An argon plasma in a magnetic cusp [1,2,3] thermalizes fast tin ions from the laser-plasma exhaust and guides their heat onto a large area beam dump. Demonstrations of plasma stability and particle control [2,3] have shown the magnetic field requirement to be modest and easily achievable with a very compact magnetic circuit. The plasma required for an extreme ultraviolet (EUV) source power of 500W has a pressure of 640Pa and is contained in a cusp magnetic well with a containment B field of only 40mT. Superconducting magnets are not required and the conventional magnet power totals only 4kW. A cusp magnetic circuit of mild steel serves as part of the source chamber wall and has very small fringing fields. In 2015 [2] stable containment was demonstrated of an argon plasma at the exhaust power, temperature and density required for this application. Without B-field containment the same degree of tin ion control would require a large gas number density, necessitating the use of hydrogen for low EUV absorption, with its associated disadvantages of dissociation, tin hydride chemistry and re-cycling concerns. Without a guide B-field, the hydrogen throughput to carry away excess plasma heat is very large and the hydrogen flow is not directional, in contrast to the flow of a magnetically-controlled plasma, making scaling of the hydrogen technology very difficult. Substantially raised EUV source efficiency (40% improvement) is achievable via plasma guidance that conforms the shape of the cusp magnetic well to the EUV collector optic, allowing a collection solid angle as high as 7 sterad. This translates into a laser power as low as 30kW for 500W of EUV power. Because the plasma braking mechanism for fast tin ions is dominantly via electron Coulomb collisions [3], non-reactive argon gas can replace hydrogen as the plasma positive ion, and its flow rate can be much lower. In the argon cusp tin LPP source, the exhaust power is guided precisely onto a ring-shaped beam dump that receives argon ions of energy too low to cause back-sputtering of tin. A typical beam dump heat loading for a 500W EUV source will be 100W cm-2 or less. This combination of low-stress components, easy argon recycling and high source efficiency makes the further development of this technology very important for high volume manufacturing.
The argon cusp plasma has been introduced [1,2] for 500W class tin LPP exhaust control in view of its high power handling, predicted low tin back-scatter from a beam dump, and avoidance of hydrogen usage. The physics of tin ion control by a plasma is first discussed. Experimentally, cusp stability and exhaust disc geometry have previously been proved at full scale [2], the equivalent of 300W-500W usable EUV. Here we verify operation of the plasma barrier that maintains a high argon density next to the collector, for its protection, and a low density in the long path toward the intermediate focus, for efficiency. A pressure differential of 2Pa has been demonstrated in initial work. Other aspects of tin LPP plasma control by the cusp have now been demonstrated using tin ions from a low Hz 130mJ CO2 laser pulse onto a solid tin surface at the cusp center. Plasma is rejected at the <0.5% level at the collector mirror location using the cusp magnetic field alone. Plasma also is rejected using a low argon density (<1x1014cm-3). We have measured the tin ion flow pattern toward the large area annular beam dump. Scaling of the cusp design to match a specified exhaust power is discussed. In view of this work, argon cusp exhaust control appears to be very promising for 500W class tin LPP sources.
Scaling the power of the tin droplet laser-produced-plasma (LPP) extreme ultraviolet (EUV) source to 500W has eluded the industry after a decade of effort. In 2014 we proposed [2] a solution: placing the laser-plasma interaction region within an argon plasma in a magnetic cusp. This would serve to ionize tin atoms and guide them to a large area annular beam dump. We have since demonstrated the feasibility of this approach. We present first results from a full-scale test plasma at power levels relevant to the generation of at least 200W, showing both that the argon cusp plasma is very stable, and that its geometrical properties are ideal for the transport of exhaust power and tin to the beam dump.
A new approach [1] to maskless EUV lithography is presented. It is based on a modulator that converts a deep ultraviolet (DUV) intensity pattern into an EUV phase pattern. The EUV phase pattern is then imaged with reduction via a 'conventional' EUV projection optic to create an intensity pattern on a wafer with pixel size of the order of 20nm and feature size of the order of 35nm [2]. The modulator consists, in one version, of a two-dimensional array of small EUV multilayer mirrors, each mounted on an elastomer pad. The required phase information is generated when the pads expand in response to the heat input pattern of the DUV programming beam. The fastest DUV writing method uses a mask, as in present day production lithography, so the proposed process is really a hybrid and is only maskless in the EUV stage. If the modulator is scanned this imaging process has the usual advantage of redundancy [3] in that as many as 100 different mirrors contribute on successive pulses to the intensity at a single feature on the wafer. Throughput is high and will be discussed for a typical case. Higher throughput may require larger DUV field size than is currently used in production. Modulator fabrication will be discussed.
Pinch discharge EUV sources tend to produce 13.5nm radiation from an elongated plasma, decreasing the possible collection efficiency for the lowest etendue collector designs. As a means to concentrate EUV emission we have investigated particle injection to locate the radiating species in a small volume at the center of the pinch. We have considered theoretically the rate of ionization and expansion of a small tin droplet in a buffer gas pinch plasma of argon or helium. The particle becomes negatively charged, with consequences for the electron and ion heat fluxes onto its surface. Photo-emission can reduce this negative potential and enhance the electron heat flux. We estimate that the 30 μm diameter tin particle that contains the minimum, or 'mass limited' number of atoms for a 5J EUV-emitting plasma, will fully ionize in less than 100nsec in a typical Star Pinch [1] plasma at the relevant electron density of 2x1019 cm-3 and temperature of 30eV. We report on the generation of high velocity, on-demand, accurately positioned tin droplets suitable for the injection pinch, and on limitations to the system reliability.
