Gohta Niimi

Development and characterization of a low current capillary discharge for X-ray laser studies

Capillary discharge experiments were carried out for soft X-ray laser studies. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for the end-on X-ray diode observation, and a Pyrex capillary has been used for side-view observation. Spike output has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 to 35 kA with a rise time of 55 ns in an argon gas pressure range from 100 to 800 mtorr. It is found that without a predischarge current, spike output has been hardly observed. Observation of spike output at a low discharge current of 9 kA provides us a possibility for design of a compact soft X-ray laser device. When the predischarge is turned off, the side-view observation of the capillary discharge clearly shows the growth of instabilities during the pinch process. This suggests that the predischarge is indispensable for achieving a uniform plasma, which is required by the soft X-ray lasing in a capillary discharge.

Observation of multi-pulse soft x-ray lasing in a fast capillary discharge

We have designed, fabricated and tested a soft x-ray device, which uses a capillary discharge to achieve neon-like argon lasing. The ceramic capillary has an inner diameter of 3 mm and a length of 150 mm. When operating the device with a current of 16 kA and dI/dt of 517.8 A ns-1 at gas pressure of 26.7 Pa, lasing has been confirmed. Multi-pulse laser output has also been observed with a slightly higher current of 17.5 kA, and this indicates that there exist several configurations suitable for the Ne-like Ar lasing during one pinch process. This is the first observation of multi-pulse laser output.

Estimation of optimum density and temperature for maximum efficiency of tin ions in Z discharge extreme ultraviolet sources

Extreme ultraviolet (EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the narrow spectrum band of 13.5±0.135 nm. A simplified collisional-radiative model and radiative transfer solution for an isotropic medium were utilized to investigate the wavelength-integrated light outputs in tin (Sn) plasma. Detailed calculations using the Hebrew University-Lawrence Livermore atomic code were employed for determination of necessary atomic data of the Sn4+ to Sn13+ charge states. The result of model is compared with experimental spectra from a Sn-based discharge-produced plasma. The analysis reveals that considerably larger efficiency compared to the so-called efficiency of a black-body radiator is formed for the electron density ≃1018 cm−3. For higher electron density, the spectral efficiency of Sn plasma reduces due to the saturation of resonance transitions.

Development of Xe‑and Sn‑fueled high‑power Z‑pinch EUV source aiming at HVM

Discharge-produced plasma (DPP) based EUV source is being developed at Gotenba Branch of EUVA Hiratsuka R&D Center. Among the several kinds of discharge scheme, Z-pinch is employed in our source. An all-solid-state magnetic pulse compression (MPC) generator is used to create a Z-pinch plasma. Low inductance MPC generator is capable of producing a pulsed current with over 50 kA of peak amplitude and about 100 ns of pulse duration at 7 kHz of pulse repetition frequency. In order to obtain sufficient output radiation power, tin-containing gas is being used as well as xenon. Due to the high spectral efficiency of tin, demonstrated EUV output power reached 645 W/2πsr within 2% bandwidth around 13.5 nm. A novel scheme of fuel gas supply led to as good output energy stability as xenon can achieve. Using a nested grazing-incidence collector, EUV power at intermediate focus point which is defined as an interface to the exposure tool reached 42 W with 3.3 mm2sr of etendue.

Performances of Ne-like Ar Soft X-ray Laser using Capillary Z-Pinch Discharge

We have designed, fabricated and tested a soft X-ray device, which uses a capillary discharge to achieve neon-like argon lasing. The ceramic capillary has an inner diameter of 3 mm and a length of 150 mm. Lasing has been confirmed when operating the device with a current of 9 to 32 kA with a rise time of 55 ns, in an argon gas pressure range from 100 to 800 mTorr. The relation between the observation of the laser spike and occurrence of the moving striation has been studied by a series of side-view observations. The appropriate starting time of the main discharge with respect to that of the predischarge current has been studied. When the predischarge is properly applied, constant laser amplification of the gain-length product of gl=12 (g=0.8 cm-1) was obtained, with a current of 32 kA at a pressure of 400 mTorr.

