This paper describes the development of laser-produced-plasma (LPP) extreme-ultraviolet (EUV) source
architecture for advanced lithography applications in high volume manufacturing. EUV lithography is
expected to succeed 193 nm immersion technology for sub-22 nm critical layer patterning. In this paper we
discuss the most recent results from high qualification testing of sources in production. Subsystem
performance will be shown including collector protection, out-of-band (OOB) radiation measurements,
and intermediate-focus (IF) protection as well as experience in system use. This presentation reviews the
experimental results obtained on systems with a focus on the topics most critical for an HVM source.
The development of a successful extreme ultraviolet light source for lithography relies on the ability to
maintain collector optic cleanliness. Cleanliness is required to maintain the reflectivity of the collector optic, thus
maintaining the light power output at the intermediate focus. In this paper, an in-situ method is explored to remove
Sn from a contaminated collector optic. Hydrogen plasma is used to promote Sn etching while maintaining the
integrity of the collector optic's multi-layer structure. The removal rate of Sn is investigated as a function of various
operational parameters including chamber pressure, plasma electron density, as well as plasma electron temperature.
Initial results are presented using an external RF-plasma source. The use of the collector optic as a RF-antenna is
also investigated to optimize the etching rate of the hydrogen plasma. Initial plasma parameter measurements reveal
electron densities on the order of 10<sup>11</sup>-10<sup>12</sup> cm<sup>-3</sup>, with electron temperatures on the order of 1-3 eV. An optimized
etch rate of ~125 nm/min off of Si was observed using 1000 W, 80 mTorr, and a flow rate of 50 sccm of H2. These
initial measurements are used as a basis for optimizing the etching rate off of the collector optic. Such results are
important in allowing the long-term usage of a single collector optic to minimize operating costs involved with
replacing the optic as well as tool downtime.
Laser produced plasma (LPP) systems have been developed as a viable approach for the EUV scanner light sources to
support optical imaging of circuit features at sub-22nm nodes on the ITRS roadmap. This paper provides a review of
development progress and productization status for LPP extreme-ultra-violet (EUV) sources with performance goals
targeted to meet specific requirements from leading scanner manufacturers. The status of first generation High Volume
Manufacturing (HVM) sources in production and at a leading semiconductor device manufacturer is discussed. The
EUV power at intermediate focus is discussed and the lastest data are presented. An electricity consumption model is
described, and our current product roadmap is shown.
This paper describes the development of laser-produced-plasma (LPP) extreme-ultraviolet (EUV) source architecture
for advanced lithography applications in high volume manufacturing. EUV lithography is expected to succeed 193nm
immersion technology for sub-22nm critical layer patterning. In this paper we discuss the most recent results from high
EUV power testing and debris mitigation testing on witness samples and normal incidence collectors. Subsystem
performance will be shown including the CO2 drive laser, debris mitigation, normal incidence collector and coatings,
droplet generation, laser-to-droplet targeting control, intermediate-focus (IF) metrology and system use and experience.
In addition, a number of smaller lab-scale experimental systems have also been constructed and tested. This
presentation reviews the experimental results obtained on systems with a focus on the topics most critical for an HVM
Laser produced plasma (LPP) systems have been developed as a viable approach for the EUV scanner light source to support optical imaging of circuit features at sub-22nm and beyond nodes on the ITRS roadmap. This paper provides a review of development progress and productization status for LPP extreme-ultra-violet (EUV) sources with performance goals targeted to meet specific requirements from leading scanner manufacturers. The status of first generation High Volume Manufacturing (HVM) sources in production and of prototype source operation at a leading scanner manufacturer is discussed. The EUV power at intermediate focus is discussed and the lastest data is presented. An electricity consumption model is described, and our current product roadmap is shown.
Improved performance and specific results are reported for several test and prototype extreme ultraviolet (EUV) light sources developed for next-generation lithography. High repetition rate and high-power CO2 laser-produced plasma sources operating on tin droplet targets are described. Details of laser architecture, source chambers and system operation are given. Stable output power, efficient light collection, and clean EUV transmission could be achieved for hours of operation. We review progress during integration of light sources with collector mirrors reaching EUV power levels at intermediate focus of 60 W and 45 W, respectively, with duty cycles of 25% and 40%. Far-field EUV images of the collected light were recorded to monitor the source output performance during extended tests of collector longevity and debris protection with system operation time exceeding 50 h. Development results on EUV spectra, out-of-band (OOB) radiation, and ion debris obtained with dedicated metrology setups are also described. Angle-resolved measurements with ion energy analyzer and Faraday cups reveal the contributions of individual ion charge states in related spectra. Our laser-produced EUV light source technology has now reached a level of maturity in full integration where prototype sources can be delivered and pilot line introduction can be prepared.
