Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/23—Arrangements of two or more lasers not provided for in groups H01S3/02 - H01S3/22, e.g. tandem arrangements of separate active media
  • H01S3/2308—Amplifier arrangements, e.g. MOPA
  • H01S3/2316—Cascaded amplifiers
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G1/00—X-ray apparatus involving X-ray tubes; Circuits therefor
  • H05G1/08—Electrical details
  • H05G1/26—Measuring, controlling or protecting
  • H05G1/54—Protecting or lifetime prediction
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
  • H05G2/001—Production of X-ray radiation generated from plasma
  • H05G2/008—Production of X-ray radiation generated from plasma involving an energy-carrying beam in the process of plasma generation
  • H05G2/0082—Production of X-ray radiation generated from plasma involving an energy-carrying beam in the process of plasma generation the energy-carrying beam being a laser beam
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
  • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/22—Gases
  • H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
  • H01S3/2232—Carbon dioxide (CO2) or monoxide [CO]
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/22—Gases
  • H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
  • H01S3/225—Gases the active gas being polyatomic, i.e. containing two or more atoms comprising an excimer or exciplex

Definitions

• the present invention relates to systems and methods for generating laser light which can be used for applications including, but not limited to, the irradiation of a target material to generate extreme ultraviolet (EUV) light, e.g., light at wavelengths of around 50 nm and below.
• EUV extreme ultraviolet
• EUV light e.g., electromagnetic radiation having wavelengths of around 50 ran or less (also sometimes referred to as soft x-rays) and including light at wavelengths at or about 13.5 ran, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
• Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range.
• a material such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam.
• LPP laser produced plasma
• LPP and other high power laser applications often require a laser source meeting specific laser output requirements. These requirements may include power
• a particular LPP configuration e.g., using tin targets, may operate efficiently using light at a wavelength of 10.6 ⁇ m, a pulse duration of 20-150ns and a pulse energy of about 10OmJ.
• a laser light source may comprise a laser oscillator producing an output beam; a first amplifier amplifying the output beam to produce a first amplified beam, and a second amplifier amplifying the first amplified beam to produce a second amplified beam.
• the first amplifier may have a gain medium characterized by a saturation energy (E s> i) and a small signal gain (g o , i); and the second amplifier may have a gain medium characterized by a saturation energy (E s> 2 ) and a small signal gain (g o, 2 ), with (g o, i) > (g o, 2 ) and (E s, 2 ) > (E s , i).
• the laser light source may further comprise a third amplifier amplifying the second amplified beam to produce a third amplified beam, the third amplifier having a gain medium characterized by a saturation energy (E Si 3) and a small signal gain (go, 3), with (go, 1) > (go, 2) > (go, 3) and (E s , 3) > (E 5 , 2 ) > (E s> ,).
• the first amplifier gain medium may comprise a gas having a first gas composition and in a particular embodiment, the second amplifier gain medium may comprise a gas having a gas composition different from the first gas composition.
• the first amplifier gain medium may be a gas at a gas pressure Pi
• the second amplifier gain medium may be a gas at a gas pressure P 2 , with Pi ⁇ P 2 .
• the gas may comprise CO 2 .
• the laser oscillator may be, e.g., a mode-locked laser oscillator, q-switched laser oscillator or other cavity dumped laser oscillator.
• the laser light source may further comprise a temporal pulse stretcher.
• the temporal pulse stretcher is positioned to receive laser pulses from the oscillator and produce stretched pulses therefrom for amplification by an amplifier.
• an EUV light source may comprise a droplet generator producing droplets containing an EUV line emitting element and a laser light source for irradiating droplets to produce an EUV emission.
• the laser light source may comprise a laser oscillator producing laser pulses; a temporal pulse stretcher stretching laser pulses; and an amplifier amplifying laser pulses.
• the amplifier may be a first amplifier amplifying pulses to produce a first amplified beam, the first amplifier having a gain medium characterized by a saturation energy (E s , i) and a small signal gain (g o , i); and the EUV light source may further comprise a second amplifier amplifying the first amplified beam to produce a second amplified beam, the second amplifier having a gain medium characterized by a saturation energy (E Si 2 ) and a small signal gain (g o , 2 ), with (go, 1) > (go, 2) and (E s , 2 ) > (E 8 , 1).
• the EUV line emitting element may be tin.
• the laser oscillator may produce laser pulses having a pulse duration in the range of 10-30ns and the temporal pulse stretcher may produce laser pulses having a pulse duration in the range of 50-150ns.
