Physical processes in EUV sources for microlithography

The light source is a critical factor when Extreme Ultra Violet Lithography (EUVL) is used. This paper shows the link between the light source and the customer's requirements. These are: Throughput, Cost of Ownership (CoO) and imaging quality (e.g. CD uniformity). Also shown is how customer requirements dictate the necessary performance specifications for light sources. To be competitive with other New Generation Lithography technologies, EUVL tools should show a potential for high throughput. This puts a high requirement on the collectable in-band power produced by an EUV source. CD control requirements, together with restrictions of reflective optics, are discussed. This means that the pulse-to-pulse repeatability, the spatial stability control and the repetition rates, must be substantially better than those of current optical systems. It is essential, in the early stages of the development of potential light sources, to understand the source limitations. It is also essential to see the way the light source or the total source/lithographic system could be improved, in the development time, to meet the lithography tool requirements. Although there are no light sources currently under development that comply with the requirements for an EUVL production tool, future improvement in the performance of light sources seems possible. This paper shows detailed analyses of the way to meet (beta) and production tool requirements, including the increase in repetition rate, for all of the sources. The decrease in energy per pulse will also be discussed. The performance requirements for the light source for the first lithographic tool, together with the timeline for its choice, will be shown.

High performance resist for EUV lithography

Organic gases cause carbon depositions on the multi-layer mirrors by Extreme Ultra Violet (EUV) light irradiations in EUV lithography tool. The dependences on organic gas species, organic gas pressure and EUV light intensity in the carbon deposition were researched in order to understand this reaction. EUV light was irradiated on a (Si/Mo) multilayer mirror sample injecting organic gas like buthane, buthanol, methyl propionate, hexane, perfluoro octane, decane, decanol, methyl nonanoate, diethyl benzene, dimethyl phthalate and hexadecane. X-ray photoelectron spectroscopy measurements revealed that organic gases with heavier molecule weight or higher boiling temperature caused faster carbon deposition rates. Carbon deposition rates increased linearly with organic gas pressures. Dependence on EUV light intensity was estimated from comparisons between an EUV light profile and carbon distributions on irradiated samples. Carbon deposition rates increased rapidly, but became saturated at higher EUV light intensities. Three chemical reactions, an adsorption, a desorption and a carbon deposition by EUV light irradiation, were taken into account to explain the behavior of the carbon deposition. Electron irradiation on a mirror sample revealed that photoelectrons emitting from the mirror surface played an important role in carbon deposition.

Extreme ultraviolet lithography (EUVL) is the leading technology for patterning at the 32 nm technology node and be-yond. EUVL light at 13.5 nm is used to print circuits. This light is produced by heating fuel (Xe, Sn) in EUV sources to a very high temperature by using either magnetic compression or laser irradiation. Today EUV source power remains the number one concern for implementation of EUVL in high volume manufacturing. Over the last few years, much pro-gress has been made in EUV source performance and availability. Today, alpha level high power (~10 W) EUV sources have been integrated in alpha level EUVL scanners. Medium and low power EUV sources are used for in-house metrol-ogy and performance studies on EUV mask blanks, EUV masks, photoresists, and optical elements. These compact dis-charge sources with medium power in the range of 10-100 mW/sr/2% bandwidth and low power EUV tubes are being used by various RD today, these low power EUV sources are instrumental in allowing in-house R&D projects. In this paper, the latest status of high power EUV sources, low and medium power metrology sources, and some of their appli-cations are described.

A model has been developed to predict the cost of extreme ultraviolet lithography (EUVL) masks. The mask blank for EUVL consists of a low thermal expansion material substrate having a square photomask form factor and is coated with reflective Mo/Si multilayers. Absorber layers are deposited on the multilayer and patterned. EUVL mask patterning will use evolutionary improvements in mask patterning and repair equipment. One of the challenges in implementing EUVL is to economically fabricate multilayer-coated mask blanks with no printable defects. The model of mask cost assigns yield and time required for each of the steps in fabricating EUVL masks from purchase of a polished substrate to shipment of a patterned mask. Data from present multilayer coating processes and present mask patterning processes are used to estimate the future cost of EUVL masks. Several of the parameters that significantly influence predicted mask cost are discussed in detail. Future cost reduction of mask blanks is expected from learning on substrate fabrication, improvements in low defect multilayer coating to consistently obtain <0.005 defects cm-2, and demonstration of multilayer smoothing which reduces the printability of substrate defects. The model predicts that the price range for EUVL masks in production will be S30-40K, which is comparable to the price of complex phase shift masks needed to use optical lithography for 70 nm critical dimension patterning.

