Extreme Ultraviolet Photoresists Research Articles

Fluorescent Labeling to Investigate Nanopatterning Processes in Extreme Ultraviolet Lithography.

Extreme ultraviolet (EUV) lithography uses 13.5 nm light to reach the sub-20 nm resolution. However, the process of pattern formation induced by this high-energy light is not well-understood. In this work, we provide an inorganic EUV photoresist with fluorescence properties by introducing a carbazole derivative as a ligand, and we study its effect on the patterning process. Using the fluorescence properties, changes in the emission of the material after EUV exposure could be tracked by means of spectroscopy and microscopy. The resist sensitivity was substantially reduced by the incorporation of the carbazole benzoate ligands, which is attributed to hole trapping and steric hindrance. After EUV irradiation of the resist films, infrared, UV-visible absorption, and fluorescence spectroscopies showed that the carbazole units were still mostly intact, although their fluorescence intensity was lowered. Our work shows that fluorescent labeling can provide relevant mechanistic insights in the patterning process of resists, potentially with a molecular resolution.

Research Progress on High Resolution Extreme Ultraviolet Photoresist

Research Progress on High Resolution Extreme Ultraviolet Photoresist

Extreme Ultraviolet Photoresist Inspection Platform in Shanghai Synchrotron Radiation Facility

Extreme Ultraviolet Photoresist Inspection Platform in Shanghai Synchrotron Radiation Facility

Chemical and structural characterization of EUV photoresists as a function of depth by standing-wave x-ray photoelectron spectroscopy

The success in the miniaturization of the electronic device constituents depends mostly on the photolithographic techniques. Recently, to achieve patterning at the sub-10-nm node, extreme ultraviolet (EUV) lithography has been introduced into high volume production. Continued scaling of EUV via increased numerical aperture to achieve nodes at 3-nm and below requires the development of fundamentally new patterning materials and new characterization methods. Current EUV-resist film thicknesses are in the 20- to 40-nm range, and further thickness reduction is required for the next generation. Therefore, interfaces become exceedingly important, and the properties of the resist film would be dominated by top and bottom interfacial effects. X-ray photoelectron spectroscopy (XPS) combined with standing-wave excitation (SW-XPS), a fairly new method in the EUV lithography field, previously had been largely applied in multilayers and superlattices for characterizing the composition and electronic structure of buried layers and interfaces as a function of depth. We applied the SW-XPS method to organic/inorganic photoresists to provide depth-selective information on their structural and chemical conditions of as a function of temperature, EUV exposure, different underlayers, and other fundamental parameters. As a first attempt, we perform an SW-XPS feasibility study on self-assembled monolayer (SAM) films after exposure to an electron beam. By SW-XPS, we determined that the interface between the Al2O3 underlayer and the SAMs is smooth, with a mean roughness of about 0.2 nm. Moreover, we determined that the SAM chains are, on average, tilted by ∼30 deg off the sample normal. The SW-XPS results also suggest that the SAM is not a perfectly aligned and uniform monolayer, with some areas having thickness higher than a single monolayer. We demonstrated that SW-XPS can provide useful information on ultrathin materials with high potential for being used as a characterization method of organic/inorganic photoresists.

Characterization of Surface Variation of Chemically Amplified Photoresist to Evaluate Extreme Ultraviolet Lithography Stochastics Effects

Extreme ultraviolet (EUV) lithography is required for advanced node semiconductor device fabrication. The stochastic effects in EUV lithography are problematic, especially with regards to pattern roughness and defect formation. In this study, we performed atomic force microscopy (AFM) on an EUV photoresist surface to determine the surface roughness, height histogram, line scan, area ratio, and power spectral density (PSD). Polymethyl methacrylate (PMMA) for nonchemically amplified resist (non-CAR), and poly(4-hydroxystyrene)(t-butyl acrylate) copolymer (PHS:tBA) and poly(4-hydroxystyrene)(polystyrene)(t-butyl acrylate) copolymer (PHSPS:tBA) with/ di-(t-butylphenyl)iodonium perfluorobutane sulfonate (TBPI-PFBS)/tetrabutylammonium lactate (TBAL) for chemically amplified resist (CAR) were examined. In this CAR system, the exposure and dark loss contributed to the surface variation of root mean square (RMS) of 1.5 nm and 0.95 nm under a nominal exposure dosage of 8 μC/cm2. The contribution of dark loss was further evaluated from the effects of backbone polymer composition and photoacid generator (PAG) loading. The dark loss induced surface roughness can be attributed to the competition of etch selectivity in the resist components. A skewness of the height histogram and change of correlation in PSD are related to the dark loss induced surface variation.

