Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

The process of high harmonic generation(HHG)enables the development of table-top sources of coherent extreme ultraviolet(XUV)light.Although these are now matured sources,they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime.Moreover,many of the emerging applications of such light sources(e.g.,photoelectron spectroscopy and microscopy,coherent diffractive imaging,or frequency metrology in the XUV spectral region)require an increase in the repetition rate.Ideally,these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously.So far,this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers.Here,a novel route with significantly reduced complexity(omitting the requirement of an external actively stabilized resonator)is demonstrated that achieves the previously mentioned demanding parameters.A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 μJ,250 fs pulses leading to,7 μJ,31 fs pulses at 10.7 MHz repetition rate.The compressed pulses are used for HHG in a gas jet.Particular attention is devoted to achieving phase-matched(transiently)generation yielding.10^(13) photons s^(-1)(.50 μW)at 27.7 eV.This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments,thus demonstrating the considerable potential of this source.

A laser-produced plasma source based on thin-film Gd targets for next-generation extreme ultraviolet lithography

We have studied laser-produced plasma based on mass-limited thin-film Gd targets for beyond the current extreme ultraviolet(EUV)light source of 13.5 nm wavelength based on tin.The influences of the laser intensity on the emission spectra centered around 6.7 nm from thin-film Gd targets were first investigated.It is found that the conversion efficiency of the produced plasma is saturated when the laser intensity goes beyond 2×10^(11)W cm^(-2).We have systematically compared the emission spectra of the laser-produced plasma with the changes in the thicknesses of the thin-film Gd targets.It is proved that a minimum-mass target with a thickness of 400 nm is sufficient to provide the maximum conversion efficiency,which also implies that this thickness is the ablation depth for the targets.These findings should be helpful in the exploration of next-generation EUV sources,as the thin-film Gd targets will reduce the debris during the plasma generation process compared with the bulk targets.

Ultraviolet Source Assisted Enhancement of Attosecond Pulse

A promising method to improve the attosecond pulse intensity has been theoretically pre- sented by properly adding an ultraviolet pulse into the orthogonal two-color field. The results show that by properly adding a 125 nm ultraviolet pulse to the orthogonal two-color field, not only the harmonic yield is enhanced by 2 orders of magnitude compared with the original orthogonal two-color field case, but also the single short quantum path, which is selected to contribute to the harmonic spectrum, results in an ultrabroad 152 eV bandwidth. Moreover, by optimizing the laser parameters, we find that the harmonic enhancement is not very sen- sitive to the pulse duration and the polarized angle of the assisted ultraviolet pulse, which is much better for experimental realization. As a result, an isolated pulse with duration of 38 as can be obtained, which is 2 orders of magnitude improvement in comparison with the original two-color orthogonal field case.

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 emis- sion 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.

Electromechanical Fields and Their Influence on the Internal Quantum Efficiency of GaN-Based Light-Emitting Diodes

The effect ofelectromechanical fields, i.e., polarization fields, on the efficiency droop of GaN-based light-emitting diodes is presented using both experimental and numerical analyses. The role of incorporating such polarization charge density in device performance is numerically investigated and further compared with the experimental results of internal quantum efficiency of three different devices in consideration.

Characteristics of ion debris from laser-produced tin plasma and mitigation of energetic ions by ambient gas

Extreme ultraviolet lithography is most promising for the next generation lithography. However, debris from laser-produced plasma, particularly energetic ions, severely decreases the lifetime of extreme ultraviolet optics. We measured the characteris- tics of ions from tin plasma by the time of flight method with a frequency-doubled Nd： YAG laser at the intensity of 3.5x1010 W/cm2 （532 nm, 8 ns）. Our measurement shows that the maximum and peak of tin ions energies from plasma under the above experimental parameters are about 4.2 and 1.8 keV, respectively. Moreover, it is found that kinetic energy angular distribution of tin ions can be fitted by cos0.8（θ）, where θ is the angle with respect to the target normal. We also investigated the mitigation effect of argon, helium gases to the tin ions, and found that tin ions from the plasma can be mitigated effectively at the pressure -38 mTorr for argon or -375 mTorr for helium, respectively.

The development of laser-produced plasma EUV light source

Extreme ultraviolet lithography(EUVL)has been demonstrated to meet the industrial requirements of new-generation semiconductor fabrication.The development of high-power EUV sources is a long-term critical challenge to the implementation of EUVL in high-volume manufacturing(HVM),together with other technologies such as photoresist and mask.Historically,both theoretical studies and experiments have clearly indicated that the CO 2 laser-produced plasma(LPP)system is a promising solution for EUVL source,able to realize high conversion efficiency(CE)and output power.Currently,ASML’s NXE:3400B EUV scanner configuring CO_(2) LPP source sys-tem has been installed and operated at chipmaker customers.Mean-while,other research teams have made different progresses in the development of LPP EUV sources.However,in their technologies,some critical areas need to be further improved to meet the requirements of 5 nm node and below.Critically needed improvements include higher laser power,stable droplet generation system and longer collector life-time.In this paper,we describe the performance characteristics of the laser system,droplet generator and mirror collector for different EUV sources,and also the new development results.