EUV mask protection against defects during use remains a challenge for EUV lithography. A stand-off protective membrane – a pellicle – is targeted to prevent yield losses in high volume manufacturing during handling and exposure, just as it is for 193nm lithography. The pellicle is thin enough to transmit EUV exposure light, yet strong enough to remain intact and hold any particles out of focus during exposure. The development of pellicles for EUV is much more challenging than for 193nm lithography for multiple reasons including: high absorption of most materials at EUV wavelength, pump-down sequences in the EUV vacuum system, and exposure to high intensity EUV light. To solve the problems of transmission and film durability, various options have been explored. In most cases a thin core film is considered, since the deposition process for this is well established and because it is the simplest option. The transmission specification typically dictates that membranes are very thin (~50nm or less), which makes both fabrication and film mechanical integrity difficult. As an alternative, low density films (e.g. including porosity) will allow thicker membranes for a given transmission specification, which is likely to improve film durability. The risk is that the porosity could influence the imaging. At imec, two cases of pellicle concepts based on reducing density have been assessed : (1) 3D-patterned SiN by directed self-assembly (DSA), and (2) carbon nanomaterials such as carbon nanotubes (CNT) and carbon nanosheets (CNS). The first case is based on SiN membranes that are 3D-patterned by Directed Self Assembly (DSA). The materials are tested relative to the primary specifications: EUV transmission and film durability. A risk assessment of printing performance is provided based on simulations of scattered energy. General conclusions on the efficacy of various approaches will provided.

EUV lithography imaging using novel pellicle membranes

EUV pellicles are needed to support EUV lithography in high volume manufacturing. We demonstrate progress in cap layer design for increased EUV transmission and infrared emission of the Polysilicon-film. In our research lab we obtained EUV transmission of 90% and good emissivity for a fully capped pSi film. We also discuss results on next generation EUV pellicle films. These include metal-silicides and graphite. Next-gen film performance is compared to the current generation pSi film. These films are expected to be stable at higher operating temperature than pSi. Metal-silicides have the advantage of sharing a similar process flow as that of pSi, while graphite shows ultimate high temperature performance at the expense of a more complicated manufacturing flow. Capping layers are needed here as well and capping strategies are discussed for these film generations.

Pellicle films supporting the ramp to HVM with EUV

To study the kinetics of phase transitions and to obtain artificial materials with improved physical properties, a set of thin-film high-sensitivity sensors for ultra-fast scanning nanocalorimetry has been constructed. To investigate the dynamics of the temperature distribution in thin-film calorimetric sensors, high-resolution high-speed infrared thermography has been applied as a tool of non-contact thermal imaging in combination with ultra-fast scanning calorimetry. The dynamic heat-transfer problem, causing the temperature distribution in a thin-film sensor at ultrafast scanning of temperature, has been solved analytically. Analytical solutions for square and circular geometry have been obtained and compared with the temperature profiles obtained by infrared thermographic measurements. A theoretical background for ultra-fast-cooling experiments has been formulated. The origin of the restrictions imposed on the maximum attainable controlled cooling rate has been investigated. It is shown that thin-film sensors can be applied for controlled ultra-fast cooling, as well as heating, at 108 K/s and even 109 K/s.To study the kinetics of phase transitions and to obtain artificial materials with improved physical properties, a set of thin-film high-sensitivity sensors for ultra-fast scanning nanocalorimetry has been constructed. To investigate the dynamics of the temperature distribution in thin-film calorimetric sensors, high-resolution high-speed infrared thermography has been applied as a tool of non-contact thermal imaging in combination with ultra-fast scanning calorimetry. The dynamic heat-transfer problem, causing the temperature distribution in a thin-film sensor at ultrafast scanning of temperature, has been solved analytically. Analytical solutions for square and circular geometry have been obtained and compared with the temperature profiles obtained by infrared thermographic measurements. A theoretical background for ultra-fast-cooling experiments has been formulated. The origin of the restrictions imposed on the maximum attainable controlled cooling rate has been investigated. It is shown that thin-film...

High-speed dynamics of temperature distribution in ultrafast (up to 108 K/s) chip-nanocalorimeters, measured by infrared thermography of high resolution

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.

Novel membrane solutions for the EUV pellicle: better or not?

