of organic contamination are chemical surface modification (i.e. hexamethyldisilazane priming), wafer box storage and extended vacuum exposure. Gate oxide integrity is evaluated electrically. The origin and/or nature of the organic contamination is seen to have different effects on the electrical breakdown. Care should be taken when exposing silicon wafers to organic contamination prior to processing. Especially when contamination occurs at the SiO2/polysilicon interface, i.e. prior to a non-oxidizing process step, organics can be extremely deleterious.
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