Design of a coupling interface for the collector mirror of an extreme ultraviolet lithography machine (EUVL). The coupling should render high repeatability and interchangeability, as the optic is to be removed periodically from the machine for cleaning.
Traditional approaches such as three spheres resting in vee-grooves do not achieve the desired level of repeatability due to the high loads (force disturbances and thermal) the system is subject to. Under these conditions problems such as frictional hysteresis arise. Therefore, a more robust solution is required.
The optical component for which the interface is being designed is the collector mirror. This is an elliptically shaped optic located at the source of the machine. The source is where the EUVL process starts. Here, a Tin (Sn) droplet is laser-pulsed into plasma state. This will provide an ultraviolet (UV) light source with a wavelength in the order of nanometers. The resulting UV light is then reflected in an elliptical shaped mirror known as the collector mirror. This redirects the light into the optical circuit of the machine.
After the UV light leaves the source it is redirected into a photomask. This is an oversized stencil of the integrated system to replicate. Several optical components shrink the pattern into the desired size, and project it over a wafer covered in light-sensitive photoresist. This way the pattern in the photomask is imprinted into the wafer.
Proper alignment of the source mirror is necessary so that the optical path is not distorted.
Here the main components that compose the source assembly are shown. The CO2 laser is generated beneath the collector, and by means of reflective optics is targeted into the primary focus of the collector mirror.
The droplet generator provides the system with Sn droplets at high frequency. Both the laser and the Tin meet at the primary focus and the UV light is generated. A camera assures the synchronization between the input of droplets and the laser pulsing.
The UV light is reflected in the collector mirror and redirected into the rest of the optical path.
Precision components must stay stationary during operation, usually the allowable displacement range is within the microns range. Additionally, a coupling interface that allows for excellent positioning of the component during the assembly is required.
To this end kinematic couplings are used. The most well know configuration consist of 3 semi-spheres attached to the component that rest on three v-grooves. This generates six points of contact that constrain the component in six degrees of freedom.
The first part of the assignment consisted in a feasibility study. The repeatability of the mentioned configuration was evaluated. To this end, a tribology study that involved a thorough study of contact mechanics, friction forces and their effect of micro displacements under disturbance forces was conducted. The results showed that the mentioned coupling interface does not render the desired levels of repeatability. Micro-displacements under frictional hysteresis would offset the optical component.
The second part of the assignment consisted on designing a coupling that would ensures high stability and repeatability of the optic. The results are under a non-disclosure agreement, and therefore no further information can be provided. This project was a collaboration between ASML, VDL Enabling technologies and Eindhoven University of Technology.
This is how the vessel looks from outside. This model is lacking plenty of the modules that are attached to it.
This assignment is under a non-disclosure agreement, and therefore no further information can be provided. This project is a collaboration between ASML, VDL Enabling Technologies and Eindhoven University of Technology.