A great deal of research effort is focused on accelerating the development of 193-nm immersion lithography because
it appears to be the most suitable lithographic solution available for 65-nm-and-below semiconductor devices.
To realize a 193-nm immersion process, we must find ways to detect and analyze immersion specific defects, and
then establish processes that let us avoid such defects.
In this paper, we examine immersion specific defects and ways to detect and eliminate them in production processes.
Through comparison of dry exposure and immersion exposure processes, we have found that "bridges" and
"water-marks" are the most significant immersion specific defects using current developable top-coats. Although we
confirmed that the current solvent-removable top-coat process is better for avoiding immersion specific defects, we also
found that the defect density with a developable top-coat was still low enough for volume production.
We also investigated the causes of immersion specific defects and hypothesized that DI water permeation and the
local topology of the top-coat play an important role in the generation of immersion specific defects. To test whether this
was so, we evaluated the change in the top-coat film thickness by the quartz crystal microbalance technique. We
confirmed that top-coat swelling caused by water permeation into the top-coat film is a major cause of immersion
The interaction between water and resist film is a very important subject to be studied in order to establish the feasibility of 193-nm immersion lithography. The water diffusion into 193nm resist films was measured by using Quartz crystal microbalance method and it showed the slow saturation after the quick water uptake in the early stage of dipping in water. Diffusion coefficient was approximated by polynomial function of diffusion time. The water diffusion was well elucidated by the single variable of diffusion coefficient, which reflects the conditions of bake or pre-soak process and molecular properties such as molecular weight. The analysis is shown to be useful to evaluate the diffusion mechanism and to develop materials for immersion lithography.
Immersion lithography has by far satisfied most expectations regarding its feasibility as the next lithographic
technique for the 65-nm node and below. To further advance 193-nm immersion lithography, a means of efficiently
controlling water as an immersion fluid and research and development concerning resist processes are necessary.
In 2004, Nikon Corporation introduced a 0.85 numerical aperture (NA) 193-nm immersion exposure tool that uses
water as the immersion liquid. This engineering evaluation tool (EET) is equipped with a highly efficient temperaturestabilized
water nozzle assembly. Selete Inc. in collaboration with Nikon Corporation has been evaluating the
performance and various characteristics of the EET while also investigating various photoresist and topcoat processes.
We selected three types of standard immersion processes that offered the best performance for our evaluation
purposes. A resolution limit of 70-nm half-pitch line-and-space (L/S) patterns has been confirmed. A 0.8-μm depth of
focus (DOF) was also verified for an 80-nm half-pitch L/S pattern. In addition, full wafer (WF) critical dimension (CD)
uniformity of less than 5 nm (3 sigma) has been demonstrated for a 90-nm half-pitch L/S pattern on a 300-mm wafer
(WF). After the implementation of various improvements to both the EET and the topcoat/resist processes, we have
achieved a total defect density of 0.23/cm<sup>2</sup>, and this defect level is low enough for pilot production.
Despite the early skepticism towards the use of 193-nm immersion lithography as the next step in satisfying Moore's law, it continuous to meet expectations on its feasibility in achieving 65-nm nodes and possibly beyond. And with implementation underway, interest in extending its capability for smaller pattern sizes such as the 32-nm node continues to grow. In this paper, we will discuss the optical, physical and lithographic properties of newly developed high index fluids of low absorption coefficient, 'Babylon' and 'Delphi'. As evaluated in a spectroscopic ellipsometer in the 193.39nm wavelength, the 'Babylon' and 'Delphi' high index fluids were evaluated to have a refractive index of 1.64 and 1.63 with an absorption coefficient of 0.05/cm and 0.08/cm, respectively. Lithographic evaluation results using a 193-nm 2-beam interferometric exposure tool show the imaging capability of both high index fluids to be 32-nm half pitch lines and spaces.
We earlier developed new monocyclic fluoropolymers (ASF-2) for F<sub>2</sub> resist materials. But, it is necessary for ASF-2 to improve of their characteristics, especially the dry-etching resistance, in order to apply for ArF and F<sub>2</sub> lithography at fine design rules. In this study, to improve the dry-etching resistance keeping good characteristics of ASF-2, we examined using two methods. The one is to co-polymerize with ASF-2; the other is to introduce protective groups. We synthesized a new series of fluorinated co-polymers (ASF-2 with various monomers, e.g., methacrylate derivatives and vinyl ester derivatives). We found that the dry-etching resistance was improved by co-polymerization. Especially, the co-polymer with methacrylates containing an adamantyl moiety had a good dry-etching resistance, 1.45 vs. conventional KrF resist. This co-polymer also kept a good transparency at 193 nm. The introduction of various protective groups to the hydroxyl group of ASF-2 was also investigated. As the result of the optimization of protective groups and a protecting ratio, the partially protected ASF-2 with CCOM protecting groups had a good transparency at 157 nm and a good etching resistance (1.42 vs. conventional KrF resist). Using partially CCOM protected ASF-2 with an appropriate protecting ratio, sub-60 nm line and space pattern in 150 nm-thick film was obtained.
