Andrew Estroff

Evanescent wave imaging in optical lithography

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.

Benefiting from polarization - effects on high-NA imaging

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.

Mask-induced polarization effects at high numerical aperture

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.

Intensive 2D SEM model calibration for 45nm and beyond

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.

Source polarization and OPC effects on illumination optimization

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.

Mask-induced 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.

ILSim: A Compact Simulation Tool for Interferometric Lithography

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.

Mask Induced Polarization Effects at High NA

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.

Hyper NA water immersion lithography at 193nm and 248nm

Immersion lithography can allow for theoretical imaging to λ∕4n (where n is the refractive index of imaging fluid). As 193nm and 248nm technology is pushed toward this limit, experimental data becomes increasingly important. This paper describes research carried out to explore the limitations of water immersion lithography and its extension to higher numerical aperture values using modifications to the imaging fluid. Resist imaging to 38nm is demonstrated using water as an imaging fluid. Several alternative fluids are presented including phosphates, sulfates, and halides, which are shown to increase the refractive index of water.

Synthesis of projection lithography for low-k 1 via interferometry

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.