Splashing by laser ablation of a liquid tin layer on a solid substrate: an investigation of substrate geometry effect on thin film splashing and an analysis of secondary droplet commutability between droplet and laser impact
In today’s world the demand for computational capacity is rising. To meet this demand, nanolithography, a method of printing on the nano scale, is rushing to lower wavelength light sources. The lower wavelength of the light allowsfor smaller objects to be printed and computer chips to increase in potential without increasing in size. One of the problems of generating low wavelength light is the availability of the extreme ultra violet (EUV) emitting tin plasma. At ASML a mass limited target is used, this method uses a continuous stream of tiny tin droplets being ablated mid flight by a laser. One of the drawbacks of this method is the instability and short lifetime of the droplet dispenser. To circumvent this problem an alternative light source is investigated. This alternative source uses the concept of a porous substrate with a liquid metal layer on top from liquid metal divertors from fusion.
A liquid tin layer is deposited on a solid tungsten substrate, this liquid is hit with a laser creating a tin plasma emitting low wavelength light. However, after the formation of the plasma, a splash of the liquid is observed. An investigation into the effect of the target geometry and analysis of the secondary droplets is performed in this work. To do this, a Nd:YAG 1064nm wavelength laser with variable laser energy and spot size is used. A target, held in place by a target holder, is impacted with the laser. The resulting splash from laser impact is captured using a CCD camera and a back light, creating a 2D image.
In this work experimentation of laser impact on thin and thick liquid tin films on a tungsten substrate is done. The first part is focused on the effect of the substrate surface roughness and geometry on the splash characteristics. The aim of the first part is to suppress the splash by laser impact. The shape, direction, and size of the resulting crown by plasma expansion of a laser produced plasma is observed to change due to spatial location of the laser impact spot on a target.
A steerability of the splash is seen in narrow deep baths. Spatially, the geometry of the target is changed due to a micro pattern present on the target surface. This geometry effect is observed for thin liquid films. However, no suppression of the splash is observed.
The second part is focused on the secondary droplet formation on thick liquid layers. The aim of the second part is to understand the secondary droplet generation of the laser impact splash and investigate its commutability with the droplet impact splash. The secondary droplets resulting from laser impact are gathered into droplet diameter distri-butions and compared to literature of droplet impact secondary droplet diameter distributions. A relation of We−1/4 for the rim diameter of the laser impact case is found for the secondary droplet diameter. For higher input energy, the droplet distribution is narrow and centered around lower droplet diameters for the initial distribution. For lower in-put energy, the droplet distribution is broad and centered around higher droplet diameters for the initial distribution.
Furthermore, evidence for prompt splashing and delayed splashing mechanisms are found in laser impact splashing by agreeable fitting of a Gauss curve from literature on the distributions in late time stages of the splash. In conclusion, commutability between the droplet and laser impact splashing is strongly suspected.
