Takuma Yokoyama

Atmospheric dc discharges with miniature gas flow as microplasma generation method

An atmospheric microplasma is generated by direct-current (dc) discharge in air with a miniature gas flow through a nozzle, which limits plasma volume. Two discharge modes appear in a nozzle-to-mesh electrode system with helium or argon. One is a repetitive pulsed discharge with a current of 10?30?mA and a short pulse width. The fast pulsed current is powered by electric charges stored in the parasitic capacitance, which depends on the spatial arrangement of the electrodes and the power leads. The pulsed discharge makes it possible to develop a discharge scheme for microplasma generation without a high-voltage pulse generator. The other is a sustained dc discharge, which develops with increasing applied voltage. In the case of helium, a glow discharge configuration is observed with a positive column and a layered structure near the cathode. The length of the positive column is affected by electrode separation and gas flow rate.

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.

High brightness extreme ultraviolet (at 13.5 nm) emission from time-of-flight controlled discharges with coaxial fuel injection

Extreme ultraviolet (EUV) emission from discharge produced plasma with the coaxial injection of fuel vapor (tin and lithium) produced by laser ablation is experimentally studied. Multiple plasma pinches preceding a strong and long recombination radiation of EUV are observed in the first half cycle of a sinusoidal discharge current. Due to the time-of-flight control type of the discharge, the shape of pinch radiation pulses is almost identical. With the coaxial injection of time-of-flight controlled discharges, the highest brightness of EUV emission (maximum extracted energy of 244.3 mJ/2π sr per pulse with the emitter size of ∼1×0.3 mm2 in full width at half maximum) is provided with efficiency exceeding 2% of deposited energy into the plasma (or 1% of dissipated energy in the discharge) due to a much better matching with the optimal plasma parameters in the recombination regime and a decrease in the off-duty factor. Stability of emitting plasma of the repetitive pinches is essentially improved with use o...

Elongation of extreme ultraviolet (at 13.5 nm) emission with time-of-flight controlled discharges and lateral fuel injection

A way toward a quasicontinuous extreme ultraviolet (EUV) radiation source is proposed and explored. Tin and lithium vapor discharges with the lateral laser-ablation injection are experimentally studied as possible efficient sources of quasicontinuous emission of EUV radiation at a wavelength of 13.5 nm. It is shown that the time-of-flight control of optimal plasma parameters by means of varying ablating laser pulse parameters provides a considerable elongation of maximal-power EUV emission with an overall efficiency of 0.1% and with an energy output exceeding 1% of the energy deposited in the discharge plasma. Along with a high average power and a stable position, such an emitter may have its size small enough to be used in the projection lithography.

Dependence of laser parameter on conversion efficiency in high-repetition-rate laser-ablation-discharge EUV source

Two projects are being conducted in EUVA under the support of NEDO and member companies; private project and national project. The private project is responsible for power improvement of a source module targeting realization of 115-W prototype. The national project covers wide area of remaining issues on a collector module to achieve sufficient reliability. In the private project, a laser-ablation-discharge-produced plasma (LADPP) is being researched as a candidate of high-power EUV source. LADPP has fascinating properties such as long lifetime, high collection efficiency, and high thermal input. More than 15 % of collection efficiency could be obtained with LADPP because of its small plasma size. Pulse repetition frequency has reached 20 kHz and 580 W/2πsr were achieved so far. In order to increase conversion efficiency (CE), detailed diagnostics of LADPP were carried out. Especially, dependence of CE on laser pulse duration is derived from the experiment. As a result, dynamics of LADPP was understood and solution to increase CE and improve frequency scalability was understood. A fundamental experiment predicted that CE can be increased 60 %.

