Numerical simulations on a stepped solid immersion lens suggested from the experimental consideration of the backflow

When the air flows round the conical solid immersion lens (SIL) due to the disk rotation, the air climbs up along the lateral surface of the conical SIL at the downstream of the SIL. The existence of this backflow, which has opposite direction of the disk rotation, is confirmed numerically and experimentally. The backflow exists below one-quarter of the gap between the rotating disk and the lateral surface of the SIL and it is known to be one of major sources which carries contaminant particles to the SIL top. A step, 40 μm in height and 25 μm below the rotating disk, is fabricated on the lateral surface of the SIL to prevent the backflow. Numerical simulation shows the step reduces the pressure difference between the bottom and the top of the SIL so that the backflow is reduced dramatically.