Extreme ultraviolet photoemission of a tin-based photoresist

Tin is a suitable element for inclusion in extreme ultraviolet photoresists because of its relatively high-absorption cross section at 92 eV. The electrons emitted after photon absorption are expected to generate secondary electrons in the solid film. In this way, several pathways lead to reactive species that cause a solubility switch. Here, we report the photoelectron spectra of tin oxo cage photoresists over the photon energy range 60–150 eV, and the relative yields of photoelectrons from the valence band of the resist, from the Sn 4d orbitals, and of inelastically scattered electrons. The experimental excitation spectra differ considerably from those predicted by commonly used database cross section values, and from the combined computed subshell spectra: the maximum efficiency of ionization of Sn 4d both in the photoresists and in Sn metal occurs near the industrially relevant EUV wavelength of 13.5 nm.

Contribution of EUV resist counting statistics to stochastic printing failures

Background: Stochastic printing failures, manifested as random defects in a patterned photoresist image, result from statistical fluctuations in photon flux and resist components and are a key issue confronting extreme ultraviolet (EUV) lithography. Empirical data indicate that photoresist composition and processing influence stochastic printing failure rates. Aim: To devise a simple and flexible model framework for assessing how changes in photoresist composition and imaging chemistry can be expected to impact the frequency of stochastic printing failures Approach: A simple physicochemical description based solely on resist component and photon statistics is combined with combinatorial calculations of resist imaging chemistry and Monte Carlo analysis to estimate rates of random printing failures. Results: This model yields results consistent with experimental observations. The method is applied to predict impacts of resist formulation, composition, and process changes on the rates of stochastic printing failures. Conclusions: This approach provides rapid assessment of the relative impact of resist materials and process modifications and is useful as a tool to advance EUV photoresist design.

Significance of plasma-photoresist interactions for atomic layer etching processes with extreme ultraviolet photoresist

Extreme ultraviolet (EUV) lithography has emerged as the next generational step in advancing the manufacturing of increasingly complex semiconductor devices. The commercial viability of this new lithographic technique requires compatible photoresist (PR) materials that satisfy both the lithographic and etch requirements of good feature resolution, chemical sensitivity, a low line edge roughness, and good critical dimension uniformity. Achieving the decreased feature pitches of modern processing nodes via EUV lithography places a limit on the available photoresist thickness for a pattern transfer process. Therefore, etch processes are required to maximize the etching selectivity of a hard mask material, such as SiO2, to an EUV photoresist. In this work, the authors evaluated the ability of an atomic layer etching (ALE) process to maximize the SiO2/EUV PR etching selectivity. Through the flexible parameter space available in an ALE process, the authors evaluated the etching behaviors as a function of the ALE parameters of ion energy, etch step length, fluorocarbon (FC) deposition thickness, and precursor gas type. The authors found that the interaction between the energetic argon ion bombardment and a deposited FC layer produces a modified surface layer on the PR material that can strongly control the PR etch rate and even produce an etch stop under some conditions. Under the same processing conditions, the etching behavior of SiO2 continues unimpeded, thus resulting in a high overall SiO2/PR etching selectivity. Secondary characterization using x-ray photoelectron spectroscopy and atomic force microscopy was used to support the conclusions derived from the ellipsometric modeling based on the surface chemistry evolution and determine the impact of the ALE process on the surface roughness of the EUV PR, respectively. Additionally, attenuated total reflection Fourier-transform infrared spectroscopy was used to track the impact on specific functional groups within the PR composition from both the argon ion bombardment and FC deposition components of the ALE process. The ALE-based PR etching concept established in this work serves as a foundation for both the understanding of the impacts of an ALE process on an EUV PR material and for future works, employing an ALE process for PR-based pattern transfer.

Unravelling the effect of fluorinated ligands in hybrid EUV photoresists by X-ray spectroscopy

A fluorinated ligand in an extreme ultraviolet hybrid photoresist yields C–F bond dissociation upon exposure, thus enhancing its overall sensitivity.

