A Primer on Reticles and Reduction Reticles

Reticles are basically a specific kind of photomask that features one or many images that will make up the final device. Usually the original design is scaled by a factor of 1x, 2x, 2.5x, 4x, 5x and 10x.  Reticles on the other hand are used in wafer lithography on step and repeat systems where the picture on the reticle is minimized through the system’s lens optics and then exposed onto the wafer substrate a lot of times in order to build up the complete array of devices that is covered on the wafer.

Reduction

According to photolithography specialists the process of reduction allows for smaller features to e integrated on the wafer as opposed to the original reticle. The problem with this however is that any mask error factors such as defects in linewidth and position are also reduced by the same magnification process.  Legacy reticles were created for exposure using G-line (436nm) and I-Line (365 nm) on step and repeat systems.

The increasing demand for tinier feature sizes also increased which also resulted in the need for shorter wavelength sources.  Reticle enhancement methods like the OPC or optical proximity correction approach and the Phase Shift Masks or PSM method assist to print smaller features than the wavelength of light, which in effect extends Moore’s Law into the sub-wavelength lithography.

Optical Proximity Correction

According to precision photolithography experts, is a method that is utilized in order to compensate for distortions that happen during sub-wavelength lithography or the printing of structures that are tinier than the wavelength of light used.

Phase Shift Masks

These are utilized to extend the resolution ability as well as the image contrast of the stepper or scanner. There are a number of different types of phase shift masks used by one method stood out to adhere to specifications required.  EaPSM or embedded attenuated PSM is used for the contact and via layers which consist of the devise pattern being imaged on a semi-transparent layer that features a chrome coating at the edge so as to support mask alignment and provide protection to the peripheral sections from damage.