Comparative study of binary intensity mask and attenuated phase shift mask using hyper-NA immersion lithography for sub-45nm era

0.35 μm pattern fabrication using quartz-etch attenuate phase-shifting mask in an I-line stepper with a 0.50 NA and a 0.60 sigma

We studied the mask error enhancement factor (MEF) for four 0.15-micrometers patterns, isolated lines, line and space (L and S), isolated holes, and dense holes, for various process conditions. The MEF for isolated lines was the smallest of all. The MEF for L and S was not as small as that of the isolated lines. We obtained a less than 15-nm wafer critical dimension (CD) variation, when we reduced the mask CD variation to 20 nm. For the isolated-hole patterns using an attenuated phase shift mask (PSM) with large mask biasing can reduce the wafer CD variation. On the other hand, it is very difficult to reduce the MEF and the wafer CD variation for the dense-hole patterns. The alternating PSM was the best of the evaluated process, but it was not good enough to reduce the mask CD variation.

In lithography, forbidden pitch refers to pitch that suffers degradation in the process window due to the application of off-axis illumination (OAI). Destructive light field interference of diffracted light from a mask at forbidden pitch causes reduction in image contrast and depth of focus (DOF) and limits the pitch range to be patterned. In this paper, a modification to conventional OAI shape is proposed to minimize the effect of forbidden pitch. The modification is based on the interaction of illumination source with pattern density. The modified source employs double annular illumination. Simulation is carried out to investigate the effect of the modified source for one dimensional line and space pattern with a pitch varying from 130to500nm. Results shows that the maximum critical dimension fluctuation is around 3% compared to 13% in conventional annular illumination. Furthermore, the degradation in DOF is within 21% of DOF compared to 49% in conventional annular illumination.

Influence of MEEF change on the mask shadowing effect in extreme ultraviolet lithography

As feature sizes approaches the resolution limit of optics (K 1 factors less than 0.5) image quality rapidly degrades. The benefits of stepper reduction are reduced and unavoidable mask errors are amplified on silicon. This phenomenon is known as Mask Error Enhancement Factor (MEEF). MEEF puts severe constraints on the mask manufacturing process. Optical Proximity Correction (OPC) for sub-150 nm designs becomes a problem. Use of strong (alternating) phase shifting masks (PSM), however, greatly reduces Mask Error Factor (MEF) and inverts MEEF into Mask Error Attenuation Factor (MEAF). Application of strong PSM enables OPC for the next IC generation.

As the ArF lithography technology is going to progress to 90nm node from 130nm node, it has been more difficult to inspect all types of mask defects, which influence wafer. Photomask for 90nm node, the aggressive OPC mask and Phase Shift Mask (PSM) might be inevitable in production devices due to the slow progresses in lithography equipment itself compared to shrinkage speed in device manufacturing. Recently, due to the similiar effect such like MEEF (Mask Error Enhancement Factor) phenomenon many mask defects become to detect difficult even printable defects even the lots of improvements mask inspection equipments. In this paper, we will present the inspection capability of advanced DUV inspection tool LM7000 (NEC) with various programmed defect masks (e.g. aggressive OPC masks, half-tone PSM, tri-tone PSM) with, and discuss the relationship between inspection sensitivity and mask defect printability.

We have studied the optical proximity effect, the depth of focus (DOF) and the mask-error-enhancement factor (MEF) of ArF attenuated phase shift masks (att. PSMs) for the application to 100-nm logic gate patterns. In the previous work, we have compared the performances of the binary mask with those of the 6% transmittance ArF att. PSM for printing 100-nm lines, and we obtained the better performances on the att. PSM with the annular illumination. In this paper, we reported the evaluation results of the higher transmittance att. PSMs. We fabricated them with 6-20% transmittance (quartz reference). Those masks have TaSiO bi-layer shifter with two tones. We did not use assist bars. An ArF scanner with NA 0.60 and a positive chemically amplified ArF resist with the thickness of 0.4 um are used. 100 nm semi-dense lines with a pitch of 250 nm are exposed without bias and 100 nm isolated lines are exposed with biases of 5 -15 nm (on wafer). Our results show that the higher transmittance att. PSMs can improve the DOF and the MEF on both semi-dense lines and isolated lines. In our experiments, the highest transmittance of 20% gives the broadest DOF of 0.4 um and the smallest MEF of 1.78 and 1.11 for semi-dense lines and isolated lines respectively. However, some higher transmittance PSMs may create the side lobes on the large patterns. From these results, we demonstrated that that 10% transmittance is suitable for the ArF att. PSMs for the fabrication of 100 nm line patterns.