This paper is devoted to the development of laser produced plasma (LPP) EUV source architecture for advanced
lithography applications in high volume manufacturing of integrated circuits. The paper describes the development
status of subsystems most critical to the performance to meet scanner manufacturer requirements for power and
debris mitigation. Spatial and temporal distributions of the radiation delivered to the illuminator of the scanner are
important parameters of the production EUV tool, this paper reports on these parameters measured at the nominal
repetition rate of the EUV source. The lifetime of the collector mirror is a critical parameter in the development of
extreme ultra-violet LPP lithography sources. Deposition of target material and contaminants as well as sputtering
and implantation of incident particles can reduce the reflectivity of the mirror coating substantially over time during
exposure even though debris mitigation schemes are being employed. We report on progress of life-test experiments
of exposed 1.6sr collectors using a Sn LPP EUV light source. The erosion of MLM coating is caused mostly by the
high-energy ions generated from the plasma. In this manuscript the ion distribution measured at small (14 degree)
and medium (45 degree) angles to the laser beam are presented. The measurements show that the chosen
combination of the CO2 laser and Sn droplet targets is characterized by fairly uniform angular ion energy
distribution. The maximum ion energy generated from the plasma is in the range of 3-3.5 keV for all incident angles
of the collector. The measured maximum energy of the ions is significantly less than that measured and simulated
for plasmas generated by short wavelength lasers (1 μm). The separation of ions with different charge states was
observed when a retarding potential was applied to the Faraday Cup detector.
Laser produced plasma (LPP) systems have been developed as a viable approach for the EUV scanner light source for
optical imaging of circuit features at sub-32nm and beyond nodes on the ITRS roadmap. This paper provides a review
of development progress and productization status for LPP extreme-ultra-violet (EUV) sources with performance goals
targeted to meet specific requirements from leading scanner manufacturers. We present the latest results on power
generation, stable and efficient collection, and clean transmission of EUV light through the intermediate focus. We
report on measurements taken using a 5sr collector optic on a production system. Power transmitted to intermediate
focus (IF) is shown. The lifetime of the collector mirror is a critical parameter in the development of extreme ultraviolet
LPP lithography sources. Deposition of target material as well as sputtering of the multilayer coating or
implantation of incident particles can reduce the reflectivity of the mirror coating during exposure. Debris mitigation
techniques are used to inhibit damage from occuring, the results of these techniques are shown. We also report on the
fabrication of 5sr collectors and MLM coating reflectivity, and on Sn droplet generators with droplet size down to 30μm
This paper provides a review of development progress for a laser-produced-plasma (LPP) extreme-ultra-violet (EUV) source with performance goals targeted to meet joint requirements from all leading scanner manufacturers. Laser produced plasma systems have been developed as a viable approach for the EUV scanner light source for optical imaging of circuit features at sub-32nm and beyond nodes on the ITRS roadmap. Recent advances in the development of the system, its present average output power level and progress with various subcomponents is discussed. We present the latest results on peak EUV and average EUV power as well as stability of EUV output, measured in burst-mode operation at the nominal repetition rate of the light source. In addition, our progress in developing of critical components, such as normal-incidence EUV collector and liquid-target delivery system is described. We also report on dose stability, plasma position stability and EUV distribution at the output region of the source. This presentation reviews the experimental results obtained on systems with a focus on the topics most critical for an HVM source.
The capability to scale LPP power by further development of the high power CO2 drive laser in order to increase duty cycle and duration of continuous light source operation is shown. Production systems with thermal management and capable of 5 sr light collection are being assembled and tested. A description of the development of a normal-incidence ellipsoidal collector is included. Improvements in substrate quality lead to increased EUV reflectance of the mirror. Results on the generation of liquid tin droplets as target material for efficient plasma generation are also described. The droplet generator serves as a key element in the precise and spatially stable delivery of small quantities of liquid tin at high repetition rates. We describe a protection module at the intermediate focus (IF) region of the source and imaging of the EUV distribution using a sub-aperture collector and a fluorescent screen. A path to meet requirements for production scanners planned well into the next decade is also presented.
