EUV Beam Research Articles

High-speed and wide-field nanoscale table-top ptychographic EUV imaging and beam characterization with a sCMOS detector.

We present high-speed and wide-field EUV ptychography at 13.5 nm wavelength using a table-top high-order harmonic source. Compared to previous measurements, the total measurement time is significantly reduced by up to a factor of five by employing a scientific complementary metal oxide semiconductor (sCMOS) detector that is combined with an optimized multilayer mirror configuration. The fast frame rate of the sCMOS detector enables wide-field imaging with a field of view of 100 µm × 100 µm with an imaging speed of 4.6 Mpix/h. Furthermore, fast EUV wavefront characterization is employed using a combination of the sCMOS detector with orthogonal probe relaxation.

EUV Beam–Foil Spectra of Germanium and a Blind-Spot Problem in Spectroscopy

Beam–foil extreme-ultraviolet survey spectra of Ge (Z=32) are presented. The data have been garnered at the performance limit of the heavy-ion accelerator available, with a correspondingly limited statistical and calibrational reliability. However, the Ge spectra have been recorded at various delays after excitation, and this technique points to a possible blind spot in some other spectroscopic techniques, and thus in the literature coverage. A similarly patchy coverage can be noted in various atomic structure computations. The experimental and theoretical gaps seem to be correlated.

\u03bb/30 inorganic features achieved by multi-photon 3D lithography

It’s critically important to construct arbitrary inorganic features with high resolution. As an inorganic photoresist, hydrogen silsesquioxane (HSQ) has been patterned by irradiation sources with short wavelength, such as EUV and electron beam. However, the fabrication of three- dimensional nanoscale HSQ features utilizing infrared light sources is still challenging. Here, we demonstrate femtosecond laser direct writing (FsLDW) of HSQ through multi-photon absorption process. 26 nm feature size is achieved by using 780 nm fs laser, indicating super-diffraction limit photolithography of λ/30 for HSQ. HSQ microstructures by FsLDW possess nanoscale resolution, smooth surface, and thermal stability up to 600 °C. Furthermore, we perform FsLDW of HSQ to construct structural colour and Fresnel lens with desirable optical properties, thermal and chemical resistance. This study demonstrates that inorganic features can be flexibly achieved by FsLDW of HSQ, which would be prospective for fabricating micro-nano devices requiring nanoscale resolution, thermal and chemical resistance.

Plasma-assisted discharges and charging in EUV-induced plasma

In the past years, EUV lithography scanner systems have entered high-volume manufacturing for state-of-the-art integrated circuits (IC), with critical dimensions down to 10 nm. This technology uses 13.5-nm EUV radiation, which is transmitted through a near-vacuum H2 background gas, imaging the pattern of a reticle onto a wafer. The energetic EUV photons excite the background gas into a low-density H2 plasma. The resulting plasma will locally change the near-vacuum into a conducting medium and can charge floating surfaces and particles, also away from the direct EUV beam. We will discuss the interaction between EUV-induced plasma and electrostatics, by modeling and experiments. We show that the EUV-induced plasma can trigger discharges well below the classical Paschen limit. Furthermore, we demonstrate the charging effect of the EUV plasma on both particles and surfaces. Uncontrolled, this can lead to unacceptably high voltages on the reticle backside and the generation and transport of particles. We demonstrate a special unloading sequence to use the EUV-induced plasma to actively solve the charging and defectivity challenges.

Full Characterization of the Mechanical Properties of 11-50 nm Ultrathin Films: Influence of Network Connectivity on the Poisson's Ratio.

Precise characterization of the mechanical properties of ultrathin films is of paramount importance for both a fundamental understanding of nanoscale materials and for continued scaling and improvement of nanotechnology. In this work, we use coherent extreme ultraviolet beams to characterize the full elastic tensor of isotropic ultrathin films down to 11 nm in thickness. We simultaneously extract the Young's modulus and Poisson's ratio of low-k a-SiC:H films with varying degrees of hardness and average network connectivity in a single measurement. Contrary to past assumptions, we find that the Poisson's ratio of such films is not constant but rather can significantly increase from 0.25 to >0.4 for a network connectivity below a critical value of ∼2.5. Physically, the strong hydrogenation required to decrease the dielectric constant k results in bond breaking, lowering the network connectivity, and Young's modulus of the material but also decreases the compressibility of the film. This new understanding of ultrathin films demonstrates that coherent EUV beams present a new nanometrology capability that can probe a wide range of novel complex materials not accessible using traditional approaches.

