Photomask Printability, Standards and Cleaning Remain Concerns
At the 28th Photomask Technology conference in Monterey, Calif., technologists discussed ways to ensure the printability of patterns at the 22 nm node, and how to avoid damage to fragile features during cleaning. Ronald Dixson of NIST gave an update on NIST's program in photomask dimensional metrology, another concern at the 22 nm generation.
Alexander E. Braun, Senior Editor -- Semiconductor International, 10/10/2008 9:14:00 AM
Lithographers attending the 28th Photomask Technology conference in Monterey, Calif., this week reported studies of the printability of 22 nm patterns, the evolution of accurate metrology standards, and how to remove particulates from masks bearing increasingly fragile features.
Seiro Miyoshi and colleagues from Toshiba Corp. (Tokyo) described a spacer patterning process for the 22 nm node. They created a mask with several programmed defects of varying sizes and two types — opaque and clear. Miyoshi said while mask defect control and defect printability have been well investigated, it remains unclear whether conventional strategies are applicable to the spacer patterning process.
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| To test mask defect printability, a 22 nm node device mask was fabricated with specific, created defects of various sizes and two types — opaque and clear. (Source: Toshiba) |
Wafers were processed using the test mask, a 193 nm immersion scanner, and a conventional resist coating. The programmed defects’ printability was investigated with a CD-SEM. “It became clear that opaque defects on the mask result in open-short defects,” Miyoshi said, “while clear defects produce shorts.”
However, the Toshiba group did not observe defects beyond those originally programmed, which led them to conclude that a conventional defect control strategy can be used. Miyoshi said the acceptable mask defect size is 55-60 nm, although the International Technology Roadmap for Semiconductors (ITRS) specs this at <40 nm.
Ronald Dixson of the National Institute of Standards and Technology (NIST, Gaithersburg, Md.) presented an update on NIST’s ongoing program in photomask dimensional metrology, describing the NIST SRM2059 master standard using traceable atomic force microscopy (AFM) metrology. He focused on NIST’s most recent Standard Reference Material (SRM) for linewidth metrology, the SRM2059. An in-house-developed tool, the NIST ultraviolet microscope, was used in transmission mode to calibrate the SRM2059 photomasks. However, because there are few optical models for determining the edge response in the UV microscope, it was used in a comparator mode. A mask was selected as a master standard and its features calibrated using traceable CD-AFM metrology.
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| NIST’s SRM2059 photomask linewidth standard has isolated linewidths and spacewidths of 0.25-32 μm, 9 nm for pitch, and its certified values are traceable to the definition of the meter with expanded (k=2) uncertainty <25 nm for linewidths and spacewidths (~14 nm at 0.25 μm CD). (Source: NIST) |
A way to increase particle removal efficiency (PRE) was the subject of, “Impact of MegaSonic Process Conditions on PRE and Sub-Resolution Assist Feature Damage,” by Stefan Helbig of HamaTech AG (Sternenfels, Germany). The presenter observed that mask features are shrinking and getting increasingly fragile, while allowable defect levels are becoming more stringent. In fact, the subresolution assist feature (SRAF) size is in the same range as the minimum allowed defect size, which complicates the use of megasonic-assisted mask cleaning without causing feature damage.
“A thorough understanding of the effect of process parameters on feature damage is essential if megasonic cleaning is to be used for future technology nodes,” Helbig said, adding that megasonic cleaning can be extended to future technology nodes through a more complete understanding of acoustic phenomena and their interaction with contaminant particle and photomask features. Key to this is a more thorough knowledge of the effects of cavitation, which is the mechanism through which photomasks are cleaned and may be damaged. “Although theoretical studies have been done on sound field propagation and its effect on cavitation, practical studies are needed to determine the optimum process window for high PRE without pattern damage.”
He outlined research showing that megasonic frequency control is a basic requirement to clean a photomask while leaving it undamaged. Among some of the results obtained, it was observed that a frequency of 3 MHz may cause less damage than 1 or 2 MHz. This appears to be a matter of synergy, since Helbig revealed that two overlapping frequencies can cause considerably more damage than a single frequency. Cavitation measurements using sonoluminescence sensors showed that a combination of 1 and 3 MHz frequencies increased cavitational events.
A result of the work was that lower concentrations of H2 gas can negatively influence the level of pattern and glass damage. “Media parameters are important,” Helbig said, giving as an example their observation that SC1 could present an advantage over a mixture of water and ammonia in terms of pattern damage.
