William N. Partlo

Extreme ultraviolet emission spectra of highly ionized xenon and their comparison with model calculations

Xenon spectra involving emission from ion species of Xe7+ to Xe12+ were analyzed for a plasma focus discharge developed for extreme ultraviolet lithography. Low and higher resolution spectra were recorded in the 8–21 nm wavelength region for different operating conditions and different He–Xe gas mixtures. The spectra have been compared with Hartree–Fock calculations. The modeling included the distribution of the various xenon ion levels at a given electron equilibrium temperature and plasma opacity effects. Spectral analysis showed that the 4d–5p transition arrays are fairly well separated in wavelength for the ions Xe8+ to Xe11+. Good agreement between experiment and calculations was obtained for line positions and intensities, in particular, for the wavelength region at around 13.5 nm.

LPP EUV source development for HVM

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.

EUV (13.5-nm) light generation using a dense plasma focus device

A dense plasma focus (DPF) device has been investigated as a source for EUV lithography. Initial characterizations have been made of a prototype DPF employing an all-solid-state pulse power drive. Using the results from a vacuum grating spectrometer combined with measurements with a silicon photo diode, it has been found that substantial amounts of radiation within the reflectance band of Mo/Si mirrors can be generated using the 13.5 nm emission line of doubly ionized Lithium. This prototype DPF converts 25J of stored electrical energy per pulse into approximately 0.76J of in- band 13.5nm radiation emitted into 4(pi) steradians. The pulse repetition rate performance of this device has been investigated up to its DC power supply limit of 200Hz. No significant reduction in EUV output was found up to this repetition rate. At 200Hz, the measured pulse-to-pulse energy stability was (sigma) equals 6 percent and no drop out pulses were observed. The electrical circuit and operation of this prototype DPF device is presented along with a description of several future modifications intended to improve stability and efficiency.

EUV discharge light source based on a dense plasma focus operated with positive and negative polarity

The application of a dense plasma focus pinch discharge as a light source for extreme ultraviolet (EUV) lithography is discussed. For operation with xenon gas, the radiation emitted at around 13.5 nm is analysed with temporal, spectral or spatial resolution. We describe and compare the operating characteristics and plasma dynamics of the device when energized at positive and negative polarity of the charging voltage. The thermal load distribution, heat deposition and wear of the electrodes are measured and compared for both configurations. High-repetition rate burst mode data show characteristic transients. Source size data are also obtained when tin powder is used as the target element. More favourable performance characteristics were generally obtained for operation of the pinch discharge at negative polarity. (Some figures in this article are in colour only in the electronic version)

Development of an EUV (13.5 nm) light source employing a dense plasma focus in lithium vapor

Initial characterization efforts of Dense Plasma Focus (DPF) technology showed that efficient conversion of electrical energy into in-band emitted radiation could be achieved. Results previously reported showed that 25 J of electrical energy can be converted into 0.38 J of in-band, 13.5 nm radiation emitted into 2 (pi) steradians. This prototype configuration demonstrated a 1.5% conversion efficiency into 2 (pi) steradians, but exhibited several major drawbacks. The two greatest issues were excessively high stored energy per pulse and poor stability of the plasma size and position. Such high input energies would limit the maximum pulse repetition rate and poor plasma stability would lead to excessive electrode erosion and large integrated source size. Recent efforts have concentrated on reducing the required input energy while simultaneously improving stability. The result of these efforts is a DPF system that exhibits table operation with as little as 1.5 J of input energy and has demonstrated pulse repetition rates as high as 2500 Hz. Once a stable, low input energy DPF was achieved, this prototype DPF device was fitted with a simple Lithium vapor delivery system. Pinhole camera images of the Lithium vapor source show that it is stable with a size of less than 350 micrometer FWHM. In this technologys present state, the potential in-band collectable EUV optical power is estimated to be 6.9 W.

LPP source system development for HVM

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.

Laser produced plasma light source for EUVL

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.

Low cost of ownership KrF excimer laser using a novel pulse power and chamber configuration

A KrF excimer laser using an all solid state Pulse Power Modulator (PPM) has been studied. This PPM configuration replaces the commonly used thyratron switch with a Silicon Controlled Rectifier (SCR) switch combined with a pulse compression-voltage multiplication circuit. Use of this PPM has extended the useful chamber life of a line-narrowed KrF excimer laser from 1.5 billion to 2.5 billion pulses. Broadband KrF laser performance, optimized for mirror based scanner systems, has also been investigated. A minimum broadband chamber life of 5 billion pulses has been demonstrated with this solid state PPM. While a thyratron-based PPM exhibits an expected lifetime of 3 billion pulses, the solid state PPM used in these experiments has been operated for greater than 6 billion pulses without any decrease in performance. Since 72% of the replacement parts cost for the ELS-4000D line-narrowed excimer laser is due to periodic chamber and PPM replacement, significant cost of ownership reduction is realized by extending the lifetime of the chamber and the PPM.

Performance results of laser-produced plasma test and prototype light sources for EUV lithography

Improved performance and specific results are reported for several test and prototype extreme ultraviolet EUV light sources devel- oped 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 perfor- mance 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 mea- surements with ion energy analyzer and Faraday cups reveal the contri- butions 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.

Direct aerial image measurement as a method of testing high numerical aperture microlithographic lenses

A method for testing high numerical aperture (NA) microlithographic lenses using direct aerial image measurements has been developed. By measuring contrast versus focus for dense line/space patterns, this system can quantify the amount of astigmatism, distortion, field curvature, and spherical aberration produced by a lens under test. Experimental results show high correlation with more traditional lens testing methods such as transmissive interferometry and evaluating exposed resist patterns. The lens used in these experiments is an 0.53 NA, 248 nm objective designed for imaging 0.35 μm features over a 31 mm diam field.