Laser-produced Plasma EUV Source Research Articles

Time-resolved spatial profiles of electron density and temperature in hydrogen plasmas induced by radiation from laser-produced tin plasmas for extreme ultraviolet lithography light sources

We observed the spatial and temporal changes of the electron density (n e) and the electron temperature (T e) of hydrogen plasmas around a laser-produced Sn plasma EUV source. The plasma parameters were measured by the laser Thomson scattering (LTS) method. In the experiment, the Sn plasmas are produced in H2 gas at a pressure of 50–200 Pa and the hydrogen plasmas were induced by radiation from the Sn plasmas. The LTS measurements were performed at distances 30–90 mm away from the Sn plasmas. In all cases, the strong bremsstrahlung radiation of the Sn plasmas easily overwhelmed the weak LTS signals. To suppress noise due to the radiation, the solid angle of radiation from the Sn plasmas was restricted. The experimental results show that the n e was in the order of 1017 m−3 and T e was around 0.7 eV.

Maskless EUV lithography, an alternative to e-beam

Background: The resolution capability of EUV lithography has reached parity with e-beam, raising the possibility that maskless EUV could supplant e-beam for mask writing and low-volume wafer patterning. Aim: We outline a maskless EUV scanner design with a 13.5-nm operating wavelength and numerical aperture of 0.55. Approach: A microlens array partitions radiation from a commercial laser-produced plasma EUV source into ∼2 million individual beams, which are focused to separate, diffraction-limited focal points on a writing surface, and the surface is raster-scanned across the focal point array as the beams are individually modulated by MEMS microshutters integrated within the microlens array to construct a digitally synthesized raster exposure image. Results: Compared to state-of-the-art mask-projection EUV lithography, the system would have ∼1000 × lower throughput, but its power requirement would also be ∼1000 × lower, the exposure dose would be ∼10 × higher, scan velocity and acceleration would be ∼1000 × lower, and it would have the advantage of maskless operation. In comparison to e-beam mask writers, a maskless EUV scanner could provide higher resolution with at least double the throughput and over 10 × higher dose. Conclusions: Maskless EUV lithography could provide significant cost and performance benefits for both direct-write applications and photomask production for mask-projection lithography.

Free-electron laser emission architecture impact on extreme ultraviolet lithography

Laser-produced plasma (LPP) EUV sources have demonstrated ∼125 W at customer sites, establishing confidence in EUV lithography (EUVL) as a viable manufacturing technology. However, for extension to the 3-nm technology node and beyond, existing scanner/source technology must enable higher-NA imaging systems (requiring increased resist dose and providing half-field exposures) and/or EUV multipatterning (requiring increased wafer throughput proportional to the number of exposure passes). Both development paths will require a substantial increase in EUV source power to maintain the economic viability of the technology, creating an opportunity for free-electron laser (FEL) EUV sources. FEL-based EUV sources offer an economic, high-power/single-source alternative to LPP EUV sources. Should FELs become the preferred next-generation EUV source, the choice of FEL emission architecture will greatly affect its operational stability and overall capability. A near-term industrialized FEL is expected to utilize one of the following three existing emission architectures: (1) self-amplified spontaneous emission, (2) regenerative amplifier, or (3) self-seeding. Model accelerator parameters are put forward to evaluate the impact of emission architecture on FEL output. Then, variations in the parameter space are applied to assess the potential impact to lithography operations, thereby establishing component sensitivity. The operating range of various accelerator components is discussed based on current accelerator performance demonstrated at various scientific user facilities. Finally, comparison of the performance between the model accelerator parameters and the variation in parameter space provides a means to evaluate the potential emission architectures. A scorecard is presented to facilitate this evaluation and provides a framework for future FEL design and enablement for EUVL applications.

