Extreme Ultraviolet Exposure Tool Research Articles

Sensitization mechanism of metal oxide nanocluster resists with carboxylic acid ligands

Metal oxide nanocluster resists are a promising candidate for enabling the high-volume production of semiconductor devices with high-numerical-aperture extreme ultraviolet exposure tools. In this study, the sensitization mechanism of metal oxide nanocluster resists was investigated by focusing on the radiation-induced reactions of carboxylic acid ligands. Radiolytic products in various solutions of unsaturated carboxylic acids, aromatic carboxylic acids, and saturated carboxylic acids were analyzed by electrospray ionization mass spectrometry, high-performance liquid chromatography, and dynamic light scattering. The conditions of the solutions were selected to control reaction paths of intermediates. The major reaction paths induced by ionizing radiation were clarified. The obtained results suggest that the generation of bridging ligands is essential to the sensitization of metal oxide nanocluster resists. Both low-energy electrons and radical cations can trigger dimerization. The diffusion of small radicals should be suppressed to prevent dimerization at undesired places.

Electron–hole pairs generated in ZrO2 nanoparticle resist upon exposure to extreme ultraviolet radiation

Metal oxide nanoparticle resists have attracted much attention as the next-generation resist used for the high-volume production of semiconductor devices. However, the sensitization mechanism of the metal oxide nanoparticle resists is unknown. Understanding the sensitization mechanism is important for the efficient development of resist materials. In this study, the energy deposition in a zirconium oxide (ZrO2) nanoparticle resist was investigated. The numbers of electron–hole pairs generated in a ZrO2 core and an methacrylic acid (MAA) ligand shell upon exposure to 1 mJ cm−2 (exposure dose) extreme ultraviolet (EUV) radiations were theoretically estimated to be 0.16 at most and 0.04–0.17 cm2 mJ−1, respectively. By comparing the calculated distribution of electron–hole pairs with the line-and-space patterns of the ZrO2 nanoparticle resist fabricated by an EUV exposure tool, the number of electron–hole pairs required for the solubility change of the resist films was estimated to be 1.3–2.2 per NP. NP denotes a nanoparticle consisting of a metal oxide core with a ligand shell. In the material design of metal oxide nanoparticle resists, it is important to efficiently use the electron–hole pairs generated in the metal oxide core for the chemical change of ligand molecules.

Analysis of line-and-space resist patterns with sub-20 nm half-pitch fabricated using high-numerical-aperture exposure tool of extreme ultraviolet lithography

The resolution of resist processes for extreme ultraviolet (EUV) lithography has been steadily improved and has reached the sub-20 nm half-pitch region. Currently, the resist materials capable of resolving 11 nm half-pitch line-and-space patterns are being developed in industrial fields. In this study, the line-and-space resist patterns with sub-20 nm half-pitches were fabricated using a high-numerical-aperture (NA) EUV exposure tool and analyzed by the Monte Carlo simulation. The scanning electron microscopy (SEM) images of resist patterns after their development were compared with the latent images calculated on the basis of the sensitization and reaction mechanisms of chemically amplified EUV resists. The approximate relationship between resist patterns and latent images was clarified for the sub-20 nm half-pitch region. For the realization of 11 nm half-pitch fabrication, the suppression of the stochastic effects in the development process is an important consideration.

Estimation of resist sensitivity for extreme ultraviolet lithography using an electron beam

It is a challenge to obtain sufficient extreme ultraviolet (EUV) exposure time for fundamental research on developing a new class of high sensitivity resists for extreme ultraviolet lithography (EUVL) because there are few EUV exposure tools that are very expensive. In this paper, we introduce an easy method for predicting EUV resist sensitivity by using conventional electron beam (EB) sources. If the chemical reactions induced by two ionizing sources (EB and EUV) are the same, the required absorbed energies corresponding to each required exposure dose (sensitivity) for the EB and EUV would be almost equivalent. Based on this theory, we calculated the resist sensitivities for the EUV/soft X-ray region. The estimated sensitivities were found to be comparable to the experimentally obtained sensitivities. It was concluded that EB is a very useful exposure tool that accelerates the development of new resists and sensitivity enhancement processes for 13.5 nm EUVL and 6.x nm beyond-EUVL (BEUVL).

