Getting Masks Clean With Minimal Optical Degradation

Cleaning and contamination issues are working their way to the forefront of photomask technology as every particle becomes more critical, presenting a greater risk of printing on finer-feature wafers. But it's not as simple as just getting a mask as clean as it can be. As traditional spin spray or bath immersion techniques clean, they also bring about significant degradation of the optical properties of phase-shift masks (PSMs) — causing losses in transmission and phase angle. At the 180 nm technology node, reticles can be cleaned only two to eight times before they are unable to meet production specifications. This issue is even further compounded as technology moves into smaller geometries.

Akrion LLC (Allentown, Pa.) has developed an immersion reticle cleaning tool that overcomes some challenges associated with traditional cleans — achieving better particle removal, better maintenance of phase-shift angle, and therefore a longer mask life.

Typical mask cleaning techniques rely on a set of chemistries similar to those used to clean wafers: SPM (a sulfuric acid/hydrogen peroxide mixture) for resist stripping, SC1 (an ammonium hydroxide/hydrogen peroxide mixture) for particle removal, and a deionized water rinse. But a "copy exact" approach cannot be applied because of material differences between wafers and masks, noted Ismail Kashkoush, Akrion's director of application and process engineering, in a presentation last month at the Photomask Technology conference in Monterey, Calif. The chemistries involved in wafer cleaning are inherently damaging to today's PSMs. Sulfuric acid, for example, has an etching effect that removes particles, but that same etching effect is what can knock the mask's phase angle and transmittance out of acceptable ranges. However, SC1 remains a preferred cleaning chemistry because of its substrate etching potential.

Ultimately, a mask cleaning solution must remove organic contaminants, remove particles, and free ionic residues and haze, Kashkoush noted. "The more you clean it, obviously you're etching, and you're changing the optical properties," he said. But the idea is to minimize the degree of etching and roughening. Akrion has patented a sulfuric megasonics technique developed specifically for cleaning reticles. To begin with, the technique uses SOM (a sulfuric acid/ozone mixture) rather than SPM for resist stripping. SPM is of concern for the mask industry because it has been shown that chromium can be etched must faster in diluted rather than concentrated sulfuric acid. So mixing and replenishing the H₂O₂ in the SPM chemistry would significantly raise the risk of chrome etching. The cleaning technique applies megasonics to SOM to enhance the stripping efficiency. Applying megasonics to SC1 can enable an optimal balance between particle removal efficiency and phase/transmittance changes.

After five cleaning cycles, the DIO3 mixture showed significantly less damage to phase angle and transmittance of EAPSMs. (Source: Akrion)

Akrion's ClearIQ tool operates with the optimal chemistries and principles verified by the research. Cleaning processes include H₂SO₄ mixtures with O₃ or H₂O₂, with direct-coupled megasonics; DIO3; and SC1 with megasonics. The system is a module-based unit that can run multiple process recipes simultaneously and process multiple substrates (one to eight reticles per batch). The system is enclosed in a Class 1 minienvironment.

ClearIQ is at least an order of magnitude better than any competing tool, asserts James Molinaro, Akrion's president, enabling a mask set to be used for 30-50% longer than with other mask cleaning tools. Of course, this all comes at a premium — the tool has an average selling price of $2M, while other reticle cleaning tools have price tags of closer to a half million dollars. But chipmakers are buying Akrion's tool nonetheless. Even in a significantly depressed semiconductor economy, Akrion has experienced a 30% revenue growth in the past year.

For additional information on lithography, go to www.semiconductor.net/lithography.