The source output power and lifetime, including the collector optics lifetime, are among the key issues for EUV lithography systems. In order to meet the requirements for the EUV collector mirror, both the reflectivity and the long-term thermal stability of its multilayer coating have been enhanced considerably during recent development efforts. Sub-aperture ellipsoidal mirrors of different substrate materials with outer diameters of about 320 mm were coated with
laterally graded high-temperature multilayers. The interface-engineered Mo/Si multilayer mirror (MLM) coatings were optimized in terms of high peak reflectivity at 13.5 nm and working temperatures above 400°C. Thin barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon and to provide long-term optical stability of the coating at elevated temperatures. A normal-incidence reflectance of R ~ 60 %
at 13.5 nm was measured on Si wafer samples after heating up to 600°C. No degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests and multiple annealing cycles. On highly polished collector substrates with improved surface roughness a reflectance for s-polarized light exceeding peak values of R = 57 % was obtained. With optimized layer gradient the degree of wavelength matching was improved, as well,
resulting in peak reflectivity values above 56 % throughout the clear aperture for a series of measurement points across the mirror. The corresponding area-weighted 2% in-band average reflectance for this collector mirror coating exceeds 52 % for unpolarized light.
One of the critical issues within extreme ultraviolet lithography is mirror lifetime and the degradation due to debris from the pinch. This research investigated and showed the efficacy of using a helium secondary plasma and heat for removal of Li debris from collecting on the surface of collector optics. A He helicon plasma, which minimizes self-biasing and sputtering, has good extreme ultraviolet (EUV) photon wavelength transmission and preferential sputtering of lithium compared to other collector optics material. Through the combined use of heating and a He secondary plasma, EUV collector sample surface roughness and surface composition was able to be maintained near as-received status. The use of the He secondary plasma while the collector optics sample is exposed to Li debris shows promise as an in situ cleaning process for collector optics and can extend the lifetime of collector optics.
This paper describes the development of laser produced plasma (LPP) technology as an EUV source for advanced scanner lithography applications in high volume manufacturing. EUV lithography is expected to succeed 193 nm immersion technology for critical layer patterning below 32 nm beginning with beta generation scanners in 2009. This paper describes the development status of subsystems most critical to the performance to meet joint scanner manufacturer requirements and semiconductor industry standards for reliability and economic targets for cost of ownership. The intensity and power of the drive laser are critical parameters in the development of extreme ultraviolet LPP lithography sources. The conversion efficiency (CE) of laser light into EUV light is strongly dependent on the intensity of the laser energy on the target material at the point of interaction. The total EUV light generated then scales directly with the total incident laser power. The progress on the development of a short pulse, high power CO2 laser for EUV applications is reported.
The lifetime of the collector mirror is a critical parameter in the development of extreme ultra-violet LPP lithography sources. The deposition of target materials and contaminants, as well as sputtering of the collector multilayer coating and implantation of incident particles can reduce the reflectivity of the mirror substantially over the exposure time even though debris mitigation schemes are being employed. The results of measurements of high energy ions generated by a short-pulse CO2 laser on a laser-produced plasma EUV light source with Sn target are presented. Droplet generation is a key element of the LPP source being developed at Cymer for EUV lithography applications. The main purpose of this device is to deliver small quantities of liquid target material as droplets to the laser focus. The EUV light in such configuration is obtained as a result of creating a highly ionized plasma from the material of the droplets. Liquid tin is the material of choice to be used as a target due to the relatively high CE of the laser energy into in-band EUV radiation. Results obtained with the droplet generator and technical challenges related to successful implementation of the device are discussed.
This paper provides a detailed review of development progress for a laser-produced-plasma (LPP) extreme-ultra-violet (EUV) source with performance goals targeted to meet joint requirements from all leading scanner manufacturers. We present the latest results on drive laser power and efficiency, source fuel, conversion efficiency, debris mitigation techniques, multi-layer-mirror coatings, collector efficiency, mass-limited droplet generation, laser-to-droplet targeting control, and system use and experience. The results from full-scale prototype systems are presented. In addition, several smaller lab-scale experimental systems have also been constructed to test specific physical aspects of the light sources. This report reviews the latest experimental results obtained on these systems with a focus on the topics most critical for a source intended for use in high volume manufacturing (HVM). LPP systems have been developed for light-sources applications to enable EUV scanners for optical imaging of circuit features at nodes of 32 nm and below on the international technology roadmap for semiconductors (ITRS). LPP systems have inherent advantages over alternate source types, such as discharge produced plasmas (DPP), with respect to power scalability, source etendue, collector efficiency, and component lifetime. The capability to scale EUV power with laser repetition rate and pulse energy is shown, as well as the modular architecture for extendability. In addition, experimental results of debris mitigation techniques and witness sample lifetime testing of coated multi-layer-mirrors (MLM) are described and used to support the useful lifetime estimation of a normal incidence collector. A roadmap to meet requirements for production scanners planned well into the next decade is also presented.