In order to shrink down the contact hole which is usually much larger than other patterns, the resist reflow process (RRP) has been widely used. Various types, shapes, and pitches of contact hole arrays are made by RRP, but RRP was limited to be used only for contact hole patterns. The same RRP method is expanded to 32 nm node arbitrary and complex patterns including dense line and space patterns. There might be simple 1-dimensional patterns, but 2-dimensional proximity conflict patterns are difficult to make in general. Specially, the data split with proximity correction needs a lot of attention for double patterning. 32 nm node arbitrary patterns can be easily made by using RRP without complex data split.

One of the most challenging issues for 45-nm-node contact hole printing is to ensure a sufficient process window required for day-to-day manufacturing, and to reduce large mask error enhancement factor (MEEF) to keep the critical dimension (CD) controllable. In this study, we propose a new phase-shifting mask for random layout contact hole printing, named Mask Enhancer. By applying the Mask Enhancer, we can enhance image contrast for both isolated and dense contact holes at the same time. In addition, the Mask Enhancer is extendable for a random layout including diagonally placed or non-square contact hole. In this paper, we show the principle of the Mask Enhancer for improving image contrast and we discuss the lithographic performance of the Mask Enhancer comparing it with that of the attenuated phase-shifting mask (Att-PSM). Furthermore, we demonstrate 0.249 µm2 static random access memory (SRAM) contact layer printing which is a 45-nm-node requirement, combining it with the immersion exposure technology on a 0.85 numerical aperture (NA) ArF immersion tool.

In EUV lithography, off-axis incident light on a mask can produce a printed image that is asymmetrical with respect to its center, even when a line and space pattern is perfectly symmetrical on the mask. A theoretical analysis of diffracted light shows that the asymmetry tends to occur, when diffracted rays of the 2nd order or higher pass through the pupil and are involved in creating the image on a wafer. In contrast, when only 0th- and 1st-order diffracted rays pass through the pupil, the printed image is always symmetrical regardless of any asymmetry in their amplitude and/or phase. In order to discover the causes of the asymmetry in aerial images, we used line-and-space line patterns with a pitch of 88 nm on a wafer, because this pattern pitch generates 0th-, 1st- and 2nd-order rays for the optical conditions of an NA of 0.25 and a σ of 0.80. Under these conditions, a thicker Ta absorber (107 nm) produces greater asymmetry than a thinner one (64 nm), especially when the target linewidth is around 44 nm (on wafer). An analysis of diffracted light and the combinations of diffraction orders involved in creating an aerial image reveals that asymmetry in a printed image is caused by 2nd-order diffracted rays.

Extreme ultra-violet lithography (EUVL) has been prepared for next generation lithography for several years. We could get sub-22 nm line and space (L/S) pattern using EUVL, but there are still some problems such as roughness, sensitivity, and resolution. According to 2007 ITRS roadmap, line edge roughness (LER) has to be below 1.9 nm to get a 22 nm node, but it is too difficult to control line width roughness (LWR) because line width is determined by not only the post exposure bake (PEB) time, temperature and acid diffusion length, but also the component and size of the resist. A new method is suggested to reduce the roughness. The surface roughness can be smoothed by applying the resist reflow process (RRP) for the developed resist. We made resist profile which has surface roughness by applying exposure, PEB and development process for line and space pattern. The surface roughness is calculated by changing parameters such as the protected ratio of resin. The PEB time is also varied. We compared difference between 1:1 L/S and 1:3 L/S pattern for 22 nm. Developed resist baked above the glass transition temperature will flow and the surface will be smoothed. As a result, LER and LWR will be much smaller after RRP. The result shows that the decreasing ratio of LER due to RRP is larger when initial LER is large. We believe that current ~ 5 nm LWR can be smoothed to ~ 1 nm by using RRP after develop.

