Line edge roughness (LER) is a common problem to most lithography approaches and is seen as the main resolution limiter for advanced technology nodes1. There are several contributors to LER such as chemical/optical shot noise, random nature of acid diffusion, development process, and concentration of acid generator/base quencher. Since interference-like lithography (IL) is used to define one directional gridded patterns, some LER mitigation approaches specific to IL-like imaging can be explored. Two methods investigated in this work for this goal are (i) translational image averaging along the line direction and (ii) pupil plane filtering. Experiments regarding the former were performed on both interferometric and projection lithography systems. Projection lithography experiments showed a small amount of reduction in low/mid frequency LER value for image averaged cases at pitch of 150 nm (193 nm illumination, 0.93 NA) with less change for smaller pitches. Aerial image smearing did not significantly increase LER since it was directional. Simulation showed less than 1% reduction in NILS (compared to a static, smooth mask equivalent) with ideal alignment. In addition, description of pupil plane filtering on the transfer of mask roughness is given. When astigmatism-like aberrations were introduced in the pupil, transfer of mask roughness is decreased at best focus. It is important to exclude main diffraction orders from the filtering to prevent contrast and NILS loss. These ideas can be valuable as projection lithography approaches to conditions similar to IL (e.g. strong RET methods).
The unique properties of metamaterials, namely their negative refractive index, permittivity, and permeability, have
gained much recent attention. Research into these materials has led to the realization of a host of applications that may
be useful to enhance optical nanolithography, such as a high pass pupil filter based on an induced transmission filter
design, or an optical superlens. A large selection of materials has been examined both experimentally and theoretically
through wavelength to verify their support of surface plasmons, or lack thereof, in the DUV spectrum via the attenuated
total reflection (ATR) method using the Kretschmann configuration. At DUV wavelengths, materials that were
previously useful at mid-UV and longer wavelengths no longer act as metamaterials. Composites bound between
metallic aluminum and aluminum oxide (Al2O3) exhibit metamaterial behavior, as do other materials such as tin and
indium. This provides for real opportunities to explore the potential of the use of such materials for image enhancement
with easily obtainable materials at desirable lithographic wavelengths.
Double patterning has been proposed as a method to extend DUV lithography to 32nm and below. Here, a
new form of double, or higher, multiple exposure technique is proposed. This new form of lithography
uses a combination of Quantum State Control (QuSC) chemistry, Amplitude Modulation Optical
Lithography (AMOL), and multiple micro-stepped exposures, without development between exposures.
Further it is proposed to use this form of lithography (called QuSC-litho), to pattern a perfect grating grid,
and to trim this grid with an earlier generation lithography tool. QuSC lithography uses short optical pulses
to modulate a photochemical pathway while an intermediate is still in a defined vibrational excited state.
This is a variation of Stimulated Emission Depletion Microscopy (STED) developed for fluorescence
microscopy. With this approach immersion tools that produce 90 nm pitch and 45 nm features should be
able to pattern levels with 22 nm features with a 1:1 line-space ratio. This approach is much less sensitive
to misalignment than present double patterning approaches. Key to successful deployment of QuSC
lithography is defining a resist photochemistry consistent with the QuSC process. There are several
approaches to Photo Acid Generator (PAG) - matrix interaction that may be consistent with this approach.
Conventional site-base model calibration approaches have worked fine from the 180nm down to the 65nm technology nodes, but with the first 45nm technology nodes rapidly approaching, site-based model calibration techniques may not capture the details contained in these 2D-intensive designs. Due to the compaction of designs, we have slowly progressed from 1D-intensive gates, which were site-based friendly, to very complex and sometimes ornate 2D-gate regions. To compound the problem, these 2D-intensive gate regions are difficult to measure resulting in metrology-induced error when attempting to add these regions to the model calibration data. To achieve the sub-nanometer model accuracy required at this node, a model calibration technique must be able to capture the curvature induced by the process and the design in these gate regions. A new approach in model calibration had been developed in which images from a scanning electron microscope (SEM) are used together with the conventional site-base to calibrate models instead of the traditional single critical dimension (CD) approach. The advantage with the SEM-image model calibration technique is that every pixel in the SEM image contributes as CD information improving model robustness. Now the ornate gate regions could be utilized as calibration features allowing the acquisition of fine curvature in the design.