• aspects of an embodiment may also include a method for generating laser light, comprising the acts and/or steps of producing a seed laser beam; amplifying the seed beam using a first gain medium to produce a first amplified beam, the first gain medium characterized by a saturation energy (E s, 1 ) and a small signal gain (g o, i); and amplifying the first amplified beam using a second gain medium to produce a second amplified beam, the second gain medium characterized by a saturation energy (E s , 2 ) and a small signal gain (go, 2), with (go, 1) > (go, 2) and (E 5 , 2 ) > (E s , 1).
• a saturation energy E s, 1
• g o, i small signal gain
• a further aspect of an embodiment of the invention may be characterized as comprising an EUV light source having a material containing an EUV line emitting element, e.g. tin; a source of at least one laser pulse, e.g., a CO2 laser; a pulse stretcher modifying the shape of the pulse to have at least three peaks, the stretcher having a beam splitter defining the relative magnitude of the peaks and a delay path establishing a temporal separation of the peaks, the modified pulse shape selected to irradiate the material and produce an EUV intensity greater than an EUV intensity generated in the absence of the pulse stretcher.
• an EUV light source having a material containing an EUV line emitting element, e.g. tin
• a source of at least one laser pulse e.g., a CO2 laser
• a pulse stretcher modifying the shape of the pulse to have at least three peaks, the stretcher having a beam splitter defining the relative magnitude of the peaks and a delay path establishing
• a first peak of the modified beam has a lower intensity than another peak of the modified beam and in a particular embodiment, one or more parameters of the first peak is selected to irradiated the material with a pre-pulse to spatially expand the line emitting element for subsequent irradiation by the second peak.
• the pulse stretcher may be a non-displacing pulse stretcher and in a particular arrangement the pulse stretcher may place each pulse peak on a different path exiting the stretcher and the material may be in the form of droplet(s).
• FIG. 1 shows a schematic view of an overall broad conception for a laser- produced plasma EUV light source according to an aspect of the present invention
• Fig. 2 illustrates one embodiment of a laser source which includes an oscillator unit and an amplification unit
• Fig. 3 illustrates another embodiment of a laser source which includes an oscillator unit, a temporal pulse stretcher and an amplification unit;
• Fig. 3A illustrates a first pulse shape that may be obtained using a pulse stretcher
• Fig. 3B illustrates another pulse shape that may be obtained using a pulse stretcher
• Fig. 3C illustrates yet another pulse shape that may be obtained using a pulse stretcher
• Fig. 3D illustrates a pulse stretcher having an optical alignment such that each successive pulse peak exiting the stretcher is directed along a slightly different path
• Fig. 4 illustrates an example of an amplification unit having two amplifiers arranged in series suitable for use in either of the Fig. 2 or Fig. 3 embodiments;
• the LPP light source 20 may include a source 22 for generating light pulses and delivering the light pulses into a chamber 26. As detailed below, the light pulses may travel along one or more beam paths from the source 22 and into chamber 26 to illuminate one or more target volumes.
• the light source 20 may also include a source material delivery system 24, e.g., delivering droplets of a source material into the interior of a chamber 26 to a target volume 28 where the source material targets will be irradiated by one or more light pulses, e.g., a pre-pulse and thereafter a main pulse, to produce a plasma and generate an EUV emission.
• the source material may include, but is not limited to, a material that includes tin, lithium, xenon or combinations thereof.
• the EUV emitting element e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/or solid particles contained within liquid droplets or any other form that delivers the EUV emitting element to the target volume.
• the light source 20 may also include a collector 30, e.g., a reflector, e.g., in the form of a truncated ellipse, e.g., a multi-layer mirror having alternating layers of Molybdenum and Silicon, with an aperture to allow the light pulses generated by the source 22 to pass through and reach the target volume 28.
• the collector 30 may be, e.g., an elliptical mirror that has a first focus within or near the target volume 28 and a second focus at a so-called intermediate point 40
• the EUV light may be output from the light source 20 and input to, e.g., an integrated circuit lithography tool (not shown).
• the light source 20 may also include an EUV light source controller system 60, which may also include a firing control system 65 for triggering one or more lamps and/or laser sources in the source 22 to thereby generate light pulses for delivery into the chamber. 26.
• the light source 20 may also include a droplet position detection system which may include one or more droplet imagers 70 that provide an output indicative of the position of one or more droplets, e.g., relative to the target volume 28 and provide this output to a droplet position detection feedback system 62, which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g. on a droplet by droplet basis or on average.
• the droplet error may then be provided as an input to the light source controller 60, which can, e.g., provide a position, direction and timing correction signal to the source 22 to control a source timing circuit and/or to control a beam position and shaping system e.g., to change the location and/or focal power of the light pulses being delivered to the chamber 26.
• the light source controller 60 can, e.g., provide a position, direction and timing correction signal to the source 22 to control a source timing circuit and/or to control a beam position and shaping system e.g., to change the location and/or focal power of the light pulses being delivered to the chamber 26.