A new direct discharge source of 13.5nm radiation addresses the heat load problem by creating the plasma remote from all surfaces. The plasma is initially formed at the intersection of many pulsed xenon beamlets. Further heating is then applied via a high current pulse to induce efficient radiation from Xe10+ ions. The plasma is compact, with a single pulse FWHM diameter of 0.7mm and length of 3mm. It is positionally stable, as illustrated by re-imaging onto a fluorescent screen sensitive to EUV and time-integrating over 250 pulses. In this mode the averaged FWHM is 0.9mm. The conversion efficiency from stored electrical energy to radiation within 2π sterad and 2% bandwidth at 13.5nm is currently 0.55%, using xenon. Power is delivered to the plasma by a solid state-switched modulator operated at a stored energy of 25J of which 10J is dissipated in the plasma plus circuit, and 15J is recovered. The EUV output in 2% bandwidth at 13.5nm is 9mJ/sterad. Repetition rate scaling of the star pinch EUV source to 1kHz there is negligible electrode erosion at 106 pulses. This is possible because the cathode for the main heating discharge is distributed into 24-fold parallel hollow cathodes, with a combined operational surface aera of approximately 20cm2. The anode is similarly distributed. The walls facing the plasma are 22mm distant from it and when scaled to 6kHz will see a heat load of less than 1kWcm-2. The cathode electrode is then expected to receive a heat load of less than 500W cm-2. The plasma is expected to clear between pulses and be reproducible at frequencies up to at least 10kHz, at which rate the usable EUV power available at a second focus, assuming colleciton in 2sterad, is predicted to be more than 80W. The star pinch has properties that favor long life and appears to scale to the 50-100W powers needed for high throughput lithography.
A xenon Z-pinch generating extreme ultraviolet radiation at the Mo-Si mirror wavelength of 13.5 nm has been scaled to emit increased power at a higher repetition frequency. The 25 mm long by 3.0 mm (FWHM) diameter pinch produces 1.5 W of EUV radiation (2.5% bandwidth) into an axial 0.1 sr solid angle when operated at 100 Hz (100 J stored). The measured average pinch liner heat load at 100 Hz is 37 W cm-2, corresponding to an average internal wall temperature of 80 degrees Celsius. Electrode and liner erosion is very slight after more than 106 pulses at 100 Hz. Source cleanliness was demonstrated via a two-mirror simulation of a condenser in which throughput was unchanged during a 106 pulse run at 50 Hz. Amplitude stability was 12% (3(sigma) ) and positional stability was less than 4% of source diameter (1(sigma) ). 13.5 nm output scaled linearly with repetition frequency to more than 150 Hz (58 J stored). The cost of ownership for this source is estimated to be no greater than for an excimer laser illuminator. A plan is outlined for continued development to > 1 kHz and usable power of 16 W.
The 1Hz helium-xenon Z-pinch previously described has been re-engineered with long-life thyratron switches to operate at 10Hz. At 10Hz, without a condenser, it currently delivers 0.2W in a 4 Angstrom band centered at 134.5A, into a solid angle of 0.03 sterad. We report on measurements of scaling of the in=-band power with stored energy, and optimization of the spectrum, either for operation at 134A with Mo-Si multilayer mirrors, or at shorter wavelengths such as 113A for use with Mo-Be mirrors. We have measured an rms pulse- to-pulse amplitude stability of 2.1 percent. No measurable loss of transmission at 134A occurred in a 250 nm silicon nitride membrane placed at 52nm axially from the plasma during a 105 pulse run at full energy, indicating that the source is clean. Electrode and pinch liner erosion is not significant in test to the 106 pulse level and the life of these components is projected to exceed 5 X 107 pulses. the possible extension to a narrow beam of at least 4W in-band 134A, via an increase of the repetition rate to 100Hz and solid angle to 0.06 sterad, is discussed.
This work addresses the possibility of a high average power coherent light source in the 4.8 micrometers atmospheric window. The approach is to frequency-double the 9.55 micrometers CO2 laser line, yielding 4.775 micrometers . In order to achieve efficient conversion, the intensity of the laser is increased by intracavity resonant modulation, or 'mode-locking'. Efficient conversion in available lengths of doubling material requires an intensity of at least 20 MWcm-2. However, the most efficient CO2 lasers operate with a pulse duration of at least 1 microsecond(s) ec, the characteristic time for energy transfer from N2 to CO2 at 1 atmosphere. Without modulation a fluence of at least 20 Jcm-2 would therefore be needed for high overall system efficiency. With modulation at a 1:10 or better mark-to-space ratio, this fluence is reduced to the 2 Jcm-2 range that typifies the surface damage threshold of available materials. The doubling material chosen for this work was AgGaSe2, silver gallium selenide. A relatively long crystal (35 mm) was used in Type 1 phase-matching. A mode-locked pulse train was generated using a TEA CO2 laser pumped for a duration of 3 microsecond(s) ec, containing 1 nsec pulses spaced by 40 nsec. The 9.55 micrometers line was selected either by injection or by the use of an intracavity grating.
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