The interaction of extreme ultraviolet (EUV) light with matter is a critical step in EUV lithographic processes, and optimization of the optical material properties of all elements in the lithographic chain (from optical coatings and pellicles to photoresists) is crucial to harnessing the full power of EUV lithography. To optimize these materials, accurate measurements of EUV absorption and reflection are needed to extract the corresponding actinic optical properties and structural parameters. Here, we report on actinic EUV metrology-based absorption and reflection measurements enabled by coherent table-top EUV sources based on high-harmonic generation. We demonstrate the capabilities and flexibility of our setup with measurements on crystalline films, photoresist systems, and carbon nanotube membranes and provide extracted optical parameters, absorption kinetics, and 2D transmission maps, respectively. These results showcase the power of lab-based actinic inspection methods based on compact, coherent EUV sources for providing crucial data for material optimization and lithographic simulation.
EUV lithography has been introduced in semiconductor fabrication and maximizing yield and throughput is extremely important. One key enabler is the use of a high-transmission pellicle to hold particles out of the focal plane and thereby minimize their impact on imaging. Imec initiated the development of a promising pellicle based on a network of carbon nanotubes (CNT). This CNT membrane offers the advantage of very high EUV transmission (> 95 %) and durability compatible with the EUV scanner power roadmap. Moreover, wafer printing with a CNT pelliclized mask on ASML’s EUV scanner at imec has been successfully demonstrated with good printing performance. Since the CNT pellicle is only a few tens of nanometers thick and suspended over an area of tens of centimeters, a major challenge of the pellicle is to control and optimize its mechanical stability and robustness when used in the EUV scanner. The pellicle rupture probability depends on a multitude of parameters, including pressure changes during mask loading and unloading, thermal expansion during exposure, initial stress/strain variations over the large pellicle, membrane degradation in the hydrogen plasma environment, and thickness of the pellicle. In this paper, the mechanical pellicle characterization as a function of the pressure changes for different CNT membranes is presented. The characterization is based on small-size sample evaluation using a bulge test method. By applying controlled plasma to such samples, it was possible to characterize the membranes not only as freshly fabricated but also after exposure to EUV scanner-like conditions. Additionally, the parameters obtained from small samples could be correlated to the actual movement during scanner manipulation. These measurements enable a fundamental understanding of CNT membranes and how they will behave in an industrial environment.
EUV lithography has been implemented in high volume wafer production. Consequently, maximizing yield is of major importance. One key component to achieve optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimize the printing of defects. The carbon nanotube (CNT) pellicle is a membrane consisting of a network of carbon nanotubes, which demonstrates EUV transmission up to 98%. The challenge is to balance the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, high durability in the scanner environment, while maintaining high transmission and low impact on imaging. While our earlier reporting on full-field CNT pellicle exposures demonstrated minimal impact on imaging, the focus of the current paper is on extended exposures on NXE:3400. In the scanner, the EUV light induces a hydrogen plasma that etches the CNTs, resulting in decreasing membrane density and increasing EUV transmission. In this work, we quantify the CNT pellicle etch rate in a real scanner environment and correlate the findings to those obtained in an offline test setup. Our exposures were performed using two different pellicles, with EUV transmission of 89% and 95%, for up to 3000 wafers. Additionally, we demonstrated the effectiveness of pellicle purification prior to mounting on the reticle, which is important to avoid contamination from the as-fabricated CNT pellicle onto the reticle surface. Current ongoing developments focus on further increasing the pellicle durability in the scanner environment. The presented results demonstrate the potential of a CNT-based pellicle at high EUV powers.
EUV lithography has been adopted worldwide for High-Volume Manufacturing (HVM) of sub-10nm node semiconductors. To support HVM, EUV pellicles were introduced by ASML in 2016. More recently, several novel pellicle materials have been developed to offer higher transmission and support higher source powers. The current focus is on two classes of pellicles to support the upcoming NXE:3800 and the associated N2 node, and beyond: Si-based composites and CNT-based pellicles. In this paper, we will give an overview of scanner integration results of current EUV pellicles, both Si-based composites and CNT-based pellicles. This overview will cover high-level aspects of transmission, imaging, robustness and lifetime; as well as underlying contributors such as EUV-uniformity, EUV-reflectivity, flare and DUV-reflectivity. Reviewing achieved performance will illustrate the suitability of these materials to support advanced nodes in production. Additionally, the anticipated future performance of EUV pellicles when used with increasing EUV source powers and high-NA scanners will be provided.