Improving resolution and focus latitude is needed in the manufacturing 0.18 micrometers LSI devices using the KrF lithography. In order to achieve this requirement, the attenuated phase shift mask (Att.PSM) has been proposed. 0.18 micrometers gate patterns can be attained using the Att.PSM with annular illumination. However it is expected to cause a differences in the printability of mask fabrication error by using a conventional illumination when annular illumination is used. Therefore, before applying our Att.PSM to practical use, we need to investigate the printability of Att.PSM fabrication errors when using annular illumination for 0.18 micrometers line patterns. Now, we clarify the printability of Att.PSM fabrication errors via simulation and experiments. Then we estimated the requirements of Att.PSM for 0.18 micrometers gate patterns. To apply the Att.PSM with annular illumination, we showed the feasibility of the Att.PSM fabrication. The other hand, we understood that it is necessary to improve technology of inspection and repair.

We have developed ArF excimer laser lithography using an alternating phase-shifting mask (PSM) for sub-0.10 micrometers logic device fabrication. The requirement of phase accuracy is discussed on the 0.10micrometers isolated and dense line patterns. It is found that the phase accuracy of +/- 2 degrees is required in order to obtain the sufficient process margin and minimize the CD difference between the nonshifter portion and the shifter portion. The 0.07micrometers by optimizing the phase shift on the dual-trench type PSM. We have also confirmed the fabrication of the 0.05micrometers isolated and semi-dense line patterns. The 0.05micrometers line patterns could be attained with the sufficient processing margin, if the resist performance is further improved. Furthermore, we have investigated the effect of mask bias to the isolated line pattern. It has been demonstrated that the DOF is improved from 0.65(Mu) m to 0.8micrometers by adding the mask bias of 0.02micrometers to the 0.10micrometers isolated line pattern, because the mask bias makes the pivotal point controllable.

Ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and low molecular weight PS and PMMA were directed to assemble on chemically patterned surfaces with hexagonal symmetry. The chemical patterns consisted of strongly PMMA preferential spots, patterned by electron-beam lithography, in a matrix of PS. The spot-to-spot spacing of the chemical patterns (L(s)) was varied between 0.9L(0) and 1.1L(0), where L(0) is the cylinder-to-cylinder spacing of the pure block copolymer in bulk. The homopolymer volume fraction of the blends (ϕ(H)) was varied between 0 and 0.3. In addition, chemical patterns were formed with selected spots missing from the perfect hexagonal array, such that the interpolation of domains between patterned spots could be examined on patterns where the polymer/pattern feature density ranged from 1:1 to 4:1. The assemblies were analyzed with top-down SEM, from which orientational order parameter (OP(o)) values were determined. The SEM analysis was complemented by Monte Carlo simulations, which offered insights into the shapes of the assembled cylindrical domains. It was found that, in comparison to pure block copolymer, adding homopolymer increased the range of L(s) values over which assemblies with high OP(o) values could be achieved for 1:1 assemblies. However, the corresponding simulations showed that in the 1:1 assemblies the shape of the cylinders was more uniform for pure block copolymer than for blends. In the case of the 4:1 assemblies, the range of L(s) values over which assemblies with high OP(o) values could be achieved was the same for all values of ϕ(H) tested, but the domains of the pure block copolymer had a more uniform shape. Overall, the results provided insights into the blend composition to be used to meet technological requirements for directed assembly with density multiplication.

Topology and defects are of fundamental importance for ordered structures on all length scales. Despite extensive research on block copolymer self-assembly in solution, knowledge about topological defects and their effect on nanostructure formation has remained limited. Here, we report on the self-assembly of block copolymer discs and polymersomes with a cylinder line pattern on the surface that develops specific combinations of topological defects to satisfy the Euler characteristics for closed spheres as described by Gauss-Bonnet theorem. The dimension of the line pattern allows the direct visualization of defect emergence, evolution, and annihilation. On discs, cylinders either form end-caps that coincide with λ+1/2 disclinations or they bend around τ+1/2 disclinations in 180° turns (hairpin loops). On polymersomes, two λ+1/2 defects connect into three-dimensional (3D) Archimedean spirals, while two τ+1/2 defects form 3D Fermat spirals. Electron tomography reveals two complementary line patterns on the inside and outside of the polymersome membrane, where λ+1/2 and τ+1/2 disclinations always eclipse on opposing sides ("defect communication"). Attractive defects are able to annihilate with each other into +1 disclinations and stabilize anisotropic polymersomes with sharp tips through screening of high-energy curvature. This study fosters our understanding of the behavior of topological defects in self-assembled polymer materials and aids in the design of polymersomes with preprogrammed shapes governed by synthetic block length and topological rules.