Extreme Ultraviolet (EUV) Lithography is currently viewed as the most promising approach for reaching the 22 nm node in the manufacture of silicon devices. One of the principal challenges in the ongoing EUVL research effort is the development of necessary at-wavelength metrology tools. EUV Technology worlds leading manufacturer of EUV metrology tools manufactures custom instrumentation for the utilization and analysis of short wavelength electromagnetic radiation - soft x-rays and extreme ultraviolet (EUV). Our company has pioneered the development of several stand-alone inspection, metrology, and calibration tools for EUV lithographic applications that can be operated in a clean room environment on the floor of a fab. An overview of necessary metrology tools for EUV Lithography will be presented, along with the challenges in developing these tools in order to support the successful implementation of EUV Lithography for the 22nm node. In addition, a detailed description of the EUV metrology tools we have delivered, their long term performance and stability of these tools along with our plans for developing a Reflectometer to achieve the HVM requirements will be discussed.

The ULE® substrates used in projection optics of extreme ultraviolet lithography (EUVL) tools are mechanically prefinished with shape accuracy of several nanometer rms (specification: under 0.15 nm rms) with high-spatial frequency roughness (HSFR) (spatial wavelength: under 1 μm) of 0.06 nm rms. The ion beam figuring is used for the final shape error correction of the substrates at low spatial wavelength of greater than 1 mm using high energy (5–10 keV) Ar+ ion beam with 1 mm beam diameter. However, the surface roughness values of the ULE® substrates when machined by 5 and 10 keV Ar+ ion beams result in 0.15 and 0.17 nm rms, respectively; also it is to be noted that these values are greater than the required 0.15 nm rms specification of HFSR. Therefore, the authors developed a method where low energy (0.3 and 0.5 keV) ion beams are used for smoothening the surface of ULE® substrates, previously treated with high energy (5 and 10 keV) ion beams for figuring. By this two-stage operation of ion beam figuring and followed by smoothening, ULE® substrates with HSFR of 0.1 nm rms were obtained. Therefore, the authors conclude that the method presented here is one of the most effective methods for the figure error correction of ULE® substrate and will be useful in EUVL projection optics.

As minimum feature size shrinks to a metal pitch of 21 nm, the current extreme ultra violet (EUV) lithographic tool with a numeric aperture (NA) of 0.33 will face resolution limit for some critical layers. High NA (0.55) EUV with anamorphic optics or EUV double patterning (DP) at 0.33 NA are being considered for the next generation of lithographic technology. Both the high NA EUV system and EUV DP will enhance resolution relative to current EUV single patterning (SP). Nevertheless, in order to be able to compare EUV DP and High NA EUV processes, important lithographic factors including image contrast, mask three dimension (M3D) effects, process variation band, stochastic effects and local critical dimension uniformity need to be investigated to understand their contributions to process variations. This study was carried out using rigorous lithographic model simulations in Sentaurus Lithography, where strong M3D effects in EUVL are computed physically. We have simulated patterns with both isomorphic and anamorphic optical proximity corrections (OPC) using the rigorous model. The study focuses on 3nm node Via layer designs. These vias need to connect to metal features which have pitches of 21 nm. Simulation results using 0.33 NA SP, 0.33 NA DP, and 0.55 NA anamorphic SP are presented. The benefit of using an alternative mask absorber and a thinner resist as well as the impact of stochastic effects have also been explored. Although a 0.55 NA EUV is expected to produce a superior image to 0.33 NA EUV and to have less impact from overlay errors and stochastic effects, an analysis of process margins of 0.33 NA EUV SD and DP versus 0.55 NA anamorphic systems helps to better understand the benefits, challenges and optimal insertion point for introducing High-NA EUV.