Reconstructing the three-dimensional latent image of extreme ultraviolet resists with resonant soft x-ray scattering

Extreme ultraviolet (EUV) lithography is one of the most promising printing techniques for high-volume semiconductor manufacturing at the 14-nm half-pitch device node and beyond. However, key challenges around EUV photoresist materials, such as the exposure-dose sensitivity or the line-width roughness, continue to impede its full adoption into industrial nanofab facilities. Metrology tools are required to address these challenges by helping to assess the impact of the EUV materials’ properties and processing conditions along different steps of the nanofabrication process. We apply the resonant soft x-ray scattering (RSoXS) technique to gain insights into the structure of patterned EUV resists before the development step takes place. By using energies around the carbon K-edge to take advantage of small differences in chemistry, the electronic density contrast between the exposed and unexposed regions of the resists could be enhanced in order to image the patterns with subnanometer precision. Critical-dimension grazing-incidence small-angle x-ray scattering is then performed at energies where the contrast is maximized, enabling the reconstruction of the three-dimensional shape of the latent image. We demonstrate the potential of RSoXS to provide a high-resolution height-sensitive profile of patterned EUV resists, which will help in quantifying the evolution of critical features, such as the line-edge roughness, at a key step of the nanofabrication process.

Mechanistic insights in Zr- and Hf-based molecular hybrid EUV photoresists

<bold>Background:</bold> Inorganic resists show promising performances in extreme ultraviolet (EUV) lithography. Yet, there is a need for understanding the exact chemical mechanisms induced by EUV light on these materials. <bold>Aim:</bold> To gain knowledge on the EUV chemistry of inorganic resists, we investigate hybrid inorganic–organic molecular compounds, metal oxoclusters (MOCs). Their molecular nature allows for the monitoring of specific structural changes by means of spectroscopy and thus for the elucidation of the mechanisms behind pattern formation. <bold>Approach:</bold> We compare the sensitivity of MOCs based on Zr and Hf, and methacrylate ligands as EUV resists. The chemical and structural changes causing the solubility switch were investigated by <italic>ex situ</italic> x-ray spectroscopy, infrared spectroscopy, ultraviolet–visible spectroscopy, and grazing incidence x-ray scattering. <bold>Results:</bold> Higher sensitivity was detected for the Hf-based material, in line with its higher absorptivity. A small fraction of the carboxylate ligands is lost at doses that yield solubility contrast, whereas aggregation of the inorganic clusters was not observed. <bold>Conclusions:</bold> These results provide evidence that, although the mechanism of solubility switch in these materials starts with decarboxylation reactions, it mainly proceeds through cross linking of the organic shells instead of aggregation of the inorganic clusters.

Effect of Oxygen on Thermal and Radiation-Induced Chemistries in a Model Organotin Photoresist.

Organotin photoresists have shown promise for next-generation lithography because of their high extreme ultraviolet (EUV) absorption cross sections, their radiation sensitive chemistries, and their ability to enable high-resolution patterning. To better understand both temperature- and radiation-induced reaction mechanisms, we have studied a model EUV photoresist, which consists of a charge-neutral butyl-tin cluster. Temperature-programmed desorption (TPD) showed very little outgassing of the butyl-tin resist in ultrahigh vacuum and excellent thermal stability of the butyl groups. TPD results indicated that decomposition of the butyl-tin resist was first order with a fairly constant decomposition energy between 2.4 and 3.0 eV, which was determined by butyl group desorption. Electron-stimulated desorption (ESD) showed that butyl groups were the primary decomposition product for electron kinetic energies expected during EUV exposures. X-ray photoelectron spectroscopy was performed before and after low-energy electron exposure to evaluate the compositional and chemical changes in the butyl-tin resists after interaction with radiation. The effect of molecular oxygen during ESD experiments was evaluated, and it was found to enhance butyl group desorption during exposure and resulted in a significant increase in the ESD cross section by over 20%. These results provide mechanistic information that can be applied to organotin EUV photoresists, where a significant increase in photoresist sensitivity may be obtained by varying the ambient conditions during EUV exposures.

