Ablation Dynamics of Tin Micro-Droplet Target for LPP-based EUV light Source

Transcription

1 1 Ablation Dynamics of Tin Micro-Droplet Target for LPP-based EUV light Source D. Nakamura, T. Akiyama, K. Tamaru, A. Takahashi* and T. Okada Graduate School of Information Science and Electrical Engineering, Kyushu University *School of Medicine, Kyushu University

2 2 EUV light source for lithography The extreme ultraviolet (EUV) light source at 13.5 nm has been developed for the next generation optical lithography below the 32nm technology node. Generation method LPP (Laser Produced Plasma) Mo-Si multilayer mirror with a reflectivity of 67 % at 13.5nm EUV light Schematic of LPP Lithographic System 180W at intermediate focus Requirements Mirror Lifetime > Pulse Intermediate focus Power > 180 Repetition rate : 7~10 khz Etendue < 3.3 mm 2 sr Debris Nd:YAG lasers(1.06 µm) or CO 2 lasers (10.6 µm) is employed as a driver laser for LPP. Sn is used as a promising target material for laser irradiation.

3 3 Problem of development of EUVL 1. The conversion efficiency from laser light to EUV light is low. Some improvement is achieved by using CO 2 laser and Sn target. 2. Debris generated by the Sn plasma damages collector optics and limits the lifetime of the optics in the lithographic system. Debris Fast ions Neutral atoms Clusters It is difficult to mitigate the neutral atoms by a shield. Approach for the solution 1. Mass limited target 2. Double pulse irradiation

4 Double pulse irradiation scheme Droplet EUV Light Dense cloud Evaporation Expansion Plasma 1. Pre-pulse irradiation Target expansion 2. Main-pulse irradiation

5 Configuration of double pulse irradiation scheme Sn is used as a promising target material for laser irradiation. Double pulses of a Nd:YAG laser(1.06 µm) and a CO 2 laser (10.6 µm) are employed as driver lasers for LPP. Mo-Si multilayer mirror R= 67 % at 13.5nm Nozzle EUV light 13.5nm 10~30 µm Lithographic System 180W at intermediate focus

6 Purpose Aim of double pulse irradiation scheme Sufficient EUV light generation Mitigation of debris such as neutral atoms and large size particulates The ablation dynamics of a Sn droplet irradiated by double laser pulses from a Nd:YAG laser and a CO 2 laser was investigated. measurement methods LIF imaging system High speed shadowgraph imaging system

7 Experimental setup for debris measurement by LIF imaging and shadowgraph imaging 7 Dye Laser GICCD Camera Beam Expander Sheet Beam Band Pass Filter Data Acquisition System Trigger Pulse Generator LD High-Speed Camera Laser Induced Fluorescence (LIF) Laser intensity : 5x10 11 W/cm 2 Nd:YAG Laser Lens Vacuum Pump Target Holder High Magnification Lens

8 The spatial distribution of Sn atoms from the Sn droplet target 8 Droplet size: 30 µmφ Delay: 100 ns 200 ns 400 ns 700 ns Laser Beam 10 mm 1 µs 1.3 µs 2 µs 3 µs Sn atoms were observed even from a small amount target such as a micro-droplet. (The droplet of this size was not ionized completely. ) The kinetic speed of the Sn atoms was estimated to be 20 km/s.

9 9 Ablation dynamics of the droplet target at a later delay time Droplet size: 30 µmφ Delay: 0 ns 200 ns 400 ns 100 µm 600 ns 800 ns 1000 ns The drifting particles concentrated to some grains at a later delay time of 800 ns to 1 µs. It was found that the grains were molten Sn target with trapping on a glass substrate mounted rear the target.

10 Influence of the pre-pulse intensity on the ablation dynamics Nd:YAG Laser Beam( t = 10 ns, Spot size: 40 µm) W/cm 2 a 200 ns 400 ns 600 ns 800 ns W/cm 2 b W/cm 2 c W/cm 2 d 100 µm The expanding speed increases with increasing the intensity of the pre-pulse. The target was hardly expanded at lower intensity of W/cm 2

11 Beam position dependence of the target expansion direction 11 The target position before irradiation Nd:YAG Laser Beam (Spot size 40 µm) Sn Droplet z (e) 60 µm (d) 30 µm (c) 15 µm (b) 0 µm (a) -15 µm The shadowgraphs of the dense cloud at the 300 ns delay from the pre-pulse irradiation when the target position was changed. a c b d e 100 µm The distribution of the dense cloud strongly depends on the beam position of pre-pulse. The arraignment of the beam and the target become a problem.

12 Shadowgraph of the droplet target irradiated by double pulses 12 Nd:YAG Laser Beam x z Sn Droplet Nd:YAG laser: W/cm 2 CO 2 laser: W/cm 2 Delay of main-pulse: 800 ns Delay: 0 ns 600 ns y CO 2 Laser Beam ICCD Camera 100 µm 800 ns 1 µs 1.2 µs After CO 2 laser irradiation the dense cloud disappeared. DPI scheme is useful the mitigation of the particulates.

13 13 Summary The behavior of debris from the Sn droplet target was investigated by the LIF imaging and the high-speed camera system. Sn atoms are ejected in all directions with a speed of about 20 km/s by LIF. It was found that the molten Sn particles remained without vaporization. After CO 2 laser irradiation, the dense cloud disappeared. DPI scheme was also useful the mitigation of the particulates. Acknowledgement A part of this work was performed under the contract subject "Leading Project for EUV lithography source development."