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.
Graphene-based devices have garnered significant attention for their potential in numerous applications, notably in integrated photonics. For graphene devices to be used in real-world systems, it is necessary to demonstrate competitive device performance, repeatability of results, reliability, and a path to large-scale manufacturing with high yield at low cost. In this study, single-layer graphene electro-absorption modulators serve as a pivotal test vehicle to facilitate wafer-scale integration in a 300mm pilot CMOS foundry, harnessing imec silicon photonics platforms along with the 6- inch graphene transfer capabilities of Graphenea. The patterning of graphene is achieved utilizing a hardmask, with tungsten-based contacts being developed via the damascene method to facilitate CMOS-compatible manufacturing. Through an extensive analysis of inline metrology data during process development along with analysis of hundreds of devices on each wafer, the impact of specific processing steps on the performance could be identified and optimized. Subsequent to optimization, a modulation depth of 50 ± 4 dB/mm is exemplified across 400 devices, measured utilizing 5 V peak-to-peak voltage, achieving electro-optical bandwidths up to 15.1 ± 1.8 GHz for 25μm-long devices. The results achieved are comparable to lab-based record-setting graphene devices of similar design and chemical vapor deposition graphene quality. By demonstrating the reproducibility of the results across hundreds of devices, this work resolves the bottleneck of graphene wafer-scale integration. Furthermore, CMOS-compatible processing enables co-integration of graphene-based devices with other photonics and electronics building blocks on the same chip, and for high-volume low-cost manufacturing.
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.
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.
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.
In this presentation we will report on our recent work on new materials that can be monolithically integrated on high-index contrast silicon or silicon nitride photonic ICs to enhance their functionality. This includes graphene and other 2D-materials for realizing compact electro-absorption modulators and non-linear devices, ferroelectric materials for realizing phase modulators and adiabatic couplers for realizing bistable switches.
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