Fifteen years has passed since ArF lithography technology transitioned to mass production. At first the node size was 130nm, however, now we are discussing one-digit nm node. This node size is one-fifteenth compared to that in the initial generation. It is obvious that generations have steadily changed.
Meanwhile, ArF projection optics has been designed with higher NA. Moreover, much higher NA or effectively short wavelength have been achieved by introducing the immersion lithography technology which injects immersion media between the light emission side of optics and the wafer surface.
Now that ten years has passed since the immersion lithography technology transitioned to mass production, the requirements for the light source have become much more demanding. This is because E95%, a typical parameter for the light source, which reduces the CD variation and influences the optical characteristics, has been required to be much shorter along with advanced node.
The spectrum characteristics of the conventional KrF 248nm-light source were defined with full-width at half maximum (FWHM). After entering ArF era, E95% was defined as more stringent parameter. This new parameter is closely linked to line narrowing and node transition. This E95% is a parameter to define optics monochromaticity and recognized as equivalent to quantified chromatic aberration.
More specifically, it is desired to reduce the chromatic aberration ideally down to zero to maximize the contrast by suppressing blur images. Provided that the blur images are caused by not only chromatic aberration but also the convoluted effect of spherical aberration, residual errors of lithography optics system and other defocus elements. In line with this we should take persistent challenges to reduce errors in designing and manufacturing lithography optics systems and drive the chromatic aberration closer to zero.
The contrast we discuss in the lithography optics systems strongly correlate to resolution, and thus it is well known that OPC bias is linear with respect to E95% when the resolution is fixed. Although this OPC bias and E95% have been firmly linear with each other, it has also been revealed that the trend of the linearity with much shorter E95% deviates from the conventional trend and shows an inflection point in the near future.
We systematically analyze the trend that linearity error tends to be greater than the conventional distribution and suggest solutions to the issue.
This systematic analysis includes:
1. Measurement errors and distribution in measuring E95% as the spectrum width
2. Correlation between raw measured spectrum intensity data and E95%
3. Linearity comparison in behaviors between when E95% is approximated by i) +/-2 sigma of, ii)+/-3 sigma of normal spectrum distribution, and iii) by another approximation.
Based on these studies, we identify the roles of the light source to contribute to 7nm-node mass production by defining the spectrum and its requirements for lithography performance.
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