Spectral lines and characteristic of temporal variations in photoionized plasmas induced with laser-produced plasma extreme ultraviolet source

Abstract Spectral lines for Kr/Ne/H2 photoionized plasma in the ultraviolet and visible (UV/Vis) wavelength ranges have been created using a laser-produced plasma (LPP) EUV source. The source is based on a double-stream gas puff target irradiated with a commercial Nd:YAG laser. The laser pulses were focused onto a gas stream, injected into a vacuum chamber synchronously with the EUV pulses. Spectral lines from photoionization in neutral Kr/Ne/H2 and up to few charged states were observed. The intense emission lines were associated with the Kr transition lines. Experimental and theoretical investigations on intensity variations for some ionic lines are presented. A decrease in the intensity with the delay time between the laser pulse and the spectrum acquisition was revealed. Electron temperature and electron density in the photoionized plasma have been estimated from the characteristic emission lines. Temperature was obtained using Boltzmann plot method, assuming that the population density of atoms and ions are considered in a local thermodynamic equilibrium (LTE). Electron density was calculated from the Stark broadening profile. The temporal evaluation of the plasma and the way of optimizing the radiation intensity of LPP EUV sources is discussed.

Mitigation of energetic ion debris from Gd plasma using dual laser pulses and the combined effect with ambient gas

For the next-generation beyond extreme ultraviolet lithography (EUVL) sources, gadolinium (Gd) plasma with emission wavelength at 6.7 nm seems to be the leading candidate. Similar to the Sn target 13.5 nm light source, ion debris mitigation is one of the most important tasks in the laser-produced Gd plasma EUV source development. In this paper, a dual-laser-pulse scheme, which uses a low energy pulse to produce a pre-plasma and a main pulse after a time delay to shoot the pre-plasma, is employed to mitigate the energetic ion generation from the source. Optimal conditions (such as pre-pulse energy and wavelength, and the time delay between the pre-pulse and the main pulse for mitigating the ion energy) are experimentally obtained, and with the optimal conditions, the peak of the ion energy is found to be reduced to 1/18 of that of a single laser pulse case. Moreover, the combined effect by applying ambient gas to the dual-pulse scheme for ion debris mitigation is demonstrated, and the result shows that the yield of Gd ions is further reduced to around 1/9 of the value for the case with dual laser pulses.

Extendibility Evaluation of Industrial EUV Source Technologies for kW Average Power and 6.x nm Wavelength Operation

Interests in the extendibility are growing after the introduction of the LPP (Laser Produced Plasma) EUV source technology in the semiconductor industry, towards higher average power and shorter wavelength, based on the basic architecture of the established LPP EUV source technology. It is discussed in this article that the power scaling of the 13.5nm wavelength source is essentially possible by a slight increase of the driving laser power, CE (Conversion Efficiency) and EUV collection efficiency by some introduction of novel component technologies. Extension of the EUV wavelength towards BEUV (Beyond EUV), namely 6.x nm is discussed based on the general rule of the UTA (Unresolved Transition Arrays) of high Z ions, and development of multilayer mirrors in this particular wavelength region. Technical difficulties are evaluated for the extension of the LPP source technology by considering the narrower mirror bandwidth and higher melting temperature of the candidate plasma materials. Alternative approach based on the superconducting FEL is evaluated in comparison with the LPP source technology for the future solution.

Cymer LPP EUV Source System Development Status

In this paper, the technologies developed by Cymer to deliver clean EUV power will be reviewed. A high average power (>20kW) pulsed CO2 laser is used to create a Sn plasma by irradiating 30μm Sn droplets at a repetition rate of approximately 50kHz. The EUV light emitted by the Sn plasma is collected using a 5sr multi-layer mirror (MLM) collector and refocused at the intermediate focus (IF). The integrity of the MLM collector is maintained over time using debris mitigation techniques based on H2. A number of diagnostics are used to characterize the EUV light past IF. We will describe the performance of multiple laser-produced plasma (LPP) EUV sources, integrated with a 5sr MLM collector and debris mitigation.