Evaluation of lithographic performance of extreme ultra violet mask using coherent scattering microscope

The authors report the application of a coherent scattering microscope (CSM) equipped with a femtosecond laser based extreme ultra violet (EUV) source to analyze aerial images for an EUV lithography mask. The CSM employs an EUV beam with a 13.5 nm wavelength and an incident angle of 6° for the wafer scanner emulation. The CSM consists of a condenser optics that delivers the EUV beam to the mask and detector that measures the field spectrum diffracted by the mask pattern. Using a pair of spherical and plane EUV mirrors, coherent EUV light is relayed to the mask and the light diffracted by the mask pattern is measured by a 1 in. size charge-coupled device detector. The integration time of the EUV beam in the detector is controlled by a high speed shutter. This device experimentally records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. These features made it possible to investigate the projected image properties, such as depth-of-focus, exposure latitude, mask error enhancement factor, horizontal–vertical critical dimension (H–V CD) bias, normalized image log slope (NILS), and so on. The device also provides a mask CD from a rigorous coupled-wave analysis algorithm modeling based on the Maxwell equation through measured diffraction patterns. NILS according to half-pitch design node was measured by CSM and simulated using a Solid-EUV simulator. Illumination conditions of NA and sigma (σ) values are 0.25 and 0.8, respectively, which are the same as those of the EUV exposure tool. The NILS measured by CSM and that simulated using the Solid-EUV have good consistency.

Throughput Increases Using EUV Assist Layers

Extreme ultraviolet (EUV) exposure is among the front-runners for single-exposure lithography for the 16-nm node and below. One major issue moving toward production is the source power for EUV, which dictates the throughput of the EUV exposure tools. Source suppliers are working to increase the source power as a possible solution. Another approach to limited source power is to use the available photons more efficiently. Previous work has shown that assist layers are critical for performing EUV lithography. Assist layers enhance the adhesion of EUV photoresists, block substrate contamination, and improve the resolution, line width roughness, and sensitivity (RLS) tradeoff. Recent research has also shown that assist layers can enhance the dose to size of a photoresist. This amplification allows the same features to be produced with a smaller exposure dose, which increases throughput. In this paper, we report the study of new assist layers with novel resins that show superior lithography performance. The results will show how to significantly improve lithography throughput by reducing the dose to size, while monitoring the trade-off often seen with line roughness and exploring the exposure latitude and depth of focus of the process.

Carbon contamination of EUV mask and its effect on CD performance

The carbon contamination on extreme ultraviolet (EUV) masks is a critical issue causing throughput degradation and unexpected effects on imaging performance. In this work, a series of carbon contamination experiments were performed on a patterned EUV mask. The impact of carbon contamination on imaging performance was analyzed using actinic EUV coherent scattering microscopy (CSM) combined with an in-situ accelerated contamination system (ICS), which was installed on 11B EUVL beam-line at Pohang Light Source (PLS). In addition, the topography of the carbon contamination on the patterned mask was inspected with a scanning electron microscope (SEM). The mask critical dimension (CD) and reflectivity were compared before and after carbon contamination through accelerated exposure. The reflectivity degradation was measured as 5.5% after 3 h exposure which caused ∼20 nm carbon deposition. A mask CD change of 88 nm line and the space pattern showed a similar trend but different absolute values as measured by CSM and CD-SEM. This difference confirms the importance of actinic inspection technique which emulates the practical imaging condition (6° incident angle) as an EUV exposure tool.

Development of Under Layer Material for EUV Lithography

For the next generation lithography (NGL), several technologies have been proposed to achieve the 22nm-node devices and beyond. Extreme ultraviolet (EUV) lithography is one of the candidates for the next generation lithography. For lithography processes, the Line width roughness (LWR) and the pattern collapse of resist are the most critical issues for NGL, because of the small target critical dimension (CD) size and high aspect ratio. In this study, we design the new concept of EUV Under layer (UL) material to meet these requirements and study the impact of polymer design for pattern collapse behavior, pattern profile and LWR control by using EUV exposure tool.

Dissolution Kinetics in Chemically Amplified EUV Resist

Polymer structure effects on the dissolution kinetics and deprotection reaction were investigated to understand inherent extreme ultraviolet (EUV) resist characteristics. The effect of protecting groups and protecting ratio on activation energy were observed. Also, the protecting ratio effect on dissolution kinetics was observed. The speed of TMAH penetration into each resist film was different for each resist as the concept of the ″penetration zone″. This indicated that the dominant rate-determining step in resists is TMAH penetration into the resist film. Thus, the dissolution rates were affected by changing the protection ratio of polymer. Moreover, the preliminary evaluation of resist profile for EUV chemically amplified resists was performed by using PROLITH and parameters obtained with EUV exposure tool. Furthermore, the resist patterns were produced by electron beam and the SEM observation results were compared with the PROLITH simulation results. It is important for EUV lithography to select appropriate protecting group and protecting ratio.