The primary requirement for the development of tools for extreme ultraviolet lithography (EUVL) has been the identification and optimization of suitable sources. These sources must be capable of producing hundreds of watts of extreme ultraviolet (EUV) radiation within a wavelength bandwidth of 2% centred on 13.5 nm, based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ∼70% at this wavelength. Since, with the exception of large scale facilities, such as free electron lasers, such radiation is only emitted from plasmas containing moderately to highly charged ions, the source development prompted a large volume of studies of laser produced and discharge plasmas in order to identify which ions were the strongest emitters at this wavelength and the plasma conditions under which their emission was optimized. It quickly emerged that transitions of the type 4p64dn − 4p54dn+1 + 4dn−14f in the spectra of Sn IX to SnXIV were the best candidates and work is still ongoing to establish the plasma conditions under which their emission at 13.5 nm is maximized. In addition, development of other sources at 6.X nm, where X ∼ 0.7, has been identified as the wavelength of choice for so-called Beyond EUVL (BEUVL), based on the availability of La/B based MLMs, with theoretical reflectance approaching 80% at this wavelength. Laser produced plasmas of Gd and Tb have been identified as potential source elements, as n = 4 − n = 4 transitions in their ions emit strongly near this wavelength. However to date, the highest conversion efficiency (CE) obtained, for laser to BEUV energy emitted within the 0.6% wavelength bandwidth of the available mirrors is only 0.8%, compared with values of 5% for the 2% bandwidth relevant for the Mo/Si mirrors at 13.5 nm. This suggests a need to identify other potential sources or the selection of other wavelengths for BEUVL. This review deals with the atomic physics of the highly-charged ions relevant to EUV emission at these wavelengths. It considers the developments that have contributed to the realization of the 5% CE at 13.5 nm which underpins the production of high-volume lithography tools, and those that will be required to realize BEUV lithography.

A system for photo-chemical analysis of EUV lithography processes has been developed. This system has consists of 3 units: (1) an exposure that uses the Z-Pinch (Energetiq Tech.) EUV Light source (DPP) to carry out a flood exposure, (2) a measurement system RDA (Litho Tech Japan) for the development rate of photo-resists, and (3) a simulation unit that utilizes PROLITH (KLA-Tencor) to calculate the resist profiles and process latitude using the measured development rate data. With this system, preliminary evaluation of the performance of EUV lithography can be performed without any lithography tool (Stepper and Scanner system) that is capable of imaging and alignment. Profiles for 32 nm line and space pattern are simulated for the EUV resist (Posi-2 resist by TOK) by using VLES that hat has sensitivity at the 13.5nm wavelength. The simulation successfully predicts the resist behavior. Thus it is confirmed that the system enables efficient evaluation of the performance of EUV lithography processes.

Development of a pellicle membrane to protect the reticle from particles for EUV source powers beyond 250 W is a subject of intensive research and is in great demand to support high volume manufacturing with EUV lithography. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability and chemical resistance within the scanner environment is extremely challenging; yet these properties are required to realize next-generation EUV pellicle solutions. This paper proposes free-standing carbon nanotube (CNT) film as an alternative next generation core pellicle material. Most of the desired pellicle characteristics can be achieved by tuning the properties of freestanding CNT films. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (0.4% half range), mechanical stability (maximum deflection ~0.08 mm at 2 Pa), thermal stability (no change under greater than 500 W equivalent EUV exposure in vacuum without hydrogen radicals) and scalability to a full pellicle size. Other important CNT membrane properties are presented and are favourable for the pellicle application: low EUV scattering, low EUV reflectivity and high transmission under DUV. The ability of the CNT film to stop particles is analysed. The only known failure of the CNT membrane is instability to hydrogen radical/ion environment within the current reticle chamber of the scanner. If changing that environment to limit hydrogen radicals near the pellicle surface is not an option, there is a need to coat the CNT structures for protection. The challenges and considerations for coating the free-standing CNT membranes are discussed.

At present, and for the last several decades, optical projection lithography has been the lithographic technique used in the high-volume manufacture of integrated circuits. The elementary line width of semiconductor device is now 90 nm, the extreme ultraviolet lithography (EUVL) with a wavelength in the range of 10 to 14 nm is vying to become the choice of the semiconductor industry for the so-called next generation of lithography that will enable future generations of integrated circuits to be printed with feature sizes of 45 nm and below around 2010. Plasma EUV source is regarded as the most promising source of EUV radiation, and a heated competition is underway over the world. The top priority issue for implementing EUVL is to upgrade the EUV power extensively. In the development of our Z-pinch plasma EUV source, xenon gas is used for the target. The Z-pinch plasma was driven by pulsed current with amplitude of 30 kA and pulse duration of 100 ns. Pinhole imaging, EUV spectrograph and in-band EUV energy monitor were used to characterize the EUV emission from the Z-pinch discharge plasma. In order to improve the convention efficiency (CE) from input electric energy to EUV radiation, also a solid tin rod was used as target material. The experimental analyses have demonstrated the CE was as high as 1.5%