High spatial resolution ZnO scintillator for an in situ imaging device in EUV region

A single shot image of a ZnO crystal excited by the EUV laser of Kansai Photon Science Institute was captured. The evaluated EUV beam waist radii from the ZnO emission pattern along the horizontal and vertical axes are 5.0 and 4.7μm, respectively. The expected focal spot size of EUV laser and the spatial resolution of the magnifier (including the Schwarzschild objectives and lenses) are however 1 and 4μm, respectively. The discrepancy on the spatial resolutions is attributed to exciton diffusion. We estimated the ZnO exciton diffusion length from the effective decay time which is shortened by exciton–exciton collision quenching and which is dependence on excitation energy density. Our results indicate that the short lifetime of ZnO is required to improve the spatial resolution.

Spatial Resolution Evaluation of ZnO Scintillator as an In-situ Imaging Device in EUV Region

We captured single shot images of ZnO emission patterns originating from excitation with the EUV laser at Japan Atomic Energy Agency (JAEA). The EUV laser was focused to a spot size of 1 μm on the ZnO crystal by a Fresnel Zone Plate (FZP). The FZP was shifted along the propagation direction of the EUV beam, essentially changing the spot size at the ZnO surface. The emission pattern was detected by a magnifier. The waist radii were determined from the fitting curve as 5.0 μm along the horizontal axis and 4.7 μm along the vertical axis. We also evaluated the spatial resolution of the magnifier, which consists of a Schwarzschild mirror, two lenses and a camera lens to be about 4 μm. These results suggest that the ZnO crystal has sub-micron spatial resolution when used as an in-situ imaging device for the diagnostics of EUV/X-ray sources.

Electron irradiation-enhanced water and hydrocarbon adsorption in EUV lithography devices

The accumulation of water and hydrocarbons molecules on pure Au smooth surfaces were monitored during 100eV electron bombardment at various beam current levels. Our studies showed that these low energy electrons could accelerate the physical adsorption processes of the gaseous contaminant molecules on the mirror surface. The 100eV electron beam was used to provide a rough simulation of the secondary electrons generated during the interaction between the EUV beam at 13.5nm wavelength and the mirror surface in an EUVL device. The adsorption enhancement phenomenon was explained in accordance with Langmuir's Adsorption model by the increase of the sticking coefficient of adsorbed molecules onto the mirror surface. We have also shown that a positive biasing of the top of mirror surface can be used for preventing the secondary electron emission from the mirror surface.

Deprotonation of Poly(4-hydroxystyrene) Intermediates: Pulse Radiolysis Study of Extreme Ultraviolet and Electron Beam Resist

Poly(4-hydroxystyrene) (PHS) has been used in current lithography as a backbone polymer and is also a promising material for EUV and electron beam (EB) lithography. PHS is efficiently deprotonated after the ionization of its radical cation at a low pKa (<0). Thus, a hydroxystyrene unit is incorporated in the chemically amplified resist formula as a proton source. The deprotonation mechanism after ionization can be characterized by using pulse radiolysis techniques. In this study, the dynamics of PHS radical cations generated upon exposure to EB were investigated in various solvents such as ketones, acetates, and nitriles. The observed deprotonation rate is controlled by intramolecular interaction in the solvents with low proton affinities. However, it is suggested that the formation of the hydrogen bonding complex between a hydroxyl group of PHS and a solvent molecule increases the proton transfer rate in dimethyl sulfoxide and pyridine with higher proton affinities (>850 kcal/mol).

Influence of the chemical structure of (co)polymer resists on their sensitivity to radiation

Resists operating via two mechanisms, i.e., disruption of the main chain and hydrolysis of side functional groups occurring upon EUV and electron beam irradiation, are studied as a function of the chemical composition of a polymer substrate. We show that the latter, belonging to the group of chemically amplified resists, are an order of magnitude more sensitive to radiation and possess outstanding contrast, as compared with disruptive type resists.

Carbon Contamination Analysis and Its Effect on Extreme Ultra Violet Mask Imaging Performance Using Coherent Scattering Microscopy/&lt;I&gt;In-Situ&lt;/I&gt; Accelerated Contamination System

The coherent scattering microscopy/in-situ accelerated contamination system (CSM/ICS) is a developmental metrology tool designed to analyze the impact of carbon contamination on the imaging performance. It was installed at 11B EUVL beam-line of the Pohang Accelerator Laboratory (PAL). Monochromatized 13.5 nm wavelength beam with Mo/Si multilayer mirrors and zirconium filters was used. The CSM/ICS is composed of the CSM for measuring imaging properties and the ICS for implementing acceleration of carbon contamination. The CSM has been proposed as an actinic inspection technique that records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. To improve the CSM measurement accuracy, optical and electrical noises of main chamber were minimized. The background noise level measured by CCD camera was approximately 8.5 counts (3 sigma) when the EUV beam was off. Actinic CD measurement repeatability was <1 A (3 sigma) at 17.5 nm line and space pattern. The influence of carbon contamination on the imaging properties can be analyzed by transferring EUV mask to CSM imaging center position after executing carbon contamination without a fine alignment system. We also installed photodiode and ellipsometry for in-situ reflectivity and thickness measurement. This paper describes optical design and system performance observed during the first phase of integration, including CSM imaging performance and carbon contamination analysis results.