We earlier developed a series of fluoropolymers (FPRs) for use as first-generation 157-nm photoresist polymers. These FPRs have a partially fluorinated monocyclic structure and provide excellent transparency. However, their etching resistance is low (half that of conventional KrF resists) and an insufficient dissolution rate in tetramethylammonium hydroxide (TMAH) solution. To improve the characteristics of these polymers, while retaining high transparency, we had to redesign the main chain fluoropolymer structure. In this paper, we describe a new monocyclic fluoropolymer structure for a second-generation 157-nm photoresist polymer. This structure also has a fluorine atom in the polymer main chain, as well as a fluoro-containing acidic alcohol group. We synthesized two types of fluoropolymers, ASF-1 and ASF-2. We found that ASF-1 had transparency of 0.18 μm<sup>-1</sup>, better than that of the FPRs, and the etching resistance was improved. Unfortunately, the dissolution rate was poor. On the other hand, ASF-2 showed even better transparency of 0.1 μm<sup>-1</sup>, improved etching resistance, and a dissolution rate of more than 600 nm/s, which is sufficient for use as a resist. The introduction of a protecting group (e.g., the methoxymethyl or adamantylmethoxymethyl group) to the hydroxyl group of ASF-2 can be done after the polymerization reaction. Using partially protected ASF-2 with an appropriate protecting group, we were able to fabricate a sub-60-nm line-and-space pattern.
Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have evaluated the potential of fluorinated polymer-based resists from the viewpoint of critical dimension (CD) control, using a 0.90 numerical aperture (NA) 157-nm micro-stepper with an alternating phase shift mask (alt-PSM). A resolution limit of 55-nm line-and-space patterns was obtained and the bake temperature dependence of the CD was found to be less than 2 nm/°C. We further evaluated these resists using a 0.80-NA FPA-5800FS1 157-nm scanner for full-field imaging with an alt-PSM. With these resists, 60-nm line-and-space patterns were resolved, and a depth of focus (DOF) of more than 400 nm for 100- and 80-nm line-and-space patterns was confirmed. The CD variation across the wafer for a 100-nm 1:1 dense line pattern was 3.3 nm (3σ). Although there is still a need to improve line edge roughness and dry etching resistance, in terms of CD control the fluorinated polymer-based resists have demonstrated sufficient potential for mass-production of 65-nm-node semiconductor devices and beyond.
Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have been studying fluorinated polymers to determine their potential for use as the base resin and have developed a new monocyclic fluorinated polymer that has high transmittance (an absorption coefficient of 0.1 μm<sup>-1</sup>) at a 157-nm exposure wavelength and high dry-etching resistance (a dry-etching rate of 1.86 times that of a KrF resist) under hard mask dry-etching conditions. Moreover, it has a high dissolution rate in standard aqueous tetramethylammoniumhydroxide developer (a dissolution rate of more than 600 nm/s). Using this polymer with adamanthylmethoxymethyl as a protecting group, we were able to resolve a 60-nm line-and-space pattern using a 0.90 numerical aperture 157-nm laser micro-stepper along with a resolution-enhancement alternating phase-shift mask technique. This polymer has enabled both high dry-etching resistance (a dry-etching rate equal to 1.43 times that of a KrF resist) and good imaging performance.
For 157-nm single-layer resists, dry etching resistance is an important issue because of the difficulty of striking a balance between 157-nm transparency and an acceptable level of dry etching resistance. To achieve an acceptable trade-off, the fluorine atom can be introduced into the resist polymer structure to obtain higher transparency, despite the fluorine atom’s high reactivity in the plasma etching process. We recently proposed a model for estimating dry-etching-resistance (the KI-model) and have shown that it can be effectively applied to the design of new fluoropolymer structures. Through simulation based on the KI-model, we were able to develop a new fluoropolymer with good dry etching resistance and high transparency. We found that a new protective group, 2-cyclohexylcyclohexanoxymethyl (CCOM), improved the characteristics of our novel fluoropolymer, compared with use of a MOM group, when used in the base resin of the resist. In this paper, we report on the usefulness of the KI-model for developing new fluorinated protective groups and new base polymers. Moreover, we have developed a new base fluoropolymer which has higher transparency and a similar degree of dry etching resistance as a monocyclic fluoropolymer with a CCOM protective group.
We have developed a new tri-layer resist process to meet requirements related to etching durability and aspect ratio of ArF process. The new phenol capped siloxane-based middle-layer does not change thickness and does not increase particles even after six months. Additionally no footing pattern formation occurs. Our middle-layer has a function as anti-reflect and simulated reflectivity in a top ArF resist layer is less than 1.0% at the tri-layer structure by controlling middle-layer and under-layer thickness. The critical dimension (CD) uniformity of 140 nm contact hole pattern is less than 6 nm (3 sigma) intra wafer. This new middle layer gives high etching sensitivity relative to under-layer and we can demonstrate pattern transfer using a contact hole pattern. We have applied this system to a dual damascene process and successfully completed a 280-nm pitch multilevel copper interconnection. We conclude that our new tri-layer resist process is suitable for use in mass production of 90-nm node LSI and below.