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Line end shortening (LES) is a classical phenomenon in photolithography, which is primarily caused by finite
resolution from the optics at the position of the line ends. The shortening varies from a couple tens of nanometers
for processes with a k1 around 0.5 to as much as 100 nanometers for advanced processes with more aggressive k1
numbers. Besides illumination, the effective resist diffusion has been found to worsen the situation. The effective
diffusion length for a typical chemically amplified resist, which has been demonstrated to be critical to the
performance of the photolithographic process, can be as much as 30 to 60 nm, which has been found to generate
some extra 30 nm LES. Experiments have indicated that wider lines have less LES effect. However, under certain
CD through-pitch condition, when the lines or spaces are very wide, the opposing line ends may even merge.
Currently, two methods have been widely used to improve the situation. One method to fix this problem is to extend
the line ends on mask, or to make them move closer toward each other to compensate for the shortening. However,
for a more conservatively defined minimum external separation rule, this method itself may not be enough to fully
offset the LES. This is because it has been found that there is a limit when the line ends are too close to each other
on mask, any perturbation on the mask CD may cause line ends to merge on wafer. The other way is to add
hammerheads, or to add wider endings. This is equivalent to the situation of an effectively wider line ends, which
has less shortening effect and can also live with a rather conservative minimum external separation. But in some
design, this luxury may not have room to implement, i.e., when the line ends are sandwiched by dense lines with
minimum ground-rules. Therefore, to best minimize the effect of LES or to completely characterize the LES effect,
one will need to study both the process window and mask error factor (MEF) under a variety of photo process
conditions, such as, illumination conditions and resist parameters. However, no such systematic study has been
reported so far. Our data indicate that the better the latent image contrast, the better the control of line end
shortening. In this paper, we will present our systematic studies in the effects of illumination condition and resist
parameters to the behavior of LES.
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