A critical issue leading to decreased mirror lifetime is the buildup of debris on the surface of the primary mirror optics that comes from the use of both Sn and Li in GDPP or LPP. While lithium can easily be evaporated from the optic surface initially, over long duration high volume manufacturing, it is experimentally observed that there can be lithium debris buildup on the optic surfaces, which can lead to shortened lifetime of the mirror optics and decreased productivity of the tool. Consequently, an in situ cleaning process is needed so as to remove and limit the surface contamination on the optics so as to extend the lifetime of the optics. The Surface Cleaning of EUV Optics by Plasma Exposure (SCOPE) experiment was developed to study the mechanism of lithium deposition and the resulting diffusion to the end goal of using a secondary plasma source to preferentially remove the lithium surface contamination while leaving the underlying optic matrix in tact. Results have shown preferential lithium debris mitigation and sputtering from the surface of the MLM optic materials through the use of a secondary plasma that does not interfere or absorb EUV photons.
We report on the approach for a high-power high-beam-quality drive laser system that is used for a laser-produced plasma (LPP) EUV source. Cymer has conducted research on a number of solutions for a multi-kW drive laser system that satisfy high volume production requirements. Types of lasers to be presented include XeF at 351 nm and CO<sub>2</sub> at 10.6 micron. We report on a high efficiency XeF amplifier with a 3rd harmonic Nd:YLF master oscillator operated in the 6 to 8 kHz range and a CO<sub>2</sub> laser system with Q-switched cavity dumped master oscillator and RF pumped fast axial flow amplifiers operated in the 10 to 100 kHz range. CO<sub>2</sub> laser short pulse gain and optical isolation techniques are reported. Optical performance data and design features of the drive laser system are discussed, as well as a path to achieve output power scaling to meet high volume manufacturing (HVM) requirements and beyond. Additionally, the electrical efficiency as a component of cost of operation is presented. Development of a drive laser with sufficient output power, high beam quality, and economical cost of operation is critical to the successful implementation of a laser-produced-plasma (LPP) EUV source for HVM applications. Cymer has conducted research on a number of solutions to this critical need. We report our progress on development of a high power system with two gas-discharge power amplifiers to produce high output power with high beam quality. We provide optical performance data and design features of the drive laser as well as a path to output power scaling to meet HVM requirements. Development of a drive laser for LPP EUV source is a challenging task. It requires multi-kW laser output power with short pulse duration and diffraction limited beam quality. In addition, this system needs to be very reliable and cost-efficient to satisfy industry requirements for high volume integrated circuit manufacturing. Feasibility studies of high power laser solutions that utilize proven laser technologies in high power optical gain modules and deliver required beam properties have been performed and are reported.
The EUV source output power and the collector optics lifetime have been identified as critical key issues for EUV lithography. In order to meet these requirements a heated collector concept was realized for the first time. An ellipsoidal collector substrate with an outer diameter of 320 mm was coated with a laterally graded high-temperature multilayer. The interface-engineered Mo/Si multilayer coating was optimized in terms of high peak reflectivity at 13.5 nm and a working temperature of 400 °C. Barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon to provide long-term optical stability of the multilayer at elevated temperatures. A normal-incidence reflectance of more than 40 % at 13.55 nm was measured after heating. After initial annealing at 400 °C for one hour, no degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests for up to 100 hours and multiple annealing cycles. The successful realization of this high-temperature sub-aperture collector mirror represents a major step towards the implementation of the heated collector concept and illustrates the great potential of high-temperature EUV multilayer coatings.
A collector subsystem has been designed, built, and tested. The subsystem consists of a 320mm diameter ellipsoidal collector coated with a graded multilayer, mounting mechanics, thermal management capability, and a collector protection system. The EUV light emission can be collected with a solid angle of 1.6 sr. Collector substrates have been developed with the goal of offering both optical surface quality to support high multilayer mirror (MLM) reflectivity and material compatibility for long-term operation in the EUV source system. An interface-engineered MLM coating capable of maintaining high normal-incidence peak reflectivity at 13.5 nm during continuous operation at 400 °C has been developed. The thermal management of the system has been engineered and tested to maintain uniform substrate temperature during operation. Lastly, protection techniques have been developed to provide the collector with a long operational lifetime. Performance data for the entire subsystem are presented. The collector was installed in the source chamber of a laser-produced-plasma EUV source during system integration experiments using a tin droplet target. First results of the collected EUV output at the intermediate focus measured with a power meter and a fluorescence-converter-based imaging system are discussed.
