Anatoly Bourov

Approaching the numerical aperture of water immersion lithography at 193-nm

As immersion nanolithography gains acceptance for next generation device applications, experimental data becomes increasingly important. The behavior of resist materials, fluids, coatings, sources, and optical components in the presence of a water immersion media presents conditions unique compared to convention “dry” lithography. Several groups have initiated fundamental studies into the imaging, fluids, contamination, and integration issues involved with water immersion lithography at 193nm. This paper will present the status and results of the next stage of the development efforts carried out at RIT. The status of two systems are presented; a small field projection microstepper utilizing a 1.05 catadioptric immersion objective lens and a 0.50 to 1.26NA interferometric immersion exposure system based on a compact Talbot prism lens design. Results of the fundamental resolution limits of resist materials and of imaging optics are presented. Additionally, an exploration into the benefits of increasing the refractive index of water is addressed through the use of sulfate and phosphate additives. The potential of KrF, 248nm immersion lithography is also presented with experimental resist imaging results.

Immersion Microlithography at 193 nm with a Talbot Prism Interferometer

A Talbot interference immersion lithography system that uses a compact prism is presented. The use of a compact prism allows the formation of a fluid layer between the optics and the image plane, enhancing the resolution. The reduced dimensions of the system alleviate coherence requirements placed on the source, allowing the use of a compact ArF excimer laser. Photoresist patterns with a half pitch of 45 nm were formed at an effective NA of 1.05. In addition, a variable NA immersion interference system was used to achieve an effective NA of 1.25. The smallest half-pitch of the photoresist pattern produced with this system was 38 nm.

Water Immersion Optical Lithography for the 45nm Node

It is possible to extend optical lithography by using immersion imaging methods. Historically, the application of immersion optics to microlithography has not been seriously pursued because of the alternative solutions available. As the challenges of shorter wavelength become increasingly difficult, immersion imaging becomes more feasible. We present results from research into 193nm excimer laser immersion lithography at extreme propagation angles (such as those produces with strong OAI and PSM). This is being carried out in a fluid that is most compatible in a manufacturable process, namely water. By designing a system around the optical properties of water, we are able to image with wavelengths down to 193nm. Measured absorption is below 0.50 cm-1 at 185nm and below 0.05 cm-1 at 193nm. Furthermore, through the development of oblique angle imaging, numerical apertures approaching 1.0 in air and 1.44 in water are feasible. The refractive index of water at 193nm (1.44) allows for exploration of the following: 1. k1 values approaching 0.17 and optical lithography approaching 35nm. 2. Polarization effects at oblique angles (extreme NA). 3. Immersion and photoresist interactions with polarization. 4. Immersion fluid composition, temperature, flow, and micro-bubble influence on optical properties (index, absorption, aberration, birefringence). 5. Mechanical requirements for imaging, scanning, and wafer transport in a water media. 6. Synthesizing conventional projection imaging via interferometric imaging.

Plasma reactive ion etching of 193 nm attenuated phase shift mask materials

This article gives details on plasma etch process development for potential attenuated phase shift masking materials for use at 193 nm. Masking films investigated include materials based on aluminum nitride, zirconium nitride, molybdenum–silicon oxide, tantalum–silicon nitride, and tantalum–silicon oxide. A variety of halogenated etch plasmas were investigated, including fluorine-based chemistries (CF4 and SF6) and chlorine-based chemistries (Cl2, CCl4) combined with oxygen, argon, and hydrogen. Thin films of TaN, MoSiO, SixNy, and TaO that allow for sufficient volatility in fluorine plasma and processes using SF6 were chosen for optimization. Fluorides of aluminum and zirconium exhibit very low vapor pressure so Cl2+Ar mixtures were chosen for study. Al and Zr chlorides can be made volatile but ion assistance is generally needed to produce sufficiently high etch rates. Because of this, selectivity to resist is generally poor. Of all the materials evaluated, attenuated phase shift mask films of TaN/Si3N4 ...

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.

Inorganic immersion fluids for ultrahigh numerical aperture 193 nm lithography

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 and low absorbance are desired. Characterization of the refractive indices and absorbance of various inorganic fluid candidates has been performed. To measure the refractive indices of these fluids, a prism deviation angle method was developed. Several candidates have been identified for 193 nm application with refractive indices near 1.55, which is approximately 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 193 nm. The effects of these fluids on photoresist performance were also examined with 193 nm immersion lithography exposure at various NAs. Half-pitch 32 nm lines were obtained with phosphoric acid as the immersion medium at 1.5 NA. These fluids are potential candidates for immersion lithography technology.

Amphibian XIS: An Immersion Lithography Microstepper Platform

Recent advances in immersion lithography have created the need for a small field microstepper to carry out the early learning necessary for next generation device application. Combined with fluid immersion, multiple-beam lithography can provide an opportunity to explore lithographic imaging at oblique propagation angles and extreme NA imaging. Using the phase preserving properties of Smith Talbot interferometry, the Amphibian XIS immersion lithography microstepper has been created for research and development applications directed toward sub-90nm patterning. The system has been designed for use at ArF and KrF excimer laser wavelengths, based on a fused silica or sapphire prism lens with numerical aperture values up to 1.60. Combined with a chromeless phase grating mask, two and four beam imaging is made possible for feature resolution to 35nm. The approach is combined with X-Y staging to provide immersion imaging on a microstepper platform for substrates ranging up to 300mm. The Amphibian system consists of single or dual wavelength sources (193nm and 248nm), a 2mm exposure field size, stage accuracy better than 1 um, polarization control over a full range from linear polarization to unpolarized illumination, full control of exposure dose and demodulation (to synthesize defocus), and the ability to image both line patterns as well as contact features. A fluid control system allows use of water or alternative fluids, with the ability to change fluids rapidly between wafers. The Amphibian system is fully enclosed in a HEPA and amine controlled environment for use in fab or research environments.

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.

In situ aberration measurement technique based on principal component analysis of aerial image

We propose a novel in situ aberration measurement technique for lithographic projection lens by use of aerial image based on principal component analysis (AMAI-PCA). The aerial image space, principal component space and Zernike space are introduced to create a transformation model between aerial images and Zernike coefficients. First the aberration-induced aerial images of measurement marks are simulated to form an aerial image space with a statistical Box-Behnken design pattern. The aerial image space is then represented by their principal components based on principal component analysis. The principal component coefficients of the aerial images are finally connected with Zernike coefficients by a regression matrix through regression analysis. Therefore in situ aberration measurement can be achieved based on the regression matrix and the principal component coefficients of the detected aerial images. The measurement performance of the proposed AMAI-PCA technique is demonstrated superior compared to that of the conventional TAMIS technique by using a lithographic simulator tool (Prolith). We also tested the actual performance of AMAI-PCA technique on a prototype wafer exposure tool. The testing results show our proposed technique can rapidly measure the aberrations up to high-order Zernike polynomial term with 1σ repeatability of 0.5 nm to 2.3 nm depending on the aberration type and range.

Design and development of thin film materials for 157-nm and VUV wavelengths: APSM, binary masking, and optical coatings applications

As optical lithography below 193 nm is explored, materials issues become more challenging. Thin film coatings that are sufficient for use at wavelengths near or above 200 nm are more likely than not to be problematic at 157 nm, 126 nm, or other potential VUV wavelengths. The situation is a concern for optical coatings, masking films, and for resist/substrate reflectivity control. Potential solutions for several film types are presented, which have been deposited and optically characterized for use as attenuated phase shift masking films, binary masking films, and optical coatings for use at 157 nm.