Spatially coherent, phase matched, high-order harmonic EUV beams at 50 kHz

By focusing a high repetition rate (50 kHz), compact, femtosecond laser system with low pulse energy (25 muJ) using a tight-focusing geometry, we demonstrate fully phase matched high-order harmonic generation for the first time at very high repetition rates, resulting in EUV light with full spatial coherence. The result is a practical, single-box, coherent source useful for applications in metrology, ultrafast spectroscopy, imaging and microscopy. The soft x-ray flux can be improved further by increasing the laser pulse energy and/or repetition rate.

Experimental Investigations on Particle Contamination of Masks Without Protective Pellicles During Vibration or Shipping of Mask Carriers

Extreme ultraviolet lithography (EUVL) is considered the next generation lithography to produce 32-nm feature-size or smaller. The challenge is that conventional pellicles are unavailable for protecting the EUVL masks against contaminant particles, because the EUV beam is easily absorbed by most solid materials. The masks are usually transported or stored in mask carriers. Without the protective pellicles, particles generated inside the mask carrier may deposit on the critical surface of the mask. It is therefore important to identify where the particles are generated inside the mask carrier during shipping. In this paper, two shipping carrier models of different mask holder designs were used. The mask carriers with quartz mask blanks inside were shaken manually, vibrated with a computer-controlled vibration table, or shipped via air freight. In order to simulate the EUVL mask shipping, no pellicles were used. Several online and offline particle detection techniques were employed to investigate particle generation inside the mask carrier during vibration or shipping. It was shown that particles were mostly generated at the contact points between the mask surface and the carrier element. The design of the mask-holding element in the mask carrier played an important role in reducing particle generation.

A novel EUV exposure station for nanotechnology studies

The soft X-ray region around 13 nm (Extreme Ultra Violet, EUV) has become very important for applications to nanopatterning. Typically, high-resolution patterning of less than 50 nm pitch lines and spaces requires optical systems that are complex and expensive. An appealing alternative is provided by interferometric patterning, whereby an EUV beam of relatively high degree of coherence is used to create a pattern by interfering two (or more) parts of the wavefront. Briefly, the radiation from a short undulator illuminates two gratings with pitch in the range of 40–100 nm. The first orders overlap, and in this region an interference pattern is generated with pitch half the original. This technique produces very high resolution 20–50 nm pitch patterns. Thus, in a collaborative effort involving CNTech and the SRC we have designed, constructed and tested a new beamline dedicated to nanopatterning using the radiation from a new undulator on the Aladdin storage ring. The beamline provides 210 mW/cm 2 average in-band power flux to the endstation. We note that the beamline is designed to provide a relatively large area exposure, with good wavefront quality in intensity and phase. The endstation supports interferometric lithography techniques, with demonstrated patterning capability down to 40 nm pitch lines and spaces. In addition to nanopattering, a branch-line experimental station is designed for studying the photo-desorption from materials during exposure. The outgassing species are captured and transferred to a gas-chromatograph for off-line characterization. This paper describes the undulator source, the beamline, and the exposure stations.

Effect of reverse flow by differential pressure on the protection of critical surfaces against particle contamination

Mask protection from particulate contaminants is one of the most serious concerns for the success of deploying extreme ultraviolet lithography (EUVL) technology for future semiconductor manufacturing. Standard pellicles are not applicable for an EUVL mask surface because of the high absorption of the EUV beam by any material. Therefore, methods such as thermophoresis and electrophoresis are desirable for protection, instead of using organic membrane pellicles. A reverse flow concept by introducing differential pressure between different operating zones (mask zone with higher pressure and optics zone with lower pressure) is introduced for protection of critical surfaces against particulate contamination. In this study, we show systematic investigations of the differential pressure effect on the protection of critical surfaces using 125nm polystyrene latex spheres at a chamber pressure of 50mTorr, whereas the critical surface zone pressure was varied up to 550mTorr. We found that a higher particle speed needs a higher differential pressure between the mask zone and the optics zone to protect the mask from incoming particles. The higher differential pressure and higher relative flow velocity for the reverse flow provide more drag force for particles to stop before reaching the critical surface. The particles having an injection speed of up to 180m∕s could be completely repelled using the differential pressure approach, but the deposition of particles with a speed of 270m∕s is hardly prevented by this reverse flow concept. Different gases showed different amounts of protection efficiency due to the change of gas properties such as density and molecular weight. Argon showed better protection efficiency against particle contamination than air. By drag force comparison, argon has about 18% higher drag force than air. The experimental results showed that argon reduced the particle deposition between 6% and 50%, depending on the particle injection speed and the differential pressures.