This paper provides a detailed review of development progress for a laser-produced-plasma (LPP) extreme-ultra-violet (EUV) source with performance goals targeted to meet joint requirements from all leading scanner manufacturers. We present the latest results on drive laser power and efficiency, source fuel, conversion efficiency, debris mitigation techniques, multi-layer-mirror coatings, collector efficiency, intermediate-focus (IF) metrology, mass-limited droplet generation, laser-to-droplet targeting control, and system use and experience. Results from several full-scale prototype systems are discussed. In addition, a multitude of smaller lab-scale experimental systems have also been constructed and tested. This paper reviews the latest experimental results obtained on these systems with a focus on the topics most critical for an HVM source. Laser produced plasma systems have been researched as probable light source candidates for an EUV scanner for optical imaging of circuit features at 32nm and beyond nodes on the ITRS roadmap. LPP systems have inherent advantages over alternative source types, such as Discharge Produced Plasma (DPP), with respect to power scalability, etendue, collector efficiency, and component lifetime. The capability to scale LPP power with repetition rate and modular design is shown. A path to meet requirements for production scanners planned well into the next decade is presented. This paper includes current testing results using a 320mm diameter near-normal-incidence elliptical collector, the first to be tested in a full-scale LPP system. With the collector in-situ, intermediate focus (IF) metrology capability is enabled, and data is presented that describes the quality of light at IF.
Metrology concepts and related results are discussed for characterization of extreme ultraviolet (EUV) light sources based on laser-produced plasmas using metal foil and droplet targets. Specific designs of narrow-band EUV detectors employing multilayer mirrors and broadband detectors for droplet steering are described. Spatially resolved plasma imaging using in-band EUV pinhole cameras is discussed. A grazing-incidence flat-field EUV spectrometer is described that has been employed for spectroscopy in the 6 nm - 22 nm range. In addition, spectroscopic data of out-of-band radiation in the ultraviolet and visible spectral regions are presented. Results obtained for different wavelengths of the incident laser radiation and for both tin- and lithium foil- and droplet- targets are discussed.
In a laser produced plasma (LPP) EUV source the multilayer mirror (MLM) collector optic will be exposed to a flux of energetic ions and neutral atoms ejected from the plasma as well as condensable vapor from excess target material. We are investigating various techniques for reducing the contamination flux and for in-situ removal of the contamination. The protection strategies under investigation must be compatible with gaseous and condensable target materials such as Xe, Sn, In, Li, and other elements. The goal is to develop MLM structures that can withstand elevated temperatures and develop protective barrier coatings that reduce erosion of the mirror surface. Results of MLM exposure to energetic ion beams and thermal atomic sources are presented. Changes in EUV reflectivity of MLM structures after exposure to ions and deposition of target material have been performed on samples cleaned by these developmental processes. In this paper, we will summarize our initial results in these areas and present techniques for mitigation of MLM damage from the source.
Efficient conversion of laser light into EUV radiation is one of the most important problems of the laser-produced plasma (LPP) EUV source. Too low a conversion efficiency (CE) increases the amount of power the drive laser will have to deliver, which, besides the obvious laser cost increase, also increases the thermal load on all the components and can lead to increased debris generation. In order to meet the requirements for a high-volume manufacturing (HVM) tool and at the same time keep the laser power requirements within acceptable limits, a CE exceeding 2.5% is likely to be required. We present our results on optimizing conversion efficiency of LPP EUV generation. The optimization parameters include laser wavelength, target material, and laser pulse shape, energy and intensity. The final choice between parameter sets that leads to the required minimum CE is dependent on the debris mitigation solutions and the laser source available for a particular parameter set.
