Extreme Ultraviolet Lithography Process Research Articles

Defect risks at interfaces of chemically amplified resists in extreme ultraviolet lithography process

In EUV lithography, stochastically generated defects (stochastic defects) are a significant issue. In this study, the formation of the latent images of line-and-space resist patterns was simulated to assess the dependence of defect risks on the conditions of resist interfaces. The protected unit distribution was calculated on the basis of the sensitization and reaction mechanisms of chemically amplified EUV resists using a Monte Carlo method. The pinching and bridging risks were calculated to be 7.4 × 10−3–2.0 × 10−2 and 1.5 × 10−3–2.6 × 10−1, respectively, depending on the boundary conditions of low-energy secondary electrons at the interfaces. Using the obtained defect risks, we roughly estimated that the impacts of interfacial effects on pinching and bridging probabilities for low-energy secondary electrons were more than one order of magnitude and more than six orders of magnitude, respectively. Controlling the low-energy electrons at the interfaces is important for the suppression of stochastic defects.

Evaluation of Ta-Co alloys as novel high-k extreme ultraviolet mask absorber

BackgroundA plausible approach for mitigating the mask 3-D (M3D) effects observed in extreme ultraviolet (EUV) lithography is to replace the existing mask absorber with alternative materials. Absorbers with a high EUV extinction coefficient k allow for lower best focus variation (BFV) through pitch and reduced telecentricity errors (TCEs).AimWe aim to evaluate Ta-Co alloys as potential high-k mask absorbers from material suitability and imaging standpoints.ApproachWe study the film morphology, surface composition, and stability of Ta-Co alloys in mask cleaning solutions and a hydrogen environment as present in the EUV scanner to assess the material suitability from an experimental aspect. Optical constants for three selected compositions, viz., TaCo, Ta2Co, and TaCo3, were determined from EUV angle-dependent reflectivity measurements. Next, utilizing rigorous simulation software, the imaging performance of Ta-Co alloys is evaluated and compared with the reference absorber. The recommended absorber thickness for Ta-Co alloy absorbers is based upon normalized image log slope (NILS) enhancement, threshold to size, and balancing of diffraction order amplitudes. A 10 nm line and space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm are used for the simulation study using high numerical aperture 0.55 EUV lithography process settings. The primary imaging metrics for through pitch evaluation include NILS, TCE, and BFV.ResultsThe Ta-Co alloys exhibit a higher EUV extinction coefficient k compared with the currently used Ta-based absorber. TaCo and Ta2Co demonstrate smooth surfaces and are stable in a hydrogen environment and in mask-cleaning solutions.ConclusionTa-Co alloys allow for a reduction in M3D effects at a lower absorber thickness compared with a 60 nm Ta-based reference absorber.

Line-Edge Roughness from Extreme Ultraviolet Lithography to Fin-Field-Effect-Transistor: Computational Study.

Although extreme ultraviolet lithography (EUVL) has potential to enable 5-nm half-pitch resolution in semiconductor manufacturing, it faces a number of persistent challenges. Line-edge roughness (LER) is one of critical issues that significantly affect critical dimension (CD) and device performance because LER does not scale along with feature size. For LER creation and impacts, better understanding of EUVL process mechanism and LER impacts on fin-field-effect-transistors (FinFETs) performance is important for the development of new resist materials and transistor structure. In this paper, for causes of LER, a modeling of EUVL processes with 5-nm pattern performance was introduced using Monte Carlo method by describing the stochastic fluctuation of exposure due to photon-shot noise and resist blur. LER impacts on FinFET performance were investigated using a compact device method. Electric potential and drain current with fin-width roughness (FWR) based on LER and line-width roughness (LWR) were fluctuated regularly and quantized as performance degradation of FinFETs.

Bottom-Up Nanofabrication with Extreme-Ultraviolet Light: Metal-Organic Frameworks on Patterned Monolayers.

The fabrication of integrated circuits with ever smaller (sub-10 nm) features poses fundamental challenges in chemistry and materials science. As smaller nanostructures are fabricated, thinner layers of materials are required, and surfaces and interfaces gain a more important role in the formation of nanopatterns. We present a new bottom-up approach in which we use the high optical resolution offered by extreme ultraviolet (EUV) lithography to print patterns on self-assembled monolayers (SAMs). Upon radiation, low-energy electrons induce chemical changes in the SAM so that the projected image is transferred to the substrate surface. We use the chemical differences between exposed and unexposed regions to promote a selective growth of hybrid structures that can act as an etch-resistant layer for further pattern transfer or can be used as functional nanostructures. The EUV doses required to promote selective growth on exposed areas are close to industrial requirements. Furthermore, this method allows for the independent tuning of different steps in the EUV lithography process (photo-induced chemistry, spatially resolved chemical contrast, and formation of nanopatterns), an advantage over current resists, in which the same material plays all roles.

