Extreme Ultraviolet Masks Research Articles

Ni-Al Alloys as Alternative EUV Mask Absorber

Extreme ultraviolet (EUV) lithography is being industrialized as the next candidate printing technique for high-volume manufacturing of scaled down integrated circuits. At mask level, the combination of EUV light at oblique incidence, absorber thickness, and non-uniform mirror reflectance through incidence angle, creates photomask-induced imaging aberrations, known as mask 3D (M3D) effects. A possible mitigation for the M3D effects in the EUV binary intensity mask (BIM), is to use mask absorber materials with high extinction coefficient κ and refractive coefficient n close to unity. We propose nickel aluminide alloys as a candidate BIM absorber material, and characterize them versus a set of specifications that a novel EUV mask absorber must meet. The nickel aluminide samples have reduced crystallinity as compared to metallic nickel, and form a passivating surface oxide layer in neutral solutions. Composition and density profile are investigated to estimate the optical constants, which are then validated with EUV reflectometry. An oxidation-induced Al L2 absorption edge shift is observed, which significantly impacts the value of n at 13.5 nm wavelength and moves it closer to unity. The measured optical constants are incorporated in an accurate mask model for rigorous simulations. The M3D imaging impact of the nickel aluminide alloy mask absorbers, which predict significant M3D reduction in comparison to reference absorber materials. In this paper, we present an extensive experimental methodology flow to evaluate candidate mask absorber materials.

Development of an EUV Microscope with Focused Coherent EUV Based on Coherent Diffraction Imaging Method for Defect Evaluation on an EUV Mask

SUMMARYFor evaluation of defects on extreme ultraviolet (EUV) masks at the blank state of manufacturing, we developed a microcoherent EUV scatterometry microscope (micro‐CSM). The illumination source is coherent EUV light with a 140‐nm focus diameter on the defect using a Fresnel zoneplate. This system directly observes the reflection and diffraction signals from a phase defect. The phase and the intensity image of the defect is reconstructed with the diffraction images using ptychography, which is an algorithm of the coherent diffraction imaging. We observed programmed phase defect on a blank EUV mask. Phase distributions of these programmed defects were well reconstructed quantitatively. We also observed actual defects which were inspected by the ABI tool. Actual amplitude defect and phase defect images were reconstructed with intensity and phase contrast. The reconstructed image indicated the defect type of these defect. The micro‐CSM is very powerful tool to evaluate an EUV phase defect.

A Novel Cleaning Approach to Remove the Carbon Contaminant from Ru Surface of EUV Mask

The surfaces of extreme ultraviolet (EUV) mask and optics are very vulnerable to carbon contamination during EUV lithography process. The top surface of EUV mask is coated with Ru capping layer to protect the mask substrate. Carbon contamination occurs on Ru surface when the EUV light of 13.5 nm interacts with the surface. The carbon contamination on mask surface is known to lose its reflectivity and change the critical dimension (CD). Thus, the removal of carbon contaminant from mask surface is required to maintain the integrity of EUV lithography process. Conventional cleaning methods such as SPM and UV cleaning are widely used for the removal of organic contaminants but can cause surface damages on EUV mask such as surface oxidation and etching of absorber pattern. In this study, a novel cleaning mechanism is proposed to remove carbon contaminant using a quaternary ammonium hydroxide solution and an organic solvent mixture solution. According to the SN2 (nucleophilic substitution) reaction, hydroxyl ions are ready to react with carbon atoms as nucleophiles. Also, polar aprotic solvents such as DMSO solvate cations to actively assist hydroxyl ion reactions. Therefore, a study on the removal of carbon contaminants was carried out using a mixture of quaternary ammonium hydroxide solution and polar aprotic solvent. There have been limited studies reported on organic solvent cleaning in a semiconductor process for removal of photoresist. However, physical cleaning such as megasonic and UV treatment were applied to the organic solvent cleaning process to improve organic removal. Few studies have also been carried out to remove the photoresist by applying an additional cleaning process such as CO2 aerosol, megasonic, and UV pretreatment to the organic solvent. So far no studies have been reported on the removal of carbon contaminants using quaternary ammonium hydroxide solution with an organic solvent. This study investigated the effect of cleaning solution on hydrocarbon film removal by using four kinds of quaternary ammonium hydroxide (TMAH, TEAH, TPAH and TBAH) as alkaline chemicals and DMSO and THF organic solvents as polar aprotic solvents. Instead of EUV carbon contaminant, a hydrocarbon film with similar properties to EUV carbon contaminant was prepared and evaluated using alkaline chemicals and organic solvents. The hydrocarbon film was effectively removed from Ru surface in the mixture of TBAH and THF. This is because of low polarity nature of both TBAH and THF. When DMSO used with quaternary ammonium hydroxide solution, hydrocarbon was also removed with longer process time. The carbon removal efficiency is improved and reduced the cleaning process time for TBAH+DMSO cleaning process with megasonic due to the easy lift-off of hydrocarbon film from Ru surface. The polarity of carbon contaminant and cleaning solution have to be considered for removal carbon contaminant from Ru surface.

