Laser cleaning is a rapidly developed technique in recent years. However, it is difficult to apply it to a slot structure, because the sidewall cannot absorb enough laser energy. Meanwhile, the focusing lens concentrates the laser beam and energy on a small spot size at the focusing position. The defocused laser beam after the focusing position would disperse and spatially enlarge. In this study, an axial laser beam (excimer laser, 248 nm) is focused at a position that is slightly in front of the slot (silicon), which makes the defocused beam propagate into the slot, and the sidewall is able to absorb the laser energy through its dispersion. The slot structure is constructed through a combination of three silicon wafers (two as sidewalls, and one as the bottom of the slot). In this manner the slot width can be controlled well during the experiment. Using this method, tiny particles (fused silica, diameter 5 µm) adhered on the slot sidewall are successfully cleaned. The cleaning threshold and efficiency for multiple slot widths (3.5–13 mm) and pulse numbers (0–40) are experimentally determined. A multi-physics model is established to understand the experimental phenomena. The electromagnetic–thermal–mechanical processes during laser cleaning are also analysed.