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.

Effect of extreme ultraviolet photoresist underlayer optical properties on imaging performance

An extreme ultraviolet lithography (EUVL) suffers from the trade-off relationship among resolution, line edge roughness, and sensitivity, which is a natural limitation for chemically amplified resists. Thus, the performance of photoresist needs to be assisted by the other materials, and the resist underlayer is one of them. Using a lithography simulation tool, the authors show that the standing wave effect can also occur in the EUVL process and is dependent on the pattern pitch ratio. Although underlayers with a lower refractive index exhibit reduced in-plane line edge roughness, the difference between the refractive index of the underlayer and photoresist should be maintained below 5% in order to optimize performance.

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.

Improved imaging properties of thin attenuated phase shift masks for extreme ultraviolet lithography

The mask shadowing effect is a unique phenomenon caused by a mirror-based mask structure and an oblique incident angle of light in the extreme ultraviolet lithography process. This results in a horizontal–vertical (H–V) bias, an ellipticity in the contact hole pattern and, eventually, a patterning limit and critical dimension (CD) nonuniformity. Reducing the absorber thickness is the most effective method to minimize the mask shadowing effect, but this can deteriorate the mask image contrast. In this paper, an attenuated phase shift mask (PSM) is proposed as a potential method for extending the patterning limit to below 16 nm. By applying Mo as a phase shift layer, which has a refractive index (n) similar to that of the TaN absorber but with a lower absorption value, thin attenuated PSMs with various reflectivity values were obtained by controlling the Mo thickness. The proposed PSM consists of a 16.5-nm-thick TaN absorber layer and a 24-nm-thick Mo phase shifter on 2-nm-thick Ru-capped Mo/Si multilayers. This attenuated PSM results in 17.98% and 27.52% reductions in the mask error enhancement factor as well as 3.65 and 1.84 nm reductions in the H–V CD bias compared with the conventional binary intensity mask with a 70-nm-thick TaN absorber for the 22 nm line and space 1:1 vertical dense pattern under 0.25 and 0.33 numerical aperture illumination conditions, respectively. Moreover, a 10%–12% improved image contrast was obtained with 11%–17% reflectivity on the absorber stack, which corresponds to a 24–29 nm Mo thickness under 0.25 NA illumination conditions.

Extreme Ultra-Violet Resists Development Concepts and Performances

Recently published experimental results indicate that current extreme ultra-violet lithography (EUVL) patterning process seems to be very hard to meet the device manufacturing specification goals, such as resolution, line-width roughness, and sensitivity (RLS) simultaneously. To overcome trade-off limitations between RLS performances of resist, we have approached the problem in several ways. Regarding materials, to make a uniform resist film we applied living radical polymerization and purification to obtain evenly interacting polymer chains. To obtain perfectly miscible resist components, such as polymer, photo acid generator (PAG) and quencher, we have optimized their structures to have similar polarity range. Acid diffusivity factors are also controlled by the resist components properties, including polymer T(g) and photo-acid polarity. In EUVL process, we applied surfactant rinse process to reduce line-width roughness and pattern collapse. In this paper, we discuss the performance of our EUV according to our material development concepts, that is, resist film homogeneity and acid diffusion control in order to meet the device manufacturing specification goals, such as resolution, line-width roughness (LWR), and sensitivity.

Highly Selective Etching of SnO<SUB>2</SUB> Absorber in Binary Mask Structure for Extreme Ultra-Violet Lithography

Among the core EUVL (extreme ultra-violet lithography) technologies for nanoscale patterning below the 30 nm node for Si chip manufacturing, new materials and fabrication processes for high-performance EUVL masks are of considerable importance due to the use of new reflective optics. In this work, the selective etching of SnO2 (tin oxide) as a new absorber material, with high EUV absorbance due to its large extinction coefficient, for the binary mask structure of SnO2 (absorber layer)/Ru (capping/etch stop layer)/Mo-Si multilayer (reflective layer)/Si (substrate), was investigated. Because infinitely high selectivity of the SnO2 layer to the Ru ESL is required due to the ultrathin nature of the Ru layer, various etch parameters were assessed in the inductively coupled Cl2/Ar plasmas in order to find the process window required for infinitely high etch selectivity of the SnO2 layer. The results showed that the gas flow ratio and V(dc) value play an important role in determining the process window for the infinitely high etch selectivity of SnO2 to Ru ESL. The high EUV-absorbance SnO2 layer, patternable by a dry process, allows a smaller absorber thickness, which can mitigate the geometric shadowing effects observed for high-performance binary EUVL masks.