• the light source 20 may include a droplet delivery control system 90, operable in response to a signal (which in some implementations may include the droplet error described above, or some quantity derived therefrom) from the system controller 60, to e.g., modify the release point of the source material from a droplet delivery mechanism 92 to correct for errors in the droplets arriving at the desired target volume 28.
• a signal which in some implementations may include the droplet error described above, or some quantity derived therefrom
• Fig. 2 illustrates one embodiment of a source 22 which includes an oscillator unit 200 and an amplification unit 202.
• the oscillator unit 200 can include a pulsed gas discharge CO 2 , excimer or molecular fluorine laser oscillators and the amplification unit 202 can include one or more gas discharge CO 2 , excimer or molecular fluorine laser units configured as oscillators or single/double pass amplifiers.
• the laser units in the amplification unit 202 may be arranged in parallel or in series.
• Fig. 3 illustrates another embodiment of a source 22' which includes an oscillator unit 200', a temporal pulse stretcher 300 and an amplification unit 202'. More details regarding temporal pulse stretchers may be found in United States Patent No.
• these optical pulse stretchers include a beam splitter 302 which passes a portion of the beam along a delay path 304 allowing the pulse shape exiting the stretcher to be controlled by selecting the proper beam splitter reflectivity and delay path length.
• the pulse stretcher 300 receives an input pulse generally having a single peak (i.e. in a plot of intensity vs. time) and outputs a pulse having a plurality of temporally spaced peaks. As shown, the stretcher 300 may be substantially lossless.
• the pulse stretcher is herein referred to as a non-displacing pulse stretcher.
• the components of the stretcher 300 can be aligned such that the pulse stretcher may be non-displacing.
• the reflectivity of the beam splitter will impact the relative magnitude of the output peaks and the length of the delay path may establish the temporal separation between peaks.
• the output pulse shape can be engineered by proper selection of the beam splitter reflectivity and the length of the delay path.
• the output pulse shape may be optimized to increase EUV output and/or conversion efficiency (i.e., the ratio of output EUV power to the EUV light source input power) and/or lower source material consumption, e.g., tin consumption, and/or reduce the formation of undesirable byproducts in the plasma chamber (e.g. debris) which can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements, e.g. mirrors, windows, etc.
• Fig. 3 A shows a shape for a pulse 310 exiting a pulse stretcher wherein the reflectivity of the beam splitter has been selected such that the second peak 312 is larger in intensity than the first peak 314, and the delay path length has been selected such that the first two peaks are separated by a time ti. It is to be appreciated that a tail corresponding to the peak 312 overlaps with a tail corresponding to the peak, and thus, the two peaks are part of the same pulse.
• Fig. 3 A shows a shape for a pulse 310 exiting a pulse stretcher wherein the reflectivity of the beam splitter has been selected such that the second peak 312 is larger in intensity than the first peak 314, and the delay path length has been selected such that the first two peaks are separated by a time ti.
• a tail corresponding to the peak 312 overlaps with a tail corresponding to the peak, and thus, the two peaks are part of the same pulse.
• FIG. 3B shows a shape for a pulse 310' exiting a pulse stretcher wherein the reflectivity of the beam splitter is roughly the same as the beam splitter corresponding to Fig. 3 A and thus the second peak 312' is larger in intensity than the first peak 314', but a longer delay path length has been employed and now the two peaks are separated by a time t 2 , with t 2 > ti.
• Fig. 3C shows, again for illustrative purposes, a shape for a pulse 310" exiting a pulse stretcher wherein the reflectivity of the beam splitter is less than the beam splitter corresponding to Fig. 3 A and thus the second peak 312" is smaller in intensity than the first peak 314".
• Fig. 3 A illustrate that a wide range of pulse shapes may be generated by varying the beam splitter reflectivity and delay path length.
• a pre-pulse peak may be used to vaporize and spatially expand the source material, e.g. tin droplet, and a subsequent main pulse used to generate an EUV emission from the expanded source material.
• the source material may, in some implementations, form a weak plasma.
• weak plasma and its derivatives means a material which includes ions but which is less than about 1% ionized.
• the irradiated material may be exposed to a main pulse peak to create a plasma and generate an EUV emission.
• a 1 - 1OmJ pre-pulse and 50 - 40OmJ main pulse may be suitable.
• a pulse stretcher 300 can be aligned such that the pulse stretcher may be nonrdisplacing.
• slight changes in the alignment of one or more of the optical components may be made to produce a pulse stretcher 300' such that each successive pulse peak is directed along a slightly different path.
• a first pulse peak may be emitted along path 320
• a second pulse peak may be emitted along path 322
• a third pulse peak may be emitted along path 324
• a fourth pulse peak may be emitted along path 326, and so on.