Background: An extreme ultraviolet (EUV)-transparent pellicle must be used during lithography to protect the photomask from fall-on particles. A pellicle made of free-standing carbon nanotube (CNT) films stops particles despite the presence of gaps while demonstrating high EUV transmission, mechanical stability, low EUV scattering and reflectivity, and DUV transmission that enables through-pellicle mask inspection.
Aim: The CNT EUV pellicle properties can be tailored based on the diversity of CNT structures and tunability of their configuration within the CNT film (density, bundle size, composition, etc.) as shown in this work. A remaining challenge is extending the CNT EUV pellicle lifetime in the scanner environment of EUV-induced hydrogen-based plasma, and the effects on different CNT films are explored here.
Approach: Optical and thermal properties of different CNT pellicles with respect to the CNT material type, density, composition, and bundle size were explored. The ability of uncoated CNT EUV pellicles to withstand high EUV powers in the hydrogen-based environment was tested. Transmission, spectroscopic, and chemical mapping of the exposed CNT membranes were performed to explore the material modifications under various exposure conditions.
Results: Uncoated CNT pellicles withstand 600-W source power equivalent in the EUV scanner-like gas environment but exhibit structural changes with prolonged exposure. Multiwalled CNT pellicles exhibit less EUV transmission change as compared to single-walled CNT pellicles under the same exposure conditions. The protection of CNT material from structural degradation by means of coating was shown.
Conclusions: These investigations add to the understanding of CNT EUV pellicle tunability for optimal performance and lifetime limiters of CNT pellicles under the influence of EUV radiation and plasma. We anticipate the need for coating the CNT pellicle to protect the CNT material against plasma damage for the current scanner conditions. Optimization of both the CNT membrane and its coating is in progress.
Background: EUV lithography has been introduced for semiconductor fabrication, which makes maximizing yield and throughput increasingly important. One key component is the use of a high-transmission pellicle to keep particles out of the focal plane and thereby minimize their impact on imaging. Imec initiated the development of a promising pellicle approach based on a network of carbon nanotubes (CNT), which has the advantage of many tunable structural parameters to form a pellicle membrane. A balance between membrane robustness and particle nonpermeability on one side and low EUV absorption and membrane scattering on the other must be found. The membrane scatter is important for EUV flare effects during wafer printing.
Aim: The experimental measurement of the flare must be determined as a function of the tunable CNT structural parameters. However, this measurement can be very challenging for the low-flare requirements involved.
Approach: The EUV scatter measurements on CNT-based pellicle membranes have been performed and optimized in a stand-alone irradiation setup at RWTH Aachen University. The measurement results were compared to flare simulations using a CNT cylinder model, which is used to improve the experimental measurements.
Results: With this approach, the flare of pellicles with different CNT structures and network parameters are investigated, as well as CNT pellicles that incorporate protective coatings.
Conclusion: The proposed flare measurement procedure can be used to test for acceptable scattering levels for EUV imaging applications.
Extreme ultraviolet (EUV) lithography was recently implemented in high-volume wafer production. Consequently, maximizing yield is gaining importance. One key component to achieving optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimizing the printing of defects. The carbon nanotube (CNT) pellicle is a membrane consisting of a network of CNTs with a demonstrated EUV transmission (EUV-T) of up to 98%. The challenge is balancing the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, low impact on imaging through scattered light, and high durability in the scanner environment, while maintaining high transmission. We report results of the first full-field CNT pellicle exposures on an EUV scanner. We demonstrate handling of the pellicles, without breakage, and provide a first assessment of their imaging behavior. Multiple single- and double-walled uncoated CNT pellicles with EUV-T of up to 97.7% were exposed on the EUV scanner at imec, and minimal impact on the imaging was confirmed. In these exposures, uncoated CNT pellicles that do not yet meet the specifications regarding lifetime were used. Therefore, ongoing developments focus on CNT durability in scanner environments. The presented demonstration proves the value of a CNT-based EUV pellicle solution.
