Construction of a High Frequency and High Reflectance Shutter for a Direct Write EUV Lithography System

Abstract

Extreme ultraviolet lithography (EUVL) is widely seen as a key technology for future semiconductor mass production. However, due to the short wavelength material properties of EUV, it is strongly absorbed by most materials. Thus if the shutter for a lithography system operates by means of absorption, one must consider the potential temperature rise due to the high energy radiation absorbed by the structure. In this paper, we propose using a high-reflectance shutter so as to resolve temperature-related precision problems in lithography systems. A single-layer molybdenum film is used to greatly reduce the quantity of absorbed radiation energy by the shutter structure (in line with Fresnel equation) by increasing the incidence angle. A green laser is used as the light source to construct an automatic measuring system for reflectance and transmittance to verify the increase of material reflectance by the incidence angle of the photosource. The obtained incidence angle is also be fixed on the multilayered piezoelectric to serve as an actuator, so as to measure the high-frequency echoed signal of the laser photosource. Results show that, when the incidence angle is 83°, the optimum energy reflectance (50%) is obtained and the switching frequency reaches a maximum of 19 kHz, verifying the feasibility of using the reflected energy as the photosource switch. Finally, experiments were conducted in Taiwan’s National Synchrotron Radiation Research Center (NSRRC) using EUV as the photosource to measure the reflectance curves of single-layer molybdenum and aluminum films with different thicknesses under different incidence angles. Experimental results show that a high degree of reflection can be produced by the proposed single-layer film structure given a large incidence angle. The reflectance also increased significantly at an incidence angle of 60° for molybdenum while 70° for aluminum, and this relatively high reflection by molybdenum with a smaller incidence angle can be used to facilitate lithography system construction.

Keywords

• EUVL, shutter, reflectance, angle of perfect reflection