Silicon based GHz acoustic lenses for time resolved acoustic microscopy

A method is shown to produce silicon based acoustic lenses in the GHz-range for the time resolved acoustic microscopy of biological cells. The method avoids the problem of disturbing echoes from the rim around the lens cavity. Isotropic wet etched lenses made of silicon have been demonstrated in the past. In this report the micro machining process is extended with a dry etching step to avoid the rim echo which enables the use of short working distance lenses with a good signal to noise ratio in time-resolved acoustic microscopy. A deepening of the lens rim of about 15 um moves the echo from the rim behind the target echo and therefore assists in overcoming one major obstacle towards the observation of biological cells with time-resolved acoustic microscopy. Acoustic lenses were produced by first etching the spherical cavity into one side of a silicon wafer and deposition of a piezoelectric zinc oxide (ZnO) transducer on the backside. To avoid the rim echo the silicon ar ound the cavity is dry etched. Applying the procedure shown below, lenses with a wide variety of focus distance, aperture angle and frequency have been produced and tested.