Absorber height effects on SWA restrictions and 'Shadow' LER
As extreme-ultraviolet lithography (EUVL) approaches introduction at the 22-nm half-pitch node, several key aspects of absorber height effects remain unexplored. In particular, sidewall angle (SWA) restrictions based on the height of the mask absorber has not yet been clearly defined. In addition, the effects of absorber height on line-edge roughness (LER) from shadowing has not been examined. We make an initial investigation into how tight SWA constraints are and the extent to which shadow LER alters basic LER. Our approach to SWA aims to find SWA restrictions based on 10% of the total CD error budget (10% of CD). Thus, we allot the SWA budget a {+-}0.2nm tolerance for 22nm half-pitch. New with EUVL is the off-axis illumination system. One potential pitfall that must be carefully monitored is the effect of mask absorber height blocking light from reaching, and therefore, correctly detecting, the base edge position of a feature. While mask features can correctly compensate sizing to target at the wafer, the effects of this shadowing on LER have not yet been investigated. Specifically, shadow LER may exacerbate or mitigate the inherent LER on the mask. Shadowing may also cause a difference in the observed LER on the right and left side of the features. We carefully probe this issue for a range of spatial frequencies. We do rigorous aerial image modeling of mask features with a nominal SWA of 80 degrees and correctly sized to target 22nm features measured at the top, 70nm TaN absorber on a 40 bilayer ML mirror with a 2.5nm Ru cap. Simulations were on a 4X system with an ideal pupil of NA = 0.32, illumination wavelength 13.4nm at 6{sup o} off-axis, and disk source shape with partial coherence factor of {sigma} = 0.50. We first implement a defocus offset to the aerial image so that best focus lies at a nominal zero defocus value. We then calculate the depth of focus (DOF) for which the image-log-slope (ILS) delivers a contrast is greater than 50%, an arbitrary standard for a good quality photoresist to print. Using this DOF, we optimize the process window and adjust the intensity threshold from it's nominal value to a new optimal value. From this optimized intensity threshold, we calculated the aerial image CD through focus for a range of angles around the nominal SWA value. From this, we can find out how much the SWA needs to be constrained by in order to keep the {+-}10% CD error budget tolerance required. For SWA of 80 degrees, we found the nominal SWA is tightly constrained by 0.5 degrees. We repeated these calculations for other nominal values of SWA 85 and 88 degrees, and found that CD sensitivity to nominal SWA is relatively independent. In examining shadow LER, we use FDTD simulations to obtain a rigorous solution for the source intensity immediately after the patterned TaN mask absorber pattern on a 2.5nm Ru capped 40 bilayer ML mirror. The optical system was comprised of an ideal pupil map, NA = 0.32, 13.4nm light at 4{sup o} off-axis, and annular source shape with partial coherence factor of {sigma} = 0.35-0.55. We assumed a 90{sup o} SWA. For the basic 2D shape of the mask that we exactly replicated through height, we created mask LER by generating a sine wave of full amplitude 4nm, equivalent to 4nm LER, (wafer dim.), confined along it's length to precisely CD=22nm, for sine waves with spatial frequencies (full period) of 25, 50, 75, 100, 125, 150, 175, and 200nm (wafer dim.), and illuminated at both {phi}=0{sup o} (perpendicular, shadowing) and {phi}=90{sup o} (parallel, no shadowing). Results show that even off-axis illumination perpendicular to the lines and spaces gives minimal difference between the left and right LER to at most 0.1nm difference at best focus. We followed up this study by repeating the data for a mask designed with 2nm LER with similar results. A plot of the transfer functions for LER of both 2nm and 4nm compared to the contrast of simple 50% duty cycle lines and spaces shows that LER has a unique transfer function as well as a linear response to LER amplitude. In the course of our investigation of SWA, we found that the nominal angle has no effect on process window size, and neither does sensitivity on changing angle alter as a function of nominal angle. For nominal 80{sup o}, 85{sup o}, 88{sup o} SWA, the amount of angle tolerance for 0.2nm change in CD was in total 0.5{sup o}. For shadow LER, we found minimal difference between left and right LER when illuminated perpendicularly to the pattern, to at most 0.1nm LER at best focus. CTFs confirm the uniqueness and linearity of an LER transfer function as opposed to the traditional MTF, as shown in a previous study. In the future we wish to expand our work on shadow LER into the regime where the k{sub 1}-factor of the optic is stressed. In particular, 16nm lines and spaces on an NA = 0.32 optic with off-axis illumination.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- Materials Sciences Division
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 1019301
- Report Number(s):
- LBNL-4566E-Poster; TRN: US201114%%742
- Resource Relation:
- Conference: SPIE Advanced Lithography, San Jose, CA, February 27 - May 3, 2011
- Country of Publication:
- United States
- Language:
- English
Similar Records
Mask roughness induced LER: geometric model at long correlation lengths
LER control and mitigation: mask roughness induced LER