This paper documents the issues of the site-base model calibration technique at the 45nm technology node and beyond. It also demonstrates the improvement in model accuracy for critical gate regions over the traditional modeling technique, and it shows the best know methods to achieve the utmost accuracy. Lastly, this paper shows how SEM-based modeling quantifies modeling error in these complex 2D regions.
New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference.
With the advent of the first immersion and hyper-NA exposure tools, source polarization quality will become a hot topic. At these oblique incident angles, unintentional source polarization could result in the intensity loss of diffraction orders possibly inducing resolution or process window loss. Measuring source polarization error on a production lithographic exposure tool is very cumbersome, but it is possible to reverse engineer any source error similarly to what has been accomplished with intensity error. As noted in the intensity maps from the source illumination, it is not safe to assume an ideal or binary source map, so model fitness is improved by emulating the real error. Likewise, by varying the source polarization types (TE, TM, Linear X and Linear Y) and ratios to obtain improved model fitness, one could deduce the residual source polarization error. This paper will show the resolution and process window gain from utilizing source polarization in immersion lithography. It will include a technique demonstrating how to extract source polarization error from empirical data using the Calibre model and will document the modeling inaccuracy from this error.
To perform a thorough source optimization during process development is becoming more critical as we move to leading edge-technology nodes. With each new node the acceptable process margin continues to shrink as a result of lowering k1 factors. This drives the need for thorough source optimization prior to locking down a process in order to attain the maximum common depth of focus (DOF) the process will allow. Optical proximity correction (OPC) has become a process-enabling tool in lithography by providing a common process window for structures that would otherwise not have overlapping windows. But what effect does this have on the source optimization? With the introduction of immersion lithography there is yet another parameter, namely source polarization, that may need to be included in an illumination optimization process. This paper explored the effect polarization and OPC have on illumination optimization. The Calibre ILO (Illumination Optimization) tool was used to perform the illumination optimization and provided plots of DOF vs. various parametric illumination settings. This was used to screen the various illumination settings for the one with optimum process margins. The resulting illumination conditions were then implemented and analyzed at a full chip level. Based on these results, a conclusion was made on the impact source polarization and OPC would have on the illumination optimization process.
Degradation in image contrast becomes a concern at higher numerical apertures (NAs) due to mask-induced polarization effects. We study how different photomask materials (binary and attenuated phase shift), feature sizes and shapes, pitch values, duty ratios (line to space), and wavelengths effect the polarization of transmitted radiation. Rigorous coupled-wave analysis (RCWA) is used to simulate the polarization of radiation by the photomask. The results show that higher NA leads to greater polarization effects in all cases. Off-axis illumination increases polarization in one of the first orders, decreasing it in the other. Nonvertical sidewall angles and rounded corners can also impact polarization, but the wavelength of incident radiation has no effect on polarization effects at the same NA values. In general, materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, whereas materials with lower refractive indices and a relatively wider range of extinction coefficients pass more TE polarized radiation. These properties can provide new design considerations for the development of next-generation masking materials.
The onset of lithographic technology involving extreme numerical aperture (NA) values introduces critical technical issues that are now receiving particular attention. Projection lithography with NA values above 0.90 is necessary for future generation devices. The introduction of immersion lithography enables even larger angles, resulting in NA values of 1.2 and above. The imaging effects from oblique angles, electric field polarization, optical interference, optical reflection, and aberration can be significant. This paper addresses polarization considerations at critical locations in the optical path of a projection system, namely in the illuminator, at the mask, and in the photoresist. Several issues are addressed including TE and azimuthal polarized illumination, wire grid polarization effects for real thin film mask materials, and multilayer resist AR coatings for high NA and polarization.
