Development and characterization of a thinner binary mask absorber for 22-nm node and beyond

Abstract

The lithography challenges posed by the 22 nm node continue to place stringent requirements on photomasks. The dimensions of the mask features continue to shrink more deeply into the sub-wavelength scale. In this regime residual mask electromagnetic field (EMF) effects due to mask topography can degrade the imaging performance of critical mask patterns by degrading the common lithography process window and by magnifying the impact of mask errors or MEEF. Based on this, an effort to reduce the mask topography effect by decreasing the thickness of the mask absorber was conducted. In this paper, we will describe the results of our effort to develop and characterize a binary mask substrate with an absorber that is approximately 20-25% thinner than the absorber on the current Opaque MoSi on Glass (OMOG) binary mask substrate.For expediency, the thin absorber development effort focused on using existing absorber materials and deposition methods. It was found that significant changes in film composition and structure were needed to obtain a substantially thinner blank while maintaining an optical density of 3.0 at 193 nm.Consequently, numerous studies to assess the mask making performance of the thinner absorber material were required and will be described. During these studies several significant mask making advantages of the thin absorber were discovered. The lower film stress and thickness of the new absorber resulted in improved mask flatness and up to a 60% reduction in process-induced mask pattern placement change. Improved cleaning durability was another benefit. Furthermore, the improved EMF performance of the thinner absorber [1] was found to have the potential to relieve mask manufacturing constraints on minimum opaque assist feature size and opaque corner to corner gap.Based on the results of evaluations performed to date, the thinner absorber has been found to be suitable for use for fabricating masks for the 22 nm node and beyond.

Critical assessment of the transport of intensity equation as a phase recovery technique in optical lithography

“…1 Here we compare the edge effects for x-polarized illumination in an OMOG mask with an attenuating phase shift mask (ATT-PSM), as shown in Figure 10.…”

“…12,13 Intensity images through -focus (1) Solve for phase Recovered Phase . The Transport of Intensity used for phase imaging of human cheek cells.…”

“…The dark regions in OMOG transmit 2% of the incident intensity without any designed phase shift and are believed to have reduced edge effects due to a thinner absorber. 1 Thick mask edge effects modify the best focus and also the through-focus behavior of the wafer plane images, and hence need to be modeled. Thin mask boundary layer models have been proposed 2-4 that can approximate diffraction along thick edges as effective boundary layers in simulations.…”

Layout optimization method to equalize the best-focus position of different patterns

“…Photomasks used in current lithographic practice are usually of either the binary or the 6% to 7% attenuated phase shifting type, fabricated with molybdenum silicide (MoSi). Various new opaque MoSi substrate materials have been introduced to improve the manufacturing performance and durability of binary blanks, 4 as well as to guarantee high opacity using thinner absorbers in order to reduce topography effects. For example, so-called thin Opaque MoSi Over Glass (thinOMOG) 5 uses a film stack as thin as 45 nm.…”