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Got a Litho Question? Ask the Experts Chris A. Mack, KLA-Tencor
Q What causes H-V (horizontal to
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vertical) bias?
A H-V bias is the difference in CD between horizontally oriented lines and vertically oriented lines printed on the wafer. There can be many causes, but two sources tend to dominate. Lens aberrations, and in particular astigmatism, can result in phase errors that are different left to right across the lens (which affect vertical lines) versus top to bottom across the lens (which affect horizontal lines). These aberrations can cause a best-focus difference and CD difference between vertical and horizontal lines. Source aberrations can cause H-V bias as well. If a conventional source is slightly elliptical rather than perfectly circular, then horizontal lines will see a different effective partial coherence than vertical lines. Since partial coherence can affect CD, this type of source aberration can certainly affect H-V bias. Top-bottom versus leftright asymmetries for off-axis illumination produce even more pronounced H-V bias than conventional illumination source errors.
How does aspect ratio affect lithography and what is the maximum aspect ratio?
A
The impact of aspect ratio (the height of a resist feature divided by its width) depends greatly on the type of feature. For long lines, the problem with high aspect ratio features is mechanical stability. After rinsing the wafer following development, the drying water creates surface tension pulling on either side of the tall, narrow line. Drying rates, however, are not spatially uniform and it is very likely that one side of the line will dry faster, and see very different surface tension forces, compared to the other. The difference in forces tends to pull the line to one side. The greater the toppling force, the taller the feature, and the more the mechanical strength of the feature reduces, the narrower its width. For very long lines, aspect ratios greater than about three tend to be mechanically unstable, leading to the phenomenon of pattern collapse. (This simple rule of thumb is quite crude, and more detailed estimates based on mechanical modeling are possible.) For lines that are not very long, the photoresist patterns connected to the ends of the line can help to hold the patterns up. And obviously, isolated trenches do not suffer from these mechanical problems at all.
Do you have a lithography question? Just e-mail lithocolumn@kla-tencor.com and have your questions answered by Chris Mack or another of our experts. Winter 2005
www.kla-tencor.com/magazine
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