Applications of a novel method for determining the rate of production of photochemical porous silicon

The rate of formation of photochemical porous silicon is measured in situ by studying the rate of increase in the radius of the circular interference patterns contained in the reflected laser beam. This technique is used to study the effects on etch rate of: (a) the composition and concentration of the etchant; and (b) the wavelength and flux of the laser irradiation. The technique is applicable over a wide range of reaction conditions and rates. The etch rate is found to be linearly dependent on the formal concentration of aqueous hydrogen fluoride (HF(aq)). The effect of increasing the flux of the incident radiation on the rate is more complex. In 48% HF(aq), the rate of etching increases up to a certain flux, the value of which is dependent on wavelength, and then exhibits saturation. In 25% HF the rate increases linearly with flux. It appears that the transport of reactive chemical species to the interface is responsible for the saturation of the etch rate. It is demonstrated that the method introduced here may be used in rate studies to investigate other factors that influence the etch rate and can be used to help elucidate the mechanism of etching.