Development of Sn-fueled high-power DPP EUV source for enabling HVM

Discharge-produced plasma (DPP)-based EUV source is being developed at Gotenba Branch of EUVA Hiratsuka R&D Center. A high-repetition-rate high voltage power supply (HVPS) was developed and put into operation on the magnetic pulse compression (MPC)-driven DPP source, enabling 8-kHz operation with 15 J/pulse of maximum charging energy and 0.11 % of stability. SnH4 gas was used as a fuel gas in order to obtain high conversion efficiency. SnH4-fueled Z-pinch source demonstrated EUV power of 700 W/2&pgr;sr within 2 % bandwidth around 13.5 nm. Using a nested grazing-incidence collector, EUV power at the intermediate focus which is defined as an interface to the exposure tool reached 62 W with 3.3 mm2sr of etendue. Tin deposition rate on the collector surface, which is the concern in any tin-fueled EUV sources, was decreased by four orders of magnitude as a result of debris-shield development. Cleaning processes were also developed to enhance total lifetime of the collector. A sequence of intentional deposition and cleaning process for the ruthenium grazing-incidence mirror sample was repeated 13 times. By measuring reflectivity of the mirror, it was confirmed that halogen cleaning process worked very effectively and did not get the mirror damaged after such a long-term cleaning experiment.

Study of low current capillary discharge for compact Soft X‐ray laser

Capillary discharge experiments were carried out to get a lasing of Ne‐like Ar. The relation between the appearance of the laser output and the instabilities of longitudinal modes has been studied by side‐view observation. And observation of lasing at a low discharge current of 9 kA provides us a possibility of designing a compact and high‐repetition‐rate soft X‐ray laser by use of a semiconductor switch instead of a gap switch.

High-power and high-repetition-rate EUV source based on Xe discharge-produced plasma

Discharge-produced plasma (DPP) based EUV source have been studied and developed at EUVA/Gotenba Branch. Among the several kinds of discharge scheme, a capillary Z-pinch has been employed in our source. An all-solid-state magnetic pulse compression (MPC) generator was used to create a Z-pinch plasma. Low inductance MPC generator provides a pulsed current with about 52 kA of peak amplitude and 120 ns of pulse duration, and allows 7-kHz operation. A water-cooled discharge head was coupled with the MPC generator. In order to evaluate the source performance, electrical energy input to the discharge, EUV radiation power, radiation spatial profile, plasma image and spectra were observed. In-band EUV power into usable solid angle obtained at 7 kHz was 93 W/2%BW. By using nested grazing-incidence collector, EUV power at intermediate focus obtained was 19 W/2%BW.

Factors affecting the pulse width of capillary discharge soft X-ray laser

Summary form only given, as follows. There are two ways to realize the population inversion in capillary discharge soft X-ray lasers: electron-collisional recombination pumping and electron-collisional excitation pumping schemes. It is possible to enhance the output of laser energy by increasing the gain-length product and the laser pulse width. While the former scheme can extract laser pulses of several tens of ns, the latter scheme can extract only several ns. Pulse widths are determined by the preservation time of optimal plasma condition and the growth time of plasma instability. The object of our study is to reveal the cause which restricts the duration of lasing in the collisional excitation scheme. The device consists of a Marx generator, a pulse transformer, a speed-up capacitor, a gap switch, a capillary and a pre-discharge circuit. The capillary is made of Pyrex glass and is 60 mm in length and 3 mm in inner diameter and is filled with an argon gas of 100-1000 mTorr. Uniformly pre-ionized argon gas in the capillary is excited with the current pulse having an amplitude of 15-60 kA and a first half cycle duration of 100 ns. The plasma photographs were taken from the side and axial directions using a high-speed camera in order to confirm its pinch and the growth of instability. The temporal evolution of the laser energy was measured with a vacuum X-ray diode. The duration of lasing is discussed based on the results of these experiments and MHD simulations of the imploding plasma.

Plasma dynamics and lasing condition of fast capillary discharges

The dependence of the lasing condition of fast capillary discharge plasmas on the initial gas pressure and rising rate of the load current; dI/dt has been experimentally investigated. The results indicate that there is an optimum condition at suitable values of initial gas pressure and dI/dt. Based on the experimental results and a simplified scaling model for the compression process, the correlation between the plasma dynamics and the lasing condition in capillary pinch plasmas is discussed.