Effect of carbon contamination on the printing performance of extreme ultraviolet masks

Carbon contamination is a significant issue with extreme ultraviolet (EUV) masks because it lowers throughput and has potential effects on imaging performance. Current carbon contamination research is primarily focused on the lifetime of the multilayer surfaces, determined by reflectivity loss and reduced throughput in EUV exposure tools. However, contamination on patterned EUV masks can cause additional effects on absorbing features and can affect the printed images. In this work, various carbon contamination experiments were performed to study the impact between contamination topography and observed imaging performance. Lithographic simulation using calculated aerial images and experimentally determined resist parameters was performed and compared to the printing results to estimate the allowed carbon thickness with critical dimension compensation applied to the mask.

EUV resist outgassing release characterization and analysis

The reduction of extreme ultraviolet (EUV) resist outgassing to minimize or prevent possible contamination of EUV exposure tools is viewed as an important factor in the development of EUV resists. In this paper, a detailed evaluation/comparison of the pressure rise method (evaluation based on the measured pressure variations due to the released resist outgassing) and the gas chromatography–mass spectroscopy (GC–MS) quantification methods is discussed. The application of compensation-values based on the factors affecting each of these methods was also investigated. The ‘relative sensitivity of ion gauge’ for the pressure rise method and an approximated amount of CO 2 (which is not collectable by desorption tubes) for GC–MS was used for the compensation of resist outgassing quantification. With the application of these compensation-values, an improved and small difference between evaluation methods of 4.7–16 times is obtained in contrast with a 34–107 times difference without compensation. Moreover, the resist outgassing difference caused by a variation in the resist chemical composition (protecting ratio, photo-acid generator (PAG) and quencher content) shows the same trend regardless of evaluation method utilized, even with compensation.

Novel negative-tone molecular resist based on polyphenol derivative for extreme ultraviolet lithography

This study investigated the extreme ultraviolet (EUV) lithographic performance of negative-tone molecular resists based on 2,7-bis[bis(2,3,5-trimethyl-4-hydroxyphenyl)methyl]naphthalene (MGR002) and their negative-tone imaging mechanism. EUV imaging experiments were performed using the high-numerical-aperture (NA=0.3), small-field EUV exposure tool (HINA). Patterning results showed the resolution of one resist to be 29nm at an EUV exposure dose of 18mJ∕cm2 and the obtainable aspect ratio to be as high as 2. Analyses by matrix-assisted laser desorption/ionization mass spectroscopy, Fourier transform infrared spectroscopy, and UV-visible absorption spectroscopy of EUV-exposed resists revealed that one reason for the good performance is that the negative-tone imaging mechanism involves both cross-linking and a change in polarity.

Absolute sensitivity calibration of extreme ultraviolet photoresists

One of the major challenges facing the commercialization of extreme ultraviolet (EUV) lithography remains simultaneously achieving resist sensitivity, line-edge roughness, and resolution requirement. Sensitivity is of particular concern owing to its direct impact on source power requirements. Most current EUV exposure tools have been calibrated against a resist standard with the actual calibration of the standard resist dating back to EUV exposures at Sandia National Laboratories in the mid 1990s. Here we report on an independent sensitivity calibration of two baseline resists from the SEMATECH Berkeley MET tool performed at the Advanced Light Source Calibrations and Standards beamline. The results show the baseline resists to be approximately 1.9 times faster than previously thought based on calibration against the long standing resist standard.

Are extreme ultraviolet resists ready for the 32nm node?

The International Technology Roadmap for Semiconductors (ITRS) insertion point of extreme ultraviolet (EUV) lithography is the 32nm half-pitch node, and significant worldwide effort is being focused toward this goal. Potential road blocks have been identified and are being addressed. Readiness of EUV photoresists is one of the risk areas. According to the ITRS (www.itrs.net), a production-worthy EUV resist at 32nm half-pitch has to have a photospeed of ∼5mJ∕cm2 and line edge roughness (3σ) of 1.4nm. Toward this goal, the joint INVENT activity (AMD, CNSE, IBM, Micron, and Qimonda) at Albany has evaluated a broad range of EUV photoresists on various EUV exposure tools worldwide, including EUV MET at Lawrence Berkeley National Laboratory, EUV MET at SEMATECH Albany, and EUV interferometer at the Paul Scherrer Institute, Switzerland. This article will give a survey of the results, assessing the strengths and weaknesses of current materials.

