Method of determining an X and Y location of a surface particle

A system comprising:

• a modulator configured to detect defects in electrical circuits, the modulator being movable in a first direction;
• a laser source positioned adjacent to a first side of the modulator, the laser source being configured to emit laser light toward a second side of the modulator in a direction parallel to the surface of the modulator and at a first angle relative to the first direction, and the laser light partially overlaps the surface of the modulator;
• at least one mirror positioned adjacent to the second side of the modulator opposite the first side, and the at least one mirror being configured to redirect the laser light toward the first side of the modulator at a second angle relative to the first direction;
• a laser sensor positioned adjacent to the first side of the modulator, the laser sensor being configured to:
• detect a first change in intensity of the laser light received from the laser source when a particle on the surface of the modulator passes through the laser light emitted by the laser source;
• and
• detect a second change in intensity in the laser light received from the laser source redirected by the at least one mirror when the particle on the surface of the modulator passes through the laser light redirected by the at least one mirror;
• and
• a processor configured to determine an X-Y location of the particle based on the first change in intensity and the second change in intensity.