Understanding EUV Photolithography and EUV Photomask

Since the mid-1980s, EUV lithography has been under development, and it is currently in the pilot phase with 0.33-NA tools at chip manufacturing facilities, with high-volume production planned soon. With increased NA and lowering wavelength, like with conventional optical lithography methods in the past, further expansion to even finer resolutions is predicted.

The 13.5 nm photons are commonly generated by a plasma source in EUV lithography instruments. An optical device called a ‘collector‘ collects EUV light from the plasma. The illumination optics are a collection of shaped optics that guide light from the collector into them. This light lights a photomask on a vacuum stage with a high scan speed. Normal incidence mirrors with multilayer coatings and grazing incidence mirrors make up the lighting optics.

What is a EUV photomask?

A patterned reflecting mask used for EUV photolithography is known as a EUV photomask. The EUV lithography mask is a crucial component of EUV lithography, a viable option for node patterning of 7 nm and smaller. Major semiconductor firms develop and utilize EUV for future device production; a EUV photomask becomes increasingly critical as feature sizes shrink.

A EUV photomask is typically a clear glass sheet on which the pattern to be transferred to the wafer surface is printed. Darkened areas of the photomask prevent penetration of light to the resist below, while transparent areas of the mask allow the resist beneath to react chemically. In the alignment stage of photolithography, the photomask is carefully aligned with the resist coated wafer. In certain commercial applications, the mask is elevated a few nanometers above the surface of the mask to minimize long-term damage from chemicals in the resist. Still, in most laboratory settings, the mask is put directly on the wafer to produce the smallest feature size in the resist pattern.

These masks have only recently begun, introducing the first full-field EUV Litho tool in 2005. Firms are beginning to emerge as the world’s largest commercial EUV provider, thanks to tight coordination with lithography equipment leaders, blank photo suppliers, reticle equipment manufacturers, multinational consortia, and numerous clients.

EUV Photomask Substrates

Before a EUV photolithography coating procedure, state-of-the-art Substrate cleaning in precision optics is critical to its performance. Any pollution of the surface reduces the coating’s adherence and causes flaws. The degree of surface cleanliness cannot be described numerically and can only be proved via appropriate tools.

A complex multi-layer film stack is the basis for a EUV photomask substrate. The films are applied to a customized 6″ by 6″ substrate with ULE/LTEM properties. Because nearly all substances absorb EUV radiation, EUV lithography must be performed in a vacuum. Absorption energy may quickly heat the mask, an engineering consequence of this necessity. The blank’s low thermal expansion quality is essential for preventing the mask from warping or distorting the picture. To enable electrostatic chucking, the backside of the substrate is coated with a conductive coating, usually chromium nitride.