In the drawings:
The EUV radiation source has a target feed device 1 which, as is shown schematically in
Solid particles 14 comprising metals or metal compounds, e.g., tin or lithium (or preferably also their oxides, SnO, SnO2, LiO, LiO2) which emit efficiently in the EUV spectral region (around 13.5 nm) and a clean (i.e., free from emitting particles) carrier gas 15, e.g., noble gases or nitrogen, are combined and mixed in the mixing chamber 11. The resulting particle-containing mixture 16 is fed to the liquefaction chamber 12, wherein liquefaction is carried out at low temperatures (T <173 K) and pressures >1 bar. Sn particles (individual particles of at most 10 μm in size) are preferably mixed in to achieve a high efficiency of EUV generation (≈3%). However, mixtures of other elements (e.g., lithium) or compounds (preferably tin compounds or lithium compounds) are also possible.
As is shown schematically in
The particle-containing liquid gas 17 is supplied to the injection unit 13 and introduced into the nozzle chamber 134. A stable continuous series 2 of droplets is dispensed along a target axis 21 in the plasma generation chamber 3 by means of a pressure modulator 132 (e.g., piezo-actuator) via the target nozzle 133 in tune with the drop breakup frequency of the liquid gas 17. An energy beam 4 is directed to the target axis 21 at the desired interaction location 41, and the successive pulses of this energy beam 4 respectively excite an individual target 23 (droplet) to form EUV-emitting plasma 5 when this individual target 23 passes the interaction location 41.
The target feed device 1 is incorporated together with the housing of the injection unit 13 in the plasma generation chamber 3. The housing of the injection unit 13 forms a nozzle antechamber 135 around the target nozzle 133 in order to adjust a higher pressure relative to the evacuated plasma generation chamber 3 so that the exit of liquid gas and the droplet formation are stabilized.
The target feed device 1 can also be introduced into the plasma generation chamber 3 at other positions, e.g., at the feed line between the liquefaction chamber 12 and the injection unit 13 or between the mixing chamber 11 and the liquefaction chamber 12.
According to
The injection of the particle-containing liquid gas 17 is carried out in such a way that droplets 23 are formed in the desired size, generally in the form of solid globules, when they reach the interaction location 41 because the liquid gas 17 expands adiabatically and freezes when injected into the vacuum of the plasma generation chamber 2, i.e., after exiting the nozzle antechamber 135 (at higher pressure).
The size of the droplets 23 is defined by the amount of mixture that is optimally excited to form a radiating plasma 5 at a given energy of an excitation pulse of the energy beam 4. The proportion of solid particles 14 in the liquid gas 17 is adjusted in such a way that the efficiency of the EUV generation and the width of the spectrum are optimized. In this way, a limiting of the amount of the Sn particles 14 assumed herein is achieved, i.e., the amount of Sn in the plasma generation chamber 3 is limited to the amount needed for generating radiation so that no excess metallic target material which, as debris, could damage the components of the radiation source as a result of insufficient excitation, remains in the plasma generation chamber 3.
The carrier gas 15 (N2 or a noble gas) can at most be potentially damaging to the optics due to the kinetic energy of its particles. A suppression of sputter processes of this kind is easily possible and is known from xenon-based EUV sources, e.g., by means of introducing a blocking gas (e.g., argon cross-flow) between the plasma 5 and the collector optics. In any case, the carrier gas 15 itself does not contain any component parts that are damaging to optics such as carbon (C) or oxygen (O2).
Because of the injection of the particle-containing mixture 16 in liquid form, a very great distance can be achieved between the generation of radiation (plasma 5) and all of the important components of the system such as the target nozzle 133, collector optics for bundling the generated EUV radiation (not shown), etc. The large distance results in a longer life of these components. In particular, the target nozzle 133 is also substantially less damaged (eroded) by heat radiation and particle radiation from the plasma 5 so that a stable target supply in the interaction location 41 can be achieved over a longer operating period.