Over the past several years, a continuous improvement of the performance parameters of discharge produced plasmas as potential sources of 13.5 nm radiation for commercial EUV lithography systems has been achieved. At Cymer we have continued developing the dense plasma focus (DPF) discharge as an EUV source. The majority of the data presented here is focused on DPF operation with xenon gas. We have recently started investigating the DPF operation with Sn, as well. A significant improvement in conversion efficiency (CE) was observed. We have investigated DPF configurations with different polarity of the drive voltage. Central to both configurations is the pulsed power system, which is being developed to operate in continuous mode at 5 kHz while delivering approximately 10 J to the load. Significant differences have been observed for the energy deposition profiles in the positive and negative polarity systems. Calorimetric data show that the fraction of energy deposited into each discharge electrode depends on the polarity. The thermal engineering of the central electrode remains a major challenge. With the present generation DPF we have demonstrated operation at 5 kHz in burst mode and at 2.3 kHz in continuous mode, with 76 W of in-band energy generated at the source. We observed that certain transient effects in the EUV output were correlated with the degree of energy coupling during the burst. However, we found that the pulsed power system is well matched to the load with >90% of the stored energy coupled to the discharge and electrodes. The conversion efficiency of the DPF operated with Xe is near 0.5% for both polarities, while measurements with Sn show a CE ~1.7%. Plasma modeling supported the optimization of the pinch dynamics and electrodes. Debris mitigation studies were also carried out and the carbon contamination was reduced.
A commercially viable light source for EUV lithography has to meet the large set of requirements of a High Volume Manufacturing (HVM) lithography tool. High optical output power, high-repetition rate, long component lifetime, good source stability, and low debris generation are among the most important parameters. The EUV source, being developed at Cymer, Inc. is a discharge produced plasma source in a dense plasma focus (DPF) configuration. Promising results with Xe as a working gas were demonstrated earlier. To scale the DPF parameters to levels required for HVM our efforts are concentrated on the following areas: (1) thermal engineering of the electrodes utilizing direct water cooling techniques; (2) development of improved pulsed power systems for > 4 kHz operation; (3) study of erosion mechanisms for plasma facing components; (4) development of efficient debris mitigation techniques and debris shields; (5) studies of plasma generation and evolution with emphasis on improving conversion efficiency and source stability; (6) development of EUV metrology techniques and instrumentation for measurements of source size; and (7) development of an optimized collector optic matched to our source parameters. In this paper, we will present results from each of these key areas. The total in-band EUV output energy now approaches 60 mJ/pulse into 2πsr and the conversion efficiency has been increased to near 0.5 %. Routine operation at 4 kHz in burst-mode, and continuous operation at 1 kHz has been demonstrated. Improved at-wavelength source metrology now allows a determination of EUV source size utilizing imaging, and monitoring of key features of the spectrum on a pulse-to-pulse basis. With effective suppression of debris generated from the anode by several orders of magnitude, UV/EUV-catalyzed carbon growth now presents the limit in producing a clean source.
Since the initial demonstration of EUV emission with Xenon as a source gas in Cymer's Dense Plasma Focus (DPF) device, significant effort has been spent exploring the parameter space for optimization of efficient generation of EUV radiation. Parameters included in this investigation are He and Xe pressure and flow rates, electrode geometries, pre-ionization characteristics, and duty factor related performance issues. In these investigations it was found that the location of the He (buffer gas) and Xe (working gas) gas injection ports as well as the pressures and flow rates of the gas mixture components had a strong impact on EUV emission efficiency. Additional constraints on the gas recipe are also derived from gas absorption of the EUV radiation and the desire to provide debris mitigation properties. Best results to date have been obtained with an axially symmetric buffer gas injection scheme coupled with axial Xe injection through the central electrode. The highest conversion efficiency obtained was 0.42 percent at 12.4 J of input energy. Measurements of energy stability show a 10 percent standard deviation at near optimum EUV output. The matching of the drive circuit to the pinch as determined by the damping of the voltage overshoot waveforms was found to depend strongly on the He and Xe pressures. Energy Dispersive X-Ray (EDX) analysis of the debris emitted from the source shows that the primary sources of the debris are the central electrode and the insulator. No evidence of cathode material has been found. In addition to efforts toward more efficient operation, first phase efforts of thermal engineering have been undertaken, which have led to continuous operation at 200 Hertz with conventional direct water cooling. The system can be operated at higher repetition rates with proportionally lower duty cycles. The data will show the distribution of thermal power throughout the whole system. This more detailed understanding of the thermal power flow allows us to better determine the ultimate high volume manufacturing potential of this source technology.
We report on room-temperature photochromism of Zn- tetrabenzoporphyrine-doped polymethylmethacrylate. A one- quantum, thermo-reversible photoreaction is initiated with the 633 nm line of a HeNe-laser. The quantum yield depends strongly on the concentration of an electron acceptor which is doped into the matrix. The optical density of the sample can be reduced by up to a factor of 5 via irradiation, leading to refractive index changes of about 0.015.