Phase Shift Masks (PSMs)
PSMs are used to extend the resolution capability and the aerial image contrast of the stepper/scanner. While researchers experiment with different types of phase shift masks, one technique has dominated production requirements: Embedded Attenuated PSM. EaPSM is typically used for the Contact and Via layers. It consists of the device pattern imaged in a semi-transparent (6%) MoSiON layer which has a chrome coating at the edge to support mask alignment and protect the peripheral area from mechanical damage. The 180° phase difference between the clear areas and the partially-transmitting MoSiON areas, enhances the edge contrast of the aerial image and improves wafer lithography.
Optical Proximity Correction (OPC)
OPC is a technique used to compensate for image distortions that occur during sub-wavelength lithography: printing structures smaller than the wavelength of light being used. These image distortions typically consist of increased corner rounding, line-end shortening, and changes in line width when located in isolated or dense environments. Lithographers can model these effects and using sophisticated software, apply corrections to the design data to correct for them.
These corrections take the form of localized line width biasing, the addition of serifs to both internal and external corners and the addition of assist bars.
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