Progress of Resist Materials and Process for hp 2x-nm Devices using EUV Lithography

Extreme ultraviolet (EUV) lithography is the leading candidate for the manufacture of semiconductor devices at the hp 22 nm technology node and beyond. The Selete program covers the evaluation of manufacturability for the EUV lithography process. Then, we have begun a yield analysis of hp 2x nm test chips using the EUV1 (Nikon) full-field exposure tool. However, the resist performance does not yet meet the stringent requirements for resolution limit, sensitivity, and line width roughness. We reported on Selete standard resist 4 (SSR4) at the EUVL Symposium in 2009. Although it has better lithographic performance than SSR3 does, pattern collapse limits the resolution to hp 28 nm. To improve the resolution, we need to optimize the process so as to prevent pattern collapse. An evaluation of SSR4 for the hp 2x nm generation revealed that a thinner resist and the use of a tetrabutylammonium hydroxide (TBAH) solution for the developer were effective in mitigating this problem. Furthermore, the use of an underlayer and an alternative rinse solution increased the exposure latitude by preventing pattern collapse when the resist is overexposed. These optimizations improved the resolution limit to hp 22 nm.

Experimental validation of full-field extreme ultraviolet lithography flare and shadowing corrections

Extreme ultraviolet lithography (EUVL) is the leading candidate for 22nm half-pitch device manufacturing. IMEC has a fully integrated 300mm EUVL process line incorporating an Alpha Demo Tool (ADT) from ASML, aimed to understand issues related to the introduction of EUV technology in high-volume manufacturing. This study experimentally investigates flare and shadowing correction strategies. Experimental characterization of the flare of the ADT is reported, as well as experimental rules for flare variation correction. With respect to shadowing, shadowing corrections are experimentally estimated. Comparisons of experimental data and rigorous simulation are also presented, and a computationally efficient methodology to generate a full-chip flare map is proposed.

Review of progress in extreme ultraviolet lithography masks

Extreme ultraviolet lithography (EUVL) is a leading next generation lithography technology. Significant progress has been made in developing mask fabrication processes for EUVL. The mask blank for EUVL consists of a low thermal expansion material substrate having a square photomask form factor that is coated with Mo/Si multilayers. SEMI standards are being developed for mask substrates and mounting. Several commercial suppliers are developing polishing processes for LTEM substrates, and they are progressing toward meeting the requirements for flatness, surface roughness, and defects defined in the a draft SEMI standard. One of the challenges in implementing EUVL is to economically fabricate multilayer-coated mask blanks with no printable defects. Significant progress has been made in developing mask blank multilayer coating processes with low added defect density. Besides lowering the added defect density, methods to reduce defect printability, such as defect compensation and buffer layer smoothing, are being developed. Experiments indicate that Mo/Si multilayers that are deposited with ion beam deposition tend to smooth substrate defects, and buffer layer films are being designed to enhance this effect. Targets for buffer layer smoothing are being defined using defect printability simulations. A method for using an electron beam to repair substrate defects after multilayer coating is also being investigated. The mask patterning process for EUVL is nearly the same as that for conventional binary optical lithography masks. EUVL mask patterning efforts are focused on developing the EUV-specific aspects of the patterning process. Eight absorbers have been evaluated against the requirements for EUVL masks, and two absorbers appear most promising. Conventional membrane pellicles are not practical for EUVL, so thermophoretic protection is being developed. Experiments have indicated that thermophoretic protection is effective for >125 nm particles down to at least 50 mTorr pressure. A removable pellicle will be used to protect the mask from defects at all times except during wafer exposure.

Theoretical estimation of absorption coefficients of various polymers at 13 nm

The linear absorption coefficients at 13 nm were calculated for more than 150 polymers. The results indicate that an aromatic substitution lowers the absorption coefficient. This is because oxygen has a larger atomic absorption than carbon or hydrogen and the substitution reduces the relative oxygen content. Furthermore, the Ohnishi parameters for the polymers were calculated in order to investigate the relationship between the absorption at 13 nm and the etching resistance. This showed that polymers with aromatic groups tend to exhibit a lower absorption and a higher etching resistance than those without aromatic groups. This suggests that, regarding resist processes for EUV (extreme ultraviolet) lithography, a single-layer resist process employing no hard-mask layer is another promising candidate in addition to one that uses both an unltrathin resist layer (∼100 nm thick) and a hard-mask layer