Tighter CD Requires Tighter Laser Bandwidth

Cymer shows research at SPIE show that sharply limits chromatic aberration for 193nm lithography.

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Concerns that the bandwidth of the light source for optical lithography can affect pattern quality are not new. No lens material is completely free from chromatic aberration: the refractive index varies with wavelength, and so different wavelengths will focus at different points.

Chromatic aberration became a much less serious concern with the replacement of broadband mercury lamps with lasers, as lasers inherently deliver a much narrower spectrum. Indeed, modern laser light sources achieve impressively narrow bandwidths already. According to Ted Cacouris, director of product management at Cymer, the company’s XLR 600ix ArF laser delivers bandwidth of 300fm ± 50fm. A femtometer is 1 x 10^-6 nanometer, so 50 fm variation on a 193nm base wavelength is about 2.6 x 10^-5 percent. And yet it isn’t enough.

In research presented at the SPIE Optical Microlithography Symposium in San Jose this week, Cymer’s Paolo Alagna and colleagues at ASML and IMEC found a CD impact of about 1nm per 100fm of bandwidth variation. Typically, line and space patterns are less sensitive to bandwidth variation, while the random features seen in logic structures are more sensitive and also more difficult to characterize.

Many factors contribute to CD variation, and many of those are beyond the light source supplier’s ability to fix. However, Cymer’s newly introduced XLR 700ix applies closed loop control on a pulse-by-pulse basis, dramatically improving bandwidth stability to 300fm ± 5fm. Moreover, the company is willing to guarantee this specification directly to the end user, independent of the stepper being used.

XLR 700 ix Bandwidth