Richard M. Ness

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)

Performance and Scaling of a Dense Plasma Focus Light Source for EUV Lithography.

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.

Evaluation of nanocrystalline materials, amorphous metal alloys, and ferrites for magnetic pulse compression applications

A magnetic pulse compressor test stand was developed to evaluate the switching and loss properties of magnetic core materials that included ferrite and several alloys of nickel-iron, Metglas, and the nanocrystalline material Vitroperm. The test stand generated 1−cos(ωt) voltage pulses across the core under evaluation to simulate the magnetic excitation encountered in pulse compressors. Pulse amplitude and repetition rate were user controlled in order to vary the initial time to saturation while keeping magnetic core temperature constant. Switching losses were calculated directly as ∫(νi)dt, and an advanced figure of merit was utilized to compare test results. The test stand, data acquisition and analysis, and test results are discussed.

Progress Toward use of a Dense Plasma Focus as a Light Source for Production EUV Lithography

Recent advances in the Dense Plasma Focus (DPR) under investigation by Cymer as an EUV light source have increased both the total energy storage capacity and the peak drive current. Previous generation DPF tools built by Cymer produced no measurable EUV radiation while employing Xenon as a source gas, but instead employed Lithium vapor due to its higher emission efficiency at low plasma temperatures. With Xenon as a source gas, this generation DPF exhibits efficiency similar to other sources employing direct electrical drive of the Xenon plasma. An emission efficiency of greater than 0.20% has been measured into 2(pi) str and 2% bandwidth centered at 13.5 nm. Other characteristics of this DPF have been measured such as energy stability, spectrum, source size, position stability, and operation at high repetition rates. In addition, the out-of-band radiation in the UV/Vis region (130nm-1300nm) has been measured and found to be only 0.38% of all radiation emitted by this source. Such low out-of-band radiation opens up the possibility of eliminating the Spectral Purity Filter and the problems associated with its use. This source concept does not use a multi-layer dielectric mirror as the first collector, but instead employs a grazing incidence collector that is more tolerant to debris. Progress has been made in the fabrication quality of this optic. Recent measurements of prototypes show that this optic will not degrade the source brightness. Also, a more efficient design has been created that consist of two shells coated with Ruthenium. This design collects and re-images 18.6% of the EUV radiation emitted into 2(pi) str.

Performance characterization for an excimer laser solid-state pulsed power module (SSPPM) after 20B shots

An experiment has been designed to characterize a solid-state pulsed power module (SSPPM) during the initial manufacturing cycle and then repeat the same characterization measurements after the module has gone through several sequences of 10B shots of normal operation in an excimer laser. The goal of such an experiment is to determine what, if any, degradation occurs during these extended periods and to assist in the development of expected module lifetimes that can then be used to estimate the cost of operation of the overall excimer laser. Initial component and subassembly measurements include the capacitance and Q of energy storage capacitors; the inductance and Q of bias, charging, and energy recovery inductors; the B-H characteristics of magnetic cores; insulation breakdown strength; connection resistance; and the general physical appearance of the unit. Operational measurements also compare the efficiency of each pulse compression stage, the repeatability and accuracy of diagnostics, thermal management parameters, and the recovery and on-state characteristics of the silicon-controlled rectifiers (SCRs) and diodes. Each of these items is monitored before testing and after each sequence of 10B shots has been completed. Results of the experiment are described.

Optimization of a dense plasma focus device as a light source for EUV lithography

Since the initial demonstration of EUV emission with Xenon as a source gas in Cymers 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.

IGBT and diode loss measurements in pulsed power operating conditions

High voltage IGBTs and series diodes are used in Cymers solid-state pulsed power commutator module. The IGBTs and series diode losses in the module affect the cooling system design and the energy transfer efficiency. It is difficult to estimate the IGBT and diode losses in the pulsed power operating condition. We have previously tried to measure the IGBT losses by measuring the voltage across the IGBT (using a high voltage differential probe) and the current through the IGBT. Since the voltage across the IGBT changes from the kilo-volt level to several volts, it is very difficult to measure the small on-state voltage accurately. Also very small internal package inductance can obscure the voltage measurement due to the high dl/dt level in the circuit. In this work, the IGBT and series diode losses were measured with a commutator module directly by measuring the water flow rate going through a cold-plate attached to the IGBT or diode and the temperature difference of the inlet and outlet water. Heat transfer through other means, such as free convection, was minimized by sealing the IGBT/diode inside a thermal isolation blanket and polyurethane foam. The loss measurement results on the dual package IGBT and dual package diode are presented in the paper

Low jitter and drift high voltage IGBT gate driver

The Cymer Master Oscillator Power Amplifier (MOPA)-based light source requires tight timing control over all operating repetition rates. The newly developed IGBT gate driver allows tight control of the IGBTs turn on/off timing jitter and drift. Its performance is significantly improved compared to the previous IGBT gate driver. Switching rate improvement of the new driver reduces the time spent in the IGBTs active switching region and therefore helps reduce losses. These improvements allow for better control of IGBT triggering and laser performance. This paper will provide the IGBT gate driver general development approaches, performance and high voltage IGBT switching test results. The new IGBT gate driver meets the requirements for Cymers MOPA-based laser systems.

Timing compensation for an excimer laser solid-state pulsed power module (SSPPM)

For certain applications, it is critical to minimize variations in the (throughput) timing between trigger and the output pulse of a magnetic modulator. A circuit is described that maintains a relatively constant delay over a large operating voltage range (600-1150 V) and temperature range (25 /spl deg/C-65 /spl deg/C) range, The circuit operates by sampling the charging voltage and magnetic switch temperature just prior to the start switch trigger. Those parameters are then used to calculate the appropriate amount of delay to add into the low-level trigger chain to ensure that the delay stays constant over the voltage and temperature operating range. Although other approaches can be conceived and implemented, this particular design is relatively simple and inexpensive and meets the desired performance goals. Data presented show that the ideal correction function is nonlinear in nature and, as a result, simple linear approximations are limited in their ability to minimize the timing variations. Improvements to the original circuit use a multiple, piece-wise, linear approach in order to obtain better performance. The results are that an initial timing variation of almost 3 /spl mu/s has been reduced to a total variation of less than 100 ns.

Evaluation of Nanocrystalline Materials, Amorphous Alloys and Ferrites for Repetitive-Magnetic Pulse Compression Applications

Recent advances in nanocrystalline magnetic materials and core insulation techniques are believed to be superior to the current magnetic cores that are employed as saturable switches in solid-state, repetitive magnetic pulse compressors. Accordingly, a magnetic pulse compressor test stand has been constructed at the University of Missouri-Columbia (UMC) to evaluate the switching properties of candidate magnetic materials and insulation schemes that cover the wide parameter space requisite to magnetic modulators. Experimental measurements were utilized to analyze and compare a wide variety of magnetic materials consisting of nanocrystalline cores, amorphous metal alloys and ferrites. The dependence of the insulating material and the core construction techniques, e.g., type and thickness of the insulation and ferromagnetic material were included in a model along with the magnetic core loss measurements. An advanced figure of merit was utilized to down-select the cores for a particular application based on the measurements and the UMC database. Final test results were analyzed to determine which core material had the best switching properties for a specific operational regime. The test stand, data acquisition equipment and methods, data processing, magnetic core materials under examination and final test results are discussed.