What is RET?

Resolution enhancement technologies (RET) are techniques used in the field of photolithography to improve the resolution and accuracy of printed images on silicon wafers.

The resolution of photolithography is determined by the wavelength of the light used to expose the photoresist. As the wavelength of light used becomes shorter, the resolution of the photolithography process improves. However, shorter wavelengths also lead to more diffraction and scattering effects, which can reduce the accuracy of the printed image. RET techniques aim to overcome these limitations and improve the resolution without sacrificing accuracy.

 

Some examples of RET techniques include:

Off-axis illumination is a technique used in photolithography to improve the resolution of printed images. In traditional photolithography, the light used to expose the photoresist is directed perpendicular to the surface of the mask, resulting in a diffraction-limited image. However, by using off-axis illumination, the angle of incidence of the light can be adjusted, which changes the diffraction pattern and improves the resolution of the printed image. Off-axis illumination can be achieved using various types of illumination systems, such as annular, quadrupole, or dipole illumination. Each of these systems has a different shape and orientation of the light source, which can be used to achieve different types of resolution enhancement.

Optical proximity correction (OPC): This technique involves modifying the layout of the mask to compensate for the diffraction and scattering effects caused by the wavelength of light used. By making small adjustments to the layout of the mask, the printed image can be improved.

Phase-shifting masks (PSM): This technique involves using a mask that has regions of different optical phases. The interference between the waves passing through these regions can improve the resolution of the printed image.

High-index immersion lithography: This technique involves using a liquid with a higher refractive index than air between the lens and the photoresist. This increases the resolution of the printed image by allowing the lens to focus the light more tightly.

These are just a few examples of the many RET techniques that are used in photolithography to improve the resolution and accuracy of printed images.