An apparatus for cleaning wafers includes a chamber, a rotatable substrate holder inside the chamber, a nozzle above the rotatable substrate holder, a cover facing downward and fluidly coupled with the nozzle. The rotatable substrate holder is configured to mount one or more semiconductor wafers on the rotatable substrate holder. The nozzle is configured to spray a cleaning medium onto the one or more semiconductor wafers. The cover is of a shape having a top edge with a top cross-sectional area and a bottom edge with a bottom cross-sectional area.