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What are the criteria that drive the development of an EUV light source?
First and foremost: safety. The manufacturing of semiconductor devices often makes use of, or generates as by-products, hazardous materials (toxic gases, strong acids, etc.). While the equipment shall be operated only by persons having proper training and qualifications, by design the equipment should be inherently safe in all circumstances. Safety encompasses:
- Safe installation/de-installation
- Safe operation
- Safe repair/maintenance
- Safety at end-of-life
The second most critical factor is system integrity, or the ability to contain natural degradation or failure. Although integrated, all active sub-systems — such as the EUV source — should be fully self-contained. Contamination beyond the source/scanner interface (Intermediate Focus [IF]) would contaminate the optics of the scanner, reduce tool lifetime and even contaminate the reticles or wafers themselves.
- XTREME’s LDP exhibits no contamination beyond the interface with the scanner [IF]. (EUVA/NEDO, SPIE Lithography 2011 San Jose, Poster 969-95)
- Because XTREME’s source does not use laser light emitting in the infra-red region (CO2 laser) and the LDP laser beam is not in the direction of the EUV light, no IR is contained in the LDP source’s spectrum.
- Furthermore, it has been demonstrated that XTREME’s LDP exhibits negligible parasitic Out-of-Band radiation (OOB) content beyond the IF. (IMEC, SPIE Lithography 2011 San Jose, Poster 969-95).
To enable a consistent lithographic process — i.e., the patterning of uniform lines of identical dimensions — EUV light output must be highly stable. Change in exposure dose from the source while the exposure tool is scanning would result in lines of variable width.
To enable the scanner to operate at maximum scanning speed hence maximum throughput, sufficient light must reach the wafer. Should the source not be powerful enough, or should the resist require a higher exposure dose, the scanner would have to slow down to ensure that sufficient photons reach the resist, thereby decreasing the throughput.
- XTREME’s LDP, unlike LPP, does not require an additional Spectral Purity Filter (SPF) for removing DUV and IR radiation at the entrance of the scanner. Current Spectral Purity Filtering (SPF) technology reduces the usable power delivered from the source to the scanner [power at IF] by more than 50%.
- To achieve the same amount of Clean Power, XTREME’s LDP needs to generate only half the number of photons in comparison to LPP, thus consuming less energy.
EUV power alone, however, is insufficient to achieve highest throughput. The source should be ready to fire when the scanner needs light: i.e., as soon as the scanner has stepped to the next exposure field. Should the source Duty Cycle (the ratio of the burst duration to the time between the beginning of two bursts) be too low, the source would not be ready to fire when needed, thus requiring the scanner to wait and lowering effective throughput. To enable maximal throughput, the duty cycle of the source should exceed the duty cycle required by the process.
The last, but not least, of the drivers is high uptime: i.e., the ability of the tool to operate for a long period without maintenance or failures.
High uptime is achieved through high reliability and the ability to readily maintain the equipment in the field. Minimizing the length of time that equipment is unproductive minimizes the Cost of Downtime (COD) — namely, the cost of owning but not operating this equipment (a major component of COO).