This PDF file contains the editorial “Special Words on Special Sections” for JM3 Vol. 1 Issue 03

This PDF file contains the editorial “Lithography for Sub-100-nm Device Fabrication” for JM3 Vol. 1 Issue 03

Recent advances have enabled exposure tool manufacturers to ship tools with numerical aperture (NA) = 0.8, and to envision optics with even larger NA. Thus the lithography community must grapple with images formed by oblique waves close to the Brewster angle. (For a typical chemically amplified resist with index of refraction n = 1.7, the Brewster angle is 59°, corresponding to NA = 0.86.) In this paper we will consider some of the surprising phenomena that occur at such high NA. Both vector diffraction simulation results and experimental results from the IBM interferometric lithography apparatus will be discussed. One of the most interesting modeling predictions is that, near the Brewster angle, the swing curve for transverse magnetic (TM) polarization is much smaller than normal, while the swing curve for transverse electric (TE) polarization is much larger than normal, and experimental measurements verify this prediction. Special image cross sections using the Flagello decoration method will also demonstrate the loss of TM image contrast due to vector imaging effects.

Newtonian design forms have been developed to explore high numerical aperture imaging systems at the wavelength of 157 nm with elements made of CaF₂ crystal. First-generation systems working at 0.60 numerical aperature (NA) are currently printing features smaller than 130 nm for resist-process development. Second-generation design forms, working with variable numerical apertures above 0.75 NA, will push feature sizes significantly below 100 nm. Several aspects of second-generation designs have been improved to accommodate the need for characterizing and enhancing imaging performance. Closed-loop methods of optimization to reduce aberrations have been developed to characterize and control the effects of crystal-related birefringence on imagery. In addition these systems are learning vehicles to enhance knowledge of aberration-image performance dependence at high numerical apertures.

157 nm lithography is being investigated for the sub-70 nm technology node of semiconductor devices. Many efforts have been reported on the exposure tool, the F₂ laser, the resist materials, the resist processing and the mask materials. A critical component for the success of this 157 nm lithography is the availability of high numerical aperture (NA) lenses that lead to higher resolution capability and higher process margin. In this paper, we describe our recent evaluation results of a high precision 157 nm Microstepper with 0.85 NA lens combined with simulation analysis of the lithographic performance. The details of the evaluation results discussed here include the resolution limit of the high NA lens and the possible effects of intrinsic birefringence upon the lithographic performance.

The discovery of a significant spatial-dispersion-induced birefringence (intrinsic birefringence) in CaF₂ at ultraviolet wavelengths has had a major impact on the design of 157 nm lithography systems, requiring complete redesign of the optics to take account of the imaging aberrations resulting from the birefringence and the accompanying index anisotropy. This intrinsic birefringence phenomena results from a symmetry-breaking effect of the finite wave vector of the photon on the symmetry of the light-matter interaction in fluorite-structure cubic crystals. As a follow-up to our original concise report of measurements and theory of the effect in CaF₂ and BaF₂, we present here a more detailed analysis of the theory, focusing on the symmetry and its consequences. We also provide the full directional dependence of the effect in useful closed forms. We analyze the implications for precision optical design with CaF₂ optical elements, and discuss qualitatively the approaches being considered to compensate for it.

We present the results of a preliminary feasibility study of liquid immersion lithography at 157 nm. A key enabler has been the identification of a class of commercially available liquids, perfluoropolyethers, with low 157 nm absorbance α₁₅₇≈10 cm^–1 base 10. With 157 nm index of refraction around 1.36, these liquids could enable lithography at numerical aperture ≈1.25 and thus resolution of 50 nm for k₁ = 0.4. We have also performed preliminary studies on the optical, chemical, and physical suitability of these liquids for use in high throughput lithography. We also note that at longer wavelengths, there is a wider selection of transparent immersion liquids. At 193 nm, the most transparent liquid measured, de-ionized water, has α₁₉₃ = 0.036 cm^–1 base 10. Water immersion lithography at 193 nm would enable resolution of 60 nm with k₁ = 0.4.