• the sequence of pulse pulses may 'follow' a moving source material 328, e.g. droplet or pre-pulsed volume, as shown moving in direction 330.
• the oscillator unit 200' may include a laser oscillator, e.g. a mode-locked laser oscillator, q-switched laser oscillator or other cavity dumped laser oscillator.
• the laser oscillator may provide pulse duration, e.g., in the range of 10 — 30ns, shorter than desired, which is then increased using the pulse stretcher. For some LPP configurations, short pulse durations may limit the conversion efficiency of the EUV light source (i.e. the ratio of EUV output to source material/laser input).
• the amplification of the long pulse is more efficient than that one for short (10ns range) pulse due to energy exchange between rotational lines in the same vibrational band of the CO2 module.
• Use of longer pulses may also avoid the complexity of a pre- pulse.
• a suitable pulse duration may be in the range of about 20-180ns and in some cases 50 — 150ns.
• Use of the pulse stretcher 300 may also have application for configuration in which a pre-pulse is employed with the pre-pulse either passing through or bypassing the pulse stretcher 300.
• the temporal pulse stretcher 300 is positioned to receive laser pulses from the oscillator unit 200 and produce stretched pulses for subsequent amplification by the amplifier unit 202.
• This arrangement lowers the peak pulse intensity prior to the amplifier unit 202, reducing high intensity damage to the amplifier unit optics.
• the pulse stretcher may be positioned after one, some or all of the laser units in the amplification unit 202'.
• Fig. 4 illustrates an example of an amplification unit 202a, 202a' having two amplifiers 400, 402 arranged in series, suitable for use in either of the Fig. 2 or Fig. 3 embodiments.
• amplifier 400 may have a gain medium characterized by a gain medium length (Li), a saturation energy (Eg , i) and a small signal gain (g o, i); and the amplifier 402 may have a gain medium characterized by a gain medium length (L 2 ), a saturation energy (E s , 2 ) and a small signal gain (go, 2 ).
• saturation energy and its derivatives means the pulse energy of an input signal pulse which leads to a reduction of the gain of a gain medium to 1/e ( ⁇ 37%) of its initial value.
• small-signal gain and its derivatives means the gain obtained for an input signal which is so weak that it does not cause any appreciable gain saturation of a gain medium.
• the ratio of output energy of the PA (E out ) to the input energy (Ej n ) is defined by 2 parameters of the gain media: the small signal gain (go) multiplied by the media length (/) and the saturation energy (E s ) level.
• RF power and high gas pressure may be used.
• Another parameter which is not covered in the Frantz-Nodvik formulation that also has an affect on go and E s is the gas mixture (e.g. the ratio of CC ⁇ N 2 :He) of the laser medium.
• a number of gain medium factors may be controlled to selectively define small signal gain (go) multiplied by the media length (/) and the saturation energy (E s ) including RF power, gas pressure, gas composition and gas pumping speed Input beam characteristics such as pulse duration and shape and intensity distribution, e.g. Gaussian, etc., may also affect the choice of parameters, e.g., gas pressure, composition and excitation voltage necessary to obtain a selected small signal gain (go) and saturation energy (E s ).
• FIG. 6 illustrates an example of an amplification unit 202b, 202b' having three amplifiers 600, 602, 604 arranged in series, suitable for use in either of the Fig. 2 or Fig. 3 embodiments.
• amplifier 600 may have a gain medium characterized by a gain medium length (Li), a saturation energy (E S) i) and a small signal gain (g o, i); and the amplifier 602 may have a gain medium characterized by a gain medium length (L 2 ), a saturation energy (E Si 2 ) and a small signal gain (g o, 2 ) and amplifier 604 may have a gain medium characterized by a gain medium length (L 3 ), a saturation energy (E 5 , 3) and a small signal gain (g Oi 3), with (go, 1 ) > (go, 2) > (go, 3) and (E s> 3 ) > (E s , 2 ) > (E s> 1).
• this arrangement can result in a larger amplification than a single amplifier having a gain medium length of Li+ L 2 + L 3 .
• a plurality of uniquely configured amplifiers may be used instead of single amplifier of equivalent length.
• the first amplifier may he configured to have maximum available go parameter (working at low pressure and power level) whereas the final output amplifier may be configured to have maximum available E s go / product.
• the intermediate segments may have gradually increasing E 5 parameter (from input to output) and decreasing go parameter.

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• 2006
  • 2006-06-14 US US11/452,558 patent/US7518787B2/en active Active
• 2007
  • 2007-05-28 TW TW096118947A patent/TWI360271B/zh active
  • 2007-06-11 WO PCT/US2007/013862 patent/WO2007146329A2/en not_active Ceased
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