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Femtosecond laser mask repair. (Feature).

Article Excerpt
Opaque defects in photomasks are being repaired using an innovative automated femtosecond laser ablation tool at IBM. The tool images the mask in the DUV with 100nm resolution and ablates defects nonthermally with ~80nm resolution. An iterative repair procedure achieves an RMS edge placement precision of ~5nm. The edge placement accuracy was <18nm at 3[sigma], with <5% loss in optical transmission at 193nm.

Over the past five years, the basic imaging performance of wafer printing tools has not kept pace with the desires of the semiconductor industry. Numerous resolution- enhancement techniques (RET) have been adopted to improve the aerial image. Most of these techniques involve modifications to the photomask, such as optical proximity correction, sub-resolution assist features, weak and strong phase shifting, and more stringent dimensional control. These mask RETs dramatically increase the complexity and cost of the photomask. They also make the mask far more susceptible to defects. Subtle variations in the optical transmission or phase over 100nm-scale distances on the photomask are now likely to cause some wafer defects.

As a result, direct fabrication of an advanced photomask containing no defects is nearly impossible. Maskmakers generally fabricate nearly perfect masks, locate the defects using automated inspection tools, and then remove these defects using mask repair tools. Mask repair has become increasingly problematic, however. Some detectable mask defects do not actually reduce wafer yield and some "repair" themselves produce repeating "killer" wafer defects. Since the geometrical and optical quality of the repairs must meet such stringent requirements, the demands placed on repair tools have increased significantly. When a defect is not repaired successfully, a new mask must be written, dramatically increasing costs, especially if it, again, requires repair.

Unfortunately, commercially available repair technologies utilizing focused ion beams [1-6] or nano/picosecond pulsed lasers [7-9] have not kept pace with the increased requirements. In particular, it has proven difficult to remove the chromium (Cr) film layers conventionally used to make reticles opaque without producing artifacts. Focused ion beams can remove the chrome, but they also reduce the transparency of repaired opaque defects by...

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