Laser-produced extreme ultraviolet (EUV) light source plasma for the next generation lithography application

Transcription

1 Laser-produced extreme ultraviolet (EUV) light source plasma for the next generation lithography application EUV light source plasma Tin icrodroplet Main pulse (CO2 laser pulse) Pre-pulse (Nd:YAG laser pulse), H. Nishimura, K. Nishimura, N. Miyanaga, Y. Izawa, K. Mima Y. Shimada*, A. Sunahara* Institute of Laser Engineering, Osaka University *Institute for Laser Technology Presented at International Congress on Plasma Physics This work is performed under the auspices of Leading Project promoted by MEXT (Japanese Ministry of Education, Culture, Sports, Science and Technology).

2 Smaller circuit pattern can be fabricated by photolithography with shorter wavelength light. Penryn processor, Minimum process size, wavelength of light (nm) Contact illumination 1:1 projection lithography g line i line Moore s law x 0.7/3 yr. Reduction projection weak super resolution strong super resolution KrF ArF F 2 EUV 13.5 nm after Intel website Year

3 All lithography optics consist of Mo/Si multilayer mirrors, which has the highest reflectivity (~ 70%) at 13.5 nm. Illumination optics Pattern mask Laser system Target supply Plasma Projection optics 28-nm hp AR = 2.07 EUV collector EUV light source system Resist + Wafer 28- -nm hp

4 High power, clean, and narrow-band EUV light source is required for the EUV lithography. Joint Requirement for EUV light source Wavelength 13.5nm 2%-bandwidth Target supply EUV Power Etendue Repetition < 3.3 mm 2 sr > 7 khz Laser Source plasma Power stability (+/- 0.3% 3σ, 50 pulses average) Life time > 30,000 hrs Spectral purity nm: < 3-7 % > 400 nm: < 0.3-3% EUV collection mirror IF point The solution to satisfy these criteria is the minimum mass fuel.

5 Summary Minimum mass of EUV fuel is a key scheme to produce a practical EUV light source system. Minimum-mass of Sn fuel Minimum number of Sn atoms required for sufficient EUV generation is only ~ atoms/pulse. Debris and Out-of-band radiation Dominant source of debris and out-of-band light is fuel in outside of the laser spot. Minimum mass fuel is a solution to reduce debris generation and suppress out-of-band radiation. Supply of minimum mass fuel Pure-Sn microdroplet is a practical minimum mass target. 4% of EUV conversion efficiency was attained with microdroplet irradiated by two-color laser pulse.

6 Optimal EUV light source is optically thin plasma, which can be produced with long-wavelength and short-duration laser pulse. Opacity of tin plasma EUV-CE for various laser conditions Transmission Wavelength (nm) Experiment (raw) Experiment (smooth) Simulation (Te = 20.9 ev) Simulation (Te = 31.0 ev) Simulation (Te = 40.3 ev) Cold opacity (Te = 0 ev) Conversion efficiency (%) Laser intensity (W/cm 2 ) et al., Phys. Rev. Lett., Vol. 95, p (2005). T. Ando, et al., Appl. Phys. Lett., Vol. 89, p (2006). CO2 laser (10.6 μm wavelength) is a practical driver. 1.2 ns pulse duration 2.3 ns pulse duration 5.6 ns pulse duration 8.5 ns pulse duration

7 Sn atoms located in 20 nm thickness layer dominantly emits EUV light x10 15 Conversion efficiency (%) nm Tin Dot Glass plate τ L = 2 ns I L = 1 x W/cm λ L = µm Sn layer thickness (nm) et al., Appl. Phys. Lett. Vol. 87, p (2005). S. Namba, et al., Appl. Phys. Lett. Vol. 88, p (2006). M. Shimomura, et al., Appl. Phys. Express, Vol. 1, p (2008). Number of Sn atoms contained in a 20 nm thickness dot is 1.5 x 10 14, which is equal of number contained in 20 µmφ pure-tin droplet. Tin Number of neutral debris (atom)

8 Dominant source of the neutral debris is periphery of the laser spot. After 1 µs After 2 µs After 20 µs Bulk Plasma Neutral Sn target Thin foil 100 nm Plasma Neutral CH target LASER Plasma Thin dot 43 nm Dye LASER 5 mm SiO 2 target M. Shimomura, et al., Appl. Phys. Express, Vol. 1, p (2008).

9 Periphery of the laser spot is the dominant source of the OOB radiation. One-dimensional spatially resolved vacuum ultraviolet (VUV) spectrograph Sn plate (0.1 µm-thickness) Sn sphere (0.1 µm-thickness) 5 mm Wavelength (nm) H. Sakaguchi, et al., Appl. Phys. Lett., Vol. 92, p (2008). S. Namba, et al., J. Appl. Phys., Vol. 104, p (2008).