Research on carbon nanotube (CNT) films for the EUV pellicle application was initiated at imec in 2015 triggered by the remarkable optical, mechanical, and thermal properties of the CNT material. Today the advancement of the CNT material synthesis together with matured methods to fabricate thin CNT membranes make free-standing CNT films a very promising EUV pellicle candidate for high volume EUV lithography. Balancing the CNT material properties for the optimal pellicle performance in EUV scanners remains the ongoing research focus. Depending on the density and morphology of the CNTs within the film and individual CNT parameters, like number of walls, bundle size, metal catalyst content, purity etc., the optical and thermal properties of the CNT pellicle can be tuned. It is critical for the pellicle to be stable in the EUV lithography scanner environment which includes hydrogen plasma and heat loads associated with high powers beyond 250 W. Different types of CNTs, i.e. single-, double-, multi-walled CNTs and their combinations, are explored as building blocks of an optimized pellicle membrane. Optical properties of different pellicles and their ability to withstand high EUV powers in the hydrogen-based environment were tested. Transmission, spectroscopic and chemical composition mapping of the exposed free-standing CNT films are used to study the material changes that occur in the scanner-like environment. A solution is needed to extend the CNT pellicle lifetime and coating is discussed as a potential approach to protect the CNT material from hydrogen plasma damage.
EUV lithography is introduced in semiconductor fabrication processes, which makes maximizing yield and throughput increasingly important. One key component is the use of a high-transmission pellicle to keep particles out of the focal plane and thereby minimize their impact on imaging. Imec initiated the development of a promising pellicle approach based on a network of carbon nanotubes (CNT), which has the advantage of many tunable structural parameters to form a pellicle membrane. A balance between membrane robustness and particle non-permeability on one side and low EUV absorption and membrane scattering on the other, must be found. The membrane scatter is important for EUV flare effects during wafer printing. Therefore, it is important to verify its magnitude experimentally as a function of the tunable CNT structural parameters. However, this measurement can be very challenging for low-flare requirements. In this work, the EUV scatter measurements on CNT-based pellicle membranes have been performed and optimized in a stand-alone irradiation setup at RWTH Aachen University. Membranes with different CNT structures and network parameters are investigated, as well as membranes with protective coatings. These measurements, in combination with scattering calculations and printing performance, can serve as a guideline on acceptable scattering levels for industrial applications.
Wafer fab mask inspection is facing new challenges in the era of Extreme Ultraviolet Lithography (EUVL). Mask qualification methods in use for ArF lithography are reticle verification based on wafer prints or defect inspection on Deep Ultraviolet (DUV) mask inspection tools. The latter is the industry preference in most cases. To implement this established methodology for EUV masks with pellicle, the pellicle must have transmission that enables inspection for defect detection. In this work we test the viability of a DUV mask inspection platform to image and inspect an EUV mask with a Carbon Nanotube (CNT) based pellicle [1]. Pellicles are used to keep particles off the patterned side of the photomask. For DUV and longer exposure wavelengths, polymers with transmission above 99% are routine. For EUV wavelengths, identifying a transparent, mechanically robust film that is compatible with EUV exposure environments has been challenging. Commercially-available EUV pellicles for lithography are p-Si based[2]. This pellicle type achieves the critical protection of the mask but induces a significant productivity loss due to EUV light absorption. The critical need for a highly-transmissive pellicle will increase with advanced nodes that put more pressure on uniformity specifications and throughput. In this arena, CNT based pellicles exceed the required transmission for High-Volume Manufacturing (HVM) and show promise towards production adoption [3]. DUV light absorption is relevant for mask inspection. DUV light is absorbed by the p-Si pellicle, while the CNT has high transmission in this wavelength region as well, making mask inspection of EUV masks with pellicles possible on existing DUV inspection tools, such as Applied Materials Aera5TM. In order to understand the impact of CNT pellicle on DUV inspection, a controlled study was performed by Applied Materials in cooperation with imec. The study consisted of various system measurements and extensive application qualification. Aera5TM inspection results on a Programmed Defect Mask (PDM) with a full field CNT pellicle will be presented. The results with pellicle are comparable to the reference no-pellicle inspection and meet the same detection sensitivity. Detection optimization Best-Known Methods (BKM) developed on EUV masks without pellicles proved to be effective. These include, but are not limited to, aperture shaping, defocus and polarization. Moreover, the imaging and main system components performance were examined, and no impact was observed.