High-radiance LDP source for mask-inspection application

High-radiance EUV source is needed for actinic mask inspection applications. LDP source for a lithography application was found to be also able to provide sufficient radiance for mask inspection purpose. Since the plasma size of LDP is properly larger than LPP, not only radiance but also power is suitable for mask inspection applications. Operating condition such as discharge pulse energy, discharge frequency and laser parameter have been tuned to maximize radiance. Introduction of new techniques and several modifications to LDP source have brought radiance level to 180 W/mm2/sr at plasma (or 130 W/mm2/sr as clean-photon radiance). The LDP source is operated at moderate power level in order to ensure sufficient component lifetime and reliability. The first lifetime test done at 10 kHz resulted in 6.5 Gpulse without failure. Debris mitigation system has been successfully installed showing optical transmission as high as 71 %.

Sn film and ignition control for performance enhancement of laser-triggered DPP source

A laser-triggered DPP source is being developed and showing considerable progress toward HVM. Performance, in terms of power and lifetime, of DPP sources has been proven by long-term usage in lithography development fields. Since high-performance debris-mitigation tools are used in DPP sources, collector lifetime is not an issue. However, it is worth developing the technology to enhance overall lifetime of the collector module. In order to suppress both neutral and ionic debris, two technologies, which can be simultaneously used in a DPP source, have been developed. First, a discharge ignition by using two lasers was developed. It was able to reduce the amount and energy of fast ions which could sputter a collector by a factor of 5. In addition to fast ion reduction, CE enhancement of 60 % was obtained. Second, an active control of liquid tin layer, which acts as a fuel material, electrode protection and cooling medium, could reduce particle debris and lower the load of a debris-mitigation tool. Implementing these technologies is considered to provide enhancement of the lifetime of the collector module and support HVM readiness.

Tin-Fueled High-Repetition-Rate Z-pinch EUV Source for Semiconductor Lithography

Summary form only given. Extreme ultraviolet (EUV) is the potential candidate for the light source used in next generation semiconductor lithography. In EUV lithography (EUVL), IC pattern as small as 32-nm pitch or below will be realized by using 13.5-nm radiation. There are two major schemes to obtain high-power EUV; laser-produced plasma (LPP) and discharge-produced plasma (DPP). DPP seems to provide more cost-effective source and easier way to obtain necessary EUV power than LPP. EUV is not a coherent radiation so that emitted radiation is collected by optics and transferred to an exposure tool. In volume production, significant amount of IC chip should be yielded. From these points of view, EUV radiation must be emitted from very small volume but have sufficient average power. In our development, Z-pinch plasma is employed to achieve such a high temperature and density micro plasma. It is very important to increase conversion efficiency (CE) of electrical energy input to 13.5-nm radiation. Xe used to be used as fuel material because of its easiness of handling and cleanliness. However, Sn is the best choice from the view point of CE. Despite its handling difficulties, Sn is now being commonly used in many EUV researches. In case of gas-discharge-produced plasma, it is necessary to feed the gas into the discharge region between the electrodes. For this purpose, we utilize SnH4 gas, which is in gaseous state at room temperature and able to be controlled like Xe. EUV source for semiconductor lithography is also required to work at pulse repetition frequency more than 7 kHz. By using high rep-rate (8 kHz) and high-average-power (120 kW) pulsed power driver, and low-inductance Z-pinch load, radiation characteristics of SnH4-fueled Z-pinch were investigated. Radiation energy, radiation stability, plasma image, temporal radiation behavior of Z-pinch were investigated. As a result, EUV power within 2 % bandwidth at 13.5 nm reached 700 W/2 pisr.

Powder as initial matter for pulsed high current discharges

Powder plasmas powered by a pulsed high current discharge was examined using high-speed cameras and laser imaging technique. Electrons created by a pre- ionization discharge collide with both an anode and powder particles, of which surfaces evaporate after then. Since vaporization of the anode far exceeds that of the particles, discharge characteristics is almost similar to that of vacuum sparks in which expanding anode plasmas are observed. Evaporation of the particle occurred in the hemisphere surface that is close to the cathode side. Influence of the effective anode area on the evaporation was examined by varying the anode shape to suppress the development of the anode plasma.