Fostered by continued advancements in the field of optical extension technologies, optical lithography continues to extend far beyond what was thought possible only a few years ago. The application of chromeless phase lithography (CPL), or "100% transmission PSM," has been used to demonstrate the potential for optical lithography to image features as small as one-quarter of the exposure wavelength at pitches that are below the exposure wavelength. The ability to print 70 nm lines through pitch using a 248 nm, 0.70 numerical aperture (NA) wafer scanner, QUASAR off-axis illumination, and a chromeless mask (CLM) has been demonstrated by Chen et al. [Chen et al., Proc. SPIE. 4346, 515-533 (2001)]. However, it was confirmed by Chen et al. that imaging complex two-dimensional (2-D) structures with high transmission CLM reticles involves very strong optical proximity effects. The need to use high NA wafer steppers with off-axis illumination in order to apply chromeless phase lithography exacerbates these effects. This phenomenon is further magnified and the interactions become more complex as the pitch between 2-D structures is decreased. The nature of the proximity effects observed with chromeless phase lithography and the means by which to correct for them using various optical proximity correction (OPC) methods are described and explained. Patterns that represent real device-like structures are used to demonstrate that data processing algorithms are feasible and can correct the induced proximity effects and thus make it possible to incorporate CPL technology for low-k1 production lithography.

The rise of low-k₁ optical lithography in integrated circuit manufacturing has introduced new questions concerning the physical and practical limits of particular subwavelength resolution-enhanced imaging approaches. For a given application, trade-offs between mask complexity, design cycle time, process latitude and process throughput must be well understood. It has recently been shown that a dense-only phase shifting mask (PSM) approach can be applied to technology nodes approaching the physical limits of strong PSM with no proximity effects. Such an approach offers the benefits of reduced mask complexity and design cycle time, at the expense of decreased process throughput and limited design flexibility. In particular, dense-only methods offer k₁<0.3, thus enabling 90 nm node lithography with high-numerical aperture 248 nm exposure systems. We present the results of experiments, simulations, and analysis designed to explore the trade-offs inherent in dense-only phase shift lithography. Gate and contact patterns corresponding to various fully scaled circuits are presented, and the relationship between process complexity and design latitude is discussed. Particular attention is given to approaches for obtaining gate features in both the horizontal and vertical orientation. Since semiconductor investment is dependent on cost amortization, the applicability of these methods is also considered in terms of production volume.

An integrated methodology has been developed for computer simulation of electromagnetic scattering from large, nonperiodic, two-dimensional layouts of advanced photomasks (masks with optical proximity correction and phase-shifting masks). The domain decomposition method consists of three steps: First, by virtue of the linearity of the Kirchhoff-Fresnel diffraction integral, the mask layout is decomposed into a set of constituent single-opening masks. Second, the rigorous electromagnetic simulation of each three-dimensional structure from the set of these single-opening masks is circumvented and, instead, the result for the scattered field is synthesized based on two two-dimensional rigorous electromagnetic simulations that model the mask geometry in two cross-sectional planes. Subsequently, compact equivalent source models are used to describe the scattered fields on a reference plane. These models are constructed in such a way as to minimize the error in the part of the diffraction spectrum that passes through the projection system, allowing accurate and efficient image simulation. The normalized mean square error of the near scattered field is typically a fraction of 1% and speed-up factors for the total simulation time in excess of 400 (compared with the rigorous mask model) are achieved. The use of a look-up table approach facilitates orders of magnitude of further speed improvement.

Photolithography is a key component of the industry's ability to continue to shrink devices according to Moore's Law, and any improvements immediately translate into large economic benefits for the industry. The relative contribution of the "wetware," i.e., the photoresists and other chemicals used in the photolithography step, versus the contribution of the exposure tool development is investigated by expressing the resist performance improvement as a numerical aperture (NA). Using this "equivalent NA" concept, it is shown that the contribution from resist improvements has historically outpaced that from stepper NA improvements. However, R&D expenditures for hardware and wetware have consistently been at greatly different levels. While part of that difference is probably intrinsic in the cost structure of the respective development efforts, it is suggested that the relative amount of spending on resist R&D has historically not been at a level that optimizes the overall return for the industry.

We describe and evaluate three kinds of pattern transfer processes that are suitable for 157-nm lithography. These transfer processes are (1) a hard mask (HM) process using SiO as a HM material, (2) a HM process using an organic bottom anti-reflecting coating/SiN structure, and (3) a bi-layer process using a silicon-containing resist and an organic film as the bottom layer. In all of these processes, the underlayer of the resist acts as an anti-reflecting layer. For the HM processes, we patterned a newly developed fluorine-containing resist using a 157-nm microstepper, and transferred the resist patterns to the hard mask by reactive ion etching (RIE) with minimal critical dimension shift. Using the HM pattern, we then fabricated a 65 nm WSi/poly-Si gate pattern using a high-numerical aperture (NA) microstepper (NA = 0.85). With the bi-layer process, we transferred a 60 nm 1:1 lines and spaces pattern of a newly developed silicon-containing resist to a 300-nm-thick organic film by RIE. The fabrication of a 65 nm 1:1 gate pattern and 60 nm 1:1 organic film pattern clearly demonstrated that 157-nm lithography is the best candidate for fabricating sub-70 nm node devices.

