Sulfate Adsorption onto and Desorption from Silicon Dioxide Films

Sulfuric acid, especially in a mixture with hydrogen peroxide, is a commonly used chemical in the surface preparation of silicon. Removal of adsorbed sulfate based contaminants by DI rinsing is an important step in processing steps using sulfuric acid. Fundamental information on the rate and extent of adsorption and desorption of sulfate on surfaces is very scarce.

This presentation will discuss sulfate adsorption onto and desorption from SiO₂ surfaces using a quartz crystal microbalance with dissipation monitoring capability (QCM-D). The amount of adsorption of sulfate was estimated from frequency shifts of a SiO₂ coated AT-cut quartz crystal. The quartz crystal, with a fundamental frequency of 5 MHz, consists of a quartz disk of 14mm diameter, and gold electrodes that are sputtered on both sides of the quartz disc. The SiO₂ coating, 300 nm thick, was deposited by PVD onto the active side of quartz crystals.

Figure 1 shows a real time frequency recording of a SiO₂ coated crystal exposed to flowing 0.2 M sodium sulfate solution. The frequency and dissipation change were measured for different harmonics of the fundamental crystal frequency at the same time. The divergent overtones and high dissipation factor indicated a “soft” adsorbed layer, rendering the Sauerbrey relation invalid for mass calculation from frequency changes. Hence, a viscoelastic model was used for conversion of frequency changes to mass changes. It can be seen the sulfate adsorption onto and desorption from SiO₂ surface happens fairly quickly as marked in the figure. At 22 C, the rate constants of desorption are about 0.06 s^-1 and 0.16–0.18 s^-1 for flow rates of 0.2 ml/min and 1.0 ml/min respectively.

The adsorption isotherm of sulfate is shown in figure 2. The equilibrium adsorption increases linearly with sulfate concentration at the range of 0.1 M to 0.6 M.