HVM LPP EUV Source System Development Status

Laser produced plasma (LPP) EUV systems have been considered as a viable and scalable approach for the EUV scanners at sub-32nm and beyond nodes on the ITRS roadmap. High EUV power and reduction of chamber contaminations inside EUV vacuum chambers or to EUV scanner optics are two main concerns for the LPP EUV source system development. In this paper, a review of Cymer LPP EUV development progress will be provided along with productization status for LPP EUV sources with performance goals targeted to meet specific requirements from leading scanner manufacturers. The latest results on high EUV power generation, debris mitigation and clean transmission of EUV light through the intermediate focus will be described. Finally, a development roadmap to achieve high LPP EUV source power will be illustrated.

Characterization and optimization of the laser-produced plasma EUV source at 13.5 nm based on a double-stream Xe/He gas puff target

The paper describes a debris-free, efficient laser-produced plasma source emitting EUV radiation. The source is based on a double-stream Xe/He gas-puff. Its properties and spectroscopic signatures are characterized and discussed. The spatio-spectral features of the EUV emission are investigated. We show a large body of results related to the intensity and brightness of the EUV emission, its spatial, temporal, and angular behavior and the effect of the repetition rate as well. A conversion efficiency of laser energy into EUV in-band energy at 13.5 nm of 0.42% has been gained. The electron temperature and electron density of the source were estimated by means of a novel method using the FLY code. The experimental data and the Hullac code calculations are compared and discussed. The source is well suited for EUV metrology purposes. The potential of the source for application in EUV lithography was earlier demonstrated in the optical characterization of Mo/Si multi-layer mirrors and photo-etching of polymers.

Laser-produced plasma EUV source based on tin-rich, thin-layer targets

In this paper a new approach to a laser-produced plasma EUV source based on a tin target is presented. A thin layer of pure tin and composite layers consisting of Sn with Si, SiO and LiF are investigated. The target composed of several thin layers produces less debris than the other targets and provides a conversion efficiency (CE) in the 13.5-nm±1% band at least comparable to the CE for the pure tin slab target. The largest CE was observed for the target composed of a mixture of Sn and LiF, due to the fact that lithium, similarly to tin, is a strong emitter at 13.5 nm.

Thin film filters for an EUV multilayer mirror optics with a laser produced plasma light source

For rejection of unwanted background component in the reflection spectrum of EUV multilayer mirrors, thin film transmission filters of Mo, Zr, and Be were evaluated with a laser produced plasma EUV source and an imaging plate detector. It is found that in addition to a standard Zr filter, a Mo filter has good rejection characteristics. In particular, for multilayer mirror use at a large angle of incidence such as the polarizing angles around 45°, Mo shows better rejection characteristics than Zr. It is also confirmed that Be transmits non-ignorable amount of UV background.

リソグラフィ用 EUV(極端紫外)光源研究の現状と将来展望 4.レーザー生成プラズマ光源 4.1 レーザー生成プラズマ光源研究の現状

The present status of the development of laser-produced plasma EUV sources throughout the world is summarized. The year 2002 will be remembered as a turning point when target material shifts from xenon to tin for high conversion efficiency. Research on the EUVL source taking place at the AIST is also briefly described.

Debris from tape-target irradiated with pulsed YAG laser

For high brightness and debris-free laser-produced plasma EUV source, EUV spectra and debris were measured. The experimental debris scattering angle distribution and the amount of piled debris were estimated in the thickness of debris on silicon wafers from tape-target irradiated with pulsed YAG laser (SHG, E=0.8 J, t=7 ns) in more than 200,000 laser shots. The shape and size of debris were observed using scanning electron microscope. EUV spectra were obtained by using the grazing-incidence XUV spectrometer with the Au-coated toroidal mirror, diffraction grating and X-ray CCD camera. In 200,000 laser shots, for copper tape-target, 50 nm thick debris at 50° from target normal and 30 nm thick debris at more than 70° from target normal were obtained. More than 20 μm in diameter debris of aluminum and less than 1 μm in diameter debris of copper were observed. Laser-produced plasma EUV source system with low debris tape-target of thin metal and polymer films, debris-shield and debris-free EUV filter has been developed. We demonstrated successive 8 h generation of EUV with the developed laser-produced plasma EUV source system for EUV lithography.