Silicon oxidation cleaning solutions

Piranha solution (cleaning) An oxidative cleaning solution based on sulfuric acid and ammonium persulfate. Used to clean silicon wafers. 

A hydrophilic surface condition has been related to the presence of a high density of silanol groups (Si-OH) or to a thin interfacial oxide film. Such an oxide can be produced chemically by hot HN03 or by solutions containing H202. The three most common cleaning solutions for silicon are based on the latter compound ... 

Thus, unless in a vacuum, the surface of silicon is never clean because of the adsorption by foreign species. As will be seen in the following sections, the type of termination, in terms of chemical nature, thickness, and composition, is a function of how the surface is prepared and cleaned. In particular, the surface cleaned with HF solutions, which are widely used for preparation of silicon surfaces, is known to be hydrogen terminated. On the other hand, in water and non-HF solutions, silicon surfaces tend to be covered with an oxide film. Employing certain treatment processes, the surface of siheon can be terminated by different species—and/or F and/or OH—using different cleaning solutions as shown in Table 2.7. ... 

According to Loewenstein et the deposition of the contaminant level of metals from cleaning solution onto the silicon or silicon oxide surface occurs by replacing the hydrogen ion on the surface silanol groups ... 

In nonaqueous solutions the formation of oxide on the surface of silicon requires the presence of water. In solvents such as acetonitrile, nitromethane, and dimethyl sulfoxide, the HF-cleaned silicon surface gradually evolves from a H-terminated passive surface to a silicon oxide-covered surface due to the residual water (-lOpprn) present... 

Formation of the first layers of oxide (i.e., native oxide) on the surface of silicon, according to Ozanam and Chazalviel, " appears to also require the presence of water even in nonaqueous solutions. On immersion into the solution the silicon surface is gradually evolving from a Fi-terminated surface (after FiF cleaning) to a silicon oxide-covered surface due to the residual water present in the nonaqueous electrolyte (10 ppm). Initially the water is molecularly adsorbed at the silicon surface, then slowly oxidizes the surface silicon atoms to form oxide islands. The oxide islands are about 0.6 nm thick and cover about 60% of the surface area after 1 week of immersion in various nonaqueous electrolytes. 

In alkaline solutions, silicon oxides etch at very slow rates which allows them to be used as masks during silicon etching in these solutions. For example, the etch rate of thermal oxide is about 0.2 A/s in 40% KOH at 85 and is less than 0.02 A/s in the standard SC-1 cleaning solution. Figure 4.8 shows that in KOH solutions the etch rate of thermal oxide increases with concentration reaching a peak value at about 35%. Figure 4.9 reveals that the etch rate in TMAH solution decreases with... 

Silicon is a rather active element and unless in a vacuum its surface is never clean because of the adsorption by foreign species. In water and aqueous solutions, the surface of silicon can be terminated by various species including hydrogen, hydroxyl, fluorine, and oxide. The specific type of termination, in terms of structure and composition, depends on how the surface is prepared and cleaned. In non-HF aqueous solutions, the silicon surface is generally covered by an oxide film and in HF solutions the silicon surface tends to be terminated by hydrogen (in the form of hydrides). The formation of a surface hydride layer or oxide layer is responsible for the stability of silicon in aqueous solutions. 

One obvious interpretation of above results is oxidation at the Si/Cu interface through voids in the Cu layer. As preliminary test to clarify whether these observations may be asssigned to copper and/or silicon oxidation, the porosity of the Cu films was inspected by immersing a Cu/Si junction into an acidic solution of CuSC>4 (pH 2) with increasing HF content. Initially, the open circuit potential (OCP) of the silicon covered by the Cu film was - 0.38V, which is equal to the rest potential of a clean Cu wire. This indicated that the junction n-Si/Cu/CuSC>4 solution was at equilibrium (Fig. 3A). Addition of a small amount of HF, up to 2%, however, induced a rapid shift of the OCP of the n-Si/Cu electrode, the value being intermediate between that of the Cu wire and that of the bare n-Si electrode in contact with the C11SO4 solution (Fig. 3B, the rest potential of bare n-Si is - 0 64 V). Since HF is known to dissolve Si oxide, the negative shift of the OCP means that HF actually reaches the Si surface, i.e. that Cu films are not ideally compact. Porosity of the Cu films is also... 

The SC-1 cleaning solution is used as part of the RCA cleaning process for silicon wafers and is a mixture of ammonium hydroxide, hydrogen peroxide, and water. This process is used to remove undesired particles and contaminants from silicon wafers. It is important to maintain the composition of the solution because an improper ratio of ammonium hydroxide to hydrogen peroxide can lead to etching of the silicon surface, ineffective cleaning of the wafer, or damage to both the thermal oxide or doped silicon dioxide layers. 

The PDMS (M = 6000 g/mol) and the carboxylated PDMS (CPDMS) were synthesized by Owens-Corning Corporation. The ionic groups (4.6 mol %) were randomly distributed along the PDMS chain. Lithium salts of the PDMS carboxylated ionomer (Li-CPDMS) were prepared by neutralizing the acid derivative with Lithium acetate dihydrate. The li ionomer was a rubbery solid at room temperature and exhibited viscous flow above 100°C. The substrates used were atomically smooth silicon wafers with a 100 orientation and one side polished. These were washed in an ultrasonic bath for 10 min. and then a 30/70 (v/v) concentrated hydrogen peroxide/concentrated sulfuric acid solution for 1 h. to clean the surface. The wafers were then rinsed thoroughly with deionized water and dried under a stream of dry nitrogen. Since silicon oxidizes very rapidly, the surface of the wafers that were used in these experiments was actually silicon dioxide (silica). 

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