A Novel Cleaning Approach to Remove the Carbon Contaminant from Ru Surface of EUV Mask

The carbon contamination on extreme ultraviolet (EUV) mask surface is known to lose its reflectivity and change the critical dimension. Conventional cleaning methods such as SPM and UV/O3 cleaning are widely used for the removal of organic contaminants but can cause surface damages on EUV mask such as surface oxidation and etching of absorber pattern. In this study, we proposed to remove carbon contaminant using quaternary ammonium hydroxides and an organic solvent mixture solution. This study investigated the effect of the cleaning solution on the hydrocarbon film removal by using four kinds of quaternary ammonium hydroxides as alkaline chemical and Dimethyl Sulfoxide (DMSO) and tetrahydrofuran (THF) organic solvents as polar aprotic solvents. The hydrocarbon film was effectively removed from Ru surface in the mixture of tetrabutylammonium hydroxide (TBAH) and THF. When DMSO was used with TBAH solution, the hydrocarbon film was also completely removed at a longer process time than THF with TBAH solution.

Nickel and Nickel Oxide Thin Films as Absorber Layer Materials of Extreme Ultraviolet Masks

Nickel and Nickel Oxide Thin Films as Absorber Layer Materials of Extreme Ultraviolet Masks

Effect of Particle Contamination on Extreme Ultraviolet (EUV) Mask and Megasonic Cleaning Process for Its Removal

Particle contaminants on EUV mask surface can cause CD error during EUV lithography process. These contaminants should be removed from EUV mask. The critical size of defect which can cause 10 % CD error is about 23~38 nm with various materials. Among them, critical defect size of SiO2 is about 30 nm. To remove this 30 nm fine silica particle, megasonic cleaning was adapted with various megasonic frequencies and with various cleaning solutions such as dilute alkali solution and gas dissolved water. At high megasonic frequency (2 MHz), though bubble cavitation effect is decreased, cavitation threshold value is higher even with the addition of gases into cleaning solution. As a result, higher particle removal efficiency of 30 nm silica particles and lower damages on PR pattern is achieved at 2 MHz megasonic cleaning in a cleaning solution of H2 gas and dilute NH4OH as compared with 1 MHz megasonic cleaning.

Effect of Particle Contamination on Extreme Ultraviolet (EUV) Mask and Megasonic Cleaning Process for Its Removal

Extreme ultraviolet lithography (EUVL) is the most promising lithography technique for the 22 nm half-pitch and beyond. The EUV patterned masks work in a reflective mode and it has 40 to 50 pairs of MoSi multilayer to reflect EUV radiation. This multilayer is capped by a 2.5 nm Ru thin film to protect the MoSi multilayer. Since EUV mask does not have a pellicle which protects mask surface from the contaminants. Therefore, EUV mask is more vulnerable to particle contamination during lithography processing which is a major yield loss in device fabrication. No study has been reported on the effect of particle size on CD variation if they exist on EUV mask. The critical defect size, which can cause 10 % CD error, has to be removed from the mask surface. When the defect is located between line and space patterns on the mask, the reflectivity will be reduced due to defect absorption. Simulation predicted that 30 nm sized silica particles can cause 10 % CD error on 16 nm line and space pattern. Thus, these contaminant particles have to be removed from EUV mask surface for preventing CD variation. In order to evaluate the 30 nm sized silica particle removal test, we adopted 30 nm standard spherical silica particles. Actually, it is a challenge to evaluate 30 nm particles removal test without inspection tools. So, we used atomic micro scope (AFM) for the evaluation of these particles. The silica particles were intentionally deposited on Ru surface using spinning method. Cleaning test was performed with twin type megasnoic. 30 nm silica particles removal efficiency was drastically decreased as compared with 137 nm silica particles at 1 MHz megasonic cleaning. Because the megasonic cleaning force was reduced to ultra-fine particles due to boundary layer thickness. Therefore, higher megasonic cleaning force is required to remove ultra-fine particles to overcome boundary layer thickness. It is well known that the large cleaning force is induced, not only particle removal efficiency but also pattern damage will be increased. Thus, megasonic cleaning process without causing pattern damage has to be optimized.In this study, we shall discuss the effect of megasonic frequency on the removal of ultra-fine particles and pattern damage. Also, gas dissolved water and dilute alkali cleaning solution are adopted to high frequency megasonic cleaning process to achieve ultra-fine particles removal without pattern damage effectively. Figure 1