EUV lithography has recently been implemented in high volume wafer production. Consequently, maximizing yield is gaining importance. One key component to achieve optimal yield is using a pellicle to hold particles out of the focal plane and thereby minimize the printing of defects. The Carbon Nano Tube (CNT) pellicle is a membrane consisting of a network of carbon nanotubes, and demonstrated EUV transmission up to 98%. The challenge is to balance the CNT material parameters for optimal performance in the EUV scanner: low probability for particles to pass, low impact on imaging through scattered light, high durability in the scanner environment, while maintaining high transmission. We report results of the first full-field CNT pellicle exposures on an NXE EUV scanner. We demonstrate handling of the pellicles on the scanner, without breakage, and provides a first assessment of their imaging behavior. Multiple single- and double-walled uncoated CNT pellicles with EUV transmission up to 97.7% were exposed on the NXE scanner at imec, and minimal impact on the imaging is confirmed. In these exposures, uncoated CNT pellicles were used which will not meet the specifications regarding lifetime. Therefore, current ongoing developments focus on CNT coating and durability in scanner environment. The presented demonstration proves the value of a CNT-based EUV pellicle solution.
EUV lithography wafer production has begun and consequently maximizing yield gains importance. One key component to high-yield lithography in manufacturing is using a pellicle to hold particles out of the focal plane and thereby minimize their impact on imaging. Using a pellicle simplifies manufacturing by eliminating wafer inspections that are used to indirectly monitor the presence of printable defects on the mask. The CNT-based pellicle – a membrane consisting of a network of carbon nanotubes – offers the advantage of very high EUV transmission and has demonstrated good durability at high EUV scanner power. Moreover, the microscopic properties of the network can be tuned by modifying several CNT membrane parameters, such as the individual CNT type and diameter, the degree of bundling, the density and the coating. The challenge is balancing these CNT material parameters for optimal performance in the EUV scanner: high transmission, low impact on imaging through scattered light, and low probability for particles to pass. Each of these areas will be addressed along with simulated and experimental data illustrating the value of a CNT-based EUV pellicle solution today and for the future.
As EUV lithography wafer volumes increase, throughput and yield require more focus. Yield can be enhanced by introducing a pellicle to hold particles out of the focal plane and minimize their impact to imaging. Using a pellicle also minimizes the extra wafer inspections required to ensure that printable mask defects do not increase over time. However, if the associated transmission loss is high, the yield advantage is offset by reduced throughput. The CNT-based pellicle offers the advantage of very high EUV transmission. CNT pellicles have also demonstrated lifetime at 300W EUV scanner power. The challenge is balancing the CNT membrane design in three areas: physical presence/the ability to stop particles, EUV transmission/imaging impact, and lifetime in the scanner/thermal tolerance. Each of these areas will be described along with simulated and experimental data illustrating the value of a CNT-based EUV pellicle solution for the future.
Background: The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. It is important to ensure that the optical characteristics of the pellicle membrane do not critically affect the reticle image quality. Aim: We want to verify the possibility to integrate pellicle inspection and characterization capabilities in reflective-mode EUV mask scanning microscope (RESCAN), our actinic mask inspection platform based on coherent diffraction imaging. Approach: We studied the impact of a few selected EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with RESCAN. Results: We measured the scattering distribution of the pellicles, and we correlated it with the mask image contrast and fidelity. We also detected the presence of a 6.5-μm-diameter fiber on the pellicle surface. Conclusions: We demonstrated that RESCAN is suitable for through-pellicle actinic mask inspection and can be also used to characterize and monitor the pellicle quality.