In Chapter 1, it was explained that the wavelength of light at which EUV lithography is practiced was chosen largely by the availability of multilayers with high reflectance. This can be contrasted with the situation in optical lithography, which has always been operated at wavelengths where there are strong sources of light with narrow bandwidths. The result is that EUV lithography has suffered from light sources that are relatively weak, and a great deal of R&D has therefore gone into the development of EUV light sources in order to improve the situation. Several methods have been used to generate EUV light. Early researchers used synchrotron light sources, but smaller light sources became needed as a practical matter as EUV lithography moved from research to development. This was true for metrology applications as well as for exposure tools, and stand-alone systems provided greater flexibility for development pilot lines. Moreover, it was eventually determined that synchrotrons of a practical size could not provide sufficient EUV light to be cost-effective, even with the use of undulators or wigglers. The EUV source used currently in manufacturing tools, the laser-produced plasma (LPP) source, is discussed in this chapter, and a source that might be used in the future for wafer exposures, the freeelectron laser (FEL), will also be described. Plasma-based sources used in a number of EUV metrology tools will also be described.

In recent years technological developments in the area of extreme ultraviolet lithography (EUVL) have experienced great improvements. So far, intense light sources based on discharge or laser plasmas, light guiding and imaging optics, as well as detection devices are already available. Currently, the application of EUV radiation apart from microlithography, such as metrology, high-resolution microscopy, or surface analysis comes more and more into focus. The aim is to make use of the strong interaction between soft x-ray radiation and matter for surface-near probing, modification or structuring techniques. In this contribution, we demonstrate the surface-near direct structuring of different polymeric materials as well as lithium fluoride crystals using EUV radiation with a wavelength of 13.5 nm. The setup consists of a table-top EUV source based on a laser-induced plasma and a modified Schwarzschild objective with a resolution down to 1 μm. The mirrors of the employed objective were coated with Mo/Si multilayers, providing a transmittance of around 42 % (reflectivity ∼65 % @ 13.5 nm per mirror). With a demagnification factor of 10 small foci are generated, leading to spot diameters of 30 μm in plasma imaging mode and down to 1 μm in mask imaging mode, respectively. The EUV energy density of ∼100mJ/cm 2 obtained in the focus is sufficient to observe direct photo-ablation of polymers, e.g. PMMA. Thus, material interaction studies are currently in progress. The investigations revealed already that in contrast to common excimer laser ablation there are no incubation pulses when using EUV radiation. For lower energies the ablation rate is found to be linear with respect to the applied dose, whereas for higher energies a saturation behavior is observed. The mechanism of the process is briefly discussed. An additional diffraction experiment revealed the potential of the setup to generate periodic interference patterns with feature sizes in the sub-μm-range By EUV irradiation of LiF samples surface-near defects within the crystal lattice are formed. These color-centers (mainly F 2 - and F 3 + -color centers) are known to be stable at room temperature. They are able to emit characteristic radiation in the visible range after optical excitation with a wavelength around 450 nm. In the future structured areas of such color centers could be used as laser-active gain medium in distributed feedback lasers.

Pellicles are an important part of the IC-manufacturing supply chain, keeping particles away from the imaging plane of the photomask to preserve wafer yield. EUV lithography poses new challenges on the pellicle membrane because the radiation must pass twice due to the reflective mask. Additionally, there are no transparent materials for EUV so the EUV pellicle must be extremely thin to keep the transmission high. Present continuous-membrane pellicle solutions will not be sufficient for source powers greater than 250 W that are anticipated for HVM EUV lithography. A possible approach to maintain strength and high transmittance is to use nano-structured materials. We report here on the EUV optical characterization of a variety of alternative membrane materials. The fine structure of etched holes or membranes made of carbon nano-tubes introduces interesting optical effects. We, therefore, not only address specular reflectance or transmittance by the optical characterization but also investigate off-specular diffuse scatter. We compare the respective optical properties of homogeneous reference membranes with etched membranes and carbon nano-tubes. Particularly the latter show a very high EUV transmittance of more than 95 % and are therefore considered being a highly promising candidate for alternative EUV pellicles.