Self-imaging marks aid chipmakers

17 Jun 2002

German researchers have developed a novel chip-alignment technique with sub-micron accuracy.

Courtesy of Opto & Laser Europe (OLE) magazine

Self-imaging

The technique, which is based on the self-imaging of special alignment marks, provides sub-micron accuracy. This could be useful to chipmakers, because lithography applications increasingly require both sides of a wafer to be processed. Bottom-emitting VCSELs, for example, can be integrated onto one side of the carrier substrate and equipped with beam-steering or beam-shaping optics on the other side.

Matthias Gruber and his colleagues used microlenses called Fresnel zone plates (FZPs) on the bottom side of a wafer. When an alignment mark on a photolithographic mask close to the surface of a wafer was illuminated, the FZPs on the bottom of the wafer reflected the image of the mark. This created an inverted image on the top of the wafer.

Owing to this inversion, when a mark on the mask was moved out of alignment on the top of the wafer, its image moved in the opposite direction. As a result, errors can be detected with twice the usual sensitivity.

The group designed an illumination module consisting of a 20 mW helium-neon laser, a beam expander and a rotating diffuser that destroys spatial coherence. This was then fitted to a Suss MA4 mask aligner in place of its white-light source.

According to Gruber, the team's technique is both simple and robust and can conveniently be implemented on laboratory mask aligners for either contact or proximity printing.

He told OLE that the technique still offered room for improvement, however: "Our cleanroom equipment did not allow us to push our method to its limits. Using state-of-the-art equipment, I would expect to see alignment accuracies in the region of 100 nm."

The technique is highly fault-tolerant because the location of the zero position is independent of the angular characteristics of the illuminating light. This, say the researchers, gives their technique advantages over other approaches for the alignment of masks for double-sided lithography.

Gruber said: "Another research group is using microlenses to generate focused light spots on the opposite side of a wafer, which requires an exactly perpendicular illumination beam. This is not needed in our method because we use the microlenses to image alignment marks."