CHEMICAL ISOTROPIC ETCHING OF SINGLE-CRYSTAL SILICON FOR ACOUSTIC LENS OF SCANNING ACOUSTIC MICROSCOPE

An acoustic lens for a scanning acoustic microscope (SAM) was newly fabricated by means of chemical isotropic etching of a silicon wafer. This novel lens fabrication process has been developed to obtain surfaces with good sphericity and smoothness, which are essential requirements for acoustic lenses. Acoustic lenses are conventionally fabricated by using grinding methods. A variety of wafers, etchants and etching conditions were investigated; the (100) wafer was the most preferable for fabrication of a symmetrical lens profile, and the optimum etchant was found to be a mixture of hydrofluoric, nitric and acetic acids with a ratio of 2:3:3 at a temperature of 50-degrees-C. A silicon acoustic lens with a radius of 140 mum designed for use at 600 MHz and a resolving power of 1.8 mum was successfully fabricated.