The objective of this paper is to study the polarization induced by mask structures. Rigorous coupled-wave analysis (RCWA) was used to study the interaction of electromagnetic waves with mask features. RCWA allows the dependence of polarization effects of various wavelengths of radiation on grating pitch, profile, material, and thickness to be studied. The results show that for the five different mask materials examined, the material properties, mask pitch, and illumination all have a large influence on how the photomask polarizes radiation.
The aerial image attained from an optical projection photolithography system is ultimately limited by the frequency information present in the pupil plane of the objective lens. Careful examination of the frequency distribution will allow the operation of such a system to be synthesized experimentally through the use of interferometric lithography. Synthesis is accomplished through single beam attenuation in a two-beam interference system, which is equivalent to adjusting the relative intensities of the primary diffraction orders in a projection system. Typical lithography conditions, such as defocus and partial coherence, can be synthesized efficiently using this technique. The metric of contrast has been utilized to assess the level of correlation between defocus in a projection system and interferometric synthesis. Simulations have shown that interferometric lithography can approximate the performance of a variety of projection system configurations with a significantly high degree of accuracy.
It is important to understand how a photomask will polarize incident radiation. This paper presents data collected on binary mask and various attenuated phase shifting mask materials, feature sizes, duty ratios, and illumination schemes via rigorous coupled wave analysis, extinction spectroscopy, and 193nm lithographic evaluation. Additionally, the result of polarization effects due to the photomask on imaging has been studied. It was found that in the majority of the cases, higher NA led to greater polarization effects. All mask materials predominantly pass the TM polarization state for the 0 order, whereas different materials and duty ratios affect the polarization of the first diffracted orders differently. The polarization effects contributed by mask materials being considered for use in high NA imaging systems need to be examined. The degree of polarization as a function of n and k is presented, providing an introduction to the desirable properties of future mask materials. Materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, which is undesirable. Materials with lower indices and relatively wide range of extinction coefficients pass more TE polarized radiation. The duty ratio, critical dimension, mask material, material thickness, and illumination scheme all influence mask induced polarization effects.
Immersion lithography has become attractive since it can reduce critical dimensions by increasing numerical aperture (NA) beyond unity. Among all the candidates for immersion fluids, those with higher refractive indices are desired. However, for many of the fluids, the strong absorption at 193nm becomes a serious problem. Therefore, it is essential to find a fluid that is transparent enough (with absorbance less than 0.5mm-1) and has high refractive index (above water, 1.44) at 193nm. Characterization of various fluid candidates has been performed and the absorbance of these fluids has been measured. To measure the absolute refractive index, a prism deviation angle method was developed. This method offers the possibility of measuring fluid refractive indices accurately. This paper also presents the obtained refractive indices of these fluids. Several candidates have been identified for 193nm application with refractive indices near 1.55, which is about 0.1 higher than that of water at this wavelength. Cauchy parameters of these fluids were generated and approaches were investigated to tailor the fluid absorption edges to be close to 193nm. The effects of these fluids on photoresist performance were also examined with 193nm immersion lithography exposure at various NA's. 1.5 NA was obtained to image 32nm lines with phosphoric acid as the immersion medium. These fluids are potential candidates for immersion lithography technology.
Interference imaging systems are being used more extensively for R&D applications where NA manipulation, polarization control, relative beam attenuation, and other parameters are explored and projection imaging approaches may not exist. To facilitate interferometric lithography research, we have developed a compact simulation tool, ILSim, for studying multi-beam interferometric imaging, including fluid immersion lithography. The simulator is based on full-vector interference theory, which allows for application at extremely high NA values, such as those projected for use with immersion lithography. In this paper, ILSim is demonstrated for use with two-beam and four-beam interferometric immersion lithography. The simulation tool was written with Matlab, where the thin film assembly (ambient, top coat, resist layer, BARC layers, and substrate) and illumination conditions (wavelength, polarization state, interference angle, demodulation, NA) can be defined. The light intensity distributions within the resist film for 1 exposure or 2-pass exposure are displayed in the graph window. It also can optimize BARC layer thickness and top coat thickness.