Because of the coating property of metallic “fuels” (solid targets), their amount must be limited to the amount necessary for generating radiation. When using tin (Sn), which has strong spectral lines at 13.5 nm, about 5·1014 Sn ions (this corresponds to an Sn volume of about 30 μm diameter) are required for an EUV source size of 0.5 mm diameter with an excitation energy of about 1 J per individual excitation. The source size is derived from the etendue requirement of EUV lithography. The small Sn volume can reasonably be adapted in size to the required source size of the emission prior to excitation by expansion with a pre-pulse of the energy beam 4. The necessary energy is on the order of 10 mJ and is carried out approximately 100 ns before introducing the high-energy pulse.
At a repetition frequency of about 10 kHz, a source with these parameters behind collector optics would reach an EUV in-band output (13.5 nm±2%) of about 100 W. The Sn consumption per day in this case is about 85 g when the quantity of Sn is limited to the amount needed for generating radiation.
The ion density (and electron density) is derived solely from the optimized EUV emission for a homogeneous volume. The electron density is too low for efficient absorption of laser radiation with a wavelength of 1 μm. Therefore, the carrier gas 15 functions additionally as an electron donor to achieve a laser absorption of almost 100%. This is ensured for nitrogen (N2) and argon (Ar) in a stoichiometric proportion of the carrier gas from about ⅔. The stoichiometric proportion is the ratio of the quantity of atoms or molecules of target material (bound in particles) and carrier gas in relation to a volume element.
In addition, by mixing in lighter carrier gases (He, Ne) the spectral bandwidth of the radiation emission of tin at 13.5 nm is reduced, whereas with pure tin it is appreciably greater than the required ±2% (J. Opt. Soc. Am. B 17 (2000) 1616, Choi et al.). Further, the proportion of radiation outside the desired EUV spectrum is likewise appreciably reduced.
A true limiting of the amount of “fuel” (solid particles 14) to the amount needed for generating radiation is only achieved when the target volumes are supplied at a frequency that exactly matches the frequency at which the energy pulses are introduced (on the order of 10 kHz), i.e., exactly one target volume is supplied to the interaction location 41 for each individual generation of radiation. In the following three examples, compared to a variant shown in
The target feed device 1 differs from that shown in
In a second variant (according to
This is realized in a reliable manner in that the nozzle antechamber 135 of the injection unit 13 downstream of the target nozzle 133 is connected to pressure compensating means 138 which are adapted to the pressure Pcarrier gas of the gas feed to the mixing chamber 11 so that the liquid target material cannot form any unwanted droplets 23 in the nozzle chamber 134 and enter the plasma generation chamber 3 without a temporary pressure increase of the pressure modulator 132. The pressure modulator 132 which can be, e.g., a piezo-actuator arranged at the nozzle chamber 134 generates pressure pulses at the frequency of the energy pulses, i.e., only individual targets 23 are supplied as needed (corresponding to the triggered pulses of the laser beam 42).
It is shown schematically in
As was already mentioned above, it is also useful to mix solid particles 14 into carrier gas 15 which has already been liquefied beforehand. An arrangement of this kind is shown in
A preferred variant of the invention is shown in
The line proceeding from the mixing chamber 11 in direction of the injection unit 13 is then tied to another carrier gas line in a connection point (+) in such a way that the gas flows can be regulated relative to one another by means of a throughflow regulator 16 prior to the connection point (+).
A measuring device 19 arranged downstream of the connection point (+) serves to determine a regulating variable. The measuring device 19 measures the actual mixture ratio, e.g., by measuring scatter light, and accordingly supplies a correcting variable for the relative adjustment of the supplied amounts of clean carrier gas 15 and particle-containing mixture 16. This additional admixing of carrier gas enables a very accurate adjustment of the proportion of solid particles 14 per volume unit of carrier gas 15 and therefore a highly accurate metering of the effective target quantity (particles 14) per droplet 23 of the liquid gas generated therefrom.
Although
While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.
Number | Date | Country | Kind |
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10 2006 017 904.8 | Apr 2006 | DE | national |