Hironobu Yabuta

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.

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.

Development of debris-mitigation tool for HVM DPP source

Debris-mitigation tools (DMTs) have been used in DPP sources and the performance has been well proven in alpha sources. In beta and HVM sources, requirement to the DMT is increasing to fulfill the power and lifetime requirements simultaneously. In order to bring DPP technology into HVM level, a high-performance DMT has been developed. It has high mitigation performance for both neutral and ionic debris, large collection angle of the collector having high optical transmission, and withstand large thermal input from the discharge source head. Experiments were carried out using mirror samples and proved sufficient performance with which no sputtering and deposition were observed.

High-radiance LDP source: clean, reliable, and stable EUV source for mask inspection

High-throughput and -resolution actinic mask inspection tools are needed as EUVL begins to enter into volume production phase. To realize such inspection tools, a high-radiance EUV source is necessary. Ushio’s laser-assisted discharge-produced plasma (LDP) source is able to meet industry’s requirements in radiance, cleanliness, stability and reliability. Ushio’s LDP source has shown the peak radiance at plasma of 180 W/mm2/sr and the area-averaged radiance in a 200-μm-diameter circle behind the debris mitigation system of 120 W/mm2/sr. A new version of the debris mitigation system is in testing phase. Its optical transmission was confirmed to be 73 %, which is 4 % lower than that of the previous version and therefore will be improved. Cleanliness of the system is evaluated by exposing Ru mirrors placed behind the debris mitigation system. Ru sputter rate was proven to be sufficiently low as 3~5 nm/Gpulse at 7 kHz, whereas frequency-dependent sputter rate was 1~3 nm/Gpulse at 5~9 kHz as previously reported. Sn deposition remained very low (< 0.05 nm) and did not grow over time. A new technique to suppress debris was tested and preliminary results were promising. Time-of-flight signal of fast ions was completely suppressed and Ru sputter rate of exposed mirrors at 3 kHz was approximately 1.3 nm/Gpulse, whereas the conventional mitigation system (new version) resulted in Ru sputter rate of 0.7 nm/Gpulse. This new technique also allows increasing the radiance efficiency by 30 %. Stability tests were done at several different discharge frequencies. Pulse energy stability was approximately 10 %. Dose energy stability dropped from approximately 2 % to 0.1 % when feedback control was activated. EUV emission position stability was studied at 3 kHz. Deviation of the plasma center of gravity was 6 μm, which is 3 % of plasma diameter and therefore considered to be negligible. Reliability tests were performed on both R and D and prototype machines and up to 200 hours of non-interrupted operation was demonstrated.

High-radiance LDP source for mask inspection and beam line applications (Conference Presentation)

High-throughput actinic mask inspection tools are needed as EUVL begins to enter into volume production phase. One of the key technologies to realize such inspection tools is a high-radiance EUV source of which radiance is supposed to be as high as 100 W/mm2/sr. Ushio is developing laser-assisted discharge-produced plasma (LDP) sources. Ushio’s LDP source is able to provide sufficient radiance as well as cleanliness, stability and reliability. Radiance behind the debris mitigation system was confirmed to be 120 W/mm2/sr at 9 kHz and peak radiance at the plasma was increased to over 200 W/mm2/sr in the recent development which supports high-throughput, high-precision mask inspection in the current and future technology nodes. One of the unique features of Ushio’s LDP source is cleanliness. Cleanliness evaluation using both grazing-incidence Ru mirrors and normal-incidence Mo/Si mirrors showed no considerable damage to the mirrors other than smooth sputtering of the surface at the pace of a few nm per Gpulse. In order to prove the system reliability, several long-term tests were performed. Data recorded during the tests was analyzed to assess two-dimensional radiance stability. In addition, several operating parameters were monitored to figure out which contributes to the radiance stability. The latest model that features a large opening angle was recently developed so that the tool can utilize a large number of debris-free photons behind the debris shield. The model was designed both for beam line application and high-throughput mask inspection application. At the time of publication, the first product is supposed to be in use at the customer site.

Sn-fueled High-brightness Compact EUV Light Source

A compact high-brightness EUV source is needed for high-throughput actinic mask inspection in EUV lithography. Ushio is developing a light source utilizing LDP technology where rotating Sn-covered electrodes, trigger lasers and pulsed electrical discharge are employed to create an EUV-emitting plasma at as high repetition rate as 10 kHz. A unique mechanical debris-mitigation system was found to be able to stop neutral particles and reduce fast-ion flux. Clean-photon EUV radiance, of which wavelength is 13.5 nm and band width is 2 %, obtained after the debris-mitigation system was 145 W/mm2/sr. Sputter rate of Ru mirror sample was 2~5 nm/Gpulse whereas Sn deposition was found not to grow with time. Up to 5 days of non-interrupted operation was successfully demonstrated.

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 %.

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.