High-Volume EUV Lithography Sources Dayton OH

provided by: Semiconductor International

Extreme ultraviolet (EUV) lithography is currently the leading option to replace ArF immersion lithography for high-volume manufacturing below the 32 nm node. Development tools will be required as early as 2009 to meet the timeline for introduction of EUV lithography at these nodes. The critical challenges for the technology are source power and lifetime, resist resolution and sensitivity, and photomask defect density and protection.

High source power is required to support high wafer throughput and meet cost-of-ownership targets. The requirements for source power have increased over time as it has become clear that high resist dose is needed to simultaneously meet resolution and linewidth-roughness (LWR) targets. It is estimated that EUV power of >115 W is required for 5 mJ/cm2 photoresist speed to enable >100 wph scanner throughput, and 180 W is needed for 10 mJ/cm2. Photoresist sensitivities above 20 mJ/cm2 could drive power requirements well above the 200 W level.

It is clear that a scalable EUV source architecture is needed to enable the evolution of EUV lithography during the lifecycle of the technology. Although experimental EUV exposure tools currently use low-power discharge-produced plasma (DPP) sources, it is unlikely that these will allow scaling to the power levels needed for high-volume manufacturing. The laser-produced plasma (LPP) source is the most promising technology to deliver the high power output needed for the production of ICs in the post-193 nm immersion era.

LPP sources generate EUV (13.5 nm) radiation by depositing laser energy into a source element, such as xenon, tin or lithium, creating a highly ionized plasma. The energetic radiation generated during de-excitation and recombination of these ions is emitted in all directions, but collected and focused to an intermediate focus (IF) point at the entrance to the scanner optical system.

Radiation is emitted over a wide range of wavelengths, but high conversion efficiency (CE) of the laser energy into in-band EUV energy is critical to meeting the required power levels. Studies of CE for several combinations of laser wavelength and source element have shown that the combination of CO2 laser radiation and tin droplet targets provides the optimum efficiency. A production-worthy source requires development and integration of several technologies, such as the CO2 laser, tin droplet generator, collection optics and debris mitigation technology, and progress in these areas is described below.

Laser

We use a CO2 laser in an RF-pumped master oscillator power amplifier (MOPA) configuration with multiple stages of amplification. The seed pulse is initiated by a Q-switched master oscillator with low energy and high repetition rate, capable of 100 kHz operation. The laser pulse is amplified, shaped and focused before entering the LPP chamber. The laser pulses are timed to impinge on an incoming liquid tin droplet stream flowing from a target delivery system at a repetition rate comparable to or larger than the laser. Steering is provided to an exact point in the vacuum chamber where the droplet is irradiated by the laser beam. The peak emission of the plasma occurs at 13.5 nm, and matches well to the reflectivity curve of multilayer-coated mirrors designed for this wavelength. In the EUV region, the coating acts as a spectral purity filter, reflecting light mainly near 13.5 nm.

The size of the EUV emitting region of the micro plasma has been measured using a pinhole camera. The plasma has overall cylindrical symmetry with respect to the optical axis with on-average ellipsoidal, near-spherical shape. This results in a small value of etendue (

0.1 mm2 sr), which can simplify the design of optical illumination devices, thus enabling the implementation of high numerical aperture (NA) and high-throughput power systems. This is a significant advantage of LPP sources over DPP sources, which typically have much larger etendue.

Droplet generator

The main requirements for the droplet generator are the capability to produce tin droplets at a controllable frequency with high droplet uniformity and stability, and to provide reliable operation over long periods of time. We currently have a third-generation version of droplet generator in operation. Stable droplets have been obtained over a range of frequencies of 20?500 kHz, with corresponding droplet diameters of 150?20 ?m.

Droplets at the minimum size contain only

1014 atoms, approaching the condition of the minimal mass required for efficient EUV generation and minimal debris generation. The total tin consumption in this high-frequency regime of operation is estimated to be only 120 mL/day. When equipped with a small-diameter nozzle, the droplet generator can produce stable tin droplets over extended periods of time. So far, 55 hours of continuous generator operation have been demonstrated, limited only by the currently installed volume of tin supply. The next-generation droplet generator will have continuous operation capability.

The stability of the droplets produced by the droplet generator is determined by the nozzle design, the parameters of the droplet generator (e.g., applied pressure and actuator driving voltage) and the overall mechanical stability of the system. Using a new nozzle design, substantial progress has recently been achieved with respect to the timing jitter of the droplets. A timing jitter of 25 nsec (0.2% of the droplet period) has been demonstrated without using any active stabilization techniques. At the plasma location, this corresponds to a spatial uncertainty of the droplet along the jet trajectory of ±0.6 ?m. The short-term positional (transverse) stability of the droplets is on the order of 5 ?m, whereas the long-term (

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