Technology - Lithography
W
hile Moore’s Law continues to guide the semiconductor
industry down an increasingly challenging photolithography path, the memory sector needs to accelerate the rate of device shrinkage beyond Moore’s Law in order to counteract the rapid decline in average sales price (ASP) for each device node (Fig. 1).
While the technology continues to evolve to
meet the requirements for smaller geometries (CD), the lithography tools continue to increase in complexity, driving costs up for outfitting a next-generation fab. Increasingly, the chip manufacturers look for ways to maximize the productivity of the process tools to leverage the growing capital investments. High wafer throughput and yield are the key drivers that enable the cost/bit to maintain pace with the market price pressures.
Process tool trends
Advanced ArF immersion scanners have been developed to continue increasing wafer throughput while improving overlay capability to enable double- and multi-patterning applications. For resolution enhancement, new freeform illumination optics are allowing designers to further extend SMO and push the limits of 193nm light. Newer resists that can achieve finer resolution are being contemplated, but require high exposure doses. These enhancements are further enabled by the light source contributions.
At the basic level, to increase efficiency, higher output power from the light source enables the use of higher exposure doses without compromising scanner throughput. Similarly, higher power can enable faster scanning speeds at the scanner for a given exposure dose, thus providing higher throughput. The Cymer XLR 600ix was designed with the basic premise of flexible power output in mind to deliver higher power when needed. Excimer laser light sources were introduced for ArF immersion applications at a 60W power level (10mJ @6kHz). This level of power has been sufficient to supply scanners with enough light to operate at wafer throughputs on the order of 150 – 180wph under typical exposure conditions. However, if higher dose resists are used, the throughput may decrease, depending on illumination conditions. Higher power from
the light source, therefore, can eliminate this constraint and enable high throughput. One of the key challenges of delivering higher power from the light source is the availability of optic materials and coatings that can withstand the high energy present within the laser and not have a detrimental impact on component lifetime. Also, higher average power introduces more severe thermal effects, and in particular, thermal transients, when the light source keeps switching on and off as is typical during a wafer exposure sequence. Such thermal transients can affect the stability of the light if the optics and their surrounding components are not sufficiently immune to thermal effects. Through a multi-year development effort, Cymer
has been able to introduce new CaF2 optics and coatings that, combined with a robust architecture, achieve high stability in order to deliver 90W output power while maintaining tight optical performance specifications. The resultant platform, the XLR 600ix, incorporates Power Optics that lead to high stability as well
Figure 1 - Average sales price (ASP) of NAND devices over time
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Figure 2 - High performance “Power Optics” tested under very high intensities in an accelerated life test show >5x improvement over conventional optics
www.euroasiasemiconductor.com Issue IV 2011
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