LER evaluation of molecular resist for EUV lithography

We have designed and synthesized a molecular resist material, which has no distribution of the protecting groups and have evaluated its performance as a molecular resist with EB and EUV exposure tool. The molecular resist attained a resolution of sub-45 nm patterning at an exposure dose of 12 mJ/cm 2. It was found that controlling the distribution of the protecting groups in a molecular resist material has a great impact on improving line edge roughness (LER). Low LER values of 3.1 nm (inspection length: L = 620 nm) and 3.6 nm ( L = 2000 nm) were achieved with this molecular resist using Extreme UltraViolet (EUV) lithography tool.

Extreme ultraviolet lithography: overview and development status

Extreme ultraviolet (EUV) lithography has emerged as the most likely successor to 193-nm lithography. We provide a technical overview of EUV lithography and a discussion of the advantages of EUV lithography over alternative technologies. The key challenges in developing EUV exposure tools for high-volume production are discussed. A brief assessment is given of the cost of ownership of EUV lithography in comparison with 193-nm immersion lithography.

Development of projection optics set-3 for high-numerical-aperture extreme ultraviolet exposure tool (HiNA)

We have developed a high-numerical-aperture extreme ultraviolet exposure tool (HiNA). HiNA is equipped with an illumination system, projection optics, a mask stage, and a wafer stage in the vacuum chamber. The projection optics consist of two aspherical mirrors (M1 and M2). The numerical aperture of the optics is 0.3. Thus far, we fabricated two sets of projection optics (set-1 and set-2). The wave-front errors of set-1 and set-2 were 7.5 and 1.9nm rms, respectively. We developed a third set of projection optics (set-3), the target wave-front error of which was less than 1nm rms. In set-3, we also attempted to reduce flare. We completed the mirror polishing, coating, and mirror adjustment of set-3. By using a recently developed polishing method, we reduced low-spatial-frequency roughness (LSFR), mid-spatial-frequency roughness (MSFR), and high-spatial-frequency roughness, simultaneously. The predicted wave-front error calculated from the LSFR number was 0.69nm rms. MSFR, which strongly affects the flare of the optics, was significantly reduced to less than 0.2nm rms. The estimated flare was 7%, which is significantly reduced to one-forth that of set-2. The wave-front error of set-3 was measured with the visible-light point diffraction interferometer after coating and assembly. The wave-front error measured after adjustment was 0.85nm rms, which is less than one-half the wave-front error of set-2. Set-3 will be completed after the cramping of the adjustment system, which is performed to prevent mirror displacement during transportation.

Experimental EUV Exposure System using a Synchrotron Source.

Extreme ultraviolet lithography (EUVL) is an candidate for lithography technology to be used for the fabrication of the future-generation semiconductor devices with the design rule of 100nm and below. Till date few extreme ultraviolet (EUV) exposure experiments, which demonstrate a high resolution of less than 100nm using small field optics, have been performed. However, such a high resolution in the case of practical large-field exposure has not yet been reported. We designed a 3-mirror ring-field projection optics, which enabled large-field exposure. An EUV exposure experiment will be performed using an EUVL beam line to be onstructed at the new synchrotron ring named New Subaru, which is now under construction by Hyogo Prefecture. In this paper the details of the EUV exposure tool will be described.

At-wavelength interferometry for extreme ultraviolet lithography

A phase-shifting point diffraction interferometer is being developed for at-wavelength testing of extreme ultraviolet lithographic optical systems. The interferometer was implemented to characterize the aberrations of a 10× Schwarzschild multilayer-coated reflective optical system at the operational wavelength of 13.4 nm. Chromatic vignetting effects are observed and they demonstrate the influence of multilayer coatings on the wave front. A subaperture of the optic with a numerical aperture of 0.07 was measured as having a wave front error of 0.090 wave (1.21 nm) root mean square (rms) at a 13.4 nm wavelength. The wave front measurements indicate measurement repeatability of ±0.008 wave (±0.11 nm) rms. Image calculations that include the effects of the measured aberrations are consistent with imaging performed with the 10× Schwarzschild optic on an extreme ultraviolet exposure tool.