Freestanding silicon nitride ($\mathrm{Si}\mathrm{N}$) mechanical resonators are of central interest in applications such as temperature and mass sensing, and for fundamental optomechanical research. Understanding thermal coupling between a membrane resonator and its environment is required to predict thermal noise, frequency noise, and sensor responses to temperature changes. In this work, we provide closed-form derivations of intrinsic thermal-coupling quantities in freestanding thin films---namely, total thermal conductance with the surroundings, thermal response time, and the relative contribution of thermal radiation. Our model is valid for any freestanding thin film anchored on all sides when small temperature differences between the film and its environment are considered. We particularly emphasize the specific case of $\mathrm{Si}\mathrm{N}$, for which spectral emissivity is thoroughly investigated as a function of thickness and temperature. We find that radiative heat exchanges can play a non-negligible role, and can even dominate thermal coupling for membranes of sizes commonly used in optomechanics experiments. We find that our model is in agreement with preliminary experimental results on radiative heat transfer between a ceramic heater and a $3\ifmmode\times\else\texttimes\fi{}3\phantom{\rule{0.2em}{0ex}}{\mathrm{mm}}^{2}$ membrane in a high vacuum.

Radiative Heat Transfer in Freestanding Silicon Nitride Membranes

Freestanding silicon nitride membranes with thicknesses down to a few tens of nanometers find use as TEM windows or soft X-ray spectral purity filters. As the thickness of a membrane decreases, emissivity vanishes, which limits radiative heat emission and resistance to heat loads. We show that thin metal layers with thicknesses in the order of 1 nm enhance the emissivity of thin membranes by two to three orders of magnitude close to the theoretical limit of 0.5. This considerably increases thermal load capacity of membranes in vacuum environments. Our experimental results are in line with classical theory in which we adapt thickness dependent scattering terms in the Drude and Lorentzoscillators.

Emissivity of freestanding membranes with thin metal coatings

Freestanding silicon nitride membranes with thicknesses down to a few tens of nanometers find use as TEM windows or soft X-ray spectral purity filters. As the thickness of a membrane decreases, emissivity vanishes, which limits radiative heat emission and resistance to heat loads. We show that thin metal layers with thicknesses in the order of 1 nm enhance the emissivity of thin membranes by two to three orders of magnitude close to the theoretical limit of 0.5. This considerably increases thermal load capacity of membranes in vacuum environments. Our experimental results are in line with classical theory in which we adapt thickness dependent scattering terms in the Drude and Lorentz oscillators.

https://www.jstage.jst.go.jp/article/ieejjia/advpub/0/advpub_21006010/_pdf

Motion Control, Mechatronics Design, and Moore's Law

EUV pellicle membranes are being pursued to protect scanner images from repeating defects caused by reticle fall-on particle defects. Because most materials highly absorb EUV, pellicle membranes must be ultrathin. In an attempt to increase the strength of the ultrathin membranes, grid-supported pellicle membranes have been proposed. In this study we compare grid-supported pellicles (GSP) over free-standing pellicles (FSP). We considered imaging, thermal, mechanical, and thermo-mechanical characteristics. Finite Element Methods (FEM) was used to investigate the thermal, and (thermo-)mechanical behavior of pellicles. The maximum temperature reached under operational conditions by the pellicle film was determined. Using a thermo-mechanical analysis wrinkling behavior was quantified. The mechanical analysis considered the influence of grid structures on the sagging behavior, on crack propagation, on the pellicle film resistance to collision with solid particles, and on the resistance to shocks on the pellicle frame. The analysis shows that GSP that meets imaging requirements will not bring any advantages over FSP.

Grid-supported EUV pellicles: A theoretical investigation for added value

In order to deploy EUV lithography as a high volume manufacturing technology an extreme control of reticle defectivity is required. Systematic defect printing is unaffordable. A potential solution being investigated for meeting the EUV reticle contamination control requirements is to utilize a suspended thin membrane (pellicle) mounted at a fixed distance in front of the reticle as a physical barrier against particles. Pellicles of the aspect ratios relevant for the scanner (~11x14 cm2, ~50 nm) that are based on polycrystalline silicon were produced and successful imaging runs were carried out. Investigations of pellicles using different materials continue to ensure having membranes capable of withstanding future high EUV source powers and to ensure larger EUV transmission values. The ideal requirements of the pellicle are: (i) maximum EUV transmission ideally above 90% single pass, (ii) chemical stability and (iii) thermo-mechanical resistance under EUV/H2. Since fulfilling all the requirements in one single layer is a challenge, different layer film architectures are proposed. This paper discusses such architectures, both silicon- and carbon-based. The base materials are complemented by nanometre thin coatings that increase IR absorption and thus enhance emissivity and that prevents oxidation of the base material occurring in high power EUV systems.

Hans Vermeulen

Papers

Emissivity of freestanding membranes with thin metal coatings

Emissivity of freestanding membranes with thin metal coatings

Motion Control, Mechatronics Design, and Moore's Law

Grid-supported EUV pellicles: A theoretical investigation for added value

Researching new EUV pellicle films for source powers beyond 250 watts