EUV Laboratories

Ambient temperature droplet targets have recently been deployed for laser plasma generation. Droplets aerosols are sub-millimeter mobiles that transport heavy metals to a target for plasma generation. When the droplets are radiated with high-powered lasers, they generate plasma that carries the potential to generate EUV. The average size of the droplet is 30microns. The major carrier for the aerosols consists of water or alcohol with pH buffers. The droplets may also contain nano-particle.

The main reason for choosing this type of target system is that it allows for a mass-limited targeting system and less generation of debris. The first real world application for this targeting system was to generate EUV for semiconductor lithography. Since the aerosols are composed mostly of low-Z solution, the concentration of ions for the heavy metals can be adjusted; this benefit cannot be generated with solid targets.

Setbacks produced by the droplet system are satellite generation and unstable droplet creation (Figure 1). These setbacks can be combated by selection correct amplitudes and frequencies that control the nozzle’s piezo for chopping the sample stream for droplet formation.

The aerosol droplets are created by forcing the droplet solution through a vibrating capillary.

It is important to fire the laser at the exact point where the beam hits the droplet in the center. The system is controlled by electronics that match the phase of the droplets and the laser pulses. Because of the setbacks of satellite formation, it is also important to image the droplets as well. The droplets are very small and travel very fast. Therefore, multiple droplets are imaged at the same time. The image is created by pulsing a high-intensity LED. This LED must also be synced with the shooting of the laser and the pulse rate of the chopper in order to create usable images.

Future development of these techniques for droplet formations will be used to create new sources for spectroscopy and to characterize the interactions with a femto-second laser pulse.