To enable high volume manufacturing with extreme ultraviolet (EUV) lithography, a pellicle membrane is needed to protect the reticle from particles at EUV source powers beyond 250 W. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability, and chemical resistance to the scanner environment is extremely challenging; yet, these properties are required to realize next-generation EUV pellicle solutions. Free-standing carbon nanotube (CNT) film as an alternative next-generation core pellicle material is proposed. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (∼0.4 % half range), mechanical stability (maximum deflection ∼0.08 mm at 2 Pa), thermal stability (no transmission change under greater than 250 W equivalent EUV power in vacuum), and scalability to a full pellicle size (∼15 × 12 cm2). The capability of the CNT membrane to withstand high EUV power in the presence of H2 for a limited time is demonstrated. Other CNT membrane properties are presented that are important for the pellicle application: low EUV scattering, low EUV reflectivity, and sufficient transmission to enable through-pellicle inspection with DUV light or electrons. The ability of the CNT film to stop particles is tested. The influence of hydrogen at higher EUV powers and prolonged exposures on the lifetime of the CNT pellicle remains the current research focus. Approaches for coating the free-standing CNT films for protection are discussed.
The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. Currently several pellicle prototypes are being developed. It is important to ensure that the optical characteristics of the pellicle membrane do not critically affect the reticle image quality. We present here a study of the impact of a few selected EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with an actinic lensless microscope.
In the early 2000s, membranes both thin enough to transmit EUV light and strong enough to be free-standing at mask dimensions did not exist. The lithography community assumed that defect control for photomasks would be achieved, not with a pellicle, but with a clean scanner environment, thermophoretic protection and a removable pellicle.1 In 2006, Intel published their research on an EUV pellicle.2 Since then, an international development effort on EUV pellicle membranes has spanned a range of materials and fabrication approaches. Not only materials, but also the requirements of the EUV pellicle membrane have evolved over time. Imec’s pellicle work based on carbon nanotubes (CNTs) started in 2015, and is placed in relation to the rich history of EUV pellicles. CNTs are one-atom-thick carbon sheets rolled into tubes. The CNTs can be single- or multi-walled and can vary in diameter and in length. These engineered CNTs can be arranged in different configurations to form membranes of different densities. Thus, the CNT membrane’s properties can be fundamentally changed to meet the EUV pellicle targets for properties like transmittance. The historical trends in EUV pellicle membrane development are presented and the CNT membranes are described in that context.
In this paper, studies on the characterization of pellicle membranes with a lab-based spectroscopic reflectometer operated in the extreme ultraviolet (EUV) spectral range from 8.7 nm to 15 nm are presented. This includes the actinic wavelength of EUV lithography at 13.5 nm for high volume manufacturing as well as its neighboring spectral bands. The tool can perform spectroscopic measurements of reflectance under adjustable incidence angles of grazing illumination, ranging from 5° to 12°. Additionally, spectroscopic measurements of transmittance under normal incidence for thin membranes ⪅ 100 nm can be performed. By acquisition of a data set of transmittance and reflectance values, membranes are characterized with respect to their optical constants and their dimensional parameters such as thickness and roughness by means of reconstruction. From reconstructed optical constants further properties such as density and stoichiometry can be derived.
