Applications of a novel method for deter-mining the rate of production of photochemical porous silicon

The rate of formation of photochemical porous silicon is measured in situ b y studying the rate of increase in the radius of the circular interference patterns contained in the reflected laser beam. This technique is used to s tudy 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 concentratio n 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 r ate of etching increases up to a certain flux, the value of which is depend ent on wavelength, and then exhibits saturation. In 25% HF the rate increas es linearly with flux. It appears that the transport of reactive chemical s pecies to the interface is responsible for the saturation of the etch rate. It is demonstrated that the method introduced here may be used in rare stu dies to investigate other factors that influence the etch rate and can be u sed to help elucidate the mechanism of etching. (C) 2000 Elsevier Science S .A. All rights reserved.