Progress in metal organic cluster EUV photoresists

Most advanced microelectronic devices are made by using 193 nm immersion lithography systems, but it is difficult to follow the rapid development of semiconductors due to their approaching physical limits. Extreme ultraviolet (EUV) lithography which uses a shorter wavelength (i.e., 13.5 nm) light source can offer a way to print features under a 20 nm HP. EUV lithography requires photoresists that utilize EUV photons because photons generated by EUV exposure are fewer than photons generated by 193 nm light exposure. In this paper, our recent progress in metal oxide photoresist research will be discussed.

Electron trapping: a mechanism for acid production in extreme ultraviolet photoresists

The photomechanism of extreme ultraviolet (EUV) exposures in chemically amplified photoresists is much different than that of previous lithographic wavelengths. Electrons generated during EUV exposure are demonstrated to be a source of acid production through a process referred to as electron trapping. Density functional theory modeling indicates that it is energetically favorable for the photoacid generator (PAG) molecule to decompose if an electron is trapped. Low-energy electrons (<10 eV) that are unlikely to produce holes and secondary electrons generate acid-indicating electron–PAG interactions that are capable of inducing decomposition. Additionally, solution phase reduction in PAGs via electrolysis is shown to produce acid. Furthermore, a more easily reduced PAG (i.e., higher likelihood of trapping an electron) produces a higher acid yield, further supporting electron trapping as a process of acid production regardless of the polymer matrix. An acid indicator, Coumarin 6, was used to determine the number of acids generated per absorbed EUV photon. The results of these measurements indicate that electron–PAG interactions are a source of acid production through electron trapping; thus, an increase in the number of electron-hole pairs available to induce chemical reactions would improve sensitivity.

Ab initio photodynamics of model EUV photoresists

Tin-oxo cages belong to a class of organometallic compounds which were recently recognized as promising photoresist materials for the extreme ultraviolet (EUV) lithography – a method proposed for the new generation of integrated circuits. The EUV exposure of tin-oxo cages causes significant structural changes; however, the molecular mechanism of these changes is to a large extent unknown. In the present work, we focus on the photoionization dynamics of the small tin-oxo molecules: the trimethlytin hydroxide (TMTH) and trihydroxymethyl stannane (THMS) molecules. We employed the surface hopping (SH) dynamics on the Floating Occupation Molecular Orbital Complete Active Space Configuration Interaction (FOMO-CASCI) potential energy surfaces accelerated with graphical processor units (GPU). After the ionization of both molecules, we observed an ultrafast relaxation (∼100 fs) into the ground and first excited electronic states followed by a rapid dissociation yielding the neutral CH3 and OH radicals and a charged remainder. However, the dissociation dynamic does not occur solely on the hot ground electronic state and reaction outcome partially depends on initially ionized state. Our work also demonstrate that the SH+FOMO-CASSCI based dynamics is suitable for electron-rich open-shell systems.

Two nanomaterial firms raise funding

Two firms specializing in nanoscale carbon materials have raised funds to help them meet the needs of customers producing a variety of electronic devices. Nano-C, an MIT spin-off founded in 2001, raised $11.5 million from investors, including Ray Stata, cofounder of integrated circuit maker Analog Devices. Meanwhile, Nantero received $29.7 million in investments from strategic partners that include the venture arms of Dell, Cisco, and Kingston Technology. The two firms are connected: Nano-C supplies carbon nanotubes to Nantero for use in a new type of computer chip called nonvolatile random access memory. In addition to memory chips, Nano-C says, its high-purity nanotubes, fullerenes, and functionalized nanomaterials will be used in printable organic photovoltaics, extreme ultraviolet lithography photoresists, and optoelectronic films used in electronic displays. Compared to more prosaic uses of nanomaterials, such as polymer reinforcement, “electronics is definitely one of the harder application development efforts,” says Anthony Schiavo, an analyst

Process development for high resolution hydrogen silsesquioxane patterning using a commercial scanner for extreme ultraviolet lithography

The semiconductor industry is transitioning toward the use of extreme ultraviolet (EUV) lithography as a next generation patterning technology. There are currently only a limited number of high resolution EUV photoresists reported with EUV patterning capabilities, and those are generally tested using EUV-interference lithography. One such resist is the more commonly known electron beam resist, hydrogen silsesquioxane (HSQ), which is also sensitive to EUV exposure. In the present work, high resolution, dense, subdense patterning of HSQ resist on 300 mm wafers was demonstrated using ASML's NXE 3300B scanner. The critical dimensions analyzed ranged from 18 to 10 nm. Resolution down to 10.0 on 21.0 nm spacing was achieved with 6.5 nm line width roughness. This demonstration of high resolution EUV patterning of HSQ on a commercial scanner makes this process potentially viable for high volume manufacturing.