Development of a pellicle membrane to protect the reticle from particles for EUV source powers beyond 250 W is a subject of intensive research and is in great demand to support high volume manufacturing with EUV lithography. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability and chemical resistance within the scanner environment is extremely challenging; yet these properties are required to realize next-generation EUV pellicle solutions. This paper proposes free-standing carbon nanotube (CNT) film as an alternative next generation core pellicle material. Most of the desired pellicle characteristics can be achieved by tuning the properties of freestanding CNT films. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (0.4% half range), mechanical stability (maximum deflection ~0.08 mm at 2 Pa), thermal stability (no change under greater than 500 W equivalent EUV exposure in vacuum without hydrogen radicals) and scalability to a full pellicle size. Other important CNT membrane properties are presented and are favourable for the pellicle application: low EUV scattering, low EUV reflectivity and high transmission under DUV. The ability of the CNT film to stop particles is analysed. The only known failure of the CNT membrane is instability to hydrogen radical/ion environment within the current reticle chamber of the scanner. If changing that environment to limit hydrogen radicals near the pellicle surface is not an option, there is a need to coat the CNT structures for protection. The challenges and considerations for coating the free-standing CNT membranes are discussed.
Pellicles are an important part of the IC-manufacturing supply chain, keeping particles away from the imaging plane of the photomask to preserve wafer yield. EUV lithography poses new challenges on the pellicle membrane because the radiation must pass twice due to the reflective mask. Additionally, there are no transparent materials for EUV so the EUV pellicle must be extremely thin to keep the transmission high. Present continuous-membrane pellicle solutions will not be sufficient for source powers greater than 250 W that are anticipated for HVM EUV lithography. A possible approach to maintain strength and high transmittance is to use nano-structured materials. We report here on the EUV optical characterization of a variety of alternative membrane materials. The fine structure of etched holes or membranes made of carbon nano-tubes introduces interesting optical effects. We, therefore, not only address specular reflectance or transmittance by the optical characterization but also investigate off-specular diffuse scatter. We compare the respective optical properties of homogeneous reference membranes with etched membranes and carbon nano-tubes. Particularly the latter show a very high EUV transmittance of more than 95 % and are therefore considered being a highly promising candidate for alternative EUV pellicles.
A protective membrane – a pellicle – must be used to prevent yield loss during EUV lithography exposure, just as it was for 193nm lithography. The pellicle must be thin enough to transmit EUV light, yet strong enough to withstand the scanner environment. Membrane solutions for ~ 80W exposure exist. Our focus is developing a membrane solution for 250W exposure power. The main pellicle challenge here is still the identification of a membrane material that has very high transmission at EUV wavelengths. Additionally, absorption during lithographic exposure results in high thermal and mechanical load for the pellicle, which can cause yield problems. The current candidates for pellicle membranes such as poly-silicon and silicon nitride cannot withstand 250W power conditions, therefore alternative materials will be required for the future HVM pellicle.
At imec, a variety of novel membrane material options are investigated for the HVM pellicle application. One promising approach is based on carbon nanotubes (CNT). In this paper we outline different CNT based process options, and report results on their optical, thermal, and mechanical performance. In addition, we will report on their uniformity and robustness towards scanner application. Finally, the family of CNT-based membrane options will be compared to promising candidates fabricated using conventional film approaches that do not have a CNT layer.
EUV lithography insertion is anticipated at the 7 nm node and below; however, defects added to the mask during use is a
lingering concern. Defectivity in the scanner is non-zero and an EUV pellicle membrane to protect the mask for high
volume manufacturing power levels does not yet exist. The EUV photons are strongly absorbed by all materials. Sibased
membranes leverage the low absorption coefficient k value (k = 0.0018 at 13.5 nm) for reasonable transmission,
but poly Si becomes fragile and wrinkles during the high temperatures associated with exposure. An alternate approach
to high transmission is deploying very thin or porous layers so that there are fewer atoms to absorb light. For example,
carbon nanomaterials have a reasonably low k value (k = 0.0069), but are strong enough to be fabricated in very thin
layers. Graphene, graphite, carbon-nanosheets and carbon nanotubes are all candidate carbon nanomaterials for this
application, but we focus here on carbon nanotubes (CNTs). Our first measurements on CNT films of ~60 nm thick were
found to have 96.5% transmission at 13.5 nm. Adding CNT layers also enhanced the strength of a thin SiN membrane
significantly. In this paper, critical pellicle metrics will be evaluated in more detail: EUV transmission, bulge test for
mechanical strength, emissivity measurements for heat management, and exposure testing in a hydrogen environment.
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