Development of Coherent Extreme-Ultraviolet Scatterometry Microscope with High-Order Harmonic Generation Source for Extreme-Ultraviolet Mask Inspection and Metrology

In extreme-ultraviolet (EUV) lithography, defect-free mask production is one of the critical issues for the high-volume manufacturing of semiconductor devices. We developed a coherent EUV scatterometry microscope (CSM), which is a simple lensless system. The CSM records diffraction from mask patterns with a charge-coupled-device (CCD) camera directly, which is illuminated with a coherent EUV light. Since a practical standalone system is required by the industry, we developed a standalone CSM system employing a high-order harmonic generation (HHG) EUV source. The 59th high-order harmonic generation of 13.5 nm wavelength is pumped by a tabletop, 6 mJ, 32 fs, Ti:sapphire laser system. The EUV output energy of 1 µW is successfully achieved. We performed the observation of an EUV mask using the HHG-CSM system. The detection limit of the line defect size is improved to 2 nm for the high output power of the HHG EUV source.

Development of Coherent Extreme-Ultraviolet Scatterometry Microscope with High-Order Harmonic Generation Source for Extreme-Ultraviolet Mask Inspection and Metrology

In extreme-ultraviolet (EUV) lithography, defect-free mask production is one of the critical issues for the high-volume manufacturing of semiconductor devices. We developed a coherent EUV scatterometry microscope (CSM), which is a simple lensless system. The CSM records diffraction from mask patterns with a charge-coupled-device (CCD) camera directly, which is illuminated with a coherent EUV light. Since a practical standalone system is required by the industry, we developed a standalone CSM system employing a high-order harmonic generation (HHG) EUV source. The 59th high-order harmonic generation of 13.5 nm wavelength is pumped by a tabletop, 6 mJ, 32 fs, Ti:sapphire laser system. The EUV output energy of 1 µW is successfully achieved. We performed the observation of an EUV mask using the HHG-CSM system. The detection limit of the line defect size is improved to 2 nm for the high output power of the HHG EUV source.

Particle Adhesion and Removal on EUV Mask Layers During Wet Cleaning

Extreme ultraviolet (EUV) masks have a very stringent cleanliness requirement that present new challenges to nanolithography industry. The cleaning of EUV mask surface is required at every exposure level due to the absence of a pellicle layer. In this study, the adhesion and removal of particles on EUV masks is investigated by calculating the interaction force and measuring the adhesion force using atomic force microscopy (AFM). Zeta potential measurements showed that the calculated interaction force was attractive on Si capping layer and Cr absorbed layer for both silica and alumina particle at all pH ranges investigated. However, the measured adhesion force of Si capping layer was similar to that of bare Si at neutral and alkaline pHs. The calculated interaction force of SiO2 buffer layer was most repulsive and the lowest adhesion force was measured. This indicates that the SiO2 buffer layer has a better cleaning efficiency at neutral and alkaline pH. The calculation of interaction force between particle and surface and measurement of adhesion force shows that a lower particle removal efficiency was expected on Cr absorber layer surface.

Simulation of Multilayer Defects in Extreme Ultraviolet Masks

We have employed a scalar simulation based on Fresnel formulas to predict approximately how strongly a multilayer defect in a reflective mask affects the aerial image in extreme ultraviolet lithography. This method enables us to obtain the field reflected from a defective mask, using considerably fewer computational resources than those required for a rigorous electromagnetic simulation. This method was applied to two-dimensional masks with multilayer defects modeled by a simple structure in which the coverage profile was identical throughout the layers. For such defects, we confirmed the validity of the scalar simulation by comparing the normalized peak intensity to that obtained from an electromagnetic simulation. The dependence of the printability of a defect on its size and position is also discussed.