Novel EUV Photoresist for Sub-7 nm Node

Extreme ultraviolet (EUV) lithography has been recognized as a promising candidate for the manufacturing of semiconductor devices as LS/CH pattern for 7 nm node and beyond. EUV lithography is ready for high volume manufacturing stage. For the high-volume manufacturing of semiconductor devices, significant improvement of sensitivity and line edge roughness (LWR) and Local CD Uniformity (LCDU) is required for EUV resist. It is well-known that the key challenge for EUV resist is the simultaneous requirement of ultrahigh resolution (R), low line edge roughness (L) and faster sensitivity (S). Especially sensitivity and good roughness is important for EUV lithography high volume manufacturing. We are trying to improve sensitivity and LWR/LCDU from many directions. From material side, we found that both sensitivity and LWR/LCDU are simultaneously improved by controlling acid diffusion length and efficiency of acid generation using novel resin and PAG. And optimizing EUV integration is one of the good solution to improve sensitivity and LWR/LCDU. We are challenging to develop new multi-layer materials to improve sensitivity and LWR/LCDU. Our new multi-layer materials are designed for best performance in EUV lithography system. From process side, we found that sensitivity was substantially improved maintaining LWR applying novel type of chemical amplified resist (CAR) and process. EUV lithography evaluation results obtained for new CAR EUV interference lithography. And, metal containing resist is one possibility to break through sensitivity and LWR trade off. In this paper, we will report the recent progress of sensitivity and LWR/LCDU improvement of JSR novel EUV resist and process.

Dynamic absorption coefficients of chemically amplified resists and nonchemically amplified resists at extreme ultraviolet

The dynamic absorption coefficients of several chemically amplified resists (CAR) and non-CAR extreme ultraviolet (EUV) photoresists are measured experimentally using a specifically developed setup in transmission mode at the x-ray interference lithography beamline of the Swiss Light Source. The absorption coefficient α and the Dill parameters ABC were measured with unprecedented accuracy. In general, the α of resists match very closely with the theoretical value calculated from elemental densities and absorption coefficients, whereas exceptions are observed. In addition, through the direct measurements of the absorption coefficients and dose-to-clear values, we introduce a new figure of merit called chemical sensitivity to account for all the postabsorption chemical reaction ongoing in the resist, which also predicts a quantitative clearing volume and clearing radius, due to the photon absorption in the resist. These parameters may help provide deeper insight into the underlying mechanisms of the EUV concepts of clearing volume and clearing radius, which are then defined and quantitatively calculated.

Low-line edge roughness extreme ultraviolet photoresists of organotin carboxylates

Pure thin films of organotin compounds have been lithographically evaluated using extreme ultraviolet lithography (EUVL, 13.5 nm). Twenty compounds of the type R2Sn(O2CR′)2 were spin-coated from solutions in toluene, exposed to EUV light, and developed in organic solvents. Exposures produced negative-tone contrast curves and dense-line patterns using interference lithography. Contrast-curve studies indicated that the photosensitivity is linearly related to the molecular weight of the carboxylate group bound to tin. Additionally, photosensitivity was found to be linearly related to free radical stability of the hydrocarbon group bound directly to tin (R=phenyl, butyl, and benzyl). Dense-line patterning capabilities varied, but two resists in particular show exceptionally good line edge roughness (LER). A resist composed of an amorphous film of (C6H5CH2)2Sn(O2CC(CH3)3)2 (1) achieved 1.4 nm LER at 22-nm half-pitch patterning and a resist composed of (C6H5CH2)2Sn(O2CC6H5)2 (2) achieved 1.1 nm LER at 35-nm half-pitch at high exposure doses (600 mJ/cm2). Two photoresists that use olefin-based carboxylates, (C6H5CH2)2Sn(O2CCH⏧CH2)2 (3) and (C6H5CH2)2Sn(O2CC(CH3)⏧CH2)2 (4), demonstrated better photospeeds (5 mJ/cm2 and 27 mJ/cm2) but worse LER.