Step and flash imprint lithography (SFIL) is an attractive method for printing sub-100 nm geometries. Relative to other imprinting processes SFIL has the advantage that the template is transparent, thereby facilitating conventional overlay techniques. In addition, the imprint process is performed at low pressures and room temperature, minimizing magnification and distortion errors. The purpose of this work was to investigate alternative methods for defining high resolution SFIL templates and study the limits of the SFIL process. Two methods for fabricating templates were considered. The first method used a very thin (<20 nm) layer of Cr as a hard mask. The second fabrication scheme attempts to address some of the weaknesses associated with a solid glass substrate. Because there is no conductive layer on the final template, scanning electron microscopy (SEM) and defect inspection are compromised. By incorporating a conductive and transparent layer of indium tin oxide on the glass substrate, charging is suppressed during SEM inspection, and the transparent nature of the final template is not affected. Using ZEP-520 as the electron beam imaging resist, features as small as 20 nm were resolved on the templates. Features were also successfully imprinted using both types of templates.

In order to produce future generations of integrated circuits (ICs), it is advantageous to have tighter coupling between process technology development and IC layout design. The first part of this paper describes a framework being developed in the technology computer aided design (TCAD) area that facilitates such coupling. The second part of the paper provides examples of coupling of product design with one of the areas within TCAD, lithography simulation. The success of optical proximity correction (OPC), for example, depends on accurate models that represent the lithography process, including photoresist performance. Some of the challenges in developing accurate resist models are addressed. In addition to OPC and the short-range effects it addresses, the impact of long-range effects is also described. Two such effects are lens aberrations and flare, with the latter being particularly dependent on the total layout.

A simple graphic analysis technique named the illumination chart method is introduced to aid the customization of the illumination aperture filter for synergistic combination with a high transmission rim-type attenuated phase-shifting mask (PSM) for deep submicron contact hole printing. This graphic method gives direct visualization of the relationship between the interference condition in the pupil and the incident angle of illumination. The working ranges of oblique illuminations with different numbers of diffraction beams taking part in imaging can be easily clarified by this graphic method, which explains the dependence of depth of focus (DOF) on pattern duty. A customized illumination aperture filter (CIF) is synthesized by collecting the effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination to dense patterns and on-axis illuminations to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-reduction mask design for dense hole pattern. We applied this integrated resolution enhancement technique to 0.17 µm contact hole printing in 248 nm wavelength, 0.55 numerical aperture optics. The experimental results show our CIF illumination not only balances the DOF enhancement throughout the pattern pitches but also suppresses the best focus shift due to spherical aberration.

We performed precise and systematic approaches to clarify what reticle flatness should be from the standpoint of focal deviation in optical lithography. The impact of reticle warpage on focus deviation was measured by an aerial image sensor to obtain any tiny shift of reticle-induced focus precisely. We clarified the criteria of reticle flatness after chucking. An optimum free-standing shape that would become the desired shape after chucking was obtained by simulation and an analytical approach. The flatness of the chucked reticle was found to be determined by both the free-standing plate shape inside the reticle holder and the shape of the plate facing the holder. Reticle flatness was redefined according to the results. Requirements with respect to the newly defined flatness for each technology node were clarified by focus budget analysis.

In the development of the electron beam projection lithography (EPL) tool, one of the most important tasks is to develop the high-speed vacuum stage system and reliable vacuum body system. Nikon has a long history of over 22 years in precision stage development for its optical lithography tools as well as over 10 years in electron beam (EB) instrument development such as EB 60 with NTT. Recently, lithography stages have been developed based on air bearing and linear motor technologies. It is desirable and of minimum risk to utilize those technologies for the EPL system in order to shorten total time period of development, but the requirements for the EB stage and body are much different from their optical counterparts and much more difficult. In this paper, development and implementation of the EPL vacuum stage system, vacuum body system, vacuum loader system, and control system are discussed and overviewed.