Reticle Enhancement Techniques
Resolution enhancement technologies (RETs) are techniques employed to modify photomasks during the lithographic processes utilized in the fabrication of integrated circuits. These methods are aimed at enhancing the resolution and accuracy of patterning on semiconductor wafers, enabling the production of smaller and more intricate features in line with the shrinking dimensions of modern semiconductor devices
As the critical dimensions of transistors continued to shrink, the fabrication of masks became increasingly challenging. To address issues related to diffraction, various Resolution Enhancement Techniques (RETs) were developed and applied to the masks. One such technique is Optical Proximity Correction (OPC), which aims to optimize the patterns on the mask and enhance their printability on the substrate, as depicted in Figure 1. OPC is employed to compensate for image inaccuracies that arise during subwavelength lithography, where patterns are smaller than the wavelength of the light used. These inaccuracies can involve changes in corner rounding, line-end shortening, and linewidth alterations in isolated or densely packed environments. Complex software is used to apply models for these effects and rectify the design data. In Figure 1, you can see that corner rounding and line-end shortening were corrected through OPC.
Phase Shift Masks (PSMs)
Another category of RET is Phase Shift Masks (PSMs), which play a role in extending the resolution capability and improving the aerial image contrast of lithographic steppers or scanners. PSMs are photomasks that utilize the interference resulting from phase differences to enhance image resolution in lithography. Two main types of phase shift masks exist Alternating Phase Shift Masks and Attenuated Phase Shift Masks. These masks leverage the principle that light passing through transparent media undergoes phase changes based on its optical thickness. This phase manipulation aids in achieving finer patterns and improved lithographic results.
Electron-beam lithography (EBL)
Electron-beam lithography (EBL) involves using a focused electron beam to create custom patterns on an electron-sensitive resist film. By exposing the resist to the electron beam, its solubility is altered, allowing for selective removal of either the exposed or unexposed areas through a solvent immersion process.
The primary advantage of EBL lies in its capability to generate custom patterns with sub-10 nanometer resolution. This direct-write approach offers exceptional precision. However, EBL comes with a drawback in terms of throughput, as the writing process can be time-consuming. This aspect raises questions about its viability as a lithography tool within the context of future technology roadmaps.
One limitation of EBL pertains to small feature patterning, where the number of electrons in the beam must be reduced. This can lead to a notable variation in dose due to shot noise effects.
The new iteration of EBL relies on secondary electrons as the primary beam. High-energy electrons induce secondary electrons by extracting electrons from an atom’s shell. These secondary electrons possess significantly lower energy levels, resulting in fewer beam-related defects.