Mitigation of fast particles from laser- produced Sn plasma for an extreme ultraviolet lithography source Y.Tao and M.S.Tillack University of California, San Diego EUV Source Workshop May 25, Vancouver, Canada

Mitigation of debris is one of the most critical issue in the development of EUV source  Besides mitigation of the total yield of debris, reduction of their kinetic energy is also important,  Lower energy particles mainly results in contamination instead of permanent damage to optics, can be cleaned.  Lower energy particles are easier to be stopped by gas. Sputtering yield as a function of ion energy  Laser-produced Sn-based plasma is one of the most hopeful candidates of EUVL source.  Debris with high velocity from LPPs could damage the optics in EUVL system, many methods have been tried to manage high energy particles.  However to date, it is still open to achieve high CE and low debris at same time at high repetition rate. The challenge is being transferred to laser! J.P.Alain et al. SPIE, 5751, 1110, (2005) 1

We proposed a novel technique to reduce the kinetic energy of particles from LP Sn plasma by introducing a low energy pre-pulse  Its basic idea is to isolate the direct interaction of pumping laser pulse with the solid density target surface.  The smooth initial ion density gradient should reduce the efficiency of ion acceleration as compared with sharp full density surface.  It may enable the use of full density Sn (higher CE) in practical EUVL source, lowering the requirement of laser. 2

ns laser 650mJ/7ns/10 Hz EKSPLA SL335/SH laser 500 mJ/130 ps/5 Hz DG535 Jitter< 0.5 ns Normaski interferometer Faraday cup E-Mon f=30cm Its feasibility has been proved at UCSD using an EUV experiment facility with comprehensive diagnostics SHG TGS Other diagnostics, Time resolved spectroscopy Silicon wafer witness plate Polarized microscope Interference Profiler WP 3 PD Cal. CCD Target chamber EUV facility

TOF of Sn ions shifts to later time by introducing a low energy pre-pulse Experimental conditions, Main pulse:  m/7 ns/ 2  W/cm 2, Pre-pulse:  m/130 ps/2 mJ Focal spot: 100  m(main), 200  m(pre) Faraday Cup: Angle: 10 degrees to normal Distance from Plasma: 15 cm Bias Voltage: -30V E-Mon: 45 degree TGS: 45 degree Target: pure Sn slab Typical TOFs of Sn ions from LP Sn plasma 4  In the case of single pulse, most the ions are located before 4  s, the flux peak is located before 1.5  s.  For the dual-pulse, the waveform shifts to later time.

More than 30 times reduction in ion energy is obtained while almost without loss of conversion efficiency 5.2 KeV 150eV Energy spectrum of ions  More than 30 times reduction in ions kinetic energy at flux peak is obtained at delay around 1  s.  Almost the same CE as compared with that obtained with single pulse is also achieved at the same delay time.  Enhancement of CE at small delay is observed. 5 CE of single pulse Delay time (ns) CE Particle energy reduction factor

A double pulse is effective in the whole angle, but more effective near the normal Ion energy at flux peak as a function of angles with respect to target normal  Uniform ions energy is achieved in the whole angle in the case of dual-pulse, it is easier to design a gas stopping. 6

Less than 1 mJ energy of pre-pulse is required Energy spectrum of ionsConversion efficiency  Pre-pulse with 2 mJ energy is enough to shift most of the fast ions from more than 5 keV to < 150 eV  Because the present focal spot of pre-pulse is much larger than that of the main pulse, the necessary pre-pulse energy could be reduced less than 1mJ. 7

Short pulse is not necessary, but it is economical to operate in practical EUVL system  High reduction factor can also be obtained with a longer pulse.  Higher pre-pulse energy is required for longer pulse duration.  The reason may come from the fact that the intensity of the pre-pulse must be above the threshold for the explosive boiling (2-3  W/cm 2 ). See page 10 & Yoo et al. APL, 76,783(2000).  Several 10 mJ may be required for an ns laser pulse  Short pulse (~20ps) laser with less 1 mJ working at 100 kHz energy is commercial available, and low cost to operate (low power). 8 The required pre-pulse energy to achieve 30 times reduction in ions energy as a function of pulse duration

9 There are two possible mechanisms dominating ion acceleration in the case of dual-pulse 1. Pre-plasma acts as a gas stopping 2. Main laser pulse is absorbed within pre-plasma, an initial ion density profile with finite gradient modifies ion acceleration. Pre-plasma Main pulse passes through pre-plasma, main plasma is generated on the solid surface, and expands into pre-plasma. The collisions with particles inside pre- plasma slower the ions. However, the stopping power of neutral gas can be simplified as, where  is velocity of target particles, predicting that gas is more effective at stopping slow ions rather than fast ions. Recalling that fast ions are more efficiently stopped than slow ions in our case, this mechanism can not explain our experiments.

840 ns1840 ns2840 ns440 ns10 ns A neutral plume is found in front of the initial surface at the delay time when large reduction factor & high CE are observed. 10 Double plumes induced by pre-pulse  The cold plume is almost neutral. Interferogram of pre- plasma obtained at delay time of 840 ns 500  m Thermal plasma cold plume  The cold plume may come from the energy transfer from the laser heated thermal plasma to target. Superheated liquid abruptly transforms into a mixture of liquid droplet and vapor via explosive boiling.

The neutral plume has a near Gaussian density profile, and the reduction of ion energy increases with its length. Typical density profile of neutral plume at a delay of 840 ns Energy reduction factor as a functions of length of the neutral plume  The neutral particles has a Gaussian density profile with a finite density gradient.  The reduction factor increases with the length of neutral plume, reaches its maximum around length of 120  m. 11

Dual-pulseSingle pulse 500  m 12 The reason can be ascribed to the interactions of the main pulse with the neutral plume instead of the sharp full density surface laser Interferograms Shadowgraphs  The main laser is mainly deposited with the pre- formed neutral plumes instead of the initial infinite sharp full density surface.  The pre-formed neutral plume provides a initial ion density profile with a finite density gradient.  The free expansion of such plasma with an finite initial density gradient accelerate ions much inefficiently as compared with that of a sharp full density surface.  The physics is still open.

1.Higher reduction in ions energy has been demonstrated while almost without loss in-band conversion efficiency by using a very low energy pre- pulse. 2.The possible reason comes from the interaction of main laser pulse with the pre-formed neutral plume with a finite density gradient instead of the full density surface. 3.The kinetic energy of neutral particles is also potentially reduced because most of the energy of neutral particles comes from plasma expansion. 4.It is simple, low cost, and easily coupled into the present design of EUVL source. 5.Combination with gas stopping, this doubling pulse makes it possible to use of mass-limited full density Sn target in practical EUVL source. 6.It is also applicable to other laser solid material interactions, like pulsed laser deposition (PLD), soft and hard x-ray sources etc. Conclusions 13

1.Characterizing the properties of neutral particles from LP Sn plasma driven by dual-pulse. 2.Demonstrating the combination of the doubling pulse with gas stopping. 3.Demonstrating a target supply for high repetition rate operation, which is based on mass-limited full density Sn utilizing the combination of dual-pulse and gas stopping. 4.Further experimental diagnostics and theoretical efforts to clarify the physics dominating ion acceleration in laser produced plasma with an initial finite density gradient. Next works 14