10 1. Minimum mass fuel contained within laser spot is a solution to reduce debris and suppress out-ofband radiation. 2. Pure-tin microdroplet (20 µm in diameter) is a practical minimum mass target. 3. Double pulse is proposed for resolving considerable mismatch between the microdroplet diameter (20 µm) and the optimal laser spot size (300 µm). 4. Double pulse irradiation enlarges density scale length of plasma and enhances laser absorption. This results in enhancing EUV conversion efficiency.

11 Diameter of the minimum-mass droplet is too small (20 µm) to be irradiated by the optimal laser spot size (295 µm). 20 µmφ Sn drop (1.5 x atoms) Pre-pulse Main pulse 0.3 J / 1 x W/cm µmφ expanded Sn drop 11 mj@13.5 nm 2%BW The droplet must be expanded prior to the main laser irradiation.

12 Young modulus of solid Sn is 50 GPa. Several GPa of pressure is necessary to deform Sn droplet. Wave velocity (km/s) Wave velocity vs. laser intensity P a = 2.5 x 10-7 I L 2/3 Criteria for shock formation Measurement Scaling curve Laser intensity (W/cm 2 )

13 Expansion behavior of prepulse irradiated Sn droplets was observed using a laser-shadowgraph technique. CCD camera Imaging lens φ36 µm Sn drop Beam expander Probe laser Nd:YAG Prepulse laser Nd: YAG Glass stalk φ6 μm carbon fiber φ36 µm Sn drop

14 To expand the microdroplet, the intensity of the Nd:YAG laser is required at least 3 x W/cm 2. Side-on shadowgraph of expanded droplet at 500 ns 5.0 x W/cm x W/cm x W/cm x W/cm µm LASER 300 µm 300 µm 300 µm (a) (b) (c) (d) Carbon fiber only (e) Transmission

15 The expanded Sn droplet was irradiated with a CO 2 laser pulse. The incident angle of the prepulse was 20 deg. EUV microscope Prepulse laser Calorimeter E-mon Nd:YAG λ =1.064 µm Spot size : φ50 µm Intensity : W/cm 2 Pulse duration : 9 ns Spectrometer Main pulse laser CO 2 λ = 10.6 µm Spot diameter : φ250 µm Pulse duration : ns

16 The highest EUV-CE of 4% was obtained for a delay of 1 µs. The EUV emission region located in low-density region. Conversion efficiency (%) Dependance of EUV-CE on delay Double pulse + droplet Single pulse + plane Delay between main- and pre-pulse (µs) Visible shadow LASER 300 m 13.5 nm emission image Transmission et al., Appl. Phys. Lett., Vol. 92, p (2008).

17 Feasibility of continuous supply of 20 µm droplet has been demonstrated presented by Dr. Endo (EUVA) in EUVL Symposium 2007.

18 180 EUV light source can be designed with 50 khz-13.4 kw CO 2 laser + 50 khz-0.63 kw Nd:YAG laser Required EUV power 545 W@source (180 W@IF*) Drop diameter 20 µmφ = 1.6 x Sn atoms Repetition 50 khz Main laser (CO 2 λ L = 10.6 µm) Intensity 1 x W/cm 2 Pulse duration 40 ns Spot size 292 µmφ Pulse energy 268 mj EUV conversion efficiency 4.0% Prepulse laser (Nd:YAG λ L = 1.06 µm) Intensity 4 x W/cm 2 Pulse duration 10 ns Spot size 20 µmφ Pulse energy 13 mj EUV conversion efficiency 1.5 % Total EUV conversion efficiency 3.9 % EUV pulse energy 10.9 mj/pulse Total Laser power 14 kw = 13.4 kw; CO kw; YAG *Assumption: collection angle 5 str. (Collection efficiency 80%), collector reflectivity 70%, debris mitigation transmission 100%, gas transmission 85%, SPF transmission 70%, etendue match 100%, Effective collection capability 33 %

19 EUV light source plasma (545 W) Tin- icrodroplet (20 μmφ) Main pulse (CO2 laser pulse) IL = 1 x W/cm 2 Pre-pulse (Nd:YAG laser pulse) IL = 4 x W/cm 2

20 Simulation predicts higher EUV-CE (5 7%) for shorter duration of laser pulse. YAG + 10 ns-co2 pulse Single 10 ns-co2 pulse K. Nishihara et al., Phys. Plasmas, Vol. 15, p (2008).

21 Conclusion Pure-tin microdroplet irradiated by two-color double pulse (Nd:YAG for prepulse and CO2 for main pulse) was demonstrated to generate clean and efficient EUV light source. 4% of EUV conversion efficiency was attained with using a practical EUV source generation scheme.